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

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 C7H14Ox (x = 05), C7H12Ox (x = 04), C7H10Ox (x = 04), CnH(2)n (n = 26), CnH(2n-2) (n = 46), CnH(2n+2)O (n = 14), CnH(2)nO (n = 16), CnH(2n-2)O (n = 26), CnH(2n-4)O (n = 46), CnH(2n+2)O(2) (n = 04, 7), CnH(2n)O(2) (n = 16), CnH(2n-2)O(2) (n = 26), CnH(2n-4)O(2) (n = 46), and CnH(2n)O(3) (n = 36). 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. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.