Study of drag torque in a two-speed dual clutch transmission electric vehicle powertrain system

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Pure electric vehicles are widely looked as a potential avenue to reduce fossil fuel consumption and emission in the long term in the transportation section. Pure electric vehicles currently being used in the market are mainly equipped with single speed transmissions, with tradeoffs between dynamic (such as climbing ability, top speed, and acceleration) and economic performance (drive range). The employment of two-speed dual clutch transmission (DCT) in electric vehicles is likely to improve average motor efficiency and range, and even can reduce the required motor size. In order to comprehensively improve the electric vehicle powertrain system efficiency, it is necessary to fully consider the proposed two-speed transmission drag torque, including its influences and potential applications. The focus of this thesis is on studying the drag torque within two-speed dual clutch transmissions. Different source of drag torque in the DCT are theoretically analysed and modelled, including torsional resistance caused by viscous shear caused between wet clutch plates and concentrically aligned shafts, gear mesh friction and windage, oil churning, and bearing losses. Then experimental works are carried out on UTS electric vehicle powertrain system test rig. Outcomes of experimentation on drag torque confirm that simulation results agree well with the test data. Then, based on the drag torque study, the thermal behaviour of the transmission is analysed via both theoretical and experimental investigation. Finally, integrated optimal design of electric vehicle powertrain system equipped with two-speed DCT is performed, considering drag torque, shift schedule, electric motor selection, gear ratio design and wet clutch design.
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