a radical way to burn

A Radical Way to Burn
By manipulating combustion parameters such as chamber geometries, valving configurations, and engine cycle pressures, engineers are developing a fundamentally different way to fire the fuel-air mixture in an engine cylinder.

By Steven Ashley,
Associate Editor

Besides compression-ignited diesels and spark-ignited gasoline power plants, there is a third way to initiate burning of the fuel-air mixture in piston-driven internal combustion engines. The technique--sometimes called radical ignition (RI) or activated radical (AR) combustion--is only now being exploited in practical power plants.

A cutaway of Honda’s EXP-2 racing-motorcycle engine shows the computer controlled exhaust valve (in yellow).
Engineers have manipulated fundamental combustion parameters such as chamber geometries, valving and porting configurations, and engine cycle pressures and timing to foster the formation of highly reactive chemical species that lower the fuel-air mixture's flash point so that even modest compression makes it self-ignite. These chemical initiators are then retained into the next cycle to start combustion, allowing the engine to run stably with no spark. Besides sparkless operation, the presence of these reactive (often oxygenated) hydrocarbon fragments in the mixture offers better gas mileage, lower exhaust emissions, and less cyclic variation.

Several engines using RI-type techniques are being developed today. One is the single-cylinder, two-cycle engine that powers the EXP-2 off-road racing motorcycle developed by research engineers at Honda Motor Co. Ltd. in Tokyo. The Honda AR combustion is based on prompt residual techniques in which a portion of the two-stroke's partially burned exhaust (residue) is reflected in a timely fashion (by a computer-controlled valve) back into the cylinder to initiate the next combustion cycle.

Elsewhere, Sonex Research Inc., an engineering firm in Annapolis, Md., has developed related but more generalized RI combustion techniques. Sonex's technology improves the fuel-combustion process by modifying the engine's combustion chamber to produce chemical and acoustic effects that combine to condition the charge before ignition, and to control both the length of ignition delay and the combustion time.

Japanese car builders are reportedly applying RI-type techniques to a new group of lean-burn, spark-ignition car engines (four-cycle), one of which is expected to be unveiled next year.

The above was adapted from an article in the August issue of Mechanical Engineering magazine. To obtain a copy of this issue, click here.