PEERING BY LASER THROUGH a quartz window in the piston of a diesel engine, researchers at Sandia National Laboratories have gotten a better look at how diesels burn fuel. That could lead to cleaner engines.

John E. Dec, Sandia's principal investigator, says hot air vaporizes diesel fuel after it has traveled just an inch from the injector. That's about half the distance previously supposed. Knowing this should allow engineers more latitude in designing the sombrero-shaped piston tops. Dec also found that large soot particles accumulate mainly in the head vortex (yellow in right-hand illustration). Breaking up the vortex could circulate soot particles to hotter regions, where they would burn up. Another finding: Lots of soot can also come from fuel that dribbles out of the injector when it shuts off too slowly.

Dec's work, at Sandia's Livermore (Calif.) branch, is partly supported by diesel-engine maker Cummins Engine Co. Next, Dec hopes to figure out how diesel engines create nitrogen oxide air pollutants.

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