Oiling the way to greater EV efficiency

  • 30-Jan-2014 05:13 EST
MillersOils01-14 Drayson EV speed record.jpg

Drayson Racing Technologies' speed-record-setting Lola B12 69/EV.

Lubricant performance may not appear to be a salient challenge for EV operation even when it comes to the achievement of world electric speed records and on-track e-racecars. Not only is there no internal-combustion engine (ICE), but most production EVs use only a single-speed reduction gearbox. Lubricants are as significant, however, as those used in ICE powertrains.

Martyn Mann, Technical Director of Millers Oils, which works closely with Drayson Racing Technologies, said that the torque characteristics of an EV powertrain—for road, track, and extreme speed attempts—create very different demands on the lubricants throughout the gearbox and final drive, compared to conventional vehicles: “Electric motors generate maximum torque from one rpm, which means that the contact pressures between each pair of teeth in a geartrain can reach a maximum before any meaningful rotation has occurred and while the system is still cold.”

This lack of rotation is very significant because lubricant drains off the surfaces while they are stationary, and the lack of temperature is also significant because conventional anti-wear additives in lubricants require high temperatures to become chemically active, he explained.

Millers Oils has overcome these issues while developing its Nanodrive lubricants, according to Mann. These contain nanoparticles sufficiently small (30 to 50 nm diameter) to infiltrate the micro-roughness that remains on even the smoothest ground surfaces, acting like miniature ball bearings. The nanoparticles also exfoliate under load, peeling like the layers of an onion, to deposit a thin wear-resistant layer that physically separates adjacent metal surfaces.

The lubricants were developed to achieve higher and more consistent film strength over a wider temperature range than existing oils, which were benchmarked during an extensive test program, Mann said: “Many of the conventional oils did not develop their full film strength until temperatures over 70°C were attained.”

The company worked closely with Drayson to help it succeed in establishing the world land speed record for lightweight EVs (subject to FIA homologation) at 330.13 km/h (205.13 mph) over a mile. CEO Lord Drayson drove a Lola B12 69/EV using specially developed Nanodrive lubricants.

Mann explains that the speed-record program proved distinctly challenging: “Lubricating the transmission in the Lola required good film strength at both extremes of temperature. With more than 520 kW from four electric motors, the torque from a standstill creates enormous loads, while at maximum speed, high temperatures were generated, requiring sophisticated thermal management in the oil’s properties.”

The electric Lola is understood to produce 4000 N·m (2950 lb·ft) of torque.

“Now, much of the knowledge gained is being channeled into the development of future lubricants for EV racecars, such as Formula E, where saving driveline weight and package space through the use of innovative fluids is a huge benefit,” he added.

Low friction properties were equally important as successful wear protection to the Drayson speed-record car, said Mann: “Maximum efficiency was essential in order to extract the greatest speed potential from the onboard battery pack, so one of our aims was to minimize friction losses. In this respect, the lubricants are equally relevant to road-car applications, where anything that improves efficiency can be used to extend the useful range.”

Road-going EVs could also benefit from a reduction in the size and weight of the transmission and driveline, made possible by using the optimum lubricant to increase load capacity, protect against wear, and reduce friction.

Mann also stated that the ongoing development of oil technology could be of particular benefit to road-going hybrid drivelines, which are required to cope with increased torque loads as electric drives are added to existing transmissions and operate in confined conditions.

The benefits that accrue from nanotechnology applied to EVs follow on conventional powertrain application, said Mann. Millers Oils has recently tested Nanodrive on a Mercedes-Benz SLS AMG GT3 racecar, and results show a power increase of 1.5% compared to the original lubricant, claims Mann. The power gain is a result of reduced internal friction despite using the same viscosity as the original oil.

“Unlocking the energy normally wasted in friction gives the cheapest power increase you can get. In a heavily restricted formula it may be the only increase easily available,” he explained. “Less friction doesn’t just provide more power, it also reduces the amount of heat produced, also reducing cooling requirements and decreasing engine wear, thus extending service intervals. Oil is often overlooked as just another consumable, but as the tests show, choosing the correct oil can make a significant difference.”

The tests were conducted on a hub dynamometer at Torque Developments International using Fortec Motorsport’s SLS GT3. Averages were taken from a series of three runs on each of two other products, both off-the-shelf SAE 5W40 engine oils.

“In a category with very few avenues to exploit, finding advantages in lubrication can make a big difference,” said Trevor Foster, Fortec Motorsport team manager. “In competitive motorsport you are continually looking for gains; most of the time they aren’t as simple as just replacing the engine oil!”

The Millers Oils formulation uses nanoparticles that produce a significantly lower friction coefficient than conventional boundary lubricants such as molybdenum disulphide. They have a greater surface area in proportion to their volume than bigger particles, making them more reactive and facilitating reduction of both friction and wear.

The use of low viscosity (thinner) oils based on high-quality synthetic base stocks is known to reduce viscous friction in oil films but is limited by wear considerations, said Mann. Thinner oil films increase the risk and frequency of contact between opposing metal surfaces, requiring the use of improved anti-wear additives. Increased contact between surfaces also increases boundary friction.

The benefits are also applicable for road cars, added Mann, with fuel-consumption reductions of up to 2.5% and complementary reductions in emissions.

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