Control optimization of a compound power-split hybrid power system for commercial vehicles

In this paper, based on two planetary gear sets, a novel compound power-split hybrid power system for commercial vehicles is presented. Firstly, mathematical models of the system dynamic torque control and the system efficiency are established by using an equivalent lever diagram and analyzing the power flow respectively. Then the system operating modes which are divided into two pure electric modes and one hybrid mode and corresponding control strategies are analyzed by the combined lever diagrams. Finally, control strategies in different modes to looking for the optimal system efficiencies are designed and validated by the bench test. In order to prolong its service life, the battery is charged with small power in the hybrid mode. The design and test results indicate that the optimal system efficiency control strategies are reasonable and reliable. The maximum value of the optimal system efficiency can reach about 0.92 in the pure electric mode, and 0.39 in the hybrid mode. And the proportion of the engine operating points with the brake specific fuel consumption (BSFC) lower than 215 g/kWh is 81%. The co-simulation results show that the maximum system efficiency can achieve 14.9% fuel consumption per 100 km drop compared with the minimum system efficiency for an 80 km/h constant speed condition, which indicates that the optimal system efficiency control strategy can greatly improve the vehicle fuel economy. This study can offer a research direction for energy management of hybrid power systems.