Hydraulic brake system for low temperatures and method of operating such a brake system - Patents.com

Abstract

The invention relates to a hydraulic brake system comprising a hydraulic pump (2) for conveying brake fluid from the suction side to the pressure side, at least one electrically controlled hydraulic valve, a brake fluid reservoir (111) connected to the suction side of the pump, and at least one control device (101, 201) for closed-loop control of the hydraulic pump (2) and the at least one hydraulic valve. To improve availability at low temperatures, the control device (101, 201) is designed to heat the brake fluid reservoir (111) by means of the at least one electrically controlled hydraulic valve, by actuating the at least one electrically controlled hydraulic valve with an electric current when the temperature falls below a threshold temperature and / or when a threshold viscosity is exceeded.

Description

[Technical field]

[0001] The present invention relates to a hydraulic brake system comprising a hydraulic pump for conveying brake fluid from a suction side to a pressure side, at least one open-loop electrically controlled hydraulic valve, a brake fluid reservoir connected to the suction side of the pump, and at least one control unit for closed-loop control of the hydraulic pump and the at least one hydraulic valve. [Background technology]

[0002] Modern redundant brake systems typically have two independent electric pressure supply devices, which are also usually operated by different control units, meaning that a minimum of delay can be ensured even if one of the pressure supply devices fails.

[0003] Such a braking system is known from WO 2017 / 144201 A1, which comprises a brake-by-wire braking system for implementing the normal braking operation and an additional module as a back-up braking system, in which a brake fluid reservoir is installed to ensure a reliable supply of brake fluid.

[0004] The problem here is the supply of braking power at very low temperatures. The problem here is the exponentially increasing viscosity of the brake fluid. This increase in viscosity has a negative effect on the volumetric flow rate to the pump suction side of the component. Summary of the Invention [Problem to be solved by the invention]

[0005] SUMMARY OF THE PRESENT EMBODIMENT It is therefore an object of the present invention to provide a braking system that reliably decelerates a vehicle even at low temperatures. [Means for solving the problem]

[0006] The above object is achieved in that the control unit is configured to heat the brake fluid reservoir by means of at least one electrically controlled hydraulic valve, by actuating the at least one electrically controlled hydraulic valve with an electric current when the temperature falls below a threshold temperature. Instead of the temperature, the viscosity of the brake fluid, which is the medium for determining the pressure rise, is taken into account directly. If this viscosity exceeds the threshold viscosity, the reservoir and thus the brake fluid therein are heated. This allows a sufficiently fast pressure rise even at low temperatures.

[0007] In one preferred embodiment of the invention, the hydraulic pump is configured as a piston pump, a linear actuator is further provided, and the control unit is configured to activate the linear actuator for pressure build-up in normal cases and to activate the piston pump for pressure build-up in case of failure of the linear actuator. In this way, the hydraulic braking system has a high degree of redundancy, thus making it possible to dispense with the need for driver-dependent hydraulic fallback levels, which is necessary for the design of braking systems with autonomous driving or so-called e-Pedals.

[0008] In one preferred embodiment of the invention, the control unit is implemented in several parts, a first control unit operates the linear actuator and a second control unit operates the piston pump. The first control unit is also called the actuator ECU. The second control unit is typically called the modulator ECU, since it also controls the wheel valves in addition to the piston pump. This means that the electronics are also redundant.

[0009] The hydraulic pump preferably has an enlarged bore on the suction side in order to further reduce the flow resistance. Preferably, the bore on the suction side may have a size of 5 mm to 15 mm, preferably 5 mm to 8 mm, particularly preferably 6.5 mm. At the same time, further bores of the brake system may be configured to be smaller than 5 mm, in particular 2 mm to 5 mm, preferably 3.3 mm to 4.46 mm.

[0010] In one preferred embodiment of the present invention, a brake fluid reservoir is installed in a housing block together with at least one open-loop electrically controlled hydraulic valve, so that the generated heat is sufficiently conducted to the reservoir. The brake fluid reservoir is preferably simply configured as a cavity in the housing block. The piston pump is also preferably installed in this housing block, so that the entire path is located in the heated housing block.

[0011] In one preferred embodiment of the invention, the brake fluid reservoir is configured as a line connection between two hydraulic units in a housing block. Such a reservoir can be manufactured particularly easily and cost-effectively.

[0012] In one preferred embodiment of the present invention, a plurality of open-loop electrically controlled hydraulic valves are installed in a housing block, and all of the valves are energized for heating and / or the valve or valves having the smallest distance from the brake fluid reservoir are energized.

[0013] In one preferred embodiment of the present invention, the volume of the brake fluid reservoir is fitted to a pressure-volume characteristic curve such that the volume mathematically allows for a delay of 2.44m / s^2.

[0014] In one preferred embodiment of the present invention, the limit temperature is between -20 and -30°C, preferably -25°C.

[0015] In one preferred embodiment of the invention, the control unit is configured to measure the viscosity. For this purpose, in particular, a linear actuator is actuated to deliver a specified volume flow rate through the outlet valve. A pressure sensor is used to measure a set pressure difference and determine the viscosity from these variables and the equation for the throttle opening of the outlet valve. Curves or tables that allow the viscosity to be determined can also be stored as variables.

[0016] In one preferred embodiment of the invention, the control unit is configured to energize the valve with a maximum current at the start of heating for a predetermined period of time or up to a predetermined temperature, and thereafter energize the valve with a low holding current, thereby quickly reaching the required temperature while at the same time preventing overheating and therefore damage to the valve.

[0017] In one preferred embodiment of the invention, the control unit is configured to energize the valve with an electric current for heating based on the deviation of the temperature from a limit temperature and / or the viscosity from a limit viscosity. In this way, the heating power is adapted to the actual situation.

[0018] In one preferred embodiment of the invention, the coil temperature is determined during heating. For this purpose, the resistance R of the coil can be estimated, for example periodically every 10 seconds, by evaluating the duty cycle dc of the pulse width modulation (PWM) and the measured current I at a given voltage U (R=U*dc / I). Finally, the coil temperature is then determined from the ratio of the current resistance to the known resistance at room temperature.

[0019] Alternatively, the coil resistance is estimated by a least squares fit of the parameters L and especially R determined from multiple dynamic measurements of the coil current and coil voltage, periodically, for example every 10 seconds. Finally, the coil temperature is determined from the ratio of the current resistance to the known resistance at room temperature.

[0020] Optionally, the resistance may be measured by some electronic equipment.

[0021] In one preferred embodiment of the invention, the heating is closed-loop controlled or open-loop controlled. In closed-loop controlled operation, a temperature sensor, particularly arranged at the pressure sensor, is evaluated accordingly. In this way, the heating is closed-loop controlled to a temperature setpoint using the temperature measurement.

[0022] In open-loop control operation, a certain required heating power can be set for a known ambient temperature, which is in particular implemented by dividing it into a "heating" phase and a "temperature maintenance" phase, whereby the heating power can be selected accordingly.

[0023] Furthermore, the above object is achieved by a method for closed-loop control of a hydraulic brake system comprising a hydraulic pump for conveying brake fluid from a suction side to a pressure side, at least one open-loop electrically controlled hydraulic valve and a brake fluid reservoir connected to the suction side of the pump, wherein the reservoir is heated by the at least one open-loop electrically controlled hydraulic valve operated by an electric current when the reservoir is below a limit temperature. [Brief description of the drawings]

[0024] [Figure 1] 1 shows a braking system according to the present invention. [Diagram 2] 3 shows an alternative embodiment of a braking system according to the present invention; [Diagram 3] 1 illustrates a schematic representation of a method according to the invention; [Figure 4] 4 shows exemplary variables in carrying out the method according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Fig. 1 very diagrammatically shows a first exemplary embodiment of a braking system according to the invention for a motor vehicle. According to this example, the braking system is arranged to operate four hydraulically actuated wheel brakes 8a-8d. However, an extension to more wheel brakes is easily possible. According to this example, the wheel brakes 8a, 8b are assigned to the rear axle (rear) of the vehicle and the wheel brakes 8c, 8d are assigned to the front axle (front) of the vehicle.

[0026] The brake system comprises, in this example, a first structural unit 100 configured as a first electro-hydraulic brake control unit (HECU1) having a valve block HCU1 and a first electronic control device 101 (ECU1) and a second structural unit 200 configured, for example, as a second electro-hydraulic brake control unit (HECU2) having a valve block HCU2 and a second electronic control device 201 (ECU2).

[0027] In the first structural unit 100 a pressure medium reservoir 4 is arranged which has two chambers, a first chamber 401 being assigned to a first container connection and a second chamber 402 being assigned to a second container connection.

[0028] In the first structural unit 100 a first electrically actuated pressure source 5 is arranged.

[0029] In the second structural unit 200, a second electrically actuated pressure source 2 and wheel-specific brake pressure regulating valves are arranged, which are designed as electrically actuated inlet valves 6a-6d and electrically actuated outlet valves 7a-7d for each wheel brake 8a-8d.

[0030] The first pressure source 5 and the second pressure source 2 are connected on the pressure side to a brake supply line 13, to which four inlet valves 6a-6d are connected. Thus, all four wheel brakes 8a-8d can be actuated by the first pressure source 5 or the second pressure source 2.

[0031] An electrically operated circuit breaker valve 40 is arranged in the brake supply line 13, so that when the circuit breaker valve 40 is closed, the brake supply line 13 is divided into a first line section 13a, to which the inlet valves 6a, 6b and the wheel brakes 8a, 8b are connected, and a second line section 13b, to which the inlet valves 6c, 6d and the wheel brakes 8c, 8d are connected. The second pressure source 2 is hydraulically connected to the first line section 13a, and the first pressure source 5 is hydraulically connected to the second line section 13b. Thus, when the circuit breaker valve 40 is closed, the brake system is separated or divided into two hydraulic brake circuits I and II. In the first brake circuit I, the pressure source 2 is connected only to the wheel brakes 8a and 8b (via the first line section 13a), and in the second brake circuit II, the first pressure source 5 is connected only to the wheel brakes 8c and 8d (via the second line section 13b). The circuit isolation valve 40 is advantageously configured to be normally open.

[0032] The braking system comprises, as already mentioned, for each hydraulically actuated wheel brake 8a-8d an inlet valve 6a-6d and an outlet valve 7a-7d, which are hydraulically interconnected in pairs via a central connector and are respectively connected to the hydraulic wheel connector of the second structural unit 200 to which the corresponding wheel brake 8a-8d is connected. A non-return valve opening to the brake supply line 13 is connected in parallel to each of the inlet valves 6a-6d. The outlet connectors of the outlet valves 7a-7d are connected via a common return line 14 to a reservoir 111 and via the reservoir 111 to the pressure medium reservoir 4 or its chamber 402. The input connectors of all the inlet valves 6a-6d can be supplied by the brake supply line 13 (i.e. when the circuit interruption valve 40 is open) with the pressure provided by the first pressure source 5 or, for example, by the second pressure source 2 in the event of a failure of the first pressure source 5.

[0033] The first electrically controlled pressure source 5 of the valve block HCU1 is configured as a hydraulic cylinder-piston arrangement (or single-circuit electro-hydraulic actuator (linear actuator)), whose piston 36 can be actuated by an electric motor 35, also shown diagrammatically, with an intermediate connection of a rotation / translation gear mechanism 39, also shown diagrammatically, and can in particular be advanced and retracted to increase and release pressure in a pressure chamber 37. The piston 36 defines the pressure chamber 37 of the pressure source 5. To actuate the electric motor, a rotor position sensor 44 is provided, which detects the rotor position of the electric motor 35 and is shown only diagrammatically.

[0034] A system pressure line section 38 is connected to the pressure chamber 37 of the first electrically controlled pressure source 5. By means of the line section 38, the pressure source 5 or the pressure chamber 37 is connected to a hydraulic connector 60 of the first structural unit 100, which in turn is connected to a hydraulic connector 61 of the second structural unit 200 via a hydraulic connecting element 80. The connection 80 is the only hydraulic pressure connection, in particular the only hydraulic connection, between the first structural unit and the second structural unit. The connection 80 is a hydraulic connection for transmitting the brake pressure for actuating the wheel brakes 8a-8d. The connecting element 80 must therefore be of a pressure-resistant construction.

[0035] The pressure chamber 37 is connected to the pressure medium reservoir 4 or its chamber 401 by a hydraulic connection 63 of the first structural unit 100 via a (refill) line 42, independent of the operating state of the piston 36. A check valve 53 is arranged in the line 42, which closes on the side of the pressure medium reservoir 4. The cylinder-piston arrangement 5 according to the present example does not have a leakage hole.

[0036] Furthermore, the pressure chamber 37 is connected to the line 42 or to the hydraulic connection 63 via the line section 38 and the second shut-off valve 23 which is according to the present example electrically operated and advantageously normally open. A non-return valve which opens on the side of the pressure chamber 37 is connected in parallel to the second shut-off valve 23.

[0037] Other than the (replenishment) connection 63 and the (pressure) connector 60, the first structural unit 100 is equipped with no hydraulic connectors.

[0038] The second open-loop electrically controlled pressure source 2 of the second structural unit 200 is according to the present example designed as two piston pumps, the two pressure sides of which are interconnected. The suction side is connected via a reservoir 111 to the return line 14 and thus to the pressure medium reservoir 4. The pressure side is connected to the first line section 13a of the brake supply line 13.

[0039] According to this example, in the second structural unit 200, in addition to the pressure source 2 and the brake pressure regulating valves 6a-6d, 7a-7d, an electrically actuated, advantageously normally open shut-off valve or switch-on valve 26 is arranged. The shut-off valve 26 is hydraulically arranged between the connector 61 and the second line section 13b of the brake supply line 13. The first pressure source 5 is thus detachably connected to the second line section 13b or to the brake supply line 13 via the shut-off valve 26.

[0040] The braking system comprises, according to the present example, a pressure sensor 19 in brake circuit II (line section 13b), which is therefore assigned to the first pressure source 5. This is advantageous for burst protection when circuit isolation is performed, i.e. when the circuit interruption valve 40 is closed. However, it is also possible to arrange the pressure sensor 19 in brake circuit I or to provide a second pressure sensor, so that each of the two brake circuits I and II can be monitored directly by a pressure sensor.

[0041] According to the present example, for leakage monitoring, the brake system is equipped with a level measuring device 50 for determining the pressure medium level in the pressure medium reservoir 4 .

[0042] According to this example, components 5, 53, 23 and line sections 38, 42 are arranged in a first valve block HCU1, and components 2, 6a-6d, 7a-7d, 26, 19 and line sections 13a, 13b (as well as the line sections between the inlet and outlet valves on the one side and the wheel connection on the other side) are arranged in a second valve block HCU2.

[0043] To each valve block HCU1, HCU2 is assigned an electronic control device 101, 201 (ECU1, ECU2). Each electronic control device 101, 201 comprises electrical and / or electronic elements (e.g. microcontrollers, power modules, valve drivers, other electronic components, etc.) for actuating the electrically actuated components of the associated valve block and, if necessary, associated sensors. The valve blocks and the electronic control devices are advantageously configured as an electro-hydraulic unit (HECU) in a known manner.

[0044] The first open-loop electronic control device 101 operates the first pressure source 5. According to this example, the first pressure source 5 is supplied with energy (from a first power source) via the first open-loop electronic control device 101.

[0045] The second open-loop electronic control device 201 operates the second pressure source 2. According to this example, the second pressure source 2 is supplied with energy (from a second power source) via the second open-loop electronic control device 201.

[0046] According to this example, the first pressure source 5 may be or is operated exclusively by the first open-loop electronic control device 101, and the second pressure source may be or is operated exclusively by the second open-loop electronic control device 201.

[0047] The braking system has a primary pressure source 5 and a secondary pressure source 2, each electrically operated by the ECU via a suction connector and a pressure connector. Brake fluid cannot flow into the pressure connector of the secondary pressure source 2 even when the system is de-energized. The primary pressure source 5 is preferably a linear actuator with a refill check valve 53 and the secondary pressure source 2 is a piston pump. The secondary pressure source 2 is preferably capable of generating a higher pressure than the primary pressure source 5.

[0048] The suction sides of the two pressure sources 2, 5 are connected to a pressure medium reservoir 4, preferably each to one of two separate chambers (402, 401).

[0049] The pressure side of the primary pressure source 5 is connected to the primary circuit node (second line section 13b) via a solenoid valve 26 (also called a pressure switch-on valve).

[0050] The pressure side of the secondary pressure source 2 is connected directly (without intermediate valve connections) to the secondary circuit node (first line section 13a). The two circuit nodes (line sections 13a, 13b) are connected to each other via a solenoid valve 40 (also called a circuit shunt valve).

[0051] In normal operation, pressure in the wheel brakes is built up by the primary pressure source 5 with the shut-off valve 23 closed. The pressure is dissipated to the primary pressure source 5 or through the shut-off valve 23. Pressure is regulated for each wheel by inlet and outlet valves as required. If required, the shut-off valve 26 is closed to allow the primary pressure source 5 to replenish further volume.

[0052] If a particularly high volumetric flow rate is required, both pressure sources 5 and 2 operate simultaneously in parallel. In this case, pressure relief is at least partially effected via the shut-off valve 23, which is preferably configured as an analog valve, i.e. the flow through it can be controlled. If a particularly high pressure is required, the shut-off valve 26 is closed and the secondary pressure source 2 increases the pressure above that of the primary pressure source 5. Outside of braking operations, atmospheric pressure equalization is permanently ensured via the shut-off valves 23 and 26.

[0053] In the event of a leak in the brake system, the circuit isolation valve 40 closes, isolating the system into two independent brake circuits I and II.

[0054] Shut-off valve 23 is preferably actuated by primary ECU 101. Shut-off valve 26 is preferably actuated by secondary ECU 201. The following description of operation in the event of a fault will be with respect to this valve allocation.

[0055] In the event of an electrical failure of the primary system, in particular the primary ECU 101 or its power supply, the secondary ECU 201 closes the shut-off valve 26 and builds up pressure via the secondary pressure source 2. The pressure is dissipated via the shut-off valve 26 or the outlet valves 7a-7d. Preferably, the inlet and outlet valves are actuated by the secondary ECU 201 so that pressure can be regulated per wheel.

[0056] In the event of an electrical failure of the secondary system, in particular the secondary ECU 201 or its voltage source, pressure is built up and dissipated as during normal operation via the primary pressure source 5 and, if necessary, the shut-off valves 23. It is necessary to do without closed-loop pressure control per wheel, but joint regulation of wheel pressures remains possible to prevent wheels from locking up and causing vehicle instability.

[0057] If a failure occurs in the ECU 1 and thus in the linear actuator 5, the piston pump 2 in the ECU 2 will also reach a speed of 2.44 m / s within 500 ms in a low temperature range of -40°C. 2A typical braking characteristic requires a delay of about 5 cm. 3 The volumetric flow rate must be set in / s.

[0058] The problem here is the exponentially increasing viscosity of the brake fluid, which affects the volumetric flow rate to the pump suction side of the following components: - the line between the reservoir and the hydraulic system of ECU2, - Lines drilled into the hydraulic system of ECU2, - Intake valve side of the pump on ECU2.

[0059] It turns out that the most negative effects occur at the pump's intake valve. It must therefore be possible to supply the pump with a volume of brake fluid preheated to a sufficiently high temperature on the intake side. The preferred solution consists of a combination of various features. These are, on the one hand, an optimized enlarged bore on the intake side of the pump in the hydraulic system of ECU2, and an additional reservoir (Res) on the intake side of the pump. In addition, the hydraulic system of ECU2 is heated to at least -25°C.

[0060] The reservoir is typically at least 2.44 m / s 3 At -25°C the pump can be set to the required volumetric flow rate.

[0061] Figure 2 shows an alternative embodiment of the braking system of Figure 1. It corresponds to the braking system of Figure 1, except for the modifications described below. Firstly, the shut-off valve 23 is connected to the brake fluid reservoir 4 via its own line 42b, which is arranged separately from the refill line 42a of the linear actuator 5. The brake fluid reservoir has its own sub-chamber 403 for this purpose. Furthermore, the pressure sensor 19 is arranged above the circuit shut-off valve 40, i.e. on the side of the piston pump 2.

[0062] Figure 3 shows the closed loop control implemented in the control device 201. If the measured temperature is below -25°C, heating is performed. This is shown diagrammatically in the "Block Heating" field. This heating affects the temperature of the whole system, shown in the "PlantHydraulicBlock" field. The set temperature is returned as the closed loop control variable.

[0063] FIG. 4 now shows an embodiment which starts when the temperature of the housing block of the brake system is −40° C. As the temperature falls below the limit of −25° C., the heating is activated. All coils are energized with maximum current, i.e. until the average winding temperature is 120° C., which is checked by a resistance measurement. This corresponds to a heating power of 200 W. As a result, the reservoir is heated at about 5 K / min. If the upper setpoint is reached, the heating is stopped until the lower limit is reached again. The heating is then activated again with a reduced power of about 50 W. This keeps the temperature within the indicated limits.

[0064] Instead of observing the temperature, the viscosity can also be observed directly, which is a direct measurement of the relevant target variable. Thus, the errors of an inaccurate temperature sensor do not come into play, which allows heating to actually be activated only if it is absolutely necessary. To measure the viscosity, a linear actuator is activated to deliver a given volumetric flow rate through the outlet valve of the rear wheel. A pressure sensor is used to measure a set pressure difference and determine the viscosity from these variables and the equation for the outlet valve opening. The applied temperature delta is determined from the ratio of the measured viscosity to the desired viscosity, based on the general relationship that (regardless of type and water content) the viscosity is halved for every 6 Kelvin increase in temperature. For example, Vmess / Vsoll=8=23, so ΔT=3*6K=18 Kelvin.

[0065] From this value, the corresponding heating power is determined and delivered as follows: P=α*Δθ, where α is the value of the thermal conductivity, which is determined by design and experiment and stored in the control unit.

[0066] The heating power is the sum of the individual powers of all valves involved. During heating, the temperature can be monitored as to when the temperature difference calculated from the viscosity is reached. Since only the temperature change is taken into account, the larger statistical errors (offsets) of the temperature sensors are eliminated, which results in improved accuracy. If necessary, the heating power P can be adjusted, for example if the external temperature changes.

[0067] If desired, the viscosity measurement can then be repeated to confirm whether the heating was successful.

[0068] Direct heating of the reservoir at the suction side of the hydraulic pump ensures acceptable pressure dynamics at low temperatures and thus availability of a fallback plane in the event of partial failure of the linear actuator. The method can be implemented without additional hardware and is therefore very cost-effective.

Claims

1. 1. A hydraulic brake system comprising a hydraulic pump for transporting brake fluid from a suction side to a pressure side, at least one electrically controlled hydraulic valve, a brake fluid reservoir connected to the suction side of the pump, and at least one control unit for closed-loop control of the hydraulic pump and the at least one hydraulic valve, characterized in that the control unit is configured to heat the brake fluid reservoir by means of the at least one electrically controlled hydraulic valve by actuating the at least one electrically controlled hydraulic valve with an electric current when a threshold temperature is below and / or when a threshold viscosity is above.

2. 2. The hydraulic brake system of claim 1, wherein the hydraulic pump is configured as a piston pump, a linear actuator is further provided, and the control unit is configured to operate the linear actuator for pressure increase in a normal case and to operate the piston pump for pressure increase in a case where a failure occurs in the linear actuator.

3. 3. The hydraulic brake system of claim 2, wherein the control unit is implemented in multiple components, a first control unit actuating the linear actuator and a second control unit actuating the piston pump.

4. A hydraulic brake system according to any one of claims 1 to 3, characterised in that the brake fluid reservoir is mounted in a housing block together with the at least one open-loop electrically controlled hydraulic valve.

5. A hydraulic brake system according to any one of claims 1 to 4, characterized in that the brake fluid reservoir is configured as a line connection between two hydraulic units in the housing block.

6. 6. A hydraulic brake system according to claim 4 or 5, characterized in that a plurality of open-loop electrically controlled hydraulic valves are installed in the housing block, all valves being energized for heating and / or the valve or valves having the smallest distance from the brake fluid reservoir being energized.

7. A hydraulic brake system according to any one of claims 1 to 6, characterised in that the volume of the brake fluid reservoir is adapted to the pressure-volume characteristic curve of the brake such that the volume mathematically allows for a delay of 2.44 m / s^2.

8. A hydraulic brake system according to any one of claims 1 to 7, characterized in that the limit temperature is between -20 and -30°C.

9. 9. A hydraulic brake system according to claim 1 , characterized in that the control unit is configured to measure the viscosity by means of a predefined volumetric flow being delivered through the outlet valve and a set pressure difference being measured.

10. 10. A hydraulic brake system according to claim 1, wherein the control unit is configured to energise the valve with a maximum current at the start of heating for a predetermined period or up to a predetermined temperature, and thereafter energise the valve with a low holding current.

11. 11. The hydraulic brake system according to claim 1, wherein the control unit is configured to energize the valve with an electric current for heating based on a deviation of the temperature from the limit temperature and / or the viscosity from the limit viscosity.

12. A hydraulic brake system according to any one of the preceding claims, characterised in that the coil temperature is determined during heating.

13. A hydraulic brake system according to any one of the preceding claims, characterized in that the heating is closed-loop or open-loop controlled.

14. 1. A method for closed-loop control of a hydraulic brake system comprising a hydraulic pump for conveying brake fluid from an intake side to a pressure side, at least one open-loop electrically controlled hydraulic valve, and a brake fluid reservoir connected to the intake side of the pump, characterized in that the reservoir is heated by the at least one open-loop electrically controlled hydraulic valve operated by an electric current when the brake fluid falls below a threshold temperature and / or when the brake fluid exceeds a threshold viscosity.

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