24th International Colloquium Tribology - January 2024 27 Dynamic Properties of Lubricants for Electric Vehicles EV fluids Yan Chen 1 and Hong Liang 1,2 * 1 Department of Materials Science & Engineering, Texas A&M University, College Station, TX 77843, USA 2 J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA * Corresponding author: hliang@tamu.edu 1. Summary & Introduction Evolving needs in lubricants requires better understanding and testing. In this presentation, the requirements in lubricants for electric, hybrid, and internal combustion engine (ICE) vehicles will be compared to identify key performance characteristics. Further discussion will be focused on our recent research. Our recent study has revealed that certain fundamental properties of lubricants alter under working and electrified conditions. Specifically, we investigated the properties of working lubricants, their electrical and thermal properties. In establishing relationship between electrical conductivity and a fluid oil film thickness, results indicated the non-ohmic behaviour of a lubricating film in the hydrodynamic regime. In probing thermal performance, we found out that thermal properties of lubricants depended on the shear that are not constant as being widely accepted. These findings are beneficial to design effective EV lubricants. 2. Working Fluids A lubricant becomes a working fluid when a mechan-ical system, such as a vehicle, is in operation. To satisfy the working conditions of an electric vehicle, new challenges arose over the electrical and thermal properties of the fluid. The current understanding about the properties of lubricants has been on the fluidic viscosity, [1-3] film formation, [4-6] and the frictional respond to shear [7, 8]. For the application to EVs, electrical and thermal conductivities are important. 3. Dynamic Properties of Working Fluids In this research, we constructed a system to success-fully examine the electrical conductivity against the oil film thickness. The thickness and electrical re-sistance can be calculated from impedance. We in-tegreate an electrochemical potential state with a disc-on-disc tribomeer. It allowed us to meaure the capacitor and resitor parallel. If we assume that ca-pacitance is fully contributed from the oil film. It thus has a dielectric constant of 2.1 [9]. The our eq-uition is like the following: (1) and (2) where R is the resistance, Z is the impedance sub-tract the impedance of the shorted measuring system Z = Z measured - Z shorted . A is the nominal area of contact, ee r is the dielectric constant, w is the angular frequency of the applied voltage. Then Re and Im take the real and imaginary part of a complex number, respectively. Our data showed that, interestingly, there was a non-ohmic behavior of the fluid in the hydrodynamic regime. Further experiments were conducted, and it showed that the properties of fluids are affected by a few factors. The study on thermal performance of a mineral oil and polyalphaolefin (PAO) was also carried out. Data gathered showed that the thermal properties of fluids are affected by the shear stress that has not been widely understood. 4. Conclusion Fluids‘ behave differently when under a share force than static. In this work, we experimentally studied the non-ohmic behavior of working fluids in they are in the hydrodynamic regime. We electrically measured the oil film thickness against its temperature. Our restuls showed that the „dynamic“ thermal conductivity of a mineral oil was 0.25mW/ K and that of a Poly-alpha-olefin (PAO) oil was 0.2mW/ K, when the speed/ load of the tribometer was set at 100cm/ Ns. These data indicated that commercial lubricants for conventional vehicles could be improved in order for them to be adapted to electric vehicles. Detailed discusison as well as thermal conductivities will be provded during presention. 28 24th International Colloquium Tribology - January 2024 Dynamic Properties of Lubricants for Electric Vehicles Figure 1, The electrical resistance against the thick-ness of an oil film. There are two regions: the linear at the lower left region and non-linear upper right. Figure is adapted from the ref. [10]. References: [1] Hamrock, B. J., and Dowson, D., 1977, “Isothermal Elastohydrodynamic Lubrication of Point Contacts: Part III-Fully Flooded Results,” J. Lubr. Technol., 99(2), pp. 264-275. [2] Okrent, E. H., 1961, “The Effect of Lubricant Viscosity and Composition on Engine Friction and Bearing Wear,” ASLE Trans., 4(1), pp. 97-108. [3] Evans, C. R., and Johnson, K. L., 1986, “The Rheological Properties of Elastohydrodynamic Lubricants,” Proc. Inst. Mech. Eng. Part C J. Mech. Eng. Sci., 200(5), pp. 303-312. [4] Jablonka, K., Glovnea, R., and Bongaerts, J., 2012, “Evaluation of EHD Films by Electrical Capacitance,” J. Phys. Appl. Phys., 45(38), p. 385301.] [5] Jablonka, K., Glovnea, R., Bongaerts, J., and Morales-Espejel, G., 2013, “The Effect of the Polarity of the Lubricant upon Capacitance Measurements of EHD Contacts,” Tribol. Int., 61, pp. 95-101. [6] Johnston, G. J., Wayte, R., and Spikes, H. A., 1991, “The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts,” Tribol. Trans., 34(2), pp. 187-194. [7] Cann, P. M., and Spikes, H. A., 1989, “Determination of the Shear Stresses of Lubricants in Elastohydrodynamic Contacts,” Tribol. Trans., 32(3), pp. 414-422. [8] Masjedi, M., and Khonsari, M. M., 2014, “Theoretical and Experimental Investigation of Traction Coefficient in Line-Contact EHL of Rough Surfaces,” Tribol. Int., 70, pp. 179-189. [9] Carey, A. A., “The Dielectric Constant of Lubrication Oils,” p. 9. [10] Y. Chen and H. Liang, “Tribological Evaluation of Electrical Resistance of Lubricated Contacts,” ASME J. Trib., 142(11), 2020. Pp: 114502. DOI: 10.1115/ 1.4045578.