24th International Colloquium Tribology - January 2024 199 Shear Stability and Thermal Performance Analysis of Engine Oils for Electric Vehicles Victor Nino, Fabio Alemanno, Deepak Halenahally Veeregowda 1 Ducom Instruments, Global Applications Team, Groningen, Netherlands * Corresponding author: fabio.a@ducom.com 1. Introduction The soaring adoption of electric vehicles (EVs) is steering the automotive industry toward a greener horizon, with profound implications for the lubricants sector. This manuscript explores the potential values of modifying lubricant testing methods to evaluate the behaviour of lubricants in the presence of changing temperature. 2. Materials and Methods The lubricant used in this study are newly formulated EV fluids given by a major lubricant manufacturer. The main properties of the fluids are listed in Table 1. Table 1. Fluids physical properties. Unit A1 B1 C1 Kinematic Viscosity (@ 40 °C) mm²/ s 40 55 18.5 Density kg/ m³ 950 950 950 Flash point °C 200 200 200 The lubricants were tested in a Four Ball Tester according to the ASTM D4172-B test method, and in a KRL Shear Stability Tester according to the CEC L-45-99 test protocol. Both the test method require a precise control of the test temperature during their execution. A modified version of both procedures was also followed. The ASTM D4172-B test method was modified by starting the test at 30 °C (instead of 75 °C as prescribed) and letting the temperature rise independently as a consequence of the friction generated at the sliding contact. The CEC L-45-99 test method was shortened, and divided into segment during which the temperature control was activated to keep the temperature to the prescribed value of 60 °C, to segments in which the temperature control system was deactivated to allow the temperature to rise spontaneously as a result of the friction generated. An example of such a cycle is reported in Figure 1. Figure 1. Temperature profile during the modified KRL test protocol. 3. Results Four Ball Tester Results The Wear Preventive (WP) tests run according to ASTM D 4172-B were analyzed to highlight differences in the lubricants in terms of friction and mean wear scar diameter on the test balls. As shown in Figure 2, the friction coefficient showed by the fluid C1 is slightly higher than the values showed by both A1 and B1, which resulted in similar friction values. Figure 2. Evolution of friction coefficient over time during ASTM D4172-B tests. The mean wear scar diameter values obtained with the three fluids showed a difference comparable to the precision of the measurement system, and were therefore considered not significantly different. Shear Stability and Thermal Performance Analysis of Engine Oils for Electric Vehicles 200 24th International Colloquium Tribology - January 2024 The average friction and wear values obtained with both the standard and the modified ASTM D4172-B test protocol are shown in Figure 3 and Figure 4, respectively. Figure 3. Average friction coefficient measured during both standard and modified ASTM d4172-B tests. Both friction and wear values measured after the modified test protocol were lower than the ones measured after the standard procedure. In particular, fluids A1 and B1 resulted in a wear reduction of about 4%, whereas C1 resulted in a 10% reduction. Figure 4. Mean Wear Scar Diameter (MWSD) measured during both standard and modified ASTM d4172-B tests. Since the temperature of the modified protocol was not controlled, the three fluids resulted in a different final temperature. Figure 5 shows the temperature change from the initial temperature of 30 °C. The temperature change for fluids A1, B1 and C1 were 31, 38 and 30 °C, respectively. Figure 5. Temperature changes with respect to the initial temperature during modified ASTM D4172-B tests. KRL Shear Stability Tester Results The conventional KRL Shear Stability tests according to CEC L-45-99 resulted in a viscosity loss of around 4% for both fluid A1 and B1, while C1 resulted in a 1.7% viscosity loss, as shown in Figure 6. Figure 6. Viscosity measurement before and after the CEC L-45-99 tests. The thermal cycling resulted in 5 cycles of spontaneous heating for each test. Figure 7 compares the average of each of the 5 cycles for each test. The temperature change for fluids A1, B1 and C1 were 18, 22 and 8 °C, respectively. Figure 7. Temperature changes with respect to the initial temperature during modified CEC L-45-99 tests. 4. Conclusions The results shows that fluids with negligible differences in their ASTM D4172-B friction and wear properties, can still be differentiated with respect to their response when one additional degree of freedom (i.e. temperature) is added to the Four Ball test setup. In the KRL test setup, differentiating the lubricants with respect to their response to a thermal stimulus helped in increasing the small differences observed with the conventional measurand (i.e. viscosity loss). The temperature increase in both the Four Ball and KRL Shear Stability tester seems to correlate with the viscosity value, with a direct proportionality between viscosity and temperature increase, whereas an inverse proportionality between viscosity and Four Ball Tester friction coefficient could be observed.