potential to enhance the long-term stability of the lubricant and the drive system under severe operating conditions. A primary objective is to mitigate friction to minimize energy consumption, thereby contributing to energy efficiency. Optimizing lubrication and protective properties, the project aims to extend maintenance intervals which not only reduces energy costs but also enhances the overall CO 2 balance, aligning sustainability goals [9]. The selection of PAG as a lubricant of choice is backed by previous studies highlighting its superior stability to temperature fluctuations and minimal changes in friction coefficient after oxidation compared to other lubricants [10]. Ultimately, the overarching goal is to strike a balance between technical performance and sustainability, ensuring that the lubricant not only meets functional requirements but also aligns with environmental objectives. TAE-Colloquium Tribology 2024 4 Tribologie + Schmierungstechnik · volume 71 · eOnly special issue 2/ 2024 1 Introduction Lubricants play a crucial role in enhancing the efficiency of vehicles and construction equipment, particularly within the automotive sector [1]. The optimization of performance requires a focus on sustainable and ecofriendly alternatives due to issues such as leakage and disposal [2]. The emergence of electric vehicles presents new hurdles, including the demand for high-speed operation exceeding 20,000 rpm at elevated temperatures [3]. Currently, a significant challenge is the limited availability of commercially available lubricants that meet criteria such as sustainability, biodegradability, non-toxicity, and performance enhancement for electromechanical drives [4]. Gear oils are instrumental in minimizing friction, wear, noise, and corrosion [5]. Sustainable gear oils aim to prolong change intervals and achieve a 0.5 % increase in powertrain efficiency, thereby reducing environmental impact. This objective aligns with the standard mandating 25 % renewable content and substantial biodegradability for sustainable lubricants [6-8]. The research project aims to investigate the rheological and tribological properties of a sustainable, biodegradable polyalkyleneglycol-based (“PAG”) lubricant in both new and aged condition in a transmission. The evaluation is carried out against a conventional mineral oil-based lubricant as a reference. It is seeked to validate the suitability of PAG as a long-term lubricant and to assess its DOI 10.24053/ TuS-2024-0028 Performance of Sustainable Lubricants in Electric Vehicle Transmissions: Ageing Effects Didem Cansu Güney, Timo König, Daniel Proksch, Prof. Dr. Markus Kley, Prof. Dr. Joachim Albrecht, Prof. Dr. Katharina Weber* Presented at 24 th International Colloquium Tribology (TAE) | submitted: 7.03.2024 accepted: 29.10.2024 This study explores the sustainable performance of polyalkyleneglycol-based (“PAG”) lubricants in electric vehicle transmissions under ageing conditions. Comparing PAG with mineral oil-based lubricants, including a specially aged variant (“PAG APS”), rheological and tribological properties are assessed in lab scale and under application conditions in a test rig. A particular “Jump Test” reveals temperaturedependent viscosity changes and regeneration times. Tribological investigations under extreme conditions highlight PAG’s lower coefficient of friction, emphasizing its potential advantages during ageing. The study aims to provide insights into sustainable lubricants, contributing to enhanced long-term stability and reduced environmental impact in electromechanical drive systems, addressing challenges in the electric vehicle sector. Keywords Sustainable lubricants, oil aging, electromechanical drive system, rheology, tribology, test rig, Polyalkyleneglycol Abstract * Didem Cansu Güney a (corresponding author) Timo König b Daniel Proksch b Prof. Dr. Markus Kley b Prof. Dr. Joachim Albrecht a Prof. Dr. Katharina Weber a a Research Institute for Innovative Surfaces FINO Aalen University, Beethovenstr. 1, D-73430 Aalen Germany b Institute of Drive Technology Aalen IAA, Aalen University, Beethovenstr. 1, D-73430 Aalen Germany 2 Experimental This experimental investigation focuses on comparing the performance of two gear oils for electromechanical drive systems: a standard mineral oil-based conventional lubricant (referred to as "Mineral”) and a polyalkyleneglycol-based lubricant (referred to as “PAG”). Additionally, the study includes a specially aged variant of PAG, denoted as “PAG APS” that undergoes ageing in a specific test rig. All lubricants share a viscosity of 220 mm 2 / s. Mineral is a widely used industrial gear lubricant as a benchmark for comparison while PAG represents an unexplored sustainable alternative under investigation. The inclusion of the PAG APS sample enables a detailed examination of how ageing affects the performance characteristics of PAG. It provides insights into its behavior compared to both the non-aged PAG and the traditional lubricant Mineral. This investigation aims to comprehensively evaluate the suitability of PAG for electromechanical drive systems and assess its performance relative to industry-standard lubricants. 2.1 Oil ageing in the transmission test rig Given the established knowledge regarding the ageing behavior of mineral oil-based lubricants, further evaluation is deemed unnecessary. Consequently, this study focuses on comparing sustainable lubricants, both in their new and aged states, with the conventional mineral oil-based lubricant in its new condition. As an innovation, it can be emphasized that the sustainable lubricant is aged via real experiments in a transmission system on a drive test rig. A concept for oil ageing is already described in publications to allow a comparison of new and aged lubricants in transmission of electric vehicles [11; 12]. The described approach is used to carry out the experiments. Therefore, a similar transmission system, the test rig setup as well as the identical load collective is drawn on to run the rig tests. The test rig setup is visualized in Figure 1. The aim here is to bring the oil up to operating temperature, so that the oil is aged in the shortest possible time. The input and output machines are on the rightand left-hand side in combination with torque TAE-Colloquium Tribology 2024 5 Tribologie + Schmierungstechnik · volume 71 · eOnly special issue 2/ 2024 DOI 10.24053/ TuS-2024-0028 Figure 1: Test rig setup for the ageing experiments [11] Figure 2: Collective with the defined load points at different speed and load conditions of the transmission system [11] measuring shafts. In addition to the torque measuring shafts, speed sensors are used. These allow to adjust the operating points. In the middle the transmission is placed, a two-stage planetary transmission with a maximum power of 8 kW. To carry out the experiments to achieve the ageing of the oil, a load collective is defined. As shown in Figure 2, load points with high torques (1-3), high powers (3-5) and high speeds (5-7) are defined to mimic the real operating conditions and driving behavior, but still with the aim to age the oil. Each load point is held for 180 s and additional 20 s are defined to take up and drive to the subsequent one. The load points are randomized, but with a repeating sequence, so that we have a total running time of 250 hours in total. 2.3 Tribological Investigation An oscillating tribometer of the “ETS” series from “Optimol Instruments Prüftechnik GmbH” in accordance with DIN 51834-2 has been used to determine the coefficient of friction, Figure 4. A spherical counter-body, specifically 100Cr6, exerted a constant force of 50 N perpendicular to the surface (also 100Cr6) lubricated with 0.3 ml of oil. The lubricated surface was subjected to a linear oscillatory motion with a stroke of 1 mm and a frequency of 50 Hz for 60 minutes. These tribological measurements were carried out under extreme conditions to gain insight into the critical performance of the lubricant. The aim was to simulate realworld scenarios with highest operational demands. The used parameters included a high normal force of 50 N, a spherical counter body with a small radius of r = 5 mm, and low relative speeds well below 0.1 m/ s. At these values, the estimated thickness of the lubricant film was in the order of 10 nm, significantly lower than the surface roughness of both the steel surface and the counterpart. Despite the intended partial lubrication, there was always solid contact between the interacting surfaces under these extreme conditions, leading to significant friction and wear. This scenario underscores the importance of understanding lubricant performance in challenging operational demands and highlights the necessity of developing lubricants capable of withstanding such harsh conditions. TAE-Colloquium Tribology 2024 6 Tribologie + Schmierungstechnik · volume 71 · eOnly special issue 2/ 2024 DOI 10.24053/ TuS-2024-0028 2.2 Rheological Investigation Rheological investigations are essential for characterizing the behavior of viscoelastic materials under varying stress and deformation conditions. A specific method employed to explore the response of such materials to sudden changes in load or deformation is the so called “Jump Test” [13]. This method facilitates the determination of crucial rheological parameters, notably the relaxation time. The Jump Test is divided into three sections, as illustrated by Figure 3 depicting the shear rate profile with markings 1, 2, and 3 corresponding to specific shear rates. The test begins by subjecting the lubricant to a constant shear rate to establish a stable state. Subsequently, there is an abrupt change of this load to examine the viscoelastic reaction of the liquid. In this experiment, a low shear rate of 0.1 1/ s is initially set, then abruptly increased to 7000 1/ s, and finally returned to the initial condition. Measurements were conducted both at 20 °C and at 60 °C, with the latter representing the operating temperature of the oil within the transmission. Throughout this process, observations are made regarding how the lubricant reacts to these changes and its behavior over time. The relaxation time, a key rheological parameter, can be derived from the time it takes for the lubricant to return to its original equilibrium state after the jump. The test has been carried out using a rotational rheometer of the model “Physica UDS 200” from “Anton Paar Germany GmbH”, using a cone-plate system with a cone radius of 25 mm and an angle of 1°. The stationary plate has an integrated Peltier element for temperature control. Figure 3: Shear-rate profile illustrating different phases marked with 1, 2, and 3, corresponding to specific shear rates: 1) Initial low shear rate of 0.1 1/ s, 2) abrupt increase to 7000 1/ s, and 3) return to the initial condition Figure 4: Coefficient of friction tested with “ETS” series tribometer: 100Cr6 spherical counter body, 50 N normal force, 1 mm oscillation stroke for 60 minutes 3 Results 3.1 Results of rheological investigation Figure 5 illustrates the Jump Tests of PAG and PAG APS from the transmission (blue curves), along with the reference oil Mineral (black). Examining the jump test results at room temperature (20 °C, Figure 5), it is clear that no regeneration is observed during this period. Initially, there is a noticeable increase in viscosity in the third section of the curve indicating a response to the abrupt change in shear rate. However, the viscosity does not fully recover to the level observed in the first section. Instead it decreases to a certain point. This decrease in viscosity further indicates a lack of regeneration at room temperature. In addition, in this context, Mineral exhibits the highest viscosity compared to the other lubricants, while the sample aged in the test rig shows the lowest viscosity, indicating a decrease due to the ageing of PAG. This contrasts to results of pure oxidation tests where an increase of the viscosity has been found (Turbine Oxidation Stability Test) [10]. This suggests that a different and more complex ageing process is taking place in this study leading to a decrease in viscosity. At operating temperature (60 °C), all lubricants regenerate after different regeneration times. During the Jump Test at this temperature, it is noticeable that the initial viscosity level observed in the first part of the curve is reached in the third part again, indicating a regeneration process for all of the lubricants. However, once this initial level of viscosity is reached, there is a slight decrease in viscosity up to a certain point. This observation suggests that although regeneration takes place, there may be a subsequent stabilisation or settling phase characterised by a slight decrease in viscosity. The viscosity of the lubricants experiences a notable decrease compared to room temperature. In this context, PAG stands out with the highest viscosity, while Mineral demonstrates the lowest viscosity among the lubricants. Moreover, the viscosity of the aged PAG is observed to be lower than that of PAG, indicating a decrease due to ageing. The data presented in Table 1 provides insights into the regeneration times of the lubricants at different temperatures. The decrease in viscosity is associated with shorter regeneration times. At 20 °C, none of the three lubricants show any regeneration. At 60 °C, PAG exhibits a regeneration time of 96 s, while PAG APS in the transmission shows a shortened regeneration time of 80 s. In comparison, Mineral at 60 °C demonstrates the lowest regeneration time of 60 s which is nearly 40 % less compared to Mineral. The regeneration time of PAG decreases with ageing in the transmission. This suggests a complex interaction between the physical properties of the lubricant and the ageing process. 3.2 Results of tribological investigation The results of the tribological investigations are shown in Figure 6, where the coefficient of friction µ is plotted over time. The graph shows significant differences between the lubricants. In the following, we distinguish between the running-in phase that is observed up to a time of t = 1000 s and the steady state, characterized by a constant value of µ from 1000 s onwards. The latter is used to calculate the average coefficient of friction. To provide further clarity on the results, Figure 7 presents the average coefficients of friction in a bar chart. TAE-Colloquium Tribology 2024 7 Tribologie + Schmierungstechnik · volume 71 · eOnly special issue 2/ 2024 DOI 10.24053/ TuS-2024-0028 Figure 5: Jump test of PAG (blue) and PAG APS (dashed blue) with dynamic viscosity over time at various shear rates (1 - 3) and temperatures (20 °C and 60 °C) compared to Mineral (black) Figure 6: Coefficient of friction analysis - The graph divides into running-in phase and steady state, with the average coefficient of friction calculated from values starting at t = 1000 s Table 1: Regeneration time of the lubricants at varying temperatures Acknowledgment The authors would like to thank Zeller + Gmelin GmbH & Co. KG for generously providing the essential oils required for the preliminary studies. Sincere thanks are also due to the German Federal Ministry of Education and Research (BMBF) for their joint support under the program ‘Forschung an Fachhochschulen’ (Grant No.: 13FH566KX1). References [1] Woydt, M., Wear, 488-489, 204134, 2022. [2] Salih, N., and Jumat, S., Biointerface Res Appl Chem, Vol. 11, 13303-13327, 2021. [3] Farfan-Cabrera, L.I., Tribology International, Vol. 138, 473-486, 2019. [4] Beyer, M., Brown, G., Gahagan, M., Higuchi, T., Hunt, G., Huston, M., Jayne, D., McFadden, C., Newcomb, T., Patterson, S., Prengaman, C., and Shamszad, M., Tribology Online, Vol. 14, 428-437, 2019. [5] Arca, M., Sharma, B.K., Perez, J.M., and Doll, K.M., International Journal of Sustainable Engineering, Vol. 6, 326-331, 2013. [6] Bartz, W.J., Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, Vol. 214, 189-196, 2000. [7] Deutsches Institut für Normung e.V., Flüssige Mineralöl- Erzeugnisse - Bio-Schmierstoffe - Empfehlungen für die Terminologie und Charakterisierung von Bio-Schmierstoffen und bio-basierten Schmierstioffen, 2011 - 10. [8] Lubricants and lubrication, 2 nd ed., Wiley-VCH-Verl., Weinheim, 850 p., 2007. [9] Bartels, T., Bock, W., Braun, J., Busch, C., Buss, W., Dresel, W., Freiler, C., Harperscheid, M., Heckler, R.-P., Hörner, D., Kubicki, F., Lingg, G., Losch, A., Luther, R., Mang, T., Noll, S., and Omeis, J., “Lubricants and Lubrication”, in: Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, p. 607, 2000. [10] Güney, D.C., Joukov, V., Albrecht, J., and Weber, K., Materialwissenschaft Werkst, Vol. 54, 1390-1399, 2023. [11] König, T., Cadau, L., Steidle, L., Güney, D.C., Albrecht, J., Weber, K., and Kley, M., Forsch Ingenieurwes, Vol. 87, 1069-1080, 2023. [12] 24 th International Colloquium Tribology, expert verlag, 2024. [13] Mezger, T., Angewandte Rheologie, 6 th ed., Anton Paar GmbH, Graz, 195 p., 2021. TAE-Colloquium Tribology 2024 8 Tribologie + Schmierungstechnik · volume 71 · eOnly special issue 2/ 2024 DOI 10.24053/ TuS-2024-0028 It is evident that Mineral exhibits the highest coefficient of friction while PAG has the lowest. Interestingly, the effect of ageing in the test rig transmission on the coefficient of friction of PAG appears to be minimal, as indicated by the barely visible increase in the coefficient of friction of PAG APS. Notably, even with ageing, the coefficient of friction for PAG APS remains lower than that of Mineral. Additionally, it is observed that viscosity decreases with ageing, while the coefficient of friction slightly increases. The impact of these findings for the wear properties of this lubricated tribological contact will be topic of further investigations. 4 Summary In summary, this paper investigates the sustainable performance of polyalkyleneglycol-based (PAG) lubricants in electric vehicle transmissions under ageing conditions. Through rheological and tribological investigations, the study evaluates temperature dependent viscosity changes, regeneration times and coefficient of friction differences between PAG and mineral oil-based lubricants. The results highlight the favorable properties of PAG, including lower viscosity and coefficient of friction, even after ageing, emphasizing its potential as a longterm lubricant solution for electromechanical drive systems. Overall, the study provides valuable insights into the development of environmentally friendly lubricants to meet the evolving needs of the automotive industry, particularly in the context of electric vehicles. Figure 7: Average coefficient of friction of the lubricants