First-stage ignition delay in the negative temperature coefficient behavior: Experiment and simulation

The existence of the first-stage ignition delay (FID) negative temperature coefficient (NTC) behavior was confirmed by rapid compression machine experiments using iso-octane and methyl-cyclohexane. The first-stage NTC behavior of iso-octane is observed in the temperature range 757-782K under 20bar and φ=1. For methyl-cyclohexane, the observed first-stage NTC temperature range is 750-785K under 15bar and φ=0.5. In further iso-octane experiments, the FID is found to be sensitive to the O₂ concentration and insensitive to the dilution gas and fuel concentrations. The effects of the FID and its NTC behavior on the total ignition delay NTC were analyzed using a detailed n-heptane mechanism. The contributions to the total ignition delay NTC from the reduced second-stage initial temperature, pressure, and less reactive species pool, together with the NTC of FID were discussed quantitatively. For the first-stage NTC behavior, five competing reactions were identified as being important based on sensitivity analysis, reaction pathway analysis, and simplified mechanism method. They are the backward reaction of second O₂ addition, RO2 ⇔ alkene+HO2, QOOH ⇔ cyclic-ether+OH, QOOH ⇔ alkene+HO2, and the beta scission reaction of the alkyl radical. Their competition with the low-temperature branching channel finally leads to the first-stage NTC behavior.