Molten Salts Could Improve Fuel Economy

(ISNS) – An emerging class of engine lubricants with a radically different chemical makeup could significantly improve the fuel economy of cars, according to a recent report from researchers at Oak Ridge National Laboratory in Tennessee and General Motors.

These new molecules, called ionic lubricants, could someday be used as additives to the base oil used in motor oil. The compounds would form only a small fraction of the chemicals in re-engineered versions of engine oil, but nevertheless could “save the U.S. tens of millions of barrels of oil annually,” said lead researcher Jun Qu.

Engine lubricants balance engine wear with fuel economy. The more free-flowing an oil is, the better the fuel economy. But the thinner the oil, the faster an engine wears down. The key to improving fuel economy is to improve the anti-wear additives that supplement base oil, explained Qu.

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Unlike oil-based molecules, ionic liquids are “molten salts” with positively and negatively charged particles combined in equal measure. The particles are held together by strong forces that keep typical salts, like the one we eat, solid at room temperature. Unlike these familiar salts, ionic liquids are made of exceptionally large molecules, the charges from their positive and negative ends spread thin over the surface. Their size and structure keeps them liquid at room temperature – like oils – but their physical and chemical properties are unique, said Qu.

Previous work with ionic liquids had shown their promise as improved anti-wear compounds. But many ionic liquids are unstable. When exposed to the elements, the positive and negative ions split up, the latter then reacting with water to form highly corrosive acids. Another problem researchers had was getting ionic liquids to dissolve in common motor oils. Much like dissolving salt in oil, or mixing oil and water, creating a true solution of ionic liquids and base oil is hard.

But implementing an entirely new kind of engine that uses only an ionic liquid – no base oil – is nearly unthinkable, said Qu. Both the looks and performance of cars have evolved over the last fifty years, but, for the most part one thing has stayed almost the same – the base oil used in engine lubricants.

Though many of these previous studies were scientifically interesting, they were “far from real practical use in the near future,” said Qu.

The researchers at Oak Ridge worked around these problems by designing an ionic lubricant with a negative ion formed with phosphorus rather than fluorine; the former is much less corrosive. Their molecule was also large enough that positive and negative charges were spread thin across its surface. This enabled it to slip into solution with oils more easily.

When added to base oil in a one percent proportion, the new molecule made for a substantially less viscous liquid. But they had to test it for engine wear.

The researchers measured its performance in industrial tests at the General Motors’ labs, using a dynamometer, an instrument where a standard 2008 Cadillac SRX engine is hooked up to sensors that measure the engine’s performance under different conditions.

Using the ionic lubricant in an engine designed for standard oils was nerve-wracking, Qu recalled.

“I was really nervous during the tests! First, I was worried it would leak because it’s such a thin liquid. Then, I worried it would wear out the engine on this million-dollar dynamometer,” he said.

However, the ionic lubricant additive in base oil did surprisingly well – even by the researchers’ expectations. The re-engineered oil met wear-and-tear standards of current synthetic motor oils. And it improved fuel economy by 2% in a standard fuel-efficiency test.

“In the automotive industry, even 0.5% is an improvement, so 2% was huge,” said Qu.

The Department of Energy’s Fuel and Lubricants program, which funded the research, aimed to improve the fuel economy of vehicles with better lubricants by 2% by the year 2015.

“It’s only 2013 and we are already near that goal,” Qu said. He and his colleagues are now preparing their results for peer-reviewed publication.

“When you look at the friction and wear characteristics in these results, the data are very strong. They show these materials really can improve energy efficiency significantly,” said Michael Lovell, who researches the lubricant properties of ionic liquids at the University of Wisconsin, Milwaukee.

The technologies should reduce our usage levels of petroleum-based products and reduce the environmental footprint of oil usage, said Lovell. But despite the extensive lab tests, it’s hard to predict how these molecules will change engine performance or fuel economy in the real world. “Until we start using these lubricants in vehicles, it’s hard to tell how they will work in the lifetime and conditions that a real car or truck experiences,” he said. “However, we should all be excited that these materials exist and have the potential to be the lubricants of the future.”

Inside Science News Service is supported by the American Institute of Physics. Jyoti Madhusoodanan is a science writer based in San Jose, Calif. She tweets at @smjyoti.