Flame Structure Visualization of Stratified Combustion in a DISI Engine via PLIF 2001-01-3649
Planar laser-induced fluorescence of 3-pentanone doped into the fuel (iso-octane) and OH, which is present in the combustion products, was performed in an optically accessible direct-injection spark-ignition (DISI) engine under stratified and homogeneous operating conditions. A wall-guided, swirl-based combustion chamber was utilized, and experiments were performed for light load, where the fuel-air equivalence ratio was 0.3, and high load conditions, with an equivalence ratio of 0.7, at speeds of 600 and 1200 rpm. The 3-pentanone images were calibrated through the use of a premixed charge condition of known equivalence ratio, with corrections applied for number density changes due to combustion. At the light load condition combustion of the highly stratified fuel cloud was directly measured for the first time. The equivalence ratio of the mixture at the flame front was found to be in the range from 0.5 - 0.8 for optimized combustion conditions in this engine. Large gradients were observed in the fuel distribution throughout combustion. Images acquired in the expansion stroke well past the end of the main heat release showed the continued presence of 3-pentanone in the end gas with an equivalence ratio near 0.4, suggesting the lean quenching of the flame. Under all operating conditions there was a weak correlation between the fuel distribution near the spark gap at the time of ignition and the combustion performance on a cycle-by-cycle basis.