DK201500271A1

DK201500271A1 - A large turbocharged two-stroke self-igniting internal combustion engine with an exhaust valve actuation system

Info

Publication number: DK201500271A1
Application number: DKPA201500271A
Authority: DK
Inventors: Kim Jensen
Original assignee: Man Diesel & Turbo Filial Af Man Diesel & Turbo Se Tyskland
Priority date: 2015-05-06
Filing date: 2015-05-06
Publication date: 2016-11-28
Also published as: JP6301998B2; KR20160131920A; KR101780206B1; DK178787B1; CN106121762A; CN106121762B; JP2016211548A

Abstract

A large turbocharged two-stroke self-igniting internal combustion engine with a plurality of cylinders (1) with scavenge ports (22) and an exhaust valve (4). The exhaust valve (4) is movable in opposite directions between a closed position and an open position. An air spring (66) with a spring piston (67) in an air spring cylinder (69) is operably connected to exhaust valve (4) and the air spring (55) is configured to urge the exhaust valve (4) towards its closed position. A hydraulic actuator (60) is configured to urge the exhaust valve (4) towards its open position when the hydraulic actuator (60) is pressurized. A controllable source of pressurized hydraulic liquid (80,82) is connected to the hydraulic actuator (60). Means (82) are provided for measuring the speed of the exhaust valve (4), and a controller (50) is operably connected to the source of pressurized hydraulic liquid (80,82). The controller (50) is in receipt of a signal representative of the measured speed of the exhaust valve (4) and the controller (50) is configured to control the speed of the exhaust valve (4) by controlling the controllable source of pressurized hydraulic liquid (80,82) in response to the measured speed of the exhaust valve (4).

Description

A LARGE TURBOCHARGED TWO-STROKE SELF-IGNITING INTERNAL COMBUSTION ENGINE WITH AN EXHAUST VALVE ACTUATION SYSTEM

TECHNICAL FIELD

The present disclosure relates to a large turbocharged two-stroke self-igniting internal combustion engine with an exhaust valve actuation system, and more particularly to a large turbocharged two-stroke self-igniting internal combustion engine with an electronically controlled exhaust valve actuation system.

BACKGROUND

Large turbocharged two-stroke self-igniting internal combustion engine are typically used as prime movers in large ocean going ships, such as container ships or in power plants.

The cylinders of these engines are provided with a single exhaust valve in the cylinder cover i.e. at the top of the cylinder and with a ring of piston controlled scavenge ports at the lower region of the cylinder liner.

Recently, most of these engines are provided with an electronically controlled and hydraulic exhaust valve actuation system. In comparison to a camshaft controlled exhaust valve actuation system the electronic control system allows for a greatly improved flexibility and adjustability allowing optimization with respect to emissions and fuel consumption over the full range of operating conditions of the engine.

Large turbocharged two-stroke self-igniting internal combustion engine with an electronically controlled exhaust valve actuation system have the disadvantage that the hydraulic system has to open the exhaust valve against the pressure in the combustion chamber that acts on the valve disk that has a considerable effective pressure area and against the force of an air spring that urges the exhaust valve towards its seat.

Thus, initially a very large force is needed to open the exhaust valve. Once the exhaust valve has a small amount of lift the force needed to open the exhaust valve force is abruptly reduced since it is only the force of the air spring that is urging the exhaust valve in the closing direction. Consequently, a very high pressure is applied to the hydraulic actuator on top of the exhaust valve in order to be able to open the exhaust valve against the pressure in the combustion chamber, but shortly thereafter this high pressure is excessive and causes the exhaust valve to accelerate to an extent that is not necessary and often causes cavitation in the hydraulic exhaust valve actuation system. This problem is particularly grave during low load conditions. The reason is that the exhaust valve actuation is calibrated to open fast capturing full load, i.e. with the valve disk being exposed to the highest combustion pressure. During low load the combustion pressure is much lower and consequently, the exhaust accelerates faster and gains more speed than intended requires a larger amount of the deceleration in the last part of its movement to its open position, thereby causing negative pressures i.e. cavitation in the hydraulic valve actuation system.

Lowering the hydraulic feed pressure is not a viable solution to reduce or overcome cavitation because this is in conflict with the need to generate a significant force at the first part of the opening movement of the exhaust valve to overcome the pressure in the combustion chamber.

Further, the characteristics of the air spring are relatively unpredictable, as they depend on various operating conditions and vary over time. Thus, it is extremely difficult if not impossible to simply calibrate the valve actuation system during engine construction for various engine loads in order to avoid the exhaust valve picking up too much speed at low loads.

This problem has been aggravated in the recent past by the tendency of shipping companies to sail their ships at speeds below the design speed, so-called slow steaming, and thereby operate the main engine for extended periods of time at a load level well below the maximum load level.

SUMMARY

In view of the above it is an object of the present invention to overcomes or at least reduce the problems mentioned above.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, there is provided a large turbocharged two-stroke self-igniting internal combustion engine, said engine comprising a plurality of cylinders with scavenge ports at their lower region and an exhaust valve at their top, an exhaust valve with a valve stem and a valve disc, said exhaust valve being movable in opposite directions between a closed position in which the valve disk rests on a valve seat and an open position, an air spring with an spring piston received in an air spring cylinder, said air spring being operably connected to the valve stem and said air spring being configured to resiliently urge the exhaust valve towards its closed position, a hydraulic actuator with a bore and a plunger slidably and sealingly received in said bore and with a pressure chamber in said bore on one side of said plunger, said plunger being connected to the stem of the exhaust valve and said hydraulic actuator being configured to urge the exhaust valve towards its open position when said pressure chamber is pressurized, a controllable source of pressurized hydraulic liquid connected to said hydraulic actuator, means for measuring the speed of the exhaust valve, and a controller operably connected to said source of pressurized hydraulic liquid, said controller being in receipt of a signal representative of the measured speed of the exhaust valve, said controller being configured to control the speed of said exhaust valve by controlling said controllable source of pressurized hydraulic liquid in response to the measured speed of the exhaust valve.

By measuring the speed of the exhaust valve and by adjusting the hydraulic power that is delivered to the hydraulic exhaust valve actuation system in response thereto, the speed that is achieved by the exhaust valve can be controlled and thereby excessive speeds that may lead to cavitation can be avoided.

According to a first implementation of the first aspect of the controller is configured to control the speed of the exhaust valve to obtain a desired maximum speed or average speed of said exhaust valve during its movement said exhaust valve during its movement from the seat to the open position.

According to a second implementation of the first aspect the desired maximum speed or average speed is dependent on the load of the engine.

According to a third implementation of the first aspect controller is configured to adjust said desired maximum speed or average speed in accordance with the actual engine load. Thus, the amount of hydraulic power that is supplied to the hydraulic valve actuation system is adjusted to the operating conditions.

According to a fourth implementation of the first aspect controller determines said desired maximum speed in accordance with the actual engine load from an equation or from a lookup table.

According to a fifth implementation of the first aspect said controllable source of hydraulic liquid includes a proportional valve.

According to a sixth implementation of the first aspect the proportional valve is connected to a source of high pressure hydraulic liquid and wherein said controller controls the speed of said exhaust valve by adjusting the opening of said proportional control valve.

According to a seventh implementation of the first aspect the desired maximum speed or average speed is set at a level high enough to ensure sufficiently fast opening of the exhaust valve and sufficiently low to avoid cavitation in said controllable source of hydraulic liquid.

According to an eighth implementation of the first aspect the speed of the exhaust valve is measured for each given number of engine cycles. The speed can be measured every engine cycle or, e.g. every second third or other suitable number of engine cycles. Since the load conditions of a large turbocharged two-stroke internal combustion engines typically change relatively slow, it is not necessary to measure the actual speed obtained by the exhaust valve every engine cycle.

According to a ninth implementation of the first aspect said controller is configured to control the controllable source of hydraulic liquid in response to the last measured speed of the exhaust valve.

The object above is also achieved according to a second aspect by providing a method for controlling the maximum or average speed of the exhaust valve of a large turbocharged two-stroke self-igniting internal combustion engine with a hydraulically actuated exhaust valve, said method comprising measuring the speed of the exhaust valve in its movement from its closed position to its open position, and adjusting the amount of hydraulic power delivered to the exhaust valve in response to the measured exhaust valve speed.

According to a first implementation of the second aspect the method further comprises the step of controlling the speed of the exhaust valve to obtain a predetermined average or maximum speed for the movement of the exhaust valve from its closed position to its open position.

According to a second implementation of the second aspect the speed of said exhaust valve is measured for each given number of engine cycles.

The object above is also achieved according to a third aspect by providing a large turbocharged two-stroke self-igniting internal combustion engine with a plurality of cylinders with scavenge ports and an exhaust valve, the exhaust valve is movable in opposite directions between a closed position and an open position, an air spring with a spring piston in an air spring cylinder is operably connected to exhaust valve and the air spring is configured to urge the exhaust valve towards its closed position, a hydraulic actuator is configured to urge the exhaust valve towards its open position when the hydraulic actuator is pressurized, a controllable source of pressurized hydraulic liquid is connected to the hydraulic actuator, means are provided for measuring the speed of the exhaust valve, and a controller is operably connected to the source of pressurized hydraulic liquid. The controller is in receipt of a signal representative of the measured speed of the exhaust valve and the controller is configured to control the speed of the exhaust valve by controlling the controllable source of pressurized hydraulic liquid in response to the measured speed of the exhaust valve.

These and other aspects of the invention will be apparent from and the embodiment (s) described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is an elevated view showing the fore end and one side of a large two-stroke self-igniting turbocharged engine according to an example embodiment,

Fig. 2 is an elevated view showing the aft end and the other side of the engine of Fig. 1,

Fig. 3 is a diagrammatic representation the engine according to Fig. 1 with its intake and exhaust systems, Fig. 4 is a sectional view of an exhaust valve of the engine according to Fig. 1,

Fig. 5 is a diagram illustrating the electrohydraulic exhaust valve actuation system of the engine of Fig. 1, Fig. 6 is a graph illustrating the opening movement of the exhaust valve and the hydraulic pressure associated therewith for the engine of Fig. 1, and

Fig. 7 a graph illustrating the opening movement of the exhaust valve and the hydraulic pressure associated therewith for a conventional (prior art) engine.

DETAILED DESCRIPTION

In the following detailed description, the large two stroke engine will be described by the example embodiments. Figs. 1 to 3 show a large low speed turbocharged two-stroke diesel engine with a crankshaft 42 and crossheads 43. Figure 3 shows a diagrammatic representation of a large low speed turbocharged two-stroke diesel engine with its intake and exhaust systems. In this example embodiment the engine has six cylinders 1 in line. Large turbocharged two-stroke diesel engines have typically between five and sixteen cylinders in line, carried by an engine frame 45. The engine may e.g. be used as the main engine in an ocean going vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 5,000 to 110,000 kW.

The engine is a diesel (self-igniting) engine of the two-stroke uniflow type with scavenge ports 22 in the form a ring of piston-controlled ports at the lower region of the cylinders 1 and an exhaust valve 4 at the top of the cylinders 1. Thus the flow in the combustion chamber is always from the bottom to the top and thus the engine is of the so called uniflow type. The scavenging air is passed from the scavenging air receiver 2 to the scavenging air ports 22 of the individual cylinders 1. A reciprocating piston 41 in the cylinder 1 compresses the scavenging air, fuel is injected, combustion follows and exhaust gas is generated. When an exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct 35 associated with the cylinder 1 concerned into an exhaust gas receiver 3 and onwards through a first exhaust conduit 18 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away through a second exhaust conduit 7. Through a shaft 8, the turbine 6 drives a compressor 9 supplied via an air inlet 10.

The compressor 9 delivers pressurized charging air to a charging air conduit 11 leading to the charging air receiver 2. The scavenging air in the conduit 11 passes through an intercooler 12 for cooling the charging air. The cooled charging air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the charging air flow in low or partial load conditions to the charging air receiver 2. At higher loads the turbocharger compressor 9 delivers sufficient compressed scavenging air and then the auxiliary blower 16 is bypassed via a non-return valve 15.

The cylinders are formed in a cylinder liner 52. The cylinder liners 52 are carried by a cylinder frame 46 that is supported by the engine frame 45.

As shown in Fig. 4, the exhaust valve 4 comprises a valve spindle 23 with a valve disk 25 at one end. The valve spindle 23 is slidably and sealingly received in a bore in a valve housing. The valve housing also defines a part of the duct 35. At its lower end the valve housing defines a circumferential valve seat 6 on which the valve disc 25 rests when the exhaust valve 4 is in its closed position that is shown in Fig. 4.

An air spring 66 that includes an air spring piston 67 is configured to resiliently urge the exhaust valve 4 to its closed position. The air spring piston 67 is secured to the valve stem 23 and the air spring piston 67 is slidably and sealingly received in a bore in the housing. A spring chamber 69 is located below the air spring piston 67 and when pressurized urges the air spring plunger 67 upwards. The spring chamber 69 is connected to a source of pneumatic pressure via a nonreturn valve, to ensure a proper pressurization of the spring chamber 69.

The top of the valve spindle 23 is provided with a plunger 61 that is slidably and sealingly received in a bore 63 in the valve housing. A pressure chamber 62 is formed in the bore 63 above the plunger 61 and in fluid communication with a port 64. The top of the plunger 61 is provided with a dampening arrangement for dampening the last portion of the closing movement of the exhaust valve 4 towards its seat 26, but will not be described in detail here.

When the pressure chamber 62 is pressurized the plunger 61 urges the exhaust valve towards its open position, i.e. in a downward direction (downward as in the orientation of Fig. 4).

With reference to Fig. 5, the exhaust valve 4 is opened hydraulically against the force of the air spring 66 and the force of the combustion pressure in the combustion chamber acting on the valve disk 25 the by means of a two-stage exhaust valve actuator 70 activated by control oil from an electronically controlled proportional valve 80. The exhaust valve 4, shown in an open position in Fig. 5, is closed again by the air spring 66 when the control oil pressure is removed.

The electrohydraulic exhaust valve actuation system comprises the two-stage exhaust valve actuator 70 that is connected to the hydraulic actuator 60 at the top of the exhaust valve via a hydraulic pressure pipe 65. One end of the hydraulic pressure pipe 65 connects to the port 64. The other end of the hydraulic pressure pipe 65 connects to a pump chamber 75 in the two-stage exhaust valve actuator 70. A pump piston 74 is received in a bore in the two-stage valve actuator 70. A pump chamber 75 is defined in the bore above the pump piston 75. The two-stage exhaust valve actuator 70 is provided with concentric first and second plungers 71,72. The first plunger 71 is directly connected to the pump piston 74, whilst the second plunger 72 slidably surrounds the first plunger 71. The first plunger 71 is active during the whole stroke of the pump piston 74, whilst the movement of the second piston 72 is limited to the start of the pump stroke. This well-known construction ensures that there is a higher pressure available during the first part of the pump stroke whilst the later part of the pump stroke is performed with a lower pressure that is only generated by the first plunger 71. A pressure chamber 77 acts on the free end of the first and second plungers 71, 72. The pressure chamber 77 is connected to a source 82 of high-pressure hydraulic fluid, such as control oil, via a proportional control valve 80. The proportional control valve 80 connects the pressure chamber 77 to the source of high-pressure hydraulic fluid 82 in one of its positions and the proportional controller 80 connects the pressure chamber 77 to tank in another one of its positions. The control valve 80 is a proportional valve i.e. the control valve 80 can assume any intermediate position between the above-mentioned positions and the control valve 80 can gradually vary the size of the opening that allows hydraulic fluid through the control valve 80.

Thus, the hvdraulic power delivered to the two-stage exhaust valve actuator 70 can be proportionally controlled and adjusted to any particular need. The control valve 80 is electronically controlled valve, i.e. the position of the control valve 80 can be changed in response to an electronic signal.

The control valve 80 is controlled by an electronic control unit 50 (controller 50). The electronic control unit 50 issues a signal to the control valve 80 via a signal cable to thereby determine the position of the control valve 80.

The exhaust valve 4 is provided with a sensor 82 for measuring the speed of the exhaust valve 4. The sensor 82 generates a signal representative of the speed of the exhaust valve 4. The sensor 82 is connected to the electronic control unit 50 via signal cable so that the electronic control unit is informed of the speed of the exhaust valve 4. The speed of the exhaust valve can also be measured indirectly, for example by measuring the flow rate through the pressure pipe 65 or by measuring the speed of the pump piston 74.

The electronic control unit 50 monitors the speed of the exhaust valve 4 during its opening movement. The electronic control unit 50 is in an embodiment configured to determine the maximum speed that the exhaust valve 4 achieves during its opening movement. In another embodiment the electronic control unit 50 is configured to determine the average speed that the exhaust valve 4 achieves during its opening movement.

In an embodiment the electronic control unit 50 is configured to measure the speed of the exhaust valve 4 for every engine cycle. However, it is understood that it can be sufficient to measure the exhaust of speed for each second, third or higher number of engine cycles.

Based on the last measured average or maximum speed achieved by the exhaust valve 4, the electronic control unit 50 determines the degree of opening of the control valve 80 for the next exhaust valve lift event.

The electronic control unit 50 is informed about the crank angle of the engine and initiates the opening of the exhaust valve 4 at the appropriate position in the engine cycle. The electronic control unit 50 issues a control signal to the control valve 82 open to the determined the degree. Thereupon, the controller 80 opens to the determine degree and high-pressure control oil pressurizes the pressure chamber 77 and the first plunger 71 and for the first part of the movement of the second plunger 72 urge the pump piston into the pump chamber 75. Thus, the pump chamber 75 is pressurized and via the pressure pipe 65 the pressure chamber 62 of the exhaust valve actuator 60 at the top of the exhaust valve 4 is also pressurized.

As described above, the pressure of the hydraulic liquid in the pressure chamber 62 causes the exhaust valve 4 to open against the pressure in the combustion chamber and against the force of the air spring 66. Since the degree of opening of the control valve 80 is adjusted to the actual operating conditions the exhaust valve 4 reaches a desired maximum for average speed during his opening movement that does not cause cavitation in the hydraulic system.

The desired maximum speed can be dependent on the engine operating conditions. In particular, the desired maximum speed for the exhaust valve 4 is normally higher for higher engine loads than for lower engine loads. For example, the desired maximum speed can be e.g. 2.2 m/s for maximum engine load and 1.8 m/s for a 4% engine load.

The electronic control unit 50 can be provided with a lookup table based on calibrated data from engine running tests or with an equation that uses calibrated data from engine running tests in order to determine the appropriate desired speed for the exhaust valve 4 during its opening movement.

The desired speed for the exhaust valve 4 is chosen such that the exhaust valve 4 opens fast enough for properly evacuating the exhaust gases but slow enough to avoid cavitation in the hydraulic exhaust valve actuating system.

The electronic control unit 50 is configured to close the exhaust valve at the appropriate time in the engine cycle. When this time has come the electronic control unit 50 commands the control valve 80 to connect the pressure chamber 77 to tank and thereupon the exhaust valve 4 returns to its closed position by the action of the air spring 66.

The embodiment described here above uses a two-stage valve actuator that is connected by a pressure pipe to a hydraulic actuator at the top of the exhaust valve. However, it is understood by those skilled in the art that the complete hydraulic arrangement including the two-staqe or sinqle staqe exhaust valve actuator and the hydraulic actuator can all be arranged at the top of the exhaust valve so that there is no need for any pressure pipe connecting these elements and all these elements can in embodiment form an integral unit at the top of the exhaust valve.

Fig. 6 shows a graph illustrating the opening movement X of an exhaust valve 4 and the hydraulic pressure P applied to the exhaust valve actuator 60 for an engine according to the present invention. Fig. 7 shows a graph illustrating the movement X of an exhaust valve and the hydraulic pressure P applied to the exhaust valve actuator for a conventional (prior art) engine.

In the engine according to the present invention the target (desired) speed has been said to 1.8 m/s. The load was set to 4% for both engines. In the conventional engine there is no control of the speed of the exhaust valve. The difference between the two graphs is clear. In the engine according to the invention the drop of the pressure after the initial opening of the exhaust valve is much less significant and does not become zero or negative whereas the pressure drop in the conventional engine after the initial opening of the exhaust valve is more significant and becomes a zero or negative and causes cavitation.

Thus, the present invention provides for an effective way for avoiding damage due to cavitation.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected bv those skilled in the art in practicina the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The reference signs used in the claims shall not be construed as limiting the scope.

Claims

1. A large turbocharged two-stroke self-igniting internal combustion engine, said engine comprising: a plurality of cylinders (1) with scavenge ports (22) at their lower region and an exhaust valve (4) at their top, an exhaust valve (4) with a valve stem (23) and a valve disc (25), said exhaust valve (4) being movable in opposite directions between a closed position in which the valve disk rests (25) on a valve seat (26) and an open position, an air spring (66) with an air spring piston (67) received in an air spring cylinder (69), said air spring (66) being operably connected to the valve stem (23) and said air spring (55) being configured to resiliently urge the exhaust valve (4) towards its closed position, a hydraulic actuator (60) with a bore (63) and a plunger (61) slidably and sealingly received in said bore (63) and with a pressure chamber (62) in said bore (63) on one side of said plunger (61), said plunger (61) being connected to the stem (23) of the exhaust valve (4) and said hydraulic actuator (60) being configured to urge the exhaust valve (4) towards its open position when said pressure chamber (62) is pressurized, a controllable source of pressurized hydraulic liquid (80,82) connected to said hydraulic actuator (60), means (82) for measuring the speed of the exhaust valve (4), and a controller (50) operably connected to said source of pressurized hydraulic liquid (80,82), said controller (50) being in receipt of a signal representative of the measured speed of the exhaust valve (4), said controller (50) being configured to control the speed of said exhaust valve (4) by controlling said controllable source of pressurized hydraulic liquid (80,82) in response to the measured speed of the exhaust valve (4).

2. An engine according to claim 1, wherein said controller (50) is configured to control the speed of the exhaust valve (4) to obtain a desired maximum speed or average speed of said exhaust valve during its movement said exhaust valve during its movement from the seat to the open position.

3. An engine according to claim 2, said desired maximum speed or average speed is dependent on the load of the engine .

4. An engine according to claim 3, wherein said controller (50) is configured to adjust said desired maximum speed or average speed in accordance with the actual engine load.

5. An engine according to claim 4, wherein said controller (50) determines said desired maximum speed in accordance with the actual engine load from an equation or from a lookup table.

6. An engine according to any one of claims 1 to 5, wherein said controllable source of hydraulic liquid includes a proportional valve (80).

7. An engine according to any one of claims 1 to 6, wherein said proportional valve (80) is connected to a source of high pressure hydraulic liquid (82) and wherein said controller (50) controls the speed of said exhaust valve (4) by adjusting the opening of said proportional control valve (80).

8. An engine according to any one of claims 1 to 7, wherein said desired maximum speed or average speed is set at a level high enough to ensure sufficiently fast opening of the exhaust valve (4) and sufficiently low to avoid cavitation in said controllable source of hydraulic liquid (80,82) .

9. An engine according to any one of claims 1 to 8, wherein the speed of the exhaust valve (4) is measured for each given number of engine cycles.

10. An engine according to any one of claims 1 to 9, wherein said controller (50) is configured to control the controllable source of hydraulic liquid (80,82) in response to the last measured speed of the exhaust valve (4) .

11. A method for controlling the maximum or average speed of the exhaust valve of a large turbocharged two-stroke self-igniting internal combustion engine with a hydraulically actuated exhaust valve (4) , said method comprising: measuring the speed of the exhaust valve (4) in its movement from its closed position to its open position, and adjusting the amount of hydraulic power delivered to the exhaust valve (4) in response to the measured exhaust valve speed.

12. A method according to claim 11, further comprising the step of controlling the speed of the exhaust valve (4) to obtain a predetermined average or maximum speed for the movement of the exhaust valve (4) from its closed position to its open position.

13. A method according to claim 11 or 12, wherein the speed of said exhaust valve (4) is measured for each given number of engine cycles.