Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures

Abstract

Autoignition characteristics of n-heptane/air, gasoline/air, and ternary surrogate/air mixtures were studied behind reflected shock waves in a high-pressure, low-temperature regime similar to that found in homogeneous charge compression ignition (HCCI) engine cycles. The range of experiments covered combustion of fuel in air for lean, stoichiometric, and rich mixtures ( Φ = 0.5 , 1.0, 2.0), two pressure ranges (15–25 and 45–60 atm), temperatures from 850 to 1280 K, and exhaust gas recirculation (EGR) loadings of (0, 20, and 30%). The ignition delay time measurements in n-heptane are in good agreement with the shock tube study of Fieweger et al. (Proc. Combust. Inst. 25 (1994) 1579–1585) and support the observation of a pronounced, low-temperature, NTC region. Strong agreement was seen between ignition delay time measurements for RD387 gasoline and surrogate (63% iso-octane/20% toluene/17% n-heptane by liquid volume) over the full range of experimental conditions studied. Ignition delay time measurements under fuel-lean ( Φ = 0.5 ) mixture conditions were longer than with Φ = 1.0 mixtures at both the low- (15–25 atm) and high- (45–60 atm) pressure conditions. Ignition delay times in fuel-rich ( Φ = 2.0 ) mixtures for both gasoline and surrogate were indistinguishable in the low-pressure (15–25 atm) range, but were clearly shorter at high-pressures (45–60 atm). EGR loading affected the ignition delay times similarly for both gasoline and surrogate, with clear trends indicating an increase in ignition delay time with increased EGR loading. This data set should provide benchmark targets for detailed mechanism validation and refinement under HCCI conditions.