The present invention relates to a common rail fuel injection system of a piston engine in accordance with claim 1 .

Background of the invention

Common rail fuel injection systems of large internal combustion engines, such as ship or power plant engines, comprise a high-pressure pump and a low- pressure pump supplying fuel to the high-pressure pump. A flow control valve is arranged between the low-pressure pump and the high-pressure pump for controlling the amount of fuel that flows to the high-pressure pump. To reduce the wear and erosive or abrasive phenomena that may occur in the valves, the flow control valves of fuel injection systems are designed for working within a narrow flow rate range. Working outside the determined flow rate range, for in- stance in partial flow conditions, is harmful for a flow control valve.

When large high-pressure pumps are used, the flow control valves need to be designed for high flow rates, which makes them expensive. Moreover, it is difficult to achieve an accurate control of the flow rate with valves that are configured for high flow rates.

Summary of the invention

An object of the present invention is to provide an improved common rail fuel injection system of a piston engine. The characterizing features of the fuel injection system according to the invention are given in claim 1 . The fuel injection system according to the invention comprises a fuel rail, at least one fuel injector connected to the fuel rail, a first fluid receiver, a second fluid receiver, at least one low-pressure pump for supplying fuel into the first fluid receiver, at least one high-pressure pump for receiving fuel from the second fluid receiver and for supplying the fuel into the fuel rail, and at least two flow control valves connected in parallel, each of the flow control valves being individually controllable. Each of the flow control valves is connected to the first fluid receiver on the upstream side of the flow control valves and to the second fluid receiver on the downstream side of the flow control valves.

Because of the fluid receivers on both sides of the flow control valves, pres- sure oscillations in the fuel injection system can be reduced and the flow control valves are protected from wear and erosive or abrasive phenomena. Since at least two flow control valves are provided, the flow control valves can work within an optimal flow rate range, which also reduces wear and the risk of damages. Also, more precise flow rate control can be achieved and less ex- pensive components are needed, since a single flow control valve does not need to work in a broad flow rate range. The fuel injection system can be easily adapted to different engines by changing the number of the flow control valves.

Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawing, which shows schematically a common rail fuel injection system of a piston engine.

Description of embodiments of the invention

In figure 1 is shown schematically a common rail fuel injection system of a piston engine 1 . 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. Four cylinders 2 are shown in the figure, but the en- gine 1 can comprise any reasonable number of cylinders 2, which are arranged, for instance, in line or in a V-configuration. The cylinder bore of the engine 1 is between 150 to 1000 mm and the rated output power 150 to 7500 kW per cylinder. The flow control arrangement and the fuel injection system according to the invention are suitable both for 2-stroke and 4-stroke engines, and the engine can thus be of either type.

The fuel injection system of the engine 1 is arranged to inject liquid fuel, such as light fuel oil, heavy fuel oil or marine diesel oil into the cylinders 2 of the en- gine 1 . Each cylinder 2 of the engine 1 is provided with at least one fuel injector 3 for injecting the fuel into the cylinder 3. In addition to the fuel injection system of the figure, the engine 1 can be provided with one or more additional fuel injection systems, for example for injecting gaseous fuel into the intake ducts or prechambers of the engine 1 . A fuel pipe 4 connects each fuel injector 3 to a fuel rail 9, where the fuel to be injected is stored under high pressure. The pressure in the fuel rail 9 is typically in the range of 800 to 3000 bar. The fuel is pressurized and supplied into the fuel rail 9 by a high-pressure pump 10, which is capable of raising the pressure to the required level. The high-pressure pump 10 is typically a cam-driven pump, which is operated by a camshaft of the engine 1 . However, the pump 10 could also be operated, for instance, hy- draulically or electrically. The fuel injection system can comprise two or more high-pressure pumps 10 and/or fuel rails 9. For instance, in a V-engine, each cylinder bank of the engine can be provided with an own fuel rail 9. In case of several fuel rails 9, the fuel injection system can be provided with one high- pressure pump 10 for each fuel rail 9. There could also be two or more high- pressure pumps 10 for each fuel rail 9. Also, one high-pressure pump 10 can be provided with two or more plungers or pistons for pressurizing the fuel. All the plungers or pistons of a high-pressure pump 10 can be arranged to supply fuel into the same fuel rail 9, or the plungers or pistons can pressurize fuel for different fuel rails 9.

The fuel to be injected is stored in a fuel tank 1 1 . The fuel is not drawn from the fuel tank directly by the high-pressure pump 10, but the fuel injection system is provided with a low-pressure pump 5. The low-pressure pump 5 works as a fuel supply pump, which supplies the fuel to the high-pressure pump 10 at a relatively low pressure. The pressure after the low-pressure pump 5 is typically up to 10 bar. The fuel injection system can comprise several low-pressure pumps 5.

For controlling the amount of the fuel that is supplied from the low-pressure pump 5 to the high-pressure pump 10, the fuel injection system is provided with a flow control arrangement. The flow control arrangement is arranged between the low-pressure pump 5 and the high-pressure pump 10. The flow control arrangement comprises at least two flow control valves 8a, 8b, 8c, which are connected in parallel. In the embodiment of the figure, the flow control ar- rangement comprises three flow control valves 8a, 8b, 8c. Each of the flow control valves 8a, 8b, 8c can be controlled individually. The flow control valves 8a, 8b, 8c are arranged between a first fluid receiver 6 and a second fluid receiver 7.

From the low-pressure pump 5, the fuel is introduced via an inlet pipe 13 into the first fluid receiver 6. The first fluid receiver 6 receives and stores the fuel that is supplied to the flow control valves 8a, 8b, 8c. The first fluid receiver 6 works as a buffer that reduces pressure variations caused by the low-pressure pump 5. The volume of the first fluid receiver 6 is 1 to 20 percent of the maximum volume that can be produced by the low-pressure pump 5 in a minute. If several low-pressure pumps 5 are connected to the same first fluid receiver 6, the volume of the first fluid receiver 6 should be selected according to the combined maximum flow rate of the low-pressure pumps 5. Each of the flow control valves 8a, 8b, 8c is connected directly to the first fluid receiver 6 via a first connecting pipe 14. The fuel from the flow control valves 8a, 8b, 8c is collected into the second fluid receiver 7, which is arranged on the downstream side of the flow control valves 8a, 8b, 8c. The second fluid receiver 7 damps pressure oscillations and reduces thus wear of the flow control valves 8a, 8b, 8c and hydraulic and mechanical stresses affecting the flow control valves 8a, 8b, 8c. In addition, the second fluid receiver 7 works as a fuel storage and pressure accumulator for supplying fuel to the high-pressure pump 10. The volume of the second fluid receiver 7 is 0.5 to 10 percent of the maximum volume that can be produced by the high-pressure pump 10 in a minute. If several high-pressure pumps 10 are connected to the same second fluid receiver 7, the volume of the second fluid receiver 7 should be selected according to the combined maximum flow rate of the high-pressure pumps 10. Each of the flow control valves 8a, 8b, 8c is connected directly to the second fluid receiver 7 via a second connecting pipe 15. An outlet pipe 16 connects the second fluid receiver 7 to the high-pressure pump 10.

The fuel injection system comprises an electronic control unit 12. The control unit 12 is arranged to control the operation of the flow control valves 8a, 8b, 8c. The control unit 12 is also arranged to control the fuel injection valves 3. However, the fuel injection system could also be provided with a separate control unit for controlling the operation of the fuel injection valves 3. Since the flow control valves 8a, 8b, 8c can be controlled independently, the flow control valves 8a, 8b, 8c can be activated according to the need. The flow control valves 8a, 8b, 8c can be identical. The flow control valves 8a, 8b, 8c are selected so that they can be operated within the optimal flow rate range. The up- per limit of the combined optimal flow rate range of the flow control valves 8a, 8b, 8c should be at least as high as the maximum flow rate of the high- pressure pump 10. Preferably, the limit should be equal or slightly greater than the maximum flow rate of the high-pressure pump 10. In the embodiment of the figure, the upper limit of the optimal flow rate range of one flow control valve 8a, 8b, 8c should thus be approximately one third of the maximum flow rate of the high-pressure pump 10.

When the engine 1 is operated at the maximum load, fuel is supplied to the high-pressure pump 10 through all three flow control valves 8a, 8b, 8c. When the fuel demand of the engine 1 is lower, one or two of the flow control valves 8a, 8b, 8c can be inactivated. Fuel is thus supplied to the high-pressure pump 10 through only one or two of the flow control valves 8a, 8b, 8c. The active flow control valves 8a, 8b, 8c can thus be operated within the optimal flow rate range, which helps to reduce wear of the flow control valves 8a, 8b, 8c and to avoid cavitation. Also more precise flow rate control can be achieved. All the flow control valves 8a, 8b, 8c can be controlled directly by the control unit 12. Alternatively, the flow control arrangement can work as a master/slave system, where one of the flow control valves 8a, 8b, 8c is arranged to control at least one of the other flow control valves 8a, 8b, 8c. The number of the flow control valves 8a, 8b, 8c can vary depending on the engine 1 . The flow control valves 8a, 8b, 8c of an engine 1 can be either identical with each other or at least one of the flow control valves 8a, 8b, 8c can have a different optimal flow rate range than the other flow control valves 8a, 8b, 8c of the engine 1 . The flow control arrangement has a modular structure, and the same flow control valves 8a, 8b, 8c can thus be used in different engines. In smaller engines, two flow control valves 8a, 8b, 8c may be sufficient, whereas in larger engines a greater number of flow control valves 8a, 8b, 8c can be used. Since identical flow control valves 8a, 8b, 8c can be used in different engines 1 , the number of different components needed for different en- gines 1 can be reduced. Furthermore, smaller and less expensive flow control valves 8a, 8b, 8c can be used also in engines 1 , where large high-pressure pumps 10 are used.

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.