Ignition delays were measured for iso-octane using a rapid compression machine at an equivalence ratio of 1, initial pressure of 300 Torr, and post-compression oxygen density of [O2]Vo=1.0, where Vo is 22,400 cm3/mole. The post-compression temperature were varied by changing specific heat ratio of mixture: this was done by blending different inert gases, i.e., CO2, N2, and Ar. Negative temperature coefficient region was observed between 750 K and 850 K. Two-stage ignition delay characteristic was observed below 830 K. Overall experimental results were found to be in good qualitative agreement with those by Shell's Thornton Research Center.
The ignition delays predicted by MIT 19 reaction reduced chemical kinetic model were compared with those from the current experiment. In the model calculation, the measured pressure was fed into the model to calculate the core temperature before there is appreciable heat release due to chemical reaction. When there is appreciable heat release, constant volume condition was used. The comparison shows that the model is quite good in differentiating the single-stage and the two-stage ignitions. It is also found that the predictions of first-stage ignition delay agree well with the experimental data. The second-stage and the pressure jump, however, are underpredicted and overpredicted respectively by the model.
The kinetic model was investigated in detail, and it is identified that the model consists of three important paths: low temperature path, intermediate temperature path A, and intermediate temperature path B. Minor changes of the kinetic constants in the intermediate temperature path lead to a better agreement, and the addition of β-scission reaction made the prediction worse.