In the last years, the research concerning hybrid electric vehicles (HEV) has been developing greatly because of some dangerous natural phenomena such as global warming, directly linked to vehicles exhaust emissions. The hybrid electric vehicles are characterized by an improved fuel economy thanks to extra degrees of freedom provided by battery energy storage and one or more electric machines which allow running a smaller combustion engine in a higher efficiency region. The aim of this thesis is to demonstrate the viability both in terms of fuel economy and performance of a powertrain downsizing when coupled with modern electrical traction motors. In this regard, at the beginning of this research work, the capability of the baseline conventional powertrain, represented by a front-wheel-drive Opel Insignia with a 2.0 l diesel engine, performed numerically.