High-pressure fuel pump, fuel injection system and method of operating a piston engine

Abstract

The high-pressure fuel pump (1) for a piston engine (20) comprises an operating fluid chamber (2), a plunger (3) protruding into the operating fluid chamber (2) and configured to be moveable to pressurize the operating fluid, a fuel chamber (4), a piston (5) having a first piston surface (5A) delimiting the fuel chamber (4) and a second piston surface (5B) facing an opposite direction and exposed to the pressure of the operating fluid chamber (2), the piston (5) being moveable to pressurize the fuel in the fuel chamber (4), and control means (10) for controlling discharging of the operating fluid from the operating fluid chamber (2) via a fluid port (6, 7) of the operating fluid chamber (2) to allow control of the stroke length of the piston (5) to adjust the amount of fuel delivered by the high-pressure fuel pump (1) during a single stroke of the plunger (3).

Description

[0001] High-pressure fuel pump, fuel injection system and method of operating a piston engine

[0002] Technical field of the invention

[0003] The invention concerns a high-pressure fuel pump for a piston engine, as defined in claim 1. The invention also concerns a fuel injection system for a piston engine. The invention further concerns a method of operating a piston engine comprising a high-pressure fuel pump.

[0004] Background of the invention

[0005] In compression ignited piston engines fuel injection pressures are typically very high, ranging from a few hundred bar to 3000 bar or even above that. The lower end of the pressure range may be used in fuel injection systems that are provided with traditional cylinder specific jerk-pumps, whereas in common rail fuel injection systems, the injection pressures are typically higher and closer to the upper end of the pressure range.

[0006] The fuel to be injected into the cylinders of the engine is pressurized by means ofone or more high-pressure fuel pumps. A traditional high-pressure fuel pump is of a jerk-type where the pump comprises a reciprocating plunger protruding into a pump chamber. The plunger is moved, i.e. jerked, by a rotating cam pushing the plunger. A retracting movement of the plunger draws fuel into the pump chamber and when the plunger is moved forward in the pump chamber by the cam, the fuel is pressurized.

[0007] Jerk-type pumps can be divided into constant stroke pumps and helix-type pumps. In the former the volume of fuel delivered by a single stroke of the plunger is controlled by flow valves. An intake valve controls the intake flow during retraction of the plunger, and towards the end of the pressurizing stroke of the plunger, a spill valve bypasses, i.e. prevents the fuel from entering into the high-pressure fuel line thus controlling the delivery volume of the pressurized fuel. In a helix-pump, the fuel volume is adjusted by rotating the plunger which alters mechanically the timing of the spill port opening via the helical groove arranged in the side of the plunger itself.

[0008] Although the prior art high-pressure fuel pumps are relatively simple and reliable, they suffer from certain drawbacks. In case the suction of fuel into the pump chamber is controlled by a flow control valve, there is a significant risk of cavitation in the pump inlet. Also, the operation of a spill valve as a separate valve in a constant stroke pump, or as integrated feature in the rotating plunger of a helix-type pump, is not without issues. As in any high-pressure pump, the high pressures require very tight mechanical tolerances and high accuracy to prevent leakages of the components including valves and plunger / chamber arrangements. Due to the high pressure in the pump chamber, the fuel can for example leak into the chamber where the drive mechanism of the pump is located.

[0009] Despite the above-mentioned problems, prior art high-pressure fuel pumps operate satisfactorily when conventional fuel, such as diesel and light fuel oil are used. The use of sustainable fuels may, however, cause additional problems and challenges. Diesel and light fuel oil have relatively good lubrication properties and higher viscosity, but for example methanol or ammonia have very poor lubrication properties and significantly lower viscosity. This requires smaller tolerances in the plunger / chamber structure and can also lead to rapid wear of the plunger and the pump chamber. Also, if such fuels leak out even internally in the pump, they may damage the pump components and pose a safety hazard.

[0010] Summary of the invention

[0011] An object of the invention is to provide an improved high-pressure fuel pump that solves at least some of the problems discussed above. Other objects of the invention are to provide a fuel injection system for a piston engine and a method of operating a piston engine comprising a high-pressure fuel pump. According to the invention, the high-pressure fuel pump for a piston engine comprises

[0012] - an operating fluid chamber having one or more fluid ports for introducing operating fluid into the operating fluid chamber and for discharging operating fluid from the operating fluid chamber,

[0013] - a plunger protruding into the operating fluid chamber, the plunger being configured to be arranged in force transmission connection with a drive element for moving the plunger in a reciprocating manner in the operating fluid chamber such that with forward motion of the plunger the operating fluid in the operating fluid chamber can be pressurized, - a fuel chamber having a fuel inlet for introducing fuel into the fuel chamber and a fuel outlet for discharging fuel from the fuel chamber, - a piston having a first piston surface delimiting the fuel chamber and a second piston surface facing an opposite direction relative to the first piston surface and exposed to the pressure of the operating fluid chamber, the piston being moveable in a reciprocating manner, wherein forward motion of the piston is configured to pressurize the fuel in the fuel chamber, and

[0014] - actively controllable control means for controlling discharging of the operating fluid from the operating fluid chamber via one or more of said one or more fluid ports of the operating fluid chamber, wherein the control means allow control of the stroke length of the piston to adjust the amount of fuel delivered by the high-pressure fuel pump during a single stroke of the plunger.

[0015] The fuel injection system according to the invention comprises a high-pressure fuel pump defined above.

[0016] The method according to the invention comprises the steps of

[0017] - monitoring a control parameter indicative of the fuel demand of the engine and / or of an individual cylinder of the engine, and

[0018] - controlling the operation of the control means of the high-pressure fuel pump based on the value of the control parameter to adjust the fuel delivery rate of the high-pressure fuel pump.

[0019] In the high-pressure fuel pump according to the invention, a restricting flow control valve is not needed on the suction side of the fuel chamber, and the risk of cavitation is thus significantly reduced. The operating fluid isolates the plunger and the drive mechanism of the pump from the fuel side of the pump, thus protecting components of the pump from the fuel and assisting in high-pressure sealing of the moving parts. This helps avoiding problems even when fuel with poor lubrication properties and / or low viscosity is pressurized by the pump.

[0020] The plunger of the pump can have a fixed stroke length, while the stroke length of the piston can be varied by adjusting the operating fluid volume in the operating fluid chamber by means of the control means of the pump. The stroke length of the piston can thus be adjusted via this trapped operating fluid volume even between two consecutive strokes of the pump to adjust the flow rate delivered by the pump.

[0021] According to an embodiment of the invention, the areas of the first piston surface and the second piston surface are non-equal. With non-equal areas, the piston of the pump can function as a pressure amplifier or pressure reducer. However, the areas of the first and second piston surfaces can also be equal. According to an embodiment of the invention, said one or more fluid ports of the operating fluid chamber comprise a fluid inlet for introducing operating fluid into the operating fluid chamber and a fluid outlet for discharging operating fluid from the operating fluid chamber, and the control means are configured to control discharging of the operating fluid from the operating fluid chamber via the fluid outlet. This allows controlling of the outflow from the operating fluid chamber for example by means of a simple flow control valve. However, the pump could also be provided with a single fluid port that is used both for introducing fluid into the operating fluid chamber and for discharging fluid from the operating fluid chamber.

[0022] According to an embodiment of the invention, the control means comprise a control valve. A control valve allows simple control of the discharging of the operating fluid. The control valve can be for example mechanically, hydraulically or electrically controlled valve.

[0023] According to an embodiment of the invention, the control valve is a solenoid valve. A solenoid valve allows quick and simple control of the discharging of the operating fluid.

[0024] According to an embodiment of the invention, the control means comprise an actuator for rotating the plunger to adjust the position of a control edge of the plunger relative to the fluid outlet. In this embodiment, a control valve is not needed to control the trapped volume of the operating fluid. The control edge of the plunger can be configured to control either opening or closing of the fluid outlet or both. If the control edge controls the opening of the fluid outlet of the operating fluid chamber, by rotating the plunger releasing of the pressure from the operating fluid chamber can be either advanced or delayed. If the control edge controls the closing of the fluid outlet, by rotating the plunger the start of the pressure build-up in the operating fluid chamber can be either advanced or delayed. With different rotational positions of the plunger, different amounts of trapped volume can be created to generate different stroke lengths of the piston even when the physical plunger stroke length remains the same.

[0025] According to an embodiment of the invention, the pump comprises a plunger biasing spring for biasing the plunger against the drive element. The drive element can be, for example, a cam. In many cases the plunger biasing spring is not necessary, for example if the plunger is fixedly connected to a drive element, such as a crankshaft of the fuel pump or a swashplate.

[0026] According to an embodiment of the invention, the pump comprises a piston biasing spring for biasing the piston towards the plunger. The piston biasing spring helps ensuring that the pressure in the operating fluid chamber is higher than the pressure in the fuel chamber to prevent leakage of fuel into the operating fluid chamber.

[0027] According to an embodiment of the invention, the pump comprises a pump chamber that is divided by the piston into the operating fluid chamber and the fuel chamber. This allows placing the plunger and the piston in the same chamber in a block forming the body of the fuel pump. However, the operating fluid chamber and the fuel chamber could also be separate chambers in the block. The chambers could even be arranged in separate blocks and the pump could comprise a connection, such as a pipe, for supplying the operating fluid to the piston of the fuel pump.

[0028] According to an embodiment of the invention, the pump comprises a connecting spring connecting the piston to the plunger. The connecting spring keeps the plunger and the piston separated when the control means allow outflow from the operating fluid chamber. However, the connecting spring is not necessary.

[0029] According to an embodiment of the invention, the pump comprises an operating fluid check valve for preventing backflow from the operating fluid chamber into an operating fluid supply line.

[0030] According to an embodiment of the invention, the pump comprises a fuel inlet check valve for preventing backflow from the fuel chamber via the fuel inlet. According to an embodiment of the invention, the pump comprises a fuel outlet check valve for preventing backflow into the fuel chamber via the fuel outlet. The check valves provide a simple way of controlling flow directions in the fuel pump. However, the check valves could also be arranged in fluid lines outside of the pump, or the functions of the check valves could be implemented by other valves.

[0031] According to an embodiment of the invention, the plunger has a fixed stroke length. As the fuel delivery rate of the pump can be adjusted using the control means, adjusting of the stroke length of the plunger is not necessary. However, the plunger could have an adjustable stroke length to provide an alternative way of adjusting the fuel delivery rate of the pump.

[0032] According to an embodiment of the invention, the operating fluid is lube oil, hydraulic oil or light fuel oil. The operating fluid could also be some other liquid that allows transmitting hydraulic force from the plunger to the piston. Preferably, the operating fluid is a liquid having good lubricating properties and high enough viscosity to both lubricate and help sealing the relevant parts. If operating fluid is selected to be fuel oil, somewhat less stringent requirements may be placed on the physical tolerances of components as a small amount of fuel oil can be allowed to leak and mix into the main fuel pressurized by the high-pressure fuel pump. In certain situations, such mixture may enhance ignition and combustion properties of the main fuel.

[0033] According to an embodiment of the invention, the fuel is alcohol-based fuel, hydrogen-based fuel or ammonia. The fuel pump according to the invention provides benefits in particular when fuels with poor lubrication properties are pumped. However, the pump could also be used for pressurizing light fuel oil or some other conventional fuel.

[0034] According to an embodiment of the invention, the pump is configured to raise the pressure of the fuel to at least 150 bar.

[0035] According to an embodiment of the invention, the fuel injection system comprises one said high-pressure fuel pump for each cylinder of the engine to pressurize fuel and to control the amount of fuel injected into the cylinder. The high-pressure fuel pump according to the invention allows quick adjustment of the fuel delivery rate and provides thus benefits when used as a cylinder- specific fuel pump. However, the high-pressure fuel pump according to the invention could also be used in a common rail fuel injection system. Multiple high-pressure fuel pump chambers could also be arranged parallel and operated sequentially to form a multi-chamber pump delivering substantially constant flow of high-pressure fuel.

[0036] According to an embodiment of the invention, the fuel delivery rate is adjusted by adjusting the amount of operating fluid discharged from the operating fluid chamber at the beginning of the forward motion of the plunger. By adjusting the amount of operating fluid discharged from the operating fluid chamber at the beginning of the forward motion of the plunger, the operating fluid is discharged at a relatively low pressure. This reduces energy losses compared to a situation where the operating fluid is released from the operating fluid chamber at the end of the forward motion of the plunger. Also, if a control valve is used for controlling the discharge of operating fluid, a less robust control valve can be used.

[0037] According to an embodiment of the invention, the control means are operated to discharge operating fluid from the operating fluid chamber during each cycle of the high-pressure fuel pump or periodically to cool down the fuel pump. By allowing certain flow of the operating fluid through the fuel pump, the service life of the fuel pump can be increased. Also, the effects of thermal expansion on for example the precision of the pump can be avoided. Instead of operating the control means such that operating fluid is discharged from the operating fluid chamber during each cycle of the high-pressure fuel pump or periodically, the fuel pump can be configured to have some inherent through-flow of the operating fluid even when the pump is operated with the maximum piston stroke length. For instance, the pump can be configured to have constant flow of operating fluid from the operating fluid chamber into the fuel chamber, especially when the operating fluid is combustible liquid, such as light fuel oil. Alternatively, or in addition, if the discharging of the operating fluid is controlled by means of a control edge of the plunger, the control edge can be configured such that even with the maximum stroke length of the piston, part of the operating fluid is discharged from the operating fluid chamber during each cycle of the plunger. Brief description of the drawings

[0038] Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

[0039] Fig. 1 shows schematically a cross-sectional viewof a high-pressure fuel pump according to an embodiment of the invention,

[0040] Fig. 2 shows schematically a partly cross-sectional view of a high-pressure fuel pump according to another embodiment of the invention, and

[0041] Fig. 3 shows schematically a fuel injection system of a piston engine according to an embodiment of the invention.

[0042] Detailed description of embodiments of the invention

[0043] Figure 1 shows schematically a simplified cross-sectional viewof a high-pressure fuel pump 1 according to an embodiment of the invention. The fuel pump 1 can be used as a fuel pump of a piston engine 20, such as a marine engine or a powerplant engine. The piston engine 20 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. Figure 2 shows schematically a simplified view of a fuel injection system of a piston engine 20 according to an embodiment of the invention.

[0044] The fuel pump 1 is configured to pressurize fuel that is supplied to the pump 1 in liquid phase. The fuel can be liquid fuel that is liquid at atmospheric pressure and at temperature of 20 °C. However, the fuel could also be gaseous at atmospheric pressure and at temperature of 20 °C but supplied to the pump 1 at a pressure that is sufficient for keeping the fuel liquid. The fuel could be, for instance, a conventional liquid fuel used in compression ignited engines, such as light fuel oil, heavy fuel oil or marine diesel oil. However, the fuel can also be sustainable fuel . Examples of sustainable fuels include alcohol-based fuels, such as methanol or ethanol, ammonia and hydrogen-based fuels.

[0045] The term “high-pressure fuel pump” refers here to a pump that is configured to raise the pressure of the fuel to at least 150 bar. This allows the fuel to be injected directly into the main combustion chambers of the engine. Alternatively, or in addition, the fuel or at least part of the fuel could be injected into prechambers of the engine. The fuel can be ignited by compression. However, the fuel could also be ignited by means of an ignition device, such as a spark plug, or by means of pilot fuel.

[0046] The fuel pump 1 comprises an operating fluid chamber 2. The operating fluid chamber 2 has one or more fluid ports 6, 7 for introducing operating fluid into the operating fluid chamber 2 and for discharging operating fluid from the operating fluid chamber 2. In the embodiment of figure 1, the fluid ports include a fluid inlet 6 for introducing operating fluid into the operating fluid chamber 2 and a fluid outlet 7 for discharging operating fluid from the operating fluid chamber 2. However, the operating fluid could be both introduced into the operating fluid chamber 2 and discharged from the operating fluid chamber 2 via a single fluid port, if the fuel pump 1 was provided with a suitable valve arrangement. The operating fluid is liquid. The operating fluid needs to be suitable for transmitting force from one piston to another piston, i.e. for use as hydraulic fluid. The operating fluid can be, for instance, light fuel oil (LFO), lube oil or hydraulic oil. Normally, the operating fluid is different from the fuel pressurized by the fuel pump 1. However, in certain situations the operating fluid could be the same as the fuel pressurized by the fuel pump 1. For instance, the fuel pump 1 could be selectively connectable to two or more fuel sources. For instance, the fuel pump 1 could be connectable to a fuel tank containing sustainable fuel, but also to a fuel tank containing light fuel oil to allow operating the engine with light fuel oil when sustainable fuel is not available. In such a situation, both the fuel and the operating fluid could be LFO.

[0047] The fuel pump 1 further comprises a plunger 3 protruding into the operating fluid chamber 2. The plunger 3 is configured to be arranged in force transmission connection with a drive element 11 for moving the plunger 3 in a reciprocating manner in the operating fluid chamber 2 such that with forward motion of the plunger 3 the operating fluid in the operating fluid chamber 2 can be pressurized. In the embodiment of figure 1, the drive element 11 is a cam. However, even some other kind of drive element could be used. For example, the drive element 11 could be a swashplate or a crankshaft of the fuel pump 1. The drive element 11 can form part of the pump 1. However, the plunger 3 can also be configured to be moved by an external drive element. The fuel pump 1 comprises a fuel chamber 4 having a fuel inlet 8 for introducing fuel into the fuel chamber 4 and a fuel outlet 9 for discharging fuel from the fuel chamber 4. The pump 1 further comprises a piston 5 having a first piston surface 5A delimiting the fuel chamber 4 and a second piston surface 5B facing an opposite direction relative to the first piston surface 5A and exposed to the pressure of the operating fluid chamber 2. The piston 5 is moveable in a reciprocating manner, wherein forward motion of the piston 5 is configured to pressurize the fuel in the fuel chamber 4. In the embodiment of figure 1, the area of the second piston surface 5B is equal to the area of the first piston surface 5A. Also, the area of each of the first piston surface 5A and the second piston surface 5B is equal to the area of the end surface of the plunger 3 in the operating fluid chamber 2. If the areas of the first piston surface 5A and the second piston surface 5B are equal, the pressures in the operating fluid chamber 2 and in the fuel chamber 4 are close to each other. The first piston surface 5A could be smaller than the second piston surface 5B, in which case the piston 5 would function as a pressure amplifier. If the first piston surface 5Awas larger than the second piston surface 5B, the piston 5 would function as a pressure reducer.

[0048] The fuel pump 1 further comprises actively controllable control means 10 for controlling discharging of the operating fluid from the operating fluid chamber 2 via one or more of the one or more fluid ports 6,7 of the operating fluid chamber 2. The control means 10 allow control of the stroke length of the piston 5 to adjust the amount of fuel delivered by the high-pressure fuel pump 1 during a single stroke of the plunger 3.

[0049] In the embodiment of figure 1, the control means 10 are configured to control discharging of the operating fluid from the operating fluid chamber 2 via the fluid outlet 7.

[0050] In the embodiment of figure 1, the control means comprise a control valve 10, which is a solenoid valve. The control valve could also be some other kind of mechanically, hydraulically or electrically controlled valve.

[0051] In the embodiment of figure 1 , the pump 1 comprises a plunger biasing spring 12 for biasing the plunger 3 against the drive element 11. The pump 1 also comprises a piston biasing spring 13 for biasing the piston 5 towards the plunger3. The pump 1 comprises a connecting spring 14 connecting the piston 5 to the plunger 3.

[0052] In the embodiment of figure 1 , the pump 1 comprises a pump chamber that is divided by the piston 5 into the operating fluid chamber 2 and the fuel chamber 4. However, the operating fluid chamber 2 and the fuel chamber 4 could also be arranged in some other way. For instance, the operating fluid chamber 2 and the fuel chamber 4 does not need to be coaxial, but the pressurized operating fluid could be supplied via a conduit to the piston 5 to apply pressure on the second piston surface 5B of the piston 5. The operating fluid chamber 2 and the fuel chamber 4 could even be arranged in separate blocks that are connected by a pipe conducting operating fluid from the operating fluid chamber 2 to the piston 5.

[0053] In the embodiment of figure 1 , the pump 1 comprises an operating fluid check valve 15 for preventing backflow from the operating fluid chamber 2 via the operating fluid inlet 6. The pump 1 further comprises a fuel inlet check valve 16 for preventing backflow from the fuel chamber 4 via the fuel inlet 8 and a fuel outlet check valve 17 for preventing backflow into the fuel chamber 4 via the fuel outlet 9. The check valves 15, 16 and 17 are not essential parts of the pump 1 , but the functions of one or more of the check valves 15, 16, 17 could also be implemented by means of check valves arranged in inlet or outlet lines connected to the pump 1 or by means of suitable pressure-controlled valves. In the fuel pump 1 according to the invention, the amount of fuel delivered by a single stroke of the plunger 3 can be adjusted by controlling the control means 10 that control outflow from the operating fluid chamber 2. Adjustment of the stroke length of plunger 3 is thus not necessary, and the plunger 3 can have a fixed stroke length. However, the stroke length could also be adjustable.

[0054] The operation of the fuel pump 1 is described below. The fuel pump 1 can be operated at least in three different modes. In a first operating mode, the control valve 10 is kept closed. As the drive element 11 moves the plunger 3 from a retracted position to a protruding position, i.e. in a forward direction, the operating fluid in the operating fluid chamber 2 is pressurized. The operating fluid check valve 15 prevents flow from the operating fluid chamber 2 into an inlet line of the operating fluid chamber 2. The control valve 10 prevents outflow from the operating fluid chamber 2 into an outlet line of the operating fluid chamber 2. Consequently, pressure is applied onto the second piston surface 5B of the piston 5, and the piston 5 is moved in the forward direction. The piston 5 pressurizes the fuel in the fuel chamber 4 and the fuel is discharged from the fuel chamber 4 via the fuel outlet 9. When the drive element 11 reaches a position where retracting movement of the plunger 3 is allowed, the plunger biasing spring 12 pushes the plunger 3 towards the retracted position and the piston biasing spring 13 pushes the piston 5 towards the retracted position. Fuel is supplied into the fuel chamber 4 via the fuel inlet 8. In the first operating mode, the stroke length of the piston 5 is equal to the stroke length of the plunger 3 and the amount of fuel delivered by a single stroke of the plunger 3 is substantially equal to the displacement volume of the plunger 3. If the sizes of the first piston surface 5A and the second piston surface 5B were not equal, the amount of fuel delivered by a single stroke of the plunger 3 would depend on the displacement volume of the plunger 3 and the ratio between the sizes of the first piston surface 5A and the second piston surface 5B. In a second operating mode, the control valve 10 is kept open. Now during the forward motion of the plunger 3, the operating fluid is discharged from the operating fluid chamber 2 via the control valve 10. Consequently, only a small pressure caused by throttling in the control valve 10 and the outlet line of the operating fluid chamber 2 is applied to the second piston surface 5B of the piston 5. The piston 5 is thus not moved and the fuel in the fuel chamber 4 is not pressurized. During the retracting movement of the plunger 3, operating fluid can flow into the operating fluid chamber 4 via the fluid inlet 6 of the operating fluid chamber 2.

[0055] In a third operating mode, the control valve 10 is actively controlled. The control valve 10 can be kept open either at the beginning or at the end of the forward motion of the plunger 3 to reduce the stroke length of the piston 5 or both at the beginning and at the end of the forward motion. If the control valve 10 is kept open at the beginning of the forward motion of the plunger 3, the forward motion of the piston 5 does not begin until the control valve 10 is closed and outflow from the operating fluid chamber 2 is prevented. If the control valve 10 is opened during the forward motion of the plunger 3, pressure is released from the operating fluid chamber 2 and consequently the forward motion of the piston 5 is terminated. In both cases, the stroke length of the piston 5 is reduced and the amount of fuel delivered by a single stroke of the plunger 3 is smaller than the displacement volume of the plunger 3.

[0056] Preferably, when the fuel flow rate of the pump 1 needs to be reduced, the control valve 10 is controlled by keeping the control valve 10 open at the beginning of the forward motion of the plunger 3. Compared to a situation where the piston stroke length is reduced by opening the control valve 10 before the end of the forward motion of the plunger 3, energy losses are reduced. Also, closing the control valve 10 when the pressure in the operating fluid chamber 2 is low causes less mechanical stress to the control valve 10 than a situation where high-pressure operating fluid is released from the operating fluid chamber 2 via the control valve 10.

[0057] In the embodiment of figure 2, the high-pressure fuel pump 1 is similar to the high-pressure fuel pump 1 of figure 1. However, in the embodiment of figure 2, the control means do not comprise a control valve. In the embodiment of figure 2, the plunger 3 is rotatable about its longitudinal axis. The pump 1 comprises an actuator (not shown) for rotating the plunger 3 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 3. The plunger 3 is provided with a control edge 28. The control edge 28 has a helical shape. By rotating the plunger 3, the position of the control edge 28 relative to the fluid outlet 7 of the operating fluid chamber 2 can be adjusted. In the embodiment of figure 2, the rotation of the plunger 3 affects the position where the operating fluid is discharged from the operating fluid chamber 2 close to the end of the forward motion of the plunger 3. By rotating the plunger 3, releasing of the pressure of the operating fluid chamber 2 can be either delayed or advanced. Instead of controlling the releasing of the pressure, the control edge 28 could be configured to control the start of the pressure build-up in the operating fluid chamber 2.

[0058] If LFO or other combustible fuel is used as the operating fluid, controlled leakage from the operating fluid chamber 2 into the fuel chamber 4 could be utilized, for example, to discharge part of the operating fluid from the operating fluid chamber 2 during each cycle of the pump 1. Fresh operating fluid is thus constantly supplied into the operating fluid chamber 2 to cool down the pump 1. Small leakage also improves lubrication of the piston 5. LFO or other combustible liquid can also enhance the combustion process in the cylinders of the engine and lubricate fuel injectors.

[0059] In the embodiment of figure 2, the control edge 28 can be configured such that part of the operating fluid is discharged from the operating fluid chamber 2 during each cycle of the plunger 3 even when the pump 1 is operated with the maximum stroke length of the piston 5 to cool down the pump 1. In the embodiment of figure 1, the control valve 10 can be operated such that part of the operating fluid is discharged from the operating fluid chamber 2 either during each cycle of the plunger 3 or periodically to cool down the pump 1.

[0060] Figure 3 shows a simplified view of a fuel injection system where the fuel pump 1 according to the invention could be used. Figure 3 shows schematically a multicylinder piston engine 20. The engine 20 can be either a two-stroke or four-stroke engine 20. Each cylinder 21 of the engine 20 is provided with at least one fuel injector 22. Each fuel injector 22 is configured to inject fuel either directly into the main combustion chamber of the respective cylinder 21 or into a prechamber. The fuel injectors 22 are configured to open when the pressure of the fuel supplied to the fuel injector 22 exceeds a predetermined limit. The fuel injection system of figure 3 comprises one high-pressure fuel pump 1 according to the invention for each cylinder 21 of the engine 20. The fuel injection system further comprises a low-pressure pump 23 for supplying fuel from a fuel tank 24 to the high-pressure fuel pumps 1. The pressure of the fuel after the low-pressure pump 23 can be, for instance, in the range of 5-20 bar.

[0061] The fuel injection system further comprises an operating fluid pump 25 supplying operating fluid from an operating fluid tank 26 to the high-pressure fuel pumps 1. The fuel injection system further comprises a control unit 27 for controlling the operation of the control valves 10 of the high-pressure fuel pumps 1. The control unit 27 can be a control unit of the engine 20. The control unit 27 can further control the operation of the operating fluid pump 25 and the low-pressure pump 23.

[0062] During operation of the engine 20, the operating fluid is kept pressurized by the operating fluid pump 25 to compensate any operating fluid discharged from the operating fluid chambers 2 of the high-pressure fuel pumps 1 via the control valves 10 and also any leakages from the operating fluid circuit. The low-pressure pump 23 constantly supplies fuel to the high-pressure fuel pumps 1. One or more control parameters indicative of the fuel demand of the engine 20 and / or of individual cylinders 21 of the engine 20 are monitored. Based on the monitoring, the control unit 27 controls the operation of the control valves 10 of the high-pressure fuel pumps 1 to adjust the fuel delivery rate of each high-pressure fuel pump 1. When the engine 20 is operated at full load and the fuel demand is at its maximum, the control valves 10 are kept closed to supply the maximum amount of fuel into the cylinders 21 of the engine 20. When the engine load and fuel demand are lower, the control unit 27 controls the control valves 10 to open before the plunger 3 has reached the fully protruding position to discharge operating fluid from the operating fluid chamber 2 and to terminate the fuel injection earlier. Alternatively, or in addition, the control valves 10 can be closed after the forward motion of the plunger 3 has started to delay fuel injection and to reduce the amount of fuel injected into the cylinder 21.

[0063] In the embodiment of figure 1, the position of the plunger 3 needs to be monitored to be able to control the opening and / or closing of the control valve 10 appropriately. The pump 1 can be provided with means for monitoring the position of the plunger 3. Alternatively, the position of the plunger 3 can be monitored indirectly, for example by monitoring the angular position of a camshaft that drives the plunger 3.

[0064] The drawings are schematic and the high-pressure fuel pump according to the invention can comprise many additional features. For instance, suitable sealing arrangements can be provided to prevent leakages from the fuel chamber into the operating fluid chamber and vice versa, if needed.

[0065] Although the invention has been described above by referring to a fuel injection system having a high-pressure fuel pump for each cylinder, the high-pressure fuel pump according to the invention could also be used as part of a common rail fuel injection system. In particular if the high-pressure fuel pump is used in a common rail fuel injection system, the fuel pump could comprise two or more plunger-piston pairs that are operated in different phases to produce more constant pressure and flow rate. For instance, if the pump comprised two plungerpiston pairs, the plungers could be operated with a 180 degree phase difference. In case of three plunger-piston pairs, the plungers could be operated with a 120 degree phase difference. The high-pressure fuel pump can be arranged in mechanical force transmission connection with the engine, but advantageously the pump is driven using an electric motor. The pump can be driven with constant rpm or with adjustable rpm. Thanks to the inherent adjustability of the pump, driving the pump with constant rpm avoids the complexity, and any associated shortcomings such as cost and durability, of variable speed pump drive arrangements. This simplicity makes the pump easier to retrofit to existing engines and allows use of different fuels which require different fuel quantity delivery even at the same engine / pump speed when a shared fuel system is used to inject two fuels with highly different lower heating values (LHV). Such a situation may be, for example injection of LFO and MeOH, or LFO and NH3 at different times through the same fuel system. The pump according to the invention, thanks to its improved control capabilities, overcomes or mitigates the cavitation issues at the suction-side as well as minimizes the energy losses at the delivery-side.

Claims

Claims:

1. A high-pressure fuel pump (1 ) for a piston engine (20), the high-pressure fuel pump (1) comprising- an operating fluid chamber (2) having one or more fluid ports (6, 7) for introducing operating fluid into the operating fluid chamber (2) and for discharging operating fluid from the operating fluid chamber (2), - a plunger (3) protruding into the operating fluid chamber (2), the plunger (3) being configured to be arranged in force transmission connection with a drive element (11) for moving the plunger (3) in a reciprocating manner in the operating fluid chamber (2) such that with forward motion of the plunger (3) the operating fluid in the operating fluid chamber (2) can be pressurized,- a fuel chamber (4) having a fuel inlet (8) for introducing fuel into the fuel chamber (4) and a fuel outlet (9) for discharging fuel from the fuel chamber (4),- a piston (5) having a first piston surface (5A) delimiting the fuel chamber (4) and a second piston surface (5B) facing an opposite direction relative to the first piston surface (5A) and exposed to the pressure of the operating fluid chamber (2), the piston (5) being moveable in a reciprocating manner, wherein forward motion of the piston (5) is configured to pressurize the fuel in the fuel chamber (4), and- actively controllable control means (10) for controlling discharging of the operating fluid from the operating fluid chamber (2) via one or more of said one or more fluid ports (6, 7) of the operating fluid chamber (2), wherein the control means (10) allow control of the stroke length of the piston (5) to adjust the amount of fuel delivered by the high-pressure fuel pump (1 ) during a single stroke of the plunger (3).

2. The high-pressure fuel pump (1) according to claim 1, wherein said one or more fluid ports (6, 7) of the operating fluid chamber (2) comprise a fluid inlet (6) for introducing operating fluid into the operating fluid chamber (2) and a fluid outlet (7) for discharging operating fluid from the operating fluid chamber (2), and the control means (10) are configured to control discharging of the operating fluid from the operating fluid chamber (2) via the fluid outlet (7).

3. The high-pressure fuel pump (1) according to claim 1 or 2, wherein the control means comprise a control valve (10).

4. The high-pressure fuel pump (1 ) according to claim 3, wherein the control valve (10) is mechanically, hydraulically or electrically controlled.

5. The high-pressure fuel pump (1) according to claim 3 or 4, wherein the control valve (10) is a solenoid valve.

6. The high-pressure fuel pump (1 ) according to claim 2, wherein the control means (10) comprise an actuator for rotating the plunger (3) to adjust the position of a control edge (28) of the plunger (3) relative to the fluid outlet (7).

7. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1) comprises a plunger biasing spring (12) for biasing the plunger (3) against the drive element (11).

8. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1) comprises a piston biasing spring (13) for biasing the piston (5) towards the plunger (3).

9. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1 ) comprises a pump chamber that is divided by the piston (5) into the operating fluid chamber (2) and the fuel chamber (4).

10. The high-pressure fuel pump (1 ) according to claim 9, wherein the pump (1) comprises a connecting spring (14) connecting the piston (5) to the plunger (3).

11. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1) comprises an operating fluid check valve (15) for preventing backflow from the operating fluid chamber (2) into an operating fluid supply line.

12. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1) comprises a fuel inlet check valve (16) for preventing backflow from the fuel chamber (4) via the fuel inlet (8).

13. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1) comprises a fuel outlet check valve (17) for preventing backflow into the fuel chamber (4) via the fuel outlet (9).

14. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the plunger (3) has a fixed stroke length.

15. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the areas of the first piston surface (5A) and the second piston surface (5B) are non-equal.

16. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the operating fluid is lube oil, hydraulic oil or light fuel oil.

17. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the fuel is alcohol-based fuel, hydrogen-based fuel or ammonia.

18. The high-pressure fuel pump (1) according to any of the preceding claims, wherein the pump (1) is configured to raise the pressure of the fuel to at least 150 bar.

19. A fuel injection system for a piston engine (20), the fuel injection system comprising a high-pressure fuel pump (1 ) according to any of the preceding claims.

20. The fuel injection system according to claim 19, wherein the fuel injection system comprises one said high-pressure fuel pump (1 ) for each cylinder (21 ) of the engine (20) to pressurize fuel and to control the amount of fuel injected into the cylinder (21).

21. A method of operating a piston engine (20) comprising a high-pressure fuel pump (1 ) according to any of claims 1 -18, the method comprising the steps of- monitoring a control parameter indicative of the fuel demand of the engine (20) and / or of an individual cylinder (21) of the engine (20), and- controlling the operation of the control means (10) of the high-pressure fuel pump (1) based on the value of the control parameter to adjust the fuel delivery rate of the high-pressure fuel pump (1 ).

22. The method of claim 21 , wherein the fuel delivery rate is adjusted by ad- justing the amount of operating fluid discharged from the operating fluid chamber (2) at the beginning of the forward motion of the plunger (3).

23. The method according to claim 21 or 22, wherein the control means (10) are operated to discharge operating fluid from the operating fluid chamber (2) during each cycle of the high-pressure fuel pump (1) or periodi- cal ly to cool down the fuel pump (1 ).

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