Method for limiting the braking power of a motor vehicle brake system

Abstract

The invention relates to a method for limiting the braking power of a motor vehicle brake system having a hydrodynamic retarder (1) that has a working chamber (4) that is arranged between two bucket wheels (2, 3) and can be or is filled with a working medium in order to hydrodynamically transmit drive power from one bucket wheel (2, 3) to the other and thereby brake the one bucket wheel (2) and a drive shaft (5) connected to the one bucket wheel (2); having an external working medium circuit (6) that is connected to the working chamber (4) via a working medium inlet (7) and a working medium outlet (8) and comprises an oil-water heat exchanger (9) with which heat is transferred from the working medium, which comprises oil, into a vehicle cooling circuit (10); wherein the braking power of the hydrodynamic retarder (1) is reduced as a function of a detected current temperature in the motor vehicle brake system. The method according to the invention is characterised in that the current temperature of the working medium is detected and, if the detected temperature of the working medium exceeds a predefined constant working medium temperature limit value, then the current braking power of the retarder (1) is reduced in such a way that the working medium temperature is set to the predefined working medium temperature limit value.

Description

[0001] Methods for limiting the braking power of a motor vehicle braking system

[0002] The present invention relates to a method for limiting the braking power of a motor vehicle braking system with a hydrodynamic retarder according to the preamble of claim 1.

[0003] Hydrodynamic retarders have been used for decades in motor vehicles, especially trucks and rail vehicles, as wear-free continuous brakes. In such hydrodynamic retarders, two axially opposed impellers form a working chamber. During braking, a working fluid is accelerated radially outwards in the first impeller, the so-called rotor, enters the second impeller, the so-called stator, is decelerated radially inwards in the stator, and then re-enters the first impeller radially inwards. The resulting hydrodynamic working fluid circuit in the working chamber transfers drive power from the first impeller to the second impeller, thereby braking the first impeller.Instead of a rotor and a stator positioned axially opposite each other, a rotor and a counter-rotating rotor can also be used in principle, with the counter-rotating rotor being driven in the opposite direction to the rotor. The operating principle, however, remains the same as described previously.

[0004] Due to the hydrodynamic working fluid circuit in the working chamber, the working fluid is heated. The heat introduced into the working fluid must be dissipated. According to one embodiment, this occurs in an external working fluid circuit connected to the working chamber via a working fluid inlet and outlet. The working fluid flows out of the working chamber and, typically, also out of the outer housing of the hydrodynamic retarder via the working fluid outlet. Within the external working fluid circuit, it is cooled outside the retarder by an oil-water heat exchanger and then re-enters the working chamber via the working fluid inlet, at least during braking operation. The secondary side of the oil-water heat exchanger typically houses the vehicle's cooling circuit, i.e., a cooling circuit with coolant or an antifreeze mixture.

[0005] If insufficient heat can be removed from the working fluid because the heat input into the vehicle cooling circuit via the working fluid or from another heat source is excessively high, the braking power of the hydrodynamic retarder must be reduced. This reduction is also known as temperature control. Conventionally, the temperature of the cooling medium in the vehicle cooling circuit is monitored, and if it exceeds a predetermined limit, the braking power of the hydrodynamic retarder is reduced. Various control strategies have been developed that, on the one hand, reduce the braking power of the hydrodynamic retarder early enough and sufficiently to prevent an undesirable temperature spike during the control process, and on the other hand, avoid unnecessary temperature control of the hydrodynamic retarder.For example, DE 102 31 610 A1 describes a method for limiting the braking effect of a retarder, wherein the braking effect of the retarder is limited depending on the temperature gradient of the cooling medium and the shape of the feedback curve or the control curve is determined by means of a pre-calculation of the available cooling capacity via a model or by evaluating parameters relevant to the course of the cooling medium temperature.

[0006] The aforementioned temperature control mechanisms of the hydrodynamic retarder aim to optimize the vehicle's cooling capacity. With known systems, limiting the retarder's braking power based on the temperature of the cooling medium in the vehicle's cooling circuit results in a highly variable temperature of the working fluid, which adjusts itself according to the retarder's current braking force. The working fluid, which consists of oil, is heated to varying degrees. This leads to a greater or lesser degree of oil degradation. Furthermore, the correspondingly preset or limiting maximum braking power is comparatively variable, which negatively impacts braking comfort.

[0007] The present invention is based on the objective of providing a method for limiting the braking power of a motor vehicle braking system with a hydrodynamic retarder, in which a significantly more constant maximum braking power is achieved and uncontrolled oil damage is avoided.

[0008] The problem according to the invention is solved by a method according to claim 1. The dependent claims describe advantageous and particularly expedient embodiments of the invention.

[0009] In the inventive method for limiting the braking power of a motor vehicle braking system with a hydrodynamic retarder, which has a working chamber arranged between two impellers, which can be filled with or is filled with a working medium in order to transfer drive power hydrodynamically from one impeller to the other and thereby brake one impeller and a drive shaft connected to one impeller, wherein furthermore an external working medium circuit is provided which is connected to the working chamber via a working medium inlet and a working medium outlet and includes an oil-water heat exchanger with which heat from the working medium, which comprises oil, is transferred to a vehicle cooling circuit, and wherein the braking power of the hydrodynamic retarder is reduced depending on a detected current temperature in the motor vehicle braking system, the current temperature of the working medium is detected and,If the measured temperature of the working fluid exceeds a predetermined constant working fluid temperature limit, the current braking power of the retarder is reduced so that the working fluid temperature adjusts to the predetermined working fluid temperature limit.

[0010] In the method according to the invention, the system is controlled not by a constant cooling medium temperature, but by a constant oil temperature. Measurements from test vehicles have shown that this results in significantly more consistent braking performance than with constant cooling medium temperature control. A higher degree of variability in the cooling medium temperature is accepted in this case.

[0011] According to an advantageous embodiment of the method according to the invention, the current temperature of a cooling medium circulating in the vehicle cooling circuit is additionally detected. The detected current temperature of the cooling medium can then be compared with a predetermined cooling medium temperature limit, and if the cooling medium temperature limit is exceeded, the control of the working medium temperature can be adjusted by reducing the predetermined constant working medium temperature limit until the current temperature of the cooling medium is equal to or less than the predetermined cooling medium temperature limit.

[0012] Thus, a second control level can be provided, which becomes effective when the constant oil temperature control according to the invention places an excessive heat load on the vehicle cooling circuit. Preferably, the smallest working medium temperature limit determined during the adjustment is maintained for a predetermined period, and subsequently a comparatively larger constant working medium temperature limit, or even the originally specified working medium temperature limit, is set again. This is advantageous if the performance of the vehicle cooling system was temporarily limited or if other components in the vehicle contributed to a temporarily higher temperature input into the vehicle cooling circuit.

[0013] Alternatively, a minimum working medium temperature limit determined during adjustment can be increased again if a current cooling medium temperature is detected that is lower than the specified cooling medium temperature limit. For example, a predetermined period of time can be allowed to pass until the current cooling medium temperature has fallen below the cooling medium temperature limit, thus preventing the need to lower the constant working medium temperature limit again soon.

[0014] The constant oil temperature control according to the invention is significantly better suited to predicting the mileage, i.e., the distance the vehicle will travel, until the next oil change based on historical oil damage and / or to dynamically adjusting the constant oil temperature control during operation to reliably achieve the mileage until the next service interval. Additionally or alternatively, the constant working medium temperature limit can be statically changed, for example, during an oil change, to achieve the required mileage again with a different oil quality. Therefore, the predetermined constant working medium temperature limit is preferably set and / or adjusted depending on at least one or more of the following parameters:

[0015] - a service interval of the motor vehicle until the next specified working fluid exchange;

[0016] - depending on the composition of the working medium;

[0017] - a number of recorded exceedances of the working medium temperature limit by the temperature of the working medium.

[0018] Particularly preferably, the service interval is defined by a distance the motor vehicle may still travel until a specified next service, furthermore, a current state of the performance of the working medium, in particular a current oil quality, a minimum performance, in particular a minimum oil quality is specified, and depending on the recorded current state, the working medium temperature limit is specified in such a way that the distance is reached or at least achievable with the current working medium while maintaining the minimum performance, in particular the minimum oil quality.

[0019] The invention will below be described by way of example using an embodiment and the figure.

[0020] Figure 1 shows a motor vehicle braking system with a hydrodynamic retarder 1, between whose impellers 2, 3 a torus-shaped working chamber 4 is formed, which can be filled with working fluid from an external working fluid circuit 6. Accordingly, the hydrodynamic retarder 1 has a working fluid inlet 7 and a working fluid outlet 8, to which the external working fluid circuit 6 is connected. Whenever a hydrodynamic circuit flow with working fluid is formed in the working chamber 4, one of the impellers 2, which is a rotor, brakes a drive shaft 5 connected to it.

[0021] The heat generated in the working chamber 4 is carried away with the working fluid from the hydrodynamic retarder 1 into the external working fluid circuit 6 and transferred in the oil-water heat exchanger 9 of the external working fluid circuit 6 to a vehicle cooling circuit 10 of the motor vehicle. In addition to the oil-water heat exchanger 9, a cooling fluid pump 11 is arranged in the vehicle cooling circuit 10, as well as a water-to-air heat exchanger (not shown), the so-called radiator, to dissipate the heat from the vehicle cooling circuit 10 to the ambient air of the motor vehicle. The vehicle cooling circuit 10 also typically includes further heat exchangers to dissipate heat from other vehicle components, for example, the drive engine, the transmission, an exhaust gas recirculation system, and / or a battery.

[0022] The current temperature of the working medium is recorded and, if the recorded temperature of the working medium exceeds a predetermined constant working medium temperature limit, the current braking power of the retarder 1 is reduced so that the working medium temperature adjusts to the predetermined working medium temperature limit.

[0023] Thus, regulation to a constant oil temperature is carried out, whereby a higher variability of the temperature of the cooling medium in the vehicle cooling circuit 10 is accepted, i.e. no regulation to a constant temperature of the cooling medium in the vehicle cooling circuit 10 takes place.

[0024] For example, a constant limit value in the range of 100 °C to 130 °C or 140 °C can be specified as the working medium temperature limit; in one embodiment, 125 °C is used. As explained, this setpoint can be changed depending on boundary conditions.

[0025] For example, the current temperature of the coolant circulating in the vehicle cooling circuit 10 is also measured. The measured current temperature of the coolant can then be compared with a predefined coolant temperature limit. If the coolant temperature limit is exceeded, the control of the working fluid temperature can be adjusted by reducing the predefined constant working fluid temperature limit until the current temperature of the coolant is equal to or less than the predefined coolant temperature limit. For example, a constant limit in the range of 90 °C to 100 °C can be specified as the coolant temperature limit; in one embodiment, this limit is 98 °C.

[0026] To detect the current temperature of the working medium, for example a temperature sensor can be provided in the external working medium circuit 6, the detected temperature value of which is supplied to a control device which is set up to carry out a method according to the invention.

[0027] To detect the current temperature of the cooling medium, for example a temperature sensor can be provided in the vehicle cooling circuit 10, the detected temperature value of which is supplied to the control device, which is set up to carry out a method according to the invention.

[0028] Reference symbol list

[0029] 1 hydrodynamic retarder

[0030] 2 paddle wheel 3 paddle wheel

[0031] 4 workroom

[0032] 5 Drive shaft

[0033] 6 Working fluid circuit

[0034] 7 Work medium inlet 8 Work medium outlet

[0035] 9 oil-water heat exchangers

[0036] 10 Vehicle cooling circuit

[0037] 11 Cooling medium pump

Claims

Patent claims 1. A method for limiting the braking power of a motor vehicle braking system with a hydrodynamic retarder (1) comprising a working chamber (4) arranged between two impellers (2, 3), which can be filled with or is filled with a working medium to hydrodynamically transfer drive power from one impeller (2, 3) to the other and thereby brake one impeller (2) and a drive shaft (5) connected to one impeller (2); comprising an external working medium circuit (6) connected to the working chamber (4) via a working medium inlet (7) and a working medium outlet (8) and comprising an oil-water heat exchanger (9) by which heat from the working medium, which comprises oil, is transferred to a vehicle cooling circuit (10); wherein the braking power of the hydrodynamic retarder (1) is reduced depending on a detected current temperature in the motor vehicle braking system;characterized in that the current temperature of the working medium is detected and, if the detected temperature of the working medium exceeds a predetermined constant working medium temperature limit, the current braking power of the retarder (1) is reduced in such a way that the working medium temperature adjusts to the predetermined working medium temperature limit.

2. Method according to claim 1, characterized in that the current temperature of a cooling medium circulating in the vehicle cooling circuit (10) is additionally detected.

3. Method according to claim 2, characterized in that the detected current temperature of the cooling medium is compared with a predetermined cooling medium temperature limit value and, if the cooling medium temperature limit value is exceeded, the control of the working medium temperature is adjusted by reducing the predetermined constant working medium temperature limit value until the current temperature of the cooling medium is equal to or less than the predetermined cooling medium temperature limit value.

4. Method according to claim 3, characterized in that a smallest working medium temperature limit value resulting from the adjustment is maintained for a predetermined period of time and subsequently a comparatively larger constant working medium temperature limit value or the originally specified working temperature limit value is specified as a constant working medium temperature limit value.

5. Method according to claim 3, characterized in that a minimum working medium temperature limit resulting from the adjustment is increased again if a current temperature of the cooling medium is detected which is lower than the predetermined cooling medium temperature limit.

6. Method according to one of claims 1 to 5, characterized in that the predetermined constant working medium temperature limit is set and / or adjusted depending on at least one or more of the following parameters: - a service interval of the motor vehicle until the next specified work medium exchange - depending on the composition of the working medium - a number of detected exceedances of the working medium temperature limit by the temperature of the working medium.

7. Method according to claim 6, characterized in that the The service interval is defined by a distance the vehicle may travel until a specified next service, the current performance status of the working medium, in particular the current oil quality, is recorded, a minimum performance of the working medium, in particular a minimum oil quality, is specified, and depending on the recorded current state, the working medium temperature limit is specified in such a way that the distance can be reached with the current working medium while maintaining the minimum performance, in particular the minimum oil quality.

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