Abstract
This study numerically investigates mechanisms of NOx reduction when pure NO is injected into a diesel engine intake. In a previous experiment, emissions were reduced successfully with NO concentrations above 600 ppm at the intake. However, the NOx suppression was significantly smaller when premixed NO concentrations were 600 ppm or below. To explain these observations, the contributions of active species O, OH, H, and N to the production of thermal NO were estimated numerically. First, the equilibrium concentrations of CO2, CO, H2O, H2, O2, O, OH, H, and N2 in flames with various equivalence ratios were calculated. The concentrations of quasi-steady-state N and the NO production rate were then calculated based on the active species concentrations. The analysis shows that the reduced NO suppression at lower premixed NO concentration is primarily due to the role of N changing from production to decomposition of NO in fuel-rich regions when the NO concentration exceeds around 1200 ppm.
