DE10123599A1 - Electro-hydraulic brake system for motor vehicles - Google Patents

Abstract

The invention relates to an electrohydraulic brake system for motor vehicles that can be controlled in a brake-by-wire mode both from the driver as well as independently of the driver. The brake system is preferably operated in the brake-by-wire mode and can also be operated in a fallback mode in which it can only be operated by the driver. The aim of the invention is to increase the operating reliability of the brake system and/or to provide a brake system which is capable of supplying sufficient hydraulic volume in all driving situations. To this end, the system has means for providing a hydraulic volume flow that ensures in the fallback mode at least the hydraulic volume required for the respective delay.

Description

The invention relates to an electrohydraulic brake system for motor vehicles, which can be controlled in a brake-by-wire operating mode both by the driver and independently of the driver, preferably operated in the brake-by-wire mode and can be operated in a fallback mode, in which is only possible for the driver to operate, with a master brake cylinder that can be actuated by means of a brake pedal and has at least one pressure chamber, a path simulator that interacts with the master brake cylinder with a simulator piston that is positioned by means of a spring, that has a simulator chamber that accommodates the spring on the one hand, and another one with one of the Pressure chambers of the master brake cylinder connected to the simulator chamber,
a pressureless pressure medium reservoir,
a hydraulic auxiliary pressure source,
an electronic control unit, and
wheel brakes that can be connected to the master brake cylinder and the hydraulic pressure source.

Such a brake system is such. B. from the international Patent application WO 00/43246 known. With the previously known Brake system is in a hydraulic line between the Path simulator and the pressure medium reservoir an electrical actuated return valve inserted that at each  Braking is closed. Occurs at the well-known Brake system a malfunction in which the control unit and / or the power supply of the electrically operated If the aggregate fails, the return valve cannot be opened be so that the path simulator is locked and no Can hold pressure medium volume. The one in the wheel brakes activated pressure causes the Simulator piston or the brake pedal in the starting position.

However, the pressure medium volume balance becomes less advantageous viewed, for example, when driving on smoother Lane sets. After the path simulator has a certain Pressure medium volume is recorded, the in the The pressure of the wheel brakes is zero due to an ABS intervention regulated down so that no resetting of the Simulator piston is possible. The known system is therefore unable to capture the path simulator To provide pressure medium volume.

It is therefore an object of the present invention to increase the operational safety of the brake system of the aforementioned To reach the genus or to propose a braking system at which is sufficient in all driving situations Pressure medium volume is made available.

This object is achieved in that means provided to provide a pressure medium volume flow are in the fallback mode at least that for the necessary delay necessary in each case Ensures pressure medium volume requirement.

To specify the concept of the invention, it is provided that the funds an active return of the path simulator absorbed volume of pressure medium in the master brake cylinder guarantee. The return is preferably carried out by means of the hydraulic auxiliary pressure source applied energy.  

An advantageous further development of the subject matter of the invention provides that the auxiliary pressure source by a rechargeable High pressure accumulator is formed, and that a valve assembly is provided, which is a in brake-by-wire mode hydraulic connection between the path simulator and the Pressure fluid reservoir releases and a hydraulic Connection between the path simulator and the high pressure accumulator shuts off while in the fallback mode hydraulic connection between the path simulator and the Shuts off pressure medium reservoir and the hydraulic Connection between the path simulator and the high pressure accumulator releases.

The simulator piston is preferably in the Fallback mode contrary to its brake-by-wire Operating mode corresponding movement direction with that of High pressure accumulator pressurized provided.

An advantageous further development of the subject matter of the invention High pressure accumulator is formed, and that a valve assembly is provided, which is a in brake-by-wire mode hydraulic connection between a hydraulic chamber Piston-cylinder arrangement and the pressure medium reservoir releases and a hydraulic connection between the Hydraulic chamber and the high pressure accumulator shut off while they in the fallback mode, the hydraulic connection between the hydraulic chamber and the pressure medium reservoir shuts off and the hydraulic connection between the Hydraulic chamber and the high pressure accumulator releases, the Hydraulic chamber is limited by a hydraulic piston the one provided by the high-pressure accumulator Pressure is applied and in the fallback mode Simulator piston contrary to its brake-by-wire Operating mode adjusted corresponding direction of movement.  

In a further embodiment of the invention, the Simulator piston through a mechanical actuator adjusted by means of a high-pressure accumulator provided tensionable return spring is operated. Part of the actuator is appropriate designed as a hydraulic piston.

It is particularly advantageous if the hydraulic Piston a first pressure chamber from a second pressure chamber separates and if a valve arrangement is provided which in the Brake-by-wire mode of operation a hydraulic connection between the first pressure chamber and the high pressure accumulator releases and a hydraulic connection between the Shuts off the pressure chamber and the pressure medium reservoir, while in the fallback mode the hydraulic Connection between the first pressure chamber and the High-pressure accumulator blocks and the hydraulic connection between the first pressure chamber and the Pressure fluid reservoir releases.

In a further advantageous variant of the The subject matter of the invention ensures an inflow an additional pressure medium volume in the Master brake cylinder, the additional pressure medium volume provided by the hydraulic auxiliary pressure source becomes.

It is also particularly useful if between the High pressure accumulator and at least one of the pressure rooms of the Master cylinder a lockable by means of a valve hydraulic connection is provided.

In the hydraulic connection, a Separating piston device can be inserted by means of a lockable line to the pressure medium reservoir connected.  

Another advantageous embodiment of the Subject of the invention provides that the master brake cylinder as a tandem master cylinder with a first and a second Pressure chamber is formed by a first and a second piston are limited, and that the valve by the Movement of the second piston can be actuated.

Further details, features and advantages of the invention go from the following description of a total of nine below Reference to the attached schematic drawing, the same reference numerals for the same components be used. The drawings show:

Fig. 1a the structure of the brake system according to a first embodiment, in the active or standby state,

FIG. 1b, the brake system of Fig. 1a in the inactive or de-energized state,

Fig. 2 shows the structure of a second embodiment of the brake system according to the invention in the inactive or de-energized state,

Fig. 3 shows the structure of a third embodiment of the brake system according to the invention in the inactive or de-energized state,

Fig. 4 shows the structure of a fourth embodiment of the brake system according to the invention in the inactive or de-energized state,

Fig. 5 shows a first modification of the embodiment shown in Fig. 4 of the brake system according to the invention in the inactive or de-energized state,

Fig. 6 shows a second modification of the embodiment shown in Fig. 4 of the brake system according to the invention in the inactive or de-energized state,

Fig. 7 shows a third modification of the embodiment shown in Fig. 4 of the brake system according to the invention in the inactive or de-energized state,

Fig. 8 shows a fifth embodiment of the brake system according to the invention, and

Fig. 9 shows a sixth embodiment of the brake system according to the invention.

The brake system is shown only schematically in the drawing essentially consists of an actuatable by a brake pedal 1, dual-circuit hydraulic pressure generator or master brake cylinder 2 in tandem design, one cooperating with the tandem master cylinder 2 travel simulator 3, a tandem master cylinder 2 associated pressure fluid reservoir 4, a hydraulic auxiliary pressure source 5 , a schematically indicated hydraulic control unit HCU 6 , which contains, among other things, all the components required for pressure control processes and to which vehicle brakes (not shown) are connected, and an electronic control and regulating unit (not shown). The tandem master cylinder 2 , which is known per se, has pressure chambers 9 , 10 which are delimited from one another by two pistons 7 , 8 and which can be connected both to the pressure medium reservoir 4 and to the vehicle brakes via the HCU 6 . The aforementioned travel simulator 3 , which conveys the familiar brake pedal feel to the driver in brake-by-wire mode, consists essentially of a simulator piston 11 and a spring 12 supported on the simulator piston 11 , which is arranged in a simulator chamber 13 delimited by the simulator piston 11 , On the other hand, the simulator piston 11 delimits a simulator chamber 14 which is connected to the first pressure chamber 9 of the tandem master cylinder 2 by means of a pressure medium channel 15 and can thus be acted upon by the hydraulic pressure introduced therein. Further, Fig. 1a and 1b can be seen that between the auxiliary pressure source 5, which is preferably formed by a high-pressure accumulator, and the simulator chamber 13 is provided a hydraulic line 40, which by means of an electromagnetically operable, preferably normally open (NO) 2 / 2-way valve 16 can be shut off or released. The simulator chamber 13 is also connected via an electromagnetically actuated, preferably normally closed (SG) 2/2-way valve 17 to a pressure-free pressure medium reservoir, which, as indicated by reference number ( 4 ), can be formed by the previously mentioned pressure medium reservoir 4 .

The mode of operation of the brake system according to the invention shown in FIGS. 1a, b is explained in more detail in the text below.

When the brake pedal 1 is not actuated, all of the components are in the position shown in FIG. 1a. In the preferred brake-by-wire operating mode, the driver deceleration request detected on the brake pedal 1 by means of a travel sensor (not shown) is converted in the aforementioned electronic control unit into a system pressure value to be controlled in the vehicle brakes, which is set by means of the HCU 6 . The tandem master cylinder 2 is separated from the vehicle brakes. By moving the first master cylinder piston 7 in the actuating direction, the simulator piston 11 is pressurized and thus shifted to the left in the drawing, so that when the brake pedal 1 is actuated, the driver is given a pedal feeling which is predetermined by the characteristic curve of the spring 12 . The volume of pressure medium displaced from the simulator chamber 13 is shifted via the open valve 17 into the pressure medium reservoir ( 4 ).

In the event of a power failure, which is caused, for example, by a battery defect, a short circuit or by switching off the ignition, the brake system according to the invention automatically switches to a first fallback operating mode in which driver braking is possible. The valves 16 , 17 are switched to the switching position shown in FIG. 1b, so that the connection between the simulator chamber 13 and the pressure medium reservoir ( 4 ) is interrupted. The end face of the simulator piston 11 facing the spring 12 is acted upon by the pressure provided by the high-pressure accumulator 5 , so that the simulator piston 11 returns to its starting position and the pressure medium volume received by the simulator chamber 14 is conveyed back into the first pressure chamber 9 of the tandem master cylinder 2 .

In the exemplary embodiment shown in FIG. 2, the path simulator 3 is functionally preceded by a piston-cylinder arrangement 18 , the piston 19 of which can be brought into a force-transmitting connection with the simulator piston 11 . The piston 19 delimits a hydraulic chamber 20 which can be connected to the high-pressure accumulator 5 or the pressure medium reservoir ( 4 ) via the valve arrangement 16 , 17 mentioned in connection with FIG. 1. The figure clearly shows that the hydraulic chamber 20 is connected to the high-pressure accumulator 5 separately from the pressure medium reservoir ( 4 ) in the switching position of the valves 16 , 17 corresponding to the fallback operating mode shown, so that the piston 19 is shifted to the right in the drawing and the Simulator piston 11 resets. The return movement of the simulator piston 11 into the starting position causes the pressure medium volume received by the simulator chamber 14 to be displaced into the master brake cylinder 2 . A displacement sensor 28 , which is only indicated schematically, enables position monitoring of the piston 19 .

In the embodiment of the subject matter shown in FIG. 3, the simulator piston 11 is reset in the fallback operating mode by the action of a return spring 22 which interacts with a mechanical actuating element 21 . The simulator piston 11 facing away from the end of the actuating element 21 is embodied as a hydraulic piston 23 that separates a first pressure chamber 24 from a second pressure chamber 25th The first pressure chamber 24 can be acted upon by the pressure provided by the high-pressure accumulator 5 , while the second pressure chamber 25 receives the return spring 22 , so that when the valve 16 is opened and the valve 17 is closed, the actuating element 21 moves to the left and the return spring 22 is tensioned. In the switching position of the valves 16 , 17 shown in Fig. 3, which corresponds to the fallback mode, the connection between the first pressure chamber 24 and the pressure medium reservoir ( 4 ) is opened, so that the return spring 22 relax and the actuator 21 in the drawing can adjust right. As a result, as in the cases described above, the simulator piston 11 is returned to its starting position.

In the example shown in FIG. 3, both the simulator chamber 13 and the second pressure chamber 25 are designed to be wet, ie filled with pressure medium. Since the pressure medium is displaced from the simulator chamber 13 and the second pressure chamber 25 during the movement of the simulator piston 11 and the actuating element 21, hydraulic connections 26 , 27 are provided between the second pressure chamber 25 and the simulator chamber 13 and between the simulator chamber 13 and the pressure medium reservoir 4 , It is of course also conceivable to dry both the simulator chamber 13 and the second pressure chamber 25 , ie to connect them to the atmosphere, so that the hydraulic connections 26 , 27 can then be omitted.

In the examples shown in FIGS. 4 to 6, the master brake cylinder 2 is supplied with an additional pressure medium volume in the fallback operating mode, which is preferably provided by the hydraulic auxiliary pressure source or the high pressure accumulator 5 . For this purpose, in the embodiment shown in FIG. 4, a hydraulic connection 29 is provided between the first pressure chamber 9 of the tandem master cylinder 2 and the high-pressure accumulator 5 , in which an electrically operable, normally open (SO) valve 30 is inserted. A merely schematically indicated, mechanically operated valve 31 in the conduit 32 between the simulator chamber 13 and the pressure fluid supply reservoir 4 enables in the fallback mode, a shut-off of the simulator chamber 13, wherein the actuation of the valve 31 beispielswese effected by the movement of the second master cylinder piston. 8 In the embodiment described, the high-pressure accumulator 5 is completely emptied in the event of a power failure. However, it is entirely conceivable to design the valve 30 inserted in the connection 29 between the high-pressure accumulator 5 and the first pressure chamber 9 of the master brake cylinder 2 as a mechanically controllable 2/2-way valve, which is actuated by means of the second master cylinder piston 8 .

The embodiment shown in FIG. 5 largely corresponds to the embodiment according to FIG. 4. The difference, however, is that in the connection 29 mentioned above, a separating piston device 33 is connected, which has a separating piston 35 preloaded by a spring 36 . The separating piston 35 delimits a pressure chamber 37 which is connected to the pressure medium reservoir ( 4 ) via an electrically switchable, preferably normally closed (SG) valve 34 . This measure ensures that the high-pressure accumulator 5 supplies a metered volume of pressure medium in the fallback operating mode and is not completely emptied.

The embodiment shown in FIG. 6 also largely corresponds to the embodiment according to FIG. 4. However, the valve 31 'inserted in the line 32 is designed as an electromagnetically actuated, normally closed (SG) 2/2-way valve, between the simulator chamber 13 and the simulator chamber 14, a pressure compensation line 38 is provided, which can be shut off or released by means of an electromagnetically actuated, preferably normally open (SO) 2/2-way valve 39 . The simulator function is switched off in the event of a power failure by pressure equalization between the simulator chamber 13 and the simulator chamber 14 . In this embodiment, too, it is conceivable to design the valve 30 inserted in the connection 29 between the high-pressure accumulator 5 and the first pressure chamber 9 of the master brake cylinder 2 as a mechanically controllable 2/2-way valve, which is actuated by means of the second master cylinder piston 8 .

In the fifth embodiment of the subject matter of the invention shown in FIG. 7, the structure of which corresponds essentially to the arrangement shown in FIG. 4, the valve 30 assigned to the high-pressure accumulator 5 is mechanically driven by the second master cylinder piston 8 , as indicated by the dotted line 41 actuated. In addition, however, a locking device 50 is provided, which blocks the simulator piston 11 when the high-pressure accumulator 5 is completely empty and thus prevents its movement in the actuating direction. The blocking device 50 essentially consists of a hydraulic pressure chamber 42 to which the storage pressure can be applied, a piston 43 coupled to a blocking element 44 and a spring 45 which prestresses the piston 43 . A sensor 46 , shown only schematically, serves to monitor the function of the locking device 50 .

In Fig. 7 all components are shown in the inactive position. If the brake-by-wire mode fails, the valve 30 is switched to its open position by the movement of the second master cylinder piston 8 , so that the high-pressure accumulator 5 is partially discharged into the first pressure chamber 9 of the master brake cylinder 2 . If, as a result of further braking operations, the high-pressure accumulator 5 is completely emptied and thus depressurized, the piston 43 or the blocking element 44 is displaced upward in the drawing by the force of the spring 45 , as a result of which the pressure medium volume received by the pressure chamber 42 is displaced back into the high-pressure accumulator 5 is and the simulator piston 11 is locked by the locking element 44 .

The embodiments shown in FIGS. 8 and 9 provide the wheel brakes, not shown, which are connected to the HCU 6 as in the examples described above, only a part of the pressure medium volume absorbed by the travel simulator 3 . For this purpose, in the embodiment shown in FIG. 8, a valve arrangement 47 is arranged between the simulator chamber 14 of the travel simulator 3 and the master brake cylinder 2 , which in the example shown is designed as an electrically operated 3/2-way valve. However, it is also conceivable to use a combination of two 2/2-way valves. In the currentless switching position of the valve arrangement 47 shown , which corresponds to the fallback operating mode, this establishes a hydraulic connection between the simulator chamber 14 and the wheel brakes of at least one vehicle axle, preferably the rear axle, while the connection between the master brake cylinder 2 and the simulator chamber 14 is interrupted , a check valve 48 opening towards the master brake cylinder 2 being connected in this connection. In the brake-by-wire operating mode, however, the master brake cylinder 2 is connected to the simulator chamber 14 and is separated from the wheel brakes.

Finally, a hydraulic connection is also provided in the embodiment shown in FIG. 9 between the simulator chamber ( 14 ) and the wheel brakes. The simulator piston 110 is designed in two parts and consists of a stepped piston 51 and an auxiliary piston 52 connected downstream of the stepped piston 51 . The area of the stepped piston 51 of larger diameter delimits the simulator chamber 14 , while its area of smaller diameter delimits an auxiliary chamber 53 . The simulator spring 12 mentioned above is supported on the auxiliary piston 52 . Between the simulator chamber 14 and the auxiliary chamber 53 there is a connection 55 which can be shut off by means of an electrically actuated shut-off valve 54 . The shut-off valve 54 is preferably designed as a normally open (SO) 2/2-way valve.

In the brake-by-wire mode, the shut-off valve 54 is closed so that when the master brake cylinder 2 is actuated, the stepped piston 51 moves synchronously with the auxiliary piston 52 and the simulator spring 12 is compressed. In the event of a power failure, the valve 54 is switched to its open position, so that pressure equalization takes place between the simulator chamber 14 and the auxiliary chamber 53 . As a result, the simulator spring 12 can relax and reset the simulator piston 110 , so that the pressure medium volume absorbed by the simulator 3 is made available to the wheel brakes.

Claims (23)

1.Electro-hydraulic brake system for motor vehicles, which can be controlled in a brake-by-wire operating mode both by the driver and independently of the driver, is preferably operated in the brake-by-wire operating mode and can be operated in a fallback operating mode in which only operation by the driver is possible with
one that can be actuated by means of a brake pedal, at least
a master cylinder having a pressure chamber,
a path simulator interacting with the master brake cylinder with a simulator piston positioned by means of a spring, which on the one hand delimits a simulator chamber receiving the spring and on the other hand a simulator chamber connected to one of the pressure chambers of the master brake cylinder,
a pressureless pressure medium reservoir,
a hydraulic auxiliary pressure source,
an electronic control unit, and
wheel brakes that can be connected to the master brake cylinder and the hydraulic pressure source, characterized in that means are provided for providing a pressure medium volume flow which, in the fallback operating mode, ensures at least the pressure medium volume requirement necessary for the respectively required deceleration. 2. Electrohydraulic brake system according to claim 1, characterized in that the means ensure a return of the pressure medium volume received by the travel simulator ( 3 ) into the master brake cylinder ( 2 ). 3. Electro-hydraulic brake system according to claim 2, characterized in that the return is carried out by means of the hydraulic auxiliary pressure source ( 5 ) applied energy. 4. Electro-hydraulic brake system according to claim 3, characterized in that the auxiliary pressure source is formed by a rechargeable high-pressure accumulator ( 5 ), and that a valve arrangement ( 16 , 17 ) is provided, which in the brake-by-wire mode of operation a hydraulic connection between the Simulator space ( 13 ) and the pressure medium reservoir ( 4 ) releases and shuts off a hydraulic connection between the simulator space ( 13 ) and the high pressure accumulator ( 5 ), while in the fallback mode it shuts off the hydraulic connection between the simulator space ( 13 ) and the pressure medium reservoir ( 4 ) and releases the hydraulic connection between the simulator room ( 13 ) and the high-pressure accumulator ( 5 ). 5. Electro-hydraulic brake system according to claim 4, characterized in that the simulator piston ( 11 ) is acted upon in the fallback operating mode against the direction of movement corresponding to the brake-by-wire operating mode with the pressure provided by the high-pressure accumulator ( 5 ). 6. Electro-hydraulic brake system according to claim 3, characterized in that the auxiliary pressure source is formed by a rechargeable high-pressure accumulator ( 5 ), and that a valve arrangement ( 16 ', 17 ') is provided, which is a hydraulic connection in the brake-by-wire mode releases between a hydraulic chamber ( 20 ) of a piston-cylinder arrangement ( 18 ) and the pressure medium reservoir ( 4 ) and shuts off a hydraulic connection between the hydraulic chamber ( 20 ) and the high-pressure accumulator ( 5 ), while in the fallback mode the hydraulic connection between the Shuts off hydraulic chamber ( 20 ) and the pressure medium reservoir ( 4 ) and releases the hydraulic connection between the hydraulic chamber ( 20 ) and the high-pressure accumulator ( 5 ), the hydraulic chamber ( 20 ) being delimited by a hydraulic piston ( 19 ) which is connected to that of the high-pressure accumulator ( 5 ) provided pressure is applied and that in the relapse ebsart adjusts the simulator piston ( 11 ) against its direction of movement corresponding to the brake-by-wire operating mode. 7. Electro-hydraulic brake system according to claim 3, characterized in that the auxiliary pressure source is formed by a rechargeable high-pressure accumulator ( 5 ), and that the simulator piston ( 11 ) in the fallback mode contrary to its brake-by-wire mode of movement corresponding to a mechanical actuating element ( 21 ) which is actuated by means of a return spring ( 22 ) which can be tensioned by the pressure provided by the high pressure accumulator ( 5 ). 8. Electro-hydraulic brake system according to claim 7, characterized in that part of the actuating element ( 21 ) is designed as a hydraulic piston ( 23 ). 9. Electro-hydraulic brake system according to claim 8, characterized in that the hydraulic piston ( 23 ) separates a first pressure chamber ( 24 ) from a second pressure chamber ( 25 ) and that a valve arrangement ( 16 , 17 ) is provided which is in the brake-by- wire operating mode releases a hydraulic connection between the first pressure chamber ( 24 ) and the high-pressure accumulator ( 5 ) and shuts off a hydraulic connection between the first pressure chamber ( 24 ) and the pressure medium reservoir ( 4 ), while in the fallback operating mode the hydraulic connection between the first Shuts off the pressure chamber ( 24 ) and the high pressure accumulator ( 5 ) and releases the hydraulic connection between the first pressure chamber ( 24 ) and the pressure medium reservoir ( 4 ). 10. Electro-hydraulic brake system according to claim 1, characterized in that the means ensure an inflow of an additional pressure medium volume into the master cylinder ( 2 ). 11. Electro-hydraulic brake system according to claim 10, characterized in that the additional pressure medium volume from the hydraulic auxiliary pressure source ( 5 ) is provided. 12. Electro-hydraulic brake system according to claim 11, characterized in that between the auxiliary pressure source ( 5 ) and at least one of the pressure chambers ( 9 , 10 ) of the master brake cylinder ( 2 ) by means of a valve ( 30 ) lockable hydraulic connection ( 29 ) is provided. 13. Electro-hydraulic brake system according to claim 12, characterized in that in the hydraulic connection ( 29 ) a separating piston device ( 33 ) is inserted, which is connected to the pressure medium reservoir ( 4 ) by means of a lockable line. 14. Electro-hydraulic brake system according to claim 12, characterized in that the master brake cylinder ( 2 ) is designed as a tandem master cylinder with a first ( 9 ) and a second pressure chamber ( 10 ), which is limited by a first ( 7 ) and a second piston ( 8 ) and that the valve ( 30 ) can be actuated by the movement of the second piston ( 8 ). 15. Electro-hydraulic brake system according to one of claims 10 to 14, characterized in that the simulator chamber ( 14 ) is connected to the simulator chamber ( 13 ) by means of a lockable line. 16. Electro-hydraulic brake system according to claim 14 or 15, characterized in that the simulator chamber ( 13 ) with the pressure medium reservoir ( 4 ) connecting line can be shut off or released by means of a shut-off valve ( 31 ) which can be actuated by the movement of the second master cylinder piston ( 8 ) is. 17. Electro-hydraulic brake system according to claim 14, characterized in that a locking device ( 50 ) is provided which prevents the movement of the simulator piston ( 11 ) in the actuating direction when the high-pressure accumulator ( 5 ) is completely empty. 18. Electro-hydraulic brake system according to claim 1, characterized in that the means in the fallback mode provide at least part of the pressure medium volume absorbed by the travel simulator ( 3 ) to the wheel brakes. 19. Electro-hydraulic brake system according to claim 18, characterized in that a valve arrangement ( 47 ) is provided which, in the brake-by-wire mode of operation, establish a connection between the master brake cylinder ( 2 ) and the simulator chamber ( 14 ) and a connection between the wheel brakes and shut off the simulator chamber ( 14 ) while in the fallback mode shut off the connection between the master brake cylinder ( 2 ) and the simulator chamber ( 14 ) and establish the connection between the wheel brakes and the simulator chamber ( 14 ). 20. Electro-hydraulic brake system according to claim 19, characterized in that in the fallback mode in the connection between the master cylinder ( 2 ) and the simulator chamber ( 14 ) or the wheel brakes a check valve ( 48 ) opening towards the master cylinder ( 2 ) is connected. 21. Electro-hydraulic brake system according to claim 19 or 20 characterized in that the wheel brakes one Vehicle axle, preferably assigned to the rear axle are. 22. Electro-hydraulic brake system according to claim 18 characterized in that the simulator chamber (14) is connected to the wheel brakes that the simulator piston (110) is designed in two parts and downstream of a stepped piston (52) and a the stepped piston (51) the auxiliary piston (52) The larger area of the stepped piston ( 51 ) delimits the simulator chamber ( 14 ), while its smaller area together with the auxiliary piston ( 52 ) delimits an auxiliary chamber ( 53 ), the simulator spring ( 12 ) being supported on the auxiliary piston ( 52 ) and a lockable connection ( 55 ) being provided between the simulator chamber ( 14 ) and the auxiliary chamber ( 53 ). 23. Electro-hydraulic brake system according to claim 22, characterized in that in the lockable connection ( 55 ) an electrically actuated, normally open (SO) 2/2-way valve ( 54 ) is inserted.