Discretely variable speed ratio control strategy for continuously variable transmission system considering hydraulic energy loss

By using the continuously variable transmission (CVT) to change the speed ratio continuously, the power resource components can operate in the high efficiency region for plug-in hybrid electric vehicle (PHEV) equipped with CVT, which improves the driving efficiency of powertrain. However, the frequent change of CVT speed ratio causes large energy loss of CVT hydraulic system and reduces the energy economy of PHEV. In view of this issue, the characteristic of energy loss for CVT hydraulic system is studied and the influence of speed ratio rate of change on energy loss of hydraulic system is obtained. Then, the simulation based on the continuously variable speed ratio control strategy (CVSRCS) is carried out and the results indicate that there is large energy loss of the CVT hydraulic system due to the frequent change of CVT speed ratio, which influences energy economy of the vehicle. Moreover, excessive driving jerk is generated, which significantly affects ride comfort of the vehicle. In order to reduce the adverse impact of frequent changes of CVT speed ratio on energy economy and ride comfort, a discretely variable speed ratio control strategy (DVSRCS) is proposed and discrete speed ratio is optimized by genetic algorithm. A comparative simulation for PHEV's performance by adopting the CVSRCS and the proposed discretely variable speed ratio control strategy is carried out under a comprehensive driving cycle. The results of this study demonstrate that the proposed control strategy can not only significantly reduce the energy loss of CVT hydraulic system and enhance the energy economy, but also improve ride comfort. (C) 2019 Elsevier Ltd. All rights reserved.