drives a variable displacement pump 4 the variable displacement pump 4 pumps the hydraulic fluid from a tank 6 in order to fill a pressure accumulator 8 the pressure accumulator 8 has a flexible membrane 12 separating a pressurized gas 10 generally containing nitrogen from the fluid which is maintained continually at a pressure approximately equivalent to that of the gas the pressurized fluid is then used by a variable displacement hydraulic machine 20 operating as motor which is connected to a differential 22 and which drives the two driving wheels 24 of a given axle of the vehicle distributing the power between these wheels in contrast during braking of the vehicle the driving wheels 24 can drive the hydraulic machine 20 operating as pump in order to refill the pressure accumulator 8 and thus to recover kinetic energy of the vehicle during the braking phases or when going downhill in order to optimize the functioning of the internal combustion engine 2 particularly in order to reduce the number of start ups of this engine and to deliver high power during its periods of operation one seeks to store the largest possible quantity of energy in the pressure accumulator 8.