The paper offers an explanation as to the nature and cause of the cyclic combustion variation in the spark ignition engine. This is done from the analysis of photographs of flame initiation, formation, and propagation in a transparent piston engine and in a steady flow transparent tube apparatus in which the turbulence level was controlled. It was found that the main cause of cyclic variation is the small-scale turbulence and the mixing that is caused by this turbulence. The ignition and the combustion process is divided into three main intervals or phases, in order to explain the influence of turbulence, mixing, and chemical kinetics on the overall cyclic variation. The small- and the medium-scale turbulence influence the mixing of the reacting mixture in the small unstable flame kernel with the reactants that surrounds it, causing variation in the reaction rate and hence variations in the delay period. The larger scale turbulence influences the variation of the flame front area in the flame propagation phase. © 1981.
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