EP4314530A1

EP4314530A1 - A piston for an engine

Info

Publication number: EP4314530A1
Application number: EP22717633.6A
Authority: EP
Inventors: Lyn Mcwilliam; Jack Johnson; Simon DRINKWATER; Christopher Harris
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2021-03-26
Filing date: 2022-03-25
Publication date: 2024-02-07
Also published as: WO2022200611A1

Abstract

A piston (54) for an engine comprising a cylinder (57), an air inlet (91a) and an exhaust outlet (92a), wherein the air inlet (91a) and the exhaust outlet (92a) are arranged about a longitudinal axis (60) of the cylinder (57). The piston (54) comprises a circular peripheral wall (141) having a central axis (142) which is aligned with the longitudinal axis (60) of the cylinder (57). The piston (54) has working surface (79) which comprises a central channel (140) that extends across the working surface (79) in a direction perpendicular to the central axis (142). The channel (140) has two ends (148a, 148b) located on opposite sides of the channel (140). Opposing sides of the channel each comprise a side wall (151, 152) which extend from a base (153) of the channel (140) to a respective side edge (149, 150) of the channel (140). The channel (140) is configured to promote tumble of air flow into the cylinder (57) from the air inlet (91a), in use during an intake stroke of the piston (54).

Description

A PISTON FOR AN ENGINE

TECHNICAL FIELD

The present disclosure relates to a piston for an internal combustion engine and, in particular, to a piston for a lean-burn gasoline engine, to a lean-burn gasoline engine and to a vehicle with such an engine.

BACKGROUND

In classic internal combustion engines, gasoline burns best when it is mixed with air in proportions of around 14.7:1 (lambda = 1) depending on the particular type of fuel. Most modern gasoline engines used in vehicles tend to operate at or near this so-called stoichiometric point for most of the time. Ideally, when burning fuel in an engine, only carbon dioxide (C02) and water (H20) are produced. In practice, the exhaust gas of an internal combustion engine also comprises significant amounts of carbon monoxide (CO), nitrogen oxides (NO ) and unburned hydrocarbons. It is desirable to increase fuel efficiency and reduce unwanted emissions. One possible route for increasing fuel efficiency is to burn the fuel with an excess of air. Burning fuel in such an oxygen-rich environment is usually called lean-burning. Typical lean-burn engines may mix air and fuel in proportions of, for example, 20:1 (lambda > 1.3) or even 30:1 (lambda > 2). Advantages of lean-burn engines include, for example, that they produce lower levels of C02 and hydrocarbon emissions by better combustion control and more complete fuel burning inside the engine cylinders. The engines designed for lean burning can employ higher compression ratios and thus provide more efficient fuel use and lower exhaust hydrocarbon emissions than conventional gasoline engines. Additionally, lean-burn modes help to reduce throttling losses, which originate from the extra work that is required for pumping air through a partially closed throttle. When using more air to burn the fuel, the throttle can be kept more open when the demand for engine power is reduced. Lean burning of fuel does, however, also come with some technical challenges that have to be overcome by providing an engine that is suitable and optimised for efficiently burning hydrocarbons in an oxygen-rich environment. For example, if the mixture is too lean, the engine may fail to combust. Especially at low loads and engine speeds, reduced flammability may affect the stability of the combustion process and introduce problems with engine misfire. Further, a lower fuel concentration leads to less output. Because of such disadvantages, lean burn is currently only used for part of the engine map and most lean-burning modern engines, for example, tend to cruise and coast at or near the stoichiometric point. In order to enable the lean burning of fuel over a larger portion of the engine map, the engine needs to be designed in such a way to enable a large air flow into the combustion chamber and to ensure a reliable combustion process that will effectively burn all fuel, despite the oxygen rich conditions.

It is an aim of the present invention to overcome or at least partially mitigate at least one of the above-mentioned problems associated with conventional combustion engines and to provide an improved engine and, in particular to provide an improved lean-burn engine.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide a piston for an engine, a cylinder head for an engine, an engine, and a vehicle with such an engine. The engine may be suitable for use with fuels including gasoline, diesel, hydrogen, LPG or any other suitable combustible fuel. The engine may be a lean-burn engine.

According to an aspect of the present invention there is provided a piston for an engine comprising a cylinder, an air inlet and an exhaust outlet, wherein the air inlet and the exhaust outlet are arranged about a longitudinal axis of the cylinder, the piston arranged to operate in the cylinder, the piston comprising: a circular peripheral wall having a central axis, wherein the peripheral wall is configured so that the central axis is substantially aligned with the longitudinal axis of the cylinder in use; and a working surface comprising a central channel extending across the working surface perpendicular to the central axis and having two ends each located on opposite sides of the central axis, wherein opposing sides of the channel each comprise a side wall which extend from a base of the channel to a respective side edge of the channel, wherein the opposing side edges of the channel are separated by the two ends of the channel, wherein the channel is configured to promote tumble of air flow into the cylinder from the air inlet, in use during an intake stroke of the piston.

The tumble promoting piston described above is advantageous as increased tumble in the air flowing into the cylinder during the intake stroke of the piston, and during the first portion of the compression stroke. This improves the homogeneity of the air/fuel mixture leading to a more complete combustion of the fuel and consequently improved efficiency of the engine.

Optionally the width of the channel varies along the length of the channel. This helps to contain the tumble motion in the centre of the chamber so that when the flow breaks down into turbulence, it is centred around the spark plug and fuel injector.

The base of the channel is optionally substantially flat for ease of manufacture with minimal impact on tumble performance. The width of the base of the channel may vary along the length of the channel.

In one example the depth of the channel varies along the length of the channel. As above, this helps to contain the tumble motion in the centre of the chamber so that when the flow breaks down into turbulence, it is centred around the spark plug and fuel injector. For example, the surface profile of the channel conforms to at least part of the surface of a three-dimensional elongated ellipsoid.

The channel is optionally asymmetrical about a longitudinal centreline of the channel extending between the two ends of the channel. Or the longitudinal centreline may be laterally offset from a parallel centreline of the circular peripheral wall of the piston.

In one example one of the side walls of the channel is steeper than the other side wall of the channel. It is beneficial to tune the shape of the channel so that air flow down the cylinder wall towards the piston is efficiently “caught” and airflow up the wall of the cylinder is efficiently “launched” back up the cylinder. Optionally at least one of the side walls is curved to promote the tumble effect.

At least a part of the side edge on a first side of the channel is optionally at a different height to at least a part of the side edge on a second side of the channel relative to a plane perpendicular to the central axis, which plane intersects the base of the central channel. Again, it is beneficial to tune the shape of the channel so that air flow is efficiently “caught” and “launched”.

The side edge of the channel on the first side of the piston may be higher than the side edge of the channel on the second side of the piston along at least part of the length of the channel.

In one example the central channel is configured to direct air flow towards a mid-point of the portion of the cylinder located above the piston when the position is located substantially at bottom dead centre in use. This maximises the tumble vortex and limits “dead zones” where there might be poor air/fuel mixing. Optionally the working surface of the piston comprises depressions for accommodating valve heads of the engine in use when the piston is at or near top dead centre to prevent contact between the valves and the piston.

The working surface optionally comprises sloped surface portions located radially outward of the channel with respect to the circular peripheral wall of the piston, wherein each sloped surface portion extends away from a side edge of the channel downwardly towards the peripheral wall of the piston.

The piston may comprise a spark bowl located in the base of the channel. In another aspect the present invention provides an engine comprising a piston as described above. Optionally the sloped surface portions of the working surface of the piston are configured to conform to at least part of a roof surface of a combustion chamber of the engine in use. This promotes direction of the air and fuel mixture into the central portion of the combustion chamber, and towards the spark plug, as the piston approaches the sloped surface portions of the combustion chamber roof. This has been found to promote efficient burn of the air fuel mixture.

In a further aspect the present invention provides an engine comprising a piston as described above, comprising a cylinder head having a combustion chamber formed therein, wherein at least part of the roof of the combustion chamber is configured to conform to the sloped surface portions of the working surface of the piston in use.ln a still further aspect the present invention provides a vehicle comprising an engine as described above.

Aspects and embodiments of the invention provide an air intake port for a lean-burn engine, a lean-burn engine and a vehicle with such an engine. The lean-burn engine may be suitable for use with gasoline as described herein. Alternatively or in addition thereto it will be appreciated that the lean-burn engine may be suitable for use with other fuels, such as hydrogen, for example. Aspects and embodiments of the invention are defined in the context of lean-burn gasoline but it will be appreciated that the fuel type can be substituted.

According to an aspect of the present invention there is provided an air intake port for an engine, such as lean-burn gasoline engine, the air intake port comprising an air inlet, two air outlets, and an air channel connecting the air inlet to the two air outlets. The air channel comprises an upstream common duct and two downstream port legs, the two downstream port legs branching off from the common duct at a bifurcation point. The upstream common duct having a duct floor and a duct ceiling, each comprising a sloped portion arranged to converge on the bifurcation point, wherein a bifurcation angle of less than 90 degrees is formed between the sloped portions of the duct floor and the duct ceiling at the bifurcation point. The terms upstream and downstream are herein used to refer to parts of the air intake port relative to flow of air through the air intake port in its normal use with a lean-burn gasoline engine. The predominant air flow direction is from an upstream position to a downstream position. It follows that in normal use the engine is downstream of the air intake port.

In previously known air intake ports the duct floor and duct ceiling remain largely in parallel up to the bifurcation point, where they are joined by either a straight wall, or by a straight wall with slightly rounded edges at the interface between the straight wall and the duct floor or duct ceiling. In such air intake ports, the bifurcation angle can be considered to be approximately 180 degrees. The inventors have observed that such traditional bifurcation points may lead to a disturbance of the air flow and a reduced flow coefficient. In addition thereto, the inventors have found that the flow coefficient can be increased and flow disturbance minimised by using the sharper bifurcation angle of the present invention. This advantage is especially relevant for a lean-burn gasoline engine which uses higher volumes of intake air per piston stroke. Furthermore, it has been found that the reduced flow disturbance contributes to the controllability of the ignition process inside the combustion chamber, which again is an important advantage, especially for lean-burn gasoline engines.

For the purpose of the present invention, the bifurcation angle is defined as the angle between the duct floor and the duct ceiling at the bifurcation point. Preferably the duct floor and the duct ceiling make an acute angle at this point in order to minimise the flow disturbance and to maximise the advantageous technical effects of the new design. In the event that, for ease of manufacturing or for any other reason, the angle between the two merging surfaces is less sharp and somewhat rounded, then the angle between the duct floor and the duct ceiling may be defined by the angle they make at a position of 5 mm in front of the bifurcation point.

In preferred embodiments, the bifurcation angle is less than 75, 55, or even 45 degrees. In general, smaller angles require a longer transition zone. While all bifurcation angles smaller than 90 degrees will provide at least some of the technical benefits of the current invention, the optimal bifurcation angle may depend on the exact design of the air intake port. Transition zone’ is herein to be interpreted as the zone at the downstream end of the upstream common duct, wherein the sloped portions of the duct floor and the duct ceiling are situated. The transition zone thus starts where one of the sloped portions starts and ends at the bifurcation point. In an embodiment of the invention, a port leg length measured from the bifurcation point to one of the two air outlets is at least twice a diameter of the respective air outlet. Advantageously, due to a smooth adjustment of the flow direction, this provides a smaller disturbance to the air flow at the bifurcation than the disturbance that would be seen with a shorter port leg length. Put another way, longer port legs allow for a smoother adjustment of the flow direction. The inventors have found that when the port legs have a length of at least twice the air outlet diameter, the disturbance is sufficiently low to not have a significant detrimental effect on the performance of the lean-burn gasoline engine. This positive effect on the reduction of air flow disturbance adds to the air flow improvement already provided by the sharp bifurcation angle according to the invention. Without this sharp bifurcation angle, longer port legs (e.g. at least three or four times the air outlet diameter) might have been needed to avoid excessive disturbance of the flow.

To increase the speed with which the air flows through the air intake port and the total volume of air that can be taken in, a total cross section of the air channel may gradually decrease between the air inlet and the two air outlets. As a further measure for not disturbing the air flow, a gradient of decrease of the cross section may be locally reduced in a region immediately upstream and/or downstream of the bifurcation point.

Known air intake ports are seen to have a decreasing cross section profile in order to accelerate the air flowing towards the combustion chamber and thereby increasing the total volume of air drawn through the air intake port. However, such air intake ports are generally designed such that the cross section decreases with a constant or approximately constant gradient of decrease. The present invention discloses that this common approach is not preferable for use with a lean-burn gasoline engine, which requires a relatively high air intake volume and may be more dependent on a precise control of the direction of flow of the intake air at the point where it enters the combustion chamber. By introducing a local reduction of the gradient of decrease of the total cross section in the region around the bifurcation point, any possible disturbance of the air flow caused by the splitting and deflecting of the air flow is minimised. Preferably, the local reduction of the gradient of decrease of the total cross section is realised in the region immediately upstream and downstream of the bifurcation point, but the desired flow enhancing effect is at least partly achieved when reducing the gradient of decrease at only one of the upstream and downstream sides of the bifurcation point.

The air channel has an average gradient of decrease of the total cross section along the length of the air channel. The optimal average gradient will usually be a compromise between different design considerations. One possible constraint is the desired maximum speed of the air flow at the entrance, or inlet, of the combustion chamber. Too high speeds may lead to excessive NVH (noise, vibration, and harshness) problems and to choking of the port flow. Cylinder size and space/packaging constraints may define the maximum cross section of the air outlets of the air intake port. Given a maximum cross section and air flow speed at the outlet, a preferable average gradient of decrease of the total cross section can be established. Further constraints on the length and width of the air intake port may also play a role when determining the preferable. In preferred embodiments, the gradient of decrease of the total cross section may, for example, be locally at least 15% or 20% below the average gradient of decrease in at least a portion of the region adjacent the bifurcation point. In other embodiments, the gradient of decrease at that position may even be more than 25%, 30%, 35%, 40%, 45%, or 50% below the average gradient of decrease of the total cross section.

In some embodiments, the gradient of decrease of the total cross section may locally be lower than or equal to zero in at least a portion of the region immediately upstream and/or downstream of the bifurcation point. This means that the cross section may even increase in the area around the bifurcation point in order to ensure an undisturbed air flow, even though the outlet cross section of the air intake port is significantly smaller than its inlet cross section. According to another aspect of the invention a lean-burn gasoline engine is provided comprising at least one air intake port as described above. While the air intake port described herein is primarily designed for use with combustion chamber having a dual intake, it could be used to serve two single intake combustion chambers too. According to another aspect of the invention, a vehicle is provided comprising a lean-burn gasoline engine with an air intake port as described above. Aspects and embodiments of the invention provide an air intake port for a lean-burn engine, a lean-burn engine and a vehicle with such an engine. The lean-burn engine may be suitable for use with gasoline as described herein. Alternatively or in addition thereto it will be appreciated that the lean-burn engine may be suitable for use with other fuels, such as hydrogen, for example. Aspects and embodiments of the invention are defined in the context of lean-burn gasoline but it will be appreciated that the fuel type can be substituted.

According to an aspect of the present invention there is provided an air intake port for an engine, the air intake port comprising an air inlet, two air outlets, and an air channel connecting the air inlet to the two air outlets and comprising an upstream common duct and two downstream port legs, the two downstream port legs branching off from the common duct at a bifurcation point. The two port legs diverge from the bifurcation point and are shaped to be parallel or converge proximal to the two air outlets. The terms upstream and downstream are herein used to refer to parts of the air intake port relative to flow of air through the air intake port in its normal use with a lean-burn gasoline engine. The predominant air flow direction is from an upstream position to a downstream position. It follows that in normal use the engine is downstream of the air intake port.

In the prior art, as well as in the air intake port according to the invention, the two port legs diverge when branching off from the common duct at the bifurcation point. In the prior art, the air flow of the air entering the combustion chamber is commonly directed outward, toward the circular wall of that combustion chamber. When approaching the far end of the combustion chamber relative to the position of the entering air, the two originally divergent air flow streams are deflected inward toward the centre of the combustion chamber and then backward toward the position of the entering air, thereby resulting in a swirl pattern that is commonly called omega swirl.

With the air intake port according to the invention, however, the direction of the omega swirl is reversed. By delivering the intake air to the combustion chamber through two port legs that do not diverge towards the air outlets of the air intake port and into the combustion chamber, the air flow of the air entering the combustion chamber will first be directed down the centre of the chamber and then splits to move outward before returning. This reverses the omega swirl compared to the prior art. The inventors have found that by reversing the omega swirl it is ensured that a larger part of the combustion will take place closer to the centre of the combustion chamber, with a small push towards the exhaust valves. As a result, this leaves the unburnt end gas under the cooler intake valves. This helps to reduce knock and thus to increase the performance and durability of the engine.

Each one of the two port legs may be defined as having a respective centre line. A tangent to the centre line of one of the two port legs at its respective air outlet makes a port exit angle with a tangent to the centre line of the other one of the two port legs at its respective air outlet. When the two port legs run in parallel when reaching the two air outlets of the air intake port, the port exit angle is 0 (zero). For converging port legs, the port exit angle is greater than 0. In exemplary embodiments of the invention, the port exit angle is larger than 5 degrees. In further embodiments, the port exit angle may be larger than 10 or 15 degrees.

In an embodiment of the air intake port according to the invention, a port leg length measured from the bifurcation point to one of the two air outlets is at least twice a diameter of the respective air outlet. Advantageously, due to a smooth adjustment of the flow direction, this provides a smaller disturbance to the air flow at the bifurcation than the disturbance that would be seen with a shorter port leg length. Put another way, longer port legs allow for a smoother adjustment of the flow direction. The present invention discloses that when the port legs each have a length of at least twice the respective air outlet diameter, the disturbance is sufficiently low to not have a significant detrimental effect on the performance of the lean-burn gasoline engine. This positive effect on the reduction of air flow disturbance adds to the air flow improvement already provided by the parallel or convergent course of the port legs when approaching the air outlets of the air intake port. Reduced air flow disturbance thereby further allow for increased control and predictability of the swirl pattern inside the combustion chamber. According to a further aspect of the invention, a lean-burn gasoline engine is provided comprising at least one air intake port as described above and a combustion chamber with two air inlets, the two air outlets of the air intake port being connected to the two air inlets of the combustion chamber.

According to another aspect of the invention, a vehicle is provided comprising a lean-burn gasoline engine with an air intake port as described above.

According to an aspect of the present invention there is provided an air intake port for an engine, the air intake port comprising an air inlet, two air outlets, and an air channel connecting the air inlet to the two air outlets and comprising an upstream common duct and two downstream port legs, the two downstream port legs branching off from the common duct at a bifurcation point. The terms upstream and downstream are herein used to refer to parts of the air intake port relative to flow of air through the air intake port in its normal use with a lean-burn gasoline engine. The predominant air flow direction is from an upstream position to a downstream position. It follows that in normal use the engine is downstream of the air intake port. A total cross section of the air intake port gradually decreases between the air inlet and the two air outlets. A gradient of decrease of the total cross section is locally reduced in a region adjacent the bifurcation point.

Known air intake ports are seen to have a decreasing cross section profile in order to accelerate the air while flowing towards the combustion chamber and thereby increasing the total volume of air drawn through the air intake port. However, such air intake ports are generally designed such that the cross section decreases with a constant or approximately constant gradient of decrease. The inventors have found that this common approach is not optimal for use with a lean-burn gasoline engine, which requires a relatively high air intake volume and may be more dependent on a precise control of the direction of flow of the intake air at the point where it enters the combustion chamber. By introducing a local reduction of the gradient of decrease of the total cross section in the region around the bifurcation point, any possible disturbance of the air flow caused by the splitting and deflecting of the air flow is minimised. Preferably, the local reduction of the gradient of decrease of the total cross section is realised in the region immediately upstream and downstream of the bifurcation point, but the desired flow enhancing effect is at least partly achieved when reducing the gradient of decrease at only one side of the bifurcation point.

The air channel has an average gradient of decrease of the total cross section along the length of the air channel. The optimal average gradient will usually be a compromise between different design considerations. One possible constraint is the desired maximum speed of the air flow at the entrance of the combustion chamber. Too high speeds may lead to excessive NVH (noise, vibration, and harshness) problems and to choking of the port flow. Cylinder size and space constraints may define the maximum cross section of the air outlets of the air intake port. Given a maximum cross section and air flow speed at the outlet, an optimum average gradient of decrease of the total cross section can be established. Further constraints on the length and width of the air intake port may also play a role when determining the optimum. In preferred embodiments, the gradient of decrease of the total cross section may, for example, be locally at least 15% or 20% below the average gradient of decrease in at least a portion of the region adjacent the bifurcation point. In other embodiments, the gradient of decrease at that position may even be more than 25%, 30%, 35%, 40%, 45%, or 50% below the average gradient of decrease of the total cross section.

Optionally, the gradient of decrease of the total cross section is approximately zero in at least a portion of the region adjacent the bifurcation point. In this embodiment, the cross section of the air intake port remains substantially constant in the region around the bifurcation point, thereby allowing the air flow to move through undisturbed. In some embodiments, the gradient of decrease of the total cross section may even be locally below zero in at least a portion of the region adjacent the bifurcation point, which means that the cross section locally increases in the region around the bifurcation point.

Preferably, the gradient of decrease of the total cross section increases downstream of the region adjacent the bifurcation point. As soon as the air flow is properly split in two branches, the cross section can be decreased again in order to further increase the air flow.

In a further embodiment, the gradient of decrease of the total cross section may be locally reduced in the region immediately upstream of the two air outlets. The air outlets of the air intake port connect to the air inlets of the combustion chamber. Like near the bifurcation point of the air intake port, there may be a risk of undesired flow disturbances when the air flow reaches the intake valves and the transition point between the air intake port and the combustion chamber. To minimise such disturbances, it may be preferred to bring the gradient of decrease of the total cross section down to or below zero in the region immediately upstream of the air outlets.

According to another aspect of the invention a lean-burn gasoline engine is provided comprising at least one air intake port as described above. While the air intake port described herein is primarily designed for use with combustion chamber having a dual intake, it could be used to serve two single intake combustion chambers too.

According to yet another aspect of the invention, a vehicle is provided comprising a lean-burn gasoline engine with an air intake port as described above.

According to an aspect of the present invention there is provided an air intake port for an engine, such as a lean-burn gasoline engine. The air intake port comprises an air inlet, at least one air outlet, and an air channel connecting the air inlet to the at least one air outlet. The air channel comprises an air channel floor and an air channel ceiling. The air channel floor is at least substantially flat in a direction of air flow in a region adjacent to the air outlet.

Prior art air intake ports are typically tubular with a circular or quasicircular cross section. The cylinder heads to which the air intake ports are attached are generally located centrally in the engine with air inlets that are often slightly inclined outward, relative to the horizontal. Air intake ports draw in air from both sides of the engine and guide it to the cylinder heads. As a consequence of the position of the air inlet of the air intake ports and the location and orientation of the air inlets of the cylinder heads, the air intake ports often comprise a bend to transition from a primarily horizontal flow direction near the inlet to a primarily downward direction near the outlet.

The inventors of the current invention have observed that with this common design a significant portion of the incoming airflow, upon leaving the air intake port, follows the internal wall of the combustion chamber. When adhering to the combustion chamber wall, this portion of the incoming air flow may move directly towards the bottom of the combustion chamber. The inventors have found that this is not the ideal air flow pattern for a lean-burn gasoline engine. Instead, the currently proposed design of the air intake port intends to create and promote a ‘tumble’ that allows a large volume of intake air to first flow along a roof of the combustion chamber towards the opposite side of the chamber. There, the air flow goes down along the rear wall to finally move up towards the air inlet, along the nearest wall (i.e. nearest to the air inlet) of the combustion chamber. With an air channel floor that is at least substantially flat in a direction of flow in a region adjacent to the air outlet, flow separation at the combustion chamber inlet significantly improved, thereby allowing the incoming air to first flow across the chamber before descending into the chamber. As a result, the desired tumble is achieved. In an embodiment of the invention, the air intake port comprises two air outlets. The air channel connects the air inlet to the two air outlets and comprises an upstream common duct and two downstream port legs. The two downstream port legs branch off from the common duct at a bifurcation point. In this embodiment, the air channel floor is at least substantially flat in a direction of flow in at least a downstream half of each of the port legs. In a preferred embodiment, the air channel floor even is at least substantially flat in a direction of flow along a full length of each of the port legs. The terms upstream and downstream are herein used to refer to parts of the air intake port relative to flow of air through the air intake port in its normal use with a lean-burn gasoline engine. The predominant air flow direction is from an upstream position to a downstream position. It follows that in normal use the engine is downstream of the air intake port.

In addition thereto, the air channel floor may be at least substantially flat in a direction of flow in at least a downstream half or even along a full length of the common duct. A uniformly flat floor throughout the air channel helps to achieve a stable and undisturbed high-volume air flow that detaches from the underlying surface and is launched into the combustion chamber when reaching the end of the air intake port.

In the foregoing, the term ‘substantially flat’ may, e.g., be defined as having a difference between a minimum inclination and a maximum inclination that is less than 5 degrees. Preferably, the flat portion of the air channel floor is designed such that the difference between the minimum and maximum inclination is less than 2, or even 1, degrees.

It is noted that a uniformly flat floor in the direction of airflow does not exclude the possibility of the floor being curved in other directions. On the contrary, as already indicated above, air intake ports are typically tubular with a circular or quasicircular cross section, which means that the floor surface is flat in the direction of air flow only.

Furthermore, in a transition zone leading to the bifurcation point where the common duct splits into the two port legs, a floor and ceiling of the common duct may be shaped to provide a gradual transition between the single common duct and the two separate port legs. As will be explained in more detail below with reference to the Figures, in this transition zone the floor of the common duct may include a curved or sloped portion that provides for a smooth separation of a common air flow in the common duct into two separate air flows in the port legs. Flowever, even if such a transition zone with a curved or sloped portion is provided, this will still allow for the floor of the common duct to be at least substantially flat in a direction of air flow. The portions that are sloped or curved form a wall or separator between the two port legs. The air flow at either side of that wall can still follow a substantially flat floor.

According to a further aspect of the invention, a lean-burn gasoline engine is provided which comprises at least one air intake port as described above. A combustion chamber with at least one air inlet being is connected to the at least one air outlet of the air intake port. The air inlet of the combustion chamber comprises a throat where the air outlet of the air intake port meets the air inlet of the combustion chamber. A movable valve is arranged to move between a closed state for closing off the air inlet of the combustion chamber and an opened state wherein intake air can flow from the air intake port into the combustion chamber.

In a preferred embodiment of this lean-burn gasoline engine, the valve comprises a bottom surface that faces the combustion chamber and a top surface that faces the air intake port. The air intake port and the valve are arranged such that when the valve is in its opened position, the complete bottom surface of the valve is positioned below the air intake port. This allows the separated air flow leaving the air intake port to flow along the roof of the combustion chamber and towards the opposite chamber wall with minimal disturbance by the valve it has to pass.

In a preferred embodiment, the air intake port and the valve are arranged such that even when the valve is half-way between its closed position and its opened position, the complete bottom surface of the valve is positioned below the air intake port. This further allows reduced flow disturbance by the valve while the valve is still opening, thereby facilitating the creation of the desired tumble as soon as the valve is opened. In alternative embodiments, the complete inlet valve face drops below the air intake port when the valve is, e.g., 75% open. In a further embodiment, the air intake port and the valve are arranged such that when the valve is in its opened position, also the complete top surface of the valve is positioned below the air intake port, which may lead to even less disturbance of the air flow and therefore a more prominent and stable tumble.

By providing an air channel with a smooth and even surface, and with a substantially constant inclination at least in the region adjacent to the air outlet, a mostly undisturbed air flow through the air channel is obtained and detachment of the air flow at the air outlet of the air intake port is promoted. In addition thereto, a sharp edge at the air channel end and/or a large enough angle with the throat may further improve the air flow detachment.

Preferably, the throat provides a sharp edge with the channel floor, such as to promote a separation of an incoming air flow from a combustion chamber wall. Without this sharp edge, there is a risk of the incoming air flow adhering to the combustion chamber wall and bending down the corner against the direction of the desired tumble. The sharp edge helps the air flow to continue in the flow direction it has at the end of the air channel and to be launched in a direction along the roof of the combustion chamber. To further increase the desired tumble motion, the throat may provide a smooth edge with the channel ceiling, such as to adhere an incoming air flow to a combustion chamber ceiling. It is noted that the throat is a circular opening that has an interface with the channel floor as well as with the channel ceiling. If a continuous circular opening that can be machined in a single cut is preferred, a compromise may need to be found between the sharpness of the edge near the air channel floor and the smoothness of the edge near the air channel ceiling.

In preferred embodiments, the angle between the channel floor and an adjacent portion of the throat is at least 225 degrees. However, angles closer to, or even beyond, 270 degrees are even more preferred. The larger the angle, the smaller the chance that the airflow will adhere to the throat surface and finds a way down into the cylinder immediately upon entering.

It is further preferred that the throat provides a smooth edge with the channel ceiling, such as to adhere an incoming air flow to a combustion chamber ceiling. By adhering to the combustion chamber ceiling, the air flow is assisted to cross the chamber towards the opposite chamber wall and thereby provide the desired tumble motion.

According to yet another aspect of the invention, a vehicle is provided comprising a lean-burn gasoline engine as described above.

Aspects and embodiments of the invention provide a lean-burn engine and a vehicle with such an engine. The lean-burn engine may be suitable for use with gasoline as described herein. Alternatively or in addition thereto it will be appreciated that the lean-burn engine may be suitable for use with other fuels, such as hydrogen, for example. Aspects and embodiments of the invention are defined in the context of lean- burn gasoline but it will be appreciated that the fuel type can be substituted.

According to an aspect of the present invention there is provided an engine, such as a lean-burn gasoline engine, comprising an air intake port, a combustion chamber, and a movable valve. The air intake port comprises an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet. The combustion chamber comprises a combustion chamber inlet which is connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet. The movable valve comprises a bottom surface that faces the combustion chamber and a tapered top surface that faces the air intake port. The movable valve is arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber. The throat comprises a tapered surface that is complementary with the tapered top surface of the movable valve. As in other lean-burn engines, one of the aims of the currently proposed design of the air intake systems is to take in large volumes of air. In addition thereto, the lean-burn gasoline engine according to the invention is configured to direct this high volume intake air flow in such a way as to create and promote a ‘tumble’ motion. This tumble motion causes the incoming air to first flow along a roof of the combustion chamber towards the opposite side of the chamber. There, the air flow goes down along the rear wall to finally move up towards the air inlet, along the wall nearest to the air inlet of the combustion chamber. This tumble is preferably kept in motion during the full intake stroke and at least a portion of the compression stroke of the piston moving through the combustion chamber.

The complementary tapered surfaces of the intake valve and the throat together ensure that during the compression stroke, when the intake valve is closed, no or little air can get trapped behind the valve or between the valve and an inner surface of the combustion chamber while tumbling through the combustion chamber.

Preferably, the tapered surface of the throat and the tapered top surface of the movable valve are configured such that when the movable valve is in its closed position, the movable valve at least partially sinks into the throat. The further the valve is allowed to sink into the throat, the less disturbance it can cause to the desired tumble. In an embodiment of the invention, the bottom surface of the movable valve may even be substantially flush with an inner surface of the combustion chamber when the movable valve is in its closed position.

Due to the tapered surface of the throat, and because the valve needs to be able to close off the air inlet, the diameter of the combustion chamber inlet is smaller than the valve diameter. The valve diameter is determined by the bottom surface of the valve. In an embodiment of the invention, the diameter of the combustion chamber inlet is less than, e.g., 95% or 90% of a diameter of the bottom surface of the movable valve. Not only does this allow for the desired taper in the throat surface, the protruding upstream portion of the throat also helps to shield of the valve edge, thereby directing the air flow over the top surface of the valve and along the roof of the combustion chamber instead of around the valve edge and down along the wall closest to the combustion chamber inlet. This effect can further be enhanced by the protruding upstream portion ending with a sharp edge that promotes detachment of the air flow. The terms upstream and downstream are herein used to refer to parts of the air intake port relative to flow of air through the air intake port in its normal use with a lean-burn gasoline engine. The predominant air flow direction is from an upstream position to a downstream position. It follows that in normal use the engine is downstream of the air intake port.

In a preferred embodiment, a deflector is provided at an inner wall of the combustion chamber and protruding radially therefrom, the deflector being positioned underneath an outer edge of the bottom surface of the movable valve. This deflector is arranged such that an air flow moving up along the inner wall of the combustion chamber is deflected radially inward and away from the outer edge of the bottom surface of the movable valve. As a result, the risk of any air being trapped behind the valve when in a closed or almost closed position is reduced. This useful deflector, on top of that, brings the additional advantage that during the intake stroke, when the valve is at least partially open and air is drawn into the combustion chamber, any air unintentionally bouncing of the top surface of the valve will be prevented from flowing down along the nearest inner wall of the combustion chamber. Instead, the deflector will block this astray air flow back into the chamber, and in the direction of the desired tumble.

In a preferred embodiment of this lean-burn gasoline engine, the air intake port and the valve are arranged such that when the valve is in its opened position, the complete bottom surface of the valve is positioned below the air intake port. This allows the separated air flow leaving the air intake port to flow along the roof of the combustion chamber and towards the opposite chamber wall with minimal disturbance by the valve it has to pass.

In a preferred embodiment, the air intake port and the valve are arranged such that even when the valve is half-way between its closed position and its opened position, the complete bottom surface of the valve is positioned below the air intake port. This further allows reduced flow disturbance by the valve while the valve is still opening, thereby facilitating the creation of the desired tumble as soon as the valve is opened. In alternative embodiments, the complete front valve face drops below the air intake port when the valve is, e.g., 60% open.

In a further embodiment, the air intake port and the valve are arranged such that when the valve is in its opened position, also the complete top surface of the valve is positioned below the air intake port, with the tapered angle of the top surface at a similar angle as the port floor, which leads to even less disturbance of the air flow, and helps to direct the air flow across the top of the chamber, with a more prominent and stable tumble as a result. The top surface may be inclined slightly upward at the point where the air flow may hit the valve in order to lift the air flow up in the direction of the chamber ceiling and/or the top end of the opposing wall.

Aspects and embodiments of the invention provide a cylinder head for an engine, an engine, and a vehicle with such an engine. The engine may be suitable for use with fuels including gasoline, diesel, hydrogen, LPG or any other suitable combustible fuel. The engine may be a lean-burn engine.

According to an aspect of the present invention there is provided a cylinder head for an engine, the cylinder head comprising: a substantially planar gasket interface surface; a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber comprises a combustion chamber roof surface having: a central domed surface portion defining a central domed portion of the combustion chamber; and a sloped surface portion defining a sloped portion of the combustion chamber, wherein the sloped surface portion comprises a substantially straight cross-section along a plane of symmetry of the combustion chamber, and a spark plug seat configured to support a spark plug, in use, such that a spark gap of the spark plug is held in a substantially fixed position within the domed portion of the combustion chamber, wherein the sloped surface portion of the combustion chamber roof is configured so that a geometric extension of the sloped surface portion is coincidental with the spark plug gap in use.

The cylinder head configuration described above is advantageous as it promotes direction of the air and fuel mixture into the central domed portion of the combustion chamber, and towards the spark plug gap, as the piston of the engine approaches the sloped surface portions of the combustion chamber roof as it moves towards top dead centre. This has been found to promote efficient burn of the air fuel mixture.

Optionally the sloped surface portion conforms to part of the surface of a cone which is a readily manufacturable shape which achieves the aim of directing the air fuel mixture towards the spark gap. The cylinder head optionally comprises two sloped surface portions located on opposite sides of the combustion chamber. Since the air fuel mixture occupies the entirety of the cylinder and combustion chamber above the piston, it is beneficial to direct the air fuel mixture towards the spark gap form both sides of the combustion chamber.

The sloped surface portions may comprise a first sloped surface portion located adjacent a combustion chamber air inlet opening, and a second sloped surface portion located adjacent a combustion chamber exhaust outlet opening. The inlet and outlet openings are typically located on opposite sides of the combustions chamber.

In one example the combustion chamber comprises a pair of air inlet openings and a pair of exhaust outlet openings, wherein the first sloped surface portion is at least partially located between the pair of air inlet openings, and wherein the second sloped surface portion is at least partially located between the pair of exhaust outlet openings. This arrangement provides symmetry between adjacent sides of the combustion chamber.

Optionally the surface area of the first sloped surface portion is less than the surface area of the second sloped surface portion to accommodate the geometry of the combustion chamber.

The length of the first sloped surface portion along the plane of symmetry of the combustion chamber may optionally be less than the length of the second sloped surface portion along the plane of symmetry of the combustion chamber.

The first sloped surface portion may comprise an innermost edge at an interface between the first sloped surface portion and the central domed portion, and wherein the second sloped surface portion comprises an innermost edge at an interface between the second sloped surface portion and the central domed portion, wherein the length of the innermost edge of the first sloped surface portion is substantially equal to the length of the innermost edge of the second sloped surface portion.

In one example the innermost edge of the first sloped surface portion is located between the pair of air inlet openings no further towards the centre of the combustion chamber than the shortest possible line joining the outermost extremities of the air inlet openings, and wherein the innermost edge of the second sloped surface portion is located between the pair of exhaust outlet openings no further towards the centre of the combustion chamber than the shortest possible line joining the outermost extremities of the exhaust outlet openings.

Optionally the ratio of: the width of a projection of the combustion chamber onto a plane parallel to the gasket interface surface measured in a direction along the plane of symmetry of the combustion chamber; and the width of a projection of the central domed portion of the combustion chamber onto a plane parallel to the gasket interface surface measured in a direction along the plane of symmetry of the combustion chamber, is about 1.7:1.

The angle between the gasket interface surface and each sloped surface portion measured along the plane of symmetry of the combustion chamber are optionally substantially equal.

The combustion chamber roof surface may comprise concave curved portions located between an outermost edge of the combustion chamber and the or each sloped surface portion.

In one example the central domed portion of the combustion chamber may be elongated in a direction perpendicular to the plane of symmetry of the combustion chamber. Optionally the spark plug seat comprises an opening in the central domed surface of the combustion chamber located such that it intersects the plane of symmetry of the combustion chamber.

The combustion chamber may optionally comprise a fuel injector seat opening in the central domed surface of the combustion chamber, wherein the fuel injector seat opening is located such that it intersects the plane of symmetry of the combustion chamber, wherein the fuel injector seat opening is positioned further towards the pair of air inlet openings than the spark plug seat opening.

In another aspect, the present invention provides a cylinder head for an engine, the cylinder head comprising: a combustion chamber extending into the cylinder head, the combustion chamber comprising a combustion chamber roof surface having a sloped surface portion, wherein the sloped surface portion conforms to part of the surface of a cone; and a spark plug seat configured to support the tip of a spark plug at a predetermined position within the combustion chamber in use, wherein the combustion chamber is configured so that the apex of a geometric extension of the sloped surface portion of the combustion chamber roof surface is located within a volume envelope that is described by a 360° rotation of the spark plug tip when the spark plug tip is supported at the predetermined position in the combustion chamber.

This arrangement promotes direction of the air and fuel mixture into the central domed portion of the combustion chamber, and towards the spark plug tip, as the piston of the engine approaches the sloped surface portions of the combustion chamber roof as it moves towards top dead centre. This has been found to promote efficient burn of the air fuel mixture.

In a further aspect the present invention provides an engine comprising a cylinder head as described above.

In a still further aspect the present invention provides an engine as described above, comprising a piston having a working surface configured to conform to at least part of the or each sloped surface portion of the combustion chamber roof surface in use.

In another aspect the present invention provides an engine as described above, wherein the gap between the sloped surface portion of the combustion chamber roof surface and the conforming part of the working surface of the piston is no less than 0.8mm and no more than 1 ,4mm when the piston is at top dead centre as measured when the engine is at substantially the same temperature as the environment.

In a further aspect the present invention provides a vehicle comprising an engine as described above.

According to an aspect of the present invention there is provided a cylinder head for an engine, the cylinder head comprising: a substantially planar gasket interface surface; and a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber comprises: a combustion chamber roof surface which intersects the gasket interface surface at a combustion chamber opening; a pair of air inlet openings located in the combustion chamber roof surface on an air inlet side of the combustion chamber; and a pair of exhaust outlet openings located in the combustion chamber roof surface on an exhaust outlet side of the combustion chamber wherein the combustion chamber roof surface comprises a plurality of machined facets, wherein: a first pair of the machined facets comprise opposing curved surfaces located on opposite sides of the combustion chamber between the air inlet openings and the exhaust outlet openings; and a second pair of the machined facets comprise opposing curved surfaces located on opposite sides of the combustion chamber, a first one of the second pair of machined facets being located on the air inlet side of the combustion chamber, and a second one of the second pair of machined facets being located on the exhaust outlet side of the combustion chamber, wherein the first pair of machined facets intersect the gasket interface surface to define a first pair of opposed curved sections of the combustion chamber opening, and the second pair of machined facets intersect the gasket interface surface to define a second pair of opposed curved sections of the combustion chamber opening.

The cylinder head described above is advantageous as the opposed curved sections helps to encourage “omega swirl” of the inflowing air as it moves from the air inlet openings towards the exhaust outlet openings and then back towards the air inlet openings; and from the centre of the combustion chamber towards the edges of the combustion chamber.

Optionally the first pair of machined facets are configured so that they are machinable by the same cutter. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

The second pair of machined facets are optionally configured so that they are machinable by the same cutter to reduce the complexity of the manufacturing process and improve manufacturing efficiency.

The combustion chamber roof surface may optionally comprise a central elongate domed portion orientated across the combustion chamber in a direction substantially parallel to an intersect plane separating the air inlet side of the combustion chamber from the exhaust outlet side of the combustion chamber, wherein the central domed portion of the combustion chamber roof surface comprises the first pair of machined facets.

In one example the elongate central domed portion of the combustion chamber roof surface comprises a third pair of machined facets, wherein a first one of the third pair of machined facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the third pair of machined facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings, wherein the third pair of machined faces comprise substantially flat surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

Optionally the first one of the third pair of machined facets is substantially parallel to the plane of the first one of the exhaust outlet openings, and the second one of the third pair of machined facets is substantially parallel to the plane of the second one of the exhaust outlet openings. This configuration is beneficial for efficient flow across the combustion chamber from one side to the other.

The third pair of machined facets are optionally configured so that they are machinable by the same cutter. Once again, this reduces the complexity of the manufacturing process and improves manufacturing efficiency.

The third pair of machined facets may be configured so that they are machinable by the same cutter used to cut the first pair of machined facets to further reduce complexity and improve efficiency.

In one example the elongate central domed portion of the combustion chamber roof surface comprises a fourth pair of machined facets, wherein a first one of the fourth pair of machined facets is located adjacent the first one of the air inlet openings on a side of the air inlet opening closest to the exhaust outlet side of the combustion chamber, and a second one of the fourth pair of machined facets is located adjacent the second one of the air inlet openings on a side of the air inlet opening closest to the exhaust outlet side of the combustion chamber, and wherein the fourth pair of machined facets comprise curved surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

Optionally the fourth pair of machined facets are configured so that they are machinable by the same cutter to reduce complexity and improve efficiency.

The fourth pair of machined facets are configured so that they are optionally machinable by the same cutter used to cut the first pair of machined facets, and the same cutter used to cut the third pair of machined facets where present to further reduce complexity and improve efficiency.

The elongate central domed portion of the combustion chamber roof surface optionally comprises a fifth pair of machined facets, wherein a first one of the fifth pair of machined facets is located adjacent the first one of the exhaust outlet openings on a side of the exhaust outlet opening closest to the air inlet side of the combustion chamber, and a second one of the fifth pair of machined facets is located adjacent the second one of the exhaust outlet openings on a side of the exhaust outlet opening closest to the air inlet side of the combustion chamber, wherein the fifth pair of machined facets comprise curved surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

The fifth pair of machined facets may be configured so that they are machinable by the same cutter to reduce complexity and improve efficiency.

In one example the fifth pair of machined facets are configured so that they are machinable by the same cutter used to cut the first pair of machined facets, and the same cutter used to cut the third pair of machined facets and/or the fourth pair of machined facets where present to further reduce complexity and improve efficiency.

Optionally the combustion chamber roof surface comprises a first squish portion located on the inlet side of the combustion chamber and a second squish portion located on the exhaust outlet side of the combustion chamber, wherein the first squish portion comprises the first one of the second pair of machined facets and the second squish portion comprises the second one of the second pair of machined facets.

The cylinder head optionally comprises a spark plug seat comprising an opening located in the elongate central domed portion of the combustion chamber roof surface, and a fuel injector seat comprising an opening located in the elongate central domed portion of the combustion chamber roof surface, wherein the spark plug seat opening and the fuel injector seat opening are positioned substantially in line with one another along a plane of symmetry of the combustion chamber perpendicular to the intersect plane separating the air inlet side of the combustion chamber from the exhaust outlet side of the combustion chamber.

In another aspect the present invention provides an engine comprising a cylinder head as described above.

Aspects and embodiments of the invention provide a cylinder head for an engine, an engine, and a vehicle with such an engine. The engine may be suitable for use with fuels including gasoline, diesel, hydrogen, LPG or any other suitable combustible fuel. The engine may be a lean-burn engine. According to an aspect of the present invention there is provided a piston for an engine comprising a cylinder, an air inlet and an exhaust outlet, wherein the air inlet and the exhaust outlet are arranged about a longitudinal axis of the cylinder, the piston arranged to operate in the cylinder, the piston comprising: a circular peripheral wall having a central axis, wherein the peripheral wall is configured so that the central axis is substantially aligned with the longitudinal axis of the cylinder in use; and a working surface having a circular periphery and comprising: a flat elongate central surface extending across the majority of the width of the working surface perpendicular to the central axis and having two ends each located on opposite sides of the central axis, wherein opposing side edges of the central surface are separated by the two ends of the central surface; and a spark plug bowl located in the flat elongate central surface substantially at the centre of the working surface.

This piston described above is advantageous as it facilitates high compression-ration operation of the engine.

Optionally the working surface comprises depressions for accommodating valve heads of the engine in use when the piston is at top dead centre so that the valve do not hit the piston in use.

The working surface optionally comprises sloped surface portions located radially outward of the central surface with respect to the circular periphery of the working surface, wherein each sloped surface portion extends away from a side edge of the flat elongate central surface downwardly towards the peripheral edge of the working surface.

In another aspect the present invention provides an engine comprising a piston as described above.

Optionally the sloped surface portions of the working surface are configured to conform to at least part of a roof surface of a combustion chamber of the engine in use. This promotes direction of the air and fuel mixture into the central portion of the combustion chamber, and towards the spark plug, as the piston approaches the sloped surface portions of the combustion chamber roof. This has been found to promote efficient burn of the air fuel mixture.

In a further aspect the present invention provides an engine as described above, comprising: a cylinder head having a combustion chamber formed therein, wherein at least part of the roof of the combustion chamber is configured to conform to the sloped surface portions of the working surface of the piston in use; and a piston as described above.

In a still further aspect the present invention provides a vehicle comprising an engine as described above.

According to an aspect of the present invention there is provided a casting for an engine, the casting comprising an air inlet opening located in a combustion chamber, wherein the air inlet opening comprises the outermost edge of an air inlet throat which extends into the casting away from the air inlet opening, wherein a portion of the air inlet throat is radially symmetrical about a central axis, and wherein the cross- section of the air inlet throat in a plane which passes through the central axis and the edge of the air inlet throat comprises a radiused or a stepped profile. The casting may comprise an exhaust outlet opening located in the combustion chamber, wherein the exhaust outlet opening comprises the outermost edge of an exhaust outlet throat which extends into the casting away from the exhaust outlet opening, wherein a portion of the exhaust outlet throat is radially symmetrical about a second central axis, and wherein the cross-section of the exhaust outlet throat in a plane which passes through the second central axis and the edge of the exhaust outlet throat comprises a radiused or a stepped profile.

Providing a radiused or a stepped profile at the air inlet throat and/or the exhaust outlet throat is advantageous to the quality of any subsequent a laser cladding process. Depending on the composition of the laser cladding material and the precise laser cladding process used, the machined profile can be tuned to facilitate the application of a high quality wear resistant cladding.

Optionally, the cross-section of the air inlet throat, and/or the exhaust outlet throat where present, comprises a concave radiused profile. It has been found that a concave radiused profile is advantageous to the application of a high quality wear resistant cladding.

The cross-section of the air inlet throat, and/or the exhaust outlet throat where present, may optionally comprise a radiused profile having a radius of between 1mm and 5mm. Optionally, between 2.5mm and 3.5mm. It has been found that a concave radiused profile of about 3mm is advantageous to the application of a high quality wear resistant cladding.

In one example, the casting is a cylinder head casting, wherein the combustion chamber comprises a combustion chamber recess which extends into the cylinder head casting away from a bottom surface of the cylinder head casting, wherein the combustion chamber recess comprises a combustion chamber roof surface, and wherein the air inlet opening, and the exhaust outlet opening where present, are located in the combustion chamber roof surface.

In another aspect, the present invention provides a method of manufacturing a component for an engine, the method comprising providing a casting, wherein the casting comprises a combustion chamber having an air inlet opening located therein, the method of manufacturing comprising machining an air inlet throat at the air inlet opening, wherein the machined air inlet throat extends into the casting, and wherein a portion of the machined air inlet throat is radially symmetrical about a central axis, wherein the cross-section of the machined air inlet throat in a plane which passes through the central axis and the edge of the machined air inlet throat comprises a radiused profile. In one example, the casting comprises a cylinder head casting.

The casting may comprise an exhaust outlet opening located in the combustion chamber and the method of manufacturing may comprise machining an exhaust outlet throat at the exhaust outlet opening, wherein the machined exhaust outlet throat extends into the casting, and wherein a portion of the machined exhaust outlet throat is radially symmetrical about a second central axis, wherein the cross-section of the machined exhaust outlet throat in a plane which passes through the second central axis and the edge of the machined exhaust outlet throat comprises a radiused profile.

A cladding may be applied to the machined air inlet throat, and/or the machined exhaust outlet throat where present, using a laser cladding process. Laser cladding can advantageously be applied to the inlet and/or exhaust throat to provide a wear resistant material at a position of high wear.

Optionally the cladding may be a Nickel Aluminium (NiAI) cladding, optionally comprising Chromium Carbide (CrC) as a hardening additive. A laser hardening process may optionally be used after the laser cladding process to further improve wear performance. Optionally a portion of the cladding may be removed in a second machining process to form a valve seat. In a further aspect, the present invention provides an engine comprising a casting as described above. In a still further aspect, the present invention provides a vehicle comprising an engine as described above. Aspects and embodiments of the invention provide a cylinder head for an engine, an engine, and a vehicle with such an engine. The engine may be suitable for use with fuels including gasoline, diesel, hydrogen, LPG or any other suitable combustible fuel. The engine may be a lean-burn engine.

According to an aspect of the present invention there is provided a cylinder head for an engine, the cylinder head comprising: a substantially planar gasket interface surface; and a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber comprises: a combustion chamber roof surface which intersects the gasket interface surface at a combustion chamber opening; a pair of air inlet openings located in the combustion chamber roof surface on an air inlet side of the combustion chamber; and a pair of exhaust outlet openings located in the combustion chamber roof surface on an exhaust outlet side of the combustion chamber wherein the combustion chamber roof surface comprises a plurality of machined facets, wherein: a first pair of the machined facets comprise curved surfaces located adjacent to the air inlet openings on a side of the air inlet openings closest to the exhaust outlet side of the combustion chamber, wherein a first one of the first pair of machined facets is located adjacent a first one of the air inlet openings, and a second one of the first pair of machined facets is located adjacent a second one of the air inlet openings; and a second pair of the machined facets comprise curved surfaces located adjacent the exhaust outlet openings on a side of the exhaust outlet openings closest to the air inlet side of the combustion chamber, wherein a first one of the second pair of machined facets is located adjacent a first one of the exhaust outlet openings, and a second one of the second pair of machined facets is located adjacent a second one of the exhaust outlet openings, wherein the first and second pair of machined facets are configured so that they are machinable by the same cutter.

The cylinder head described above is advantageous as the first and second pair of machined facets are machined by the same cutter in the manufacturing process. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

Optionally the combustion chamber roof surface comprises a third pair of machined facets, wherein a first one of the third pair of machined facets is located between the first one of the air inlet openings and the first one of the exhaust outlet openings, and a second one of the third pair of machined facets is located between the second one of the air inlet openings and the second one of the exhaust outlet openings, wherein the third pair of machined facets comprise substantially flat surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

The first one of the third pair of machined facets is optionally substantially parallel to the plane of the first one of the exhaust outlet openings, and the second one of the third pair of machined facets is optionally substantially parallel to the plane of the second one of the exhaust outlet openings. This configuration is beneficial for efficient flow across the combustion chamber from one side to the other.

The third pair of machined facets may be configured so that they are machinable by the same cutter used to cut the first and second pair of machined facets. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

In one example the combustion chamber roof surface comprises: a fourth pair of machined facets comprising opposing curved surfaces located on opposite sides of the combustion chamber between the air inlet openings and the exhaust outlet openings such that the fourth pair of machined facets extend between the air inlet side of the combustion chamber and the exhaust outlet side of the combustion chamber; and a fifth pair of machined facets comprising opposing curved surfaces located on opposite sides of the combustion chamber, a first one of the fifth pair of machined facets being located on the air inlet side of the combustion chamber, and a second one of the fifth pair of machined facets being located on the exhaust outlet side of the combustion chamber, wherein the fourth pair of machined facets intersect the gasket interface surface to define a first pair of opposed curved sections of the combustion chamber opening, and the fifth pair of machined facets intersect the gasket interface surface to define a second pair of opposed curved sections of the combustion chamber opening.

The opposed curved sections help to encourage “omega swirl” of the inflowing air as it moves from the air inlet openings towards the exhaust outlet openings and then back towards the air inlet openings; and from the centre of the combustion chamber towards the edges of the combustion chamber.

Optionally the fourth pair of machined facets are configured so that they are machinable by the same cutter. Once again, this reduces the complexity of the manufacturing process and improves manufacturing efficiency.

The fourth pair of machined facets are optionally configured so that they are machinable by the same cutter used to cut the first and second pairs of machined facets, and the same cutter used to cut the third pair of machined facets where present. This further reduces complexity and improves efficiency.

The fifth pair of machined facets may be configured so that they are machinable by the same cutter to further reduce complexity and improve efficiency.

In one example the combustion chamber roof surface comprises a sixth pair of machined facets, wherein a first one of the sixth pair of machined facets is located between the air inlet openings, and a second one of the sixth pair of machined facets is partially located between the exhaust outlet openings, wherein the sixth pair of machined facets comprise curved surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

Optionally the sixth pair of machined facets are configured so that they are machinable by the same cutter to reduce complexity and improve efficiency of the manufacturing process.

The combustion chamber roof surface optionally comprises a central elongate domed portion orientated across the combustion chamber in a direction substantially parallel to an intersect plane separating the air inlet side of the combustion chamber from the exhaust outlet side of the combustion chamber, wherein the central domed portion of the combustion chamber roof surface comprises the first, second and fourth pairs of machined facets, and the third pair of machined facets where present, and the sixth pair of machined facets where present.

The combustion chamber roof surface may comprise a first squish portion located on the inlet side of the combustion chamber and a second squish portion located on the exhaust outlet side of the combustion chamber, wherein the first squish portion comprises the first one of the fifth pair of machined facets and the second squish portion comprises the second one of the fifth pair of machined facets.

In one example the cylinder head comprises a spark plug seat comprising an opening located in the combustion chamber roof surface, and a fuel injector seat comprising an opening located in the combustion chamber roof surface, wherein the spark plug seat opening and the fuel injector seat opening are positioned substantially in line with one another along a plane of symmetry of the combustion chamber perpendicular to the intersect plane separating the air inlet side of the combustion chamber from the exhaust outlet side of the combustion chamber. In another aspect the present invention provides an engine comprising a cylinder head as described above. In a further aspect the present invention provides a vehicle comprising an engine as described above.

According to an aspect of the present invention there is provided a cylinder head for an engine, the cylinder head comprising: a substantially planar gasket interface surface; and a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber comprises: a combustion chamber roof surface which intersects the gasket interface surface at a combustion chamber opening; a pair of air inlet openings located in the combustion chamber roof surface on an air inlet side of the combustion chamber; and a pair of exhaust outlet openings located in the combustion chamber roof surface on an exhaust outlet side of the combustion chamber, wherein the combustion chamber roof surface comprises a central elongate domed portion orientated across the combustion chamber in a direction substantially parallel to an intersect plane separating the air inlet side of the combustion chamber from the exhaust outlet side of the combustion chamber, wherein the central elongate domed portion of the combustion chamber roof surface comprises a plurality of machined facets, wherein a first pair of the machined facets comprise curved surfaces, wherein a first one of the first pair of machined facets is located between the air inlet openings, and a second one of the first pair of machined facets is at least partially located between the exhaust outlet openings, wherein the first pair of machined facets are configured so that they are machinable by the same cutter.

The cylinder head described above is advantageous as the first pair of machined facets are machined by the same cutter in the manufacturing process. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

Optionally the elongate central domed portion of the combustion chamber roof surface comprises a second pair of machined facets, wherein a first one of the second pair of machined facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the second pair of machined facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings, wherein the second pair of machined facets comprise substantially flat surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

The first one of the second pair of machined facets is optionally substantially parallel to the plane of the first one of the exhaust outlet openings, and the second one of the second pair of machined facets is optionally substantially parallel to the plane of the second one of the exhaust outlet openings. This configuration is beneficial for efficient flow across the combustion chamber from one side to the other.

The second pair of machined facets may be configured so that they are machinable by the same cutter. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

In one example the elongate central domed portion of the combustion chamber roof surface comprises a third pair of machined facets, wherein a first one of the third pair of machined facets is located adjacent the first one of the air inlet openings on a side of the air inlet opening closest to the exhaust outlet side of the combustion chamber, and a second one of the third pair of machined facets is located adjacent the second one of the air inlet openings on a side of the air inlet opening closest to the exhaust outlet side of the combustion chamber, and wherein the third pair of machined facets comprise curved surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

Optionally the third pair of machined facets are configured so that they are machinable by the same cutter to reduce complexity and improve efficiency of the manufacturing process.

The third pair of machined facets are optionally configured so they are machinable by the same cutter used to cut the second pair of machined facets where present to further reduce complexity and improve efficiency.

The elongate central domed portion of the combustion chamber roof surface may comprise a fourth pair of machined facets, wherein a first one of the fourth pair of machined facets is located adjacent the first one of the exhaust outlet openings on a side of the exhaust outlet opening closest to the air inlet side of the combustion chamber, and a second one of the fourth pair of machined facets is located adjacent the second one of the exhaust outlet openings on a side of the exhaust outlet opening closest to the air inlet side of the combustion chamber, wherein the fourth pair of machined facets comprise curved surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

In one example the fourth pair of machined facets are configured so that they are machinable by the same cutter to reduce complexity and improve efficiency of the manufacturing process.

Optionally the fourth pair of machined facets are configured so that they are machinable by the same cutter used to cut the second pair of machined facets where present, and/or the same cutter used to cut the third pair of machined facets where present to further reduce complexity and improve efficiency.

The combustion chamber roof surface optionally comprises: a fifth pair of machined facets comprising opposing curved surfaces located at opposite ends of the elongate central domed portion of the combustion chamber roof surface between the air inlet openings and the exhaust outlet openings such that the fifth pair of machined facets extend between the air inlet side of the combustion chamber and the exhaust outlet side of the combustion chamber; and a sixth pair of machined facets comprising opposing curved surfaces located on opposite sides of the combustion chamber, wherein a first one of the sixth pair of machined facets is located on the air inlet side of the combustion chamber, and a second one of the sixth pair of machined facets is located on the exhaust outlet side of the combustion chamber, wherein the fifth pair of machined facets intersect the gasket interface surface to define a first pair of opposed curved sections of the combustion chamber opening, and the sixth pair of machined facets intersect the gasket interface surface to define a second pair of opposed curved sections of the combustion chamber opening.

The fifth pair of machined facets may be configured so that they are machinable by the same cutter to reduce complexity and improve efficiency of the manufacturing process.

In one example the fifth pair of machined facets are configured so that they are machinable by the same cutter used to cut the second pair of machined facets where present, and/or the same cutter used to cut the third pair of machined facets where present, and/or the same cutter used to cut the fourth pair of machined facets where present to further reduce complexity and improve efficiency. Optionally the combustion chamber roof surface comprises a first squish portion located on the inlet side of the combustion chamber and a second squish portion located on the exhaust outlet side of the combustion chamber, wherein the first squish portion comprises the first one of the sixth pair of machined facets and the second squish portion comprises the second one of the sixth pair of machined facets.

The cylinder head optionally comprises a spark plug seat comprising an opening located in the elongate central domed portion of the combustion chamber roof surface, and a fuel injector seat comprising an opening located in the elongate central domed portion of the combustion chamber roof surface, wherein the spark plug seat opening and the fuel injector seat opening are positioned substantially in line with one another along a plane of symmetry of the combustion chamber perpendicular to the intersect plane separating the air inlet side of the combustion chamber from the exhaust outlet side of the combustion chamber. In another aspect the present invention provides an engine comprising a cylinder head as described above. In a further aspect the present invention provides a vehicle comprising an engine as described above.

According to an aspect of the present invention there is provided a method of machining a combustion chamber roof surface in a cylinder head for an engine, wherein the machined cylinder head comprises: a substantially planar gasket interface surface; and a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber roof surface intersects the gasket interface surface to define a combustion chamber opening, the method comprising: using a cutter to machine a first pair of facets of the combustion chamber roof surface, wherein the first pair of facets are located on opposite sides of the combustion chamber between a pair of air inlet openings located on an air inlet side of the combustion chamber and a pair of exhaust outlet openings located on an exhaust outlet side of the combustion chamber such that each one of the first pair of facets extend between the air inlet side of the combustion chamber and the exhaust outlet side of the combustion chamber; and using the cutter to machine a second pair of facets of the combustion chamber roof surface, wherein a first one of the second pair of facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the second pair of facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings.

This method is advantageous as the first and second pair of machined facets are machined by the same cutter in the manufacturing process. This reduces the complexity of the manufacturing process and improves manufacturing efficiency.

Optionally the first pair of facets intersect the gasket interface surface.

The method optionally comprises using the cutter to machine a third pair of facets of the combustion chamber roof surface, wherein a first one of the third pair of facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the third pair of facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings. Again, this method is advantageous as the third pair of machined facets are machined by the same cutter which reduces the complexity of the manufacturing process and improves efficiency.

The method may comprise using the cutter to machine a fourth pair of facets of the combustion chamber roof surface, wherein a first one of the fourth pair of facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the fourth pair of facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings. As above, this method is advantageous as the fourth pair of machined facets are machined by the same cutter to reduce complexity and improve efficiency. In one example the first pair of facets are machined before the second pair of facets. Optionally the first pair of facets are machined before the second pair of facets, and the third pair of facets are machined after the second pair of facets.

The first pair of facets are optionally machined before the second pair of facets, and the third pair of facets are optionally machined after the second pair of facets, and the fourth pair of facets are optionally machined after the third pair of facets. The second pair of facets may comprise a pair of flat surfaces. In one example the third pair of facets, or the fourth pair of facets where present, comprise a pair of flat surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

Optionally a first one of the pair of flat surfaces is substantially parallel to the plane of the first one of the exhaust outlet openings, and the second one of the pair of flat surfaces is substantially parallel to the plane of the second one of the exhaust outlet openings. This configuration is beneficial for efficient flow across the combustion chamber from one side to the other.

The flat surfaces are optionally located between curved surfaces which boarder the air inlet openings and the exhaust outlet openings respectively.

The second pair of facets may comprise a pair of curved surfaces located immediately adjacent the air inlet openings or immediately adjacent the exhaust outlet openings. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

In one example the third pair of facets, or the fourth pair of facets where present, comprise a pair of curved surfaces located immediately adjacent the air inlet openings or immediately adjacent the exhaust outlet openings. To help open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

Optionally the third pair of facets are located immediately adjacent the air inlet openings, and the fourth pair of facets are located immediately adjacent the exhaust outlet openings.

The first pair of facets may optionally comprise opposing curved surfaces to help encourage “omega swirl” of the inflowing air as it moves from the air inlet openings towards the exhaust outlet openings and then back towards the air inlet openings; and from the centre of the combustion chamber towards the edges of the combustion chamber. The first pair of facets and the gasket interface surface may define a first pair of opposed curved sections of the combustion chamber opening. In another aspect the present invention provides a cylinder head comprising a combustion chamber roof surface machined as described above. In a further aspect the present invention provides an engine comprising a cylinder head as described above. In a still further aspect the present invention provides a vehicle comprising an engine as described above.

According to an aspect of the present invention there is provided a method of machining a combustion chamber roof surface in a cylinder head for an engine, wherein the machined cylinder head comprises: a substantially planar gasket interface surface; and a combustion chamber extending into the cylinder head away from the gasket interface surface, wherein the combustion chamber roof surface intersects the gasket interface surface to define a combustion chamber opening, the method comprising: using a first cutter to machine a first pair of facets of the combustion chamber roof surface, wherein a first one of the first pair of facets is located at least partially between a pair of air inlet openings in the combustion chamber roof located on an air inlet side of the combustion chamber, and a second one of the first pair of facets is located at least partially between a pair of exhaust outlet openings in the combustion chamber roof located on an exhaust outlet side of the combustion chamber; using a second cutter to machine a second pair of facets of the combustion chamber roof surface, wherein the second pair of facets are located between the pair of air inlet openings and the pair of exhaust outlet openings such that each one of the second pair of facets extends between the air inlet side of the combustion chamber and the exhaust outlet side of the combustion chamber; and using the second cutter to machine a third pair of facets of the combustion chamber roof surface, wherein a first one of the third pair of facets is located between a first one of the air inlet openings and a first one of the exhaust outlet openings, and a second one of the third pair of facets is located between a second one of the air inlet openings and a second one of the exhaust outlet openings.

This method is advantageous as the second and third pairs of machined facets are machined by the same cutter in the manufacturing process. This reduces the complexity of the manufacturing process and improves manufacturing efficiency. Optionally the first pair of facets are machined before the second pair of facets. The second pair of facets are optionally machined before the third pair of facets.

The method may comprise using the second cutter to machine a fourth pair of facets of the combustion chamber roof surface, wherein a first one of the fourth pair of facets is located between the first one of the air inlet openings and the first one of the exhaust outlet openings, and a second one of the fourth pair of facets is located between the second one of the air inlet openings and the second one of the exhaust outlet openings.

Again, this method is advantageous as the fourth pair of machined facets are machined by the same cutter used for the second and third pairs of facets which reduces the complexity of the manufacturing process and improves efficiency.

In one example the method comprises using the second cutter to machine a fifth pair of facets of the combustion chamber roof surface, wherein a first one of the fifth pair of facets is located between the first one of the air inlet openings and the first one of the exhaust outlet openings, and a second one of the fifth pair of facets is located between the second one of the air inlet openings and the second one of the exhaust outlet openings. As above, this method is advantageous as the fifth pair of machined facets are machined by the same cutter used for the second to fourth pairs of cuts to reduce complexity and improve efficiency.

Optionally the method comprises using a third cutter to machine a sixth pair of facets of the combustion chamber roof surface, wherein a first one of the sixth pair of facets is located at least partially between the pair of air inlet openings, and a second one of the sixth pair of facets is located at least partially between the pair of exhaust outlet openings, wherein each one of the sixth pair of facets intersect the gasket interface surface to define a first pair of opposing sections of the combustion chamber opening. The fourth pair of facets are optionally machined after the third pair of facets. The fifth pair of facets may be machined after the fourth pair of facets. In one example the sixth pair of facets are machined after the fifth pair of facets.

Optionally the third pair of facets comprise a pair of flat surfaces. This helps to open up the combustion chamber and remove material between the inlet side and the outlet side which may otherwise hinder flow from one side of the combustion chamber to the other.

The first one of the third pair of facets is optionally substantially parallel to the plane of the first one of the exhaust outlet openings, and the second one of the third pair of facets is substantially parallel to the plane of the second one of the exhaust outlet openings. This configuration is beneficial for efficient flow across the combustion chamber from one side to the other. Each one of the third pair of facets may be located between curved surfaces which boarder the air inlet openings and the exhaust outlet openings respectively.

In one example the second pair of facets comprise opposing curved surfaces to help encourage “omega swirl” of the inflowing air as it moves from the air inlet openings towards the exhaust outlet openings and then back towards the air inlet openings; and from the centre of the combustion chamber towards the edges of the combustion chamber.

Optionally the intersection of the first pair of facets and the gasket interface surface define a second pair of opposing sections of the combustion chamber opening. The intersection of the sixth pair of facets and the gasket interface surface optionally define a second pair of opposing sections of the combustion chamber opening. In another aspect the present invention provides a cylinder head comprising a combustion chamber roof surface machined as described above. In a further aspect the present invention provides an engine comprising a cylinder head as described above. In a still further aspect the present invention provides a vehicle comprising an engine as described above.

Preferably, aspects and embodiments of the invention provide a cylinder head for a lean-burn gasoline engine, a lean-burn gasoline engine and a vehicle with such an engine.

According to an aspect of the present invention there is provided a lean-burn gasoline engine comprising: an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet; a combustion chamber having a combustion chamber inlet being connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet; a movable valve, or air inlet valve, comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber; a valve guide opening in a wall of the air channel opposite the top surface of the movable valve; and a valve guide passage extending into the wall and away from the valve guide opening, the valve guide passage housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, the valve guide having a first end proximate the movable valve, the first end being positioned within the valve guide passage such that airflow through the air channel is not impeded by the valve guide.

In this way, by positioning the valve guide such that airflow through the air channel is not impeded by the valve guide, a problem that the valve guide can disrupt air intake through the air intake port can be avoided, or at least ameliorated. The valve guide may for example be positioned such that it extends to or proximate the opening in the wall of the air channel, but does not protrude (or at least does not substantially protrude) into the air channel.

Preferably, at least part of the first end of the valve guide is within less than approximately 5 mm of the opening in the wall of the air channel. More preferably, at least part of the first end of the valve guide is within less than approximately 1 mm of the opening in the wall of the air channel. In some embodiments, at least part of the first end of the valve guide is substantially flush with the wall of the air channel. In this case, optionally the first end of the valve guide is substantially flush with the wall of the air channel at all edges of the opening.

The opening may comprise a first edge distal from the air intake port outlet and a second edge proximal to the air intake port outlet.

The air channel may comprise an upper wall having a substantially straight portion which transitions to a curved portion, the curved portion curving towards the combustion chamber, wherein the opening in the wall is provided in the upper wall at or near the transition. In this case, the first edge of the opening may be on the substantially straight portion of the upper wall. The second edge of the opening may be at or near the transition. Alternatively, the second edge of the opening may be on the substantially straight portion of the upper wall. With these geometries, airflow passing the first edge will generally pass by the second edge too, rather than striking an internal surface of the valve guide channel adjacent the second edge. The valve guide passage may have a substantially uniform diameter about its central axis inwardly of the passage from the first edge. The walls of the passage about the valve guide may continue in the same direction beyond the valve guide to the edges of the opening. The valve guide may extend closer to the air channel at the first edge of the opening than at the second edge of the opening.

The opening may be elliptical, and have a major axis of: where 0_P is the diameter of the valve guide passage and a is the angle of the passage with respect to the wall of the air channel.

A volume V of free space defined between the opening, the interior walls of the passage, and the valve guide may be less than or equal to 1e ⁶ m³, and is preferably less than 5e⁷ m³, and still more preferably less than or equal to 3.7e⁷ m³. These volumes are small enough so as not to significantly disrupt airflow past the entrance to the valve guide passage.

The first edge of the opening may define a sharp transition, at a first angle, between the wall of the air channel within which the opening is provided, and a wall of the valve guide passage. The valve guide passage may be inclined at a second angle with respect to the air channel, wherein the first angle and the second angle are substantially the same. The first edge may have a radius of curvature of between zero and 3mm, and preferably between zero and 1 mm. The first angle may be acute. The first angle may for example be less than 60°, greater than 15°, and more preferably between 20° and 30°. According to another aspect of the invention, there is provided a vehicle comprising a lean-burn gasoline engine according to the above.

According to another aspect of the invention, there is provided a cylinder head for a lean-burn gasoline engine, the cylinder head comprising: an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet, the ain intake port outlet being connectable to a combustion chamber of a lean-burn gasoline engine having a combustion chamber inlet being connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet; the cylinder head being configured to receive a movable valve comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber; a valve guide opening in a wall of the air channel opposite the top surface of the movable valve; and a valve guide passage extending into the wall and away from the valve guide opening, the valve guide passage housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, the valve guide having a first end proximate the movable valve, the first end being positioned within the valve guide passage such that airflow through the air channel is not impeded by the valve guide.

According to another aspect of the present invention, there is provided a method of manufacturing a (preferably) lean-burn gasoline engine, the engine comprising an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet, and a combustion chamber having a combustion chamber inlet being connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet, a movable valve comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber, a valve guide opening in a wall of the air channel opposite the top surface of the movable valve, and a valve guide passage extending into the wall and away from the valve guide opening, the valve guide passage housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, the valve guide having a first end proximate the movable valve, the first end being positioned within the valve guide passage such that airflow through the air channel is not impeded by the valve guide, the method comprising: providing a cast part; cutting through the wall of the air channel, to form the valve guide opening and the valve guide passage.

Two main implementations are envisaged. In the first implementation, the cutting step comprises a first cutting step carried out from a side of the cast part away from the air channel and cutting towards the air channel, the first cutting step forming a first cut of the valve guide passage, followed by a second cutting step carried out through the throat and the air channel and into the first cut, which forms a second cut of the valve guide passage. In this case, the first and second cuts are carried out from and in opposite directions, but along substantially the same axis. A region of the cast part into which the cutting tool is to initially cut may be configured to be generally perpendicular to the direction of cut to reduce the likelihood of the cutting tool sliding from a desired cutting axis. The first cut therefore provides the valve guide passage in the correct place, whereas the second cut finishes the passage cleanly, and ensures accurate alignment with the throat of the air intake port (which is cut in the same step, or with a tool coaxially aligned with the tool used to cut the valve guide passage.

In a second implementation, a portion of the wall of the air channel at which the opening is to be formed is provided with a cast formation which extends into the air channel, the formation having a target surface substantially or generally perpendicular to an intended orientation of the valve guide passage starting at the target surface. In this case, the cutting step comprises a first cutting step, starting at the target surface, cutting away the formation, and forming the valve guide passage. It will be appreciated that the first cutting step in the second implementation is carried out in the opposite direction to the first cutting step in the first implementation. In the second implementation, the cutting step comprises a second cutting step, carried out through the throat and the air channel and into the first cut made by the first cutting step, which forms a second cut of the valve guide passage. It will be appreciated that the second cutting step is substantially the same for both implementations.

Also commonly to both implementations, the method may further comprise: inserting the valve guide into the valve guide passage; and machining the valve guide to form or enlarge a bore through the valve guide while the valve guide is disposed within the valve guide passage. It will be appreciated that, in the case of the bore being formed by this step, the valve guide may be inserted as a solid cylinder, whereas in the case of the bore being enlarged by this step, the valve guide may be inserted as a cylinder having a central bore extending part or entirely through it. In either case, the result of the machining step will be a bore which is dimensioned to receive the valve stem, and oriented such that the valve accurately aligns with the valve seat. It will be appreciated that the cutting steps may be carried out at an angle with respect to the upper wall of the air passage which achieves the desired sharp transition of the first edge, while the insertion step may be carried out such as to achieve the desired positioning of the valve guide. In each case achieving the desired airflow properties with respect to the valve guide passage.

According to another aspect, there is provided an engine comprising: an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet; a combustion chamber having a combustion chamber inlet being connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet; a movable valve comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber; a valve guide opening in a wall of the air channel opposite the top surface of the movable valve; and a valve guide passage extending into the wall and away from the valve guide opening, the valve guide passage housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, the valve guide having a first end proximate the movable valve, the first end being positioned within the valve guide passage such that airflow through the air channel is not impeded by the valve guide.

According to an aspect of the present invention there is provided a lean-burn gasoline engine comprising: an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet; a combustion chamber having a combustion chamber inlet being connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet; a movable valve, or air inlet valve, comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber; and a valve guide opening in an upper wall of the air channel opposite the top surface of the movable valve, and a valve guide passage extending into the upper wall and away from the valve guide opening, the valve guide passage having a passage wall and housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, wherein the valve guide opening comprises a first edge distal from the air intake port outlet and a second edge proximal to the air intake port outlet, the first edge defining a sharp transition, at a first angle, between the upper wall and the passage wall of the valve guide passage.

This sharp transition serves to detach the air flow from the valve guide opening and reduce the reverse flow into any cut-out or clearance provided at the valve guide opening, and thus reduce disturbances in the air flow past the valve guide opening and valve guide towards the combustion chamber. The valve guide passage may be inclined at a second angle with respect to the air channel, wherein the first angle and the second angle are substantially the same.

The first edge may have a radius of curvature of between zero and 3mm, and preferably between zero and 1 mm.

The first angle may be acute. For example, the first angle may be less than 60°, preferably greater than 15°, and more preferably between 20° and 30°.

The first end of the valve should not extend into the air channel. In this way, by positioning the valve guide such that airflow through the air channel is not impeded by the valve guide, a problem that the valve guide can disrupt air intake through the air intake port can be avoided, or at least ameliorated. The valve guide may for example be positioned such that it extends to or proximate the opening in the wall of the air channel, but does not protrude (or at least does not substantially protrude) into the air channel.

The air channel may comprise an upper wall having a substantially straight portion which transitions to a curved portion, the curved portion curving towards the combustion chamber, wherein the opening in the wall is provided in the upper wall at or near the transition. In this case, the first edge of the opening may be on the substantially straight portion of the upper wall. The second edge of the opening may be at or near the transition. Alternatively, the second edge of the opening may be on the substantially straight portion of the upper wall. With these geometries, airflow passing the first edge will generally pass by the second edge too, rather than striking an internal surface of the valve guide channel adjacent the second edge.

The valve guide passage has a substantially uniform diameter about its central axis inwardly of the passage from the first edge.

The walls of the passage about the valve guide may continue in the same direction beyond the valve guide to the edges of the opening.

The valve guide may extend closer to the air channel at the first edge of the opening than at the second edge of the opening.

A first end of the valve guide may extend to or proximate the opening in the wall of the air channel. For example, at least part of the first end of the valve guide extends to within less than 5 mm of the opening in the wall of the air channel. More preferably, at least part of the first end of the valve guide extends to within less than 1 mm of the opening in the wall of the air channel. In some cases, at least part of the first end of the valve guide may be substantially flush with the wall of the air channel. In one example, the first end of the valve guide is substantially flush with the wall of the air channel at all edges of the opening.

A volume V of free space defined between the opening, the interior walls of the passage, and the valve guide is less than or equal to 1e ⁶ m³, and is preferably less than 5e⁷ m³, and still more preferably less than or equal to 3.7e⁷ m³. These volumes are small enough so as not to significantly disrupt airflow past the entrance to the valve guide passage.

According to another aspect of the invention, there is provided a vehicle comprising a lean-burn gasoline engine according to the above. According to another aspect of the invention, there is provided a cylinder head for a lean-burn gasoline engine, the cylinder head comprising: an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet, the air intake port outlet being connectable to a combustion chamber inlet of a combustion chamber, the combustion chamber having a throat where the air intake port outlet meets the combustion chamber inlet; a movable valve comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber; and a valve guide opening in an upper wall of the air channel opposite the top surface of the movable valve, and a valve guide passage extending into the upper wall and away from the valve guide opening, the valve guide passage having a passage wall and housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, wherein the valve guide opening comprises a first edge distal from the air intake port outlet and a second edge proximal to the air intake port outlet, the first edge defining a sharp transition, at a first angle, between the upper wall and the passage wall of the valve guide passage.

Also commonly to both implementations, the method may further comprise: inserting the valve guide into the valve guide passage; and machining the valve guide to form or enlarge a bore through the valve guide while the valve guide is disposed within the valve guide passage. It will be appreciated that, in the case of the bore being formed by this step, the valve guide may be inserted as a solid cylinder, whereas in the case of the bore being enlarged by this step, the valve guide may be inserted as a cylinder having a central bore extending part or entirely through it. In either case, the result of the machinging step will be a bore which is dimensioned to receive the valve stem, and oriented such that the valve accurately aligns with the valve seat.

It will be appreciated that the cutting steps may be carried out at an angle with respect to the upper wall of the air passage which achieves the desired sharp transition of the first edge, while the insertion step may be carried out such as to achieve the desired positioning of the valve guide. In each case achieving the desired airflow properties with respect to the valve guide passage.

According to another aspect of the present invention, there is provided an engine comprising: an air intake port having an air intake port inlet, an air intake port outlet, and an air channel connecting the air intake port inlet to the air intake port outlet; a combustion chamber having a combustion chamber inlet being connected to the air intake port outlet, the combustion chamber inlet having a throat where the air intake port outlet meets the combustion chamber inlet; a movable valve comprising a valve head bottom surface that faces the combustion chamber, a valve head top surface that faces the air intake port and a valve stem, the movable valve being arranged to move between a closed state for closing off the combustion chamber inlet and an opened state wherein intake air can flow from the air intake port into the combustion chamber; and a valve guide opening in an upper wall of the air channel opposite the top surface of the movable valve, and a valve guide passage extending into the upper wall and away from the valve guide opening, the valve guide passage having a passage wall and housing a valve guide arranged to guide the valve stem and permit movement of the movable valve between the closed state and an opened state, wherein the valve guide opening comprises a first edge distal from the air intake port outlet and a second edge proximal to the air intake port outlet, the first edge defining a sharp transition, at a first angle, between the upper wall and the passage wall of the valve guide passage.

According to an aspect of the present invention there is provided a piston for an engine comprising a cylinder, an air inlet and an exhaust outlet, wherein the air inlet and the exhaust outlet are arranged about a longitudinal axis of the cylinder, the piston comprising: a circular peripheral wall having a central axis, wherein the peripheral wall is configured so that the central axis is substantially aligned with the longitudinal axis of the cylinder in use; and a working surface comprising: a central dished portion; an outer sloped portion, wherein the outer sloped portion surrounds the central dished portion; a first pair of valve pockets located in the outer sloped portion on a first side of the central dished portion; and a second pair of valve pockets located in the outer sloped portion on a second side of the central dished portion opposite the first side of the central dished portion, wherein the central dished portion comprises a ramp protuberance located on the first side of the central dished portion between the first pair of valve pockets, and wherein the central dished portion is configured to promote tumble of air flow into the cylinder from the air inlet, in use during an intake stroke of the piston.

The piston described above is advantageous as increased tumble in the air flowing into the cylinder during the intake stroke of the piston, and during the first portion of the compression stroke. This improves the homogeneity of the air/fuel mixture leading to a more complete combustion of the fuel and consequently improved efficiency of the engine.

Optionally the central dished portion comprises a second ramp protuberance located on the second side of the central dished portion between the second pair of valve pockets. It is beneficial to tune the shape of the working surface of the piston so that air flow down the cylinder wall towards the piston is efficiently “caught” and airflow up the wall of the cylinder is efficiently “launched” back up the cylinder. The first and second protuberances help in this regard.

The central dished portion is optionally centred on the central axis of the piston such that the distance between the central axis of the piston and the intersection of the central dished portion with the outer sloped portion on the first side of the central dished portion is equal to the distance between the central axis of the piston and the intersection of the central dished portion with the outer sloped portion on the second side of the central dished portion.

The central dished portion may be offset from the central axis of the piston such that the distance between the central axis of the piston and the intersection of the central dished portion with the outer sloped portion on the first side of the central dished portion is not equal to the distance between the central axis of the piston and the intersection of the central dished portion with the outer sloped portion on the second side of the central dished portion.

It is beneficial to tune the position of the dished portion on the working surface so that air flow is efficiently “caught” and “launched” to better promote tumble.

In one example the surface of the central dished portion conforms to a portion of the surface of a sphere. Alternatively, the surface of the central dished portion may conform to a portion of the surface of a prolate or oblate spheroid. Both of these shapes help to contain the tumble motion in the centre of the chamber so that when the flow breaks down into turbulence, it is centred around the spark plug and fuel injector.

The central dished portion optionally comprises a flat base portion surrounded by a curved wall portion for ease of manufacture with minimal impact on tumble performance.

The surface of the central dished portion is optionally asymmetrically curved about the central axis of the piston.

The piston may comprise a spark bowl located in the central dished portion.

In one example the outer sloped portion of the piston conforms to the surface of a cone.

In another aspect, the present invention provides an engine comprising a piston as described above.

The engine may comprise a cylinder head, wherein the cylinder head comprises: a combustion chamber extending into the cylinder head, the combustion chamber comprising a combustion chamber roof surface having a sloped surface portion which is configured to conform to the outer sloped portion of the piston in use; and a spark plug seat configured to support a spark plug at a predetermined position within the combustion chamber in use, wherein the combustion chamber is configured so that the apex of a geometric extension of the sloped surface portion of the combustion chamber roof surface is located within a volume envelope that is described by a 360° rotation of the spark plug when the spark plug is supported at the predetermined position in the combustion chamber by the spark plug seat.

This engine configuration promotes direction of the air and fuel mixture into the central portion of the combustion chamber, and towards the spark plug, as the piston approaches the sloped surface portions of the combustion chamber roof. This has been found to promote efficient burn of the air fuel mixture.

Optionally the gap between the sloped surface portion of the combustion chamber and the sloped outer portion of the piston is no less than 0.8mm and no more than 1 ,4mm when the piston is at top dead centre as measured when the engine is at substantially the same temperature as the environment.

In a still further aspect the present invention provides a cylinder head for an engine, the cylinder head comprising: a combustion chamber extending into the cylinder head, the combustion chamber comprising a combustion chamber roof surface having a sloped surface portion; and a spark plug seat configured to support a spark plug at a predetermined position within the combustion chamber in use, wherein the combustion chamber is configured so that the apex of a geometric extension of the sloped surface portion of the combustion chamber roof surface is located within a volume envelope that is described by a 360° rotation of the spark plug when the spark plug is supported at the predetermined position in the combustion chamber.

Optionally the sloped surface portion of the combustion chamber roof conforms to part of the surface of a cone.

In another aspect the present invention provides an engine as described above.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a vehicle in which the invention may be used;

Figure 2 shows an air intake port according to an embodiment of the invention;

Figure 3 schematically shows a bottom view of the air intake port of Figure 2;

Figures 4a and 4b schematically show side views from the plane IV-IV as shown in Figure 3, into the inside of the air intake port of Figures 2 and 3;

Figure 5a schematically shows a cross-sectional view on a combustion chamber to which an air intake port according to the prior art is attached;

Figure 5b schematically shows a cross-sectional view on a combustion chamber to which an air intake port according to an embodiment of the invention is attached;

Figure 6 schematically shows a bottom view of the air intake port of Figure 2, together with a diagram indicating the cross section at different positions along its length;

Figure 7 shows a cross section of a combustion chamber with a retracted piston and a closed inlet valve;

Figure 8 shows a close-up of the inlet valve of Figure 7;

Figure 9a shows the inlet valve of Figure 8 in a partially opened position;

Figure 9b shows the inlet valve of Figures 8 and 9a, in a more open position;

Figure 10 shows a plan view of the roof surface of the combustion chamber of Figure 7;

Figure 11 shows a second cross section of the engine block and cylinder head of Figure 7 with the piston at top dead centre;

Figure 12 shows a magnified view of the cross section of Figure 11 ;

Figure 13 shows a schematic drawing of the combustion chamber roof surface and piston of Figure 7 superimposed with an alternative combustion chamber roof surface geometry and alternative piston geometry;

Figure 14 shows a plan view of the underside of the cylinder head in the “as cast” condition;

Figure 15a shows a plan view of the underside of the cylinder head after a first machining process has been carried out;

Figure 15b shows a plan view of the underside of the cylinder head highlighting the facets cut by the first machining process;

Figure 15c shows a sectional view of the cylinder head highlighting the facets cut by the first machining process;

Figure 15d shows the volume envelope traversed by the cutter in the first machining process;

Figure 16a shows a plan view of the underside of the cylinder head after a second machining process has been carried out;

Figure 16b shows a plan view of the underside of the cylinder head highlighting the facets cut by the second machining process;

Figure 16c shows a sectional view of the cylinder head highlighting the facets cut by the second machining process;

Figure 16d shows the volume envelopes traversed by the cutter in the second machining process;

Figure 17a shows a plan view of the underside of the cylinder head after a third machining process has been carried out;

Figure 17b shows a plan view of the underside of the cylinder head highlighting the facets cut by the third machining process;

Figure 17c shows a sectional view of the cylinder head highlighting the facets cut by the third machining process;

Figure 17d shows the volume envelopes traversed by the cutter in the third machining process;

Figure 18a shows a plan view of the underside of the cylinder head after a fourth machining process has been carried out;

Figure 18b shows a plan view of the underside of the cylinder head highlighting the facets cut by the fourth machining process;

Figure 18c shows a sectional view of the cylinder head highlighting the facets cut by the fourth machining process;

Figure 18d shows the volume envelopes traversed by the cutter in the fourth machining process;

Figure 19a shows a plan view of the underside of the cylinder head after a fifth machining process has been carried out;

Figure 19b shows a plan view of the underside of the cylinder head highlighting the facets cut by the fifth machining process;

Figure 19c shows a sectional view of the cylinder head highlighting the facets cut by the fifth machining process;

Figure 19d shows the volume envelopes traversed by the cutter in the fifth machining process;

Figure 20a shows a plan view of the underside of the cylinder head after a sixth machining process has been carried out; Figure 20b shows a plan view of the underside of the cylinder head highlighting the facets cut by the sixth machining process;

Figure 20c shows a sectional view of the cylinder head highlighting the facets cut by the sixth machining process;

Figure 20d shows the volume envelope traversed by the cutter in the sixth machining process;

Figure 21 shows a plan view of the underside of the cylinder head after a seventh and eighth machining process has been carried out; Figure 22 shows the volume envelopes traversed by the cutters in the first to the eighth machining processes superimposed on one another, and

Figure 23 shows the volume envelopes traversed by the cutters in the first to the sixth machining processes in a flow diagram;

Figure 24 shows a plan view of the working surface of the piston of Figure 7;

Figure 25 shows an isometric view of an alternative piston according to the invention.

Figure 26 shows a cross section of the engine block and cylinder head of Figure 7 with the piston of Figure 25 near top dead centre.

Figure 27a shows a plan view of the working surface of a further alternative piston according to the invention;

Figure 27b shows an isometric view of the working surface of the piston of Figure 27a;

Figure 27c shows a side view of the working surface of the piston of Figure 27a;

Figure 28 shows a detailed cross-sectional view of the air inlet side of the cylinder head after an intermediate machining step has been completed;

Figure 29 shows a detailed view of the machined surface of Figure 28;

Figure 30a shows cladding depths achieved on a first curved machined profile;

Figure 30b shows cladding depths achieved on a second curved machined profile;

Figure 31 shows a detailed cross-sectional view of the air inlet side of the cylinder head after an alternative intermediate machining step has been completed;

Figure 32 shows a detailed view of the machined surface of Figure 31 ;

Figure 33a shows cladding depths achieved on a stepped machined profile;

Figure 33b shows cladding depths achieved on the curved machined profile;

Figure 34 shows a cross section of the engine block and cylinder head of Figure 7 with an alternative piston near top dead centre;

Figure 35 shows an isometric view of the working surface of the piston of Figure 34;

Figure 36 shows an isometric view of another alternative piston according to the invention;

Figure 37 shows an isometric view of a further alternative piston according to the invention;

Figure 38a shows an isometric view of a still further alternative piston according to the invention;

Figure 38b shows a schematic drawing of the combustion chamber roof surface with the piston of Figure 38a near top dead centre;

Figure 39a shows an isometric view of a further alternative piston according to the invention;

Figure 39b shows a schematic drawing of the combustion chamber roof surface with the piston of Figure 39a near top dead centre;

Figure 40a shows an isometric view of a still further alternative piston according to the invention;

Figure 40b shows a schematic drawing of the combustion chamber roof surface with the piston of Figure 40a near top dead centre;

Figures 41 A to 41 C show a further view of a valve guide passage and valve guide, and two variations thereof; and Figures 42A to 42F show a series of manufacturing steps for forming an engine in accordance with the invention.

DETAILED DESCRIPTION

Figure 1 shows a vehicle 100 in which the invention may be used. In this example, the vehicle 100 is a car, but the invention is equally applicable to other vehicles driven by a lean-burn gasoline engine 110. As mentioned above, it is to be noted that air intake port according to the invention and as described herein can be advantageously used in engines burning other fuels or fuel mixtures than gasoline. For example, the air intake port would be useful in a hydrogen burning internal combustion engine. In this vehicle 100, the lean-burn gasoline engine 110 is positioned in the front and coupled to a drivetrain to drive the front and/or rear wheels of the vehicle 100. The energy needed for driving the vehicle 100 is provided by burning fuel in the engine’s cylinders causing the cylinder pistons to drive a crankshaft that is mechanically connected to the vehicle’s drivetrain. Compared to classic internal combustion engines, the lean-burn engine 110 of this vehicle 100 burns the fuel with an excess of air in the air- fuel mixture. Lean-burn engines may mix air and fuel in proportions of, for example, 20:1 (lambda > 1.3) or even 30:1 (lambda > 2). Advantages of lean-burn engines include more efficient fuel use and lower exhaust hydrocarbon emissions than conventional gasoline engines.

In order to enable the lean burning of fuel over a large portion of the engine map, i.e. in a large range of different engine speeds as well as engine output power or torque, the engine 110 is designed in such a way to enable a large air flow into the combustion chamber and a good mixing with the relatively small amount of fuel that is to be burnt to ensure a reliable combustion process that will effectively burn all fuel, despite the oxygen rich conditions.

Figure 2 shows an air intake port 10 according to an embodiment of the invention. The air intake port 10 has an air inlet 14 and two air outlets 15a, 15b. An air channel connects the air inlet 14 to the two air outlets 15a, 15b. The first, upstream portion of the air channel, starting at the air inlet 14 forms a common duct 11. At a bifurcation point 13, at a downstream end of the common duct 11 , the common duct 11 branches off in two port legs 12a, 12b that provide the two respective air outlets 15a, 15b. The terms upstream and downstream are used to refer to parts of the air intake port 10 relative to flow of air through the air intake port 10 in its normal use with a lean-burn gasoline engine 110. The predominant air flow direction is from an upstream position to a downstream position. It follows that in normal use the engine 110 is downstream of the air intake port 10. The air outlets 15a, 15b are configured to connect to two respective inlets of the combustion chamber. Near the downstream ends of the port legs 12a, 12b, two valve guides 16a, 16 are provided, each being configured to receive a valve stem that is used for controlling the valve that selectively opens and closes the combustion chamber inlets.

The port legs 12a, 12b diverge from the bifurcation point 13 to provide for two separate air flow channels to two separate combustion chamber air inlets. At some point in between the bifurcation point 13 and the air outlets 15a, 15b, the port legs 12a, 12b stop diverging and start running in parallel, or may even converge. These directional changes are preferably designed such that any disturbance of the air flow is avoided or minimised. The advantages of the non-diverging port legs 12a, 12b and the thus obtained non-diverging air flows will be discussed in more detail below with reference to Figures 5a and 5b.

To ensure a smooth air flow between the air inlet 14 and the two air outlets 15a, 15b, the air intake port 10 is designed such that any disturbances by the bifurcation are minimised. A key aspect of this minimisation of the air flow disturbance is the sharp bifurcation angle at the bifurcation point 13. In this embodiment, the sharp bifurcation angle 13 turns into a sloped portion 131 of the ceiling of the common duct 11. A similar sloped portion may be provided in the bottom surface of the common duct 11 (see Figures 3 and 4). In other embodiments, the floor and ceiling of the common duct 11 may be flat and the sharp bifurcation angle 13 may be formed by structures provided entirely inside the common duct 11.

Additionally, the air flow disturbance may be reduced by having port legs 12a, 12b that are long enough for splitting and redirecting the incoming air flow in a gradual way. Preferably, a port leg length, measured from the bifurcation point 13 to one of the two air outlets 15a, 15b, is at least twice a diameter of the respective air outlet 15a, 15b. Without the sharp bifurcation angle, however, longer port legs 12a, 12b may be necessary to obtain the air intake flow that is needed for the desired lean combustion process. It thus is an advantage of the sharp bifurcation angle that the relatively short port legs 12a, 12b allow for a more compact design of the air intake port 10 and the engine 110.

Figure 3 schematically shows a bottom view of the air intake port 10 of Figure 2. In addition to what has already been shown in and described with reference to Figure 2, Figure 3 shows the air outlets 15a, 15b, and a sloped portion 132 in the common duct floor which leads to the bifurcation point 13. Figure 3 further shows a plane IV-IV through the air intake port 10, from which the views on the inside of the air intake port 10 as shown in Figures 4a and 4b are taken. In addition thereto, Figure 3 indicates, with three arrows 33, the direction from which the cross section is viewed in the view of Figures 4a and 4b. Figure 3 further indicates the respective longitudinal axes 111, 112a, 112b of the common duct 11 and the port legs 12a, 12b. Looking at the longitudinal axes 112a, 112b of the port legs 12a, 12b, it can be seen how they first diverge and then bend towards each other until they slightly converge near the air outlets 15a, 15b. As will be explained in more detail below with reference to Figures 5a and 5b, this converging configuration provides an important advantage by directing the airflow as it enters the combustion chamber.

The side views shown in Figures 4a and 4b thus show the inside of the air intake port 10 as seen from the plane IV-IV indicated in Figure 3. As will be explained below, Figures 4a and 4b show two different embodiments of the sharp bifurcation angle 133 according to the invention. From this viewpoint inside the common duct 11 , we look directly upon a side wall 43 of the common duct 11 and a side wall 44 of one of the leg ports 12a. In addition to a side walls 43, 44, the common duct 11 and the port legs 12a, 12b may have a ceiling 41 , a floor 42, and another side wall (not shown). It is noted that the common duct 11 and the port legs 12a are preferably not rectangularly shaped. Depending on the exact shape of the air intake port 10, the boundaries of its floor 42, side walls 43, 44, and ceiling 41 may not be easy to define. The common duct 11 and the port legs 12a, 12b may, e.g., be tubular, oval, rectangular with rounded corners, or have flat floors 42 and/or ceilings 41 with curved side walls. Combinations and variations of such shapes are possible too. In preferred embodiments, however, at least the floor 42 of the common duct 11 is substantially flat.

In prior art air intake ports with one air intake and two air outlets, the bifurcation point is typically formed as a straight and substantially vertical wall or pillar that connects the air intake floor 42 to the air intake ceiling 41. This vertical wall is situated centrally in the air intake port 10, at the end of the common duct 11. From there, the two port legs 12a, 12b and there opposing inner walls diverge.

In this case, as can be seen in the views of Figures 4a and 4b, the bifurcation is a more gradual transition and not, as in the prior art, a straight wall perpendicular to the air flow 34 through the common duct 11. In a transition zone 134 at the downstream end of the common duct 11. When approaching the bifurcation point 13, and in or around the centreline of the common duct 11, the ceiling 41 and the floor 42 of the common duct 11 start approaching each other, until the sloped portions 131, 132 of the ceiling 41 and the floor 42 meet each other in the bifurcation point 13. If these sloped portions 131, 132 are sufficiently long, they make a sharp angle 133 at this bifurcation point 13. The inventors have found that with such a sharp angle, the air flow 34 is allowed to split in two, with far less disturbance than if the bifurcation is formed by a simple vertical wall (or an approximation thereof). In order to achieve this advantageous effect, a bifurcation angle 133 of less than 90° is preferred, however even better results may be obtained with even sharper angles of, e.g., less than 75° 55°, or 45°.

In this example, the bifurcation point 13 is located centrally in the common duct 11, i.e. midway between the two side walls and at equal distances from the floor 42 and the ceiling 41. Flowever, other, less symmetric configurations may be provided without departing from the scope of the invention. For example, the bifurcation point 13 may be positioned somewhat closer to the floor 42, the sloped portion 131 at the ceiling 41 being steeper and/or longer than the sloped portion 132 near the floor 42. In other embodiments the bifurcation point 13 may be somewhat rounded to further reduce air flow disturbances and/or because manufacturing constraints. It is noted that in the event of a slightly rounded bifurcation point 13, the bifurcation angle 133 may be defined as the angle between the duct floor 42 and the duct ceiling 41 measured at a point beyond the rounded edge, e.g. at a position of 5 mm in front of the bifurcation point.

Although the sloped portions 131 , 132 are shown as straight lines with a constant slope in Figure 4a, the actual slope of these sloped portions 131, 132 may vary. For example, the sloped portions 131, 132 may have a curved profile as shown in Figure 4b with a larger slope near the bifurcation point 13 than where the sloped portions 131 , 132 meet the ceiling 41 or the floor 42 of the common duct 11. In addition to a slope in the longitudinal direction, i.e. in the direction of the air flow 34, the sloped portions 131 , 132 are preferably sloped in the transverse direction too, thereby forming an aerodynamically shaped wedge-like structure.

According to the invention, the air channel floor 42 of the air intake port 10 of Figures 4a and 4b is at least substantially flat in a direction of flow in a region adjacent to the air outlet 15a, 15b, but preferably along the whole port leg 15a, 15b and part or the whole common duct 11 too. The purpose of this flat and even air channel floor 42 is to achieve a stable and undisturbed high-volume air flow that detaches from the underlying surface 42 and is launched into the combustion chamber when reaching the end of the air intake port 10. The term ‘substantially flat’ may herein, e.g., be defined as having a difference between a minimum inclination and a maximum inclination that is less than 5 degrees.

Preferably, the flat portion of the air channel floor is designed such that the difference between the minimum and maximum inclination is less than 2, or even 1, degrees. In the example shown, the flat air channel floor 42 is a completely straight floor 42 with a constant inclination. In the event of a non-rectangular air channel, it may be difficult to distinguish the exact transition between the floor 42, walls 43, 44 and ceiling 41 of the air channel. To obtain the described benefits of the described flat floor 42, at least the central and lowest portion of the air channel is designed to be flat. Preferably, however, the floor 42 has a similar flatness in the direction of flow over at least half or even the full width of the air intake port 10. With an air channel floor 42 that is at least substantially flat in a direction of flow in a region adjacent to the air outlet 15a, 15b flow separation at the combustion chamber inlet significantly improved, thereby allowing the incoming air to first flow across the chamber before descending into the chamber. As a result, the desired tumble is achieved. This tumble is shown and discussed in more detail with reference to Figure 7.

It is noted that while the embodiments shown in Figures 4a and 4b, have this sloped or curved portion 132 that provides for a smooth transition towards the bifurcation point 13, this will still allow for the floor 42 of the common duct 11 to be at least substantially flat in a direction of air flow. The portions 132 that are sloped or curved are part of the side wall 44 separating the two port legs 12a, 12b. The air flow at either side of that side wall 44 can still follow a substantially flat floor 42 in the air flow direction. It is further to be noted that the now presented design of the bifurcation 13 and the substantially flat duct floor 42 both help to provide a stable and undisturbed high-volume air flow that detaches from the underlying surface 42 and is launched into the combustion chamber 50 when reaching the end of the air intake port 10. Both measures add to the same technical effect that is already obtained by the use of a substantially flat floor 42 in at least a downstream portion of the port legs 12a, 12b. Flowever, the advantageous effects of a substantially flat floor 42 in the common duct can also be obtained with a vertical wall type bifurcation.

Figure 5a schematically shows a cross-sectional view on a combustion chamber 50 to which an air intake port according to the prior art is attached. Like most known air intake ports with a single air inlet and two air outlets, this one has two straight port legs 42a, 42b that branch off and extend in a straight line from the bifurcation point. As a result, the air flow of the air entering the combustion chamber 50 is directed outward, toward the circular wall of that combustion chamber 50. When hitting or approaching the opposite chamber wall, the two air flow streams are then deflected inward and backward, thereby resulting in a swirl pattern that is commonly called omega swirl. Also shown in Figure 5a are two exhaust outlets 56 through which the exhaust air is expelled by the piston stroke following the combustion. Exhaust valves close off these exhaust outlets 56 before and during combustion.

Figure 5b schematically shows a cross-sectional view on a combustion chamber 50 to which an air intake port 10 according to an embodiment of the invention is attached. With this air intake port 10, the direction of the omega swirl is reversed. By delivering the intake air to the combustion chamber 50 through two port legs 12a, 12b that converge towards the air outlets 15a, 15b of the air intake port 10, the air flow of the air entering the combustion chamber 50 will first be directed down the centre of the chamber 50 and then splits to move outward. The inventors have found that by reversing the omega swirl it is ensured that a larger part of the combustion will take place closer to the centre of the combustion chamber 50, with a small push towards the exhaust valves. As a result, this leaves the unburnt end gas under the cooler intake valves. This helps to reduce knock and thus to increase the performance and durability of the engine 110.

Each one of the two port legs 12a, 12b defines a respective centre line 112a, 112b. A tangent to the centre line 112a, 112b of one of the two port legs 12a, 12b at its respective air outlet makes a port exit angle 34 with a tangent to the centre line 112a, 112b of the other one of the two port legs 12a, 12b at its respective air outlet 15a, 15b. In exemplary embodiments of the invention, the port exit angle 34 is larger than 5 degrees. In further embodiments, the port exit angle 34 may be larger than 10 or 15 degrees. It is, however, to be noted that the desired reversal of the omega swirl direction has also be obtained with a port exit angle 34 just above or even as small as zero degrees, i.e. when the port legs 12a, 12b run in parallel when approaching the combustion chamber 50.

As described above, the air flow disturbance may be further reduced by having port legs 12a, 12b that are long enough for splitting and redirecting the incoming air flow in a gradual way. Preferably, a port leg length, measured from the bifurcation point 13 to one of the two air outlets 15a, 15b, is at least twice a diameter of the respective air outlet 15a, 15b. This positive effect on the reduction of air flow disturbance adds to the air flow improvement already provided by the parallel or convergent course of the port legs when approaching the air outlets of the air intake port. Reduced air flow disturbance thereby further allows for increased control and predictability of the swirl pattern inside the combustion chamber. the end of the two port legs 12a.12b. The decrease of the cross section does not follow a continuous and linear profile but is specifically designed to provide preferable air flow conditions with an aim to provide an undisturbed, high speed and high-volume flow of air at the outlets 15a, 15b of the air intake port 10. It is noted that, if the common duct 11 and the leg ports 12a, 12b are tubular or have a constant height- width ratio, the change in cross-section size may alternatively be visualised by showing the development of the radius, height, or width between the air inlet 14 and the air outlets 15a, 15b. Even though the overall profile of the cross section does not follow a linear pattern, the cross section may decrease linearly over parts of the common duct 11 and or the port legs 12a, 12b. This may particularly happen in sections where, e.g., the width of the common duct 11 or leg ports 12a, 12b is kept constant while the height decreases linearly (or vice versa).

As can be seen in the Figure 6, the gradient of decrease of the total cross section is locally reduced in a region 31 adjacent the bifurcation point 13. The present invention discloses that by introducing this local reduction of the gradient of decrease of the total cross section in the region 31 around the bifurcation point 13, any possible disturbance of the air flow caused by the splitting and deflecting of the air flow is minimised. Preferably, the local reduction of the gradient of decrease of the total cross section is realised in the region immediately upstream and downstream of the bifurcation point 13, but the desired flow enhancing effect is at least partly achieved when reducing the gradient of decrease at only one side of the bifurcation point 13.

The air channel has an average gradient of decrease of the total cross section. The preferable average gradient will usually be a compromise between different design considerations. One possible constraint is the desired maximum speed of the air flow at the entrance of the combustion chamber. Too high speeds may lead to excessive NVH (noise, vibration, and harshness) problems and to choking of the port flow. Cylinder size and space constraints may define the maximum cross section of the air outlets of the air intake port. Given a maximum cross section and air flow speed at the outlet, a preferable average gradient of decrease of the total cross section can be established. Further constraints on the length and width of the air intake port may also play a role when determining the preferred. In preferred embodiments, the gradient of decrease of the total cross section may, for example, be locally at least 20% below the average gradient of decrease in at least a portion of the region adjacent the bifurcation point. In other embodiments, the gradient of decrease at that position may even be more than 25%, 30%, 35%, 40%, 45%, or 50% below the average gradient of decrease of the total cross section.

Optionally, like in the embodiment shown in Figure 6, the gradient of decrease of the total cross section is locally about zero in at least a portion of the region 31 adjacent the bifurcation point 13. In this embodiment, the cross section of the air intake port 10 remains substantially constant in the region around the bifurcation point, thereby allowing the air flow to move through undisturbed. In some embodiments, the gradient of decrease of the total cross section may even be locally below zero in at least a portion of the region 31 adjacent the bifurcation point 13, which means that the cross section locally increases in the region 31 around the bifurcation point 13. Preferably, the gradient of decrease of the total cross section increases downstream of the region adjacent the bifurcation point 13. As soon as the air flow is split in two branches 12a, 12b, the cross section can be decreased again in order to further increase the air flow.

In the embodiment shown in Figure 6, the gradient of decrease of the total cross section is also locally reduced in the region 32 immediately upstream of the two air outlets. The air outlets 15a, 15b of the air intake port 10 connect to the air inlets of the combustion chamber. Like near the bifurcation point 13 of the air intake port 10, there may be a risk of undesired flow disturbances when the air flow reaches the intake valves and the transition point between the air intake port 10 and the combustion chamber. To minimise such disturbances, it may be preferred to bring the gradient of decrease of the total cross section down to or below zero in the region 32 immediately upstream of the air outlets 15a, 15b.

Figure 7 shows a cross section of a portion of an engine block 52 and a cylinder head 53. The engine block 52 comprises a cylinder 57 which houses a piston 54 shown at or near bottom dead centre (BDC). The cylinder head 53 comprises a combustion chamber 50 which extends into the cylinder head 53 away from a gasket interface surface 58, which may be substantially planar. As will be described in detail below with reference to Figure 24 and beyond, other non-planar configurations may bring considerable advantages and improved combustion efficiency. A head gasket 80 is located between the engine block 52 and cylinder head 53. The cylinder head 53 is typically made of a cast aluminium alloy which is machined in critical areas to ensure geometrical accuracy.

An inlet poppet valve 51 controls the opening and closing of the first air inlet opening 91a, and an exhaust poppet valve 55 controls the opening and closing of the first exhaust outlet opening 92a. An equivalent inlet valve (not shown) controls the opening and closing of the second air inlet opening 91b, and an equivalent exhaust valve (not shown) controls the opening and closing of the second exhaust outlet opening 92b. In this Figure, the inlet valve 51 and the outlet valve 55 are both closed. A dotted line 59 provides a simplified 2D representation of the preferred air flow into and through the cylinder 57. The air flow path 59 is not possible with the inlet valve 51 in the closed position as shown. Nonetheless, the preferred air flow path 59 is shown for the purpose of illustration.

With the valve 51 and air inlet design of this embodiment, it is possible to create a tumble motion of the incoming air, first along the roof of the combustion chamber 50 towards the opposite wall, under the outlet valve 55 that closes off the exhaust outlet 56, and then down along that opposing wall, back over the top surface of the piston 54 and up along the combustion chamber wall in the direction of the inlet valve 51 again. As will be described in greater detail below, the design of the working surface 79 of the piston 54 may also optimised to create this tumble motion. The tumble is preferably kept in motion during the full intake stroke and at least a portion of the compression stroke of the piston 54 moving through the combustion chamber 50. The thus produced tumble helps to obtain an optimal distribution of air and fuel inside the combustion chamber 50 that can then break down into turbulence to facilitate the subsequent combustion process. Wherein, in this context, “turbulence” refers to a flow state having chaotic changes in velocity and pressure and no necessarily clear flow directions as is well known in the art.

In order to create the desired tumble, the valve 51 and the air inlet of the combustion chamber 50 are designed such that the air flow entering the combustion chamber 50 is promoted to detach from the floor of the port leg 12a, 12b of the air intake port 10 and to flow along the ceiling of the combustion chamber 50. Some of the specific design features that can help to promote the desired tumble are discussed below with reference to Figures 8, 9a, and 9b.

Figures 8, 9a, and 9b show a close-up of the inlet valve 51 of Figure 7. As can be seen in all these Figures, the air channel floor 42 of the port leg 12a, 12b is flat in the full region up to the air outlet 15a, 15b of the air intake port 10. The flat air channel floor 42 promotes the detachment of the air flow as soon as it leaves the air intake port 10 and enters the combustion chamber 50, which contributes to the desired tumble. The movable valve 51 comprises a bottom surface 61 that faces the combustion chamber 50 and a tapered top surface 62 that faces the air inlet passage 49a of the air intake port 10. The tapered top surface 62 is also referred to as a valve head top surface 62. The air inlet valve 51 is provided at the end of a valve stem 63 which is moveable within a valve guide insert 65 located within a valve guide opening 66 in the cylinder head 53. The valve guide passage is provided in a wall of the air channel opposite the top surface of the movable valve. The valve guide passage 66 extends into the wall and away from a valve guide opening 69 in the wall of the air channel. The valve guide passage 66 houses the valve guide 65 arranged to guide the valve stem 63 and permit movement of the movable valve between the closed state and an opened state. The valve guide insert 65 and the valve guide passage 66 share a common axis 67 along which the valve stem 63 moves in use. The inlet valve 51 is arranged to move by controlling the position of the valve stem 63. The movable valve 51 may be moved between a closed state (Figure 8) for closing off the combustion chamber inlet and an opened state (Figures 9a and 9b) wherein intake air can flow from the air intake port 10 into the combustion chamber 50.

The air inlet passage 49a extends into the cylinder head 53 away from the air inlet opening 91a. The portion of the air inlet passage 49a located proximate the air inlet opening 91a comprises an inlet throat 68. The throat 68 comprises a tapered surface 71 that is complementary with the tapered top surface 62 of the movable valve 51 , such that when the movable valve 51 is in its closed position, the movable valve 51 at least partially sinks into the throat 68. At least the flat surface 71 of the inlet throat 68 is radially symmetrical about the central axis 67 of the valve guide opening 66. The tapered flat surface 71 which forms the valve seat may be provided in a valve seat insert or may be machined directly into a wear resistant cladding which has been applied to the throat area 68 of the inlet passage 49a prior to machining of the flat valve seat surface 71.

The exhaust poppet valve 55 (Figure 7) has substantially the same construction and operation as the air inlet valve 51 described above. The exhaust outlet valve 55 moves along exhaust valve axis 69 in use to open and close the exhaust outlet passages 56a, 56b. The skilled person will appreciate that the features and construction of the air inlet valve 51, and its seating surface 71, can equally be applied to the exhaust outlet valve 55.

The created tumble is preferably kept in motion during the full intake stroke and at least a portion of the compression stroke of the piston 54 moving through the combustion chamber 50. The complementary tapered surfaces 62, 71 of the intake valve 51 and the throat together ensure that during the compression stroke, when the intake valve 51 is closed, no or little air can get trapped behind the valve 51 or between the valve 51 and an inner surface of the combustion chamber 50 while tumbling through the combustion chamber 50. The further the valve 51 is allowed to sink into the throat, the less disturbance it can cause to the desired tumble. In an embodiment of the invention, the bottom surface 61 of the movable valve 51 may even be substantially flush with an inner surface of the combustion chamber 50 when the movable valve 51 is in its closed position.

Due to the tapered surface of the throat, and because the valve 51 needs to be able to close off the air inlet, the diameter of the combustion chamber inlet is smaller than the valve diameter. The valve diameter is determined by the bottom surface 61 of the valve 51. In an embodiment of the invention, the diameter of the combustion chamber inlet is less than, e.g., 95% or 90% of a diameter of the bottom surface 61 of the movable valve 51. Not only does this allow for the desired taper 71 in the throat surface, the protruding upstream portion of the throat also helps to shield of the valve edge, thereby directing the air flow over the top surface 62 of the valve 51 (see Figure 9a) and along the roof of the combustion chamber 50 instead of around the valve edge and down along the wall closest to the combustion chamber inlet.

This effect can further be enhanced by the protruding upstream portion ending with a sharp edge 73 that promotes detachment of the air flow. In this example, the sharp edge 73 coincides with the outer end of the air channel floor 42 at the air outlet 15a, 15b of the air intake port 10. While this is the preferred embodiment, the channel floor 42 may alternatively end at a position in front of or behind the sharp edge 73. In preferred embodiments, the angle between the channel floor 42 and an adjacent portion of the throat is at least 225 degrees. Flowever, angles closer to, or even beyond, 270 degrees are even more preferred. The larger the angle, the smaller the chance that the airflow will adhere to the throat surface and finds a way down into the combustion chamber 50 immediately upon entering.

Additionally, an optional deflector 72 is provided at an inner wall of the combustion chamber 50 and protruding radially therefrom. The deflector 72 is positioned underneath an outer edge of the bottom surface 61 of the movable valve 51. This deflector 72 is arranged such that an air flow moving up along the inner wall of the combustion chamber 50 is deflected radially inward and away from the outer edge of the bottom surface 61 of the movable valve 51. As a result, the risk of any air being trapped behind the valve 51 when in a closed or almost closed position is reduced. This useful deflector 72, on top of that, brings the additional advantage that during the intake stroke, when the valve 51 is at least partially open and air is drawn into the combustion chamber 50, any air unintentionally bouncing of the top surface 62 of the valve 51 will be prevented from flowing down along the nearest inner wall of the combustion chamber 50. Instead, the deflector 72 will block this astray air flow back into the chamber 50, and in the direction of the desired tumble.

In a preferred embodiment of this lean-burn gasoline engine 110, the air intake port 10 and the valve 51 are arranged such that when the valve 51 is in its opened position, the complete bottom surface of the valve 51 is positioned below the air intake port 10. This allows the separated air flow leaving the air intake port 10 to flow along the roof of the combustion chamber 50 and towards the opposite chamber wall with minimal disturbance by the valve 51 it has to pass. In an even more preferred embodiment, the complete bottom surface 61 of the valve 51 is already positioned below the air intake port 10 when the valve 51 is only half-way between its closed position and its opened position. This further allows reduced flow disturbance by the valve 51 while the valve is still opening, thereby facilitating the creation of the desired tumble as soon as the valve 51 is opened. In alternative embodiments, the complete bottom surface 61 drops below the air intake port 10 when the valve is, e.g., 60% open.

In a further embodiment, the air intake port 10 and the valve 51 are arranged such that when the valve 51 is in its opened position, also the complete top surface 62 of the valve 51 is positioned below the air intake port 10, with the tapered angle of the top surface 62 at a similar angle as the port floor, which leads to even less disturbance of the air flow, and helps to direct the air flow across the top of the chamber, with a more prominent and stable tumble as a result. The top surface 62 may be inclined slightly upward at the point where the air flow may hit the valve 51 in order to lift the air flow up in the direction of the chamber ceiling and/or the top end of the opposing wall.

As shown in Figure 10, a pair of air inlet passages 49a, 49b open into the combustion chamber 50 on an air inlet side of the combustion chamber. The air inlet passages 49a, 49b provide a path for a flow of air to the combustion chamber 50 in use. A pair of exhaust outlets 56a, 56b are located on an exhaust outlet side 21 of the combustion chamber 50. The exhaust outlets 56a, 56b provide an exhaust path for the combustion products exiting the combustion chamber 50 in use.

The air inlet passages 49a, 49b connect to respective air inlet openings 91a, 91b located in the roof surface 90 on the air inlet side 20 of the combustion chamber 50, and the exhaust outlets 56a, 56b connect to respective exhaust outlet openings 92a, 92b located in the roof surface 90 on the exhaust outlet side 21 of the combustion chamber 50. The first air inlet opening 91a and the first exhaust outlet opening 92a are located on a first side 93a of the combustion chamber 50, and the second air inlet opening 91b and the second exhaust outlet opening 92b are located on a second side 93b of the combustion chamber 50. The first 93a and second 93b sides of the combustion chamber 50 are located on either side of a plane of symmetry 87 of the combustion chamber 50. The cross section of Figure 7 is taken along section A-A of Figure 10 which passes through the first air inlet opening 91a and the first exhaust outlet opening 92a on the first side 93a of the combustion chamber 50.

Figure 10 shows a plan view of the underside of the machined roof surface 90 of the combustion chamber 50 and Figure 11 shows a cross sectional view of the engine block 52 and cylinder head 53 along section B-B shown in Figure 10. Section B-B corresponds with the plane of symmetry 87 of the combustion chamber 50 such that every feature on the first side 93a of the combustion chamber 50 is a mirror image of every feature of the second side 93b of the combustion chamber 50.

The combustion chamber roof surface 90 extends into the cylinder head 53 away from the gasket interface surface 58. The intersection between the combustion chamber roof surface 90 and the gasket interface surface 58 comprises a combustion chamber opening 86 in the gasket interface surface 58. The pair of air inlet openings 91a, 91b, and the pair of exhaust outlet openings 92a, 92b are formed in the combustion chamber roof surface 90. For the avoidance of doubt, the internal surfaces of the air inlet passages 49a, 49b, and exhaust outlets 56a, 56b seen in Figure 10 do not form part of the combustion chamber roof surface 90.

As best shown in Figure 11, a central domed surface portion 99 of the combustion chamber roof surface 90 defines a central domed portion 88 of the combustion chamber 50. In this embodiment the central domed surface portion 99 is elongate such that it extends from one side of the combustion chamber 50 to the other in a direction substantially perpendicular to the plane of symmetry 87. In an alternative embodiment the central domed surface portion 99 may be substantially circular or oval in plan view. Note that the central domed surface portion 99 of the combustion chamber roof surface 90 is not a single smooth surface, but rather is a surface made up of a plurality of facets formed during casting of the cylinder head or made by different machine cutters during manufacture or formed during casting of the cylinder head.

Two sloped surface portions 94, 95 of the combustion chamber roof surface 90 define a sloped portion 89 of the combustion chamber 50. In this embodiment the sloped surface portions 94, 95 each have a shape which conforms to the surface of a single cone. That is to say, the sloped surface portions 94, 95 each form part of the surface of the same conical shape. In alternative embodiments, each of the sloped surface portions 94, 95 may conform to the surface of two different conical shapes such that curvature and slope of the first sloped surface 94 does not match the slope and curvature of the second sloped surface 95. In a further alternative embodiment, the sloped surfaces 94, 95 may be planar with equal or different slopes depending on design choice. As best shown in Figure 11 , the combustion chamber roof surface 90 between the sloped surface portions 94, 95 and the combustion chamber opening 86 comprises curved portions which extend from the sloped surface portions 94, 95 to the combustion chamber opening 86.

The skilled person will understand that whether the sloped surface portions 94, 95 conform to the surface of a cone, or whether they are planar, the cross sections of the sloped surface portions 94, 95 will be substantially straight along the plane of symmetry 87 of the combustion chamber 50.

A spark plug seat 75 and a fuel injector seat 76 are located in the cylinder head. Both the spark plug seat 75 and fuel injector seat 76 open into the domed surface portion 99 of the combustion chamber roof surface 90. The spark plug seat 75 opens into roof surface 90 at the approximate centre of the combustion chamber 50, and the fuel injector seat 76 opens into the roof surface 90 substantially adjacent to the spark plug seat opening further towards the air inlet openings 91a, 91b than the spark plug seat opening. Both the spark plug seat opening and the fuel injector seat opening are located substantially on the plane of symmetry 87 of the combustion chamber 50.

The spark plug seat 75 is configured so that the tip 78 of the spark plug 82 is supported towards the centre of the central domed portion 88 substantially on the plane of symmetry 87 of the combustion chamber 50. The fuel injector seat 76 is configured to support the tip 77 of the fuel injector 81 proximate the combustion chamber roof surface 90 substantially in line with the tip 78 of the spark plug 82.

Referring now to Figure 12, the slope of the sloped surface portions 94, 95 of the combustion chamber roof surface 90 along the plane of symmetry 87 is illustrated by dotted lines 84, 85. The dotted lines 84, 85 therefore represent a geometric extension of the sloped surface portions 94, 95 along the plane of symmetry 87. In this embodiment, the sloped surface portions 94, 95 have a shape which conforms to the surface of a cone which has its apex at the spark gap 83. The sloped surface portions 94, 95 are therefore configured so that the spark gap 83 of the spark plug 82 is substantially coincidental with the geometric extension of the sloped surface portions 94, 95. As discussed above, in an alternative embodiment the sloped surface portions 94, 95 do not conform to a single conical surface, but instead conform to two separate conical surfaces. In cases such as this the sloped surface portions 94, 95 may conform to conical surfaces each of which has its apex at the spark gap 83. Alternatively, one or both of the sloped surface portions 94, 95 may conform to conical surfaces which do not have an apex coincidental with the spark gap 83. In such cases, at least the geometric extension of the sloped surface portions along the plane of symmetry 87 of the combustion chamber 50 are coincidental with the spark gap 83.

In the further alternative discussed above, the sloped surface portions 94, 95 may be planar. In such cases, the geometric extension of the sloped surface portions along the plane of symmetry 87 of the combustion chamber 50 are coincidental with the spark gap 83. Planar sloped surface portions may have the same or different slopes.

The piston 54 comprises a working surface 79 which has a central scooped portion 140 and outer sloped portions 96, 97. As shown most clearly in Figure 11 , the outer sloped portions 96, 97 of the working surface 79 conform to the shape of the sloped surface portions 94, 95 of the combustion chamber roof surface 90.

As discussed above, during the intake stroke of the piston 54, and during the early stages of the compression stroke of the piston 54, the air flow path tumbles as illustrated by the dotted line 59 in Figure 7. As the piston 54 moves through the later stages of the compression stroke this tumble airflow pattern breaks down into a turbulent flow which helps to maximise combustion efficiency and flame front speed. As the air and fuel mixture is compressed into the combustion chamber 50 by the rising piston 54, the air fuel mixture is forced into the central domed portion 88 of the combustion chamber 50 as the sloped portions 96, 97 of the working surface 79 approach the sloped surface portions 94, 95 of the combustion chamber roof surface 90; this is known as “squish”. Because the sloped surface portions 94, 95 slope towards the spark gap 83 of the spark plug 82, the air fuel mixture is directed towards the spark gap 83 where it is ignited by a spark just before the piston 54 reaches top dead centre.

The sloped surface portions 94, 95 of the combustion chamber roof 90 and the sloped portions 96, 97 of the working surface 79 of the piston 54 are configured so that the maximum separation between them when the piston 54 is at top dead centre is around 1.2 mm (measured normal to the surfaces when the engine is cold). It has been found in practice that the gap between the sloped surface portions 94, 95 of the combustion chamber roof 90 and the sloped portions 96, 97 of the working surface 79 should be greater than about 0.8 mm and less than about 1.4 mm when the piston 54 is at top dead centre (measured normal to the surfaces when the engine is cold). A gap of less than about 0.8 mm risks the piston 54 hitting the cylinder head 53, and a gap any greater than about 1.4 mm results in poor combustion and insufficient “squish”. The skilled person will understand that “cold” in the above description means substantially at the same temperature as the environment.

Specific characteristics of the embodiment shown in the Figures are described below by way of example with particular reference to Figure 10. The surface area of the first sloped surface portion 94 is less than the surface area of the second sloped surface portion 95, and the length of the first sloped surface portion 94 along the plane of symmetry 87 of the combustion chamber 50 is less than the length of the second sloped surface portion 95 along the plane of symmetry 87.

The length of the intersection 135 between the first sloped surface portion 94 and the central domed portion 99 of the combustion chamber roof surface 90, and the length of the intersection 136 between the second sloped surface portion 95 and the central domed portion 99 are substantially equal. The intersection 135 between the first sloped surface portion 94 and the central domed portion 99 is located further towards the combustion chamber opening 86 than the shortest possible line joining the outermost extremities of the air inlet openings 91a, 91b. The intersection 136 between the second sloped surface portion 95 and the central domed portion 99 is located further towards the combustion chamber opening 86 than the shortest possible line joining the outermost extremities of the exhaust outlet openings 92a, 92b. The ratio of the width of the combustion chamber 50 in plan view measured along the plane of symmetry 87 and the width of the central domed portion 88 of the combustion chamber 50 measured in a direction along the plane of symmetry 87 is about 1.7:1.

Figure 13 shows a schematic drawing of the combustion chamber roof surface 90 with the piston 54 near top dead centre. An alternative combustion chamber roof surface 515 geometry with a conforming alternative piston geometry 510 is also shown superimposed with the piston 54 and combustion chamber roof surface 90.

The piston 510 has a wider central scooped portion 540 than the central scooped portion 140 of the piston 54 such that the edges of the central scooped portion 540 of the piston 510 are further towards the periphery of the piston 510 than the edges of the central scooped portion 140 of the piston 54. As a result, in order to maintain the minimum gap of between 0.8 mm and 1.4 mm between the sloped surface portions of the combustion chamber roof surface and the outer sloped portions of the working surface of the piston, the slope of the outer sloped portions 513, 514 of the working surface 79 of the piston 510 are steeper than the outer sloped portions 96, 97 of the working surface 79 of the piston 54. Consequently, the sloped surface portions 512, 516 of the combustion chamber roof surface 515 of the piston 510 are steeper than the sloped surface portions 94, 95 of the combustion chamber roof surface 90. As a result, the geometric extensions 517 of the sloped surface portions 512, 516 of the combustion chamber roof surface 515 have a common apex 518 at a different position to the common apex (at the spark gap 83) of the geometric extensions 84, 85 of the sloped surface portions 94, 95 of the combustion chamber roof surface 90. An increase of steepness 616 of about 1.6 degrees may be measurable between the geometric extensions 85 and 517. Other increases of steepness may be useful. Nonetheless, the apex 518 is located between the opening of the spark plug seat 75 in the combustion chamber roof 515 and the tip 78 of the spark plug 82 so that the air fuel mixture is directed towards the vicinity of the tip 78 of spark plug 82 where it is ignited by a spark just before the piston 510 reaches top dead centre.

As will be clear to a person skilled in the art, there are many possible configurations for a combustion chamber and associated piston and each particular engine geometry and fuel combination will require slightly different tuning of the working surface configuration and associated combustion chamber roof geometry. It has been found in practice that it is desirable for the “squish” to be aimed at the lower end of the spark plug in use. As demonstrated by the piston 510 described above, it is possible to aim the “squish at a slightly different position in the space below the spark plug. It is preferable to aim the “squish” so that the apex of a geometric extension of the sloped surface portions of the combustion chamber roof surface are located within a volume envelope that is described by a 360° rotation of the spark plug 82 when the spark plug 82 is supported by the spark plug seat 76 in the combustion chamber 50. This envelope illustrated in Figure 6 by dotted line 640. It will be clear to the skilled person that the spark plug 82 does not actually rotate in its seat 76 in use, but rather is held in a predetermined position. Flowever, the skilled person will understand that nonetheless, a notional 360° rotation of the spark plug 82 when the spark plug 82 is held at the predetermined position in the combustion chamber 50 will describe a defined volume.

In the embodiment described above the outer sloped portions 512, 516 of the combustion chamber roof surface 515 conform to the shape of a single cone such that the geometric extensions 517 of the sloped surface portions 512, 516 have a common apex. In an alternative embodiment the sloped surface portions of the combustion chamber roof surface may conform to different cones which may share a common apex, or which may have different apex locations. In such cases the apex of the geometric extensions of the different conforming conical surfaces of the combustion chamber roof surface are nonetheless located within the volume 640 described by a 360° rotation of the spark plug 82.

Although the spark plug 82 and fuel injector 81 are shown in line along the plane of symmetry 87 of the combustion chamber 50, it will be appreciated that the spark plug 82 and fuel injector 81 may in other embodiments be located sided by side in a plane perpendicular to the plane of symmetry 87 or in any other suitable position. Referring once again to Figure 10, broadly speaking the combustion chamber 50 has two zones, a central domed portion 88 and an outer sloped portion 89. The central domed portion 88 is bounded by the central domed roof surface portion 99 of the combustion chamber roof surface 90, and the outer sloped portion 89 is bounded by two sloped surface portions 94, 95 of the combustion chamber roof surface 90. In this embodiment the sloped surface portions 94, 95 each have a shape which conforms to the surface of a single cone. That is to say, the sloped surface portions 94, 95 each form part of the surface of the same conical shape. Thus, the cross sections of the sloped surface portions 94, 95 are straight along the plane of symmetry 87 of the combustion chamber 50. As best shown in Figure 12, the combustion chamber roof surface 90 between the sloped surface portions 94, 95 and the combustion chamber opening 86 comprises curved portions 98 which extend from the sloped surface portions 94, 95 to the combustion chamber opening 86.

The machining steps taken to achieve the finished profile of the combustion chamber roof surface 90 will now be described with reference to Figures 13 to 22. Figure 13 shows a plan view of the underside of the cylinder head 53 in the “as cast” condition. That is to say that the profile of the underside of the cylinder head is entirely determined by the casting process and no machining process has yet been undertaken. As shown in Figure 13, holes to form the air inlet passages 49a, 49b and exhaust outlets 56a, 56b are formed in the casting process. In addition, holes to form the spark plug seat 75 and fuel injector seat 76 are formed during the casting process. A recess 180 is formed in the underside of the cylinder head 53 in the casting process. Flowever, the recess 180 does not comprise any of the features of the finished combustion chamber roof surface 90.

Figure 15a shows a plan view of the underside of the cylinder head 53 after a first machining process has taken place. In the first machining process material is removed from the cylinder head 53 by a ball nose cutter to form the first cuts of the central domed portion 99 of the combustion chamber roof surface 90. The first machining process cuts surfaces 181, 182. Figure 15d shows the envelope 183 traced by the ball nose cutter during the first machining process. The shadow of the envelope 183 is also shown in plan in Figure 15b and in side view in Figure 15c.

Figure 15b shows a plan view of the completed combustion chamber roof surface 90. Here it can be seen that after all of the machining processes have been completed, only small portions of the first machining process cut surfaces 181 , 182 remain between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b. These remaining portions 181, 182 of the first machining process cuts form part of the central domed surface portion 99 of the combustion chamber roof 90.

Figure 16a shows a plan view of the underside of the cylinder head 53 after a second machining process has taken place. In the second machining process material is removed from the cylinder head 53 by a radiused cutter to form the second cuts of the central domed portion 99 of the combustion chamber roof surface 90. The second machining process cuts surfaces 185a, 185b. Figure 16d shows the envelope 184 traced by the radiused cutter during the second machining process. The shadow of the envelope 184 is also shown in plan in Figure 16b and in side view in Figure 16c.

Figure 16b shows a plan view of the completed combustion chamber roof surface 90. Here it can be seen that after all of the machining processes have been completed, only the end portions of the second machining process cut surfaces 185a, 185b remain at either end of the central domed portion 99 of the combustion chamber roof surface 90. These end portions 185a, 185b formed by the second machining process form part of the central domed surface portion 99 of the combustion chamber roof 90. The end portions 185a, 185b may be described as a first pair of machined facets comprising opposing curved surfaces.

The cuts made by the radiused cutter in the second machining process are beneficial as they form curved wall portions of the central domed portion 99 of the combustion chamber roof 90. This encourages an “omega swirl” flow path as the inflowing air moves from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b and then back towards the air inlet openings 91a, 91b. and from the centre of the chamber 50 towards the edges of the chamber 50. The “omega swirl” flow path is illustrated in Figure 16b by the dotted lines 175. It should be noted that the “omega swirl” flow pattern 175 is superimposed with the “tumble” flow pattern (illustrated by dotted line 59 in Figure 7) in the operating engine 110.

The cut surfaces 185a, 185b made by the radiused cutter in the second machining process form the characteristic curved portions 176a, 176b of the combustion chamber opening 86 located between the air inlet openings 91 a, 91 b and the exhaust outlet openings 92a, 92b respectively. These curved portions 176a, 176b may be described as a first pair of opposed curved sections of the combustion chamber opening 86.

Figure 17a shows a plan view of the underside of the cylinder head 53 after a third machining process has taken place. In the third machining process material is removed from the cylinder head 53 by the same radiused cutter as that used to form the second cuts 185a, 185b. The third cuts form part of the central domed portion 99 of the combustion chamber roof surface 90. The third machining process cuts surfaces 186a, 186b. Figure 17d shows the envelope 187 traced by the radiused cutter during the third machining process. The shadow of the envelope 187 is also shown in plan in Figure 17b and in side view in Figure 17c.

Figure 17b shows a plan view of the completed combustion chamber roof surface 90. Flere it can be seen that after all of the machining processes have been completed, portions of the third machining process cut surfaces 186a, 186b remain between the air inlet openings and the exhaust outlet openings. These remaining portions 186a, 186b of the third machining process cuts form part of the central domed surface portion 99 of the combustion chamber roof 90. The surfaces 186a, 186b may be described as a third pair of machined facets comprising substantially flat surfaces.

With particular reference to Figure 17c, it can be seen that the envelope 187 traced by the radiused cutter during the third machining process has a substantially flat top profile that is substantially parallel to the plane of the exhaust outlet opening 92a. As a result of this, the cut surfaces 186a, 186b formed by the third machining process are substantially flat in the region of the third cut surfaces 186a, 186b located between the air inlet openings and the exhaust outlet openings. Only the air inlet opening 91a and the exhaust outlet opening 92a are shown in Figure 17c. The substantially flat cut made by the radiused cutter in the third machining process are beneficial as they remove material between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b so that inflowing air may flow across the top of the combustion chamber 50 from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b without interference. By substantially matching the flat portions of the machined surfaces 186a, 186b located between the air inlet openings and the exhaust outlet openings to the plane of the exhaust outlet openings 92a, 92b, uninterrupted flow between the air inlet openings and the exhaust outlet openings can be maximised. This is beneficial to the creation of a tumble flow pattern in the cylinder 57 during the intake stroke of the piston 54.

Figure 18a shows a plan view of the underside of the cylinder head 53 after a fourth machining process has taken place. In the fourth machining process material is removed from the cylinder head 53 by the same radiused cutter as that used to form the second cuts 185a, 185b and the third cuts 186a, 186b. The fourth cuts form part of the central domed portion 99 of the combustion chamber roof surface 90. The fourth machining process cuts surfaces 188a, 188b. Figure 18d shows the envelope 189 traced by the radiused cutter during the fourth machining process. The shadow of the envelope 189 is also shown in plan in Figure 18b and in side view in Figure 18c.

Figure 18b shows a plan view of the completed combustion chamber roof surface 90. Flere it can be seen that the fourth machining process cut surfaces 188a, 188b are located adjacent the air inlet openings 91a, 91b. The fourth machining process cuts 188a, 188b form part of the central domed surface portion 99 of the combustion chamber roof 90. The surfaces 188a, 188b may be described as a fourth pair of machined facets comprising curved surfaces.

The cuts made by the radiused cutter in the fourth machining process are beneficial as they remove material between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b so that inflowing air may flow across the top of the combustion chamber 50 from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b without interference. In particular, the cuts 188a, 188b made in the fourth machining process help to open up the roof 90 of the combustion chamber to promote tumble flow.

Figure 19a shows a plan view of the underside of the cylinder head 53 after a fifth machining process has taken place. In the fifth machining process material is removed from the cylinder head 53 by the same radiused cutter as that used to form the second cuts 185a, 185b, the third cuts 186a, 186b and the fourth cuts 188a, 188b. The fifth cuts form part of the central domed portion 99 of the combustion chamber roof surface 90. The fifth machining process cuts surfaces 190a, 190b. Figure 19d shows the envelope 191 traced by the radiused cutter during the fifth machining process. The shadow of the envelope 191 is also shown in plan in Figure 19b and in side view in Figure 19c.

Figure 19b shows a plan view of the completed combustion chamber roof surface 90. Flere it can be seen that the fifth machining process cut surfaces 190a, 190b are located adjacent the exhaust outlet openings 92a, 92b. The fifth machining process cuts 190a, 190b form part of the central domed surface portion 99 of the combustion chamber roof 90. The surfaces 190a, 190b may be described as a fifth pair of machined facets comprising curved surfaces

The cuts made by the radiused cutter in the fifth machining process are beneficial as they remove material between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b so that inflowing air may flow across the top of the combustion chamber 50 from the air inlet openings 91a ,91b towards the exhaust outlet openings 92a, 92b without interference. In particular, the cuts 190a, 190b made in the fifth machining process help to open up the roof 90 of the combustion chamber to promote tumble flow.

Figure 20a shows a plan view of the underside of the cylinder head 53 after a sixth machining process has taken place. The sixth machining process forms the outer sloped portion 89 of the combustion chamber 50 by cutting sloped surface portions 94, 95. Figure 20d shows the envelope 192 traced by the cutter during the sixth machining process.

Figure 20b shows a plan view of the completed combustion chamber roof surface 90. Flere it can be seen that the sloped surfaces 94, 95 are located between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b respectively. The sloped surfaces 94, 95 may be described as a second pair of machined facets comprising opposing curved surfaces.

Figure 20c shows a sectional view of the cylinder head 53 along the plane of symmetry 87 of the combustion chamber. Flere it can be seen that the sloped surfaces 94, 95 are directed towards the tip of the spark plug 82. This orientation of the sloped surfaces 94, 95 is beneficial as the slope of the sloped surface 94, 95 conform to the sloped surfaces 96, 97 of the piston 54 such that as the piston 54 approaches top dead centre, the air fuel mixture is squeezed out of the sides of the combustion chamber 50 towards the tip of the spark plug 82. This helps to facilitate a more complete combustion of the air fuel mixture.

The outermost edges of the sloped surfaces 94, 95 made by the sixth machining process form curved portions of the combustion chamber opening 86 located between the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b respectively. These curved portions may be described as a second pair of opposed curved sections of the combustion chamber opening 86.

Figure 21 shows a plan view of the underside of the cylinder head 53 after seventh and eighth machining processes have taken place. The seventh machining process forms surface 193 in the vicinity of the fuel injector seat 76, and the eighth machining process forms surface 194 in the vicinity of the spark plug seat 75. Both the seventh and the eighth cuts help to open up the roof 90 of the combustion chamber 50 to allow for better flow and more complete combustion. Although the spark plug 82 and fuel injector 81 are shown in line along the plane of symmetry 87 of the combustion chamber 50, it will be appreciated that the spark plug 82 and fuel injector 81 may in other embodiments be located sided by side in a plane perpendicular to the plane of symmetry 87 or in any other suitable position.

Figure 22 shows the volume envelopes traversed by the cutters in the first to the eighth machining processes superimposed on one another, and Figure 23 shows the volume envelopes traversed by the cutters in the first to the sixth machining processes in a flow chart.

It is envisaged that the order in which the cuts are made may vary from that described above and shown in Figure 23. For example, the cuts 94, 95 made by the sloped surface cutter moving through envelope 192 may be made at the beginning of the machining process rather than near the end. Similarly, the cuts 181, 182 made by the ball nose cutter moving through the envelope 183 may be made towards the end of the machining process rather than at the beginning. In fact, any of the cuts described above may be made in any order. Flowever, it is preferred that the machining processes be ordered as illustrated in Figure 23 as this sequence provides an optimal balance between the amount of material to be removed in any given machining process, and the amount of “fresh air” that a cutter moves through. It is well known in the art that it is desirable for a cutter to continuously remove a small amount of material in a cutting process without biting too deeply into the material as this can cause imperfections in the cut surface and too much load on the cutter. It is also desirable to avoid the cutter repeatedly lifting off and re-contacting the material.

With exception of the “sixth” machining process described above with reference to Figures 20a to 20d - and represented by envelope 192 - the sequence of machining processes from the “first” to the “fifth” is preferably as described above. The “sixth” machining process is preferably made after the “fifth” machining process as described above, or it may preferably be made at the beginning of the machining processes. The “seventh” and “eighth” cuts described above with reference to Figure 21 , are preferably the last cuts to be made. Flowever, they may optionally be made at any other suitable position in the machining sequence.

It is beneficial to make all cuts made by the same tool in sequence. For example, the sequence of cuts made by the radiused cutter moving through the envelopes 184, 187, 189, 191. These machining processes may optionally be made in any suitable order. Flowever, the order shown in Figure 23, and described above with reference to Figures 16a to 19d, is preferred because it provides the above-described optimal balance between the amount of material to be removed in any given machining process, and the amount of “fresh air” that the cutter moves through.

In the description above, the completed roof surface 90 of the combustion chamber 50 comprises only machined- surfaces. In an alternative arrangement, some of the roof surface 90 of the combustion chamber 50 may be “as cast” such that no material is removed from certain areas of the roof of the recess 180.

Figure 24 shows a plan view of the working surface 79 of the piston 54. Referring to Figure 11 and Figure 24, the piston 54 comprises a circular peripheral wall 141 having a central axis 142 which is substantially aligned with the longitudinal axis 60 of the cylinder 57 when the piston 54 is arranged for operation in the cylinder 57 of the lean burn engine 110.

The working surface 79 of the piston 54 comprises a central channel 140 which extends across the working surface 79 in a direction perpendicular to the central axis 142 of the piston 54. The position of the central channel 140 on the working surface 79 is configured so that when the piston 54 is arranged for operation in the cylinder 57 of the lean burn engine 110, the central channel 140 extends across the cylinder 57 in a direction perpendicular to the plane of symmetry 87 of the combustion chamber 50.

The central channel has two ends 148a, 148b located at either end of a longitudinal centreline 160 of the central channel 140. The two ends 148a, 148b separate opposing first and second side edges 149, 150 of the central channel 140. The opposing sides of the central channel 140 comprise first and second curved side walls 151, 152 which extend from the base 153 of the central channel 140 to respective first and second side edges 149, 150. The first side wall 151 and first side edge 149 are located on an air inlet side 22 of the piston 54, and the second side wall 152 and second side edge 150 are located on an exhaust outlet side 23 of the piston 54. Note, the air inlet side 22 and the exhaust outlet side 23 of the piston 54 are with respect to the orientation of the piston 54 when arranged for operation in the cylinder 57 of the lean burn engine 110.

In this embodiment, the base 153 of the central channel 140 is substantially flat and the width of the base 153 varies along the length of the central channel 140 such that the base 153 is narrowest at each end 148a, 148b of the central channel 140, and widest at the midpoint of the central channel 140 as depicted by dotted lines 146, 147. In this embodiment, the intersections of the side walls 151, 152 with the base 153, depicted by the dotted lines 146, 147, are curved to help contain the tumble motion in the centre of the chamber 50 so that when the flow breaks down into turbulence, it is centred around the spark plug 82 and fuel injector 81. As shown most clearly in Figures 7 and 11 , the first side edge 149 on the air inlet side 22 of the piston 54 is higher relative to the base 153 than the second side edge 150, and the curve of the first side wall 151 is steeper than that of the second side wall 152.

The working surface 79 of the piston 54 comprises first and second outer sloped portions 96, 97 located radially outward of the central channel 140. As shown most clearly in Figure 11, the first and second outer sloped portions 96, 97 of the working surface 79 conform to the shape of the first and second sloped surface portions 94, 95 of the combustion chamber roof surface 90.

The first sloped portion 96 of the working surface 79 is substantially located between cut outs 144a, 144b which provide depressions in the working surface 79 for accommodating the inlet valves 51 of the lean-burn gasoline engine 110 in use when the piston 54 is at or near top dead centre. Similarly, the second sloped portion 97 of the working surface 79 is substantially located between cut outs 145a, 145b which provide depressions in the working surface 79 for accommodating the exhaust valves 55 of the lean-burn gasoline engine 110 in use when the piston 54 is at or near top dead centre. Because the cut outs 144a, 144b, 145a, 145b overlap the central channel 140, the side edges 149, 150 of the central channel 140 are discontinuous such that a centremost portion 154 of the first side edge 149 is located further towards the peripheral wall 141 of the piston 54 than the outermost portions 161a, 161b of the first side edge 149, and a centremost portion 155 of the second side edge 150 is located further towards the peripheral wall 141 of the piston 54 than the outermost portions 162a, 162b of the second side edge 150. The central portion 154 of the first side edge 149 is formed at the intersection of the first sloped portion 96 of the working surface 79 and the central channel 140, and the central portion 155 of the second side edge 150 is formed at the intersection of the second sloped portion 97 of the working surface 79 and the central channel 140.

As discussed above and illustrated in Figure 7, during the intake stroke of the piston 54, and during the early stages of the compression stroke of the piston 54, the air flow path tumbles as illustrated by the dotted line 59. The profile of the central channel 140 helps to create this tumble by “catching” the air flow as it moves down the inner wall of the cylinder 57 on the exhaust outlet side 23 of the piston 54, and then by “launching” the air flow upward towards inner the inner wall of the cylinder 57 on the air inlet side 22 of the piston 54.

It has been found in trials/computational fluid dynamic (CFD) modelling that increased tumble in the air flowing into the cylinder 57 during the intake stroke of the piston 54, and during the first portion of the compression stroke, improves the homogeneity of the air/fuel mixture leading to a more complete combustion of the fuel and consequently improved efficiency of the engine. In a preferred configuration, the slope of the first side wall 151 of the channel 140 is chosen so that the air flow is “launched” towards a mid-point 64 of the cylinder 57 (see Figure 7) when the piston 54 is at or near BDC. This maximises the tumble vortex and limits “dead zones” where there might be poor air/fuel mixing.

As discussed above, the second side edge 150 of the central channel 140 is lower than the first side edge 149 with respect to the base 153 of the central channel 140, and the second side wall 151 is not as steep as the first side wall 151. This arrangement is beneficial as the lower/shallower second side wall 152 is shaped to “catch” the downward flow of air and direct it across the top of the channel 140 without interfering with the flow of air by creating a barrier to the flow. The higher/steeper configuration of the first side wall 151 is beneficial as it helps to “launch” the airflow back up the inner wall of the cylinder 57.

The central portions 154, 155 of the first and second side edges 149, 150 of the central channel 140 are spaced further apart from one another than the outermost portions 161a, 161b, 162a, 162b of the first and second side edges 149, 150. As best shown in Figure 11, the central portions 154, 155 of the first and second side edges 149, 150 are substantially aligned with the spark plug 82 when the piston 54 is arranged for operation in the cylinder 57 of the engine 110. The greater separation of the central edges 154, 155 provides greater first and second side wall 151, 152 area in the central region of the working surface 79 of the piston 54. This is beneficial as the tumble of the air flow can be assisted to a greater extent by the increased wall surface in the vicinity of the spark plug 82.

It is not essential that the side walls 151, 152 of the central channel 140 be located at different heights above the base 153 of the central channel 140. Nor is it necessary that the side walls 151, 152 are of different steepness. Depending on the design of the engine 110, the side walls 151, 152 may be of equal height above the base 153 of the central channel 140, or the second side wall 152 may be higher than the first side wall 151 such that the second side edge 150 is higher than the first side edge 149. Similarly, the side walls 151, 152 may have equal or differing steepness depending on design choice.

In the embodiment described above, the outermost portions 161a, 161b of the first side edge 149 are higher than the outermost portions 162a, 162b of the second side edge 150, and the central portion 154 of the first side edge 149 is higher than the central portion 155 of the second side edge 150. It is not essential that every part of the first side edge 149 be higher than every part of the second side edge 150 and in some embodiments some pars of the second side edge may be higher than the corresponding part of the opposing first side edge 149.

In further alternative embodiments (not shown), the base 153 of the central channel 140 may be curved or any other suitable profile. The surface of the central channel 140, comprising the base 153 and first and second side walls 151 , 152 may advantageously conform to part of the surface of an elongate ellipsoid such as a rugby ball type of shape. A central channel having a surface which conforms to part of the surface of an elongate ellipsoid is advantageous as this shape of central channel is particularly effective at promoting the desire tumble of the airflow in the intake stroke of the piston. Flowever, it is complicated in practice to machine such a shape into the working surface of a piston. The shape of the channel 140 shown in Figure 24, formed by the base 153 and first and second side walls 151, 152, is an approximation of an elongate ellipsoid. The advantage of the shape of the central channel 140 of Figure 24 is that it is easier to manufacture than an elongate ellipsoid but benefits from a similar same overall shape so that tumble of the incoming airflow is promoted.

It is not essential that the central channel 140 be symmetrical, nor that it be centred on a centreline of the circular peripheral wall 141 of the piston 54. The longitudinal centreline 160 of the central channel 140 may offset from the centreline of the piston 54 such that it is located further towards the air inlet side 22, or the exhaust outlet side 23, of the piston 54. The cross-section of the base 153 of the central channel 140 in a plane perpendicular to the longitudinal axis 160 of the central channel 140 may be asymmetrical about the longitudinal axis 160.

The first side and/or second side walls 151, 152 of the central channel 140 may be substantially planar. Additionally, the base 153 of the central channel 140 may comprise one or more substantially planar facets.

Figure 25 shows an alternative piston 165 for use in a high compression ratio lean burn engine. A high compression ratio lean burn engine is one which operates with a compression ratio of at least 15:1. The high compression ratio piston 165 is similar in most respects to the piston 54 described above with the exception of the features mentioned below. For consistency, like numerals have been used to identify like components throughout this specification. The high compression ratio piston 165 comprises a central channel 140 which extends across the working surface 79 in a direction perpendicular to the central axis 142 of the piston 54. The surface of the central channel 140 defines a central surface 143. The position of the central channel 140 on the working surface 79 is configured so that when the piston 165 is arranged for operation in the cylinder 57 of the high-pressure lean burn engine, the central channel 140, and hence the central surface 143, extends across the cylinder 57 in a direction perpendicular to the plane of symmetry 87 of the combustion chamber 50.

Figure 26 shows the high compression ratio piston 165 arranged for operation in the cylinder 57 of a high compression ratio lean burn engine. In order to achieve a higher compression ratio than the piston 54 discussed above, it is necessary to compress the volume of air and fuel drawn into the cylinder 57 during the intake stroke of the piston 165 into a smaller volume than in the equivalent lean burn engine 110 discussed above. This is achieved by reducing the volume of the combustion chamber 50 when the piston 165 is at or near top dead centre. In particular, the volume of the central domed portion 88 of the combustion chamber 50 is reduced by the greater volume of the piston 165 which extends into the central domed portion 88 more than compared to the volume of the piston 54 that extends into the central domed portion 88 in the lean-burn engine 110. As a result, the air fuel mixture is compressed into a smaller volume by the high compression ratio piston 165 than by the piston 54.

In order to avoid arcing of the spark from the end of the spark plug 82 to the working surface 79 of the piston 165, a spark plug bowl 166 is provided substantially at the centre of the working surface 79. When the piston 165 is at or near top dead centre, the spark plug bowl provides sufficient space underneath the tip of the spark plug 82 to prevent arcing or flame quenching occurring.

Figures 27a to 27c show the working surface 79 of an alternative configuration for a high compression ratio piston 167. In this embodiment, a central elongate surface portion 168 of the working surface 79 surrounds and extends away from the spark plug bowl 166. The central elongate surface portion 168 is substantially flat such that the intersection of the sloped portions 96, 97 of the working surface 79 and the central elongate surface portion 168 define the plane of the central elongate surface portion 168. In this embodiment, the central elongate surface portion 168 extends across the working surface 79 perpendicular to the central axis 142 of the piston. The central elongate surface portion 168 has a first curved end 170a and a second opposite curved end 170b.

Although the spark plug 82 and fuel injector 81 are shown in line along the plane of symmetry 87 of the combustion chamber 50, it will be appreciated that the spark plug 82 and fuel injector 81 may in other embodiments be located sided by side in a plane perpendicular to the plane of symmetry 87 or in any other suitable position.

Figure 14 shows a plan view of the bottom surface 58 of the cylinder head 53 in the “as cast” condition. That is to say before any machining processes have been carried out. In the “as cast” condition, the cylinder head 53 comprises a combustion chamber recess 180 which extends into the cylinder head 53 away from the bottom surface 58 and which comprises a roof surface 390. The roof surface 390 of the recess 180 is machined to form the roof surface 90 of the combustion chamber 50 of the completed cylinder head. In the example of Figure 10, the whole of the surface 390 of the cast recess 180 is machined away to form the combustion chamber roof surface 90. Flowever, in other embodiments some of the cast roof surface 390 may remain after the machining processes are complete so that part of the finished combustion chamber roof surface 90 comprises sections of the original cast roof surface 390.

The air inlet passages 49a, 49b open into the cast roof surface 390 at air inlet openings 391a, 391b, and the exhaust outlet passages 56a, 56b open into the cast roof surface 390 at exhaust outlet openings 392a, 392b. It is clear that the air inlet openings 391a, 391b and the exhaust outlet openings 392a, 392b in the roof surface 390 of the “as cast” combustion chamber recess 180 will have a different shape to the air inlet openings 91a, 91b and the exhaust outlet openings 92a, 92b in the machined roof surface 90 of the combustion chamber 50. Flowever, for the purpose of this description, both are referred to as “air inlet openings” and “exhaust outlet openings” respectively and refer to the opening in the roof surface 90 of the machined combustion chamber 50, or the roof surface 390 of the cast combustion chamber recess 180, as the case may be. In addition, as discussed below, there may be an intermediate machining step in which a specific profile is machined into the throat portion of the air inlet passages 49a, 49b and/or the exhaust outlet passages 56a, 56b before a cladding layer is applied and subsequently machined to form the final valve seat profile. In such cases, the openings in the chamber roof surface 90, or the cast roof surface 390, are still referred to as “air inlet openings” and “exhaust outlet openings” regardless of the fact that they will be cladded and further machined in subsequent manufacturing processes.

It is well known in the art of laser cladding that the conditions and parameters used during the laser cladding process are critical to the weld quality and durability of the resulting laser clad material. In particular, it is important to reduce voids and porosity in the laser clad material which may lead to failure of the laser clad material in use. It is also important to minimise the heat affected zone (HAZ) thereby reducing the dilution rate in order to avoid incorporating an alloying layer between the cladding material and the material of the cylinder head 53. It is also important to ensure that the geometric characteristics of the laser clad material, such as thickness and length, are sufficient to ensure that the correct geometry can be achieved when machining the valve seat surface 71 into the laser clad material.

One factor that can affect the quality and geometric characteristics of the laser clad material is the surface profile to which the laser cladding process is applied. To investigate possible beneficial pre-clad surface profiles, the following investigations were undertaken.

Investigation 1 : Nickel Aluminium (NiAI) cladding applied to a radiused profile.

Figure 28 shows a cross-sectional view through an air inlet 49a of a cylinder head casting 53 before any cladding material has been applied. In this example, the air inlet 49a meets the roof 90 of the combustion chamber 50 at air inlet opening 393. The region of the air inlet 49a proximate the air inlet opening 393 defines a throat, or air inlet throat, 368 which extends into the cylinder head casting 53 away from the air inlet opening 393. The throat 368 has a radiused profile in a plane which passes through the central axis of the throat 368 and the edge, which may be called the outermost edge, of the throat 368. In Figure 28 it can be seen that the central axis of the throat 368 corresponds to the valve guide axis 67.

In investigation 1 , two radii for the throat 368 were tested, a radius of 3mm and a radius of 1 ,5mm. Figure 29 shows a schematic view of the throat 368 in which the two test radii are schematically illustrated by R3 and R1.5 respectively.

Nickel Aluminium (NiAI) cladding was applied to both test throat profiles in a laser cladding process. Figure 30a and Figure 30b show plots of the resulting cladding height above the pre-clad throat profile position R3 and R1.5 respectively. It will be understood by the person skilled in the art that the pre-clad profiles R3 and R1.5 do not exist after the cladding has been applied since the material of the cylinder head casting 53 is melted into the laser cladding material at the heat affected zone.

Figures 30a and 30b each show four plot lines for measurements of cladding height taken at 90° angles around the throat 368. Each plot also shows a target line 300 indicating the height of the final valve seat above the pre-clad profiles R3 and R1.5. As can be seen in Figures 30a and 30b, the throat 368 with the 3mm radiused profile resulted in cladding most closely matching the target line 300 for Nickle Aluminium (NiAI) cladding.

The cladding of each test piece was subjected to a laser hardening (re-melting) process and the results were compared to the “as welded” cladding. Flere it was found that the re-melting process had little effect on the cladding height, but micrographs of the cladding material revealed increased melting of the material of the cylinder head casting 53 in both test pieces and increased porosity in the 3mm radius test piece. For the “as welded” samples, the electron micrographs revealed no obvious porosity issues within the cladding or at the interface of the cladding and the material of the cylinder head casting 53. Investigation 2: Nickel Aluminium with Chromium Carbide hard phase (NiAI-CrC) cladding applied to a 3mm radiused profile and to a stepped profile.

In Investigation 2, two profiles for the throat 368 were tested, a radiused profile of 3mm as described above for Investigation 1 , and a stepped profile as shown in Figures 31 and 32 described below.

Figure 31 shows a cross-sectional view through an air inlet 49a having a throat 368 with a stepped profile 369, and Figure 32 shows a schematic view of the stepped profile 369. As before, the air inlet 49a meets the roof 90 of the combustion chamber 50 at air inlet opening 393. The region of the air inlet 49a proximate the air inlet opening 393 defines a throat 368 which extends into the cylinder head casting 53 away from the air inlet opening 393. The throat 368 has a stepped profile 369 in a plane which passes through the central axis of the throat 368 and the edge of the throat 368. In Figure 31 it can be seen that the central axis of the throat 368 corresponds to the valve guide axis 67. The stepped profile 369 comprises two steps 370a, 370b (Figure 32). It is envisioned that other numbers of step may be used.

Nickel Aluminium - Chromium Carbide (NiAI - CrC) cladding was applied to both test throat profiles in a laser cladding process. Figure 33a and Figure 33b show plots of the resulting cladding height above the pre-clad throat profile 369 and R3 respectively. As above, it will be understood by the person skilled in the art that the pre-clad profiles do not exist after the cladding has been applied since the material of the cylinder head casting 53 is melted into the laser cladding material at the heat affected zone.

Figures 33a and 33b each show four plot lines for measurements of cladding height taken at 90° angles around the throat 368. Each plot also shows a target line 300 indicating the height of the final valve seat above the pre-clad profiles. As can be seen in Figures 33a and 33b, both test profiles resulted in cladding height above the target line 301 for Nickle Aluminium - Chromium Carbide (NiAI - CrC) cladding.

The cladding of each test piece was subjected to a “double pass” cladding process and the results were compared to the “single pass” cladding. Flere it was found that the double pass process showed low variation between the weld profile and the cladding height for the stepped profile test piece, whereas for the radiused test piece the double pass cladding showed undulations near the centre of the cladding which reduced cladding depth. Micrographs of the “single pass” cladding material revealed that the step of the stepped profile had been melted away and that there were regions of porosity. For the radiused profile test piece, the electron micrograph of the “single pass” showed no evidence of porosity.

Investigation 3: Transition Zone.

The transition zone, or heat affected zone (FIAZ), was measured from electron micrographs of the cladded materials. For the 1 ,5mm radiused profile Nickel Aluminium (NiAI) cladded workpiece of Investigation 1 , and for the stepped profile Nickel Aluminium - Chromium Carbide (NiAI- CrC) cladded workpiece of Investigation 2, the FIAZ varied between 150 and 200pm in depth giving a dilution rate of 12%. For the 3mm radiused profile Nickel Aluminium - Chromium Carbide (NiAI-CrC) cladded workpiece of Investigation 2 the depth of the FIAZ was measured at approximately 35pm giving a dilution rate of 2%. An EDX elemental map of the 3mm radiused profile Nickel Aluminium - Chromium Carbide (NiAI-CrC) cladded workpiece of Investigation 2 showed a clear distinction between the cladding and base material of the cylinder head casting indicating minimal dilution of compositional elements between the base material and the cladding.

The above description has been given in the context of a cylinder head to be used in conjunction with a cylinder block as is well known in the art. Flowever, the skilled person will be aware of other internal combustion engine constructions having combustion chamber air inlets and exhaust outlets located elsewhere than in the roof of a combustion chamber recess located in a cylinder head, for example, side valve engine designs. It will be clear to the person skilled in the art that the above disclosure is equally applicable to other internal combustion engine designs and is not solely limited to internal combustion engines comprising air inlets and/or exhaust outlets located in a cylinder head. Figure 34 shows a cross sectional view of the engine block 52 and cylinder head 53 along section B-B shown in Figure 10. Section B-B corresponds with the plane of symmetry 87 of the combustion chamber 50 such that every feature on the first side 93a of the combustion chamber 50 is a mirror image of every feature of the second side 93b of the combustion chamber 50.

The combustion chamber roof surface 90 extends into the cylinder head 53 away from the gasket interface surface 58. The intersection between the combustion chamber roof surface 90 and the gasket interface surface 58 comprises a combustion chamber opening 86 in the gasket interface surface 58. The pair of air inlet opening s 91a, 91b, and the pair of exhaust outlet openings 92a, 92b are formed in the combustion chamber roof surface 90. For the avoidance of doubt, the internal surfaces of the air inlet passages 49a, 49b, and exhaust outlet passages 56a, 56b seen in Figure 10 do not form part of the combustion chamber roof surface 90. As best shown in Figure 34, a central domed surface portion 99 of the combustion chamber roof surface 90 defines a central domed portion 88 of the combustion chamber 50. A sloped surface portion 495 of the combustion chamber roof surface 90 defines a sloped portion 89 of the combustion chamber 50. In this embodiment, the sloped surface portion 495 of the combustion chamber roof surface 90 comprises four sections 494a, 494b, 494c, 494d. The sloped surface portion 495 - and therefore each of the four sections 494a, 494b, 494c, 494d of the sloped surface portion 495 - has a shape which conforms to the surface of a single cone. That is to say, each of the four sections 494a, 494b, 494c, 494d of the sloped surface portion 495 form part of the surface of the same conical shape.

A spark plug 82 is located in a spark plug seat 75, and a fuel injector 81 is located in a fuel injector seat 76, both being located in the cylinder head 53 such that the tip 78 of the spark plug 82 and the tip 77 of the fuel injector 71 are located in the domed portion 88 of the combustion chamber 50. The spark plug seat 75 is configured to support the tip 78 of the spark plug 75 at a predetermined position within the combustion chamber.

Figure 35 shows an isometric view of the working surface 79 of the piston 454. Referring to Figure 34 and Figure 35, the piston 454 comprises a circular peripheral wall 141 having a central axis 142 which is substantially aligned with the longitudinal axis 60 of the cylinder 57 (see Figure 7) when the piston 454 is arranged for operation in the cylinder 57 of the engine 110.

The working surface 79 of the piston 454 comprises a central dished portion 440 which is surrounded by an outer sloped portion 496. The outer sloped portion 496 comprises four sections 497a, 497b, 497c, 497d. The outer sloped portion 496 of the working surface 79 - and therefore each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496 - is configured to conform to the sloped surface portion 495 of the combustion chamber roof surface 90 when the piston 454 is installed for use in the cylinder 57. Therefore, in this embodiment, the outer sloped portion 496 - and therefore each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496, has a shape which conforms to the surface of a single cone. That is to say, each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496 form part of the surface of the same conical shape.

The surface 453 of the central dished portion 440 has a shape which conforms to the surface of a sphere with its spherical axis of symmetry in line with the central axis 142 of the piston 454. The central dished portion 440 meets the outer sloped portion 496 at a chamfered edge 450. In this embodiment, the distance between the central axis 142 of the piston 454 and the intersection of the central dished portion 440 with the outer sloped portion 496 on the air inlet side 22 of the central dished portion 440 is equal to the distance between the central axis 142 of the piston 454 and the intersection of the central dished portion 440 with the outer sloped portion 496 on the exhaust outlet side 23 of the central dished portion 440.

Two valve pockets 444a, 444b are located in the outer sloped portion 496 of the working surface 79 on an air inlet side 22 of the piston 454, and two valve pockets 445a, 445b are located in the outer sloped portion 496 of the working surface 79 on an exhaust outlet side 23 of the piston 454. Each of the two valve pockets on the air inlet side and the two valve pockets on the exhaust outlet side may be referred to as pairs. These may be termed a first pair and a second pair. References to the air inlet side 22 and the exhaust outlet side 23 of the piston 454 refer to the orientation of the piston 54 when installed for use in the cylinder 57.

The valve pockets 444a, 444b provide room to accommodate the inlet valves 51 when they are open and the piston 454 is at or near top dead centre. Similarly, the valve pockets 445a, 445b provide room to accommodate the exhaust valves 55 when they are open and the piston 454 is at or near top dead centre. Because of the different sizes and swept volumes of the air inlet valves 51 as compared to the exhaust valves 55, the valve pockets 444a, 444b located in the outer sloped portion 496 on the air inlet side 22 overlap the central dished portion 440 to define a ramp protuberance 449 located between the valve pockets 444a, 444b. By contrast, the valve pockets 445a, 445b located in the outer sloped portion 496 on the exhaust outlet side 23 do not overlap the central dished portion 440 so that the section 497c of the outer sloped portion 496 is continuous with the neighbouring section 497b, 497d of the outer sloped portion 496.

As discussed above and illustrated in Figure 7, during the intake stroke of the piston 454, and during the early stages of the compression stroke of the piston 454, the air flow path tumbles as illustrated by the dotted line 59. The profile of the central dished portion 440 helps to create this tumble by “catching” the air flow as it moves down the inner wall of the cylinder 57 on the exhaust outlet side 23 of the piston 454, and then by “launching” the air flow upward towards inner the inner wall of the cylinder 57 on the air inlet side 22 of the piston 454. It has been found in trials/computational fluid dynamic (CFD) modelling that increased tumble in the air flowing into the cylinder 57 during the intake stroke of the piston 454, and during the first portion of the compression stroke, improves the homogeneity of the air/fuel mixture leading to a more complete combustion of the fuel and consequently improved efficiency of the engine.

The ramp protuberance 449 maintains the efficacy of the tumble promoting nature of the central dished portion 440 despite the incursion into the central dished portion 440 by the valve pockets 444a, 444b.

Referring again to Figure 34, the slope of the sloped surface portion 495 of the combustion chamber roof surface 90 is illustrated by dotted lines 84 which represent a geometric extension of the sloped surface portion 495. As discussed above, in this embodiment the sloped surface portion 495 has a shape which conforms to the surface of a cone. As can be seen in Figure 34, the geometric extension 84 of the sloped surface portion 495 has its apex 85 located between the opening of the spark plug seat 75 in the combustion chamber roof 90 and the tip 78 of the spark plug 82.

During the intake stroke of the piston 454, and during the early stages of the compression stroke of the piston 454, the air flow path tumbles as illustrated by the dotted line 59 in Figure 7. As the piston 454 moves through the later stages of the compression stroke this tumble airflow pattern breaks down into a turbulent flow which helps to maximise combustion efficiency and flame front speed. As the air and fuel mixture is compressed into the combustion chamber 50 by the rising piston 454, the air fuel mixture is forced into the central domed portion 88 of the combustion chamber 50 as the outer sloped portion 496 of the working surface 79 approaches the sloped surface portion 495 of the combustion chamber roof surface 90; this is known as “squish”. Because the sloped surface portion 495 of the combustion chamber roof surface 90 has its apex 85 located between the opening of the spark plug seat 75 in the combustion chamber roof 90 and the tip 78 of the spark plug 82, the air fuel mixture is directed towards the vicinity of the tip 78 of spark plug 82 where it is ignited by a spark just before the piston 454 reaches top dead centre.

The sloped surface portion 495 of the combustion chamber roof surface 90 and the outer sloped portion 496 of the working surface 79 of the piston 454 are configured so that the maximum separation between them when the piston 454 is at top dead centre is around 1.2 mm (measured normal to the surfaces when the engine is cold). It has been found in practice that the gap between the sloped surface portion 495 of the combustion chamber roof surface 90 and the outer sloped portion 496 of the working surface 79 should be greater than about 0.8mm and less than about 1 ,4mm when the piston 454 is at top dead centre (measured normal to the surfaces when the engine is cold). A gap of less than about 0.8mm risks the piston 454 hitting the cylinder head 53, and a gap any greater than about 1 ,4mm results in poor combustion and insufficient “squish”. The skilled person will understand that “cold” in the above description means substantially at the same temperature as the environment.

Figure 36 and Figure 37 show alternative pistons suitable for use in the engine 110 comprising the engine block 52 and the cylinder head 53. Like reference numerals have been used throughout to identify like components and features.

Figure 36 shows an isometric view of the working surface 79 of an alternative piston 500. The piston 500 comprises a circular peripheral wall 141 having a central axis 142 which is substantially aligned with the longitudinal axis 60 of the cylinder 57 (see Figure 7) when the piston 500 is arranged for operation in the cylinder 57 of the engine 110.

The working surface 79 of the piston 500 comprises a central dished portion 501 which is surrounded by an outer sloped portion 496. The outer sloped portion 496 comprises four sections 497a, 497b, 497c, 497d. The outer sloped portion 496 of the working surface 79 - and therefore each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496 - is configured to conform to the sloped surface portion 495 of the combustion chamber roof surface 90 when the piston 500 is installed for use in the cylinder 57. Therefore, in this embodiment, the outer sloped portion 496 - and therefore each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496, has a shape which conforms to the surface of a single cone.

The surface 502 of the central dished portion 501 has a shape which conforms to the surface of a prolate spheroid such as a rugby ball shape. The surface 502 of the central dished portion 501 is centred about the central axis 142 of the piston 500 such that the distance between the edges 503 of the central dished portion 501 in a direction across the working surface 79 from a point on the edge 503 at the mid-point of the air intake side 22 to an opposing point on the edge 503 at the mid-point of the exhaust outlet side 23 is equally bisected by the central axis 142, and the distance between the points on the edge 503 which intersect a plane separating the piston 500 equally between the air inlet side 22 and the exhaust outlet side 23 is equally bisected by the central axis 142.

Two valve pockets 444a, 444b are located in the outer sloped portion 496 of the working surface 79 on an air inlet side 22 of the piston 500, and two valve pockets 445a, 445b are located in the outer sloped portion 496 of the working surface 79 on an exhaust outlet side 23 of the piston 500. The valve pockets 444a, 444b located in the outer sloped portion 496 on the air inlet side 22 overlap the central dished portion 501 to define a ramp protuberance 449 located between the valve pockets 444a, 444b. By contrast, the valve pockets 445a, 445b located in the outer sloped portion 496 on the exhaust outlet side 23 do not overlap the central dished portion 501 so that the section 497c of the outer sloped portion 496 is continuous with the neighbouring section 497b, 497d of the outer sloped portion 496.

Figure 37 shows an isometric view of the working surface 79 of a further alternative piston 505. The piston 505 comprises a circular peripheral wall 141 having a central axis 142 which is substantially aligned with the longitudinal axis 60 of the cylinder 57 (see Figure 7) when the piston 505 is arranged for operation in the cylinder 57 of the engine 110.

The working surface 79 of the piston 505 comprises a central dished portion 506 which is surrounded by an outer sloped portion 496. The outer sloped portion 496 comprises four sections 497a, 497b, 497c, 497d. The outer sloped portion 496 of the working surface 79 - and therefore each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496 - is configured to conform to the sloped surface portion 495 of the combustion chamber roof surface 90 when the piston 505 is installed for use in the cylinder 57. Therefore, in this embodiment, the outer sloped portion 496 - and therefore each of the four sections 497a, 497b, 497c, 497d of the outer sloped portion 496, has a shape which conforms to the surface of a single cone.

In this embodiment, the surface 507 of the central dished portion 506 comprises a spark bowl 166 which is located at the centre of the working surface 79 such that the central axis 142 of the piston 505 is located at the centre of the spark bowl 166. The surface 507 of the dished portion 506 is asymmetrical about the plane which separates the piston 505 equally between the air inlet side 22 and the exhaust outlet side 23 such that the distance between the edges 508 of the central dished portion 506 in a direction across the working surface 79 from a point on the edge 508 at the centre of the air intake side 22 to an opposing point on the edge 508 at the centre of the exhaust outlet side 23 is unequally bisected by the central axis 142. However, the surface 507 is symmetrical about a plane which passes through the central axis 142 of the piston 505 and which is perpendicular to the plane that separates the piston equally between the air inlet side 22 and the exhaust outlet side 23 such that the distance between the points on the edge 508 which intersects the plane separating the piston equally between the air inlet side 22 and the exhaust outlet side 23 is equally bisected by the central axis 142.

The base 451 of the surface 507 is substantially flat from the edges of the spark bowl 166 to a peripheral wall 452 which extends from the base 451 to the edges 508 of the dished portion 506. The peripheral wall 452 is curved with the degree of curvature varying about the central axis 142 of the piston 505 such that the peripheral wall is steepest at the mid-point of the air inlet side 22 of the piston 505 and shallowest along the plane separating the piston equally between the air inlet side 22 and the exhaust outlet side 23. The curvature of the peripheral wall 452 at the mid-point of the exhaust outlet side 23 being less than that of the point of the peripheral wall 452 at the opposing mid-point of the air inlet side 22, and greater than that of the peripheral wall 452 along the plane separating the piston equally between the air inlet side 22 and the exhaust outlet side 23. This configuration allows the working surface 79 of the piston 505 to be tuned to promote the tumble of the air flow in the cylinder 57. Preferably, the curvature of the peripheral wall 452 at the mid-point of the air inlet side 22 of the piston 505 is chosen so that the air flow is “launched” towards a mid-point 64 of the cylinder 57 (see Figure 7) when the piston 505 is at or near BDC. This maximises the tumble vortex and limits “dead zones” where there might be poor air/fuel mixing. The curvature of the peripheral wall 452 at the mid-point of the exhaust outlet side 23 of the piston 505 is chosen so that the air flow is “caught” as it moves down the wall of the cylinder 57 towards the piston 505.

Two valve pockets 444a, 444b are located in the outer sloped portion 496 of the working surface 79 on an air inlet side 22 of the piston 505, and two valve pockets 445a, 445b are located in the outer sloped portion 496 of the working surface 79 on an exhaust outlet side 23 of the piston 505. The valve pockets 444a, 444b located in the outer sloped portion 496 on the air inlet side 22 overlap the central dished portion 506 to define a ramp protuberance 449 located between the valve pockets 444a, 444b. By contrast, the valve pockets 445a, 445b located in the outer sloped portion 496 on the exhaust outlet side 23 do not overlap the central dished portion 506 so that the section 497c of the outer sloped portion 496 is continuous with the neighbouring section 497b, 497d of the outer sloped portion 496.

Figure 38a shows an isometric view of a still further alternative piston 510 and Figure 38b shows a schematic drawing of the combustion chamber roof surface with the pistons of Figures 35 and 38a near top dead centre. The piston 510 of Figure 38a is similar in all respects to the piston 454 of Figure 35 except that the central dished portion 511 of the working surface 79 is wider than that of the piston 454 of Figure 35. As a result, the valve pockets 445a, 445b on the exhaust outlet side 23 of the piston 510 overlap the central dished portion 511 to define a second ramp protuberance 448 located between the valve pockets 445a, 445b.

As best shown in Figure 38b, the piston 510 of Figure 38a has a wider central dished portion 511 than the central dished portion 440 of the piston 454 of Figure 35 such that the edge 513 of the central dished portion 511 is further towards the circular peripheral wall 141 of the piston 510 than the edge 450 of the central dished portion 440 of the piston 454 of Figure 35. As a result, in order to maintain the minimum gap of between 0.8mm and 1 ,4mm between the sloped surface portion of the combustion chamber roof surface and the outer sloped portion of the working surface of the piston, the slope of the outer sloped portion 514 of the working surface 79 of the piston 510 is steeper than the outer sloped portion 496 of the working surface 79 of the piston 454 of Figure 35. Consequently, the sloped surface portion 516 of the combustion chamber roof surface 515, which is configured to conform to the outer sloped portion 514 of the working surface 79 of the piston 510, is steeper than the sloped surface portion 495 of the combustion chamber roof surface 90 which is configured to conform to the outer sloped portion 476 of the working surface 79 of the piston 454. As a result, the geometric extension 517 of the sloped surface portion 516 of the combustion chamber roof surface 515 has its apex 518 at a different position to the apex 85 of the geometric extension 84 of the sloped surface portion 495 of the combustion chamber roof surface 90. Nonetheless, the apex 518 is still located between the opening of the spark plug seat 75 in the combustion chamber roof 515 and the tip 78 of the spark plug 82 so that the air fuel mixture is directed towards the vicinity of the tip 78 of spark plug 82 where it is ignited by a spark just before the piston 510 reaches top dead centre.

Figure 39a shows an isometric view of a still further alternative piston 520 and Figure 39b shows a schematic drawing of the combustion chamber roof surface with the pistons of Figures 35 and 9a near top dead centre. The piston 520 of Figure 39a is similar in all respects to the piston 454 of Figure 35 except that the surface 522 of the central dished portion 521 has an elongated curved shape centred about the central axis 142 of the piston 520 such that the distance between the edges 523 of the central dished portion 521 in a direction across the working surface 79 from a point on the edge 523 at the mid-point of the air intake side 22 to an opposing point on the edge 523 at the mid-point of the exhaust outlet side 23 is equally bisected by the central axis 142, and the distance between the points on the edge 523 which intersect a plane separating the piston 520 equally between the air inlet side 22 and the exhaust outlet side 23 is equally bisected by the central axis 142.

As best shown in Figure 39b, the central dished portion 521 of the piston 520 is substantially the same width as the central dished portion 440 of the piston 454 of Figure 35. Flowever, the edge 523 of central dished portion 521 is higher at the mid-points of the air inlet 22 and exhaust outlet 23 sides than the edge 450 of the central dished portion 440 of the piston 454. As a result, the valve pockets 445a, 445b on the exhaust outlet side 23 of the piston 520 overlap the central dished portion 521 to define a second ramp protuberance 448 located between the valve pockets 445a, 445b.

In order to maintain the minimum gap of between 0.8mm and 1.4mm between the sloped surface portion of the combustion chamber roof surface and the outer sloped portion of the working surface of the piston, the slope of the outer sloped portion 524 of the working surface 79 of the piston 520 is steeper than the outer sloped portion 496 of the working surface 79 of the piston 454. Consequently, the sloped surface portion 526 of the combustion chamber roof surface 525, which is configured to conform to the outer sloped portion 524 of the working surface 79 of the piston 520, is steeper than the sloped surface portion 495 of the combustion chamber roof surface 90 which is configured to conform to the outer sloped portion 476 of the working surface 79 of the piston 454. As a result, the geometric extension 527 of the sloped surface portion 526 of the combustion chamber roof surface 525 has its apex 528 at a different position to the apex 85 of the geometric extension 84 of the sloped surface portion 495 of the combustion chamber roof surface 90. Nonetheless, the apex 528 is still located between the opening of the spark plug seat 75 in the combustion chamber roof 525 and the tip 78 of the spark plug 82 so that the air fuel mixture is directed towards the vicinity of the tip 78 of spark plug 82 where it is ignited by a spark just before the piston 520 reaches top dead centre.

Figure 40a shows an isometric view of a still further alternative piston 530 and Figure 40b shows a schematic drawing of the combustion chamber roof surface with the pistons of Figures 35 and 40a near top dead centre. The piston 530 of Figure 40a is similar in all respects to the piston 454 of Figure 35 except that the surface 532 of the central dished portion 531 conforms to the shape of a prolate spheroid centred about the central axis 142 of the piston 530.

As best shown in Figure 40b, the central dished portion 531 of the piston 530 is narrower than the width of the central dished portion 440 of the piston 454 of Figure 35, and the edge 533 of central dished portion 521 is higher at the mid-points of the air inlet 22 and exhaust outlet 23 sides than the edge 450 of the central dished portion 440 of the piston 454. As a result, the valve pockets 445a, 445b on the exhaust outlet side 23 of the piston 530 adjoin the central dished portion 531.

In order to maintain the minimum gap of between 0.8mm and 1.4mm between the sloped surface portion of the combustion chamber roof surface and the outer sloped portion of the working surface of the piston, the slope of the outer sloped portion 534 of the working surface 79 of the piston 530 is steeper than the outer sloped portion 496 of the working surface 79 of the piston 454. Consequently, the sloped surface portion 536 of the combustion chamber roof surface 535, which is configured to conform to the outer sloped portion 534 of the working surface 79 of the piston 530, is steeper than the sloped surface portion 495 of the combustion chamber roof surface 90 which is configured to conform to the outer sloped portion 476 of the working surface 79 of the piston 454. As a result, the geometric extension 537 of the sloped surface portion 536 of the combustion chamber roof surface 535 has its apex 538 at a different position to the apex 85 of the geometric extension 84 of the sloped surface portion 495 of the combustion chamber roof surface 90. Nonetheless, the apex 538 is still located between the opening of the spark plug seat 75 in the combustion chamber roof 535 and the tip 78 of the spark plug 82 so that the air fuel mixture is directed towards the vicinity of the tip 78 of spark plug 82 where it is ignited by a spark just before the piston 530 reaches top dead centre.

As will be clear to a person skilled in the art, there are many possible configurations for a piston having a central dished portion surrounded by an outer sloped portion and each particular engine geometry and fuel combination will require slightly different tuning of the working surface configuration and associated combustion chamber roof geometry. It has been found in practice that it is desirable for the “squish” to be aimed at the lower end of the spark plug in use. As demonstrated by the pistons described above, it is possible to aim the “squish at slightly different positions in the space below the spark plug. It is preferable to aim the “squish” so that the apex of a geometric extension of the sloped surface portion of the combustion chamber roof surface is located within a volume envelope that is described by a 360° rotation of the spark plug 82 when the spark plug 82 is supported by the spark plug seat 76 in the combustion chamber 50. This envelope illustrated in Figure 40b by dotted line 540. It will be clear to the skilled person that the spark plug 82 does not actually rotate in its seat 76 in use, but rather is held in a predetermined position. Flowever, the skilled person will understand that nonetheless, a notional 360° rotation of the spark plug 82 when the spark plug 82 is held at the predetermined position in the combustion chamber 50 will describe a defined volume.

In the embodiments described above the outer sloped portions 496, 514, 524, 534 of the pistons have all conformed to the shape of a single cone such that the geometric extensions 84, 517, 527, 537 of the sloped portions 496, 514, 524, 534 all have a common apex. In an alternative embodiment the outer sloped portion of the piston may have sections which conform to different cones which may share a common apex or which may have different apex locations. In such cases the apex of the geometric extensions of the different conforming conical surfaces of the combustion chamber roof are nonetheless located within the volume 540 described by a 360° rotation of the spark plug 82.

In a further alternative the outer sloped portion of the piston may comprise planar facets. In such cases, the geometric extensions of the different conforming flat surfaces of the combustion chamber roof are aimed at volume 540 described by a 360° rotation of the spark plug 82.

It will be understood that the different configurations of the working surface 79 of the pistons 454, 50, 505, 510, 520, 530 described above are examples only and that may different configurations are possible, In particular, it will be understood that the dished surface portions may be centrally located about the central axis 142 of the piston or may be off set from centre, may be symmetrical or asymmetrical, may have a flat or curved base, and may comprise a spark bowl.

Referring again to Figure 8, 9a, and 9b, the air intake port comprises an air channel defined between a top wall/ceiling 41 and a bottom wall/floor 42 (and side walls, not shown in Figure 8). The air intake port comprises an air intake port inlet 44 and an air intake port outlet. The combustion chamber inlet is coterminous with the air intake port outlet. To achieve the tumble motion described above, it is beneficial to minimise disruption to the air flow into the combustion chamber as it passes through the air channel, particularly as it approaches the air inlet opening 91a. The valve guide 65 and the valve guide passage 66 are configured such that they minimise disruption to airflow through the air channel. Specifically, the valve guide 65 has a first end proximate the movable valve, the first end being positioned within the valve guide passage 66 such that airflow through the air channel is not impeded by the valve guide 65. This non-impedance is achieved by providing that the valve guide 65 does not protrude into the air channel (which would form an obstacle to disrupt airflow), and by providing that the valve guide 65 is not significantly recessed into the upper wall 41 of the air channel (which would cause a large volume/cross section increase in the air channel just before the valve, again disrupting airflow). Given the positioning of the valve guide 65 and valve guide passage 66 just before the entrance to the combustion chamber 50, such disruptions to airflow would have significant effects on the desired tumble motion shown in Figure 7.

The valve guide 65 is positioned such that it extends to or proximate the opening in the upper wall 41 of the air channel but does not protrude (or at least does not substantially protrude) into the air channel. That is, the valve guide 65 may be provided entirely within the valve guide passage 66 and thus outside of the air channel. While the valve guide 65 may be flush with the upper wall 41 of the air channel, it is acceptable for it to be set back slightly, for example due to manufacturing tolerances. For example, at least part of the first end of the valve guide 65 may be within less than 5 mm of the opening in the upper wall 41 of the air channel, but more preferably, within less than 1 mm of the opening in the upper wall 41 of the air channel. In some embodiments, at least part of the first end of the valve guide is substantially flush with the wall of the air channel. In this case, optionally the first end of the valve guide 65 is substantially flush with the wall of the air channel at all edges of the opening. It will be appreciated that this would require the first end of the valve guide 65 to be shaped with an angled end, in the case that the valve guide passage 66 is at a non-perpendicular angle with respect to the upper wall 41.

The opening can be seen to comprise a first edge distal from the air intake port outlet and a second edge proximal to the air intake port outlet. The air channel can be seen to comprise an upper wall having a substantially straight/flat portion (generally opposite to a substantially straight/flat floor of the air channel) which transitions to a curved portion, the curved portion curving towards the combustion chamber. The opening in the wall is provided in the upper wall at or near the transition from the straight portion to the curved portion. The first edge of the opening is on the substantially straight portion of the upper wall, whereas the second edge of the opening is at or near the transition.

Referring to Figure 41a, the valve guide 65 and passage 66 are shown in more detail. The valve guide passage 66 can be seen to have a substantially uniform diameter about its central axis inwardly of the passage from the first edge. The walls of the passage 66 about the valve guide 65 continue in the same direction beyond the valve guide 65 to the edges 46a, 46b of the opening. Because the valve guide passage 66 is at an acute angle to the upper wall 41 of the intake port channel, and because the valve guide 65 is cylindrical, the valve guide 65 extends closer to the air channel at a first edge 46a of the opening than at a second edge 46b of the opening. The first edge is distal from the air inlet 49a, while the second edge is proximal the air inlet 49a. If the valve guide passage 66 is perpendicular to the wall of the intake port channel, or if at least one end of the valve guide is angled, then the valve guide may instead be flush with, or at the same distance from, the upper wall 41 of the air channel at all edges of the opening.

Due to the valve guide passage 66 being at an angle to the upper wall 41 of the intake port channel, the size and shape of the opening are not the same as the diameter of the valve guide passage itself (about its central axis). In particular, the opening will be an ellipse rather than a circle, and will have a minor axis which substantially matches the diameter of the valve guide passage, and a major axis (generally in the direction of airflow within the intake port channel) of:

0_d = Op / Sin a, where 0_P is the diameter of the valve guide passage and a is the angle of the passage with respect to the upper wall 41 of the air channel.

The described geometry gives rise to a relatively small volume V of free space defined between the opening, the interior walls of the passage, and the valve guide (referred to generally herein as the valve guide cut-out / clearance). This volume may for example be less than or equal to 1e ⁶ m³, and preferably less than 5e⁷ m³, and still more preferably less than or equal to 3.7e⁷ m³. It can be seen from Figures 41a to 41c that the first edge 46a distal from the air intake port outlet defines a sharp transition, at a first angle, between the upper wall 41 of the air intake port channel and the passage wall of the valve guide passage 66. This sharp transition serves to detach the air flow from the valve guide opening 69 and thus reduce disturbances in the air flow past the valve guide opening 69 and valve guide 65. In general terms, the valve guide passage 65 may be inclined at a second angle with respect to the air channel, with the second angle being either the same or different to the first angle. Usually, the first and second angles will be substantially the same, representing the case where the walls of the passage continue to the upper wall 41 without narrowing or widening at the opening. This is the case shown in Figures 41a. That is, a single angled transition is provided from the upper wall of the air intake port to the passage wall of the valve guide passage. This geometry is the simplest to machine and ties the sharpness of the transition to the angle of the passage with respect to the air channel. In alternative arrangements, the first and second angles may be different. Two examples of such are shown in Figure 41b and Figure 41c. In Figure 41b, the first angle is smaller than the second angle. This provides for a sharper transition at the first edge 46a than would be provided in the case of a valve guide passage 66 which is at a larger angle with respect to the air intake port channel. In contrast, in Figure 41 b the first angle is larger than the second angle. This provides for a less sharp transition but may be beneficial in the case where the second angle of the valve guide passage with respect to the air intake port channel is sufficiently acute that the first edge 46a would be too sharp/thin and fragile to be formed by machining, or to survive ongoing use.

In any of these cases, the sharp corner 46a in the direction of flow in the roof 41 of the channel minimises reverse flow into the valve guide cut out (empty portion of passage, where the valve guide does not extend to the opening). It will be appreciated that in the case of a valve guide which is entirely flush with the opening, there will be no valve guide cut out/clearance into which reverse flow could occur, and no corner.

While the present technique utilises a sharp edge/transition at corner 46a to achieve the desired effects, some degree of roundedness may be permitted without losing these benefits. It is therefore envisaged that the first edge may thus have a radius of curvature of between zero and 3mm, and preferably between zero and 1 mm. Preferably though, the first edge is formed (for example using the cutting techniques described subsequently) without actively adding a rounded corner at the first edge.

The first angle is preferably acute. The first angle may for example be between 60° and 0°. Preferably the first angle is greater than 15°. Still more preferably, the first angle is between 20° and 30°. Generally, the more acute the angle, the more detached the air flow, and the less reverse flow will occur into the valve guide cut-out/clearance. It will however be appreciated that very small angles may not be possible due to manufacturing limitations, structural integrity and other operational reasons.

Referring to Figures 42a to 42f, a manufacturing method for forming a cylinder head of an engine, and in particular for forming the valve guide and valve guide passage, as described above, is schematically illustrated.

As shown in Figure 42a, a cast part 190 of the cylinder head is provided at a first step. In the cast part, a portion of the upper wall 41 of the air channel at which the opening to the valve guide passage is to be formed is provided with a cast formation 191b which extends into the air channel. That is, where the valve guide passage opening is to be formed, the upper wall 41 of the air channel in the cast part has a formation which protrudes into the air channel. The formation 191b has a target surface substantially or generally perpendicular to an intended orientation (angle with respect to the air channel) of the valve guide passage.

As can be seen in Figure 42b, a first cut of the valve guide passage 66 is then carried out at a second step. In particular, starting at the target surface, a drill or other cutting tool is used to cut away the formation, and into the wall of the air channel, to form the valve guide opening and the valve guide passage. Since the target surface is perpendicular to the intended orientation of the valve guide passage, and thus to the cutting direction of the cutting tool, the cutting tool is less likely to deviate from its intended cutting axis than would be the case if it were to attempt to cut at an acute angle against a wall of the air channel. This improves the accuracy in the positioning of the valve guide passage, the quality of the cut, and reduces the likelihood of tool breakage due to shear forces. It will be appreciated that the desired orientation of the valve guide passage is on a cutting axis which passes through the mouth of the air intake port - permitting the cutting tool access via the mouth. It is apparent from Figure 42b that the protruding formation in the original cast part 190 has been completely removed by the second step.

In an alternative embodiment, referring to Figure 42a the cast part is not provided with a formation 191b extending into the air channel, and instead the upper wall of the air channel is continuous in this region. In this case, the passage cut to form the valve guide passage 66 shown in Figure 42b is cut from above (as shown in Figure 42b), that is from a side of the cast part opposite to the air intake port, and air inlet. In order that the cut be accurate and the cutting tool not wander on first contact with the cast part 190, a formation 191 a is used, having a target surface substantially perpendicular to the direction of the cut.

In Figure 42c, the passage cut in Figure 42b is widened out by a further cutting process at a third step. Since an existing bore is present (irrespective of which direction it has been cut from), a reamer may be used for this further cutting process, to widen out and tidy up the passage. Comparison of Figures 42b and 42c reveals the diameter of the valve guide passage in Figure 42c is greater than in Figure 42b. In addition to providing a wider passage, this second cut cleans up (smooths) the interior of the passage, ready to receive the valve guide.

In Figure 42d, a cladding 192 is added to the mouth of the air inlet, generally in the form of a bead of weld at a fourth step. The cladding 192 provides the valve seat for the intake valve.

In Figure 42e, the valve guide 65, which may be of stainless steel, is inserted (pressed) into the valve guide passage 66 formed in the above steps, at a fifth step. The inserted valve guide 66 is generally cylindrical, with a predrilled pilot hole extending through it (or at least part way through it) at its longitudinal axis. In the present example, the valve guide passage extends entirely through the cylinder head, and so the valve guide may be inserted from either direction. The valve guide 65 is dimensioned to provide a tight fit within the valve guide passage 66, and thus may need to be forcibly urged into place within the valve guide passage. As shown in Figures 41a to 41c and 42e and 42f, one end of the valve guide is provided with a sloped or chamfered exterior surface to aid its insertion into the valve guide passage. The chamfered end is located proximate the intake port channel once the valve guide is fully inserted into the valve guide passage.

In Figure 42f, a further machining step is carried out at a sixth step while the valve guide is in situ within the valve guide passage. The further machining step cuts a desired diameter hole/through bore about the pilot hole in the valve guide. As a result of the final bore through the valve guide being cut while the valve guide is in situ within the valve guide passage, the alignment of the bore (which will subsequently receive the stem of the valve, as per Figures 7 and 8) with respect to the valve seat (via which the cut is made) can be more accurately achieved, while permitting the original manufacturing of the valve guide (with only a pilot bore) to be carried out with less stringent manufacturing tolerances, reducing cost.

In carrying out the above steps, the channel is machined at the second and third steps, the guide inserted with a pilot hole in it at a fifth step, and then the final machining of the valve seat and the inner diameter of the valve guide are carried out at the final, sixth, step with the same tool, and from the inside (through valve mouth) so that the valve guide inner and the valve seat are concentric. This in turn reduces any valve seat wear from the valves landing off centre and being dragged back into the seat during use.

In addition to the improved airflow characteristics achieved by both the positioning of the valve guide within the valve guide passage, and the sharp angle transition of the valve guide passage from the intake port channel, these features additionally result in reduced opportunities for material (such as fuel or debris) to travel back from the combustion chamber (due to the undisturbed airflow) and fewer recesses into which such fuel or debris can become trapped. Due to the undisturbed air flow, any material carried out from the combustion chamber may be carried back in by the airflow in the next intake cycle, since the fuel or debris cannot readily become trapped where the airflow cannot dislodge it.

Generally, the features described in relation to the intake are not necessarily applied on the outlet side. This is because the flow characteristics on the outlet are not relevant to defining the tumble motion desired in the present application.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A piston for an engine comprising a cylinder, an air inlet and an exhaust outlet, wherein the air inlet and the exhaust outlet are arranged about a longitudinal axis of the cylinder, the piston arranged to operate in the cylinder, the piston comprising: a circular peripheral wall having a central axis, wherein the peripheral wall is configured so that the central axis is substantially aligned with the longitudinal axis of the cylinder in use; and a working surface comprising a central channel extending across the working surface perpendicular to the central axis and having two ends each located on opposite sides of the central axis, wherein opposing sides of the channel each comprise a side wall which extend from a base of the channel to a respective side edge of the channel, wherein the opposing side edges of the channel are separated by the two ends of the channel, wherein the channel is configured to promote tumble of air flow into the cylinder from the air inlet, in use during an intake stroke of the piston.

2. A piston as claimed in claim 1 , wherein the width of the channel varies along the length of the channel.

3. A piston as claimed in claim 1 or claim 2, wherein the base of the channel is substantially flat, optionally wherein the width of the base of the channel varies along the length of the channel.

4. A piston as claimed in any preceding claim, wherein the depth of the channel varies along the length of the channel.

5. A piston as claimed in claim 1 , wherein the surface profile of the channel conforms to at least part of the surface of a three- dimensional elongated ellipsoid.

6. A piston as claimed in any preceding claim, wherein the channel is asymmetrical about a longitudinal centreline of the channel extending between the two ends of the channel.

7. A piston as claimed in any preceding claim, wherein a longitudinal centreline of the channel extending between the two ends of the channel is laterally offset from a parallel centreline of the circular peripheral wall of the piston.

8. A piston as claimed in any preceding claim, wherein one of the side walls of the channel is steeper than the other side wall of the channel.

9. A piston as claimed in any preceding claim, wherein at least one of the side walls is curved.

10. A piston as claimed in any preceding claim, wherein at least a part of the side edge on a first side of the channel is at a different height to at least a part of the side edge on a second side of the channel relative to a plane perpendicular to the central axis, which plane intersects the base of the central channel, optionally wherein the side edge of the channel on the first side of the piston is higher than the side edge of the channel on the second side of the piston along at least part of the length of the channel.

11. A piston as claimed in any preceding claim, wherein the central channel is configured to direct air flow towards a mid-point of the portion of the cylinder located above the piston when the position is located substantially at bottom dead centre in use.

12. A piston as claimed in any preceding claim, wherein the working surface of the piston comprises depressions for accommodating valve heads of the lean-burn gasoline engine in use when the piston is at or near top dead centre.

13. A piston as claimed in any preceding claim, wherein the working surface comprises sloped surface portions located radially outward of the channel with respect to the circular peripheral wall of the piston, wherein each sloped surface portion extends away from a side edge of the channel downwardly towards the peripheral wall of the piston.

14. A piston as claimed in any preceding claim, comprising a spark bowl located in the base of the channel.

15. An internal combustion engine comprising a piston as claimed in any one of claims 1 to 14, optionally wherein the engine is a lean-burn gasoline engine.

16. A piston as claimed in claim 14, wherein the sloped surface portions of the working surface are configured to conform to at least part of a roof surface of a combustion chamber of the lean-burn gasoline engine in use.

17. An internal combustion engine comprising a piston as claimed in claim 15, comprising a cylinder head having a combustion chamber formed therein, wherein at least part of the roof of the combustion chamber is configured to conform to the sloped surface portions of the working surface of the piston in use.

18. A vehicle comprising an internal combustion engine according to any one of claims 15 or 17.