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Direct measurements of intermediates of ignition are challenging experimental objectives, yet such measurements are critical for understanding fuel decomposition and oxidation pathways. In the current work, a new gas-sampling system is used to provide quantitative discrete measurements of 30 hydrocarbon and oxygenate species during rapid compression facility studies of isooctane ignition. Two target conditions and equivalence ratios (based on molar fuel to oxygen ratio) were studied (P = 5.2 atm, T = 1000 K, φ = 0.4 and P = 4.8 atm, T = 975 K , φ = 1.2). The results are compared with model predictions that use the detailed reaction mechanism developed by Curran et al. (Combust Flame 2002, 129, 253-280). In general, the model predictions are in excellent agreement with the experimental data, including several trace species. Isobutene (i-C 4 H 8 ) and propene (C 3 H 6 ) were the major olefin species identified in the experiments. The results are consistent with an intermediate temperature reaction path sequence, where isooctane is consumed by H-atom abstraction to yield isooctyl radicals that undergo β-scission to form olefin and alkyl radical species.

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High-speed imaging combined with the optical access provided by a research engine offer the ability to directly image and compare ignition and combustion phenomena of various fuels. Such data provide valuable insight into the physical and chemical mechanisms important in each system. In this study, crank-angle resolved imaging data were used to investigate homogeneous charge compression ignition (HCCI) operation of a single-cylinder four-valve optical engine fueled using gasoline, indolene, and iso-octane. Lean operating limits were the focus of the study with the primary objective of identifying different modes of reaction front initiation and propagation for each fuel. HCCI combustion was initiated and maintained over a range of lean conditions for various fuels, from ϕ=0.69 to 0.27. The time-resolved imaging and pressure data show that high rates of heat release in HCCI combustion correlate temporally to simultaneous, intense volumetric blue emission. Lower rates of heat release are characteristic of spatially resolved blue emission. Gasoline supported leaner HCCI operation than indolene. Iso-octane showed a dramatic transition into misfire. Similar regions of preferential ignition were identified for each of the fuels considered using the imaging data.