On the crossover temperature and lower turnover state in the NTC regime

The thermodynamic and chemical kinetic behaviors of the lower turnover state of the negative temperature coefficient (NTC) regime were studied. n -Butane and n -heptane which represent typical short and long straight-chain alkanes exhibiting the NTC were the fuels selected. The first and second stage ignition delays were approximately equal to each other at the lower turnover states under various pressures such that the total ignition delay shared similar temperature dependence with the first stage ignition delay. To further characterize the second ignition stage the cool flame temperature rise at the end of the first stage decreased linearly with increasing initial temperature. Comparison of the results with the literature experimental data of the n-heptane ignition delay time showed good agreement.