Torque-fill control and energy management for a four-wheel-drive electric vehicle layout with two-speed transmissions

This paper presents a novel four-wheel-drive electric vehicle layout consisting of one on-board electric drivetrain per axle. Each drivetrain includes a simplified clutch-less two-speed transmission system and an open differential to transmit the torque to the wheels. This drivetrain layout allows eight different gear state combinations at the vehicle level, thus increasing the possibility of running the vehicle in a more energy-efficient state for the specific wheel torque demand and speed. To compensate for the torque gap during gearshifts, a 'torque-fill' controller was developed that varies the motor torque on the axle not involved in the gearshift. Experimental tests show the effectiveness of the developed gearshift strategy extended with the torque-fill capability. Energy efficiency benefits are discussed by comparing the energy consumptions of the case study vehicle controlled through a constant front-to-total wheel torque distribution and conventional gearshift maps, and the same vehicle with an energy management system based on an offline optimization. Results demonstrate that the more advanced controller brings a significant reduction of the energy consumption at constant speed and along different driving cycles.