Fuel injection system, piston engine, method of operating a piston engine, and method of retrofitting a piston engine

Abstract

The fuel injection system comprises a first low-pressure pump (11) for pressurizing a first fuel, a second low-pressure pump (12) for pressurizing a second fuel, a fuel injector (3), a high-pressure pump (13) for the first fuel, and a pressure booster arrangement comprising a fuel chamber (4) in fluid communication with the second low-pressure pump (12), a booster chamber (15), and a booster piston (16), wherein the first fuel pressurized by the high-pressure pump (13) can be supplied into a first fuel portion (15A) of the booster chamber (15) to exert a force onto a first piston surface (16A) of the booster piston (16), and a second fuel portion (15B) of the booster chamber (15) is in fluid communication with the fuel chamber (4) such that when the force is exerted onto the first piston surface (16A), the pressure of the second fuel in the fuel chamber (4) is raised.

Description

[0001] Fuel injection system, piston engine, method of operating a piston engine, and method of retrofitting a piston engine

[0002] Technical field of the invention

[0003] The present invention concerns a fuel injection system for a multi-cylinder piston engine, as defined in claim 1 . The invention further concerns a piston engine comprising such a fuel injection system, a method of operating a piston engine and a method of retrofitting a piston engine.

[0004] Background of the invention

[0005] Large piston engines, such as marine or power plant engines, are often operated using high-pressure fuel injection into the cylinders or prechambers. High- pressure fuel injection systems can be divided into common rail systems and systems comprising jerk-type fuel pumps. Common rail systems keep the fuel constantly at a high pressure and the high-pressure fuel flows to fuel injectors, which are actively controlled to inject the fuel into individual cylinders. In fuel injection systems with jerk-type fuel pumps, each cylinder is provided with an own high-pressure pump providing pulsed high-pressure fuel flow. A typical jerk-type fuel pump comprises a rotatable plunger that is provided with a helix for controlling the fuel supply rate of the pump. Each cylinder is further provided with a fuel injector that receives fuel from the high-pressure pump and is configured to open when the pulsed fuel pressure exceeds a predetermined level set by the injector mechanics.

[0006] There is a growing need to be able to operate piston engines with sustainable fuels, such as methanol or ammonia, instead of conventional fuels, such as light fuel oil.

[0007] Many sustainable fuels differ significantly from conventional fuels in terms of lubrication properties and / or viscosity. An example of such a fuel is methanol. Compared to light fuel oil, methanol has poor lubrication properties and also lower viscosity. These properties make pressurizing of methanol challenging, in particular when high injection pressures are needed.

[0008] Many existing high-pressure fuel pumps are not suitable for pressurizing methanol, but pumps having a special construction and materials are needed. Such pumps are expensive. The high cost of high-pressure pumps that are suitable for pressurizing methanol and other sustainable fuels with similar properties is a problem that concerns all engines operated using such fuels, but the problem is even worse when existing engines are converted for use with sustainable fuels. The high-pressure fuel pumps may still have a significant part of their expected lifecycle left, and replacing the pumps with new high-pressure pumps is unnecessary wastage. The existing high-pressure pumps also form an integral part of the original engine structure and have connections to other systems as well. Therefore, the removal or replacement of those pumps would require changes into the structure of the engine and / or relevant auxiliary systems.

[0009] Summary of the invention

[0010] An object of the present invention is to provide an improved fuel injection system for a multi-cylinder piston engine. Another object of the invention is to provide an improved method of operating a multi-cylinder piston engine. Further objects of the invention are to provide a method of operating a piston engine and a method of retrofitting a piston engine.

[0011] The fuel injection system according to the invention comprises

[0012] - a first low-pressure pump for pressurizing a first fuel,

[0013] - a second low-pressure pump for pressurizing a second fuel,

[0014] - for each cylinder of the engine a fuel injector configured to inject fuel into the cylinder,

[0015] - for each cylinder of the engine a high-pressure pump for raising the pressure of the first fuel pressurized by the first low-pressure pump, and

[0016] - for each cylinder of the engine a pressure booster arrangement configured to raise the pressure of the second fuel pressurized by the second low-pressure pump prior to injection through the fuel injector.

[0017] The pressure booster arrangement comprises

[0018] - a fuel chamber arranged in fluid communication with the second low- pressure pump to receive the second fuel pressurized by the second low-pressure pump,

[0019] - a booster chamber, and

[0020] - a booster piston arranged in the booster chamber, the booster piston dividing the booster chamber into a first fuel portion and a second fuel portion. The pressure booster arrangement is configured to allow supply of the first fuel pressurized by the high-pressure pump into the first fuel portion of the booster chamber to exert a hydraulic force onto a first piston surface of the booster piston, and the second fuel portion of the booster chamber is arranged in fluid communication with the fuel chamber such that when said hydraulic force is exerted onto the first piston surface of the booster piston, the pressure of the second fuel in the fuel chamber is raised.

[0021] The piston engine according to the invention comprises a fuel injection system defined above.

[0022] The method according to the invention for operating a piston engine defined above comprises the steps of

[0023] - pressurizing the first fuel by means of the first low-pressure pump,

[0024] - raising the pressure of the first fuel by means of the high-pressure pumps,

[0025] - pressurizing the second fuel by means of the second low-pressure pump,

[0026] - introducing the second fuel into the fuel chambers,

[0027] - introducing the first fuel pressurized by the high-pressure pumps into the first fuel portions of the booster chambers to raise the pressure of the second fuel, and

[0028] - injecting the second fuel into the cylinders of the engine.

[0029] The fuel injection system according to the invention thus comprises cylinderspecific fuel injectors and high-pressure pumps. The fuel injectors can be controlled by fuel pressure. The fuel injector can thus open when the fuel pressure in the fuel injector exceeds a predetermined threshold. The fuel injector can comprise an injector needle that is spring-biased to a closed position and opened by a sufficient fuel pressure. The fuel injectors can be arranged to inject fuel into prechambers or directly into main combustion chambers of the cylinders of the engine. The engine can be operated at least by using the second fuel, which can be a sustainable fuel, such as ammonia, ethanol or methanol. The engine thus has at least one operating mode, in which the engine is operated using the second fuel. The second fuel is injected into the cylinders of the engine at a high pressure. However, the second fuel is not pressurized directly by the high-pressure pumps, but by using the first fuel as a hydraulic fluid that is used for operating the pressure booster arrangements. The second fuel is therefore not in contact with the components of the high-pressure pumps, and the high-pressure pumps do not need to be designed and constructed such that they can pump the second fuel, which may have poor lubricating properties and / or low viscosity. This makes the fuel injection system suitable in particular for retrofits, as many existing components of an existing fuel injection system can be used without modifications or with minor modifications. For instance, high-pressure pumps configured for light fuel oil can be now used for indirectly pressurizing the second fuel, which is the fuel powering the engine.

[0030] Furthermore, the pressure booster arrangement can be located in the proximity of the fuel injector. Only relatively short high-pressure pipes or no high- pressure pipes at all are needed for conveying the second fuel because the conversion to high-pressure takes place in the booster chamber. Many sustainable fuels are toxic and / or highly volatile, inflammable and / or explosive. Often double-wall pipes are thus required for conveying the fuel at a high pressure and special measures may be needed to prevent and monitor leaks. High- pressure piping becomes thus expensive and also occupies extra space and requires special installation practices. By pressurizing the second fuel close to the injectors, the need for double-wall pipes and other expensive constructions can be minimized.

[0031] According to an embodiment of the invention, the pressure booster arrangement comprises an inlet check valve, via which the second fuel can be introduced from the second low-pressure pump into the fuel chamber. With the inlet check valve complicated control of the flow of the second fuel into the fuel chamber is not needed, but the fuel chamber can be in continuous fluid communication with the second low-pressure pump, and the inlet check valve prevents backflow from the fuel chamber to the second low-pressure pump during pressure boosting.

[0032] According to an embodiment of the invention, the fuel chamber forms part of the fuel injector. The fuel chamber can be formed within a conventional fuel injector. In retrofit solutions separate fuel chambers are thus not necessary, but the pressure of the second fuel can be raised within the fuel injector. The inlet check valve can be arranged in connection with the fuel injector. According to an embodiment of the invention, the high-pressure pump is a jerktype fuel pump. The high-pressure pumps can be conventional jerk-pumps that are configured to pressurize LFO or similar fuels. As the second fuel is not in contact with the high-pressure pumps, no modifications or only minor modifications of the high-pressure pumps are needed.

[0033] According to an embodiment of the invention, the high-pressure pump comprises a rotatable helix-plunger for controlling fuel supply rate of the high-pressure pump. The rotatable plunger can be used for controlling the amount of the second fuel injected into the cylinders of the engine.

[0034] According to an embodiment of the invention, the fuel injection system comprises for each cylinder of the engine a control valve for controlling the supply of the first fuel from the high-pressure pump into the first fuel portion of the booster chamber. By controlling the supply of the first fuel into the booster chamber, the injection timing and the injection amount of the second fuel can be controlled. The control valve can be used instead of the helix-plunger or in addition to that to control the injection timing and amount. In case the second fuel is a fuel with different combustion properties and / or energy content than LFO or other conventional fuel typically used with jerk-type high-pressure fuel pumps, the control valve can extend or add on to control capabilities of the high-pressure pump allowing the fuel injection to be tailored based on the properties of the fuel in question.

[0035] According to an embodiment of the invention, the control valve is a solenoid valve. A solenoid valve allows quick and precise control of the flow of first fuel.

[0036] According to an embodiment of the invention, the control valve has a first position connecting a fuel outlet of the high-pressure pump to a drain line and a second position connecting the fuel outlet of the high-pressure pump to the first fuel portion of the booster chamber. The temperature of the first fuel rises when the first fuel is pressurized by the high-pressure pump. By releasing the excess first fuel into the drain line, the control valve and the high-pressure pump constantly receive fresh fuel, which helps to manage the thermal load of the control valve and the high-pressure pump.

[0037] According to an embodiment of the invention, the control valve and the booster chamber are arranged in a pressure booster unit that is arranged between the high-pressure pump and the fuel injector. This minimizes the modifications needed in the fuel injector and the high-pressure pump.

[0038] According to an embodiment of the invention, the booster chamber is arranged at least partly within the fuel injector. By arranging the booster chamber at least partly within the fuel injector, no high-pressure pipe is needed between the booster chamber and the fuel injector. This makes the construction of the fuel injection system simple and minimizes the number of possible leakage positions in the fuel injection system. An existing fuel injector can be modified to be used in the fuel injection system according to the invention.

[0039] According to an embodiment of the invention, the booster chamber is arranged in a booster chamber block that is arranged between the high-pressure pump and the fuel injector. The booster chamber arranged in a separate module can be easily installed between the high-pressure pump and the fuel injector.

[0040] According to an embodiment of the invention, the second low-pressure pump is configured to raise the pressure of the second fuel to 4-50 bar. The pressure produced by the second low-pressure pump can be adjustable. Depending on the compressibility of the second fuel, by adjusting the pressure produced by the second low-pressure pump, the injection amount can be adjusted.

[0041] According to an embodiment of the invention, the pressure booster arrangement is configured to raise the pressure of the second fuel such that the injection pressure exceeds the cylinder pressure during the injection by at least 50 bar. A sufficient difference between the injection pressure and the cylinder pressure ensures reliable injection.

[0042] According to an embodiment of the invention, the booster arrangement is configured to raise the pressure of the second fuel to at least 250 bar.

[0043] According to an embodiment of the invention, the booster piston has a second piston surface delimiting the second fuel portion of the booster chamber, and the area of the second piston surface is 5-50 percent larger than the area of the first piston surface. According to an embodiment of the invention, the area of the second piston surface is 10-30 percent larger than the area of the first piston surface. When the second piston surface is slightly larger that the first piston surface, the pressure of the first fuel is slightly higher than the pressure of the second fuel. Fuel leakage thus occurs mainly from the first fuel portion of the booster chamber into the second fuel portion of the booster chamber and the first fuel lubricates the piston. Controlled leakage of the first fuel may also improve ignition of the second fuel.

[0044] According to an embodiment of the invention, the booster piston and / or the booster chamber is provided with an annular groove and the fluid injection system comprises a fluid channel that is connected to the annular groove. The annular groove and the fluid channel can be used for collecting leaking first and second fuel or for separating the first fuel and the second fuel by a sealing liquid.

[0045] According to an embodiment of the invention, the fluid channel is configured to discharge the first fuel and the second fuel leaking from the first fuel portion and the second fuel portion of the booster chamber into the annular groove. Depending on the areas of the first and the second piston surfaces and the pressures on each side of the booster piston, the leaking fuel may be mainly the first fuel or the second fuel. The fluid channel may be connected to a nonpressurized tank. The annular groove reduces mixing of the first fuel and the second fuel in the fuel injection system.

[0046] According to an embodiment of the invention, the fluid channel is configured to supply pressurized sealing liquid into the annular groove. The sealing liquid can be fuel, such as the same fuel as the first fuel. Depending on the pressures of the sealing liquid, the first fuel and the second fuel, the sealing liquid can prevent the first fuel from flowing into the second fuel portion of the booster chamber or the second fuel from flowing into the first fuel portion of the booster chamber or both.

[0047] According to an embodiment of the invention, the second fuel is ethanol, methanol or ammonia. Ethanol, methanol and ammonia have poor lubrication properties and low viscosity, which makes them incompatible with many conventional high-pressure pumps. With the fuel injection system according to the invention, conventional high-pressure pumps can be used for indirectly pressurizing ethanol, methanol and ammonia.

[0048] According to an embodiment of the invention, the first fuel is LFO. LFO is safe to pressurize using conventional high-pressure pumps. When LFO is used as the hydraulic fluid in the pressure booster arrangement, the same fuel can also be used for operating the engine, for instance when the second fuel is not available. Furthermore, minor leakages from the first fuel portion of the booster chamber into the second fuel chamber do not disturb combustion, but may have even a positive effect on the ignition and combustion in the respective cylinder of the engine. This provides also a possibility to design the booster chamber to allow a predetermined amount of leakage to keep the booster piston and downstream components of the injector lubricated.

[0049] According to an embodiment of the invention, the method comprises the steps of monitoring a parameter indicative of the fuel demand of the engine and / or individual cylinders of the engine and controlling the amount of the second fuel introduced into the cylinders of the engine based on the value of the parameter.

[0050] According to an embodiment of the invention, the method comprises the step of using control valves that control the amount of the first fuel introduced into the first fuel portions of the booster chambers to control the amount of the second fuel introduced into the cylinders.

[0051] According to an embodiment of the invention, the method comprises the step of controlling the high-pressure pumps to control the amount of the second fuel introduced into the cylinders.

[0052] According to an embodiment of the invention, the method comprises the step of controlling the second low-pressure pump to control the amount of the second fuel introduced into the cylinders.

[0053] The control strategies based on the use of the control valves, controlling of the high-pressure pumps and controlling of the low-pressure pump can be used either separately or together to control the amount of the second fuel introduced into the cylinders.

[0054] In the method according to the invention of retrofitting a multi-cylinder piston engine the engine comprises

[0055] - a first low-pressure pump for pressurizing a first fuel,

[0056] - for each cylinder of the engine a fuel injector configured to inject fuel into the cylinder, and

[0057] - for each cylinder of the engine a high-pressure pump for raising the pressure of the first fuel pressurized by the first low-pressure pump. The method comprises the steps of

[0058] - providing the engine with one or more second low-pressure pumps for pressurizing a second fuel,

[0059] - connecting each fuel injector to one of said one or more second low- pressure pumps via a check valve,

[0060] - providing the engine for each cylinder of the engine with a booster chamber and a booster piston arranged in the booster chamber, the booster piston dividing the booster chamber into a first fuel portion and a second fuel portion,

[0061] - connecting a fuel outlet of each high-pressure pump to the first fuel portion of the respective booster chamber, and

[0062] - connecting the second fuel portion of each booster chamber to the respective fuel injector such that the pressure of the second fuel introduced into the fuel injector can be raised by means of the booster piston.

[0063] In the retrofitting method according to the invention, the fuel injection system of the engine is thus modified to allow the engine to be operated using a second fuel instead of or in addition to the first fuel. Many existing components of the engine can thus be utilized. The high-pressure pumps of the engine do not need to be replaced, but the high-pressure pumps are connected to the booster chambers to allow the second fuel to be pressurized indirectly using the high-pressure pumps to pressurize the first fuel and using the pressurized first fuel as a hydraulic fluid to drive the booster pistons. The high-pressure pumps are thus not exposed to the second fuel.

[0064] In the retrofitting method according to the invention, existing mechanical injectors or injectors of a corresponding type can be modified to accept the arrangements required for receiving the second fuel from the second low-pressure pump and boosting the second fuel to the high injection pressure either within the fuel injector or in close proximity of the fuel injector.

[0065] According to an embodiment of the invention, the method of retrofitting a multicylinder piston engine comprises the step of arranging a control valve between each high-pressure pump and the respective booster chamber to control the amount of first fuel supplied from the high-pressure pump into the first fuel portion of the booster chamber. The control valves allow more precise control of the amount of the second fuel injected into the cylinders of the engine. Brief of the

[0066] Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

[0067] Fig. 1 shows schematically a fuel injection system and a piston engine according to an embodiment of the invention,

[0068] Fig. 2 shows schematically a control valve of a fuel injection system according to an embodiment of the invention,

[0069] Fig. 3 shows the control valve of figure 2 in a different position,

[0070] Fig. 4 shows a high-pressure pump and a fuel injector according to an embodiment of the invention,

[0071] Fig. 5 shows schematically a high-pressure pump and a fuel injector according to another embodiment of the invention, and

[0072] Figs. 6A and 6B show schematically different booster chamber and piston arrangements.

[0073] Detailed description of embodiments of the invention

[0074] Figure 1 shows schematically a simplified view of a piston engine 1 that is provided with a fuel injection system according to an embodiment of the invention. The piston engine 1 can be a large piston engine having a cylinder diameter of at least 150 mm and / or a rated power of at least 150 kW / cylinder. The engine 1 can be, for instance, a marine engine or a powerplant engine. The engine 1 is a multi-cylinder engine 1. In figure 1 four cylinders 2 are shown, but the engine 1 can comprise any reasonable number of cylinders 2 arranged in line or in a V-configuration.

[0075] The fuel injection system comprises a first low-pressure pump 11 for supplying and pressurizing a first fuel. The first fuel is a liquid fuel that is liquid at atmospheric pressure and at temperature of 20 °C. The first fuel can be, for example, light fuel oil (LFO) or marine diesel oil. The first low-pressure pump 11 supplies the first fuel from a first fuel tank 18. The first low-pressure pump 11 can be configured to raise the pressure of the first fuel for example to the range of 5- 20 bar. Each cylinder 2 of the engine 1 is provided with an own fuel injector 3. Each fuel injector 3 is configured to inject the fuel into the respective cylinder 2. The fuel can be injected directly into the main combustion chamber of the cylinder 2. Alternatively, the fuel injector 3 can be configured to inject the fuel into a prechamber. Figure 4 shows an example of a fuel injector 3 that can be used in the fuel injection system of figure 1 . The fuel injector 3 comprises an injector needle 30. The injector needle 30 is spring-biased to a closed position. The injector needle 30 is configured to be lifted when the pressure of the fuel in the fuel injector 3 reaches a predetermined level. When the injector needle 30 is lifted, the nozzle openings of the fuel injector 3 open and fuel is injected. The fuel injector 3 is thus a mechanical injector, which is controlled by the fuel pressure.

[0076] Each cylinder 2 of the engine 1 is provided with an own high-pressure pump 13. Each high-pressure pump 13 is configured to raise the pressure of the first fuel pressurized by the first low-pressure pump 11. The term “high-pressure pump” refers here to a pump that is configured to raise the pressure of the first fuel to at least 150 bar, which would allow the fuel to be injected directly into the main combustion chambers or prechambers of the engine 1. The high- pressure pumps 13 can be configured to raise the pressure of the first fuel for example to at least 250 bar or 500 bar.

[0077] The high-pressure pumps 13 can be jerk-type fuel pumps. Figure 4 shows an example of a high-pressure pump 13 that is suitable for the fuel injection system of figure 1. A jerk-type high-pressure fuel pump 13 comprises a plunger 31. The plunger 31 protrudes into a pump chamber 32. The plunger 31 is moved in a reciprocating manner. The movement of the plunger 31 is timed with respect to the movement of the respective piston of the engine. Each high- pressure pump 13 may thus be in a different phase relative to the other high- pressure pumps 13. During a retracting motion of the plunger 31 , fuel is introduced into the pump chamber 32. Forward motion of the plunger 31 pressurizes the fuel in the pump chamber 32. The plunger 31 can be driven, for example, by means of a rotating cam. Each high-pressure pump 13 provides a pulsed high-pressure flow of the first fuel for the respective cylinder 2 of the engine 1 .

[0078] The plunger 31 of the high-pressure pump 13 can be a rotatable helix-plunger that can be used for controlling the fuel supply rate of the high-pressure pump 13. The plunger can thus be rotatable about its longitudinal axis. The high- pressure pump 13 can comprise an actuator for rotating the plunger about its longitudinal axis. The actuator could be, for instance, a hydraulic cylinder, an electrical linear actuator or an electric motor that is connected for example by means of a rack and a pinion to the plunger. A helix-plunger is provided with a control edge having a helical shape. By rotating the plunger, the position of the control edge relative to a fuel discharge opening of the pump chamber 32 of the high-pressure pump 13 can be adjusted. For instance, the rotation of the plunger can affect the position where the fuel is discharged from the pump chamber 32 via the fuel discharge opening close to the end of the forward motion of the plunger. By rotating the plunger, releasing of the pressure from the high-pressure pump 13 can be either delayed or advanced. Consequently, the pumped fuel volume is affected. Instead of controlling the releasing of the pressure, the control edge could be configured to control the start of the pressure build-up in the high-pressure pump 13. A helix-plunger is not necessary, but the fuel injection timing and rate can be controlled in the fuel injection system according to the invention using alternative means, as will be described later.

[0079] The engine 1 is configured to be operable using a second fuel. The engine 1 thus has at least one operating mode, in which the second fuel is injected into the cylinders 2 of the engine 1 . The fuel injection system comprises a second low-pressure pump 12 for pressurizing the second fuel. The second low-pressure pump 12 can be configured to raise the pressure of the second fuel for example to the range of 4-50 bar. The second fuel can be supplied from a second fuel tank 19.

[0080] The second fuel is different from the first fuel. Also the second fuel can be liquid fuel. However, the second fuel could also be gaseous at atmospheric pressure and at temperature of 20 °C but pressurized by means of the second low-pressure pump 12 to a pressure that is sufficient for keeping the second fuel liquid. The second fuel can be a sustainable fuel. Examples of sustainable fuels include alcohol-based fuels, such as methanol or ethanol, ammonia and hydrogen-based fuels.

[0081] Each cylinder 2 of the engine 1 is further provided with a pressure booster arrangement that is configured to raise the pressure of the second fuel pressurized by the second low-pressure pump 12 prior to injection through the fuel injector 3.

[0082] The pressure booster arrangement comprises a fuel chamber 4 arranged in fluid communication with the second low-pressure pump 12 to receive the second fuel pressurized and supplied by the second low-pressure pump 12, a booster chamber 15, and a booster piston 16 arranged in the booster chamber 15. The booster piston 16 divides the booster chamber 15 into a first fuel portion 15A and a second fuel portion 15B. The booster piston 16 has a first piston surface 16A delimiting the first fuel portion 15A and a second piston surface 16B delimiting the second fuel portion 15B of the booster chamber 15.

[0083] The pressure booster arrangement is configured to allow supply of the first fuel pressurized by the high-pressure pump 13 into the first fuel portion 15A of the booster chamber 15 to exert a hydraulic force onto the first piston surface 16A of the booster piston 16. The second fuel portion 15B of the booster chamber 15 is arranged in fluid communication with the fuel chamber 4 such that when the hydraulic force is exerted onto the first piston surface 16A of the booster piston 16, the pressure of the second fuel in the fuel chamber 4 is raised.

[0084] In the embodiments of figures 1 and 4, each fuel chamber 4 forms part of the respective fuel injector 3. In the embodiments of figures 1 and 4, the pressure booster arrangement is provided with an inlet check valve 5. The second fuel is introduced from the second low-pressure pump 12 into the fuel chamber 4 via the inlet check valve 5. The inlet check valve 5 prevents backflow to the second low-pressure pump 12. The inlet check valve 5 can be arranged in connection with the fuel injector 3, as shown in figure 4.

[0085] With the fuel injection system according to the invention, the second fuel is pressurized indirectly by the high-pressure pumps 13. The second fuel is thus not in contact with the components of the high-pressure pumps 13, but the first fuel is used as a hydraulic medium that transmits force from the high-pressure pumps 13 to the booster pistons 16. That allows high-pressure pumps 13 that are designed and constructed for use with LFO or similar fuels to be used for indirectly pressurizing methanol or other fuels with poor lubricating properties and / or low viscosity. Furthermore, the pressure booster arrangement can be arranged close to the fuel injector 3. Only short high-pressure pipes or no high-pressure pipes at all are needed for conveying the second fuel, which may be toxic and / or highly volatile, inflammable and / or explosive. By pressurizing the second fuel close to the fuel injectors 3, the need for double-wall pipes and other expensive constructions can be minimized.

[0086] In the embodiment of figure 1 , a control valve 17 is arranged between each high-pressure pump 13 and the respective booster chamber 15 to control the supply of the first fuel from the high-pressure pump 13 into the first fuel portion 15A of the booster chamber 15. In the embodiment of figure 1 , the control valves 17 are solenoid-operated valves. The control valve 17 has a first position connecting a fuel outlet of the high-pressure pump 13 to a drain line 21 and a second position connecting the fuel outlet of the high-pressure pump 13 to the first fuel portion 15A of the booster chamber 15. In figure 1 , the control valves 17 are in the first position.

[0087] When the control valves 17 are in the first position, the first fuel pressurized by the high-pressure pumps 13 is returned to the first fuel tank 18. The second fuel pressurized by the second low-pressure pump 12 flows to the fuel injectors 3, but the pressure of the second fuel is not sufficient for lifting the injector needles 30 of the fuel injectors 3, and the second fuel is not injected into the cylinders 2 of the engine 1 .

[0088] When a control valve 17 is switched to the second position, the first fuel pressurized by the respective high-pressure pump 13 can flow into the first fuel portion 15A of the booster chamber 15. The second fuel portion 15B of the booster chamber 15 is filled with the second fuel. The pressure in the second fuel portion 15B is close to the pressure produced by the second low-pressure pump 12.

[0089] The pressurized first fuel exerts a force onto the first piston surface 16A of the booster piston 16. The force moves the booster piston 16, and as a result the pressure in the second fuel portion 15B of the booster chamber 15 is raised. In the embodiment of figure 1 , each booster chamber 15 is provided with a biasing spring 25, which is configured to bias the booster piston 16 towards the first fuel portion 15A of the booster chamber 15. The pressure of the second fuel depends on the areas of the first and second piston surfaces 16A, 16B and the spring force of the biasing spring 25. If the areas of the first and second piston surfaces 16A, 16B are equal and the spring force is relatively small, the pressure of the second fuel is close to the pressure of the first fuel. As the pressure of the second fuel is raised by the movement of the booster piston 16, the injector needle 30 of the fuel injector 3 is lifted and the second fuel is injected into the cylinder 2.

[0090] When the fuel has been injected, the fuel injector 3 is again filled with the second fuel supplied by the second low-pressure pump 12. As the fuel injector 3 is in fluid communication with the second fuel portion 15B of the booster chamber 15, the second fuel exerts together with the biasing spring 25 a force onto the booster piston 16, which is moved towards the first fuel portion 15A of the booster chamber 15.

[0091] The first and second piston surfaces 16A, 16B of the booster piston 16 can have equal areas. This allows the second fuel to be injected substantially at the same pressure as the pressure produced by the high-pressure pump 13. However, the second piston surface 16B could have a smaller area than the first piston surface 16A. That would decrease the amount of the second fuel injected by a single stroke of the booster piston 16, but increase the injection pressure. For instance, if the diameter of the first piston surface 16A is twice the diameter of the second piston surface 16B, the injection pressure is approximately for times as high as the pressure produced by the high-pressure pump 13. The pressure booster arrangement thus functions as a pressure amplifier. To have a sufficient fuel injection flow rate, the high-pressure pump 13 needs to produce a relatively large volume flow. As the pressure of the second fuel is higher than the pressure of the first fuel, the second fuel may leak from the second fuel portion 15B of the booster chamber 15 into the first fuel portion 15A of the booster chamber 15.

[0092] If the second piston surface 16B had a larger area than the first piston surface 16A, the injection pressure would be decreased but the injection amount could be increased. For instance, if the diameter of the second piston surface 16B is twice the diameter of the first piston surface 16A, the pressure of the second fuel is approximately one fourth of the pressure of the first fuel. Smaller volume flow of the first fuel is required to produce a certain volume flow of the second fuel and fuel leakage occurs mainly from the first fuel portion 15A of the booster chamber 15 into the second fuel portion 15B of the booster chamber 15. In certain cases, it may be beneficial if the second piston surface 16B is slightly larger than the first piston surface 16A. In that case, the pressure of the first fuel is slightly higher than the pressure of the second fuel. Fuel leakage thus occurs mainly from the first fuel portion 15A of the booster chamber 15 into the second fuel portion 15B of the booster chamber 15 and the first fuel lubricates the piston 16. Controlled leakage of the first fuel may also improve ignition of the second fuel. The area of the second piston surface 16B could be, for example, 10-30 percent larger than the area of the first piston surface 16A.

[0093] Figures 2 and 3 show part of a pressure booster arrangement according to an embodiment of the invention. In the embodiment of figures 2 and 3, the control valve 17 and the booster chamber 15 are arranged in a pressure booster unit 24. The pressure booster unit 24 can be arranged between the high-pressure pump 13 and the fuel injector 3. The pressure booster unit 24 comprises a fuel inlet 22 that can be connected to a fuel outlet of the high-pressure pump 13. The pressure booster unit 24 comprises a fuel outlet 23 that can be connected to the drain line 21. Figure 2 shows the control valve 17 in the first position. When the control valve 17 is in the first position, the fuel inlet 22 of the pressure booster unit 24 is connected to the fuel outlet 23, and the first fuel received from the high-pressure pump 13 can flow through the control valve 17 into the drain line 21 . Flow of the first fuel into the booster chamber 15 is prevented. When the control valve 17 is switched to the second position shown in figure 3, flow from the fuel inlet 22 to the fuel outlet 23 is prevented and flow from the fuel inlet 22 into the booster chamber 15 is allowed. The first fuel can thus exert a hydraulic force onto the first piston surface 16A of the booster piston 16, and the pressure of the second fuel in the second fuel portion 15B of the booster chamber 15 is raised. The second fuel can flow from the booster chamber 15 into the fuel chamber 4, from which it is injected into the cylinder 2 of the engine 1 . After the injection, the second fuel supplied by the second low- pressure pump 12 returns the booster piston 16 to the position shown in figure 2.

[0094] The pressure booster unit 24 allows several functionalities of the pressure booster arrangement to be arranged in a single module. The pressure booster unit 24 is thus easy to install for example in retrofit solutions. However, components of the pressure booster arrangement can also be arranged in many alternative ways. Figure 4 shows another embodiment of the invention. In the embodiment of figure 4, the booster chamber 15 is arranged within a fuel injector 3. This arrangement has the benefit that the pressure of the second fuel is raised by means of the pressure booster arrangement within the fuel injector 3. The risk of leaking second fuel is thus minimized. A control valve 17 is arranged between the high-pressure pump 3 and the booster chamber 15 in a similar way as in the embodiment of figure 1 . In the embodiment of figure 4, the first fuel is supplied to the high-pressure pump 13 by means of a first low-pressure pump (not shown) in a similar way as in the embodiment of figure 1 .

[0095] In the embodiment of figure 4, a conventional fuel injector 3 has been modified to accommodate the booster chamber 15 and to allow a chamber within the fuel injector 3 to be used as the fuel chamber 4. The fuel injector 3 has been equipped with the inlet check valve 5, which prevents backflow to the second low-pressure pump 12. In addition, a fuel inlet of the fuel injector 3 has been enlarged to form the booster chamber 15 and to accommodate the booster piston 16.

[0096] In the fuel injection system according to the invention, small amounts of the first fuel can leak from the first fuel portion 15A of the booster chamber 15 into the second fuel portion 15B of the booster chamber 15. The leaking first fuel can further flow into the fuel chamber 4 and be injected into a cylinder 2 of the engine 1 . Otherwise the engine 1 can be configured to be operated solely with the second fuel. The first fuel can thus be used only as a hydraulic medium. However, the engine 1 can be configured to be operable using both the first fuel and the second fuel.

[0097] In the embodiment of figure 4, the fuel injection system is provided with a third low-pressure pump 26. The third low-pressure pump 26 is arranged to supply the first fuel to the fuel injectors 3. The fuel injection system comprises a fuel switch valve 27 that allows to selectively connect either the second low-pressure pump 12 or the third low-pressure pump 26 to the fuel injectors 3. The engine 1 can thus be operated using selectively either the first fuel or the second fuel. When the engine 1 is operated using the first fuel, the pressure of the first fuel is raised by means of the pressure booster arrangement in the same way as the pressure of the second fuel. Instead of providing the fuel injection system with the third low-pressure pump 26, the first low-pressure pump could be arranged to supply the first fuel both to the high-pressure pumps 13 and to the fuel injectors 3.

[0098] Instead of using the pressure booster arrangement to pressurize the first fuel when the engine 1 is operated using the first fuel, the fuel injection system could be configured to allow the first fuel to be supplied directly into the fuel injectors 3. The pressure booster arrangement could thus be bypassed when the engine 1 is operated using the first fuel.

[0099] Figure 5 shows a further embodiment of the invention. In the embodiment of figure 5, the pressure booster arrangement comprises a booster chamber block 28 and a separate control valve 17. The booster chamber block 28 comprises the booster chamber 15 and the booster piston 16. The booster chamber block 28 can be attached to a cylinder head.

[0100] Figures 6A and 6B show arrangements for controlling leakages of the first and second fuel. In the arrangement of figure 6A, the booster piston 16 is provided with an annular groove 33. The fuel injection system comprises a fluid channel 29 that opens into the booster chamber 15 in a position where the fluid channel 29 is connected to the cavity formed between the annular groove 33 and the wall of the booster chamber 15. Figure 6B shows an arrangement, where the booster chamber 15 is provided with an annular groove 33’ and a fluid channel 29 is connected to the annular groove 33’.

[0101] In the arrangements of figures 6A and 6B, the fluid channel 29 can be a leakage line. The first and / or second fuel leaking from the first and second fuel portions 15A, 15B of the booster chamber 15 into the annular groove 33, 33’ can thus be supplied via the fluid channel 29 into a tank. That reduces the mixing of the first and second fuel in the fuel injection system.

[0102] Alternatively, the fluid channel 29 could be used for supplying sealing liquid into the annular groove 33, 33’. The sealing liquid could be, for example, the same fuel as the first fuel. The sealing liquid can be supplied into the annular groove at a higher pressure than the pressures in the first and second fuel portions 15A, 15B of the fuel chamber 15. The sealing liquid reduces leakages between the first and second fuel portions 15A, 15B of the fuel chamber 15. Instead, the sealing liquid may leak into the first and second fuel portions 15A, 15B and lubricate the piston 16. In the method according to the invention, the engine 1 is operated by pressurizing the first fuel by means of the first low-pressure pump 11 . The pressure of the first fuel is then raised by means of the high-pressure pumps 13. The second fuel is pressurized by means of the second low-pressure pump 12, and the second fuel is introduced into the fuel chambers 4. The first fuel pressurized by the high-pressure pumps 13 is introduced into the first fuel portions 15A of the booster chambers 15 to raise the pressure of the second fuel and the second fuel is injected into the cylinders 2 of the engine 1 . The fuel injected into the cylinders 2 of the engine 1 can be ignited by compression. However, the injected fuel could also be ignited by means of an ignition device, such as a spark plug, or by means of pilot fuel.

[0103] The method can further comprise the steps of monitoring a parameter indicative of the fuel demand of the engine 1 and / or individual cylinders 2 of the engine 1 and controlling the amount of the second fuel introduced into the cylinders 2 of the engine 1 based on the value of the parameter.

[0104] The control valves 17 can be used to control the amount of the second fuel introduced into the cylinders 2. In addition to or instead of using the control valves 17, the high-pressure pumps 13 can be controlled to control the amount of the second fuel introduced into the cylinders 2. For instance, if the high- pressure pumps 13 are jerk-type fuel pumps with helix-plungers, the rotational positions of the plungers can be adjusted to control the amount of fuel delivered by the high-pressure pumps 13.

[0105] Also, the method can comprise the step of controlling the second low-pressure pump 12 to control the amount of the second fuel introduced into the cylinders 2. By raising the pressure produced by the second low-pressure pump 12, the amount of second fuel trapped in the fuel chamber 4 can be increased, provided that the second fuel is compressible at least to a certain extent.

[0106] As the fuel injection system according to the invention allows using many conventional fuel injection system components for injecting fuels that have poor lubrication properties and / or low viscosity, the fuel injection system according to the invention is beneficial in particular as a retrofit solution.

[0107] Many existing engines are provided with a fuel injection system comprising a low-pressure pump 11 for pressurizing a liquid fuel, such as LFO, cylinder- specific fuel injectors 3 configured to inject the fuel into the cylinders 2, and cylinder-specific high-pressure pumps 13 for raising the pressure of the fuel pressurized by the low-pressure pump 11 .

[0108] In a retrofitting method according to the invention, such an engine 1 is provided with one or more additional low-pressure pumps 12 for pressurizing a second fuel, each fuel injector 3 is connected to one of the additional low-pressure pumps 12 via a check valve 5, the engine 1 is provided for each cylinder 2 of the engine 1 with a booster chamber 15 and a booster piston 16 arranged in the booster chamber 15, the booster piston 16 dividing the booster chamber 15 into a first fuel portion 15A and a second fuel portion 15B, a fuel outlet of each high-pressure pump 13 is connected to the first fuel portion 15A of the respective booster chamber 15, and the second fuel portion 15B of each booster chamber 15 is connected to the respective fuel injector 3 such that the pressure of the second fuel introduced into the fuel injector 3 can be raised by means of the booster piston 16.

[0109] Furthermore, a control valve 17 can be arranged between each high-pressure pump 13 and the respective booster chamber 15 to control the amount of first fuel supplied from the high-pressure pump 13 into the first fuel portion 15A of the booster chamber 15.

[0110] In certain situations, some modifications of the engine may be needed in the retrofitting method. For instance, the profiles of the cams that drive the high- pressure pumps may need to be modified to make the timing of the high-pressure pumps suitable for the injection of the second fuel.

[0111] Various modifications of the invention are possible. For instance, the fuel injection system according to the invention could comprise two or more first low- pressure pumps and / or two or more second low-pressure pumps. The additional low-pressure pumps could be arranged in parallel for redundancy, or each low-pressure pump could supply fuel only to some high-pressure pumps or fuel injectors of the engine.

Claims

Claims:1 . A fuel injection system for a multi-cylinder piston engine (1 ), the fuel injection system comprising- a first low-pressure pump (11 ) for pressurizing a first fuel,- a second low-pressure pump (12) for pressurizing a second fuel,- for each cylinder (2) of the engine (1 ) a fuel injector (3) configured to inject fuel into the cylinder (2),- for each cylinder (2) of the engine (1 ) a high-pressure pump (13) for raising the pressure of the first fuel pressurized by the first low-pressure pump (11 ), and- for each cylinder (2) of the engine (1 ) a pressure booster arrangement configured to raise the pressure of the second fuel pressurized by the second low-pressure pump (12) prior to injection through the fuel injector (3), wherein the pressure booster arrangement comprises- a fuel chamber (4) arranged in fluid communication with the second low-pressure pump (12) to receive the second fuel pressurized by the second low-pressure pump (12),- a booster chamber (15), and- a booster piston (16) arranged in the booster chamber (15), the booster piston (16) dividing the booster chamber (15) into a first fuel portion (15A) and a second fuel portion (15B), wherein the pressure booster arrangement is configured to allow supply of the first fuel pressurized by the high-pressure pump (13) into the first fuel portion (15A) of the booster chamber (15) to exert a hydraulic force onto a first piston surface (16A) of the booster piston (16), and the second fuel portion (15B) of the booster chamber (15) is arranged in fluid communication with the fuel chamber (4) such that when said hydraulic force is exerted onto the first piston surface (16A) of the booster piston (16), the pressure of the second fuel in the fuel chamber (4) is raised.

2. The fuel injection system according to claim 1 , wherein the pressure booster arrangement comprises an inlet check valve (5), via which the second fuel can be introduced from the second low-pressure pump (12) into the fuel chamber (4).

3. The fuel injection system according to claim 1 or 2, wherein the fuel chamber (4) forms part of the fuel injector (3).

4. The fuel injection system according to claims 2 and 3, wherein the inlet check valve (5) is arranged in connection with the fuel injector (3).

5. The fuel injection system according to any of the preceding claims, wherein the high-pressure pump (13) is a jerk-type fuel pump.

6. The fuel injection system according to claim 5, wherein the high-pressure pump (13) comprises a rotatable helix-plunger for controlling fuel supply rate of the high-pressure pump (13).

7. The fuel injection system according to any of the preceding claims, wherein the fuel injection system comprises for each cylinder (2) of the engine (1 ) a control valve (17) for controlling the supply of the first fuel from the high-pressure pump (13) into the first fuel portion (15A) of the booster chamber (15).

8. The fuel injection system according to claim 7, wherein the control valve (17) is a solenoid valve.

9. The fuel injection system according to claim 7 or 8, wherein the control valve (17) has a first position connecting a fuel outlet of the high-pressure pump (13) to a drain line (21 ) and a second position connecting the fuel outlet of the high-pressure pump (13) to the first fuel portion (15A) of the booster chamber (15).

10. The fuel injection system according to any claims 7-9, wherein the control valve (17) and the booster chamber (15) are arranged in a pressure booster unit (24) that is arranged between the high-pressure pump (13) and the fuel injector (3).

11. The fuel injection system according to any of claims 1 -9, wherein the booster chamber (15) is arranged at least partly within the fuel injector (3).

12. The fuel injection system according to any of claims 1 -9, wherein the booster chamber (15) is arranged in a booster chamber block (28) that is arranged between the high-pressure pump (13) and the fuel injector (3).

13. The fuel injection system according to any of the preceding claims, wherein the second low-pressure pump (12) is configured to raise the pressure of the second fuel to 4-50 bar.

14. The fuel injection system according to any of the preceding claims, wherein the pressure booster arrangement is configured to raise the pressure of the second fuel such that the injection pressure exceeds the cylinder pressure during the injection by at least 50 bar.

15. The fuel injection system according to any of the preceding claims, wherein the pressure booster arrangement is configured to raise the pressure of the second fuel to at least 250 bar.

16. The fuel injection system according to any of the preceding claims, wherein the booster piston (16) has a second piston surface (16B) delimiting the second fuel portion (15B) of the booster chamber (15), and the area of the second piston surface (16B) is 5-50 percent larger than the area of the first piston surface (16A).

17. The fuel injection system according to claim 16, wherein the area of the second piston surface (16B) is 10-30 percent larger than the area of the first piston surface (16A).

18. The fuel injection system according to any of the preceding claims, wherein the booster piston (16) and / or the booster chamber (15) is provided with an annular groove (33, 33’) and the fluid injection system comprises a fluid channel (29) that is connected to the annular groove (33, 33’).

19. The fuel injection system according to claim 18, wherein the fluid channel (29) is configured to discharge the first fuel and the second fuel leaking from the first fuel portion (15A) and the second fuel portion (15B) of the booster chamber (15) into the annular groove (33, 33’).

20. The fuel injection system according to claim 18, wherein the fluid channel (29) is configured to supply pressurized sealing liquid into the annular groove (33, 33’).

21. The fuel injection system according to any of the preceding claims, wherein the second fuel is methanol, ethanol or ammonia.

22. The fuel injection system according to any of the preceding claims, wherein the first fuel is LFO.

23. A piston engine (1 ) comprising a fuel injection system according to any of the preceding claims.

24. A method of operating a piston engine (1 ) according to claim 23, the method comprising the steps of- pressurizing the first fuel by means of the first low-pressure pump (11 ).- raising the pressure of the first fuel by means of the high-pressure pumps (13),- pressurizing the second fuel by means of the second low-pressure pump (12),- introducing the second fuel into the fuel chambers (4),- introducing the first fuel pressurized by the high-pressure pumps (13) into the first fuel portions (15A) of the booster chambers (15) to raise the pressure of the second fuel, and- injecting the second fuel into the cylinders (2) of the engine (1 ).

25. The method according to claim 24, wherein the method comprises the steps of monitoring a parameter indicative of the fuel demand of the engine (1 ) and / or individual cylinders (2) of the engine (1 ) and controlling the amount of the second fuel introduced into the cylinders (2) of the engine (1 ) based on the value of the parameter.

26. The method according to claim 25, wherein the method comprises the step of using control valves (17) that control the amount of the first fuel introduced into the first fuel portions (15A) of the booster chambers (15) to control the amount of the second fuel introduced into the cylinders (2).

27. The method according to claim 25 or 26, wherein the method comprises the step of controlling the high-pressure pumps (13) to control the amount of the second fuel introduced into the cylinders (2).

28. The method according to any of claims 25-27, wherein the method comprises the step of controlling the second low-pressure pump (12) to control the amount of the second fuel introduced into the cylinders (2).

29. A method of retrofitting a multi-cylinder piston engine (1 ) comprising- a first low-pressure pump (11 ) for pressurizing a first fuel,- for each cylinder (2) of the engine (1 ) a fuel injector (3) configured to inject fuel into the cylinder (2), and- for each cylinder (2) of the engine (1 ) a high-pressure pump (13) for raising the pressure of the first fuel pressurized by the first low-pressure pump (11 ), the method comprising the steps of- providing the engine (1 ) with one or more second low-pressure pumps (12) for pressurizing a second fuel,- connecting each fuel injector (3) to one of said one or more second low-pressure pumps (12) via a check valve (5),- providing the engine (1 ) for each cylinder (2) of the engine (1 ) with a booster chamber (15) and a booster piston (16) arranged in the booster chamber (15), the booster piston (16) dividing the booster chamber (15) into a first fuel portion (15A) and a second fuel portion (15B),- connecting a fuel outlet of each high-pressure pump (13) to the first fuel portion (15A) of the respective booster chamber (15), and- connecting the second fuel portion (15B) of each booster chamber (15) to the respective fuel injector (3) such that the pressure of the second fuel introduced into the fuel injector (3) can be raised by means of the booster piston (16).

30. The method according to claim 29, wherein the method comprises the step of arranging a control valve (17) between each high-pressure pump (13) and the respective booster chamber (15) to control the amount of first fuel supplied from the high-pressure pump (13) into the first fuel portion (15A) of the booster chamber (15).

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