Investigation of Flame-Front Equivalence Ratio during Stratified Engine Combustion

Abstract

Stratified engine combustion was investigated using simultaneous imaging of the fuel distribution and flame front in an optically accessible direct-injection spark-ignition engine. Planar laser-induced fluorescence of 3-pentanone doped into iso-octane and the OH radicals naturally occurring in the combustion products were imaged with two intensified CCD cameras. The 3-pentanone images provide a quantitative measure of fuel concentration and the OH images allowed for the position of the flame front to be accurately determined. These results represent the first data taken during stratified combustion using a two-camera technique. Using the image data a novel method was developed to determine the flame-front equivalence ratio during stratified combustion. The results of the method provide insights into the stratified combustion process. Additionally, engine-out NOx and CO measurements are presented and an effort to determine a correlation between the flame-front equivalence ratio and measured emissions is made where the flame-front equivalence ratio is thought to be a major factor in pollutant emission formation during stratified combustion. The effects of engine speed, engine load, spark timing, and ignition timing were investigated. The data indicate that a wide range of equivalence ratios are present along the flame front. The limited field of view was found to significantly influence the data. The flame-front equivalence ratio data taken for conditions with varying injection and varying spark timing at equivalence ratios of ? = 0.32 and ? = 0.42 at 600 rpm showed little correlation with the measured emissions. However, the NOx data did clearly reflect the trends of peak pressure. The available field of view may have been one cause for the lack of correlation, but the pressure trends and emissions data also indicate that combustion phasing has a strong influence on NOx emissions with changes in spark timing of 10 crank angle degrees causing almost a factor of two change in measured engine-out NOx.