The effect of molecular structures of alkylbenzenes on ignition characteristics of binary n-heptane blends

Abstract

Alkylbenzenes are major aromatic constituents of real transportation fuels and important surrogate components. In this study, the structural impact of nine alkylbenzenes on their ignition characteristics is experimentally and computationally investigated with particular emphasis on the blending effect with significantly more reactive normal alkanes. Experimental comparisons of mono-alkylbenzenes (toluene, ethylbenzene, n-propylbenzene, iso-propylbenzene) from a modified CFR engine showed that the difference in pure alkylbenzene reactivity significantly diminished when blended with n-heptane, as the strength of the radical scavenging effect of all three alkylbenzenes is similar. Among C8H10 isomers, the reactivity of pure ethylbenzene and o-xylene and their blends with n-heptane showed a complex competing effect between the difference in CH bond energy and the existence of intermediate/low-temperature chemistry caused by adjacent methyl pairs. A similar structural impact was also observed for C9H12 isomers and their blends with n-heptane, while the influence of CH bond energy was more noticeable than C8H10 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 CH2O, which accelerates the oxidation chemistry of more reactive n-heptane. This study provides knowledge on the blending effect of alkylbenzene compounds with n-heptane on their ignition characteristics that is useful to develop surrogates that can better mimic the reactivity of real fuels.