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Figure 2: Animation of the complex cooling circuit in the AUDI e-tron Twin-Koax motor


In addition to creating specialised EV lubricants for performance purposes, the developing lubricants must also be compatible with the electrical components within EVs such as electric modules, cable insulations, sensors, and circuits. The use of ceramic hybrid ball bearings makes the specification much more demanding for the formulation of EV lubricants. Since the electric motor generates heat during usage and charging that must be insulated from the electric modules, a myriad of cooling concepts and thermal management requirements must be considered when developing a compatible lubricant. Premium segment and sport cars differentiate themselves by offering “lasting peak endurance” power, which require cooled battery packs and motors. These cooling circuits have up to 45 litres and must comply with a much wider range of different materials than the cooling circuits in internal combustion engines. Such cooling circuits are complex and cool the stator and the rotor (Figure 2) including power electronics.


The most popular lubricants today include mineral- based oils and synthetic-based oils due to their improved lubricity and thermal and oxidative stability.


Hybrid electric vehicles (HEV) have seen similar growth as EVs and share many of the same requirements. With smaller combustion engines alongside an electric motor, researchers are concerned over the heat and aging stability in HEVs. To create a viable lubricant for both EVs and HEVs, strong thermal insulation properties are essential.


Various tests are performed to ensure specifications are met. Oil viscosity is developed according to load, speed, and operating temperature of the application. While viscosity should be reduced to minimise friction loss, too low-a-viscosity hinders durability and causes the lubricant to leak out of the vehicle’s bearings. This brings oxidation properties and dropping point to play when undergoing extreme temperatures. Additives are added to modify these properties, but some additives may be counterintuitive and shorten the life span of the grease. Lastly, the lubricant must maintain electrical properties such as volume resistivity, dissipation factor, and dielectric strength to avoid electrical losses in the system. Overall, the lubricant must be formulated to balance all these requirements.


Continued on page 14 LUBE MAGAZINE NO.162 APRIL 2021 13


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