March 2007

The Incredible Shrinking Engine

Continued from page 1

By Kevin Bullis

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An alternative way to prevent knock is to use a fuel other than gasoline; although gasoline packs a large amount of energy into a small volume, other fuels, such as ethanol, resist knock far better. But a vehicle using ethanol gets fewer miles per gallon than one using gasoline, because its fuel has a lower energy density. Cohn and his colleagues say they've found a way to use both fuels that takes advantage of each one's strengths while avoiding its weaknesses.

The MIT researchers focused on a key property of ethanol: when it vaporizes, it has a pronounced cooling effect, much like rubbing alcohol evaporating from skin. Increased turbocharging and cylinder compression raise the temperature in the cylinder, which is why they lead to knock. But Cohn and his colleagues found that if ethanol is introduced into the combustion chamber at just the right moment through the relatively new technology of direct injection, it keeps the temperature down, preventing spontaneous combustion. Similar approaches, some of which used water to cool the cylinder, had been tried before. But the combination of direct injection and ethanol, Cohn says, had much more dramatic results.

The researchers devised a system in which gasoline would be injected into the combustion chamber by conventional means. Ethanol would be stored in its own tank or compartment and would be introduced by a separate direct-injection system. The ethanol would have to be replenished only once every few months, roughly as often as the oil is changed. A vehicle that used this approach would operate around 25 percent more efficiently than a vehicle with a conventional engine.

A turbocharger and a direct-injection system would add to the cost of an engine, as would strengthening its walls to allow for a higher level of turbocharging. The added equipment costs, however, would be partially offset by the reduced expense of manufacturing a smaller engine. In total, an engine equipped with the new technology would cost about $1,000 to $1,500 more than a conventional engine. Hybrid systems, which are expensive because they require both an internal-combustion engine and an electric motor powered by batteries, add $3,000 to $5,000 to the cost of a small to midsize vehicle--and even more to the cost of a larger vehicle.

When the MIT group first hatched its idea, Bromberg created a detailed computer model to estimate the effect of using ethanol to enable more turbocharging and cylinder compression. The model showed that the technique could greatly increase the knock-free engine's torque and horsepower. Subsequent tests by Ford have shown results consistent with the MIT computer model's predictions. And since the new system would require relatively minor modifications to existing technologies, it could be ready soon. Ethanol Boosting Systems, a company the researchers have started in Cambridge, MA, is working to commercialize the technology. Cohn says that with an aggressive development program, the design could be in production vehicles as early as 2011.

While Cohn applauds the benefits of hybrids and says his technology could be used to improve them, too, he notes that the popularity of hybrid technology is still limited by its cost. Cheaper technology will be adopted faster, he suggests, and will thus reduce gasoline consumption more rapidly. "It's a lot more useful," he says, "to have an engine that a lot of people will buy."

The Incredible Shrinking Engine

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