What is the working principle of a Hydrodynamic Retarder (e.g. Voith / ZF Intarder), and how is the generated braking energy dissipated?

1. Oil is pumped between a rotating impeller (rotor) and a stationary housing (stator); fluid friction decelerates the rotor, converting kinetic energy into heat, which is transferred directly into the engine's coolant circuit via a heat exchanger
2. Friction brake pads press against a water-cooled rotor disc at each wheel hub
3. Electromagnets reverse electric current into the vehicle battery
4. Compressed exhaust air is blasted into the turbocharger to stall the engine