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Millers Oils' Technical Director Martyn Mann at work in the company's R&D center. His team is applying its experience in race car lubrication to the challenges of stop-start-equipped production vehicles. 

Racing to solve the lubrication challenges of stop/start-equipped engines

“There is a direct parallel between the lubricant requirements of stop-start systems and those of electrically powered motorsport vehicles,” observed Martyn Mann, Technical Director of low friction lubricant specialist, Millers Oils.

That may sound like one of the more unlikely links between diverse areas of automotive engineering because the one thing that a race car doesn’t want to do is to stop/start on the track. But Mann explained: “With stop-start, the engine is typically stopped and started around 500,000 times in its lifetime compared to 50,000 for a conventional vehicle. The challenge for us is that when the engine stops for longer than four seconds, the low-viscosity oils used in today’s high-efficiency engines drain off bearings and other components, leaving them vulnerable."

There's a similar challenge in motorsport, Mann noted. Electric sports cars are fun to drive because maximum torque is available immediately the accelerator pedal is floored. But this means that gear tooth pressure is at maximum before any rotation, when there is little lubrication available, leaving components unprotected. He said working on these challenges in parallel has already led to superior and more robust lubricant solutions.

“The development of ultra-high-performance electric vehicles in motorsport is helping provide a greater depth of understanding in a real-world validation laboratory. That has fed directly back to solve stop-start lubrication issues for vehicle manufacturers,” he explained.

Stop-start systems were making a tentative appearance in production cars long before electric race cars and electric dragsters (an ideal application of instant maximum torque availability) became formally established in their own right. Volkswagen offered the technology on its production Passat in 1980, but it was not until a combination of improved technology and ever-tighter legislation accelerated its availability over the past three years that stop-start really gained an increasing market share. Currently about 40% of all new cars sold in Europe have the feature.

And as the auto industry’s philosophy turned a brighter shade of green, so electric motor racing emerged. Formula E, the world’s first full electric race series under FIA auspices, started last year. But despite many examples of technology migration from motorsport to volume production, the link is not always one way. In fact the electric race car/stop-start link is very much a two-way street.

Mann said that his company's Nanodrive lubricants migrated from the racetrack to the road and now the increasingly tough duty cycles faced by road car powertrains are forcing the pace of lubrication development across a wider front than just motorsport. While both want the lowest friction under high loads, the road car also dictates longer drain intervals, a broader temperature range, compliance with exhaust emission limits, good NVH performance and compatibility with hybrid or stop-start operation, he explained.

Overlooked potential

But the particular attraction of development through motorsport continues to be the compression of timescales, with small numbers of vehicles and rapid feedback often delivering faster progress to an optimum solution.

According to Mann, a hybrid road car may have to accommodate additional torque loads from an electric motor through its existing transmission, often at much lower shaft speeds than the corresponding torque from the combustion engine. Extending the capability of the transmission and driveline through improved lubricant performance rather than mechanical redesign is an attractive prospect, but the potential is sometimes overlooked.

"We are often surprised by the lack of awareness, even within highly reputable companies, regarding the opportunities presented by improved lubricant performance,” he said.

A particular issue for road vehicle drivelines is to maintain acceptable NVH while achieving low wear and reduced friction under high load operation. Mann cites as example a rear differential within the transaxle on a high performance car. The final drive pinion was mounted well above the oil level and the hypoid mesh geometry involved high levels of sliding contact, creating heavy demands on the lubricant. The additive package was formulated with precise slip characteristics because the differential used a clutch pack to control wheel slip, and low NVH during operation was essential for refinement.

The solution was to reduce the oil temperature by 15ºC to 20°C through reduced friction, Mann said, while meeting both 150,000 mile durability requirements and NVH targets. Prior to Millers Oils’ work, the axle supplier believed that mechanical changes to the clutch pack would be required.

While transmission applications may include restrictions on the slip characteristics of oil additive formulations, they are at least free of the direct effects of emissions legislation. In contrast, engine lubricants are constrained to use a low SAP (sulfur, ash and phosphorous content) specification in order to prevent catalyst poisoning.

“Innovations like the use of nanoparticles have proved invaluable in delivering the required protection for both road cars and motorsport vehicles, within the permitted specification,” Mann noted.

Additive packages for modern road engine applications can account for up to 20% lubricant by volume, necessitating extensive screening to eliminate undesirable interactions. Because different constituents become chemically active at different temperatures, much of Mann’s focus is on maintaining the most uniform film strength possible, across the widest temperature range, to protect running surfaces under all conditions. The priority is consistent oil film strength across temperatures from cold start to fully warm.

30% friction reduction

In benchmarking trials, Mann's development team has has seen some oils “surprisingly deficient” in this respect, a situation suggesting imperfectly matched additive combinations. They believe this causes different additives to ‘fight’ each other for grip on the metal surfaces at certain temperatures. It is also why aftermarket additives should under no circumstances whatever be mixed with a balanced formulation, Mann said.

Millers Oils approaches low temperature protection in two ways. Nanoparticles provide rolling elements that separate contacting surfaces until the temperatures rise sufficiently to trigger the anti-wear additives in the formulation, while deposits laid down during the previous loaded operation remain effective, providing insurance until they are replenished.

Most of the initial additive-pack development for a new application is carried out in the laboratory, using a combination of rolling and sliding tests (including up to 100% sliding) to evaluate friction and wear performance. Typical improvements obtained include 20% to 30% lower friction and up to 50% reduction in wear scar area, compared to standard benchmark oils. The most promising formulations are then evaluated in the vehicle.

As for the lubricant technology transfer between road and track, both use reduced wear rates to extend component life, while the efficiency gain from lower friction provides a fuel economy advantage in road cars and a performance benefit in motorsport.

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