A cool flame is a flame having a typical temperature of about 400 °C (752 °F).[1] In contrast to an ordinary hot flame, the reaction is not vigorous and releases little heat, light, or carbon dioxide. Cool flames are difficult to observe and are uncommon in everyday life, but they are responsible for engine knock – the undesirable, erratic, and noisy combustion of low-octane fuels in internal combustion engines.[2][3][4]