Shock tube measurements of branched alkane ignition delay times

Ignition delay times for three branched alkanes: 2,4-dimethylpentane, 2,5-dimethylhexane and isooctane, were measured behind reflected shock waves. The ignition delay time measurements cover the temperature range of 1313-1554 K, with pressures near 1.5 and 3 atm, equivalence ratios of 0.5 and 1 in 4% oxygen/argon. Regression analyses of the data over the limited range of conditions studied yield the following correlations for ignition delay time as a function of temperature (K), pressure (atm), and equivalence ratio: 2,4- DMP: tau(sign)[s] = 8.4 x 10 P-11 (0.61)Phi(1.03)exp(46.6[kcal//mol]/RT) 2,5- DMH: tau(sign)[s] = 2.1 x 10 P-10 (0.60)Phi(0.99)exp(43.4[kcal/mol]/RT) Iso-octane: tau(sign)[s] = 1.1 x 10 P-10 (0.47)Phi(0.86)exp(45.7[kcal/mol]/RT) Comparing the current ignition delay time data of branched alkanes with published values for their normal alkane isomers, it was confirmed that increasing the degree of carbon chain branching lowers the reactivity of the fuel and increases the ignition delay time. In addition, longer ignition delay times were observed for 2,4-dimethylpentane than 2,5-dimethylhexane, confirming the influence on reactivity by changing the straight carbon chain by one carbon for symmetric branched hydrocarbon fuels. The low reactivity and long ignition delay times for branched alkanes were attributed to the high concentrations of propene and iso-butene formed when branched alkanes decompose, as propene and iso-butene reduce the radical pool by consuming OH, O and H to form less-reactive species like allyl radical and allene. The ignition delay times of the fuels studied were also seen to increase monotonically with octane number under the current test conditions. (C) 2013 Elsevier Ltd. All rights reserved.