CONTROLLING ROLE OF COOL FLAMES IN 2-STAGE IGNITION

Experiments have been carried out in a rapid compression machine, with 2-methylpentane, n-hexane, 2,2-dimethylbutane, 2,3-dimethylbutane, iso-octane and primary reference fuels 25, 60 and 80 as fuels, to determine the chemical mechanism of the two-stage spontaneous ignition of alkane-air mixtures under engine conditions, and to elucidate the parameters of this process which have to be controlled if knock is to be avoided. It has been established for six of these fuels that the delay between the cool flame and hot ignition, τ2, is related to Δp, the pressure rise during the cool flame, by the equation τ2 = kΔp-1.5±-0.2. The cool flame thus controls the hot-ignition process; if Δp can be reduced sufficiently τ2 will be increased to the point where knock is avoided. The chemical mechanism of the cool-flame step involves the chain-propagation sequence, CnH2n+1· a ̊rrCnH2n+1OO· a ̊rr·CnH2nOOH a ̊rrP+·OH a ̊rr CnH2n+1·, where P represents a variety of stable or moderately stable products. Certain classes of these products are potential agents of degenerate branching, and at least one of them is necessary to the development of hot ignition. © 1969.