Ionization Rates and Charge Production in Hydrocarbon Fuels

In fuel air mixtures the electron energy distribution function (EEDF) is dominated by the effects of the nitrogen momentum transfer and inelastic collisions. However the ionization potential of the hydrocarbons, n-octane and n-decane, are about 11.7 eV and 10.2 eV respectively whereas the nitrogen ionization potential is 15.5eV. Therefore the total ionization rate at higher fuel ratios can be dominated by ionization of the fuel. On the other hand in vitiated air, nitric oxide is produced and it has an even lower ionization potential. If NO (ionization potential 9.25 eV) is present in small quantities, it will also be an important source of ionization even though the EEDF is still controlled by nitrogen. The charge transfer rates also lead to ionization of the lowest ionization potential species. Charged-particle collisions contribute to the ignition process by producing radicals, cracking the fuel molecules, and increasing the gas temperature through heat released from exothermic reactions. This paper presents our studies using Fourier-transform mass spectrometry (FTMS) on the fuel compounds, n-octane and decane and nitromethane. We have measured their electron impact ionization and gas-phase ion-molecule reactions. Calibration using argon allowed absolute ionization cross-sections and charge exchange rates of cracked ions with the parent molecules to be obtained. Ion-molecule reactions proceed much more rapidly than neutral radical-molecule reactions and therefore will be important at sufficient ionization fraction in the breakdown and expansion phases of the spark ignition process. The power into the plasma is given by the product of the voltage (time) and current (time) and it is important to measure accurately the phase between these two parameters in order to quantify the energy input. See also ADM001739. Presented at the Final Conference Proceedings on Thermochemical Processes in Plasma Aerodynamics held in St. Petersburg, Russia on 28-31 July 2003. EOARD-CSP-03-5031.