Retarder

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A retarder is a device that enables long-term and wear-free braking without decreasing its braking performance.

While a normal service brake is not suitable for continuous operation and tends to overheat during prolonged operation and thus to reduce braking or even to brake failure ( fading ), the continuous brake has a supporting function to relieve the service brake, for example when driving downhill for long periods. However, the permanent brake cannot bring the vehicle to a complete standstill; this is done solely by the service brake. According to German regulations (StVZO §41 Paragraph 15), the permanent brake must at least be sufficient to hold the fully loaded vehicle on a distance of 6 km and a gradient of 7% at a speed of 30 km / h.

As a rule, the permanent brakes are activated via the service brake during normal braking. If the driver presses the brake pedal lightly, only the retarder is usually activated in the first stage; the mechanical brakes only take effect when the brakes are applied more strongly. However, all systems can also be operated separately using a foot switch or stepped hand lever (motor brakes) or a stepped hand lever (retarder).

One differentiates:

  • Engine brakes (a pure engine brake does not meet the regulations according to § 41 (15) StVZO). By closing the intake flap, the flow in the intake duct is interrupted and thus a drag is achieved; this applies to all gasoline engines . The normal engine brake has a braking power of 5 to 7 kW per liter of displacement - see also the main article engine brake .
    • Engine dust brake with exhaust flap . The exhaust line is closed by means of a flap using a foot-operated solenoid valve. At the same time, the injection system is brought to zero delivery (stop position). This variant is most frequently used in trucks / buses and achieves a braking power of 14–20 kW per liter of displacement.
    • Engine vacuum brake with exhaust flap and constant throttle . Here, the targeted decompression of the 2nd and 3rd work cycle is also used for braking performance by a valve. The braking power that can be achieved is 30–40 kW per liter of displacement.
  • Hydrodynamic retarder or flow brake. A Föttinger coupling on the cardan shaft converts the mechanical energy into thermal energy of the fluid. The heated hydraulic oil needs to be cooled by a heat exchanger. The retarder is controlled by a pump. The disadvantage is that the braking power is heavily dependent on the shaft speed (see also Föttinger principle ).
  • Electromagnetic retarder or eddy current brake . A rotor (iron disk), which usually sits on the cardan shaft (it can also be installed in the axles of a trailer), rotates in an adjustable magnetic field and is slowed down by the eddy currents that arise . If there is a risk of overheating, the wind-cooled excitation coils are partially switched off. The braking torque depends on the size of the rotor coils. A retarder can achieve a braking torque of up to 3,000 Nm, comparable to a constant throttle.

The various retarders are divided into primary and secondary retarders .

Primary retarders work via the engine's crankshaft and are therefore speed-dependent. The primary retarders include exhaust flap systems, valve systems, turbo systems and aquatarders.

Secondary retarders act on the cardan shaft and are therefore dependent on the speed. The secondary retarders include the hydrodynamic flow brake and the electromagnetic eddy current brake.

swell

StVZO §41 brakes and wheel chocks

literature

  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3
  • Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4