This invention relates to an internal combustion engine operating on a two stroke cycle.

Background of the Invention

Internal combustion engines tend to operate on either a two stroke or a four stroke cycle. A four stroke cycle means that a combustion event only occurs once in four back and forth movements of the piston. Two stroke engines by comparison perform a combustion event in only two back and forth movements of the piston, meaning that two stroke engines are often lighter for a given power output. This makes two stroke engines particularly useful in smaller applications where improved power cannot be easily achieved by simply increasing the size of the engine. Example uses of two stroke engines include hand held power tools, small bikes and remotely operated light aircraft.

It is an object of the invention to provide a two stroke engine that offers improved power to weight performance.

Statements of Invention

According to the invention, there is provided a two stroke internal combustion engine comprising at least one cylinder, a piston disposed within the cylinder, a crankshaft and a connecting rod that connects the piston to the crankshaft, wherein the piston divides the cylinder into first and second combustion chambers and is reciprocal within the cylinder to perform a power stroke producing work on a crankshaft while moving towards or away from the crankshaft, and wherein the engine is configured to introduce air into the first combustion chamber prior to, and/ or during, each occasion that the piston moves away from the crankshaft and into the second combustion chamber prior to, and/ or during, each occasion that the piston moves toward the crankshaft. The engine may further comprise a blower configured to alternately introduce air into the first and second combustion chambers.

The blower may comprise an electrically driven fan or compressor. The blower may comprise a fan or compressor mechanically coupled to the crankshaft The piston may comprise a first end defining a wall of the first combustion chamber and a second end defining a wall of the second combustion chamber, such that, as the piston moves in reciprocal fashion within the cylinder, the first and second combustion chambers alternately increase and decrease in volume, and wherein a skirt separates the first and second ends of the piston and is configured to alternately block off and open up an air inlet into the first and second combustion chambers, the air inlet being in fluid communication with the blower.

The skirt may be configured to alternately block off or open up an exhaust outlet communicating with the first and second combustion chambers.

The engine may comprise two air inlets and two exhaust outlets spaced apart along a longitudinal axis of the cylinder. The engine may further comprise a separation plate that extends across an end of the cylinder to seal the second combustion chamber, the connecting rod passing through a sealed opening of the separation plate.

The separation plate may be configured to slide across the end of the cylinder to accommodate lateral sway of the connecting rod as it turns the crankshaft.

The sealed opening may comprise a joint having a bore through which the connecting rod extends, the joint comprising a curved outer surface configured to ensure contact with an outer seal disposed between the separation plate and the joint, irrespective of the angle of the connecting rod relative to the separation plate.

The sealed joint may further comprise an inner seal disposed between the bore and the connecting rod and wherein the outer and inner seals are selected from any of:

a split ring compression seal;

a split ring expansion seal;

a gapless expansion seal;

a gapless compression seal;

a labyrinth seal; and

a brush seal. A separation plate seal may be provided between the second combustion chamber and the separation plate.

The engine may further comprise a crankcase housing the crankshaft, the crankcase being configured to retain the separation plate against the cylinder.

A seal maybe disposed between the crankcase and the separation plate.

The piston may be connected to the crankshaft by a pin, and wherein one end of the pin comprises an oil outlet that communicates with a pressurised oil gallery extending through the connecting rod to deliver a film of oil between the piston and cylinder wall.

The piston may comprise first and second oil control rings, the oil outlet being located between the oil control rings.

The pin further may comprise an oil scavenging port communicating with an oil return gallery in the connecting rod, configured to return excess oil to the crankcase.

Brief Description of the Figures

Fig. lA shows a schematic two stroke engine with its piston at bottom dead centre;

Fig. lB shows another schematic of the two stroke engine with its piston at top dead centre;

Fig. 2 shows a schematic of another two stroke engine;

Fig. 3 shows a schematic of a separation plate sealing arrangement;

Fig. 3A shows a detail view of the separation plate sealing arrangement of Fig. 3;

Fig. 4 shows a schematic of another two stroke engine;

Fig. 5 shows a detail view of a separation plate sealing arrangement of the engine of Fig. 4;

Fig. 6 shows a detail view of a seal of the separation plate sealing arrangement of Fig. 5; Fig. 7 shows a detail view of another separation plate sealing arrangement;

Fig. 8 shows a detail view of another separation plate sealing arrangement;

Fig. 9 shows a schematic of another two stroke engine;

Fig. 10 shows a schematic of the two stroke engine of Fig. 9 from another angle;

Fig. 11 shows a schematic of a piston lubricating system; and

Fig. 12 shows a cross section of another piston lubricating system. Detailed Description of the Invention

With reference to Fig. lA and Fig. lB, there is shown a two stroke internal combustion engine no according to the present invention. The engine no comprises a cylinder block n mounted on a crankcase 12, which serves as a sump. For the sake of convenience only a single cylinder 13 is shown but the block 11 could house any number of cylinders as is desired for a particular engine

configuration. The cylinder 13 is divided into first and second combustion chambers 14 & 15 by a reciprocal piston 16. Piston rings 161 ensure a gas tight seal between the piston 16 and the cylinder wall 13.

The piston 16 is connected to a crankshaft 21 by a connecting rod 17 in the usual way.

The piston 16 is a double acting piston. The term "double acting" means that a power stroke for the engine 11 can be performed in either direction of movement of the piston 16. In other words the piston 16 performs a power stroke producing work on the crankshaft 21 irrespective of whether the piston 16 is moving towards or away from the crankshaft 21, as will be explained in more detail below.

In a conventional combustion engine, the volume of the combustion chamber alternately increases and decreases as the piston moves in reciprocal fashion within the cylinder. As the piston moves toward a cylinder head, air and fuel in the cylinder is compressed between the piston and the cylinder head for combustion. Following combustion, the resultant expanding gases perform work on the piston by forcing it back down the cylinder.

In the present invention, both first and second combustion chambers 14, 15 are configured to compress fuel and air for combustion. In the illustrated example of Figs. lA and lB, the first combustion chamber 14 compresses air and fuel as the piston 16 moves toward a cylinder head 112, closing a first end of the cylinder 13; and in the second combustion chamber 15 air and fuel is compressed as the piston 16 moves toward a separation plate 18, closing a second end of the cylinder 13. Because the first and second combustion chambers 14, 15 are defined either side of the piston 16, it shall be appreciated that the volume of the first combustion chamber 14 decreases as the volume of the second combustion chamber 15 increases and vice versa. Combustion events occur in both combustion chambers 14, 15 on a two stroke cycle as follows: As the piston 16 moves toward the separation plate 18, combustion, exhaust and intake occur in the first combustion chamber 14, while fuel and air in the second combustion chamber 15 are compressed for combustion. Then, as the piston 16 moves back toward the cylinder head 112, combustion exhaust and intake occur in the second combustion chamber 15 while fuel and air in the first combustion chamber 14 are compressed for combustion. The process is then repeated.

The operational cycle of the two chambers 14, 15 will now be explained in more detail, with reference to Figs. lA and lB. In this example, the cylinder 13 is provided with a single inlet port 114 and a single exhaust port 116. As the piston 16 moves towards and away from the crankshaft 21 it acts as a valve, alternately opening the inlet port 114 and exhaust port 116 to the first and second combustion chambers 14, 15, respectively.

In the illustrated example, the inlet and exhaust ports 114, 116 are disposed midway along the length of the cylindrical wall of the cylinder 13. Therefore, as the piston 16 moves between the extremes of its range of movement, a cylindrical wall 166 of the piston - herein referred to as the piston skirt 166 - moves over the inlet and exhaust ports 114, 116, alternately opening them into the first and second combustion chambers 14, 15· As the piston 16 moves to a first extreme as shown in Fig. lA, herein referred to as bottom dead centre (BDC), the first combustion chamber 14 is enlarged to its maximum volume and the second combustion chamber 15 is reduced to its minimum volume. With the piston at BDC, the exhaust and inlet ports 114, 116 communicate with the first combustion chamber 14. This allows exhaust gasses to exit the first combustion chamber 14 through the exhaust outlet 116, and a fresh charge of air to enter through the inlet port 114. Fuel is simultaneously introduced with the fresh charge of air, either through the inlet port 114 by entrainment in the air, or the fuel may be separately injected by an injector (not shown). As the piston 16 moves back along the cylinder from BDC to its other extreme as shown in Fig. lB, herein referred to as top dead centre (TDC), the piston 16 passes over the inlet and exhaust ports 114, 116, blocking them off and sealing the first combustion chamber 14. The fresh charge of air and fuel is compressed and, when the piston 16 is at about TDC, ignited by a spark plug 26. Ignition forces the piston 16 back along the cylinder toward BDC. The exhaust and inlet ports 116, 114 are again uncovered and the process is repeated.

This cycle is mirrored in the second combustion chamber 15, so that, when the piston 16 is at TDC, the inlet and exhaust ports 114, 116 communicate with the second combustion chamber 15 to allow a fresh charge of air and fuel in and exhaust gases out.

As the piston 16 moves back along the cylinder 13 from TDC to BDC, the piston 16 passes over the inlet and exhaust ports 114, 116, blocking them off and sealing the second combustion chamber 15. The fresh charge of air and fuel is compressed and, when the piston 16 is at about BDC, ignited by a spark plug 27. Ignition forces the piston 16 back along the cylinder 13 toward TDC. The exhaust and inlet ports 116, 114 are again uncovered and the process is repeated.

The air inlet 114 is pressurized using a blower 117 so that the pressure at the air inlet 114 is greater than the pressure in the respective combustion chamber 14, 15 when the air inlet 114 is uncovered by the piston 16. Without a blower 117, air would be unable to enter the combustion chambers 14, 15, making the cycle inoperable. Furthermore, the blower 117 ensures that the pressure at the exhaust outlet 116 is less than the pressure at the air inlet 114 so that exhaust gases exit the respective combustion chamber 14, 15 as air enters.

The blower 117 may be any suitable compressor, for example a positive displacement pump or a fan. The blower 117 may be driven by a mechanical linkage to the crankshaft 21 such as a chain or a belt, or it may be driven by a separate motor. It shall be appreciated that the inventive engine is not limited to a single inlet 114 and single exhaust port 116, as described above, but may comprise first and second inlet and exhaust ports 114a, 114b, 116a, 116b as shown in Fig. 2. In this example the first inlet port 114a and the first exhaust port 116a communicate only with the first combustion chamber 14, and the second inlet port 114b and the second exhaust port 116b communicate only with the second combustion chamber 15. The first and second inlet ports 114a, 114b and the first and second exhaust ports 116a, 116b are offset along the length of the cylinder 13. Therefore, as the piston 16 moves between TDC and BDC, the piston skirt 166 alternately moves over the first and second inlet and exhaust ports 114a, 114b, 116a, 116b to open them into the first and second combustion chambers 14, 15, respectively.

As the piston 16 moves toward BDC it covers the second inlet and exhaust ports 114b, 116b and opens the first inlet and exhaust ports 114a, 116b into the first combustion chamber 14. This allows exhaust gasses to exit the first combustion chamber 14 through the first exhaust outlet 116a, and a fresh charge of air in through the first inlet port 114a. Fuel is simultaneously introduced either through the first inlet port 114a or by injector (not shown).

As the piston 16 moves back along the cylinder 13 toward TDC the piston 16 passes over the first inlet and exhaust ports 114a, 116a, blocking them off and sealing the first combustion chamber 14. The fresh charge of air and fuel is compressed and, when the piston 16 is at about TDC, ignited by the spark plug 26. Ignition forces the piston 16 back along the cylinder 13 toward BDC. The first exhaust and inlet ports 116a, 114a are again uncovered and the process is repeated.

This cycle is mirrored in the second combustion chamber 15. As the piston 16 moves between BDC toward TDC it first covers the first inlet and exhaust ports 114a, 116a and then uncovers the second inlet and exhaust ports 114b, 116b, allowing them to communicate with the second combustion chamber 15 to allow a fresh charge of air and fuel in and exhaust gases out.

As the piston 16 moves back along the cylinder from TDC to BDC, the piston passes over the second inlet and exhaust ports 114b, 116b, blocking them off and sealing the second combustion chamber 15. The fresh charge of air and fuel is compressed and, when the piston 16 is at about BDC, ignited by the spark plug 27. Ignition forces the piston 16 back along the cylinder 13 toward TDC. The second exhaust and inlet ports 116b, 114b are again uncovered and the process is repeated.

Preferably, when the piston 16 is moving from BDC to TDC, the covering of the first inlet and exhaust ports 114a, 116a and the uncovering of the second inlet and exhaust ports 114b, 116b occurs simultaneously. Likewise, it is preferable that with the piston moving from TDC to BDC, the covering of the second inlet and exhaust ports 114b, 116b and the uncovering of the first inlet and exhaust ports 114b, 116b also occurs simultaneously. Simultaneous covering and uncovering of the respective inlet and exhaust ports 114a, 116a, 114b, 116b confers an advantage over the example engine described with reference to Figs. lA and lB, in which the single inlet port and single exhaust port 114, 116 remain covered by the piston 16 as it passes. The example of Fig. 2 is therefore beneficial for longer stroke engines in which intake of a fresh charge of air can occur for the first combustion chamber 14 throughout the compression stroke of the second combustion chamber 15, and vice versa.

In the illustrated example of Fig. 2, a single blower 117 provides pressurised air to both the first and second inlet ports 114a, 114b. However, each inlet port 114a, 114b may be provided with its own blower, if desired. Although in the examples described above, the piston 16 acts as a valve to cover and open up the inlet and exhaust ports 114, 114a, 114b, 116, 116a, 116b, it shall be appreciated that the invention is not intended to be limited to such arrangements and separate valves maybe provided to selectively open and close the inlet and exhaust ports 114, 114a, 114b, 116, 116a, 116b. For example, exhaust and inlet valves 22, 23, 24, 25 may be provided in the cylinder head 112 that communicate with the first combustion chamber 14; and in the cylinder wall to communicate with the second combustion chamber 15, as shown in Fig. 4. Alternatively a sleeve valve may be provided to control opening to any arrangement of inlet and exhaust ports 114, 114a, 114b, 116, 116a, 116b in the cylinder wall. In another alternative, any combination of inlet valve, exhaust valve, sleeve valve or piston controlled opening may be used.

One advantage of the example inventive engines 110 described above is that the crankcase 12 can serve as an oil reservoir, as per the convention with four stroke engines. On most two stroke engines this is not possible as the crankcase is used for pumping and circulating the air-fuel mixture to the combustion chamber. In this case, the crankcase is not required for pumping air to the combustion chamber as blowers 117 are used instead.

The separation plate 18 of the above examples is configured to accommodate lateral movement of the connecting rod 17 as it turns the crankshaft 21. The term“lateral movement” means movement away from a longitudinal axis of the cylinder 13, A- A. In the example of the engines no illustrated by Figs. lA, lB and 2, the lateral movement of the connecting rod 17 is accommodated by an aperture 113 in the separation plate 18. In other words, the connecting rod 17 can move within the aperture 113 without fouling the separation plate 18.

The aperture 113 is closed by a slide portion 118 which is sealed to the connecting rod 17 by seals 115. These seals 115 maybe brush seals, labyrinth seals or similar, which allow the connecting rod 17 to pass through the slide portion 118. The slide portion 118 extends over the aperture 113 and slides across the separation plate 18 with the lateral movement of the connecting rod 17 to prevent gases escaping through the aperture 113 and into the sump 12 during operation of the engine.

A different sealing arrangement is shown in the example of Fig. 3A and 3B, in which a pair of spring loaded seals 41, 42 are located in the aperture 113 in separation plate 18. The connecting rod 17 may bear against the seals, or may contact bearing guides 43 mounted against the seals 41 & 42 respectively. The seals 41, 42 reciprocate in the aperture 113 to seal around the moving connecting rod 17. In the example of the engine 110 illustrated by Fig. 4, the separation plate 18 is configured to move across the second end of the cylinder 13. The separation plate 18 is located in a guide 121 disposed between the cylinder block 11 and the sump 12, or machined into one or the other. The length of the guide 121 transverse to the axis A-A of the cylinder 13 is greater than corresponding dimension of the separation plate 18 so that it is free to move laterally within the guide 121.

As the connecting rod 17 turns the crankshaft 21 the angle of inclination of the connecting rod 17 relative to the separation plate 18 changes. This changing angle is accommodated by a sealing joint 50. The sealing joint 50 is provided directly in the separation plate 18.

Referring to Fig. 5, the joint 50 is located in an opening 183 of the separation plate and comprises a bore 51 through which the connecting rod 17 extends and a curved outer surface to allow rotation of the joint 50 within the opening 183. Preferably the joint 50 comprises a spherical outer surface to accommodate slight rotation in other axes that might result from manufacturing tolerances. The joint 50 is retained by a curved inner edge of the separation plate 18 opening 183, which is shaped to prevent the joint 50 escaping the separation plate 18.

Referring still to Fig. 5, two seals are provided: An outer seal 181, disposed between the outer surface of the joint 50 and the separation plate, or slide portion; and an inner seal 52, disposed between the connecting rod 17 and the bore 51. Both seals 181, 52 have to accommodate movement of underlying surfaces. The inner seal 52 must accommodate the connecting rod 17 as it moves through the bore 51; while the outer seal 181 must accommodate rotation of the joint 50 and the associated relative movement of the outer surface and the separation plate 18 or sliding portion 118. Lubrication of the joint 50 may be effected by natural dispersion of oil during rotation of the crankshaft 21 as oil is picked up from the sump 12 and thrown against the separation plate 18, or,

alternatively, oil maybe sprayed from a nozzle (not shown) provided in the sump 12.

Oil may alternatively be mixed in with fuel, as is common on conventional two stroke engines.

The inner and outer seals 52, 181 are split ring compression seals. In the illustrated example, the inner seal 52 comprises two split ring compression seals 52 spaced apart along the length of the bore 51, each being located in a corresponding groove 53 in the bore wall. The split nature of the seals allows them to decrease in diameter under compression to provide a seal about their inner edge against the connecting rod 17. During operation of the engine 110, when combustion occurs in the second combustion chamber 15, combustion gasses expand into the bore 51 and grooves 53, compressing each seal 52 against the connecting rod 17 and simultaneously pushing each seal 52 onto a seat of the corresponding groove 53. This cuts off the bore 51 from fluid communication to prevent combustion gases from escaping into the sump 12. The provision of two split ring compression seals 52 ensures that the split parts of each seal 52 can be offset to further prevent the escape of gasses during combustion. However, it shall be appreciated that it is equally feasible to use gapless compression seals, in which case only a single compression seal 52 is required. Gapless compression seals may comprise a sleeve which extends over the split portion of the seal or be arranged so as to have overlapping free ends.

The outer seal 181 also comprises split ring compression seals 181, of which there are preferably two. Each seal 181 is located in a groove 182 provided about the inner edge of the opening 183. During operation of the engine 110, when combustion occurs in the second combustion chamber 15, combustion gasses expand into the opening 183 and grooves 53, compressing each seal 181 against the outer surface of the joint 50 and simultaneously pushing each seal 181 onto a seat of the corresponding groove 182. This cuts off the opening 183 from fluid communication to prevent combustion gases from escaping into the sump 12. As with the inner seal 52, it is possible to use a gapless compression seal, in which case only a single split ring compression seal 183 is required.

In an alternative example, the outer and inner compression seals are replaced with outer and inner labyrinth seals. An example labyrinth seal is shown in Fig. 6, located in a groove 53 of the bore 51. Each labyrinth seal comprises a castellated inner edge, the castellations being arranged in the axial direction of the seal to make a tortuous path for escaping combustion gasses. In the illustrated example, a castellated inner edge of an inner seal 53 is shown in abutting relation with the connecting rod 17. Where a labyrinth seal is used for the outer seal 181, the castellated surface will be arranged in abutting relation with the outer surface of the joint 50.

In another alternative example, the outer and inner compression seals are replaced with outer and inner brush seals (not shown). Each brush seal comprises thousands of fine wires that extend from a supporting ring. The densely packed arrangement of these wires forms a barrier to escaping combustion gases whilst accommodating excursions, thermal movements of misalignments of the underlying surfaces that would otherwise reduce the efficiency of a labyrinth seal. Compression seals, brush seals or labyrinth seals are ideally suited for dealing with the combustion forces experienced during operation of the engine. It is also possible to use split ring expansion seals where the seals are located in grooves of the other of the respective components: For example, the outer seal 181 is located in a groove in the outer surface of the joint and expands under the influence of combustion gases to seal against the inner edge of the opening 183.

The separation plates 18 of the example of Fig. 4 is supported in its guide 121 by a hydrostatic oil bed. Drilled oil galleries 122 that communicate with the guide 121 allow oil to well up into the space between the separation plate 18 and the guide walls, as shown in Fig. 7. Oil pressure is maintained by an oil pump (not shown) in the conventional manner. The hydrostatic oil bed provides lubrication and protects the separation plate 18 and guide walls from premature wear by separating the two components by a film of pressurised oil.

A seal 123 is provided between the separation plate and the guide 121 in which it is located to prevent combustion gasses escaping around the edges of the separation plate 18 and into the sump 12. The seal 123 provides the further advantage of restricting oil transfer from the guide 121 into the second combustion chamber 15. As illustrated, a channel 124 in a wall of the guide 121 is provided to retain the seal 123. The channel 124 is located inward of edges of the separation plate 18 so that the seal 123 remains in contact with the separation plate 18 as it moves laterally with the throw of crankshaft 21. The illustrated seal 123 is a labyrinth seal having a castellated surface in contact with the separation plate 18 to create a tortuous path for combustion gasses; although it shall be appreciated that any conventional sealing method maybe used, including a brush seal. Preferably the seal is located on an upper surface of the separation plate 18, by which it is meant that the surface facing the second combustion chamber 15. A spring 125 may be provided to maintain the seal 123 in contact with the separation plate 18.

Alternatively, in another example illustrated in Fig. 8, the seal 123 has an inclined inner edge which directs combustion gas or compressed air into the channel 124 to force the seal 123 down and onto the separation plate 18, sealing the second combustion chamber 15 during compression and combustion strokes. The seal 123 may also be configured to lift off of the separation plate 18 during exhaust and intake strokes to reduce friction. For example, an extension spring 127, as illustrated, or a magnet (not shown) may be provided to lift the seal 123 off of the separation plate 18, or, alternatively, the seal 123 may comprise an inclined outer edge (not shown) so that the seal 123 is lifted by the hydrodynamic effect of the film of oil. It is important the inclined outer edge is configured so that the hydrodynamic effect is easily overcome by the force of combustion gas or compressed air during compression and combustion strokes so that the combustion chamber 15 remains sealed during this time.

The illustrated seal of Fig. 8 also comprises a lip 126 which extends about its outer edge and provides a seat for an additional labyrinth seal 123a, although this is merely optional. Yet another construction of engine 120 according to the present invention, is shown in Figs. 9 and 10. This engine is similar to the engines 110 described above excepting that the second compression chamber 15 includes a portion of the sump 12 in which valves 23 & 25 and spark plug 27 are located in the wall thereof. The second combustion chamber 15 extends only into a portion 213 of the sump and is sealed by bearings/ seals 212 around the respective portion of the crankshaft 21. In a preferred condition, the total extended volume of the second combustion chamber 15 including the respective portion 213 of the sump equates with the effective working volume of the first combustion chamber 14.

In conventional combustion engines, the cylinder wall and piston are lubricated by the natural dispersion of oil during rotation of the crankshaft, as oil is picked up from the sump and thrown into the cylinder. In more recent engines, oil is sprayed into the cylinder from a nozzle adjacent the connecting rod. In the presently described examples of the engine 110, 120, the presence of the second combustion chamber 15 prevents such forms of lubrication.

Therefore, lubrication for the presently described engines 110 may include the use of self-lubricating fuels which may comprise added lubricants. Or, alternatively lubrication may be achieved by high pressure lubrication systems pumping lubricant along internal bores in the crankshaft 21 and connecting rods 17 and associated pins and bearings.

In one example, the lubrication system comprises an oil pump (not shown) which draws oil from the sump 12 and feeds it through a series of oil galleries that channel oil along the crankshaft 21 and up through an oil bore in the connecting rod 17. The oil bore opens onto the pin 30. Further oil galleries provided in the pin 30 transfer oil to piston galleries 163 (see Fig. 11) from where the oil may pass out of openings 164 in the piston cylindrical wall to provide a film of oil on the cylinder wall.

Careful management of this film of lubricant is necessary to prevent excessive oil combustion and to ensure sufficient lubrication of the piston rings 161. The proposed solution may use any combination of the oil distribution control techniques set out below: Each of the openings 164 on the cylindrical wall of the piston may be provided with a valve 162 configured to regulate the oil film thickness on the cylinder wall. For example, the valve 162 maybe configured so that when the hydostatic oil pressure of the film of oil between the cylindrical wall of the piston 16 and the cylinder 13 drops below the oil pressure in the piston galleries 163, the valve 162 opens and oil passes out, replenishing the oil film. In the illustrated example each valve 162 comprises a ball bearing located in a countersunk mouth of the opening 164.

Alternatively, valves are omitted and the oil film thickness is instead regulated simply by careful design of the diameter of each opening 164.

As a further measure to control the oil film the piston skirt 166 is further provided with oil scavenging ports (not shown), through which excess oil can flow back into the galleries. The oil scavenging ports comprise one way valves, such as calibrated spring loaded stem valves, to ensure oil back into the oil galleries only when the hydrostatic oil pressure exceeds a predetermined value.

In another example shown in Fig. 12, an oil bore 171 in the connecting rod 17

communicates with an oil gallery 301 in the pin 30. The oil gallery 301 opens directly into the cylinder 13 from an opening 302 in one end of the pin 30 to provide a film of oil on the cylinder wall. The oil is pressurized and flows around the piston 16 to another opening 304 in the other end of the pin 30. Opening 304 leads into a scavenging gallery 303 which communicates with an oil return bore 172 in the connecting rod 17 to return excess oil to the crankshaft 21 and into the sump 12.

In each of the above examples the piston 16 comprises first and second piston rings 161. The openings of the oil galleries 163, 302 are located between the piston rings 161. Further oil control rings 165 are provided to retain the oil film, as much as possible, between the piston rings 161. The oil control rings 165 are provided outwardly of the piston rings 161, that is to say nearer end surfaces of the piston 16. The oil control rings 165 scrape excess oil from the cylinder walls to prevent excessive oil remaining in the combustion chamber during combustion.

The engine may also use sleeved cylinders having oil porous walls and oil drainage may be provided for the removal of excess oil. The use of oil porous metals which are pre-impregnated with oil may be possible for short life engine for example but without limitation, racing engines which are stripped between races. The oil may also acts as a coolant for the engine.