Brake booster
The brake booster (BKV) allows to reduce the operating force on the brake of a vehicle , which is required to achieve the desired braking effect.
In the vacuum brake booster, which is predominantly installed in passenger cars and light commercial vehicles , the auxiliary power is generated by means of a pressure difference (atmospheric pressure to vacuum). For medium to heavy commercial vehicles (usually from 7.49 t) such as B. Trucks , the braking force is generated by compressed air ( pneumatics ) (external power brake system ). Only the parking or hand brake is actuated there by means of spring force and opened by compressed air. The operating pressure is always in a defined, regulated range, e.g. B. between 7.2 and 8 bar . Hydraulic or electric brake boosters are also possible.
history
The first cars with brake boosters include the Mercedes 300 SL and 300 b as well as the Borgward P 100 . At that time the system was as Bremshelf or brake Help called. It spread in the course of the 1960s, initially in higher-quality cars, and later in all vehicle classes. Until well into the 1980s, smaller motor vehicles such as the Fiat Panda , the VW Beetle , the VW Polo , the VW Golf or the Renault Rapid were still manufactured without brake booster. Either the brakes themselves were z. B. designed as a self- energizing drum brake or the pedal travel was correspondingly large so that the pedal force was sufficient even without a brake booster. One of the few advantages of vehicles without a brake booster is that the usual full braking power is available even when the engine is switched off.
Vacuum booster
functionality
As in conventional brake systems without a booster, the brake pedal acts directly on the master brake cylinder , which presses the brake fluid into the line system to the brakes (see Pascal's law ). This pressure is supported by a working diaphragm , both sides of which (in the rest position) are under negative pressure . By actuating the brake pedal, the pedal-facing side of the membrane is subjected to atmospheric pressure via a valve , so that the pressure difference then creates a force that supports the force applied to the pedal in the same direction. Due to the relatively small pressure difference, the area of the membrane must be relatively large in order to achieve a sufficiently high force effect (force equals pressure difference times membrane area). Accordingly, the housing of the brake booster has a diameter of up to 11 inches (approximately 28 cm). If the installation space is small (e.g. with the Smart ), so-called tandem BKVs are also used, in which two brake boosters "connected in series" provide the necessary membrane area.
The valves in the brake booster are designed so that the auxiliary force of the piston is always proportional to the pedal force. Without negative pressure, for example when towing the car without the engine running, the pedal force alone has to generate the braking force. In order to achieve the same braking effect as when the brake booster is working, a much greater effort is then required, which as a rule considerably exceeds the force that the driver can apply with his leg muscles.
Negative pressure generation
In conventional gasoline engines , a throttle valve is used to reduce the pressure in the intake tract ( intake manifold ) at partial load so that less fuel-air mixture enters the cylinders. This negative pressure supplies the pneumatic energy for the brake booster. A separate vacuum pump (suction pump) is required for diesel engines and more modern gasoline engines with variable intake timing, such as the Valvetronic from BMW or the Multiair system from Fiat, which no longer have a throttle valve . A check valve is installed in the line between the brake booster and the vacuum source, which is used to maintain the vacuum at full load and the engine is not running.
Functional test
The function of the brake booster is checked as follows: Switch off the engine, press the brake pedal several times until a strong resistance can be felt, ie the vacuum still present in the system is "used up". Keep the brake pedal depressed, start the engine. If the brake pedal now gives way, the amplifier is OK. A leaky membrane can be recognized if the motor is switched off while the brake is depressed: If the pedal comes up against the leg force after a short time, the membrane is leaking. In vehicles that take the negative pressure from the intake tract, this error can influence the lambda control of the engine and, in the event of severe leaks, adjust it to the control limit, since the air sucked in here does not flow through the air mass meter (this is also called "false air"). In such a case, the engine control unit stores an error message relevant to the UMA , the cause of which is not always easy to find, even for specialist workshops, due to its rarity.
See also
literature
- Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology . 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4
Individual evidence
- ^ Wolfgang Appel; Hermann Brähler; Ulrich Dahlhaus; Jochen Gräfenstein ;: Commercial vehicle technology basics, systems, components . 2013th edition. Springer-Verlag, (Berlin Heidelberg New York), ISBN 978-3-322-96930-9 , pp. 121 .
