HYDRAULIC BRAKE SYSTEM FOR LOW TEMPERATURES AND METHOD FOR OPERATING SUCH A BRAKE SYSTEM

Description

[0001] The 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 electrically controlled hydraulic valve, and a brake fluid reservoir which is connected to the suction side of the pump, as well as at least one control unit for regulating the hydraulic pump and the at least one hydraulic valve.

[0002] Modern redundant braking systems typically feature two independent electrical pressure supply devices, which are usually controlled by different control units. This ensures minimal deceleration even if one of the pressure supply devices fails.

[0003] Such a braking system is known from WO 2017 / 144201 A1, featuring a brake-by-wire braking system for implementing normal braking and an additional module as a backup braking system. This additional module incorporates a brake fluid reservoir to reliably supply it with brake fluid.

[0004] The problem here is providing braking power at extremely low temperatures. The problem here lies in the exponentially increasing viscosity of the brake fluid. This increased viscosity impairs the flow rate to the suction side of the pump in the components.

[0005] DE 100 59 348 A1 discloses a braking system in which electrically controlled hydraulic valves with coils are activated for the purpose of electrically heating the supports accommodating these coils.

[0006] Furthermore, a similar method is known from WO 02 / 066306 A1, in which electrically controlled hydraulic valves with coils are controlled for the purpose of electrically heating the carriers receiving these coils, in which a first heating phase is carried out, followed by a heating pause and at least one further heating pulse.

[0007] It is therefore an object of the present invention to provide a braking system which ensures vehicle deceleration even at low temperatures.

[0008] This task is solved by the control unit being configured to heat the brake fluid reservoir by means of at least one electrically controlled hydraulic valve when the temperature falls below a certain threshold, by activating the valve with an electric current. As an alternative to temperature, the viscosity of the brake fluid, which is the decisive medium for pressure buildup, is directly considered. If this viscosity exceeds a certain threshold, the reservoir and thus the brake fluid within it are heated. This enables a sufficiently rapid pressure buildup even at low temperatures.

[0009] According to the invention, the hydraulic pump is designed as a piston pump, and a linear actuator is also provided. The control unit is configured to control the linear actuator to build up pressure in a fault-free situation and to control the piston pump to build up pressure in the event of a fault in the linear actuator. The hydraulic braking system thus has a high degree of redundancy and can thus operate without a driver-dependent hydraulic fallback level. This is necessary for autonomous driving or the implementation of the braking system with a so-called ePedal.

[0010] In a preferred embodiment of the invention, the control unit is designed in multiple parts, with a first control unit controlling the linear actuator and a second control unit controlling the piston pump. The first control unit is also referred to as the actuator ECU. Since the second control unit typically controls the wheel valves in addition to the piston pump, it is referred to as the modulator ECU. Thus, the electronics are also designed redundantly.

[0011] It is preferred that the hydraulic pump has enlarged bores on the suction side to further reduce flow resistance. The bores on the suction side can preferably have a size of 5 mm to 15 mm, preferably 5 mm to 8 mm, particularly preferably 6.5 mm. The other bores of the brake system can simultaneously be smaller than 5 mm, in particular 2 mm to 5 mm, preferably 3.3 to 4.46 mm.

[0012] In a preferred embodiment of the invention, the brake fluid reservoir is installed in a housing block together with the at least one electrically controlled hydraulic valve. This ensures efficient conduction of the generated heat to the reservoir. The brake fluid reservoir is preferably simply designed as a cavity in the housing block. Preferably, the piston pump is also installed in this housing block, so that the entire path lies within the heated housing block.

[0013] In a preferred embodiment of the invention, the brake fluid reservoir is designed as a line connection between two hydraulic units within the housing block. Such a reservoir can be manufactured particularly easily and cost-effectively.

[0014] In a preferred embodiment of the invention, several electrically controlled hydraulic valves are installed in the housing block, wherein all valves are energized for heating and / or the valve(s) with the smallest distance to the brake fluid reservoir are energized.

[0015] In a preferred embodiment of the invention, the volume of the brake fluid reservoir is matched to a pressure-volume characteristic curve in such a way that the volume mathematically enables a deceleration of 2.44 m / s^2.

[0016] In a preferred embodiment of the invention, the limit temperature is between -20 and -30°C, preferably -25°C.

[0017] In a preferred embodiment of the invention, the control unit is configured to measure viscosity. For this purpose, the linear actuator is specifically controlled to deliver a predetermined volume flow through an exhaust valve. A pressure sensor measures the resulting pressure difference, and the viscosity is determined from these variables and the orifice equation of the exhaust valve. Curves or tables can also be stored for these variables, allowing viscosity to be determined.

[0018] In a preferred embodiment of the invention, the control unit is configured to supply the valves with a maximum current at the beginning of heating for a predetermined period of time or until a predetermined temperature is reached, and then with a lower holding current. This allows the required temperature to be reached quickly while simultaneously preventing overheating and thus potential damage to the valves.

[0019] In a preferred embodiment of the invention, the control unit is configured to supply the heating valves with a current based on the deviation of the temperature from the limit temperature and / or the viscosity from the limit viscosity. Thus, the heating output is adapted to the actual conditions.

[0020] In a preferred embodiment of the invention, the coil temperature is determined during heating. For this purpose, the resistance R of the coils can be estimated by evaluating, for example, the duty cycle dc of the pulse width modulation (PWM) and the measured current I at a given voltage U cyclically every 10 seconds. R = U * dc lI , and finally, the coil temperature is determined from the ratio of the current resistance to the known resistance at room temperature.

[0021] Alternatively, the resistance of the coils is estimated by determining the parameters L and especially R from several dynamic measured values ​​of coil current and coil voltage using the least squares method, e.g., cyclically every 10 seconds. Finally, the coil temperature is determined from the ratio of the current resistance to the known resistance at room temperature.

[0022] Optionally, the resistance can be measured by the provided electronics. In a preferred embodiment of the invention, the heating is controlled or regulated. In controlled operation, a temperature sensor, which is located in particular in the pressure sensor, is evaluated accordingly. Thus, the heating is regulated to a setpoint temperature using the measured temperature value.

[0023] In controlled operation, a predetermined required heating output can be set at a known ambient temperature. This can be done, in particular, by dividing the heating output into "heating" and "maintaining temperature" phases, in which the heating output is selected accordingly.

[0024] The object is also achieved by a method for controlling a hydraulic brake system comprising a hydraulic pump for conveying brake fluid from a suction side to a pressure side, at least one electrically controlled hydraulic valve, and a brake fluid reservoir which is connected to the suction side of the pump, wherein when a limit temperature is undershot, the reservoir is heated by means of the at least one electrically controlled hydraulic valve by controlling it with an electrical current. Fig. 1 shows a braking system according to the invention, Fig. 2 shows an alternative embodiment of a brake system according to the invention, Fig. 3 shows schematically the method according to the invention, Fig. 4 shows exemplary sizes during the implementation of the method according to the invention;

[0025] In Fig. 1 A first embodiment of a braking system according to the invention for a motor vehicle is shown highly schematically. For example, the braking system is designed to actuate four hydraulically actuated wheel brakes 8a-8d; expansion to more wheel brakes is easily possible. For example, the wheel brakes 8a, 8b are assigned to the rear axle and the wheel brakes 8c, 8d to the front axle of the vehicle.

[0026] The brake system comprises a first structural unit 100, which is designed, for example, as a first electro-hydraulic brake control unit with a valve block and a first electronic control device 101 (ECU1), and a second structural unit 200, which is designed, for example, as a second electro-hydraulic brake control unit with a valve block and a second electronic control device 201 (ECU2).

[0027] A pressure medium reservoir 4 with two chambers is arranged on the first structural unit 100, wherein a first reservoir connection is assigned to the first chamber 401 and a second reservoir connection is assigned to the second chamber 402.

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

[0029] In the second structural unit 200, a second electrically actuated pressure source 2 and wheel-individual brake pressure modulation valves are arranged, which are designed as an electrically actuated inlet valve 6a-6d and an electrically actuated outlet valve 7a-7d per 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 the four inlet valves 6a-6d are connected. Thus, all four wheel brakes 8a-8d can be actuated by the first pressure source 5 or by the second pressure source 2.

[0031] An electrically actuated circuit isolation valve 40 is arranged in the brake supply line 13, so that when the circuit isolation valve 40 is closed, the brake supply line 13 is separated 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. When the circuit isolation valve 40 is closed, the brake system is thus separated or divided into two hydraulic brake circuits.In the first brake circuit, pressure source 2 (via the first line section 13a) is connected only to the wheel brakes 8a and 8b, and in the second brake circuit, the first pressure source 5 (via the second line section 13b) is connected only to the wheel brakes 8c and 8d. The circuit isolation valve 40 is advantageously designed to be open when de-energized.

[0032] As already mentioned, the braking system comprises an inlet valve 6a-6d and an outlet valve 7a-7d for each hydraulically actuated wheel brake 8a-8d. These valves are hydraulically interconnected in pairs via central connections and each connected to a hydraulic wheel connection of the second assembly 200, to which the corresponding wheel brake 8a-8d is connected. A check valve opening toward the brake supply line 13 is connected in parallel to each of the inlet valves 6a-6d. The outlet connections of the outlet valves 7a-7d are connected via a common return line 14 to a reservoir 111 and, via this, to the pressure fluid reservoir 4 or its chamber 402. The inlet connections of all inlet valves 6a-6d can be supplied by means of the brake supply line 13 (i.e. with the circuit isolation valve 40 open) with a pressure which is provided by the first pressure source 5 or, e.g. in the event of failure of the first pressure source 5, by the second pressure source 2.

[0033] The first electrically controllable pressure source 5 of the valve block is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electro-hydraulic actuator (linear actuator)), the piston 36 of which can be actuated by a schematically indicated electric motor 35 with the interposition of a rotation-translation gear 39, also shown schematically, in particular can be moved forwards and backwards in order to build up and reduce pressure in a pressure chamber 37. The piston 36 delimits the pressure chamber 37 of the pressure source 5. To control the electric motor, a rotor position sensor 44, which detects the rotor position of the electric motor 35 and is only indicated schematically, is provided.

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

[0035] Pressure chamber 37 is connected, regardless of the actuation state of piston 36, to the pressure fluid reservoir 4 or its chamber 401 via a (suction) line 42 with a hydraulic connection 63 of the first structural unit 100. A check valve 53 closing toward the pressure fluid reservoir 4 is arranged in the line 42. The cylinder-piston assembly 5, for example, has no sniffing holes.

[0036] Furthermore, pressure chamber 37 is connected, for example, to line 42 or the hydraulic connection 63 via line section 38 and an electrically actuated, advantageously de-energized, second isolating valve 23. A check valve opening toward the pressure chamber 37 is connected in parallel to the second isolating valve 23.

[0037] Apart from the (suction) connection 63 and the (pressure) connection 60, the first structural unit 100 does not comprise any further hydraulic connections.

[0038] The second electrically controllable pressure source 2 of the second assembly 200 is designed, for example, as a two-piston pump whose two pressure sides are interconnected. The suction sides are connected via the reservoir 111 to the return line 14 and thus to the pressure fluid reservoir 4. The pressure sides are connected to the first line section 13a of the brake supply line 13.

[0039] In addition to the pressure source 2 and the brake pressure modulation valves 6a-6d, 7a-7d, an electrically actuated, advantageously normally open, isolating valve or connecting valve 26 is arranged in the second structural unit 200, for example. The isolating valve 26 is hydraulically arranged between the connection 61 and the second line section 13b of the brake supply line 13. Thus, the first pressure source 5 is separably connected to the second line section 13b or the brake supply line 13 via the isolating valve 26.

[0040] The brake system, for example, includes a pressure sensor 19 in the second brake circuit (line section 13b), which is thus assigned to the first pressure source 5. This is advantageous for burst protection during active circuit separation, i.e., when the circuit separation valve 40 is closed. However, pressure sensor 19 can also be arranged in the first brake circuit, or a second pressure sensor can be provided, so that each of the two brake circuits can be directly monitored by means of a pressure sensor.

[0041] For example, the brake system for leakage monitoring comprises a level measuring device 50 for determining a pressure medium level in the pressure medium reservoir 4.

[0042] For example, the components 5, 53, 23 and the line sections 38, 42 are arranged in the first valve block and the components 2, 6a-6d, 7a-7d, 26, 19 and the line sections 13a, 13b (and the line sections between the inlet and outlet valves on the one hand and the wheel connections on the other hand) are arranged in the second valve block.

[0043] Each valve block is assigned an electronic control unit 101, 201 (ECU1, ECU2). Each electronic control unit 101, 201 comprises electrical and / or electronic elements (e.g., microcontrollers, power units, valve drivers, other electronic components, etc.) for controlling the electrically actuated components of the associated valve block and, if applicable, the associated sensors. The valve block and electronic control unit are advantageously designed as an electrohydraulic unit, as is known in the art.

[0044] The first electronic control device 101 controls the first pressure source 5. For example, the first pressure source 5 is supplied with energy (from a first electrical energy source) via the first electronic control device 101.

[0045] The second electronic control device 201 controls the second pressure source 2. For example, the second pressure source 2 is supplied with energy (from a second electrical energy source) via the second electronic control device 201.

[0046] For example, the first pressure source 5 can be or is controlled exclusively by the first electronic control device 101 and the second pressure source 2 can be or is controlled exclusively by the second electronic control device 201.

[0047] The brake system has a primary pressure source 5 and a secondary pressure source 2, each electrically operated by an ECU and having a suction port and a pressure port. Brake fluid cannot flow into the pressure port of the secondary pressure source 2, even when de-energized. Preferably, the primary pressure source 5 is a linear actuator with a suction check valve 53, and the secondary pressure source 2 is a piston pump. Preferably, the secondary pressure source 2 can generate 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 a primary circuit node (second line section 13b) via an electromagnetic valve 26, also called a pressure connection valve.

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

[0051] During normal operation, the pressure in the wheel brakes is built up by primary pressure source 5 with isolation valve 23 closed. The pressure is released into primary pressure source 5 or via isolation valve 23. The pressure is modulated for each wheel as needed by the inlet and outlet valves. If necessary, isolation valve 26 is closed so that primary pressure source 5 can draw in additional volume.

[0052] If a particularly high flow rate is requested, both pressure sources 5 and 2 operate in parallel. In this case, the pressure reduction occurs at least partially via the isolating valve 23, which is preferably designed as an analog valve, thus being able to control its flow rate. If a particularly high pressure is requested, the isolating valve 26 is closed, and the secondary pressure source 2 increases the pressure above the pressure of the primary pressure source 5. Outside of braking situations, atmospheric pressure equalization is permanently ensured via the isolating valve 23 and the isolating valve 26.

[0053] If there is a leak in the brake system, the circuit isolation valve 40 is closed and the system is divided into two independent brake circuits.

[0054] Preferably, the isolation valve 23 is controlled by the primary ECU 101. Preferably, the isolation valve 26 is controlled by the secondary ECU 201. The following description of operation in the event of a fault refers to this valve assignment.

[0055] If the primary system fails electrically, specifically the primary ECU 101 or its power supply, the secondary ECU 201 closes the isolation valve 26 to build up pressure via the secondary pressure source 2. Pressure is released via the isolation valve 26 or via the exhaust valves 7a-7d. Preferably, the inlet and exhaust valves are controlled by the secondary ECU 201, allowing the pressure to be modulated for each individual wheel.

[0056] If the secondary system fails electrically, specifically the secondary ECU 201 or its voltage source, the pressure is increased and decreased as in normal operation via the primary pressure source 5 and, if applicable, the isolation valve 23. Individual wheel pressure control is not required, but joint modulation of the wheel pressures remains possible to prevent the vehicle from being destabilized by locking wheels.

[0057] In the event of a failure of ECU 1 and thus of the linear actuator 5, the piston pump 2 in ECU 2 should be able to displace enough volume within 500 ms, even in low-temperature ranges down to -40 °C, to achieve a deceleration of 2.44 m / s. For typical brake characteristics, a volume flow of approximately 5 cm3 / s must be set.

[0058] The problem here is the exponentially increasing viscosity of the brake fluid. This increased viscosity impairs the flow rate to the suction side of the pump in the components: Line between reservoir and hydraulics of ECU2 The drilled lines in the hydraulics of ECU2 At the suction valves of the pump of ECU2

[0059] It has been shown that the greatest negative impact occurs at the pump's suction valves. Therefore, it is essential to provide the pump with a sufficient volume of preheated brake fluid on the suction side. The preferred solution consists of a combination of various features. These include optimized, enlarged bores on the pump's suction side within the ECU2 hydraulics and an additional reservoir on the pump's suction side. In addition, the ECU2 hydraulics are heated to at least -25°C.

[0060] The reservoir typically has the volume required to achieve a deceleration of at least 2.44 m / s. At -25°C, the pump can adjust to the required flow rate.

[0061] Fig. 2 shows an alternative embodiment of the braking system of the Fig. 1 . Except for the changes to the braking system described below, this corresponds to the Fig. 1 Firstly, the isolation valve 23 is connected to the brake fluid reservoir 4 via a dedicated line 42b, which is separate from the suction line 42a of the linear actuator 5. The brake fluid reservoir has its own subchamber 403 for this purpose. Furthermore, the pressure sensor 19 is arranged above the circuit isolation valve 40 and thus on the side of the piston pump 2.

[0062] Fig. 3 shows the control implemented in control device 201. If the measured temperature is below -25°C, heating is activated. This is shown schematically in the "Blockheating" field. This heating influences the temperatures in the entire plant, which are shown in the "PlantHydraulicBlock" field. The resulting temperature is fed back as a controlled variable.

[0063] Fig. 4shows an embodiment that begins at a temperature of the brake system housing block of -40°C. Since the temperature is below the limit of -25°C, heating is activated. All coils are supplied with maximum current, i.e. until they reach an average winding temperature of 120°C, which is checked by a resistance measurement. This corresponds to a heating output of 200W. This heats the reservoir at approximately 5K / min. Once the upper setpoint is reached, the heating is deactivated until a lower limit is again undershot. The heating is then activated again at a reduced output of approximately 50W. This keeps the temperature within the shown limits.

[0064] Instead of looking at the temperature, the viscosity can also be looked at directly, which means the relevant target variable is measured directly. Errors in the inaccurate temperature sensor are therefore irrelevant, meaning that the heater is only activated when absolutely necessary. To measure the viscosity, the linear actuator is controlled to deliver a specified volume flow through an exhaust valve on a rear wheel. A pressure sensor measures the resulting pressure difference, and the viscosity is determined from these values ​​and the orifice equation of the exhaust valve. From the typical relationship that the viscosity is halved for every 6 Kelvin increase in temperature (regardless of type and water content), the temperature delta to be applied is determined from the ratio of measured to desired viscosity. e.g. V meas / V target = 8 = 2 3<, ΔT = 3 * 6K = 18 Kelvin.

[0065] From this value, an appropriate heating output is determined and provided: P = α * Δϑ with α as thermal conductivity, which is determined by design and experiments and stored in the control unit.

[0066] The heating power is calculated as the sum of the individual powers for all involved valves. During heating, the temperature can be monitored to determine when the temperature delta calculated from the viscosity is reached. Since only temperature changes are considered, the larger static errors (offset) of the temperature sensor are eliminated, thus improving accuracy. If necessary, the heating power P can be adjusted, for example, if the external temperature changes.

[0067] Afterwards, the viscosity measurement can be repeated if necessary to check whether the heating had the desired effect.

[0068] By heating the reservoir directly on the suction side of the hydraulic pump, acceptable pressure dynamics can be ensured even in the event of a partial failure of the linear actuator, thus ensuring the availability of the fallback level at low temperatures. The process can be implemented without additional hardware and is therefore very cost-effective.

Claims

1. Hydraulic brake system comprising a hydraulic pump (2) for conveying brake fluid from a suction side to a pressure side, at least one electrically controlled hydraulic valve (6, 7, 40, 26), and a brake fluid reservoir (111), which is connected to the suction side of the pump (2), and at least one control unit (101, 201) for regulating the hydraulic pump (2) and the at least one hydraulic valve (6, 7, 40, 26), wherein the control unit (101, 201) is designed to heat the brake fluid reservoir (111) by means of the at least one electrically controlled hydraulic valve (6, 7, 40, 26) when a limit temperature is undershot and / or a limit viscosity is exceeded, by the hydraulic valve being actuated with an electric current, characterized in that the hydraulic pump (2) is designed as a piston pump and, furthermore, a linear actuator (5) is provided, wherein the control unit (101, 201) is configured to actuate the linear actuator (5) for pressure build-up in the fault-free state and, in the event of a fault in the linear actuator (5), to actuate the piston pump (2) for pressure build-up.

2. Hydraulic brake system according to Claim 1, characterized in that the control unit is designed in several parts, wherein a first control unit (101) actuates the linear actuator and a second control unit (201) actuates the piston pump.

3. Hydraulic brake system according to either of the preceding claims, characterized in that the brake fluid reservoir (111) is installed, together with the at least one electrically controlled hydraulic valve (6, 7, 40, 26), in a housing block (200).

4. Hydraulic brake system according to one of the preceding claims, characterized in that the brake fluid reservoir (111) is formed as a line connection between two hydraulic units within the housing block (200).

5. Hydraulic brake system according to either of Claims 3 and 4, characterized in that that a plurality of electrically controlled hydraulic valves (6, 7, 40, 26) are installed in the housing block (200), wherein all valves (6, 7, 40, 26) are energized for heating purposes and / or the valve or valves (6, 7, 40, 26) at the smallest distance from the brake fluid reservoir (111) is / are energized.

6. Hydraulic brake system according to one of the preceding claims, characterized in that the volume of the brake fluid reservoir (111) is matched to a pressure-volume characteristic curve of the brakes in such a way that the volume mathematically allows a deceleration of 2.44 m / s^2.

7. Hydraulic brake system according to one of the preceding claims, characterized in that the limit temperature is between -20 and -30°C.

8. Hydraulic brake system according to one of the preceding claims, characterized in that the control unit (101, 201) is configured to measure the viscosity by a predetermined volumetric flow being conveyed through an outlet valve (7) and the pressure difference which sets in being measured.

9. Hydraulic brake system according to one of the preceding claims, characterized in that the control unit (101, 201) is configured to energize the valves (6, 7, 40, 26) at the beginning of heating with a maximum current for a predetermined period of time or up to a predetermined temperature and then to energize them with a lower holding current.

10. Hydraulic brake system according to one of the preceding claims, characterized in that the control unit (101, 201) is configured to energize the valves (6, 7, 40, 26) for heating purposes with a current based on the deviation of the temperature from the limit temperature and / or the viscosity from the limit viscosity.

11. Hydraulic brake system according to one of the preceding claims, characterized in that the temperature of the coils of the valves (6, 7, 40, 26) is determined during heating.

12. Hydraulic brake system according to one of the preceding claims, characterized in that the heating is regulated or controlled.

13. Method for controlling a hydraulic brake system according to Claim 1 comprising a hydraulic pump (2) for conveying brake fluid from a suction side to a pressure side, at least one electrically controlled hydraulic valve (6, 7, 40, 26), and a brake fluid reservoir (111) which is connected to the suction side of the pump (2), characterized in that, when a limit temperature is undershot and / or a limit viscosity is exceeded, the reservoir (111) is heated by means of the at least one electrically controlled hydraulic valve (6, 7, 40, 26) by the latter being actuated with an electric current.