Original Date: 01/23/1995
Revision Date: 01/18/2007

Sandia National Laboratories partnered with Cummins Engine Company to construct a more accurate model of the dynamics occurring during diesel engine combustion in response to a need by American diesel engine manufacturers to develop a clean, efficient diesel engine without reducing performance or reliability. Sandia designed and built an optically accessible engine that retained the basic geometry of a production, heavy- duty diesel engine. Optical portholes on the engine enabled Sandia researchers to direct a two-dimensional laser sheet into the combustion chamber and quantitatively measure fuel-vapor concentrations and relative soot particle distributions.

The results showed that diesel combustion and soot particle formation proceed in a completely different manner from what was previously believed. The diesel engine design community believed that the liquid-phase fuel jet penetrated far into the combustion chamber and that both liquid and vapor-fuel were present in the combustion zone. It was also believed that the combusting fuel jet consisted of a pure-fuel core with a diffusion flame around the periphery and that large soot particles formed on the fuel-rich side of the diffusion flame in a shell around the periphery. Tests showed that in fact, under typical conditions, all liquid fuel has vaporized 25 mm from the fuel injector and that the fuel in the main combustion zone was in the vapor phase. It was also determined that soot formation occurred throughout the cross section of the leading portion of the jet with a higher concentration, and larger soot particles toward the leading edge in the head vortex region. This information and the results of other studies in the optically accessible diesel engine have contributed to the significant reduction in diesel emissions achieved by engine designers in recent years. From 1978 to 1994, particulate emissions have been reduced by a factor of 10, and the emissions of oxides of nitrogen have been reduced by a factor of 2.

By successfully imaging the in-cylinder combustion process, through the application of planar laser Rayleigh scattering and laser-induced incandescence, results were obtained that directly contributed to the development of a more accurate model of diesel engine combustion. This new model gives diesel engine designers the ability to make more intelligent design decisions in their efforts to further reduce emissions and increase efficiency while maintaining performance and reliability at acceptable levels.

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