The search for clean automotive propulsion systems has brought to study, besides the full electric solution and the traditional hybrid schemes, a family of new hybrid powertrains, called SIPRE and introduced by PARMENIDE s.r.l. (Rome) some years ago; SIPRE is the acronym of Sistema lbrido di Propulsione con Ruotismo Epicicloidale (in Italian language), i.e.: hybrid propulsion systems with planetary gear train. Such kind of systems have been studied for four years and five different versions have been developed.The main aim of this research is the realisation of hybrid powertrains with improved performances and efficiencies; analysing the traditional series and parallel hybrid systems is manifest that an enhanced hybrid equipment can be achieved developing a system layout with the minimum number of machines and reducing the number of energy conversions between the thermal unit and the vehicle traction system. The development of the SIPRE systems has been focused in order to obtain steady state operation mode of the i.c. engine, typical of series powertrains, together with mechanical traction, distinctive of parallel systems; it is achievable with the utilisation of a planetary gear set (pgs from now on) for the linkage of the different machines that compound the power unit; the layout of the connections and the transmission ratios of such a system could be various.The main advantage of the SIPRE systems is the good energy efficiency due to the i.c. engine operation mode and the optimised power flow between electrical machines and battery pack. Further, the low demand of battery pack capacity allows the reduction of system weight.In this paper the SIPRE 3 system, actually the most interesting of the SIPRE group, will be described and the principles of design will be discussed; moreover the results of an in-depth study of the systems behaviour will be presented together with the description of machines control strategies. The analysis here reported has been fulfilled developing a SIPRE 3 computer model of simulation rich of details on the electrical machines behaviour and on the control logic. For the simulation the MATLAB with SIMULINK™ software development kit has been utilised.