Massachusetts Institute of Technology researchers are developing a half-sized gasoline engine that performs like its full-sized counterpart but offers fuel efficiency approaching that of today's hybrid engine system — at a far lower cost. The key? Carefully controlled injection of ethanol directly into the engine's cylinders when there's a hill to climb or a car to pass.
MIT says spending an extra $1,000 and adding a couple of gallons of ethanol every few months would give consumers an engine that can go as much as 30% farther on a gallon of fuel than an ordinary engine can. Moreover, the little engine performs well without high-octane gasoline.
The impact on U.S. oil consumption could be substantial. Researchers figure if all cars had the new engine, current U.S. gasoline consumption of 140 billion gallons per year would drop by more than 30 billion gallons.
"There's a tremendous need to find low-cost, practical ways to make engines more efficient and clean and to find cost-effective ways to use more biofuels in place of oil," says Daniel R. Cohn, senior research scientist in the MIT Laboratory for Energy and the Environment and the Plasma Science and Fusion Center (PSFC).
For decades, efforts to improve the efficiency of the conventional spark-ignition (SI) gasoline engine have been stymied by a barrier known as the knock limit. Changes that might have made engines far more efficient would have caused knock — spontaneous combustion that makes a metallic clanging noise and can damage engines. Sophisticated computer simulations let the MIT team determine how ethanol could suppress spontaneous combustion and essentially remove the knock limit.
When the engine is working hard and knock is likely, a small amount of ethanol goes directly into the hot combustion chamber where it quickly vaporizes, cooling the fuel and air and making spontaneous combustion much less likely. Simulations predict the engine won't knock even when pressure inside the cylinder is three times higher than in conventional SI engines. Engine tests at Ford Motor Co. produced results consistent with the model's predictions.
Eliminating knock would let researchers incorporate two operating techniques that help make diesel engines efficient, but without their notoriously high emissions levels. First, the engine is highly turbocharged. Incoming air is compressed to fit more air and fuel in the cylinder so the engine produces more power. Second, the engine can work at a higher compression ratio. Burning gases expand more in each cycle, extracting more energy from a given amount of fuel.
The combined changes could double engine power, compared to ordinary ICEs. Rather than seeking higher vehicle performance — the trend in recent decades — MIT researchers shrank their concept to half the size.
If all goes as expected, vehicles with the new engine could be on the road within five years.