Smoothing the way to greater efficiency01 March 2012
If there's a common concern that unites the automobile industry, it's the quest to improve fuel efficiency and thus reduce CO2 emissions, says Tony Lewin
Every component of a vehicle must play its part – and that includes the lubricants that reduce friction to allow the engine and driveline to operate smoothly. Estimates vary when it comes to just how much energy is lost to friction. Within the engine, it certainly ranks close to noise, vibration and rejected heat as a parasitic loss, while in the transmission it can be level with rolling resistance and electrical losses as an unwelcome absorber of energy. Clearly, lubricant-led improvements in any area will contribute to an overall efficiency improvement.
"Certainly, with the engine, there is a larger amount of friction, so it presents slightly more potential for reductions than the transmission," says Simon Dunning, global technical manager for passenger car motor oils at Shell. "It very much depends on the conditions under which you measure efficiency: at high loads and high torques, transmissions are very efficient – but, at other parts of the driving cycle, they are less efficient and there could be more to be gained."
Alain Bouffet, of TOTAL's research division, believes fine-tuning viscosities and traction coefficients within the gearbox can improve overall efficiency by as much as 4.5%. An example of the energy-saving potential within the engine is provided by Shell's recent co-operation with Gordon Murray Design (GMD) on the revolutionary lightweight T25 city car. The oil scientists worked hand in hand with Murray's engine specialists to develop a 'concept' oil to push the boundaries of friction reduction still further. The result, says Dunning, was an efficiency improvement of more than 6% in the urban cycle. "This product was designed specifically for fuel economy: the key thing is also to maintain durability. One of the things we are working on all the time is to get that balance between fuel efficiency, whilst delivering the durability that the OEMs and customers require."
Part of the secret of the new oil is a significant drop in viscosity, clearly demonstrating the direction of travel of Shell's research teams. "Our goal in the research area is to provide the lowest viscosity lubricant, which still provides the required engine protection and durability," confirms Selda Gunsel, Shell's vice president of global commercial technology.
Shell does concede that the concept oil for the GMD project is so low in viscosity that the SAE does not yet have a category into which it can be placed. "It's somewhere between 0 and 10 to 20-ish" is the closest to which anyone will admit.
"The prime thing was tailoring the viscosity to the engine to give us the best fuel economy," explains Dunning. Instrumental in this is Shell's new GTL (gas to liquid) technology, which provides a low-viscosity base oil at a lower cost than in the past and meets manufacturers' specifications for parameters such as volatility. Yet, he stresses, it is much more than just a drop in viscosity: key, too, is an understanding of rheology – how the viscosity varies with temperature, pressure and shear. All these elements have allowed the chemists to formulate
an oil which, claims Dunning, has provided much better protection
than was initially expected from a lubricant so specifically targeted at fuel economy.
Both Dunning and Gunsel point to the importance of new-generation lubricants being formulated to deal with the new materials, such as lightweight alloys, magnesium and titanium, that are beginning to replace the ferrous components in traditional engines; diamond-hard coatings, too, demand different lubricant technologies.
While the three-cylinder Mitsubishi engine in GMD's T25 might be a relatively straightforward unit, other downsized gasoline engines are highly boosted, run at higher temperatures and place higher stresses on the oil.
Diesels, likewise, place higher loadings on their bearings, making some OEMs very conservative about the oil viscosities they are prepared to specify. All this, says Dunning, makes it doubly important for the engine and the lubricant designers to work together for the most advantageous result.
Heavy-duty diesels are another important area of lubricant research: again, working holistically with Daimler engineers on the latest-generation Actros trucks, Shell helped achieve fuel savings of up to 7%, using its Rimula M6 HME heavy-duty engine oil.
The transmission was traditionally home to many of the heaviest oils on the vehicle, but again a shift to lower viscosities has brought greater efficiencies. Complicating the picture is the proliferation of gearbox and launch mechanism types, each bringing its own specific challenges.
But one thing is all too clear, as far as lubricants are concerned: the oils of the future will move to still lower viscosities, perhaps 0-20 or even below.
Significant, too, will be the pressure on chemistries to avoid elements such as phosphorus and sulphur, which harm emissions systems. Getting the base oil right will be vital, concludes Shell's Selda Gunsel: "Higher quality base oils, with low volatility, high viscosity index, narrow boiling range and higher oxidation stability, will be really critical in designing advanced lubricants – and GTL certainly fits into this category."
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