Friday, August 03, 2007

A More Efficient Engine

A new type of engine could be relatively inexpensive.

By Kevin Bullis

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Clean burning: A new sparkless gas engine could significantly reduce fuel consumption. In a conventional engine (top), a mixture of fuel and air is ignited by a spark plug. In a diesel (middle), the fuel ignites when it's injected into hot, compressed air. An alternative to both is called homogeneous charge compression ignition (bottom). Here, a mixture of fuel and air is compressed until it combusts. Because the fuel and air are premixed, they burn more evenly than they do in a diesel engine, producing much less soot and nitrogen oxide.
Credit: MIT Laboratory for Energy and the Environment

A new version of the internal combustion engine, which could significantly cut gas consumption, might be surprisingly practical and easy to deploy, according to recent findings by researchers at MIT. Tests on a prototype based on the technology, which allows engines to switch between conventional technology and the new gas-saving type of combustion, show that it does not require a special fuel, and engines using the technology can be cheaply made out of conventional auto parts.

The gas-saving technology, called homogeneous charge compression ignition, or HCCI, uses a form of combustion that is much more efficient than conventional spark ignition. Under some conditions, it can reduce fuel consumption by 25 percent, says William Green, a professor of chemical engineering at MIT who was coauthor of the new study. That's very similar to the efficiency of a diesel engine, which also achieves combustion by compression rather than a spark. But unlike diesel engines, HCCI results in a more uniform combustion and is thus much cleaner. A system that combines HCCI with conventional combustion could improve fuel economy by a few miles per gallon on average, Green says.

Several research groups are working on the new type of combustion. Volvo, for example, has built a hybrid system that can switch between conventional spark ignition and HCCI. Some experts, however, had expected that the new type of engine would require special fuel.

The MIT research shows that an HCCI engine can operate with any of the varieties of gasoline sold in North America, making a special fuel unnecessary. The researchers tested a range of different gasolines made at different refineries. They found that the HCCI engine "was less sensitive to the fuel than people had feared," says Green.

While the HCCI has several performance limitations, these can be addressed using a hybrid approach, in which an engine could switch between HCCI and conventional spark ignition. Using already mass-produced parts could make it relatively inexpensive to build such a hybrid, Green says.

In conventional gasoline engines, a spark ignites a mixture of fuel and air in a combustion chamber, creating an explosion that drives a piston. While this happens very efficiently when the engine is working hard, it's less efficient at lower loads, such as during cruising, when less gasoline is being pumped into the combustion chamber. At these times, to keep the ratio of fuel to oxygen optimized, a partial vacuum is created in the chamber. It takes extra energy to make this vacuum, which decreases the engine's efficiency.

The HCCI technology avoids the use of an energy-wasting vacuum. Instead, hot gases from a previous combustion cycle remain in the chamber; the engine uses a combination of heat from these hot gases and heat generated by compressing the mixture to raise temperatures high enough that the mixture explodes.

A More Efficient Engine

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