Analysis of high-speed broadband flame chemiluminescence imaging in a SI engine

Abstract

This study investigates the characteristics of flame propagation in a gasoline S.I engine through a combined thermodynamic and optical approach. Two high-speed CMOS camera were used together with an endoscopic system to visualize the early flame in a spark-ignited engine. An unintensified high-speed (20 kHz) imaging of early flame kernel formation and turbulent flame propagation for hundreds of consecutive combustion cycles has been analysed and compared to pressure-derived heat release rates and mass fraction burn (MFB) profiles. At very low light levels, the patterned read-out noise on the detector becomes significant. Filtering in the Fourier domain was effective in suppressing this noise component to acceptable levels. In this paper, a previously-developed algorithm with automatic dynamic thresholding was used to separately detect spark ignition and flame kernel in the image sequences. The detected flame boundary was post-processed to compute basic flame characteristics such as flame area and turbulent flame speed based on spherical flame propagation assumption. Ignition and flame propagation are compared for fast combustion cycles and slow combustion cycles based on the optical flame speed and crank angle resolved mass fraction burned profile. The good burn (fast) cycles show higher spark stretch caused by local tumble flow-field, greater early flame growth rate, and nearly spherical flame propagation at the center of combustion chamber. In contrast, the poor (slow) cycles show slower flame kernel growth rate, and mostly asymmetric flame propagation near the spark-plug and pent-roof of the combustion chamber. The analysis shows a good correlation between thermodynamics and optical data.