Stored braking energy saves fuel

Storing braking energy in a hydraulic accumulator, then releasing it to assist acceleration, could improve heavy vehicle fuel economy by up to 35%. The system is called the Regenerative Drive System (RDS). In its basic form, the driveshaft is equipped with a pump/motor. When a vehicle is braking, braking energy is used to pump oil into a hydraulic accumulator, pressurised with nitrogen. When the vehicle subsequently accelerates, the compressed gas in the accumulator pushes oil through the pump/motor to assist the engine by adding energy to the drive train. The control system simultaneously reduces engine output while the stored energy is returned in order to maximise fuel saving.

Source: 26 March 2007