Better than Hybrids

A proposed engine design approaches the efficiency of gas-electric hybrids, but could be far cheaper.

Consumers hoping to cut gasoline spending, with average gas prices nearing $3 a gallon, could opt for hybrids. But even with gas prices high, the added cost of hybrid cars can cancel money saved at the pump, suggesting the need for lower-cost alternatives.

A new type of ethanol-boosted, turbocharged gasoline engine could be the answer. The engine would be almost as efficient as gas-electric hybrids, but cost much less, according to its MIT inventors -- Leslie Bromberg and Daniel Cohn, plasma science and fusion center researchers, and John Heywood, professor of mechanical engineering.

The new engine would improve efficiency in two ways. The first is to decrease the size of the engine, which reduces friction, thus saving fuel at light engine loads, such as during city driving. When more power is needed, a turbocharger kicks in. It uses exhaust flow to compress air, making it possible to combust more air and fuel in a smaller space.

The second approach is to engineer the engine to have a higher compression ratio -- the ratio of the volume of air and fuel before and after it is compressed in an engine. A higher compression ratio "makes the engine more efficient, because you expand the burned gases more and extract more energy out of them," Heywood says.

Neither of these are new ideas. But in the past, such efforts have been limited by a phenomenon called knock: high compression ratios and extreme turbocharging cause gasoline to spontaneously combust when the engine is under heavy loads, such as during acceleration or at high speeds, potentially causing serious damage. The MIT researchers have found a way to prevent knock, allowing them to crank up the turbocharger and increase the compression ratio -- and thereby increase the power of an engine by 250 percent.

If this increase in power is taken advantage of to reduce the size of the engine -- which would go against long-time trends emphasizing performance over fuel economy -- it could save gas. "This allows very large pressure turbocharging, very large downsizing of the engine, and makes it possible to have a small engine with much higher efficiency," Cohn says.

The researchers solved the knocking problem by injecting into combustion chambers precisely controlled amounts of ethanol at moments when the engine is working hard enough to cause knock. Compared with gasoline, ethanol has higher octane, a rating of how much a fuel can be compressed before it combusts spontaneously, that is, before it causes knocking. The injected ethanol also cools the mixture, so it effectively increases the octane rating of the fuel mix to about 130 -- as good as high-performance racing fuels, Cohn says.