Torque Distribution Strategy for a Front- and Rear-Wheel-Driven Electric Vehicle

Electric vehicles (EVs) with a distributed drive train configuration offer great potential and flexibility for improving system efficiency, performance, reliability, and safety. This paper investigates a torque distribution scheme for a front- and rear-wheel-driven microsized EV to improve drive train efficiency over a wide torque and speed range. The loss model of the traction permanent-magnet (PM) motor is characterized in both the constant-torque and flux-weakening regions. The relationship between motor efficiency and torque at a given speed is then derived. It has been shown that maximum efficiency is achieved if the total torque required by the vehicle is equally shared between the two identical motors. In addition, the distribution of the energy consumption over a New European Driving Cycle (NEDC) is analyzed, and the regions of high speed and low torque are identified to have a high level of energy consumption; in these regions, motor efficiency improvement is the most important. Therefore, this paper further proposes to operate just one motor to provide the total required torque in the low-torque region. A clutch may be employed between one motor and gearbox (differential), thus “switching off” its idle loss (no-load loss and flux-weakening loss) and improving drive train efficiency. An online optimized torque distribution algorithm has been devised based on the motor efficiency map to determine whether the second motor should be disengaged by the clutch in the low-torque region. With the proposed optimization scheme, drive train efficiency can be improved by 4% over the NEDC. Experimental test results validate the proposed torque distribution strategy.