Downshift decision and process optimal control of dual clutch transmission for hybrid electric vehicles under rapid braking condition

Hybrid electric vehicles (HEVs) equipped with dual clutch transmission (DCT) can benefit from engine auxiliary braking force and driving motor regenerative braking force in braking conditions. This can effectively reduce the workload of hydraulic mechanical braking systems and recovery braking energy, which will improve the fuel economy. In the downshift process, the driving motor is involved to response the change of torque/speed demand, which can further improve the shift quality of DCT without affecting the engine emissions and fuel economy. The combined braking condition of HEVs with DCT allows for the variation of power distribution between the front and rear axle, as well as the electric and hydraulic braking force. In this study, the downshift decision strategy is designed based on the two-parameter shift schedule, while considering the driver's subjective driving intention. Subsequently, the mathematical model developed to investigate the downshift process of HEVs with dual clutch transmission has been applied to skip downshift process, to analyze the control sequence and implement different control algorithms according to respective control objects. Specifically, the sliding mode variable structure optimization control algorithm is used to allow the engine speed to track the target value in the torque phase. In addition, the linear quadratic tracking controller and the model reference adaptive system are used, based on adjustable parameters, to realize the speed synchronization of the clutch driving and driven plate in the inertial phase. According to the results of electro-hydraulic braking force distribution, the braking forces of an integrated starter generator (ISG) motor and hydraulic brake system are controlled to switch to the demand level. Then, the simulation model established on a Matlab/Simulink platform is used to validate the skip downshift control strategy. Finally, the designed control strategies are verified on the test bench. The test results demonstrate that the established shift control strategy can achieve the target power downshift under braking conditions, and have a good downshift quality, which lays a foundation for subsequent development and real vehicle tests.