The present invention refers both to a process of the kind indicated in the preamble to claim 1, and to a control system of the kind indicated in the preamble to claim 5.

It is concerned with limiting the maximum braking effect which may be utilised from a hydrodynamic auxiliary brake, a so-called retarder in an engine-driven vehicle, preferably a truck or a bus.

State of the art

It is known for motor vehicles, especially heavy duty trucks, to be provided with a so-called hydrodynamic retarder, which is a hydrodynamic brake which can be used as a brake to help the vehicle's ordinary brakes to reduce the vehicle's speed. The retarder therefore provides the possibility, to a greater or lesser extent, of sparing the ordinary brakes during braking of the vehicle. The more the necessary braking work can be performed by the retarder, the less the vehicle's ordinary brakes will need to be used and hence the less wear there will be on their linings.

The retarder is placed at a suitable position in the driveline between the engine and wheels of the vehicle and is therefore drivingly connected to the vehicle's power transmission or its wheels. The braking effect in the retarder is engendered by a hydraulic fluid flow acting on parts which are drivingly connected to the power transmission or the vehicle's wheels.

A major problem with hydraulic retarders is that the braking work which is done in the retarder results in heating of the hydraulic oil in the retarder, with a consequent need to remove this heat from the hydraulic oil. This is achieved by the hydraulic oil being cooled by means of coolant contained in the ordinary cooling system of the vehicle's engine. The coolant flow, i.e. the heat removal capacity, of that cooling system depends on the effect of the coolant pump, which depends on the speed at which the pump is run. This speed depends in its turn on the speed of the vehicle's engine.

When the driver wishes to brake the vehicle and depresses the brake pedal, it is natural for him to release the accelerator, resulting in the engine speed decreasing in step with the vehicle speed reduction resulting from the braking, and if the driver then disengages the clutch the engine speed will decrease to idling speed. Such a low engine speed entails a correspondingly low coolant pump speed with consequently low pump effect resulting in minimum coolant flow and minimum heat removal capacity precisely in the situation which requires the greatest possible coolant flow for achieving as great a cooling effect as possible and being able to "cool away" the heat which develops in the retarder when the latter is used for braking. For this reason it is usual, when braking with a retarder, to employ a special driving technique which involves first changing down to a lower gear in order to increase the engine speed and hence also increase coolant flow in the coolant circuit of the cooling system.

To ensure that this will under no circumstances result in local boiling anywhere in the coolant circuit, it is essential to limit the maximum braking effect which the retarder can be allowed to develop. This limitation to prevent boiling entails the maximum braking effect value which the retarder can reach having to be set at a relatively low level with respect to the value represented by the maximum braking effect which the retarder could otherwise provide. ^* *

On long runs it is therefore not possible to use the full braking effect of the retarder (particularly not at high engine speeds) without having an unnecessarily inferior or ineffective retarder brake because of having to meet the braking effect limitation requirement in order to reliably eliminate any risk of local boiling in the cooling system circuit. This result does of course directly conflict with an expressed wish to spare the vehicle's ordinary brakes by making the maximum possible use of the retarder for providing the desired braking effect.

Object of the invention

The object of the invention is to be able as far as possible to eliminate the abovementioned severe limitation on the maximum braking effect which may be utilised from the retarder, and to achieve this without any risk of simultaneous boiling of the coolant in the cooling system. In other words, the aim is to achieve automatic prevention of boiling at the same time as being able to apply a substantially higher maximum limit to the braking effect which may be utilised from the retarder than has previously been possible.

Summary of the invention

The aforesaid object is achieved according to the invention, on the one hand by a process of the kind indicated in the introduction and distinguished by adoption of the measures indicated in the characterising portion of claim 1, and on the other hand by a control system of the kind indicated in the introduction and exhibiting the features indicated in the characterising portion of claim 5.

The independent claims 2, 3 and 4 indicate further developments of the process, and claims 6 and 7 indicate advantageous embodiments of the control system.

The idea on which the invention is based may be said to be that the maximum braking effect which may be utilised from the retarder is controlled by means of at least one primary operating parameter, viz. engine speed, possibly also in combination with one or more secondary operating parameters such as, for example, coolant temperature.

Brief description of the drawing

The invention will now be further explained and clarified with reference to an embodiment depicted in the accompanying drawing.

The single block diagram in the drawing depicts schematically a control system for limitation by engine speed of the maximum braking effect may be utilised from a retarder (a hydrodynamic auxiliary brake) in an engine-driven vehicle.

Description of the embodiment

The vehicle concerned is only schematically represented in the form of the block 10, which is delineated by a broken line and within which appear the parts of the vehicle which are of primary interest in the present context, viz. the vehicle engine 12, the gearbox 14 adjacent thereto and a hydrodynamic retarder 16 which in this case is fitted adjacent to the gearbox. This retarder constitutes an auxiliary brake for the vehicle, with the task of helping the vehicle's ordinary brakes, such as wheel brakes, to brake the vehicle. The retarder is depicted here as fitted to the gearbox, and the retarder's impeller is drivingly connected to the output shaft of the gearbox via an undepicted gear. The retarder impeller is also permanently drivingly connected to the vehicle's propeller shaft and its rotation speed is proportional to that of the propeller shaft. The stator portion interacting with the impeller is fixed in the retarder housing. The retarder could also inherently be conceived as being placed elsewhere in the driveline between the engine/gearbox and the vehicle's powered wheels 18. Part of this driveline is denoted in the diagram by reference 20.

When the vehicle is braked by means of the retarder 16, the braking effect developed is converted into heating of the hydraulic operating medium of the retarder, usually a suitable type of hydraulic oil. It is therefore necessary to achieve cooling of the hydraulic oil, which in the present case is done by means of a coolant, usually water, which is contained in and flows through the engine's ordinary cooling system, which is represented in the diagram by a radiator 22 connected to the engine 12 and to the retarder 16. The left end of the retarder 16 as seen in the diagram constitutes the retarder's oil cooler 24, in which heat exchange takes place between the retarder's hydraulic oil and the cooling system's coolant, which is then cooled in the usual manner in the radiator 22. The flow of coolant through the cooling system is maintained in a conventional manner by means of an undepicted coolant pump which is driven by the engine 12 and whose speed and cooling effect therefore vary with the engine speed.

We now go on to consider the electrical control circuit which controls and limits the maximum braking effect which may be utilised from the retarder 16. This control circuit incorporates an electrical control unit 26 which is connected to the retarder 16 via the signal line 28. This control unit 26 is connected by signal lines 30, 32 and 34 to an engine speed

sensor 36, a propeller shaft speed sensor 38 and a cooling water temperature sensor 40. The control unit 26 is also provided with or connected to an electrical programme control 42 which on the basis of the engine speed sensed by the engine speed sensor 36 adjusts the output signal from the control unit 26 to the retarder 16 (via the line 28) so that the maximum braking effect which can be derived from the retarder will vary with the engine speed in accordance with the graph 44 depicted in the diagram and stored in the control block 42.

In this embodiment, control of the braking effect of the retarder 16 is exerted indirectly by controlling its braking torque. For the control unit to be able to calculate the braking torque which corresponds to a given braking effect, it also needs information on the retarder speed at the time. As mentioned above, the retarder is permanently drivingly connected to the vehicle's propeller shaft, which makes the retarder speed proportional to the propeller shaft speed, which is therefore used to represent the retarder speed. The control unit is connected via the line 32 to the propeller shaft speed sensor 38 so that the control unit 26 can use signals from the latter for calculating the maximum braking torque which the retarder may be allowed so as to correspond to the maximum braking effect which has previously been calculated for the relevant engine speed according to the graph 44 in the control block 42. Control of the retarder's braking torque is provided in practice by means of a valve arrangement in the retarder which adjusts the oil pressure in the retarder in proportion to the voltage supplied by the control unit 26 via the signal line 28.

As already discussed above in the general portion of the description, the basic principle of the invention is that the engine speed is used as control parameter for determining the maximum braking effect the retarder 16 is allowed to develop. It is thus possible at any engine speed value to apply optimum retarder effect without any risk of coolant in the cooling system reaching boiling point anywhere in the cooling medium circuit. The invention thus ensures automatic prevention of boiling.

Using the engine speed to achieve maximum braking effect limitation according to the invention makes it possible at high engine speeds and correspondingly high cooling medium flows to utilise a greater retarder effect than is possible in practice by previously known techniques whereby the maximum utilisable retarder braking effect is unavoidably limited to a

predetermined relatively very low maximum value which is independent of engine speed and is chosen so as to reliably prevent boiling irrespective of engine speed.

Reference will now be made finally to the programme control 42 and more specifically to the graph 44 depicting the maximum utilisable retarder braking effect p as a function of the engine speed n.

Our consideration of the graph or braking effect curve 44 will proceed from its right end in the diagram to its left end corresponding to a notional zero speed of the vehicle engine.

As may be seen, the control system has the characteristic that a constant maximum braking effect p, is always available when the engine speed n exceeds a predetermined first break¬ point speed n,. This braking effect p, corresponds to the maximum which the retarder can provide during normal operating conditions with proper cooling of its hydraulic oil.

Reducing the engine speed n results in cooling capacity being reduced correspondingly by the reduced speed of the coolant pump. At the break-point speed n„ the maximum available braking effect is controlled so as to decrease in a linear manner with the engine speed n down to a minimum braking effect value p^.. at a predetermined second break-point speed n ₂ . This speed n ₂ may itself be set at zero, corresponding to complete engine standstill. In this example, however, the second break-point speed n ₂ has been set at a very low speed, e.g. 250 revolutions per minute, which is below the engine's idling speed.

The reason for this is as follows. Should the speed sensor 36 or its connection line 30 cease to function, this would result in an indicated engine speed of zero. This would in its turn mean that the maximum braking effect of the retarder would be reduced to an insignificant or low value even if the engine was actually running at a substantially higher speed. To ensure a certain braking effect in such a situation, the retarder is set to produce a certain theoretical braking effect P _n0 if the engine speed indicated is zero (n ₀ = O), which corresponds to interruption of operating parameter control by means of engine speed.

To make the braking effect variation between the break-point speed n ₂ and the zero speed n ₀ continuous, the retarder's maximum braking effect p is set so as to increase in an linear

manner with decreasing speed n between those speeds up to the braking effect limit value Pno-

More precise control of the retarder's braking effect setting can be achieved in modified embodiments of the invention by sensing and applying further operating parameters. One such operating parameter may be the engine water temperature, which is sensed by means of a water temperature sensor 40 connected to the control unit via the line 34. An indicated high water temperature denotes a reduced boiling risk margin and can be applied to shift downwards the whole or parts of the control curve 44. A low water temperature can be applied correspondingly to shift the curve 44 upwards so as to achieve a higher maximum braking effect.

The embodiment described is primarily concerned with the operation of the retarder when using maximum braking effect. The retarder may of course also be used for braking the vehicle with smaller effects in an essentially conventional manner.