BEV AWD EDU Gear Ratio Selection for Efficiency

The single speed gearbox is a largely preferred solution for electric drive units (EDU) in hybrid vehicle P4 and Battery Electric Vehicles (BEV). A proper gear ratio selection is required to meet the power and speed performance requirements, and also to minimize the parasitic losses over the life of the vehicle. Conventional wisdom holds that a higher ratio power dense gear box design for an EDU is lighter, more packaging friendly, and more efficient. This study examines the validity of this assumption for an auxiliary EDU installed in the rear of a BEV CUV providing AWD functionality. This paper outlines the power loss analysis for a single speed EDU propelled by an induction electric machine, and provides a useful approach for gear ratio selection at the concept phase of a design. Specifically, this paper determines the range, mass and cost tradeoffs for two auxiliary EDU design options without disconnect function, with gear ratios of 7 and 10 for a given duty cycle, and for two different induction motor material choices (aluminum and copper). We show that the lower gear ratio design option with aluminum rotor saves approximately 450 kWh energy compared to the higher gear ratio over the life of the vehicle. For an assumed battery miles/kWh, this saving translates into approximately $30 lower overall cost (battery+EDU). Alternately, for a given battery pack capacity, the ratio 7 design with aluminum rotor is expected have a longer range (+2.13 miles) for an estimated $31 cost and 6.5 kg mass increase.