Downshifting strategy of plug-in hybrid vehicle during braking process for greater regenerative energy

The P2 configuration plug-in hybrid electric vehicle (P2-PHEV) equipped with a multi-speed transmission has a high potential for recovering more regenerative energy, as the shifting strategy can be employed to adjust the working zone of the electric motor (EM). However, the existing shifting strategy designed for normal driving conditions cannot achieve optimal regenerative energy recovery. In this study, a shifting strategy used in the regenerative braking process is proposed. First, to make the EM provide more regenerative force during braking, a braking force distribution algorithm is devised while simultaneously considering braking stability and safety. Second, to realize maximum regenerative braking energy recovery, an optimal shifting strategy is designed for regenerative braking. Third, the classical braking process is analyzed and six thresholds are abstracted and optimized to establish a rule for restraining frequent gearshifts raised in the proposed optimal shifting strategy. Finally, the proposed strategy is verified under three standard cycles, results show that the proposed shifting strategy can recover considerable regenerative energy without frequent gearshifts.