A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems, usually on heavy vehicles. Retarders serve to slow vehicles, or maintain a steady speed while traveling down a hill, and help prevent the vehicle from "running away" by accelerating down the hill. They are not usually capable of bringing vehicles to a standstill, as their effectiveness diminishes as vehicle speed lowers. They are usually used as an additional "assistance" to slow vehicles, with the final braking done by a conventional friction braking system. As the friction brake will be used less, particularly at higher speeds, their service life is increased, and since in those vehicles the brakes are air-actuated, it helps to conserve air pressure too. Friction-based braking systems are susceptible to "brake fade" when used extensively for continuous periods, which can be dangerous if braking performance drops below what is required to stop the vehicle – for instance if a truck or bus is descending a long decline. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based. Retarders are not restricted to road motor vehicles, but may also be used in railway systems. The British prototype Advanced Passenger Train (APT) used hydraulic retarders to allow the high-speed train to stop in the same distance as standard lower speed trains, as a pure friction-based system was not viable. Engine brake Diesel-powered vehicles Diesel engines regulate power output purely by the volume and timing of fuel injected into the combustion chambers. The engine braking generated by creating partial vacuum with a closed throttle at each intake stroke in petrol/gasoline engines does not apply to vehicles equipped with diesel engines, as such engines are quite "free-running". However Clessie L. Cummins, founder of Cummins Engine Company, realized 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 coming from the transmission used to compress the air, hence slowing 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. In a compression