FROM SPARK-IGNITION TO FLAME INITIATION

The process of spark ignition and the subsequent flame propagation in an internal combustion engine have been investigated. A unique theoret ical 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 r adial symmetry. The model employs also a detailed chemical reaction sc heme 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 whi le including the various energy modes stored in the mixture particles. The appropriate conservation equations are solved numerically by usin g an integration of the PHOENICS and the CHEMKIN codes. It was conclud ed from the numerical results that the spark kernel growth can be desc ribed as a two-step process. The early short stage (1-5 mu s), which i nvolves a pressure wave emission, is followed by a much longer (1-10 m s) diffusive period. In the early stage the mass and energy transfer p rocesses are very much dominated by the pressure wave and the violentl y expanding plasma kernel, with only negligible contribution of the ch emical reactions to the kernel development. During the diffusive stage , when the contribution of the chemical reactions to the kernel expans ion is sensible, an inflammation zone is created. Based on a parametri c study it was concluded that the spark kernel expansion can be enhanc ed by increasing the spark power during the early stage of its develop ment, decreasing the electrodes' diameter and increasing their gap dis tance as these reduce substantially the energy losses to the electrode s.