The propulsion system comprises a heat engine, a transmission unit for transmitting to the vehicle wheels power supplied by the heat engine, an auxiliary power unit for storing and supplying energy, a planetary gear set interposed between the heat engine and the transmission unit and connected to the auxiliary power unit, a locking device associated with the planetary gear set and shiftable between a first position, wherein the auxiliary power unit does not exchange power with the planetary gear set, and a second position, wherein the auxiliary power unit exchanges power with the planetary gear set, and a braking device which is interposed between the planetary gear set and the heat engine and is configured to modulate the motion resistance torque of the heat engine.