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Experiments were conducted on a small SI engine with the objective to investigate the relationship between air/fuel-ratio and the ionic current signal measured across the spark plug electrodes. The propane-operated, single-cylinder engine has been equipped with sensors to monitor its operating conditions and a CDI ignition system with a built-in ionic current sensing circuit. More than 50 features of the ionic current signal have been examined in a preliminary analysis. Three features of the cycle-averaged signal (signal peak, signal integral in a crank angle window around the peak position, signal amplitude after a preset delay) were found to correlate reasonably well with air/fuel-ratio for the full range of operating conditions. After an evaluation of cycle-to-cycle variations of these features, simple algorithms for an application of the ionic current - mixture strength relationship are introduced.