Abstract: Due to the transient characteristics of engines, motors, and clutches, the output torque fluctuations in hybrid powertrains and the resulting vehicle jerks may occur during the transition from pure electric mode to parallel mode for series-parallel hybrid electric vehicles. A coordinated control strategy for a series-parallel hybrid electric bus is proposed to improve drivability during the transition from pure electric mode to parallel mode. The mode-transition control strategy is developed according to the operating status of the clutch, and on the consideration of the transient response characteristics of the engine and motor. In this control strategy, a fuzzy adaptive system is developed to estimate the uncertainties by varying parameters and the difference between the actual and target engine torques. Then, the estimated differences are utilized to regulate the control variables of the sliding mode controller to improve the control accuracy and robustness. Simulation and experimental results show that the range of torque fluctuations in the powertrain and the maximum vehicle jerk are reduced by 85% and 78% respectively while satisfying the driver’s torque request during the mode transition with the proposed control strategy. Thus, the drivability of the hybrid bus can be significantly improved.

Key words: Experimental validation, Fuzzy adaptive sliding mode control, Mode transition, Series-parallel hybrid electric bus, Transient response