Optimal power distribution strategy for series-parallel hybrid electric vehicles

The series-parallel hybrid electric vehicle (HEV), which employs a planetary gear set to combine one internal combustion engine and two electric motors, can take advantage of both series and parallel hybrid systems by controlling power flows. In this system, the control of power flows means adjusting not only the amount and direction of mechanical and electrical power but also the operating point of the engine. This study introduces a power distribution strategy to minimize the loss in power flow and to operate the engine at an efficient point. The strategy employs control maps to reduce the calculation load in deriving the most efficient operating point. Furthermore, an energy strategy to decide the optimal charging/discharging power of a battery is established using the results of the power distribution strategy. The control structure, in which the power distibution strategy and the energy strategy are merged, assures efficient driving of the HEV. A series of simulation and vehicle tests were performed to verify the effectiveness of the strategies and the results show a distinct improvement in fuel economy.