According to a recent study of friction in passenger cars, one in every three litres of fuel consumed by cars – more than 208,000m litres of fuel in 2009 – is used to overcome friction in the engine, transmission, tyres and brakes (Tribology International, doi:10.1016/j.triboint.2011.11.022). Not surprisingly, reducing these friction-related losses has therefore become a top priority for researchers looking to make efficiency gains. Lead author Kenneth Holmberg of the VTT Technical Research Centre in Finland estimates that friction losses could be reduced by 18% over the short term (five-10 years) and by 61% in the long term (15-25 years).
Holmberg and his coworkers applied calculations to an averaged car with today’s advanced commercial tribological technology; a car with today’s best advanced technology based on recent R&D (Lab 2010); and a car with the best technology forecast in the next 10 years (Car 2020). Lab 2010 and Car 2020 gave economic savings of €174,000m and €576,000m and CO2 emissions reductions of 290m and 960m t, respectively.
‘In the research labs there are very interesting new kinds of techniques that can reduce friction,’ Holmberg says. One technique is to apply low friction coatings based on diamond-like carbon or molybdenum-based composites to some engine components, controlling the coating structure at the nano level. Already, the friction of components with new advanced surface coatings has fallen by more than 90% for dry contacts and 10-50% for boundary-lubricated contacts.
Another technique is to change the surface topography, Holmberg explains. ‘You tailor and use surface texturing by laser techniques so you can influence the lubricant flow in contact zones and you can create some micro-measures to keep components apart.’ Published research shows that reconfiguring the surface topography of gears to be smoother reduces friction typically by 30%.
Product development where rubber hits the road can also make a significant contribution to friction reduction. As tyres roll below a car, the elastomer suffers repeated cycles of deformation and recovery and dissipates the resulting ‘hysteresis’ energy loss as heat. ‘There is a lot of work by tyre companies on producing low hysteresis materials and lower energy dissipation tyres. You have to do things like make sure the little particles of silica or carbon black in the tyre don’t slip against the elastomer surfaces,’ says Hugh Spikes, tribology expert at Imperial College London, UK. Carbon black is the predominant filler, but other fillers also include silica-silane and carbon black, which reduce rolling resistance without comprising traction or tread wear.
Hysteresis losses depend on tyre materials, geometry and construction, together with the load, velocity, air pressure and temperature. In terms of geometry, the outer diameter, rim diameter and tyre width all affect the rolling resistance. Rubber additives and the size, shape and number of tread blocks also matter.
‘Over the last 10 years, our computer-based modeling capabilities have grown significantly,’ to take all of these different variables into account, says Chris Helsel, technology director at The Goodyear Tire & Rubber Company in Akron, Ohio, US. The company has been collaborating with Sandia National Laboratory in Albequerque, New Mexico, US, to develop an advanced simulation system to assist product development work, he continues.
Since 2005, Goodyear claims to have succeeded in reducing rolling resistance of high performance summer tyres for Europe by 18%. A 13% improvement equals a 2% increase in fuel economy, notes Helsel, which can save billions of gallons of gasoline annually in the US. New European tyre labels to be implemented in Europe later in 2012 will display a fuel economy grade demonstrating efficiency for consumers.
Bearings and oil
Edward Holweg, director of product and systems development at Swedish car components supplier SKF, says the automotive sector has been strongly focused on carbon dioxide reduction and fuel consumption reduction, pushed by legislation. SKF’s energy efficient product offerings include optimised bearings and seals and their arrangements, which promise to decrease friction, usually by 20 to 30%.
SKF’s Vehicle Environmental Performance Simulator allows the company to model a complete vehicle with all critical components and then replace certain bearing related components or systems to see what impact this will have. ‘That allows us to say you can reduce friction in those types of bearings by, say 30%, which will save so many grams of CO2 in your vehicle,’ Holweg explains.
Improving lubricants is another means of reducing friction within passenger cars. Essential for avoiding solid contacts between engine parts, forming a protective chemical film on surfaces and preventing corrosion, churning oil also leads to energy losses due to the viscosity of the lubricant. Using lower viscosity lubricating oils will lessen energy losses but the key is to find the ‘optimum viscosity,’ Spikes explains, as inadequate viscosity leads to wear and tear. ‘You have to get clever with the chemicals you add to the oil to reduce what is called boundary friction, which is the friction you get when the high spots on the metal surface rub together,’ he says.
Spikes has reduced friction in lubricating oils using organic friction modifiers and developed additive packages containing little or no sulphated ash, phosphorus or sulphur anti-wear additives. Friction modifiers and anti-wear additives are also emerging based on nanoparticles, such as MoS2 and H3BO3, but Spikes says there is so far little evidence to show they will be able to cope in an engine. ‘The main interests are in carbon fullerenes and inorganic fullerenes,’ he notes.
Around 95% of a lubricant is comprised of base oil. Today, this is usually made by cracking all the hydrocarbons in mineral oil and reforming them to an almost entirely new set of molecules. Lubricants derive from the mostly heavily reformed blends or so-called Group 3 oils, from which most of the aromatic and volatile components have been removed. Volatility is undesirable because it can mean heavier lubricating molecules getting into the exhaust resulting in more hydrocarbon emissions. However, Spikes says that industry is currently holding its breadth for the emergence of a gas-to-liquid (GTL) base oil, which Shell has already started to produce and ship to blending plants.
The more tightly controlled GTL process results in more iso-paraffins which are particularly good for lubricant base oils. The advantages of iso-paraffins include compatibility with after-treatment catalysts and their lower viscosity at low temperatures, which will offer fuel economy benefits for short trips and cold start drives, according to Shell.
Spikes adds: ‘Nobody knows what they are going to be like but it is thought they are going to be somewhere between the Group 3 and purely synthetic polyolefins, but compared with the latter there may be a relatively large volume.’
A new dawn?
So what then of the future? There will be continued focus on energy savings, Spikes predicts. ‘Thirty years ago, the research in lubrication was primarily concerned with durability, making machines and components last longer. But now the emphasis is definitely on efficiency gains, reducing friction, while maintaining acceptable durability,’ he says. Future gains in the durability of engine oils might not happen, but oils for improved energy efficiency will.
It is possible, meanwhile, that the diesel or petrol internal combustion engine’s battle against friction may peter out if a replacement technology takes the lead. With fewer moving parts, the friction energy losses in an electric car are only about half those in an internal combustion passenger car, Holmberg’s study concludes.