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Mixture preparation in spark-ignition (SI) engines has been observed to influence nitric oxide ( N O ) emissions signijicantly. 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 nonhomogeneity 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 signiJcant and is chiefly responsible for the experimentally observed effect of mixture preparation methods on N O emissions. The degree of nonhomogeneity and not the scale was observed to influence N O predominantly. The calculated trends of N O emissions with increase in charge non-homogeneity correlate well with the experimental observations.NOTATION [ = stoichiometric air-fuel ratio, (F/A),, 1 actual air-fuel ratio, F/A Subscripts and superscriptaverage or mean max related to maximum combustion pressure st stoichiometric mixture -The M S was

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