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manufacturers would still normally recommend fluid change each 60,000 to 100,000 km. Most EV transmissions, on the other hand, will probably be really “filled for life.” For instance, for old Tesla Model S cars, it was recommended to replace the fluid at intervals, but there are no such recommendations for newer cars after MY2016. This raises requirements for fluid longevity even higher, making it necessary to implement fluid filtration.
New joint industry projects initiated by the world-leading research institutions, such as Automobil- Prüftechnik Landau GmbH (APL) and Southwest Research Institute, aim to advance our understanding of the unique stressors placed on EV fluids, helping development and optimisation of EV powertrains by bringing together expertise from automotive OEMs and lubricant and additive companies.
should be equivalent to 17/14. As shown by numerous studies, useful life of gears lubricated by clean oil with ISO cleanliness of 14/11 or better is two to four time longer than with dirty oil of ISO cleanliness 20/17 or worse. The impact of oil cleanliness on mechatronic devices is even greater.
Though the vast majority of EV transmission fluids in today’s use are formulated using API Group II and III base oils, synthetic products using polyalphaolefin (PAO), esters and oil-soluble Polyalkylene Glycol (PAG) are gradually gaining momentum, in particular in ULV grades. There are also a number of novel non-conventional fluids in the research-in-progress phase, including water-PAG systems and ionic fluids, the use of which allows one to achieve ultralow viscosities, KV100 < 3 cSt, while improving efficiency, heat transfer and electrical conductivity.
Figure 2: Relative significance of various properties for conventional ATF and EV transmission fluids.
Industry pays a lot of attention to lubricant cleanliness. The ISO 4406 Cleanliness Code used to quantify particulate contamination levels for lubricants is expressed in three numbers, e.g. 19/17/14, each number representing the contaminant level – proportional to the number of particles – for the particle size greater than 4 µm, 6 µm, and 14 µm. Each time a code increases the quantity of particles doubles: for instance, the code 19 corresponds to the particle concentration range of 2,500 to 5,000 particles/mL; and the next code, 20, already to the range 5,000 to 10,000 particles/mL. However, in the automotive industry, an older two-number system is still in use, e.g. 19/16, with the first number referring to the particles larger than 5 μm, and the second, referring to the particles larger than 15 μm. From a practical viewpoint, the last two numbers in the three-number code will not differ much from the two numbers in the two-number code, hence, for laymen use, 19/17/14
14 LUBE MAGAZINE NO.173 FEBRUARY 2023
Many new additive types are coming into use as well. For instance, conventional sulphurised olefins, which are known to be copper-corrosive, are replaced by less corrosive hindered dialkyl polysulfides, thiophosphoric esters, aminophosphates and borates. The latter allow to achieve high scuffing resistance at low treat levels, while minimising chemical polishing. Also, yellow metal deactivators such as benzotriazole as often used to reduce the risk of printed circuit board (PCB) corrosion. Disappearance of the clutch slippage problem opens the door to broader use of various friction modifiers for improved energy efficiency. New antioxidant types are also used, either alongside or instead of zinc dialkyldithiophosphates (ZDDP). Finally, new shear-stable viscosity index improvers should be mentioned.
When it comes to improving gear tribology specifically for electric vehicles, everything basically boils down to proper gear design and geometric optimisation, which includes selecting right materials and methods for gear manufacture and post-processing. Extensive previous knowledge from cross-field applications of high-speed gears in motor sports, gas turbines and turbo-compressors comes very handy.
In practice, we can never get perfect gears – good enough is the best. Gear accuracy is regulated by ISO 1328 and a number of national standards. To give the reader some feeling of numbers: For the gear diameter from 50 to 200 mm found in transmissions, the highest ISO 1328 accuracy grade sets tolerances down to a micron level, which is comparable to the thermal
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