DK201670059A1

DK201670059A1 - A large turbocharged two-stroke compression-ignited internal combustion engine with blow-off control

Info

Publication number: DK201670059A1
Application number: DKPA201670059A
Authority: DK
Inventors: Mads Røgild; Poul Cenker
Original assignee: Man Diesel & Turbo Filial Af Man Diesel & Turbo Se Tyskland
Priority date: 2016-02-03
Filing date: 2016-02-03
Publication date: 2017-08-28
Also published as: JP2017137866A; DK179120B1; CN107061007B; JP6188975B2; CN107061007A; KR20170092472A; KR101780212B1

Abstract

A large turbocharged two-stroke compression-ignited internal combustion engine with crossheads (43), a number of cylinders (1) that serve as combustion chambers (27), the cylinders being (1) provided with a cylinder cover (22), with an exhaust valve (4) placed centrally in the cylinder cover (22), and with an exhaust duct (35) connecting the exhaust valve (4) to an exhaust gas receiver (3). The cylinder cover (22) is provided with a blow-off 10 valve (50) with an inlet (57) of the blow-off valve (50) being fluidically connected to the combustion chamber (27) and an outlet (58) of the blow-off valve (50) being fluidically connected to a discharge conduit. The blow-off valve (50) is provided with a with movable valve member (52) that is moveable between a closed position and a fully open position with a range of intermediate positions there between. The blow-off valve (50) allows gas flow from the inlet (57) to the outlet (58) when the movable valve member (52) is in any of the intermediate positions or in the fully open position and the blow-off valve (50) prevents gas flow from the inlet (57) to the outlet (58) when the movable valve member (52) is in the closed position. The movable valve member (52) is provided with a first effective pressure surface (58) that is exposed to the pressure in the combustion chamber (27) so that the pressure in the combustion chamber (27) urges the movable valve member (52) towards the fully open position, and the movable valve member (52) has a second effective pressure surface exposed to a hydraulic pressure for urging the movable valve member (52) towards the closed position.

Description

TITLE

A LARGE TURBOCHARGED TWO-STROKE COMPRESSION-IGNITED INTERNAL COMBUSTION ENGINE WITH BLOW-OFF CONTROL

TECHNICAL FIELD

The disclosure relates to control of blow-off events in a large turbocharged two-stroke compression-ignited internal combustion engine with crossheads.

BACKGROUND

Large two-stroke turbo charged compression-ignited internal combustion 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 internal combustion engines. Their exhaust valves may weigh up to 400 kg, pistons have a diameter up to 100 cm 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 e.g. 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. Cylinder cover lift is an explosive gas leakage in which uncontrolled gas is released under a loud bang of up to 170db. Any bystanders could be seriously hurt when such a sideward blow-off occurs due to exposure to a jet of hot gas, often in the form of a flame. Further, the extremely hot and high pressure gas erodes the precisely machined mating surfaces of the cylinder liner and the cylinder cover and damages the sealing ring that is placed between the top of the cylinder liner and the bottom of the cylinder cover. Therefore, a blow-off event will typically require that these surfaces are machined and that the sealing ring is replaced in order to obtain the required fluid tightness. Consequently, the reparation costs are significant after a blow-off. Moreover, 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.

Large ship insurers (Classification societies) require that large marine engines must be protected with safety measures against damage from excessive pressure in the combustion chamber

Hereto, some prior art engines are provided with a rupture disk in the wall of the combustion chamber that is designed to fail in order to protect the engine against possible damage from excessive pressure in the combustion chamber. A disadvantage of these ruptured discs is that they weaken over time from the exposure to the fluctuating pressure in the combustion chamber and eventually fail already at a relatively small excess pressure, e.g. due to a small misfire. Thus, rupture discs tend to fail prematurely. This is problematic, especially since the engine has to be stopped in order to replace the failed rupture disc with a with a new disk. Consequently, the presently most commonly used measure with the rupture discs is not optimal.

Other engines have been provided with security valves in order to comply with the requirements that are supposed to open for evacuating gases from the combustion chamber when excessive pressures occur in the combustion chamber. These are spring loaded poppet valves. However, the explosive nature of blow-off occurrences render these poppet valves relatively ineffective since their maximum opening is insufficient for relieving the pressure sufficiently fast. Thus, these security valves cannot effectively provide the required opening area in sufficiently short time and only have the effect of an indicator or a whistle in advance of a cylinder cover lift but these known valves cannot prevent cylinder cover lift.

Thus, there is a need for an improved blow-off control system for large two-stroke compression-ignited internal combustion engines of the crosshead type.

SUMMARY

It is an object of the invention to provide large two-stroke compression-ignited internal combustion engine of the crosshead type that overcomes or at least reduces 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 compression-ignited internal combustion engine with crossheads, comprising: a number of cylinders that serve as combustion chambers, the cylinders being provided with a cylinder cover, with an exhaust valve placed centrally in the cylinder cover, and with an exhaust duct connecting the exhaust valve to an exhaust gas receiver, the cylinder cover being provided with a blow-off valve with an inlet of the blow-off valve being fluidically connected to the combustion chamber and an outlet of the blow-off valve being fluidically connected to a discharge conduit, the blow-off valve being provided with a with movable valve member that is moveable between a closed position and a fully open position with a range of intermediate positions there between, the blow-off valve allows gas flow from the inlet to the outlet when the movable valve member is in any of the intermediate positions or in the fully open position and the blow-off valve preventing gas flow from the inlet to the outlet when the movable valve member is in the closed position, the movable valve member is provided with a first effective pressure surface that is exposed to the pressure in the combustion chamber so that the pressure in the combustion chamber urges the movable valve member towards the fully open position, and the movable valve member having a second effective pressure surface exposed to a hydraulic pressure for urging the movable valve member towards the closed position.

By providing a blow-off valve with a movable member that is urged to its open position by the pressure in the combustion chamber and urged to its closed position by a hydraulic pressure, an arrangement can be achieved that can open both fast enough and with a sufficiently large opening for accommodating the relief that is required for preventing damage to the engine in an excessive pressure event in the combustion chamber. By using hydraulic pressure to balance the movable member of the blow-off valve the pressure at which the blow-off valve opens can be reliably controlled, and a construction can be provided that opens fast and wide providing for a large flow-through area for the gases of the combustion chamber to escape.

In a first possible implementation form of the first aspect the second effective pressure surface has a first size when the movable valve member is in the closed position and a second size that is less than the first size in a first range of positions of the movable valve member, the first range of positions extending from the fully open position to a predetermined intermediate position.

In a second possible implementation form of the first aspect the predetermined position is closer to the closed position than to the fully open position.

In a third possible implementation form of the first aspect the second effective pressure surface is formed by a first surface of the movable valve member and by a second surface of the moveable valve member that is smaller than the first surface and oppositely directed to the first surface.

In a fourth possible implementation form of the first aspect the first surface is pressurized by the hydraulic pressure in all of the positions of the movable valve member and wherein the second surface is pressurized by the hydraulic pressure in the first range of positions.

In a fifth possible implementation form of the first aspect the movable valve member opens a port that allows the second surface to be pressurized by the hydraulic pressure in the first range of positions.

In a sixth possible implementation form of the first aspect the movable valve member is provided with a valve disk that cooperates with a valve seat, the valve seat being arranged in a blow-off bore that extends through the cylinder cover.

In a seventh possible implementation form of the first aspect a main plane of the valve seat is arranged squint relative to a main direction of the blow-off bore.

In an eighth possible implementation form of the first aspect the movable valve member comprises a valve disk connected to a valve stem and an actuation piston operatively-connected to the valve stem.

In a ninth possible implementation form of the first aspect the first surface is arranged on one side of the actuation piston and the second surface is arranged on the opposite side of the actuation piston

In a tenth possible implementation form of the first aspect the blow-off bore is connected to the exhaust duct or to the exhaust gas receiver via a blow-off pipe in order to bypass the exhaust valve.

In a eleventh possible implementation form of the first aspect the blow-off valve is provided with cooling means, the cooling means preferably including path for a cooling medium through the blow-off valve.

In a twelfth possible implementation form of the first aspect the engine comprises a hydraulic system with a pressure that increases with increasing engine load and decreases with decreasing engine load and wherein the second effective pressure surface is pressurized with the hydraulic pressure of the hydraulic system.

In a thirteenth possible implementation form of the first aspect the hydraulic system powers the fuel injection valves .

In a fourteenth possible implementation form of the first aspect the hydraulic system powers the exhaust valve actuating system.

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 a front view of a large two-stroke diesel engine according to an exemplary embodiment,

Fig. 2 is a side view of the large two-stroke engine of Fig. 1,

Fig. 3 is a diagrammatic representation the large two-stroke engine according to Fig. 1,

Fig. 4 is a side view of a cylinder cover and exhaust valve with a blow-off valve according to an embodiment,

Fig. 5 is a sectional view along the lines A-A of Fig. 4, Fig. 6 is an elevated view of a blow-off valve according to an embodiment,

Figs. 7 to 9 are sectional view of the blow-off valve of Fig. 6, with a movable valve member of the blow-off valve in different positions,

Fig. 10 is a different sectional view of the blow-off valve of Fig. 6, and

Fig. 11 is a schematic representation of a hydraulic system of the engine of Figs. 1 to 3.

DETAILED DESCRIPTION

In the following detailed description, a large two-stroke turbocharged compression-ignited (diesel) internal combustion engine with crossheads will be described by example embodiments. Figs. 1 to 3 show a large low-speed turbocharged two-stroke compression-ignited engine with a crankshaft 42 and crossheads 43. Fig. 3 shows a diagrammatic representation of the engine with its intake and exhaust systems. In this example embodiment the engine has six cylinders 1 in line. Large two-stroke turbocharged compression-ignited internal combustion 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 marine 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 compression-ignited engine of the two-stroke uniflow type with scavenge ports 19 at the lower region of the cylinders 1 and an exhaust valve 4 centrally arranged in the cylinder cover 22 at the top of the cylinders 1. The main direction of the gas flow though the cylinder is from the scavenge ports 19 at the bottom of the cylinder 1 to the exhaust valve 4 at the top of a cylinder 1, hence the name "uniflow". The charging air is passed from a charging air receiver 2 in the form of a large generally hollow cylindrical body to the scavenging air ports 19 of the individual cylinders 1. A piston 41 in the cylinder 1 compresses the charging air, fuel is injected from fuel injection valves (not shown) in the cylinder cover 22 and 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 the exhaust gas receiver 3 in the form of a large hollow cylindrical body 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 to the atmosphere. Through a shaft 8, the turbine 6 drives a compressor 9 supplied with air 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 intake air in the conduit 11 passes through an intercooler 12 for cooling the charging air - that leaves the compressor at approximately 200 °C - to a temperature between 36 and 80 °C.

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

Figs. 4 and 5 show the exhaust valve 4 and cylinder cover 22 in greater detail in side- and sectional view, respectively. The exhaust valve 4 is securely bolted to the cylinder cover 22 with the exhaust valve spindle 44 with its integral valve disk arranged in the central opening in the cylinder cover 22. The exhaust valve spindle 44 is shown in its closed position with the disk of the exhaust valve spindle 44 resting on the valve seat. When exhaust valve 4 is open the combustion chamber 27 is connected to an exhaust duct 35. The exhaust duct 35 is in an embodiment directly connected to the exhaust gas receiver 3.

The cylinder cover 22 forms the upper portion of the combustion chamber 27. The cylinder cover 22 is provided with a plurality of cooling channels that are not visible in the figures. Further, fuel injection valves (typically three fuel valves for single fuel engines for each cylinder and six fuel valves for dual fuel engines for each cylinder)(fuel valves not shown) are received bores in the cylinder cover 22 with the nozzle of the fuel valve protruding into the combustion chamber 27.

The exhaust valve 4 is provided with a hydraulic exhaust valve actuator 47 that includes a hydraulic pressure chamber 38 at the top of the valve spindle 44. An air spring 37 urges the valve spindle 44 upwards (as upwards as in Fig. 5), i.e. in the closing direction and the hydraulic actuator 47 urges the valve spindle 44 in the opening direction when the hydraulic actuator 47 is pressurized. Thus, lift of the exhaust valve spindle 44 is achieved by applying hydraulic pressure to the hydraulic actuator 47 and the air spring 37 ensures return of the valve spindle 44 to its closed position.

The engine is provided with a blow-off conduit that extends (as shown) from the combustion chamber 27 to the exhaust duct 35. Alternatively, the blow-off conduit extends from the combustion chamber 27 to the exhaust gas receiver 3. The cross-sectional area of the blow-off conduit is sufficiently large for relieving the pressure in the combustion chamber 27 sufficiently fast in case of a misfire or other event that causes excessive pressure in the combustion chamber 27. A blow-off valve 50 controls the opening and closing of the blow-off conduit and the blow-off valve 50 can open sufficiently fast with a sufficiently large opening for relieving the pressure in the combustion chamber 27 in case of an excessive pressure event to prevent damage to the engine. A portion of the blow-off conduit is formed by a blow-off bore 29 in the cylinder cover 22. A blow-off pipe 36 connects the blow-off bore 29 to the exhaust duct 35 or to the exhaust gas receiver 3.

Figs. 6 to 10 illustrate the blow-off valve 50 in greater detail. The blow-off valve 50 can be provided with its own housing 51, (as shown) so that it can be used as a cartridge that is inserted into a suitable bore 28 (Fig. 5) in the cylinder cover 22 or (not shown) the blow-off valve can be an integral part of the cylinder cover 22.

The blow-off valve 50 is inserted into the bore 28 in the cylinder cover 22 with a bracket 68 protruding from the cylinder cover 22. The bracket 68, can be bolted to the housing 51 or be an integral part of the housing 51. The bracket 68 is provided with bores for receiving bolts (Fig. 5) that secure the blow-off valve 52 to the cylinder cover 22. The housing 51 is provided with large outlet (openings) 58 for allowing evacuation of exhaust gas when the blow-off valve 50 is open.

The blow-off valve 50 is provided with a movable valve member 52 that can move between a closed position shown in Fig. 7 and a fully open position shown in Fig. 9, i.e. the movable valve member 52 can move back and forth over the range of positions between the closed position of Fig. 7 and the fully open position of Fig. 9. In Fig. 8 the movable valve member 52 is shown in an intermediate position. The blow-off valve 50 allows gas flow from its inlet 57 to its outlet 58 when the movable valve member 52 is not in its closed position. A hollow 63 in valve housing 52 connects the inlet 57 to the outlets 58. A valve seat 55 is provided around the inlet 57.

The movable valve member 52 includes a spindle 53 that is provided with a valve disk 54 at one of its longitudinal ends. The other opposite longitudinal end of the spindle 53 is provided with a damper element 69. The spindle 53 is slidably received in a longitudinal bore in the housing 51.

The end of the spindle 53 that is provided with the valve disk 54 is extends form the longitudinal bore into the hollow 63 in the valve housing 51. The valve disk 54 rests on the valve seat 55 when the movable valve member 52 is in its closed position, as shown in Fig. 7 and 10 with the surface of the valve disk 54 that is exposed to the opening 57 forming the first effective pressure surface 59.

When the movable valve member 52 has lift, as shown in Figs. 8 and 9 the valve disk 54 is located in the hollow 63 and provides a substantial flow area for gas to flow from the inlet (opening) 57 to the outlet (opening) 58. In an embodiment the main plane Z of the valve seat 55 is arranged squint relative to the main direction to the blow-off bore 29 in order to minimize the restriction to flow through the blow-off bore 29. When the blow-off bore 29 is straight the main direction is the longitudinal direction of the flow of bore 29. When the blow-off bore 29 is curved, its main direction is the local direction at the curve at the position where the seat 55 intersects with the blow-off bore 29.

The hollow 63 is provided at one side of the longitudinal bore that guides the valve spindle 53. On the other side of this longitudinal bore there is provided a cylindrical chamber in which an actuation piston 56 is received. The actuation piston 56 device the cylindrical chamber in a first chamber 60 and a second chamber 66. The actuation piston 56 is secured to the spindle 53 in order to be operatively connected to the movable valve member 52. In an embodiment the piston 56 can also be an integral part of the movable valve member 52. The movable valve member 52 has a first surface 61 and an oppositely directed second surface 62. The first surface 61 is exposed to pressure in the first chamber 64 and urges the movable valve member 52 to its closed position when the first chamber 60 is pressurized. The second surface 62 is exposed to pressure in the second chamber 66 and urges the movable member 52 to its fully open position when the second chamber 66 is pressurized. The area of the first surface 61 is larger than the area of the second surface 62. Thus, the movable valve member 52 is urged towards closed position when the pressure in the first chamber 60 is equal to the pressure in the second chamber 66. The first surface 61 and the second surface 62 form together the second effective pressure surface. The pressure in the combustion chamber 27 acting on the first effective pressure surface 59 urges the movable valve member 52 to is closed position with a force that corresponds to the area of the first effective pressure surface 59 multiplied by the pressure in the combustion chamber 27.

The first chamber 60 is connected to an inlet port 80 in the blow-off valve 50 via supply bore 82 and a damper chamber 65. The inlet port 80, the supply bore 82 and the damper chamber 65 are in this embodiment arranged in the bracket 68, but could just as well be located in the housing 51.

The inlet port 80 is connected to a hydraulic system of the engine 88 that at least provides a continuous minimum level of hydraulic pressure. Preferably, the hydraulic system 88 provides a constant pressure or provides a pressure that increases with increasing engine load and decreases with increasing engine.

The pressure at the port 80 is applied to the first chamber 60 via the damper chamber 65 and the supply conduit 82 and this pressure urges the movable valve member 52 to is closed position with a force that corresponds to the area of the first surface 61 multiplied by the pressure in the first chamber 61. A secondary conduit 64 connects the second chamber 66 to port 80 via a compensation port 71 when the actuation piston 56 is in a position above the compensation port 71 (above as in the orientation of the blow-off valve 50 as in Figs. 7-10) . When the movable valve member 52 is in its closed position shown in Fig. 7 and 10, the compensation port 71 is blocked by the actuation piston 56.

In this situation shown in Fig. 7 and 10 the second chamber 66 is pressurized at a low pressure, i.e. a pressure significantly lower than the pressure at port 80 via cooling inlet port 75. The cooling inlet port 75 allows a flow of cooling oil (hydraulic liquid) into chamber 66 for cooling the blow-off valve 50. A cooling outlet port 74 allows an outflow of cooling oil (hydraulic liquid). A flow restriction (not shown) is provided upstream of the cooling inlet port 75 and downstream of the cooling outlet port 74 for allowing the pressure in the second chamber 66 to be momentarily higher than the relatively cooling liquid pressure .

When an excessive pressure in the combustion chamber 27 occurs the force of the pressure in the combustion chamber 27 acting on the first effective pressure surface 59 will exceed the opposing force of the pressure created by the first pressure chamber on the movable valve member 52 and the movable valve ember 52 will start moving towards its fully open position, as shown in Fig. 8.

At a certain amount of lift of the movable valve member 52 the secondary port 71 will open and the second pressure chamber 66 will be connected to the higher pressure at port 80. The restrictions associated with the cooling inlet port 75 and with the cooling outlet port 74 ensure that the pressure in chamber 66 is not lost through these respective ports 74,75. Thus, second surface 62 is pressurized with the pressure from port 80 and creates an additional force that urges the movable member towards its fully open position. The closing force acting on the movable valve member 52 is thus significantly reduced. This is reduction in closing force when the movable valve member 52 is somewhat open serves to ensure a stable opening of the blow-off valve 50 during a blow-off event. The opening force of the pressure of the gas in the combustion chamber 27 is namely significantly reduced the moment that the valve disk 54 has lift form the seat 55 due to pressurizing of the hollow 63. Without countermeasures this reduction in opening force loss could lead to undesirable repetitive opening and closing of the blow-off valve 50 during a single excessive pressure event. The closing force is reduced when the secondary port 71 is unobscured by the actuation piston 56 thereby providing a stable opening procedure for the blow-off valve 50.

Thus, the second effective pressure surface acting in the closing direction on the movable valve member 52 that is pressurized with the pressure from the hydraulic system 88 has a first size in the closed position of the movable valve member 52 and a second size that is less than said first size in a first range of positions of the movable valve member 52, the first range of positions extends from the fully open position to a predetermined intermediate position. The intermediate position depends on the position of the secondary port 71 relative to the actuation piston 56 and is preferably closer to said closed position than to said fully open position, but this depend on circumstances and the skilled person will be able to determine the optimum position at which the closing force is reduced. Other means to ensure that the closing force is reduced once the movable valve member 52 has some lift could also be used.

When the movable valve member 52 moves all the way towards its fully open position its movement is dampened by a the cooperation of a dampening element 69 at the free end of spindle 53 with the dampening chamber 65. The dampening element 69 is a cylindrical element with diameter suitable for it to be slidably received in the cylindrical dampening chamber 65 and ensures that the movable valve member 52 lands softly in its fully open position shown in Fig. 9.

The hydraulic system 88 is described in greater detail with reference to Fig. 11 and comprises a pump or pump station 41 that supplies a supply conduit 45 with pressurized hydraulic liquid. The pressure in the supply conduit 45 can be anywhere between 50 and 500 bar. The supply conduit 45 can be a common rail or a pseudo-common rail that supplies various consumers such as the fuel injection 42, the exhaust valve actuation system 48 and the blow-off valves 50 with pressurized hydraulic liquid. A common return line 49 connects the various consumers of hydraulic liquid to tank.

The MAN B&W ® (Brand) engines of the ME line the hydraulic system is a pseudo common rail in which local accumulators are used to accommodate for sudden local consumption peaks. The hydraulic system of the ME engines powers the fuel valves and the exhaust valves. In a typical ME engine the pressure in the hydraulic system in relation to the engine load is as follows:

Thus, the hydraulic pressure increases with increasing engine loads and decreases with decreasing engine closed. It is noted that the numbers in the table above are merely by way of example and the actual pressure values may vary from engine to engine of a given type and from engine type to engine type. Further, de-rated engines, i.e. engines in which load and speed are different from the LI load for the engine type concerned will also have different, typically slightly lower system pressure.

The normal maximum pressure in the combustion chamber 27 also increases with increasing engine load and decreases with decreasing engine load, with a profile that is guite similar to that of the pressure of the hydraulic system as shown in the table here above. Thus, by dimensioning the size of the area of the first surface 61 relative to the size of the area of the first effective pressure surface 59 the blow-off valve 50 can be dimensioned to open at a pressure that is higher with a margin than the normal pressure in the combustion chamber 27 for the actual engine load. The margin could be a constant pressure differential or increase with increasing engine load and vice versa.

The blow-off valve 50 can also be operated with a constant pressure supplied to port 80. In this case the pressure should be such that the blow-off valve 50 opens when the highest pressure in the combustion chamber 27 during normal engine operation is exceeded by a margin, e.g. the blow-off valve 50 is set to open at a pressure of 230 Bar when the maximum expected pressure (i.e. at 100% engine load) is 200 Bar. The blow-off valve 50 can be set to open at the required pressure by adjusting the ratio between the first effective pressure surface 59 and the first surface 61 in view of the pressure applied at port 80.

In an embodiment (not shown) the blow-off valve 50 is coupled to a controller of the engine and issues an alarm upon a blow-off event. Hereto, the blow-off valve 50 is in an embodiment provided with a pressure sensor that senses the pressure in the second chamber 66. If the pressure in the second chamber 66 exceeds the cooling oil pressure the blow-off valve 50 has opened and an alarm is issued. Alternatively, the movable valve member 52 can be monitored with a movement sensor and an alarm is issued when the moveable valve member 52 moves away from its closed position .

Resilient means, such a disc springs or the like that urge the movable valve member 52 to away from the closed position can in an embodiment (not shown) be provided to exercise the blow-off valve 50 when the engine is stopped.

In an embodiment, the balance of forces that determines whether the movable valve member 52 is urged to the closed position or to the fully open position is only determined by the balance of the force of the pressure of the gas in the combustion chamber 27 acting on the movable valve member 52 and the force of the pressure of the hydraulic liquid supply to the blow-off valve acting on the movable-valve member 52. Thus, if there is any resilient means for exercising the blow-off valve when the engine stopped these resilient means will have such a minor strength that they do not have any significant influence on the movement of the movable valve member 52 during engine operation.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing 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 .

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

Claims

1. A large turbocharged two-stroke compression-ignited internal combustion engine with crossheads (43), comprising: a number of cylinders (1) that serve as combustion chambers (27), said cylinders being (1) provided with a cylinder cover (22), with an exhaust valve (4) placed centrally in said cylinder cover (22), and with an exhaust duct (35) connecting said exhaust valve (4) to an exhaust gas receiver (3), said cylinder cover (22) being provided with a blow-off valve (50) with an inlet (57) of said blow-off valve (50) being fluidically connected to said combustion chamber (27) and an outlet (58) of said blow-off valve (50) being fluidically connected to a discharge conduit, said blow-off valve (50) being provided with a with movable valve member (52) that is moveable between a closed position and a fully open position with a range of intermediate positions there between, said blow-off valve (50) allows gas flow from said inlet (57) to said outlet (58) when said movable valve member (52) is in any of said intermediate positions or in said fully open position and said blow-off valve (50) preventing gas flow from said inlet (57) to said outlet (58) when said movable valve member (52) is in said closed position, said movable valve member (52) is provided with a first effective pressure surface (58) that is exposed to the pressure in said combustion chamber (27) so that the pressure in the combustion chamber (27) urges the movable valve member (52) towards said fully open position, and said movable valve member (52) having a second effective pressure surface exposed to a hydraulic pressure for urging the movable valve member (52) towards said closed position.

2. An engine according to claim 1, wherein said second effective pressure surface has a first size when said movable valve member (52) is in said closed position and a second size that is less than said first size in a first range of positions of said movable valve member (52), said first range of positions extending from said fully open position to a predetermined intermediate position.

3. An engine according to claim 2, wherein said predetermined position is closer to said closed position than to said fully open position.

4. An engine according to claim 2 or 3, wherein said second effective pressure surface is formed by a first surface (61) of said movable valve member (52) and by a second surface (62) of said moveable valve member that is smaller than said first surface (61) and oppositely directed to said first surface (61).

5. An engine according to claim 4, wherein said first surface (61) is pressurized by said hydraulic pressure in all of the positions of said movable valve member (52) and wherein said second surface (62) is pressurized by said hydraulic pressure in said first range of positions.

6. An engine according to claim 5, wherein said movable valve member (52) opens a port (71) that allows said second surface (62) to be pressurized by said hydraulic pressure in said first range of positions.

7. An engine according to any one of claims 1 to 6, wherein said movable valve member (52) is provided with a valve disk (54) that cooperates with a valve seat (55), said valve seat being arranged in a blow-off bore (29) that extends through said cylinder cover (22).

8. An engine according to any one of claims 1 to 7, wherein a main plane of said valve seat (55) is arranged squint relative to a main direction of a said blow-off bore (29).

9. An engine according to any one of claims 1 to 8, wherein said movable valve member (52) comprises a valve disk (54) connected to a valve stem (53) and an actuation piston (56) operatively connected to said valve stem (53).

10. An engine according to any one of claims 1 to 9, wherein said first surface (61) is arranged on one side of said actuation piston (56) and said second surface (62) is arranged on the opposite side of said actuation piston (56) .

11. An engine according to any one of claims claim 1 to 10, wherein said blow-off bore (29) is connected to said exhaust duct (35) or to said exhaust gas receiver (3) via a blow-off pipe (36) in order to bypass said exhaust valve (4) .

12. An engine according to any one of claims 1 to 11, wherein said blow-off valve (50) is provided with cooling means, said cooling means preferably including path for a cooling medium through the blow-off valve (50).

13. An engine according to any one of claims 1 to 12, wherein said engine comprises a hydraulic system (88) with a pressure that increases with increasing engine load and decreases with decreasing engine load and wherein said second effective pressure surface (61,62) is pressurized with the hydraulic pressure of said hydraulic system.

14. An engine according to claim 13, wherein said hydraulic system (88) powers the fuel injection valves (42).

15. An engine according to claim 13 or 14, wherein said hydraulic system (88) powers the exhaust valve actuating system (48).