Analytical loss model for single- and two-speed electric vehicle gearboxes

This paper investigates load-dependent and load-independent power losses of clutchless dual-stage single- and two-speed gearboxes for a subcompact electric vehicle (EV). The load-dependent loss includes gear meshing and bearing friction losses, whereas the load-independent loss encompasses gear churning, bearing drag and oil lip seal sources of loss. The methodology proposed estimates these losses based on the geometric parameters and dynamic characteristics of the gears and bearings and lubricant properties. A single-speed production EV gearbox was disassembled to determine the bearing, gear, and lubrication characteristics. The proposed analytical model is applied to estimate power losses for this gearbox as a function of electric machine speed and torque. Additionally, a second gear is added, making the gearbox two-speed, resulting in increased loss but providing the opportunity for increased electric machine and inverter efficiency. The single-speed gearbox has a 7.05:1 final ratio, while the two-speed variant has 7.05:1 and 4.2:1 for the first and second gears. At lower torques, where an EV would operate most, the single-speed gearbox has an efficiency between 94 and 97%, and the two-speed gearbox has between 0.1% and 0.7% lower efficiency when operating in first gear (the same 7.05:1 ratio as for the single-speed gearbox). When at low torque in second gear, the gearbox is about 1 percent more efficient for highway driving conditions, an important contribution towards extending vehicle range. The analysis also shows gear oil immersion depth significantly affects churning loss, and oil temperature has a small effect on overall gearbox loss.