Effect of Mixture Heterogeneity on the Formation of Nitrogen Oxides in Lean-Burn Gas Engines.

Abstract

This study reveals that the mixture heterogeneity significantly affects the formation of nitrogen oxides in spark-ignition engines. The experimental results for a variety of engine types and sizes under lean-burning operation show that a homogeneous lean mixture permits much greater reduction of the exhaust NOx concentration than a heterogeneous mixture. In addition, the histories of temperature distribution and NO concentration are calculated using a themodynamic model based on the measured rates of heat release for the mixture conditions having different degrees of heterogeneity. It is shown that a higher NO concentration results from the hot burnt gas ignited initially. Thus, lean burning of spark-ignition engines with a more homogenous mixture is desirable. It is also suggested that a significant reduction of NO concentration may be achieved once the burnt gases are mixed during combustion so as to make the burnt zone more uniform.