Synchrotron vacuum ultraviolet photoionization mass spectrometry and modeling study of iso-octane low temperature oxidation at varied pressures

Abstract

Because of its excellent anti-knock properties, iso-octane has been selected as the main component of gasoline surrogates, and a thorough understanding of its low temperature oxidation process is essential for the development of kinetic models. Although iso-octane has been studied for decades, a comprehensive understanding of its low temperature oxidation chemistry has not been realized, and kinetic models were not well developed. In this work, the previously developed, variable pressure jet-stirred reactor-synchrotron vacuum ultraviolet photoionization mass spectrometry system was used to study the low temperature oxidation of iso-octane at five bar, with an initial fuel mole fraction of 0.01, residence time of 2 s, and equivalence ratio of 0.25. Dozens of oxidation intermediates were qualitatively and quantitatively analyzed, helping to reveal the low temperature oxidation reaction processes of iso-octane and examine the kinetic models. Comparing iso-octane studies at high pressure in the literature, a highly detailed species pool was probed, which included H2O2, carbonyl acids, alkyl hydroperoxides, and keto-hydroperoxides (KHP). By combining the atmospheric pressure data in the literature, the recently developed model of iso-octane low temperature oxidation was examined. Further improvement of the kinetic model was achieved by carefully tuning the reaction rate constant of the KHP decomposition. Finally, according to the variation of mole fraction of products at different pressures, the pressure effects on key reaction pathways were discussed, and the selectivity of products at varied pressure conditions clarified.