710 Traction control system

With a conventional final drive differential the torque output from each driving wheel is always equal. Thus if one wheel is driven over a slippery patch, that wheel will tend to spin and its adjacent sun (side) gear will not now be able to provide the reaction torque for the other (opposite) sun (side) gear and driving road wheel. Accordingly, the output torque on the other wheel which still has a good tyre to surface grip will be no more than that of the slipping wheel and it doesn't matter how much the driver accelerates to attempt to regain traction, there still will be insufficient reaction torque on the spinning side of the differential for the good wheel to propel the vehicle forward.

One method which may be used to overcome this loss of traction when one wheel loses its road grip, is to simply apply the wheel brake of the wheel showing signs of spinning so that a positive reaction torque is provided in the differential; this counteracts the delivery to the good wheel of the half share of the driving torque being supplied by the combined engine and transmission system in terms of tractive effort between the tyre and ground.

To achieve this traction control an electronic control unit (ECU) is used which receives signals from individual wheel speed sensors, and as soon as one of the driving wheels tends to accelerate (spin due to loss of tyre to ground traction), the sensor's generated voltage change is processed by the ECU computer, and subsequently current is directed to the relevant traction solenoid valve unit so that hydraulic brake pressure is transmitted to the brake of the wheel about to lose its traction. As soon as the braked wheel's speed has been reduced to a desirable level, then the ECU signals the traction solenoid valve unit to release the relevant wheel brake.

7.10.1 Description of system (Fig. 7.43)

This traction control system consists of: an electric motor driven hydraulic pump which is able to generate brake pressure independently to the foot brake master cylinder and a pressure storage accumulator; a traction boost unit which comprises a cylinder housing, piston and poppet valve, the purpose of which is to relay hydraulic pressure to the appropriate wheel brake caliper and at the same time maintain the traction boost unit circuit fluid separate from the foot brake master cylinder fluid system; a pair of traction solenoid valve units each having an outlet and return valve regulates the cut-in and -out of the traction control; an electronic control unit (ECU) is provided and individual wheel speed sensors which monitor the acceleration of both driven and non-driving wheels. Should the speed of either of the driven wheels exceed the mean wheel speed of the non-driven wheels by more than about 20%, then the ECU will automatically apply the appropriate wheel brake via the traction solenoid valves and traction boost device.

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