Nitric Oxide Formation in Spark-Ignition Engines with in-Cylinder Charge Non-Homogeneity of a Random Nature

Mixture preparation in spark-ignition (SI) engines has been observed to influence nitric oxide (NO)emissions significantly. As mixture preparation affects charge homogeneity, its influence on nitric oxide emissions in conventional SI engines was analysed on the basis that a non-homogeneous mixture with random spatial and temporal variations of mixture strength exists in the cylinder. Charge non-homogeneity was considered to influence NO emissions through (a) increased combustion variations and (6) combustion of spatially distributed charge elements of differing mixture strength. While the combustion variation effect is small, the latter is significant and is chiefly responsible for the experimentally observed effect of mixture preparation methods on NO emissions. The degree of non-homogeneity and not the scale was observed to influence NO predominantly. The calculated trends of NO emissions with increase in charge non-homogeneity correlate well with the experimental observations.