Hybrid electric vehicle powertrains are inherently complex due to the numerous possibilities which such systems allow. Not only can the choice of components be varied, but the relative sizes and control strategy of the components will have a significant impact on the performance of the powertrain. The application of the vehicle will also have an enormous effect, since this will govern the mass of the vehicle and the performance which is required. In order to optimise the vehicle, the most important performance parameters must be clearly understood, be they highest efficiency, greatest range, lowest emissions, greatest acceleration performance or lowest cost.
Given the number of variables, it is sensible to assess different options by means of simulation of the various powertrains. It is also prudent to include computational mechanisms for automatic sizing of the components for different design cases such that the solution time is minimised.
The paper describes the sizing of powertrain components for optimal performance in hybrid electric traction systems for two basic cases. These incorporate either a diesel generator or a small gas turbine generator. The two powertrains are optimised for highest efficiency given that a minimum acceleration performance and range is met. Although the diesel generator has a greater efficiency than the gas turbine, some of this advantage is eroded since it has a greater weight which also increases the sizes of all the other components. This weight increase has an adverse effect on overall vehicle efficiency