HEAT TRANSFER AND FLAME SURFACE STRETCH EFFECTS DURING EARLY COMBUSTION IN S.I. ENGINES
This study concerns experimental and theoretical analysis of early stages of the flame kernel development. The simultaneous measurement of engine operating conditions, pressure traces and sequences of combustion images has been made in a single cylinder four stroke engine. The early stages of the combustion have been analysed using a new stochastic technique of image analysis. This technique can measure the total kernel growth, the thermal expansion part, the local translational velocity of the centroid, stretching of the flame kernel surface and its roughness. The theoretical results of the study are based on a fundamental thermodynamic balance which includes the effect of enthalpy gain due to combustion, heat loss to the unburned mixture and electrode surface area as well as the effects of reduction in flame-front area due to flame stretch and plug electrode interference. A correlation of the flame kernel development/extinction has been derived to show the effects of flame kernel surface stretch, heat transfer to the electrodes, electrode geometry, turbulent intensity and turbulent burning speed. This analysis shows that the flame stretch, the flame front area supporting propagation, and the turbulent intensity have a major effect on the flame kernel self-sustain condition. The heat transfer to the electrodes, through the contact area with the flame kernel, is found to have minor effect only.