EXPERIMENTAL INVESTIGATION OF BULK FLAME QUENCHING IN A DIRECT-INJECTION SPARK IGNITION ENGINE
Strand, Tyson E.
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The following thesis describes planar laser-induced fluorescence (PLIF) experiments that investigate bulk flame quenching in the lean periphery of a stratified fuel cloud during light-load operation of a direct-injection spark-ignition (DISI) engine. PLIF of both 3-pentanone doped into the fuel (iso-octane) and OH present naturally in the combustion products were imaged on an intensified CCD camera. The OH images show the progression of the flame front and the expansion of the product zone. The 3-pentanone images provide visualization of the progression of the flame front through the consumption of fuel, as well as allowing quantification of the local equivalence ratio in the stratified, unburned mixture. Under stratified operating conditions, using an overall equivalence ratio of ? = 0.3 and an engine speed of 600 rpm, quenching of the flame in the lean periphery of the fuel cloud was observed. The combustion product zone (OH fluorescence) showed a period of rapid growth shortly after ignition. The flame front propagation stopped before the edge of the piston bowl, and the product zone ceased expansion. Images of the fuel region (3-pentanone fluorescence) demonstrate the consumption of the fuel, the propagation and stalling of the flame front, and a region of unburned fuel present long after the end of heat release (as late as 70� ATDC). Advancing the combustion phasing by varying the injection and ignition timings within a window of acceptable combustion characteristics was not sufficient to alleviate quenching. However, quenching was alleviated when the air intake pressure was reduced to 69 kPa while the mass of injected ii fuel was held constant. The cause for this behavior is believed to be a combination of increased homogeneity of the fuel cloud and higher end gas temperatures, which decreases the lean flammability limit of the fuel.