Ignition Quality Tester (IQT) Investigation of the Negative Temperature Coefficient Region of Alkane Autoignition

Abstract

The negative temperature coefficient (NTC) region of alkane autoignition was observed for the first time in the Ignition Quality Tester (IQT). The C7 isomers studied included n-heptane, 3-ethylpentane, 2,4-dimethylpentane, 2,3-dimethylpentane, and 2,2,3-trimethylbutane. The temperatures of the fuel–air mixture ranged from 650 to 1023 K with pressures of 0.5, 1.0, and 1.5 MPa at equivalence ratios between 0.8 and 1.0. The longer autoignition times of increasingly branched isomers allowed the reacting mixtures sufficient time to reach a pseudohomogeneous state, so that the kinetic behavior was similar to that observed in homogeneous rapid compression machine (RCM) and shock tube experiments. Although the IQT produced longer ignition delays than RCM data, the order of ignition delays for the various isomers was the same; that is, isomers with more branching had reduced reactivity and the location of the methyl group among equally branched isomers also affected reactivity. The characteristic NTC region was observed from all of the fuels at 0.5 MPa, except for n-heptane which had ignition delays too short to overcome the effects of fuel–air heterogeneity on autoignition. However, reducing the pressure to 0.2 MPa further increased the ignition delay so that NTC behavior was observed for n-heptane. A computational fluid dynamics model was used to study fuel evaporation and fuel–air mixing, and a 0-D homogeneous batch reactor was used to model the ignition of the C7 isomers. The latter produced reasonable levels of agreement with experiments across the temperature range. The 0-D chemical kinetic model also successfully modeled hexadecane autoignition in the IQT at long ignition delays (>20 ms). However, coupled computational fluid dynamics/kinetic model may be required at short ignition delays (<20 ms), because the ignition process is affected by spray dynamics and mixture heterogeneity effects. NTC behavior for the low-volatility fuel 2,2,4,4,6,8,8-heptamethylnonane (isocetane) was also measured experimentally for the first time. These results suggest that IQT ignition delay measurements at conditions (pressure and temperature) producing sufficiently long times (>20 ms) have the potential to provide meaningful data to assist in the validation of combustion kinetic mechanisms.