From Spark Ignition to Flame Initiation

ABSTRACT

The process of spark ignition and the subsequent flame propagation in an internal combustion engine have been investigated. A unique theoretical model which considers the various physical and chemical phenomena associated with the ignition process has been developed. It employs a two-dimensional cylindrical coordinate system and assumes axial and radial symmetry. The model employs also a detailed chemical reaction scheme for a methane-air mixture which contains 29 chemical species and 97 reactions. The thermodynamic and transport properties are evaluated by using statistical thermodynamics and molecular theory approach while including the various energy modes stored in the mixture particles. The appropriate conservation equations are solved numerically by using an integration of the PHOENICS and the CHEMKIN codes. It was concluded from the numerical results that the spark kernel growth can be described as a two-step process. The early short stage (1–5 μs), which involves a pressure wave emission, is followed by a much longer (1–10 ms) diffusive period. In the early stage the mass and energy transfer processes are very much dominated by the pressure wave and the violently expanding plasma kernel, with only negligible contribution of the chemical reactions to the kernel development During the diffusive stage, when the contribution of the chemical reactions to the kernel expansion is sensible, an inflammation zone is created. Based on a parametric study it was concluded that the spark kernel expansion can be enhanced by increasing the spark power during the early stage of its development, decreasing the electrodes' diameter and increasing their gap distance as these reduce substantially the energy losses to the electrodes.