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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.