New internal combustion engines (ICE) are characterized by increasing maximum efficiency, thanks to the adoption of strategies like Atkinson cycle, downsizing, cylinder deactivation, waste heat recovery and so on. However, the best performance is confined to a limited portion of the engine map. Moreover, electric driving in urban areas is an increasingly pressing request, but battery electric vehicles use cannot be easily spread, due to limited vehicle autonomy and recharging issues. Therefore, hybrid propulsion systems are under development, in order to reduce vehicle fuel consumption, by decoupling the ICE running from road load, as well as to permit energy recovery and electric driving. This paper analyses a new-patented solution for power split hybrid propulsion system with gearbox. The system comprises an auxiliary power unit, adapted to store and/or release energy, and a planetary gear set, which is interposed between the ICE and the gearbox. A further device, coupled with the ICE to modulate the resistance torque, in order to use the auxiliary power unit also for regenerative braking, characterizes the system. The choice to maintain a gearbox in the propulsion gives the possibility to achieve different purposes. It allows propelling the vehicles only with ICE, also during the initial motion, maximizing the direct use of mechanical energy. Moreover, it requires only one electric motor/generator (MG), unlike the widespread power split systems, which use two electric motors, for managing the vehicle speed and traction torque. Finally, there are other interesting features of the patented system, as the possibility of operating, only with the ICE in the case of failure of the auxiliary power unit, or keeping the driving style of vehicle with gearbox.