The Graded Ratio Control Strategy For Plug-in Hybrid Electric Vehicle Equipped With CVT

Plug-in hybrid electric vehicle(PHEV)is a combination of the advantages of pure electric vehicles and hybrid electric vehicle.The power batteries can be recharged by external power grid.Therefore,it can greatly reduce the dependence on fossil fuels.So PHEV is get a lot of attention.With continuously variable transmission(CVT)as the transmission device used in plug-in hybrid system,the vehicle the power supply can work in high efficient area,and the energy consumption economy is increased.However,the efficiency of CVT is low.Especially the energy loss of hydraulic system is large in the process of speed ratio change.So it is necessary conducted research for CVT speed ratio control,considering the hydraulic system damage,formulating reasonable speed ratio control strategy,so as to achieve the aim of improving the vehicle performance.In this paper,a single-axle parallel plug-in hybrid electric vehicle with CVT is used as the research object.The following research contents are mainly carried out:(1)By combining theory with experimental data,the engine model,motor model and battery model are built respectively.According to the working mode of the vehicle,with the required torque and the value of SOC as the switching conditions,a rule-based vehicle energy management strategy was developed.And vehicle model is completedl.(2)With the analysis of force and power of transmission system under different modes of PHEV,the formula of system efficiency is obtained.With the goal of optimal system efficiency,for each mode was optimized by using discrete exhaustive optimal goal in CVT speed ratio.Through the analysis of CVT transmission system dynamics and typical driving conditions CVT speed ratio distribution,the CVT speed ratio change rate of the limited area is obtained.The working characteristics of CVT hydraulic system are analyzed and the mathematical model is established.Simulation results under WLTP condition show that the CVT hydraulic system generates power loss due to the change of CVT speed ratio during vehicle driving.(3)In order to unified the electricity and fuel consumption,the economic function of energy consumption was developed.The driving conditions which are suitable for different modes of plug-in hybrid electric vehicle are formulated.And the shift schedule which is based on the system efficiency of the model is obtained.Then the relationship between energy consumption economy and transmission shaft is obtained.Finally based on geometric series method,it is obtained that the shaft of numbers of each mode.(4)Build a composite driving conditions to energy consumption economy as the optimization goal,genetic algorithm is adopted to optimize each point of different modes under the composite condition.And the best speed ratio points are obtained for the better energy consumption economy.Based on MATLAB/Simulink simulation platform,the CVT graded ratio control strategy and continuously control strategy is adopted to simulate the energy consumption economy and ride comfort.The results show that when adopting the ratio grading control strategy the power loss of CVT hydraulic system has drastically reduce,energy consumption economy is improved,and the life of CVT is prolonged,at the same time the degeree of impact is reduce.