Knowledge of flame propagation enables a more detailed description of the combustion processes as well as assessing the influences of engine operating conditions and design parameters, particularly in view of efficiency improvements and pollutant reduction. Flame propagation in a single-cylinder spark-ignition engine is studied by means of a measuring technique using lightconductors coupled with photo-multipliers. The propagation process is monitored through a large number of optical probes arranged in a matrix in the combustion chamber. The spatial flame contour, the flame volume, and the flame front velocity as a function of time are furnished from these measurements. The investigations show the formation of quench zones ("flame quenching") not penetrated by the flame directly above the piston towards the end of the expansion phase at extremely lean operating conditions. This results from conditions of the mixture for lean operation which are unable to completely sustain the combustion process. The flame front velocity is sharply reduced through this flame quenching, thereby impairing the flame front in pursuing the piston during the expansion phase. A sharp drop in efficiency as well as an incrase in unburned hydrocarbon emissions are registered for these conditions.