Effects of branch structure of alkylbenzenes on spray auto-ignition of n-decane and alkylbenzenes blends

Abstract

Spray auto-ignition characteristics of the blends of n-decane and several alkylbenzenes were carried out on a heated constant-volume spray combustion chamber. The derived cetane numbers of the fuel blends were determined, and the temperature-dependent ignition delay times and combustion durations were measured across a range of temperatures from 808 to 911 K. The results reveal that blending alkylbenzene to n-decane inhibits fuel spray auto-ignition propensity. For mono-alkylbenzenes, the fuel blend containing toluene has a higher derived cetane number than those with ethylbenzene and n-propylbenzene, but has a lower derived cetane number than the fuel blend containing n-butylbenzene. For those binary fuels containing ethylbenzene, n-propylbenzene and n-butylbenzene, their derived cetane numbers increase with the side alkyl chain length. The derived cetane numbers of the fuel blends with C8H10 isomers follow the trend of n-decane/ o-xylene > n-decane/ethylbenzene > n-decane/ m-xylene ∼ n-decane/ p-xylene, given the alkylbenzene blending fraction. For the blends with C9H12 isomers, those containing 1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene have the highest and lowest derived cetane numbers, respectively, while the fuel blends containing 1,2,4-trimethylbenzene, n-propylbenzene and i-propylbenzene have comparatively intermediate derived cetane numbers. The blending effects of alkylbenzenes on ignition delay time are consistent with the observation on fuel derived cetane numbers. Both the number and proximity of substituted methyl groups significantly affect fuel auto-ignition propensity, and the adjacent methyl groups could increase the auto-ignition propensity. The combustion duration for the test fuels, except for n-decane and the n-decane/ n-butylbenzene blend, monotonically decreases with increased temperature. The non-monotonic dependence of combustion duration on temperature, for neat n-decane and the n-decane/ n-butylbenzene blend, may result from the increased diffusive burnt fraction. Finally, the comparison between gas-phase and spray auto-ignition reactivity of the test fuels highlights the contribution of both fuel physics and chemistry in spray auto-ignition.