Torque request during wheelspin
If one of the drive wheels exceeds the specified limit of wheel spin, the TC/ABS control module will request a lower engine torque from the engine control module. The engine control module will then regulate the throttle in the throttle body to provide a rapid reduction of torque. The boost pressure will subsequently be reduced if necessary.
Torque request during engine braking
During engine braking, it is possible that the speed of the front wheels becomes lower than the speed of the rear wheels. If the vehicle speed exceeds 25 km/h and the engine torque is below -50 Nm, the TC/ABS control module will request increased torque to maintain directional stability.
There are two levels for the request for increased torque
- If the speed of the front wheels is between 2 and 5 km/h lower than the rear wheels, the TC/ABS control module will request an engine torque of -10 Nm.
- If the speed of the front wheels is more than 5 km/h lower than the rear wheels, the TC/ABS control module will request an engine torque of +10 Nm.
Higher torque is not requested at speeds below 25 km/h.
Scheme 150
The return pump is utilized in different ways, depending on whether the TC/ABS control module is working in ABS mode or TCS mode. The function of the pump when the control module is in TCS mode will be described here.
The return pump is continuously active during TCS modulation with braking.
Pressure is built up by closing the pressure relief valve and opening the pressure increase valve to supply the return pump with brake fluid. The control module closes the pressure increase valve when a preset pressure is reached.
The pressure is then modulated as the control module opens the pressure increase valve to increase the pressure, keeps the pressure increase and pressure relief valves closed (to maintain pressure) and opens the pressure relief valve to reduce the pressure. The surplus brake fluid resulting from opening the pressure relief valve is returned to the master cylinder.
The return pump is described more fully in .
Scheme 151
The TCS function in the TC/ABS control module employs both reduction of engine torque and 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. When either of the drive wheels rotates at a higher speed than the rear wheels, this is described as wheelspin. The magnitude of this wheelspin 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 wheelspin is always allowed. This varies with the speed of the car, the friction between tyres 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.
Scheme 152
When starting on a slippery surface the following will happen
The drive wheel having the lowest friction begins to spin first. When the wheelspin reaches 8.5 km/h (5.2 mph), TCS modulation starts by applying the brakes.
When the wheel is braked, additional tractive effort is transferred to the other wheel which still has a grip on the road. If the road surface does not have much friction, the other wheel may also start to spin and when it reaches 6.5 km/h (4 mph), the TC/ABS control module sends a request to the Trionic for engine torque limitation which then prevents further wheelspin.
The optimum combination of tractive power and steering ability is thus achieved as well as the same traction as with the use of a differential brake.
The upper limit for permitted wheel spin before modulation is engaged then gradually drops until a vehicle speed of 22 km/h (13.5 mph) is attained and then increases again for engine torque limitation and brake application.
A relatively high degree of wheelspin is accepted when starting in order to retain the feeling of sportiness and also enable the wheels to dig themselves down to a firmer surface whenever conditions make this necessary.
Scheme 153
At 22 km/h (13.5 mph), the TCS function changes its way of working and starts to modulate using engine torque limitation of the wheel that first starts to spin, i.e. the wheel with the lowest friction.
The limit for modulation by brake application lies somewhat higher, which is to say that braking is used on the spinning wheel when engine torque limitation is inadequate.
Braking is used at speeds up to 100 km/h (62 mph).
Primary limitation of engine torque means that no extra tractive force is transferred to the outer wheel when cornering. The outer wheel then has a sufficient margin of grip to take up steering forces to the full. The driver avoids being taken by surprise as the front wheels suddenly lose traction.
At 22 km/h (13.5 mph), wheelspin is allowed to reach 2 km/h (1.2 mph) or about 9%. After that, the curve rises in a straight line to 150 km/h (93 mph) where a wheelspin of 5.2 km/h (3.2 mph) or about 3.5% is allowed. At 150 km/h (93 mph), the curve breaks again and permissible wheelspin will follow the increase in speed proportionally at about 3.5%.
Scheme 154
The valve block, which is an integral part of the hydraulic unit, modulates the brake pressure to the calipers when ABS braking and TCS modulation with braking are activated.
The valve block contains 12 solenoid valves: four inlet valves, four outlet valves, two pressure increase valves and two pressure relief valves. These are arranged as follows: one inlet and one outlet valve for each of the four wheels and one pressure increase and one pressure relief valve for each of the front wheels. A description of the inlet and outlet valves and their accumulator chambers is given under the heading "Valve block, ABS function". The valves concerning the TCS function are described here.
The pressure increase valve is normally closed but is opened by the control module to supply the return pump with brake fluid for building up pressure in connection with active TCS modulation.
The pressure-reducing valve is normally open but will be closed during pressure build-up and pressure maintenance, and opened for reducing pressure to maintain the brake pressure, calculated by the control module, at the wheel concerned in connection with active TCS modulation.
The valve block contains 2 check valves for the TCS function. If the driver brakes while TCS modulation is in progress, the TCS function with braking will be disengaged and the braking function will be chosen instead. During the delay which may arise, the check valve mounted parallel with the pressure-reducing valve prevents the braking function from being affected because the check valve leads the brake pressure out to the wheel.
The check valve mounted after the outlet valve prevents the accumulator from being filled with brake fluid.
The return pump is continuously active during TCS modulation.
See also:
• SELF-DIAGNOSTICS