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Oils and electric motors Effective performance tests for the next generation of electric vehicle additives and lubricants are helping to mitigate the risks associated with drivetrain electrification Engineers and chemists typically approach the same problem from radically different perspectives. Runaway progress in drivetrain electrification is now forcing these uneasy allies to work ever closer together to find safe, reliable solutions to shared e-mobility challenges. Applying lubricant as a direct coolant to the e-motor is growing in popularity. Using lubricant in this way improves thermal management and increases vehicle efficiency, helping battery size or range. When the direct cooling system shares the same sump and pump as the lubricant for gears, power electronics and any shifting devices, e-axles can also be made lighter and more compact with no danger of fluid cross-contamination. But these benefits are only realized when enabled by suitable lubricant technology, so chemists are pushing to establish exactly what is being asked of lubricants in second- and third-generation EVs. Electrical properties, corrosion prevention and cooling capabilities
are all relevant, but feedback suggests OEMs’ priorities vary widely, likely driven by different experiences and design challenges. To many engineers’ annoyance, lubricant is one critical system component that cannot be CAD-designed. Given that the lubricant is critical to performance, engineers will be concerned by risks not fully in their control when working with new components and systems. Mitigating risk is all about effective testing. Well-balanced additive chemistry delivers the majority of an advanced lubricant’s performance, and for this reason Afton Chemical invests in developing robust tests to control risk. The company looks at the oil itself and its impact on parts, systems and the whole vehicle. However, even extensive validation programs can’t always replicate extremes of real environments, duty cycles, or especially abusive drivers. Afton Chemical therefore uses accelerated testing, exposing oils to higher-than-normal temperatures,
Vapor phase
loads and contamination. This better differentiates fluid performance, while also creating a safety margin for engineers. Some well-defined tests already exist for lubricant electrical properties, such as ASTM D2624 electrical conductivity, but many of these tests were designed for static power transformers and turbines, not the AC motors of today’s electric vehicles. Industry standardization through formal industry bodies is a long way off, which means most auto manufacturers are busy designing hardware, fluid specs and tests in isolation. This is where relationships can make all the difference. Smart engineers are able to leverage the expertise and growing experience of specialist additive and lubricant chemists. Copper and other metal parts are becoming increasingly sensitive to transmission function, and Afton Chemical believes the standard ASTM D130 copper corrosion test is insufficient. The company recommends extending test duration from just three hours to
Broken wire
Fluid level Liquid phase Afton conducted copper corrosion tests, using an ATF used by a large OEM, where the wiring was destroyed by the lubricant vapor
92 / September 2019 // Transmission Technology International
four weeks, at higher temperatures, and also measuring the dissolved copper in the oil. This more severe test differentiates lubricant technologies much better, matching the increased importance of corrosion in EVs. While formation of corroded layers is bad, especially if it is conductive, components that disappear are worse. Vapor phase corrosion is a growing concern and has been shown to cause immediate vehicle failure. Afton has examined this too, testing wound copper wires connected to a current in sealed oil tubes. It was found that with some lubricants, even if they generally protected the submerged copper, wire sections exposed to air were completely destroyed. This has clear implications for e-motor hairpins, sensors and mechatronics. Testing fresh and aged oil is important as wear metals, oxidation products and water ingress all increase lubricant conductivity. Direct cooling can also diminish performance in other, unexpected ways, so teasing out the complexities is essential. Afton-developed tests have demonstrated that friction and heat/cool cycle stresses have synergistic effects, degrading fluid performance faster than otherwise expected. In certain situations, e-motors can get very hot, meaning low volumes of lubricant can experience high transient temperatures and thermal load despite reasonable average sump temperatures. New and extended tests, developed by expert chemists, can offer a robust and reliable framework for mitigating engineering risk. FREE READER INQUIRY SERVICE To learn more about Afton Chemical, visit: www.ukimediaevents.com/info/tr
INQUIRY NO. 516