GB2214569A

GB2214569A - Free-piston I.C engine

Info

Publication number: GB2214569A
Application number: GB8801281A
Authority: GB
Inventors: Barry John Rymer
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-01-21
Filing date: 1988-01-21
Publication date: 1989-09-06
Anticipated expiration: 2008-01-21
Also published as: GB8801281D0; GB2214569B

Abstract

An engine comprises first and second interconnected pistons 7, 8 which define four variable volume chambers in a block 1. Gas in the chamber 21 is compressed by the second piston 8 during the combustion stroke of the engine to provide resilient means for urging the first piston 7 back along the cylinder during the return stroke. During normal running, gas drawn into chamber 17 through manifold 26 is compressed for storage in a cross scavenging reservoir 9, to scavenge the combustion chamber 4 after every combustion stroke. During start up however, pressurised gas from line 33 is supplied to the chamber 17 so as to move the second piston 8 to the left to compress the gas in chamber 21, and suitable operation of solenoids 34 and 36 causes reciprocation of the pistons 7, 8 until normal running is established. Chamber 27 forms a compressor chamber in which gas drawn in via manifold 10 is, in normal running, compressed for supply to a reservoir via line 41, 33 or, during start up, exhausted to atmosphere via line 45. <IMAGE>

Description

INTERNAL COMBUSTION PISTON ENGINE The invention described herein relates to the field of internal combustion piston engines.

Conventional piston engines have at least one piston which reciprocates in a cylinder bore in the engine block. The piston is coupled by a connecting rod to a crankshaft, the crankshaft itself being rotatably mounted in the engine block via main bearings. The piston is coupled to one end of the connecting rod by a gudgeon pin and the crankshaft crankpin is coupled to the other end of the connecting rod by a big end bearing.

The crankshaft and connecting rod have a number of functions in-a conventional piston engine.

The energy released through the combustion of the fuel is converted-nto mechanical work by rotating the crankshaft against'a torque. In addition, a piston engine only converts energy during the combustion stroke of the engine operation cycle and energy must be expended to cause the piston to move through the remaining strokes of the cycle which include the compression of the air/fuel mixture.

In a conventional piston engine the rotational kinetic energy of the crankshaft provides a source of energy to move the piston through the strokes of the cycle other than the combustion stroke including providing the energy to compress the air/fuel mixture.

The conventional crankshaft and connecting rod arrangement does however suffer from a number of technical problems. In operation the bearings at both ends of the connecting rod and the crankshaft main bearings are subject to considerable stress.

Consequently these components have to be manufactured to close tolerances and provided with a complex and expensive lubrication system. Also the crankshaft and connecting rod must themselves be sufficiently strong to withstand the imposed stresses thereby resulting in a disadvantageous increase in the weight of the engine. The crankshaft and connecting rod also increase the overall size of the piston engine which in many applications (e.g. a motor vehicle) is a disadvantage.

Viewed from one aspect the invention provides an internal combustion piston engine in which a resilient means provides the restoring force for the return stroke of the piston(s).

The engine may thus have a first piston movable along a first cylinder bore, the first piston and the first cylinder bore together forming a combustion chamber for containing a combustible mixture wherein, in use, the resilient means urges the first piston to move along the first cylinder bore during the return stroke so as to compress the combustible mixture in the combustion chamber.

Such an arrangement can provide a number of advantages over known internal combustion engines, including a reduction in the number of moving parts with consequential cost and reliability benefits.

The provision of a resilient restoring means for the piston represents a new departure from traditional internal combustion engines in which the momentum of the crankshaft carries the piston back to a top dead centre position during the return or compression stroke of the engine.

The invention is applicable to all forms of internal combustion engine, including diesel and petrol engines etc. In all cases the operation of the engine and the associated fuel inlet/outlet means may conveniently be controlled by a microprocessor responsive to means arranged to sense the position of the piston or pistons. In the case, for example, of a petrol engine, the microprocessor may also control the ignition circuit for the spark plugs.

The invention is particularly applicable to a twostroke diesel engine, which requires neither a timed ignition means nor a complex arrangement of inlet/outlet values and which consequently requires less complex control electronics.

The form of the resilient restoring means may vary, and it is envisaged that such may include one or more compression springs arranged to-urge the piston(s) towards the cylinder head in the compression stroke. In a particularly advantageous embodiment, however, the resilient restoring means comprises a piston return chamber, a piston being arranged to compress a gas in the return chamber during the combustion stroke of the engine so as to provide the required resilient restoring force.

In a simple form of the invention, the piston return chamber may be defined on the side of the or each main piston of the engine remote from the combustion chamber.

In this case the engine may comprise a single cylinder (or a plurality of cylinders in a multicylinder engine) divided into two sealed chambers by a single piston, namely a combustion chamber and a piston return chamber. An appropriate resilience of the restoring means can be selected by providing a suitable compression ratio and gas pressure in the piston return chamber.

In a preferred embodiment, however, the piston return chamber is separate from the main cylinder and the engine includes a second or subsidiary piston movable in the return chamber and drivingly coupled to the main piston via a suitable connecting means such as a connection rod.

In a preferred such embodiment, the engine includes one or more subsidiary chambers bounded by cylinder bores and the end faces of one or more subsidiary pistons slidable within the bores, the motion of the or each piston being coupled to the motion of the first piston. One or more of the subsidiary chambers may form the piston return chambers, the piston return chamber(s) forming the resilent means in that gas pressure within the piston return chamber(s), in use, urges the first piston to move along the first cylinder bore and compress the combustible mixture in the combustion chamber.

Preferably, the gas pressure within the piston return chamber is maintained above a predetermined minimum value.

In one embodiment of the invention adapted for use as a compressor a further subsidiary chamber forms a compression chamber for, in use, compressing gas as the first piston reciprocates in the first cylinder bore. In such embodiment means are preferably provided whereby the compression chamber does not compress gas whilst the engine is starting.

In a preferred embodiment a further subsidiary chamber forms a starter chamber with gas pressure in the starter chamber urging the first piston to mqve along the first cylinder bore against the resilient means during starting of the engine.

According to a further preferred feature of the invention a subsidiary chamber forms a cross scavenging chamber for compressing gas for storage in a cross scavenging reservoir whilst said engine is running, compressed gas from said cross scavenging reservoir being fed to the combustion chamber as part of the combustable mixture.

In a particularly preferred embodiment the same subsidiary chamber forms both the starter chamber and the cross scavenging chamber, such chamber preferably being defined on the side of the return piston remote from the piston return chamber.

Thus, whilst it is envisaged that the preferred engine may comprise more than one subsidiary piston movable with the main piston and associated with respective starter, cross-scavenge, piston return, compression and combustion chambers, a particularly preferred arrangement comprises only a main piston coupled to a single subsidiary piston (or a plurality of main pistons and respective subsidiary pistons).

The pistons are located in respective main and subsidiary cylinders which are thus divided into variable volume chambers , the chambers associated with the main piston being the combustion chamber (as with traditional engines) and a chamber available for use as a compressor, and the chambers associatd with the subsidiary piston being the piston return chamber and a chamber used for starting and subsequently for providing cross-scavenging pressure.

The preferred engine is described for use as a compressor, although there may be other applications for engines in accordance with the invention.

For example the piston may be coupled to a rotating crankshaft in a manner similar to traditional engines, in which case the resilient piston restoring means may avoid the need e.g. for a heavy fly wheel to ensure the piston is returned to the top of its stroke.

The internal combustion engine of the preferred embodiment has a number of advantages. The engine has essentially only one mechanical moving part which reduces production and maintainance costs and increases reliability. One possible application of the preferred embodiment would be as a compressor for providing compressed gas via a swirl tank to a turbine powering a vehicle. The engine, the turbine and the swirl tank could all be positioned separately improving the weight distribution of the vehicle.

The means discussed above whereby fresh gas from a cross-scavenge chamber under pressure is introduced into the combustion chamber leads to improved efficiency of the engine and is thus advantageous independently of the resilient restoring means in accordance with the first aspect of the invention.

Accordingly, viewed from a second aspect, the invention provides an internal combustion engine comprising a main piston drivingly coupled to or associated with a variable volume subsidiary piston chamber, the subsidiary chamber providing pressurised gas for scavenging the combustion chamber of the main piston after each combustion stroke thereof.

The subsidiary chamber may be associated with the main piston itself (i.e. it may be defined in the main cylinder on the side of the main piston remote from the combustion chamber) although preferably the engine comprises a subsidiary piston drivingly coupled to the main piston and located in a separate cylinder bore.

Preferably, an outlet from the subsidiary chamber feeds gas under pressure via a first one way valve means to an intermediate pressure reservoir which in turn communicates with the combustion chamber via a port in the side wall thereof, such port being closed by the main piston when in the upper part of its stroke and being open when the piston is at or adjacent its lower dead centre position. The operation of such an engine is as follows.

During the combustion stroke of the engine gas e.g. fresh air, is drawn into the subsidiary chamber via a further one way valve means. During the return stroke such air is forced under pressure into the intermediate reservoir whose outlet port is at that time blocked by the main piston. After the next combustion stroke, such pressurised gas is thereby released into the combustion chamber when the port is opened thus assisting in ejecting exhaust gases from the combustion chamber and increasing efficiency.

A specific embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a detailed cross section of the engine; Figures 2a-2d show the running cycle of the engine; Figures 3a-3d show the starting cycle of the engine.

Figure 1 shows a detailed cross section of an internal combustion piston engine in accordance with the present invention. The engine comprises a cylinder block 1 having a first piston 7 and a second piston 18 slidable along the cylinder bore. First piston 7 and second piston 18 are connected by connecting rod 16. Connecting rod 16 passes through baffle block 28 via seal 15.

One end of the cylinder block 1 is closed by combustion chamber cylinder head 6 and the other end of the cylinder block 1 is closed by piston return chamber cylinder head 19. The cylinder block 1, the cylinder heads 6 and 19, the baffle block 28 and the first and second pistons 7 and 18 thus define four chambers within the engine. The cylinder block l, the combustion chamber cylinder head 6 and the first piston 7 define a combustion chamber 4, and the cylinder block 1, the baffle block 28 and the first piston 7 define a compression chamber 27. The cylinder block 1, the piston return chamber cylinder head 19 and the second piston 18 define a piston return chamber 21, and, finally, the cylinder block 1, the baffle block 28 and the second piston 18 define a starter/cross scavenging chamber 17. The functions of the four chambers will be described further below.

The engine is connected via first pressure line 20 to a swirl tank (not shown). Piston return chamber 21 is connected to first pressure line 20 via a regulator 29 and first non-return valve 30. First non-return valve 30 only allows gas to pass into piston return chamber 21.

The starter/cross scavenging chamber 17 has a first inlet 31 and a first outlet 32. The first inlet is connected to pressure line 20 via a second pressure line 33. Second pressure line 33 may be closed using a first solenoid valve 34. The first inlet 31 is also connected to the atmosphere via first inlet manifold 26. A second non-return valve 35 is position within first inlet manifold 26 and only permits gas to flow into starter/cross scavenging chamber 17. The first outlet 32 is connected to a cross scavenging reservoir 9 via a first exhaust manifold 11 which may be closed by second solenoid valve 36. First exhaust manifold 11 also contains a third non-return valve 37 which only allows gas to flow out of starter/cross scavenging chamber 17 into cross scavenging reservoir 9.

The compression chamber 27 has a second inlet 38 and second outlet 39. Second inlet 38 is connected to the atmosphere via second inlet manifold 10.

Positioned within second inlet manifold 10 is a fourth non-return valve 40 which only allows gas to pass into the compression chamber 27. Second outlet 39 is connected to second pressure line 33 via a second outlet manifold 41. Positioned within second outlet manifold 41 is a fifth nonreturn valve 42, which only allows gas to pass out of compression chamber 27 into second pressure line 33.

The combustion chamber 4 has a third inlet 43 and a third outlet 44, the third inlet 43 being connected to cross scavenging reservoir 9 via third inlet manifold 8. Third outlet 44 is connected to the atmosphere via third exhaust manifold 2.

The embodiment illustrated has a diesel fuel injector 5 positioned in the combustion chamber cylinder head 5. Alternatives to this arrangement would be indirect diesel fuel injection, petrol fuel injection and spark ignition, petrol carburation and spark ignition and other internal combustion engine fuel supply systems known in the art.

Second outlet manifold 41 and third exhaust manifold 2 are linked by third pressure line 45, which communicates with the second exhaust manifold 41 between fifth non-return valve 42 and second outlet 39. Positioned within third pressure line 45 is a third solenoid 46. A combustion sensing heat sensor 3 is mounted on third exhaust manifold 2.

The first piston 7 and the second piston 18 have conventional piston rings 13 around the circumference. First piston 7 has an oil control ring 14 around its circumference to one side of the piston rings 13 at the end of the first piston 7 closest to the combustion chamber 4. The engine is lubricated by cylinder lubricator 12 which provides an oil-mist through the cylinder block 1 to lubricate the second piston 18 and into the second inlet manifold 10 to lubricate the first piston 7. An array of position sensors 23 is disposed in the wall of the cylinder block 1 and provide signals to a micro-computer (not shown) in response to the position of the second piston 18. The lowest point 24 of outlet manifold 41 includes an automatic bleeder 25 to clear any condensation from the system.

The bleeder is connected to the exhaust manifold 2 via a small passage 100. The engine may be water cooled by fitting a water jacket, electric water pump and radiator (not shown) or air cooled by utilising exhaust compressed air.

It is envisaged that the relative positions of the chambers of the engine could be changed and such other configurations are within the scope of this disclosure.

The function of the illustrated engine is to pressurise a swirl tank (not shown). The compressed air in such a swirl tank could then be used, for example, to drive a turbine. As described in detail above, the engine consists of four chambers: combustion chamber 4, compression chamber 27, piston return chamber 21 and starter/cross scavenging chamber 17. The function of piston return chamber 21 is to urge the second piston 18 together with the connecting rod 16 and first piston 7 towards the combustion chamber cylinder head 6 thereby compressing the combustable mixture in the combustion chamber 4 ready for the next combustion stroke. The piston return chamber 21 operates in this manner in each return stroke during both starting and running of the engine.

The starter/cross scavenging chamber 17 has two functions. During starting of the engine the starter/cross scavenging chamber 17 is pressurised thereby pushing the piston assembly towards piston return chamber 21 to cause the first piston 7 to move away from the combustion chamber cylinder head 6 as required. The starter/cross scavenging chamber 17 is pressurised by closing second solenoid valve 36 and opening first solenoid valve 34.

Compressed gas from the swirl tank (or, in other applications, from any suitable pressure source) then flows down second pressure line through first inlet 31 into starter/cross scavenging chamber 17. The motion of the second piston 18 towards the piston return chamber cylinder head 19 is sensed by the position sensors 23 and when the piston assembly has reached the required position the first solenoid valve 34 is closed and the second solenoid valve 36 is opened under the control of the micro-computer allowing the starter/cross scavenging chamber 17 to depressurise through third non-return valve 37 and first exhaust manifold 11 into cross/scavenging reservoir 9.The gas pressure in the starter/cross scavenging chamber 17 must be higher than the gas pressure in the piston return chamber 21 in order that the second piston 18 will move in the direction of the piston return chamber cylinder head 19.

When however the first solenoid valve 34 is closed and the second solenoid valve 36 is open the piston assembly will move towards the combustion chamber cylinder head 5 due to the gas pressure in piston return chamber 21 acting upon second piston 18.

When the second piston 18 has reached the required position as sensed by position sensors 23 then the first solenoid valve 34 will again be opened and the second solenoid valve 36 closed under the control of the micro-computer so that the starting cycle can be repeated, if necessary.

Regulator 29 and first non-return valve 30 have the effect of ensuring that the minimum gas pressure within the piston return chamber 21 does not fall below a value determined by the setting of regulator 29 with the effect that when second piston 18 is at its most distant position from piston return chamber cylinder head 19 then any gas which has escaped past second piston 18 into starter/cross scavenging chamber 17 will be replaced by gas flowing through the regulator 29 and first non-return valve 30.

The action of the piston return chamber 21 and the starter/cross scavenging chamber 17 under control of the micro-computer has the effect of causing the first piston 7 to reciprocate in the cylinder block 1. During starting, third solenoid valve 46 is held open by the micro-computer so that compression chamber 27 does not compress gas during starting. As the first piston 7 reciprocates air is drawn into the compression chamber 27 through second inlet manifold 10 and fourth non-return valve 40 and is then exhausted through second outlet 39 and third pressure line 45 through the open third solenoid valve 46 into third exhaust manifold 2.

When the first piston 7 is at its maximum displacement from the combustion chamber cylinder head 6, third inlet 43 is uncovered and compressed gas from the cross/scavenging reservoir 9 flows -into the combustion chamber 4. Under the action of the piston return chamber 21 the first piston 7 is forced towards the combustion chamber cylinder head 6 compressing the gas in the combustion chamber.

When the appropriate position of first piston 7 is sensed by the position sensors 23 then the microcomputer issues a signal which causes the diesel fuel injecter 5 to inject fuel into the heated and compressed gas in the combustion chamber.

If combustion then occurs, first pistion 7 will be forced away from the combustion chamber cylinder head 6. As the first piston 7 moves the third outlet 44 becomes uncovered allowing the exhaust gases to escape through third exhaust manifold 2. A fresh charge of gas will then enter from the cross-scavenge reservoir through third inlet 43 to replace the gas burnt. The micro-computer senses that combustion has occurred using a heat sensor 3 which is sensitive to the high temperature exhaust gases which flow through exhaust manifold 2 when combustion is occurring. The micro-computer will then switch from its starting mode of control t a running mode of control. If combustion did not occur then the starting cycle is simply repeated, using the fresh charge of gas in the combustion chamber 4. Starting may be aided by the use of a conventional glow-plug (not shown) positioned in the combustion chamber cylinder head 6 to preheat the cylinder.

When the engine is running the combustion chamber 4 and the piston return chamber 21 operate in the same manner as described during the starting mode. The operation of the starter/cross scavenging chamber 17 and the compression chamber 27 are however different.

First solenoid valve 34 is held closed and second solenoid valve 36 is held open throughout the entire running engine cycle. As the second piston 18 reciprocates air is drawn through first inlet manifold 26, second non-return valve 35 and first inlet 31 into starter/cross scavenging chamber 17. This air is then compressed and forced through third non-return valve 37 into cross/scavenging reservoir 9 as the second piston moves towards baffle block 28. Cross/scavenging reservoir 9, thus provides a source of compressed, fresh gas for the combustion chamber 4 thus resulting in an increase in the efficiency in which the fresh gas is introduced into and the exhaust products are expelled from the combustion chamber 4.

When the engine is running, third solenoid valve 46 is closed and thus as the first piston 7 reciprocates air is drawn in through second inlet manifold 10 and forth non-return 40 into the compression chamber 27 and then this gas is forced out under pressure through fifth non-return valve 42, second exhaust manifold 41, second pressure 33 and first pressure line 20 to the swirl (not shown).

The engine speed as sensed by the array position sensors 23 is controlled by the micro-computer which alters the fuel injection amount to change the engine speed in a manner which maintains the pressure in the swirl tank constant. The amount of oil-mist produced by cylinder lubricator 12 may also be varied in dependance upon the sensed engine speed so as to increase the amount of lubrication with increasing engine speed.

Referring now to figures 2a-2d these show schematically the flow of gas with the engine running.

In figure 2a the first piston 7 is at the top of its compression stroke and ignition of the air/fuel mixture has just occurred. As the first piston 7 moves away from the combustion chamber cylinder head 6 the gas in the compression chamber 27 is forced out into the swirl tank, gas is drawn into starter/cross scavenging chamber 17 from the atmosphere and the gas in piston return chamber 21 is compressed.

In figure 2b the first piston 7 is in an intermediate position during its combustion stroke and the flow of gases continues in the same manner shown in figure 2a. In figure 2c the first piston 7 has just started its compression stroke. The burnt gases are being replaced by fresh gas in the combustion chamber, gas is starting to be drawn from the atmosphere into the compression chamber 27 and the gas in starter/cross scavenging chamber 17 is starting to be compressed into the cross/scavanging reservoir 9. It is gas pressure within piston return chamber 21 which causes the piston assembly to move towards the combustion chamber cylinder head 6. In figure 2d the first piston 7 is in an intermediate position during its compression stroke and the flow of gas continues as in figure 2c except that no gas now flows either into or out of the combustion chamber 4.The first piston 7 then returns to the position shown in figure 2a and the cycle is repeated.

Figures 3a-3d show the engine starting cycle.

In figure 3a first piston 7 is just starting to move away from combustion chamber cylinder head 6 under the influence of gas from the swirl tank flowing into the starter/cross scavenging chamber 17 causing the second piston 18 to compress the gas in the piston return chamber 21. The gas in the compression chamber 27 is exhausted at low pressure into the exhaust manifold of the combustion chamber 4. In figure 3b the first piston 7 is at an intermediate position in it movement away from combustion chamber cylinder head 6 and the flow of gas is as described for figure 3a. In figure 3c the first piston 7 is just starting its compression stroke. The flow of gas into starter/cross scavenging chamber 17 from the swirl tank has been stopped and the gas from starter/cross scavenging chamber 17 now flows into cross/scavenging reservoir 9. The piston assembly is being forced to move towards the combustion chamber cylinder head 6 by the gas pressure in piston return chamber 17.

Gas is being drawn into compression chamber 27 and gas is flowing through combustion chamber 4 from the cross/scavenging reservoir 9 to the exhaust manifold. In figure 3d the first piston is at an intermediate position during its compression stroke and the flow of gas is the same as shown in figure 3c except that no gas now flows into or out of combustion chamber 4. The first piston 7 will continue to moves towards combustion chamber cylinder head 6 and the engine will then return to the state shown in figure 3a.

Claims

CLAIMS:

1. An internal combustion engine wherein a resilient means provides the restoring force for the return stroke of the piston or pistons.

2. An internal combustion engine as claimed in claim 1 and having a first piston movable along a first cylinder bore, the first piston and the first cylinder bore together forming a combustion chamber for containing a combustible mixture wherein, in use, the resilient means urges the first piston to move along the first cylinder bore during the return stroke so as to compress the combustible mixture in the combustion chamber.

3. An internal combustion engine as claimed in claims 1 or 2 wherein said resilient means includes one or more compression springs arranged to urge the said piston(s) towards the cylinder head in the compression stroke.

4. An internal combustion engine as claimed in any preceding claim wherein said resilient restoring means comprises a piston return chamber, a piston being arranged to compress a gas in the return chamber during the combustion stroke of the engine so as to provide the required resilient restoring force.

5. An internal combustion engine as claimed in claim 4 wherein the piston return chamber is defined on the side of the or each main piston of the engine remote from the combustion chamber.

6. An internal combustion engine as claimed in claim 5 wherein the engine comprises a single cylinder, (or a plurality of cyliners in a multi-cylinder engine) divided into two sealed chambers by a single piston, namely a combustion chamber and a piston return chamber.

7. An internal combustion engine as claimed in any one of claims 4 to 6 wherein an appropriate resilience of the restoring means is selected by providing a suitable compression ratio and gas pressure in the piston return chamber.

8. An internal combustion engine as claimed in claims 4 or 5 wherein the piston return chamber is separate from the main cylinder and the engine includes a second or subsidiary piston movable in the return chamber and drivingly coupled to the main piston via a suitable connecting means such as a connection rod.

9. An internal combustion engine as claimed in claim 8, including one or more subsidiary chambers bounded by cylinder bores and the end faces of one or more subsidiary pistons slidable within the bores, the motion of the or each subsidiary piston being coupled to the motion of the first piston(s).

10. An internal combustion engine as claimed in claim 9, wherein one or more of the subsidiary chamber(s) forms the piston return chamber(s), the piston return chamber(s) forming the resilient means in that gas pressure within the piston return chamber(s), in use, urges the first piston to move alone the first cylinder bore and compress the combustible mixture in the combustion chamber.

11. An internal combustion engine as claimed in claim 10 wherein the gas pressure within the piston return chamber is maintained above a predetermined minimum value.

12. An internal combustion engine as claimed in any one of claims 2 to 11 comprising a subsidiary chamber which forms a compression chamber for, in use, compressing gas as the first piston reciprocates the first cylinder bore.

13. An internal combustion engine as claimed in claim 12, comprising means whereby the compression chamber does not compress gas whilst the engine is starting.

14. An internal combustion engine as claimed in any one of claims 2 to 13 comprising a subsidiary chamber which forms a starter chamber, with gas pressure in the starter chamber urging the first piston to move along the first cylinder bore against the resilient means during starting of the engine.

15. An internal combustion engine as claimed in any one of claims 2 to 14, comprising a subsidiary chamber which forms a cross scavenging chamber for compressing gas for storage in a cross scavenging reservoir whilst said engine is running, compressed gas from said cross scavenging reservoir being fed to the combustion chamber as part of the combustable mixture.

16. An internal combustion engine as claimed in claim 15 when dependent upon claim 14 wherein the same subsidiary chamber forms both the starter chamber and the cross scavenging chamber.

17. An internal combustion engine as claimed in claim 16, wherein the said subsidiary chamber is defined on the side of return piston remote from the piston return chamber.

18. An internal combustion engine comprising a main piston drivingly coupled to or associated with a variable volume subsidiary piston chamber, the subsidiary chamber providing pressurised gas for scavenging the combustion chamber of the main piston after each combustion stroke thereof.

19. An internal combustion engine as claimed in claim 18, wherein the subsidiary chamber is defined in the main cylinder on the side of the main piston remote from the combustion chamber.

20. An internal combustion engine as claimed in claim 18 comprising a subsidiary piston drivingly coupled to the main piston and located in a separate cylinder bore.

21. An internal combustion engine as claimed in any one of claims 15 to 20 wherein an outlet from the subsidiary chamber feeds gas under pressure via a first one way valve means to an intermediate pressure reservoir which in turn communicates with the combustion chamber via a port in the side wall thereof, such port being closed by the main piston when in the upper part of its stroke and being open when the piston is at or adjacent its lower dead centre position.

22. An internal combustion engine according to any preceding claim comprising microprocessor control means responsive to means arranged to sense the position of the piston or pistons.

23. An internal combustion engine substantially as described herein with reference to the accompanying drawings.