In the present work, the layout design of a high power density engine for range extender application in hybrid vehicles is introduced. After a brief overview of the legislative framework, the input data for the design are provided. In the following, the attention is on the first design of crankshaft, bearings and connecting rod. The methodology followed for crankshaft and connecting rod design is the same: the starting point is a CAD based on geometrical considerations, to assess the feasibility of the desired design. The first step is then the choice of the appropriate material for the component, followed by a fatigue calculation of the component.