Hydraulic circuit and method for controlling said circuit

1. A method of controlling a hydraulic circuit for a twin-clutch transmission for motor vehicles, wherein the hydraulic circuit comprises first and second clutches that can be hydraulically actuated, the pressures of said clutches being controlled with respective pressure control valves, and wherein a safety circuit creates a safe state for the hydraulic circuit if a malfunction occurs, the method comprising:

when a partial malfunction affecting only one of the two clutches occurs, disengaging the affected clutch; and

when a total malfunction affecting both clutches occurs, disengaging the clutch with the lower clutch pressure at the time of the total malfunction, and maintaining the clutch with the higher clutch pressure at the time of the total malfunction engaged, wherein the transfer of torque via said clutch is maintained.

2. The method as claimed in claim 1 , disengaging either or both of the first and second clutches comprises disengaging the connection between the clutch to be disengaged and the assigned pressure control valve to reduce the clutch pressure.

3. The method as claimed in claim 2 , wherein the clutch pressure is reduced when disengaging the clutch by depressurizing the clutch.

4. The method as claimed in claim 2 , wherein the clutch pressure is gradually reduced when disengaging the clutch.

5. The method as claimed in claim 1 , wherein at least one clutch is disengaged with a mechanical or hydraulic energy store.

6. The method as claimed in claim 5 , wherein disengaging the at least one clutch comprises the energy store actuating a valve.

7. The method as claimed in claim 1 , further comprises comparing pressures of the first and second clutches when a total malfunction occurs by applying the pressures of the first and second clutches to oppositely acting control surfaces of a logic valve.

8. A hydraulic circuit for a twin-clutch transmission, the circuit comprising:

first and second clutches that can be hydraulically actuated;

first and second clutch circuits, wherein the first clutch circuit comprises a first pressure control valve configured to control a first clutch pressure for the first clutch, and the second clutch circuit comprises a second pressure control valve configured to control a second clutch pressure for the second clutch; and

a safety circuit, configured to disengage the first clutch if the first clutch is affected by a malfunction and the second clutch is not affected by the malfunction, to disengage the second clutch if the second clutch is affected by a malfunction and the first clutch is not affected by the malfunction, and if the first and second clutches are both affected by a malfunction, to disengage the clutch having a lower clutch pressure, wherein the clutch having a higher clutch pressure remains engaged.

9. The hydraulic circuit as claimed in claim 8 , wherein the first and second clutch circuits each comprise a release valve which is arranged between the pressure control valve of the clutch circuit and the clutch associated with the clutch circuit and each clutch circuit is configured to engage or disengage the connection between the clutch associated with the clutch circuit and the pressure control valve of the clutch circuit.

10. The hydraulic circuit as claimed in claim 9 , wherein each release valve is biased with an energy store to a disengaged position in which the release valve disengages the connection between the clutch associated with the clutch circuit and the pressure control valve of the clutch circuit.

11. The hydraulic circuit as claimed in claim 10 , wherein, in the disengaged position, each of the release valves connect the clutch associated with the clutch circuit with another valve configured to selectively connect the clutch associated with the clutch circuit with a tank and with a pressure maintaining device depending on whether a malfunction affects both clutches or a single clutch.

12. The hydraulic circuit as claimed in claim 11 , wherein each of the other valves respectively comprise a control valve which is formed separately from the release valve associated with the clutch circuit.

13. The hydraulic circuit as claimed in claim 11 , wherein the other valves are integrated into a single valve unit.

14. The hydraulic circuit as claimed in claim 8 , wherein the safety circuit comprises a pressure maintaining device configured to generate a retaining pressure for the clutch with the higher clutch pressure at the time of malfunction to maintain the transfer of torque via said clutch.

15. The hydraulic circuit as claimed in claim 14 , wherein the pressure maintaining device comprises a pressure comparison device which compares pressures of the first and second clutches.

16. The hydraulic circuit as claimed in claim 15 , wherein the pressure comparison device comprises a directional control valve which can be actuated by oppositely arranged control surfaces which are connected with the first and second clutch pressures, respectively.

17. The hydraulic circuit as claimed in claim 16 , wherein the control surfaces are adapted to the transfer behavior of the assigned clutch.

18. The hydraulic circuit as claimed in claim 14 , wherein the pressure maintaining device comprises a throttle device to soften the fall in pressure of the clutch to be disengaged if a malfunction occurs.