New vehicles and especially heavy vehicles, such as for example lorries, often have faster end gears (differential gears), i. e. end gears having a higher gear ratio, than older vehicles.

Faster end gears makes it possible to dimension the engine to be driven at lower engine speed, which allows the possibilities to reduce the fuel consumption and improve the economy.

The braking of a vehicle is performed, among others, by means of the wheel brakes of the vehicle, which, for example, may be drum brakes or disc brakes. During long applications of the brakes, such as during applications of the brakes in long downhill slopes, a risk of so called fading exists, i. e. a reduction of the brake action which occurs when the brake linings of the wheel brakes are too strongly heated. In order to reduce the wear of the linings of the wheel brakes and in order to reduce the risk of fading, additional auxiliary brakes have been developed which act independently of the wheel brakes. The auxiliary brakes may act during long periods and, thereby, the brake linings of the wheel brakes may be kept cold for emergency situations or other heavy applications of the brakes, and furthermore the wear of the linings are reduced since the wheel brakes do not need to be actuated just as frequently.

A common kind of auxiliary brake is a retarder and a special kind of retarder is a hydraulic retarder, which generates a braking moment by means of viscous friction, by means of oil between a stator and a rotor. The retarder is connected to the power train of the vehicle, for example, at the propeller shaft of the vehicle in connection to the gear box of the vehicle for making a braking of the driving wheels of the vehicle possible. The retarder may be directly provided to the propeller shaft or via a gear unit. The braking effect of the retarder increases

with the rotary speed of the propeller shaft and the output braking moment of the retarder is substantially proportional to the number of revolutions of the propeller shaft. Consequently, at a certain number of revolutions of the propeller shaft a certain maximum output braking moment may be obtained. However, the retarder may be adjusted by a brake regulator in such a manner that the driver of the vehicle may obtain an output braking moment, which is lower than the maximum output braking moment. The space between the stator and the rotor is called torus. The output braking moment depends on the coefficient of fullness of oil in this space. Consequently, by controlling the oil flow to said space, the output braking moment may be adjusted. The application time of the retarder depends on the previous use of the same. Before the retarder begins to produce an output braking moment, the space around the stator and the rotor has to be filled with oil. The retarder comprises herefore a pump and an oil reservoir. The lower the velocity of the propeller shaft of the vehicle, the longer time is required to fill the space around the stator and the rotor. In order to accelerate this operation, an oil accumulator is often used in connection to the oil reservoir.

It is to be noted that a lorry may comprise a number of auxiliary brakes. An example of a further auxiliary brake is the exhaust brake which is provided in the exhaust pipe and which is connected to the engine of the heavy vehicle. The exhaust brake comprises any kind of valve device, which may be arranged to be driven by an air cylinder. The function of the exhaust brake is to increase the natural brake action of the engine by throttling the exhaust flow from the engine. The braking moment produced by the exhaust brake depends, among others, on the kind of engine and the actual gear of the vehicle. The exhaust brake is most effective at large exhaust flows, which usually occurs when the vehicle is driven with low gears. Another auxiliary brake, which increases the natural brake action of the engine, is the compression brake, which transforms the compression work of the engine to braking work.

Both newer lorries of the kind mentioned above and older lorries may be equipped with a retarder. However, during braking by means of the retarder, the speed reduction becomes lower for a new lorry of the above mentioned kind than the speed reduction of an older lorry, if the both lorries have the same velocity before the braking. This depends, among others, on the fact that the engine speed of the newer lorry, and consequently the number of revolutions of the propeller shaft, is lower for the newer lorry than of the older lorry at the same velocity, wherein a lower output braking moment is obtained for the newer lorry.

However, a further reason is that the output braking moment generated by the retarder is changed to a lower braking force of the driving wheels of the newer lorry since the newer

lorry is provided with a faster end gear, i. e. a higher gear ratio between the propeller shaft and the driving shaft.

As mentioned above, lorries may be provided with a brake regulator, for example a lever by which the driver controls the moment that the retarder has to generate and consequently for braking the propeller shaft. Said lever may be provided in connection to the instrument panel and is a separate member. The lever may be graduated in steps, for example between 1 to 5, corresponding to different grades of braking moment, wherein step 1 may correspond to 1000 Nm, step 2 may correspond to 2000 Nm, step 3 may correspond to 3000 Nm etc. It ought to be noted that the retarder of certain lorries may be activated by the travel brake pedal, i. e. the pedal which activates the wheel brakes. When the driver presses the pedal, the first retarder is activated, which corresponds to a play of the pedal, and first at a continuing pressing of the pedal, the wheel brakes are activated. However, it may be difficult to control a certain braking moment to the retarder by the travel brake pedal.

Furthermore, it is possible that the driver presses the pedal so far down that the wheel brakes are also activated, wherein the risk of fading exists.

When the driver of an older lorry provided with a retarder changes to a new lorry provided with a retarder and with a faster end gear, the driver often experiences that the retarder of the new lorry has a lower braking moment. A reason to this, it is that the driver expects, when the retarder lever is located in a certain position and the lorry has a certain velocity, that the retarder will give the same output braking moment as it did in the older lorry, which the driver is used to. However, it is not the case since the new lorry is provided with a faster end gear. This is experienced as very unsatisfactory to a driver and it may create a reaction of the driver, wherein the driver activates the wheel brakes in such a manner that an unnecessary heavy brake action is obtained, so-called over braking.

SUMMARY OF THE INVENTION The object of the present invention is to eliminate the above-mentioned problem.

Especially, a device is aimed at, which contributes to the retarder generating an output braking moment which substantially corresponds to the speed reduction, which the driver desires to obtain, i. e. the retarder generating an output braking moment, which substantially corresponds to the desired braking force at the driving wheels.

This object is achieved by the initially mentioned device, which has the features defined in the characteristic part of claim 1.

By such a device comprising means arranged to make it possible for the driver of the vehicle to define a desired braking force at the driving wheels and that the device comprises a control unit, which is arranged to calculate the braking action of the first auxiliary brake, which substantially corresponds to the desired braking force and thereby to control the first auxiliary brake in such a manner that the calculated braking action substantially is achieved, the above mentioned problem is eliminated. In an advantageous embodiment, said means may comprise a brake regulator, by which the driver may specify the braking force he/she desires at the driving wheels and said first auxiliary brake may be a retarder. Since the driver specifies a desired braking force at the driving wheels instead of a desired output braking moment of the retarder, which often is the case today, the problem that the driver experiences that the retarder of a new lorry provided with a faster end gear has a lower braking moment than an old lorry, does not arise.

According to an embodiment of the invention, the control unit is arranged to calculate the brake action by means of the actual number of revolutions of the propeller shaft, the actual velocity of the vehicle and said desired braking force. Furthermore, the control unit is arranged to calculate the proportion between the actual number of revolutions of the propeller shaft and the actual velocity of the vehicle, wherein this proportion states the total gear ratio between the propeller shaft and the driving wheels. Consequently, the device takes into consideration the gear reduction, which is obtained, via the end gear, the driving shafts and the driving wheels, at the calculation of the braking action, which the retarder has to generate for the achievement of the braking force that the driver desires at the driving wheels.

According to a further embodiment of the invention, the first auxiliary brake consists of a retarder, preferably of a hydraulic retarder, which is directly connected to the propeller shaft. It is also possible that the retarder is connected to the propeller shaft in a driving power transmitting manner via a gear unit. This makes it possible to drive the retarder with a higher number of revolutions than the propeller shaft and to make the retarder more effective and/or with smaller dimensions. If the retarder is connected to the propeller shaft, via a gear unit, consideration has to be taken to the gear change by this gear unit for calculating which braking action the retarder has to generate for obtaining the desired braking force at the driving wheels. Hereby, the gear change of said gear unit may be stored

in the control unit, wherein the control unit also takes into consideration said gear change for calculating the braking action, which the retarder has to generate for obtaining a desired braking action at the driving wheels.

According to a further embodiment of the invention, the device comprises a display member arranged to show the real braking force at the driving wheels. By showing the real braking force on the display member, the driver's satisfaction will further be increased.

Possibly, one may also allow the display member to show the desired braking force.

Hereby, the driver may compare the result, which was obtained.

The vehicle may comprise one or several further auxiliary brakes beside said first auxiliary brake, which may be a retarder. The vehicle may, for example, comprise an exhaust brake and/or a compression brake, which also consist of alternative embodiments of the first auxiliary brake.

In the cases that the first auxiliary brake consists of a retarder, it is able to generate a maximum braking action, which is substantially proportional to the number of revolutions of the propeller shaft, at a certain velocity of the vehicle, and the retarder consequently has an upper limit for the braking moment to be generated at a certain number of revolutions of the propeller shaft. According to a further embodiment of the invention, the control unit is arranged to control the difference, between said maximum brake action and the calculated brake action, to any other auxiliary brake in the cases when the output brake action of the retarder is not sufficient. It is possible, that the driver states a desired braking force at the driving wheels, which is larger than what it is possible to obtain by the retarder. The control unit could also be arranged in such a manner that it, when necessary, also activated the wheels brakes. However, it is advantageous if only the auxiliary brakes are activated since these may act during long periods without being overheated.

The objects are also achieved with the initially mentioned brake system which has the features defined in the characteristic part of claim 10.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be explained by means of embodiments described as examples and with reference to the attached drawings, in which

Fig 1 shows schematically how the main brake and the auxiliary brakes are provided in a heavy vehicle, such as a lorry, Fig 2 shows schematically a brake system according to a first embodiment of the invention, and Fig 3 shows schematically a brake system according to a second embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Fig 1 shows schematically the location of available brakes of a heavy vehicle, such as a lorry. The vehicle comprises a main brake, which comprises brake members 1 a-f arranged to act against the wheels 2a-f of the vehicle. The braking members la-f may comprise pneumatic disc brakes, wherein the main brake comprises the equipment which is required for allowing the brake linings of the respective disc brake to be applied against the disc which rotates with the wheel 2a-f in question. The vehicle comprises a first auxiliary brake comprising a hydraulic retarder 3, which generates an output braking moment by viscous friction. The retarder 3 is connected to the propeller shaft 4 and to the gearbox 5 of the vehicle and only brakes the driving wheels 2c-d of the vehicle. As is shown by Fig 1, the retarder is placed directly on the propeller shaft but it ought to be noted that the retarder may be connected to the propeller shaft, via a gear unit. Furthermore, the vehicle comprises a further auxiliary brake comprising an exhaust brake 6, which may be of the kind described in the initial description. The exhaust brake 6 is connected to the engine 7 of the vehicle. The driving wheels 2c-2d are provided to a respective driving shaft 8, which, via a differential gear (end gear) 9, are connected to the propeller shaft 4.

Fig 2 shows a brake system 10 according to a first embodiment of the invention. The brake system 10 comprises a control unit 11, the retarder 3 and the exhaust brake 6, and a hand regulator 12 arranged to make it possible for the driver of the vehicle to define a desired braking force at the driving wheels 2c-d, i. e. the driver states by the hand regulator 12 the speed reduction which he/she desires to obtain. It ought to be noted that the brake system 10 also could comprise other brakes. Furthermore, the brake system 10 comprises a first sensor 13, a second sensor 14, a third sensor 15 and a fourth sensor 16. In alternative embodiments of the invention, corresponding signals may, instead for the separate sensors 13-16, be obtained, directly or indirectly, from other units in the vehicle where the sensed parameters of the respective sensors are available. The brake system 10 comprises also a display member 17. How the included parts of the brake system 10 co-operate will be

described more exactly later on in the description. Furthermore, it ought to be noted that the brake system 10 according to the invention also is applicable to other kinds of vehicles, as, for example, other kinds of cargo-carrying vehicles, buses and similar commercial vehicles, and all kinds of vehicles where similar problems and conditions exist.

The hand regulator 12 may be provided in connection to the instrument panel (not showed) of the vehicle. The hand regulator 12 may be graduated in steps, for example between 1 to 5, wherein each step corresponds to a certain braking force at the driving wheels 2c-d.

Fig 3 shows a brake system 10 according to a second embodiment of the invention, which only differs from the brake system in Fig 2 in that it instead for a hand regulator 12, arranged to make it possible for the driver of the vehicle to define a desired braking force at the driving wheels 2c-d, comprises a manoeuvre panel 18 with a set of buttons 19 arranged to make it possible for the driver of the vehicle to define a desired braking force at the driving wheels 2c-d. Also, the manoeuvre panel 18 may be provided in connection to the instrument panel of the vehicle. The manoeuvre panel 18 may comprise a display 20, which shows the braking force that the driver desires at the driving vehicle 2c-d.

Said first sensor 13 in Fig 2 is arranged to sense in which step the hand regulator 12 is located, which corresponds to the braking force the driver desires at the driving wheels 2c- d. The first sensor 13 transmits a signal corresponding to the desired braking force to the control unit 9, which comprises a computer, wherein the control unit 9 is arranged to calculate the brake action that has to be generated by the retarder 3 so that the driver will obtain the desired braking force at the driving wheels 2c-d. The control unit 11 is arranged to calculate the brake action by means of the actual number of revolutions of the propeller shaft 4 and the actual velocity of the vehicle and the braking force desired by the driver.

Hereby, said second sensor 14 is arranged to sense the actual number of revolutions of the propeller shaft 4 and transmit a signal corresponding to the actual number of revolutions of the propeller shaft 4 to a control unit 11 and said third sensor 15 is arranged to sense the actual velocity of the vehicle and to transmit a signal corresponding to the actual velocity of the vehicle.

The control unit 11 is arranged and comprises means for controlling the retarder 3 in such a manner that the calculated brake action substantially is achieved with the aid of the signals obtained from the respective sensors 13-15. The display member 17, which is provided at the instrument panel of the vehicle, is arranged to show the real braking force at the driving

wheels 2c-d to the driver after the control unit 11 has controlled the retarder 3 in such a manner that the calculated braking action substantially is achieved. Hereby, said fourth sensor 16 may be arranged to sense the retardation of the vehicle and to transmit a signal corresponding to the retardation to the control unit 11, which processes this signal and thereafter transmits the processed information as a signal to the display member 17, which shows the real braking force at the driving wheel 2c-d. The real braking force may be calculated by means of the weight of the vehicle, the inclination of the road, the travel resistance and the retardation. A further way to find out the real braking force at the driving wheels is to use the tables which may be stored in the control unit 11, which tables state which real braking force one obtains when a retarder generates a certain output braking moment and when one knows the gear change between the propeller shaft and the driving wheels. Consequently, it may exist a detector which senses the output braking moment of the retarder 3, wherein one by means of the output braking moment of the retarder 3 and the above mentioned tables may find out the real braking force at the driving wheels 2.

Possibly, the desired braking force at the driving wheels 2c-d also may be shown on the display member 17.

Since the generated braking action from the retarder 3 is substantially proportional to the number of revolutions of the propeller shaft 4 it exists a maximum braking action which the retarder 3 may generate at a certain velocity of the vehicle. This maximum braking action, i. e. the maximum output moment, which the retarder 3 generates at said velocity, is changed to a real braking force at the driving wheels 2c-d, via the differential gear 9. If the real braking force differs from the desired braking force, it may be such that the retarder 3 is not able to generate such a great output braking moment corresponding to the braking moment desired by the driver at the driving wheels 2c-d. If that is the case, the control unit 11 may be arranged to distribute the difference between the calculated braking action and the maximum braking action, which the retarder 3 may generate at the actual velocity of the vehicle to the exhaust brake 6 in such a manner that the braking force desired by the driver at the driving wheels 2c-d substantially is achieved. It ought to be noted that the brake system 10 also could comprise the main brake with its brake members la-f, wherein the control unit 11 could be arranged to distribute the difference between the calculated brake action and the maximum brake action, which the retarder 3 may generate at the actual velocity of the vehicle, to the brake members la-f of the main brake in such a manner that the braking force desired by the driver at the driving wheels 2c-d substantially is achieved.

However, it may be advantageous if only the auxiliary brakes of the vehicle are activated since the brake linings of the wheel brakes thereby may be kept cool for emergency

situations or heavy applications of the brakes. The auxiliary brakes have the advantage that they may act during long periods without being overheated.

The invention is not restricted to the showed embodiment but may be varied and modified within the scope of the following claims. In the example of the description, a retarder has been defined, which in the first place concerns a hydraulic retarder, but which in alternative embodiments of the invention may be an electric retarder working as an eddy current brake.