Development of Transaxle Fluid for Electrification Vehicles: Design of Novel Additive Formulation

To achieve carbon neutrality by reducing carbon dioxide (CO₂) emissions, vehicles with an internal combustion engine have started to be replaced by electrification vehicles such as hybrid electric vehicles (HEVs), plug-in HEVs (PHEVs), and battery EVs (BEVs) worldwide, which have motors in their transaxles (T/As). Reducing transmission torque loss in the transaxles is effective to reduce CO₂ emissions, and lowering the viscosity of lubrication fluids in T/As is a promising method for reducing churning and drag loss. However, lowering viscosity generally leads to thin oil films and makes the lubrication condition severe, resulting in worse anti-fatigue and anti-seizure performance. To deal with these issues, we made improvements on the additive formulation of fluid, such as the addition of an oil-film-forming polymer, chemical structure change of calcium detergents, and an increase of anti-wear additives including phosphorus and sulfur. As a result, we succeeded in developing a novel fluid with greater lubricity than a conventional automatic transmission fluid (ATF), despite the fact that the viscosity of the new fluid was lowered by around 50% compared with that of the ATF. In addition, the electrical insulating and anti-foaming performance of this fluid, which are required for T/A fluid in electrification vehicles, were equal to or greater than those of the ATF. The fuel economy in the Toyota Hybrid System (THS) was improved by 1.0% and more compared with the ATF because of the significant viscosity decrease in the developed fluid. This fluid will be widely used for HEVs, PHEVs, BEVs, and fuel cell electric vehicles (FCEVs) and can contribute to the realization of a carbon-neutral society.