High-Speed Imaging of OH* and Soot Temperature and Concentration in a Stratified-Charge Direct-Injection Gasoline Engine

Abstract

In-cylinder soot formation and oxidation in a stratified-charge spark-ignited direct-injection (SIDI) engine have been studied with a high-speed (9000 frames/s) camera system with three intensified detectors. Continuous records of soot temperature and relative soot concentration within individual engine cycles are evaluated from images of soot radiation at two wavelengths (650 and 750 nm). Combustion is followed simultaneously by imaging OH* chemiluminescence (306 nm). The spatially and temporally resolved data allow the two most important soot sources to be quantified separately for a typical part-load operating condition. (1) Soot first forms in partially premixed flame propagation through rich zones, but the high soot temperatures (_2000–2400 K) and rapid mixing with surrounding hot lean regions lead to rapid soot oxidation and burnout. (2) Soot formation in pool fires (diffusion flames fed by thin films of liquid fuel on the piston) begins later and continues until late in the cycle, when further significant soot oxidation is unlikely (soot temperatures have dropped to _1500 K, and OH* chemiluminescence is no longer detectable). For this SIDI engine design and operating condition, pool fires are the dominant source of engine-out soot.