TCS function: Notes

Fig 1: Identifying TCS Function Graphical Representation (Phases 1-3)

Courtesy of SAAB-SCANIA OF AMERICA, INC.

The TCS function in the control module employs both reduction of engine torque (request to engine control module) brake application on the driving wheels in order to utilize the friction of the road surface in all driving conditions.

To understand the TCS function it is best to assume that the road surface is slippery, with varying degrees of friction under the two drive wheels.

The rotational speed of the rear wheels is used as a reference for comparing the speed of the drive wheels individually. Wheelspin is described as the result of when either of the drive wheels rotates at a higher speed than the rear wheels. The magnitude of this wheelspin and the speed of the car are decisive with regards to system functionality.

At low speeds, traction is given priority and the system operates primarily by braking. The purpose of this is to provide a certain amount of braking torque to the drive wheel having the lowest friction (the wheel that spins first).

This allows more power to be transferred to the other drive wheel, which then has maximum traction. Through this distribution of the power applied to the wheels, the available friction can be utilized to maximum effect.

Wheelbrakes provide a quick and forceful way to retard spinning wheels while engine torque limitation is the gentler, more comfortable way. These processes often operate simultaneously and certain driving cases demand braking, for example starting on an incline with different friction on both drive wheels. Braking is applied to the wheel with the least friction to such a degree that the traction is transferred to the other drive wheel, which achieves better traction due to the differential brake.

A certain degree of wheelspin is always allowed so that the sporty feel and handling of the car will still be retained. This varies with the speed of the car, the friction between the tyres and the road surface and how "aggressively" the car is being driven (position of the accelerator pedal).

If the driver brakes during TCS modulation, the TCS function with brake application will be switched off and braking function will be selected. The control module will continue TCS modulation using engine torque limitation.