Additive gives improved mileage, less smog.

A chemical normally known for its role in the manufacture of synthetic rubber may lead to cleaner-burning, more-fuel-efficient cars and trucks.

Automobiles using the polymer additive, called polyisobutylene, decrease their emissions by 70 percent, says Paul F. Waters of American University in Washington, D.C. What's more, mileage goes up 20 percent, while horsepower increases 10 percent, he says.

Due to the emissions improvements and fuel savings, Waters says, polyisobutylene "reduces the number of gases that potentially lead to global warming."

Waters reported the results in late August at the 220th national meeting of the American Chemical Society in Washington, D.C.

Gasoline is a menagerie of hydrocarbon molecules that differ in size and shape. Short molecules generally burn very quickly, causing temperature and pressure inside an engine to rise in a dramatic spike. That creates engine knock and emissions of nitrous oxide pollution. Meanwhile, longer hydrocarbons burn more slowly and incompletely, which raises exhaust temperature and leaves remnants of hydrocarbons that contribute to soot and other forms of pollution.

Polyisobutylene appears to slightly delay the burning of short hydrocarbon molecules, which then hasten the burning of the longer hydrocarbons, Waters says. With the additive, therefore, the fuel burns more completely and engine and exhaust temperatures drop significantly.

Waters suspects that polyisobutylene changes the surface tension of fuel droplets, preventing short hydrocarbons from immediately vaporizing when gasoline is sprayed into the engine. The delay apparently is "long enough, so that ... the two components evaporate more or less in the same physical region in space and then they burn more or less together," says Waters.

"I think Paul has some intriguing results that could be very promising for the automotive industry," comments Graham Swift, a polymer chemist and independent consultant outside of Philadelphia. "It sounds delightfully simple and promising to me. We tend not to look for simple solutions."

"What I like about his approach was that he looked at combustion, and he looked at what controls combustion, and then applied it to the internal combustion engine," says Swift. With better control over the fuel droplets, Swift says, "the better your combustion, the better your fuel consumption, thee better your automotive horsepower, the less incomplete combustion you get, and that means you've got less noxious fumes coming out."

So far, Waters and his colleagues have tested the additive on a dozen automobiles in three states and several other countries. Waters points out that 50 other cars also have shown improved performance with the additive.

Waters says the mileage benefits could pay for the cost of the fuel additive--perhaps 10 cents a gallon, he says. Polyisobutylene works well in diesel engines, too, he adds.

"This is of course useful for global warming, but it's also useful for the vehicle and the engine itself," says Waters. "When the engine operates at a lower overall temperature, there's less wear and tear on all the parts. The less temperature that you impose on any of these devices, the longer they're going to last, and in principle, the less your repair bill is going to be."