Retarder (mechanical engineering)

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

An additional benefit retarders are capable of providing is an increase in the service life of the friction brake, as it is subsequently used less frequently, particularly at higher speeds.

Diesel engines regulate power output purely by the volume and timing of fuel injected into the combustion chambers.

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.

When retardation is required, fluid (oil or water) is pumped into the chamber, and the viscous drag induced will slow the vehicle.

When retardation is required, the electrical windings in the stator receive power from the vehicle battery, producing a magnetic field through which the rotor moves.

A hybrid vehicle drivetrain uses electrical retardation to assist the mechanical brakes, while recycling the energy.

Regenerative braking might not be classified as a retarder as it uses no extra physical hardware in addition to the existing rotor/stator pair of the motor.

It effectuates braking by using the electric field created by the rotational inertia in the rotor/stator that is delivered into the rotor by the momentum of the vehicle(wheels).

In contrast, eddy current retarder brakes comprise a distinct and purpose-built static armature and rotor that are explicitly made and added to a vehicle for braking and dissipation of heat and not for motive power; it is a purpose-built system distinct from the motor.

Whereas an eddy brake relies on eddy currents to create magnetic resistance some of which is incidentally dissipated as heat, rheostatic braking relies on controller circuitry resistors which directly dissipate current-borne electric energy as heat.

In regenerative braking, the electric current created is typically fed back into the power supply (i.e. overhead catenary, third rail), and can be used by other locomotives or stored for later use.