23rd International Colloquium Tribology - January 2022 41 Base Oil Benchmarking for Gear Oils in Electric Vehicle Drivetrains Dr. Steffen Glänzer Clariant Produkte Deutschland GmbH, Clariant Innovation Center, August-Laubenheimer-Strasse 1, 65926 Frankfurt 1. Summary The market penetration of electric vehicles is a major global trend resulting from regulations intended to reduce greenhouse gas emissions. OEMs, suppliers and researchers are looking for base oils that meet new e-fluid needs which happen to be different than for lubricants used in internal combustion engine vehicles. So far, few systematic studies benchmark crucial properties of e-fluid base oils. This paper will compare base oil properties of polyalphaolefins and various new polyalkylene glycole solutions covering the following topics: 1) energy efficiency, 2) cooling performance, 3) electrical properties, and 4) sustainability. One major finding is that polyalkylene glycols offer the possibility to optimize and finetune these properties over a wide range. In summary, friction can be reduced down to the level of superlubricity (coefficient of friction below 0.01) and heat transfer efficiency can be increased up to 30%. In addition, Clariant Polyglykols offer the possibility to formulate hazard label-free and readily biodegradable fluids. 2. Introduction Improving energy efficiency is one of the main goals in today’s electric vehicle powertrain development. The gear lubricant can contribute to achieve higher energy efficiency, which results in a longer range of the electric vehicle. Secondly, increasing energy density of the electric motor requires more efficient cooling concepts. Direct cooling of the electric motor using a gear lubricant with excellent heat transfer properties can be an excellent solution.[1] In addition, the lubricant for electric vehicle reduction gears should have suitable electric properties. It should be thermo-oxidatively stable, and it should be label free and readily biodegradable. As only limited data is available in comparing different base oil solutions for electric vehicle drivetrains, Clariant conducted a benchmarking study launching four new base oils. The following base oils were selected, all having the same viscosity at 100°C of 3-4 mm2/ s: Polyglykol E01 P Polyglykol E02 A Polyglykol E03 A Polyglykol E04 E PAO 3.6 Viscosity at 40 °C (mm 2 s -1 ) 13 12 13 22 15 Viscosity at 100 °C (mm 2 s -1 ) 3.2 3.6 3.4 4.1 3.6 3. Results Values on energy efficiency were determined using an elastohydrodynamic (EHD) ball-on-disc tribometer. As parameters, different pressures (800 N mm-2 and 1200 N mm-2), temperatures (40°C and 80°C), and a sum speed of 1.6 m/ s were selected. The slide-to-roll ratio of the ball and disc were varied between 0.0 (pure rolling) and 1.0 (high amount of sliding). Selected results are shown in Graph 1. Surprisingly, the coefficient of friction varies dramatically depending on the selection of the base oil. POLYGLYKOL E04 E showed a outstandingly low coefficients of friction. E.g. at 800 N mm-2, 40°C, sum speed of 1.6m/ s and a SRR of 0.6 the coefficient of friction is less than half compared to PAO (0.021 vs. 0.047). Notably, the coefficient of friction of POLYGLYKOL E04 E is significantly lower, even though the viscosity of POL- YGLYKOL E04 E is higher compared to PAO at 40°C. POLYGLYKOL E02 A and POLYGLYKOL E03 A show coefficients of friction lower than PAO 3.6, whereas the coefficient of friction of POLYGLYKOL E01 P is higher compared to PAO 3.6 Conditions: Pressure: 800 N mm -2 Temperature: 40°C sum speed: 1.6 m/ s Graph 1: Coefficient of friction of different base oils 42 23rd International Colloquium Tribology - January 2022 Base Oil Benchmarking for Gear Oils in Electric Vehicle Drivetrains As a second important property heat transfer of the different base oils was evaluated. For forced turbulent flow conditions, the Mouromtseff Number Mo applies [2]. The higher Mo the more effective the heat transfer. Mo is influenced by the density of the oil, the thermal conductivity, the heat capacity and the dynamic viscosity. All parameters were evaluated and Mo was determined at 40°C and 100°C for all fluids. Remarkably, the heat transfer Mo of POLYGLYKOL E03 A and POLYGLYKOL E04 E are between 10% and 30% higher compared to PAO 3.6 (Graph 2). At 40°C the best heat transfer properties are observed for the base oils POLYGLYKOL E02 A, POLYGLYKOL E03 A and POLYGLYKOL E04 E. At 100°C the highest heat transfer is observed for POLYGLYKOL E04 E. PAO 3.6 showed the lowest Mo at 40°C and the second lowest Mo at 100°C. Graph 2: Relative heat transfer Morel of different base oils. Furthermore, the electric conductivity of the different base oils was investigated. For the application the electric conductivity should be high enough to avoid electro-erosion in bearings because of high voltage build up and subsequent electrical discharge. On the other hand, the electric conductivity should be low enough to avoid short circuits. Minimum and maximum values for the required conductivity are still under discussion. In the benchmarking study PAO 3.6 revealed the lowest conductivity of <1pS/ m and Polyglykol E04 E showed the highest conductivity of approx. 20mS/ m. As the last aspect, sustainability properties were compared. PAO 3.6 is not readily biodegradable, whereas POLYGLYKOL E01 P, POLYGLYKOL E03 A and POL- YGLYKOL E04 E are readily biodegradable. Moreover, POLYGLYKOL E01 P, POLYGLYKOL E02 A, POLY- GLYKOL E03 A and POLYGLYKOL E04 E are hazard label-free according to GHS. Literature [1] McGuire, N. (2021), The brave new world of electric vehicle fluids, Tribology Lubrication Technology [2] Simons, Robert E. (2006), Comparing Heat Transfer Rates of Liquid Coolants Using the Mouromtseff Number, Electronics cooling.