Millers Oils' lubrication expertise helped in development of the Performance Electric Vehicle Transmission (PEVT) by Hewland. (Hewland)
Tackling the lubrication challenges of electric vehicles
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Although electric vehicles (EVs) often are extolled as “low-maintenance” alternatives to conventional internal-combustion (IC) propulsion and its brew of critical fluids that require routine maintenance, EV drivetrains also rely on proper lubrication—and in many senses pose performance challenges at least equal to those of IC drivetrains.
Not the least of electric drivetrains’ unique stresses on lubrication: electric motors can deliver maximum torque at zero rpm.
With EVs poised to proliferate in many automakers’ product portfolios, Martyn Mann, Technical Director of U.K. specialist oil developer and supplier Millers Oils, warns of the need for greater understanding of gear lubrication in EV transmissions. Millers, founded in 1887, specializes in developing specialized lubricants for vehicles ranging from classic cars to Le Mans racers.
“Today’s EV designers must maximize (propulsion) efficiency and range by using thinner oils and reduced contact areas. This requires careful formulation of a suitable (lubrication) additive pack to ensure achievement of adequate oil-film strength under all operating conditions, combined with minimum friction.”
Immediate maximum torque
Electric motors’ ability to generate maximum torque at zero rpm poses a particular challenge for lubrication. Mann explained that a single-speed EV usually has a train of reduction gears to convert the electric motor’s speed to road-wheel speed, with each pair of teeth in the gear train experiencing maximum contact pressure before any meaningful rotation has occurred—and while the system is still cold. “The lack of rotation matters, because lubricant can drain off the surfaces while they are stationary, leaving them unprotected. The lack of temperature is significant because conventional anti-wear additives in lubricants require high temperatures before they become chemically active,” Mann said.
To achieve consistent lubricant performance over the full range of temperatures and contact pressures, Millers Oils uses a combination of conventional zinc, phosphorous or sulfur chemistry for higher temperatures and nano-chemistry for a range of temperatures to provide high levels of protection throughout an EV propulsion system’s operating range.
If this balance is not achieved there can be “alarming gaps” at certain temperatures where individual additives seem to be competing with each other for reactivity with the metal surfaces they are supposed to be protecting, said Mann. He likens the complementary performance of well-chosen additives to that of a relay team carrying the baton sequentially as the temperature in the transmission rises. Any gap in protection over a certain temperature range is analogous to “dropping the baton” instead of a smooth handoff.
Transferring racing experience
Millers Oils’ experience working with EV manufacturers began with the successful Drayson Racing’s bid on EV land speed record for 1000-kg (2205-lb) vehicles, which was achieved at 204.185 mph (328.6 km/h) in 2013 and led to a subsequent collaboration with transmission company Hewland to support the Formula E electric-car racing series.
The need for the EV industry to integrate the design of the e-machine and its transmission more closely to reduce weight, cost, and package size—while permitting more efficient, bespoke designs—will present further lubricant challenges, notably when it serves as the coolant for the e-machine because of additional heat flow.
“Service intervals are increasing too, so the oil has to do more for longer,” adds Mann. “The move to electric power has made lubricants an even more-critical component of the vehicle’s powertrain, with the need for EV developers of new-generation systems to be fully appreciative from the full potential that can be unlocked by using the appropriate lubricant.”
But development of lubricants for EVs is not disassociated from conventional IC-powered road or racing vehicles. The real-world development and testing experience gained working with Hewland in Formula E led to the formulation of Millers Oils’ Nanodrive transmission lubricant marketed as Hewland UPB. Millers Oils describes it as a fully synthetic transmission oil with friction modifiers for LSDs (limited slip differentials).
The new UPB lubricants now are an integral part of Hewland’s products for EVs. Applications range from low-volume, high-performance transmissions to high-volume EV transmissions manufactured via a joint venture between Hewland and Hero Motors. Steve Robins, CEO of Hewland, said, “A successful EV drivetrain requires a systems approach to the transmission and motor design and the lubricant is an integral part of that system.”
The benefit of Millers Oils collaboration with Hewland was demonstrated at the 2017 Le Mans 24-hour race, explained Mann: “After the event, transmission oil samples from several LMP2 cars were tested and showed the (parts per million) of ferrous materials to be as much as 42% lower than comparable samples of the same grade.”
Another innovation is Millers Oils’ “bio” oil, described as a sustainable, bio-degradable lubricant. Although the company has not revealed full details, the base oil is described as being “a byproduct of the sugar-making process” from the food industry. The company has successfully completed a full test program on the oil, taking it to production readiness.
“We believe we are the first company to combine this base stock with its highly-desirable, low-friction properties and to take it to marketable status," Mann declared.
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