WO2012143614A1

WO2012143614A1 - Hydraulic system and operating method

Info

Publication number: WO2012143614A1
Application number: PCT/FI2012/050388
Authority: WO
Inventors: Ilari Kallio; Johan Lillhannus; Sami Jussila
Original assignee: Wärtsilä Finland Oy
Priority date: 2011-04-21
Filing date: 2012-04-19
Publication date: 2012-10-26
Also published as: EP2699805B1; FI20115392A0; KR20140034195A; KR101941723B1; CN103477088A; EP2699805A1; CN103477088B

Abstract

The hydraulic system comprises a first subsystem (23) working within a first pressure range and a second subsystem (24) working within a second pressure range, where the upper limit is higher than the upper limit of the first pressure range. The system com- prises a pump (2) for supplying hydraulic fluid into the system and two pressure intensifiers (10, 10') for increasing the pressure in the second subsystem. The pressure intensifiers (10, 10') are piston-type pressure intensifiers with at least two alternative intensification ratios. The invention also concerns a method for operating a hydraulic system.

Description

Hydraulic system and operating method

Technical field of the invention

The present invention relates to a hydraulic system according to the preamble of claim 1. The invention also concerns a method for operating a hydraulic system.

Background of the invention

Efficiency and low component costs are important features of hydraulic systems. Many different machines, such as large internal combustion engines, may involve several hydraulic circuits with different pressure and flow requirements. Often this means that several pumps are needed to produce hydraulic fluid flows at different pressure levels, which increases the costs of the systems. Another problem is that pumps that are capable of producing high pressures and flows often have low efficiency. Even though high- efficiency pumps may be available, they are much more expensive. In addition to having several hydraulic circuits with different pressure requirements, it is also common that a certain hydraulic circuit requires different pressures and flows in different operating modes, making the system more complicated.

Summary of the invention

An object of the present invention is to provide an improved hydraulic system. The characterizing features of the system according to the present invention are given in the characterizing part of claim 1. Another object of the invention is to provide an improved method for operating a hydraulic system. The characterizing features of the method are given in the characterizing part of the other independent claim. A hydraulic system according to the present invention comprises at least a first subsystem working within a first pressure range and a second subsystem working within a second pressure range. The upper limit of the second pressure range is higher than the upper limit of the first pressure range. The hydraulic system further comprises a pump for supplying hydraulic fluid into the system at a first pressure level, and two pressure intensifiers for increasing the pressure in the second subsystem to a second pressure level. The pressure intensifiers are piston-type pressure intensifiers with at least two alternative intensification ratios.

With the hydraulic system in accordance with the invention, the same pump can be used for supplying hydraulic fluid into two subsystems that have different pressure requirements. The maximum output pressure of the pump can be chosen according to the lower pressure requirement, which reduces the costs of the system. Due to two pressure intensifiers with alternative intensification ratios, the second subsystem can be supplied with steady flow at different pressures.

The method according to the present invention concerns operation of a hydraulic system, which comprises at least a first subsystem working within a first pressure range, and a second subsystem working within a second pressure range, where the upper limit of the second pressure range is higher than the upper limit of the first pressure range. The method comprises at least a first operating mode comprising a first phase, in which first phase hydraulic fluid is introduced into a first chamber of a first pressure intensifier for moving a plunger of the pressure intensifier and pressurizing fluid in a second chamber of the pressure intensifier and supplying it to the second subsystem, and hydraulic fluid is introduced into a second chamber of a second pressure intensifier for moving a plunger of the pressure intensifier for emptying a first chamber and a third chamber of the pressure intensifier. The first operating mode further comprises a second phase, in which second phase hydraulic fluid is introduced into the first chamber of the second pressure intensifier for moving the plunger of the pressure intensifier and pressurizing fluid in the second chamber of the pressure intensifier and supplying it to the second subsystem, and hydraulic fluid is introduced into the second chamber of the first pressure intensifier for moving the plunger of the pressure intensifier for emptying the first chamber and a third chamber of the pressure intensifier.

In the method according to the invention, steady flow at a constant pressure can be sup- plied to the second subsystem, since one of the plungers of the two pressure intensifiers is always used for supplying fluid to the second subsystem unless the pressure intensifiers are in a by-pass mode. The emptying phase of the first and third chambers can be used either for reloading the pressure intensifier or for supplying fluid from the third chamber to the second subsystem.

According to an embodiment of the present invention, each pressure intensifier of the system comprises a plunger comprising a first pressure surface, a second pressure surface, and a third pressure surface. The first pressure surface and the third pressure surface are on the same side of the plunger in the moving direction of the plunger. The walls of the pressure intensifier define a first chamber with the first pressure surface, a second chamber with the second pressure surface, and a third chamber with the third pressure surface. Each of the chambers is connectable to the outlet of the pump for receiving hydraulic fluid. The second chamber and the third chamber are further connected to the second subsystem for supplying hydraulic fluid into it.

According to another embodiment of the invention, the ratio between the area of the first pressure surface and the area of the second pressure surface differs from the ratio between the area of the second pressure surface and the area of the third pressure surface two percent at most. With this selection, the movement of the plunger in both directions can be used to produce flow at an essentially same pressure level. According to another embodiment of the invention, the ratio between the combined area of the first pressure surface and the third pressure surface and the area of the second pressure surface differs from the ratio between the area of the second pressure surface and the area of the third pressure surface two percent at most. According to another embodiment of the invention, the system comprises means for opening and closing the flow communication between the pump and the first chamber of the pressure intensifier, and means for opening and closing the flow communication between the pump and the third chamber. According to another embodiment of the invention, the first and second subsystems comprise components of an internal combustion engine. According to another embodiment of the invention, the first subsystem comprises gas-exchange valves of the engine and the second subsystem comprises fuel injectors of the engine. According to another embodiment of the invention, the pump is a variable displacement pump. According to an embodiment of the invention, the method comprises a second operating mode. The second operating mode comprises a first phase, in which first phase hydraulic fluid is introduced into the first chamber and the third chamber of the first pressure intensifier for moving the plunger of the pressure intensifier and pressurizing fluid in the second chamber of the pressure intensifier and supplying it to the second subsystem at a higher pressure than in the first operating mode, and hydraulic fluid is introduced into the second chamber of the second pressure intensifier for moving the plunger of the pressure intensifier for emptying the first chamber and the third chamber of the pressure intensifier. The second operating mode further comprises a second phase, in which second phase hydraulic fluid is introduced into the first chamber and the third chamber of the second pressure intensifier for moving the plunger of the pressure intensifier and pressurizing fluid in the second chamber of the pressure intensifier and supplying it to the second subsystem at a higher pressure than in the first operating mode, and hydraulic fluid is introduced into the second chamber of the first pressure intensifier for moving the plunger of the pressure intensifier for emptying the first chamber and the third chamber of the pressure intensifier.

According to another embodiment of the invention, in the second operating mode the third chamber is emptied and filled through the same line. When the fluid is not released into the tank via a return line, the energy stored by the fluid in the third chamber can be recovered.

Brief description of the drawings

Fig. 1 shows a hydraulic system according to an embodiment of the invention.

Figs. 2a-2e show part of the system of Fig. 1 at different stages of the working cycle. Fig. 3 shows a hydraulic system according to another embodiment of the invention. Detailed description of the invention

Embodiments of the invention are now described in more detail with reference to the accompanying drawings. In figure 1 is shown a hydraulic system according to an embodiment of the invention. The hydraulic system is used for operating fuel injectors and gas exchange valves of a large internal combustion engine, such as an engine that is used as a main or auxiliary engine of a ship or for producing electricity at a power plant. The hydraulic system comprises a tank 1 for storing hydraulic fluid and a pump 2 for pressurizing the hydrau- lie fluid and supplying it into the hydraulic circuit. The pump 2 is a variable displacement pump that allows adjustment of the flow. The system is also provided with a first pressure accumulator 4 for reducing pressure fluctuations in the circuit and helping thus in maintaining a stable pressure in the system. There is also a pressure relief valve 3 in the system for preventing overpressure.

The gas exchange valves form a first subsystem 23 that requires a first pressure range that is 235 to 350 bar. The required flow may then be approximately 64 1/min. The fuel injectors form a second subsystem 24 that requires a second pressure range that is 250 to 700 bar. The upper limit of the pressure range required by the second subsystem 24 is thus higher than the upper limit of the pressure range required by the first subsystem 23. The average flow needed in the second subsystem 24 may be approximately 36 1/min. The pump 2 is chosen to fulfill the pressure requirement of the first subsystem 23 and the flow requirement of the whole hydraulic system. For increasing the pressure of the hydraulic fluid that goes to the second subsystem 24, the system is provided with a first pressure intensifier 10 and a second pressure intensi- fier 10' . To the first subsystem 23, the hydraulic fluid goes at the output pressure of the pump 2. Each pressure intensifier 10, 10' comprises a reciprocating plunger 11, I V . The walls of the pressure intensifier 10, 10' and the plunger 11, 11 ' define a first cham- ber 12a, 12a', a second chamber 12b, 12b' , and a third chamber 12c, 12c' . The first chamber 12a, 12a' and the third chamber 12c, 12c' are on the same side of the plunger 11, 11 ' in the moving direction of the plunger 11 , I V . The second chamber 12b, 12' is on the opposite side. Each chamber 12a, 12a' , 12b, 12b' , 12c, 12c' is provided with a fluid port 10a, 10a', 10b, 10b', 10c, 10c' for introducing fluid into the chamber and out of it. A first pressure line 18, 18' that is provided with a first on-off valve 6, 6' connects the first chamber 12a, 12a' to the pump 2. The first chamber 12a, 12a' is also connected to the tank 1 with a first return line 19, 19' that is provided with a second on-off valve 7, 7' . The second chamber 12b, 12b' is connected to the pump 2 with a second pressure line 20, 20', which is provided with a first check valve 13, 13'. The third chamber 12c, 12c' is connected to the pump 2 with a third pressure line 21, 2 that comprises a third on-off valve 8, 8'. A third return line 22, 22' connects the third chamber 12c, 12c' to the tank 1. The pressure intensifier 10, 10' also comprises a fourth chamber 12d, 12d', which is opposite to the first chamber 12a, 12a' . In the embodiment described here, the fourth chamber 12d, 12d' is not used for pressure intensification. However, also the fourth chamber 12d, 12d' could be provided with a pressure line, if more alternative intensification ratios were needed. A leak line 17, 17' connects the fourth chamber 12d, 12d' to the tank 1.

A second check valve 14, 14' is arranged in the line 28, 28' leading from the second chamber 12b, 12b' to the fuel injectors and a third check valve 15, 15' is arranged in the line 29, 29' leading from the third chamber 12c, 12c' to the fuel injectors. A fourth check valve 16, 16' is arranged between the third chamber 12c, 12c' and the third on-off valve 8, 8'.

The plunger 11, 11' has three separate pressure surfaces. The first pressure surface Al, Α is in contact with the fluid in the first chamber 12a, 12a' . The second pressure surface A2, A2' is in contact with the fluid in the second chamber 12b, 12b', and the third pressure surface A3, A3' is in contact with the fluid in the third chamber 12c, 12c'. The sizes of the pressure surfaces are chosen so that the area of the first pressure surface Al divided by the area of the second pressure surface A2 equals, within manufacturing tolerances, the area of the second pressure surface A2 divided by the area of the third pressure surface A3, i.e. A1/A2 = A2/A3. The allowable difference depends on the applica- tion. In the embodiment described here, the difference between ratios A1/A2 and A2/A3 should be two percent at most, more preferably less than one percent, and most preferably less than 0.5 percent. The working principle of the hydraulic system is now described in more detail by referring to figures 2a-2e.

In figure 2a is shown a situation where the plunger 11 of the first pressure intensifier 10 is moving upwards, i.e. towards the second fluid port 10b. The term "upwards" refers here only to the figures, the real pressure intensifier 10 can be arranged to work in any direction. The pump 2 supplies at a constant pressure hydraulic fluid from the tank 1 into the hydraulic circuit. In this example, the pressure is in the range of 235 to 350 bar. The first on-off valve 6 in the pressure line 18 of the first chamber 12a is open allowing flow to the pressure intensifier 10. The fluid enters the first chamber 12a of the pressure intensifier 10 through the first fluid port 10a. The second on-off valve 7 that is arranged in the return line 19 of the first chamber 12a is kept closed for preventing the fluid from flowing directly into the tank 1. Also the third on-off valve 8 in the pressure line 21 of the third chamber 12c is kept closed. The fourth on-off valve 9 in the return line 22 of the third chamber 12c is open for allowing fluid being sucked from the tank 1 into the third chamber 12c to fill the void space created by the upwards motion of the plunger 11. It should be noted that differently from the embodiment illustrated in the figures, the fluid source from which the fluid is supplied into the third chamber 12c can also be other than the tank 1. For example, in an internal combustion engine the source of fluid can be a fluid line connected to a lubrication pump. The pressurized hydraulic fluid in the first chamber 12a pushes the plunger 11 upwards. Consequently, the pressure in the second chamber 12b increases. The ratio between the pressures of the first chamber 12a and the second chamber 12b is inversely proportional to the ratio between the area of the first pressure surface Al and the area of the second pressure surface A2. In this ex- ample, the pressure is theoretically raised to a level of 341-508 bar. The first check valve 13 prevents the fluid that is at higher pressure than the pressure produced by the pump 2 from flowing to the pump 2. The second check valve 14 allows fluid flow to the fuel injectors.

Figure 2b shows a situation where the plunger 11 is moving downwards, i.e. towards the third fluid port 10c. Now the first on-off valve 6 is closed preventing flow in the pressure line 18 of the first chamber 12a. Also the third on-off valve 8 is closed preventing flow in the pressure line 21 of the third chamber 12c. The first check valve 13 allows fluid flow through the second pressure line 20 and the second fluid port 10b into the second chamber 12b. The fluid in the second chamber 12b pushes the plunger 11 downwards. The second on-off valve 7 in the return line 19 of the first chamber 12a is kept open for allowing the fluid to flow freely from the first chamber 12a into the tank 1. Pressure level in the first chamber 12a is thus the same as the pressure in the tank 1, i.e. the ambient pressure. The fourth on-off valve 9 in the return line 22 of the third chamber 12c is closed for preventing fluid flow from the third chamber 12c into the tank 1. The fluid in the third chamber 12c thus flows through the third check valve 15 to the fuel injectors. The fourth check valve 16 protects the third on-off valve 8 from the high-pressure of the pressure intensifier 10. The third on-off valve 9 can thus have lower allowed maximum pressure, which reduces costs of the hydraulic system. Since the ratio between the areas of the first and second pressure surfaces Al, A2 equals the ratio between the areas of the second and third pressure surfaces A2, A3, the pressure at the fuel injectors is the same as in the situation of figure 2a. In figures 2a and 2b the pres- sure intensifier 10 works in a medium-pressure mode. In the medium-pressure mode, the pressure intensifiers 10, 10' work as two-way intensifiers, which supply pressurized fluid to the system in their both moving directions.

In figure 2c is shown a situation where the plunger 11 is moving upwards. Now both the first on-off valve 6 in the pressure line 18 of the first chamber 12a and the third on-off valve 8 in the pressure line 21 of the third chamber 12c are open. Hydraulic fluid from the pump 2 can thus flow into the first chamber 12a and the third chamber 12c. The second on-off valve 7 in the return line 19 of the first chamber 12a and the fourth on-off valve 9 in the return line 22 of the third chamber 12c are closed for preventing the fluid from flowing directly into the tank 1. The fluid in the first chamber 12a and the third chamber 12c pushes the plunger 11 upwards. The pressure in the second chamber 12b is thus increased. The pressure increase is proportional to the ratio between the combined area of the first and third pressure surfaces Al, A3 and the area of the second pressure surface A2. The pressure is thus higher than in the situations of figures 2a and 2b, in this example 503-749 bar. The first check valve 13 prevents flow from the second chamber 12b to the pump 2, and the fluid thus flows through the second check valve 14 to the fuel injectors. Figure 2d shows a situation where the plunger 11 is moving downwards. The first on- off valve 6 in the pressure line 18 of the first chamber 12a and the third on-off valve 8 in the pressure line 21 of the third chamber 12c are closed. The fluid thus flows from the pump 2 into the second chamber 12b and pushes the plunger 11 downwards. The second on-off valve 7 in the return line 19 of the first chamber 12 and the fourth on-off valve 9 in the return line 22 of the third chamber are kept open, and the fluid can thus flow freely from the first chamber 12a and the third chamber 12b into the tank 1. This a reloading phase, in which phase the pressure intensifier 10 does not produce any pressure for the fuel injectors. The operation of the pressure intensifier 10 in figures 2c and 2d form a high-pressure mode. In the high-pressure mode, the pressure intensifiers 10, 10' work as one-way intensifiers, which supply pressurized fluid to the system in one moving direction only, and the other moving direction of the plunger 11, 11 ' is used for reloading the pressure intensifiers 10, 10'. In the situation of figure 2e the plunger 11 is at its bottom position, i.e. at the end where the third fluid port 10c is located. The first on-off valve 6 is closed for preventing flow into the first chamber 12a. The third on-off valve 8 is open and would allow flow into the third chamber 12c, but since the fluid can also flow through the first check valve 13 into the second chamber 12b and the area of the second pressure surface A2 is larger than the area of the third pressure surface A3, the plunger 11 does not move upwards. The fourth on-off valve 9 is closed for preventing the fluid from flowing from the pressure line 21 of the third chamber 12c through the return line 22 of the third chamber 12c into the tank 1. The fluid can thus flow through two routes to the fuel injectors: through the first and second check valves 13, 14 and through the third on-off valve 8 and the fourth and third check- valves 16, 15. The third on-off valve 8 could also be closed. In that case, the fluid would flow through the first and second check valves 13, 14 only. The second on-off valve 7 could also be open. This is a by-pass mode where the pressure at the fuel injectors is the same as the pressure at the pump 2, providing that pressure losses in the system are ignored. If the first on-off valve 6 is not opened after the downward movement of the plunger 11, the upward movement of the plunger 11 is prevented and the pressure intensifier 10 is switched to the by-pass mode. Only the functioning of the first pressure intensifier 10 was described above. The second pressure intensifier 10' works in the same manner, but it is arranged to work in a different phase than the first pressure intensifier 10' . If a medium-high pressure is needed at the fuel injectors, the pressure intensifiers 10, 10' work in the operating mode of figures 2a and 2b. When the pressure line 18 to the first chamber 12a of the first pressure intensifier 10 is open and the plunger 11 is moving upwards, the pressure lines 18' , 2 to the first and third chambers 12a', 12c' of the second pressure intensifier 10' are closed and the plunger 1 1 ' of the second pressure intensifier 10' is moving downwards. Also the return line 22' of the third chamber 12c' of the second pressure intensifier 10' is closed. Both pressure intensifiers 10, 10' supply fluid to the fuel injectors at the same pressure. The second pressure intensifier 10' could also be operated in the way described in figure 2d. In that case, the return line 22' of the third chamber 12c' of the second pressure intensifier 10' would be open, and the second pressure intensifier 10' would not produce any pressure for the fuel injectors. When the plungers 11, 11 ' reach their end positions, the relevant valves are switched to other positions to change the moving directions of the plungers 11, I V . Because of the second pressure accumulator 5, the plungers 11, 11 ' of the first and the second pressure intensifiers 10, 10' can be in opposite phases and change their moving direction simultaneously. The second pressure accumulator 5 ensures that the fluid supply to the second subsystem 24 is not interrupt- ed. However, the pressure intensifiers 10, 10' can also be arranged to work so that the plungers 11, 11 ' do not reach their end positions at the same time. This way the interruption in fluid supply that is caused by the change of the moving directions of the plungers 11, 11 ' can be avoided. When high pressure is needed at the fuel injectors, the pressure intensifiers 10, 10' work in the operating mode of figures 2c and 2d. When the pressure lines 18, 21 to the first and third chambers 12a, 12c of the first pressure intensifier 10 are open and the plunger 11 of the first pressure intensifier 10 is moving upwards for supplying high-pressure fluid to the fuel injectors, the second pressure intensifier 10' is in the reloading phase of figure 2d. The pressure lines 18' , 2 to the first and third chambers 12a', 12c' of the second pressure intensifier 10' are thus closed and the plunger 11 ' of the second pressure intensifier 10' is moving downwards. When the positions of the relevant valves are switched, also the moving directions of the plungers 11, 11 ' are changed. The change from the pressure supplying phase into the reloading phase can be done when the plunger 11, 11 ' reaches its end position. The duration of the reloading phase is shorter than the duration of the pressure supplying phase, and therefore the plunger 11, 11 ' can be left in the by-pass mode of figure 2e after the reloading phase until the other plunger 11, 1 , which is in the pressure supplying phase, reaches its end position. The pressure intensifiers 10, 10' can be provided with position sensors that are used for determining appropriate timing for switching the positions of the relevant on-off valves.

If no pressure intensifying is needed, the plungers 11, 11 ' of the both pressure intensifi- ers 10, 10' can be operated in the by-pass mode of figure 2e.

A second pressure accumulator 5 is arranged upstream from the fuel injectors between the first and the second pressure intensifiers 10, 10' . One purpose of the second pressure accumulator 5 is to reduce pressure fluctuations and to prevent interruptions in the fluid supply when the plungers 11, 11 ' of the pressure intensifiers 10, 10' change their moving directions.

The embodiment shown in figure 3 works in the same manner as the embodiment shown in figures l-2e. In figure 3 the first pressure accumulator 4 and the pressure re- lief valve 3 are not shown, but also this embodiment could be provided with these devices. The main difference between the embodiments is that in the system of figure 3, the return line 22, 22' from the third chamber 12c, 12c' is provided with a fifth check valve 25, 25' that does not allow flow into the tank 1. The return line 22, 22' is thus only used for sucking fluid into the third chamber 12c, 12c' when the plunger 11, 11 ' is driven upwards by introducing fluid into the first chamber 12a. 12a' . There is thus no need for the fourth on-off valve 9, 9' . In the reloading phase, the third chamber 12c, 12c' is emptied through the third pressure line 21, 21 ' . An advantage of this arrangement is that the energy stored by the fluid in the third chamber 12c, 12c' is recovered. Since the third pressure line 21, 21 must allow flow in both directions, it is not provided with the fourth check valve 16, 16' between the third on-off valve 8, 8' and the pressure intensifier 10, 10' . Instead, there is a sixth check valve 26, 26' on the other side of the third on-off valve 8, 8' . A throttle valve 27, 27' is arranged in parallel with the sixth check valve 26, 26' for equalizing the flow from the pump 2 to both plungers 11, I V . The functionality of the throttle valve 27, 27' prevents a situation where the majority of the pump flow is guided only to one of the plungers 11, I V . The sixth check valve 26, 26' allows flow from the pump 2 to the pressure intensifier 10, 10' , but not in the other direction. When the third chamber 12c, 12c' is emptied during the reloading phase, the fluid flows through the throttle valve 27, 27' .

According to a further embodiment of the invention, the sizes of the pressure surfaces are chosen so that the combined area of the first pressure surface Al and the third pressure surface A3 divided by the area of the second pressure surface A2 equals, within manufacturing tolerances, the area of the second pressure surface A2 divided by the area of the third pressure surface A3, i.e. (Al+A3)/A2 = A2/A3. With this selection, the pressure intensifiers 10, 10' can supply high-pressure fluid to the fuel injectors 24 during both the upward and downward movement of the plunger 11, I V . Medium-pressure fluid is supplied to the fuel injectors 24 when fluid is introduced into the first chamber 12a, 12' of the pressure intensifier 10, 10' . Figures 2b and 2c would thus show the high- pressure mode of the pressure intensifier 10 and figures 2a and 2d the medium-pressure mode. In this embodiment, the pressure intensifiers 10, 10' work as two-way intensifiers in the high-pressure mode and as one-way intensifiers in the medium-pressure mode. 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. For instance, the hydraulic system does not need to be used in an internal combustion engine, but it can be used for operating any hydraulic devices that require different pressure levels. It is also possible to choose the areas of the pressure surfaces differently from the way described above. It is thus possible to obtain more than two different intensification ratios.

Claims

1. A hydraulic system comprising at least

a first subsystem (23) working within a first pressure range, and

a second subsystem (24) working within a second pressure range,

the upper limit of the second pressure range being higher than the upper limit of the first pressure range, the hydraulic system further comprising

a pump (2) for supplying hydraulic fluid into the system at a first pressure level, and

a first pressure intensifier (10) and a second pressure intensifier (10') for in- creasing the pressure in the second subsystem to a second pressure level, characterized in that the first pressure intensifier (10) and the second pressure intensifier (10') are piston-type pressure intensifiers with at least two alternative intensification ratios.

2. A hydraulic system according to claim 1, characterized in that each pressure intensifier (10, 10') comprises a plunger (11, 11'), which plunger (11, 11 ') comprises a first pres sure surface ( A 1 , A 1 ' ) ,

a second pressure surface (A2, A2'), and

a third pressure surface (A3, A3'),

the first pressure surface (Al, Α ) and the third pressure surface (A3, A3') being on the same side of the plunger (11, 11 ') in the moving direction of the plunger (11, 11'), and the walls of the pressure intensifier (10, 10') defining

a first chamber (12a, 12a') with the first pressure surface (Al, Α ),

a second chamber (12b, 12b') with the second pressure surface (A2, A2'), and - a third chamber (12c, 12c') with the third pressure surface (A3, A3'), each of the chambers (12a, 12a', 12b, 12b', 12c, 12c') being connectable to the outlet of the pump (2) for receiving hydraulic fluid, the second chamber (12b, 12b') and the third chamber (12c, 12c') being further connected to the second subsystem (24) for supplying hydraulic fluid into it.

3. A hydraulic system according to claim 2, characterized in that the ratio between the area of the first pressure surface (Al, Α ) and the area of second pressure surface (A2, Α2') differs from the ratio between the area of the second pressure surface (A2, A2') and the area of the third pressure surface (A3, A3') two percent at most.

4. A hydraulic system according to claim 2, characterized in that the ratio be- tween the combined area of the first pressure surface (Al, Α ) and the third pressure surface (A3, A3') and the area of the second pressure surface (A2, A2') differs from the ratio between the area of the second pressure surface (A2, A2') and the area of the third pressure surface (A3, A3') two percent at most.

5. A hydraulic system according to any of claims 2-4, characterized in that the system comprises means (6, 6') for opening and closing the flow communication between the pump (2) and the first chamber (10a, 10a') of the pressure intensifier (10, 10'), and means (8, 8') for opening and closing the flow communication between the pump (2) and the third chamber (10c, 10c').

6. A hydraulic system according to any of the preceding claims, characterized in that the first and second subsystems (23, 24) comprise components of an internal combustion engine.

7. A hydraulic system according to claim 6, characterized in that the first subsystem (23) comprises gas-exchange valves of the engine and the second subsystem (24) comprises fuel injectors of the engine.

8. A hydraulic system according to any of the preceding claims, characterized in that the pump (2) is a variable displacement pump.

9. A method for operating a hydraulic system, which system comprises at least a first subsystem (23) working within a first pressure range, and

a second subsystem (24) working within a second pressure range,

the upper limit of the second pressure range being higher than the upper limit of the first pressure range,

characterized in that the method comprises at least a first operating mode comprising a first phase, in which first phase hydraulic fluid is introduced into a first chamber (12a) of a first pressure intensifier (10) for moving a plunger (11) of the pressure intensifier (10) and pressurizing fluid in a second chamber (12b) of the pressure intensifier (10) and supplying it to the second subsystem (24), and

- hydraulic fluid is introduced into a second chamber (12b') of a second pressure intensifier (10') for moving a plunger (11 ') of the pressure intensifier (10') for emptying a first chamber (12a') and a third chamber (12c') of the pressure intensifier (10'),

the first operating mode further comprising a second phase, in which second phase

- hydraulic fluid is introduced into the first chamber (12a') of the second pressure intensifier (10') for moving the plunger (1 ) of the pressure intensifier (10') and pressurizing fluid in the second chamber (12b') of the pressure intensifier (10') and supplying it to the second subsystem (24), and

hydraulic fluid is introduced into the second chamber (12b) of the first pressure intensifier (10) for moving the plunger (11) of the pressure intensifier (10) for emptying the first chamber (12a) and a third chamber (12c) of the pressure intensifier (10).

10. A method according to claim 9, characterized in that the movement of the plunger (11, I V) emptying the first chamber (12a, 12a') and the third chamber (12c,

12c') is used for pressurizing fluid in the third chamber (12c, 12c') and supplying it to the second subsystem (24).

11. A method according to claim 9, characterized in that the method comprises a second operating mode comprising a first phase, in which first phase

hydraulic fluid is introduced into the first chamber (12a) and the third chamber (12c) of the first pressure intensifier (10) for moving the plunger (11) of the pressure intensifier (10) and pressurizing fluid in the second chamber (12b) of the pressure intensifier (10) and supplying it to the second subsystem (24) at a higher pressure than in the first operating mode, and

hydraulic fluid is introduced into the second chamber (12b') of the second pressure intensifier (10') for moving the plunger (1 ) of the pressure intensifier (10') for emptying the first chamber (12a') and the third chamber (12c') of the pressure intensifier (10'),

the second operating mode further comprising a second phase, in which second phase hydraulic fluid is introduced into the first chamber (12a') and the third chamber (12c') of the second pressure intensifier (10') for moving the plunger (1 ) of the pressure intensifier (10') and pressurizing fluid in the second chamber (12b') of the pressure intensifier (10') and supplying it to the second subsystem (24) at a higher pressure than in the first operating mode, and

hydraulic fluid is introduced into the second chamber (12b) of the first pressure intensifier (10) for moving the plunger (11) of the pressure intensifier (10) for emptying the first chamber (12a) and the third chamber (12c) of the pressure intensifier (10).

12. A method according to claim 11, characterized in that in the second operating mode the third chamber (12c, 12c') is filled and emptied through the same line (21, 21').