b) Hydrodynamic Retarder
The hydrodynamic retarder (Figure above) works according to the functional principle of the Föttinger clutch.
The mechanical energy of the drive shaft is converted over the pump wheel into kinetic energy of the fluid. This energy is converted into heat in the turbine wheel which is rigidly fixed to the vehicle body. A cooling of the working fluid hence becomes necessary. The brake moment is controlled by the volumetric efficiency of the retarder. The brake effect of retarders can be very large, in heavy trucks greater than 3,000 Nm or 400 kW.
The retarder is usually positioned behind the transmission in the cardan-shaft loop. New developments integrate it directly into the transmission. In automatic commercial vehicle transmissions, the existing converter can be used as a retarder. This way, the retarder characteristic can to a large extent be varied by torque conversion in the transmission.
A relatively high constructional expense opposes low weight. The magnitude of the braking torque is independent of the service temperature. The braking heat can be dissipated over an oil-water heat exchanger to the engine cooling.