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, methanol, ethanol, DME, methane, ethane or LPG, 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 comprising fuel supply and return lines and 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 from the fuel supply and return lines.

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, methanol, ethanol, DME, methane, ethane or LPG 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 and US9732713 BB describes a dual fuel engine where gaseous fuel, such as a low flash point fuel as ammonia, is admitted or injected into the combustion zone during gas mode operation. In both documents, the fuel line is purges by means of a vacuum pump system. A vacuum pump for ammonia exists, but it will not be either simple or cheap for use in connection with a large turbocharged two-stroke uniflow crosshead internal combustion engine. Further, a prolonged purging time may be expected due to small amounts of liquid in form of NH3 possibly with a little amount of water left in the pipes will have to be boiled off. In addition, problems may arise keeping the piping system of the low flash point fuel system tight due to the cycling between negative pressure during purging and overpressure during gas mode operation, which will probably stress all seals.

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, methanol, ethanol, DME, methane, ethane or LPG, 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 comprising fuel supply and return lines and 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 from the fuel supply and return lines, and being characterized in that said purging system comprises means for directing liquid, such as water or diesel, through at least the fuel supply and return lines of the low flash point fuel system.

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. 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.

Furthermore, some low flash point fuels, such as ammonia, methanol, ethanol and DME are soluble in water, whereas others, such as methane, ethane and LPG are not. Accordingly, if the liquid used for in the purging system is water, the water directed through the low flash point fuel system, would either mix with first mentioned or simply push last mentioned in front and carry no contamination from a purging gas.

In case the low flash point fuel are insoluble in water, such as methane, ethane and LPG the liquid used for in the purging system may be diesel or a similar liquid.

The means for directing liquid, such as water or diesel, through the fuel supply and return lines of the low flash point fuel system may comprise any suitable means, however it is preferred that said means comprise a liquid reservoir, a liquid pipe line, which connects the liquid reservoir with the low flash point fuel system and a valve, such as an on/off valve, a butterfly valve or similar, for opening and closing said liquid pipe line.

In one embodiment of the invention, the liquid reservoir may be arranged at a height that provides the liquid with sufficient potential energy to direct it through the low flash point fuel system. In such embodiment, the means for directing liquid through the low flash point fuel system may in addition comprise a liquid pump arranged in the liquid pipe line. In an alternative embodiment, the liquid reservoir may be arranged at any suitable height and the means for directing liquid through the low flash point fuel may comprise a liquid pump arranged in connection with the liquid pipe line.

In an embodiment of the invention it may be advantageous to drain off some of the fuel from at least the fuel supply and return lines of the low flash point fuel system before liquid is directed through these lines, especially in cases where fuel is in liquid form and/or at a certain positive pressure. In such an embodiment, the purging system may comprise means for depressurization and drainage of liquid fuel from the low flash point fuel system to a tank. Such means for depressurization and drainage of liquid fuel from the low flash point fuel system to the tank 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 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 to conduct liquid low flash point fuel to the tank and the second pipe line may be connected to a low flash point fuel absorption system in case the gaseous low flash point fuel is toxic, such as gaseous ammonia, or to a vent in case the gaseous low flash point fuel is not toxic.

In cases where the low flash point fuel would be gaseous at atmospheric conditions the evacuation and drainage of the low flash point fuel from the fuel system may be conducted in a two-step routine, where the evacuation in the first step is partly driven by gravity, due to sloping fuel lines and/or 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 this depressurization and drainage step the fuel system will often 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 part of the low flash point fuel has been evacuated from the fuel system by drainage to the tank the remaining will be forced out of the low flash point fuel system by directing liquid, such as water or diesel, through the system in the second evacuation step and forwarded to the tank. In case the low flash point fuel is liquid at atmospheric pressure and usual ambient temperatures, like methanol or ethanol, it may simply be drained to a tank in the first step driven by gravity.

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. Before atmospheric air is forced through the fuel lines, it may be advantageous to drain out as much water as possible by gravity to a drainage 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 expanded low flash point fuel. Such an embodiment is especially relevant for low flash point fuels, such ammonia, LPG and DME. 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, the purge system may comprise means for conducting the gaseous low flash point fuel fraction to an low flash point fuel absorption system, which is of particular relevance when the low flash point fuel is ammonia. Thus, in case the low flash point fuel is ammonia, the low flash point fuel absorption system may comprise at least one pressure vessel that during use is at least partially filled with water for absorbing ammonia into the water to form ammonia water. Ammonia water, also referred to as aqueous ammonia, is a solution of ammonia in water. The low flash point fuel 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.

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, where the low flash point fuel is ammonia.

Fig. 5 is a diagrammatic representation of the engine with a low flash point fuel system and a low flash point fuel purging system, where the low flash point fuel is methanol or ethanol.

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 may both be 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, or of the low- pressure type in which fuel is admitted at relatively low pressure when the piston is on its way towards the top dead center. In the detailed description with reference made to Fig. 4, the low flash point fuel is ammonia, and with reference made to Fig. 5, the low flash point fuel may either be methanol or ethanol.

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. The engine has at least one low flash point fuel mode in which the engine is operated on a low flash point fuel, such as ammonia, methanol, ethanol, DME, methane, ethane or LPG 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. In the shown embodiment, fuel in form of a low flash point fuel, such as ammonia, methanol, ethanol, DME, methane, ethane or LPG in the low flash point fuel 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. In an alternative embodiment, the low flash point fuel fuel may be admitted through fuel valves 50’ arranged in the cylinder liner somewhere between the scavenging ports 18 and TDC. Combustion follows and exhaust gas is generated. Each cylinder cover 22 is provided with 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 injecting both low flash point fuel, such as e.g. ammonia and suitable for injecting 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. Further, additional, typically smaller fuel valves (not shown) are in an embodiment provided in the cylinder cover for injecting ignition fluid, for ensuring reliable ignition of the low flash point 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 emissions, in particular 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.

With reference to Fig. 4, the low flash point fuel is ammonia, thus the engine is in the low flash point fuel 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.

The ammonia fuel system 30 including a purging system is disclosed in greater detail in Fig. 4. 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 an 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 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 expands 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 water directed through the ammonia fuel supply and return lines 36, 38 from a water reservoir 40 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 a possible first evacuation step where the low flash point fuel, e.g. in form of ammonia, is partly driven by gravity, due to sloping fuel lines and partly driven by the partial pressure of the fuel being above 1 bar absolute, a valve 74 in the third purge line 47 is opened. In this way any gaseous ammonia in the knock out drum 46 may be driven by surplus pressure to the ammonia absorption system 60.

After as much of the ammonia as possible, if any, has been evacuated from the ammonia fuel system by gravity and surplus pressure, valve 41 is opened and water is allowed to flow from the water reservoir 40 via a water line 75 to and through the ammonia fuel system. In this way, the remaining ammonia fuel from the ammonia fuel system and purging system, comprising ammonia fuel lines 36, 38, purge lines 42, 44, is directed to the knockout drum 46. A water pump 73 may as shown be arranged in connection with the water line 75 to add additional energy to the water. Any gas phase ammonia in the knockout drum 46 is conducted via 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 knockout drum 46 and possibly 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 pressurized air source 81 and a valve 82 opening into the ammonia fuel system.

Before atmospheric air is forced through the fuel lines, it may be advantageous to drain out as much water (with dissolved residual low flash point fuel) as possible by gravity to a drainage tank 79. For this purpose, the purging system may, as shown in Fig. 4., comprise a water drainage line 77 and a valve 58 to drain water from purge line 47 to the drainage tank 79, and a valve 78 to drain water from fuel lines 36, 38 and purge line 42 to the drainage tank 79.

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. With reference to Fig. 5, the low flash point fuel is methanol or ethanol, thus the engine is in the low flash point fuel mode operated with methanol or ethanol as the main fuel which is supplied to the fuel valves 50 at a substantially stable pressure and temperature.

The methanol or ethanol fuel system 30 shown in Fig. 5 corresponds to the ammonia fuel system 30 described in connection with Fig. 4 including fuel storage tank 31, supply line 32, low-pressure feed pump 33, filter arrangement 34 and medium pressure feed pump 35, which forces the fuel to the fuel supply line 36.

When operation on methanol or ethanol fuel is discontinued, the methanol or ethanol fuel system 30 is purged to remove the methanol or ethanol from the fuel system. Hereto, a purging system as further explained below is utilized.

The purging operation is performed in three steps. In the first purging step, purging valves 92, 93 and 86 are opened, and the residual methanol or ethanol fuel is driven from the methanol or ethanol fuel supply and return lines 36, 38 into a first drainage tank 87 by gravity and possibly pressure feed pump 35, if not by-passed. Then purging valve 86 is closed. In the second purging step, venting valves 85 and 91 in venting lines 84 and 90, respectively are opened and water is directed to the methanol or ethanol fuel supply and return lines 36, 38 from a water reservoir 40, possibly assisted by water pump 73 arranged in water line 75, so that the methanol or ethanol fuel supply and return lines 36, 38 are completely filled with water. Thereafter, in the third purging step, purging valve 88 is opened and the mixture of water and methanol or water and ethanol is drained from the methanol or ethanol fuel supply and return lines 36, 38 into a second drainage tank 89 by gravity and possibly pressure feed pump 35, if not by-passed. After complete drainage of the fuel supply and return lines 36, 38, purging valve 88 is closed. Purging step two and three may be repeated as necessary to obtain the required purging of the fuel supply and return lines 36, 38.

The first drainage tank 87 will contain approximately clean methanol or ethanol fuel, whereas the second drainage tank 89 will contain water with a low concentration of methanol or ethanol. The clean methanol or ethanol fuel in the first drainage tank may advantageously be returned to the fuel storage tank 31 for use as fuel. The mixture of water and methanol or ethanol in the second drainage tank 89 may also be returned to the fuel storage tank 31. In one embodiment of the invention, the system may only comprise one drainage tank.

In case the low flash point fuel is insoluble in water, such as methane, ethane and LPG the liquid used for in the purging system may be diesel or a similar liquid, and the purging system may essentially correspond to the system shown in Fig. 4 except that the tank 40 should instead contain diesel or a similar liquid. Further, the need of an absorption system is limited and may be omitted.