Drag loss and efficiency analysis of an electric vehicle gearbox using water-based lubricants

Water-based lubricants (WBLs) have demonstrated promising frictional behaviour in component-level tribological studies; however, their performance under full-scale gearbox conditions remains largely unexplored. In this study, a modified commercial electric vehicle (EV) gearbox was employed to experimentally evaluate the influence of three fully formulated WBLs on drag losses, gearbox efficiency, thermal response, and post-test fluid stability, using a conventional gear oil as a reference. The results show that the WBLs achieved a relative efficiency improvement compared to the reference gear oil, with a maximum peak efficiency gain of 1.95% across the tested conditions. In addition, the WBLs exhibited enhanced thermal performance, reducing bearing temperature rise by up to 8 °C under equivalent load conditions. Post-test characterisation indicated no significant degradation or oxidative deterioration in the WBL samples, although minor water evaporation was detected in certain cases. Overall, the findings highlight a clear efficiency advantage of WBLs, while also identifying certain limitations that require consideration for the continued development of WBLs as potential e-fluids.