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Cool technology The next step change for electric vehicles, the compact eAxle, is poised to add appeal to BEVs thanks to direct cooling and multiple speeds Bringing together electric motors, power electronics, reduction gearing and differentials in a compact eAxle saves space, improves efficiency and increases vehicle range. Innovations such as multi-speed further enhance efficiency but a major challenge for any eMotor is adequate cooling. Cooling is not just the key to unlocking extended driving range or reduced battery costs, it can also prevent long-lasting heat damage to permanent magnet eMotors. Cooling technology is therefore evolving fast, in parallel to eAxle development. Early electric vehicles employ indirect cooling, also called dry cooling, whereby the eMotor is cooled by air and water-glycol. An example is the Nissan Leaf, in which oil is used only to lubricate the reduction gears. By contrast, direct cooling – also known as wet cooling – involves lubricant coming into direct contact with the eMotor, a system used in Tesla vehicles. By transferring heat directly from the source in the coils, direct cooling is more efficient than indirect
cooling and enables better control of eMotor temperatures. Keeping an eMotor operating under peak conditions for more of the time results in an optimization of both efficiency and dynamics. “Although contact between an electric motor and lubricant can bring certain challenges, none of these are insurmountable with the right lubricant additive formulation,” explains Dr Susie Hurley, senior adviser in ATF R&D at Afton. “To enable direct cooling, we firstly need a dielectric fluid that has good compatibility with all eMotor materials, including copper and the various types of coating.” When an eMotor cooling system is shared with the whole eAxle, the lubricant not only needs to meet the demands of the motor and power electronics, it must also protect mechanical parts such as gears and bearings. “Antiwear properties and friction characteristics must be carefully balanced with the eMotor’s needs,” says Dr Hurley. “As the advantages of direct cooling become more widely acknowledged, we’re also seeing
Lubricants for eAxles may need to directly cool eMotors and serve shifting devices
148 // July 2020 // www.electrichybridvehicletechnology.com
Cooling of an electric vehicle motor can help unlock greater driving range as well as reduce battery costs and prevent long-lasting damage to its permanent magnets
a shift towards more multi-speed electric vehicles in development,” says Dr Harald Maelger, Afton’s senior OEM relationship manager and eMobility expert. “eMotors have a broad operating window of speed, but for greater efficiency they still benefit from two to three speeds, especially for more varied driving cycles,” he adds. The first two-speed eAxle was developed by GKN in 2014 for use in the production BMW i8. OEMs such as ZF and Porsche have also launched multi-speed systems, and many more are in development for higher-volume applications. The key to unlocking the full potential of these new eAxles resides within the lubricant used. “We’ve been working closely with OEMs to develop lubricant additives for direct-cooled multi-speed eAxle modules,” says Dr Maelger. “Blending all the right performance characteristics into one fluid unlocks the power of direct cooling.” To facilitate direct-cooled multi-speed eAxle technology, the
lubricant must handle significant thermal load and still retain the right balance of properties throughout its working life. Thermal degradation of fluid has a detrimental impact on electrical properties, and on clutch friction performance in particular. “Our testing shows that heat cycles and clutch engagements combine to harm clutch friction performance, so the right initial formulation and appropriate drain intervals are both essential to protect hardware and prevent shifting deterioration,” describes Dr Hurley. Afton’s data shows that direct cooling can increase system efficiency significantly. To minimize battery costs or offer enhanced driving range, Afton will be launching the world’s first additive package designed specifically for direct-cooled eMotors with multi-speed. FREE READER INQUIRY SERVICE To learn more about Afton Chemical, visit: www.magupdate.co.uk/PEHV