Experiments under two intake air swirl levels (swirl ratio of 0.55 and 5.68) were conducted to investigate the early flame development of combustion in a single-cylinder spark-ignition direct-injection engine. The engine was equipped with a quartz insert in the piston which provided an optical access to its cylinder through the piston. The crank angle resolved combustion images through the piston window and in-cylinder pressure measurements of 250 cycles were recorded simultaneously for both swirl levels at a specified engine speed and low load condition. The early development, size and spatial characteristics extracted from the flame images were analyzed as a function of crank angle degrees after the ignition. Experimental results revealed that the early flame development was strongly influenced by the highly directed swirl motion of intake-air into the combustion cylinder. The location of the start of flame kernel relative to the spark plug position also changed intermittently at different swirl levels. While the structure of the early flame was found to be similar for both swirl levels, the starting location of the flame showed vast difference in how the flame progressed. In general, the flame kernel was formed 2 crank-angle degrees after spark timing for the high swirl level, which was 4 crank-angle degrees earlier than that of low swirl case. For low swirl flow, the early combustion showed more cycle-to-cycle variation in terms of both flame size and centroid location. It was quantitatively shown that increasing swirl ratio from 0.55 to 5.68 could reduce the cycle-to-cycle variation of early flame structure, resulting in about 3 to 4 crank-angle degrees advance of peak pressure location and 1% improvement for COV of IMEP.

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