A More Efficient Engine

A new type of engine could be relatively inexpensive.

By Kevin Bullis

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.

But if the engine's temperature is too low, such as when it's being started or being operated under very low loads, the mixture doesn't get hot enough to combust. And at high loads, when the temperature is high, the mixture can combust too early, out of sync with the cycling of the engine, causing a potentially damaging phenomenon called knock. Differences in fuels can also affect precisely when the mixture combusts.

The hybrid system switches between the two forms of combustion. To do this requires changing the way the engine deals with combusted gases. During spark combustion, the gases are forced out through an open valve. In HCCI, the timing of the opening of that valve is changed so that it closes before the gases completely escape, trapping them inside.

John Heywood, a professor of mechanical engineering at MIT who was not involved with this work, says that HCCI could eventually provide even greater benefits as researchers find ways to adapt the engine so that they can use it for a wider range of loads. What's more, it could be used in combination with other gas-saving technologies already available on many vehicles. The extent to which HCCI can be combined with other approaches could determine how widely it's adopted, suggests Heywood.