Available online at www.sciencedirect.com Proceedings of the Combustion Institute 37 (2019) 4681–4689 www.elsevier.com/locate/proci The effect of molecular structures of alkylbenzenes on ignition characteristics of binary n-heptane blends Dongil Kang a , Doohyun Kim b , Kwang Hee Yoo b , Angela Violi a,b,c , André Boehman b,∗ a Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, United States b Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, United States c Department of Macromolecular Science Engineering, Biophysics Program, Applied Physics, University of Michigan, Ann Arbor, MI 48109, United States Received 1 December 2017; accepted 16 June 2018 Available online 5 July 2018 Abstract Alkylbenzenes are major aromatic constituents of real transportation fuels and important surrogate com- ponents. In this study, the structural impact of nine alkylbenzenes on their ignition characteristics is experi- mentally and computationally investigated with particular emphasis on the blending effect with signifcantly more reactive normal alkanes. Experimental comparisons of mono-alkylbenzenes (toluene, ethylbenzene, n-propylbenzene, iso-propylbenzene) from a modifed CFR engine showed that the difference in pure alkyl- benzene reactivity signifcantly diminished when blended with n-heptane, as the strength of the radical scav- enging effect of all three alkylbenzenes is similar. Among C 8 H 10 isomers, the reactivity of pure ethylbenzene and o-xylene and their blends with n-heptane showed a complex competing effect between the difference in C–H bond energy and the existence of intermediate/low-temperature chemistry caused by adjacent methyl pairs. A similar structural impact was also observed for C 9 H 12 isomers and their blends with n-heptane, while the infuence of C–H bond energy was more noticeable than C 8 H 10 molecules. Kinetic simulations of the alkylbenzene/n-heptane blends highlighted the effect caused by adjacent methyl pairs that is referred to as the “ortho effect”. Analysis of ethylbenzene and o-xylene showed that o-xylene’s intermediate/low-temperature pathways initiated by benzylperoxy radical – benzylhydroperoxide isomerization (RO2 – QOOH) produce additional active radicals such as OH and CH 2 O, which accelerates the oxidation chemistry of more reactive ∗ Corresponding author. E-mail address: boehman@umich.edu (A. Boehman). https://doi.org/10.1016/j.proci.2018.06.128 1540-7489 © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.