The autoignition of hydrocarbon fuels at high temperatures and pressures—Fitting of a mathematical model

Abstract

A systematic study has been carried out of the autoignition of a number of hydrocarbon fuels under engine-like conditions in a rapid compression machine. The fuels were selected on the basis of their octane quality and sensitivity as measured in standard octane tests, and included a number of primary and toluene reference fuels and the olefin 2-methylhex-2-ene. The results confirm that there is a strong correlation between the performance of these fuels in engines and their autoignition properties measured in a rapid compression machine. The importance of charge temperature and density in “de-rating” sensitive fuels under “severe” engine conditions is demonstrated. The mathematical model of hydrocarbon auto-ignition described previously has been fitted to the experimental results for each of the fuels studied. This model is based on a relatively simple chemical mechanism, but is capable of giving a good simulation of all the experimentally observed trends. It is not possible to obtain a unique fit and thereby to determine the Arrhenius parameters for individual rate coefficients or groups of coefficients within the chemical model, but some qualitative pointers towards the nature of the chemical mechanism are possible using this approach. In particular the results suggest that degenerate-branching agent is formed by a radical reaction whose kinetic competition with the alternative main chain propagation reaction exhibits first-order dependence on oxygen concentration rather than on fuel concentration.