EP1282764B1

EP1282764B1 - Improved two-stroke internal combustion engine, with increased efficiency and low emission of polluting gas

Info

Publication number: EP1282764B1
Application number: EP00935466A
Authority: EP
Inventors: Vito De Gregorio
Original assignee: Baschieri & Pellagri SpA
Current assignee: Baschieri & Pellagri SpA
Priority date: 2000-05-17
Filing date: 2000-05-17
Publication date: 2004-07-28
Anticipated expiration: 2020-05-17
Also published as: EP1282764A1; WO2001088350A1; ATE272167T1; DE60012585D1; AU2000251004A1; DE60012585T2; ES2223528T3

Abstract

The improved two-stroke internal combustion engine (1) with improved efficiency and reduced exhaust emissions comprises a crank mechanism (5, 6, 7, 8, 9), with a crank pin (7) and a connecting rod (8), the latter being connected to a guide piston (10) that moves up and down in a sleeve (11), and with a piston (20) connected to the guide piston (10). The piston (20) moves up and down in a cylinder (15) that is completely separate from the area of the crank mechanism (5, 6, 7, 8, 9), the latter being lubricated with oil that can be recycled. The piston (20) is substantially disc shaped and may comprise a plurality of cooling fins (20c). The lower part of the engine (1) may be used as a compressor to feed fresh air to the exhaust or to supercharge the engine, or as a pneumatic pump for direct fuel injection.

Description

Technical Field

The present invention relates to an improved two-stroke internal combustion engine with improved efficiency and reduced exhaust emissions. In particular, the invention relates to two-stroke reciprocating internal combustion engines, without limitations on power, cylinder capacity and type of fuel used, whether of the carburation or injection type, whether ignited by sparking (as in the otto cycle) or by compression (as in the diesel cycle), whether fed by natural induction or supercharged.

Background Art

In all internal combustion engines, the charge of combustible gas, that is to say, the mixture of air and fuel, must be renewed at the end of each thermodynamic cycle. Therefore, the burnt gas must be expelled and a new charge of combustible gas fed into the engine cylinder.

The expulsion (or exhaust) of the burnt gases and intake of new gas must take place after the expansion stroke and before the compression stroke of the cylinder.

In four-stroke engines, the renewal of the charge of air-fuel mixture is achieved by including two additional piston strokes between the expansion stroke and the compression stroke, the first to expel the burnt gases, the second to take in new fuel. These two additional strokes reduce the efficiency of the engine because the pumping action of the piston during these two strokes does not produce any power and indeed subtracts power from the engine.

To overcome this drawback, the two-stroke engine was developed. A two-stroke engine produces more power than a four-stroke engine of the same cylinder capacity because it doubles the frequency of the working strokes performed by the piston.

It is extremely important to optimise the flows of air, air-fuel mixture and burnt gases within the engine under all operating conditions, and it is essential to have an efficient lubricating system.

Numerous disadvantages have therefore prevented two-stroke engines from being adopted on a larger scale. The main drawbacks are the following:

1) loss of unburnt mixture through the exhaust;

2) incomplete intake and scavenging;

3) dirtying of spark plug by the oil mixed with the petrol;

4) clogging up and dirtying of the exhaust pipe caused by oil burnt together with the fuel;

5) air pollution caused by oil burnt with the fuel;

6) unreliability of crank parts;

7) uneven wear of cylinder, piston and packing rings.

Disclosure of the Invention

The two-stroke engine made according to the present invention is of the type with unidirectional scavenging, that is to say, one with transfer or inlet ports located at the base of the cylinder, corresponding to the bottom dead centre of the piston, and exhaust ports at the top of the cylinder, close to the cylinder head or on the cylinder head itself. This creates a scavenging flow from the base to the top of the cylinder without any reverse flow. This type of scavenging is called unidirectional scavenging.

In a preferred embodiment of the two-stroke engine made according to the present invention, the burnt gases are pushed out through a controlled valve located in the cylinder head. The exhaust cycle may be asymmetrical because it is started by a command that opens the exhaust valve and does not depend simply on the action of the piston. It is therefore much more efficient than the exhaust cycle of traditional two-stroke engines.

The two-stroke engine made according to the present invention also has an intake and precompression chamber that is separate from the crank mechanism, allowing the crank mechanism to be lubricated in the same way as a four-stroke engine, with recycling of lubricating oil, and thus without escape of lubricating oil through the engine exhaust.

The engine made according to the present invention can be advantageously applied not only to vehicles of many different kinds, such as motor cars, two- or three-wheeled vehicles, mopeds, motorcycles and scooters, but also to tools and implements such as power saws, lawn mowers and so on.

Further, the present invention can also be advantageously used in the field of marine engines such as outboard motors.

The main advantages of conventional two-stroke engines are that they are easier to build, lighter and have a high specific power output potential.

Their principal disadvantages are high fuel consumption, the fact that they generate considerable quantities of exhaust emissions in the form mainly of carbon monoxide and unburnt hydrocarbons and the limited life of the fit between cylinders and pistons and other moving parts.

These drawbacks, especially the pollution caused by emissions, are so serious that current and future legislation on the subject of pollution, has practically prohibited the use of conventional two-stroke engines.

Attempts to solve these problems include the application of catalytic converters or feeding by direct injection of the fuel mixture into the combustion chamber.

These solutions have not, however, been able to overcome these drawbacks satisfactorily.

Indeed, catalytic converters are only partly effective in eliminating emissions and two-stroke engines equipped with catalytic converters cannot meet the requirements of the strictest anti-pollution regulations. Moreover, in a two-stroke engine, a catalytic converter has a limited life on account of the large quantity of oil expelled by the engine and as a result, the converter has to be changed relatively frequently. This creates an additional problem linked to the disposal of used converters.

As for direct injection into the combustion chamber, this is uneconomical since it involves high-pressure injection and because the injection cycle must be performed in a very short space of time, which makes it difficult to control the quantity of mixture fed to the engine, especially in small engines at partial loads. Further, in engines with direct injection, the scavenging cycle is performed by air and oil, the oil being necessary for lubrication, and the problem of pollution caused by the burnt oil therefore remains. Moreover, scavenging the combustion chamber with abundant air is disadvantageous in terms of efficiency because the pumping work required is substantially wasted.

In the two-stroke engine made according to the present invention, the combustion chamber is effectively scavenged without unnecessary loss of air to the exhaust.

Yet another drawback of two stroke engines equipped with a direct injection system of the type mentioned above is that, even if they generate less emissions to start with, they are unable to maintain the same low level of emissions after operating for some time on account of the uneven wear on the contact surface between cylinder and piston. This contact surface in two-stroke engines is subject to uneven wear mainly because of the ports in the cylinder wall and especially because of the large size of the exhaust port or ports and the fact that lubrication in current two-stroke engines has to be minimised, using as little oil as possible on account of the difficulty of treating the lubricating oil.

It should be remembered that lubrication is very important for internal combustion engines, since it improves mechanical efficiency, reducing the power required to overcome the passive resistance of moving parts while maintaining their mechanical properties, and preventing or reducing wear on reciprocating engine parts. In a four-stroke engine, oil is pumped from the crankcase to the different parts to be lubricated and excess oil returns to the crankcase. The crankcase of the four-stroke engine can therefore be filled with an abundant supply of heavy oil.

As mentioned above, today's two-stroke engines do not have a lubricating system that can be filled with such an abundant supply of oil because the lubricating oil is mixed with the fuel and burnt or otherwise lost in the combustion chamber.

The burnt lubricating oil is driven out through the exhaust and contributes to increasing the quantity of exhaust emissions and thus producing a lot of pollution.

To reduce the emissions generated by the oil, current two-stroke engines must make sparing use of lubrication, but this necessity significantly reduces the reliability of the crank parts.

The present invention therefore provides an improved two-stroke engine in an attempt to overcome the above mentioned drawbacks.

According to one aspect of it, the present invention provides a two-stroke engine as specified in claim 1.

The dependent claims relate to preferred, advantageous embodiments of the invention.

Brief Description of the Drawings

The invention will now be described with reference to the accompanying drawings, which illustrate preferred embodiments of it purely by way of example and in which:

Figure 1 illustrates a cross section of the two-stroke engine made according to the present invention;
Figure 2 shows the guide piston of the engine illustrated in Figure 1, partly in cross section;
Figure 3 is a top view of the guide piston of the engine illustrated in Figure 1;
Figure 4 shows a cross section of the piston of the engine illustrated in Figure 1;
Figure 5 is a bottom view of the piston of the engine illustrated in Figure 1;
Figure 6 illustrates another embodiment of the engine made according to the present invention;
Figures 7 and 8 show, in cross section and in a bottom view, another embodiment of the piston in the engine illustrated in Figures 1 and 6;
Figure 9 shows details of the lower part of the engine illustrated in the other figures, with some parts cut away in order to better illustrate others.

Detailed Description of the Preferred Embodiments of the Invention

With reference to the accompanying drawings, the two-stroke engine is labelled 1 as a whole. The engine 1 comprises a base 2 closed at the bottom by a sump 3 designed to contain lubricating oil 4.

The crankcase 2 houses a crankshaft 5 that turns about pins 6. The crankshaft 5 has a crank pin 7 to which a connecting rod 8 is attached in a known manner. The top of the connecting rod 8 is joined to a piston pin 9 to which a guide piston 10 is also joined.

The guide piston 10 moves up and down in a cylindrical sleeve 11 and has grooves 12 made in it to allow air and lubricant to pass freely into the section above it. At the top 13 of the guide piston 10 there is a rod 14 which is fixed to the guide piston itself or which is made as integral part of it.

The inside of the rod 14 comprises a hollow section 14a not only for lightness but also to allow oil to reach the piston 20.

This helps cool the piston 20 and in particular helps cool the upper part of the piston 20.

Above the sleeve 11 there is a cylinder 15, having an axis 15a, that is completely separated from the crankcase 2 and from the area of the crankshaft 5 by a diaphragm or separating wall 16, thus creating two sections: an upper hot section and a lower cold section. The diaphragm or separating wall 16 also has a hole 17 for the passage of the rod 14.

The hole 17 is surrounded by a collar 18 and if necessary the collar 18 can be provided with gaskets (not illustrated) fitted between it and the rod 14. There is a very small amount of clearance between the sides of the hole 17 and the rod 14. Thus, even if the collar 18 is not fitted with gaskets, a good seal is obtained during the reciprocating motion of the rod 14 in the hole 17 and collar 18. The collar 18 comprises an upper end 18a that is flared in the direction of the hole 17.

The flared end 18a is designed to collect the lubricating oil that is carried up past the hole 17 and to facilitate its flowing back into the crankcase 2.

A cylinder head 19 closes the top of the cylinder 15 within which a substantially disc shaped piston 20 runs. The piston 20 is connected to the rod 14 and has an upper packing ring 21a and a lower packing ring 21b to prevent leakage between piston and cylinder. There are no oil scraper rings.

The piston 20 divides the cylinder 15 into an upper section 22, where the combustion chamber is, and a lower section 23 where there is an intake and precompression chamber, for example a crankcase/pump. The two sections communicate through a plurality of transfer ports 24, also referred to simply as ports 24, made in the side wall 25 at the base of the cylinder 15.

The cylinder head 19 comprises a spark plug 26 and at least one exhaust pipe 27, equipped with a closing valve 28, which may be controlled by conventional means such as, for example, a shaft and cam 29.

As the piston 20 approaches its bottom dead centre, the pressure in the lower section 23 of the cylinder 15 increases until it is higher than the pressure in the crankcase 2. As a result, any oil that has made its way up into the lower section 23 through the space between the rod 14 and the collar 18 is sucked back into the crankcase 2.

The piston 20 does not have a skirt and therefore requires much less lubricant to enable it to move up and down within the cylinder 15.

To further reduce the lubricant requirement, there may be a relatively large clearance between the piston 20 and the cylinder 15 since the seal is guaranteed by the packing rings 21a and 21b which are the only parts in contact with the cylinder wall.

In the two-stroke engine made according to the present invention, the piston skirt is not necessary because the piston 20 does not have to sustain the sideways thrusts due to the crank mechanism and does not therefore have to act as a guide for the connecting rod 8 and crankshaft 5 since the thrusting actions are borne directly by the guide piston 10. The latter, being located in the area of the crankcase 2, has an abundant supply of oil either by splash lubrication or by force feed lubrication through a pump (not illustrated).

The absence of sideways thrusts by the piston 20 also has another important advantage, and that is that the cylinder 15 is not subject to ovality and that means the close, sealed fit between the cylinder 15 and the piston 20 lasts much longer.

That is a very important advantage in reducing engine emissions even after long periods of operation because there is practically no wear caused by contact between the cylinder 15 and piston 20, which means that the burnt gases cannot leak into the lower section 23 of the cylinder 15.

The piston 20 never actually comes into direct contact with the cylinder 15, even at high temperatures, because the tolerance of fit between the two parts is much greater than in conventional two-stroke engines.

The risk of seizing of the piston 20 is therefore eliminated not only for the reasons stated above but also because the piston 20 has no skirt and its guiding function is performed instead by the guide piston 10.

To further improve the cooling of the upper part of the piston 20, in addition to the cooling effect provided by the rod 14, and to create the turbulence required to facilitate mixing in the lower section 23, the underside of the piston 20 comprises a plurality of radial fins 20c over which fresh air is blown during each engine cycle, and which transfer heat from the crown of the piston 20 to the lower section 23 to reduce the high temperatures reached by the crown of the piston 20.

The lower section 23 communicates directly, through an induction manifold 30 equipped with reed valves 31, with a carburettor 32 of known type designed to supply the required mixture of air and fuel. The carburettor 32 is equipped with a filter 33 that communicates with the crankcase 2 area through a connecting pipe 34. The oil vapour in the crankcase 2 is sucked into the cylinder 15 and contributes sufficiently to the lubrication of the piston. Thus, the upper part of the engine, also known as the "dry area", is lubricated in the same way as a four-stroke engine.

The piston 20 can therefore be lubricated by the oil vapour sucked in from the crankcase 2 and/or by-the oil film transferred by the rod 14 during its reciprocating motion through the crankcase 2.

By varying only a few simple features of the engine, such as oil vapour flow and the clearance between the hole 17 and the rod 14, the lubrication of the upper part of the engine can be adapted to different requirements.

Further, as stated above, the tolerance of fit between the piston 20 and the cylinder 15 may be very large and hence the oil requirement is much smaller than that of any other type of piston, with a significant advantage in terms of reduced emissions.

It should also be noted that since the valve 28 is an exhaust valve, it always operates at relatively high pressures and hence the lubricating oil on the valve stem does not tend to leak through the valve guide.

Below is a description of the two-stroke operation of the engine made according to the present invention.

The air-fuel mixture is sucked in from the carburettor 32 and passes through the valve 31 into the lower section 23. As the piston 20 moves towards its bottom dead centre, it drives the air-fuel mixture from the lower section to the upper section 22 through the ports 24. At the same time, the exhaust valve 28 is opened by the cam 29. The fresh charge of air fuel mixture displaces the gases burnt during the previous combustion cycle and drives them towards the exhaust pipe 27 but without mixing with them. When the air fuel mixture is near the exhaust valve 28, the latter closes and prevents fuel from being lost through the exhaust.

A timing diagram typical of the present engine shows a symmetrical transfer cycle around the bottom dead centre. Transfer is, however, very efficient because the lower section 23, acting like a pump, has a small clearance volume and is almost completely swept by the piston 20.

Since the lower section 23 can reach very high levels of precompression, the transfer ports 24 can be increased in number and they may be very narrow and hence relatively undersized, as shown in Figure 9.

Also, it is also easier to increase the number of transfer ports because there is no need for an exhaust port in the wall of the cylinder 15.

The special shape of the transfer ports 24 facilitates turbulence in the combustion chamber. In another embodiment (not illustrated), the transfer ports 24 may consist of a plurality of small holes distributed uniformly in one row around the wall of the cylinder 15.

On account of the small size of the transfer ports and the absence of an exhaust port, there is practically no wear on the contact surface between the cylinder 15 and piston 20 and on the packing rings 21a and 21b.

This configuration considerably improves the performance of the engine compared to the crankcase/pump of a conventional two-stroke engine, which has a large clearance volume on account of the presence of the crankshaft. It also has the advantage of speeding up gas flows which is very useful for two-stroke engines, where the various different cycles must be performed in a very limited time.

The transfer ports 24 may be arranged at a constant angle relative to the cylinder wall 25 and there are enough ports to enable the size of each single port to be reduced. The transfer port 24, on the side of the induction manifold 30 may be connected with the latter.

To improve turbulence and make the air-fuel mixture from the transfer ports 24 follow a rotational motion, the transfer ports may be oriented in an oblique direction (not illustrated) relative to the axis 15a of the cylinder 15.

The exhaust cycle is asymmetrical because it is achieved by opening the exhaust valve 28 whose operation is independent of the position of the piston 20.

Being asymmetrical, the exhaust cycle is much more efficient than that of a conventional two-stroke engine.

In fact, the closing of the exhaust valve 28 can be sufficiently advanced to prevent the escape of the air-fuel mixture fed in through the transfer ports 24.

To further improve the efficiency of the scavenging and exhaust cycle, the exhaust valve 28 may be tuned in such a way as to be opened to a lesser or greater degree so as to obtain engines with different operating characteristics, such as torque at low revolutions per minute or higher maximum power. The exhaust valve 28 may also be tuned in such a way that it opens or closes with different timing, that is to say, the exhaust valve 28 opens or closes at different times relative to the position of the piston 20 and the related crankshaft 5. This may be achieved by a device for varying the timing of the cam 29. At low revolutions per minute and high loads, the exhaust timing can be advanced by a few degrees to facilitate the expulsion of burnt gases or closed in advance to prevent the escape of fresh gases.

Figures 2 and 3 show the guide piston 10 of the engine illustrated in Figure 1, where the rod 14 is integral with the guide piston. There are no grooves because the guide piston 10 fulfils additional functions as explained below.

Figures 4 and 5 show the piston 20 of the engine illustrated in Figure 1. The piston has no skirt and has cooling fins 20c. Figures 7 and 8 illustrate another embodiment of the piston shown in Figures 4 and 5.

In this embodiment, the piston 20 comprises one or more unidirectional valves consisting of holes 51 closed by corresponding poppet valves. Each poppet valve has a stem 53 that moves up and down in a seat 54 made in the crown 55 of the piston 50 and may be equipped with a return valve 56. The holes 51 and the corresponding valves 52 make it possible to completely eliminate the need for the transfer ports 24.

With the piston 50 equipped with valves 52, the transfer cycle does not have a fixed timing. Transfer occurs only when the pressure in the lower section 23 of the cylinder 15 exceeds by a defined value the pressure in the upper section 22 of the cylinder 15.

The transfer cycle is thus automatically adjusted to the operating conditions of the engine. It follows therefore that the transfer cycle may also be asymmetrical relative to the bottom dead centre. The transfer cycle will therefore be a function of the pressure in the upper section 22, the pressure in the lower section 23, the force of the return springs 56 of the poppet valves 52 and the pressure loss through the holes 51.

Figure 6 illustrates an embodiment of the engine where the lower section acts as a reciprocating compressor. The guide piston 10 has no grooves 12 in it and thus works like an ordinary piston moving in the sleeve 11, which is shaped like a cylinder, sucking air in through a first tube 40 and pushing air out through a second tube 41 to the engine's exhaust pipe 27. The first tube 40 is equipped with a filter 42.

During operation of the motor illustrated in Figure 6, the air compressed by the guide piston 10 is pushed out through the tube 41 to the exhaust pipe 27, facilitating the oxidation of the combustion products. This permits a considerable reduction of exhaust emissions without using a catalytic converter.

In another embodiment of the engine that is not illustrated, the air compressed by the guide piston 10 is driven into the intake of the cylinder 15 in such a manner as to supercharge the engine.

In yet another embodiment of the engine that is not illustrated, the lower section acts as a pneumatic pump for injecting the fuel directly into the combustion chamber.

As described above, the guide piston 10 and the related sleeve 11 may therefore perform the different functions of compressor, supercharger and pneumatic injection pump without requiring the addition of a lot of other parts and thus greatly simplifying engine design.

The invention has important advantages.

The fact that the piston does not have a skirt acting as a guide for the crank mechanism allows it to be fitted in the cylinder with a larger tolerance of fit, avoiding contact between the piston and the cylinder and eliminating the risk of piston seizure.

The piston moves within the cylinder in a straight, axial direction only, thus greatly reducing friction and, consequently, the need for their lubrication is also much less.

Another advantage is that it has fewer parts compared to an equivalent four-stroke engine. In fact, a four-stroke engine delivering the same power output as the engine made according to the present invention must have twice as many cylinders and pistons and related crank mechanism. In addition, the two-stroke engine disclosed here requires only one exhaust valve for each cylinder, while a four stroke engine must also have an intake valve.

As described above, the lower part of the two-stroke engine made according to the present invention may serve as compressor, a supercharger and a pneumatic injection pump with the addition of practically no other parts, thus greatly simplifying engine design.

Yet another advantage of the present invention is the fact that the lubricating oil is recovered and re-used as in a four-stroke engine, thus increasing the reliability of the engine and reducing pollution due to burnt oil.

Key

1: engine
2: crankcase
3: oil sump
4: lubricating oil
5: crankshaft
6: pins of shaft 5
7: crank pin
8: connecting rod
9: piston pin
10: guide piston
11: sleeve
12: grooves
13: upper part of guide piston 10
14: rod
14a: hollow section of rod 14
15: cylinder
15a: cylinder axis
16: diaphragm or separating wall
17: hole for the passage of rod 14
18: collar
18a: flared end
19: cylinder head
20: piston
20c: fins
21a, 21b: packing rings
22: upper section of cylinder 15
23: lower section of cylinder 15
24: transfer ports
25: wall of cylinder 15
26: spark plug
27: exhaust pipe
28: exhaust valve
29: cam
30: induction manifold
31: reed valve
32: carburettor
33: air filter
34: connecting pipe
40: first tube
41: second tube
42: filter
43,44: unidirectional valves
50: piston
51: holes in piston
52: poppet valve
53: stem
54: housing for stem 53
55: crown of piston 50
56: valve 52 return spring

Claims

A two-stroke internal combustion engine (1) with unidirectional scavenging comprising a crank mechanism (5, 6, 7, 8, 9) connected to a guide piston (10) that runs in a sleeve (11) , a piston (20; 50), whose lower end is connected to the guide piston (10) by a (14), the piston (20; 50) being substantially disc shaped and moving in a cylinder (15), having an axis (15a), the cylinder (15) being completely separated from the area of the crank mechanism (5, 6, 7, 8, 9) by a diaphragm or separating wall (16) , the engine being characterised in that the piston (20; 50) comprises a plurality of radial cooling fins (20c) protruding from the underside of the piston (20; 50).
The engine according to claim 1, characterised in that it comprises a cylinder head (19) located above the cylinder (15) and designed to close the latter, the cylinder head (19) including an exhaust pipe (27) and an exhaust valve (28) for the expulsion of the burnt gases.
The engine according to any of the foregoing claims, characterised in that the piston (20; 50) forms in the cylinder (15) a lower section (23) acting as an intake and precompression chamber that is separate from the crank mechanism (5, 6, 7, 8, 9) and that enables the oil used to lubricate the crank mechanism to be recovered and re-used without being lost to the atmosphere.
The engine according to any of the foregoing claims, characterised in that the guide piston (10) is fitted closely in and runs in the sleeve (11) , which is made like a cylinder, the. sleeve (11) communicating with a first tube (40) equipped with a unidirectional valve (43) through which air is sucked into the sleeve (11), and with a second tube (41) equipped with a unidirectional valve (44) through which air is forced out into the exhaust pipe (27) of the engine in order to facilitate oxidation of the exhaust gases passing through the exhaust pipe (27).
The engine according to any of the foregoing claims from 1 to 3, characterised in that the guide piston (10) is fitted closely in and slides in the sleeve (11), which is made like a cylinder, the sleeve (11) communicating with a first tube (40) equipped with a unidirectional valve (43) through which air is sucked into the sleeve (11), and with a second tube (41) through which the air compressed by the guide piston (10) is forced into the intake (30, 31, 32, 33) of the cylinder (15} in order to supercharge the engine.
The engine according to any of the foregoing claims from 1 to 3, characterised in that the guide piston (10) is fitted closely in and runs in the sleeve (11), which is made like a cylinder, the sleeve (11) communicating with a first tube (40) equipped with a unidirectional valve (43) through which air is sucked into the sleeve (11), and with a second tube (41) through which the air compressed by the guide piston (10) is delivered to a pneumatic pump that injects fuel directly into the combustion chamber.
The engine according to any of the foregoing claims from 1 to 6, characterised in that the exhaust valve (28) is opened by a cam (29) whose opening and closing timing can be varied relative to the position of the piston (20; 50) and the related crankshaft (5).
The engine according to any of the foregoing claims from 1 to 7, characterised in that its exhaust cycle is asymmetrical.
The engine according to any of the foregoing claims, characterised in that the piston (50) comprises a plurality of holes (51), each closed by a valve (52), said valves (52) enabling the transfer cycle to be performed when the pressure in the lower section (23) of the cylinder (15) is greater than that in the upper section (22) of the cylinder (15).