Analysis of the Ionization Equilibrium in the Post-Flame Zone

A combined experimental and theoretical effort has been made to identify the most important contributors to equilibrium ionization in the post-flame gas. In the past, nitric oxide (NO) has always been assumed to be the main electron donor in the compressed hot post-flame gases. However, correlations observed between the amount of NO in the exhaust gases and the current amplitude may be deceiving due to the fact that both the formation of NO and the ionization process are strongly temperature dependend.

The temperature-current relationship in data from various experiments in constant volume combustion chambers and engines was utilized to check the hypothesis that NO acts as the major contributor to ionization. Based on a well-motivated model for the current, the effect of temperature and electron donor concentration has been separated. The results indicate that species with much lower ionization energy than NO make a significant contribution to the conductivity of the gas in the post-flame zone. Assuming realistic concentrations of the most abundant alkali metals in air, an analysis of the ionization equilibrium has been performed. The results substantiate the experimental findings.