n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine

2018 
Abstract This work identifies classes of cool flame intermediates from n -heptane low-temperature oxidation in a jet-stirred reactor (JSR) and a motored cooperative fuel research (CFR) engine. The sampled species from the JSR oxidation of a mixture of n -heptane/O 2 /Ar (0.01/0.11/0.88) were analyzed using a synchrotron vacuum ultraviolet radiation photoionization (SVUV-PI) time-of-flight molecular-beam mass spectrometer (MBMS) and an atmospheric pressure chemical ionization (APCI) Orbitrap mass spectrometer (OTMS). The OTMS was also used to analyze the sampled species from a CFR engine exhaust. Approximately 70 intermediates were detected by the SVUV-PI-MBMS, and their assigned molecular formulae are in good agreement with those detected by the APCI-OTMS, which has ultra-high mass resolving power and provides an accurate elemental C/H/O composition of the intermediate species. Furthermore, the results show that the species formed during the partial oxidation of n -heptane in the CFR engine are very similar to those produced in an ideal reactor, i.e. , a JSR. The products can be classified by species with molecular formulae of C 7 H 14 O x ( x  = 0–5), C 7 H 12 O x ( x  = 0–4), C 7 H 10 O x ( x  = 0–4), C n H 2 n ( n  = 2–6), C n H 2 n −2 ( n  = 4–6), C n H 2 n +2 O ( n  = 1–4), C n H 2 n O ( n  = 1–6), C n H 2 n −2 O ( n  = 2–6), C n H 2 n −4 O ( n  = 4–6), C n H 2 n +2 O 2 ( n  = 0–4, 7), C n H 2 n O 2 ( n  = 1–6), C n H 2 n −2 O 2 ( n  = 2–6), C n H 2 n −4 O 2 ( n  = 4–6), and C n H 2 n O 3 ( n  = 3–6). The identified intermediate species include alkenes, dienes, aldehyde/keto compounds, olefinic aldehyde/keto compounds, diones, cyclic ethers, peroxides, acids, and alcohols/ethers. Reaction pathways forming these intermediates are proposed and discussed herein. These experimental results are important in the development of more accurate kinetic models for n -heptane and longer-chain alkanes.
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