METHOD FOR CONTROLLING A HYDRAULIC BRAKE SYSTEM DURING A REGENERATIVE BRAKING PROCESS, HYDRAULIC BRAKE SYSTEM, COMPUTER PROGRAM PRODUCT, CONTROL UNIT AND MOTOR VEHICLE

Abstract

A method for controlling a hydraulic brake system during a regenerative braking process which utilizes a generator braking torque effected by an electric machine is provided. A hydraulic fluid is displaced in the direction of a wheel brake by means of a brake cylinder, and at least a volume fraction of the hydraulic fluid is conducted into an accumulator. The method comprises the step whereby at least a volume fraction of the hydraulic fluid is conveyed from the accumulator in the direction of the wheel brake by means of a pump, in order to realize an increase of a hydraulic braking torque effected by the wheel brake, if a braking torque demand is higher than a braking torque limit of the electric machine.

Claims

1. A method for controlling a hydraulic brake system during a regenerative braking process which utilizes a generator braking torque effected by an electric machine, wherein a hydraulic fluid is displaced in the direction of a wheel brake by means of a brake cylinder, and wherein at least a volume fraction of the hydraulic fluid is conducted into an accumulator, wherein the method comprises the step whereby at least a volume fraction of the hydraulic fluid is conveyed from the accumulator in the direction of the wheel brake by means of a pump, in order to realize an increase of a hydraulic braking torque effected by the wheel brake, if a braking torque demand is higher than a braking torque limit of the electric machine.

2. The method as defined in claim 1, wherein a pressure dissipation valve is adjusted in a direction away from a closed state in order to conduct at least a volume fraction of the hydraulic fluid back into the accumulator via the pressure dissipation valve, and wherein an isolation valve is adjusted in the direction of a closed state in order to at least partially hydraulically isolate the wheel brake from the brake cylinder and from the accumulator.

3. The method as defined in claim 2, wherein the pressure dissipation valve is adjusted in the direction away from the closed state and, subsequently or simultaneously, the isolation valve is adjusted in the direction of the closed state.

4. The method as defined in claim 2, wherein the pressure dissipation valve is adjusted in the direction away from the closed state and the isolation valve is adjusted in the direction of the closed state in order to set a differential pressure between a region positioned downstream of the isolation valve and a region positioned upstream of the isolation valve and thereby meter the hydraulic braking torque effected by the wheel brake.

5. The method as defined in claim 4, wherein the pressure dissipation valve and/or the isolation valve is adjusted by means of at least one associated actuator, in order to set the differential pressure, by virtue of the at least one actuator being activated by means of an electrical voltage signal and/or electrical current signal, for example utilizing closed-loop and/or open-loop control.

6. The method as defined in claim 5, wherein the actuator is activated by means of a pulse-width-modulated electrical signal.

7. The method as defined in claim 4, wherein the differential pressure is determined by virtue of the pressure present in the region positioned upstream being measured and the pressure present in the region positioned downstream being estimated.

8. The method as defined in claim 2, wherein, during the adjustment of the isolation valve and/or during the adjustment of the pressure dissipation valve, a conveying action of the pump is maintained.

9. A hydraulic brake system for a motor vehicle, comprising: a brake cylinder and a wheel brake which are hydraulically connected to one another via a feed line, wherein the brake cylinder is configured to displace a hydraulic fluid in the direction of the wheel brake, and the wheel brake is configured to impart a hydraulic braking torque by means of the hydraulic fluid; an isolation valve which is fluidically assigned to the feed line and which is configured to close the feed line; a return line for returning at least a volume fraction of the hydraulic fluid from a region positioned downstream of the isolation valve into a region positioned upstream of the isolation valve; a pressure dissipation valve, a pump and an accumulator, which are fluidically assigned to the return line, wherein the pump is configured to convey at least a volume fraction of the hydraulic fluid, the accumulator is configured to store at least a volume fraction of the hydraulic fluid, and the pressure dissipation valve is configured to open the return line; a control unit which is connected in signal-exchanging fashion to the isolation valve, the pressure dissipation valve and the pump and which is configured such that, in the presence of an actuation of the brake cylinder and in the presence of a generator braking torque of an electric machine, said control unit activates the pressure dissipation valve for adjustment in the direction of a closed state and activates the pump to impart a conveying action, in order to realize an increase of the hydraulic braking torque effected by the wheel brake, if a braking torque demand is higher than a braking torque limit of the electric machine.

10. The brake system as defined in claim 9, wherein the control unit is configured such that, after the adjustment of the pressure dissipation valve in the direction of the closed state, said control unit activates the pressure dissipation valve for adjustment in the direction away from the closed state, in order to conduct at least a volume fraction of the hydraulic fluid back into the accumulator, and activates the isolation valve for adjustment in the direction of a closed state, in order to at least partially hydraulically isolate the wheel brake from the brake cylinder and the accumulator.

11. The brake system as defined in claim 10, wherein the control unit is configured such that, after the adjustment of the pressure dissipation valve in the direction of the closed state, said control unit activates the pressure dissipation valve for adjustment in the direction away from the closed state and subsequently or simultaneously activates the isolation valve for adjustment in the direction of the closed state.

12. The brake system as defined in claim 10, wherein the control unit is configured such that, after the adjustment of the pressure dissipation valve in the direction of the closed state, the control unit activates the pressure dissipation valve for adjustment in the direction away from the closed state and activates the isolation valve for adjustment in the direction of the closed state in order to set a differential pressure between the region positioned downstream and the region positioned upstream and thereby meter the hydraulic braking torque effected by the wheel brake.

13. The brake system as defined in claim 12, wherein the pressure dissipation valve and/or the isolation valve is assigned at least one actuator which is connected in signal-exchanging fashion to the control unit and which is configured to be activated by means of electrical voltage signals and/or electrical current signals, in particular pulse-width-modulated electrical signals, and wherein the control unit is configured to activate the at least one actuator in order to set the pressure difference.

14. The brake system as defined in claim 12, wherein the control unit is configured to determine the differential pressure from measured values and estimated values, wherein the measured values relate to the region positioned upstream and the estimated values relate to the region positioned downstream.

15. The brake system as defined in claim 9, wherein the isolation valve and/or the pressure dissipation valve and/or the pump and/or the accumulator are a constituent part of an anti-lock braking system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Further details and features of the present disclosure will emerge from the following description of two exemplary embodiments on the basis of the drawing. In the drawing:

[0052] FIG. 1 shows a possible embodiment of a hydraulic brake system, which is suitable for carrying out a regenerative braking process, in a schematic illustration, and

[0053] FIG. 2 shows a further possible embodiment of a hydraulic brake system, which is suitable for carrying out a regenerative braking process, in a schematic illustration.

DETAILED DESCRIPTION

[0054] FIG. 1 shows a possible embodiment of a hydraulic brake system 10 which is used for example in a motor vehicle. In FIG. 1, the hydraulic brake system 10 is illustrated by way of example in conjunction with one vehicle wheel 100. The hydraulic brake system 10 is configured to be able to perform a regenerative braking process. In the regenerative braking process, the kinetic energy of the motor vehicle is utilized in order to drive an electric machine 50 in generator mode and thereby generate electrical energy. The electrical energy can be utilized for example to charge an electrical energy store of the motor vehicle. By way of example, in FIG. 1, the electric machine 50 is assigned to the vehicle wheel 100 in order to illustrate that the electric machine 50 is driven by the movement of the vehicle, that is to say by the rotation of the vehicle wheel 100. The electric machine 50 is preferably a constituent part of an electric drive of the motor vehicle, which serves for example for driving the vehicle wheel 100. During a regenerative braking process, said electric drive is utilized as a generator.

[0055] The hydraulic brake system 10 comprises, for example, a brake cylinder 16 and a wheel brake 28, which are hydraulically connected to one another via a feed line 20. The brake cylinder 16 is configured to displace a hydraulic fluid in the direction of the wheel brake 28. The wheel brake 28 is configured to exert a braking force, for example in the form of a friction force, on the vehicle wheel 100 by means of the hydraulic fluid. The hydraulic brake system 10 is preferably assigned a brake pedal 12, by means of which the brake cylinder 16 is to be actuated. The brake cylinder 16 is preferably assigned a reservoir 18 for the purposes of storing hydraulic fluid for the hydraulic brake system 10 in said reservoir. The reservoir 18 may have an inlet opening in order to be refilled or filled via said inlet opening.

[0056] To boost an actuating force input by means of the brake pedal 12, for example by a driver of the motor vehicle, a brake force booster 14 may be provided. The brake force booster 14 preferably boosts the actuating force in a known manner in accordance with a pneumatic, electrohydraulic or electromechanical principle. In order, for automatic vehicle control, to actuate the brake cylinder independently of an actuation of the brake pedal by the driver, it is also possible for an electrically controlled brake force booster (EBB; Electronic Brake Booster) to be provided.

[0057] The hydraulic brake system 10 preferably furthermore comprises an isolation valve 22 which is fluidically assigned to the feed line 20 and which is configured to close the feed line. For example, it is the intention in this way for the wheel brake 28 to be able to be at least partially or entirely hydraulically isolated from the brake cylinder 16. The isolation valve 22 is preferably provided for adjustment between a closed position and an open position in order to close or shut off, in particular entirely or at least partially close or shut off, the feed line 20. Preferably, in the closed position of the isolation valve 22, the feed line 20 is shut off, in particular fully shut off or at least largely or substantially shut off, and, in the open position, the feed line 20 is open, in particular substantially open or fully open.

[0058] Preferably, the hydraulic brake system 10 furthermore comprises a return line 32 which serves for returning at least a volume fraction of the hydraulic fluid from a region positioned downstream of the isolation 22 valve into a region positioned upstream of the isolation valve 22. For example, the return line 32 is connected in terms of flow by means of one end to the feed line 20 in a region between the isolation valve 22 and the wheel brake 28. Preferably, the return line 32 is connected in terms of flow by means of another end to the feed line 20 in a region between the isolation valve 22 and the brake cylinder 16. In this way, at least a volume fraction of the hydraulic fluid can be returned from the wheel brake 28 into the feed line 20, bypassing the isolation valve 22.

[0059] Preferably, the return line 32 is fluidically assigned a pressure dissipation valve 34, a pump 38 and an accumulator 42. The pump 38 is configured to convey at least a volume fraction of the hydraulic oil, in particular in a return direction 70. Preferably, by means of a conveying action of the pump 38 in the return direction 70, the at least one volume fraction of the hydraulic fluid is conveyed in the direction of the region positioned upstream. The accumulator 42 is configured to store at least a volume fraction of the hydraulic fluid, in particular to store the same under pressure, in particular to buffer-store the same.

[0060] The pressure dissipation valve 34 is configured to open and close the return line 32. The pressure dissipation valve 34 is preferably provided for adjustment between a closed position and an open position in order to open, in particular entirely or at least partially open, the return line 32. Preferably, in the open position of the pressure dissipation valve 34, the return line 32 is open, in particular at least partially open or fully open, and, in the closed position, the return line 32 is closed or shut off, in particular entirely shut off or at least largely or substantially shut off. Preferably, as viewed in the return direction 70 of the hydraulic fluid, the pressure dissipation valve 34, the pump 38 and the accumulator 42 are arranged in the sequence in which the pressure dissipation valve 34 comes first, and is followed either by the pump 38 or the accumulator 42. By opening the return line 32, the accumulator 42 is thus filled with the returned volume fraction of the hydraulic fluid.

[0061] Preferably, the hydraulic brake system 10 furthermore comprises a control unit 48, in particular an electrical control unit, for activating the isolation valve 22 and/or the pressure dissipation valve 34 and/or the pump 38. For example, for this purpose, the control unit 48 is connected in signal-exchanging fashion to the isolation valve 22 and/or to the pressure dissipation valve 34 and/or to the pump 38 via a corresponding signal line 61 or 62 or 63 respectively, in particular electrical signal line. Preferably, the isolation valve 22 and/or the pressure dissipation valve 34 and/or the pump 38 has in each case one electrical receiver unit in order to process the control signals transmitted by the control unit 48 and initiate or perform a corresponding actuation of the isolation valve 22 or of the pressure dissipation valve 34 or of the pump 38 respectively.

[0062] For example, for this purpose, the pump 38 may have a corresponding actuating device, such as for example an electric drive motor M, which is activated by the control line 63 and which acts on the pump 38, in particular on a working cylinder of the pump 38, via a mechanical and/or hydraulic and/or electromagnetic actuation connection 65. Preferably, both control signals and state signals, for example signals with information items regarding monitored or detected parameters, are to be transmitted via the signal lines 61, 62, 63.

[0063] The control unit 48 is preferably connected in signal-exchanging fashion to the electric machine 50 for example via a signal line 60, in order to transmit control signals from the control unit 48 to the electric machine 50 and/or conversely in order to transmit control signals or signals containing information items regarding an operating state of the electric machine 50, for example, to the control unit 48. For this purpose, the electric machine 50 may have a control unit 52 which communicates via the signal line 60 with the control unit 48 and which activates, in particular directly activates, the electric machine 50.

[0064] Preferably, the control unit 48 is furthermore connected in signal-exchanging fashion via a signal line 64 to a sensor element assigned to the brake pedal 12, in particular a pedal travel sensor 46. The pedal travel sensor 46 serves for detecting a pedal travel of the brake pedal 12. Via the signal connection between the pedal travel sensor 46 and the control unit 48, the control unit 48 can take into consideration information items relating to the pedal travel.

[0065] The control unit 48 is preferably configured such that, in the presence of an actuation of the brake cylinder 16 and in the presence of a generator braking torque of the electric machine 50, said control unit activates the pressure dissipation valve 34 for adjustment in the direction of a closed state, for example its closed position, and furthermore activates the pump 38 to impart a conveying action, in order to realize an increase of the hydraulic braking torque effected by the wheel brake 28, if a braking torque demand is higher than a braking torque limit of the electric machine (50), that is to say the above-described blending phase is present. It is thus made possible, by means of the hydraulic braking torque, to cover a gap that exists in the blending phase between the braking torque demand and the presently provided overall braking torque.

[0066] The control unit 48 is preferably furthermore configured such that, after the adjustment of the pressure dissipation valve 34 in the direction of the closed state, said control unit activates the pressure dissipation valve 34 for adjustment in the direction away from the closed state, for example its open position, in order to conduct at least a volume fraction of the hydraulic fluid into the accumulator again 42, and said control unit furthermore activates the isolation valve 22 for adjustment in the direction of a closed state in order to at least partially hydraulically isolate the wheel brake 28 from the brake cylinder 16 and/or from the accumulator 42. For example, the control unit 48 is configured such that, after the adjustment of the pressure dissipation valve 34 in the direction of the closed state, said control unit activates the pressure dissipation valve 34 for adjustment in the direction away from the closed state and subsequently or simultaneously activates the isolation valve 22 for adjustment in the direction of the closed state.

[0067] In order to provide the above-described closed-loop recirculation control, the control unit 48 is configured such that, after the adjustment of the pressure dissipation valve 34 in the direction of the closed state, said control unit activates the pressure dissipation valve 34 for adjustment in the direction away from the closed state and furthermore activates the isolation valve 22 for adjustment in the direction of the closed state, in order to set a differential pressure between the region positioned downstream and the region positioned upstream and thereby meter the hydraulic braking torque effected by the wheel brake 28. For this purpose, the pressure dissipation valve 34 and/or the isolation valve 22 may be assigned at least one actuator which is connected in signal-exchanging fashion to the control unit 48 and which is configured to be activated by means of electrical voltage signals and/or electrical current signals, in particular pulse-width-modulated electrical signals. Furthermore, the control unit 48 may be configured to activate the at least one actuator in order to set the pressure difference. Also, the control unit 48 may be configured to determine the differential pressure from measured values and estimated values, wherein the measured values relate to the region positioned upstream and the estimated values relate to the region positioned downstream.

[0068] Before a regenerative braking process begins, the hydraulic brake system 10 is in an initial state. Preferably, in the initial state, the isolation valve 22 is in an open position (FIG. 1), such that the feed line 20 is open, that is to say a hydraulic connection exists between the wheel brake 28 and the brake cylinder 16. Preferably, in the initial state, the pressure dissipation valve 34 is in a closed state (FIG. 1), such that the return line 32 is closed or shut off. Preferably, in the initial state, no conveying of hydraulic fluid is performed by the pump 38, that is to say the pump 38 does not impart a conveying action. The accumulator 42 is preferably in a discharged or at least substantially discharged state (FIG. 1).

[0069] The hydraulic brake system 10 permits functioning as described below on the basis of the example of a motor vehicle equipped with the hydraulic brake system, wherein, by way of example, reference is made only to the one vehicle wheel 100 of FIG. 1:

[0070] The motor vehicle performs a travelling movement, for example with an accelerator pedal actuated. If the electric machine 50 is utilized as a drive, the electric machine 50 is in a motor mode. Furthermore, the hydraulic brake system 10 is in the initial state described above. In order to initiate a braking process, it is for example the case that the actuation of the accelerator pedal is ended and, for example, an actuation of the brake pedal 12 is commenced. The electric machine 50 is preferably prepared for use as a generator, for example is switched into the generator mode.

[0071] Preferably, the actuation of the brake pedal 12 or the onset of an actuation of the brake pedal 12 is identified or detected by the control unit 48 of the hydraulic brake system 50. For example, the pressure dissipation valve 34 is hereupon activated by the control unit 48 for adjustment into an open position, and an opening of the return line 32 occurs. As a result of the actuation of the brake pedal 12, a displacement of a hydraulic fluid from the brake cylinder 16 in the direction of the wheel brake 28 is effected via the feed line 20. Owing to the opened return line 32, at least a volume fraction of the hydraulic fluid is conducted into the accumulator 42, such that a hydraulic braking force corresponding to the displacement of the hydraulic fluid is not generated at the wheel brake 28.

[0072] By means of the actuation of the brake pedal 12, a braking torque demand is input, which must be matched by generation of a braking torque, for example of a braking torque at the vehicle wheel 100. For this purpose, the drag torque originating from the electric machine 50 is utilized, which acts as a braking torque on the moving system, that is to say in the present case on the motor vehicle or the vehicle wheel 100.

[0073] In the present open position of the pressure dissipation valve 34, the generator braking torque effected by the electric machine 50 can, with rising braking torque demand, basically be utilized until such time as the braking torque limit of the electric machine 50 has been reached. Only then is a hydraulic braking torque required or must a hydraulic braking torque be increased. For example, this is then performed by means of an adjustment of the pressure dissipation valve 34 in the direction of a closed state. For this purpose, the pressure dissipation valve 34 is correspondingly activated by the control unit 48. By means of the hydraulic braking torque, it is then possible, together with the generator braking torque, for an overall braking torque to be provided which covers the braking torque demand.

[0074] Proceeding from the open position of the pressure dissipation valve 34, a situation may also arise in which the provided generator braking torque has not yet reached the braking torque limit of the electric machine 50 but the braking torque demand is already no longer covered by the provided overall braking torque. This situation may be present if the braking pressure increase and thus the braking torque increase in the wheel brake 28 has too shallow a gradient. Such a situation is detected or identified by the control unit 48. Then, the pressure dissipation valve 34 is hereupon activated by the control unit 48 in order to effect an early adjustment of the pressure dissipation valve 34 in the direction of the closed state and thus realize an early increase of the hydraulic braking torque effected by the wheel brake 28. If the control unit 48 detects or identifies that, as a result of the adjustment of the pressure dissipation valve 34 in the direction of the closed state, the provided overall braking torque is higher than the braking torque demand, the control unit 48 triggers a situation in which the utilized generator braking torque is reduced, in particular is correspondingly reduced.

[0075] In a further situation, the braking torque demand is likewise not covered by the provided overall braking torque, but the pressure dissipation valve 34 has already been adjusted in the direction of the closed state to such an extent, in particular adjusted into the closed state, that the gap between the overall braking torque and the braking torque demand cannot be closed in this way. Furthermore, the fraction of the generator braking torque in the provided overall braking torque has already reached the maximum, that is to say has already reached the torque limit of the electric machine 50. Such a situation is detected or identified by the control unit 48. Then, the pump 38 is hereupon activated by the control unit 48 to impart a conveying action in order to convey at least a volume fraction of the hydraulic fluid stored in the accumulator 42 in the direction of the wheel brake 28 and thus realize an increase of the hydraulic braking torque effected by the wheel brake 28.

[0076] As a result of conveyance of the hydraulic fluid by means of the pump 38, the hydraulic braking torque effected by the wheel brake 28 can become so great that the provided overall braking torque overshoots the braking torque demand. This situation arises for example if the required conveying power of the pump 38 is lower than that provided by the pump 38 when it is operating at its lower limit rotational speed, that is to say the pump 38 conveys too much hydraulic fluid. For example, for this situation, the pressure dissipation valve 34 and/or the isolation valve 22 may be utilized for closed-loop recirculation control. For example, for this purpose, the pressure dissipation valve 34 is adjusted in the direction away from the closed state, and/or the isolation valve 22 is adjusted in the direction of the closed state and, in this way, a differential pressure is set between the region positioned downstream and the region positioned upstream in relation to the isolation valve 22.

[0077] By means of this setting, the hydraulic braking torque effected by the wheel brake 28 is metered in targeted fashion such that the overall braking torque that is to be predefined or that is sought, such as for example the braking torque demand, is attained. By means of the adjustment of the pressure dissipation valve 34 in the direction away from the closed state and/or the adjustment of the isolation valve 22 in the direction of the closed state, it is likewise possible for at least a volume fraction of the hydraulic fluid to be conducted from the wheel brake 28 back into the accumulator 42 again, and this is then available again in order to be fed once again to the wheel brake 28 in order to increase the brake pressure.

[0078] The setting of the differential pressure is performed for example by virtue of the at least one actuator being activated by means of an electrical voltage signal and/or an electrical current signal, for example utilizing closed-loop and/or open-loop control. For example, the differential pressure is determined by virtue of the pressure in the region positioned upstream being measured and the pressure in the region positioned downstream being estimated.

[0079] FIG. 2 shows a further possible embodiment of a hydraulic brake system 10 which is suitable for performing a regenerative braking process and which may be used for example in a motor vehicle. In the hydraulic brake system 10, two hydraulically mutually separate brake circuits are provided. There are preferably interactions between the two brake circuits. For example, a pressure equalization takes place via a common brake cylinder 16, such that the same brake pressure prevails in both brake circuits. Below, only one of the brake circuits will be referred to, wherein the other brake circuit may be of identical and/or functionally identical construction. For the sake of simplicity and for better clarity, any signal lines that are present have been omitted in FIG. 2.

[0080] The hydraulic brake system 10 of FIG. 2 is a brake system as described in WO 2014/082885 A1. In this respect, with regard to the construction and the functionality of the hydraulic brake system 10, reference is made to the disclosure of WO 2014/082885 A1, which is hereby incorporated in its entirety into the description.

[0081] The above-described components of the hydraulic brake system 10 of FIG. 1 may likewise be present in the hydraulic brake system 10. The hydraulic brake system 10 comprises for example a brake pedal 12, a brake force booster 14, a brake cylinder 16, a reservoir 18, a feed line 20, an isolation valve 22, a wheel brake 28, a return line 32, a pressure dissipation valve 34, a pump 38, an accumulator 42, a pedal travel sensor 46, a control unit 48, an electric machine 50 and a control unit 52. These components may be structurally identical and/or functionally identical to the corresponding components of the hydraulic brake system 10 of FIG. 1.

[0082] For example, the brake pedal 12 may correspond and/or be structurally identical and/or functionally identical to the brake pedal 12, the brake force booster 14 may correspond and/or be structurally identical and/or functionally identical to the brake force booster 14, the brake cylinder 16 may correspond and/or be structurally identical and/or functionally identical to the brake cylinder 16, the reservoir 18 may correspond and/or be structurally identical and/or functionally identical to the reservoir 18, the feed line 20 may correspond and/or be structurally identical and/or functionally identical to the feed line 20, the isolation valve 22 may correspond and/or be structurally identical and/or functionally identical to the isolation valve 22, the wheel brake 28 may correspond and/or be structurally identical and/or functionally identical to the wheel brake 28, the return line 32 may correspond and/or be structurally identical and/or functionally identical to the return line 32, the pressure dissipation valve 34 may correspond and/or be structurally identical and/or functionally identical to the pressure dissipation valve 34, the pump 38 may correspond and/or be structurally identical and/or functionally identical to the pump 38, the accumulator 42 may correspond and/or be structurally identical and/or functionally identical to the accumulator 42, the pedal travel sensor 46 may correspond and/or be structurally identical and/or functionally identical to the pedal travel sensor 46, the control unit 48 may correspond and/or be structurally identical and/or functionally identical to the control unit 48, the electric machine 50 may correspond and/or be structurally identical and/or functionally identical to the electric machine 50, and the control unit 52 may correspond and/or be structurally identical and/or functionally identical to the control unit 52, of the hydraulic brake system 10 of FIG. 1. In this respect, reference is made to the description relating to the hydraulic brake system 10 of FIG. 1.

[0083] FIG. 2 illustrates four vehicle wheels, which are each assigned a wheel brake. The brake circuit under consideration comprises not only the wheel brake 28 but also a further wheel brake 30, which is assigned to a different vehicle wheel. The two vehicle wheels with the associated wheel brakes 28 and 30 may be present at a common axle or may be assigned to different axles, for example to the front axle and to the rear axle of a motor vehicle. FIG. 2 shows, by way of example, an assignment of the vehicle wheels to the front axle and to the rear axle in a diagonal configuration, wherein VR denotes the front right vehicle wheel, VL denotes the front left vehicle wheel, HR denotes the rear right vehicle wheel, and HL denotes the rear left vehicle wheel. By way of example, in FIG. 2, the electric machine 50 is assigned to the rear axle. The electric machine 50 interacts with the vehicle wheel at the rear left. For example, a further electric machine may be provided which interacts with the vehicle wheel at the rear right. It is also possible for the rear axle to be assigned an electric machine which is common to both vehicle wheels.

[0084] The two wheel brakes 28 and 30 are jointly hydraulically connected to the feed line 20, wherein, at one end, the brake cylinder 16 is present and, at another end, the feed line 20 divides into two line portions 20.1 and 20.2, which are in each case hydraulically connected to one of the wheel brakes 28 and 30. The line portion 20.1 is assigned the isolation valve 22, and the line portion 20.2 is assigned a separate isolation valve 24. The isolation valves 22 and 24 are preferably structurally identical and/or functionally identical with respect to one another.

[0085] The return line 32 provided in the case of the hydraulic brake system 10 of FIG. 1 at least partially corresponds to the return line 32, which is assigned the pump 38 and the accumulator 42. As viewed in the direction of the wheel brakes 28 and 30, the return line 32 divides into two line portions 32.1 and 32.2, which are in each case hydraulically connected to one of the wheel brakes 28 and 30. Aside from the pressure dissipation valve 34, a further pressure dissipation valve 36 is provided, which are assigned in each case to one of the line portions 32.1, 32.2 of the return line 32. By means of the isolation valves 22 and 24, each of the two wheel brakes 28 and 30 can be separately hydraulically isolated. By means of the pressure dissipation valves 34 and 36, it is possible, separately for each of the wheel brakes 28 and 30, for a volume fraction of a hydraulic fluid displaced by means of the brake cylinder 16 to be conducted onward in the associated line portion 32.1, 32.2 of the return line 32 in order to be stored in the accumulator 42.

[0086] Preferably, the control unit 48 is of extended functional scope in relation to the control unit 48 of the hydraulic brake system 10 in FIG. 1 such that, aside from the isolation valve 22 and the pressure dissipation valve 34, which are assigned to the wheel brake 28, it is additionally also possible for the isolation valve 24 and the pressure dissipation valve 36, which are assigned to the wheel brake 30 to be activated. The isolation valve 24 and the pressure dissipation valve 36 are preferably activatable by the control unit 48 in the same way as the isolation valve 22 and the pressure dissipation valve 34. For example, the isolation valves 22, 24 and the pressure dissipation valves 34, 36 are a constituent part of an anti-lock braking system which is provided by means of the hydraulic brake system 10. For example, the control unit 48 is additionally configured for executing the hydraulic brake system 10 during an anti-lock braking process.

[0087] As regards the regenerative braking process described with reference to FIG. 1, with regard to the embodiment of FIG. 2, a distinction is to be made between the rear vehicle wheels HL, HR and the front vehicle wheels VL, VR and the respectively assigned wheel brakes, wherein, for the sake of simplicity, only the vehicle wheel VR and the vehicle wheel HL and the assigned wheel brakes 28, 30 will be considered below.

[0088] In the case of the hydraulic brake system 10 of FIG. 2, the functions of the hydraulic brake system 10 of FIG. 1, as have been described above, are preferably implemented at the front vehicle wheels VL, VR and the assigned wheel brakes. Preferably, the control unit 48 is therefore configured such that, in the presence of an actuation of the brake cylinder 16 and in the presence of a generator braking torque effected by the electric machine 50, said control unit activates the pressure dissipation valve 34 for adjustment in the direction of a closed state, and activates the pump 38 to impart a conveying action, if a braking torque demand is higher than a braking torque limit of the electric machine 50, that is to say a blending phase is present. The aim of the adjustment of the pressure dissipation valve 34 in the direction of the closed state and the conveyance of hydraulic fluid by means of the pump 38 is to realize an increase of the hydraulic braking torque effected by the wheel brake 28. In this way, a gap between the braking torque demand and the presently provided overall braking torque can be compensated. Also, the above-described closed-loop recirculation control can be realized by means of the control unit 48 and by means of the isolation valve 22 and/or the pressure dissipation valve 34 and/or the pump 38.

[0089] The pressure dissipation valve 36, which is assigned to the wheel brake 30 at the rear vehicle wheel HL, preferably remains in a closed state. Preferably, the isolation valve 24 is activated by the control unit 48 for adjustment in the direction of a closed state, in particular for adjustment into the closed state, in order to at least partially or entirely hydraulically isolate the wheel brake 30 from the brake cylinder 16. In this way, the hydraulic braking torque provided during the generator braking process in the above-described braking phases is generated primarily or exclusively by the front wheel brakes, which are thus assigned to the front vehicle wheels VR, VL. It is basically self-evidently also possible for the wheel brakes of the rear vehicle wheels HR, HL to effect the hydraulic braking torque or at least a fraction of the hydraulic braking torque. The respectively associated isolation valve and/or pressure dissipation valve must then be correspondingly adjusted, for example in accordance with the method implementation at the wheel brakes of the front vehicle wheels VR, VL.

[0090] As can also be seen from FIG. 2, the feed line 20 may be assigned a further isolation valve 26, which is arranged in the feed line between the division into the line portions 20.1, 20.1 and the brake cylinder 16. Furthermore, a supply valve 40 may be assigned to the return line 32. By means of the supply valve 40, the return line 32 can be hydraulically connected, bypassing the further isolation valve 26, to a region positioned upstream of said isolation valve 26. For example, the isolation valve 26 and the supply valve 40 are a constituent part of a driving dynamics control system (ESP). For example, the control unit 48 is additionally configured for executing the hydraulic brake system 10 during a driving dynamics control process.

[0091] In the present description, the reference to a particular aspect or a particular embodiment or a particular refinement means that a particular feature or a particular characteristic described in conjunction with the respective aspect or the respective embodiment or the respective refinement is comprised at least therein but need not necessarily be comprised in all aspects or embodiments or refinements of the present disclosure. It is expressly pointed out that any combination of the various features and/or structures and/or characteristics described with regard to the present disclosure are encompassed by the present disclosure unless this is expressly or positively ruled out by the context.

[0092] The use of individual or all examples or of an exemplary phrasing in the text is intended merely to illustrate the present disclosure and does not constitute a limitation with regard to the scope of the present disclosure, unless stated otherwise. Also, no phrasing or wording of the description is to be understood as referring to an element which is not claimed but which is essential for the practical implantation of the present disclosure.