Ignition two-stage

They exhibit two-stage ignition, which may be related to the cool flame phenomenon. 

Unified theory of explns, cool flames, and two-stage ignitions)... 

For aliphatic hydrocarbons a close relationship exists between knock and low-temperature, two-stage ignition (110). In both cases two induction periods are observed. One, Ti, extends up to cool flame formation. The other, r2, follows ti and lasts up to autoignition. 

Certain compounds do not show two-stage ignition. These include methane, acetylene, benzene, hydrogen, methanol, formaldehyde, and glyoxal (106, 108). Higher aromatics show the effects of their aliphatic side chains and act like aliphatics. 

Cool flames are observed at pressures lower than those necessary for two-stage ignition. These non-isothermal events occur at intervals of time during an otherwise almost isothermal reaction. The majority of workers consider cool-flame propagation to be the central part of reaction during which the bulk of the fuel is incompletely oxidized. Shtern, however, considers the cool flame to be a minor process which plays little part in the overall reaction, since the cool-flame oxidation and the slow oxidation are very similar in their chemical nature. Indeed, the pressure--time curves Shtern obtained for the cool-flame oxidation of propane have the same S-shape as those for the slow oxidation if the non-isothermal events are ignored, as can be seen in Fig. 3. 

The phenomenon of two-stage ignition had not been extensively studied, but it had been suggested [14] that the temperature rise accompanying the passage of the cool flame is sufficient to cause rapid further oxidation in the high temperature region which leads to a thermal... 

Vovelle et al. [83] showed that the cool-flame region for diethyl ether was considerably extended by the addition of di-tert-butyl peroxide. The products of the normal combustion, cool flames and two-stage ignition were all examined [84] and found to include carbon monoxide and dioxide, water, acetaldehyde, methanol and methane, ethylene and acetylene. 

Fig. 5.42. Typical experimental records in two-stage ignition region III. (Reprinted with permission from reference [75], Royal Society of London.)...

Fig. 6,7. Pressure and light output records typical of those observed from the two-stage ignition of hydrocarbons. These results were obtained by the authors during the combustion of n-pentane in a rapid compression machine. The duration of the compression stroke was 22 ms, as shown in the lower pressure record. The first- and second-stage time intervals, measured from the end of compression, are marked as ti and T2 respectively. Compressed gas temperature derived from (6.16) was 740 K [49].

Vertical, upward flow, laminar flow reactors which have been used to study stabilized cool-flames and two-stage ignitions are a development of the atmospheric pressure, vertical, laminar flow reactors, pioneered at the Naval Research Laboratories in Washington [71]. These types of systems have been used by other research groups [72-74]. The flow tube that was... 

Heated flat-flame burners also have been used for the stabilization of cool-flame and two-stage ignitions. These systems were pioneered by Agnew et al. [76], and a definitive study of the spatially resolved chemistry of diethyl ether combustion has been reported [77]. Ballinger and Ryason [78] have described this type of burner in detail and have reported the relative behaviour of some n-alkanes, PRF fuel mixtures, and the effect of anti-knock additives. 

The light output which accompanies the final stage of two-stage ignition is considerably more intense. It is characteristic of the emission from premixed flames of hydrocarbons and arises predominantly from chemiluminescent reactions, mainly giving electronically excited CH, C, col and OH [123]. 

Major molecular products identified during the ignition delay of two-stage ignition of n-butane (T = 730 K p,>0.8MPa) [21]... 

Fig. 6.26. Concentration - time profiles for intermediate products measured during the two-stage ignition of n-butane in a rapid compression machine. The concentrations are based on the carbon balance. For the purposes of numerical modelling the abscissa is normalized to the duration of the ignition delay. Experimental data are marked as points, numerical results are shown as solid lines. (After Minetti et al. [22]).

Much of the outstanding chemical investigation of the low-temperature combustion of alkanes was performed on the C5 and alkanes, in the late 1960s and early 1970s, mainly by Cullis and co-workers [137, 189-193] and Fish and co-workers [99, 194-199]. The quality and extent of the chemical analyses in these studies has rarely been equalled in subsequent work, and the data obtained provide very strong evidence, not only for the importance of alkylperoxy radical formation and isomerization in the low-temperature chemistry, but also the role of these reactions in the development of cool-flames and two-stage ignitions. However, one constraint on quantitative application is that much of the information was obtained under markedly non-isothermal conditions in the absence of any record of the reactant temperature. Moreover, the effects of convection on the movement of cool-flame combustion waves within an unstirred reaction vessel were not appreciated at the time [52], which casts doubt on some of the mechanistic interpretations of the evolution of multiple cool-flames that were made [195]. 

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