Measurements of the auto-ignition of n-heptane/toluene mixtures using a rapid compression machine

Ignition delay times of undiluted stoichiometric n-heptane/toluene mixtures have been measured in a rapid compression machine (RCM) for pure n-heptane and for an increasing concentration of toluene by liquid volume (25–100%) in the mixture, mainly at two different molar densities (150 and 180 mol/m3). Initial pressures and temperatures were varied respectively in the 0226–0298 bar range and in the 299–352 K range, in order to allow compressed pressures and temperatures respectively in the 7.95–10.38 bar range and in the 710–814 K range. No ignition was observed for pure toluene due most probably to the variation of the final temperature and pressure in the combustion chamber during the very long ignition times of pure toluene at the experimental conditions considered in this work. The ignition delay variation at fixed temperature and the relative variation of the two-stage ignition times are reported. It is shown how the mixture behavior seems to be governed by the n-heptane chemistry for low toluene amount, while an increase of toluene concentration causes longer ignition delay times. Numerical modeling of the experiments with a recent Lawrence Livermore chemical mechanism for surrogate fuels shows that the results agree fairly well in the case of low and mid proportion of toluene by liquid volume. The single-zone model has allowed to extend the temperature range of investigation, showing that the low temperature range is the most influenced by the presence of toluene.