WO2007142512A1

WO2007142512A1 - A two-cycle internal combustion engine, a valve ring, a piston, and a piston hole cover assembly

Info

Publication number: WO2007142512A1
Application number: PCT/NL2006/050132
Authority: WO
Inventors: Gerrit Hendrik Gerrits
Original assignee: Sevilla Beheer Bv; Noordbruis, Lambertus, Jan
Priority date: 2006-06-02
Filing date: 2006-06-02
Publication date: 2007-12-13

Abstract

The invention relates to a two-cycle internal combustion engine (1). The engine (1) comprises a stepped piston (6) so as to form a pumping chamber (15). An upper piston part (7) has a piston skirt (9) which includes a projection (23) that closes-off an exhaust port (22) when the piston (6) is in its top dead centre. The projection (23) fits into a recess (26) of a valve ring (19) when the piston (6) is in its bottom dead centre. Due to these features the pumping chamber (15) has an optimised pumping capacity. The valve ring is also provided with inlet and outlet valves (18, 19). The engine (1) also includes features for varying the compression ratio so as to increase the engine efficiency and features for optimising lubrication of the piston (6) in the cylinder (3) and to minimize oil consumption.

Description

A two-cycle internal combustion engine, a valve ring, a piston, and a piston and piston hole cover assembly

The present invention relates to a two-cycle internal combustion engine according to the preamble of claim 1.

Such an engine is known from DE 42 05 663 Al. This prior art engine is provided with a stepped piston which has an upper part and a lower part . The upper part has a larger diameter than the lower part. The piston is movable within an upper cylinder part. A top side of the upper piston part is in contact with the combustion chamber, whereas a bottom side of the upper piston part next to the lower piston part as viewed in longitudinal direction of the cylinder is in contact with a pumping chamber. A disadvantage of the presence of the pumping chamber is that the construction height of this type of engine is relatively high.

It is an object of the present invention to provide a two-cycle engine having a reduced construction height.

To obtain this object, the engine according to the invention comprises the features of the characterizing portion of claim 1.

Due to these features the displaceable volume within the pumping chamber is increased as the volume in the pumping chamber which may be present between the piston skirt and the lower piston part can be compressed as a consequence of receiving the portion of the piston skirt by the recess, whereas a higher maximal volume is created in the pumping chamber by reducing the height of the piston skirt in the region spaced from the exhaust port, hence increasing the capacity of the pumping chamber. This provides an opportunity to reduce the height of the pumping chamber and thereby decrease the construction height of the engine. An embodiment of the piston skirt may have a substantially flat lower edge facing away from the combustion chamber, and may include a projection closing-off the exhaust port when the piston is in its top dead centre, which projection may fit at least partly into the recess when the piston is in its bottom dead centre. The advantage of the configuration of this embodiment is that it is relatively simple to manufacture.

The base part may comprise at least an inlet valve and an outlet valve which function as the inlet and the outlet of the pumping chamber, respectively. The base part may be formed by a valve ring comprising a circular valve ring housing, in which at least the inlet and outlet valve are disposed, which valve ring is fixed to the engine block such that it envelopes the lower cylinder part. As this embodiment of the base part integrates the recess as well as the inlet and the outlet valves the number of components for assembly in the engine are reduced. This is also advantageous in the case of maintenance work, for example, if the base part has to be replaced due to wear of the valves . The base part has an upper surface facing a bottom surface of the upper piston part, which surfaces may substantially correspond to each other such that they are adjacent to each other over substantially the entire surfaces when the piston is in its bottom dead centre. This feature minimizes the minimum volume of the pumping chamber when the piston is in its bottom dead centre. As a consequence, the capacity of the pumping chamber is increased or, in the case of sufficient capacity, the pumping chamber can be reduced while maintaining the capacity, which enables the engine designer to reduce the construction height of the engine further.

For lubricating the piston in the cylinder lubrication oil can be transferred from a crank case to a hollow space in the piston where it is transferred from further to a circumferential wall of the piston. Therefore, the piston comprises at least a piston ring groove for receiving a piston ring, which groove may communicate with the hollow space in the piston through groove holes, and the groove holes are spaced from the exhaust port in circumferential direction of the piston. The oil may be transferred through the groove holes to a cylinder wall and back. When the groove holes are located such as specified above, excessive oil transfer from the hollow space in the piston to the exhaust port will be avoided. This prevents excessive oil loss into the exhaust gases when the piston passes the exhaust port . The piston may include a piston hole aligned with a hole of a connecting rod, wherein a piston pin is provided in the holes for connecting the piston to the connecting rod, and the piston hole may be closed by piston hole covers fixed to the piston. This prevents the lubrication oil from flowing out of the piston pin hole, which could lead to oil loss into the pumping chamber resulting in excessive oil consumption and harmful exhaust gas emissions. On the other side, the piston hole covers avoid gas flow from the pumping chamber into the hollow space of the piston and further to the crank case. The piston hole covers can be made of plastic plugs fitting into the piston hole. This minimizes the weight of the covers.

The piston pin may be a hollow pin so as to reduce the weight of moving parts in the engine further. In an alternative embodiment of the engine the upper cylinder part is displaceable in axial direction with respect to the lower cylinder part. This feature makes it possible to vary the compression ratio of the engine, which is particularly advantageous in part-load conditions of the engine. A relatively simple construction is achieved with the embodiment in which the upper cylinder part is connected to the lower cylinder part through a flexible member and wherein the upper and lower cylinder part are displaceable with respect to each other by a cam-follower mechanism. The invention also relates to a valve ring for a two- cycle internal combustion engine, wherein the valve ring comprises a circular valve ring housing in which at least an inlet valve and at least an outlet valve are disposed, which inlet and outlet valve can be opened in opposite direction of each other in axial direction of the valve ring.

Furthermore, the invention relates to a piston for a two-cycle internal combustion engine, comprising an upper piston part and a lower piston part, wherein the diameter of the lower piston part is smaller than that of the upper piston part, and the upper piston part comprises a piston skirt, wherein the piston skirt includes a projection extending axially in the direction of the lower piston part and circumferentially over less than half of the circumference of the upper piston part. The invention also relates to a piston and piston hole cover assembly for an internal combustion engine, comprising a piston which includes a piston hole for receiving a piston pin so as to enable fixing the piston to a connecting rod, and a piston hole cover for closing-off the piston hole when the piston pin is in a mounted condition, wherein the piston and the piston hole cover are adapted to be fixed to each other.

Further details and advantages of the invention will now be explained with reference to the very schematic drawings showing two embodiments of the invention.

Fig. 1 is a very schematic sectional view of an embodiment of a two-cycle internal combustion engine according to the invention.

Fig. 2 is a partially cut away side view of the engine of Fig. 1 and a partially cross-sectional view along the line H-II in Fig. 1.

Fig. 3 is a very schematic perspective view of the piston and valve ring as shown in Figs. 1 and 2.

Fig. 4 is a very schematic perspective view of the valve ring according to the invention.

Fig. 5 is a plan view of the valve ring of Fig. 4.

Fig. 6 is a partial view of the valve ring of Fig. 4, illustrating a part of the ring.

Fig. 7 is a cross-sectional view of Fig. 6 along the line VII-VII in Fig. 6.

Fig. 8 is a plan view of a valve ring guide.

Fig. 9 is a very schematic sectional view of an alternative embodiment of a two-cycle internal combustion engine according to the invention. An embodiment of a two-cycle internal combustion engine

1 according to the invention is very schematically shown in Fig. 1 and 2. The engine 1 comprises an engine block 2 which is provided with a cylinder 3. The cylinder 3 such as shown in Fig. 1 includes an upper cylinder part 4 and a lower cylinder part 5. The upper cylinder part 4 has a larger inner diameter than the lower cylinder part 5.

The engine 1 further comprises a stepped piston 6 including an upper piston part 7 and a lower piston part 8. The upper piston part 7 has a larger outer diameter than the lower piston part 8. The outer diameter of the upper piston part 7 substantially corresponds with the inner diameter of the upper cylinder part 4, and the outer diameter of the lower piston part 8 substantially corresponds with the inner diameter of the lower cylinder part 5, such that the upper piston part 7 movably fits into the upper cylinder part 4 and the lower piston part 8 movably fits into the lower cylinder part 5. The upper piston part 7 has a piston skirt 9 which is at least formed by a circumferential portion of the upper piston part 7. The piston skirt 9 has a lower edge 9a which forms a part of a bottom surface 7a of the upper piston part 7. In Fig. 1 and 2 the lower edge 9a of the main part of the piston skirt 9 is aligned with the bottom surface 7a of the upper piston part 7. In practice, it is also possible that the entire piston skirt 9, such as the case in most conventional piston engines, projects axially from the bottom surface 9a in the direction of the lower piston part 8. In such a configuration a portion of the bottom surface 9a extends radially between the piston skirt 9 and the lower piston part 8. During operation of the engine 1 the piston 6 moves between a top dead centre and a bottom dead centre along a centre line 10. Referring to Fig. 1 and 2 the top dead centre is defined as the highest vertical position of the piston 6 and the bottom dead centre is defined as the lowest vertical position of the piston 6 with respect to the engine block 2.

In Fig. 1 a combustion chamber 11 in the cylinder 3 is defined as the cylinder volume between a cylinder head 12 and the upper piston part 7. Thus, the volume of the combustion chamber 11 varies during engine operation. In Fig. 1 the piston 6 is in its top dead centre, hence the combustion chamber 11 has the smallest volume.

The piston 6 is connected to a crankshaft 13 through a connecting rod 14, such as known for piston-driven engines. The engine 1 in Fig. 1 is provided with a pumping chamber 15, which is enclosed by the upper piston part 7, the lower piston part 8, the upper cylinder part 4 and a base part. The base part in this embodiment comprises a valve ring 16, which is fixed to the engine block 2. During engine operation the volume of the pumping chamber 15 varies inversely with the volume of the combustion chamber 11. The pumping chamber 15 is applied to pump air or combustible mixture from an inlet duct 17 to the combustion chamber 11. If it only pumps air to the combustion chamber 11, fuel for combustion is added between the pumping chamber 15 and the combustion chamber 11 or directly into the combustion chamber 11.

During engine operation the air or combustible mixture is received in the pumping chamber 15 through an inlet valve 18. In this case the inlet valve 18 is a membrane which can only open inwardly of the pumping chamber 15. When the piston 6 moves from bottom to top dead centre the pressure in the pumping chamber 15 tends to decrease as a consequence of the increasing volume of the chamber 15. This creates a positive pressure difference over the inlet valve 18 which will automatically open to allow air or combustible mixture to flow into the pumping chamber 15. When the piston 6 is in its top dead centre the expansion of the pumping chamber 15 will stop such that the pressure difference over the inlet valve 18 will decrease. As a consequence, the inlet valve 18 will close automatically. When the piston 6 moves from top dead centre to bottom dead centre the air or combustible mixture trapped in the pumping chamber 15 will be compressed creating a pressure difference over an outlet valve 19 which can automatically open when the pressure in the pumping chamber 15 is higher than the pressure in a scavenging channel 20 located downstream of the outlet valve 19. The scavenging channel 20 ends at a scavenging port 21 in the upper cylinder part 4, see Fig. 1. The scavenging port 21 is located above the upper piston part 7 when this is in bottom dead centre. This allows the compressed air or combustible mixture to flow from the pumping chamber 15 through the outlet valve 19, through the scavenging channel 20 and via the scavenging port 21 into the combustion chamber 11.

Fig. 2 shows that the upper cylinder part 4 is provided with an exhaust port 22. In top dead centre this port 22 is closed-off by the piston skirt 9 so as to avoid that the pumping chamber 15 communicates with the exhaust port 22. This would cause mixing of exhaust gases with the air or combustible mixture present in the pumping chamber 15 or preliminary discharge of the trapped gases in the pumping chamber 15 to an exhaust system (not shown) of the engine 1. During an expansion stroke of the piston 6 after a combustion cycle the piston 6 moves from top dead centre to bottom dead centre and after a certain displacement of the piston 6 the exhaust port 22 starts communicating with the combustion chamber 11, thereby discharging exhaust gases, which were created during combustion, via the exhaust port 22 to the exhaust system. After further displacement to bottom dead centre the upper piston part 7 will pass the scavenging port 21, hence allowing air or combustible mixture to flow into the combustion chamber 11.

When the piston 6 moves from bottom to top dead centre the air or combustible mixture trapped in the combustion chamber 11 will be compressed after the piston 6 has passed the exhaust port 22 thereby closing-off this port 22, which is a well-known process in the art of two-cycle combustion engines. In the case of direct fuel injection into the cylinder 3, fuel may be injected during the mentioned air compression process.

According to the invention the piston skirt 9 is shaped such that, when the piston 6 is in its top dead centre, at least a portion of the piston skirt 9 closing-off the exhaust port 22 extends beyond a main part of a remaining portion of the piston skirt 9 when viewed in a direction away from the combustion chamber 11. Such a configuration is shown in the embodiment of Figs. 1 and 2. The piston skirt 9 has an axial projection 23 which is located such that the exhaust port 22 is closed-off when the piston 6 is in its top dead centre. A part of the piston skirt 9 spaced from the projection 23 in circumferential direction is shaped such that the lower edge 9a of this part facing away from the combustion chamber 11 lies above a lower edge 25 of the exhaust port 22 when the piston 6 is in its top dead centre. Due to this feature the volume of the pumping chamber 15 has increased without additional construction height, whereas the exhaust port 22 is still closed-off in top dead centre . The projection 23 of the piston skirt 9 may extend axially in the direction of the lower piston part 8 and circumferentially over less than half of the circumference of the upper piston part 7. Preferably, the projection 23 is shaped such that it just closes-off the exhaust port 22 when the piston 6 is in its top dead centre. This means that the projection 23 may have a shape and size corresponding to that of the cross- section of the exhaust port 22.

Furthermore, Figs. 1 and 2 show that the valve ring 16 comprises a recess 26 which is located and shaped such that the projection 23 of the piston skirt 9 fits therein when the piston 6 is in its bottom dead centre.

As the projection 23 in Figs. 1 and 2 disappears in the recess 26 when the piston 6 is in its bottom dead centre and the main part of the piston skirt 9 is shorter than the projection 23 as viewed from the combustion chamber 11 the compressible volume in the pumping chamber 15 is maximized. If the capacity of the pumping chamber is higher than necessary the construction height of the engine 1 can be minimized due to this configuration.

Preferably, the valve ring 16 has an upper surface 27 facing the bottom surface 7a of the upper piston part 7, which surfaces 27, 7a substantially correspond to each other such that they are adjacent to each other over substantially the entire surfaces 27, 7a when the piston 6 is in its bottom dead centre. This condition is achieved with the embodiment of the engine 1 in Figs. 1 and 2. Both the bottom surface 7a of the upper piston part 7 and the upper surface of the valve ring 16 have a substantially similar conical shape. Fig. 3 shows a perspective view of the stepped piston 6 and the valve ring 16 in a position between top and bottom dead centre. It can be seen that the piston skirt 9 has a substantially flat lower edge 9a facing the valve ring 16 and that the skirt 9 includes only one projection 23. In the case of an engine having more than one exhaust port 22 a wider projection 23 or more than one projection 23 per piston 6 is conceivable .

Figs. 4 to 7 show an embodiment of the valve ring 16 in more detail. The valve ring 16 comprises a circular valve ring housing 29, in which several inlet valves 18 and outlet valves 19 are disposed in a circular pattern. The valves 18, 19 are formed by membranes 32, which function as one-way valves. When closed, the membranes 32 are in engagement with seats 30. It can be seen that the seats 30 for the different valves 18, 19 are configured in a different manner. The reason for this is that the membrane of the inlet valve 18 can only open in +Y direction and the outlet valve 19 can only open in -Y direction.

One of the outlet valves 19 is indicated by "A" and one of the inlet valves 18 by "B". Fig. 6 shows a part of the valve ring 16 including the membranes 32 of the outlet A and inlet valve B. It can be seen that the membrane 32 of the inlet valve B can only open in +Y direction and the membrane 32 of the outlet valve A can only open in -Y direction. When the engine 1 is in operation the membranes 32 of the inlet valves 18 open when the pressure in the pumping chamber 15 is lower than upstream of the inlet valve 18, and the membranes 32 of the outlet valves 19 open when the pressure in the pumping chamber

15 is higher than downstream of the outlet valve 19. The membranes 32 are secured to a securing element 31 which is fixed to the valve ring housing 29. The membranes 32 can be fixed by screws, for example, to this securing element 31. The membranes 32 of the inlet and outlet valves 18, 19 can be secured from the upper side of the valve ring 16. This facilitates replacement of membranes during repair or maintenance of the engine.

As can be seen in Fig. 5 the valve ring housing 29 comprises a partially circular recess 26 which is disposed at the outer side of the membranes 32 as viewed in outward radial direction of the ring 16. As explained hereinbefore a projection 23 of the piston 6 can be received by the recess 26 when mounted to the engine.

The valve ring 16 is fixed to the engine block 2 such that it envelopes the lower cylinder part 5, such as shown in Fig. 2. A way of fixing the ring 16 to the engine block 2 is applying a valve ring guide 33, such as shown in Fig. 8. The guide is secured to the engine block 2 through guide securing elements 34, such as bolts, for example, whereas the valve ring

16 is secured to the guide by placing the ring 16 into the valve ring guide 33 and fixing them together by ring securing means 35. These means may be bolts, for example, which are screwed through the ring 16 into the engine block 2. As can be seen in Fig. 8 the valve ring guide 33 has such a shape that it can receive the valve ring 16. The piston 6 of the engine 1 according to the invention comprises piston ring grooves 36 for receiving piston rings 37, such as shown in the embodiment of Fig. 1-3, for example. The piston rings 37 having holes 38 in Figs. 1 and 2 may be oil control rings which are used to wipe excessive oil from a cylinder wall and to transfer oil from a crank case 39 through a hollow space in the piston 6 to the cylinder wall, such as known in the art. In order to transfer oil from and to the hollow space in the piston 6 the piston ring grooves 38 communicate with the hollow space in the piston 6 through groove holes 40 located in the ring grooves 36, such as shown in Fig. 3. In this case the groove holes 40 are spaced from the projection 23 of the piston 6 in circumferential direction thereof so as to prevent excessive oil flow from the inner space of the piston 6 to an outer piston edge which passes the exhaust port 22 during engine running. The groove holes 40 are located in the ring grooves 36 of the upper piston part 7 as well as in the lower piston part 8.

An alternative embodiment of the two-cycle internal combustion engine 1 is shown in Fig. 9. In this embodiment the upper cylinder part 4 is displaceable in axial direction with respect to the lower cylinder part 5. Due to this feature the compression ratio of the engine 1 can be varied. This is advantageous because the engine efficiency can be increased in part-load condition when the compression ratio is increased, for example. In the embodiment of Fig. 9 the upper cylinder part 4 is connected to the lower cylinder part 5 through a flexible member 41 and the upper 4 and lower cylinder part 5 are displaceable with respect to each other by a cam-follower mechanism. The cam-follower mechanism comprises a cam 42 which is rotatably connected to the engine block 2 and a follower 43 which is fixed to the upper cylinder part 4. Rotating the cam 42 results in a displacement of the follower 43, such that the upper cylinder part 4 displaces with respect to the lower cylinder part 5.

Figs. 2 and 9 show that the connecting rod 14 is connected to the lower piston part 8 through a piston pin 44 which corresponds to a piston hole 45 in the piston 6. It is also shown that the piston hole 45 is closed-off at the ends of the piston pin 44 by a piston hole cover 46. This avoids that lubrication oil passing between the piston pin 44 and the piston 6 in outward direction of the piston 6 flows into the pumping chamber 15. A piston hole 45 having open ends would lead to increased oil consumption and oil loss into the air or combustible mixture causing harmful exhaust gas emissions. The piston hole covers 46 can be made of plastic plugs which are pressed or screwed into the piston hole 45 or can be attached to the piston hole 45 in another way. The piston pin 44 may be a hollow pin, such as illustrated in Fig. 2. This reduces the weight of the piston pin 44, hence reducing the mass forces of the moving parts of the engine 1.

From the foregoing it will be clear that the invention provides a two-cycle internal combustion engine having a pumping chamber, whose capacity is maximized so as to enable a construction height reduction of the engine. Furthermore, the integration of the inlet and outlet valves of the pumping chamber and the recess to receive the piston skirt projection in one valve ring reduces the number of components to assemble. The engine also includes features for varying the compression ratio so as to increase the engine efficiency and features for optimising lubrication of the piston in the cylinder and to minimize oil consumption. The invention is not restricted to the above-described embodiment as shown in the drawings. It may be apparent that various changes can be made in the embodiment without departing from the scope of the claims. It is, for example possible to apply another type of valves in the valve ring than membranes . It is also possible to use another way of securing the valve ring to the engine block, for example by securing the ring directly to the engine block in which already a ring guide is moulded, for example.

Claims

1. A two-cycle internal combustion engine (1), comprising an engine block (2) , at least one cylinder (3) disposed in the engine block (2) , a stepped piston (6) including an upper piston part (7) having a piston skirt (9) and a lower piston part (8) having a smaller outer diameter than the upper piston part (7) , an upper cylinder part (4) having a diameter corresponding to that of the upper piston part (7) and within which the upper piston part (7) and at least a portion of the lower piston part (8) are movable along a centre line (10) of the cylinder (3) between a bottom dead centre and a top dead centre, a lower cylinder part (5) having a diameter corresponding to that of the upper piston part (7) , a pumping chamber (15) for pumping air or combustible mixture to a combustion chamber (11) adjacent to the upper piston part (7), which pumping chamber (15) is enclosed by at least the upper (7) and lower piston part (8) , the upper cylinder part (4) and a base part (16) fixed to the engine block (2), wherein the pumping chamber (15) includes at least an inlet (18) for receiving air or combustible mixture into the pumping chamber (15) and an outlet (19) for discharging the air or combustible mixture, said inlet (18) and outlet (19) being both located near a portion of the upper piston part (7) adjacent to the pumping chamber (15) when the piston (6) is in its bottom dead centre, said upper cylinder part (4) being provided with at least an exhaust port (22), characterized in that the piston skirt (9) is shaped such that, when the piston (6) is in its top dead centre, at least a portion (23) of the piston skirt (9) closing-off the exhaust port (22) extends beyond the main part of a remaining portion of the piston skirt (9) when viewed in a direction away from the combustion chamber (11) , and the base part (16) comprises at least a recess (26) which is located and shaped such that at least the portion (23) of the piston skirt (9) fits at least partly into the recess (26) when the piston (6) is in its bottom dead centre.

2. A two-cycle internal combustion engine (1) according to claim 1, wherein the piston skirt (9) has a substantially flat lower edge (9a) facing away from the combustion chamber (11) , and the portion (23) of the piston skirt (9) is formed by a projection (23) closing-off the exhaust port (22) when the piston (6) is in its top dead centre.

3. A two-cycle internal combustion engine (1) according to claim 1 or 2 , wherein the base part (16) comprises at least an inlet valve (18) and an outlet valve (19) which function as the inlet (18) and the outlet (19) of the pumping chamber (15) , respectively.

4. A two-cycle internal combustion engine (1) according to claim 3, wherein the inlet valve (18) is a one-way valve which opens when the pressure in the pumping chamber (15) is lower than the pressure upstream of the inlet valve (18) , and the outlet valve (19) is a one-way valve which opens when the pressure in the pumping chamber (15) is higher than the pressure downstream of the outlet valve (19) .

5. A two-cycle internal combustion engine (1) according to claim 4, wherein the inlet (18) and outlet valve

(19) are each formed by a membrane (32) which is in engagement with a seat (30) when the valve is closed, which seat (30) is arranged such that the inlet valve (18) can only open inwardly and the outlet valve (19) can only open outwardly of the pumping chamber (15) .

6. A two-cycle internal combustion engine (1) according to one of the claims 3 - 5, wherein the base part (16) is formed by a valve ring (16) comprising a circular valve ring housing 29, in which the inlet (18) and outlet valve (19) are disposed, which valve ring (16) is fixed to the engine block (2) such that it envelopes the lower cylinder part (5) .

7. A two-cycle internal combustion engine (1) according to one of the preceding claims, wherein the base part

(16) has an upper surface (27) facing a bottom surface (7a) of the upper piston part (7), which surfaces (27, 7a) substantially correspond to each other such that they are adjacent to each other over substantially the entire surfaces (27, 7a) when the piston (6) is in its bottom dead centre.

8. A two-cycle internal combustion engine (1) according to one of the preceding claims, wherein the piston (6) comprises at least a piston ring groove (36) for receiving a piston ring (37) , which groove (36) communicates with a hollow space in the piston (6) through groove holes (40) , and the groove holes (40) are spaced from the exhaust port (22) in circumferential direction of the piston (6) .

9. A two-cycle internal combustion engine (1) according to one of the preceding claims, wherein the piston (6) includes a piston hole (45) aligned with a hole of a connecting rod (14) , and a piston pin (44) is provided in the holes for connecting the piston (6) to the connecting rod (14) , and the piston hole (45) is closed by piston hole covers (46) fixed to the piston (6) , which piston hole covers (46) can be made of plastic plugs fitting into the piston hole (45) .

10. A two-cycle internal combustion engine (1) according to claim 9, wherein the piston pin (44) is a hollow pin.

11. A two-cycle internal combustion engine (1) according to one of the preceding claims, wherein the upper cylinder part (4) is displaceable in axial direction with respect to the lower cylinder part (5) .

12. A two-cycle internal combustion engine (1) according to claim 11, wherein the upper cylinder part (4) is connected to the lower cylinder part (5) through a flexible member (41) and the upper and lower cylinder part (4, 5) are displaceable with respect to each other by a cam- follower mechanism.

13. A two-cycle internal combustion engine (1) according to claim 12, wherein a cam (42) is rotatably connected to the engine block (2) and a follower (43) is fixed to the upper cylinder part (4) .

14. A valve ring (16) for a two-cycle internal combustion engine (1) , characterized in that the valve ring (16) comprises a circular valve ring housing (29) in which at least an inlet valve (18) and at least an outlet valve (19) are disposed, which inlet and outlet valve (8, 19) can open in opposite direction of each other in axial direction of the valve ring (16) .

15. A valve ring (16) according to claim 14, wherein the valve ring housing (29) is provided with an at least partially circular recess (26) in axial direction of the valve ring (16) for receiving a piston projection (23) when mounted to an engine (1) , which recess (26) extends beyond the inlet and outlet valve (18, 19) as viewed from a centre line of the valve ring (16) .

16. A valve ring (16) according to claim 14 or 15, wherein the inlet and outlet valve (18, 19) are each formed by a membrane (32) which is in engagement with a seat (30) when the valve is closed.

17. A piston (6) for a two-cycle internal combustion engine (1) , comprising an upper piston part (7) and a lower piston part (8) , the diameter of the lower piston part (8) being smaller than that of the upper piston part (7) , the upper piston part (7) comprising a piston skirt (9) , characterized in that the piston skirt (9) includes a projection (23) extending axially in the direction of the lower piston part (8) over less than half of the circumference of the upper piston (6) part.

18. A piston (6) for a two-cycle internal combustion engine (1) according to claim 17, wherein the piston (6) comprises a piston ring groove for receiving a piston ring, which groove communicates with a hollow space in the piston (6) through groove holes, which are spaced from the projection (23) in tangential direction thereof.

19. A piston (6) and piston hole cover (46) assembly for an internal combustion engine (1) , comprising a piston (6) including a piston hole (45) for receiving a piston pin (44) so as to enable to fix the piston (6) to a connecting rod (14) , and a piston hole cover (46) for closing-off the piston hole (45) when the piston pin (44) is in a mounted condition, wherein the piston (6) and the piston hole cover (46) are adapted to be fixed to each other.