WO2022199839A1

WO2022199839A1 - A piston for a lean-burn gasoline engine

Info

Publication number: WO2022199839A1
Application number: PCT/EP2021/057925
Authority: WO
Inventors: Jack Johnson; Lyn Mcwilliam; Simon DRINKWATER
Original assignee: Jaguar Land Rover Limited
Priority date: 2021-03-26
Filing date: 2021-03-26
Publication date: 2022-09-29
Also published as: EP4314529A1

Abstract

A piston (54) for a lean-burn gasoline 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 a lean-burn gasoline engine

TECHNICAL FIELD

The present disclosure relates to a piston for a lean-burn gasoline engine, to a lean-burn gasoline engine comprising the piston 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_x) and unburned hydrocarbons.

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 better performance, 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 for 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 knock. 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 airflow 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 provide an improved lean-burn gasoline engine.

SUMMARY OF THE INVENTION

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 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.

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

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 a cross section of portion of an engine block and cylinder head with a piston according to the invention shown near bottom dead centre;

Figure 3 shows a plan view of the roof surface of the combustion chamber of Figure 2;

Figure 4 shows a second cross section of the engine block and cylinder head of Figure 2 with the piston near top dead centre;

Figure 5 shows a plan view of the working surface of the piston of Figure 2;

Figure 6 shows an isometric view of an alternative piston according to the invention;

Figure 7 shows a cross section of the engine block and cylinder head of Figure 2 with the piston of Figure 6 near top dead centre;

Figure 8a shows a plan view of the working surface of a further alternative piston according to the invention;

Figure 8b shows an isometric view of the working surface of the piston of Figure 8a; and Figure 8c shows a side view of the working surface of the piston of Figure 8a.

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. 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 and let the cylinder pistons 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. 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 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, 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 a cross section of a portion of an engine block 52 and a cylinder head 53 of the lean burn engine 110. The engine block 52 comprises a cylinder 57 which houses a piston 54 shown near bottom dead centre (BDC) in Figure 2. The cylinder is circular in cross-section in a plane perpendicular to the longitudinal axis 60 of the cylinder. The cylinder head 53 comprises a combustion chamber 50 which extends into the cylinder head 53 away from a gasket interface surface 58. A head gasket 80 is located between the engine block 52 and cylinder head 53.

Referring to Figure 3, a pair of air inlets 49a, 49b are located on an air inlet side 20 of the combustion chamber 50. The air inlets 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 inlets 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 2 is taken along section A-A of Figure 3 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.

Referring once again to Figure 2, an inlet valve 51 controls the opening and closing of the first air inlet opening 91a, and an exhaust 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. The inlet valve 51 and the exhaust valve 55 are shown in the closed position in Figure 2. A dotted line provides a simplified 2D representation of the preferred air flow path 59 into and through the combustion chamber 50 and cylinder 57 during the intake stroke of the piston 54. As noted above, the inlet valve 51 is shown in the closed position in Figure 2. 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.

As will be described in greater detail below, the design of the working surface 79 of the piston 54 helps to create a tumble motion of the incoming air, first along the roof 90 of the combustion chamber 50 towards the opposite wall of the cylinder 57, under the outlet valves 55 that close off the exhaust outlet openings 92a, 92b, and then down along that opposite wall of the cylinder 57, back over the working surface 79 of the piston 54 and up along the other wall of the cylinder 57 in the direction of the inlet valves 51 again. This 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 cylinder 57. The thus produced tumble helps to obtain an optimal distribution of air and fuel inside the cylinder 57 and combustion chamber 50 that can then break down in the latter stages of the compression stroke 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.

Figure 3 shows a plan view of the roof surface 90 of the combustion chamber 50 and Figure 4 shows a cross sectional view of the engine block 52 and cylinder head 53 along section B- B shown in Figure 3. 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 inlets 49a, 49b, and exhaust outlets 56a, 56b seen in Figure 3 do not form part of the combustion chamber roof surface 90.

As best shown in Figure 4, 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 between the first side 93a of the combustion chamber 50 and the second side 93b of the combustion chamber 50 in a direction substantially perpendicular to the plane of symmetry 87. 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 made by different machine cutters during manufacture.

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. As best shown in Figure 4, 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.

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 spark plug 82 and fuel injector 71 are located in the domed portion 88 of the combustion chamber 50.

Figure 5 shows a plan view of the working surface 79 of the piston 54. Referring to Figure 4 and Figure 5, 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 2 and 4, 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 4, 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 2, 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 portion of the cylinder 57 (see Figure 2) located above the piston 54 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 4, 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 parts 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. However, 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 5, 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 5 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 6 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 7 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 8a to 8c 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.

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 a lean-burn gasoline 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.

4. A piston as claimed in claim 3, wherein the width of the base of the channel varies along the length of the channel.

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

6. 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.

7. 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.

8. 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.

9. 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.

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

11. 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.

12. A piston as claimed in any claim 12, 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.

13. 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.

14. 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.

15. 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.

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

17. A lean-burn gasoline engine comprising a piston as claimed in any one of claims 1 to 16.

18. A piston as claimed in claim 15, 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.

19. A lean-burn gasoline engine comprising a piston as claimed in claim 18, 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.

20. A vehicle comprising a lean-burn gasoline engine according to any one of claims 17 or 19.