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

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

Main article: Engine braking

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.

This type of retarder is known to North American heavy vehicle drivers as a Jake brake, named after such a system produced by the Jacobs Manufacturing Company. A disadvantage of this system is that it is very noisy in operation, such that some stretches of road ban its use.

The exhaust brake

Main article: Exhaust brake

The exhaust brake is simpler in operation than an engine brake. Essentially, the exhaust pipe of the vehicle is restricted by a valve. This raises the pressure in the exhaust system, forcing the engine to work harder on the exhaust stroke of its cylinders, so again the engine is acting as an air compressor, with the power required to compress the air being taking from the transmission, and therefore retarding the vehicle. A disadvantage of this system is that the exhaust pipe has to be engineered to accommodate the high pressures generated by this method of retardation.

The hydraulic retarder

Hydraulic retarders use the viscous drag forces between dynamic and static vanes in a fluid-filled chamber to deliver their retardation. There are several different types which can use standard transmission fluid (oil), separate oil, or water.

A simple retarder would use vanes attached to a transmission driveshaft between the clutch and roadwheels. They can also be driven separately via gears off a driveshaft. The vanes would be enclosed in a static chamber with small clearences to the chamber's walls (which will also be vaned), as in an automatic transmission. When retardation is required, fluid (oil or water) is pumped into the chamber, and the viscous drag induced will slow down the vehicle. The working fluid will heat up, and will usually be circulated through a cooling system. The degree of retardation can be varied by adjusting the fill level of the chamber.

Hydraulic retarders are extremely quiet in operation compared to engine brakes.

The electric retarder

Main article: Electromagnetic brake

The electric retarder uses electromagnetic induction to provide a retardation force. An electric retardation unit can be placed on an axle, transmission, or driveline and consists of a rotor attached to the axle, transmission, or driveline and a stator securely attached to the vehicle chassis. There are no contact surfaces between the rotor and stator, and no working fluid. When retardation is required, the electrical windings in the stator are powered up from the vehicle battery, producing magnetic fields alternating in polarity for the rotor to move in. This induces eddy currents in the rotor, which slows down the rotor, and hence the axle, transmission or driveshaft to which it is attached. The rotor is engineered to provide its own air-cooling, so no load is placed on the vehicles cooling system, and the operation of the system is extremely quiet.