Electrified four-ball testing – A potential alternative for assessing lubricants (E-fluids) for electric vehicles

Abstract

The increasing worldwide demand for hybrid and electric vehicle technology has brought new challenges for the global lubricant industry. The new lubricants for electric vehicles, also called E-fluids or EV-fluids, are expected to meet a new set of requirements including withstanding the much severe operating conditions of EV powertrains. High starting torques, high RPMs and uncontrollable shaft currents passing through the contact interfaces are some of the most challenging powertrain conditions that can impact the performance of these lubricants. Although there have been some papers already reporting on significant alteration of tribological properties of lubricants under electrification, so far there are no standard test protocols aimed at the fast and reliable screening of base oils, additives and formulated lubricants under such conditions. Thus, this research work focuses on exploring the popular four-ball ASTM-D4172 standard method to evaluate the tribological behavior of lubricants under electrified conditions. Specifically, a conventional four-ball tester was instrumented with a DC power source and a resistance data logger was used to explore the friction and wear performance of various lubricants (including a neat base oil, two automatic transmission fluids (ATF-3 and ATF-5) and a gear oil) with different dielectric strength under the conditions of ASTM-D4172 standard with and without DC currents (0, 1.5 and 3 A). The changes in average wear scar diameter, specific wear modes and mechanisms, friction coefficients, electrical contact resistance and temperature were analyzed and reported. Besides, the chemical changes in oils during the electrified tests were evaluated via IR spectroscopy. The main findings suggest that passing current through the four-ball contact interfaces significantly alters friction, wear and temperature in comparison to the conventional standard method without electrification. Thus, the proposed test method with electrification could be a potential new alternative for testing E-fluids under electrical environments.