Modelling the chemical kinetics of combustion of higher hydrocarbon fuels in air

The chemical aspects of the oxidation of the typical higher normal alkane hydrocarbon, n-heptane, in air was investigated analytically using computer simulation with a detailed chemical kinetic mechanism made up of 1950 elementary reactions and 380 species. The simulation covered lean to stoichiometric mixtures under isothermal, adiabatic constant volume and constant pressure processes, motored engines and an externally heated steady flow reactor.;Comparison of the results of the simulation, which displayed the major characteristics of the combustion of higher normal alkanes such as single and multi-stage ignition processes, with the available experimental data showed good qualitative agreement; however some quantitative differences were found. The main sources for such differences in both the modelling as well as the experimental results are discussed.;Better estimation of the minimum autoignition compression ratio in motored engine simulation was obtained when the calculated adiabatic core temperature of the charge was used. The presence of some groups of species in the residual gases at the commencement of a cycle was shown to increase the reactivity of the charge (e.g. heptyl hydroperoxides), while other species (e.g. formaldehyde) have inhibiting effects.;Reduced versions of the kinetic scheme were formulated for the reactions of n-heptane and its mixtures with alkanes of up to n-butane. The use of such reduced schemes produced excellent agreement in the key operational parameters and the major species concentrations with the corresponding values obtained using the full scheme while effecting significant reductions in computing times (e.g. up to 80%).;The nature and extent of any chemical interaction that may take place between binary mixtures of n-heptane and methane were investigated under engine-like conditions. This was extended to consider the influence of typical variations in the composition of natural gas on the ignition process under such conditions, and especially in relation to changes in the relatively small concentrations of high molar mass alkanes that are usually present in the processed pipeline natural gas. It was shown that such variations can have significant effect on the reactivity of natural gas and its suitability for engine applications. Since reduced schemes are still too lengthy to be used in engine simulation, a "propane equivalent" concept is suggested to serve as a tool for the simulation of combustion of complex fuel mixtures such as natural gases in engine simulations.