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A thermodynamic model of spark ignition engine combustion, with multiple burned gas zones, has been extended to permit the different burned gas zones to have different mixture strengths. The NO formation is predicted in each burned gas zone using the extended Zeldovich mechanism. The model has been used to study stratified charge spark ignition engine combustion, in order to investigate the influence of overall equivalence ratio and degree of stratification on the NO emissions and the engine brake specific fuel consumption. For fixed throttle operation, it is concluded that the best trade-off is with an overall weak mixture that is close to homogeneous. For maximum power output using a slightly rich of stoichiometric mixture, then the mixture should also be close to homogeneous.

A recently suggested technique of combustion analysis has been applied to experimental data from a spark ignition engine operated at different compression ratios and over a range of spark timing and air-fuel ratio. The results of the analysis show that compression ratio has a strong effect on incompleteness of combustion and that this is detected both from unburned hydrocarbon emissions and from the new analysis method. The use of the analysis technique may give useful insights into the proportion of unburnt hydrocarbon sourced from incomplete combustion associated with cycle-by-cycle combustion variability and that sourced from crevices and other quench regions.