BACKGROUND TO THE INVENTION AND STATE OF THE ART The invention relates to an arrangement and a method for controlling the braking of a hydrodynamic auxiliary brake in a vehicle according to the preambles to patent claims 1 and 11.

Auxiliary brakes such as hydraulic retarders contain circulating hydraulic oil which substantially absorbs the heat energy generated during a braking process. The hydraulic oil is usually cooled by the motor vehicle's ordinary cooling system before it can be used in the retarder again. A known way of preventing coolant boiling in the cooling system is to use information about the coolant temperature as a basis for limiting the braking effect which the driver calls for from the auxiliary brake. Such limitation of the hydraulic retarder's braking effect may begin at a coolant temperature of 95°C and increase in a linear manner to complete limitation of the hydraulic retarder's braking effect at a coolant temperature of 110°C.

Such a control system has to incorporate a suitably increased safety margin to ensure that no local boiling occurs in the cooling system. Such a safety margin appreciably reduces the braking effect obtainable from the retarder.

A way of reducing this safety margin is referred to in SE 507 807, in which a control system limits the braking effect called for from the hydrodynamic auxiliary brake by using information about the coolant temperature and the vehicle engine speed.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method and an arrangement which make possible a substantially minimum limitation of the braking effect called for from a hydrodynamic auxiliary brake during braking of a vehicle, without any risk of incipient

local boiling of coolant in a cooling system which is designed to remove the heat generated by the hydrodynamic brake.

This object is achieved with the method and arrangement mentioned in the introduction which are characterised by what is indicated in the characterising parts of patent claims 1 and 11. Such a maximum temperature is usually the temperature at which there is risk of local boiling of coolant in the cooling system. Obtaining information not only about the coolant temperature but also about its rate of increase makes it possible to predict future coolant temperature with good precision without affecting the braking effect. The braking effect of the hydrodynamic auxiliary brake can thus be limited in situations where the coolant tends to exceed the maximum coolant temperature. With such refined control there is no need to apply such a large safety margin for ensuring that the maximum coolant temperature is not exceeded. Such a control system means that the hydrodynamic brake can brake the vehicle over a considerably longer distance before any limitation of braking effect has to be applied.

According to a preferred embodiment of the present invention, limitation of the braking effect called for is possible on the basis of information about the temperature of the medium. By taking also into account the temperature of the medium before it is cooled by the coolant it is possible to estimate the coolant temperature trend with greater precision. With advantage, the medium is prevented from exceeding a maximum temperature. For safety reasons it is desirable that such a maximum medium temperature should not be exceeded. With advantage, limitation of the braking effect called for is possible on the basis of information about the rate of increase of the temperature of the medium. Taking this factor also into account further enhances the precision of estimating the coolant temperature trend when a braking effect is called for from the hydrodynamic brake. If information about a rapidly rising medium temperature is received, it may be supposed that the coolant temperature will also follow a corresponding pattern.

A further preferred embodiment of the present invention allows activation of functions which counteract a temperature increase of the cooling medium. Such a function may be to allow an increased coolant flow in the cooling system. This may be achieved by control of thermostats in the cooling system to provide them with a greater degree of

opening and create an increased coolant flow. Another function may be to allow increased cooling of the coolant. This may be achieved by controlling a cooling fan so that the radiator of the cooling system is provided with an increased air flow. With advantage, activation of an alternative auxiliary brake which does not load the cooling system is allowed. Heavy motor vehicles often incorporate two or more auxiliary brakes.

Such an auxiliary brake may be an exhaust brake or a compression brake. Such auxiliary brakes do not load the cooling system and their activation is therefore very appropriate when the braking effect called for from the hydrodynamic brake has to be limited.

A further preferred embodiment of the present invention allows limitation of the braking effect called for, at a value which is related to a calculated value according to a mathematical algorithm. Such a mathematical algorithm takes into account the coolant temperature and its rate of increase and, with advantage, the medium temperature and its rate of increase in such proportions as to provide a substantially minimum downward adjustment of the braking effect called for from the hydrodynamic brake, without the coolant exceeding the maximum coolant temperature. At the same time it is also possible, depending on the calculated value of the algorithm, to allow activation of the functions which counteract a coolant temperature increase. At a certain value of the algorithm the cooling system thermostats may be caused to open so that the coolant flow increases. The degree of opening of the thermostats may be related to the value of the algorithm. The cooling fan may correspondingly be activated at a certain algorithm value. The cooling fan speed may also be related to the value of the algorithm.

A further preferred embodiment of the present invention makes it possible in a crisis situation to allow the coolant temperature to exceed the maximum temperature, and to confine limitation of the braking effect called for to preventing the medium temperature from exceeding the maximum medium temperature. In a crisis situation when, for example, the ordinary brakes on a downhill run lose the whole or part of their braking function, the braking effect called from the hydrodynamic auxiliary brake is only limited if there is risk of the medium reaching a temperature above the maximum medium temperature at which there is risk of fire or explosion. In such situations the coolant is allowed to rise beyond its maximum coolant temperature. Boiling of coolant in the cooling system is of little importance in a crisis situation. A crisis situation may be

identified in various ways, e. g. the vehicle's acceleration may be greater than 1 m/s2 while at the same time the brake pedal is more than 70% depressed. A crisis situation may also be identified by the medium being at a temperature above the maximum medium temperature, thereby causing risk of fire and explosion. This may occur if coolant leaks from the cooling system.

BRIEF DESCRIPTION OF THE DRAWING A preferred embodiment of the invention is described below by way of example with reference to the attached drawing in which : Fig. 1 depicts schematically an embodiment of an arrangement according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 depicts schematically a motor vehicle with a number of selected parts appropriate in this context. The motor vehicle incorporates non-driving wheels l and driving wheels 2 which are in contact with a running surface 3. The vehicle incorporates not only ordinary brakes (not depicted in Fig. 1) but also a hydrodynamic first auxiliary brake in the form of a hydraulic retarder 4 and a second auxiliary brake in the form of an exhaust brake 5. The hydraulic retarder 4 incorporates a stator and a rotor which form a toroidal space. The toroidal space is designed to be filled with a medium in the form of hydraulic oil when it is desired to apply a braking action to the vehicle. The hydraulic retarder 4 is fitted adjacent to an output shaft from a gearbox 6 which is connected to the vehicle's engine 7. The rotor of the hydraulic retarder 4 is provided with driving power by the vehicle's driveline, which includes inter alia a propeller shaft 8 for transmission of driving torque to the vehicle's driving wheels 2. The hydraulic retarder 4 thus only provides braking action to the vehicle's driving wheels 2. A first brake control in the form of a brake pedal 9 and a second brake control in the form of a hand control 10 are arranged to apply braking action to the vehicle. The brake controls 9,10 are settable by the driver so that they initiate a braking effect called for from the hydraulic retarder 4.

The braking effect called for from the hydraulic retarder 4 is transmitted in the form of a signal to the control unit 12 via a signal line 11. Using information about such a braking effect called for, the control unit 12 is designed to send a control signal to the hydraulic retarder 4 via a signal line 13.

The circulating hydraulic oil of the hydraulic retarder 4 substantially absorbs the heat energy generated during a braking process. The heated hydraulic oil is cooled in a heat exchanger 14 before it can be used again. The heat exchanger 14 is connected to the vehicle's ordinary cooling system. The hydraulic oil is cooled by the cooling system's circulating coolant passing through the heat exchanger 14. The coolant is thus heated and is led to the engine's ordinary radiator 15 in order to be cooled. The coolant is cooled in the radiator 15 by an air flow which passes through the radiator 15.

The control unit 12 is designed to prevent the coolant from exceeding a maximum coolant temperature. The maximum coolant temperature is the coolant temperature at which there is risk of the coolant beginning to boil locally in the cooling system. Such a temperature is about 110°C in a conventional cooling system. A first sensor 16 is designed to measure the coolant temperature after the coolant has passed through the heat exchanger 14. The first sensor 16 is designed to send to the control unit 12 a signal which is related to the coolant temperature detected. A second sensor 17 is designed to measure the temperature of the hydraulic oil after the latter has left the hydraulic retarder 4. The second sensor 17 is designed to send to the control unit a signal which is related to the hydraulic oil temperature detected. On the basis of said signals, the control unit 12 is designed to determine coolant and hydraulic oil temperature values at constant intervals of time. The control unit 12 stores the coolant and hydraulic oil temperature values for at least one period of time. The stored temperature values are used for estimating a rate of increase of the temperatures of the coolant and the hydraulic oil. The rate of increase is estimated by calculating the difference between the latest temperature value determined and at least one previously determined temperature value and then dividing by the temperature difference between said temperature values.

The control unit 12 is connected to the exhaust brake 5 by a signal line 18. The control unit 12 can thus control the activation of the exhaust brake 5. The control unit 12 is also

connected by a signal line 19 to at least one thermostat 20 in the cooling system. The control unit 12 can thus control the degree of opening of the thermostat 20 and vary the coolant flow in the cooling system. The control unit 12 is also connected by a signal line 21 to a cooling fan 22. The control unit 12 can thus control the speed of the cooling fan 22 and vary the air flow through the radiator 15. The control unit 12 can thus increase the cooling of the coolant in the radiator 15.

The control unit 12 is thus designed to prevent the coolant exceeding a maximum coolant temperature. In crisis situations requiring maximum utilisation of the vehicle's braking capacity to prevent an accident, however, the control unit 12 is designed to allow the coolant temperature to exceed the maximum coolant temperature. In crisis situations only, the control unit 12 allows a limitation of the braking effect called for, in order to prevent the hydraulic oil temperature exceeding a maximum hydraulic oil temperature.

The maximum hydraulic oil temperature is the temperature at which there is risk of the hydraulic oil overheating. Such overheating entails risk of fire and explosion. The control unit 12 is designed to allow limitation of the braking effect called for, at a value which is related to a calculated value according to a mathematical algorithm. An example of such an algorithm is set out below. r=C)-te+C2-tf/+C3-tm'+S-C4-tm where r = a reference value, C i, C2, C3, C4 = constants toc = coolant temperature, tc'= rate of increase of coolant temperature, tm = hydraulic oil temperature, tm'= rate of increase of hydraulic oil temperature, and s = 0 in all situations other than a crisis situation, in which s = 1.

Such an algorithm provides a calculated reference value r which is related to the extent to which the braking effect called for has to be limited to prevent the coolant exceeding the maximum coolant temperature. The reference value r thus increases with a rising coolant

temperature te and the latter's rate of increase tc'and a rising rate of increase tm'of cooling medium temperature. The control unit 12 calculates the value of the reference value r at continuous intervals of time during a braking process. At a low reference value r within a reference value range up to a first limit value ri, the risk of the coolant temperature tc reaching the maximum coolant temperature is negligible. In this situation the braking effect called for by the driver is passed on unmodified by the control unit to the hydraulic retarder 4. At a higher reference value r within a range from the first limit value ri up to a second limit value r2, the control unit 12 has to respond actively to ensure that the coolant temperature tc does not exceed the maximum coolant temperature. The control unit 12 activates the thermostat 20 to provide a greater degree of opening. The degree of opening of the thermostat 20 is related to the reference value r. A greater degree of opening results in an increased coolant flow in the cooling system, thereby counteracting a rising coolant temperature tc. Within this reference value range the control unit 12 also activates the cooling fan 22. The cooling fan 22 is activated in relation to the reference value r. The coolant is thus subjected to greater cooling in the radiator, thereby also counteracting a rising coolant temperature tc. The control unit 12 thus in the first place activates functions which increase the performance of the cooling system to prevent the coolant temperature tc exceeding the maximum coolant temperature.

If these measures are not sufficient to stop the pace of increase of the coolant temperature tc, a yet higher reference value r is reached within a reference value range which extends from the second limit value r2 up to a third limit value rs. Within this reference value range the control unit 12 has gradually to limit the retarder braking effect called for by the driver. At the same time, the control unit 12 activates the exhaust brake 5 so that at least part of the braking effect called for by the driver can be replaced. Within this range, the retarder braking effect called for is limited in relation to the reference value r. If the reference value exceeds the third reference value rB, the control unit 12 limits the whole braking effect called for from the hydraulic retarder 4. As the reference value r takes into account not only the coolant temperature tc but also its rate of increase tc', such an algorithm results in a substantially minimum limitation of the retarder braking effect called for, without the coolant temperature tc exceeding the maximum coolant temperature.

In a crisis situation, the control exercised by the control unit 12 is modified so that the retarder's braking effect is only limited if there is risk of the hydraulic oil temperature tm exceeding a maximum hydraulic oil temperature. A crisis situation is defined, for example, by vehicle acceleration exceeding 1 m/s2 and the brake pedal 9 being more than 70% depressed. In such a situation the vehicle accelerates uncontrolledly despite the driver trying to brake the vehicle. The situation requires the utilisation of all available braking resources to prevent an accident. In such a crisis situation, s assumes the value 1 in the algorithm. In a crisis situation the coolant temperature tc is allowed to exceed the maximum coolant temperature. A crisis situation also obtains if for any reason the hydraulic oil temperature tm exceeds the maximum hydraulic oil temperature. This may occur if coolant leaks from the cooling system, in which case s = 1. With s = 1 and a hydraulic oil temperature tm above the maximum hydraulic oil temperature, the reference value r will be more than r3. The control unit 12 therefore completely limits the braking effect of the hydraulic retarder 4.

The present invention is in no way limited to the embodiment depicted in Fig. 1 but may be varied freely within the scopes of the patent claims. The control unit may, for example, use information about the coolant temperature and its rate of increase as a basis for allowing a limitation of the braking effect called for from the hydraulic brake, without using a mathematical algorithm, and if such an algorithm is used it is not limited to that described above.