the engine and the recharging of the accumulator similarly the control unit controls the variable displacement of the second hydraulic machine 22 as a function of the required torque on the drive wheels 8 and the pressure difference between the high and low pressure circuits generating the torque when the combustion engine 2 stops driving continues in pure hydraulic mode due to the flow supplied by the pressure accumulator 24 which is indicated by arrow 52 without emission of polluting gas fig3 is a diagram of the powertrain in a serial operating mode arranged for recovering energy or for starting the combustion engine 2 during a deceleration of the vehicle or when driving downhill the kinetic energy of the vehicle can be recovered by adjusting the variable displacement of the second hydraulic machine 22 used as pump which is driven by the drive wheels 8 to recharge the pressure accumulator 24 as indicated by arrow 62 the variable displacement of the second hydraulic machine is adjusted as a function of the pressure difference between high and low pressure circuits in this way a braking torque is applied on the drive wheels 8 the power of which is modulated by regulation of the variable displacement hydraulic machine 22 energy also draws independently as indicated by arrow 60 from the pressure accumulator 24 for driving the first hydraulic machine 20 used as motor and for starting the combustion engine 2 when the vehicle starts driving or automatically after stopping.