Part 2

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 tires and the road surface and how "aggressively" the car is being driven (position of the accelerator pedal).

If the driver brakes while TCS regulation is in progress, the control module will continue TCS regulation with engine torque limitation.

TCS regulation
When starting on a slippery surface, the following will happen:

The drive wheel with the lowest friction will start to spin first. When wheel spin is detected through the wheel sensors, a request is sent via the bus to the engine control module (ECM) for engine torque limitation to modulate the wheel spin.

This procedure will achieve the best traction and steering force and provide accessibility that compares with the use of a differential brake. A limitation of engine torque also means that no extra traction will be transferred to the outer wheel when cornering so it will have a reserve remaining for absorbing steering forces. The driver will not be surprised by the front end suddenly loosing grip.

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.

TCS function with ESP control module

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 degree 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.

Wheel brakes 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 tires 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.

Low speed

When starting on a slippery surface, the following will happen:

The drive wheel having the lowest friction begins to spin first. When wheelspin is detected by the control module through the wheel sensors, TCS modulation starts by applying the brakes.

When the wheel is braked, additional tractive power 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 begin to spin. When wheelspin occurs, the control module will send an engine torque limitation request to the ECM 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.

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.

High speed
At speeds higher than 40 km/h (25 mph), the TCS function changes its mode of operation and starts modulation with engine torque limitation of the wheel that first starts to spin, i.e. the wheel with the lowest friction.

Modulation with brake application is used to brake the spinning wheel when engine torque limitation is inadequate.

Braking is used at speeds up to 40 km/h (25 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.

TCS regulation with ESP system
Since TCS modulation braking occurs without depression of the brake pedal, the hydraulic unit must build up and modulate the pressure to the driven wheel which is to be braked if it develops wheelspin in excess of the permitted limit.

1 Pressure Increase Valve Front Left
2 Pressure Increase Valve Front Right
3 Pressure Relief Valve Left Front
4 Pressure Relief Valve Front Right
5 Pump
6 Wheel sensor, front left
7 Wheel sensor, front right
8 Wheel sensor, rear left
9 Wheel sensor, rear right
10 Inlet valve, front left (ABS)
11 Inlet valve, front right (ABS)
12 Inlet valve, rear left (ABS)
13 Inlet valve, rear right (ABS)
14 Outlet valve, front left (ABS)
15 Outlet valve, front right (ABS)
16 Outlet valve, rear left (ABS)
17 Outlet valve, rear right (ABS)
18 Accumulator chamber
19 Pressure chamber

The control module performs TCS modulation with braking when the following criteria are met

^ The driver does not depress the brake pedal, the brake light switch input is not active.
^ Road speed is below 40 km/h.
^ One of the driven wheels is spinning more than the permissible limit.

When TCS modulation with braking occurs, the vacuum servo, primary piston and secondary piston are in their rest positions. In the valve block the inlet and outlet valves are in their rest positions, that is to say the inlet valves are open and the outlet valves are closed.

The control module starts the pump while the pressure relief valve is closed in order to allow pressure build-up to occur. Pressure is built up by opening the pressure increase valve in order to supply the return pump with brake fluid. The control module closes the pressure increase valve when the pre-determined pressure has been reached. The pressure is then modulated as the control module opens the pressure increase valve to increase pressure, keeps both the pressure increase and pressure relief valves closed to retain pressure and opens the pressure relief valve to reduce pressure. The excess brake fluid resulting from opening the pressure relief valve is returned to the master cylinder.

When TCS modulation starts, a "filling pulse" is generated, which means that a small amount of pressure (same for both wheels) is built up in the wheel cylinders for both driven wheels irrespective of the wheel for which modulation with braking will occur.

Pressure will be maintained on the front wheel for which braking will not occur, i.e. both the pressure increase and pressure relief valves will be closed. This is in order to prepare for an application of the brakes if necessary.

The pressure increase valve on the wheel which is spinning will open. This causes an increase in the pressure to the wheel cylinder and the wheel is braked.

Modulation continues until:

^ The friction between wheels and road surface changes so that wheelspin does not exceed the limit.
^ The driver brakes (brake lights switch input active).
^ Modulation is interrupted due to the risk for overheating the brakes.

When modulation is interrupted, the control module stops the pump, closes the pressure increase valve and opens the pressure relief valve, i.e. the valves and the return pump resume their rest positions.

If TCS modulation is interrupted by the driver braking, TCS modulation with braking will be disengaged and the braking function will be chosen instead. Any delay that may then arise will not affect the braking function since the brake fluid can flow through the check valve which is mounted parallel with the pressure relief valve.

Interruption of TCS modulation with braking due to a risk for overheating the brakes occurs as a result of continuous recording of the aggregate duration of TCS modulation with braking over a certain period of time by the control module.

The time is compared with a pre-programmed maximum value and modulation with braking is disengaged when this is exceeded.

Braking phases

Phase I Pressure build-up on both front wheels (filling pulse and holding pressure) The driver does not brake.

Filling pulse
When TCS modulation starts, a "filling pulse" is generated, which means that a small amount of pressure (same for both wheels) is built up in the wheel cylinders for both driven wheels irrespective of the wheel for which modulation with braking will occur. The pump starts, the pressure relief valve closes and the pressure increase valve opens (pre-determined amount of time).

The inlet and outlet valves in the valve block are in their rest positions, i.e. the inlet valves are open and the outlet valves are closed.

Holding pressure
When the filling pulse is concluded, the respective inlet valves close in order to prepare for individual modulation for each wheel.

For wheels which will not be braked with TCS modulation: Pressure holding will occur by closing the pressure increase and pressure relief valves at each circuit and closing the inlet and outlet valves at each wheel. This is to prepare for possible application of the brakes.

Phase 2 Pressure increase in the circuit with wheelspin
The pressure increase valve opens in order to supply the pump with brake fluid, which is pumped into the circuit, and the pressure relief valve is closed. The inlet valve for the wheel to be braked is opened until the conditions for pressure build-up have been met.

This means that the pressure out to the wheel cylinder is also increased and the wheel is braked. The pump is running.

Phase 3 Pressure reduction in the circuit with wheelspin

The pressure increase valve is closed and the pressure relief valve is opened in order to relieve pressure in the circuit.

The inlet valve is closed and the outlet valve is opened in order that the pressure out at the wheel is reduced.

Brake fluid is returned from the wheel outlet valve through the pressure relief valve out to the master cylinder. The pump is running.

Phase 4 Release
The criteria for TCS modulation have ceased. All valves resume their rest positions, i.e. the pressure relief valves are open, the pressure increase valves are closed, the inlet valves are open and the outlet valves are closed. The pump stops.

ESP function