Research On Multi-source Optimal Energy Management Of Four-wheel Independent Drive Electric Vehicle

Developing pure electric vehicle(PEV)technology is a critical factor in the strategic decision-making of science and technology of China,which is also the most effective way to solve the problems of environmental pollution and energy shortage at present.The economy problems of the electric vehicle is the core reason which restricts its popularization and rapid development,and the development cost,driving range and kinetic performance are closely related to the economy of PEV.Energy storage system(ESS)and driving system are the key elements of automobile development,which are regarded as two breakthroughs to solve the economy problems of PEVs.With the advantages of compact chassis structure,considerable energy efficiency and flexible control characteristics,four-wheel independent driving electric vehicle(4WID-EV)has become the research focus of PEV technology at present.Combined with the natural advantages of 4WID-EV,starting with the energy storage system and the driving system,taking ‘Broadening source' and ‘Reducing outflow' as the guidance,the comprehensive performance of PEV can be improved from multiple sources through the means of energy optimization management,which is also the focus of this paper to solve the economic problem of PEV in essence.In order to break through the economy multi-source optimization problem of 4WID-EV,a dual-battery hybrid energy storage system(HESS)based on lithium iron phosphate battery and lithium titanate battery is proposed in this paper.On the basis of this,the optimal torque allocation strategy considering torque change rate for in-wheel motor(IWM)system energy saving is proposed.Firstly,the 4WID-EV dynamics model for energy management is established in MATLAB/Simulink,including longitudinal dynamics model,hybrid battery system(HBS)model and IWM system model.The HBS model includes lithium-ion battery model,lithium battery degradation model and lithium battery thermal model,and IWM model includes IWM efficiency model,power calculation model and energy consumption calculation model.Secondly,the parameters of the equivalent circuit model are identified offline by pulse current experiment,which makes the model suitable for different SOC and current rate.To solve the problem of power allocation in HBS,power allocation strategies based on power frequency threshold and fuzzy logic control are proposed to give full play to the performance advantages of the two kinds of batteries,which give full play to the performance advantages of the two batteries and improve energy utilization and service life simultaneously.Taking the total cost and service life of HBS as the optimization objectives,the multi-objective optimization of the parameters is carried out on the basis of the power allocation strategies,and the fuzzy power allocation controller is optimized by employing the adaptive mutation differential evolution algorithm.Thus,the service life of the energy storage system is further improved.Finally,aiming at the torque allocation problem of IWM system,an energy-saving optimal torque allocation strategy is proposed,and the axle torque allocation coefficients of optimal energy consumption is obtained by off-line optimization.In order to solve the problem of current oscillation caused by motor torque change,an energy-saving torque optimal allocation strategy considering torque change rate is proposed,and the axle torque allocation coefficients of motor efficiency and torque change rate are obtained by on-line optimization.At the same time,the average torque change rate and peak torque change rate are reduced while ensuring optimal energy consumption.