23rd International Colloquium Tribology - January 2022 165 Improving Gear and Thermal Efficiency of Electric Vehicle Fluids Using Group V Base Stocks Gareth Moody Corresponding author: gareth.moody@croda.com Bethan Warren Nicholas Weldon Croda Europe Ltd, Snaith, United Kingdom 1. Introduction The formulating process to produce dedicated transmission fluids for electric vehicles is ongoing, with a focus on several key parameters where standard ATF formulations fell short. New formulations deal primarily with material compatibility (copper, insulating materials and elastomers) and having a low level of electrical conductivity. There has also been a recent drive to maximise thermal properties of these fluids and lower traction to improve gear efficiency. Here, Group V base fluids will be shown to have exceptionally low levels of traction and high levels of thermal conductivity whilst being fully compatible with key materials and Group 1 - 4 base oils. These fluids can be mixed with other base oils to enhance their properties and improve vehicle efficiency when formulated into an electric vehicle gear fluid. Improving gear efficiency has the effect of increasing driving range. It is well established that range anxiety is a key reason why many people would not consider purchasing an electric vehicle. Battery costs are still high ($100-$150 per kW) and because of this, any additional range which can be gained via fluid optimisation can potentially be justified. Further extensions in range can be achieved by reducing fluid viscosity which reduces churning and drag but this must not be at the expense of wear protection and cooling ability. 2. Results The Group V base oils used in this work are esters in the viscosity range 2.5 - 5.5 cSt at 100C. Table 1: viscosity of esters used (low viscosity) Name KV 40 cSt KV 100 cSt VI Group III 3 cSt 12.0 3.2 115 Group IV PAO2 5.0 1.7 238 Fluid 1 9.6 2.9 158 Fluid 2 7.7 2.4 135 Fluid 3 11.5 3.2 149 Figure 1: Traction curves of low vis ester fluids 1-3 vs Group III and Group IV base oils at 40°C and 100°C 166 23rd International Colloquium Tribology - January 2022 Improving Gear and Thermal Efficiency of Electric Vehicle Fluids Using Group V Base Stocks Table 2: viscosity of esters used 4-6 cSt Name KV 40 cSt KV 100 cSt VI Group III 4 cSt 19.3 4.2 122 Group IV PAO4 18.6 4.1 124 Fluid 4 19.0 4.5 163 Fluid 5 20.0 4.4 140 Fluid 6 26.0 5.4 157 The traction curves in Figure 1 were created at a high speed of 2.2 m/ s and load of 16N across a SRR of 0 - 100%. The oils in Figure 1 are neat ester with no additives. Here, all esters have lower levels of traction than the Group III mineral oil and all except one are lower than the PAO. The first point here is that esters can have very low traction, but not all esters. The structure required to achieve this must be considered. Of all the esters tested Fluid 1had the lowest level of traction. Figure 2: Traction curves of low vis ester fluids 4-6 vs Group III and Group IV base oils at 40°C and 100°C Again, at 100°C, the pattern is the same with the ester fluids having lower traction than Group III base oil. From the results it can be seen that using an ester can reduce traction and therefore theoretically improve gear efficiency in a system at high, constant speeds similar to conditions on a highway. The next step was to create more representative formulations which could be used in vehicles. As Fluid 1 showed good low traction properties it was selected for further evaluation. This involved firstly combining the fluids with an additive package to see the influence of this on the traction and then secondly using 20% of Fluid 1 alongside either Group III or Group IV basestock and an additive package used at the recommended treat rate. These were then compared against the same formulation without Fluid 1. Figure 3: Fully formulated Fluid 1 vs Group III 3 cSt traction curves at 40°C and 100°C. The ester fluid has significantly lower traction than the Group III base oil and the additive package appears to have no influence on its performance. At 40°C, the reduction is around 35% and at 100°C the reduction is around 40%. The significance of the reduction is such that the performance of the ester based formulation at 40°C is similar to the performance of the Group III at 100°C despite there being around a threefold difference in viscosity (11.4 cSt for Fluid 1 and 3.4 cSt for Group III). This has important implications not only for efficiency but also potentially for longevity of parts. As a general trend, lubricants have been decreasing in viscosity to improve efficiency and reduce energy consumption but in this case, the same level of traction can be achieved with a higher viscosity fluid. Although efficiency gains have been shown at high speeds, testing drive cycles such as WLTP and indeed real-life driving will also involve much lower speed travelling along with stopping and setting off. Here, the conditions are much harsher and the lubricant is more likely to be in boundary or mixed conditions. To test this, another MTM method was used with low speeds (0.2 m/ s) and higher loads (25N). The influence of a base oil on friction is closely related to its effect on film thickness which will be discussed during the presentation. As well as gear efficiency, thermal properties of fluids are also very important. It is not uncommon for the electric motor and gear system to be incorporated into a single unit and utilise the same fluid for both gear lubrication and motor cooling. In this case, a higher thermal conductivity of a fluid would aid cooling. To assess this, the thermal conductivity of the fluids was tested with the esters in general having a higher thermal conductivity than Group III and IV at 40°C and 80°C. 23rd International Colloquium Tribology - January 2022 167 Improving Gear and Thermal Efficiency of Electric Vehicle Fluids Using Group V Base Stocks Table 3: Thermal conductivity values of fluids Name Thermal cond. 40°C W/ mK Thermal cond. 80°C W/ mK Group III 3cSt 0.124 0.119 Group IV PAO2 0.128 0.120 Fluid 1 0.139 0.130 Fluid 2 0.133 0.126 Fluid 3 0.139 0.133 Group III 3cSt 0.124 0.119 Group IV PAO2 0.128 0.120 Fluid 1 0.139 0.130 Fluid 2 0.133 0.126 Fluid 3 0.139 0.133 3. Conclusion Some esters haven been shown to have exceptionally low traction compared to Group III and Group IV fluids. In particular at high speeds, the traction reduction was up to 30-40% at both 40°C and 100°C The addition of an additive package did not have a negative effect on the performance of Fluid 1. Esters in general have a higher thermal conductivity than mineral oil or PAO. References [1] https: / / www2.deloitte.com/ uk/ en/ insights/ focus/ future-of-mobility/ electric-vehicle-trends-2030. html [2] Kurihara, Isao, and Osamu Kurosawa. “Design and Performance of Low-Viscosity ATF.” SAE Transactions, vol. 116, SAE International, 2007, pp. 805-12, [3] De Laurentis, N., Cann, P., Lugt, P.M. et al. The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/ Sliding Concentrated Contacts. Tribol Lett 65, 128 (2017). https: / / doi.org/ 10.1007/ s11249-017-0908-7