Cold flame

Cool flame is a flame having maximal temperature below about 400 °C (752 °F). 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.

Cool flames were accidentally discovered in the 1810s by Sir Humphry Davy, who noticed that certain types of flame did not burn his fingers or ignite a match. He also found that those unusual flames could change into conventional ones and that at certain compositions and temperatures, they did not require an external ignition source, such as a spark or hot material.Harry Julius Emeléus was the first to record their emission spectra, and in 1929 he coined the term "cold flame".

Cool flame can occur in hydrocarbons, alcohols, aldehydes, oils, acids, waxes and even methane. The lowest temperature of a cool flame is poorly defined and is conventionally set as temperature at which the flame can be detected by eye in a dark room (cool flames are hardly visible in daylight). This temperature slightly depends on the fuel to oxygen ratio and strongly depends on gas pressure – there is a threshold below which cool flame is not formed. A specific example is 50% n-butane–50% oxygen (in volume percent) which has a cool flame temperature (CFT) of about 300 °C at 165 mmHg (22.0 kPa). One of the lowest CFTs (156 °C) was reported for a C₂H₅OC₂H₅ + O₂ + N₂ mixture at 300 mmHg (40 kPa). The CFT is significantly lower than the auto-ignition temperature (AIT) of conventional flame (see table).

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