Study of ignition and combustion characteristics of consecutive injections with iso-octane and n-heptane as fuels

Abstract

Gasoline compression ignition (GCI) engines have potential to improve fuel economy and reduce emissions harmful to health and the environment compared with conventional diesel combustion engines. The underpinning knowledge of key phenomena of fuel–air mixture formation, ignition, combustion, and pollutant formation, however, is lacking at present. This work investigated the ignition and combustion interaction processes between two consecutive jets of iso-octane (gasoline surrogate) and n-heptane (diesel surrogate) inside a quiescent steady environment with an ambient density of 22.8 kg/m3, an O2 concentration of 21 vol % but with ambient temperatures of 950 and 780 K, respectively. Three injection schedules were tested, consisting of a double injection that follows a pilot–main injection duration (2.6 ms/6.5 ms) with dwell times of 0.2, 0.7, and 1.2 ms, in addition to single injections with long (9.1 ms) and short (2.6 ms) injection durations as reference cases. Combined high-speed schlieren imaging, pressure trace measurements, combustion luminosity detection using photodiode and closed-homogeneous reactor simulations revealed that under the 950 K condition for iso-octane and 780 K condition for n-heptane, pilot-injection-induced local temperature and flame intermediate changes can considerably affect the preignition reactions of the main injections that followed. It is also found that the interaction effects are dependent on the ignition and combustion characteristics of the fuels involved as well as the temporal separation between the injections.