TCS function: Notes
The TCS function in the TC/ABS control module both reduces engine torque (request to Trionic) and applies the brakes on the driving wheels so that the surface friction of the road is utilized optimally under 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. When either of the drive wheels rotates at a higher speed than the rear wheels, this is described as wheel spin. The magnitude of this wheel spin and the speed of the car are decisive for the manner in which the system works.
Traction is given priority at speeds below 22 km/h (13.5 mph) and the system then works primarily by braking, the purpose being to give a certain amount of braking torque to the drive wheel having the lowest friction (the wheel that first spins).
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.
At higher speeds, priority is given to the transfer of lateral force so that steering control is maintained. The system then works primarily by limiting engine torque.
So that the sporty feel and handling of the car will still be retained when driving on slippery roads, and so that the available friction will be utilized to the full, a certain degree of wheel spin is always allowed. This varies with the speed of the car, the friction between tires and road surface, and how "aggressively" the car is being driven (position of throttle pedal).
If the driver brakes during TCS modulation, the TCS function with brake application will be turned off and the braking function will be chosen. The TC/ABS control module will continue with TCS modulation using engine torque limitation.