A detailed chemical kinetic reaction mechanism for the oxidation of iso-octane and n-heptane over an extended temperature range and its application to analysis of engine knock

Abstract

A detailed chemical kinetic reaction mechanism is developed to describe the oxidation of n -heptane, iso-octane, and their mixtures over a wide range of operating conditions. In addition to a high temperature submechanism, reaction paths are included to describe the lower temperature regimes in which the rate and intermediate products of oxidation are controlled by addition of molecular oxygen to alkyl and isomerized alkylperoxy radicals, internal H atom abstractions, and reactions involving O-heterocyclic species. This overall reaction mechanism is validated through comparisons between computed results and experimental data from shock tubes, turbulent flow reactor, and low temperature static and stirred reactors. The mechanism is then used to study the influence of fuel composition on knocking in internal combustion engines. Autoignition of mixtures of iso-octane and n -heptane is examined, in which experimentally measured variations of engine pressure with time were used to simulate the conditions encountered by the end-gases responsible for knocking operation. The computations reproduce the variations of autoignition delay time with octane number and these variations are interpreted in terms of detailed differences in the structure of the two primary reference fuels. Sensitivity analyses of the computations are presented, indicating those portions of the reaction mechanisms which have the greatest influence on the model results.