BACKGROUND AND STATE OF THE ART There are various ways for a motor vehicle driver, particularly in the case of heavy motor vehicles such as trucks, to address difficult situations such as steep upgrades and running surfaces with poor grip. Vehicles may for example be equipped with systems for drive power control ("Traction Control") and/or systems for load transfer (known as"Electronic Level Control"in the case of vehicles with air suspension).

The vehicle normally has a driving axle with two powered wheels. The system for drive power control detects whether either or both of the powered wheels are slipping.

If either of the powered wheels is slipping, the drive power control system increases the brake pressure on the slipping wheel, thereby transferring braking power to the other wheel, which is gripping. If both powered wheels are slipping, the engine power applied is reduced, which may enable the powered wheels to grip.

The vehicle normally has not only a driving axle but also a loadbearing auxiliary axle.

The system for load transfer usually uses air bellows in a suspension system for the vehicle in order to raise and lower the loadbearing auxiliary axle, thereby regulating the pressure on the driving axle. If both of the powered wheels are slipping, the driver can thus raise the loadbearing auxiliary axle to increase the pressure on the driving axle, thereby enabling the powered wheels to gain a grip.

The various types of systems described above which exist in the state of the art may nevertheless give rise to certain problems. When the drive power control system reduces the engine power applied so that both of the powered wheels slip, this may

lead to the power from the engine being insufficient to move the vehicle away from the running surface which is for any reason troublesome, thereby still further increasing the risk of the vehicle becoming immobilised. The load transfer system entails the risk that the driveline, i. e. the engine, gearbox, universal shaft and rear axle, may be overloaded, with consequent damage. As the known load transfer systems are controlled entirely manually, the driver has to decide how much load can be imposed upon the driving axle without causing damage, a decision which he/she is usually ill- equipped to take.

SUMMARY OF THE INVENTION The object of the present invention is to provide motor vehicle drive power adaptation which remedies the problems mentioned above.

This is achieved by a method for motor vehicle drive power adaptation in the case of a vehicle which comprises at least one driving axle with two powered wheels, a system for drive power control and a system for load transfer, which load transfer system can regulate a pressure on the driving axle, while a system for supervising the drive power control and load transfer systems, when the drive power control system indicates that both of the powered wheels are slipping, automatically orders the load transfer system to increase the pressure on the driving axle until the drive power control system no longer indicates that both of the powered wheels are slipping.

The fact that the system for supervising the other two systems links the latter together results automatically in correct pressure on the driving axle, enabling the vehicle to gain grip without the engine power applied being reduced and without the driver having had to put at risk the strength of the driveline.

Preferably, the load transfer system can only increase the pressure on the driving axle automatically up to a maximum pressure, although a driver of the vehicle may manually allow the load transfer system to exceed the maximum pressure.

This means that the driver can always be certain that the driving axle pressure will not become excessive, but there still remains, in particularly difficult situations such as an emergency, a possibility of providing the vehicle with grip by means of extra-high driving axle pressure.

The pressure on the driving axle is advantageously increased by reducing the proportion of the load which is supported by a loadbearing auxiliary axle, which is an easy way of increasing the driving axle pressure, particularly in the case of vehicles equipped with an air suspension system which can be used to raise or lower, for example, the loadbearing auxiliary axle.

The invention also relates to a device according to claim 4, with preferred embodiments according to claims 5-6, for implementing the aforesaid method.

DESCRIPTION OF THE DRAWING The invention will now be described with reference to the attached drawing, in which: Fig. 1 depicts schematically a motor vehicle as seen from above, comprising a device according to the invention.

DESCRIPTION OF EMBODIMENTS Fig. 1 depicts a motor vehicle 1, in this case a truck, with a bogie, with a driving axle 2 with two powered wheels 3,4 and a loadbearing auxiliary axle 5 with two supporting wheels 6,7. The vehicle 1 also comprises a drive power control system TC and a load transfer system ELC. The vehicle also comprises a supervisory system IDA for the two aforesaid systems TC, ELC. The broken lines represent communication paths between systems, and between systems and the vehicle. Communication may for example be via the can-bus in the vehicle.

The drive power control system TC (Traction Control) monitors whether either or both of the powered wheels 3,4 are slipping, in order to make it easier for the driver to deal with difficult situations, such as steep upgrade or running surface with poor grip. If either of the powered wheels 3,4 is slipping, the TC system increases the brake pressure on the slipping wheel, thereby transferring the braking power via the differential to the other wheel. When neither of the powered wheels is slipping any longer, the braking power is reduced again on the previously slipping powered wheel.

The load control system ELC (Electronic Level Control) uses a suspension system with air bellows in the vehicle to keep the vehicle at a desired level above the ground.

When load transfer occurs, the ELC system is used to make it easier for a driver to deal with difficult situations, such as steep upgrade or running surface with poor grip, by allowing the ELC system to increase the driving axle pressure. This is accomplished by the loadbearing auxiliary axle 5 being raised, thus reducing the proportion of load supported by the loadbearing auxiliary axle 5 and increasing the proportion of load supported by the driving axle 2, and hence the pressure on the driving axle 2. The factors which control the available drive power allowed by the running surface are the normal force and the running surface friction on the powered wheels 3,4. The normal force increases when the driving axle pressure increases, thereby increasing the available drive power. However, there are certain limitations, both legal and technical, on the extent to which the driving axle pressure can be increased. The loads on the driveline, which comprises the engine, gearbox, universal shaft and drive shaft, may become extremely large, with consequent risk of damage.

The IDA (Intelligent Drivepower Adaptation) system for supervising the two systems TC, ELC described above receives information from the drive power control system TC. If the information indicates that both of the powered wheels 3,4 are slipping, the IDA system automatically orders the load transfer system ELC to increase the driving axle pressure, and hence the available drive power, until the drive power control system TC no longer indicates that both of the powered wheels are slipping. The IDA system for supervising the other two systems TC, ELC thus links together these two other systems TC, ELC. The increase in the driving axle pressure was previously

controlled manually by the driver, who had difficulty in deciding how much load could be imposed on the driving axle without damaging the driveline. In the present invention, this is done automatically as necessary and the pressure is increased until the available drive power is sufficient to cause the vehicle to move on. The increase in the drive pressure is discontinued, however, when the pressure reaches a maximum permissible level. It is nevertheless possible for the driver, in particularly difficult situations such as emergencies, to use some kind of means 8, such as a control on the instrument panel, to remove a barrier and enable the load transfer system to increase the driving axle pressure further. It has then to be made clear to the driver, e. g. by some form of acoustic or light signal, that there is great risk of overloading the driveline and causing consequent damage.

The foregoing should not be regarded as limiting the invention but only as a possible embodiment of it. For example, the invention may be applicable to vehicles other than trucks, e. g. buses or passenger cars equipped with systems for drive power control and load transfer, and the load transfer system may use means other than the air suspension system. Nor need the drive power control system send information continuously to the supervisory system, as it may for example be sufficient that the drive power control system informs the supervisory system that both of the powered wheels are slipping.

The systems, which comprise electrical control systems and computer units, may each be situated separately, as illustrated in Fig. 1, or they may be built together with one another. Many other alternative solutions also fall within the scope of what is indicated in the ensuing claims.