DE10105638B4 - Method for protecting the brakes of a motor vehicle against thermal overload - Google Patents

Abstract

Method for protecting the brakes of a motor vehicle against thermal overload, in which, depending on vehicle operating parameters, a braking-critical driving condition in which, during a full braking on the brakes of the motor vehicle as a result of the energy input associated with a possible braking, a strong decrease of the friction coefficient of the brake would occur detected and the brake critical driving condition characterizing signal is generated, characterized in that upon detection of such a critical braking driving condition, the acceleration capability of the engine is reduced by engaging in the engine management, and after a termination of the critical braking driving condition, the reduction in the acceleration capability of the engine is withdrawn.

Description

The invention relates to a method for protecting the brakes of a motor vehicle from thermal overload.

At the brakes of a motor vehicle very high temperatures can occur under intense stress. The performance of the brakes is determined by the coefficient of friction between the brake pads and the brake discs. In conventional vehicle brakes, the friction coefficient of the brake pads at temperatures above 600 degrees Celsius decreases very sharply, so that at such temperatures on the brakes deteriorates the braking power and thus extends the braking distance. This effect is also known as brake fading. In vehicles with an anti-lock device, it may come in this case to suspend the brake control.

The temperature of the brake discs results from the introduced energy during braking and the cooling during unrestrained operation. In the course of increasing performance, the braking capacity must be adapted to the higher speeds and accelerations of the vehicle to ensure safe braking from each driving condition. The dimensioning of the vehicle brakes is to be oriented to the maximum loads occurring. In particular, it must be ensured that at extreme, engine-related loads, eg. B. in a sporty driving style, follow-stops, that is, repeated strong braking in a row, or targeted attempts at the power limit of the brakes too high temperatures are avoided on the brake discs. It further takes into account that with an increased engine and a utilization of the engine power, the average speed level of the vehicle increases, so that in the middle of a higher energy at the brakes is converted into heat With higher accelerations and thus shortened acceleration times is also the cooling time between successive Reduced braking, which also tends to increase the mean brake disc temperature.

To avoid a thermal overload of a vehicle brake manifold solutions are already known. For example, the performance of a disc brake can be increased by using the largest possible brake discs. However, any enlargement of the brake discs are set clear limits by the vehicle wheels. Moreover, an increase in the number of brake discs is associated with an increase in production costs.

Furthermore, it is known to cool the brake discs by constructive measures on the brake discs themselves or on the body of the vehicle by means of air.

The use of new materials, such as brake discs made of carbon fiber composites with a SiC coating can be a significant increase in performance can be achieved. However, such brake discs are very expensive to manufacture and thus cost a multiple of conventional brake discs made of steel and similar materials.

From the prior art, it is well known to prevent overloading of the brakes by an intervention in the engine management. This approach is based on the consideration to prevent such driving conditions of the vehicle for a certain period of time, from which can no longer be braked in a desired manner at an intensive use of the vehicle brake.

In the DE 197 52 345 A1 a method for limiting the operating temperature of motor vehicle brakes is described in which conclusions are drawn from the operating parameters of the vehicle to the driving behavior. If a mode of operation characterized by frequent braking and acceleration is detected, the driving speed of the vehicle is automatically limited to an upper limit speed so that the driver can not exceed a certain speed value in order to protect against thermal overstressing of the vehicle brake. The speed limit is reached there by an intervention in the engine management. For this purpose, for example, the amount of fuel supplied to the engine or of the air / fuel mixture can be influenced. In this method, the actual temperature is measured by means of a temperature sensor on at least one brake.

Furthermore, from the DE 40 20 693 A1 a vehicle with monitoring of the brake temperature known in which the temperature of the brakes is not measured, but is calculated solely by using existing signals on the vehicle. In the DE 40 20 693 A1 For this purpose, use is made of the signals which are used in an antilock device. Based on the wheel speeds information about the change of the speed of the vehicle is determined, from which is concluded on a braking process or acceleration process. These are in a continuously updated temperature profile for the brake temperature increasing or decreasing temperature increments for the Brake temperature determined and added to the continuous brake temperature. When a limit temperature is exceeded, the engine torque is reduced, so that the driving speeds are reduced to a value at which still a braking to a stop is possible without overheating the brake. The limit temperature is determined for each vehicle type and taken into account when calculating the increments by vehicle-specific parameters.

Against this background, the invention has for its object to provide a method of the type mentioned above, on the one hand without great design effort, a thermal overloading of the vehicle brake is avoided, on the other hand abruptly abrupt changes in the driving behavior of the vehicle.

For this purpose, a method for protecting the brakes of a motor vehicle against thermal overload is proposed, in which, depending on vehicle operating parameters, a braking-critical driving state in which at a full braking on the brakes of the motor vehicle due to the energy input associated with a possible braking a strong decrease in the coefficient of friction Brake would occur detected and a brake critical driving condition indicative signal is generated upon detection of such a critical braking driving condition, the acceleration capability of the engine is reduced by engaging in the engine management, and after termination of the braking critical driving condition, the reduction in the acceleration capability of the engine is withdrawn.

The inventive method allows adequate protection of the brakes of the motor vehicle from thermal overload. By reducing the acceleration ability critical shortening of the cooling time between two consecutive braking is avoided, so that a sufficient cooling is ensured while driving, since the vehicle reaches a higher speed later in the case of reduced acceleration. In contrast to conventional methods, this does not directly limit the achievable travel speed or regulate it to a specific speed level. The handling of the vehicle is thus less affected, so that the driver is less surprised when detecting a critical braking condition compared to a turn-off on the driving speed intervention. Moreover, the intervention in the driving behavior is limited only to those cases in which the driver wants to further accelerate the vehicle. Otherwise, the intervention is independent of the driving speed based on the driving behavior not noticeable.

In an advantageous embodiment of the invention, a holding time interval is switched between the time of termination of the braking critical driving condition and the withdrawal of the reduction of the acceleration capability of the engine, within which the engine is further operated with reduced acceleration capability. This can be particularly well ensured that a maximum load of the brakes from the full performance of the vehicle out only when the brakes are cooled with high certainty to a sufficiently low temperature level. In addition, over the holding time interval any possible variations can be eliminated which result from the fact that the temperature at the brakes is not measured directly.

To reduce the ability to accelerate an additional operating map is provided for the engine, is switched to the determination of a brake-critical driving condition. The engine is operated with this additional operating map until the reduction in engine acceleration capability is withdrawn. Thereafter, the map for normal operation is switched back. By using a characteristic map control during the braking-critical driving state, the acceleration capability can be very specifically tuned to selected driving conditions in which a thermal overloading of the vehicle brake can occur. Thus, it is possible, for example, to close on the basis of the torque and the speed of a high-speed operation or downhill. Thus, an influence on the driving behavior of the vehicle to avoid the overloading of the brakes can be very narrow to the actual critical driving conditions, so that the driver is protected from surprises.

In continuation of this consideration, in a particularly advantageous embodiment of the invention, the acceleration capability of the engine is reduced only in the range of high speeds, but not in the range of medium or low speeds.

For this purpose, for example, in the region of high rotational speeds, the engine torque is lowered compared to the normal operation. At medium and lower speeds, d. H. when starting or during normal driving, however, the original engine torque is maintained. In a map control, the respective areas of the map for the normal operation and the additional map for the operation correspond to the reduction in the acceleration capability.

The reduction of the acceleration capability can also be made by a dependent on the vehicle acceleration lowering of the engine torque. For example, it is possible at a high vehicle deceleration as a result of braking to reduce the engine torque relatively strong, with a lower vehicle deceleration, however, make a less severe reduction. Moreover, the acceleration capability can also be made dependent on the driver's request, for example, by accelerations up to a certain limit size, possibly depending on the map, are allowed.

The brake-critical driving state can be described, for example, by an information conglomerate, in the engine parameters, for example the engine speed, engine speed, engine acceleration, fuel consumption and / or engine torque, frequency of actuation of the vehicle brake, vehicle speed, vehicle acceleration, wheel speeds, Enter wheel speeds and / or wheel accelerations. From this information or just a selected group of this information, a comparison variable can be determined by means of a vehicle-typical calculation model, which represents a braking-critical driving state in which an intervention in the engine management is to take place.

With regard to the temperature dependence of the coefficient of friction between the brake linings and the brake discs, it is particularly advantageous to adopt a brake-critical driving condition if a predetermined maximum temperature, for example 600 ° Celsius, would be exceeded during full braking on at least one of the brakes.

The actuation of the brakes can be derived from the deceleration behavior of the motor vehicle. However, sometimes such deceleration processes are interpreted as braking, in which no actuation of the brakes and thus no energy input into the brake discs occurs. Preferably, therefore, the determination of the brake-critical driving condition takes place while detecting and taking into account the times of the actual actuation of the brakes. This ensures that an increase in the brake temperature is only accepted if such an operation has actually taken place. The detection of the time allows, in particular for several consecutive braking processes, an accumulation of the energy input, taking into account intermediate cooling effects. Thus, the temperature profile in the brakes can be computationally determined very accurately, so that the presence of a brake-critical driving condition can be determined with high accuracy. A reduction in the ability to accelerate remains limited to the few actual needs.

For example, the determination of the brake-critical driving state in an engine control device using the engine speed and / or derived therefrom variables, in particular the rotational speed and / or the rotational acceleration.

Alternatively or additionally, the determination of the braking-critical driving condition can be made using a variable characterizing the fuel consumption.

Furthermore, it is possible to detect a braking-critical driving state in a control device of an anti-lock device using the wheel speeds and / or variables derived therefrom, in particular the wheel speeds and / or wheel accelerations. This assumes, of course, that the vehicle is equipped with such an anti-lock device. If such is not present, an additional control device may alternatively be provided which cooperates with suitable devices for determining the wheel speeds. Instead of wheel speeds, this additional control unit can also evaluate the information present in the engine management system, for example the engine speed and / or fuel consumption, and generate a corresponding signal when a braking-critical driving condition is detected.

In an advantageous embodiment of the invention, the braking-critical driving state is determined with the aid of a temperature calculation model for the vehicle brakes. The signal representing the brake-critical driving state is generated when the temperature value determined with the temperature calculation model exceeds a predetermined temperature value.

To determine the current temperature at the brakes, for example, in the DE 44 18 768 C2 described method can be used. The corresponding disclosure content of DE 4418 768 C2 is hereby expressly included in this application. Alternatively, for example, in the DE 40 20 693 A1 described method used to determine the current brake temperature. Also, the corresponding disclosure of the DE 40 20 693 A1 is hereby expressly included in this application. The determination of a braking-critical driving condition then takes place taking into account the current brake temperature and the current driving condition of the vehicle. Based on the Driving condition, the temperature increase occurring during any emergency braking can be determined. If the sum of the current temperature and the temperature increase exceeds a predetermined limit value, then the current driving state is assessed as a braking-critical driving state and, accordingly, a reduction in the acceleration capability of the engine is undertaken.

In a further advantageous embodiment of the invention, the braking-critical driving condition is made perceptible to the driver until the reduction of the acceleration capability is withdrawn. This can be done, for example, acoustically or visually perceptible. Thus, the driver can adjust better during a brake-critical driving condition to the change in the driving behavior of the vehicle and adjust its behavior accordingly. This is conducive to driving safety.

The invention will now be explained in more detail with reference to an embodiment and the accompanying drawings. The drawing shows in:

1 a diagram illustrating the course of the temperature at a disc brake of a vehicle, wherein a braking critical driving condition is achieved, and in addition the corresponding course of the coefficient of friction μ between the brake disc and a brake pad, and in

2 a schematic representation of an additional engine map for a vehicle engine to reduce the acceleration capability of the engine.

The exemplary embodiment relates to a method for protecting the brakes of a motor vehicle against thermal overstress. This method is based on the idea of temporarily increasing the inter-brake time interval by allowing the brakes to cool down, ie by temporarily reducing the engine's acceleration capability between successive full braking brakes in the region of maximum braking power , H. to increase the length of the minimum cooling interval. A reduction in the acceleration capability of the engine has the consequence that after a full braking or very strong deceleration, a longer period is required until the vehicle again reaches a high driving speed and can be braked again with a high heat input into the brakes. A limitation of the maximum speed is not connected with this, but rather, this is only later, d. H. achieved after a longer period of time.

The acceleration capability is reduced only when a brake-critical driving condition is achieved. Such is assumed if, taking into account the current temperature of the brakes and the current driving condition at a time made full braking the coefficient of friction μ between the brake pad and the brake disc at least one brake of the vehicle would drop significantly. This decrease in the coefficient of friction depends on the material of the brake lining and the brake disc and on the temperature of the brake. In the present embodiment it is assumed that this waste occurs at temperatures above 600 degrees Celsius, based on steel brake disks. For other material combinations, this decrease in coefficient of friction may also be at lower or higher temperatures. Exceeds the aforementioned threshold during a braking operation, the temperature of the brake, this leads to a significant extension of the braking distance. In the worst case, the braking effect can even be canceled almost completely.

In order to avoid this, such braking-critical driving states are determined during the driving operation as a function of vehicle parameters and an actuation of the brake. If a braking-critical driving condition occurs, a characteristic signal is generated, which is supplied to the engine control in order to reduce the acceleration capability by engaging in the engine management of the vehicle engine. After completion of the brake-critical driving state, that is, a current driving state in which a full deceleration out a significant drop in the coefficient of friction of the brake would no longer occur, the reduction in the acceleration capability of the engine is withdrawn, so that this again taking advantage of its full power potential, especially with high accelerations can be operated.

1 shows by way of example the course of the coefficient of friction μ of a vehicle brake at a deceleration from a high speed by utilizing the power potential of the brake. Furthermore, parallel to this, the course of the temperature T and a mean temperature T _{m is shown} on the associated brake disc.

In addition to the course of the coefficient of friction μ, a limit value μ _limit for the friction value is also entered, which represents the state from which the vehicle can safely be brought to a standstill within an acceptable braking distance during full braking from the maximum speed. It takes, how 1 shows, the coefficient of friction μ during the braking process tends to. This decrease essentially results from the 1 also shown temperature increase due to the frictional heat at the brake disc. With the completion of the braking process, the temperature drops again, which is accompanied by an increase in the coefficient of friction μ. The cooling of the brake disc is favored by a subsequent driving movement, with higher driving speeds cause a faster cooling. If, however, such braking operations are carried out repeatedly in succession in a short succession, the cooling effect is insufficient to prevent a thermal overloading of the brake. Namely, the brake is in a state with already reduced coefficient of friction μ, or increased temperature, so again a strong deceleration or full braking led to a significant and thus undesirable extension of the braking distance. In 1 This applies to all driving conditions in which the limit value μ _limit for the friction coefficient μ is undershot on the brake. This state is not to be confused with the even lower value for the coefficient of friction μ, in which a brake failure or a very strong extension of the braking distance occurs.

Within the period in which the current coefficient of friction μ is below the limit μ _grenz , therefore, the acceleration capability of the engine is withdrawn. If the driver attempts to greatly accelerate the vehicle within this time period by rapidly depressing the accelerator pedal, this is prevented by the engine management system until the brake-critical state has ended. In the illustrated embodiment, the termination of the brake critical condition is followed by a hold time interval t _H during which the reduction in the acceleration capability of the engine is maintained. Only after expiration of this holding time interval t _H is the full acceleration capacity available again. This ensures that the brakes are cooled in each case to a suitable for a sufficient coefficient of friction μ temperature level. This hold time interval is waited for regardless of the ignition, so that even after switching off and restarting the ignition protection against thermal overload remains.

With the reduction of the acceleration capacity of the engine, the driving behavior of the vehicle changes in case of acceleration. This can irritate the driver and lead to misjudgments of driving situations. Therefore, acceleration performance is reduced only if it protects the driver from loss of controllable braking ability of the vehicle.

For this purpose, the acceleration capability of the engine is reduced only in the range of high speeds, but not in the range medium or low speeds. The brakes are dimensioned to such that at medium and low speeds, even with repeatedly made full braking in a short succession always sufficiently short braking distance is achieved. The engine-side measures the performance of the entire system of drive and brake at high speeds is improved without the need for constructive measures on the brake or the vehicle body would be required. Rather, a relatively small and therefore cost-effective brake can be installed by the temporary reduction in the acceleration capability of the engine to the vehicle in question despite high engine power.

In order to reduce the risk of irritating the driver as far as possible, the latter is informed about the braking-critical driving state. For this purpose, for example, a warning display is installed in the field of vision of the driver, via which the brake-critical driving condition is visually perceptible. Alternatively or additionally, it is also possible to inform the driver via an acoustic signal about the braking-critical driving state. The brake-critical driving condition is made perceptible to the driver until the reduction in the acceleration capability is withdrawn.

In the illustrated embodiment, an additional operating map is provided to optimize the reduction of the acceleration capability for the engine, which is switched to the determination of a brake-critical driving condition. This switching is effected by the aforementioned characteristic signal.

2 shows such an additional operating map, which here largely agrees with the operating map for the normal operation of the engine on the basis of the full power potential. Only the in 2 hatched area, which indicates high engine speeds, differs from the map of normal operation by a reduced engine torque. The engine-side measures required for reducing the torque are familiar to the person skilled in the art and require no further explanation here. Exemplary is in 2 the torque curve M _n in normal operation and the torque curve M _{k drawn} during a brake critical state. The latter shows only in the range of high speeds a significant reduction.

In principle, a uniform lowering of the engine torque or of the engine power can take place in the hatched area. On the map control, however, an adjustment of the acceleration capacity as a function of the speed is possible. Moreover, alternatively or additionally, a torque dependency can also be provided in the characteristic field.

The determination of the braking-critical driving condition takes place without a direct temperature measurement on the brakes. Rather, this is replaced by a temperature calculation model for the vehicle brakes. With the aid of the temperature calculation model, which is adapted to the respective vehicle, the brakes used and the vehicle engine, a brake-critical driving state representing signal is generated, which serves to switch between the aforementioned maps. The temperature calculation model assumes a braking-critical driving state when a predetermined maximum temperature would be exceeded during full braking. This maximum temperature is fixed in the embodiment with 600 degrees Celsius.

When the brake-critical driving condition is determined, the times of the actual actuation of the brakes are detected and the heat input generated during braking is calculated into the brakes. From the heat input into the brakes can then be concluded that the change in temperature and thus in turn to the change in the coefficient of friction. Moreover, from further parameters characterizing the driving behavior, the cooling of the brakes is determined, so that their friction properties or temperature conditions are always known during operation of the vehicle.

This temperature calculation model is mapped in a computing device, which is implemented here in a motor control device. The determination of the braking-critical driving condition takes place in the engine control device using the information known there anyway. By way of example, such from the engine speed and derived variables can be determined. Furthermore, conclusions about the engine torque are drawn from a variable characterizing the fuel consumption and thus closed to the driving state.

The temperature calculation model can also be accommodated in a control device of an anti-lock braking device (ABS), optionally in combination with a traction control system (ASR) and an electronic differential lock (EDS), the braking-critical driving state then preferably being determined on the basis of parameters known there or anyhow determined becomes. For this purpose, the wheel speeds and / or variables derived therefrom, in particular the wheel speeds and / or the wheel accelerations are preferably used.

LIST OF REFERENCE NUMBERS

μ

friction

μ _border

Limit value of the coefficient of friction

t

Time

t _bkrit

brake-critical period

t _H

Hold time interval

T _border

Limit of temperature

T

brake temperature

M

engine torque

M _n

Motor torque in normal operation

M _k

Engine torque in braking-critical driving condition

n

Engine speed

Claims (15)

Method for protecting the brakes of a motor vehicle from thermal overload, in which, depending on vehicle operating parameters, a braking-critical driving condition in which, during full braking on the brakes of the motor vehicle, as a result of the energy input associated with any braking, a strong decrease in the coefficient of friction of the brake would occur; detected and the brake critical driving condition characterizing signal is generated, characterized in that upon detection of such a critical braking driving condition, the acceleration capability of the engine is reduced by engaging in the engine management, and after a termination of the critical braking driving condition, the reduction in the acceleration capability of the engine is withdrawn. A method according to claim 1, characterized in that between the time of termination of the braking critical driving condition and the withdrawal of the reduction of the acceleration capability of the engine, a holding time interval is switched, within which the engine is further operated with reduced acceleration capability. A method according to claim 1 or 2, characterized in that an additional operating map is provided for the engine, is switched to the determination of a brake-critical driving condition. Method according to one of Claims 1 to 3, characterized in that the acceleration capability of the engine is reduced only in the range of high speeds but not in the range of medium or low speeds. Method according to one of claims 1 to 4, characterized in that in the range high Speeds the engine torque is lowered compared to the normal operation. Method according to one of claims 1 to 5, characterized in that the reduction of the acceleration capability is performed by a dependent of the vehicle acceleration lowering of the engine torque. Method according to one of Claims 1 to 6, characterized in that a braking-critical driving condition is assumed if a predetermined maximum temperature was exceeded on at least one of the brakes during full braking. A method according to claim 7, characterized in that a brake-critical driving condition is assumed if at a full braking at least one of the brakes, a temperature of more than 600 degrees Celsius would occur. Method according to one of claims 1 to 8, characterized in that the determination of the braking-critical driving condition takes place under detection and consideration of the times of the actual actuation of the brakes. Method according to one of claims 1 to 9, characterized in that the determination of the brake-critical driving condition in an engine control device using the engine speed and / or derived therefrom variables, in particular the rotational speed and / or the rotational acceleration. Method according to one of claims 1 to 10, characterized in that the determination of the braking-critical driving condition is carried out using a fuel consumption indicative size. Method according to one of claims 1 to 9, characterized in that the determination of the braking-critical driving condition in a control device of an anti-lock device using the wheel speeds and / or derived variables, in particular the wheel speeds and / or wheel accelerations takes place. Method according to one of Claims 1 to 12, characterized in that the braking-critical driving state is determined with the aid of a temperature calculation model for the vehicle brakes, and the signal representing the braking-critical driving state is generated when the temperature value determined using the temperature calculation model exceeds a predetermined temperature value. Method according to one of Claims 1 to 13, characterized in that the braking-critical driving condition is made perceptible to the driver until the reduction in the acceleration capability is withdrawn. Motor vehicle comprising an apparatus for carrying out the method according to one of the preceding claims.

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