WO2019150172A1

WO2019150172A1 - Two-stroke engine and method of operating engine

Info

Publication number: WO2019150172A1
Application number: PCT/IB2018/050693
Authority: WO
Inventors: Thomas THURNHEER
Original assignee: Wärtsilä Services Switzerland Ltd
Priority date: 2018-02-05
Filing date: 2018-02-05
Publication date: 2019-08-08

Abstract

The method of operating a two-stroke piston engine comprises the steps of determining at least one pressure difference value, which represents the difference between the cylinder pressure of one or more cylinders (1) at a predetermined crank angle or in a predetermined crank angle range during one or more engine cycles and the pressure of the fresh charge, and adjusting the opening timing of each exhaust valve (6) of the engine based on a determined pressure difference value.

Description

Two-stroke engine and method of operating engine Technical field of the invention

The present invention relates to a method of operating a multi-cylinder two- stroke piston engine in accordance with claim 1. The invention also concerns a multi-cylinder two-stroke piston engine as defined in the other independent claim.

Background of the invention

In large two-stroke engines, such as marine engines, each cylinder of the en gine is typically provided with an exhaust valve. The exhaust valve is located in a cylinder head. During the downward movement of the piston, the exhaust valve is opened, and discharging of exhaust gas from the combustion chamber is allowed. Fresh intake air is introduced into the cylinder when the piston is close to bottom dead center. The wall of the cylinder is provided with inlet ports and when the piston is below the inlet ports, flow of intake air into the cylinder is allowed. The pressure of the intake air should normally be equal to or slightly higher than the cylinder pressure when the piston is below the inlet ports, and the intake air flow thus pushes the remaining exhaust gas out of the cylinder.

In many large two-stroke engines, the exhaust valve timing is cam-controlled. The timing thus depends only on the cam geometry and the angular position of the cam and the exhaust valve timing cannot be changed during the operation of the engine. Also electronically controlled exhaust valves are known. This al lows changing of the exhaust valve timing. However, in the known control methods, an optimal timing is not achieved in all operating conditions.

Summary of the invention

An object of the present invention is to provide an improved method of operat ing a multi-cylinder two-stroke piston engine, where each cylinder comprises at least one exhaust valve with variable opening timing and at least one inlet port, which inlet port is arranged in the wall of the cylinder and opens for allowing flow of fresh charge into the cylinder when the piston in the cylinder has moved below the upper edge of the inlet port. The characterizing features of the method according to the invention are given in claim 1. Another object of the invention is to provide an improved multi-cylinder two-stroke piston engine, where each cylinder comprises at least one exhaust valve with means for vari able opening timing and at least one inlet port, which inlet port is arranged in the wall of the cylinder and opens for allowing flow of fresh charge into the cyl inder when the piston in the cylinder has moved below the upper edge of the inlet port. The characterizing features of the engine are given in the other inde pendent claim.

The method according to the invention comprises the steps of determining at least one pressure difference value, which represents the difference between the cylinder pressure of one or more cylinders and the pressure of the fresh charge at a predetermined crank angle or in a predetermined crank angle range during one or more engine cycles, and adjusting the opening timing of each exhaust valve of the engine based on a determined pressure difference value.

The engine according to the invention comprises means for determining at least one pressure difference value, which represents the difference between the cylinder pressure of one or more cylinders and the pressure of the fresh charge at a predetermined crank angle or in a predetermined crank angle range during one or more engine cycles, and means for adjusting the opening timing of the exhaust valve based on the pressure difference value.

With the method and the engine according to the invention, optimal exhaust valve timing can be used in all operating conditions of the engine. Good scav enging is thus ensured while minimizing thermal losses and maximizing the thermal efficiency of the engine.

According to an embodiment of the invention, the predetermined crank angle or the predetermined crank angle range is in the proximity of the crank angle at which the inlet port opens. This allows to accurately determine the pressure difference between the cylinder pressure and the pressure of the fresh charge at the moment when the inlet ports open and to achieve a desired pressure dif ference by adjusting the opening timing of the exhaust valve. According to an embodiment of the invention, the predetermined crank angle or the predetermined crank angle range is within 5 degrees from the crank an gle at which the inlet port opens.

According to an embodiment of the invention, the predetermined crank angle or the predetermined crank angle range is within 2 degrees from the crank an gle at which the inlet port opens.

According to an embodiment of the invention, the predetermined crank angle or the predetermined crank angle range is before the crank angle at which the inlet port opens. This ensures that the opening of the inlet port does not affect the cylinder pressure when the pressure difference value is determined.

According to an embodiment of the invention, the opening timing of the ex haust valve is advanced in case the pressure difference value is above a first predetermined limit value. The opening timing of the exhaust valve is thus cor rected in case the cylinder pressure is too high compared to the pressure of the fresh charge. This prevents blow-back of exhaust gas.

According to an embodiment of the invention, the first predetermined limit val ue is in the range of -0.1 - 0.1 bar. According to another embodiment of the in vention, the first predetermined limit value is in the range of -0.1 - 0.0 bar. Normally a slightly negative pressure difference is desired, but in some cases even a slightly positive pressure difference can be accepted.

According to an embodiment of the invention, the opening timing of the ex haust valve is delayed in case the pressure difference value is below a second predetermined limit value. According to an embodiment of the invention, the second predetermined limit value is in the range of -0.2 - 0.0 bar. According to another embodiment of the invention, the second predetermined limit value is in the range of -0.2 - -0.1 bar. Too early opening of the exhaust valve is thus corrected, which increases the thermal efficiency of the engine. By setting the threshold to be slightly negative, effective scavenging is ensured.

According to an embodiment of the invention, an individual pressure difference value is determined for each cylinder of the engine, and the opening timing of the exhaust valve is adjusted individually in each cylinder of the engine based on the respective individual pressure difference value. Due to manufacturing tolerances and wear, the cylinder pressures of different cylinders may differ from each other. By controlling the exhaust valve opening timing individually in all cylinders of the engine, each cylinder can be operated with optimal timing.

According to an embodiment of the invention, each individual pressure differ ence value is based on the cylinder pressure in the respective cylinder. According to an embodiment of the invention, the pressure difference value is determined by means of at least one differential pressure sensor that is ar ranged in the cylinder liner of a cylinder.

According to an embodiment of the invention, the pressure difference value is determined by means of at least one sensor measuring pressure in a cylinder and at least one sensor measuring the pressure of the fresh charge.

According to an embodiment of the invention, the pressure difference value is determined on the basis of the pressure of the fresh charge in the proximity of the inlet port.

According to an embodiment of the invention, the exhaust valves are electro- hydraulically actuated.

According to an embodiment of the invention, in the engine the means for de termining the pressure difference value comprise a differential pressure sen sor. According to an embodiment of the invention, the differential pressure sensor is arranged in the wall of the cylinder in the proximity of the inlet port. According to an embodiment of the invention, each cylinder of the engine is provided with a differential pressure sensor.

According to an embodiment of the invention, the engine comprises at least one sensor for determining the absolute pressure of the fresh charge. This helps to increase the accuracy of the pressure measurements.

Brief description of the drawings

Embodiments of the invention are described below in more detail with refer ence to the accompanying drawings, in which Fig. 1 shows a cross-sectional view of an upper part of a cylinder of a large two-stroke engine,

Fig. 2 shows a cross-sectional view of a lower part of the cylinder of Fig. 1 ,

Fig. 3 shows a similar view as Fig. 2 but the piston being at a lower position,

Fig. 4 shows a similar view as Fig. 3 but the piston being at an even lower po sition, and

Fig. 5 shows an example of the development of the cylinder pressure during the downwards movement of the piston.

Description of embodiments of the invention

Figure 1 shows a cross-sectional view of an upper part of a cylinder 1 of a pis ton engine, i.e. a reciprocating internal combustion engine. The engine is a large low-speed two-stroke engine, such as a marine engine. The engine can be used for driving a propeller of a ship. The maximum rotation speed of the engine can be in the range of 50-220 rpm. The engine can be a crosshead engine. The cylinder bore of the engine can be in the range of 30-1 10 cm. The rated power of the engine can be in the range of 1-100 MW. The engine can be a diesel engine, which is operated using liquid fuel and the Diesel cycle. The engine could also be a dual-fuel engine, which can be operated either in a gas mode using gaseous fuel, such as natural gas, or in a liquid fuel mode us ing liquid fuel. In the gas mode, the engine can be operated using the Otto cy cle. In the liquid fuel mode, the engine can be operated using the Diesel cycle. The gaseous fuel can be ignited by injecting liquid pilot fuel into the cylinders 1.

The engine comprises a plurality of cylinders 1. A piston 5 is arranged to move in a reciprocating manner in the cylinder 1. In figure 1 , the piston 5 is at a top dead center (TDC) position. The TDC position of the piston 5 corresponds to the crank angle of 0 degrees. The wall of the cylinder 1 is formed by a cylinder liner 2, which is arranged inside an engine block 7. A cylinder head 3 closes the upper end of the cylinder 1. The cylinder liner 2, piston 5 and cylinder head 3 delimit a combustion chamber 4. The cylinder 1 is provided with at least one exhaust valve 6. In the embodiment of figure 1 , the cylinder 1 is provided with one exhaust valve 6, but there could be also several exhaust valves 6 for each cylinder 1. The exhaust valve 6 is arranged in the cylinder head 3. The exhaust valve 6 opens and closes an exhaust port 10. Via the exhaust port 10, exhaust gas is discharged from the combustion chamber 4 into an exhaust duct 15.

The engine is provided with means for variable exhaust valve timing. In the embodiment of the figures, the means for variable exhaust valve timing com prise valve actuators 12. Each exhaust valve 6 of the engine is provided with an own valve actuator 12. The valve actuator 12 can be e.g. an electro- hydraulic actuator. With the valve actuator 12, at least the opening timing of each exhaust valve 6 can be individually adjusted. Preferably, also the closing timing can be adjusted. Instead of an electro-hydraulic actuator, also other kinds of actuating means could be used. For instance, the exhaust valves 6 could be cam-operated and provided with hydraulic means for adjusting the valve timing. Also electrical or hydraulic valve actuators could be used. In the embodiment of the figures, the valve actuator 12 is connected to a control unit 13, which controls the opening and closing timing of the exhaust valve 6. The control unit 13 is configured to transmit a control signal to the valve actuator 12 for opening and closing the exhaust valve 6.

Figure 2 shows a lower part of the cylinder 1 of figure 1. The cylinder 1 is pro vided with a plurality of inlet ports 8, which are distributed along the perimeter of the cylinder liner 2. The inlet ports 8 allow flow of fresh charge into the com bustion chamber 4 when the piston 5 is below the upper ends of the inlet ports 8. In practice, limited flow of fresh charge into the combustion chamber 4 may start when the uppermost piston ring 16 is below the upper edges of the inlet ports 8. Flowever, usually this flow is insignificant due to the small clearance between the piston 5 and the wall of the cylinder 1 and also due to the small difference between the cylinder pressure and the pressure of the fresh charge when the piston 5 is close to the inlet ports 8. Opening timing of the inlet ports 8 refers to a crank angle, at which the piston 5 is in a position, where fluid communication between the combustion chamber 4 and the space outside the inlet ports 8 opens. In most cases, the crank angle at which the top of the pis ton 5 has moved below the upper ends of the inlet ports 8 can be considered as the opening timing of the inlet ports 8. In figure 2, the piston 5 is above the inlet ports 8 and flow of fresh charge into the combustion chamber 4 is pre vented. The inlet ports 8 are thus closed. The expression“fresh charge” refers to the gas which flows into the cylinder 1 when the inlet ports 8 are open. The fresh charge can consist of pressurized intake air, but the fresh charge can al- so comprise for example recirculated exhaust gas and/or gaseous fuel mixed with intake air.

The cylinder 1 is provided with one or more fuel injectors 9, which are arranged in the cylinder head 3. Fuel is injected into the combustion chamber 4 when the piston 5 is close to the TDC position. As the piston 5 moves towards bot tom dead center (BDC), which corresponds to the crank angle of 180 degrees, the exhaust valve 6 is opened. Typically, the exhaust valve 6 is opened within the crank angle range of 100-130 degrees. This allows initial blow-down of the exhaust gas from the cylinder 1. Consequently, the cylinder pressure drops faster after the opening of the exhaust valve 6. The development of the cylin der pressure during part of the downwards movement of the piston 5 is shown in figure 5. The vertical lines show the opening crank angles of the exhaust valve 6 (EVO) and the inlet ports 8 (IPO). The dash-dotted line shows the cyl inder pressure and the dashed line shows the pressure of the fresh charge. The term“PUS” refers to piston underside space. The piston underside space comprises both the space or volume under the piston 5 and the space outside the cylinder liner 2 connected by the inlet ports 8 to the space within the cylin der liner 2. In the example of Fig. 5, the exhaust valve 6 is opened when the crank angle is approximately 1 15 degrees. The pressure of the fresh charge is approximately 5 bar. Before the piston 5 has moved below the inlet ports 8, the pressure in the cylinder 1 has dropped below the pressure of the fresh charge. As the inlet ports 8 open, fresh charge can flow into the cylinder 1. In the ex ample of Fig. 5, the inlet ports 8 are opened when the crank angle is approxi mately 145 degrees. Since the pressure of the fresh charge is higher than the cylinder pressure, the fresh charge pushes the remaining exhaust gas out of the cylinder 1 via the open exhaust port 10. The cylinder pressure gradually approaches the pressure of the fresh charge.

The opening timing of the exhaust valve 6 affects the development of the cyl inder pressure during the rest of the downwards movement of the piston 5. The earlier the exhaust valve 5 is opened, the lower is the cylinder pressure at the time the inlet ports 8 open. When the inlet ports 8 open, the cylinder pressure should normally not be higher than the pressure of the fresh charge. Cylinder pressure that is higher than the pressure of the fresh charge leads to exhaust gas being pushed into the piston underside space (blow-back). This heats up the piston underside space, which is detrimental to engine performance and also increases NOx emissions. Furthermore, the exhaust gas dirt contaminates the piston underside space, which results in a need of increased cleaning ef fort. Dirt on the surfaces of the piston underside space can even lead to clog ging of the parts of the piston underside space or the inlet ports 8. However, in certain cases a cylinder pressure that is slightly higher than the pressure of the fresh charge can be acceptable.

With an early opening of the exhaust valve 6, blow-back of exhaust gas is pre vented and effective scavenging is achieved. However, if the exhaust gas is discharged from the cylinder 1 earlier than necessary, the energy of the ex haust gas is not converted into mechanical work and the thermal efficiency of the engine decreases. It is thus desirable to optimize the opening timing of the exhaust valve 6 in order to prevent blow-back of exhaust gas while maximizing the thermal efficiency of the engine and achieving the lowest possible fuel con sumption.

An engine according to the invention comprises means for determining at least one pressure difference value, which represents the difference between a cyl inder pressure and the pressure of the fresh charge. The cylinder pressure to which the pressure of the fresh charge is compared can be an instantaneous pressure of a single cylinder 1 at a predetermined crank angle during a single engine cycle, an average of instantaneous pressures of several cylinders 1 at a predetermined crank angle during a single engine cycle, an average of in stantaneous pressures at a predetermined crank angle in a single cylinder 1 during several engine cycles, or an average of instantaneous pressures of several cylinders 1 at a predetermined crank angle during several engine cy cles. Alternatively, the cylinder pressure can be an average pressure based on several measurements in a predetermined crank angle range in a single cylin der 1 or in a plurality of cylinders 1 during one or more engine cycles. By taking several measurements in a single cylinder 1 in a certain crank angle range or during several engine cycles, noise of the measurements can be reduced. Tak ing an average of the measurements of several cylinders 1 has the same ef fect, but in that case differences in the cylinder-wise pressures cannot be taken into account in the adjustment of the opening timings of the exhaust valves 6. The engine further comprises means 12, 13 for adjusting the opening timing of the exhaust valve 6 based on the pressure difference value.

The method according to the invention comprises the steps of determining at least one pressure difference value, which represents the difference between the cylinder pressure of one or more cylinders 1 at a predetermined crank an gle or in a predetermined crank angle range during one or more engine cycles and the pressure of the fresh charge, and adjusting the opening timing of each exhaust valve 6 of the engine based on a determined pressure difference val ue.

The pressure difference value is determined for a crank angle or a crank angle range in the proximity of the crank angle at which the inlet port 8 opens. The predetermined crank angle or the crank angle range can be before the opening of the inlet port 8, preferably immediately before the opening of the inlet port 8. However, since the cylinder pressure changes relatively slowly just after the crank angle at which the opening of the inlet ports 8 starts, the measurement of the cylinder pressure can take place or continue even after the opening of the inlet ports 8. The predetermined crank angle or the predetermined crank angle range can be, for instance, within 5 degrees from the crank angle at which the inlet port 8 opens. Preferably, the predetermined crank angle or the predetermined crank angle range is within 2 degrees from the crank angle at which the inlet port 8 opens.

In the embodiment of the figures, each cylinder 1 of the engine is provided with a differential pressure sensor 11. The differential pressure sensor 1 1 is ar ranged in the cylinder liner 2. The differential pressure sensor 1 1 is arranged in the proximity of the inlet ports 8 above the inlet ports 8. The differential pres sure sensor 1 1 can measure the pressure difference between the cylinder 1 and the piston underside space, i.e. the pressure difference between the cylin der pressure and the pressure of the fresh charge. The differential pressure sensor 11 is connected to the control unit 13. On the basis of the measurement data received from the differential pressure sensor 1 1 , the control unit 13 transmits a control signal to the valve actuator 12 for achieving optimal exhaust valve opening timing. Since the differential pressure sensor 1 1 is located close to the inlet ports 8, it is protected from the highest combustion chamber tem peratures. Instead of the differential pressure sensor 1 1 , the engine could be provided with a separate cylinder pressure sensor and a pressure sensor for measuring the pressure of the fresh charge.

Because the engine is provided with means for individually determining a pres sure difference value for each cylinder 1 , the exhaust valve opening timing can be adjusted individually in each cylinder 1. However, it would also be possible to monitor the cylinder pressure in a single cylinder 1 only, and to adjust the opening timing of all the exhaust valves 6 of the engine based on the pressure difference value of the single cylinder 1. Alternatively, cylinder pressure of sev eral cylinders 1 can be monitored and the pressure difference value can be an average value based on the cylinder pressures of several cylinders 1 and the pressure of the fresh charge.

In the embodiment of the figures, the engine further comprises an absolute pressure sensor 14, which is arranged to measure the pressure of the fresh charge. Also the absolute pressure sensor 14 is connected to the control unit 13. By means of the absolute pressure sensor 14, the accuracy of the cylinder pressure measurement can be increased. It can also be used for many other purposes, such as for calculation of indicated mean effective pressure (IMEP).

The opening timing of the exhaust valve 6 of each cylinder 1 can be controlled by comparing the determined pressure difference value to a target value or target value range or to one or more limit values. The values referred above can be fixed or they can depend on engine type, load, angular speed or other parameters. The target value or limit values are preferably set in such a way that significant blow-back is prevented while fuel consumption is minimized under all engine operating conditions.

The cylinder pressure should normally be equal to or slightly lower than the pressure of the fresh charge when the inlet ports 8 start to open. If the cylinder pressure is in a desired range, the current opening timing of the exhaust valve 6 can be maintained. If the cylinder pressure is too high compared to the pres sure of the fresh charge, the opening timing of the exhaust valve 6 is ad vanced. The phase of faster dropping of the cylinder pressure thus starts earli er during the downwards movement of the piston 5. Consequently, the cylinder pressure is at a lower level when the inlet ports 8 open. If the cylinder pressure is too low compared to the pressure of the fresh charge, the opening timing of the exhaust valve 6 is delayed.

According to an embodiment of the invention, the pressure difference value is compared to a first predetermined limit value, and the opening timing of the exhaust valve 6 is advanced in case the pressure difference value is above a first predetermined limit value. The first limit value is thus an upper limit for the pressure difference. If the cylinder pressure is higher than the pressure of the fresh charge, the pressure difference is positive. Normally a negative pressure difference value is desired for preventing blow-back of exhaust gas. The first predetermined limit value can thus be, for example, in the range of -0.1 - 0.0 bar. However, also a slightly positive pressure difference may be allowed, in which case the first predetermined limit value could be, for example, in the range of -0.1 - 0.1 bar.

According to an embodiment of the invention, the pressure difference value is compared to second predetermined limit value, and the opening timing of the exhaust valve 6 is delayed in case the pressure difference value is below a second predetermined limit value. The second limit value is thus a lower limit for the pressure difference. If the cylinder pressure is much lower than the pressure of the fresh charge when the opening of the inlet ports 8 starts, ther mal energy has been wasted. The second predetermined limit value can be, for example, in the range of -0.2 - 0.0 bar, or in the range of -0.2 - -0.1 bar.

The pressure difference value is determined during each engine cycle. The opening timing of the exhaust valves 6 can thus be continuously adjusted based on a closed-loop control. The pressure difference can thus be main tained at an optimal level in all operating conditions of the engine and over the whole life cycle of the engine. For instance, the wear of different engine com ponents can be taken into account. For example aging of the turbocharging components can lead to decreasing intake air pressure and wear of the fuel in jection components can affect the cylinder pressure. With the closed-loop con trol based on the difference between the cylinder pressure and the pressure of the fresh charge, these effects are automatically compensated.

If the measurement of the pressure difference fails for some reason, the en gine can be switched to use an open-loop control of the exhaust valve timing. For instance, the opening timing can be based on the engine speed.

The differential pressure sensor 1 1 also allows collection of different data. For instance, the pressure measurements could allow monitoring of the condition of the fuel injection system or the turbocharging system. In the situation of fig ure 3, the differential pressure sensor 1 1 measures the pressure difference be tween the piston ring pack and the fresh charge. This information could possi bly be used for monitoring the condition of the piston rings. The differential pressure sensor 11 could also comprise an integrated capacitive sensor, which could measure the distance to the piston rings. This could be used for measur ing the thickness of the cylinder liner 2.

It will be appreciated by a person skilled in the art that the invention is not lim ited to the embodiments described above, but may vary within the scope of the appended claims.

Claims

1. A method of operating a multi-cylinder two-stroke piston engine, where each cylinder (1 ) comprises at least one exhaust valve (6) with variable open ing timing and at least one inlet port (8), which inlet port (8) is arranged in the wall of the cylinder (1 ) and opens for allowing flow of fresh charge into the cyl inder (1 ) when the piston (5) in the cylinder (1 ) has moved below the upper edge of the inlet port (8), the method comprising the steps of

- determining at least one pressure difference value, which represents the difference between the cylinder pressure of one or more cylinders (1 ) and the pressure of the fresh charge at a predetermined crank angle or in a predetermined crank angle range during one or more engine cycles, and

- adjusting the opening timing of each exhaust valve (6) of the engine based on a determined pressure difference value.

2. A method according to claim 1 , wherein the predetermined crank angle or the predetermined crank angle range is in the proximity of the crank angle at which the inlet port (8) opens.

3. A method according to claim 1 or 2, wherein the predetermined crank an gle or the predetermined crank angle range is within 5 degrees from the crank angle at which the inlet port (8) opens.

4. A method according to any of claims 1-3, wherein the predetermined crank angle or the predetermined crank angle range is within 2 degrees from the crank angle at which the inlet port (8) opens.

5. A method according to any of the preceding claims, wherein the prede termined crank angle or the predetermined crank angle range is before the crank angle at which the inlet port (8) opens.

6. A method according to any of the preceding claims, wherein the opening timing of the exhaust valve (6) is advanced in case the pressure difference value is above a first predetermined limit value.

7. A method according to claim 6, wherein the first predetermined limit value is in the range of -0.1 - 0.1 bar.

8. A method according to claim 7, wherein the first predetermined limit value is in the range of -0.1 - 0.0 bar.

9. A method according to any of the preceding claims, wherein the opening timing of the exhaust valve (6) is delayed in case the pressure difference value is below a second predetermined limit value.

10. A method according to claim 9, wherein the second predetermined limit value is in the range of -0.2 - 0.0 bar.

11. A method according to claim 10, wherein the second predetermined limit value is in the range of -0.2 - -0.1 bar.

12. A method according to any of the preceding claims, wherein an individual pressure difference value is determined for each cylinder (1 ) of the engine, and the opening timing of the exhaust valve (6) is adjusted individually in each cyl inder (1 ) of the engine based on the respective individual pressure difference value.

13. A method according to claim 12, wherein each individual pressure differ ence value is based on the cylinder pressure in the respective cylinder (1 ).

14. A method according to any of the preceding claims, wherein the pressure difference value is determined by means of at least one differential pressure sensor (1 1 ) that is arranged in the cylinder liner (2) of a cylinder (1 ).

15. A method according to any of claims 1-13, wherein the pressure differ ence value is determined by means of at least one sensor measuring pressure in a cylinder (1 ) and at least one sensor measuring the pressure of the fresh charge.

16. A method according to any of the preceding claims, wherein the pressure difference value is determined on the basis of the pressure of the fresh charge in the proximity of the inlet port (8).

17. A method according to any of the preceding claims, wherein the exhaust valves (6) are electro-hydraulically actuated.

18. A multi-cylinder two-stroke piston engine, where each cylinder (1 ) com prises at least one exhaust valve (6) with means (12) for variable opening tim- ing and at least one inlet port (8), which inlet port (8) is arranged in the wall of the cylinder (1 ) and opens for allowing flow of fresh charge into the cylinder (1 ) when the piston (5) in the cylinder (1 ) has moved below the upper edge of the inlet port (8), wherein the engine further comprises means (1 1 ) for determining at least one pressure difference value, which represents the difference be tween the cylinder pressure of one or more cylinders (1 ) and the pressure of the fresh charge at a predetermined crank angle or in a predetermined crank angle range during one or more engine cycles, and means (12, 13) for adjust ing the opening timing of the exhaust valve (6) based on the pressure differ- ence value.

19. An engine according to claim 18, wherein the means for determining the pressure difference value comprise a differential pressure sensor (1 1 ).

20. An engine according to claim 19, wherein the differential pressure sensor (1 1 ) is arranged in the wall of the cylinder (1 ) in the proximity of the inlet port (8).

21. An engine according to claim any of claims 18-20, wherein each cylinder (1 ) of the engine is provided with a differential pressure sensor (11 ).

22. An engine according to any of claims 18-21 , wherein the engine com prises at least one sensor (14) for determining the absolute pressure of the fresh charge.