On the Correlation between NOx Emissions and the Diesel Premixed Burn 2004-01-1401

It is generally accepted that exhaust NO_X emissions of diesel engines increase with the degree of premixed burning. Although several mechanisms proposed in the literature are likely responsible for some aspects of the correlation, taken together, they cannot explain all observations of this correlation. In this study, thermodynamic analyses and optical/imaging diagnostics were employed in an optically-accessible, heavy-duty DI diesel engine to examine the in-cylinder mechanisms by which fuel/air premixing affects engine-out NO_X emissions. Exhaust NO and NO_X emissions were measured and correlated with observations of soot luminosity and jet penetration as the intake-temperature and injection timing were varied. The engine was operated at low-load conditions, for which the premixed burn was a significant fraction of the total heat released.

As injection timing was retarded to the misfire limit, dramatic reductions in soot luminosity accompanied increased exhaust NO_X emissions, even as the calculated adiabatic flame temperatures decreased. Since thermal NO formation increases with temperature, the reduction of the cooling effect of soot radiative heat transfer may increase the actual flame temperature, and thus NO formation, as the jets become less sooty. Compression heating of the reactant mixture by the large pressure rise during premixed combustion was found to be insufficient to explain exhaust NO_X trends. Compression of post-flame burned-gases, however, could be responsible for some aspects of the correlation. Though the products of the premixed burn are normally too rich to form significant thermal NO, under very long ignition-delay conditions, portions of the mixture at ignition may be lean enough for significant thermal NO formation.