Flame imaging studies of combustion completion in a SI four-stroke engine

Direct flame imaging through a transparent cylinder liner has been used to study the final phase of lean combustion in a spark-ignition four-stroke optical engine. Two image-intensified video cameras record instantaneous lean flame radiation at two preset times to examine the structure and development of the flame as it approaches the wall to complete the flame travel. There is one peripheral spark plug on the side of the cylinder opposite the cameras, so that combustion completion images capture only one flame thickness and provide a unique map of flame intensity. These flames have highly structured intensity patterns on all of the scales associated with in-cylinder turbulence. Although the large-scale pattern of burning results from the flame shape early in the cycle, intensity variations of the flame images indicate that the burning rate varies significantly over the flame area on both large and small scales, indicating that the flame front is not a uniformly propagating sheet.