Development of Transaxle Fluid for Electrified Vehicles: Validating Optimized Viscosity through Targeted Hardware Testing 2022-01-1103

Reducing powertrain losses is an important technical challenge to further improve the efficiency of electric vehicles as part of measures toward achieving carbon neutrality. One effective method of accomplishing this goal is to reduce the viscosity of transaxle lubricating oil. However, it is generally known that lowering viscosity can cause durability issues such as wear and seizure if the thickness of the lubricating oil film on metal sliding surfaces is insufficient. In gears and bearings, reducing the oil film thickness can increase direct contact with the base metal and may cause surface fatigue peeling.

A new additive formulation for lubricating oil specifically for electrified vehicles has been designed in anticipation of the wider adoption of such vehicles in the future. The result has been a new transaxle fluid that ensures unit durability while reducing viscosity of 40°C to 12.2[mm2/sec]. This low viscosity fluid raises not only the fuel efficiency of hybrid electric vehicles (HEVs) by 1.0% or more under test cycle driving conditions, but also effective in improving the cooling performance of the motor. In addition, it was confirmed that this low viscosity oil satisfied anti-wear and anti-seizure performance of the gear in the high-speed durability test and the differential durability test. Furthermore, it was confirmed in the gear and bearing unit fatigue test that fatigue life was better than the conventional oil.

The developed fluid can contribute to the realization of carbon neutrality by finding widespread use in other electrified vehicles, including plug-in hybrid (PHEVs), battery (BEVs), and fuel cell electric vehicles (FCEVs).