The autoignition of practical fuels at HCCI conditions: High-pressure shock tube experiments and phenomenological modeling

Delay times for first- and second-stage ignition of n-heptane and two practical kerosene-like fuels have been measured in a heated high-pressure shock tube at conditions similar to those found in homogeneous charge compression ignition (HCCI) engines. Initial reflected shock conditions covered temperatures from 700 to 1100 K, pressures from 20 to 65 bar, equivalence ratios of 0.5, 0.67, and 1, and exhaust gas recirculation rates (EGR) of 0%, 30%, and 50%. EGR is simulated by introducing additional N2 in the test gas mixture. Because detailed chemical kinetics models are not available for practical fuels so far, we propose a global ignition time correlation that is fitted to the measured data. The empirical model describes both first- and second-stage ignition delay as a function of temperature, pressure, equivalence ratio and EGR. It is based on a three-stage Arrhenius approach, which has been extended to capture the first-stage ignition times and the influence of EGR. For each of the fuels, even for the fuels that contain cyclo-alkanes and aromatics, good agreement between model and experiment is observed, especially at lean and high-EGR conditions relevant for HCCI. Based on this extensive set of ignition data, interesting correlations between fuel composition and ignition behavior could be identified, which may prove useful in matching the fuel to the engine application.