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The applicability of the Ionic Current sensing method in production application is severely limited by the extent of cycle-to-cycle variations in the signal trace. In an attempt to investigate the sources for these cyclic signal fluctuations, experiments were conducted on a single cylinder 4-stroke, propane-operated engine. The ionic current signal was measured at two locations. The flame development phase in the vicinity of the spark plug gap was optically monitored by means of a high-speed CCD camera. The results showed that only the second part of the Ionic Current signal is directly impacted by cyclic variations in combustion parameters. The optical results provided evidence that the characteristics of the early flame development phase have a strong impact on the features in the first part of the signal.

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.