The use of high efficiency engines is an unavoidable necessity in the transportation field. The reduction of gas emissions and the optimisation of conversion efficiency cannot be entrusted only to engine characteristics, but it is necessary to work with integrated propulsion systems, which have to manage the production and transmission of the energy in an optimal way. In this paper a hybrid propulsion system made of a parallel type hybrid engine and a hydrostatic transmission is presented. The operational flexibility of the hydrostatic transmission allows the thermal engine to operate at a constant speed, hence at maximum efficiency and/or minimum emission conditions. This propulsion system was applied to the case of a “12 m class” bus for passenger transportation. An Italian hybrid vehicle was taken as the reference solution for performance comparison and as a technical database for the vehicle design aid. Starting from the Italian laws regarding buses, the design and sizing of the propulsion group components were performed. Then, in order to verify the ability of the hybrid scheme to satisfy the design targets, a simulation of the system was carried out using a dynamic simulation code. The simulation showed that the combination of the hydrostatic transmission with the hybrid engine leads to energy savings and a pollutant reduction in comparison with the reference vehicle.

Keywords: vehicular hybrid propulsion, hydrostatic transmission