A Comprehensive Experimental and Kinetic Study of Laminar Flame Characteristics of H2 and CO Addition to Oxygenated Gasoline

Abstract

Following stringent regulations enforced by environmental regulatory authorities, various steps have been recently implemented to ensure the clean combustion of gasoline with minimum emissions by including additives to gasoline. This kinetic and experimental study has been conducted to explore the effect of H2 and CO addition to Halterman gasoline at stoichiometric conditions of 358 K and 1 bar. Two different mechanisms, a gasoline surrogate (iso-octane, n-heptane, toluene, and ethanol), LLNL, and a KAUST TPRFE (primary reference fuel, toluene, and ethanol), were used to provide a detailed comparative kinetic understanding of gasoline. Via a spherical flame propagating in a constant-volume combustion chamber, the unstretched, adiabatic laminar burning velocity, SLo, was measured. H2 and CO were added (as unitary and binary additives) to the Haltermann gasoline, in proportions of 1, 2.5, 5, and 10% by mass. Adding hydrogen enhanced the SLo significantly, while CO addition had only a slight effect on SLo. The maximal mole fractions of OH and H were increased with the H2 addition, while adding CO raised the O radical peak mole fraction. A strong correlation between SLo and the sum of the O, H, and OH peak mole fractions was evident. The OH radical was identified to be a kinetics indicator for SLo of gasoline/air mixtures at these conditions; a higher fraction of ethanol in Haltermann gasoline was the precursor for high OH concentration. The addition of a binary additive (10% H2–10% CO) significantly enhanced the consumption of iso-octane compared to other fuel species, strengthening the H-abstraction of iso-octane, 99% compared to 71% with neat Haltermann gasoline. The simulated flame speed showed that the primary chain branching reaction (H + O2 = O + OH) rate was much higher for the LLNL mechanism than for the KAUST-TPRFE mechanism, and thus, the LLNL overpredicted SLo for the Haltermann gasoline surrogate.