Water-Based Lubricants for Electric Vehicle Transmission Applications: Properties, Tribological Performance and Efficiency.

Water-based lubricants (WBLs) are emerging as promising alternatives to conventional oil-based lubricants in electric vehicle (EV) transmission systems, driven by increasing demands for energy efficiency, sustainability, thermal management, and environmental compatibility. OEMs and researchers are striving to minimise frictional, thermal, and power losses in EV gearboxes to maximise driving range and system durability, and WBLs have the potential to meet this demand. Moreover, WBLs offer flexibility in viscosity tuning, higher specific heat capacity, and superior heat transfer capability compared with oil-based lubricants. These characteristics create opportunities to improve cooling performance and support the development of a single e-fluid concept. However, their successful implementation requires a comprehensive understanding of film formation, friction and wear behaviour, system-level efficiency, and material compatibility. This thesis investigates the feasibility of WBLs for EV transmissions through a series of interconnected studies. It begins with the characterisation of elastohydrodynamic (EHL) film formation and pressure–viscosity relationships, revealing the distinctive film-forming behaviour of WBLs. The effects of water content and evaporation sensitivity on the pressure–viscosity coefficient are examined, and the applicability of classical predictive models, including the Hamrock–Dowson equation, is reassessed. Friction and wear analyses demonstrate that fully formulated WBLs can achieve near-superlubricity with minimal shear heating, facilitated by robust surface–additive interactions under rolling/sliding contact. These laboratory findings are validated through full-scale EV gearbox testing, where WBLs reduce power losses and thermal load, improving overall gearbox efficiency by at least 1.5%. Finally, durability is evaluated via tribocorrosion analysis of bearing steel, highlighting the synergistic interaction between mechanical and chemical wear in aqueous environments. Overall, this work positions WBLs as viable high-efficiency e-fluids for future sustainable transport, provided that challenges related to water loss and wear are effectively addressed through advanced formulation and system design.