GB2526279A

GB2526279A - Reciprocating steam engine

Info

Publication number: GB2526279A
Application number: GB1408826.4A
Authority: GB
Inventors: John David Tetlow
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-05-19
Filing date: 2014-05-19
Publication date: 2015-11-25
Also published as: GB201408826D0

Abstract

There is provided a reciprocating steam engine 100 and a method of operating a reciprocating steam engine. The steam engine provides improved efficiency and maximizes the heat transfer from the high pressure steam. The steam engine comprises a cylinder head 20 slidably arranged along a path of reciprocation and the method includes moving the cylinder head towards a piston 40 moving away from the cylinder head. The engine may further include a piston with a valve 46 arranged to open and close a passageway 42 such that fluid communication either side of the piston is possible. This allows spent steam to pass through the piston and, on the injection of coolant into the cylinder space, suck the piston toward bottom dead centre by reducing pressure in the cylinder chamber.

Description

Reciprocating Steam Engine

Field of Invention

The invention relates to a reciprocating steam engine and method of operating a steam engine particularly, although not exclusively, for use in stationary or moving power plants for industrial or commercial use.

Background to the Invention:

A reciprocating steam engine is an external combustion engine. Heat energy is transferred to mechanical work using steam as the working fluid. The steam may be obtained from the combustion of traditional fossil fuels, such as solid fossil fuels, like coal, liquid fossil fuels, like diesel or gasoline and gaseous fossil fuels, like natural gas. The heat energy generated from renewable energy sources, such as biofuels, solar energy, wood pellets or geothermal power, can also be utilised as long as the water can be turned into steam. The operating principle of steam engines is well established and well known. However, there is a need to improve the efficiency of all types of power generation, such as reciprocating steam engines, in order to help tackle the global energy crisis.

Traditionally, a reciprocating steam engine employs a double acting cylinder, whereby steam is supplied alternatively to each side of the piston and alternatively released from the other. In such engines, the pipes or tubes in fluid communication with each side of the piston, are alternately heated and cooled in one cycle. This allows the design of the engine to be simple but means that the high pressure, high temperature steam that is initially supplied to the cylinder is then exited through similar passageways out of the cylinder as relatively low pressure, low temperature spent steam. This heating and cooling effect is detrimental to the efficiency and performance of the engine. Furthermore, the useful work out of a traditional reciprocating steam engine is reduced by the work required by the intake steam to push the spent steam (via the piston) out of the cylinder. So called uniflow reciprocating steam engines aim to reduce the relative heating and cooling of the cylinder walls, by separating the inlet and outlet ports. However, a high degree of spent steam tends to be trapped from previous cycles which results in higher compression work. Another type of reciprocating steam engine designed to lessen the magnitude of the heating and cooling effect, is the compound steam engine steam whereby the steam is expanded in two or more stages. The re-routed steam requires dependence between cylinders and the intricate design of passageways, which increases the heat loss. Furthermore, the compound steam engine is space inefficient and has the common disadvantage of successive heating and cooling of either the piston and/or passageways routing the steam into, around and out of the engine.

It is an aim of the present invention to maximise the expansion energy of the steam. It is a further aim to reduce the parasitic loses associated with a steam engine so that the efficiency is improved. For instance, it is an object to reduce the pumping work.

There is a need to improve the efficiency of reciprocating steam engines. More particularly, there is a need to extract more heat energy from the steam in reciprocating piston steam engines. Therefore, it is a further aim to provide a reciprocating steam engine with an improved energy recovery system. It is desired to provide an engine with reduced heat loss.

It is an object of the present invention to attempt to overcome at least one of the above or other identified problems. It is a further aim to provide a reciprocating steam engine that is capable of running under various operating conditions. The steam engine may have several variables in order better control and optimise the efficiency and/or the performance of the heat conversion process. Furthermore, it is an aim to reduce the amount of working fluid that needs to be replenished in the steam engine so that the engine requires less refilling whilst in operation. It is a further aim to improve the performance of the engine such that the torque can be maximised, particularly at very low engine speed.

Summary of the Invention

According to the present invention, there is provided a two-stroke reciprocating steam engine and a method of operating a two-stroke reciprocating steam as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims

and the description which follows.

A two-stroke reciprocating steam engine is provided. The steam engine comprises a cylinder, a cylinder head and a piston. The piston is configured to move reciprocally within the cylinder such that a first in-cylinder volume between opposing faces of the piston and cylinder head is smaller than or equal to a reference volume at a given stroke position. The reference volume is defined as the sum of a clearance volume and a sweeping volume. The clearance volume is defined as the volume between the opposed faces of the piston and cylinder head at the maximum stroke position. The sweeping volume is defined as the volume calculated by multiplying the distance between the maximum stroke position and the given stroke position by the in-cylinder cross-sectional area. This configuration is advantageous because when steam is injected late into the first in-cylinder volume (i.e., when the piston moves towards the minimum stroke position, i.e. a proximal end), the steam can affect the piston to a greater extent. This is because the piston is already on the return stroke. By reducing the first in- cylinder volume over a reference volume, improved control of the rate of change of the first in-cylinder volume can be achieved, which helps to improve the heat conversion efficiency because less steam is required for a given torque output. Furthermore, the maximum torque output can be increased. The steam does not have to work as hard on the piston because the piston has already moved away from the maximum piston stroke position (i.e. the distal position). This helps to maximise the heat energy from the steam and transfer as much heat energy as possible into rotational mechanical work. The steam engine provided is thus more controllable which allows the efficiency to be optimised.

Alternatively, the piston may be reciprocally moveable within the cylinder in a radial direction to an axis of rotation, wherein a first in-cylinder volume exists between the cylinder head and piston and the piston is arranged to move between a distal position and a proximal position from the axis of rotation to effect the first in-cylinder volume wherein a rate of change of the first in-cylinder volume is arranged independently of, or in addition to, the piston position.

The rate of change of the first in-cylindervolume may be arranged to be asymmetric between strokes. The distal position is equivalent to the maximum stroke position and the proximal position is equivalent to the minimum stroke position.

Preferably, the steam engine is a low speed engine. The engine may be arranged to operate at a maximum speed of 500 revolutions per minute (rpm) or lower. Preferably, the maximum engine speed is around 300rpm or lower. However, most preferably, the maximum engine speed is between about 120rpm and about 200rpm. Particular parts of the engine, such as the injection events, could be mechanically or electronically controlled. Electronic control helps to improve efficiency. For instance, an electronic control unit or ECU may be used to monitor sensors and control parameters across the engine, such as inlet valve opening and closing timings relative to the crankshaft rotation position, gas/liquid injection timings and cylinder head displacement starting points.

The rate of change of the first in-cylinder volume may be arranged to be asymmetric between strokes. This may allow for the first in-cylinder volume to be lower around a start of steam injection after the piston reaches top dead centre (or TDC, previously described as the distal end) compared to an equivalent reciprocating position, or crank angle position, before TDC. This helps to maintain low pumping work as the piston approaches TDC but allows the first in-cylinder volume to be reduced after TDC in order to improve the heat conversion efficiency.

The first in-cylinder volume may be arranged to be substantially constant as the piston moves away from the maximum stroke position and toward a predetermined crank angle position. Further, the first in-cylinder volume may be arranged to be substantially constant around TDC or substantially constant after TDC. This predetermined crank angle position may be determined by the crankshaft position and may coincide with the start of steam injection, the end of steam injection or any crank angle position between these positions.

At least part of the cylinder head of the two-stroke reciprocating steam engine may be slidably arranged along a path of reciprocation of the piston. The cylinder head may be fully or partly arranged in the cylinder and slidable therein. This allows the rate of change of the first in-cylinder volume to be varied based on the position of the slidable cylinder head and not only the position of the reciprocating piston. The maximum cylinder head displacement may be up to about 10% of the piston stroke. Preferably, the displacement is up to about 7.5% of the piston stroke. The cylinder head may be coupled to a cylinder head camshaft which is driven by a timing belt connected to the crankshaft. Therefore, one rotation of the crankshaft would be equal to one rotation of the cylinder head camshaft. The cylinder head may be biased away from the piston, such that in a rest position the cylinder head is forced away from the piston to avoid collision with the piston when at TDC. The cylinder head may have a reduced mass by providing a cylinder head with long skirts to provide stability. The long skirts create a tubular part of the cylinder head with an open first end and a closed second end. Cut outs may be provided in the skirts for the camshaft to pass through on rotation of the camshaft.

The rotation of the camshaft relative to the crankshaft may be altered by adjusting the effective length of a timing belt coupled between the camshaft and crankshaft. The cylinder head movement may be arranged to be mechanically coupled and driven by the crankshaft or may be driven independently of the crankshaft by electronic sensors and actuators. The cylinder head may be arranged to move toward the piston as the piston moves towards the minimum stroke position (i.e. the proximal position). Alternatively, the rate of change of the first in-cylinder volume may be affected by manipulation of the piston. For instance, the piston may be varied in geometry as the piston travels between the distal and proximal ends.

The steam engine may comprise a single or a plurality of cylinders and cooperating pistons. The piston may be made to be longer than the piston stroke in order to reduce heat loss. Each piston may drive the same crankshaft where rotation is made smoother and oscillations dampened out by the use of a flywheel. The flywheel ensures that the stored kinetic energy can be used to exert energy against friction. The noise, vibration and harshness of a steam engine can be reduced by generally increasing the number of cylinders and associated parts. Each cylinder may be orientated in a horizontal or vertical arrangement such that each piston moves in a side-by-side or up-and-down fashion. However, it is anticipated that the pistons may not be arranged radially fully around a single crankshaft. A single crankshaft may be provided which can collaborate with a single or a plurality of pistons.

The pistons may be coupled to the crankshaft by a connecting rod. Although a single cylinder and piston may be provided, preferably the number of cylinders and pistons are an even number. Furthermore, the cylinders may be arranged in-line or may be in a V or W arrangement in order to provide scaling up of the engine. The cylinder head camshaft may be mounted on top of the steam engine, i.e. overhead, or mounted on the side. The overhead orientation may be more suited to operation on small engines whereas a side mounted camshaft with at least one push rod can be provided on larger engines. A small engine is considered to be an engine with an engine displacement of less than 6,000 cubic centimetres (6 litres) and a large engine may have a displacement of at least 6,000 cubic centimetres (6 litres) wherein the engine displacement is determined by the bore and stroke of the engine and can be described as the total volume displaced from BDC to TDC. The cylinder head provided may be effectively a floating cylinder head that is pushed down the cylinder bore and returned with an elastic member such as a spring, in order to allow steam to be injected after TDC.

Furthermore, the engine arrangement may provide for a cylinder bore diameter to be equal to the piston stroke, i.e. the engine is a "square" type arrangement. For instance, the cylinder bore may have a diameter around 300mm and the piston stroke may also be around 300mm.

Therefore, the displacement of a single cylinder engine may be around 21.2 litres. A square type engine helps to maintain heat separation between opposing sides of the piston so that piston cooling is reduced. Furthermore, the engine may be an "undersquare" type engine, i.e. the cylinder bore may be equal to or smaller than the piston stroke (distance between the proximal BDC position and the distal TDC position). Therefore, the bore-to-stroke ratio may be less than or equal to 1.

The engine may have a steam inlet that is arranged on the cylinder wall. The steam inlet may be blocked by the piston as the piston travels towards the distal end. A steam valve allows high pressure steam to pass through the steam inlet and enter the first in-cylinder volume. The steam may be injected by at least one valve or at least one injector. Each valve or injector may be mechanically controlled. However, it is preferable that each valve or injector is electronically controlled so that the timing, duration and other parameters can be varied. In an alternative example, the cylinder head may be charged with high pressure steam before the steam is injected into the cylinder. In this example, the steam may be injected toward the piston face as opposed to across the piston face when the inlet is arranged in the cylinder wall.

Preferably, the steam is operated around Sbar absolute pressure but is most preferably more than Sbar absolute pressure. For instance a head of steam around 11.Sbar absolute pressure may be provided. The steam can be injected at the best possible crank position such that the maximum amount of work can be done with the least amount of steam. For instance, the crank angle position of the start of injection may be around 35 after IDO. Therefore, the relative movement of the cylinder head and piston is arranged to continue until at least a start of steam injection.

A seal may be provided between the piston and side wall of the cylinder in order to help reduce blowby and prevent gas escape past the piston in order to improve the efficiency of the engine. The seal may be a piston ring that clamps around the piston through cut outs in the side wall of the piston. Further seals may be provided on the cylinder head and side wall of the cylinder so that the cylinder head can slide without causing a leak. Alternatively, the seal may be provided on the cylinder due to the lower degree of movement of the cylinder head compared to the piston stroke.

Another embodiment of a two-stoke reciprocating steam engine is provided. The steam engine comprises a cylinder having first and second in-cylinder volumes and a piston having a piston valve. The piston is arranged to separate the first and second in-cylinder volumes and the piston valve is arranged between an open and closed position such that in the open position, the in-cylinder volumes either side of the piston are in fluid communication with each other through a passageway in the piston. The advantage of this arrangement is increased efficiency. Spent steam from a previous cycle can pass from one side of the piston to the other side via the passageway such that the pumping losses can be reduced and further energy from the spent steam can be used later in the cycle. Therefore, the steam engine can further maximise energy from the steam through multiple stage operation. When the piston valve is arranged to be open and the steam is arranged to pass through the piston, the steam may be retained in at least one tube, so that the steam is configured to be re-directed towards the bottom vacuum cylinder known as the low pressure or second in-cylinder volume. This helps to control the direction of the steam in order to prevent vapour build-up in the piston cavity or passageway. Furthermore, a drain may be fitted within the passageway in order to allow any moisture build-up to escape.

The piston valve of the steam engine may be arranged in the open position when the piston is arranged to travel towards the cylinder head, i.e. towards a maximum stroke position, otherwise known as top dead centre (TDC) or the distal position. This allows steam to pass through the passageway as the piston is driven away from a crankshaft bearing in order to allow the steam to move from the first in-cylinder volume to the second in-cylinder volume.

Therefore, a high pressure steam side and a low pressure condenser side may be produced on either side of the piston. The opening of the passageway may be sufficient to allow the pumping work to be reduced. Furthermore, the difference in pressure between the first and second in-cylinder volumes allows the spent steam to pass between the volumes in an efficient manner whilst reducing the effect of heat loss from the first in-cylinder volume. The piston valve may be driven by a piston valve cam. The piston valve may be opened by a lobe on the end of the connecting rod. The movement of a cam follower may multiply the movement of the cam to open the valve. The valve may be arranged to open around BDC and then to close around TDC. The piston valve may be slidably held within a piston rod to protect the valve.

The piston may be a one piece arrangement and may have a split around the top to allow a void for the steam to pass through and separate the relatively hot top face of the piston from the relatively cool bottom face. However, the piston may consist of more than one part and be coupled together on assembly. The piston may be made from a metal such as steel, aluminium or iron and may be cast or machined. The depth of the piston may be greater than or equal to the stroke in order to allow the first and second in-cylinder columns to be separated as much as possible. This helps to keep the relatively hot and cold surfaces as far apart as possible.

The steam engine may be provided with a liquid injector that is arranged to inject liquid into the second in-cylinder volume such that spent steam arranged in the second in-cylinder volume, is arranged to condense and lower the pressure in the second in-cylinder volume.

This allows the reduced pressure of the condensed spent steam in the second in-cylinder volume to effectively draw the piston away from the cylinder head and to the bottom dead centre (BDC) position. The temperature of the second in-cylinder volume, or low pressure side may be kept approximately below lOOt. The timing of the liquid coolant may be variable and may occur before TDC. This helps to reduce the lag in sufficiently cooling the spent steam so that the effect on the piston is increased. The liquid injection may be electronically controlled (i.e. timing, duration, multiple bursts, split-injection) and/or may be operated by a pump on a separate circuit. The liquid injected into the second in-cylinder volume may be the same in composition as the gas injected as steam into the first in-cylinder volume. The working fluid (i.e. liquid and gas injected into the respective in-cylinder volumes) may be water. However, the working fluid may be a partial glycol (25-75% glycol in water by volume when mixed at 25t). The glycol acts as a corrosion inhibitor and helps to increase temperature the range of the liquid phase so that less liquid can be injected. The working fluid used as a coolant may be injected at a pressure of around l7Obar. The coolant may enter the second in-cylinder volume in a fine mist in order to improve the distribution of droplets and increase the cooling effect. The cooling of the spent steam allows the piston to be worked on both sides at the same time and allow further energy to be extracted from the steam prior to the steam leaving the cylinder. The reduced pressure creates a suction effect on the underside of the piston in order to draw down the piston. Therefore, the piston is acted on by forces on both sides simultaneously.

A condensate exhaust may be provided in the engine. For instance, at least one vacuum brake having a non-return valve outlet may allow the condensate to drain away from the engine. The exhaust may be arranged to be closed when the piston is arranged to move toward TDC so that the spent steam is retained in the second in-cylinder volume. The at least one vacuum brake may be located in a base of the cylinder. In a sideways mounted cylinder (i.e. the piston reciprocates back and forth in a horizontal direction), the at least one vacuum brake may be mounted at a low position so that the condensate is fed out by gravity. The inlet and exhaust are separated to reduce the heating and cooling effect so that the efficiency is

B

raised. Furthermore, the working fluid of the engine may be recirculated in a closed loop. This allows the working fluid to be re-used once cooled or fed through a heat exchanger to re-heat the working fluid reservoir. The engine may be provided with a drain that allows any cooled working fluid to bleed off and prevent hydraulic lock when the engine is turned off Any type of mechanically or electronically operated valve may be used to extract the working fluid.

Any aspect of the first embodiment may be incorporated with any aspect of the second embodiment. For instance, a slidable cylinder head may be provided with the piston arrangement as previously described. This helps to maximise the efficiency of the steam engine and improve engine performance.

A method of operating a two-stroke reciprocating steam engine is further provided. The method comprises injecting steam into a first in-cylinder volume of a cylinder, whereby the first in-cylinder volume is arranged between opposing faces of a piston and a cylinder head. This allows steam to expand by reciprocal movement of the piston within the cylinder from a maximum stroke position towards a minimum stroke position such that the first in-cylinder volume is smaller than or equal to a reference volume at a given stroke position. The reference volume is defined as the sum of a clearance volume and a sweeping volume. The clearance volume is defined as the volume between the opposed faces of the piston and cylinder head at the maximum stroke position. The sweeping volume is defined as the volume calculated by multiplying the distance between the maximum stroke position and the given stroke position by the in-cylinder cross-sectional area. This helps to improve efficiency and performance. For instance, by moving the piston away from the distal position, the first in- cylinder volume can be similar to that when at the distal position. This allows the first in-cylinder volume to be maintained at a relatively lower level when the steam is injected than if the movement was regular. The method may comprise slidably moving at least part of the cylinder head along a path of reciprocation of the piston such that the rate of change of the first in-cylinder volume can be controlled. Furthermore, the method may comprise injecting the steam (i.e. starting the steam injection process) after the piston leaves the maximum stroke position (i.e. moves away from the distal position). This helps to maximise efficiency. The method may include moving a piston valve arranged within the piston between an open and closed position to allow steam to pass through a passageway in the piston and enter a second in-cylinder volume on the other side of the piston. The method may comprise injecting liquid into the second in-cylinder volume such that spent steam in the second in-cylinder volume condenses to lower the pressure in the second in-cylinder volume. The method may comprise exerting forces on either side of the piston in the same direction when the piston travels towards the proximal position. For instance, the steam injected into the top of the cylinder may push the piston towards the proximal position while at the same time, the condensation effect in the second in-cylinder volume draws the piston towards the proximal position.

The method may further comprise retracting the cylinder head towards the distal position as piston moves away from the proximal position. The gap between the cylinder head and piston may be maintained as the piston moves away from the distal position to a S predetermined position which may be about the start of injection. The method may comprise a start or end of injection timing of around 35 degrees after the piston reaches the distal position (i.e. top dead centre).

The steam engine as previously described may be used in light-duty or heavy-duty motor vehicles, commercial freight transportation such as land, water or air transport. For instance, the engine may be used in small-scale applications such as lawnmowers to large marine propulsion and power plant applications. Most preferably, the application of this engine is suited to large engine applications, such as trains, power generation sets (e.g. generators).

Therefore, the engine is preferably used in a steady-state engine operating mode (i.e. engine speed is held substantially constant), although it may be beneficial to allow the engine speed to vary and provide transient use of the engine. Therefore, the engine is designed with variables that can be controlled and optimised for specific applications. It is anticipated that the head of steam may be generated from traditional fossil fuels such as diesel, gasoline, natural gas or the heat energy generated from renewable energy sources, such as biofuels, solar energy, wood pellets or geothermal power.

Brief DescriQtion of the Drawings

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which: Figure 1 shows a schematic cross-sectional side view through a two-stroke reciprocating steam engine; Figure 2a shows a schematic cross-sectional side view of an example piston valve mechanism; Figure 2b shows a piston with internal tubes, integral valve and a long skirt; Figure 3 shows a schematic cross-sectional side view of an example cylinder head mechanism; Figures 4a to 4d show a cross-sectional view of the reciprocating steam engine at key stages in the engine operating cycle; and Figures Sa to Se show the full operating cycle including the key stages of operation.

Detailed Descririlion of the Invention Figure 1 shows a cross-section through a two-stroke reciprocating steam engine 100.

Each stroke is the distance between top dead centre (TDC) and bottom dead centre (BDC) or the greatest linear distance the piston can travel. Therefore, a two stoke engine requires that a piston is moved twice between TDC and BDC while the crankshaft is rotated once. The working fluid may be water or a composition of water and glycol. Therefore, the working fluid may be a partial glycol. The term steam is herein used to describe the gas phase of the working fluid and the term condensate is used to describe the liquid phase. The steam engine is provided with several key components such as a cylinder 10, a cylinder head 20, a high pressure steam inlet 30 and a piston 40. Although not shown, the high pressure steam inlet 30 could be provided with a valve or injector that is controlled mechanically. However, it is preferably that the valve or injector is controlled electronically. The steam inlet 30 is located on the side of the cylinder 10. Therefore, as the piston 40 passes towards bottom dead centre (BDC), the steam can be injected into the cylinder volume above the piston 40. Although not shown in Figure 1, the cylinder head 20 cannot pass the steam inlet 30 although the piston 40 can. Although the cylinder head 20 is located within the cylinder 10, the cylinder head 20 may be part mounted on the cylinder 10. Fasteners may be used to secure the cylinder head 20 to the cylinder 10, such as cylinder head bolts (not shown). The cylinder head 20 can slide within the cylinder 10 such that the cylinder head 20 can move towards the piston 40 along a line of reciprocation. The cylinder head 20 is driven by a cam shaft 22 having cam lobes arranged eccentrically on the cam shaft 22 in order to depress and activate a cam follower 24 and subsequently lower the cylinder head 20. Once the furthest point on the cam lobe (away from the centre of rotation the cam shaft 22) has rotated pass the cam follower 24, the cylinder head 20 can be raised away from the piston 40. Although not shown, the cylinder head 20 is biased away from the piston 40 such that the cam follower 24 works against the biasing when depressing the cylinder head 20. The biasing member can be a spring, such as a return spring like a tension spring. Alternatively, a compression spring may be used.

The piston 40 arrangement is shown in Figure 1 and further in Figures 2a and 2b. The piston 40 has a seal 44, a valve 46 and a valve retum spring 48. The piston 40 shown is substantially T-shaped in side view and is a fixed one-piece component. The piston 40 is shown with a piston rod 46b which contains a piston valve stem 46a operable therein. The piston rod 4Gb extends toward the crankshaft from a piston head. The piston head contains a passageway 42 for allowing fluid communication between the in-cylinder volumes either side of the piston 40. The piston seal 44 ensures that steam does not pass alongside the piston walls or piston skirt in order to minimise blowby and raise the heat transfer efficiency. The piston valve 46 and piston valve return spring 48 work in cooperation with a connecting rod cam 56. The connecting rod cam 56 opens and closes the piston valve 46 as the connecting rod 54 is rotated by the rotation of a crankshaft 50 and big end bearing 52. A flywheel 70 is also shown to help smooth vibrations of the steam engine 100. Although a single cylinder 10 and single piston 30 arrangement is shown, it is envisaged that a plurality of cylinders 10 and cooperating pistons 30 can be used. The piston 30 is sealed at the piston seats in the piston which are provided as cut outs on the piston skirt, in order to prevent gas escaping into the passageway 42. Cooling injectors 60 are shown at the bottom of the cylinder 10 in order to condense the spent steam and reduce pressure on the underside of the piston 40 away from the cylinder head 20. The cooling fluid injected through the cooling injectors 60 has a substantially similar composition to the gas injected through the steam inlet 30. The lower pressure created when the cooling fluid is injected into the cylinder volume on the underside of the piston 40, helps to draw the piston 40 down the cylinder 10 by effectively sucking the piston 40 away from the cylinder head 20 so that more energy can be extracted from the working fluid. Although two cooling injectors are shown, a single cooling injector 60 may be used. The start of injection timing, duration and number of injections can be varied and controlled electronically and will be adjusted depending on the engine load and engine speed.

A fine mist is preferable in order to avoid fluid contact with internal component surfaces.

Although not shown, the crankshaft 50 can be run in a sump whereby oil is pumped around to lubricate the crankshaft 50 and bearings. The top end of the engine where the cylinder head camshaft 22 resides may not need lubrication. The cylinder head camshaft 22 and cam follower 24 can be run on sealed bearings (not shown) in order to reduce the complexity of the lubrication system.

The piston arrangement 40 is shown in more detail in Figures 2a and 2b. The piston 40 is shown at top dead centre (TDC), such that the piston head is arranged at the furthest location away from the centre of rotation of the crankshaft 70. In the TDC position, the piston components and connecting rod 54 are arranged in a collinear fashion such that the piston valve 46 is closed in order to prevent fluid communication between opposing sides of the piston 40 through the passageway 42. The various forces on the piston 40 cause the piston to move towards bottom dead centre (BDC) and the crankshaft 50, in this example, is rotated in an anti-clockwise manner. This causes the connecting rod 54 to swing to one side and reduce contact between the piston valve cam 56 and cam follower 57 such that the piston valve 46 remains closed on the downward stroke. The return spring 48 is a compression spring held in place by a spring retainer 48a such that the piston valve 46 is biased away from the cylinder head 20 as the spring 48 resists compression. At BDC, the piston components and connecting rod 54 are also collinear. As the crankshaft 50 is rotated away from BDC, the connecting rod 54 is swung to the opposing side as the piston 40 begins to travel upwards within the cylinder 10. At this point, the eccentric cam lobe on the cam 56 exerts a force on the cam follower 57 in order for the piston valve 46 to open. The cam follower 57 may magnify the movement of the cam 56 by extending the pivot point. This causes the return spring 48 to move to a compressed state such that the piston valve 46 opens.

As shown in Figure 2a, the piston rod 46b is fixed relative to the piston head such that any rocking and rolling of the connecting rod 54 caused by the rotation of the crankshaft 50, is removed or at least reduced such that internal component clatter is avoided within the cylinder bore 10. The piston rod 46b may be fixed to a housing comprising at least one component of the piston 40 arrangement such as the return spring 48, the cam follower 57 and/or the cam 56. The fixing may be a screw thread 46c and a lock nut 46d.

Figure 2b shows a further piston 40 arrangement. The piston 40 is shown with a greater height H than width W in order to reduce the amount of heat transfer between the in-cylinder volumes 12,14 either side of the piston 40. The piston 40 shown is a one-piece construction that may be fabricated and then machined. The piston 40 has a first part 40a and a second part 40b which are integrally formed but include seals 44 around the outer edge to contact the cylinder bore. The piston has cutaways between the first and second parts 40a,40b in order to reduce contact with the cylinder bore and help to reduce transfer to and/or from the piston 40 to the cylinder 10. The passageway 42 within the piston 40 is shown by the tubes 42 either side of the piston valve stem 46a. This allows the steam to pass between the high and low pressure sides 12,14 of the cylinder 10 during operation. When the piston has a width of around 300mm, 8 tubes may be used that are each 20mm in diameter. This helps to stop most of the heat transfer to the cylinder walls 10 and send the steam directly to the low pressure side 14 of the cylinder 10. The piston rod would then be coupled to piston 40 at the low pressure side 14 of the piston 4Oso that a relatively small rod can pass through the middle of the piston 40 in order to open the valve 46a. This improves the connection to the connecting rod 54 by moving the return spring 48 internally of the piston 40.

Figure 3 shows an example arrangement of the cylinder head 20 and cylinder head camshaft 22. Here, the cylinder head 20 is shown with long skirts 21 that slide within the cylinder 10. This helps to reduce the mass of the cylinder head 20 such that the cylinder head can be slid in an efficiency manner. The cam follower 24 is therefore shown to be at least partly located between the skirts 21 in order to provide a more compact arrangement. The cylinder head cam shaft 22 is shown with an eccentric cam lobe 23 that can pass through a cut out 11 in the cylinder 10. The cut out is only provided in one side of the cylinder in a specific location as the cam lobe 23 passes through. The rotation action of the camshaft 22 may be driven by a timing belt (not shown) which is connected to the crankshaft 50.

Figures 4a through to 4d show the key stages of operation of the reciprocating steam engine 100 and the movement of the piston 40 between top dead centre (TDC) and bottom dead centre (BDC). Areas highlighted by spots show the working fluid in gaseous form, for instance the presence of steam, and areas highlighted by a wavy pattern show the working fluid in liquid form, i.e. the presence of condensate. The relative density of spots provides an appreciation of the relative change in pressure between the stages. For instance, a higher density of spots represents a relatively higher pressure. Arrows are used to show the general flow of steam and condensate throughout the system.

In Figure 4a, the steam engine 100 is shown at the end of a previous cycle (one cycle is equal to two strokes of the piston 40 displacement from TDC to BDC), whereby the piston 40 is positioned at TDC. In the TDC position, the cylinder head 20 is fixed at the furthest point away from the piston 40. Although the cylinder head 20 may be charged with high pressure steam which enters though the steam inlet 30, in this example, the steam inlet 30 is provided in the cylinder walls. In the TDC position, the inlet 30 is blocked by the walls of the piston 40 (i.e. piston skirt). This steam is generated by a boiler (not shown). The boiler heats the working fluid to turn the working fluid into a gaseous phase. The boiler may be powered by non-renewable (i.e. coal, diesel) or renewal fuel sources (i.e. solar energy, geothermal energy). In the TDC position, the spent steam from the previous cycle is trapped below the piston 40 in the lower side of the cylinder chamber 10. Therefore, the cylinder 10 is shown with two working in-cylinder volumes. The first in-cylinder volume 12 is herein called the high pressure side and the second in-cylinder volume 14 is called the low pressure side. The spent steam from the previous cycle is retained within the cylinder 10 in the low pressure side 14 in order to generate reduced pressure and pull the piston 40 down towards BDC in an attempt to recover more heat energy from the steam. Therefore, the process is a multi-stage process. The process is a two-stage process with a high pressure and a low pressure stage which are able to work simultaneously on either side of the piston 40. At the stage shown in Figure 4a, a gap G1 is created between the cylinder head 20 and the upper surface of the piston 40. This in-cylinder volume is said to be the clearance volume and is the minimum in-cylinder volume achievable in the high pressure side 12. At TDC, the piston valve 46 is in the closed position such that there is no communication between the first 12 and second 14 in-cylinder volumes via the passageway 42. Just after TDC, the cylinder head 20 begins to move towards the piston 40 as the piston 40 moves towards BDC. The cylinder head 20 may enter the swept volume, that is, the volume that the piston sweeps through when travelling from TDC to BDC.

Figure 4b shows the steam engine 100 as the piston travels away from TDC and towards BDC by momentum of the flywheel 70 and movement of the crankshaft 50 due to the heat energy transferred from the previous cycle. The cylinder head camshaft 22 depresses the cam follower 24 which causes the cylinder head 20 to lower toward the piston 40 such that a gap G2 is created between the cylinder head 20 and piston 40. This gap G2 may be substantially the same as the previous gap Gi at TDC. Therefore, between TDC and a predetermined angle, the in-cylinder volume on the high pressure side 12 may be maintained and be substantially constant although the piston 40 is moving towards BDC. This allows the injection of steam to occur after TDC in order to maximise the heat energy transfer. This helps to improve performance and efficiency. For instance, higher engine torque is produced and less working fluid provided in gaseous form to drive the piston down. The engine torque (i.e. load), is described as the turning force at the crankshaft angle position and is normally measured in Nm. At a predetermined angle, the start of steam injection begins. Here, high pressure steam from the steam inlet 30 enters the high pressure side 12 of the cylinder 10 as shown by the arrow. At this point away from TDC, the piston valve 46 remains closed so that the high pressure steam can be used effectively to push the piston 40 down. The pressure exerted by the steam on the piston 40 therefore causes the piston 40 to be driven towards BDC. Before this crank angle position, or at a substantially similar crank angle position, the working fluid in liquid form (i.e. water or partial glycol) may be injected through at least one injector 60 in the low pressure side 14 of the cylinder 10. An early injection before TDC may be required due to the lag in condensing the spent steam from the previous cycle. This causes the spent steam trapped within the low pressure side 14 to condense, which subsequently reduces the pressure and acts on the piston to pull the piston 40 toward the proximal position (BDC). A vacuum on one side of the piston 40 may therefore be created.

Accordingly, simultaneous forces are acting on the piston 40. A push force is acting on the high pressure side 12 and a pull force is exerted on the piston 40 at the low pressure side 14.

This helps to improve performance and efficiency of the steam engine 100. It is anticipated that any lag caused by the condensation process in the low pressure side 14 or the steam injection process on the high pressure side 12 can be overcome by the coolant injector timing and/or the steam inlet 30 timing, either of which may be electronically controlled. However, the coolant (i.e. water or partial glycol) injector timing and/or cam timing may be driven mechanically, for instance from the crankshaft 50 by a drive belt or chain, or may be driven electronically using sensors located on the steam engine 100.

The piston 40 arrangement is shown at the BDC location in Figure 4c. Here, the cylinder head 20 is shown to remain on full cam, i.e. at full displacement towards the piston 40.

The high pressure steam has entered fully into the high pressure side 12 and the condensate in the low pressure side 14 is starting to be removed from the steam engine 100 through the vacuum brakes 60. The vacuum brakes 60 may be controlled by the varying pressures within the low pressure side 14 in order to open and close a non-return valve (not shown). At this point, the passageway 42 within the piston 40 does not allow open fluid communication between either side of the piston 40. However, this communication starts as the piston moves away from BDC and towards TDC.

S Figure 4d shows the piston 40 travelling away from BDC. At this point, the piston valve 46 is open and allows fluid communication to the high 12 and low 14 pressure sides of the cylinder 10 on either side of the piston 40. This allows the spent steam to pass though the passageway 42 within the piston 40 and enter the low pressure side 14 as the piston 40 moves towards the cylinder head 20. This helps to improve performance and/or efficiency because the relatively lower pressure of the steam is utilised on the next cycle such that a pull force is also provided on the piston 40. At this stage, the cylinder head 20 is also shown to set to the rest position, whereby the return spring has drawn the cylinder head 20 back and above TDC. However, the cylinder head 20 may gradually return to the "off cam" position as the piston 40 moves towards TDC. As the piston 40 is raised towards TDC, the piston valve 46 may close. The piston valve 46 closing timing can occur as late as possible but at least before the minimum in-cylinder volume is reached. This allows as much steam as possible to pass from the high pressure side 12 to the low pressure side 14 and to help raise efficiency.

An example of the two-stroke reciprocating steam engine cycle is shown in Figures 5a to Se. The cycle comprises one full rotation of a crankshaft (360 degrees) and two piston strokes (i.e. back and forth, or up and down) from TDC to BDC. The piston 40 displacement relative to TDC and BDC is shown against crank angle position or crank timing (labelled as degrees after TDC, or aTDC). The crankshaft timing may be achieved by a timing belt or electronically through the use of sensors. The cycle is a two-stroke engine cycle and a full 360 degrees are therefore shown which represents piston travel away from and back to TDC.

Therefore, the reciprocating displacement of the piston 40 is plotted against the corresponding angular displacement of the crankshaft 50 and flywheel 70. The key stages along the steam engine cycle are shown by the cross-marks which generally correspond in order of Figures 4a to 4d. At TDC (0aTDC), as shown by the first cross-mark 310, the cylinder head 20 is "off cam". At this point, the cylinder head 20 may be fully charged with high pressure steam if steam enters the cylinder head 20. However, it is anticipated that a steam inlet 30 is provided in the cylinder wall. The pressure of the steam may be varied according to the engine speed and power required. However, it is anticipated that the steam provided is substantially constant and the timing and duration of steam injection is varied. The return spring is biasing the cylinder head 20 away from the piston 40 so that the piston 40 can travel fully to TDC.

Although Figure Sb shows the cylinder head 20 is displaced by the cylinder head cam 22 at around 35aTDC, the cylinder head 20 may be displaced towards the piston 40 at any point from TDC as long as the cylinder 20 does not hit the piston 40. Therefore, the cylinder head and piston 40 may maintain a substantially constant in-cylinder volume 12 away from TDC to a predetermined angle. This predetermined angle may be the angle equivalent to the start or end of steam injection. The cylinder head then remains "on cam" 350 until around BDC as shown at cross-mark 330 and slowly returns to the "off-cam" position. The cylinder head cam 22 may have such a profile that the cylinder head 20 catches up with the piston 40. When the crankshaft 50 is at the ideal position to gain most work from a small injection of steam, the steam inlet 30 opens and releases steam into the high pressure side 12, i.e. the gap between the piston 40 and the cylinder head 20. At the start of the cycle at TDC, the piston valve 46 may remain open from the previous cycle to allow previous spent steam to migrate towards the low pressure side 14 of the cylinder chamber 10. Although Figure 4a shows a closed piston valve 46, the piston valve 46 can equally be left open at this point, as shown by Figure 5c.

However, enough clearance must be given so that the piston valve 46 does not come into contact with the cylinder head 20. The piston valve 46 must be closed when the high pressure steam enters the high pressure side 12 of the cylinder chamber 10.

At point 320, the cylinder head 20 is shown to chase towards the piston 40. However, this can occur at any time after TDC, as long as the piston 40 or piston valve 46 do not collide with the cylinder head 20. The piston valve 46 closes as the piston 40 travels away from TDC to close the passageway 42 to both sides of the piston 40. The piston 40 can close after TDC to allow further spent steam to enter the low pressure side 14 due to in-cylinder momentum.

However, the piston valve 46 must be closed before the high steam pressure steam is injected. Around 35aTDC, the start of steam injection may occur through the inlet 30. On the high pressure side 12, high pressure steam is injected to push the piston 40 down towards BDC, whereas on the low pressure side 14, water is injected to condense the previous spent steam and draw the piston 40 down away from TDC. However, coolant (i.e. liquid working fluid) may be injected to the low pressure 14 side before TDC in order to start the condensation process. As shown in Figures Sd and Se, the water and steam injection phases 370,380 start concurrently but end at different crankshaft positions. The water injected into the bottom of the cylinder 10 is relatively cold compared to the spent steam and this causes the spent steam to condense and draw the piston 40 down the cylinder 10. A temperature probe fitted in the condensate return may help to control the amount of coolant that is needed to be injected in order to reduce the spent steam to around 95t or at least less than lOOt or at least than the boiling point of the working fluid. This retains the temperature of the cylinder 10 in order to maximise the heat energy extracted from the steam. The duration of the coolant and steam injections 370,380 may be different due to the output demand. The coolant injection 370 results in reduced pressure on the low pressure side 14 which helps to pull the piston 40 down towards BDC. At the same time, the high pressure steam entering the high pressure side 12 acts directly on the top of the piston 40 in order to push the piston 40 down the cylinder bore 10. Therefore, the piston 40 is acted on concurrently from either side and the double acting forces occur simultaneously in orderto help improve efficiency and performance (i.e. engine torque).

As the piston falls towards BDC, and the heat energy from the high pressure steam has worked on the piston 40, the piston valve 46 opens 360, as shown in Figure Sc. This allows the spent steam to pass into the low pressure side 14 in order to be further utilised on the next cycle. The condensate is allowed to exit the cylinder 10 from the low pressure side 14. The condensate exits (i.e. escapes) the steam engine 100 through vacuum brakes 62, whereby a non-return valve may open. This has the advantage that a reduced amount of working fluid is required as steam because more energy is extracted. The steam engine 100 may operate on a closed loop system such that the working fluid is recirculated. Further working fluid may be added to the engine 100 but it is anticipated that the heat recovering method will further utilise the spent steam so that the steam can be reused. The condensate can be reused in the boiler and re-enter the cylinder head 20 as high pressure steam. Around BDC, the cylinder head 20 may return to being "off cam" at this stage but this can equally occur at a later stage and the cylinder head 20 can remain on full cam (i.e. "on cam") but is getting ready to move back to the "off cam" position as the piston 40 travels back up the cylinder bore 10. Equally, the cylinder head 20 may retract steadily as the piston moves from BDC to TDC. As the piston 40 travels towards TDC as shown by cross-mark 340, the piston valve 46 remains open via the connecting rod 54 in order to allow spent steam to pass through the passageway 42 in the piston 40 and leave the high pressure side 12. The piston valve 46 is opened through the use of a connecting rod cam 56 and is also biased by a return spring 46. The piston 40 without the piston valve 46 can be described as a one piece arrangement with a split around the top. This allows a void for the steam to pass through and help separate the hot top face of the piston from the cooled bottom cylinder. The passageway 42 is blocked during the high pressure stage but when open allows fluid communication between the high and low pressure sides 12,14. The piston 40 is designed to have a particular depth so that the high pressure and low pressure sides can be separated to avoid steep temperature gradients across the piston 40.

The action of opening the fluid passageway 42 also helps to reduce pumping losses. The only resistance is friction from the at least one piston seal 44. As the piston 40 returns towards TDC, the two-stroke cycle begins once more. Therefore, improved efficiency, performance and/or variability can be provided.

The industrial application of the invention is readily appreciated from the description herein. In particular, the reciprocating steam engine 100 is capable of being made and used in industry, especially in the energy sector.

Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

Claims

Claims 1. A two-stroke reciprocating steam engine, comprising: a cylinder; a cylinder head; and a piston; wherein the piston is arranged to be reciprocally moveable within the cylinder from a maximum stroke position towards a minimum stroke position such that a first in-cylinder volume between opposing faces of the piston and cylinder head is smaller than or equal to a reference volume at a given stroke position; wherein the reference volume is defined as the sum of a clearance volume and a sweeping volume; wherein the clearance volume is defined as the volume between the opposed faces of the piston and cylinder head at the maximum stroke position; and the sweeping volume is defined as the volume calculated by multiplying the distance between the maximum stroke position and the given stroke position by the in-cylinder cross-sectional area.
2. The two-stroke reciprocating steam engine as claimed in claim 1, wherein the first in-cylinder volume is substantially constant as the piston moves away from the maximum stroke position and towards a predetermined stroke position.
3. The two-stroke reciprocating steam engine as claimed in any preceding claim, wherein at least a part of the cylinder head is slidably arranged along a path of reciprocation of the piston.
4. The two-stroke reciprocating steam engine as claimed in claim 3, wherein the cylinder head is biased away from the piston.
5. The two-stroke reciprocating steam engine as claimed in claim 3 or 4, wherein the cylinder head movement is arranged to be mechanically coupled and driven by a crankshaft.
6. The two-stroke reciprocating steam engine as claimed in any preceding claim, wherein the cylinder head is arranged to move toward the piston as the piston moves towards the minimum stroke position.
7. A two-stroke reciprocating steam engine, comprising: a cylinder having first and second in-cylinder volumes; and a piston comprising a piston valve; wherein the piston is arranged to separate the first and second in-cylinder volumes and the piston valve is arranged between an open and closed position such that in open position, the in-cylinder volumes either side of the piston are in fluid communication with each other through a passageway in the piston.
8. The two-stroke reciprocating steam engine as claimed in claim 7, wherein the piston valve is arranged in the open position when the piston is arranged to travel towards a maximum stroke position.
9. The two-stroke reciprocating steam engine as claimed in any of claims 7 orB, wherein a liquid injector is arranged to inject liquid into the second in-cylinder volume such that spent steam arranged in the second in-cylinder volume is arranged to condense and lower the pressure in the second in-cylinder volume.
10. The two-stroke reciprocating steam engine as claimed in any of claims 1 to 6, comprising any of the features of claims 7 toY.
11. A method of operating a two-stroke reciprocating steam engine, comprising: injecting steam into a first in-cylinder volume, the first in-cylinder volume being arranged between opposing faces of a piston and a cylinder head; expanding the steam by reciprocal movement of the piston within the cylinder from a maximum stroke position towards a minimum stroke position such that the first in-cylinder volume is smaller than or equal to a reference volume at a given stroke position; wherein the reference volume is defined as the sum of a clearance volume and a sweeping volume; wherein the clearance volume is defined as the volume between the opposed faces of the piston and cylinder head at the maximum stroke position; and the sweeping volume is defined as the volume calculated by multiplying the distance between the maximum stroke position and the given stroke position by the in-cylinder cross-sectional area.
12. The method of operating a two-stroke reciprocating steam engine as claimed in claim 11, comprising slidably moving at least part of the cylinder head along a path of reciprocation of the piston.
13. The method of operating a two-stroke reciprocating steam engine as claimed in claim 12, comprising injecting the steam after the piston leaves the maximum stroke position.
14. The method of operating a two-stroke reciprocating steam engine as claimed in claims 12 or 13, comprising moving a piston valve arranged within the piston between an open and closed position to allow steam to pass through a passageway in the piston and enter a second in-cylinder volume on the other side of the piston.
15. The method of operating a two-stroke reciprocating steam engine as claimed in claim 14, comprising injecting liquid into the second in-cylinder volume such that spent steam in the second in-cylinder volume condenses to lower the pressure in the second in-cylinder volume.