Gabriel CALDERON-SALMERON1, Johan LECKNER1,2, Sergei GLAVATSKIH1,3,4
1KTH, Stockholm, Sweden
2Axel Christiernsson International AB, Nol, Sweden
3Ghent University, Ghent, Belgium
4University of New South Wales, Sydney, Australia
Electric vehicles (EV) are changing the automobile industry and represent one of the main alternatives to contribute to the UN's sustainable development goals. However, the success of this alternative depends on overcoming the different challenges created by using an electric motor as the vehicle propulsion system. Such challenges are mainly related to improving the vehicle's efficiency and reliability, parameters that strongly depend on the design of the lubrication system (lubricant, components, materials, etc.).
Among the different components of an EV, the rolling element bearings that support the motor rotor (usually grease-lubricated) are exposed to extremely harsh conditions. These conditions include a wide range of constantly fluctuating speeds (low and very high) and exposition to electric discharges caused by the charge build-up when the lubricant act as a capacitor. Therefore, greases that enable vehicle operation under these conditions are needed. Conductive greases could mitigate the electric discharge problem since the grease conductivity allows a controlled current flow through the different bearing contacts [1,2,3]. However, the lubrication efficiency and reliability (evaluated through the tribological response) are still a point of concern.
Ionic liquids (ILs) have shown good potential as grease additives due to their high thermal stability, low vapour pressure, and good surface adsorption[3]. Additionally, they can provide different conductivity levels in the lubricant depending on their chemistry [4]. These characteristics made them suitable candidates for EV motor bearings. Yet, to include ILs in a grease formulation, several design considerations need to be taken into account during the grease and IL design phase.
In this work, we discuss the applicability of ILs in greases in rolling bearings operating in conditions similar to the ones of an EV, the considerations that must be included from the grease design phase, and proper methodologies to evaluate the energy-saving potential that could be achieved with the use of different ILs.
References:
[1] Klueber, Ionic liquids – innovative lightning conductor in e-mobility, // Klüber Lubrication. Available at: https://www.klueber.com/us/en/company/newsroom/news/ionic-liquids-innovative-lightning-conductor-in-e-mobility/ (Accessed: November 24, 2022).
[2] Suzumura, Junichi. "Prevention of electrical pitting on rolling bearings by electrically conductive grease." Quarterly Report of RTRI 57.1 (2016): 42-47.
[3] Wade Millar, Zachary M. Aman, Rob Atkin, Hua Li, Graphite infused ionic liquid greases, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 653, 2022, 130017, ISSN 0927-7757, https://doi.org/10.1016/j.colsurfa.2022.130017.
[4] Reddy, Akepati Bhaskar, et al. "On Electric Conductivity of Greases." (2022).