Procedure for controlling a braking system of a motor vehicle

Abstract

Method for controlling a braking system (1) of a motor vehicle comprising an electro-regenerative brake and a number of hydraulic friction brakes (2) driven by a brake pressure generating means via a brake medium (B), the total deceleration of which is composed of deceleration components of the friction brakes (2) and the electro-regenerative brake, wherein when the braking torque of the electro-regenerative brake decreases, the desired total deceleration is at least partially maintained by additional brake pressure build-up for the friction brakes (2) with a further power-assisted brake pressure generating means, wherein the braking torque of the electro-regenerative brake is limited to a predetermined maximum value, and wherein the braking torque of the electro-regenerative brake is limited in such a way thatthat the change in the deceleration component of the electro-regenerative brake when the vehicle speed changes does not exceed a predetermined maximum value, characterized in that the maximum value for the electro-regenerative brake is predetermined such that the deceleration contribution achievable by it does not exceed approximately 0.1g, preferably approximately 0.07g.

Description

[0001] The invention relates to a method for controlling a braking system, in particular for controlling a regenerative braking system with a number of friction brakes and an electric regenerative brake for a motor vehicle.

[0002] The purpose of regenerative braking systems in motor vehicles is to store at least some of the energy generated during braking within the vehicle and reuse it for propulsion. This reduces the vehicle's overall energy consumption, increases efficiency, and thus makes operation more economical. Motor vehicles with a regenerative braking system typically have various types of brakes, also known as brake actuators.

[0003] These systems typically employ hydraulic friction brakes, as found in conventional motor vehicles, and an electro-regenerative brake. The braking pressure for the friction brakes is applied, as with conventional friction brakes, via a brake pressure generator or by the movement of the brake pedal. The electro-regenerative brake is generally designed as an electric generator, which provides at least a portion of the total braking power. The generated electrical energy is fed into or back into a storage medium, such as an on-board battery, and reused to power the vehicle via a suitable drive system.

[0004] In general, regenerative braking systems can be implemented as so-called serial regenerative concepts, in which the proportion of the braking torque applied by the generator is as high as possible. In contrast, parallel and so-called drag-torque-based regenerative concepts are also known, in which the braking torque is distributed among the brake actuators in predetermined ratios. Furthermore, hybrid concepts of these braking concepts are known. All systems have in common that, at least in some ranges of the required braking torque, braking is carried out simultaneously by several brake actuators, so that the total deceleration is composed of the deceleration contributions of the brake actuators.

[0005] The distribution of braking energy between the friction brakes and the electric generator, particularly in series-type systems, generally depends on the target braking torque, the battery's state of charge, and especially the generator's operating range and other specific characteristics. Therefore, in regenerative braking systems, the brake pressure is built up at least partially independently of the brake pedal's hydraulic influence due to this distribution of braking energy.

[0006] From EP 1327556 ​​A1, it is known to divide the braking torque in a braking system with friction brakes and regenerative braking into a low-frequency part and a high-frequency part. The high-frequency part is requested by the drive unit as regenerative braking. The low-frequency part is either from the regenerative braking or the hydraulic braking.

[0007] In contrast, conventional braking systems, which only use a friction brake, build up hydraulic brake pressure depending on the position of the brake pedal. The pressure of a brake fluid, which is absorbed by the friction brake, is generated by the position of the brake pedal, with or without auxiliary power. The pedal position thus corresponds to the braking behavior of the vehicle. Exceptions may include the use of electronic safety systems, such as an electronic stability program (ESP), which may include devices for automatically building up brake pressure, independent of the brake pedal position.

[0008] In comparison to conventional braking behavior, a regenerative braking system, consisting of a combination of an electrically regenerative brake and a conventional, hydraulically actuated friction brake, has the disadvantage that the position of the brake pedal does not necessarily correspond to the braking behavior of the vehicle. For example, during an increase in braking deceleration, the position of the brake pedal may remain constant, which provides a very unfamiliar braking sensation for the driver, resulting in insufficient braking comfort.

[0009] As a solution to this problem, so-called "brake-by-wire" systems can be used. In these braking systems, electronic sensors detect the driver's braking request at the brake pedal. Electronically controlled valves then direct brake fluid from a high-pressure accumulator to the friction brakes. The high-pressure accumulator is filled with brake fluid and pressurized by a motor-driven pump unit. With such "brake-by-wire" systems, the required braking pressure at the friction brakes is generally supplied exclusively by the high-pressure accumulator, except in the event of a system component failure. Therefore, the brake pedal is inherently decoupled from the friction brakes, as the control and delivery of brake fluid to the friction brakes is electronically regulated and controlled.Decoupling the brake pedal from the friction brakes allows for the integration of a pedal control system suitable for regenerative braking, resulting in an acceptable brake pedal feel. However, "brake-by-wire" systems such as electronic brakes (EHB) or electromechanical brakes (EMB) are very expensive and can only be implemented with considerable effort.

[0010] Therefore, in an alternative braking system to achieve an acceptable pedal feel with a regenerative braking system, it may be provided that brake fluid is diverted into a pressure accumulator, particularly a low-pressure accumulator, when braking with an electric-regenerative brake. This simulates additional braking by the friction brakes to obtain a pedal movement or pedal force corresponding to the driving behavior.

[0011] Such a braking system can be controlled by an electronic control unit regulating electrically controlled hydraulic valves. In addition to suitable low-pressure accumulators, another pressure-generating device, such as a motor-driven pump unit, is used to build up and regulate the pedal pressure and to transport brake fluid from the low-pressure accumulator to the friction brakes via suitable hydraulic lines. The braking system is controlled by predefined control processes, using the brake fluid pressure of the friction brakes as well as the position and pressure of the brake pedal as control variables. Unlike a "brake-by-wire" system, the hydraulic unit of the braking system is deliberately coupled to the brake pedal to allow for adjustments to the pedal's response based on the desired and actual deceleration.

[0012] For high system dynamics, a further embodiment of the braking system described above incorporates a number of high-pressure accumulators in the hydraulic system. This ensures that, in the event of a significant increase in braking torque, such as during a so-called "after-pedaling" of the brake pedal, the system can respond to the increased braking demand with sufficient speed. A similar principle applies to situations where the driver quickly releases the brake pedal, resulting in a sudden decrease in the current braking pressure. In such a situation, the brake pedal is returned to its original position via the high-pressure accumulators.

[0013] To smooth out pressure pulsations on the low-pressure side of motor pump units, the brake system can also have a number of additional low-pressure accumulators that have a damping effect.

[0014] The problem with the braking system described above is that, while it achieves high system dynamics with acceptable pedal feel, the system is very complex. This leads to particularly complicated control of the braking system, and can result in problematic control transitions from a control engineering perspective.

[0015] Due to its high complexity and numerous system components, especially the additional high- and low-pressure accumulators, the braking system is comparatively expensive and therefore has an unfavorable cost-benefit ratio, so that its use in larger quantities has not yet been realized.

[0016] The invention is therefore based on the objective of providing a regenerative braking system and a method for controlling a braking system which, in addition to high braking comfort and high system dynamics, has a particularly favorable cost-benefit ratio.

[0017] This problem is solved according to the invention by providing that, in a braking system of a motor vehicle comprising an electro-regenerative brake, in particular a generator and a number of hydraulic friction brakes driven by a brake pressure generating means via a brake medium, the total deceleration of which is composed of deceleration components of the friction brakes and the electro-regenerative brake, the desired total deceleration is at least partially maintained when the braking torque of the electro-regenerative brake decreases, by providing additional brake pressure build-up for the friction brakes with a further power-assisted brake pressure generating means.

[0018] The invention is based on the premise that by deviating from the premise of a serial regenerative braking concept, the requirements for the braking system are significantly reduced. This generally leads to lower braking system costs and thus a more favorable cost-benefit ratio. A parallel braking system, a drag torque-based system, or a hybrid of these systems is conceivable. While these braking systems do not achieve optimal brake energy recovery, they also require fewer system components and control processes.

[0019] Thus, the braking behavior, and especially the brake pedal feel, of non-series regenerative braking systems is generally unproblematic across most of the deceleration range. Only at low speeds, particularly below approximately 15 km / h, is a significant drop in braking torque from the electric regenerative brake problematic. To achieve the same deceleration at this speed as with the preceding braking, the driver would have to abruptly reapply the brake pedal, which is very disruptive during targeted braking and has a critical impact on safety.

[0020] To compensate for the decreasing braking torque of the electro-regenerative brake in this speed range, additional brake pressure is applied to the friction brakes via a further power-assisted brake pressure generating means, according to the invention. The decreasing braking torque of the electro-regenerative brake is thus compensated for by a stronger braking torque from the friction brakes, with the necessary additional brake pressure being applied by the further brake pressure generating means.

[0021] For a simple and cost-effective braking system, it is advantageous to build up additional brake pressure for the friction brakes only at vehicle speeds below a maximum speed. This means that in other or higher speed ranges, the braking system can be essentially unregulated, or no additional regulation takes place.

[0022] This maximum speed is set at a limit beyond which the braking torque and braking power of the electro-regenerative brake decrease particularly sharply, preferably at a maximum speed of about 15 km / h.

[0023] The brake fluid required to increase the brake pressure of the friction brakes is conveniently supplied by the auxiliary brake pressure generating system to the hydraulic lines leading to the friction brakes. The required brake fluid can be stored in a reservoir of the actual brake pressure generating system.

[0024] To prevent this pressure increase from affecting the brake pressure generating device connected to a brake pedal, the hydraulic connection between the actual brake pressure generating device and a friction brake is throttled, preferably while additional brake pressure is being built up for the friction brakes using another power-assisted brake pressure generating device. This can be achieved via a throttleable hydraulic valve.

[0025] To restore the braking feel via the friction brakes after a braking operation, pressure equalization between a hydraulic line leading to a friction brake and a pressure generating medium is preferably achieved after braking by the controlled opening of a hydraulic valve. The valve can initially be opened slowly for pressure equalization and then remain open for a subsequent braking operation.

[0026] To prevent the braking feel from becoming too decoupled from the actual braking behavior or vehicle deceleration when braking simultaneously with an electro-regenerative brake and the friction brakes, due to the varying braking torque of the electro-regenerative brake depending on the speed, the braking torque of the electro-regenerative brake is limited to a predetermined maximum value according to the invention. In particular, the peak power and braking torque of the electro-regenerative brake are filtered out. Depending on the state of charge of the vehicle's battery, in which the energy generated by the electro-regenerative brake is stored, the limiting predetermined maximum braking torque is reached at different vehicle speeds.

[0027] For a suitable braking torque limitation of the electrically regenerative brake, the maximum value for the electrically regenerative brake is specified according to the invention such that the deceleration contribution achievable by it does not exceed approximately 0.1g, preferably 0.07g.

[0028] To prevent an excessively abrupt change in deceleration to the specified maximum value when the specified maximum braking torque of the electrically regenerative brake is reached, a smoothing function is preferably provided to the electrically regenerative brake when its braking torque approaches the aforementioned maximum value.

[0029] To prevent the varying braking torque of the regenerative brake, or the resulting varying deceleration component, from becoming so pronounced that it negatively impacts the braking feel, the braking torque of the regenerative brake is preferably limited so that the change in the deceleration component when the vehicle speed changes does not exceed a predetermined maximum value. Mathematically, this corresponds to specifying a maximum derivative for the braking torque curve of the regenerative brake with respect to speed or time.

[0030] In an additional or alternative advantageous embodiment, the smoothing can also be designed such that not only the change in the deceleration component of the electro-regenerative brake, but also the change in this change when the vehicle speed changes, does not exceed a predetermined maximum value. This limits not only the first, but also the second derivative of the braking torque curve with respect to vehicle speed or time.

[0031] For the control of the braking system, in addition to the components already described, this advantageously has a hydraulic unit with controllable hydraulic valves and hydraulic lines that connect the components of the braking system, wherein the braking system is advantageously controllable with the control unit via the controllable hydraulic valves.

[0032] To precisely determine the desired braking force, a target braking torque or pressure is determined via the brake pedal of the braking system. For this purpose, the brake pedal preferably has a pedal position sensor and / or a pressure sensor positioned in the hydraulic line leading to the pressure-generating device or the brake pedal. For increased operational reliability, the pressure sensor and the pedal position sensor can advantageously be redundant. The target braking torque or pressure can be determined by measuring the position of the brake pedal (determined by the position sensor) and the applied brake pedal pressure (determined by the pressure sensors).

[0033] In order to be able to apply the intended pressure generation processes with the brake pressure generation means, it is advantageous for one brake pressure generation means to be a power-operated master brake cylinder, in particular a tandem master cylinder, and for the other pressure generation means to be an electrically controlled hydraulic pump.

[0034] To measure the brake fluid pressure at the friction brakes, and especially to control this braking process, a pressure sensor is advantageously positioned in several hydraulic lines leading to the friction brakes. By measuring the brake fluid pressure, the required additional brake pressure can be calculated and adjusted.

[0035] For this purpose, the electrically controlled hydraulic valves can preferably be controlled in such a way that pressure regulation can be carried out via a hydraulic valve.

[0036] For high operational reliability, a check valve is advantageously assigned to an additional pressure generating device, either on the inlet or outlet side. This check valve allows brake fluid to flow only in the direction of flow from the pressure generating device. In the event of a failure of the additional brake pressure generating device, there is no loss of brake fluid pressure in the hydraulic lines of the friction brakes, and the braking system can continue to operate.

[0037] The additional hydraulic line, equipped with a shut-off valve, establishes a further direct connection between the active components of the brake system and the brake fluid reservoir. Failure of individual components, and in particular of the additional shut-off valve or the upstream check valve, could therefore lead to leaks and unwanted backflow of fluid. To effectively prevent this, regular test cycles are preferably performed during operation of the brake system to detect any existing or developing leaks.

[0038] Such a test cycle is preferably performed only when the driver has not requested the braking system, so that the driving experience is not affected. Alternatively or additionally, such a test cycle is performed automatically, without requiring active intervention or initiation by the driver, and advantageously in a global test phase for the vehicle, preferably before the start of a journey (so-called "Pre Drive Check") or after the end of a journey (so-called "Post Drive Check").

[0039] The advantages achieved with the invention lie particularly in the possibility of providing a regenerative braking system with high system dynamics and a pedal feel corresponding to the deceleration behavior, which has a simple design with few system components. Due to the simple design, the necessary control processes are also comparatively simple and can be carried out without the occurrence of control problems. The braking system is therefore comparatively inexpensive, so that it has an acceptable cost-benefit ratio.

[0040] An example embodiment is explained with the aid of a drawing. The drawing shows: Fig. 1. The basic circuit diagram of a braking system and Fig. 2 an example of the braking torque curve of an electric generator of the braking system according to Fig. 1 depending on the speed of a motor vehicle.

[0041] In Fig. Figure 1 shows a schematic diagram of a braking system 1 for a motor vehicle. The braking system 1 is a regenerative braking system 1, which, in addition to the four friction brakes 2, also includes an electric generator 4 for generating electrical energy that is stored in an on-board battery (not shown) to power the motor vehicle. The braking system 1 is designed as a non-series braking system 1, meaning that the maximum possible proportion of braking torque is not applied via the generator 4. This allows the braking system 1 to be designed relatively simply, as described below.

[0042] To apply the main braking torque for the friction brakes 2, the brake system 1 is equipped with a brake booster, which is designed as a tandem master cylinder 8 with auxiliary power and is connected to a brake pedal 6. The brake pressure applied by the tandem master cylinder 8 is transmitted to the friction brakes 2 via hydraulic lines 10 with a brake fluid B.

[0043] To regulate the braking system 1, particularly for the most comfortable braking feel and for typical driving stability functions, it features an electronic control unit 28. This unit allows the braking system 1, and especially its hydraulic system, to be regulated via suitable, electronically controlled hydraulic valves. Valves 14 and 16 are designated according to whether they are open or closed when de-energized (i.e., without electrical actuation). The SO valve 14 is open when de-energized, and the SG valve 16 is closed when de-energized. The EU valve 18 corresponds to an electronic changeover valve.

[0044] The following describes the sequence of a braking process using the following examples: Fig. 1. This is explained in more detail, whereby this is only done on one part of the brake system 1, specifically on each friction brake 2 with its associated system components. The system components of the other friction brakes 2 function accordingly. The target braking torque and the brake fluid pressure at the friction brakes 2 serve as input and control variables for the control of the brake system. As can be seen from Fig. As can be seen from 1, the target braking torque is first determined by the electronic control unit 28 via a position sensor 24, which is positioned on the brake pedal 6, and a pressure sensor 26, which is located on the hydraulic line 10 leading to the tandem master cylinder 8. The brake fluid pressure at the friction brakes 2 is determined by a pressure sensor 26, which is positioned in a hydraulic line of a friction brake 2.

[0045] In Fig.Figure 2 shows the braking torque curve of a generator 4 during braking from 130 km / h. The deceleration component or braking torque of the friction brakes 2 acting simultaneously with the generator is not shown in the figure. The generator 4 is an asynchronous generator exhibiting the characteristics shown. The theoretically achievable braking torque of this generator 4 during braking with a sufficiently discharged vehicle battery is shown in line 2, and the theoretically achievable generator power is shown in line 3. The change in the generator braking torque occurs due to the change in rotational speed during the vehicle's deceleration and falls to zero below a speed of approximately 15 km / h, regardless of the vehicle battery's state of charge.

[0046] To maintain maximum braking comfort during this braking process and to prevent the pedal feel from becoming too decoupled from the actual deceleration behavior, the generator braking torque is limited to a maximum braking torque corresponding to a generator deceleration of 0.07 g, as shown in line 1. To prevent the change in braking torque, or the resulting deceleration component, from changing too drastically during vehicle deceleration, which would negatively impact braking comfort, the generator braking torque is limited so that the change in the generator's deceleration component when the vehicle speed changes does not exceed a predetermined maximum value. Line 1 is limited by a maximum gradient.

[0047] Furthermore, the generator braking torque gradually approaches the maximum braking torque, which corresponds to 0.07 g, using a smoothing function, resulting in a smooth transition to the maximum braking torque of generator 4.

[0048] At the start of the braking process, at 130 km / h, brake fluid B is initially supplied via the brake pedal 6 or the tandem master cylinder 8 through the open ASR valve 20 and the open SO valve 14 into the hydraulic line 10 of the friction brake 2 when the target braking torque is increased.

[0049] When the vehicle has decelerated to 25 km / h, the braking torque of generator 4 is gradually reduced to mitigate the abrupt decrease in generator braking torque at 15 km / h and to ensure a smooth transition. Simultaneously, the entire braking torque is transferred to the friction brakes 2. For this purpose, additional brake fluid B is pumped from the reservoir of the tandem master cylinder 8 into the hydraulic line 10 of a friction brake 2 via an additional shut-off valve 21, which is connected to a hydraulic line 10 directly linked to the brake fluid reservoir. This additional brake fluid is generated by the hydraulic pump 22, which in this case acts as a motor-driven pump unit and serves as an additional brake pressure generator. To prevent excessive feedback to the brake pedal 6 or to prevent the pedal from being pushed back, the ASR valve 20 is temporarily throttled or held closed.

[0050] The additional shut-off valve 21 is protected by a check valve 40 connected in series with it in the hydraulic line 10.

[0051] When the vehicle comes to a standstill, or even before, the ASR valve 20 is opened in a controlled manner to restore pressure equalization between the friction brake pressure and the pressure of the tandem master cylinder 8. The ASR valve 20 opens in a throttled manner. At the end of the braking process, the brake pedal 6 is returned to its initial position, and excess brake fluid B is returned to the reservoir of the tandem master cylinder 8.

[0052] The brake system 1 is designed for a particularly high level of operational safety. Particular attention has been paid to the fact that the additional hydraulic line 10, equipped with the shut-off valve 21, creates a further direct connection between the active components of the brake system 1 and the brake fluid reservoir. This means that if individual components, and especially the shut-off valve 21 or the check valve 40 upstream of it, fail, leaks and undesired backflow of the fluid could occur. To effectively prevent this, regular test cycles are preferably carried out during the operation of the brake system 1 to detect any existing or developing leaks.

[0053] Such a test cycle is preferably performed only when the driver has not requested braking system 1, so that the driving experience is not affected. Alternatively or additionally, such a test cycle is performed automatically, without requiring active intervention or initiation by the driver, and advantageously in a global test phase for the vehicle, preferably before the start of a journey (so-called "Pre Drive Check") or after the end of a journey (so-called "Post Drive Check").

[0054] During such a test cycle, a condition is first created in which the hydraulic pump 22 is not supplied with brake fluid B and therefore should not be able to build up brake pressure on the friction brakes 2. For this purpose, the supply lines to the pressure-generating medium or to the hydraulic pump 22 are closed by actuating the corresponding valves. To completely decouple the inlet from the brake fluid reservoir, the ASR valve 20 is also closed. Subsequently, the hydraulic pump 22 is activated, and as a result of the decoupling, no brake pressure should build up at the friction brakes 2, as designed. If, however, pressure buildup is detected at the friction brakes 2, a leak in the supply system, and in particular in the shut-off valve 21, can be concluded. Reference symbol list 1. Braking system 2 friction brakes 4 Generator 6 Brake pedal 8 tandem master cylinders 10 Hydraulic lines 12 pressure accumulators 14 SO valve 16 SG valve 18 EU valve 20 ASR valve 21 valve 22 Hydraulic pump 24 wayfinders 26 Pressure sensor 28 Control unit 40 Check valve B Brake fluid

Claims

[1] Method for controlling a braking system (1) of a motor vehicle, which has an electrically regenerative brake and a number of hydraulic friction brakes (2) driven by a braking pressure generating means via a braking means (B), the total deceleration of which is composed of deceleration components of the friction brakes (2) and the electrically regenerative brake, wherein when the braking torque of the electrically regenerative brake decreases, the desired total deceleration is at least partially maintained by additional brake pressure build-up for the friction brakes (2) with a further auxiliary power-assisted brake pressure generating means, wherein the braking torque of the electrically regenerative brake is limited to a predetermined maximum value and wherein the braking torque of the electrically regenerative brake is limited in such a way thatthat the change in the deceleration component of the electric regenerative brake does not exceed a specified maximum value when the vehicle speed changes, characterized by that the maximum value for the electric regenerative brake is specified in such a way that the deceleration contribution achievable by it does not exceed about 0.1 g, preferably about 0.07 g. [2] Method according to claim 1, in which additional brake pressure for the friction brakes (2) is only built up at a vehicle speed below a maximum speed [3] Method according to claim 1 or 2, in which a speed of approximately 25 km / h is specified as the maximum speed. [4] Method according to one of claims 1 to 3, in which, for the additional brake pressure build-up for the friction brakes (2), brake medium (B) is conveyed from an auxiliary power-assisting brake pressure generating means into the hydraulic lines (10) leading to the friction brakes (2). [5] Method according to one of claims 1 to 4, in which the hydraulic connection of the actual brake pressure generating means with the friction brakes is temporarily throttled during the additional brake pressure build-up for the friction brakes (2) with the further auxiliary power-assisted brake pressure generating means. [6] Method according to one of claims 1 to 5, in which at the end and / or after a braking operation, a pressure equalization between a hydraulic line (10) leading to a friction brake (2) and a pressure generating means is carried out by the controlled opening of a hydraulic valve (20). [7] Method according to claim 5 or 6, in which a smoothing function is specified for the electrically regenerative brake when its braking torque approaches the above-mentioned maximum value. [8] Method according to one of claims 1 to 7, in which the change in the deceleration component of the electric regenerative brake is subjected to smoothing. [9] Method according to one of claims 1 to 8, in which the braking torque of the electric regenerative brake is limited in such a way that the change in the deceleration component of the electric regenerative brake does not exceed a predetermined maximum value when the vehicle speed changes. [10] Braking system (1) of a motor vehicle, comprising a brake pressure generating means with or without auxiliary energy, which is connected to a brake pedal (6) for generating hydraulic brake pressure via a brake means (B) for friction brakes (2) of the braking system (1), an electronically controllable electric regenerative brake for generating electrical energy from the kinetic energy of the motor vehicle, an electronic control unit (28), a hydraulic unit with electrically controllable hydraulic valves (14, 16, 18, 20, 21), and hydraulic lines (10) which establish the connection between hydraulic valves (14, 16, 18, 20, 21), brake pressure generating means and the friction brakes (2), wherein when the braking torque of the electric regenerative brake is reduced, an additional brake pressure build-up for the friction brakes (2) can take place with a further auxiliary power-assisted brake pressure generating means,wherein the braking torque of the electric regenerative brake is limited to a predetermined maximum value and wherein the braking torque of the electric regenerative brake is limited such that the change in the deceleration component of the electric regenerative brake does not exceed a predetermined maximum value when the vehicle speed changes, characterized by that the maximum value for the electric regenerative brake is specified in such a way that the deceleration contribution achievable by it does not exceed about 0.1 g, preferably about 0.07 g. [11] Brake system (1) according to claim 10, wherein the brake fluid (B) for the additional brake pressure build-up can be removed directly from the brake fluid reservoir via a hydraulic line (10) and an additional shut-off valve (21). [12] Brake system (1) according to claim 11, wherein a check valve (40) is connected in series with the shut-off valve (21) in the hydraulic line (10). [13] Brake system (1) according to one of claims 10 to 12, one of which brake pressure generating means is a master brake cylinder operated by auxiliary power and the further pressure generating means is an electrically controllable hydraulic pump (22). [14] Brake system (1) according to one of claims 10 to 13, wherein a pressure sensor (26) for measuring the pressure of a braking medium (B) is positioned in a number of hydraulic lines (10) leading to the friction brakes (2). [15] Brake system (1) according to one of claims 10 to 14, in which electrically controllable hydraulic valves (14, 16, 18, 20, 21) can be controlled in such a way that pressure control can be carried out via a hydraulic valve (14, 16, 18, 20, 21). [16] Braking system (1) according to one of claims 10 to 15, whose brake pedal (6) has a number of displacement sensors (24). [17] Braking system (1) according to one of claims 10 to 16, comprising a number of pressure sensors (26) for measuring the pressure of a braking medium (B), wherein a pressure sensor (26) is positioned in a hydraulic line (10) leading to a braking pressure generating means. [18] Method for operating a braking system (1) according to one of claims 10 to 17, in which the shut-off valve (21) is subjected to regular test cycles. [19] Method according to claim 18, in which a test cycle is carried out only when there is no driver request. [20] Method according to claim 18 or 19, in which a test cycle is carried out automatically. [21] Method according to claim 20, in which a test cycle is carried out in a global test phase before the start of the journey or after the end of the journey. [22] Method according to one of claims 18 to 21, in which, during a test cycle, the valves connected in the supply lines to the pressure generating means (22) and the ASR valve (20) are closed and then the pressure generating means (22) is activated, wherein, if a pressure build-up is detected at the friction brakes (2), a leak in the supply line system is concluded. [23] Method according to one of claims 18 to 22, in which, during a test cycle, the valves connected in the supply lines to the pressure generating means (22) and the ASR valve (20) are closed and then the pressure generating means (22) is controlled, wherein, if an increased power consumption in the pressure generating means (22) is detected, it is concluded that there is a leak in the supply line system. [24] Method according to one of claims 18 to 21, in which, during a test cycle, the valves connected in the supply lines to the pressure generating means (22) and the SO valve (14) provided as an inlet valve for the friction brake (2) are closed and the ASR valve (20) is throttled if necessary and then the pressure generating means (22) is activated, wherein if an increased power consumption in the pressure generating means (22) is detected, it is concluded that there is a leak in the supply line system.

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