Method for operating a brake system, brake systems in which the method is carried out and motor vehicles comprising said brakes systems

1. A method for operating a brake system having two brake circuits, with a recuperative brake comprising an electrical generator, a master brake cylinder and a brake pedal, the method comprising:

opening at least one outlet valve so that brake fluid is conducted into at least one reservoir with an associated volume of brake fluid in an event of a driver's braking demand with an associated desired braking torque and if a first condition is met, and

closing the at least one outlet valve if a second condition is met,

wherein the first condition is met if a braking variable corresponding to the braking demand is not less than a specified minimum value, and if an available generator braking torque (Tq_rcap_ 0 ) available from the generator is greater than a specified minimum generator braking torque (Tq_min),

wherein the second condition is met if the volume of brake fluid has flowed into the reservoirs.

2. The method as claimed in claim 1 , wherein the first condition is met if a change against time of the braking variable does not exceed a specified maximum value.

3. The method as claimed in claim 1 , wherein, if the desired braking torque exceeds the available generator braking torque (Tq_rcap_ 0 ) at a start of braking, operating brakes of the brake system are operated and the volume of brake fluid is dimensioned in such a way that for a present displacement of the brake pedal a sum of a generator braking torque (Tq_rcap) and the braking torque generated by the operating brakes corresponds to the braking torque of a conventional hydraulic brake system for an equivalent pedal displacement.

4. The method as claimed in claim 1 , wherein for decreasing generator braking torque isolating valves are closed and at least one hydraulic pump is operated and in this way the brake fluid is conducted from the reservoir into wheel brakes of the brake system.

5. The method as claimed in claim 1 , wherein the braking variable is a travel (s_tmc) of a master brake cylinder or a pedal displacement (s_prd) of the brake pedal.

6. The method as claimed in claim 1 , wherein the method is used for operating an electrohydraulic brake system, and wherein operation of the brake system takes place hydraulically by operation of the brake pedal and/or by operating at least one hydraulic pump.

7. An electrohydraulic brake system having two brake circuits and associated brakes, with a recuperative brake, comprising an electrical generator, a master brake cylinder and a brake pedal, a hydraulic control and regulating unit, with at least one reservoir and outlet valves, whose opening allows brake fluid to be conducted into the at least one reservoir, wherein the method as claimed in claim 6 is implemented in an electronic control and regulating unit and wherein the at least one reservoir is a low pressure reservoir.

8. A motor vehicle having a brake system comprising two brake circuits and associated brakes, with a recuperative brake, comprising an electrical generator, a master brake cylinder and a brake pedal, a hydraulic control and regulating unit, with at least one reservoir and outlet valves, whose opening allows brake fluid to be conducted into the at least one reservoir, wherein the method as claimed in claim 6 is implemented in an electronic control and regulating unit and wherein the at least one reservoir is a low pressure reservoir.

9. The method as claimed in claim 1 , wherein the method is used for operating a brake-by-wire brake system and wherein brakes of the brake system are actively operated by a braking force booster.

10. A brake-by-wire brake system having two brake circuits and associated brakes, with a recuperative brake, in particular an electrical generator, a master brake cylinder and a brake pedal, with at least one reservoir and outlet valves, whose opening allows brake fluid to be conducted into the reservoir, wherein the method as claimed in claim 9 is implemented in an electronic control and regulating unit and wherein the at least one reservoir is a low pressure reservoir.

11. The method as claimed in claim 1 , wherein the method is used for operating a combined brake system with two electromechanically operated brakes associated with a first vehicle axle and two hydraulic brakes associated with a second vehicle axle.

12. A combined brake system having two brake circuits, wherein a first brake circuit of the two brake circuits comprises two electromechanically operated brakes and a second brake circuit of the two brake circuits comprises two electrohydraulically operated brakes, and with at least one reservoir, wherein the method as claimed in claim 11 is implemented in an electronic control and regulating unit, and for which the at least one reservoir is a low pressure reservoir.

13. A method for operating a brake system having two brake circuits, with a recuperative brake comprising an electrical generator, a master brake cylinder and a brake pedal, the method comprising:

opening at least one outlet valve so that brake fluid is conducted into at least one reservoir with an associated volume of brake fluid in an event of a driver's braking demand with an associated desired braking torque and if a first condition is met, and

closing the at least one outlet valve if a second condition is met,

wherein the first condition is met if a braking variable corresponding to the braking demand is not less than a specified minimum value,

wherein the second condition is met if the volume of brake fluid has flowed into the reservoirs, and

wherein, if the desired braking torque is less than a currently available generator braking torque (Tq_rcap_ 0 ), an increase in a generator braking torque (Tq_reap) to be anticipated during braking is taken into account during a calculation of the volume of brake fluid.

14. A method for operating a brake system having two brake circuits, with a recuperative brake comprising an electrical generator, a master brake cylinder and a brake pedal, the method comprising:

opening at least one outlet valve so that brake fluid is conducted into at least one reservoir with an associated volume of brake fluid in an event of a driver's braking demand with an associated desired braking torque and if a first condition is met, and

closing the at least one outlet valve if a second condition is met,

wherein the first condition is met if a braking variable corresponding to the braking demand is not less than a specified minimum value,

wherein the second condition is met if the volume of brake fluid has flowed into the reservoirs, and

wherein following a braking process at least one electrical diverter valve is operated and the brake fluid is conducted from the at least one reservoir back into at least one brake circuit.

15. A method for operating a brake system having two brake circuits, with a recuperative brake comprising an electrical generator, a master brake cylinder and a brake pedal, the method comprising:

opening at least one outlet valve so that brake fluid is conducted into at least one reservoir with an associated volume of brake fluid in an event of a driver's braking demand with an associated desired braking torque and if a first condition is met, and

closing the at least one outlet valve if a second condition is met,

wherein the first condition is met if a braking variable corresponding to the braking demand is not less than a specified minimum value,

wherein the second condition is met if the volume of brake fluid has flowed into the reservoirs,

wherein following a braking process at least one electrical diverter valve is operated and the brake fluid is conducted from the at least one reservoir back into at least one brake circuit, and

wherein an operating time of the at least one electrical diverter valve is selected depending on a temperature and a volume of the brake fluid stored in the reservoir.