Impact of branched structures on cycloalkane ignition in a motored engine: Detailed product and conformational analyses
Lilik, Gregory K.
Dillstrom, Vernon Tyler
Agudelo, John Ramiro
Boehman, André Louis
KAUST DepartmentClean Combustion Research Center
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AbstractThe ignition process of ethylcyclohexane (ECH) and its two isomers, 1,3-dimethylcyclohexane (13DMCH) and 1,2-dimethylcyclohexane (12DMCH) was investigated in a modified CFR engine. The experiment was conducted with intake air temperature of 155. °C, equivalence ratio of 0.5 and engine speed of 600. rpm. The engine compression ratio (CR) was gradually increased in a stepwise manner until autoignition occurred. It was found that ECH exhibited a significantly higher oxidation reactivity compared to its two isomers. The autoignition criterion was based on CO emissions and the apparent heat release rates. Ethylcyclohexane (ECH) indicated noticeable two stage ignition behavior, while less significant heat release occurred for the two isomers at comparable conditions. The mole fractions of unreacted fuel and stable intermediate species over a wide range of compression ratios were analyzed by GC-MS and GC-FID. Most of the species indicated constant rates of formation and the trends of relative yield to unreacted fuel are well in agreement with the oxidation reactivity in the low temperature regime. The major intermediate species are revealed as a group of conjugate olefins, which possess the same molecular structure as the original fuel compound except for the presence of a double carbon bond. Conjugate olefins were mostly formed through (1,4) H-shift isomerization during the low temperature oxidation of alkylcyclohexanes. Conformation analysis explains the reactivity differences in the three isomers as well as the fractions of intermediate species. The hydrogen availability located in alkyl substituents plays an important role in determining oxidation reactivity, requiring less activation energy for abstraction through the (1,5) H-shift isomerization. This reactivity difference contributes to building up the major intermediate species observed during oxidation of each test fuel. 12DMCH, whose ignition reactivity is the lowest, less favors β-scission of C-C backbone of cyclic ring, thereby resulting in lower concentrations of small olefins and higher concentrations of conjugate olefins and large oxygenated species in the low temperature regime, prior to autoignition.
SponsorsThe authors gratefully acknowledge financial support from the KAUST Global Collaborative Research (GCR) program, the Spanish Ministry of Education, for the financial support for Prof. Lapuerta's for stay at the EMS Energy Institute at Penn State University (PR2010-0419), and the University of Antioquia for the financial support for Prof. Agudelo's for stay at the EMS Energy Institute at Penn State University. The authors also gratefully acknowledge the assistance in the quantum mechanical calculations provided by Professor Angela Violi's group at the University of Michigan.
JournalCombustion and Flame