EP3262294B1

EP3262294B1 - Fuel injection system

Info

Publication number: EP3262294B1
Application number: EP15707406.3A
Authority: EP
Inventors: Antti VUOHIJOKI
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2015-02-23
Filing date: 2015-02-23
Publication date: 2019-05-01
Anticipated expiration: 2035-02-23
Also published as: EP3262294A1; WO2016135369A1

Description

Technical field of the invention

The present invention relates to a fuel injection system for a piston engine in accordance with the preamble of claim 1.

Background of the invention

A common rail fuel injection system of a piston engine comprises a fuel rail, where the fuel is stored in a high pressure and from where it is supplied to fuel injectors. The fuel injectors are typically electrically controlled and allow accurate control of the fuel injection timing and the amount of the injected fuel. The fuel injection system further comprises one or more high-pressure pumps, which are arranged to supply the fuel to the fuel rail in a very high pressure, typically in the range of 800 to 3000 bar. The fuel rail is provided with a safety valve that is configured to open when a certain pressure level in the fuel rail is exceeded to prevent overpressure in the fuel rail. One or more low-pressure pumps feed the fuel from a tank to the high-pressure pumps. The fuel flow rate to the high-pressure pumps is controlled with electrical flow control valves, which are arranged between the low-pressure pump and the high-pressure pumps.
Especially in ship engines, it is important that the engine can function also in the case the electrical signal controlling the flow control valves is lost. Therefore, current fuel injection systems are provided with flow control valves that ensure full fuel delivery by the high-pressure pumps even in the case the control of the flow control valves is for some reason lost. In the absence of a control signal, the flow control valves are fully open. This kind of valves are called normally open valves.
If the control of one or more of the flow control valves is lost, the high-pressure pumps concerned supply fuel to the fuel rail at the maximum flow rate. As a result, the pressure in the fuel rail starts to raise and eventually the safety valve opens. The safety valve is configured to hold a certain pressure difference between the fuel rail and a low-pressure fuel system. Typically the pressure difference is approximately 1500 bar. This ensures that the pressure in the fuel rail is kept at a level allowing a safe operation of the engine in a so called limp home mode. Depending on the load of the engine, a certain amount of the fuel that is delivered by the high-pressure pumps needs to flow out of the fuel rail through the safety valve. If the engine load is high, only a small amount of fuel flows through the safety valve, but if the engine load is low, the amount of fuel flowing through the safety valve can be significant. When the pressure of the fuel drops while it flows through the safety valve, potential energy stored in the pressurized fuel is transformed to heat, which is absorbed by the safety valve and the fuel that flows through the valve. Depending on the flow rate of the excess fuel, the amount of the absorbed energy can be very high, which can cause thermal problems. In large engines, the rate of the energy transformation can be tens of kilowatts.
Such a fuel injection system is for example known from EP 2 148 982 A2 .

Summary of the invention

An object of the present invention is to provide an improved fuel injection system for a piston engine The characterizing features of the fuel injection system according to the invention are given in the characterizing part of claim 1.
The fuel injection system according to the invention comprises at least one low-pressure pump for raising the pressure of fuel to a first pressure level, at least one high-pressure pump for raising the pressure of the fuel from the first pressure level to a second pressure level that is higher than the first pressure level, a fuel accumulator that is connected to the high-pressure pump(s) for receiving fuel at the second pressure level, a fuel injector that is connected to the fuel accumulator for receiving pressurized fuel from the fuel accumulator and for injecting the fuel into a cylinder of the engine, and at least two flow control valves, each of the flow control valves being arranged between a low-pressure pump and a high-pressure pump for controlling the amount of the fuel supplied from the low-pressure pump to the high-pressure pump. At least one of the flow control valves is a normally open valve and at least one of the flow control valves is a normally closed valve.
The fuel injection system according to the invention ensures that safe operation of the engine in a so called limp home mode is possible in case there is a failure in the control of the flow control valves, but at the same time energy losses occurring in a safety valve and heating of the safety valve are reduced. According to an embodiment of the invention, the system comprises at least three flow control valves of which each is arranged between a low-pressure pump and a high-pressure pump for controlling the amount of the fuel supplied from said low pressure pump to said high pressure pump. By increasing the number of the flow control valves, the ratio between the normally open and normally closed valves can be better adjusted. For example 20 to 50 percent of the flow control valves can be normally closed valves and the rest normally open valves. This ensures that enough engine power is available and the heat formation in the safety valve is not excessive.
According to an embodiment of the invention, the system comprises at least two high-pressure pumps and at least one flow control valve is arranged between each high-pressure pump and a low-pressure pump for controlling fuel flow to said high-pressure pump.
According to an embodiment of the invention, at least two flow control valves are arranged between each high-pressure pump and a low-pressure pump. According to an embodiment of the invention, at least three flow control valves are arranged between each high-pressure pump and a low-pressure pump. By arranging several flow control valves before each high-pressure pump, the flow rates to the high-pressure pumps can be controlled more precisely during normal operation of the engine and smaller flow control valves can be used.
According to an embodiment of the invention, the system comprises a first high-pressure pump and a second high-pressure pump and a first flow control valve is arranged to control fuel flow to the first high-pressure pump and a second flow control valve is arranged to control fuel flow to the second high-pressure pump. According to an embodiment of the invention, the first flow control valve is a normally open valve and the second flow control valve is a normally closed valve. The system can comprise a third high-pressure pump and a third flow control valve arranged to control fuel flow to the third high-pressure pump. The third flow control valve can be a normally open valve.
According to an embodiment of the invention, in the closed position of the normally closed flow control valves a small leakage flow through the valve is allowed. This ensures that the high-pressure pump that is arranged downstream from the flow control valve is sufficiently lubricated even in the case the flow control valve is closed.
According to an embodiment of the invention, all the fuel accumulators of the fuel injection system are in fluid communication with each other. This ensures that all the fuel injectors of the system receive fuel even in the case that one of several high-pressure pumps is not producing volume flow.

Brief description of the drawings

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

Fig. 1 shows a fuel injection system according to a first embodiment of the invention,
Fig. 2 shows a fuel injection system according to a second embodiment of the invention,
Fig. 3 shows a fuel injection system according to a third embodiment of the invention,
Fig. 4 shows a fuel injection system according to a fourth embodiment of the invention, and
Fig. 5 shows a fuel injection system according to a fifth embodiment of the invention.

Description of embodiments of the invention

Figures 1 to 5 show schematically fuel injection systems for a piston engine 1 according to different embodiments of the invention. The engine 1 is a large internal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. The cylinder bore of the engine 1 is at least 150 mm. The maximum output per cylinder is at least 150 kW.
The figures show a common rail fuel injection system, which is configured to inject liquid fuel, such as light fuel oil, marine diesel oil or marine gas oil into the cylinders 2 of the engine 1. In addition to the fuel injection system of the figures, the engine 1 can be provided with an additional fuel injection system, for instance a fuel system that is configured for gaseous fuel. In the embodiments of the figures, the engine 1 comprises six cylinders 2 that are arranged in line, but the engine 1 can comprise any reasonable number of cylinders 2 and the cylinders 2 could also be arranged for instance in a V-configuration.
The fuel injection system comprises one fuel injector 3 for each cylinder 2 of the engine 1. The fuel injectors 3 are arranged to inject the fuel directly into the cylinders 2 of the engine 1. The fuel injectors 3 are controlled electrically. The fuel injectors 3 are controlled by an electronic control unit 12. The fuel injection system further comprises at least one fuel accumulator 4, 4a, 4b, 4c for storing pressurized fuel. In the embodiments of the figures 1 to 4, the fuel accumulator is a fuel rail 4 and all the fuel injectors 3 are connected directly to the fuel rail 4. In the embodiment of figure 5, the fuel injection system comprises three fuel rails 4a, 4b, 4c and each fuel rail 4a, 4b, 4c serves two fuel injectors 3. The fuel rails 4a, 4b, 4c are in fluid communication with each other. Instead of the fuel rails shown in the figures, each fuel injector 3 could be provided with an own fuel accumulator, which would be in fluid communication with the fuel accumulators of the other fuel injectors 3 of the fuel injection system. In all embodiments shown in the figures, the fuel injection system either comprises only one fuel accumulator or the fuel accumulators belonging to the fuel injection system are in fluid communication with each other. This ensures that in case the fuel injection system is provided with several high-pressure pumps and even if one of the high-pressure pumps is not producing volume flow, all the fuel injectors 3 of the fuel injection system receive pressurized fuel. If the engine 1 is provided with several fuel accumulators and more than one high-pressure pump supplies fuel to each fuel accumulator, then the fuel accumulators do not need to be in fluid communication with each other. For instance, in a V-engine each bank of the engine could be provided with an own fuel rail and two or more high-pressure pumps.
The fuel injection system is provided with at least one high- pressure pump 5, 5a, 5b, 5c for pressurizing the fuel and at with least one low- pressure pump 7, 7a, 7b, 7c for supplying fuel from a tank 8 to the high-pressure pumps 5, 5a, 5b, 5c at a lower pressure. The expression "high-pressure pump" means here that the pump is capable of raising the pressure of the fuel to a level that is required for fuel injection in a common rail fuel injection system and the expression "low-pressure pump" refers to a pump that can work as a feed pump for a high-pressure pump of a common rail fuel injection system. After the low-pressure pumps 7, 7a, 7b, 7c the pressure of the fuel is less than 30 bar, typically the pressure is in the range of 5 to 10 bar. After the high-pressure pumps 5, 5a, 5b, 5c the pressure of the fuel is at least 500 bar. Typically the fuel pressure in the fuel accumulator is 800 to 3000 bar. The expression "pump" can mean either a separate pump or a part of a pump module that comprises two or more separate chambers which are provided with own outlets and plungers so that each plunger produces an own volume flow of pressurized fuel. The high-pressure pumps 5, 5a, 5b, 5c are preferably cam-driven.
The fuel injection system further comprises at least two flow control valves 6a, 6b, 6c, 6d, 6e, 6f for controlling the amount of the fuel that is supplied from the low-pressure pumps 7, 7a, 7b, 7c to the high-pressure pumps 5, 5a, 5b, 5c. The flow control valves 6a, 6b, 6c, 6d, 6e, 6f are actively controllable valves. Preferably the flow control valves 6a, 6b, 6c, 6d, 6e, 6f can be controlled electrically. The flow control valves 6a, 6b, 6c, 6d, 6e, 6f can be controlled by the same electronic control unit 12 as the fuel injectors 3, or a separate control unit can be provided. The flow control valves 6a, 6b, 6c, 6d, 6e, 6f are capable of adjusting the flow rate from the low-pressure pumps 7, 7a, 7b, 7c to the high-pressure pumps 5, 5a, 5b, 5c. The flow control valves 6a, 6b, 6c, 6d, 6e, 6f can be either proportional valves or on/off valves. The flow control valves 6a, 6b, 6c, 6d, 6e, 6f can be servo valves. With proportional valves, the flow rate can be steplessly adjusted. However, a sufficiently accurate flow control could also be achieved with on/off valves, at least if a sufficient number of flow control valves 6a, 6b, 6c, 6d, 6e, 6f is provided.
At least one of the flow control valves 6a, 6b, 6c, 6d, 6e, 6f is a normally open valve and at least one of the flow control valves is a normally closed valve. The expression "normally open valve" means here that when the valve does not receive a control signal, it remains in an open position allowing full flow through the valve. The expression "normally closed valve" means here that when the valve does not receive a control signal it remains in a closed position. However, the normally closed flow control valves are preferably designed so that there is a small leakage flow through the valve even in the closed position to ensure lubrication of the high-pressure pump that is arranged downstream of the flow control valve. The closed position can thus be a minimum flow rate position, in which the flow rate through the flow control valve 6a, 6b, 6c, 6d, 6e, 6f is smaller than the flow rate required for idling of the engine 1. The leakage flow can be for example 1 to 3 percent of the maximum flow rate through the valve. The flow control valves 6a, 6b, 6c, 6d, 6e, 6f can be provided for instance with a spring, which keeps a normally open valve in the open position and a normally closed valve in the closed position when the valve is not actuated.
The fuel injection system is provided with a safety valve 9, which is connected to the fuel accumulator 4, 4a, 4b, 4c. The fuel injection system could also comprise two or more safety valves that are connected to the fuel accumulator 4, 4a, 4b, 4c. The safety valve 9, or in case of several safety valves 9 the valves together, have at least two functions: During normal operation of the engine, the safety valve 9 works as a pressure relief valve, which opens when the pressure in the fuel accumulator 4, 4a, 4b, 4c exceeds a predetermined limit value. The safety valve 9 thus limits the pressure in the fuel accumulator 4, 4a, 4b, 4c. In case of a failure of the high-pressure pump control, the safety valve 9 is configured to hold the pressure of the fuel accumulator 4, 4a, 4b, 4c at a level that is high enough for safe operation of the engine 1. In this "limp home mode", a controlled leakage through the safety valve 9 is allowed. The excess fuel that is not injected into the cylinders 2 of the engine 1 flows thus out of the fuel accumulator 4, 4a, 4b, 4c. The pressure difference over the safety valve 9 in the limp home mode depends on the engine 1, but can be set to be for instance in the range of 1000 to 2000 bar. The flow rate through the safety valve 9 depends on the load of the engine 1. The smaller the load, the greater portion of the fuel flows through the safety valve 9. The flow rate through the safety valve 9 also depends on the flow control valve configuration. The higher the proportion of the normally closed flow control valves is, the smaller is the volume flow of the high-pressure pumps 5, 5a, 5b, 5c and consequently also the volume flow through the safety valve 9.
In the embodiment of figure 1, the fuel injection system comprises one low pressure pump 7 and three high-pressure pumps: a first high-pressure pump 5a, a second high-pressure pump 5b and a third high-pressure pump 5c. The low-pressure pump 7 thus supplies fuel to all high- pressure pumps 5a, 5b, 5c. Each high- pressure pump 5a, 5b, 5c is connected to the same fuel rail 4. The fuel rail supplies 4 fuel to all fuel injectors 3 of the fuel injection system. The fuel injection system comprises three flow control valves: a first flow control valve 6a, a second flow control valve 6b and a third flow control valve 6c. The first flow control valve 6a is arranged between the low-pressure pump 7 and the first high-pressure pump 5a, the second flow control valve 6b is arranged between the low-pressure pump 7 and the second high-pressure pump 5b and the third flow control valve 6c is arranged between the low-pressure pump 7 and the third high-pressure pump 5c. The first and the third flow control valves 6a, 6c are normally open valves and the second flow control valve 6b is a normally closed valve.
During normal operation of the engine 1, each of the flow control valves 6a, 6b, 6c is actively controlled to regulate the amount of fuel that is supplied from the low-pressure pump 7 to the high- pressure pumps 5a, 5b, 5c. Preferably, the flow rate through each of the flow control valves 6a, 6b, 6c is approximately the same. The safety valve 9 remains closed. If the control of the flow control valves 6a, 6b, 6c is lost, the first and the third flow control valves 6a, 6c will automatically open, whereas the second flow control valve 6b will automatically close. Full flow rate through the first and the third flow control valve 6a, 6c is allowed, but only a small leakage flow can flow through the second flow control valve 6b. The engine 1 is switched into the limp home mode, in which the high- pressure pumps 5a, 5b, 5c produce a constant volume flow. Since the second flow control valve 6b is closed, the second high-pressure pump 5b produces practically no volume flow. The first and the third flow control valves 6a, 6c are fully open, and the first and the third high- pressure pumps 5a, 5c produce maximum volume flow. Since one of the three flow control valves 6a, 6b, 6c is closed, the flow rate produced by the high- pressure pumps 5a, 5b, 5c is approximately 67 percent of the full capacity of the flow control valves 6a, 6b, 6c. The flow control valves 6a, 6b, 6c and the high- pressure pumps 5a, 5b, 5c are sized so that the high- pressure pumps 5a, 5b, 5c can produce a volume flow that is slightly greater than the volume flow that is needed for the maximum power of the engine 1. The maximum engine power in the limp home mode is thus slightly more than two thirds of the maximum power.
If the control of only one or two of the flow control valves 6a, 6b, 6c is lost, the maximum power available depends on which of the flow control valves 6a, 6b, 6c are working. For example, in case only the second flow control valve 6b can be controlled, the first and the third high- pressure pumps 5a, 5c produce the maximum flow and by controlling the second flow control valve 6b, the flow can be adjusted between 67 and 100 percent of the maximum flow. If one of the first or the third flow control valve 6a, 6c has failed, the volume flow can be regulated in the range between 33 to 100 percent of the maximum flow. In case only the second flow control valve 6b has failed, the flow can be adjusted between 0 to 67 percent of the maximum flow.
If one or more of the flow control valves 6a, 6b, 6c cannot be controlled, the high- pressure pumps 5a, 5b, 5c may produce more flow than is consumed by the fuel injectors 3. In that case, the excess fuel flow from the high- pressure pumps 5a, 5b, 5c must be conducted via the safety valve 9 into a tank. Preferably the fuel injection system is provided with a separate tank 11, which allows the fuel to cool down before being recirculated into the fuel tank 8 for fuel injection. The safety valve 9 is configured to maintain a constant pressure difference between the fuel rail and the tank 11, for instance 1500 bar. Compared to a fuel injection system where all the flow control valves 6a, 6b, 6c are normally open valves, the maximum flow rate through the safety valve 9 is significantly smaller. This reduces the rate at which potential energy of the fuel is converted to heat when the excess fuel flows through the safety valve 9. Energy losses and problems caused by the heat are thus reduced. The fuel injection system of figure 1 could be provided with more than three flow control valves 6a, 6b, 6c and high- pressure pumps 5a, 5b, 5c. If the fuel injection system comprised for instance four flow control valves and high-pressure pumps, either one or two of the flow control valves could be normally closed valves. The flow rate produced by the high-pressure pumps in the limp home mode would thus be either 50 or 75 percent of the maximum flow rate. A higher number of the normally closed valves reduces the maximum power of the engine in the limp home mode, but it also reduces energy losses and heat formation. The suitable proportion of the normally closed flow control valves depends on the application, and could be in the range of 20 to 50 percent.
Figure 2 shows a fuel injection system according to a second embodiment of the invention. In the embodiment of figure 2, the fuel injection system is provided with only one high-pressure pump 5. Also in this embodiment, the fuel injection system is provided with a first, second and third flow control valves 6a, 6b, 6c. The fuel injection system comprises a single low-pressure pump 7 feeding the high-pressure pump 5 through the flow control valves 6a, 6b, 6c. The first and the third flow control valves 6a, 6c are normally open valves and the second flow control valve 6b is a normally closed valve. During normal operation of the engine and at maximum load, fuel is supplied to the high-pressure pump 5 via three flow control valves 6a, 6b, 6c. When the fuel demand of the engine 1 is lower, one or two of the flow control valves 6a, 6b, 6c can be inactivated. Fuel is thus supplied to the high-pressure pump 5 through only one or two of the flow control valves 6a, 6b, 6c. The active flow control valves 6a, 6b, 6c can thus be operated within the optimal flow rate range, which helps to reduce wear of the flow control valves 6a, 6b, 6c and to avoid cavitation. Also more precise flow rate control can be achieved. In the embodiment of figure 2, on/off valves could be used as the flow control valves 6a, 6b, 6c. In case of on/off valves, the cross-sectional flow area through each of the valves 6a, 6b, 6c could be different, allowing more precise control of the flow rate.
If the control signal of the flow control valves 6a, 6b, 6c is lost, the normally open flow control valves 6a, 6c are switched to the open position and the normally closed flow control valve 6b to the closed position. Full flow through the first and the third flow control valves 6a, 6c is thus allowed. In this embodiment, a leakage flow through the second flow control valve 6b is not needed, since the single high-pressure pump 5 receives fuel from the other flow control valves 6a, 6c and is thus sufficiently lubricated. However, the second flow control valve 6b can still be designed to allow the leakage flow. The engine 1 is switched to the limp home mode, in which approximately 67 percent of the maximum power of the engine is available. The excess fuel that is not needed for the fuel injection can flow into a tank 11 via a safety valve 9. Also in this embodiment, more than three flow control valves can be provided. The fuel injection system would work also with only two flow control valves, of which one is a normally closed valve and the other a normally open valve. By increasing the number of the flow control valves, also the accuracy of the flow control during normal operation of the engine would be improved, since flow control valves with smaller capacity could be used.
The embodiment of figure 3 is similar to the embodiment of figure 1. However, in this embodiment the fuel injection system is provided with a first low-pressure pump 7a and a second low-pressure pump 7b. Both low- pressure pumps 7a, 7b supply fuel to all three high- pressure pumps 5a, 5b, 5c. Otherwise the fuel injection system of figure 3 functions in the same way as the fuel injection system of figure 1.
Figure 4 shows an embodiment, where the fuel injection system is a modification of the fuel injection system of figure 2. The fuel injection system comprises a first high-pressure pump 5a and a second high-pressure pump 5b. Both high- pressure pumps 5a, 5b supply pressurized fuel to the same fuel rail 4. A first low-pressure pump 7a supplies fuel to the first high-pressure pump 5a via a first, second and third flow control valve 6a, 6b, 6c. A second low pressure pump 7b supplies fuel to the second high-pressure pump 5b via a fourth, fifth and sixth flow control valve 6d, 6e, 6f. The second and the fifth flow control valves 6b, 6e are normally closed valves and the rest of the flow control valves are normally open valves. The flow to the high- pressure pumps 5a, 5b is controlled during the normal operation of the engine 1 in the same way as in the fuel injection system of figure 2. Because of the multiple flow control valves that are provided for each high- pressure pump 5a, 5b, valves with smaller capacity can be used and the flow control valves 6a, 6b, 6c, 6d, 6e, 6f can operate within an optimal flow rate range. In case the controllability of the flow control valves 6a, 6b, 6c, 6d, 6e, 6f is lost, the engine 1 is switched to a limp home mode, where the second and fifth flow control valves 6b, 6e are closed and the rest of the flow control valves are open. Each high- pressure pump 5a, 5b thus receives approximately 67 percent of the maximum flow. Also in this embodiment, a higher number of flow control valves could be provided for allowing more accurate flow control during the normal operation of the engine and/or for providing a different maximum flow rate in the limp home mode. Instead of two low- pressure pumps 7a, 7b, a single low-pressure pump could feed both high- pressure pumps 5a, 5b. On/off valves could be used also in this embodiment as the flow control valves 6a, 6b, 6c, 6d, 6e, 6f.
The embodiment of figure 5 differs from the embodiment of figure 1 in that the fuel injection system comprises three low- pressure pumps 7a, 7b, 7c. A first low-pressure pump 7a is arranged to supply fuel to a first high-pressure pump 5a, a second low-pressure pump 7b is arranged to supply fuel to a second high-pressure pump 5b and a third low-pressure pump 7c is arranged to supply fuel to a third high-pressure pump 5c. An additional difference is that instead of a single fuel rail, the fuel injection system is provided with three fuel accumulators 4a, 4b, 4c. The first high-pressure pump 5a is arranged to supply fuel to a first fuel accumulator 4a, the second high-pressure pump 5b is arranged to supply fuel to a second fuel accumulator 4b, and the third high-pressure pump 5c is arranged to supply fuel to a third fuel accumulator 4c. Each fuel accumulator supplies fuel to two fuel injectors 3 of the engine 1. The fuel accumulators 4a, 4b, 4c are in fluid communication with each other via connecting pipes 10. The fuel injection system of figure 5 works in the same way as the fuel injection system of figure 1. In the limp home mode, only the first and the third high- pressure pump 5a, 5c supply fuel to the fuel accumulators. Since the fuel accumulators 4a, 4b, 4c are in fluid communication with each other, also the fuel injectors 3 that are connected to the second fuel accumulator 4b receive pressurized fuel and the fuel injection works in all cylinders 2 of the engine 1.
It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.

Claims

A fuel injection system for a piston engine (1), the fuel injection system comprising
- at least one low-pressure pump (7, 7a, 7b, 7c) for raising the pressure of fuel to a first pressure level,

- at least one high-pressure pump (5, 5a, 5b, 5c) for raising the pressure of the fuel from the first pressure level to a second pressure level that is higher than the first pressure level,

- a fuel accumulator (4, 4a, 4b, 4c) that is connected to the high-pressure pump(s) (5, 5a, 5b, 5c) for receiving fuel at the second pressure level,

- a fuel injector (3) that is connected to the fuel accumulator (4, 4a, 4b, 4c) for receiving pressurized fuel from the fuel accumulator (4, 4a, 4b, 4c) and for injecting the fuel into a cylinder (2) of the engine (1), and

- at least two flow control valves (6a, 6b, 6c, 6d, 6e, 6f), each of the flow control valves (6a, 6b, 6c, 6d, 6e, 6f) being arranged between the at least one low-pressure pump (7, 7a, 7b, 7c) and the at least one high-pressure pump (5, 5a, 5b, 5c) for controlling the amount of the fuel supplied from the low-pressure pump (7, 7a, 7b, 7c) to the high-pressure pump (5, 5a, 5b, 5c),
characterized in that at least one of the flow control valves (6a, 6b, 6c, 6d, 6e, 6f) is a normally open valve and at least one of the flow control valves (6a, 6b, 6c, 6d, 6e, 6f) is a normally closed valve.
A fuel injection system according to claim 1, wherein the system comprises at least three flow control valves (6a, 6b, 6c, 6d, 6e, 6f) of which each is arranged between a low-pressure pump (7, 7a, 7b, 7c) and a high-pressure pump (5, 5a, 5b, 5c) for controlling the amount of the fuel supplied from said low pressure pump (7, 7a, 7b, 7c) to said high pressure pump (5, 5a, 5b, 5c).
A fuel injection system according to claim 1 or 2, wherein 20 to 50 percent of the flow control valves (6a, 6b, 6c, 6d, 6e, 6f) are normally closed valves and the rest are normally open valves.
A fuel injection system according to any of the preceding claims, wherein the system comprises at least two high-pressure pumps (5, 5a, 5b, 5c) and at least one flow control valve (6a, 6b, 6c, 6d, 6e, 6f) is arranged between each high-pressure pump (5, 5a, 5b, 5c) and a low-pressure pump (7, 7a, 7b, 7c) for controlling fuel flow to said high-pressure pump (5, 5a, 5b, 5c).
A fuel injection system according to any of the preceding claims, wherein at least two flow control valves (6a, 6b, 6c, 6d, 6e, 6f) are arranged between each high-pressure pump (5, 5a, 5b, 5c) and a low-pressure pump (7, 7a, 7b, 7c).
A fuel injection system according to claim 5, wherein at least three flow control valves (6a, 6b, 6c, 6d, 6e, 6f) are arranged between each high-pressure pump (5, 5a, 5b, 5c) and a low-pressure pump (7, 7a, 7b, 7c).
A fuel injection system according to any of the preceding claims, wherein the system comprises a first high-pressure pump (5a) and a second high-pressure pump (5b) and a first flow control valve (6a) is arranged to control fuel flow to the first high-pressure pump (5a) and a second flow control valve (6b) is arranged to control fuel flow to the second high-pressure pump (5b).
A fuel injection system according to claim 7, wherein the first flow control valve (6a) is a normally open valve and the second flow control valve (6b) is a normally closed valve.
A fuel injection system according to claim 7 or 8, wherein the system comprises a third high-pressure pump (5c) and a third flow control valve (6c) is arranged to control fuel flow to the third high-pressure pump (5c).
A fuel injection system according to claim 9, wherein the third flow control valve (6c) is a normally open valve.
A fuel injection system according to any of the preceding claims, wherein in the closed position of the normally closed flow control valves a small leakage flow through the valve is allowed.
A fuel injection system according to any of the preceding claims, wherein all the fuel accumulators (4, 4a, 4b, 4c) of the fuel injection system are in fluid communication with each other.
A fuel injection system according to any of the preceding claims, wherein the flow control valves (6a, 6b, 6c, 6d, 6e, 6f) are actively controllable valves.
A fuel injection system according to any of the preceding claims, wherein the flow control valves (6a, 6b, 6c, 6d, 6e, 6f) are proportional valves.
A piston engine comprising a fuel injection system according to any of the preceding claims.