Autoignition delay

Previous work by Sahetchian et al. [177] on the oxidation of n-heptane and n-butane in a motored CFR engine, with particular attention to the isomerization reactions and the autoignition delay, showed that the concentration of the hydroperoxide formed in the isomerization reaction reaches a maximum concentration just preceding autoignition at a compression ratio of CR = 9.6 1 (T = 640 K). However, in similar experiments on n-butane oxidation no hydroperoxide was formed and autoignition was not observed for compression ratios up to 16 1. This may be regarded as a consequence of the different chain lengths of the two hydrocarbons, and probably relates to the different tendencies for isomerization reactions, and dihydroperoxide formation to occur. 

Griffiths [85] and Schrieber et al. [86] have shown how the Muller scheme can be modified to incorporate these features by slightly expanding the low temperature part of the mechanisms and adding a further intermediate. While mass and energy balances were intrinsic in the formulation, an empirical approach was adopted both for the form of the additional reaction and for all the rate constants. The latter, for instance, included pressure dependent terms. Good fits were obtained to rapid compression machine and shock-tube autoignition delay-times for heptane, iso-octane, and their mixtures. 

Fig. 7.27. Calculated autoignition delay-times (solid lines) and excitation times (dashed lines) for stoichiometric methane, acetylene, ethylene and ethane with air at (a) 1 atm, (b) 1500 K. Initial radius of the centre is 1 mm. Geometry is cylindrical. From [194].

R. Minetti, M. Ribaucour, M. Carlier and L.R. Sochet, Autoignition Delays of a Series of Linear and Branched Chain Alkanes in the Intermediate Range of Temperature, Comb. Sci. Tech. 113-114 (1996) 179. 

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When a reaction mixture is under the same conditions of temperature and pressure as in Figure 2, where an autoignition is likely to develop, there is a certain time lapse, called the autoignition delay, before this becomes effective. This delay is important in the Diesel engine but also in the safety of industrial combustion reactions. The measurement of these delays is carried out mainly using batch reactors, rapid compression machines and shock tubes. 

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