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Modeling the Formation of Nox and Soot Emissions in a Diesel Engine at Different Humidity

Water has been found to be useful to reduce NOx emissions. In this research, numerical investigations are conducted to understand the effect of air humidity on NOx and soot emissions. A modeling study was carried out by using KIVA-3V code with the Zeldovich thermal NOx model and an improved soot oxidation model based on oxygen and OH oxidation. Good agreement between predictions and measurements of engine-out NOx and soot emissions was achieved. The computations confirm that air humidity allows significant NOx reduction with a little soot penalty except heavy and full load conditions. The computations show that air humidity with small level of (e.g., 2%) can significantly reduce 30% NOx production while increasing soot formation levels appreciably at light and middle loads. When the equivalence relative humidity is 6%, the NOx reduction of more than 70% can be achieved while the engine-out soot emissions increase by more than 30%. The reduction in NOx emissions by air humidity does not totally depend on the lower in-cylinder mean temperature. However, the in-cylinder temperature has direct effect on the formation of O radicals, and has significant effect on the NOx emissions. On the whole, humidity has dual effects on soot formation.