24th International Colloquium Tribology - January 2024 195 Oil Aging on a Test Rig to Introduce Sustainable Lubricants in Electric Vehicle Transmissions Timo Koenig 1* , Marco Kohnle 1 , Luca Cadau 1 , Lukas Steidle 1 , Didem Cansu Gueney 2 , Katharina Weber 2 , Joachim Albrecht 2 , Markus Kley 1 1 Aalen University, Institute of Drive Technology Aalen IAA, Aalen, Germany 2 Aalen University, Research Institute for Innovative Surfaces FINO, Aalen, Germany * Corresponding author: timo.koenig@hs-aalen.de 1. Introduction There is currently a growing demand for electric vehicles. In order to make them even more environmentally friendly, there is the need to replace the conventional transmission oil with sustainable alternatives that must have at least similar properties. The demand for sustainable lubricants is increasing due to the finite fossil oil resources and the release of conventional lubricants into the environment. The sustainable lubricants, which are biodegradable, could reduce future damage when used in transmissions. This paper compares the properties of sustainable oils at different stages of aging. The properties of the oils are measured using rheological investigations. An innovation of this publication is that the aging of the sustainable oil is achieved by an standardized oxidation test, the so-called TOST test, according to DIN EN ISO 4263-1 [1] and real test rig experiments with an electric vehicle transmission. The aged oils are compared with the new oil in terms of sustainability to allow a better understanding of the aging behaviour. 2. State of the Art This publication shows a comparison between new, oxidized and aged oil in a transmission. As a reference for aging of lubricants, a publication is used which shows a concept for comparison of new and aged lubricants in transmissions and a method for oil aging [2]. The focus here is to highlight the differences between the aged oils in comparison to the new oil. High torques and speeds as well as a lot of other factors are among the biggest stress factors that can affect the performance of lubricants in electric vehicles [3]. For this reason, it is necessary to develop and apply special lubricants that are adapted to the conditions and requirements of electric vehicles. For most applications, conventional, mineral oil-based lubricants are still used. In order to meet the sustainability goals, also in the area of lubricants, it is essential to use sustainable oils. [2] These oils generally must have a suitable viscosity, be oxidation-resistant and thermally stable. High load capacity and corrosion protection are also requirements as well as the friction coefficient. These are crucial points for all lubricants, regardless of the sustainability. [2] It is a well-known point that the properties of lubricants deteriorate with increasing operating time. The aging behavior of a biodegradable lubricant, in this application a polyalkylene glycol-containing lubricant with an amine phosphate as an additive to reduce friction, is therefore to be investigated. The requirements of the sustainable lubricant are adapted to those of the conventional oil used so far. It can be emphasized that in the literature a direct comparison between new, oxidized oil and oil aged in a transmission has been made only to a limited extent. 3. Oil aging process and oil analysis Two different approaches are used for the aging of the sustainable oil in this paper. The aged lubricants are compared with each other as well as with the new oil. The first test is a standardized oxidation test, the so-called TOST test with a total running time of 312 h, and the second test is represented by realistic aging on a test rig in a transmission. For the tests on the drive test rig, a load collective is defined to reflect the real operating conditions of the transmission, so that the oils can be aged on the test rig under conditions that are as real as possible (Cf. with figure 1). The load points are randomized but with a repeating sequence so that the lubricant can be aged for 250 h in total. [2] Figure 1: Defined load collective for oil aging [2] The aged lubricants, each with an almost identical aging time, and the new lubricant are analyzed with different rheological tests at different temperatures. The conducted rheological experiments are described in more detail in a previous paper [2]. In the following diagrams, new oil is denoted in blue (- -), oxidized oil in red (- -), and real aged oil in green (- -). The curves shown are each calculated from three individually performed tests. The flow behavior of the oils is visualized in figure 2. The viscosity at constant temperature T is determined as a function of shear rate γ. There is a clear contrast between oxidized and real aged oil in a transmission. The analyzed oils show a shear-thinning behavior at 20-°C and an ideal viscous behavior at 60-°C. The viscosity of real aged oil remains significantly lower than that of oxidized oil throughout. 196 24th International Colloquium Tribology - January 2024 Oil Aging on a Test Rig to Introduce Sustainable Lubricantsin Electric Vehicle Transmissions Figure 2: Flow Behavior Figure 3 shows the temperature behavior of the analyzed sustainable oil. Here, the temperature is increased at a constant shear rate (100 1/ s) and the viscosity h is measured at different temperatures T. The curves of the oxidized and the new oil are almost identical, whereas the curve of the real aged oil shows lower viscosities over the temperatures. However, the temperature dependent behavior is identical in all tests. Figure 3: Temperature Behavior Figure 4 illustrates the jump test. The study investigates the regeneration and recovery of the structure after a sudden, strong shear loading of the specimen. The findings are presented as a function of the viscosity h over the time t. Here, it is also shown that the real aged oil has a lower viscosity, although the regeneration can be evaluated equally in comparison to all samples. Figure 4: Jump Test Figure 5 presents the frequency test. The time-dependent behavior of the oil in the non-destructive range is analyzed by changing the oscillation frequency while keeping the amplitude constant. The storage moduli G 9 and the loss moduli G 0 are depicted in relation to the angular frequency w. All samples exhibit nearly the same intersection point. The intersection points of G 9 and G 0 shift to the right at higher temperatures. Figure 5: Frequency Tests at 20-°C and 60-°C As can be seen in the figures, there are notable differences between the real aging and the standardized TOST test. The results of the laboratory oxidation tests do not fully match the results of the rig tests for the sustainable oil. In the real aging curves, the viscosity is significantly lower than in the calculated curves in new condition and after the oxidation test. This decrease in viscosity can possibly be attributed to moisture absorption during rig testing. 4. Conclusion In summary, differences can be observed in rheological tests according to different aging methods. Therefore, the TOST test may not adequately represent the real aging of the sustainable oils as reflected in the differences in viscosity, and new laboratory test procedures could be adjusted for the new generation of sustainable gear oils. References [1] DIN EN ISO 4263-1: 2004. Mineralölerzeugnisse und verwandte Produkte - Bestimmung des Alterungsverhaltens von inhibierten Ölen und Flüssigkeiten - TOST Verfahren - Teil-1: Verfahren für Mineralöle [2] König, T., Cadau, L., Steidle, L., Güney, D. C., Albrecht, J., Weber, K. and Kley, M.: A concept for comparison of new and aged lubricants in transmissions of electric vehicles and a method of oil aging on a test rig. Forschung im Ingenieurwesen (2023), doi: 10.1007/ s10010-023-00705-3. [3] Aguilar-Rosas, O. A., Farfan-Cabrera, L. I., Erdemir, A. and Cao-Romero-Gallegos, J. A.: Electrified fourball testing - A potential alternative for assessing lubricants (E-fluids) for electric vehicles. Wear 522 (2023), pp.-204676, doi: 10.1016/ j.wear.2023.204676.