An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions

Autoignition of iso-octane was examined using a rapid compression facility (RCF) with iso-octane, oxygen, nitrogen, and argon mixtures. The effects of typical homogeneous charge compression ignition (HCCI) conditions on the iso-octane ignition characteristics were studied. Experimental results for ignition delay times, τign, were obtained from pressure time-histories. The experiments were conducted over a range of equivalence ratios (ϕ=0.25-1.0), pressures (P=5.12-23atm), temperatures (T=943-1027K), and oxygen mole fractions (χO2=9-21%), and with the addition of trace amounts of combustion product gases (CO2 and H2O). It was found that the ignition delay times were well represented by the expression τign=1.3×10−4P−1.05ϕ−0.77χO2−1.41exp(33,700/R(cal/mol/K)T), where P is pressure (atm), T is temperature (K), ϕ is the equivalence ratio (based on iso-octane to O2 molar ratios), χO2 is the oxygen mole percent (%), and τign is the ignition delay time (ms). Carbon dioxide was found to have no chemical effect on τign. Water was found to systematically decrease τign by a small amount (less than 14% for the range of conditions studied). The maximum uncertainty in the measured τign is ±12% with an average uncertainty of ±6%. The performance of several proposed chemical reaction mechanisms (including detailed, reduced, and skeletal mechanisms) was evaluated in the context of the current experimental results.