CN110998151B

CN110998151B - Hydraulic system

Info

Publication number: CN110998151B
Application number: CN201880052597.0A
Authority: CN
Inventors: S·F·沃德哈克; A-I·普罗丹; D·J·G·保鲁森; D·范莱文
Original assignee: Punch Powertrain NV
Current assignee: Nanjing Bangqi Automatic Transmission Co ltd
Priority date: 2017-06-21
Filing date: 2018-06-21
Publication date: 2022-03-04
Anticipated expiration: 2038-06-21
Also published as: BE1025334A1; DE112018003221T5; WO2018234432A1; CN110998151A; BE1025334B1

Abstract

A hydraulic system for a vehicle transmission having at least one primary and secondary friction element for coupling and transmitting engine power to the wheels of the vehicle through the friction element driven via the hydraulic system, wherein the hydraulic system includes a pump system having at least two outlet lines for supplying hydraulic fluid to a hydraulic control unit providing line pressure, the pump system having a bypass circuit arranged to provide a reduced pressure in one of the outlet lines when the bypass circuit is open to allow flow therethrough, the pump system further including a single flow control valve for controlling the flow in the outlet line and the bypass circuit.

Description

Hydraulic system

Technical Field

The present invention relates to a hydraulic system for a vehicle transmission having at least one primary and secondary friction element for coupling and transmitting engine power to the wheels of a vehicle through actuation of the friction elements via the hydraulic system.

Background

Typically, transmissions provide a controlled application of engine power by converting speed and torque from a power source, such as an internal combustion engine or an electric motor, to the wheels of the vehicle. In a vehicle transmission, such as one having at least one primary and one secondary friction element for coupling and transmitting engine power to the wheels of the vehicle, the friction elements are actuated via a hydraulic system. Vehicle transmissions comprising friction elements are known, for example as continuously variable transmissions incorporating friction elements or dual clutch transmissions incorporating clutches. The friction element may be embodied as a pulley, between which a flexible element, such as a chain or a belt, may be clamped by means of friction.

The hydraulic system comprises a hydraulic control unit which controls the flow and/or pressure in the hydraulic system for actuating the friction element.

The hydraulic control unit is supplied with pressurized hydraulic fluid provided by a pump system. The pump system may comprise a single pump having two pump chambers, or may comprise two pumps each comprising one pump chamber. The pump system has two outlet lines connected to the hydraulic control unit for supplying hydraulic fluid from the pump system to the hydraulic control unit. Furthermore, the pump system may comprise a bypass circuit for bypassing the flow of the second pump or the second pump chamber from the line pressure circuit when the second pump flow is not required in the line pressure circuit. Two or more valves are typically positioned between the pump system and the hydraulic control unit to control flow from the pressure system to the hydraulic control unit and to prevent backflow when the system is shut down. However, these multiple valves make the system rather complex, expensive and relatively bulky. Moreover, these valves can be easily damaged and subject to failure, which makes the hydraulic system relatively unreliable.

Accordingly, there is a need to provide a valve system that eliminates at least one of the above-mentioned problems. In particular, efforts may be made to achieve a reliable and/or compact valve system.

Disclosure of Invention

To this end, a hydraulic system according to the following is provided.

By providing a single flow control valve for controlling the flow in the outlet line and the bypass circuit; wherein the single flow control valve is adjustable between a first position in which hydraulic fluid is prevented from flowing from the hydraulic control unit back to the pump system, a second position in which the bypass circuit is closed to restrict flow therethrough, and a third position in which the bypass circuit is open to allow flow therethrough, the desired valve functions being combined in the single valve. This improves reliability and reduces the required cost and space due to the use of fewer components.

In particular, a first position of the control valve is provided in which hydraulic flow is prevented from flowing from the hydraulic control unit back to the pump system. In this way a one-way mode of the valve is obtained, draining of the consumer is prevented and the pressure in the hydraulic control unit can be kept stationary, which is advantageous for example during start-stop. Thus, more energy may be saved, and the hydraulic system, and thus the transmission, may become more energy efficient and/or cost effective.

Also, a second position is provided in which the bypass circuit is closed to restrict flow therethrough. This is a so-called normal operation mode, in which at least two pumps and/or at least two pump chambers both supply a pressurized flow to the hydraulic control unit. The normal operating mode is typically the default operating mode of the control valve. This mode is used in situations where a high flow of the hydraulic control unit is required, for example during a rapid shift of the vehicle transmission or for setting the pressure on the secondary friction element lower than the line pressure, etc.

Furthermore, a third position is provided in which the bypass circuit is open to allow flow therethrough, which is the so-called bypass mode. The bypass mode is used when the full capacity of the pump system is not required. The second pump or second pump chamber is then connected to the bypass circuit. In this way, pump drive torque is reduced, which reduces transmission actuation losses and increases the fuel economy of the vehicle.

According to the present invention, these three functions or modes are combined into a single valve element, thus requiring fewer components to combine the functions.

Advantageously, the flow control valve comprises a valve housing and a valve piston system which is adjustable in at least three positions in the valve housing. By providing a single valve housing with a single valve piston system, a compact and reliable design of the valve can be obtained. Typical lengths of such valve housings are about 10Cm to about 15 Cm. Of course, other dimensions are possible.

Further, the flow control valve includes: a first inlet port connectable to the first pump outlet line, a second inlet port connectable to the second pump outlet line, a first control port for receiving a control input in respect of the first pump outlet line, a second control port for receiving an input from the hydraulic control unit, a first outlet port connectable to the hydraulic control unit, and a second outlet port connectable to the bypass circuit. In this way, the input from the pump, the control lines from the hydraulic control unit and the pump, and the outputs to the hydraulic control unit and the bypass circuit are integrated into a single valve element. The input ports are connectable to the first pump outlet and the second pump outlet, respectively. The control port is configured to receive input from a hydraulic control unit (which may be provided via a hydraulic line), or may be provided electrically, where an electrical signal converts line pressure to an input signal for the control port, or may be provided mechanically via a mechanical connection. The control port may be connected to a hydraulic control unit to provide a signal associated with the line pressure, which is also referred to as pilot pressure if the signal is provided by hydraulic pressure.

Advantageously, the valve piston system is configured to block and/or open at least one of the ports in each of the three positions. In the first position, the valve piston system is positioned to block the first inlet port and the first outlet port such that the second inlet port is in fluid connection with the second outlet port to prevent flow back to the pump system, thus obtaining a one-way mode. In the second position, the valve piston system is positioned to fluidly connect the first and second inlet ports with the first outlet port to close the bypass circuit, thus providing a normal operating mode. In the third position, the valve piston system is positioned to fluidly connect the first inlet port with the first outlet port such that the second inlet port is fluidly connected with the second outlet port to open the bypass circuit, thus obtaining the bypass mode. Thus, relatively simple and compact components can be used for the adjustment in the three positions.

Preferably, the valve piston system comprises a valve piston rod provided with a first valve body and a second valve body (also referred to as first valve face and second valve face, respectively), and further comprises a free valve body (also referred to as piston). The first valve body is disposed between the first end of the valve housing and the second valve body. The second valve body is arranged between the first valve body and the free valve body. The free valve body is arranged adjacent to the second end of the valve housing and is configured to receive an input from the second input port, preferably also arranged adjustable relative to the valve piston rod. The second valve fluid is configured to close a fluid connection between the first inlet port and the first outlet port in the first position of the flow control valve. The first valve body is configured such that in the second position and the third position of the flow control valve a connection chamber between the first inlet port and/or the second inlet port and the first outlet port is formed, which connection chamber is defined by the first valve body, the second valve body and the valve housing. By providing a single valve piston system comprising a valve piston rod on which two valve bodies, i.e. valve faces, are mounted and a free valve body, i.e. a piston, a compact structure movable in a valve housing can be obtained.

Preferably, the valve piston rod further comprises a third valve body, which is advantageously arranged between the first valve body and the first end of the valve housing. By providing such a third valve body, the valve piston system may be more stable and/or more rigid. In this way, the valve piston system may react more accurately to control signals from the first control port and/or the second control port.

More preferably, the diameter of the third valve body is smaller than the diameter of the first valve body, even if the pressure of the control signal of the first control port is relatively high (such that if provided hydraulically), the control signal from the first control port entering between the third valve body and the first valve body provides sufficient generated force to move the valve piston system.

Preferably, the flow control valve comprises a biasing element biasing the valve piston system into the first position. In this way, when the pressure drops or the pump idles, the pressure in the hydraulic circuit remains and the discharge of the hydraulic control unit is prevented. Advantageously, the biasing element is arranged between the second valve body and the free valve body.

By further providing a stop element to stop the free valve body in the third position, the pilot pressure may be increased for controlling other components and/or valves in the hydraulic control unit without interfering with the function of the single flow control valve.

The invention also relates to a flow control valve and a vehicle transmission comprising such a flow control valve.

Furthermore, the invention relates to a method for controlling a flow in a hydraulic system. Advantageously, a self-switching of the valve piston system from the third position to the second position is provided when a pressure difference between the first control port and the second control port exceeds a predefined threshold value. This is advantageous for a dual chamber pump to control the pressure differential between the two chambers to improve the life and/or reliability of the pump.

Further advantageous embodiments are indicated in the dependent claims.

Drawings

The invention will be further elucidated with reference to the drawing. The figures in the accompanying drawings are given by way of example of embodiment. In the drawings:

FIG. 1A shows a schematic hydraulic diagram of a portion of a hydraulic system having a control valve according to the present disclosure;

FIG. 1B shows a schematic cross-sectional view of a first embodiment of a control valve according to the present disclosure;

FIG. 2A shows a schematic hydraulic diagram of a portion of the hydraulic system with the control valve in a first position;

FIG. 2B shows a schematic cross-sectional view of the control valve of FIG. 1B in a first position;

FIG. 3A shows a schematic hydraulic diagram of a portion of the hydraulic system with the control valve in a second position;

FIG. 3B shows a schematic cross-sectional view of the control valve of FIG. 1B in a second position;

FIG. 4A shows a schematic hydraulic diagram of a portion of the hydraulic system with the control valve in a third position;

FIG. 4B shows a schematic cross-sectional view of the control valve of FIG. 1B in a third position;

FIG. 5A shows a schematic cross-sectional view of a second embodiment of a control valve according to the present invention in a first position;

FIG. 5B shows a schematic cross-sectional view of the second embodiment of the control valve according to the present invention in a second position;

fig. 5C shows a schematic cross-sectional view of the second embodiment of the control valve according to the invention in a third position.

Detailed Description

In the drawings, only the figures are given as schematic illustrations of the invention. Corresponding elements are denoted by corresponding reference numerals.

Fig. 1A shows a part of a hydraulic diagram of a hydraulic system 1 for a vehicle transmission. The hydraulic system 1 comprises a pump system 2, which pump system 2 comprises two pump or pump chamber chambers P1 and P2 having at least two outlet lines 3, 4, the outlet lines 3, 4 being used for supplying hydraulic fluid to a Hydraulic Control Unit (HCU) 5 via a supply line 6. The hydraulic control unit 5 is typically configured to provide line pressure to control hydraulic pressure on friction elements of a vehicle transmission.

The pump system 2 has a bypass circuit 7, which bypass circuit 7 is arranged to provide a reduced pressure there when the bypass circuit is open to allow flow through one of the outlet lines 3, 4. Furthermore, a sump 8 is provided, in which sump 8 drained hydraulic fluid can be received and from which sump 8 hydraulic fluid can be supplied to the pump system 2. Furthermore, a single flow control valve 9 is provided for controlling the flow in the outlet lines 3, 4 and the bypass circuit 7. The flow control valve 9 is adjustable between a first position, shown in fig. 2A, 2B, in which hydraulic flow is blocked from flowing from the hydraulic control unit 5 back to the pump system 2, a second position, shown in fig. 3A, 3B, in which the bypass circuit 7 is closed to restrict flow therethrough, and a third position, shown in fig. 4A, 4B, in which the bypass circuit 7 is opened to allow flow therethrough.

The dashed lines in FIG. 1A show the control lines. There is a second control line 10 which provides an input from the hydraulic control unit 5. The second control line 10 may be electrical or hydraulic or mechanical. If the second control line 10 is hydraulic, it provides a pressure determined by the hydraulic control unit 5, the so-called pilot pressure. Alternatively, the second control line 10 may be electrical, providing an electrical signal from the hydraulic control unit 5 for controlling the valve 9. Alternatively, the second control line 10 may be implemented mechanically, providing a mechanical connection for inputting control signals from the hydraulic control unit 5 to the control valve 9, e.g. via a mechanical actuator. A further control line 11 provides an input from the first pump outlet line 3 to the valve 9. In addition, the further control line 11 may be electrical or hydraulic or mechanical. As will be elucidated below, the control line 10 may be connected to the second control port 21 for receiving input from the hydraulic control unit 5, and is therefore referred to as the second control line 10. The control line 11 may be connected to a first control port 20 for receiving an input from the first pump line 4, and is therefore referred to as the first control line 11.

Shown in FIG. 1B: the flow control valve 9 comprises a valve housing 12 and a valve piston system 13. The valve piston system 13 is adjustable in at least three positions in the valve housing 12. The valve housing 12 has a first end 14 and a second end 15. The second end 15 is here embodied as a plug 16 in the valve housing 12 and can be held in place, for example, by a clamping element 17, but other valve ends can also be provided. The first end 14 may be integrally formed with the valve housing 2 or may be implemented with a plug. Many variations are possible. A sump port 33 may also be provided in the first end 14. This sump port 33 provides a connection to the sump 8, particularly when the valve body 25 is a distance from the first end 14, such as in the second position of fig. 2B or the third position of fig. 4B, to prevent pressure buildup between the first end 14 and the valve body 25.

The valve housing 12 includes various openings or ports that may be connected to a hydraulic system. The first inlet port 18 is connectable to the first pump outlet line 3, the second inlet port 19 is connectable to the second pump outlet line 4, the first control port 20 is for receiving a control input in respect of the first pump outlet line 3 and is connectable to the first control line 11, and the second control port 21 is for receiving an input in respect of the line pressure and is connectable to the second control line 10. The input regarding the line pressure may be provided by means of an electrical signal or a mechanical connection, or preferably hydraulically. Furthermore, there is a first outlet port 22 connectable to the hydraulic control unit 5 and a second outlet port 23 connectable to the bypass circuit 7.

The valve piston system 13 comprises a valve piston rod 24, which valve piston rod 24 is provided with a first valve body 25 and a second valve body 26, also referred to as valve faces. The valve piston rod 24 is provided with a protrusion 24A, which protrusion 24A is a distance holder of the valve piston rod 24 with respect to the first end 14. By providing this protrusion 24A as a distance holder, input, in particular hydraulic fluid, from the first control port 20 may flow between the valve piston rod 24 and the first end 14.

The valve piston system 13 further comprises a free valve body 27. The free valve body 27 or the piston 27 is provided with a projection 27a, which projection 27a is a distance holder of the free valve body 27 relative to the second end 15. By providing this protrusion 27a as a distance holder, input from the second control port 21, in particular hydraulic fluid, can flow between the free valve body 27 and the second end 15. Furthermore, a biasing element 28 is arranged in the valve housing 12. The first valve body 25 is disposed between the first end 14 of the valve housing 12 and the second valve body 26.

The second valve body 26 is arranged on the valve piston rod 24 between the first valve body 25 and the free valve body 27. The first valve body 25 and the second valve body 26 are fixedly connected to the valve piston rod 24. The second valve body 26 is configured to close the fluid connection between the first inlet port 18 and the first outlet port 22 in the first position of the flow control valve 9. The first valve body 25 is configured such that in the second and third positions of the flow control valve 9 a connection chamber 37 between the first and/or

second inlet port

18, 19 and the first outlet port 22 is formed, which connection chamber 37 is defined by the first valve body 25, the second valve body 26 and the valve housing 12.

The free valve body 27 is arranged adjacent the second end of the valve housing 12. The free valve body 27 is configured to receive an input from the second control port 21. This means that a hydraulic or electrical signal or mechanical input can be provided via the second control port 21 to move the free valve body 27 from its position near the second end 15 towards the first end 14. The biasing element 28 is arranged between the second valve body 26 and the free valve body 27, so that a control signal in relation to the free valve body 27 may need to overcome the biasing force of the biasing element 28, typically a spring element. The biasing element 28 is configured to bias the valve piston system to a first position. The free valve body 27 is adjustable relative to the valve piston rod 24. By moving the valve piston system 13 in the housing 12, the valve piston system 13, in particular the

valve bodies

25 and 26, can block and/or open at least one of the

ports

18, 19, 22, 23 in each of the three positions.

The valve housing 12 further comprises a stop element 29 against which stop element 29 the free valve body 27 can abut when actuated by the second control port 21. The stop element 29 may be, for example, a diameter step of the valve housing 12.

In fig. 2A, 2B, 3A, 3B, 4A, 4B, the valve piston system 13 is shown in a first position, a second position and a third position in the valve housing 12.

In the first position, as shown in fig. 2A and 2B, the valve piston system 13, in particular the second valve body 25, is positioned to block the first inlet port 18 and the first outlet port 22 such that the second inlet port 19 is in fluid communication with the second outlet port 23 to prevent flow from the hydraulic control unit 5 back to the pump system 2. In this way, flow in the hydraulic control unit 5 can be prevented from returning to the pump system 2 when the pump system 2 is idling, for example during start-stop. This improves fuel economy and saves time because when the motor and thus the pump system 2 is restarted, the pump system 2 can avoid pumping the otherwise displaced fluid to the hydraulic control unit 5 again.

In the second position, as shown in fig. 3A and 3B, the valve piston system 13 is positioned to fluidly communicate the first and

second inlet ports

18 and 19 with the first outlet port 22 to close the bypass circuit, the first outlet port 22 being connected to the hydraulic control unit 5. In particular, the first valve body 25 and the second valve body 26 define a connection chamber 37, the connection chamber 37 being positioned so as to fluidly connect the first inlet port 18 and the second inlet port 19 with the first outlet port 22 via said connection chamber 37. In this second position, the second valve body is configured to disconnect the connecting chamber 37 from the second outlet port 23, closing the bypass circuit. Via a first control port 20 (providing an input as to the pressure delivered by the first pump or pump chamber), hydraulic fluid enters the valve housing 12. In this first embodiment, the first control port 20 is fluidly connected to a stem receiving space 34 arranged in the first end 14 of the valve 9. The valve stem receiving space 34 is configured to receive and enclose an end of the valve piston rod 24. The diameter of the stem receiving space 34 corresponds to the diameter of the valve stem 24, the diameter of the receiving space 34 being smaller than the diameter of the

valve face

25 or 26. In the valve stem receiving space 34, the hydraulic fluid pushes against the end surface 30 of the valve piston rod 24. When the pressure provided is sufficiently high and above the biasing force of the biasing element 28, the valve piston system 13 moves toward the second end 15 until the fluid pressure on the surface 30 equals the biasing force of the biasing element 28. The first valve body 25 is then positioned on the first end side of the first outlet port 22 and the first inlet port 18 so that they are in fluid communication with each other. To prevent pressure buildup in the chamber 38 between the first end 14 and the land 25, for example from hydraulic fluid that may leak from the first control port along the valve stem 24, a sump port 33 is provided to drain hydraulic fluid. Any pressure build up in this chamber 38 may reduce the effective input of pressure from the end surface 30 of the rod. The second valve body 26 is then positioned at a second end side of the second inlet port 19 such that the second inlet port 19 is also in fluid connection with the first inlet port 18 and the first outlet port 22. In practice, the first valve body 25 and the second valve body 26 are positioned so that the connecting chamber 37 is fluidly connected with the first and

second inlet ports

18, 19 and with the first outlet port 22. Thus, both pumps or pump chambers may supply hydraulic fluid to the hydraulic control unit 5. It is used in a so-called normal operating mode, in which a large flow is required and both pump or pump chambers supply hydraulic fluid to the hydraulic control unit 5. The flow control valve 9 may also be configured to switch to this second position in a self-switching mode. This is particularly important when the pump system 2 comprises a single pump with two pump chambers. When the pressure difference between the two outlet lines 3, 4 of the pump system 2 becomes greater than a predefined threshold value, the control valve 9 switches to this second position as a safety mode of the pump system 2. In particular, in the second position of the control valve 9, the first control port 20 provides an input to push the valve piston system 13 towards the second end 15 when the pressure in the first outlet line 3 is greater than a predefined threshold value exceeding the pressure in the second outlet line 4. In this way, a safe mode for the pump is provided and the pilot pressure connected to the second control port 21 can still be used without affecting the control valve 9 or the pump 2.

In the third position, as shown in fig. 4A and 4B, the valve piston system 13 is positioned to fluidly connect the first inlet port 18 with the first outlet port 22 via the connection chamber 37, such that the first pump or first pump chamber supplies hydraulic fluid to the hydraulic control unit 5. Furthermore, the valve piston system 13 is positioned to fluidly connect the second inlet port 19 with the second outlet port 23 to open the bypass circuit 7. In this third position, the second valve body 26 is configured to decouple the first inlet port 18 from the second inlet port 19, creating two parallel fluid connections in the flow control valve 9. The second pump or second pump chamber is connected to a bypass circuit 7. In this way, since less driving torque is required to supply the hydraulic fluid to the hydraulic control unit 5, the driving loss of the pump can be reduced.

To obtain the third position, the pilot pressure is increased and supplied via the second control port 21. The pilot pressure fluid presses on the end surface 31 of the free valve body 27 and pushes the free valve body 27 towards the first end 14 of the valve housing 12 and also the valve piston rod 24 via the biasing element 28. The free valve body 27 is pushed until it abuts against the stop element 29. The second valve body 26 is then positioned between the first outlet port 22 and the second inlet port 19 to fluidly isolate the first inlet port 18 from the second inlet port 19. The first inlet port 18 is then fluidly connected to the first outlet port 22. The second inlet port 19 is then fluidly connected to the second outlet port 23 and the bypass circuit 7 is thus closed.

The pilot pressure of the hydraulic control unit 5 can be further increased, for example to control other valves and/or for example to reduce the line pressure on the secondary friction element below the line pressure on the main friction element, without the free valve body 27 moving further. The free valve body 27 is restricted from further movement by the stop element 29.

It will be appreciated that the movement of the free valve body 27 may also be controlled electrically or mechanically, for example. An electrical input signal may then be provided for actuation of the valve body 27 to move the valve body 27 and the valve piston rod 24 towards the first end of the valve housing 12. Alternatively, a mechanical connection may be provided for actuation of the valve body 27.

Alternatively, the movement of the first valve body 25 can also be controlled electrically or mechanically in a similar manner.

Fig. 5A, 5B and 5C show schematic cross-sectional views of a second embodiment of a control valve according to the invention in a first position, a second position and a third position, respectively. The first and

second inlet ports

18, 19, the first and

second control ports

20, 21 and the first and

second outlet ports

22, 23, and the free valve body 27 and biasing element 28 are connected and function in substantially the same manner as the first embodiment described above. Here, the control signals from the

control ports

20, 21 may also be hydraulic or electrical or mechanical. The second valve end 15 is configured as in the first embodiment. In this second embodiment, the first end 14 of the valve housing 12 is differently configured in that the first end 14 does not comprise a reduced valve stem receiving space 34 (as shown in fig. 3B and 4B) arranged to receive an end section of the valve piston rod 24. Instead, the valve piston rod 24 is provided with a third valve body 32 or a third valve face, which is arranged adjacent to the first end 14 of the valve housing 12 at or near the end surface 30 of the valve piston rod 24.

By providing the chamber 38 for the first valve end 14, the difference between the diameter of the chamber 38 and the inner diameter of the valve housing 12 is small and manufacturing and/or mechanical control of the valve is relatively easy. Furthermore, by providing the third valve face to the valve piston rod, a relatively stable and/or stiff valve system may be obtained, which may react relatively accurately to the control signal from the control port.

Here, in this embodiment, the sump port 33 is provided at the first end 14 of the valve housing 12, and has the same function as in the first embodiment. Any hydraulic fluid leaking from the chamber 38 along the third land 32 is drained to sump 8 via sump port 33 to avoid pressure build-up on the land 32.

The diameter of the third land 32 is slightly smaller than the diameter of the first land 25, because the inner diameter of the first end 14 is smaller than the inner diameter of the valve housing 12, the third land 32 is movable in the first end 14 and the first land 25 is movable in the valve housing 12. Depending on the minimum pressure required to move the valve piston system 13 and the pressure of the hydraulic fluid supplied via the first control port 20, the diameter difference between the first 25 and third 30 valve faces may be determined. This difference in diameter then corresponds to a step "s" in the inner radius of the valve housing 12 at the first end 14 and the cavity 37. The step s may be relatively small, between about 5 microns or less and about 5mm, the size of the step s depending on the pressure from the first control port 20 and the force required to move the valve stem 24 with the valve face, such as the biasing force against the biasing element 28.

Between the first valve body 25 and the third valve body 32 there is a hydraulic fluid receiving chamber 38, hydraulic fluid entering said chamber 38 via the first control port 20. The hydraulic fluid pushes against the first valve body 25 and against the third valve body 32. Due to the high pressure of the fluid entering from the first control port 20, the force generated over a small area of radius s is sufficient to move the valve piston system 13 towards the second end 15. Hydraulic fluid entering chamber 36 pushes against the surface of third land 32 and in the opposite direction against the surface of first land 25. Due to the slight difference in diameter, there is a resulting force on the surface of the first valve face 25 that is large enough to overcome the biasing force of the biasing element 28, urging the valve piston system 13 toward the second end 15 into the normal operating position of fig. 5B.

Adjustment from the second position shown in fig. 5B to the third position shown in fig. 5C is performed in a similar manner to that in the first embodiment. After the inlet flow from the second control port 21, the free valve body 27 may be pushed towards the first end 14 until it abuts against the stop element 29, the inlet flow may enter between the free valve body 27 and the second valve end 15 due to the projection 27 a.

Also, this second embodiment provides for self-switching of the control valve 9 to the normal operating mode. For example, in case of operation in bypass mode, when the control valve 9 is in the third position shown in fig. 5C, the balance of forces acting on the valve piston system 13 via the valve faces 25, 26, 32 changes when the pressure of the hydraulic fluid supplied via the first control port 20 is above a predefined threshold value. The valve piston system 13 is then moved towards the second valve end 15 into the second position of the normal operating mode, as shown in fig. 5B. The threshold value for "automatic" movement of the valve piston system 13 (i.e. the threshold value for automatic switching between the bypass mode and the normal operation mode) is determined by the diameters of the valve faces 25, 32 and the valve piston 27, and by the pressure level at the second control port 21. Such self-switching of the control valve 9 is advantageous for a dual chamber pump, when the control of the pressure difference between the two chambers of the pump is related to the reliability of the pump.

For purposes of clarity and conciseness of description, features are described herein as part of the same or separate embodiments, however, it is to be understood that the scope of the invention may include embodiments having combinations of all or some of the features described. It is to be understood that the illustrated embodiments have identical or similar components, except as described differently.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words "a" and "an" should not be construed as limited to "only one," but rather are used to mean "at least one," and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. Many variations will be apparent to those skilled in the art. All such modifications are intended to be included within the scope of this invention as defined in the following claims.

List of reference numerals

1. Hydraulic system

2. Pump system

3. First pump outlet line

4. Second pump outlet line

5. Hydraulic Control Unit (HCU)

6. Supply line

7. Bypass circuit

8. Storage tank

9. Flow control valve

10. A first control line

11. A second control line

12. Valve housing

13. Valve piston system

14. First end

15. Second end

16. Plug for bottle

17. Clamping element

18. A first inlet port

19. Second inlet port

20. A first control port

21. A second control port

22. A first outlet port

23. Second outlet port

24. Valve piston rod

Protrusion of 24A valve piston rod

25. First valve body

26. Second valve body

27. Free valve body

27a projection of free valve body

28. Biasing element

29. Stop element

30. End surface of valve piston rod

31. End surface of free valve body

32. Third valve body

33. Reservoir port

34. Valve stem receiving space

35. Sump oil receiving space

36. Hydraulic fluid receiving chamber

37. Connecting chamber

Claims

1. A hydraulic system for a vehicle transmission having at least a primary friction element and a secondary friction element for coupling and transmitting engine power to wheels of the vehicle through actuation of the friction elements via the hydraulic system, characterized in that the hydraulic system has:

-a pump system having at least two outlet lines for supplying hydraulic fluid to a hydraulic control unit providing a line pressure, the at least two outlet lines comprising a first pump outlet line and a second pump outlet line;

-the pump system has a bypass circuit arranged to provide a reduced pressure in one of the outlet lines when the bypass circuit is open to allow flow through a flow control valve;

-further comprising a single flow control valve for controlling the flow out of the port line and the bypass circuit; wherein the single flow control valve is adjustable between a first position in which hydraulic fluid is prevented from flowing from the hydraulic control unit back to the pump system, a second position in which the bypass circuit is closed to restrict flow through the flow control valve, and a third position in which the bypass circuit is open to allow flow through the flow control valve,

the flow control valve includes:

a valve housing and a valve piston system adjustable in at least three positions in the valve housing: a first inlet port (18) connectable to the first pump outlet line, a second inlet port (19) connectable to the second pump outlet line, a first control port (20) for receiving a control input in respect of the first pump outlet line, a second control port (21) for receiving an input from the hydraulic control unit, a first outlet port (22) connectable to the hydraulic control unit, and a second outlet port (23) connectable to the bypass circuit,

in the first position, the valve piston system is positioned to block the first inlet port and the first outlet port such that the second inlet port is fluidly connected with the second outlet port to prevent flow back to the pump system,

in the second position, the valve piston system is positioned to fluidly connect the first and second inlet ports with the first outlet port to close the bypass circuit,

in the third position, the valve piston system is positioned to fluidly connect the first inlet port with the first outlet port such that the second inlet port is fluidly connected with the second outlet port to open the bypass circuit.

2. The hydraulic system according to claim 1, characterized in that the valve piston system comprises a valve piston rod provided with a first valve body and a second valve body and further comprises a free valve body.

3. The hydraulic system of claim 2, wherein the first valve body is disposed between the first end of the valve housing and the second valve body.

4. The hydraulic system according to claim 2 or 3, characterized in that the second valve body is arranged between the first valve body and the free valve body.

5. The hydraulic system of claim 2, wherein the free valve body is disposed adjacent the second end of the valve housing and is configured to receive an input from the second inlet port and is further disposed to be adjustable relative to the valve piston rod.

6. The hydraulic system of claim 2, wherein the valve piston rod further comprises a third valve body.

7. The hydraulic system of claim 6, wherein the third valve body is disposed between the first valve body and the first end of the valve housing.

8. The hydraulic system of claim 7, wherein a diameter of the third valve body is smaller than a diameter of the first valve body.

9. The hydraulic system of claim 7 or 8, wherein a first control port is provided between the third valve body and the first valve body.

10. The hydraulic system of claim 2, wherein the valve piston system further comprises a biasing element to bias the valve piston system to the first position.

11. The hydraulic system of claim 10, wherein the biasing element is disposed between the second valve body and the free valve body.

12. The hydraulic system of claim 2, further comprising a stop element to stop the free valve body in the third position.

13. A flow control valve for controlling flow in a hydraulic system of a vehicle transmission, the flow control valve comprising a valve housing and a valve piston system arranged in the valve housing, wherein the valve piston system is adjustable between a first position in which hydraulic fluid is prevented from flowing from a hydraulic control unit back to a pump system, a second position in which a bypass circuit is closed to restrict flow through the valve piston system, and a third position in which the bypass circuit is open to allow flow through the valve piston system,

the flow control valve includes a first inlet port, a second inlet port, a first control port, a first outlet port, and a second outlet port,

in the first position, the valve piston system is positioned to block the first inlet port and the first outlet port such that the second inlet port is fluidly connected with the second outlet port to prevent flow back to the pump system

In the second position, the valve piston system is positioned to fluidly connect the first and second inlet ports with the first outlet port to close the bypass circuit, an

14. A vehicle transmission comprising a hydraulic system according to any one of claims 1 to 12.

15. A method for controlling flow in a hydraulic system, the method comprising providing a flow control valve comprising a valve housing and a valve piston system adjustably disposed in the valve housing, wherein the valve piston system is adjustable between a first position in which hydraulic fluid is prevented from flowing from a hydraulic control unit back to a pump system, a second position in which a bypass circuit is closed to restrict flow through the valve piston system, and a third position in which the bypass circuit is open to allow flow through the valve piston system as a function of pump pressure input from a first control port and a line pressure from a second control port,

16. The method for controlling flow in a hydraulic system of claim 15, further comprising self-switching a valve piston system from the third position to the second position when a pressure differential between the first control port and the second control port exceeds a predefined threshold.