A Fundamentally Based Correlation Between Alkane Structure and Octane Number

Abstract

At temperatures below about 850 K, the autoignition of alkane/air mixtures takes place by a degenerate chain mechanism. We postulate here that the differences in ease of autoignition between alkanes are due predominantly to the different rates at which the kinetic branching agent is formed. This process involves two successive internal hydrogen abstractions (isomerisations) in peroxy radicals and is very structure dependent. Its rate has been calculated from literature generic isomerisation rate constants and we find that it correlates with Research Octane Number for most alkanes. When allowance is made for decomposition of certain highly substituted alkyl radicals. a good linear relationship is obtained (correlation coefficient = 0.94). This suggests that the “low temperature” mechanism of hydrocarbon oxidation involving peroxy radical isomerisations provides a possible explanation for the autoignition behavior under the conditions occurring in the engine during the octane rating test.