Automotive lubricants are under attack, besieged not only by advancing requirements to reduce friction losses across many aspects of the powertrain but also by the impact of associated actions, including the effects of stop-start systems, downsizing and extreme boosting, which push up thermal and mechanical loads on lubricated surfaces.
And a further threat comes from extended service intervals that bring demands for oil to remain within specification for periods of use far in excess of those of even a few years ago. So with friction now typically accounting for some 20% of engine efficiency loss, lubricants are going to remain in the front line for the foreseeable future.
“Oil films, which need to be thicker than the roughness of the surfaces they lubricate or the diameter of any wear debris in the oil, become thinner as temperatures rise, as loads increase, and as shaft rotation slows down. This means that all the trends in engine design tend to produce thinner oil films, and the choice of lower viscosity oil grades simply compounds the issue,” said Martyn Mann, Technical Director of Millers Oils, a specialist in low-friction lubricants.
Different engine components operate under different lubrication mechanisms, depending on the oil film thickness that can be sustained. Because the film thickness varies with load, speed and temperature, even the same component can experience several different regimes during engine operation. Mann cited the example of the piston ring pack, which "may ride on a full oil film at mid-stroke but, as the piston slows down at the extremes of stroke, the motion necessary to generate the film is briefly lost,” he noted.
The traditional approach to dealing with the growing challenge of improving efficiency while satisfying durability requirements has been to progressively reduce viscosity and, in the process, reduce film thickness. But now that is simply not sufficient to meet what may be seen as extremes of downsizing in the enduring campaign to lower CO2 levels in parallel with reduced fuel consumption but with no performance penalty.
Added to this are W (Winter) viscosities now at zero; Mann said new answers are needed in the ongoing quest for reduced losses. “Maintaining wear protection has increasingly relied on complex additive packs," he reported, "but these have to be carefully designed as modern after-treatment systems can be poisoned if some of these compounds are present in too great a concentration.”
It all adds up to the need for a lubrication campaign to contribute to the achievement of reduced friction losses via a fresh approach to oil formulation, in order to address friction under all conditions and to provide good wear resistance, said Mann.
Millers Oils has five decades of experience working with OEM powertrain teams to maximize lubricant performance and system durability. Its range of oils currently are approved by eight vehicle manufacturers. But it has also developed nanotechnology for use by race cars (see http://articles.sae.org/11201/) which the company regarded as having production car potential, too.
Now, that potential is seen by Millers as absolutely apposite for the downsize/friction loss philosophy of most OEMs—spanning their premium to budget products—that has rapidly gained momentum in recent years.
Nanotechnology provides the required combination of low friction and good wear resistance without contaminating the exhaust after-treatment system, stated Mann. He explained that Millers’ Nanodrive oils are formulated to a low SAPS (sulfur, ash and phosphorous content) specification.
“Nanoparticles are small enough to infiltrate the micro-roughness that remains on even the smoothest ground surfaces, acting like miniature ball bearings between adjacent surfaces,” he said. “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. In fact, they adhere so strongly to metal surfaces that they remain in place after oil drainage and impart a dark coloration to the engine components.”
Mann described this as a significant breakthrough in reducing wear under conditions such as stop-start that are challenging for conventional low viscosity lubricants, claiming that nanotechnology could avoid the need for more costly materials solutions.
Increased film strength during warm-up
Millers Oils’ R&D teams have also looked at the benefits for cylinder lubrication and conducted significant testing of long-term durability capability. Typically, around 40% of an engine’s frictional losses occur within the cylinder liners and ring pack and 25-35% in the valve train. To properly research these issues, Millers has invested in equipment which simulates these different conditions under laboratory conditions with a high degree of repeatability, Mann noted.
By simulating the sliding friction of engine cylinders and valve trains, together with the combination of rolling and sliding friction in gear trains, at specific test temperatures and loads, it was possible to establish and quantify the potential for improvement.
“Compared to an industry recognized, standard fuel efficient formulation, our low friction Nanodrive oil reduced sliding friction by almost half, while maintaining a more substantial oil film during engine warm-up,” claimed Mann.
The enhanced film strength during warm-up is a key benefit. "Many conventional oils do not develop their full film strength until temperatures over 70°C are attained, but most engine wear take place during this initial period," he stated. "We developed the Nanodrive lubricants to have higher, and more consistent, film strength over a wider temperature range than existing benchmark oils, in order to provide enhanced protection during this crucial phase.”
At higher temperatures, more typical of normal engine operation, the highly reactive nanoparticles actually reduce friction levels between adjacent surfaces, a highly desirable characteristic for “real world” efficiency improvement. Test results for component wear rate showed a reduction in wear of 40% to 50% when using Nanodrive lubricants instead of conventional oils (measured by industry standard methods), said Mann.
The enduring battle for higher powertrain efficiency has “raised the bar” for all oil developers, not just Millers Oils. “Low viscosity oils, higher loads, smaller bearings and start-stop operation all increase the chances, and the frequency, of boundary contact between surfaces," Mann noted, adding that approaches such as nanotechnology help to address both the friction and wear issues associated with thinner oil films.