1. Field of the Invention
The invention is based on a brake system for a vehicle.
2. Description of the Prior Art
From the prior art, brake systems are known that include various safety systems, such as an anti-lock system (ABS), electronic stability program (ESP), and so forth, and perform various safety functions, such as an anti-lock function, traction control (TC), and so forth. FIG. 1 shows a brake system with which various safety functions can be performed. As can be seen from FIG. 1, a conventional brake system 1 for a vehicle includes a master cylinder 2, a fluid control unit 3, and four wheel brakes 4.1 through 4.4, which each have an associated wheel brake cylinder. Two each of the four wheel brakes 4.1 through 4.4 are associated with a brake circuit 10, 20, and each brake circuit 10, 20 is connected to the master cylinder 2. Hence a first wheel brake 4.1, which is disposed for instance on the left rear axle of the vehicle, and a second wheel brake 4.2, which is disposed for instance on the right front axle of the vehicle, are assigned to a first brake circuit 10, and a third wheel brake 4.3, which is disposed for instance on the left front axle of the vehicle, and a fourth wheel brake 4.4, which is disposed for instance on a right rear axle of the vehicle, are assigned to a second brake circuit 20. One inlet valve 13.1, 13.2, 23.1, 23.2 and one outlet valve 14.1, 14.2, 24.1, 24.2 is assigned to each wheel brake 4.1 through 4.4, and via the various inlet valves 13.1, 13.2, 23.1, 23.2, pressure in the corresponding wheel brake 4.1 through 4.4 can be built up, and via the various outlet valves 14.1, 14.2, 24.1, 24.2, pressure in the corresponding wheel brake 4.1 through 4.4 can be reduced. As can also be seen from FIG. 1, the first wheel brake 4.1 is assigned a first inlet valve 13.1 and a first outlet valve 14.1; the second wheel brake 4.2 is assigned a second inlet valve 13.2 and a second outlet valve 14.2; the third wheel brake 4.3 is assigned a third inlet valve 23.2 and a third outlet valve 24.2; and the fourth wheel brake 4.4 is assigned a fourth inlet valve 23.1 and a fourth outlet valve 24.1. Moreover, the first brake circuit 10 has a first intake valve 11, a first switchover valve 12, a first fluid reservoir 16, and a first recirculating pump 15. The second brake circuit 20 additionally has a second intake valve 21, a second switchover valve 22, a second fluid reservoir 26, and a second recirculating pump 25, and the first and second recirculating pumps 15, 25 are driven by a common electric motor 35. The fluid control unit 3, for ascertaining the actual brake pressure, also includes a sensor unit 30. For brake pressure modulation in the first brake circuit 10, the fluid control unit 3 uses the first switchover valve 12, the first intake valve 11, and the first recirculating pump 15, and for brake pressure modulation in the second brake circuit 20, it uses the second switchover valve 22, the second intake valve 22, and the second recirculating pump 25.
The recirculating pumps 15, 25 of the two brake circuits can be embodied for instance as piston pumps or geared pumps. During an ESP regulating action, a brake pressure of up to 140 bar can be adjusted by the intake valves 11 and 21 that have been switched open, and with that pressure the suction side of the corresponding recirculating pump 15, 25 is loaded when braking has to be done in the system. In a partly active system state as well, the recirculating pump 15, 25 can be subjected on the suction side to up to 140 bar. A pilot pressure on the suction side of the recirculating pump 15, 25 can occur as well, if the pressure of the master cylinder 2 is conducted via the open switchover valves 12 and 22 to the recirculating pumps 15, 25 and is then boosted, up to the wheel pressure necessary for the regulating action, via the corresponding recirculating pump 15 or 25. In a version of the recirculating pumps 15, 25 as piston pumps, this high pressure, which acts on a seal on the cam side of the recirculating pump 15, 25, can lead to very high wear, extrusion, and hence increased leakage. If a geared pump is used as the recirculating pump 15, 25, then this high pressure puts a burden on wave sealing rings of the recirculating pumps 15, 25, which can lead to increased friction and as in the piston pump to increased wear of the seals, and high-pressure-resistant wave sealing rings are quite expensive.