WO2024032899A1

WO2024032899A1 - A large turbocharged two-stroke uniflow crosshead internal combustion engine

Info

Publication number: WO2024032899A1
Application number: PCT/EP2022/072636
Authority: WO
Inventors: Stefan Mayer
Original assignee: Man Energy Solutions, Filial Af Man Energy Solutions Se, Tyskland
Priority date: 2022-08-12
Filing date: 2022-08-12
Publication date: 2024-02-15

Abstract

Described is a large turbocharged two-stroke uniflow crosshead internal combustion engine, which has at least one mode of operation in which the main fuel is a low flash point fuel, such as ammonia, ethane, LPG or DME, which main fuel is injected at high pressure according to the diesel principle, the engine comprising at least one cylinder with a cylinder liner (1), a reciprocating piston (10) therein and cylinder cover (22) covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston (10) and the cylinder cover (22), a low flash point fuel system (30) configured for supplying pressurized fuel to at least one fuel valve (50) that is arranged in the cylinder cover (22) or in the cylinder liner (10), and a purging system for evacuation of the low flash point fuel. The engine is peculiar in that said purging system comprises means (42, 43, 44, 45) for depressurization and drainage of liquid fuel from the low flash point fuel system to a tank (57) in a first evacuation step and a vacuum pump system (71) for lowering the pressure and pumping out the remaining fuel from the low flash point fuel system to a tank (61, 57) in a second evacuation step. Hence, the low flash point fuel, such as ammonia, can be evacuated from the fuel system in a safe and economic way, where the low flash point fuel is collected in a tank for reuse as fuel and a costly and complicated handling of a mixture of nitrogen and fuel is avoided, as no inert gas, such as nitrogen is used for purging. Furthermore, the vacuum pump system pumps pure fuel gas with no contamination from a purging gas, and accordingly, said fuel gas can still be used as a fuel, if requested.

Description

A LARGE TURBOCHARGED TWO-STROKE UNIFLOW CROSSHEAD INTERNAL

COMBUSTION ENGINE

TECHNICAL FIELD

The present invention relates to a large turbocharged two-stroke uniflow crosshead internal combustion engine, which has at least one mode of operation in which the main fuel is a low flash point fuel, such as ammonia, ethane, LPG or DME, which main fuel is injected at high pressure according to the diesel principle, the engine comprising at least one cylinder with a cylinder liner, a reciprocating piston therein and cylinder cover covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston and the cylinder cover, a low flash point fuel system configured for supplying pressurized fuel to at least one fuel valve that is arranged in the cylinder cover or in the cylinder liner, and a purging system for evacuation of the low flash point fuel.

BACKGROUND OF THE INVENTION

Large turbocharged two-stroke uniflow crosshead internal combustion engine are typically used as prime movers in large ocean going ships, such as container ships or in power plants. Very often, these engines are operated with heavy fuel oil or with fuel oil, such as diesel.

Recently, there has been a demand for large two-stroke diesel engines to be able to handle alternative types of fuel, such as ammonia, ethane, LPG, DME and/or other similar fuels. An engine, which is operable in both a fuel oil mode, in which it is operated only on fuel oil and an alternative fuel mode, in which it is operated on alternative fuel and pilot fuel oil, is often referred to as a dual fuel engine.

Dual fuel engines for low flash point fuels all have separate fuel systems for these highly volatile fuels. In all cases equipment needs to be present to completely drain the fuel from the fuel system, i.e. fuel pipes, valves, pumps, etc., in order to make the engine safe for shutdown, fuel shift to fuel oil or for maintenance.

Known dual fuel engines comprise a purging system for evacuation of the low flash point fuel from the fuel system. Such purging system ensures initially a depressurization of the fuel system and then followed by purging with nitrogen. The purging with nitrogen has the disadvantage that a mixture of nitrogen and fuel gas needs to be handled and disposed. Furthermore the nitrogen needs to be produced on board, which has a cost boost in terms of OPEX and CAPEX.

EP3203053 Bl describes a gas turbine application, where a gaseous fuel is injected. The fuel line is purges by a vacuum pump. US9732713 BB describes a dual fuel engine where the gas mode is run at low pressure, where gaseous fuel is admitted in the proximity of air intake ports. Thus, the known gas turbine and the known dual fuel engine run on gaseous fuel and in both the combustion is of the premixed type, where the gaseous fuel is admitted at low pressure into the combustion chamber while the piston is on its way towards the TDC. In the case of ammonia, ethane, LPG and DME, these secondary or alternative fuels are gaseous at atmospheric pressure. Accordingly, depressurization alone is enough to evacuate most of the gas out of the fuel system. In order to ensure that also the remaining gaseous fuel gas is removed, a vacuum pump is employed to evacuate all remaining fuel from the fuel system, instead of purging with nitrogen, as common practice.

Opposite to the engines described in the above prior art, in the internal combustion engine according to the present invention, the alternative fuel in form of a low flash point fuel, such as ammonia, ethane, LPG and DME, is injected at high pressure according to the diesel principle and is in liquid state at the injection pressure. Hence, as the low flash point fuel is in liquid state it would require a relatively large amount of energy to evacuate it solely by means of a vacuum pump, because all the liquid fuel should be evaporated by such method in order to be evacuated from the fuel system.

The invention also relates to a method for operating a large turbocharged two-stroke uniflow crosshead internal combustion engine as described above and claimed in the attached claims.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a large turbocharged two-stroke uniflow crosshead internal combustion engine of the kind mentioned in the introduction, where the above mentioned challenges relating to purging of the low flash point fuel from the fuel system are at least significantly reduced.

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 uniflow crosshead internal combustion engine, which has at least one mode of operation in which the main fuel is a low flash point fuel, such as ammonia, ethane, LPG and DME, which main fuel is injected at high pressure according to the diesel principle, the engine comprising at least one cylinder with a cylinder liner, a reciprocating piston therein and cylinder cover covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston and the cylinder cover, a low flash point fuel system configured for supplying pressurized fuel to at least one fuel valve that is arranged in the cylinder cover or in the cylinder liner, and a purging system for evacuation of the low flash point fuel, and being characterized in that said purging system comprises means for depressurization and drainage of liquid fuel from the low flash point fuel system to a tank in a first evacuation step and a vacuum pump system for lowering the pressure and pumping out the remaining fuel from the low flash point fuel system to a tank in a second evacuation step.

Hence, the low flash point fuel can be evacuated from the fuel system in a safe and economic way, where the low flash point fuel may be collected in a tank for reuse as fuel and a costly and complicated handling of a mixture of nitrogen and fuel is avoided, as no inert gas, such as nitrogen is used for purging. Furthermore, the vacuum pump system pumps pure fuel gas with no contamination from a purging gas, and accordingly, said fuel gas can still be used as a fuel, if requested. In the case the low flash point fuel is ammonia the lack of an inert purging gas, such as nitrogen is particularly advantageous, because the subsequent scrubbing is made much easier, as the scrubber do not have to be dimensioned for ventilation with nitrogen and can therefore be made much smaller and cheaper. An additional advantage is that start of engine running on low flash point fuel will be much less tricky, simply because no nitrogen/air is present, when low flash point fuel is allowed into the fuel system. Hence it is not necessary to de-aerate the fuel lines and therefore the inclusion and design of design a return or recirculating fuel supply line to the fuel gas valves can be made unnecessary. The means for depressurization and drainage of liquid fuel from the low flash point fuel system to the tank during the first evacuation step may comprise any suitable means, however it is preferred that said means comprise a valve, such as an on/off valve, a butterfly valve or similar, which opens into a pipe line that connects the low flash point fuel system to the tank.

The vacuum pump system for lowering the pressure and pumping out the remaining fuel from the low flash point fuel system to the tank in the second evacuation step may comprise any suitable vacuum pump, however it is preferred that said vacuum pump is of positive displacement type, such as a rotary vane pump or similar.

The purging system may comprise a first and a second pipe line, which both are connected to the low flash point fuel system, where the first pipe line is used during the first evacuation step to conduct liquid low flash point fuel to the tank and the second pipe line is connected to the vacuum pump and is used during the second evacuation step.

The first and the second pipe line are joint into a single pipe line, which is connected to a common tank or are individually connected to a common . In the latter the gaseous low flash point fuel in the second pipe line needs to be liquefied and accordingly the second pipe line may comprise liquefying means, e.g. in form of pressurizing or cooling means, to liquefy gaseous low flash point fuel.

The first and a second pipe line may instead be individually connected to each their tank.

In one embodiment the purging system may comprise a knockout drum, that is configured to separate the liquid fraction from the gaseous fraction of the low flash point fuel. In such embodiment, the purge system may also comprise means for conducting both low flash point fuel fractions to same or separate tank(s). In such an embodiment, the means for conducting the gaseous low flash point fuel fraction may comprise liquefying means, e.g. in form of pressurizing and/or cooling means.

However, in such an embodiment comprising a knockout drum and in case the low flash point fuel is ammonia or another toxic fuel, the purge system may comprise means for conducting the gaseous ammonia fraction to an ammonia absorption system. The ammonia absorption system may comprise at least one pressure vessel that during use is at least partially filled with water for absorbing the ammonia into the water to form ammonia water. Ammonia water, also referred to as aqueous ammonia, is a solution of ammonia in water. The ammonia absorption system may also or instead comprise at least one absorption tank. If it comprises more than one, these are preferably arranged as a cascade in series.

In case the low flash point fuel is a non-toxic fuel, such as ethane, LPG or DME. The gas fraction from the knockout drum may simply be vented to the atmosphere.

The evacuation and drainage of the low flash point fuel from the fuel system will in the first step be partly driven by gravity, due to sloping fuel lines and partly driven by the partial pressure of the fuel being above 1 bar absolute. Typically, the pressure in the low flash point fuel system is about 30 to 80 bar pressure during operation of the engine on said fuel. At the end of the depressurization and drainage step the fuel system will be filled with a two-phase blend of fuel with wet inner pipe surfaces and pockets of liquid fuel at places, which may not be drained by gravity. Once the main part of the low flash point fuel has been evacuated from the fuel system by drainage to a tank the remaining liquid fuel in the fuel system adhered on inner pipe surface, present in liquid pockets, valves etc. will be evaporated due to the lower pressure being established in the fuel system by the vacuum pump system in the second evacuation step and is pumped out of the fuel system to a tank.

To completely emptying the fuel system from low flash point fuel, the purge system may further comprise a ventilating system for forcing atmospheric air through the fuel lines, said ventilating system comprising least an valve to an air inlet or to a source of pressurized air or inert gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more details with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is an elevated front view of a large two-stroke diesel engine according to an example embodiment. Fig. 2 is an elevated side view of the large two-stroke engine of Fig. 1.

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

Fig. 4 is a diagrammatic representation of the engine with a low flash point fuel system and a low flash point fuel purging system.

DETAILED DESCRIPTION

In the following detailed description, the invention will be described for a large turbocharged two-stroke uniflow crosshead internal combustion engine, but it is understood that the internal combustion engine could be of another type. The large turbocharged two-stroke uniflow crosshead internal combustion engine is of the high-pressure type in which fuel is injected at or near top dead center of the pistons and is compression ignited typically by means of a pilot ignition with an ignition fluid, e.g. fuel oil, for ensuring reliable ignition. In the detailed description, the low flash point fuel is referred to as ammonia, however it may as well be ethane, LPG or DME.

Figs. 1, 2, and 3 show a large low-speed turbocharged two-stroke diesel engine with a crankshaft 8 and crossheads 9. Fig. 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 in line. Large low-speed turbocharged two-stroke diesel engines have typically between four and fourteen cylinders in line, carried by a cylinder frame 23 that is carried by an engine frame 11. The engine may e.g. be used as the main engine in a 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 1,000 to 110,000 kW.

The engine is in this example embodiment a dual-fuel compression-ignited engine of the two- stroke uniflow type with scavenging ports 18 in the lower region of the cylinder liners 1 and a central exhaust valve 4 at the top of each cylinder liner 1. In this shown example, the engine has at least one ammonia mode in which the engine is operated on ammonia fuel or an ammonia- based fuel and at least one conventional fuel mode in which the engine is operated on conventional fuel, e.g. fuel oil (marine diesel), or heavy fuel oil.

The scavenge air is passed from the scavenge air receiver 2 to the scavenge ports 18 of the individual cylinders 1. A piston 10 that reciprocates in the cylinder liner 1 between the bottom dead center (BDC) and top dead center (TDC) compresses the scavenge air. Fuel in form of ammonia in the ammonia mode is injected through high-pressure fuel valves 50 that are arranged in the cylinder cover 22 into the combustion chamber in the cylinder liner 1 at or near TDC. Combustion follows and exhaust gas is generated. Each cylinder cover 22 is in the shown embodiment provided with two valves 50, however it may as well be provided with only one or more fuel valves 50. The fuel valves 50 are either configured to inject only one specific type of fuel, e.g. ammonia and in this case, there will also be one or more fuel valves 50 for injecting conventional fuel into the combustion chamber. Hence, the engine will have two or more fuel valves. In case the fuel valves 50 are suitable for inj ecting both ammonia and suitable for inj ecting conventional fuel there can be one or more fuel valves 50 for each cylinder. The fuel valves 50 are arranged in the cylinder cover 22 around the central exhaust valve 4. Timed ignition at or near TDC is triggered by spark, laser, ignition fluid injection, or the like. In one embodiment, typically smaller fuel valves (not shown) are provided in the cylinder cover for injecting ignition fluid, for ensuring reliable ignition of the ammonia fuel. The ignition fuel is ordinary diesel fuel but can also be another form of ignition enhancer, such as dimethyl ether (DME) or hydrogen. Since the engine can be a dual-fuel engine it can also be provided with a conventional fuel supply system (not shown) for supplying the conventional fuel to the fuel valves 50. In another embodiment this main fuel injection system can also be used to ensure the ignition of the ammonia fuel.

When an exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct associated with the cylinders into the exhaust gas receiver 3 and onwards through a first exhaust conduit 19 via a Selective Catalytic Reduction (SCR) reactor 28 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away through a second exhaust conduit via an economizer 20 to an outlet 21 and into the atmosphere. The SCR reactor reduces NOx emissions.

Through a shaft, the turbine 6 drives a compressor 7 supplied with fresh air via an air inlet 12. The compressor 7 delivers pressurized scavenge air to a scavenge air conduit 13 leading to the scavenge air receiver 2. The scavenge air in the scavenge air conduit 13 passes an intercooler 14 for cooling the scavenge air.

The cooled scavenge air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the scavenge air flow when the compressor 7 of the turbocharger 5 does not deliver sufficient pressure for the scavenge air receiver 2, i.e. in low or partial load conditions of the engine. At higher engine loads the turbocharger compressor 7 delivers sufficient compressed scavenge air and then the auxiliary blower 16 is bypassed via a non -return valve 15 and the electric motor 17 is deactivated.

The engine is in the ammonia mode operated with ammonia as the main fuel which is supplied to the ammonia valves 50 at a substantially stable pressure and temperature. The ammonia is supplied to the ammonia valves 50 in the liquid phase and may be aqueous ammonia (ammonia- water blend).

The conventional fuel system is well known and not shown and described in further detail. The ammonia fuel system 30 supplies the fuel valves 50 with liquid phase ammonia at a medium supply pressure (e.g. 30 to 80 bar pressure). The engine is of the compression-igniting type and the fuel valves 50 comprise a pressure booster that significantly raises the pressure of the ammonia fuel from the medium pressure to a high pressure to allow the ammonia fuel to be injected at a pressure well above the compression pressure of the engine. Typically, the injection pressure for an ignition-compressing engine is above 300 bar.

With reference to Fig. 4, the ammonia fuel system 30 including a purging system is disclosed in greater detail. Ammonia is stored in the liquid phase in a pressurized storage tank 31 at approximately 17 bar. Ammonia can be stored in the liquid phase at a pressure above 8.6 bar and an ambient temperature of 20°C in the ammonia storage tank 31. However, ammonia is preferably stored at approximately 17 bar or higher to keep it in the liquid phase when the ambient temperature increases.

A low-pressure ammonia supply line 32 connects an outlet of the ammonia storage tank 31 to the inlet of a medium pressure feed pump 35. A low-pressure feed pump 33 forces the liquid phase ammonia from the storage tank 31, through a filter arrangement 34 to an inlet of the medium pressure feed pump 35. The medium pressure feed pump 35 forces the liquid ammonia through a medium pressure ammonia supply line 36 to the fuel valves 50. A portion of the liquid ammonia that is supplied to the fuel valves 50 is injected into the combustion chambers of the engine whilst another portion of the liquid ammonia that is supplied to the fuel valve 50 is returned to an ammonia return line 38 that connects a return port of the fuel valves 50 to the low-pressure supply line 32. Thus, a portion of the liquid ammonia fuel is recycled to the inlet of the medium pressure feed pump 35.

When operation on ammonia fuel is discontinued, for example due to a failure in the ammonia fuel system 30, or another reason for switching to conventional fuel, the ammonia fuel system 30 is purged to remove the ammonia from the system. Hereto, a purging system as further explained below is utilized.

A first purge line 42 that includes a first purge valve 43 connects the medium pressure ammonia supply line 36 to a knockout drum 46. Optionally, if a return line is installed to the system, as shown in Fig. 4., a second purge line 44 that includes a second purge valve 45 connects the ammonia return line 38 to the knockout drum 46. In a purging operation, the first and second purging valves 43, 45 are opened, and the residual ammonia fuel is driven from the ammonia fuel supply and return lines 36, 38 into the knockout drum 46 first by surplus pressure and gravity in the ammonia fuel supply system 30 in a first evacuation step and then by means of a vacuum pump 71 in a second evacuation step. The knockout drum 46 is configured to separate the liquid phase ammonia fuel from gas phase ammonia. A liquid phase ammonia outlet is arranged in the lower area of the knockout drum 46 and connects to a recovery tank 57 via a pipe line 72 and a valve 59. A gaseous ammonia vent line 48, which includes a valve 49 connects the interior of the knockout drum 46 to the surroundings for venting gaseous ammonia from the recovery tank 57 back to the knockout drum 46. The liquid ammonia in the recovery tank 57 is in an embodiment, not shown, conveyed to the ammonia storage tank 31 for being used as ammonia fuel. A gas phase ammonia outlet of the knockout drum 46 is connected via third purge line 47 to the ammonia absorption system 60.

During the first evacuation step a valve 73 in the third purge line 47 is opened and a valve 58 arranged in connection with the vacuum pump 71 is closed. In this way any gaseous ammonia in the knock out drum 46 may be driven by surplus pressure to the ammonia absorption system 60. During the second evacuation step, valve 73 is closed and valve 58 is opened. Then the vacuum pump 71 is started in order to pump the remaining ammonia fuel from the ammonia fuel system and purging system, comprising ammonia fuel lines 36, 38, purge lines 42, 44, knockout drum 46 and purge line 47, to the ammonia absorption system 60.

In order to completely emptying the ammonia fuel system and forcing any remaining ammonia to the ammonia absorption system 60, the purge system may further comprise a ventilating system for forcing atmospheric air through the fuel lines 36, 38, purge lines 42, 44, 47 and knockout drum 46, said ventilating system comprising at least a valve 41 opening to an air inlet or to a pressurized source 40 of air or inert gas. It might be necessary several times to repeat the second evacuation step lowering the pressure again by means of the vacuum pump system for and then forcing atmospheric air through the system.

The ammonia absorption system 60 comprises in the shown embodiment a cascade of three absorption tanks 61, 63 and 65 in series, which during use is at least partially filled with water for absorbing the ammonia into the water to form ammonia water.

Ammonia water, also referred to as aqueous ammonia, is a solution of ammonia in water.

During purging operation gas phase ammonia is routed via purge line 47 to the cascade of three absorption tanks in series, comprising a first absorption tank 61, a middle absorption tank 63, and a last absorption tank 65.

The last absorption tank 65 is provided with a fourth vent 66 that connects the last absorption tank 65 to the surroundings. In an embodiment, there are be more than three absorption tanks to obtain a lower concentration of ammonia in the atmosphere above the water in the last absorption 10, and hence a lower concentration of ammonia in the gases vented through the fourth vent 66.

The absorption efficiency of the cascade of absorption tanks is maintained by regular replacement of the water in the last absorption tank 65 from the pressurized source of (fresh) water 71 and reusing the slightly contaminated water in the upstream tanks. Hence, the water in the last tank 65 in which some ammonia is absorbed that is replaced by water from the source of water 71 is transported to the middle absorption tank 63 through a first water return line 67 that is controlled by a first water return valve 68. Similarly, water from the middle absorption tank 63 is transported to the first absorption tank 61 through a second water return line 69 that is controlled by a second water return valve 70. The system is configured to compensate for the evaporation of water in the absorption tanks 61, 63, 65, and 4 ammonia water removed from the first ammonia tank 61, i.e. the water level in the absorption tanks 61, 63, 65 is kept between a minimum and maximum indicated by the dashed lines in Fig. 4.

The ammonia fumes above the water in the first absorption tank 61 flow via a first ammonia vent line 62 to the middle absorption tank 63. The ammonia fumes above the water in the middle absorption tank 63 flow via a second ammonia vent line 64 to the last absorption tank 65. The process is preferably driven by the pressure of the purge process.

The ammonia concentration in the fourth vent 66 is sufficiently low to allow venting to the surroundings. However, if needed to comply with regulations the ammonia emissions from the vent mast can be further decreased by introducing an additional absorption column where the absorbing media is an acid. The acid protonates the NH3(aq) under the formation of ammonium hydroxide and thus reduces the ammonia that is vented atmosphere.

The resulting ammonia concentration in the water in the first absorption tank 61 is higher than the ammonia concentration in the water in the middle absorption tank 63 and the ammonia concentration in the water in the middle absorption tank 63 is higher than the ammonia concentration in the water in the last ammonia absorption tank 65.

The ammonia water of the first absorption tank 61 is removed from the first absorption tank 61 through a first ammonia water return line 51 that includes a return pump 52. A second ammonia water return line 53 that includes a return pump 52’ and a first return valve 54 connects the first ammonia water return line 51 to the low-pressure ammonia supply line 32. Thus, when the first return valve 54 is opened the ammonia water with the relatively high ammonia concentration originating from the first absorption tank 61 is mixed with the fuel coming from the ammonia storage tank 31 and the ammonia that is absorbed by the ammonia absorption system 60 is thereby be reused as fuel for the engine. A third ammonia water return line 55 that includes a second return valve 56 connects the first return 51 to a reductant inlet associated with the SCR reactor 28. The reductant inlet can be part of the SCR reactor 28 or be arranged in the exhaust gas flow path upstream of the SCR reactor 28. Thus, when the second return valve 56 is open the ammonia that is absorbed by the ammonia absorption system 60 is used as a reductant in the SCR reactor 28.

Cascade of water tanks 61, 63, 65 system is fully passive, i.e. that no pumps or other axillary systems need to be available when a shutdown absorption of ammonia is required. Hence, the system will inherently be reliable and available when needed.

The low-pressure ammonia fuel line 32, the medium pressure ammonia fuel line 36, and the ammonia return line 38 are in embodiment completely or partially constructed of double-walled piping with a space between an inner pipe and an outer pipe. Preferably, a detection system is provided that detects the presence of ammonia in the space between the inner and outer tubing, allowing for discontinuation of the operational ammonia if ammonia is detected in the space, followed by a subsequent purging of the ammonia fuel system and absorption of the residual ammonia into the absorption purging system 60.

An electronic control unit 100 is connected via signal lines or wirelessly to the pumps and valves of the fuel system 30, the purging system and the and the ammonia absorption 60. The electronic control unit 100 is configured to control these components, e.g. by regulating the speed of the pumps and by controlling the opening and closing of the respective valves, to ensure the operation of the fuel system and the purging and absorption system as described above.

Claims

1. A large turbocharged two-stroke uniflow crosshead internal combustion engine, which has at least one mode of operation in which the main fuel is a low flash point fuel, such as ammonia, ethane, LPG or DME, which main fuel is injected at high pressure according to the diesel principle, the engine comprising at least one cylinder with a cylinder liner (1), a reciprocating piston (10) therein and cylinder cover (22) covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston (10) and the cylinder cover (22), a low flash point fuel system (30) configured for supplying pressurized fuel to at least one fuel valve (50) that is arranged in the cylinder cover (22) or in the cylinder liner (10), and a purging system for evacuation of the low flash point fuel, characterized in that said purging system comprises means (42, 43, 44, 45) for depressurization and drainage of liquid fuel from the low flash point fuel system to a tank (57) in a first evacuation step and a vacuum pump system (71) for lowering the pressure and pumping out the remaining fuel from the low flash point fuel system to a tank (61, 57) in a second evacuation step.

2. The engine according to claim 1, characterized in that the means for depressurization and drainage of liquid fuel from the low flash point fuel system to the tank (57) during the first evacuation step comprises a valve (43, 45), such as an on/off valve, a butterfly valve or similar, which opens into a pipe line that connects the low flash point fuel system to the tank (57).

3. The engine according to claim 1, characterized in that the vacuum pump system (71) comprises a vacuum pump of a positive displacement type, such as a rotary vane pump or similar.

4. The engine according to claim 1, characterized in that the purging system comprises a first and a second pipe line (72, 47), which both are connected to the low flash point fuel system (30), where the first pipe line (72) is used during the first evacuation step to conduct liquid low flash point fuel to the tank (57) and the other pipe line (47) is connected to the vacuum pump and is used during the second evacuation step.

5. The engine according to claim 4, characterized in that the first and the second pipe line (72, 47) are joint into a single pipe line, which is connected to a common tank.

6. The engine according to claim 4, characterized in that the first and the second pipe line (72, 47) are individually connected to a common .

7. The engine according to claim 6, characterized in that the second pipe line (47) comprises liquefying means, e.g. in form of pressurizing or cooling means, to liquefy gaseous low flash point fuel.

8. The engine according to claim 4, characterized in that the first and a second pipe line (72, 47) are individually connected to each their tank (57, 61).

9. The engine according to any of the preceding claims, characterized in that the purging system comprises a knockout drum (46), that is configured to separate the liquid fraction from the gaseous fraction of the low flash point fuel.

10. The engine according to claim 9, characterized in the purge system comprise means (47) for conducting the gaseous low flash point fuel fraction, such as ammonia to an ammonia absorption system (60), which comprises at least one absorption tank (61, 63, 65).