Driveline & Transmission

A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems of (usually) heavy vehicles. more...

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Friction-based braking systems are susceptible to 'fade' when used extensively, and this can become dangerous if the braking performance drops below that required to stop a vehicle -- for instance if a lorry or coach is descending a long incline. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based.

Retarders are not restricted to road vehicles, but may also be used also in railway systems. The British prototype Advanced Passenger Train used hydraulic retarders to allow the high-speed train to stop in the same distance as standard lower speed trains, as a purely friction-based system was not viable.

Retarders serve to slow vehicles down, or maintain a steady speed on inclines. They are usually not capable of bringing vehicles to a standstill, as their effectiveness diminishes at low speeds. They are usually used to slow vehicles down, with the final braking being carried out by a friction brake. As the friction brake does not then need to be used so much, particularly at higher speeds, the service lifetime of friction brakes is enhanced.

The engine brake

Petrol-engined vehicles

Most petrol-driven car drivers are familiar with the use of so-called engine braking to descend inclines: put the car in low gear and do not use the accelerator. The retardation effect is not caused by friction in the engine (although that does make a contribution), but by the fact that with the throttle closed, air cannot enter the cylinder on the intake stroke of the pistons. Essentially, a partial vacuum is being created at each intake stroke, and the energy required to create this partial vacuum comes from the transmission, hence retarding the motion of the vehicle.

Diesel-engined vehicles

Diesel-engined vehicles do not have a throttle, as they regulate power output purely by the volume of fuel sprayed into the cylinders, so the engine braking generated by creating partial vacua at each intake stroke in petrol engines does not apply to diesel engined vehicles -- they are quite 'free-running'. However Clessie M. Cummins, founder of Cummins Engine Company, realised that by opening the cylinder exhaust valves when the piston reached top dead centre, rather than at the end of the power stroke the accumulated compressed air in the cylinder could be vented before it could act as a 'spring' to drive the piston back down again. By doing this, the engine acts as an air compressor, with the energy used to compress the air coming from the transmission, hence retarding the vehicle. The amount of power extracted from the transmission can be up to 90% of the rated power of the engine for certain engines.

Read more at Wikipedia.org