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, (tau)ign, were obtained from pressure time-histories. The experiments were conducted over a range of equivalence ratios (phi=0.25-1.0), pressures (PH=5.12–23 atm), temperatures (T=943-1027 K), and oxygen mole fractions (XO2=921%), 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(tau)ign=1.3*10^-4p^-1.05(theta)^0.77S^-1.41 O2 exp(33,700/R(cal/mol/K)T)where P is pressure (atm), T is temperature (kappa), phi is the equivalence ratio (based on isooctane to O2 molar ratios), XO2 is the oxygen mole percent (%), and (tau)ign is the ignition delay time (ms). Carbon dioxide was found to have no chemical effect on (tau)ign. Water was found to systematically decrease (tau)ign by a small amount (less than 14% for the range of conditions studied). The maximum uncertainty in the measured (tau)ign is (plus-minus)12% with an average uncertainty of (plus-minus)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.