Cool flame is a flame having maximal temperature below about 400 °C (752 °F).[1] It is usually produced in a chemical reaction of a certain fuel-air mixture. Contrary to conventional flame, the reaction is not vigorous and releases very little heat, light and 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]