Post-Flame Oxidation and Unburned Hydrocarbon in a Spark-Ignition Engine

Abstract

<div class="htmlview paragraph">Many recent publications indicate that spark ignition (SI) engines equipped with the conventional port-injection fuel system (PIF) seem to have serious fuel-maldistribution problems, including the formation of liquid layers over the combustion chamber surfaces. It is reasonable to expect that such a maldistribution is an unfavorable condition for the flame propagation in the cylinder.</div> <div class="htmlview paragraph">The in-cylinder flame behaviors of a PIF-SI engine as fueled with gasoline are investigated by using the Rutgers high-speed spectral infrared imaging system. These results are then compared with those obtained from the same engine operated by gaseous fuels and other simple fuels.</div> <div class="htmlview paragraph">The results from the engine operated by gasoline reveal slowly burning fuel-rich local pockets under both fully warmed and room-temperature conditions. The local pockets seem to stem from the liquid layers formed over the surfaces during the intake period. The (invisible) post-flame oxidation of the rich pockets is observed to continue even after the exhaust valve opens. On the contrary, the same engine run with a gaseous fuel exhibits some predictable and “clean” flame propagations.</div> <div class="htmlview paragraph">The new results obtained from the present study suggest that such a late oxidation of locally fuel-rich liquid pockets may be a significant cause for the emission of the engine-out unburned hydrocarbon (UHC). The sluggish consumption of the fuel there may also be a factor for reducing the thermal efficiency of the engine. A parametric study of this observation is performed to obtain a better understanding of the findings.</div>