Vacuum servo
The brake booster ( BKV ) enables the reduction of the operating force to the brake of a vehicle, which is required to achieve the desired braking effect.
In the passenger cars and light commercial vehicles primarily built vacuum brake booster the assist force is generated (atmospheric pressure to low pressure) by means of a pressure difference. For medium to heavy duty vehicles ( generally from 7.49 t ), such as a truck, the braking force is generated by spring force ( power brake system ), which controlled loosening with compressed air ( pneumatic ). The operational pressure is at about 8 bar; the pedal force is less than the spring pressure. Also, hydraulic or electric brake booster are possible.
Vacuum generation
Gasoline engines
Vehicles with gasoline engines require at part load the use of a throttle valve to generate a combustible fuel-air mixture. As a side effect is created in the intake manifold after the throttle valve ( intake manifold ), a negative pressure which serves as a power for BKV.
Some of the current gasoline engines with direct fuel injection, such as BMW vehicles with Valvetronic, is the systemic elimination of the throttle a separate suction pump or vacuum pump (also vacuum pump) as required in diesel engines. In the connecting line between BKV and vacuum source, a check valve is installed, which serves to maintain the negative pressure at full load and the engine stopped.
Diesel engines
A diesel engine has inherently no throttle. Here, the negative pressure by a suction pump or vacuum pump is generated (also vacuum pump). This vacuum represents a pressure differential with the outside air, which can be exploited in the brake booster to increase the braking force.
Function
As in conventional brake systems without booster the brake pedal acts directly on the master cylinder that forces the brake fluid in the piping system to the brakes (see Pascal 's law). This pressure is supported by a working diaphragm, both sides of which are ( at rest ) under vacuum. By the actuation of the brake pedal is the pedal -directed side of the membrane at atmospheric pressure is applied via a valve, so that due to the prevailing pressure difference, a force which supports the applied braking force on the pedal in the same direction. Due to the relatively small pressure difference, the surface of the membrane must be relatively large in order to achieve a sufficiently high dynamic effect. Accordingly, the housing of the brake booster has a diameter of up to 11 inches (note: about 28 cm, the diameter at BKV is always given in inches. ). In a small space (eg when Smart) also called tandem BKV are used, in which is provided by two " connected in series " brake booster for the necessary surface.
F = force [ N]
P = pressure [ Pa], here the pressure difference between the atmosphere (about 101.300 Pa) and generated negative pressure
A = area [m² ]
The design of the valves in the brake booster, the auxiliary force of the piston is always proportional to the pedal force. Without vacuum, for example when towing the car without the engine running, the pedal force must apply the braking force alone. In order to achieve the same braking action is performed with power brakes, a much larger force is then necessary, which greatly exceeds that in the rule that power which can muster with his leg muscles of the drivers.
The function of the brake booster is tested as follows: Turn off the engine, press brake pedal several times until a strong resistance is felt, therefore, the existing still in the system vacuum " used up". Keep it pushed the brake pedal, start the engine. When the brake pedal now gives way, the amplifier is in order.
History
Until well into the 1980s different (smaller) motor vehicles, such as the Fiat Panda, the Volkswagen Beetle, the VW Polo or Renault Rapid were still made without this component. Either the brakes themselves were carried out, for example, as a self-energizing drum brake or pedal had a size that the pedal force was sufficient even without brake booster.