Determination of Flame-Front Equivalence Ratio During Stratified Combustion

Combustion under stratified operating conditions in a direct-injection spark-ignition engine was investigated using simultaneous planar laser-induced fluorescence imaging of the fuel distribution (via 3-pentanone doped into the fuel) and the combustion products (via OH, which occurs naturally). The simultaneous images allow direct determination of the flame front location under highly stratified conditions where the flame, or product, location is not uniquely identified by the absence of fuel. The 3-pentanone images were quantified, and an edge detection algorithm was developed and applied to the OH data to identify the flame front position. The result was the compilation of local flame-front equivalence ratio probability density functions (PDFs) for engine operating conditions at 600 and 1200 rpm and engine loads varying from equivalence ratios of 0.89 to 0.32 with an unthrottled intake. Homogeneous conditions were used to verify the integrity of the method. Early injection cases were found to give flame-front equivalence ratio PDFs comparable to the homogeneous condition, albeit slightly wider. With late injection timings the flame-front equivalence ratio was found to deviate widely from a homogenous state, ranging from 0 to 2.5. Conditions that exhibited very rich flame-front equivalence ratios were also conditions for which spark plug fouling was observed. The data were found to be strongly affected by the time of image acquisition relative to the overall heat release history, with data acquired near the end of the heat release showing a large fraction of pixels below the lean flammability limit. These results represent the first measurements of the local flame-front equivalence ratio in stratified engine combustion known to these authors.