WO2009030253A1

WO2009030253A1 - Large two-stroke diesel engine with outwardly opening exhaust valves

Info

Publication number: WO2009030253A1
Application number: PCT/EP2007/007689
Authority: WO
Inventors: Henning Jorgensen; Christian Adriansen; Allan Gerner Christiansen
Original assignee: Man Diesel A/S
Priority date: 2007-09-04
Filing date: 2007-09-04
Publication date: 2009-03-12
Also published as: CN101495718B; JP4566277B2; JP2009536996A; CN101495718A; KR20090110216A; KR101012110B1

Abstract

A large two-stroke diesel engine with an exhaust valve actuating system that moves the exhaust valves in an outward direction in order to allow the exhaust gases to be evacuated from the combustion chambers. The exhaust valve is provided with a relief system that allows the exhaust valve to open automatically if excessive cylinder pressures occur.

Description

LARGE TWO-STROKE DIESEL ENGINE WITH OUTWARDLY OPENING EXHAUST VALVES

FIELD OF THE INVENTION

The present invention relates to a large two-stroke diesel engine and in particular to the exhaust valve system of a large two-stroke diesel engine.

BACKGROUND OF THE INVENTION

Large two-stroke diesel engines of the cross-head type are for example used for propulsion of large oceangoing vessels or as primary mover in a power plant. Not only due to sheer size, these two-stroke diesel engines are constructed differently from any other combustion engines. Their exhaust valves may weigh up to 400 kg, pistons have a diameter of more than 1 meter and the maximum operating pressure in the combustion chamber is typically several hundred bar. The forces involved at these high pressure levels and piston sizes are enormous.

Due to erroneous fuel injection timing or amount, excessive pressure may occur on rare occasions in one of the cylinders. In order to accommodate these excessive pressures, the force with which the cylinder cover is pressed onto the top of the cylinder liner is carefully controlled by the tension that is applied to the stay bolts that connect the cylinder cover to the bedplate and keep the engine construction together. Thus, when excessive pressure occurs the cylinder cover is lifted and the excessive pressure is blown out between the top of the cylinder liner and the bottom of the cylinder cover. This system - that is generally used in the art - is not without problems. Firstly, any bystanders could be seriously hurt when such a sideward blow-off occurs. Secondly, the extremely hot and high pressure gases significantly erode the precisely machined mating surfaces of the cylinder liner and the cylinder cover, and therefore a blow off event will require that the surfaces are machined in order to obtain the required fluid tightness. Thus, the reparation costs are significant after a blow off. Thirdly, the tension in the stay bolts varies due to temperature changes of the engine and the environment and can therefore not be very accurately controlled. If a blow-off occurs at a moment that the tension in the stay bolts is relatively high, the forces on the piston and the crankshaft have in the past caused damaged big ends and other expensive engine components. Such an occurrence is even more expensive than a better controlled blow off.

Most engines are also provided with security valves that are supposed to open evacuating gases from the combustion chamber when excessive pressures occur in the engine. However, the explosive nature of these occurrences render these valves relatively ineffective since their maximum opening is insufficient for relieving the pressure sufficiently fast. Thus, these security valves can not effectively provide the required opening area in sufficiently short time.

Thus there is a desire for improved blow-off control system for large two-stroke diesel engines. DISCLOSURE OF THE INVENTION

On this background, it is an object of the present invention to provide a large two-stroke diesel engine with an improved system for handling excessive cylinder pressure incidents.

This object is achieved by providing a large two-stroke diesel engine of the crosshead type comprising a number of cylinders that serve as combustion chambers, each cylinder being provided with at least one exhaust valve, and an exhaust valve actuation system for opening and closing the exhaust valves synchronously with the engine cycle, the exhaust valve actuation system opens the respective exhaust valve in an outward direction relative to the combustion chamber for controlled evacuation of the exhaust gases after combustion, the exhaust valve actuation system closes the respective exhaust valve in an inward direction relative to the combustion chamber before combustion, and the exhaust valve actuation system allowing the respective exhaust valve to open in an outward direction relative to the combustion chamber when excessive pressure occurs in the cylinder concerned irrespective of the current phase of the engine cycle.

By arranging that the exhaust valves move away from the combustion chamber for opening and by allowing the exhaust valve to open up automatically when excessive pressure occurs in the cylinder concerned, a number of advantages are obtained:

One of the advantages is that the inertia of the exhaust valve spindle is small compared with the inertia of the cylinder cover. This results in a faster response and relief when an excessive cylinder pressure incident occurs.

Another advantage is that the flow area with which the exhaust valves open upon the relief situation is large compared to the narrow gap between the cylinder cover and cylinder liner. Thus, the gases can escape quicker and the pressure buildup is lower which in turn reduces the risk of damage to engine components such as the big end or the crankshaft.

Yet another advantage is that the relief pressure setting of the exhaust valve can be regulated relatively precisely and can therefore be placed relatively close to the peak pressure during normal operation. Many components of the engine are dimensioned on basis of the pressure when relief takes place. Thus, it becomes possible to design these components with a smaller safety margin allowing a lighter engine construction with the same kind of reliability.

A further advantage is that in a blow out the hot gases pass components that are designed for the purpose of transporting hot gases and there should be no damage to these components when a blow out occurs. Thus, once the error that caused the excessive cylinder pressure has been corrected a continuation of normal engine operation should be possible. In the prior art this is impossible because the mating surfaces of the cylinder cover and the cylinder liner would first need to be machined, i.e. normal operation can only take place again after a relatively complicated repair operation. Another advantage is that in a relief incident the gases are let out into the exhaust system, not into the machine room.

Another advantage is that the exhaust valve is helped by the pressure in the combustion chamber to make its opening movement, in contrast to the prior art where the exhaust valve has to open against the pressure in the combustion chamber. This means that the force that is required to open the exhaust valve is less, and consequently the load on the valve actuation system is lower. Thus, the energy required to operate the exhaust valve actuation system is reduced compared to the prior art systems that are equipped with inwardly opening exhaust valves.

The exhaust valves may be urged in an outward direction for opening the exhaust valve by a gas spring that acts on the stem of the exhaust valves.

A hydraulic actuator may urge the exhaust valve in an inward direction for closing the exhaust valve.

The exhaust valve may include a valve head that interacts with the cylinder cover on top of the respective cylinder for sealing the combustion chamber when the exhaust valve is in its closed position.

The valve head may fit with sealing engagement within an annular opening in the cylinder cover. Thus, a range of closed positions can be obtained. Also, the exhaust valve can pick up speed before opening, thereby minimizing the period of time in which there is a narrow opening gap and the resulting high velocity gas flow with consequent heat loading of the valve seat and throttling losses.

The inner surface of the annular opening may be provided with one or more sealing rings.

The circumferential surface of the valve head may be provided with one or more sealing rings.

The sealing surfaces on the cylinder cover and the valve seat may have a normal with only a radial component.

The exhaust valve may not have a fixed seated position, but a range in which the valve is closed.

The valve head may abut with a valve seat in the cylinder cover.

The valve seat may have a surface with a normal that has substantial axial component.

The axial component is directed in an outward direction relative to the combustion chamber.

The valve seat may be conical.

The valve actuation system may comprise a camshaft, and a cam driven actuator pump that is operatively connected to the hydraulic actuator.

The cam driven pump may be connected to the actuator via a pressure conduit with a timed cutoff valve being placed in the pressure conduit. The exhaust valve concerned may be blocked from moving when the cutoff valve associated therewith is closed, except when excessive pressure occurs in the associated combustion chamber.

The cutoff valve is only open in the period where exhaust valve lift takes place in order to protect the cam driven pump and the camshaft from the high pressures that the hydraulic actuator is exposed to during combustion.

The cam driven pump may be connected to the actuator via a pressure conduit with a timed changeover valve being placed in the pressure conduit.

In a first position the changeover valve may connect a pressure chamber in the hydraulic actuator directly to the cam driven pump during opening and closing of the valve and may connect in a second position the pressure chamber to the cam driven pump via a pressure amplifier during the period in which the exhaust valve is closed.

The cam on the cam shaft that acts on the cam driven pump may have a progressive profile that causes the cam driven pump to continuously deliver at least a small amount of hydraulic fluid in the period of the engine cycle where the exhaust valve is closed.

The changeover valve may be provided with a non return valve element in the second position The changeover valve may in a first position connect a pressure chamber in the hydraulic actuator directly to the cam driven pump during opening and closing of the valve and may in a second position connect another source of high pressure hydraulic fluid to the pressure chamber in the hydraulic actuator.

The other source of high pressure hydraulic fluid may be a hydraulic pump.

The valve actuation system may comprise a high pressure hydraulic pump that is operatively connected to the exhaust valve actuators and via pressure conduits and control valves.

A relief valve may be operatively connected to the hydraulic actuator, the relief valve being configured to open when excessive pressure occurs in the combustion chamber.

The relief valve may be configured to open when the pressure in the hydraulic actuator exceeds a predetermined threshold.

The relief valve may be of the type that opens completely once the predetermined threshold has been exceeded and remains open thereafter.

Further objects, features, advantages and properties of the large two-stroke diesel engine according to the invention will become apparent from the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawings, in which:

figure 1 is a cross-sectional view of an engine according to the present invention, figure 2 is a longitudinal-sectional view of one cylinder section of the engine shown in figure. 1, figure 3 is a diagrammatic representation of a first embodiment of the exhaust valve actuating system according to the present invention, figure 4 is a variation of the embodiment shown in figure

3, figure 5 is a diagrammatic representation of a second embodiment of the exhaust valve actuating system according to the present invention, and figure 6 is a diagrammatic representation of a third embodiment of the exhaust valve actuating system according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figures 1 and 2 show an engine 1 according to a preferred embodiment of the invention in cross-sectional view and longitudinal-sectional view (for one cylinder) respectively. The engine 1 is a uniflow low-speed two- stroke crosshead diesel engine of the crosshead type, which may be a propulsion system in a ship or a prime mover in a power plant. These engines have typically from 4 up to 14 cylinders in line. The engine 1 is built up from a bedplate 2 with the main bearings for the crankshaft 3. The crankshaft 3 is of the semi-built type. The semi- built type is made from forged or cast steel throws that are connected with the main journals by shrink fit connections .

The bedplate 2 can be made in one part or be divided into sections of suitable size in accordance with production facilities. The bedplate consists of side walls and welded cross girders with bearing supports. The cross girders are in the art also referred to as "transverse girders". The oil pan 58 is welded to the bottom of the bedplate 2 and collects the return oil from the forced lubricating and cooling oil system.

The connecting rods 8 connect the crankshaft 3 to the crosshead bearings 22. The crosshead bearings 22 are guided between vertical guide planes 23.

A welded design A-shaped frame box 4 is mounted on the bedplate 2. The frame box 4 is a welded design. On the exhaust side the frame box 4 is provided with relief valves for each cylinder, while on the camshaft side the frame box 4 is provided with a large hinged door for each cylinder. The crosshead guide planes 23 are integrated in the frame box 4.

A cylinder frame 5 is mounted on top of the frame box 4. Staybolts 27 connect the bedplate 2, the frame box 4 and the cylinder frame 5 and keep the structure together. The staybolts 27 are tightened with hydraulic jacks.

The cylinder frame 5 is cast in one or more pieces eventually with an integrated camshaft housing 25, or it is a welded design. According to another embodiment (not shown) the camshaft 28 is housed in a separate camshaft housing that is attached to the cylinder frame.

The cylinder frame 5 is provided with access covers for cleaning the scavenge air space and for inspection of scavenge ports and piston rings from the camshaft side. Together with the cylinder liner 6 it forms the scavenge air space. The scavenge air receiver 9, is bolted with its open side to the cylinder frame 5. At the bottom of the cylinder frame there is a piston rod stuffing box, which is provided with sealing rings for scavenge air, and with oil scraper rings which prevent exhaust products to penetrate into the space of the frame box 4 and the bedplate 2 and in this way protects all the bearings which are present in this space.

The piston 13 includes a piston crown and piston skirt. The piston crown is made of heat-resistant steel and has four ring grooves which are hard-chrome plated on both the upper and the lower surfaces of the grooves.

The piston rod 14 is connected to the crosshead 22 with four screws. The piston rod 14 has two coaxial bores (not visible in the drawings) which, in conjunction with a cooling oil pipe, forms the inlet and outlet for cooling oil for the piston 13.

The cylinder liners 6 are carried by the cylinder frame 5. The cylinder liners 6 are made of alloyed cast iron and are suspended in the cylinder frame 5 by means of a low situated flange. The uppermost part of the liner is surrounded by cast iron cooling jacket. The cylinder liners 6 have drilled holes (not shown) for cylinder lubrication. The cylinders are of the uniflow type and has scavenge air ports 7 located in an airbox, which from a scavenge air receiver 9 (Fig. 1), is supplied with scavenge air pressurized by a turbocharger 10 (Fig. 1) .

The engine is fitted with one or more turbochargers 10 arranged on the aft end of the engine for 4-9 cylinder engines and on the exhaust side for 10 or more cylinder engines.

The air intake to the turbocharger 10 takes place directly from the engine room through an intake silencer (not shown) of the turbocharger. From the turbocharger 10, the air is led via a charging air pipe (not shown) , air cooler (not shown) and scavenge air receiver 9 to the scavenge ports 7 of the cylinder liners 6.

The engine is provided with electrically-driven scavenge air blowers (not shown) . The suction side of the blowers is connected to the scavenge air space after the air cooler. Between the air cooler and the scavenge air receiver non-return valves (not shown) are fitted which automatically close when the auxiliary blowers supply the air. The auxiliary blowers assist the turbocharger compressor at low and medium load conditions.

Fuel valves 40 are mounted concentrically in a cylinder cover 12. At the end of the compression stroke the injection valves 40 inject fuel at high pressure through their injection nozzles as a fine mist into the combustion chamber 15. The fuel can be diesel oil, heavy fuel oil or gas. When the engine is gas driven it is usually provided with a fuel oil injection system and a gas injection system (not shown) so that the engine can be operated with either type of fuel. An exhaust valve 11 is mounted centrally in the top of the cylinder in the cylinder cover 12. The exhaust valve 11 comprises a valve head 11a and a valve stem lib At the end of the expansion stroke the exhaust valve 11 opens upwardly before the engine piston 13 passes down past the scavenge air ports 7, whereby the combustion gases in the combustion chamber 15 above the piston 13 flow out through an exhaust passage 16 opening into an exhaust receiver 17 and the pressure in the combustion chamber 15 is relieved. The exhaust valve 11 closes again with a downward movement towards the cylinder and combustion chamber 15 during the upward movement of the piston 13. The exhaust valve 11 is hydraulically activated.

Figure 3 shows a first embodiment of the exhaust valve actuating system according to the present invention. The exhaust valve actuating system is for all of the embodiments illustrated with respect for a single cylinder. In a multi-cylinder engine there will be the same provisions for each cylinder. The exhaust valve actuating system includes the camshaft 28 with cams 29 (only one is shown) . A roller 30 follows the surface of the cam 29 and is connected to the piston of a cam driven positive displacement pump 32. The positive displacement pump 32 is connected to an exhaust valve actuator 34 via a conduit 36. The exhaust valve actuator 34 comprises a cylindrical pressure chamber and a piston that is slidingly received in the cylindrical pressure chamber and acts on the stem lib of the exhaust valve 11. A gas spring 38 is also connected to the stem lib of the exhaust valve and the gas pressure in the pressure chamber of the gas spring 38 urges the exhaust valve 11 in the opening direction away from the combustion chamber 15. When the valve actuator 34 is pressurized it urges the exhaust valve 11 in the closing direction towards the combustion chamber 15. The valve head 11a fits in this embodiment in a sealing manner inside on annular opening in the cylinder cover 12. The circumferential surface of the valve head 11a has normal with a radial direction. The same applies to the inner surface of the annular opening, which has a normal with a radial direction.

The position of the exhaust valve is in a not shown embodiment measured by the sensor that is connected to an electronic control system of the engine.

A cutoff valve 41 is placed in the connection between the cam driven pump 32 and the pressure chamber of the hydraulic valve actuator 34. The electronically controlled cutoff valve 41 has in the present embodiment two positions. In a first position in the cutoff valve connects the pressure chamber of the hydraulic valve actuator 34 to the cam driven hydraulic pump 32 via pressure conduit 36. In a second (shown) position the cutoff valve 41 closes the connection between the pressure chamber of the hydraulic exhaust valve actuator 34 and the cam driven hydraulic pump 32.

A large bore in the wall of the pressure chamber of the hydraulic exhaust valve actuator 34 connects the pressure chamber to a pressure relief valve 43. The pressure relief valve 43 is configured to open to tank (not shown) when the pressure in the pressure chamber exceeds a predetermined threshold. This threshold set slightly above the maximum (peak) pressure that occurs in the pressure chamber of the hydraulic exhaust valve actuator 34 during normal engine operation. Typically this peak pressure will correspond to the phase in the engine cycle in which combustion takes place. The pressure relief valve 43 is of a type that opens completely once the set pressure has been reached and remains open thereafter. A primer pump 42 replenishes oil lost through leakage.

In operation, the camshaft 28 rotates in unison with the crankshaft of the engine. The profile of the cam 29 determines the movement of the positive displacement pump 32. When the positive displacement pump 32 moves upwards, hydraulic fluid is forced into the valve actuator 34 via conduit 36. The actuator 34 forces the exhaust valve 11 to close against the pressure in the gas spring 38. During this period the cutoff valve 41 is in the first position and allows the fluid from the cam driven positive displacement pump 32 to flow into the pressure chamber of the hydraulic exhaust valve actuator 34. When the exhaust valve 11 has reached its closed position that is shown by the uninterrupted lines in figure 3 the cutoff valve 41 moves to the second (shown) position and renders any fluid communication between the pressure chamber in the hydraulic exhaust valve actuator 34 and the cam driven positive displacement pump 32 impossible. Thus, the exhaust valve 11 is locked in its closed position with the circumferential edge of the valve head 11a forming a fluid tight seal in collaboration with the projection 47 of cylinder cover 12. The projection 47 is in the shown embodiment provided with a sealing ring 49. In other embodiments (not shown) the sealing ring or sealing rings 49 are provided in the circumferential surface of the valve head 11a. When it is time to open the exhaust valve (after the combustion has taken place) the cutoff valve 41 moves back to its first position to connect the pressure chamber in the hydraulic exhaust valve actuator 34 to the cam driven positive displacement pump 32. The cam driven positive displacement pump 32 moves downwards and the gas spring 38 in combination with the pressure in the combustion chamber that acts on the valve head 11a urges the exhaust valve 11 and the exhaust valve actuator 34 to move upwards and thereby the fluid in the exhaust valve actuator 34 flows back into the cam driven positive displacement pump 32. Some of the energy that was transferred to the exhaust valve actuator 34 during the opening movement of the exhaust valve 11 is returned to the camshaft 28 by the pressure that is created in the positive displacement pump 32 during the return stroke of the exhaust valve actuator 34. Thus, only a small portion of the hydraulic energy that is needed to open the exhaust valve 11 is dissipated.

Should an excessive pressure occur in the combustion chamber 15, this will cause a high force urging the exhaust valve 11 upwards thereby causing high pressures in the pressure chamber of the hydraulic exhaust valve actuator 34, i.e. pressures that are higher than those that will occur during normal engine operation. Thereupon, the pressure relief valve 43 will fully open and allow the fluid in the pressure chamber of the hydraulic exhaust valve actuator 34 to be evacuated quickly. The excessively high pressure in the combustion chamber in combination with the force of the gas spring 38 cause the exhaust valve 11 to open rapidly in the direction away from the combustion chamber 15. With the exhaust valve 11 open the gas in the combustion chamber 15 can be evacuated via the normal channels that are designed to lead the exhaust gases away from the combustion chamber 15 to the environment. This process will normally not cause any damage to the engine 1 because the threshold pressure at which the pressure relief valve 34 opens can be set accurately. Thus, the exhaust valve 11 will open in before any inadvertent pressure build up in the combustion chamber can damage engine components. Once the exhaust is open, the gas is passed to the environment through channels that are designed to handle these gases and will cope with this flow of gases without incurring any damage.

In a not shown variation of the first embodiment the cutoff valve is not electronically controlled but instead it is controlled by a camshaft. This can be the camshaft

28 with additional lobes for the control of the cutoff valve or it can be a separate camshaft that has as its sole purpose to control the cutoff valves associated with the various cylinders.

The enlarged portion of figure 3 shows a detail of a variation of the first embodiment. In this embodiment in the pressure with which the sealing ring 49 engages the valve head 11a is variable. A conduit 50 connects the outer circumference of the sealing ring 49 to the combustion chamber 15. With increasing pressure in the combustion chamber 15 the sealing ring is urged towards the valve head 11a with an increasing pressure. Thus, with low pressures in the combustion chamber 15 when the valve head 11 is moving in and out of contact with the sealing ring 49 there is little or no pressure between these two elements, thereby reducing wear and tear. When the pressures in the combustion chamber 15 are high, for example during combustion the sealing ring 49 is urged with a high pressure towards the valve head 11a, thereby ensuring that the high-pressure gases in the combustion chamber 15 do not leak past the sealing ring 49.

Figure 4 illustrates a variation of the embodiment shown in figure 3. The embodiment shown in figure 4 is essentially identical with the embodiments described with reference to figure 3, except that the cylinder cover 12 is includes a cylindrical portion that is provided with ports 60 and a ring Channel 62 that surrounds the ports. When the exhaust valve is in its upper position (as indicated by the interrupted line) the exhaust gases can escape from the combustion chamber 15 through the ports 60 and the ring channel 62. The ring Channel 62 is connected to the exhaust gas receiver. The provision of the cylindrical portion with the ports 60 ensures that the sealing rings on the valve head 11a are continuously in contact with a wall surface thereby avoiding possible where that is caused by a sealing ring getting into and out of contact with the wall surface.

Figure 5 shows a third embodiment of the exhaust valve actuating system according to the invention that is essentially identical with the embodiment described with reference to figure 3 except for the following differences. The cutoff valve 41 has been replaced by a changeover valve 42. The electronically controlled changeover valve 42 has in the present embodiment two positions. In a first position the changeover valve 42 connects the conduit 36 to the pressure chamber of the exhaust self actuator 34. In the first position a pressure amplifier 44 is connected to tank. In the second and shown position in the changeover valve 42 connects the conduit 36 to the pressure amplifier 44 to the conduit 36. This arrangement ensures that the cam driven positive displacement pump 32 is not exposed to the high pressures in the pressure chamber of the hydraulic exhaust valve actuator 34 during the combustion phase.

In this embodiment the exhaust valve 11 is provided with a conical exhaust valve seat 48 in the projection 47. The conical valve seat is provided on the upper side of the protrusion 47 of the cylinder cover 12, and the normal to the seat surface has a substantial component in the axial direction of the valve shaft lib in a direction away from the combustion chamber 15. The valve head 11a has a corresponding conical edge that is designed to go into sealing abutment with the valve seat 48. In a this embodiment the profile of the cam 29 is progressive (as indicated by the interrupted line on the cam 29) so that the cam driven positive displacement pump 32 keeps on delivering a small amount of fluid during the phase in which the exhaust valve is in its closed position. Thus, despite any leakage of hydraulic fluid, a substantial pressure is ensured for acting on the hydraulic exhaust valve actuator 34 via the pressure amplifier 44.

A pressure limiting valve 45 allows any surplus hydraulic fluid delivered by the cam driven positive displacement pump 32 to flow to tank.

During operation the changeover valve 42 is in the second shown position when the exhaust valve 11 is closed and rests on its seat 48. During the opening and closing movement and during the period that the exhaust valve 11 is open the changeover valve is in the (not shown) first position.

Operation of the exhaust valve actuation system according to the second embodiment is both during normal engine operation and during excessive pressure incidents in the combustion chamber 15 identical with the operation as described with reference to the first embodiment.

Figure 6 shows a fourth embodiment of the invention. In this embodiment the hydraulic valve actuating system also uses a hydraulic push rod system including a positive displacement pump 32 connected to a hydraulic valve actuator 34 by a conduit 36. However, in this embodiment the changeover valve 42 alternatively connects the pressure chamber in the hydraulic exhaust valve actuator 34 to the cam driven positive displacement pump 32 via conduit 36 and to a hydraulic pump 54. In its first position (not shown) the electronically controlled changeover valve 42 connects the pressure chamber in the hydraulic exhaust valve actuator 34 to the cam driven positive displacement pump 32. In its second position the changeover valve 42 connects to the pressure chamber in the hydraulic exhaust valve actuator 34 to the hydraulic pump 54. The hydraulic pump 54 can be an electrically or mechanically driven positive displacement pump. Any unused capacity of the hydraulic pump 54 is let to tank via a pressure regulation valve 55.

During normal engine operation, the changeover valve 42 is in its first (not shown) position during the opening and closing movement of the exhaust valve 11. When the exhaust valve 11 is in its closed position the changeover valve 42 is in its second shown position, in which the pressure chamber in the hydraulic exhaust valve actuator 34 is pressurized by the pump 54. By means of this pressure the exhaust valve 11 is securely pressed against its seat 48 during the combustion phase of the engine cycle. This embodiment is also provided with a pressure relief valve 43 that operates in the same manner as described above with reference to the first and second embodiment .

According to another embodiment (not shown) the exhaust valve actuation system may also be of the kind that does without a camshaft and instead operates with hydraulic pressure from a continuous high pressure source that is controlled by control valves that lead the hydraulic fluid to the respective hydraulic exhaust valve actuators at the desired time of opening and closing the exhaust valves (such as a common rail system) . Also in this embodiment the exhaust valve is kept closed by the pressure in the pressure chamber of on hydraulic exhaust valve actuator, and a pressure relief valve is connected to the pressure chamber of the hydraulic exhaust valve actuator, with a set pressure that allows the exhaust valve to open when an excessive pressure in the combustion chamber occurs.

The term "comprising" as used in the claims does not exclude other elements. The term "a" or "an" as used in the claims does not exclude a plurality.

Although the present invention has been described in detail for purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the scope of the invention.

Claims

CLAIMS :

1. A large two-stroke diesel engine of the crosshead type comprising:

a number of cylinders that serve as combustion chambers, each cylinder being provided with at least one exhaust valve, and

an exhaust valve actuation system for opening and closing said exhaust valves synchronously with the engine cycle,

said exhaust valve actuation system opens the respective exhaust valve in an outward direction relative to the combustion chamber for controlled evacuation of the exhaust gases after combustion,

said exhaust valve actuation system closes the respective exhaust valve in an inward direction relative to the combustion chamber before combustion,

and said exhaust valve actuation system allowing the respective exhaust valve to open in an outward direction relative to said combustion chamber when excessive pressure occurs in the cylinder concerned irrespective of the current phase of the engine cycle .

2. An engine according to claim 1, wherein said exhaust valves are urged in an outward direction for opening the exhaust valve by a gas spring that acts on the stem of the exhaust valves.

3. An engine according to claim 2, wherein a hydraulic actuator urges the exhaust valve in an inward direction for closing the exhaust valve.

4. An engine according to any of claims 1 to 3, wherein the exhaust valve includes a valve head that interacts with the cylinder cover on top of the respective cylinder for sealing the combustion chamber when the exhaust valve is in its closed position.

5. An engine according to claims 4, wherein the valve head fits with sealing engagement within an annular opening in the cylinder cover.

6. An engine according to claim 5, wherein the inner surface of said annular opening is provided with one or more sealing rings.

7. An engine according to claim 5 or 6, wherein the circumferential surface of said valve head is provided with one or more sealing rings.

8. An engine according to any of claims 5 to 7, wherein the sealing surfaces on the cylinder cover and the valve seat have a normal with only a radial component.

9. An engine according to any of claims 5 to 8, wherein the exhaust valve has no fixed seated position, but a range wherein it is closed.

10. An engine according to claim 4, wherein the valve head abuts with a valve seat in the cylinder cover.

11. An engine according to claim 10, wherein the valve seat has a surface with a normal that has substantial axial component.

12. An engine according to claim 11, wherein said axial component is directed in an outward direction relative to the combustion chamber.

13. An engine according to any of claims 10 to 12, wherein said valve seat is conical.

14. An engine according to any of claims 5 to 9, wherein said valve actuation system comprises a camshaft, and a cam driven actuator pump that is operatively connected to said hydraulic actuator.

15. An engine according to claim 14, wherein said cam driven pump is connected to said actuator via a pressure conduit with a cutoff valve being placed in said pressure conduit . (

16. An engine according to claim 15, wherein the exhaust valve concerned is blocked from moving when the cutoff valve associated therewith is closed, except when excessive pressure occurs in the associated combustion chamber.

17. An engine according to claim 15 or 16, wherein the cutoff valve is only open in the period where exhaust valve lift takes place in order to protect the cam driven pump and the camshaft from the high pressures that the hydraulic actuator is exposed to during combustion.

18. An engine according to any of claims 10 to 13, wherein said valve actuation system comprises a camshaft, and a cam driven actuator pump that is operatively connected to said hydraulic actuator.

19. An engine according to claim 18, wherein said cam driven pump is connected to said actuator via a pressure conduit with a timed changeover valve being placed in said pressure conduit.

20. An engine according to claim 19, wherein said changeover valve in a first position connects a pressure chamber in said hydraulic actuator directly to said cam driven pump during opening and closing of said valve and in a second position connects the pressure chamber to said cam driven pump via a pressure amplifier during the period in which the exhaust valve is closed.

21. An engine according to claim 20, wherein the cam on said cam shaft that acts on said cam driven pump has a progressive profile that causes the cam driven pump to continuously deliver at least a small amount of hydraulic fluid in the period of the engine cycle where the exhaust valve is closed.

22. An engine according to claim 20 or 21, wherein said changeover valve is provided with a non return valve element in said second position

23. An engine according to claim 19, wherein said changeover valve in a first position connects a pressure chamber in said hydraulic actuator directly to said cam driven pump during opening and closing of said valve and a second position connects in another source of high pressure hydraulic fluid to the pressure chamber in the hydraulic actuator.

24. An engine according to claim 23, wherein said other source of high pressure hydraulic fluid is a hydraulic pump.

25. An engine according to any of claims 1 to 13 wherein said valve actuation system comprises a high pressure hydraulic pump that is operatively connected to the exhaust valve actuators and via pressure conduits and control valves.

26. An engine according to any of claims 14 to 25, wherein a relief valve is operatively connected to the hydraulic actuator, said relief valve being configured to open when excessive pressure occurs in the combustion chamber.

27. An engine according to claim 26, wherein the relief valve is configured to open when the pressure in the hydraulic actuator exceeds a predetermined threshold.

28. An engine according to claim 27, wherein the relief valve is of the type that opens completely once the predetermined threshold has been exceeded and remains open thereafter.