Piston engine and method of operating a piston engine

Abstract

The piston engine (1) is operable in a first operating mode us- ing first fuel supplied via a first fuel supply line (10), and in a second operating mode using gaseous fuel supplied via a sec- ond fuel supply line (20), the engine (1) comprises a controlla- ble venting valve (9) connected to the second fuel supply line (20) upstream from gas admission valves (5) and configured to allow venting of the second fuel supply line (20), and at least one of the gas admission valves (5) of the engine (1) is con- figured to allow flow of pressurized intake air via the gas ad- mission valve (5) into the second fuel supply line (20) and fur- ther out of the second fuel supply line (20) via the venting valve (9) when the engine (1) is operated in the first operating mode to warm up the second fuel supply line (20).

Description

[0001] Piston engine and method of operating a piston engine

[0002] Technical field of the invention

[0003] The present invention concerns a piston engine, as defined in claim 1. The invention also concerns a method of operating a piston engine.

[0004] Background of the invention

[0005] The use of gaseous fuels for operating piston engines brings many benefits, such as reduced emissions. However, leaking gaseous fuel poses a safety risk, and therefore certain protective measures are needed in engines that are operable using gaseous fuels. For instance, typically gaseous fuels need to be conducted in double-wall pipes. A double-wall pipe comprises an inner pipe, in which the fuel is conducted, and an outer pipe forming a protective sheath around the inner pipe. If there is a leakage in the inner pipe, the leaking fuel is captured by the outer pipe. The space between the inner pipe and the outer pipe is typically ventilated to a safe place. In addition, the space can be monitored by a gas detector and an alarm is triggered if gas leakage is detected. In certain cases the space between the inner pipe and the outer pipe may be filled with inert gas.

[0006] Gaseous fuels should not be allowed to condensate in the fuel gas supply system. Condensation of the fuel is a risk in particular if ammonia, which has a relatively high boiling point, is used as fuel. To avoid condensation of the gaseous fuel, the temperature of the gas and the pipes the gas is in contact with need to be high enough. When the engine is operating using the gaseous fuel, condensation can be avoided by heating the gaseous fuel before it is introduced into the fuel supply system. This will maintain a high enough pipe temperature to avoid condensation. However, prior starting of the operation using the gaseous fuel, the temperature of the pipes may be lower than the condensation point of the gas. When the gas enters a cold pipe, the gas cools down and condensation might occur on the surface of the pipe. One way to avoid this problem is to arrange trace heating of the pipe with an electrical heating wire around the pipe. This can be complicated to arrange for double wall pipes, especially for the pipes located on the engine. Another solution is to heat air with a dedicated heater and circulate the heated air through the annular space of the double wall pipe. However, this makes the system complicated and is also problematic if the annular space is filled with inert gas.

[0007] Summary of the invention

[0008] An object of the invention is to provide an improved piston engine. Another object of the invention is to provide an improved method of operating a piston engine.

[0009] The piston engine according to the invention comprises a first fuel supply line for supplying first fuel into the cylinders of the engine for operating the engine using the first fuel as main fuel, and a second fuel supply line for supplying second, gaseous fuel from a gas source into the cylinders of the engine for operating the engine using the gaseous fuel as main fuel, the second fuel supply line comprising for each cylinder of the engine a gas admission valve that is configured to allow the gaseous fuel to be introduced directly or indirectly into the cylinder. The engine is configured to be operable at least in a first operating mode, in which mode the engine is operated using the first fuel supplied into the cylinders of the engine via the first fuel supply line as main fuel, and in a second operating mode, in which mode the engine is operated using gaseous fuel supplied into the cylinders of the engine via the second fuel supply line as main fuel. The engine comprises a controllable venting valve that is connected to the second fuel supply line upstream from the gas admission valves and configured to allow venting of the second fuel supply line, and at least one of the gas admission valves is configured to allow flow of pressurized intake air of the engine via said gas admission valve into the second fuel supply line and further out of the second fuel supply line via the venting valve when the engine is operated in the first operating mode to warm up the second fuel supply line.

[0010] The method according to the invention for operating the engine defined above comprises the simultaneous steps of operating the engine in the first operating mode, keeping the venting valve open to allow flow out of the second fuel supply line, and allowing flow of pressurized intake air via at least one of the gas admission valves into the second fuel supply line.

[0011] The temperature of the intake air of the engine rises as the intake air is pressurized for example by means of one or more turbochargers. By introducing the warm air into the second fuel supply line, the temperature of the second fuel supply line can be increased before the engine is switched to operation in the second operating mode, and condensing of the gaseous fuel on the surfaces of the second fuel supply line can thus be avoided. The invention provides a very simple way of heating the second fuel supply line. The gas admission valves may be actively controlled to supply pressurized intake air into the second fuel supply line. However, in some cases inherent leakage of the gas admission valves creates sufficient backflow to warm up the second fuel supply line.

[0012] According to an embodiment of the invention, the second fuel is ammonia. For example at a pressure of 10 bar, the boiling point of ammonia is 25 °C, and ammonia can thus easily condense on the surfaces of the second fuel supply line. This can be avoided by the present invention.

[0013] According to an embodiment of the invention, the first fuel is liquid fuel.

[0014] According to an embodiment of the invention, the gas admission valves are arranged to introduce the gaseous fuel into the cylinders via the inlet ports of the cylinders.

[0015] According to an embodiment of the invention, the gas admission valves are arranged in the cylinder heads of the engine. Alternatively, the gas admission valves could be arranged to introduce the gaseous fuel into inlet ducts between a charge air receiver and the cylinder heads.

[0016] According to an embodiment of the invention, the venting valve is connected to the second fuel supply line between the gas admission valves and a main shut-off valve that is configured to control gas flow from the gas source to the cylinders of the engine. The venting valve is preferably connected to the second fuel supply line close to the main shut-off valve to allow a major part of the second fuel supply line to be heated.

[0017] According to an embodiment of the invention, the engine comprises a flow control unit configured to regulate the pressure and / or the flow rate of the gaseous fuel to the gas admission valves, and the venting valve is connected to the second fuel supply line upstream from the flow control unit. According to an embodiment of the invention, at least part of the second fuel supply line is a double-walled fuel supply line comprising an inner pipe for conducting the gaseous fuel and an outer pipe arranged around the inner pipe so that an annular leakage space is formed between the inner pipe and the outer pipe. If a double-walled fuel supply line is used, alternative ways of heating, such as electrical heat tracing, are more difficult to use.

[0018] According to an embodiment of the invention, the engine comprises control means configured to allow operation of the engine in a warm-up mode, in which mode the engine is operated in the first operating mode, the venting valve is kept open, and flow of pressurized intake air through at least one of the gas admission valves is allowed. The warm-up mode could be actuated either automatically or manually. For instance, if the engine is switched from the first operating mode to the second operating mode, the warm-up mode could be automatically initiated before the engine is allowed to switch to use the gaseous fuel. Alternatively, the control means could be configured to allow flow of pressurized intake air via the gas admission valve and the venting valve whenever the engine is operated in the first operating mode to keep the second fuel supply line ready for conveying the gaseous fuel.

[0019] According to an embodiment of the invention, the control means are configured to operate the engine in the warm-up mode for a predetermined period of time or until predetermined conditions are met before switching the engine to operate in the second operating mode.

[0020] According to an embodiment of the invention, the engine is operated according to the warm-up mode for a predetermined period of time or until predetermined conditions are met.

[0021] According to an embodiment of the invention, the engine is operated according to the warm-up mode until the charge air, the second fuel supply line or the air flowing through the venting valve reaches a predetermined temperature.

[0022] According to an embodiment of the invention, at least one of the gas admission valves is actively controlled to regulate the flow of pressurized intake air into the second fuel supply line. Active control of the gas admission valves may be needed if the gas admission valves do not leak in the backflow direction. This may be the case in particular if the gas admission valves are poppet valves configured to introduce the gaseous fuel directly into the main combustion chamber. Even if there is inherent leakage in the backflow direction of the gas admission valves, active controlling may be used to increase the flow of pressurized intake air into the second fuel supply line.

[0023] According to an embodiment of the invention, the venting valve is actively controlled to regulate the flow through the venting valve. By controlling the venting valve, excessive flow of the pressurized intake air into the second fuel supply line can be prevented.

[0024] According to an embodiment of the invention, the flow of pressurized charge air in the second fuel supply line is controlled based on the charge air pressure.

[0025] According to an embodiment of the invention, the engine is provided with at least one wastegate valve controlling the supply of exhaust gas into the turbine of a turbocharger, and the operation of the wastegate valve is controlled to compensate the pressure loss caused by the flow of pressurized intake air into the second fuel supply line.

[0026] Brief description of the drawings

[0027] Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

[0028] Fig. 1 shows schematically part of a piston engine according to an embodiment of the invention,

[0029] Fig. 2 shows schematically one cylinder of the engine of figure 1 ,

[0030] Fig. 3 shows schematically part of the piston engine of figure 1 , and

[0031] Fig. 4 shows as a flowchart the method according to the invention.

[0032] Detailed description of embodiments of the invention

[0033] Figure 1 shows schematically part of a piston engine 1 according to an embodiment of the invention. Figure 2 shows schematically one cylinder 2 of the engine 1 of figure 1 and figure 3 shows part of a fuel supply system of the engine 1 of figure 1 . The engine 1 is a large piston engine. The expression “large piston engine” refers here to an engine having a cylinder diameter of at least 150 mm. The engine can be, for instance, an engine that is used as a main or an auxiliary engine in a ship or an engine that is used at a power plant for driving a generator for producing electricity. The engine is a four-stroke engine. Figure 1 shows four cylinders 2, which are arranged in line. However, the engine 1 could comprise any reasonable number of cylinders 2, which could also be arranged, for instance, in a V-configuration.

[0034] The engine 1 is configured to be operable using at least two different fuels, a first fuel and a second fuel, as main fuel. The expression “main fuel” refers to a fuel that is the main source of energy and which is thus responsible for the major part of heat release in the cylinders 2. The first fuel can be liquid fuel. The liquid fuel could be, for instance, light fuel oil, marine diesel or heavy fuel oil. The liquid fuel could also be a mixture of different liquid fuels. When the engine 1 is operated using the first fuel as main fuel, it can be operated using solely the main fuel. However, some other fuel could be used as liquid pilot fuel to facilitate the ignition and / or combustion of the main fuel.

[0035] The second fuel is gaseous fuel. The expression “gaseous fuel” refers here to a fuel that is gaseous in atmospheric pressure and at a temperature of 20 °C. The gaseous fuel could be, in particular, ammonia (NHs). However, the gaseous fuel could also be some other fuel. The gaseous fuel can be a mixture of two or more types of gaseous fuel.

[0036] When the engine 1 is operated using the second fuel, it can utilize liquid fuel, such as light fuel oil, as pilot fuel facilitating the ignition and / or combustion of the gaseous fuel. However, that is not necessary, but the gaseous fuel could be self-igniting or spark plugs or other ignition means could be used for igniting the gaseous fuel.

[0037] The engine 1 can be configured to be operable using selectively two or more different liquid fuels and / or two or more different gaseous fuels. The engine can thus be, a dual-fuel engine or a multi-fuel engine.

[0038] For introducing the first fuel into the cylinders 2 of the engine 1 , the engine 1 is provided with a first fuel supply system. In the embodiment of figure 1 , the first fuel is liquid fuel and the first fuel supply system is a common rail fuel injection system. The first supply system comprises a first fuel supply line 10. The first fuel supply system comprises a low-pressure pump 8, which supplies fuel from a liquid fuel source, such as a fuel tank 4, to a high-pressure pump 13. The low-pressure pump 8 supplies fuel to the high-pressure pump 13 at a first pressure level, which can be, for instance, in the range of 5 to 15 bar. The high-pressure pump 13 raises the fuel pressure to a second level, which is suitable for direct fuel injection into the cylinders 2 of the engine 1 . The second level can be, for instance, in the range of 500 to 3000 bar.

[0039] The pressurized fuel from the high-pressure pump 13 is supplied to a fuel rail 6. Each cylinder 2 of the engine 1 is provided with a fuel injector 14, which is configured to inject the liquid fuel into the cylinder 2. In the embodiment of the figures, each fuel injector 14 is configured to inject the liquid fuel directly into a main combustion chamber 19 of the cylinder 2. Instead of injecting all the liquid fuel directly into the main combustion chambers 19 of the cylinders 2, part of the liquid fuel could be injected into prechambers.

[0040] In the embodiment of figure 1 , all the fuel injectors 14 are connected to the same fuel rail 6. However, the first fuel supply system could be constructed in many other ways. For instance, the first fuel supply system could comprise several low-pressure and / or high-pressure pumps 8, 13. Instead of one fuel rail 6, the first fuel supply system could comprise two or more fuel rails. Instead of a fuel rail serving several cylinders 2 of the engine 1 , each cylinder 2 could be provided with an own fuel accumulator. Instead of a common rail system, each cylinder 2 of the engine 1 could be provided with an own high-pressure fuel pump.

[0041] Each fuel injector 14 can be individually controlled. The fuel injectors 14 are connected to a control unit 12, which can be used for adjusting the timing and duration of the fuel injection. Also the amount of the injected liquid fuel can thus be controlled.

[0042] For introducing the second, gaseous fuel into the cylinders 2 of the engine 1 , the engine 1 is provided with a second fuel supply system. The second fuel supply system is configured to introduce the gaseous fuel into the cylinders 2 of the engine 1 . The second fuel supply system comprises a second fuel supply line 20. Each cylinder 2 of the engine 1 is provided with a gas admission valve 5, which can be used for controlling the supply of the gaseous fuel into the cylinder 2. In the embodiment of the figures, the gaseous fuel is not introduced directly into the cylinders 2, but each gas admission valve 5 is configured to introduce the gaseous fuel into an inlet channel of the engine 1 between a charge air receiver 16 and the main combustion chamber 19 of the cylinder 2. However, the second fuel supply system could be configured to introduce the second fuel directly into the cylinders 2 of the engine 1 . Each cylinder head 17 of the engine 1 could thus be provided for example with a poppet valve configured to control the supply of the gaseous fuel into the cylinders 2 of the engine 1. The gaseous fuel could be introduced into main combustion chambers 19 and / or into prechambers.

[0043] In the embodiment of the figures, each gas admission valve 5 is configured to introduce the gaseous fuel into an inlet port 15 that is an integral part of the cylinder head 17 of the respective cylinder 2. However, the gas admission valve 5 could also be configured to introduce the gaseous fuel into an inlet duct 18 connecting the inlet port 15 of the cylinder head 17 to the charge air receiver 16. The gas admission valves 5 are connected with a wired or wireless connection to the control unit 12, which controls opening and closing of the gas admission valves 5.

[0044] The engine 1 comprises at least one turbocharger 25, which pressurizes the intake air of the engine 1 and supplies the pressurized intake air into the charge air receiver 16, from which it is supplied via the inlet ducts 18 and inlet ports 15 into the main combustion chambers 19 of the engine 1 . The engine 1 could comprise two or more turbochargers, which could be arranged in series and / or in parallel. The engine 1 could further comprise one or more charge air coolers. The engine 1 is provided with a wastegate valve 29. The wastegate valve 29 can be controlled to bypass the turbine of the turbocharger 25 to control the pressure difference over the turbine. By controlling the wastegate valve 29, the pressure of the intake air can be controlled.

[0045] The gaseous fuel is supplied to the cylinders 2 of the engine 1 from a gas source, such as a gas tank 3, via the second fuel supply line 20. The gas source could also be, for instance, a pipe line or an industrial process producing combustible gas. The second fuel supply system is provided with a main shut-off valve 7 for controlling the flow of the gaseous fuel to the gas admission valves 5 of the cylinders 2. The pressure of the gaseous fuel should be higher than the pressure of the intake air in the charge air receiver 16. The pressure of the gaseous fuel could be, for instance, in the range of 8-20 bar. The second fuel supply line 20 can be provided with different means for controlling the pressure of the gaseous fuel. The means can comprise, for instance, a pressure regulating valve and / or a compressor. In the embodiment of the figures, the second fuel supply system comprises a flow control unit 26, which can be called as a gas ramp. The flow control unit 26 is arranged between the main shut-off valve 7 and the gas admission valves 5 and configured to regulate the pressure and / or the flow rate of the gaseous fuel to the gas admission valves 5. The flow control unit 26 can comprise one or more pressure regulating valves and / or flow control valves. The flow control unit 26 can also comprise one or more shut-off valves and / or pressure relief valves. The flow control unit 26 can further comprise other safety devices. The flow control unit 26 could further comprise a filter.

[0046] The engine 1 can further comprise means for heating the gaseous fuel. The means for heating the gaseous fuel could comprise, for instance, a heat exchanger arranged in the second fuel supply line 20 and configured to heat the fuel flowing in the second fuel supply line 20. The heat exchanger could be arranged between the gas source 3 and the main shut-off valve 7. Alternatively, the gaseous fuel could be heated for example by means of an electrical heating cable arranged around part of the second fuel supply line 20.

[0047] Instead of introducing all the gaseous fuel into the inlet ports 15 or into the inlet ducts 18, part of the gaseous fuel could also be injected directly into the cylinders 2 of the engine 1 or into prechambers. Each cylinder 2 can be provided with a prechamber valve for controlling the supply of the gaseous fuel into the prechamber.

[0048] When the engine 1 is operated using the gaseous fuel as main fuel, Otto combustion process could be utilized. The gaseous fuel is introduced into the cylinders 2 of the engine 1 during the intake stroke. The gaseous fuel is mixed with the intake air to form a homogenous mixture. The mixture can be a lean mixture containing more air than is needed for complete combustion of the fuel that is introduced into the cylinders 2. The mixture of air and the gaseous fuel could be ignited using liquid pilot fuel or a spark plug or some other ignition means for triggering the combustion. In addition to the first and second fuel supply systems, the engine 1 could comprise one or more additional fuel supply systems. For instance, the engine 1 could comprise a pilot fuel supply system for introducing liquid pilot fuel into the main combustion chambers 19 or prechambers of the engine 1 . The pilot fuel supply system could be similar to the first fuel supply system. The first fuel supply system could also function as a pilot fuel supply system, or some components of the first fuel supply system could also be used in the pilot fuel supply system .

[0049] In the embodiment of the figures, the second fuel supply line 20 is a doublewalled fuel supply line 20. The second fuel supply line 20 could also be only partly double-walled. For instance, the second fuel supply line 20 could be a single-walled fuel supply line outside of the engine room.

[0050] The double-walled part of the second fuel supply line 20 comprises an inner pipe 21 that is configured to convey the fuel, and an outer pipe 22 arranged around the inner pipe 21 so that an annular leakage space 23 is formed between the inner pipe 21 and the outer pipe 22. The purpose of the leakage space 23 is to receive any fuel possibly leaking from the inner pipe 21. The leakage space 23 could be open to the outside of the leakage space 23 for continuous venting of the leakage space 23. Alternatively, the leakage space 23 could be gastight. A gastight leakage space 23 could be filled either with air or with inert gas, such as nitrogen. The leakage space 23 is configured to vent any leakage from the inner pipe 21 into the leakage space 23 to a safe place, such as to the atmosphere sufficiently far from the engine 1 and outside of any closed space surrounding the engine 1 .

[0051] The engine 1 can be operated at least in a first operating mode, in which mode the engine 1 is operated using the first fuel supplied into the cylinders 2 of the engine 1 via the first fuel supply line 10 as main fuel, and in a second operating mode, in which mode the engine 1 is operated using the second fuel supplied into the cylinders 2 of the engine 1 via the second fuel supply line 20 as main fuel. In the first operating mode, no second fuel is supplied into the cylinders 2 of the engine 1 via the second fuel supply line 20. The main shut-off valve 7 is thus kept closed. In the embodiment of the figures, the first operating mode is a liquid fuel mode and the second operating mode is a gas mode. If the engine 1 is operated in the first operating mode for a longer period of time or the engine 1 has been shut down for a longer period of time, the temperature of the second fuel supply line 20 is close to the ambient temperature. Depending on the ambient temperature, the pressure of the gaseous fuel and the condensation temperature of the gaseous fuel at the pressure used for the supply of the gaseous fuel, there is risk that the gaseous fuel condensates on the surfaces of the second fuel supply line 20 when the engine 1 is started or switched to the second operating mode.

[0052] To avoid condensing of the gaseous fuel in the second fuel supply line 20, at least one of the gas admission valves 5 is configured to allow flow of pressurized intake air of the engine 1 into the second fuel supply line 20. All the gas admission valves 5 can be configured to allow flow of pressurized intake air of the engine 1 into the second fuel supply line 20. If the gas admission valves 5 are arranged to supply the fuel indirectly into the cylinders 2 via the inlet ports 15 or via the inlet ducts 18, the gas admission valves 5 may have, depending on the type of the valves, inherent leakage in the reverse flow direction. Pressurized intake air may thus leak into the second fuel supply line 20 even when the gas admission valves 5 are closed.

[0053] If the gas admission valves 5 are arranged to introduce the gaseous fuel directly into the main combustion chambers 19 or into prechambers, significant backflow should not occur when the gas admission valves 5 are closed. In that case, the gas admission valves 5 need to be actively controlled to allow flow of pressurized intake air via the gas admission valves 5 into the second fuel supply line 20.

[0054] The engine 1 is also provided with a controllable venting valve 9 that is connected to the second fuel supply line 20 and configured to allow venting of the second fuel supply line 20. In the embodiment of the figures, the engine 1 is provided with a first venting valve 9 that is connected to the second fuel supply line 20 upstream from the gas admission valves 5. The first venting valve 9 is connected to the second fuel supply line 20 upstream from the flow control unit 26. The first venting valve 9 is arranged in a first venting line 11 . Preferably, at least the gas admission valve 5 located farthest from the first venting valve 9 is configured to allow supply of pressurized intake air of the engine 1 into the second fuel supply line 20. The first venting valve 9 can be provided with an actuator that is connected with wired or wireless communication means to the control unit 12. The control unit 12 can thus control opening and closing of the first venting valve 9.

[0055] In the embodiment of the figures, the engine 1 is provided with a second venting line 28 that is connected to the second fuel supply line 20 downstream from the gas admission valves 5. The second venting line 28 is provided with a second venting valve 27 for controlling flow through the second venting line 28.

[0056] By introducing warm charge air into the second fuel supply line 20 and letting the warm charge air to flow through the second fuel supply line 20 to the first venting valve 9 and out of the second fuel supply line 20, the second fuel supply line 20 can be warmed up when the engine 1 is operated in the first operating mode. As the first venting line 11 is connected to the second fuel supply line 20 upstream from the gas admission valves 5, the second fuel supply line 20 is heated up to the first venting line 11 . The second venting valve 27 is kept closed during heating of the second fuel supply line 20 to prevent escape of the warm air via the downstream end of the second fuel supply line 20.

[0057] If there is inherent leakage in the backflow direction of the gas admission valves 5, opening of the gas admission valves 5 is not necessarily needed to supply a sufficient amount of pressurized intake air into the second fuel supply line 20 to warm up the second fuel supply line 20. However, even in that case one or more of the gas admission valves 5 can be actively controlled to increase the flow of pressurized intake air into the second fuel supply line 20.

[0058] When the engine 1 is operated in the second operating mode, the gaseous fuel can be heated with heating means, such as a heat exchanger arranged in the second fuel supply line 20.

[0059] In the method according to the invention, the engine 1 is operated in the first operating mode (step 101 ) and simultaneously the venting valve 9 is kept open to allow flow out of the second fuel supply line 20 (step 102) and flow of pressurized intake air via at least one of the gas admission valves 5 is allowed to supply pressurized intake air into the second fuel supply line 20 (step 103). Pressurized intake air can thus flow either as leakage flow or via one or more open gas admission valves 5 from the intake port 15 or from the cylinder 2 into the second fuel supply line 20. The operation according to steps 101 -103 can be continued until the charge air temperature or the temperature of the second fuel supply line 20 reaches a predetermined temperature. Alternatively, the engine 1 could be operated according to steps 101 -103 for a predetermined period of time.

[0060] The control unit 12 of the engine 1 can be configured to have a warm-up mode, in which the engine 1 is operated according to steps 101 -103. The warm-up mode could be actuated either automatically or manually. For instance, if the control unit 12 receives a command to switch the engine 1 from the first operating mode to the second operating mode, the control unit 12 could initiate the warm-up mode before opening the main shut-off valve 7 and allowing supply of gaseous fuel to the gas admission valves 5. The control unit 12 could be configured to operate the engine 1 in the warm-up mode for a predetermined period of time. Alternatively, the control unit 12 could be configured to operate the engine 1 in the warm-up mode until predetermined conditions are met. For instance, the engine 1 could be provided with means for monitoring the temperature of the second fuel supply line 20 or the temperature of the air flowing through the first venting valve 9 and the engine 1 could be operated in the warm-up mode until the temperature of the second fuel supply line 20 or the temperature of the air reaches a predetermined temperature.

[0061] If the engine 1 has been shut down, the engine 1 could be started in the first operating mode and operated in the warm-up mode for a predetermined period of time or until the temperature of the second fuel supply line 20 or the temperature of the air flowing through the first venting valve 9 reaches a predetermined temperature. Alternatively, the engine 1 could be operated in the warmup mode until the charge air temperature reaches a predetermined limit.

[0062] Instead of using the warm-up mode only immediately prior to switching the engine 1 to the second operating mode, the control unit 12 could be configured to allow flow of pressurized intake air via the gas admission valve 5 and the venting valve 9 whenever the engine 1 is operated in the first operating mode to keep the second fuel supply line 20 ready for conveying the gaseous fuel.

[0063] The first venting valve 9 could be an on-off valve. Alternatively, the first venting valve 9 could be controlled in steps or it could be continuously adjustable. That would allow controlling of the flow through the second fuel supply line 20 when the engine 1 is operated in the first operating mode. The flow could also be controlled by the gas admission valves 5, either by controlling the opening degree of one or more of the gas admission valves 5 or by controlling the number of open gas admission valves 5. The engine 1 could also be provided with separate flow regulating means arranged in the first venting line 11 . The flow regulating means could comprise, for example, an adjustable throttle. By controlling the air flow in the second fuel supply line 20, excessive drop of the charge air pressure can be prevented.

[0064] If the engine 1 is provided with the flow control unit 26 and the flow control unit 26 comprises shut-off valves, the shut-off valves need to be kept open during the warm-up mode to allow the flow of pressurized intake air to the first venting valve 9. The valves of the flow control unit 26 can be controlled by the control unit 12 of the engine 1 .

[0065] The venting valve 9 and the venting line 11 could also be used as a degassing valve and degassing line when the engine 1 is shut down or switched from the first operating mode to the second operating mode. However, in the embodiment of the figures, the engine 1 is provided with the second venting valve 27 that can be used as a degassing valve for degassing the second fuel supply line 20 via the second venting line 28.

[0066] To prevent excessive dropping of the charge air pressure, the flow of air in the second fuel supply line 20 can be controlled based on the charge air pressure. The flow can be limited by means of the gas admission valves 5, the first venting valve 9 and possible other flow regulating means. Furthermore, the pressure loss caused by the flow of pressurized intake air into the second fuel supply line 20 can be compensated by controlling the wastegate valve 29 of the turbocharger 25.

Claims

Claims:1 . A piston engine (1 ) comprising- a first fuel supply line (10) for supplying first fuel into the cylinders (2) of the engine (1 ) for operating the engine (1 ) using the first fuel as main fuel, and- a second fuel supply line (20) for supplying second, gaseous fuel from a gas source (3) into the cylinders (2) of the engine (1 ) for operating the engine (1 ) using the gaseous fuel as main fuel, the second fuel supply line (20) comprising for each cylinder (2) of the engine (1 ) a gas admission valve (5) that is configured to allow the gaseous fuel to be introduced directly or indirectly into the cylinder (2), wherein- the engine (1 ) is configured to be operable at least in a first operating mode, in which mode the engine (1 ) is operated using the first fuel supplied into the cylinders (2) of the engine (1 ) via the first fuel supply line (10) as main fuel, and in a second operating mode, in which mode the engine (1 ) is operated using gaseous fuel supplied into the cylinders (2) of the engine (1 ) via the second fuel supply line (20) as main fuel,- the engine (1 ) comprises a controllable venting valve (9) that is connected to the second fuel supply line (20) upstream from the gas admission valves (5) and configured to allow venting of the second fuel supply line (20), and- at least one of the gas admission valves (5) is configured to allow flow of pressurized intake air of the engine (1 ) via said gas admission valve (5) into the second fuel supply line (20) and further out of the second fuel supply line (20) via the venting valve (9) when the engine (1 ) is operated in the first operating mode to warm up the second fuel supply line (20).

2. The piston engine (1 ) according to claim 1 , wherein the second fuel is ammonia.

3. The piston engine (1 ) according to claim 1 or 2, wherein the first fuel is liquid fuel.

4. The piston engine (1 ) according to any of claims 1 to 3, wherein the gas admission valves (5) are arranged to introduce the gaseous fuel into the cylinders (2) via the inlet ports (15) of the cylinders (2).

5. The piston engine (1 ) according to any of the preceding claims, wherein the gas admission valves (5) are arranged in the cylinder heads (17) of the engine (1 ).

6. The piston engine (1 ) according to any of the preceding claims, wherein the venting valve (9) is connected to the second fuel supply line (20) between the gas admission valves (5) and a main shut-off valve (7) that is configured to control gas flow from the gas source (3) to the cylinders (2) of the engine (1 ).

7. The piston engine (1 ) according to any of the preceding claims, wherein the engine (1 ) comprises a flow control unit (26) configured to regulate the pressure and / or the flow rate of the gaseous fuel to the gas admission valves (5), and the venting valve (9) is connected to the second fuel supply line (20) upstream from the flow control unit (26).

8. The piston engine (1 ) according to any of the preceding claims, wherein at least part of the second fuel supply line (20) is a double-walled fuel supply line comprising an inner pipe (21 ) for conducting the gaseous fuel and an outer pipe (22) arranged around the inner pipe (21 ) so that an annular leakage space (23) is formed between the inner pipe (21 ) and the outer pipe (22).

9. The piston engine (1 ) according to any of the preceding claims, wherein the engine (1 ) comprises control means (12) configured to allow operation of the engine (1 ) in a warm-up mode, in which mode the engine (1 ) is operated in the first operating mode, the venting valve (9) is kept open, and flow of pressurized intake air through at least one of the gas admission valves (5) into the second fuel supply line (20) is allowed.

10. The piston engine (1 ) according to claim 9, wherein the control means (12) are configured to operate the engine (1 ) in the warm-up mode for a predetermined period of time or until predetermined conditions are met before switching the engine (1 ) to operate in the second operating mode.

11. A method of operating a piston engine (1 ) according to any of the preceding claims, the method comprising the simultaneous steps of- operating the engine (1 ) in the first operating mode (101 ),- keeping the venting valve (9) open to allow flow out of the second fuel supply line (20) (102), and- allowing flow of pressurized intake air via at least one of the gas admission valves (5) into the second fuel supply line (20) (103).

12. The method according to claim 11 , wherein the engine (1 ) is operated according to steps (101 )-(103) for a predetermined period of time or until predetermined conditions are met.

13. The method according to claim 12, wherein the engine (1 ) is operated according to steps (101 )-(103) until the charge air, the second fuel supply line (20) or the air flowing through the venting valve (9) reaches a predetermined temperature.

14. The method according to any of claims 11 to 13, wherein at least one of the gas admission valves (5) is actively controlled to regulate the flow of pressurized intake air into the second fuel supply line (20).

15. The method according to any of claims 11 to 14, wherein the venting valve (9) is actively controlled to regulate the flow through the venting valve (9).

16. The method according to any of claims 11 to 15, wherein the flow of pressurized charge air in the second fuel supply line (20) is controlled based on the charge air pressure.

17. The method according to any of claims 11 to 16, wherein the engine (1 ) is provided with at least one wastegate valve (29) controlling the supply of exhaust gas into the turbine of a turbocharger (25), and the operation of the wastegate valve (29) is controlled to compensate the pressure loss caused by the flow of pressurized intake air into the second fuel supply line (20).

18. The method according to any of claims 11 to 17, wherein the engine (1 ) comprises a second venting valve (27) connected to the second fuel supply line (20) downstream from the gas admission valves (5), and duringoperation of the engine (1 ) according to steps (101 )-(103) the second venting valve (27) is kept closed.

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