A shock tube study of iso-octane ignition at elevated pressures: The influence of diluent gases

The ignition of iso-octane/air and iso-octane/O2/Ar (~20% O2) mixtures was studied in a shock tube at temperatures of 868–1300 K, pressures of 7–58 atm, and equivalence ratios Φ=1.0, 0.5, and 0.25. Ignition times were determined using endwall OH* emission and sidewall piezoelectric pressure measurements. Measured iso-octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the influence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.