24th International Colloquium Tribology - January 2024 189 Correlation of MTM Striebeck Curves with Efficiency Data for Predictive Analysis of Coaxial EV Gearbox Performance An Experimental Study Dmitriy Shakhvorostov 1* , Mirjam Bäse 2 1 Oil Additives, Evonik Operations GmbH, Kirschenallee 64293 Darmstadt, Germany 2 Magna Powertrain GmbH & Co KG, Plant Lannach, Lannach, Austria * Corresponding author: dmitriy.shakhvorostov@evonik.com 1. Introduction The transition towards fully electric or hybrid powertrains in passenger vehicles has become the preferred solution to meet greenhouse gas emission reduction targets [1]. Ongoing optimization efforts are focused on achieving maximum efficiency and durability for all the components, including the mechanical transmission, electric motor, and power electronics. Improving efficiency not only reduces CO 2 emissions during the vehicle‘s use phase, but also extends the range of electric vehicles. Additionally, it can help reduce costs and CO 2 emissions associated with production, such as by reducing the size of the battery. This research study investigates the correlation between Mini Traction Machine (MTM) Stribeck curves and efficiency data obtained from a coaxial EV gearbox, aiming to predict and enhance the overall efficiency of the transmission system. By employing both ball-on-disk and low-pressure barrel-on-disk setups, the Stribeck curves were measured for eight different lubricating fluids. Notably, the study revealed that the low-pressure barrel-ondisk contact configuration provided a better correlation between the MTM Stribeck curves and efficiency data compared to the ball-on-disk contact configuration. The obtained data was then compared to efficiency measurements from the Worldwide Harmonized Light Duty Testing Procedure (WLTP) to establish correlation using the Pearson coefficient. 2. Results The results showed a significant correlation, enabling the utilization of the MTM methodology for predicting the efficiency of the transmission in a WLTP driving cycle. Additionally, specific operating points in terms of torque and speed demonstrated satisfactory correlation, although a few operating points did not exhibit the same level of correlation. The temperatures utilized in the MTM tests were precisely aligned with the operating temperatures within the transmission. The precision of the transmission tests and the MTM predictions, calibrated using five fluids with established correlation, were within the range of +- 0.02%. Notably, the correlation performed well at 40-°C operating temperature (see Fig 1), while the correlation at 80-°C operating temperature was relatively less robust. Fig. 1 Result of correlation of efficiency improvement in electric vehicle transmission in WLTP simulated cycle to a relevant domain averaged friction coefficient in a MTM with low pressure barrel-on-disk contact. 3. Conclusion While our study shows that it is useful to apply MTM (Fig.-1) for additives and base oils selection for transmission efficiency optimization, we could identify limits of the methodology at conditions resembling an intensive mixed lubrication regime, where MTM specimen surfaces do not replicate the surfaces in the real application (transmission). Special care needs to be taken for proper conditions selection (unlike conditions selection approach seen in the literature [2-3]) for a meaningful screening. References [1] UNECE Vehicle Regulations. [Online] https: / / unece. org/ transport/ vehicle-regulations. [2] Cañellas, G.; Emeric, A.; Combarros, M.; Navarro, A.; Beltran, L.; Vilaseca, M.; Vives, J. Tribological Performance of Esters, Friction Modifier and Antiwear Additives for Electric Vehicle Applications. Lubricants 2023, 11, 109. [3] Costello, Michael T. “Effects of basestock and additive chemistry on traction testing.” Tribology Letters 18 (2005): 91-97.