Torque Allocation Control Of Four-wheel Drive EVs Considering Energy Efficiency Optimization

As a solution to the energy crisis and environmental pollution problems,new energy vehicles have become a main development direction of the current automotive industry.Among new energy vehicles with multiple driving modes and vehicle frame,the four-wheel drive electric vehicle driven by separate wheel motors for each wheel respectively has become a research hotspot in the field of electric vehicles due to its compact structure,flexible driving form and high transmission efficiency.One of the key research issues of four-wheel-drive electric vehicles is the optimal control of the motor torque.In this paper,the torque distribution strategy of four-wheel drive electric vehicles is studied from the perspective of vehicle energy efficiency optimization.Through a reasonable torque distribution method,the energy loss of the motors and wheels is reduced,and the economy of the entire vehicle is effectively improved.Firstly,this paper analyzes the characteristics of the energy loss of the motor and tire slippage,according to energy consumption characteristics of the four-wheel drive electric vehicle.In this regard,the structural characteristics and energy consumption analysis of the in-wheel motor are introduced,and the energy loss model of the in-wheel motor is established.Second,the wheel model and tire model are introduced,and the tire slip model based on the brush model is explained.It lays a foundation for constructing the torque distribution optimization problem.Then,the motor energy loss and tire slip energy loss are taken as performance index function to establish the optimal torque distribution control problem.On the basis of the system dynamics model,the state equation,system constraint,system boundary condition and the optimization objective function are established.The system constraints are non-linear,which increases the difficulty of solving the problem.So this optimization problem is transformed into a non-linear programming problem,and it is solved numerically using the sequential quadratic programming method(SQP).Using MATLAB software,the off-line simulation model of the four-wheel drive electric vehicle system is constructed from the parameter selection of the vehicle,the drive motor and other modules.In numerical processing,the relationship between the peak torque and the speed of the motor is fitted into a mathematical expression.The working efficiency of the motor and the inverter is related to the operating point,and there is no mathematical expression that can directly characterize the relationship between the working efficiency of the motor and the torque and speed of the motor.Therefore,the relationship between the energy loss performance index function and motor torque and motor speed is established through the numerical fitting method.Finally,the off-line simulation results of three different torque distribution methods(including motor energy loss and tire slip loss,motor energy loss only and axial load distribution)are compared under different simulation conditions.The results show that,compared with the existing torque distribution strategies,the torque distribution method considering both motor energy loss and tire slip energy loss can better improve vehicle energy efficiency and effectively reduce the energy consumption of the entire vehicle.In addition,the torque distribution method that considers the energy consumption of the motor and the tire slip at the same time will greatly reduce the tire wear than the torque distribution method that only considers the energy loss of the motor,and effectively increase the tire life.