March 2007

The Incredible Shrinking Engine

A new engine design could significantly improve fuel efficiency for cars and SUVs, at a fraction of the cost of today's hybrid technology.

By Kevin Bullis

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At MIT's Sloan Automotive Laboratory, Daniel Cohn (pictured above) stands behind an engine equipped with test instruments (in yellow) and an injection system that sprays fuel directly into the engine's combustion chambers.
Credit: Porter Gifford

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•	View a demo of the new engine deisgn built by Daniel Cohn.

The following article appears in the March/April 2007 issue of Technology Review.

For Daniel Cohn, a senior research scientist at MIT's Plasma Science and Fusion Center, the century-old internal-combustion engine is still a source of inspiration. As he strides past the machinery and test equipment in the MIT Sloan Automotive Laboratory, his usually reserved demeanor drops away. "An engine this size," he says, pointing out an ordinary-looking 2.4-liter midsize gasoline engine, "would be a rocket with our technology."

By way of explaining that technology, he shows off a turbocharger that could be bolted to the 2.4-liter engine; the engine, he adds, uses direct fuel injection rather than the port injection currently found in most cars. Both turbocharging and direct injection are preëxisting technologies, and neither looks particularly impressive. Indeed, used separately, they would lead to only marginal improvements in the performance of an internal-combustion engine. But by combining them, and augmenting them with a novel way to use a small amount of ethanol, Cohn and his colleagues have created a design that they believe could triple the power of a test engine, an advance that could allow automakers to convert small engines designed for economy cars into muscular engines with more than enough power for SUVs or sports cars. By extracting better performance from smaller, more efficient engines, the technology could lead to vehicles whose fuel economy rivals that of hybrids, which use both an electric motor and a gasoline engine. And that fuel efficiency could come at a fraction of the cost.

Cohn says that his colleagues--Leslie Bromberg, a principal research scientist at the Plasma Science and Fusion Center, and John Heywood, a professor of mechanical engineering and director of the Sloan Auto Lab--considered many ways to make internal-combustion engines more efficient. "And then, after a lot of discussion, it just sort of hit us one day," Cohn recalls. The key to the MIT researchers' system, he explains, was overcoming a problem called "knock," which has severely limited efforts to increase engine torque and power.

In gas engines, a piston moves into a cylinder, compressing a mixture of air and fuel that is then ignited by a spark. The explosion forces the piston out again. One way to get more power out of an engine is to design the piston to travel farther with each stroke. The farther it travels, the more it compresses the air-fuel mixture, and the more mechanical energy it harvests from the explosion as it retreats. Overall, higher compression will lead to a more efficient engine and more power per stroke. But increasing the pressure too much causes the fuel to heat up and explode independently of the spark, leading to poorly timed ignition. That's knock, and it can damage the engine.

To avoid knock, engine designers must limit the extent to which the piston compresses the fuel and air in the cylinder. They also have to limit the use of turbocharging, in which an exhaust-driven turbine compresses the air before it enters the combustion chamber, increasing the amount of oxygen in the chamber so that more fuel can be burned per stroke. Turning on a car's turbocharger will provide an added boost when the car is accelerating or climbing hills. But too much turbocharging, like too much compression, leads to knock.

The Incredible Shrinking Engine

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