Effects of Lubricant Oil Properties on Efficiency and Gear Friction Loss in an Electric Transaxle

Enhancing electric-transaxle (e-axle) efficiency is essential for extending the driving range of battery electric vehicles. This study investigated how base-oil properties and additive-controlled boundary friction affect e-axle efficiency and load-dependent gear friction loss. Four lubricants were evaluated using a production oil-cooled e-axle, and their gear-mesh friction characteristics were further investigated using a high-speed back-to back gear test rig. At a constant kinematic viscosity of 11.8 mm2/s at 40 °C, varying the base-oil type produced a 0.72-percentage-point difference in WLTC-weighted efficiency. The API Group IV-based lubricant exhibited higher thermal conductivity and a 60% lower traction coefficient than the Group I-based lubricant, consistent with improved motor cooling and reduced fluid-film shear losses. Compared with the Group II-based lubricant, it reduced gear friction loss by an average of 15% over the investigated speed range. Reducing the block-on-ring friction coefficient from 0.093 to 0.022 increased WLTC-weighted efficiency by approximately 0.07 percentage points; however, this difference was comparable to the observed repeat-to-repeat variation. Nevertheless, the low-friction formulation improved e-axle efficiency under low-speed, high-torque conditions and substantially reduced gear friction loss at pitch-line velocities below 5 m/s. Gear friction loss decreased rapidly up to approximately 10 m/s and then approached a plateau. The increasing difference between the additive formulations at λ ≤ 1 indicated a growing contribution of boundary lubrication. These results demonstrate that jointly optimizing thermal conductivity, fluid-film traction, and additive-derived boundary friction is essential for maximizing e-axle efficiency across practical operating conditions.