Effects of energy deposition schedule on minimum ignition energy in spark ignition of methane/air mixtures

A two-dimensional numerical analysis with elementary reactions including ion-molecule reactions has been performed to investigate the effect of energy deposition schedule on the minimum ignition energy in the composite spark ignition of methane/air mixtures. The chemical reaction scheme for a methane/air mixture, which has 27 species including 5 ion molecules and electrons and 81 elementary reactions with ion-molecule reactions, is employed in the calculation. Experiments are also carried out, and a qualitative comparison between experiment and calculation is made. The calculated result showed that the optimum ratio of capacity spark energy, for which the minimum ignition energy has a lowest value, exists. The increase in the capacity spark energy leads to a decrease in the formation of radicals during the spark discharge, while it also leads to an active reaction after spark discharge, which is due to the enhanced supply of the unburned mixture into the flame kernel by the inward flow along electrodes. The qualitative trend of calculated and experimental results shows good agreement for the relationship between the ratio of capacity spark energy and the minimum ignition energy. The minimum ignition energy obtained from the calculation with ion-molecule reactions is slightly larger than that without ion-molecule reactions. This may be due to the consumption of energy for ionization.