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 . The invention also concerns a method for damping pressure fluctuations of a fuel injection system of a piston engine, as defined in the other independent claim.

Background of the invention Pressures in the fuel injection systems of piston engines can be up to 3000 bar. However, the pressure in the fuel injection system is not constant, but the operation of the components of the fuel injection system causes pressure fluctuations. For instance, opening of a fuel injector causes a sudden pressure drop on the high-pressure side of the fuel injection system, and when the fuel injector closes again, a pressure peak occurs. Different components of the fuel injection system cause pressure fluctuations with different frequencies and amplitudes. At some frequencies, the amplitudes are higher than at other frequencies. Pressure fluctuations at frequencies that are close to the natural frequencies of the fuel injection system are particularly harmful, since they cause resonance, which may lead to failure of the system. Different pressure dampers for damping the pressure fluctuations are known, but most of the pressure dampers either disturb the flow in the fuel injection system or are not effective.

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 . Another object of the invention is to provide a method for damping pressure fluctuations of a fuel injection system of a piston engine, as defined in the other independent claim. The fuel injection system according to the invention comprises a high-pressure pump for pressurizing liquid fuel for fuel injection and a low-pressure pump for feeding fuel to the high-pressure pump, the downstream side of the high- pressure pump forming a high-pressure side of the fuel injection system. The high-pressure side of the fuel injection system is provided with a pressure damper, which is configured to have a natural frequency corresponding to a selected frequency at which pressure fluctuations on the high-pressure side of the fuel injection system occur.

The method according to the invention comprises the steps of determining a harmful pressure fluctuation frequency of the high-pressure side of the fuel injection system, providing a pressure damper having a natural frequency that corresponds to the determined frequency of pressure fluctuations, and connecting the pressure damper to the high-pressure side of the fuel injection system. The pressure damper starts resonating when it receives pressure waves at the selected frequency and therefore effectively absorbs energy from the pressure fluctuations having this frequency. The pressure damper therefore protects the rest of the fuel injection system from resonation.

According to an embodiment of the invention, the natural frequency of the pressure damper is configured to be the opening frequency of the fuel injectors of the fuel injection system. Opening and closing of the fuel injectors is a significant source of pressure fluctuations, and therefore it is beneficial to configure the pressure damper for this frequency.

According to an embodiment of the invention, the pressure damper comprises a body and a piston that is resiliently attached to the body. The piston can be connected to the body by means of a spring. According to an embodiment of the invention, the piston comprises a first piston surface having a first area and a second piston surface having a second area, the second area differing from the first area. The mass of the piston, the stiffness of the spring and the ratio between the areas of the first piston surface and the second piston surface are parameters that can be used for adjusting the natural frequency of the pressure damper.

According to an embodiment of the invention, the pressure damper is arranged between the high-pressure pump and a fuel injector. The pressure damper can be for example between a fuel rail and a fuel injector. The fuel injectors are a major source of pressure fluctuations, and therefore it is beneficial to arrange the pressure damper close to the fuel injectors.

According to an embodiment of the invention, the pressure damper is arranged in a dead end of the fuel injection system. For, instance, the pressure damper can be attached to a pipe that is branched from a fuel rail or from a high- pressure pipe downstream from the high-pressure pump. By arranging the pressure damper in a dead end, disturbances to the fuel flow are minimized.

According to an embodiment of the invention, the fuel injection system com- prises at least two pressure dampers. For instance, one pressure damper can be arranged between each fuel injector and a fuel rail. If the fuel injection system comprises two or more pressure dampers, the pressure dampers can be configured to have different natural frequencies. Pressure fluctuations at different frequencies can thus be effectively attenuated.

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 of a piston engine, Fig. 2 shows a pressure damper for the fuel injection system of figure 1 ,

Fig. 3 shows a fuel injection system according to another embodiment of the invention, and

Fig. 4 shows a pressure damper for the fuel injection system of figure 3.

Description of embodiments of the invention

In figure 1 is shown a 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. The cylinder bore of the engine 1 is at least 150 mm. The rated power of the engine 1 is at least 150 kW/cylinder. Figure 1 shows 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 embodiment of figure 1 , 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 a fuel accumulator 4 for storing pressurized fuel. The fuel accumulator is a fuel rail 4 and all the fuel injectors 3 are connected directly to the fuel rail 4 via connecting pipes 22. Instead of the fuel rail 4 shown in figure 1 , each fuel injector 3 could be provided with an own fuel accumulator, or the fuel injection system could be provided with two or more fuel rails 4, each of the fuel rails 4 serving a group of fuel injectors 3. The fuel injection system of figure 1 is provided with two high-pressure pumps 5a, 5b for pressurizing the fuel and with a low-pressure pump 7 for supplying fuel from a tank 8 to the high-pressure pumps 5a, 5b at a lower pressure. The high-pressure pumps 5a, 5b supply the fuel to the fuel rail 4 via fuel supply lines 23. 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 pump 7 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 5a, 5b the pressure of the fuel is at least 500 bar. Typically the fuel pressure in the fuel accumulator 4 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 5a, 5b are preferably cam-driven. The part of the fuel injection system that is downstream from the high-pressure pumps 5a, 5b forms the high-pressure side 13 of the fuel injection system.

The fuel injection system further comprises flow control valves 6a, 6b for controlling the amount of the fuel that is supplied from the low-pressure pump 7 to the high-pressure pumps 5a, 5b. The flow control valves 6a, 6b are actively controllable valves. Preferably the flow control valves 6a, 6b can be controlled electrically. The flow control valves 6a, 6b 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 are capable of adjusting the flow rate from the low-pressure pump 7 to the high-pressure pumps 5a, 5b. The flow control valves 6a, 6b are not necessary, but the flow from the low-pressure pump 7 to the high-pressure pumps 5a, 5b could be controlled in some other way, for instance by adjusting the flow produced by the low-pressure pump 7.

The fuel injection system is provided with a safety valve 9, which is connected to the fuel accumulator 4 via a pressure release line 24. The fuel injection system could also comprise two or more safety valves that are connected to the fuel accumulator 4. The safety valve 9 works as a pressure relief valve, which opens when the pressure in the fuel accumulator 4 exceeds a predetermined limit value. The safety valve 9 thus limits the pressure in the fuel accumulator 4. The fuel from the safety valve 9 is returned to a tank 1 1 .

Instead of the arrangement of figure 1 , the fuel injection system of figure 1 could also be provided with only one high-pressure pump. There could also be more than two high-pressure pumps in the fuel injection system. The fuel injection system could also comprise two or more low-pressure pumps. The low- pressure pumps could be arranged in parallel to supply fuel to the same high- pressure pumps, or each low-pressure pump could feed only one or some of the high-pressure pumps.

For attenuating pressure fluctuations on the high-pressure side 13 of the fuel injection system, the fuel injection system is provided with one or more pres- sure dampers 14. In the embodiment of figure 1 , the fuel injection system comprises one pressure damper 14 for each fuel injector 3 of the engine 1 . The pressure dampers 14 are arranged between the fuel rail 4 and the fuel injectors 3. The pressure dampers 14 are thus located downstream from the high-pressure pumps 5a, 5b. Since the fuel injectors 3 are a major source of pressure pulsations, it is beneficial to arrange the pressure dampers 14 close to the fuel injectors 3. The fuel flows from the fuel rail 4 through the pressure dampers 14 and the connecting pipes 22 to the fuel injectors 3. Instead of the pressure dampers 14 between the fuel rail 4 and the fuel injectors 3 or in addi- tion to them, the fuel injection system could be provided with one or more pressure dampers 14 that are arranged between the high-pressure pumps 5a, 5b and the fuel rail 4.

Figure 2 shows a pressure damper 14 according to an embodiment of the invention. The pressure damper 14 has a first end, which is open and can re- ceive fuel, and a second end. The first end of the pressure damper 14 is connected to the fuel rail 4 and the second end is connected to a fuel injector 3. The pressure damper 14 comprises a body 20 and a piston 16 that is resiliently attached to the body 20. In the embodiment of figure 2, the pressure damper 14 comprises a helical spring 19 that is used for attaching the piston 16 to the body 20, but also some other means could be used instead of a spring. The piston 16 could also be attached to the body 20 via two or more springs 19. The piston 16 can reciprocate inside the pressure damper 14 in the direction of the fuel flow, i.e. towards the first end and the second end of the pressure damper 14. The piston 16 is provided with a first piston surface 17 and a sec- ond piston surface 18. The piston surfaces 17, 18 are arranged at opposite ends of the piston 16. The first piston surface 17 faces the first end of the pressure damper 14 and the second piston surface 18 faces the second end of the pressure damper 14. The first piston surface 17 has a first area and the second piston surface has a second area. The second area differs from the first area. In the embodiment of figure 2, the second area is greater than the first area, and a certain pressure thus creates a greater force when applied onto the second piston surface 18 than when applied onto the first piston surface 17. The piston 16 is provided with a conduit 22, which allows flow through the piston 16. Instead of the conduit 22 of the piston 16 or in addition to it, the body 20 of the pressure damper 14 could be provided with one or more conduits allowing flow to the other side of the piston 16. Because of the construction of the pressure damper 14, it does not significantly disturb flow in the fuel injection system.

When a pressure wave arrives into the pressure damper 14 through the first end of the pressure damper 14, it encounters the first piston surface 17 of the piston 16. The pressure wave pushes the piston 16 towards the second end of the pressure damper 14. The spring 19 resists the movement of the piston 16 towards the second end of the pressure damper 14. The conduit 22 allows the pressure wave to continue through the piston 16 and to reflect from any obstacle in the fuel injection system downstream from the pressure damper 14. The pressure wave then encounters the second piston surface 18 of the piston 16 and pushes the piston 16 towards the first end of the pressure damper 14.

The pressure damper 14 has a certain natural frequency. When the pressure damper 14 receives pressure waves at a frequency that is close to its natural frequency, it starts to resonate. The natural frequency of the pressure damper 14 depends on the mass of the piston 16, the stiffness of the spring 19 and the areas of the first piston surface 17 and the second piston surface 18. By adjusting these parameters, the natural frequency of the pressure damper 16 can be changed. When the pressure damper 14 resonates, it absorbs effectively energy from pressure waves. By configuring the pressure damper 14 to have a certain natural frequency, it can thus be used for absorbing energy particularly from pressure fluctuations having the same or nearly the same frequency as the natural frequency of the pressure damper 14. It can thus effectively protect the rest of the fuel injection system from resonation.

The natural frequency of the pressure damper 14 is selected by first determin- ing the frequency of the most harmful pressure fluctuations in the fuel injection system. These are usually the pressure fluctuations having the highest amplitude. Typically, the opening and closing of the fuel injection valves 3 is a significant source of pressure fluctuations. The frequency of these pressure waves corresponds to the opening frequency of the fuel injectors 3. The pressure damper 14 can thus be configured to have a natural frequency that is equal to the opening frequency of the fuel injectors 3.

Figure 3 shows a fuel injection system according to another embodiment of the invention. The fuel injection system of figure 3 works otherwise in the same way as the fuel injection system of figure 1 , but the pressure damper 14 is ar- ranged differently. In the embodiment of figure 3, the pressure damper 14 is arranged in a dead end of the fuel injection system. The pressure damper 14 is thus in a separate branch of the fuel injection system, through which the fuel is not supplied to any components of the fuel injection system. The pressure damper 14 does therefore not disturb the fuel flow, as is the case for example if the pressure damper 14 is arranged in a fuel supply line 23 between a high- pressure pump 5a and the fuel rail 4. In the embodiment of figure 3, the pressure damper 14 is connected to the fuel rail 4. However, the pressure damper 14 could also be connected to a high-pressure pipe of the fuel injection system, such as to the fuel supply lines 23, the connecting pipes 22 or the pres- sure release line 24. In the embodiment of figure 3, the pressure damper 14 is connected to the fuel rail via a pipe 15.

A pressure damper 14 for the fuel injection system of figure 3 is shown in figure 4. The pressure damper 14 of figure 4 is similar to the pressure damper 14 of figure 2, but the second end of the pressure damper 14 is closed with a flange 21 . The pressure wave 14 thus enters the pressure damper 14 via the first end, flows through the conduit 22 of the piston 16, and reflects from the closed second end of the pressure damper 14. The fuel injection system of figure 3 could be provided with two or more pressure dampers 14. Each of the pressure dampers 14 could thus have a different natural frequency and pres- sure fluctuations at different frequencies could thus be effectively attenuated. It is also possible that the fuel injection system of figure 1 is additionally provided with one or more pressure dampers 14 that are arranged in a dead end of the fuel injection system, as described above.

It will be appreciated by a person skilled in the art that the invention is not lim- ited to the embodiments described above, but may vary within the scope of the appended claims. For instance, the invention is not limited to common rail fuel injection systems, but could also be applied to fuel injection systems with individual fuel injection pumps.