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

Abstract

Water has been found to be useful to reduce NO_x emissions. In this research, numerical investigations are conducted to understand the effect of air humidity on NO_x and soot emissions. A modeling study was carried out by using KIVA-3V code with the Zeldovich thermal NO_x model and an improved soot oxidation model based on oxygen and OH oxidation. Good agreement between predictions and measurements of engine-out NO_x and soot emissions was achieved. The computations confirm that air humidity allows significant NO_x 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% NO_x production while increasing soot formation levels appreciably at light and middle loads. When the equivalence relative humidity is 6%, the NO_x reduction of more than 70% can be achieved while the engine-out soot emissions increase by more than 30%. The reduction in NO_x 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 NO_x emissions. On the whole, humidity has dual effects on soot formation.

Keywords:

• Diesel engine
• Humidity
• NO_x
• Soot
• Model

ACKNOWLEDGMENTS

This project was supported and funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and Research and Innovation Project for College Graduates of Jiangsu Province (No. CXZZ11_0563).