Chemical and spectroscopic studies of blue flames in the auto-ignition of methane

A blue pre-ignition glow has been found with weak methane + air mixtures, using a motored compression-ignition engine with fixed compression ratio and speed; the temperature was controlled by preheating the inlet air. The limits of the glow and its transition to normal flame have been studied. Analyses of exhaust products and records of pressure and luminosity have been made. The spectrum of the glow shows formaldehyde bands, normally associated with cool flames. As the normal ignition limit is approached this spectrum changes smoothly, with increasing temperature or mixture strength, to a normal flame spectrum showing OH, CH and HCO bands. Using a stroboscope we have found slight indication that the CH2O bands precede the CH and OH. Less lean mixtures show yellow luminosity, associated with carbon particles. The possibilities that the CH2O emission results from polymerization of the methane, from partial oxidation to methyl alcohol, or directly from a cool-flame phenomenon in methane, are discussed in relation to conditions in the combustion chamber. The new observation of CH2O bands from methane may be interpreted in favour of its formation by reactions of methoxy radicals. Methane is known to knock in an engine at high compression ratios. Present results suggest that low-temperature oxidation processes may, as with other hydrocarbons, contribute to this knock.