TRIBOLOGY
Tribology of EV transmissions
Powertrain electrification has been a growing trend in the automotive industry. Electric motors used in Battery Electric Vehicles (BEVs) operate at high speeds – 3,000 to 16,000 rpm – with high performance motors reaching over 20,000 rpm. This is significantly higher than internal combustion engines used in passenger cars (600-6,000 rpm), though some Formula 1 engines could go up to 20,000 rpm. The efficiency of the electric motor reaches a maximum close to its maximum speed. Small high-revving motors are also lighter and cheaper to manufacture. However, higher speeds bring new challenges with lubrication, heat management, seals, and noise. Gear meshing is the primary source of intrusive whining noise that irritates most drivers. This noise is usually linked to the transmission error and geometric imperfections of gears [1].
Most EVs use only reduction gears – a sort of one-speed EV reduction transmission/ transaxle.
Figure 1: The gearing mechanism of a high speed EV transmission (Courtesy of Vicura AB).
Luxury models, such as Porsche Taycan, Audi e-tron GT and BMW i8 use 2-speed e-drives developed by ZF. The 2-speed e-drive improves the range and driving characteristics since it allows one to achieve both a high initial torque and a high top speed. Quite recently, a three-speed ZF transmission has been presented.
Proper gear lubrication is critical for transmission operation. Lubricant viscosity is the first parameter to look at. Industry standards such as ISO 6336 and AGMA 6011 provide recommendations for selecting the right viscosity grade depending on application. A higher gear speed favors the use of lower viscosity lubricants.
Boris Zhmud, Technical Director, Nuspec Oil Linus Everlid, Development Engineer, Tribonex Ellen Bergseth, Associate Professor, Department of Machine Design, KTH Royal Institute of Technology
At the moment, automatic transmission fluids (ATFs) are often used in EV reduction transmissions, despite the fact that, design-wise, EV transmissions are quite different. Common ATFs such as MERCON® DEXRON®
LV, VI, Toyota T4, Honda DW-1, have KV100 in
the range 6 to 8 cSt. This roughly corresponds to the SAE J306 viscosity grades 70-75. Ultra low viscosity (ULV) ATFs go down to KV100 3.5 to 4.5 cSt and are formulated using synthetic base oils, such as poly alpha olefins (PAO), esters and oil-soluble polyalkylene glycols (OSP). The market demand for ULV ATFs has been very low so far. At the same time, viscosity wise, ULV ATFs are a better fit for high-speed EV reduction transmissions. Cooling and resistance to shocks can be further improved by using forced lubrication. While sufficient lubrication is essential, excessive lubricant flow can lead to excessive heat generation and power losses, especially for high-speed gears. Hence, a total system approach is important – the gearbox and gear oil should fit together.
Proper selection of fluids for EV transmissions is complicated by a wide diversity of EV hardware. In general, EV transmission fluids call for a different spectrum of properties compared to conventional ATFs. Some properties are universally important for both: efficiency, durability, seal compatibility, foaming tendency, wide operating range, environment, health and safety (EHS) profile. Some other properties such as oxidation stability, copper corrosion and electrical conductivity gain increased significance. On the contrary, traction properties – which are critical for proper function of wet clutches – are no longer important.
Common oxidation tests for ATFs include Aluminium Beaker Oxidation Test (ABOT) used by Ford, DEXRON® Oxidation and Cycling Tests used by GM, Indiana Stirring Oxidation Test (ISOX) used by Asian OEMs, and the CEC L-48 Test used by European OEMs. It is interesting that the new Information Report J3200 on Electric Drivetrain Fluids (EDF) released by SAE includes CEC L-48 [2]. Also, even though many automatic transmissions are labelled as “filled for life”, the
Continued on page 14 LUBE MAGAZINE NO.173 FEBRUARY 2023 13
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