Advances in Drivetrain Lubricating Fluid Technology for Hybrid Electric Vehicles

The automotive transmission market has seen an increase in the number of hybrid electric vehicles (HEV), and forecasts predict additional growth. As the design of HEV drivetrain hardware evolves; so must the lubrication technology to anticipate this pace. The reason is that in HEVs the hybrid drivetrain hardware may increasingly combine electric motor, friction devices, gearbox, electro-hydraulics and the control unit all of which will be expected to be lubricated by a suitable drivetrain fluid. There is an extra challenge posed by incorporating an e-motor within the transmission housing while being constrained to ever smaller packaging dimensions, since the e-motor provides an extra source of heat which would be expected to be managed by the lubricating fluid. This leads to the need for an understanding and optimization of corrosion protection and thermal management characteristics of the fluid, and the development of new high-performing lubricating fluids suitable to the challenges is therefore paramount. There are further new challenges to lubricants. In addition to ensuring the lubricant will be durable in the harsher thermal environment, it must also be compatible with new plastics and able to protect them from degradation. There are ramifications that the hotter transmission design and environment has had on shaping the latest lubricant technology for electrified transmissions. The heat transfer characteristics of lubricants and the fundamental factors that impact them and fluid and transmission design are reviewed. The critical factors of protection against corrosion, including the ability of the fluid to allow both fluid and vapour-phase corrosion protection of copper-based componentry is addressed by the fluid technology. A secondary impact on the design of other fluid attributes, such as minimizing noise vibration and harshness (NVH), is addressed and the new fluid technology formulation approach to deal with such challenges is highlighted.