An experimental and kinetic modeling study of the autoignition of 3-methylheptane, a compound representative of the high molecular weight lightly branched alkanes found in large quantities in conventional and synthetic aviation kerosene and diesel fuels, is reported. Shock tube and rapid compression machine ignition delay time measurements are reported over a wide range of conditions of relevance to combustion engine applications: temperatures from 678 to 1356 K; pressures of 6.5, 10, 20, and 50 atm; and equivalence ratios of 0.5, 1.0, and 2.0. The wide range of temperatures examined provides observation of autoignition in three reactivity regimes, including the negative temperature coefficient (NTC) regime characteristic of paraffinic fuels. Comparisons made between the current ignition delay measurements for 3-methylheptane and previous results for n-octane and 2-methylheptane quantifies the influence of a single methyl substitution and its location on the reactivity of alkanes. It is found that the three C8 alkane isomers have indistinguishable high-temperature ignition delay but their ignition delay times deviate in the NTC and low-temperature regimes in correlation with their research octane numbers. The experimental results are compared with the predictions of a proposed kinetic model that includes both high- and low-temperature oxidation chemistry. The model mechanistically explains the differences in reactivity for n-octane, 2-methylheptane, and 3-methylheptane in the NTC through the influence of the methyl substitution on the rates of isomerization reactions in the low-temperature chain branching pathway, that ultimately leads to ketohydroperoxide species, and the competition between low-temperature chain branching and the formation of cyclic ethers, in a chain propagating pathway.
Bibliographical noteFunding Information:
The Rensselaer group was supported by the U.S. Air Force Office of Scientific Research (Grant No. FA9550-11-1-0261 ) with Dr. Chiping Li as technical monitor. The work at LLNL was performed under the auspices of the US Department of Energy under Contract DE-AC52-07NA27344. The LLNL group was supported by the Office of Naval Research (program manager Sharon Beermann-Curtin) and the US Department of Energy, Office of Vehicle Technologies (program manager Gurpreet Singh). Co-author SMS acknowledges fellowship support from NSERC of Canada.
- Chemical kinetic modeling
- Rapid compression machine
- Shock tube
ASJC Scopus subject areas
- Chemical Engineering(all)
- Mechanical Engineering
- Physical and Theoretical Chemistry