Development of Ultra-Low Viscosity Transaxle Fluid for Electrification Vehicles
The automotive industry is accelerating the development of electrified vehicles such as hybrid electric (HEVs), plug-in hybrid (PHEVs), battery (BEVs), and fuel cell electric vehicles (FCEVs) to help reduce CO2 emissions and prevent global warming. Toyota Motor Corporation (TMC) aims to achieve 100% carbon neutrality by 2050, and has set a sales target of eight million electric vehicles per year in 2030 and an annual production target of 3.5 million BEVs for 30 models. In addition to accelerating the spread of electrified vehicles, improvement of fuel efficiency of electrified vehicles will be an important technical initiative. The transaxle accounts for a relatively high proportion of the energy loss that occurs in an electrified vehicle. For this reason, reducing transaxle loss is an effective method of improving the efficiency of all electrified vehicles. 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 because 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. To counter the negative impact of lowering viscosity, 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. To prevent surface pitting and the seizure of gears and bearings, the protectivity of the tribo-film was improved by controlling the adsorption of additives on metal surfaces and by reviewing the anti-wear agents, extreme pressure agents, and detergents. This tribo-film is densely formed on metal surfaces and has a smoothing effect that fills in the fine surface asperity.
As a result, a new transaxle fluid has been completed that ensures unit durability while reducing viscosity by 50% (the kinematic viscosity at 100℃ is about 3.3 mm2/s) compared to conventional fluids. With this new lubricating oil, the fuel efficiency of HEVs can be increased by 1.0% or more under test cycle driving conditions, and the cooling performance of the motor, which is an important element of electrified vehicle transaxles, can be improved. This new lubricating oil is planned to be gradually rolled out in PHEVs, BEVs and FCEVs, starting with the next generation of HEV vehicles. This is a fundamental technology for the electrification of TMC’s lineup in the future, and an essential technology that greatly contributes to the realization of carbon neutrality.
