Abstract
Increasing automotive powertrain electrification is impacting drivetrain complexity and the profiles of the fluids needed. Since the millennium, drivetrain fluid viscosities have been reduced for better efficiency, but this new challenge is driving them to unprecedented low levels. This paper assesses some of the potential implications of ultra-low viscosity fluids on drivetrain functionality and durability. Model formulations have been prepared from a variety of base fluids combined with additive packages. These have been evaluated in typical automotive drivetrain rig tests, as well as with some selected functional tests. In addition, the thermo-oxidative stability and electrical and thermal properties of the fluids were compared. Based on the results, the impact of low viscosity fluids on drivetrain functionality and durability varies depending on the performance parameter evaluated. For example, gear scuffing and bearing wear is highly dependent on additives, whilst gear and bearing fatigue is mainly affected by fluid viscosity. However, by carefully balancing base fluids and additives, acceptable component and fluid durability can be achieved. With respect to new electric drivetrain performance needs, the thermal properties of the finished fluid are essentially dependent on the base fluid composition, whilst its electrical properties are more influenced by additive chemistry, with some secondary impact from base fluid composition.
Highlights
Automotive powertrains are undergoing a major transformation, posing significant challenges for drivetrain component and fluid manufacturers alike
For drivetrain fluids, recent changes to the SAE J306 Automotive Gear Lubricant Viscosity Classification [1] are a good reflection of this trend
Friction element performance in general is strongly connected to lubricating oil properties
Summary
Automotive powertrains are undergoing a major transformation, posing significant challenges for drivetrain component and fluid manufacturers alike. With its most recent revision from 2019, new lower viscosity classifications have been introduced It is to be noted though, that ongoing drivetrain development projects include ultra-low viscosity (ULV) fluids with even lower viscosity levels than defined in SAE J306. Balancing this trend without compromising other fluid properties such as anti-wear protection, volatility or flash point remains a continuous challenge for both drivetrain component designers and fluid formulators. Electric powertrains by design generate heat, potentially exposing fluids and components to significantly higher temperatures than previously observed in conventional drivetrain applications. Based on the general fluid performance data, test candidates were selected for the stage of testing, to evaluate the reduced fluid viscosity impact on the functionality and durability of some transmission components: Synchronizer units Rolling bearings
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