Early flame development in a spark-ignition engine

Abstract

The effect of changing the compression ratio from 7 to 3.5 and of different fuels, viz., propane, methane, and isooctane, on early flame development in a spark-ignition engine has been studied using an optical technique. This early phase of combustion is very crucial since cyclic variations in combustion and hence pressure development originate during this phase. The average flame speed increases under the influence of turbulence as the flame grows and appears to reach a fully developed value by the time the flame radius has reached about 11 mm in the engine studied. The evolution of the average flame velocity in this early stage appears to be spherically symmetrical in the engine considered. For the same operating conditions, propane flames are the fastest, followed by those of isooctane and finally of methane, as one would expect from their respective laminar burning velocities. Decreasing the compression ratio reduces the flame velocity sharply, mainly through the increase in residual mass fraction. The estimated initial burning velocity, S 0, differs from the laminar burning velocity, S L, calculated from previously published correlations. There is considerable cyclic variation in combustion and this decreases as S 0 or S L increases.