WO2005080166A1

WO2005080166A1 - Arrangement and method for braking a motor vehicle

Info

Publication number: WO2005080166A1
Application number: PCT/SE2005/000225
Authority: WO
Inventors: Johnny Rickman; Tomas Selling
Original assignee: Scania Cv Ab (Publ)
Priority date: 2004-02-19
Filing date: 2005-02-17
Publication date: 2005-09-01
Also published as: SE0400383L; SE0400383D0; DE112005000272T5

Abstract

The present invention relates to an arrangement and a method for braking a vehicle (1). The invention comprises a control device (10) intended to be operated by a driver when it is desired to maintain on a downhill run a substantially constant speed (v0) of the vehicle, braking means (7, 8), comprising a first supplementary brake (7), for braking the vehicle on the downhill run, a cooling system (12) for cooling the first supplementary brake (7), and a control unit (9) adapted to controlling the activation of said braking means (7, 8). The control unit (9) is adapted to activating said braking means (7, 8) with the braking effect (B0) required for maintaining the desired speed (v0) during the whole downhill run, when this is possible without the cooling system becoming overloaded, and, if this is not possible, to activating the supplementary brake with a smaller braking effect (B1) which it is possible to maintain during the whole downhill run without the cooling system (12) becoming overloaded.

Description

Arrangement and method for braking a motor vehicle

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The invention relates to an arrangement and a method for braking a motor vehicle according to the preambles of claims 1 and 11.

A known practice in heavy vehicles with supplementary brakes is the use of an automatic braking process which provides the vehicle with a constant downhill speed by activation of at least one supplementary brake of the vehicle. Activating the automatic braking process entails the vehicle being caused to assume a desired speed, followed by the driver operating a control device, which may be a brake pedal, a knob or a lever, to initiate activation of the automatic braking process. An electrical control unit is adapted to estimating the braking effect required for maintaining the speed at which the vehicle was travelling downhill at the time of initiating the process, and to activating the supplementary brake with that braking effect. Where applicable, two or more supplementary brakes may be activated to ensure that the vehicle is provided with the estimated braking effect. The automatically controlled braking process ceases when the driver activates the vehicle's accelerator pedal.

In most cases, a hydrodynamic retarder is used primarily for maintaining the vehicle's automatic braking process. A hydrodynamic retarder comprises a stator, a rotor and a braking medium in the form of an oil which flows at high velocity between the stator and the rotor when the retarder is activated. During the braking process, the kinetic energy of the oil converts to thermal energy. In many cases the cooling system for the vehicle's combustion engine is used for cooling this oil. However, said cooling system is primarily dimensioned for cooling the combustion engine, and cooling said oil often involves a greater cooling effect than cooling the vehicle's engine. To avoid overheating of the cooling system, the retarder' s braking effect is reduced when the coolant of the cooling system reaches a maximum acceptable temperature. When the automatic braking process is activated on a long and steep downhill run, it is not unusual that the temperature of the coolant of the cooling system rises to the maximum acceptable level, resulting in a reduction in the retarder' s braking effect. Such a reduction in the retarder' s braking effect leads to it being impossible for the constant speed to be maintained and to the vehicle suddenly assuming a higher downhill speed.

DE 197 16 919 and DE 100 09 959 refer to methods for controlling a hydrodynamic retarder during a vehicle's braking process. The cooling system which cools the vehicle's engine is used for cooling the hydrodynamic retarder. When the retarder is activated, the temperature of the coolant of the cooling system is measured before and after it passes through the retarder. The cooling effect imparted to the retarder from the circulating coolant of the cooling system can therefore be determined substantially instantaneously on the basis of information about, inter alia, this temperature difference. The retarder' s braking effect is then regulated as a function of the available cooling effect in the cooling system. The retarder can therefore be controlled so that, when necessary, it provides a substantially optimum braking effect without the cooling system becoming overloaded.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement and a method whereby a driver, when initiating an automatic braking process in order to maintain a desired constant vehicle speed on a downhill run, is provided with an early indication as to whether the desired vehicle speed can be maintained for the whole downhill run or not. The object stated above is achieved with the arrangement mentioned in the introduction which is characterised by what is indicated in the characterising part of claim 1. The control unit is thus adapted, after the control device has been operated, to deciding whether it is possible to maintain during the whole downhill run the speed desired by the driver. Information about the capacity of the cooling system for cooling the first supplementary brake can be used for determining the braking effect with which the first supplementary brake can be activated without the cooling medium of the cooling system reaching a temperature higher than the maximum acceptable value. The control unit can thus decide whether said braking means can supply the braking effect required for maintaining the desired vehicle speed on the downhill run without the cooling system becoming overloaded. If this is not possible, the control unit causes activation of said braking means with a smaller braking effect which it is possible to maintain during the whole downhill run without the cooling system becoming overloaded. The smaller braking effect may be the greatest braking effect value which it is possible to maintain without the cooling system becoming overloaded. In this latter case, the vehicle therefore already provides at the time of initiation of the automatic braking process a smaller braking effect than that required for maintaining the desired speed. The vehicle will therefore immediately accelerate to a higher speed than the desired speed. The driver will thus perceive, immediately after operating the control device, whether the desired speed can be maintained on the downhill run or not. The driver may accept continuation of the higher speed during the whole downhill run. Alternatively, the driver may temporarily activate the vehicle's ordinary wheelbrakes during the downhill run in order to hold the speed at a lower level. With the present invention, the supplementary brake can therefore only be activated with the braking effect which it is possible to maintain substantially continuously throughout the downhill run. The possibility of the vehicle's suddenly acquiring downhill acceleration after a desired speed has been maintained during a first portion of the downhill run is thus avoided.

According to an embodiment of the present invention, the control unit is adapted to estimating the cooling system's available cooling effect for cooling the first supplementary brake, which is the cooling system's cooling effect when the temperature of said cooling medium of the cooling system is at the maximum acceptable value, and to activating the first supplementary brake with a braking effect corresponding to not more than said available cooling effect. A cooling system in a vehicle usually comprises a radiator in which the cooling medium undergoes the bulk of its cooling. The cooling effect which the radiator provides depends inter alia on the temperature difference between the air flow being led through the radiator and said cooling medium. The radiator thus has a maximum cooling effect on the cooling medium when the latter is at a temperature corresponding to the maximum acceptable value. It is thereby ensured that the supplementary brake will not impart more heat to the cooling system than the radiator is capable of giving off. The supplementary brake can therefore provide this braking effect during downhill runs of indefinite length without the cooling medium reaching a temperature which exceeds the maximum acceptable value. According to one alternative, the control unit may provide information, e.g. from a GPS unit, concerning the length of the downhill run. If the length of the downhill run is known, the control unit can activate the supplementary brake with a braking effect which exceeds the cooling system's available cooling effect for cooling the supplementary brake, but not more than whereby the temperature of the cooling medium is allowed to rise from an original temperature to a temperature at the end of the downhill run which does not exceed the maximum acceptable temperature value. According to another embodiment, the control unit may be adapted to, when necessary or when a certain braking requisite exists, momentarily activating the first supplementary brake with a braking effect which exceeds the cooling system's available cooling effect for cooling the supplementary brake when the temperature of the cooling medium is below the maximum acceptable value. If the temperature of the cooling medium is below the maximum acceptable value, there is scope for rapidly raising the temperature of the cooling medium to the acceptable value. The first supplementary brake may thus, during an initial stage of the braking process, provide a greater braking effect than the cooling system's available cooling effect.

According to an embodiment of the present invention, the arrangement comprises at least two temperature sensors for measuring the difference between the respective temperatures of the cooling medium at two different positions in the cooling system, whereby the control unit is adapted to estimating the cooling system's available cooling effect on the basis of information from said temperature sensors. A cooling system thus usually comprises a radiator in which the cooling medium undergoes the bulk of its cooling. In such cases the temperature sensors are with advantage situated in such a way that at least the temperature drop of the cooling medium in the radiator can be determined. The cooling medium is usually circulated in the cooling system by means of a cooling medium pump driven by the vehicle's engine. In most such cases the cooling medium flow can be estimated on the basis of knowledge of the speed of the engine. The specific heat of the cooling medium is a known parameter.

Knowledge of the parameters indicated above can be used to determine the cooling medium's current cooling effect as the product of the cooling medium flow, the measured temperature drop and the coolant's specific heat. As the cooling effect of the radiator varies inter alia with the temperature of the air being led through the radiator in order to cool the cooling medium, the radiator's current cooling effect needs estimating at quite frequent intervals. Knowledge of the radiator's current cooling effect and the difference between the current temperature of the cooling medium and the maximum acceptable temperature value can be used to determine the available cooling effect which thus is obtained when the cooling medium has reached the maximum acceptable temperature value.

According to another embodiment of the invention, the control unit is adapted to registering the current speed of the vehicle, when the control device has been operated, as the desired downhill speed. In this case the driver causes the vehicle to assume the desired downhill speed before the control is operated. The control device unit may be adapted to estimating the braking effect required for maintaining the desired downhill speed as the vehicle's current braking effect when the vehicle initially assumes the desired constant downhill speed. When the driver has caused the vehicle to assume a desired speed and has operated the control device, a very short initial activation of the vehicle's ordinary brakes is effected in order to maintain the initiated speed. In this situation the control unit provides information about the braking effect required for maintaining the desired constant speed. It is thus easy to obtain an estimate of the necessary braking effect. Alternatively, the arrangement may comprise a gradient sensor which detects the downhill gradient, and at least one load sensor for estimating the vehicle's weight, whereby the control unit is adapted to using information from said gradient sensor and said load sensor in order to estimate the braking effect required for maintaining a desired downhill speed.

According to an embodiment of the present invention, the control unit is adapted to controlling, when necessary or when a certain braking requisite exists, at least one component of the vehicle in such a way that the cooling system is provided with a greater cooling effect for cooling the first supplementary brake so that the latter can provide an increased braking effect. In this case the control unit may for example engage a lower gear in the vehicle's gearbox. The cooling medium of the cooling system is usually circulated by a pump driven by the vehicle's engine. Engaging a lower gear in the gearbox results in a higher engine speed and hence a higher pump speed and a greater cooling medium flow in the cooling system. An alternative possibility is to increase the speed of a cooling fan which leads air through the radiator of the cooling system. In either case the cooling system can provide a greater cooling effect and the first supplementary brake can thus have a corresponding increased braking effect.

According to an embodiment of the present invention, the arrangement comprises a second supplementary brake which is not cooled by said cooling system, whereby the control unit is adapted, when said first supplementary brake cannot supply the necessary braking effect for maintaining a desired constant downhill speed, to activating a second supplementary brake to impart to the vehicle a further braking effect which, if possible, together with the braking effect of the first supplementary brake, provides the vehicle with said braking effect. Such a second supplementary- brake may be an exhaust brake or a compression brake.

According to an embodiment of the present invention, said cooling system also has the function of cooling an engine of the vehicle. When a driver operates the control device in order to provide a desired constant downhill speed, no driving torque is required from the vehicle's engine, which thus changes over to no-load running. In this situation the engine's cooling requirement is substantially negligible. In such cases it is therefore appropriate to use the cooling system for cooling the first supplementary brake. The first supplementary brake may be a hydrodynamic retarder, in which case the cooling system comprises a heat exchanger for cooling a cooling medium of the hydrodynamic retarder. Hydrodynamic retarders are effective supplementary brakes commonly used in heavy vehicles for supplying a long-time braking effect, particularly on downhill runs. Wear on the vehicle's wheelbrakes is thereby reduced. In such cases the cooling system's cooling medium is circulated through the heat exchanger in order to cool the hot braking medium coming from the retarder. The retarder' s braking medium is usually an oil with suitable characteristics.

The object stated above is also achieved with the method mentioned in the introduction which is characterised by what is indicated in the characterising part of claim 11.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is described below by way of example with reference to the attached drawings, in which:

Fig. 1 depicts schematically an arrangement according to the present invention, Fig. 2 depicts the cooling system in Fig. 1 in more detail and

Fig. 3 depicts a flowchart illustrating a method according to the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Fig. 1 schematically depicts selected parts of a heavy vehicle and an arrangement for maintaining a constant downhill speed of the vehicle. The vehicle comprises pneumatically operable wheelbrakes 1 both for the vehicle's unpowered wheels 2 and for the vehicle's powered wheels 3. The wheelbrakes 1 comprise undepicted conventional brake cylinders with friction linings designed to be applied to brake discs in order to cause braking of the vehicle. The vehicle has an engine 4 which in this case is a diesel engine, and a driveline which comprises inter alia a gearbox 5 and a universal shaft 6. A first supplementary brake in the form of a hydrodynamic retarder 7 is connected to the universal shaft 6. The hydrodynamic retarder 7 provides, when activated, braking of the universal shaft 6 and hence of the vehicle's powered wheels 3. A second supplementary brake in the form of an exhaust brake 8 is arranged in connection with the vehicle's engine 4.

In this case, an electrical control unit 9 is adapted to controlling a braking process of the vehicle in order to maintain a constant downhill speed of the vehicle. The control unit 9 may be a computer unit provided with software stored on a data support or data carrier 9a. A control device 10, here in the form of a lever, is situated at a suitable location in the vehicle's driving cab. The control device 10 is intended to be operated by the driver in order to initiate activation of said braking process. The control device 10 need not necessarily be a lever, as it may be a knob or the like. When the driver operates the control device 10, a signal is sent to the control unit 9, which registers the vehicle's current speed vo. The control unit 9 is also connected to a gearchange mechanism 11 in order to be able, when necessary or when a certain braking requisite exists, to initiate engagement of a lower gear in the gearbox 5. A cooling system 12 used for cooling both the vehicle's engine 4 and the retarder 7 is depicted symbolically.

Fig. 2 depicts selected parts of the cooling system 12 in Fig. 1. The cooling system 12 contains a coolant which is circulated in the cooling system 12 by a coolant pump 13. The coolant pump 13 is driven by the vehicle's engine 4 via a driving belt or the like. Coolant is led from the coolant pump 13 to the engine 4, in which it is circulated through cooling ducts to cause cooling of the engine 4. After passing through the engine 4, the coolant is led to a heat exchanger 14 in which it is applied to cooling the oil used as braking medium in the hydrodynamic retarder 7. The coolant of the cooling system 12 is led from the heat exchanger 14 to a radiator 15 in which it is intended to give off the thermal energy absorbed from the engine 4 and the heat exchanger 14. A first temperature sensor 16 is adapted to detecting the temperature of the coolant before the latter is led into the engine 4. A second temperature sensor 17 is adapted to detecting the temperature of the coolant after the latter has passed through the heat exchanger 14.

Fig. 3 depicts a flowchart illustrating a method for maintaining a constant vehicle speed on a downhill run. When the vehicle reaches the beginning of the downhill run, the driver has the possibility of initiating activation of the automatic braking process in order to maintain a constant downhill speed. If the driver intends to use the automatic braking process, the vehicle is initially caused to assume the speed v₀ which the driver wishes to maintain on the downhill run. When the vehicle is at the desired speed v₀, the driver operates the control device 10 so that the automatically controlled braking process begins, which takes place at 20 in the diagram. When the control device 10 is operated, a control signal is sent to the control unit 9. At 21, the control unit 9 registers the vehicle's current speed v₀. Thereafter the control unit 9 activates the wheelbrakes so that the desired speed v₀ is maintained for a short while on the downhill run. At 22, the control unit 9 thus receives information about the braking effect B₀ required for maintaining the desired downhill speed v₀. It is desirable that the necessary braking effect B₀ be primarily provided by activation of the retarder 7, inter alia because the latter is considerably quieter in operation than the exhaust brake 8. However, the braking capacity of the retarder 7 is limited by the cooling capacity of the cooling system 12. At 23, the control unit 9 therefore estimates the current cooling effect Eco of the cooling system 12. To this end, the control unit 9 receives signals from the temperature sensors 16, 17 concerning the temperatures of the coolant as measured before and after the radiator 15. The temperature drop across the radiator 15 can thus be determined. The control unit 9 also receives information concerning the speed of the engine 4. Knowledge of the speed of the engine 4 can be used to determine the speed of the coolant pump 13 and hence the cooling medium flow in the cooling system 12. The specific heat of the cooling medium is a known parameter. The current cooling effect Ec of the cooling system 12 can therefore be estimated as the product of the cooling medium flow, the temperature drop across the radiator and the coolant's specific heat. At 24, the difference between the coolant's maximum acceptable temperature T_max and the coolant's current temperature T₀ is estimated. When the retarder 7 is activated, the point where the coolant is at its highest temperature in the cooling system 12 is substantially immediately after the heat exchanger 14. If the control unit 9 receives temperature values from the sensor 17 which indicate that the coolant's current temperature T₀ is below the maximum acceptable value T_max, there is scope for raising the temperature of the cooling medium. Raising the temperature of the coolant to the maximum acceptable value Tβ also results in a greater cooling effect in the radiator 15 which is related to the temperature difference between the coolant and the air which flows through the radiator 15. When the coolant is at a temperature corresponding to the maximum acceptable temperature value, the cooling system reaches a maximum value here referred to as the cooling system's available cooling effect Ec for cooling the retarder 7. The control unit 9 is therefore adapted, at 25, to estimating the available cooling effect Ec of the cooling system 12. Such estimation is possible on the basis of information inter alia about the cooling system's current cooling effect Eco and the temperature difference between the maximum acceptable value T_max and the coolant's current temperature To. The retarder 7 can thus constantly maintain a braking effect B_R corresponding to the cooling system's available cooling effect Ec without the coolant reaching a temperature in the cooling system 12 which exceeds the maximum acceptable value T_max. The retarder 7 can thus provide this braking effect B_R on a downhill run of indefinite length without the cooling system 12 becoming overloaded.

At 26, the control unit 9 decides whether the necessary braking effect B₀ is less than or equal to the aggregate of the available cooling effect Ec of the cooling system 12 for cooling the retarder 7 and the exhaust brake's maximum braking effect B_Amax- If such is the case, it is possible, at 27, to provide the necessary braking effect Bo by activating the retarder 7 with a braking effect B_R and the exhaust brake 8 with a braking effect BA. In this situation the control unit activates the retarder 7 primarily in order to maintain the necessary braking effect Bo and, when necessary, the exhaust brake 8 to maintain any remaining braking effect, so that they together provide the necessary braking effect BQ. If the necessary braking effect Bo is greater than the aggregate of the available cooling effect Ec of the cooling system 12 and the maximum braking effect B_AIMX of the exhaust brake 8, the control unit is adapted, at 28, to controlling one or more components of the vehicle in such a way that the cooling system 12 provides a greater available cooling effect Ec₊. In this case the control unit 9 initiates engagement of a lower gear in the gearbox 5. A downward gearchange provides increased engine braking of the vehicle but, above all, a higher engine speed. Higher engine speed results in higher speed of the coolant pump 13 and hence increased coolant flow in the cooling system 12. A downward gearchange therefore results in the cooling system 12 providing a greater available cooling effect Ec₊ and the consequent possibility of the retarder 7 being activated with a corresponding greater braking effect B_R without the cooling system 12 becoming overloaded. Another alternative for obtaining a greater available cooling effect Ec₊ is to control an undepicted cooling fan in such a way that it causes a higher air velocity through the radiator 15. At 29, the control unit 9 decides whether the necessary braking effect B₀ is less than or equal to the aggregate of the cooling system's greater cooling effect Ec₊ for cooling the retarder 7 and the maximum available braking effect BA_ITOX of the exhaust brake 8. If such is the case, the control unit 9 activates, at 30, the retarder 7 with the braking effect BR and the exhaust brake with the braking effect BA, SO that they together provide the vehicle with the necessary braking effect B₀ and hence the desired vehicle speed v₀ during the whole downhill run.

In cases where the necessary braking effect B₀ is greater than the aggregate of the available cooling effect Ec₊ of the cooling system 12 and the maximum braking effect B_Amax of the exhaust brake 8, it may be found impossible to maintain the desired speed vo during the whole downhill run. The control unit therefore estimates, at 31, the maximum braking effect _\ which it is possible to supply without the cooling system 12 becoming overloaded. The control unit 9 thereafter activates the retarder 7 with a braking effect B_R which corresponds to the available coolmg effect Ec₊ of the cooling system 12, and the exhaust brake with the maximum braking effect BAmax, so that the vehicle is provided with the braking effect B₁. As the braking effect B₁ is not sufficient for maintaining the desired speed vo, the vehicle acquires acceleration to a higher speed γ_\ substantially immediately afterwards the control device 10 has been operated. The driver thus perceives substantially immediately that it is impossible to maintain the desired speed v₀ during the whole downhill run. The driver may therefore choose to accept the higher constant speed v throughout the downhill run or to temporarily activate the ordinary wheelbrakes 1 in order to hold the speed at a lower level. If on the other hand the vehicle keeps to the speed v₀ after the control device 10 has been operated, the driver can be certain that the vehicle will be able to maintain the speed v₀ throughout the downhill run whatever its length. When the driver activates the vehicle's accelerator pedal, the braking process ceases. When the driver again operates the control device 10, the automatic braking process starts again at 20.

All the process steps, and any desired partial sequences of steps, illustrated in Fig. 3 can of course be controlled by a computer programme which is directly loadable to the internal memory of a computer and comprises suitable software for controlling the necessary steps when the programme is run on the computer. In addition, even if the embodiment of the invention described with reference to the drawings is software- controlled by means of a computer and processes performed by computer, the invention extends also to computer programmes, particularly such computer programmes stored on a data support adapted to utilising the invention. The programme may be in the form of source code, object code, a code which takes the form of a level between source code and object code (e.g. in partly compiled form), or in whatever other form may be suitable for use in implementing the method according to the invention. The data support may be any desired entity or device capable of storing the programme, e.g. the data support may comprise a storage medium such as ROM (Read Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash or EEPROM (Electrically EPROM). The data support may also take the form of a transferable support, such as an electrical or optical signal capable of being transferred via an electric or optical cable or by radio or by some other means. If the programme is comprised in a signal which can be led directly through a cable or some other device or means, the data support may take the form of such a cable, device or equipment. Alternatively, the data support may be an integrated circuit in which the programme is stored, whereby the integrated circuit is adapted to performing, or being used in the performance of, relevant processes. The invention is in no way limited to the embodiment described, as it may be varied freely within the scopes of the claims. In many cases it will be appropriate to use the engine's cooling system for cooling the supplementary brake, but it is also possible for the supplementary brake to be cooled by a separate cooling system. The embodiment exemplified above uses an exhaust brake which is not cooled by the cooling system, and a component control which pertains to providing a greater cooling capacity of the cooling system, in order to make possible an increased braking effect so that a desired vehicle downhill speed can be maintained. Said braking means may comprise any desired number of supplementary brakes and component controls in any desired combinations for providing the vehicle with a greater braking capacity with the object of maintaining the desired downhill speed. There is nevertheless no need to use braking means other than the hydrodynamic retarder in order to effect the automatic braking process.

Claims

1. An arrangement for braking a motor vehicle, whereby the arrangement comprises a control device (10) intended to be operated by a driver when it is desired to maintain on a downhill run a substantially constant speed (v₀) of the vehicle, braking means, comprising a first supplementary brake (7), for braking the vehicle on the downhill run, a cooling system (12) with a circulating cooling medium for cooling the first supplementary brake (7), and a control unit (9) adapted to controlling the activation of said braking means and to providing information concerning the braking effect (Bo) required for maintaining the desired downhill speed (vo), characterised in that the control unit (9) is adapted to deciding, when said control device (10) is operated, whether it is possible to maintain the necessary braking effect (Bo) over the whole length of the downhill run by activation of said braking means without the first supplementary brake (7) having to be activated in such a way that said cooling medium of the cooling system (12) reaches a temperature exceeding a maximum acceptable value (T_max), after which the control unit (9) is adapted to activating said braking means with the necessary braking effect (B₀) when this is possible, and, when this is not possible, to substantially immediately activating said braking means with a smaller braking effect (Bi) which it is possible to maintain over the whole length of the downhill run without the first supplementary brake (7) having to be activated in such a way that said cooling medium of the cooling system (12) reaches a temperature exceeding the maximum acceptable value (T_max).

2. An arrangement according to claim 1, characterised in that the control unit is adapted to estimating the available cooling effect (Ec, Ec₊) of the cooling system (12) for cooling the first supplementary brake (7), which is the cooling system's coolmg effect when the temperature of said cooling medium of the cooling system (12) is at the maximum acceptable value (T_max), and to activating the first supplementary brake (7) with a braking effect (BR, B_R+) which does not exceed said available cooling effect

3. An arrangement according to claim 1 or 2, characterised in that the control unit is adapted to, when a braking requisite exists, momentarily activating the first supplementary brake with a braking effect (B_R, B_R+) which exceeds the available cooling effect (Ec, Ec₊) of the cooling system (12) for cooling the supplementary brake (7) when the cooling medium's temperature (To) is below the maximum acceptable value (T_max).

4. An arrangement according to any one of the foregoing claims, characterised in that the arrangement comprises at least two temperature sensors (16, 17) for measuring the difference between the respective temperatures of the cooling medium at two different positions in the cooling system (12), whereby the control unit (9) is adapted to estimating the available cooling effect (Ec, Ec₊) of the cooling system (12) on the basis of information from said temperature sensors (16, 17).

5. An arrangement according to anv one of the foregoing claims, characterised in that the control unit (9) is adapted, when the control device (10) has been operated, to registering the vehicle's current speed as the desired downhill speed (vo).

6. An arrangement according to claim 5, characterised in that the control unit (9) is adapted to estimating the braking effect (Bo) required for maintaining the desired downhill speed (vo) as the vehicle's current braking effect when the vehicle initially assumes the desired constant downhill speed (vo).

7. An arrangement according to any one of the foregoing claims, characterised in that the control unit (9) is adapted to, when a braking requisite exists, controlling at least one component (11) of the vehicle in such a way that the cooling system (12) acquires a greater available cooling effect (Ec₊) so that the first supplementary brake (7) can provide a corresponding greater braking effect (B_R+).

8. An arrangement according to any one of the foregoing claims, characterised in that said braking means comprise a second supplementary brake (8) not cooled by said cooling system (12), whereby the control unit (9) is adapted, when said first supplementary brake (7) cannot supply the necessary braking effect (B₀) for maintaining a desired constant downhill speed (vo), to activating the second supplementary brake (8) in order to impart to the vehicle a further braking effect (B_A) which, if possible, together with the braking effect (B_R) of the first supplementary brake, provides the vehicle with said necessary braking effect (Bo).

9. An arrangement according to any one of the foregoing claims, characterised in that said cooling system (12) also has the function of cooling an engine (4) of the vehicle.

10. An arrangement according to anv one of the foregoing claims, characterised in that the first supplementary brake is an hydrodynamic retarder (7), in which case the cooling system (12) comprises a heat exchanger (14) for cooling a braking medium which is used by the hydrodynamic retarder (7).

11. A method for braking a motor vehicle, whereby the vehicle comprises a control device (10) intended to be operated by a driver when it is desired to maintain on a downhill run a substantially constant speed (v₀) of the vehicle, braking means, comprising a first supplementary brake (7), for braking the vehicle on the downhill run, a cooling system (12) with a circulating cooling medium for cooling the first supplementary brake (7), whereby the method comprises the steps of controlling the activation of said braking means and of providing information concerning the braking effect (Bo) required for maintaining the desired downhill speed (v₀), characterised by the steps of deciding, when said control device (10) is operated, whether it is possible to maintain the necessary braking effect (B₀) over the whole length of the downhill run by activation of said braking means without the first supplementary brake (7) having to be activated in such a way that said cooling medium of the cooling system (12) reaches a temperature exceeding a maximum acceptable value (T_max), and of activating said braking means with the necessary braking effect (Bo) when this is possible, and, when this is not possible, of substantially immediately activating said braking means with a smaller braking effect (Bi) which it is possible to maintain over the whole length of the downhill run without the first supplementary brake (7) having to be activated in such a way that said cooling medium of the cooling system (12) reaches a temperature exceeding the maximum acceptable value (T_max).

12. A method according to claim 11, characterised by the steps of estimating the available cooling effect (Ec, Ec₊) of the cooling system (12) for cooling the first supplementary brake (7), which is the cooling system's cooling effect when the temperature of said cooling medium of the cooling system (12) is at the maximum acceptable value (T_max), and of activating the first supplementary brake (7) with a braking effect (B_R, B_R+) which does not exceed said available cooling effect (Ec, Ec₊).

13. A method according to claim 11 or 12, characterised by the step of, when a braking requisite exists, momentarily activating the first supplementary brake with a braking effect (B_R, B_R+) which exceeds the cooling system's available cooling effect (Ec, Ec₊) for cooling the supplementary brake (7) when the cooling medium's temperature (To) is below the maximum acceptable value (T_max).

14. A method according to any one of claims 11 to 13 above, whereby the vehicle comprises at least two temperature sensors (16, 17) for measuring the temperature of the cooling medium at different points in the cooling system (12), characterised by the step of estimating the cooling system's available cooling effect (Ec, Ec₊) for cooling the supplementary brake (7) on the basis of information from said temperature sensors (16, 17).

15. A method according to any one of claims 11-14 above, characterised by the step, when the control device (10) has been operated, of registering the vehicle's current speed as the desired downhill speed (v₀).

16. A method according to claim 15, characterised by the step of estimating the braking effect (B₀) required for maintaining the desired downhill speed (v₀) as the vehicle's current braking effect when the vehicle initially assumes the desired constant downhill speed (v₀).

17. A method according to any one of claims 11 to 16 above, characterised by the step of, when a braking requisite exists, controlling at least one component (11) of the vehicle in such a way that the cooling system (12) acquires a greater cooling effect (Ec₊) for cooling the first supplementary brake (7) so that the latter can provide an increased braking effect (B_R+).

18. A method according to any one of claims 11 to 17 above, whereby said braking means comprise a second supplementary brake (8) not cooled by said cooling system (12), characterised by the step, when said first supplementary brake (7) cannot supply the necessary braking effect (Bo) for maintaining a desired constant downhill speed (vo), of activating the second supplementary brake (8) in order to impart to the vehicle a further braking effect (B_A) which, if possible, together with the braking effect (B_R) of the first supplementary brake, provides the vehicle with said necessary braking effect (Bo).

19. A method according to any one of claims 11 to 18 above, characterised by the step of also using said cooling system (12) to cool an engine (4) of the vehicle.

20. A method according to any one of claims 11 to 19 above, characterised by the step of using a heat exchanger (14) of the cooling system (12) to cool a braking medium which is used by the first supplementary brake (7).

21. A computer programme directly loadable to the internal memory of a computer and comprising software for controlling the method according to any one of claims 11 - 20 when the programme is run on a computer.

22. A computer-readable medium (9a) on which is stored a programme which is suitable for enabling a computer to control the method according to any one of claims 11 - 20.

Fig 1

Fig 2