Laminar Flame Speeds and Ignition Delay Times of Gasoline/Air and Gasoline/Alcohol/Air Mixtures: The Effects of Heavy Alcohol Compared to Light Alcohol

The oxidation of gasoline and gasoline/alcohol blends is studied in a shock tube and a spherical reactor. A commercial oxy-free gasoline and two alcohols (ethanol and iso-pentanol) were used in this study. The spherical reactor experiments were conducted at an initial temperature of 483 K, initial pressure of 0.1 MPa, and equivalence ratios from 0.65 to 1.36. Ignition delay times were measured behind reflected shock waves. The shock tube experiments were conducted at 2 MPa over a temperature range from 955 to 1284 K and for two equivalence ratios (0.5 and 1). The experimental measurements indicate that replacing ethanol by iso-pentanol on a gasoline/alcohol blend results in flame speeds which are closer to the ones of a commercial gasoline at nearly stoichiometric conditions (an in-engine applications). On the other hand, the ignition delay times are more affected by the presence of iso-pentanol than the ethanol case. Two different surrogate fuels composed of n-heptane, iso-octane, and toluene were also tested against the newly obtained experimental results (oxy-free gasoline and mixtures with ethanol) using kinetic modeling with a reduced model (Cai, L.; Pitsch, H. Combust. Flame 2015 162, 1623-1637). Although a good agreement between real fuel and surrogate properties was observed for the laminar flame speeds, discrepancies were obtained between the measured and calculated ignition delay times especially in the lower temperature range of our study. Additional simulation analyses on the ignition delay times were performed with a different chemical kinetic model (detailed LLNL model for gasoline surrogates) and a more complex four-component surrogate. The results show a considerable improvement in the prediction capabilities of the ignition properties of the oxy-free gasoline and the oxy-free gasoline + ethanol mixtures. The ignition delay time data are also in agreement with the correlations provided in the literature for gasoline fuels and their surrogates.