Efficiency improvement potential of a BEV gearbox using a mono fluid circuit

Abstract In Battery Electric Vehicles (BEVs), energy dissipation can potentially be decreased by combining the cooling of electrical components and lubrication of the gearbox in a mono fluid circuit. This can be achieved by using novel fluids with good lubrication and cooling properties. In a mono fluid circuit, the direct use of the heat generated by power electronics and the electric machine can be employed to increase the gearbox temperature. This can result in a reduction in fluid viscosity, which can improve the gearbox efficiency. This study demonstrates the potential of a mono fluid circuit to increase the efficiency of the gearbox in a high-speed electric drive unit. Maps of the gearbox efficiency and the heat flow rate from power electronics and electric machine were derived on a test rig and used in driving cycle simulations. The simulations were conducted for an electric drive unit with a mono fluid circuit and compared to a fluid circuit with separated gearbox lubrication. The results indicate that the mono fluid circuit has the potential to decrease the energy demand of the gearbox by 2.2% for the WLTP class 3b driving cycle.