Visualization of late-cycle phenomena in a direct injection spark ignition engine

Abstract

Planar laser-induced fluorescence measurements of 3-pentanone doped into the fuel and the hydroxyl (OH) radical were separately performed in a direct injection spark ignition engine operating unthrottled at 600 r/min with an overall fuel-air equivalence ratio W of 0.3. It was found that the premixed flame, identified by OH fluorescence, did not propagate fully to the outermost edge of the combustion chamber regardless of image timing. Further, late in the cycle after the heat release had completed, fuel fluorescence was identified at the combustion chamber periphery. The fuel concentration at these late times was found to be lean, with little fuel at W>0.5. These findings strongly suggest that the flame has quenched at the edge of the fuel cloud, confirming a long-held belief about stratified combustion. A series of additional experiments were performed to examine the factors controlling the observed quenching phenomenon. Changes in the end gas conditions resulting from advanced combustion phasing were not sufficient to alleviate the existence of late-cycle fuel fluorescence. Throttling the intake to 69 kPa while maintaining the same mass of fuel injected resulted in complete flame propagation in the investigated field of view, as evidenced by the OH fluorescence, and late-cycle fuel fluorescence was not present in the field of view. Time-resolved exhaust hydrocarbon concentration measurements were found to be lower at the throttled condition.