Antilock Braking System

Function

The electronic antilock braking system should avoid heavy slipping of the wheels, keeping the car manoeuvrable.

How it works

The electronic antilock braking four port system consists of two magnet valves (see figure) per wheel brake cylinder and two recirculation pumps with one drive. The two recirculation pumps, both driven by one electric engine, are necessary, because the two hydraulic cycles of a dual circuit brake system have to be separated. The pumps have a low throughput, but may temporarily obtain very high pressures of up to 150 bars. Out of 8 magnet valves normally 4 are closed and 4 are open; the springs of the magnet valves ensure that this holds in case of a power failure too. The system operates like a normal dual-circuit brake system.
If the difference of the number of revolutions between the wheels - while braking - is too large, the pipe to the wheel brake cylinder of the slower or locking wheel is blocked with a magnet valve. If this is not sufficient to tackle the difference in numbers of revolution, the pressure is decreased by opening the second magnet valve. Simultaneously, the recirculation pump starts, pumping the brake fluid into the pipe which connects the master cylinder with the magnet valve.
The driver feels this pumping as pulsation in the brake pedal. It may occur up to ten times per second. Naturally, the reaction time of the braking system at the wheel is much too slow to perform a full standstill or a full easing of the brake in accordance with the fast pulsation. In practice the swing will be between 20 and 35 percent slip. The front wheels are always regulated separately, for the rear ones it varies. With a pneumatic brake there is no necessity for a recirculation pump, because the assistant system always provides enough air pressure.

At the front suspension the wheels are always regulated separately, for the rear suspension it varies. With an air brake there is no necessity for a recirculation pump, because the assistant system always provides enough air pressure. The released air is replaced imediately.