This study concerns flame speeds during the ignition and combustion processes in a spark-ignition engine. These have been analysed from both combustion images in the early stages of the flame kernel development, and from the pressure time history. Simultaneous measurements of engine operating conditions, pressure traces and sequences of combustion images have been made in a single-cylinder four-stroke engine. The early stages of the combustion have been analyzed using stochastic image analysis techniques. These techniques can measure the total kernel growth, the local translational velocity of the centroid, the stretching of the flame kernel surface and its roughness. In addition, the fraction of the flame surface area supporting propagation has been determined from the directional variation of flame propagation between successive image frames. From the pressure time history the actual instantaneous flame speed has been derived for the main burn phase and thereafter extrapolated for the ignition phase. Good agreement has been demonstrated between the average value of the derived flame speed and well known correlations. For the ignition phase and early stages of combustion, the results demonstrate strong evidence that the flame speed is biased by both ignition effects and by the stretch of the flame kernel surface area. However, after the establishment of a fully developed turbulent flame, the spherical assumption is rather reasonable.
