PISTON ARRANGEMENT AND INTERNAL COMBUSTION ENGINE

Abstract

As disclosed herein, a piston arrangement may include a cylinder, a piston head movable along a piston axis within the cylinder, a con rod, and a track. The con rod has a first end which is coupled to the piston head and a second end which is coupled to the track. The track is adapted to be moved relative to the cylinder and is shaped such that, as the track moves relative to the cylinder, the piston head moves in reciprocating motion along the piston axis in accordance with a path of the track. The path of the track may be shaped such that piston head displacement is non simple harmonic with respect to displacement of the track relative to the cylinder.

Claims

1. An internal combustion engine having a piston arrangement, the piston arrangement comprising: a cylinder; a piston head movable along a piston axis within the cylinder; a con rod; a swinging arm pivotally attached to a fixed point on the engine and to the con rod; and a track having a path; wherein the con rod has a first end which is coupled to the piston head and a second end which is coupled to the track; wherein the track is adapted to be moved relative to the cylinder and is shaped such that, as the track moves relative to the cylinder, the piston head moves in reciprocating motion along the piston axis in accordance with the path of the track; and wherein the path of the track is shaped such that piston head displacement is non simple harmonic with respect to displacement of the track relative to the cylinder.

2. An internal combustion engine according to claim 1, wherein the track has a path shape which is not sinusoidal.

3. An internal combustion engine according to claim 1, wherein the track has a first local minimum and a second local minimum different to the first local minimum such that as the piston head moves in reciprocating motion along the piston axis it passes through a first bottom dead center position corresponding to the first local minimum of the track and subsequently passes through a second bottom dead center position corresponding to the second local minimum of the track, wherein the piston head is at a different displacement with respect to the cylinder when it is in the second bottom dead center position to when it is in the first bottom dead center position.

4. An internal combustion engine according to claim 1, wherein the track includes at least one portion of reduced gradient at a location between a local minimum of the track and an adjacent local maximum of the track such that as the piston head moves between a bottom dead center position corresponding to the local minimum and a TDC position corresponding to the local maximum it experiences at least one period of reduced speed.

5. An internal combustion engine according to claim 1, wherein the track includes a prolonged portion of low gradient or zero gradient at or near a local maximum or local minimum of the track such that, as the piston head moves through a top dead center position corresponding to the local maximum or through a bottom dead center position corresponding to the local minimum, it experiences a substantial period of dwell time.

6. An internal combustion engine according to claim 1, wherein the track forms a closed loop and is provided on a rotating body.

7. An internal combustion engine according to claim 1, wherein the track is provided on a drum or disk which rotates relative to the cylinder.

8. An internal combustion engine according to claim 7, wherein the track is provided on an inner or outer radial surface of a drum, or on an end surface of a drum, or on an outer radial surface of a disk or on an end surface of a disk.

9. An internal combustion engine according to claim 8, wherein the track is recessed into and/or protrudes from a surface of the drum or disk.

10. An internal combustion engine according to claim 1, wherein the con rod has one or more followers which engage the track to control displacement of the con rod.

11. An internal combustion engine according to claim 10, wherein the con rod has at least two followers which are spaced apart from each other in a direction parallel to the cylinder axis.

12. An internal combustion engine according to claim 10, wherein the con rod has at least two followers mounted to the con rod via a bogie which is pivotally mounted to the con rod.

13. An internal combustion engine according to claim 10, wherein the track includes a first surface which engages one or more of the followers to move the piston head in a first direction along the piston axis and a second surface opposing the first surface which engages one or more of the followers to move the piston head in a second direction opposite to the first direction along the cylinder axis.

14. An internal combustion engine according to claim 1, wherein the con rod is constrained to move substantially in the direction of the cylinder axis without rotating relative to the piston head.

15. An internal combustion engine according to claim 1, including at least two piston arrangements which are each said piston arrangement recited in claim 1, wherein the respective con rods of the at least two piston arrangements are coupled to a common track.

16. An internal combustion engine according to claim 1, comprising first and second piston arrangements which are each said piston arrangement recited in claim 1, wherein con rods of the first and second piston arrangements are respectively coupled to separate first and second tracks, wherein the first and second tracks are moveable with respect to each other to vary the relative timing of the respective piston heads of the first and second piston arrangements.

17. An internal combustion engine comprising first and second piston arrangements which are each said piston arrangement recited in claim 1, wherein the first and second piston arrangements respectively comprise first and second piston heads movable within a common cylinder, wherein the first and second piston heads oppose each other such that a chamber is formed between the first and second piston heads.

18. An internal combustion engine according to claim 1, wherein the con rod is coupled to the track via a bogie.

19. An internal combustion engine according to claim 18, wherein the swinging arm is attached to the con rod at the bogie.

20. An internal combustion engine according to claim 18, wherein the bogie has a wheel mounted on the bogie for engaging the track.

21. An internal combustion engine according to claim 20, wherein the wheel is received in the track.

22. An internal combustion engine according to claim 18, wherein the swinging arm reacts a side load exerted on the bogie by the track.

23. An internal combustion engine according to claim 1, wherein the swinging arm has an oil delivery passage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0054] FIG. 1 shows a cross section through an engine according to a first embodiment of the invention;

[0055] FIG. 2 shows the con rod to track interface of FIG. 1 in greater detail;

[0056] FIGS. 3a and 3b show schematic representations of a track from the engine shown in FIG. 1;

[0057] FIGS. 4a to 4f show schematic views of the operating cycle of the engine of FIG. 1;

[0058] FIGS. 5a to 5e show alternative con rod to track coupling arrangements;

[0059] FIGS. 6a to 6c show a three way valve used in an alternative embodiment of the invention;

[0060] FIGS. 7a and 7b show schematic views of a transfer piston arrangement used in an alternative embodiment of the invention;

[0061] FIGS. 8 and 9a to 9f show an engine according to a second embodiment of the invention;

[0062] FIGS. 10 and 11 a to 11e show an engine according to a third embodiment of the invention; and

[0063] FIGS. 12a and 12b show variable length con rod arrangements.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0064] FIG. 1 shows a cross section taken through an internal combustion engine according to a first embodiment of the invention. The engine comprises a combustion cylinder 1 having a main piston 2 which reciprocates within the combustion cylinder, and a compression cylinder 3 having a compression piston 4 which reciprocates within the compression cylinder. The compression cylinder 3 is fluidically connected to the combustion cylinder 1 by a transfer duct 5 which is operated by a transfer valve 6 to open and close the transfer duct.

[0065] The engine further comprises an HCCI compression ignition cylinder 7 having an HCCI compression ignition piston which reciprocates within the HCCI compression ignition cylinder. The HCCI compression ignition cylinder 7 has an open end which opens into the head wall or upper wall of the combustion cylinder 1. The engine further comprises a water and/or steam injector device 9 adapted to inject water and/or steam into the combustion cylinder through the head wall, and an exhaust port 10 formed in the bore of the combustion cylinder operated by an exhaust valve.

[0066] The above-mentioned components and systems are all located between a central shaft 11 and a hollow outer drum 12. The shaft 11 and drum 12 are rotationally fixed with respect to each other by a linking wall 11 a but are adapted to rotate with respect to the engine cylinders about a central axis of the engine 13. The engine may comprise any number of similar piston arrangements arranged radially between the shaft 11 and the drum spaced apart circumferentially around the central axis 13. For increased clarity only one arrangement is shown in the drawings and described in detail.

[0067] The main piston 2 is connected to a con rod 14 which is slidably received in a fixed sleeve 23 which constrains the con rod to move substantially only in a direction aligned with the combustion cylinder axis without significant rotation with respect to the main piston 2. The con rod is coupled at its opposite end to a bogie 15, shown in more detail in FIG. 2, which carries a first pair of bearings acting as wheels 16a, 16b on a first side of the bogie and a second pair of bearings acting as wheels 17a, 17b on a second side of the bogie. The upper wheels 16a, 17a are mounted on a first axle 18 and the lower wheels 16b, 17b are mounted on a second axle 19. It should be noted that the terms upper and lower are used for illustrative purposes only and the actual spatial location of the wheels will vary according to the orientation of the engine. The con rod 14 is connected to the bogie 15 at the first shaft 18, and the bogie is rotatable about the axis of the first shaft 18 with respect to the con rod.

[0068] The bogie 15 and its wheels 16a, 16b, 17a, 17b are received in a track 20 mounted to the inner surface of the drum 12 and extending around the circumference of the drum in a continuous closed loop. The track 20 includes a first arm carrying a projecting portion 21 which extends between the first pair of wheels 16a, 16b and a second arm carrying a projecting portion 22 which extends between the second pair of wheels 17a, 17b. The projecting portions 21, 22 engage the upper wheels 16a, 17a such that the track can push the piston in a direction extending away from the track under the action of the rotating track and the piston can push the track such that the track rotates under the action of the reciprocating piston moving towards the track. Similarly the projecting portions 21, 22 engage the lower wheels 16b, 17b such that the track can pull the piston in a direction extending towards the track under the action of the rotating track and the piston can push the track such that the track rotates under the action of the reciprocating piston moving away from the track. The upper wheels are required to transmit greater loads than the lower wheels and so are larger and stronger.

[0069] The projecting portions have a height H in a direction extending away from the inner surface of the drum 12 which varies around the circumference of the track 20 such that, as the drum rotates thereby moving the track relative to the combustion cylinder 1, the main piston 2 moves in reciprocating motion within the combustion cylinder along its axis in accordance with the varying height H of the track. The height H of the track 20 has a non-sinusoidal shape with respect to the angle of rotation of the drum 12 such the main piston 2 does not follow simple harmonic motion with respect to the angle of rotation of the track.

[0070] The approximate path of the track is shown in FIGS. 3a and 3b. The size of the track has been exaggerated and other components have been omitted from these figures to more clearly illustrate the changes in the height of the path of the track corresponding to the different phases of the engine cycle. The actual track leaves sufficient space inboard of its inner-most point to allow one or more of the piston arrangements as shown in FIG. 1 to be arranged radially within the confines of the track. FIG. 3a shows the shape of the track 20 as it would appear when viewed from a direction aligned with the engine axis 13, and FIG. 3b shows how the shape of the track would appear if the track could be unwound from its curved shape to more clearly illustrate the track path. The reasons for the shape of the track will become apparent in view of the engine operating cycle described below. The track shown in FIG. 3a has a continuous path shape corresponding to a single engine cycle for each complete revolution of the track. In other embodiments, a track may have a path shape corresponding to multiple cycles for each revolution of the track. It should be noted that since the wheels 16a, 16b, 17a, 17b are mounted on a rotating bogie the projecting portions can have a constant (or approximately constant) thickness T around the circumference of the track.

[0071] The compression piston 4 is coupled via a con rod 24 to a second track 25 also provided on the inner surface of the drum 12. The transfer valve 6, HCCI compression ignition piston 8 and exhaust valve are each coupled via respective con rods 26, 28, 30 to respective tracks 27, 29, 31 provided on the outer surface of the shaft 11. The coupling arrangements and drive mechanisms of the compression piston 4, the transfer valve 6, the HCCI compression ignition piston 8 and the exhaust valve are each similar to that described in relation to the main piston 2 and so are not shown or described separately in detail.

[0072] The track 27 of the HCCI compression ignition piston arrangement is arranged to be rotated on the shaft 11 in use to vary the position of the HCCI compression ignition piston track with respect to the other tracks 20, 25, 29, 31. For example, the HCCI compression ignition piston arrangement track may be held stationary in a first position during a first period of operation of the engine, and then rotated about the central axis 13 by a required number of degrees to advance or retard operation of the HCCI compression ignition piston by that amount and held stationary in its new position for a second period of operation. This allows the timing of the HCCI compression ignition piston 8 to be varied compared to the other pistons such that ignition timing may be varied according to the operating conditions of the engine. Movement of the track may be controlled by any suitable actuation system, for example a rack and pinion system. Operation of the HCCI compression ignition piston is explained below.

[0073] The engine functions according to the following operating cycle: [0074] a) a charging phase in which charge is transferred from the compression cylinder 3 into the combustion cylinder 1 via the transfer duct 5 while the main piston 2 is at or near a top dead center or TDC position (FIGS. 4a to 4b); [0075] b) a power phase in which the charge burns and the main piston 2 is moved towards a bottom dead center or BDC position and extracts work (FIGS. 4b to 4c); [0076] c) an exhaust phase in which the main piston 2 moves back towards a TDC position thereby expelling the burnt charge from the combustion cylinder via the exhaust port 10 (FIGS. 4c to 4d); [0077] d) a secondary steam power phase in which steam and/or water is injected into the combustion cylinder by the injector device 9 and the main piston 1 is moved towards a BDC position and extracts work (FIGS. 4d to 4e); and [0078] e) a steam recovery phase in which the main piston 2 moves back towards a TDC position thereby expelling the expanded steam from the combustion cylinder back to the steam injector device 9 which recirculates the steam for use in a following steam cycle (FIGS. 4e to 4f).

[0079] During the charging phase, the transfer valve 6 is open, the main piston 2 remains at or near TDC after completion of the previous steam recovery phase, and the compression piston 4 is approaching TDC having compressed a homogeneous charge of well mixed fuel and air. The tracks 20 and 25 of the main piston arrangement and the compression piston arrangement each have a prolonged portion of low or zero gradient corresponding to this phase of the operating cycle such that the main piston 2 and the compression piston 4 each experience a substantial period of dwell time, thereby allowing a more complete transfer of compressed charge into the combustion cylinder 1.

[0080] Following charging of the combustion cylinder 1, the transfer valve 6 closes thereby sealing the combustion cylinder 1, and the HCCI compression ignition piston moves from a BDC position (that is an outer position with respect to the combustion cylinder corresponding to a maximum volume of the HCCI compression ignition cylinder) into a TDC position (that is an inner position further towards or even extending into the combustion chamber corresponding to a minimum volume of the HCCI compression ignition cylinder) thereby reducing the overall volume of the combustion chamber and causing a rapid increase in pressure. The rapid increase in pressure causes auto ignition of the homogeneous fuel/air mixture and the main piston 2 moves towards a BDC position in a power stroke (see FIGS. 4b to 4c) in which the main piston extracts work and drives the drum 12 via the track 20, thereby transferring work to the shaft 11 in the power phase. The HCCI compression ignition piston 8 may be actuated shortly after the main piston 2 has started to move away from its TDC position such that it is already moving towards BDC when peak pressure occurs. The track 20 is steeper over the first part of the power stroke such that piston speed is higher during the first half of the stroke when cylinder pressure is highest. At a point after auto ignition of the fuel/air mixture (when substantially all the fuel has burnt) the injector device 9 injects water and/or steam into the combustion cylinder 1, which evaporates and expands, thereby cooling the engine and doing further work on the main piston 2 which is transferred to the shaft via the track 20 and drum 12 until the main piston reaches a BDC position (FIG. 4c). During the power phase the compression piston 4 moves towards a BDC position thereby drawing fresh charge into the compression cylinder 3.

[0081] At the end of the power phase, after the main piston 2 has reached BDC (FIG. 4c), the exhaust valve opens the exhaust port and the track pushes the main piston 2 back towards TDC in the exhaust phase, thereby scavenging the combustion cylinder of burnt fuel and air and expanded vapour from the steam phase (FIGS. 4c to 4d).

[0082] At the end of the exhaust phase when the main piston 2 is at or near TDC (FIG. 4d) the exhaust valve closes and steam and/or water is injected into the combustion cylinder for a second time by the injector device 9. Again, the steam and/or water evaporates and expands, thereby cooling the engine and doing further work on the main piston which moves away from TDC (FIGS. 4d to 4e) in a secondary steam power phase towards a second BDC position. The second BDC position is at a different location to the first BDC position reached at the end of the main power phase since the piston displacement required for the secondary steam power phase is less than that required for the combined combustion and steam power phase. The different BDC positions are made possible by a track shape including a first local minimum at a first height H corresponding to the BDC position at the end of the combined combustion and steam power phase and a second local minimum at a second height H different to the first corresponding to a different BDC position at the end of the secondary steam power phase. Following the secondary steam power phase the main piston then moves back towards TDC (FIG. 4f) in a steam recovery phase scavenging the combustion cylinder of the expanded vapour from the secondary steam power phase. The expanded steam is collected by the injector device 9 (or alternatively via a separate steam exhaust port) so that it can be recirculated and used in further steam cycles.

[0083] Following the steam recovery phase, the main piston 2 is at or near TDC and ready to begin the next engine cycle, starting with the following charging phase and moves slightly away from TDC to create space in the combustion cylinder 1 for fresh compressed charge. During the power phase, exhaust phase, secondary steam power phase and steam recovery phases of the main piston 2, the compression piston 4 continues to move towards a BDC drawing charge into the compression cylinder via an inlet port. An inlet valve closes around BDC and the compression piston then moves back towards its TDC position, thereby compressing the charge. The track 25 includes significant portions of low or zero gradient corresponding to the BDC and TDC positions to allow time for a more complete transfer of charge into the compression cylinder during its intake phase and from the compression cylinder into the combustion cylinder during the charging phase. The compression piston 4 approaches TDC around the time the main piston 2 finishes the steam recovery phase, so that the transfer valve 6 may be opened to transfer the compressed charge into the combustion cylinder 1 in the charging phase of the following engine cycle.

[0084] In the above-described embodiment the engine is an HCCI compression ignition type engine including a dedicated HCCI compression ignition piston. In alternative embodiments the engine may not include this extra piston and ignition may be achieved by compression under the action of the main piston 2 alone, or by a spark plug mounted in the head of the combustion cylinder 1. Where ignition is achieved by compression under the action of the main piston alone the main piston may slow or pause its travel towards TDC or even briefly reverse its direction of travel to allow charge to be transferred into the combustion chamber 1, before performing a rapid movement towards TDC after the transfer valve 6 has closed to create the required final increase in pressure to cause ignition of the charge.

[0085] In the above-described embodiment the con rod 14 is coupled to the track by a bogie arrangement 15 with pairs of longitudinally spaced wheels on each side of the bogie, as shown in FIG. 2. In alternative embodiments the wheels 16a, 16b, 17a, 17b may be coupled to the bogie 15 on spindles and the con rod 14 may be mounted by a separate shaft, as shown in FIG. 5a. Alternatively the wheels may be attached directly to the con rod, as shown in FIG. 5b. In this case the thickness T of the projecting portions 21, 22 of the track 20 will vary significantly along the length of the track as the gradient of the track changes to account for the distance between the upper 16a, 17a and lower 16b, 17b wheels. Alternatively wheels may provided only on one side of a bogie or con rod, as shown in FIG. 5c. Alternatively a bogie or con rod may not include longitudinally spaced pairs of wheels each sandwiching a projecting portion of a track but rather a single wheel may be employed on one or both sides of a bogie or con rod, as shown in FIG. 5d or concentric wheels may be employed on one or both sides of a bogie or con rod, as shown in FIG. 5e.

[0086] In the above-described embodiment the con rod 14 is substantially constrained to translational movement along the direction of the axis of the combustion cylinder by the sleeve 23. Alternatively or in addition a swinging arm may be pivotally attached to a fixed point in the engine and pivotally attached to the con rod 14, for example at the bogie 15 or near to the bogie. As well as helping to constrain movement of the con rod, a swinging arm may also be adapted to react some or most or substantially all of the side load exerted on the bogie arrangement by the track, thereby significantly reducing the load transferred to the con rod. A swinging arm may also have an oil delivery passage extending along its length which may be used to deliver lubricating oil to the various bearing surfaces located around the distal end of the con rod and the bogie arrangement.

[0087] The steam phases may provide sufficient cooling for the engine that no further cooling of the combustion cylinder 1 is required. The steam phases may be activated or disabled as required, for example if the engine is not running at a sufficiently high temperature for the steam phases to function adequately one or both of the steam phases may be disabled. In alternative embodiments the initial steam phase and/or the secondary steam power phase may be omitted. If the secondary steam power phase is omitted, the main piston 2 only performs 2 piston strokes per engine cycle instead of the four described above since the engine is ready to begin the next charging phase following completion of the exhaust phase.

[0088] In the above-described embodiment the transfer valve 6, the HCCI compression ignition piston 8 and the exhaust valve are all actuated by con rod and track arrangements as used for the main piston 2 and compression piston 4. In alternative embodiments one or more of the transfer valve 6, the HCCI compression ignition piston 8 and the exhaust valve may be actuated by any known alternative actuation system for valves or pistons.

[0089] In the above-described embodiment the engine includes a transfer valve 6 in a transfer duct 5 connecting the compression cylinder 3 to the combustion cylinder 1 and also a separate exhaust port. In an alternative embodiment the transfer valve may be replaced by a three-way valve 106 (as shown in FIGS. 6a to 6c) allowing a single port to act as both an intake port and an exhaust port for a combustion cylinder 101. The valve 106 has a first position as shown in FIG. 6a in which a first duct 106a is aligned with the combined intake and exhaust port 105a of the combustion cylinder 101 and a transfer duct 105b of a compression cylinder 103 such that charge may be passed through the three-way valve from the compression cylinder into the combustion cylinder. The valve 106 further has a second position as shown in FIG. 6b in which a closing portion 106b is aligned with the combined intake and exhaust port 105a of the combustion cylinder 101 such that the combined intake and exhaust port is substantially sealed. The valve 106 further has a third position as shown in FIG. 6c in which a second duct 106c is aligned with the combined intake and exhaust port of the combustion cylinder 101 and opens into an exhaust system 110 such that burnt charge may be passed through the three-way valve from the compression cylinder into the exhaust system. In the embodiment shown in FIGS. 6a to 6c the valve moves linearly between its three positions and the closed position is located between the intake position and the exhaust positions, but in other embodiments the valve may rotate instead of translating and the intake, closed and exhaust positions may be in any order. This valve arrangement advantageously reduces the number of ports and valves required to operate a combustion cylinder, and may equally be applied to an engine arrangement without a dedicated combustion cylinder but instead having a conventional intake system which is fluidically connected to the combustion cylinder when the three-way valve is in its first position. It should be noted that the three way valve arrangement is not limited to use on an engine as shown in FIG. 1 but may generally be employed for any piston system where a piston is required to have both an inlet and an exhaust port.

[0090] In the above-described embodiment the compression cylinder 3 is connected to the combustion cylinder 1 by a transfer duct 5 operated by a transfer valve 6. In alternative embodiments the transfer valve may be replaced by a transfer cylinder arrangement as shown in FIGS. 7a and 7b. The transfer cylinder 201 has a bore which opens into the head of the combustion cylinder 1 and a transfer port 202 fluidically connects the transfer cylinder to the compression cylinder 3. A transfer piston 203 is moveable in reciprocating motion within the bore of the transfer cylinder 201 to open and close the transfer port 202, thereby controlling the flow of compressed charge into the combustion cylinder. The transfer piston 203 moves between a TDC position as shown in FIG. 7a in which the transfer port 202 is sealed and a BDC position as shown in FIG. 7b in which the transfer port is open. The transfer piston 203 moves towards its BDC position to uncover the transfer port 202 thereby allowing compressed charge to flow from the compression cylinder 3 into the combustion cylinder 1 to initiate the charging phase. The transfer piston 203 then moves towards its TDC position to cover the transfer port 202 at the end of the charging phase, thereby sealing the combustion cylinder during its power phase(s) and exhaust phase(s) and sealing the compression chamber 3 during its intake and compression phases. The transfer piston may be adapted to function as an HCCI compression ignition piston with the final part of its movement towards its TDC position causing a pressure spike to initiate auto ignition of a homogeneous fuel/air mixture. In this case there is no need to include a separate HCCI compression ignition cylinder arrangement 7 as shown in FIG. 1. The transfer piston 203 may be actuated by a con rod and track mechanism similar to that described in relation to the main piston 2, or alternatively may be actuated by any other known piston actuation arrangement.

[0091] FIGS. 8 and 9a to 9f show schematic views of an internal combustion engine 500 according to a second embodiment of the invention. The engine 500 comprises common cylinder 501 having an intake port 502 formed in its bore, an exhaust port 503 formed in its bore and spaced apart from the intake port along a longitudinal axis of the common cylinder, and a spark plug 504 provided at an intermediate location between the intake port and the exhaust port. The engine 500 further comprises a first piston 505 moveable in reciprocating motion within the common cylinder and a second piston 506 also moveable in reciprocating motion within the common cylinder and opposing the first piston such that a combustion chamber is formed between the pistons. The first and second pistons 505, 506 are both connected via con rods 507, 508 to tracks 509, 510 provided on rotating disks 511, 512. The disks 511, 512 are rotationally fixed with respect to each other and rotate relative to the common cylinder 501 about a central axis of the engine 513. Rotation of the disks 511, 512 drives the pistons 505, 506 during some phases of the operating cycle the disks 511, 512 are driven by linear motion of the pistons in other phases, as will become apparent with reference to the following description of an operating cycle. The con rod and track arrangements of the first and second pistons are similar to those already described in relation to the main piston arrangement in the first embodiment of the invention and so are not shown or described in detail. The shapes of the tracks 509, 510 are, however, significantly different to the shape of the track 20 used for the main piston in the first embodiment due to the different piston displacement profiles required for each of the first and second pistons.

[0092] The engine 500 of the second embodiment functions according to the following operating cycle: [0093] a) an intake phase in which the first and second pistons 505, 506 are located on alternate sides of the intake port 502 and the second piston 506 moves away from the first piston 505 in a first direction parallel to the common cylinder axis such that the volume of the chamber is increased and charge is drawn into the chamber (FIGS. 9a to 9b); then [0094] b) a compression phase in which the first piston 505 moves in the first direction towards the second piston 506 such that the volume of the chamber is decreased and the charge is compressed (FIGS. 9b to 9c); then [0095] c) a power phase in which the charge burns following ignition by the spark plug 504 and the second piston 506 moves in the first direction away from the first piston 505 and extracts work (FIGS. 9c to 9d); then [0096] d) an exhaust phase in which the first and second pistons 505, 506 are located on alternate sides of the exhaust port 504 and the first piston 505 moves in the first direction towards the second piston 506 such that the volume of the chamber is decreased and the burnt charge is expelled from the chamber via the exhaust port (FIGS. 9d to 9e); then [0097] e) a recovery phase in which the first and second pistons 505, 506 both move in a second direction parallel and opposite to the first direction back towards the intake port 502 in preparation for the intake phase of the following cycle (FIGS. 9e to 9f).

[0098] The use of the above described con rod and track power transfer mechanism allows the first and second pistons 505, 506 to follow unusual displacement profiles (which would not be possible with conventional power transfer mechanisms such as crank shaft and con rod mechanisms) thereby enabling the above operating cycle. In this case the first piston 505 is slow or stationary around BDC at the start of the cycle and during the intake phase, moves in the first direction during the compression phase, slows significantly (or even pauses or moves slightly in the second direction) during the power phase, moves in the first direction in the exhaust phase, and then moves in the second direction in the recovery phase. During the power phase the first piston moves more slowly than during the compression and exhaust phases, thus experiencing a period of reduced piston speed compared to its speed during the compression and exhaust phases.

[0099] The operating cycle may further include a steam phase in which water and/or steam is injected into the chamber and work is extracted by the first and/or second piston heads. The steam phase may occur during the power phase and/or between the exhaust phase and the recovery phase. If a steam phase occurs during the power phase then a water and/or steam injector is provided in the bore of the common cylinder 501 at a location between the spark plug 503 and the exhaust port 504. In this case water and/or steam is injected into the combustion chamber as the second piston 506 is moving away from the first piston 505 and extracting work, and the water and/or steam evaporates and expands thereby doing further work on the second piston and cooling the engine. If a steam phase occurs after the exhaust phase then a water and/or steam injector is provided in the bore of the common cylinder at a location above the exhaust port. In this case, following the exhaust phase the second piston 506 moves away from the first piston and water and/or steam is injected into the chamber, evaporating and expanding thereby doing further work of the second piston and cooling the engine. In this case the expanded steam may be recollected via a steam collection port and recirculated. Following the additional steam phase both pistons then return to their starting point in a recovery phase.

[0100] In an alternative embodiment the engine 500 may comprise a fuel injector instead of a spark plug and function using direct injection. An another alternative embodiment the engine 500 may be an HCCI compression ignition type engine and an homogeneous charge of well mixed fuel and air may be caused to auto ignite by movement of the first piston 505 towards the second piston 506 and/or movement of the second piston towards the first piston.

[0101] FIGS. 10 and to 11a to 11e show schematic views of an internal combustion engine 600 according to a third embodiment of the invention. The engine 600 comprises common cylinder 601 having an intake port 602 formed in its bore, an exhaust port 603 formed in its bore at a substantially similar location along a longitudinal axis of the common cylinder, and a spark plug 604 also provided at a substantially similar location along the longitudinal axis of the common cylinder. The engine 600 further comprises a first piston 605 moveable in reciprocating motion within the common cylinder and a second piston 606 also moveable in reciprocating motion within the common cylinder and opposing the first piston such that a combustion chamber is formed between the pistons. The first and second pistons 605, 606 are both connected via con rods 607, 608 to tracks 609, 610 provided on rotating disks 611, 612. The disks 611, 612 are rotationally fixed with respect to each other and rotate relative to the common cylinder 601 about a central axis of the engine 613. Rotation of the disks 611, 612 drives the pistons 605, 606 during some phases of the operating cycle the disks 611, 612 are driven by linear motion of the pistons in other phases, as will become apparent with reference to the following description of an operating cycle. The con rod and track arrangements of the first and second pistons are similar to those already described in relation to the main piston arrangement in the first embodiment of the invention and so are not shown or described in detail. The shapes of the tracks 609, 610 are, however, significantly different to the shape of the track 20 used for the main piston in the first embodiment due to the different piston displacement profiles required for each of the first and second pistons.

[0102] The engine 600 of the second embodiment functions according to the following operating cycle: [0103] a) an intake phase in which the inlet valve 602 is open and the first piston 605 moves in a first direction away from the second piston 606 and the second piston 606 moves in a second direction away from the first piston 605 thereby drawing charge into the chamber formed between the first and second pistons (FIG. 11a to 11b); [0104] b) a compression phase in which the first piston 605 moves in the second direction towards the second piston 606 and the second piston 606 moves in the first direction towards the first piston 605 thereby compressing the charge (FIG. 11b to 11c); [0105] c) a power phase in which the first piston 605 moves in the first direction away from the second piston 606 and the second piston 606 moves in the second direction away from the first piston 605 thereby extracting work (FIGS. 11c to 11d); and [0106] d) an exhaust phase in which the exhaust valve 603 is open and the first piston 605 moves in the second direction towards the second piston 606 and the second piston 606 moves in the first direction towards the first piston 605 thereby expelling the burnt charge from the chamber (FIG. 11d to 11e).

[0107] The BDC positions reached by the first and second pistons 605, 606 at the end of the intake phase (FIG. 11b) may be at the same or different locations to the BDC positions reached at the end of the power phase (FIG. 11d). The TDC positions reached by the first and second pistons 605, 606 at the end of the compression phase (FIG. 11c) may be at the same or different locations to the TDC positions reached at the end of the exhaust phase (FIG. 11e).

[0108] Combustion is initiated by the spark plug 604. In alternative embodiments the engine may be a compression ignition engine or an HCCI compression ignition engine. In this case ignition may be achieved by movement of the first piston 605 towards the second piston 606 and/or movement of the second piston towards the first piston.

[0109] In alternative embodiments the intake and exhaust ports may be replaced by a combined intake and exhaust port operated by a three-position valve as shown in FIGS. 6a to 6c. In alternative embodiments the common cylinder may be supplied with compressed intake air provided by a compression cylinder similar to that described above in reference to other embodiments. If a compression cylinder is included that the intake and compression phases may be eliminated and replaced by a single charging phase in which compressed charge in transferred from the compression cylinder into the common cylinder. The charging phase may be accompanied by a slight movement of the first and second pistons 605, 606 away from each other to assist with the transfer of compressed charge. The compressed change may then be ignited by the spark plug or alternatively by movement of the pistons towards each other to cause compression ignition or auto ignition in HCCI embodiments.

[0110] A steam phase may be included either during the power phase or in a separate steam power stroke after the exhaust phase.

[0111] Any of the above described engines may be modified to have a variable length con rod. The variable length con rod may be adjusted during operation of the engine to allow the piston displacement profile to be shifted towards or away from the cylinder head, for example to change the compression ratio.

[0112] FIG. 12a shows a first arrangement in which a piston head 701 is coupled to a track 702 by a variable length con rod 703. The variable length con rod 703 comprises a piston mounting section 704 coupled to the piston head, a track following section 705 coupled to the track via a follower 705a, and an intermediate section 706 having threaded connections 707 and 708 respectively with the piston mounting section and the track following section. The piston mounting section 704 and the track following section 705 each comprise protrusions 709 and 710 which are slidably received within position control grooves in the engine which prevent the piston mounting section and the track following section from rotating about the con rod axis 711. The intermediate section 706 has a protrusion 712 which is slidably received within a piston control groove provided in a rotatable body. The rotatable body may be rotated about the con rod axis 711, for example using an electric motor, thereby rotating the intermediate section. When the intermediate section is rotated in a first direction the threaded connections 707 and 708 move the piston mounting section 704 and the track following section 705 away from each other, thereby increasing the length of the con rod 703. When the intermediate section is rotated in a second direction opposite to the first direction the threaded connections 707 and 708 move the piston mounting section 704 and the track following section 705 towards each other, thereby decreasing the length of the con rod 703.

[0113] FIG. 12b shows a second arrangement in which a piston head 801 is coupled to a track 802 by a variable length con rod 803. The variable length con rod 803 comprises a piston mounting section 804 coupled to the piston head, and a track following section 805 coupled to the track via a follower 805a. The piston mounting section 804 has a threaded connection 807 with the track following section 805. The track following section 805 comprises a protrusion 810 which is slidably received within a position control groove in the engine which prevents the track following section from rotating about the con rod axis 811. The piston head has an extended skirt with a protrusion 809 which is slidably received within a piston control groove 820 provided in a rotatable body 821 mounted below the fixed bore of the cylinder. The rotatable body may be rotated about the con rod axis 811 to rotate the piston mounting section 804 relative to the track following section 805, thereby lengthening or shortening the con rod 803 as desired.

[0114] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.