Measurements of the Influence of Soot Radiation on In-Cylinder Temperatures and Exhaust NOx in a Heavy-Duty DI Diesel Engine

It is generally accepted that thermal (Zeldo'vich) chemical kinetics dominate NO formation in diesel engines, so control of temperature is critical for reducing exhaust NOx emissions. Recent optical engine data revealed that when the start of injection (SOI) was retarded to very late timings, combustion luminosity decreased while exhaust NOx emissions increased, causing a "NOx bump." This data suggested that changes in radiative heat transfer from soot may affect in-cylinder temperatures and subsequent NOx formation. In this study, soot thermometry measurements of in-cylinder temperature and radiative heat transfer were correlated with exhaust NOx to quantify the role of radiative heat transfer on in-cylinder temperatures and NOx formation. The engine was operated at low-load conditions, for which the premixed burn was a significant fraction of the total heat release. Soot thermometry showed that radiative cooling reduced flame temperatures by 25-50 K for early SOI, reducing exhaust NOx by 12-25%. Near the NOx bump, radiative cooling was essentially absent, and the NOx reduction was thus removed. Radiative cooling alone, however, was found to be insufficient to be solely responsible for the observed changes in NOx emissions. Thermodynamic analysis showed that in the slow-mixing limit, compression-heating of burned gases for the large premixed-burn conditions near the NOx bump can also increase temperatures by about 25 K, for a further 12% increase in NOx. The data also shows that these two factors can contribute to other exhaust NO observations, such as the intake temperature optimum for minimizing NOx emissions. Finally, although it was not directly measured, the rate of post-flame mixing was identified and examined as a third primary factor affecting in-cylinder temperatures and NOx formation in diesel engines.