Method for adjusting the brake pressures of a motor vehicle by controlling a pressure control valve, brake system for carrying out the method and motor vehicle

Abstract

Method for adjusting brake pressures of a motor vehicle (6), wherein a control unit (17) actuates a pressure control valve (16) with an inlet valve (19) for venting and with an outlet valve (20) for venting the brake pressure line (38, 39) controlled by the pressure control valve (16), taking into account determined dynamic state variables of the wheels (4) to be braked, wherein the control times (tA) of the outlet valve (20) are determined as a function of the control of the inlet valve (19), characterized in that the number (28) of actuations of the inlet valve (19) during the build-up of brake pressure is counted and the control times (tA) of the outlet valve (20) are determined as a function of the number (28) of actuations of the inlet valve (19) that have already taken place.

Description

[0001] The invention relates to a method for adjusting the brake pressure of a motor vehicle according to the preamble of claim 1. The invention further relates, according to claim 12, to a brake system for carrying out the method for adjusting the brake pressure and, according to claim 16, to a motor vehicle with such a brake system for carrying out the method.

[0002] To brake a motor vehicle, the vehicle's wheels are braked, usually by actuating the wheel brake cylinders. In commercial vehicles, the required brake pressure is typically generated pneumatically. If the driver requests braking via a brake pedal, the braking system translates this request into a corresponding brake pressure. The brake pedal either acts via a service brake valve or is designed as a signal-generating position sensor.

[0003] Anti-lock braking systems (ABS) are control devices in the braking system that prevent the wheels from locking up during braking, thus maintaining steering control and driving stability. A control unit adjusts the brake pressure in the connected brake cylinders, taking into account the dynamic parameters of the wheels being braked, via a pressure control valve at each wheel. These dynamic parameters are often determined from the measurements of wheel speed sensors. If a wheel shows a tendency to lock up, the deceleration and wheel slip increase significantly. If the determined dynamic parameter exceeds certain critical values, the control unit intervenes by means of appropriate control measures via the pressure control valve.During the intervention, the stability and instability of the wheel movement are repeatedly detected alternately and, through a sequence of pressure build-up, pressure holding and pressure release phases, are usually adjusted in the slip range with maximum braking force.

[0004] EP 1 028 043 B1 discloses such a pressure control device for commercial vehicles, wherein the pressure control valve comprises a 2 / 2-way valve acting as an inlet valve and a 2 / 2-way valve acting as an outlet valve. In the known device, the inlet valve is actuated during the pressure build-up phase. Under certain conditions, the inlet valve is also actuated during the pressure maintenance phase or even during the pressure reduction phase. The outlet valve is primarily actuated during the pressure reduction phase. However, in the known device, the outlet valve can also be actuated during the pressure maintenance phase or during the pressure build-up phase under certain conditions. Actuating the outlet valve during the pressure build-up phase, or the inlet valve during the pressure reduction phase, is intended to enable counter-regulation, i.e., to reduce overshoot tendencies during rapidly changing pressure.

[0005] German patent DE 2406690 A1 describes a control valve comprising a cylindrical housing with coaxially arranged solenoid coils for ease of operation. The solenoid coils act on solenoid valves coupled to diaphragms positioned above the housing, which control the pressure level in the valve chamber. One diaphragm is directly coupled to a coaxial inlet valve to allow brake fluid, while the other diaphragm is coupled to a coaxial vent valve. For complete modulation control, both sets of valves operate in opposite directions.

[0006] The pressure control valve, with its combination of inlet and outlet valves, is used by the control unit for interventions within the anti-lock braking system. The inlet valve is normally open (continuously open) when de-energized, while the outlet valve is closed. This allows the brake pressure to be built up via a service brake valve during normal braking operation, based on the driver's request.

[0007] Conventionally, the control units of anti-lock braking systems (ABS) adjust the brake pressure according to so-called internal braking requirements, meaning control interventions within the framework of the ABS system. In addition to the control interventions within the ABS system determined independently of the driver by the control unit, the internal braking requirements can also take the driver's wishes into account, particularly if a driver braking request is generated by an electronic brake signal transmitter and transmitted to the control unit.

[0008] It has been shown that even when the brake pressure is regulated by the pressure control valves of the anti-lock braking system (ABS), large pressure fluctuations frequently occur, leading to unsatisfactory control of the brake pressure and ultimately to poor vehicle deceleration. This is particularly true when the pressure differential between the supply pressure at the inlet valve and the regulated brake pressure is large, resulting in unacceptably large pressure changes. This is especially true at the start of braking when an external brake application is triggered.

[0009] Especially with small volumes to be ventilated, the pressure fluctuations are so large that only a completely unsatisfactory level of control can currently be achieved. Small volumes frequently occur, for example, in towing vehicles with small brake cylinders or vehicle combinations where the trailer's braking system is controlled by the towing vehicle.

[0010] Furthermore, small pressure increments are often desirable, for example, when adjusting brake pressure based on external braking demands. Besides internal braking requirements, particularly the interventions of the anti-lock braking system (ABS), the vehicle's braking system must often also respond to external braking demands. These external braking demands can originate from other driver assistance systems. Especially in commercial vehicles, various driver assistance systems are typically housed in separate units. Each of these systems can detect a braking requirement under certain conditions, which the braking system must then address.

[0011] The brake pressure is typically not regulated in a closed-loop system when external braking is required, but rather simply controlled. When controlling the brake pressure, no setpoint / actual value comparison takes place, as no actual brake pressure measurement is provided or available. It is known to control the brake pressure using a 3 / 2-way valve. However, such a 3 / 2-way valve only offers two switching states: one for increasing and one for decreasing the brake pressure. During control, there is frequent switching between pressure increases and decreases, each with maximum gradients. The gradient depends on the current pressure levels, specifically the ratio of the available supply pressure to the controlled brake pressure, and the ratio of the controlled brake pressure to the reference pressure (atmosphere).Furthermore, the gradient is influenced by the volume of the brake cylinder to be ventilated.

[0012] The pressure control valves of anti-lock braking systems (ABS) are typically optimized for the purposes of the ABS and feature large inlet and outlet cross-sections. However, precisely because of these large cross-sections, the pressure regulation becomes unsatisfactory when external braking demands are applied. Therefore, 3 / 2-way valves are often preferred for controlling brake pressure under external braking conditions and are actuated accordingly to adjust the brake pressure.

[0013] Small brake volumes, especially when used with trailers, require regular venting if the trailer's braking system is indirectly controlled by the towing vehicle via a brake pressure line. In this case, a relatively small volume between the pressure control valve and another valve before the trailer's brake system inlet needs to be vented.

[0014] The present invention is based on the objective of enabling a more precise and faster adjustment of the brake pressure while reducing the air volume requirement.

[0015] This problem is solved according to the invention by a method for adjusting the brake pressure with the features of claim 1. The problem is further solved by a brake system according to claim 12 and according to claim 16 by a motor vehicle with such a brake system for carrying out such a method.

[0016] According to the invention, the control timing of the outlet valve of the pressure control valve is determined as a function of the control of the inlet valve. The system is designed accordingly for determining the control timing of the outlet valve as a function of the control of the inlet valve. The invention recognizes that a pressure control valve of an anti-lock braking system can be controlled not only as a single unit of combined valves, but also as two separately controllable valves. Due to the inventive dependence of the control timing of the outlet valve on the control of the inlet valve, even with small volumes to be vented, the small pressure changes required for high control accuracy can be achieved.

[0017] The invention is particularly suitable for adjusting the brake pressure in brake systems that have comparatively small volumes to be vented or vented via the pressure control valve. In an advantageous embodiment, the control unit, in an external braking mode (i.e., for implementing an external braking request), determines the control timing of the outlet valve of the pressure control valve as a function of the control of the inlet valve. Accordingly, the control unit is designed to receive an external braking request independent of the driver's braking request and to determine the control timing of the outlet valve as a function of the control of the inlet valve.

[0018] Driver assistance systems that trigger an external braking request include, for example, roll stability control (RSC) and adaptive cruise control (ACC). The invention enables very small pressure fluctuations and thus high control accuracy by controlling the exhaust valve, optionally simultaneously with the inlet valve. For example, in combination stability control (CSC), the invention allows for the smallest possible pressure fluctuations via the exhaust valve, starting from zero and increasing according to a predefined function. In particular, the braking system of the trailer is controlled with the smallest possible pressure fluctuations via the pressure control valve with inlet and exhaust valves and via the control valve that actuates the trailer's braking system.Even in a braking system with individual wheel braking, adjusting the brake pressure by determining the timing of the outlet valve of the pressure control valve as a function of the inlet valve control is advantageous. Furthermore, high control accuracy is also achieved during anti-lock braking (ABS) intervention by determining the timing of the outlet valve of the pressure control valve according to the invention, as a function of the inlet valve control.

[0019] In an advantageous embodiment, a trailer's braking system can be activated indirectly via a brake pressure line for the trailer and a pressure control valve located in this brake pressure line. The brake pressure line for the trailer, with the pressure control valve located in this line, is connected to a pneumatic coupling head to which the trailer's braking system can be coupled. In this way, only a small volume in the brake pressure line between the pressure control valve and another valve at the inlet to the trailer's braking system needs to be pressurized or vented.

[0020] In one embodiment of the invention, the timing of the exhaust valve is determined by adding a predetermined time interval, for example three milliseconds, to the timing of the preceding timing of the intake valve.

[0021] If, at least in the lower brake pressure range, the timing of the inlet valve and the exhaust valve overlap at least partially during pressure increases, excessive pressure jumps can be prevented even with small volumes to be ventilated.

[0022] In an advantageous embodiment of the invention, the number of inlet valve actuations during brake pressure build-up is counted, and the exhaust valve timing is determined based on the number of inlet valve actuations already performed. During braking, the inlet valve is always actuated several times at different intervals. By adjusting the exhaust valve timing to the number of inlet valve openings, the pressure surge caused by the inlet valve opening can be mitigated, or excessive pressure surges can be prevented. The number of inlet valve actuations is counted and stored, and the sequential number of the inlet valve actuations thus far is used to determine the corresponding exhaust valve timing.

[0023] In a further advantageous embodiment, the exhaust valve timing is determined based on the intake valve timing, so that immediately after the intake valve is actuated, the exhaust valve is always actuated with timing dependent on the preceding intake valve timing. It is possible and intended that the exhaust valve actuation occurs simultaneously with the actuation of the intake valve, or with a time delay before or after the start of the intake valve actuation, always immediately after the end of the intake valve actuation, or also with a time delay after the end of the actuation.

[0024] In a further advantageous embodiment, a correction factor is determined during external braking mode. This factor is used to weight the intake valve timing to determine the exhaust valve timing. The exhaust valve timing is then calculated accordingly using this correction factor. Advantageously, the correction factor is continuously determined according to a predefined procedure, based on a measured vehicle deceleration. Unlike setting a constant correction factor, this approach takes into account the dynamic state of the vehicle and the remaining braking force during the braking process.

[0025] In an advantageous embodiment, the correction factor for determining the exhaust valve timing, as a function of the intake valve timing, is itself determined as a function of the sum of the timings of previous intake valve actuations and those exhaust valve timings previously determined in connection with a reduction of brake pressure, according to a predefined characteristic curve. The previous exhaust valve timings considered are those timings determined outside of the external braking mode in connection with a brake force reduction control and stored for determining the correction factor.

[0026] In a further embodiment for determining the correction factor as a function of the sum of the inlet and exhaust valve timings, a predetermined time constant is subtracted from each of the recorded timings. The time constant is determined in advance in such a way as to compensate for any dead time or reaction time immediately after a valve opens.

[0027] The pressure control valves are advantageously arranged in brake pressure lines, through which a brake cylinder is vented or pressurized on a side of the pressure control valve located beyond a pressure reservoir. The pressure control valves thus control the brake pressure line downstream of the pressure control valve, i.e., on the side of the pressure control valve located beyond the pressure reservoir.

[0028] Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. The drawing shows: Fig. 1. A pneumatic and electrical diagram of an anti-lock braking system in a commercial vehicle, Fig. 2. A pneumatic and electrical diagram of an anti-lock braking system in a commercial vehicle with a trailer. Fig. 3 a graphical diagram of the time courses of the control of intake valves and exhaust valves, Fig. 4 a flowchart of a first embodiment of a method for adjusting the brake pressure in external braking mode, Fig. 5 a flowchart of a second embodiment of a method for adjusting the brake pressure in external braking mode, Fig. 6 a flowchart of a third embodiment of a method for adjusting the brake pressure in external braking mode, Fig. 7 a flowchart of a fourth embodiment of a method for adjusting the brake pressure in external braking mode.

[0029] Fig. Figure 1 shows an electro-pneumatic diagram of a braking system 1 of a motor vehicle 6, in particular a commercial vehicle. Electrical lines are represented by solid lines and pneumatic lines by dotted lines. In the illustrated embodiment, the motor vehicle 6 comprises two axles, namely a front axle 2 and a rear axle 3, on each of which wheels 4 are arranged on both sides. To brake the wheels 4, each wheel 4 is assigned a brake cylinder 5, which exerts a braking force on the rotating wheel according to the applied pneumatic brake pressure. Brake cylinders 5 with spring accumulators 7 are provided on the wheels 4 of the rear axle 3, which serve as a parking brake.

[0030] During operation of the motor vehicle 6, the driver can apply pneumatic pressure to the brake cylinders 5 by pressing a brake pedal 8. In the illustrated embodiment, the brake pedal 8 is coupled to a service brake valve 9, which opens when the brake pedal 8 is pressed. In other embodiments, an electric driving signal transmitter is provided, the electrical actuation signal of which is used to apply pneumatic pressure.

[0031] In the illustrated embodiment, the brake system 1 of the motor vehicle 6 has two brake circuits. A first brake circuit 10 can be connected to a first pressure reservoir 11 via the service brake valve 9, and a second brake circuit 12 can be connected to a second pressure reservoir 13. The first brake circuit 10 includes a first relay valve 14, which is arranged between the service brake valve 9 and the connected brake cylinders 5. Similarly, in the second brake circuit 12, a second relay valve 15 is arranged between the service brake valve 9 and the connected brake cylinders 6 of the second brake circuit 12.

[0032] The braking system 1 of the motor vehicle 40 includes an anti-lock braking system (ABS) which intervenes in the braking process of a wheel 4 that tends to lock up, as needed, via pressure control valves 16. Each wheel 4 is assigned a pressure control valve 16, which is located directly upstream of the respective brake cylinders 6, i.e., in the illustrated embodiment, between the relay valves 14, 15 and the respective brake cylinders 5, 6. Each pressure control valve 16 is connected to the respective brake cylinder 5 via a brake pressure line 38. The pressure control valves 16 can be electrically controlled by a control unit 17, depending on a dynamic state variable of the braking wheels. For this purpose, each wheel is assigned a sensor, which in the illustrated embodiment is a speed sensor 18. The speed sensor 18 thus senses the state of motion of the wheel.If a wheel shows a tendency to lock up, i.e., if the determined values ​​exceed predetermined limits, the control unit 17 sends electrical control commands to the pressure control valve 16 of the wheel 4 that is prone to locking up.

[0033] The pressure control valves 16 are a combination of at least two solenoid valves, namely an inlet valve 19 and an outlet valve 20. The inlet valve 19 serves primarily to increase or maintain the pressure in the brake cylinder 5, while the outlet valve 20 opens to reduce the brake pressure and vents the respective connected brake cylinder 5. In this embodiment, the inlet valve 19 and the outlet valve 20 are 2 / 2-way valves, which can be electrically controlled by the control unit 17.

[0034] The inlet valves 19 can be connected to the pressure reservoirs 11 and 13 of the respective brake circuits 10 and 12 via the relay valves 14 and 15. This means that the pressure control valves 16 of the first brake circuit 10 of the front axle 2 can be fluidically connected to the first pressure reservoir 11 via the first relay valve 14, and the pressure control valves 16 of the second brake circuit 12 can be connected to the second pressure reservoir 13 via the second relay valve 15.

[0035] The control unit 17 is designed and configured to automatically influence the braking process, independent of the driver's wishes, depending on the dynamic state variables of the wheels to be braked, and opens the connection between the pressure control valves and the respective connected pressure reservoirs 11, 13 as required. The control unit 17 controls a 3 / 2-way valve 22, which is arranged between the control input 21 of the relay valve 14 and a third pressure reservoir 23.

[0036] The service brake valve 9 and the 3 / 2-way valve 22 of the anti-lock braking system are coupled to the control input 21 of the relay valve 14 via a double check valve 24. Similarly, a 3 / 2-way valve 22 is provided in the second brake circuit 12, which is coupled to the relay valve 15 of the second brake circuit 12 via a double check valve 24 of the second brake circuit 12.

[0037] The control unit 17 has an input for external braking requests 25 and, when an external braking request is received, sets the corresponding braking pressure in a designated external braking mode by controlling the pressure control valves 16. An external braking request is typically a braking need detected by an external driver assistance system 6 of the vehicle. Such driver assistance systems of the vehicle 6 often have their own control electronics and sensors and can be connected to the control unit 17 of the anti-lock braking system, for example, via a data bus. However, the term "external braking request" is not limited to braking performance requests communicated by external driver assistance systems, but can encompass any type of braking request that is not among the internal braking requests of the anti-lock braking system.

[0038] In the exemplary embodiment according to Fig. Figure 1 shows a towing vehicle. Fig. Figure 2 shows an electro-pneumatic diagram of a brake system 1 of a motor vehicle 6, in particular a commercial vehicle or towing vehicle of a vehicle combination, to which a trailer 40 or its brake system can be connected. Electrical lines are shown with solid lines and pneumatic lines with dotted lines. With the exception of the features noted below, the design of the brake system 1' corresponds to the design of the brake system 1 of the commercial vehicle according to [reference to figure]. Fig. 1.

[0039] A third brake circuit 41 is provided for activating the braking system of the trailer 40. Similar to the first brake circuit 11 and the second brake circuit 12, the third brake circuit 41 has a pressure control valve 16, a double check valve 24, and a 3 / 2-way valve 22. The pressure control valve 16 of the third brake circuit 41, specifically its inlet valve 19 and outlet valve 20, is controllable by the control unit 17. Unlike the first brake circuit 11 and the second brake circuit 12, a brake pressure line 39 is connected downstream of the pressure control valve 16 to a control valve 43, which controls the connection between the fourth pressure reservoir 42 and a pneumatic coupling head 44. The braking system of the trailer 40 can be coupled to the coupling head 44.

[0040] In the illustrated embodiment, the control valve 43 is pilot-operated by the pressure in the brake pressure line 39, so that the brake system can be indirectly activated or controlled via the brake system 1' of the commercial vehicle 6. In the illustrated embodiment, the brake system of the trailer 40 is supplied from a fourth pressure reservoir 42, which can be released by the pilot-operated valve. In this design, the pressure control valve 16 only needs to vent or depressurize a very small volume, namely exclusively the volume of the brake pressure line 39. The brake pressure in the brake pressure line is adjusted as described below.

[0041] Examples of how to adjust the brake pressure in external braking mode according to an external braking request 25 are shown below. Fig. 3 to 7 explained in more detail.

[0042] According to the invention, the control timing of the outlet valve is determined by the control unit depending on the control of the inlet valve of the respective pressure control valve, whereby very small pressure jumps can be controlled. Fig. Figure 3 shows the time-dependent switching states of the inlet and outlet valves for two examples during an external braking mode. In these time-dependent graphs, switching state "1" represents the switched-on state of the respective valve, and switching state "0" represents the switched-off state. For each embodiment, one switching state E of the inlet valve and one switching state A of the outlet valve are shown.

[0043] During braking, in the illustrated embodiment due to an external braking request, the pressure control valves for implementing the braking request in the lower brake pressure range are controlled in both embodiments such that the opening times of the inlet valve and the outlet valve overlap. In the first embodiment according to Fig. 3. The exhaust valve is actuated approximately simultaneously with the inlet valve. The other embodiment shows the exhaust valve being actuated with a time delay after the inlet valve. In further embodiments not shown, the actuating occurs immediately after the inlet valve has finished or with a slight time delay after the inlet valve has finished its actuating. Preferably, each pressure control valve 22 can perform one of the above-mentioned actuating actions independently of the actuating actions of the other pressure control valves 22.

[0044] Fig. Figure 4 shows a method for determining the exhaust valve timing tA as a function of the intake valve control. If the control unit 17 detects an external brake request 25, the control unit switches to external brake mode 26, whereby the brake pressure is set by actuating the pressure control valve and the exhaust valve timing is determined as a function of the intake valve control. The multiple actuations of the intake valve during the build-up of brake pressure are counted in a counting step 27. The exhaust valve timing tA is determined as a function of the number 28 of the intake valve actuations that have already occurred.

[0045] Fig. Figure 5 shows a further embodiment of a method for adjusting the brake pressure in external brake mode 26, wherein the same reference numerals are used for identical features as in Figure 5. Fig. 4. In the exemplary embodiment according to Fig. 5. The exhaust valve timing tA is determined as a function of the intake valve timing tE. For this purpose, the intake valve timing tE is determined in external braking mode 26 and taken into account when determining the exhaust valve timing tA. In a determination step 29, the exhaust valve timing tA is determined taking into account a correction factor 30. The intake valve timing tE is weighted by the correction factor 30 in order to determine values ​​for the exhaust valve timing tA. The correction factor is determined according to the exemplary embodiment. Fig. 5 is continuously determined as a function of a measured vehicle deceleration z according to a predetermined determination procedure 31. The vehicle deceleration z is advantageously derived from the measured values ​​of the speed sensors 18 ( Fig. 1) derived.

[0046] Fig. Figure 6 shows a further embodiment of a method for adjusting the brake pressure, wherein the same reference numerals are used for the same features as in Figure 6. Fig. 4 and Fig. 5 are used. As in the embodiment according to Fig. 5 is also used in the exemplary embodiment according to. Fig. 6. The control time tA of the exhaust valve is determined as a function of the control time tE of the intake valve and a correction factor 30. In contrast to the embodiment according to Fig. In step 5, the correction factor 30 is determined according to a predefined characteristic curve 33, based on the sum of the valve timings tE of previously performed inlet valve actuations and those of the exhaust valve previously determined in connection with a reduction of brake pressure. For this purpose, information IE on the valve timings of previously performed inlet valve actuations and information IA on those of the exhaust valve previously determined and stored in connection with a reduction of brake pressure outside of the external brake mode are combined in a summation step 32. Using the sum value 37 formed in the summation step, the correction factor 30 is determined from a predefined characteristic curve 33. This factor is used to weight the valve timing tE of the inlet valve in order to determine the valve timing tA of the exhaust valve.

[0047] Fig. Figure 7 shows a further embodiment of a method for adjusting the brake pressure and, in particular, for determining the timing tA of the exhaust valve as a function of the timing tE of the inlet valve. The embodiment according to Fig. 7 corresponds to the embodiment according to the following, except for the following differences. Fig. 6, that is to say in particular that a correction factor 30 is determined as a function of the sum of the timings of already carried out actuations of the inlet valve and such timings of the exhaust valve which were previously determined in connection with a reduction of the brake pressure, according to a given characteristic curve 33.

[0048] In contrast to the embodiment according to Fig.In summing step 32, a predetermined time constant 34 is subtracted from the previously stored induction valve timings tE and exhaust valve timings tA in a correction step 35. This time constant accounts for an initial dead time or reaction time, during which no brake pressure is applied in addition to the effective valve opening time, when the induction and exhaust valve timings differ. Accordingly, the time constant 34 to be subtracted is determined in advance and made available for correction step 35 in external brake mode 26. Reference numeral list (part of the description) 1, 1' Brake system 2 Front axle 3 Rear axle 4-wheeler 5 brake cylinders 6 Motor vehicle 7 spring accumulators 8 Brake pedal 9 Service brake valve 10 First brake circuit 11 First print run 12 Second brake circuit 13 Second print run 14 First relay valve 15 Second relay valve 16 Pressure control valve 17 Control unit 18 Speed ​​sensor 19 Inlet valve 20 Exhaust valve 21 Tax receipt 22 3 / 2-way valve 23 Third print run 24 Double check valve 25 External brake request 26 External braking mode 27th counting step 28 Number 29 Determination step 30 correction factor 31 Investigation Regulation 32 summation step 33 Characteristic curve 34 Time constant 35 Correction step 36 Determination step 37 Total value 38 Brake pressure line 39 Brake pressure line trailer 40 trailer 41 Third brake circuit 42 Fourth print run 43 Control valve 44 Coupling head tE Intake valve timing tA exhaust valve timing E Switching state of inlet valve A Switching state of the exhaust valve z vehicle delay IE Information Inlet Valve IA Information Exhaust Valve

Claims

[1] Method for adjusting brake pressures of a motor vehicle (6), wherein a control unit (17) takes into account determined dynamic state variables of the wheels to be braked (4) and controls a pressure control valve (16) with an inlet valve (19) for venting and with an outlet valve (20) for venting the brake pressure line (38, 39) controlled by the pressure control valve (16), wherein the control times (tA) of the outlet valve (20) are determined as a function of the control of the inlet valve (19), characterized by , that the number (28) of actuations of the inlet valve (19) is counted during the build-up of brake pressure and the timing (tA) of the exhaust valve (20) is determined as a function of the number (28) of actuations of the inlet valve (19) that have already taken place. [2] Method according to claim 1, characterized by, that the control unit (17), upon receiving an external brake request (25) independent of the driver's brake request, sets the brake pressure in the brake pressure line (38, 39) corresponding to the external brake request (25) in an external brake mode (26) provided for this case by determining the control times (tA) of the outlet valve (20) depending on the control of the inlet valve (19). [3] Method according to claim 1 or 2, characterized by , that the timing (tE) of the inlet valve (19) and the timing (tA) of the exhaust valve (20) overlap at least partially. [4] Method according to any one of the preceding claims, characterized by , that the timing (tA) of the exhaust valve (20) is determined as a function of the timing (tE) of the intake valve (19). [5] Method according to claim 4, characterized by, that during a braking process a correction factor (30) is determined, with which the timing (tE) of the inlet valve (19) is weighted to determine the timing (tA) of the exhaust valve (20). [6] Method according to claim 5, characterized by , that the correction factor (30) is continuously determined as a function of a measured vehicle deceleration (z) according to a predetermined determination procedure (31). [7] Method according to claim 5, characterized by , that the correction factor (30) is determined according to a predetermined characteristic curve (33) as a function of the sum of the control times (tE) of the inlet valve already actuated and of the control times of the exhaust valve (20) previously determined in connection with a reduction of the brake pressure. [8] Method according to claim 7, characterized by, that a predetermined time constant (34) is subtracted from each of the total timing (tE) of the inlet valve (19) and the timing (tA) of the exhaust valve (20). [9] Method according to any one of the preceding claims, characterized by , that a brake cylinder (5) is vented or ventilated via the brake pressure line (38) on a side of a pressure control valve (16) located beyond a pressure reservoir (11, 13). [10] Method according to any one of the preceding claims, characterized by , that a braking system of a trailer vehicle (40) can be activated indirectly via a brake pressure line (39) for the trailer vehicle (40) and a pressure control valve (16) arranged in this brake pressure line (39). [11] Brake system (1) of a motor vehicle (6) for carrying out the method according to one of claims 1 to 10, with a brake cylinder (5) and a pressure control valve (16) per wheel (4), which has an inlet valve (19) for ventilation and an outlet valve (20) for venting a brake pressure line (38, 39) connected directly or indirectly to the brake cylinder (5), which can be controlled by a control unit (17) taking into account determined dynamic state variables of the wheels (4) to be braked, wherein the control unit (17) is designed to determine the control times (tA) of the outlet valve (20) depending on the control of the inlet valve (19). [12] Brake system according to claim 11, characterized by , that the control unit (17) is designed to receive an external brake request (25)) that is independent of the driver's brake request. [13] Brake system according to claim 11 or 12, characterized by, that the brake pressure lines (38) are connected to a brake cylinder (5) on a side of a pressure control valve (16) located beyond a pressure reservoir (11, 13). [14] Brake system according to one of claims 11 to 13, characterized by , that a brake pressure line (39) for a trailer vehicle (40) is connected to a pneumatic coupling head (44) with a pressure control valve (16) arranged in this brake pressure line (39), to which a brake system of the trailer vehicle (40) can be coupled and activated via the brake pressure line (39) for the trailer vehicle (40). [15] Motor vehicle (6) with a braking system (1) according to claim 11 for carrying out a method according to any one of claims 1 to 10.

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