Autoignition Characteristics of Primary Reference Fuels and their Mixtures

This study investigates the autoignition of Primary Reference Fuels (PRFs) using a detailed kinetic model. The chemical kinetics software CHEMKIN is used to facilitate solutions in a constant volume reactor and a variable volume reactor, with the latter representing an IC engine. Experimental shock tube and HCCI engine data from literature is compared with the present predictions in these two reactors. The model is then used to conduct a parametric study in the constant volume reactor of the effect of inlet pressure, inlet temperature, octane number, fuel/air equivalence ratio, and exhaust gas recirculation (EGR) on the autoignition of PRF/air mixtures. A number of interesting characteristics are demonstrated in the parametric study. In particular, it is observed that PRFs can exhibit single or two stage ignition depending on the inlet temperature. The total ignition delay, whether single or two stage, is correlated withn-C₇H₁₆/O₂ ratio. Thermodynamic and chemical effects are identified for the EGR constituents CO₂, H₂O, and N₂. The former effect is due to the difference in specific heats and is well known. Introduction of each diluent reduces the mole fraction of O₂ which has the chemical effect of delaying ignition. In addition, H₂O as a diluent is shown to directly affect key species and reactions involved in high temperature kinetics, resulting in the enhancement of the ignition process relative to N_2.