Flame initiation and development from glow discharges—Effect of electrode deposits

Abstract

The predominant discharge mode in the spark from a conventional automobile coil ignition system is a glow discharge. It is also known that certain gasoline additives—alkali and alkaline earth metal compounds known as “spark-aiders”—enhance the ignition process in spark ignition engines by forming deposits on the spark plug electrodes. This work describes experiments performed in a combustion bomb to study flame initiation and development from glow discharges, and the effects of electrode deposits on these phenomena. The two phenomena, which are intimately linked, are shown to be profoundly influenced by the presence of electrodes and to depend on electrode gap, material, shape and size as well as spark power, energy, mixture characteristics, and pressure. Marginal conditions for flame initiation can result from a suitable combination of any of these parameters. Crystalline deposits on the electrodes make breakdown of the gap easier and more repeatable, but breakdown itself plays a relatively unimportant role in flame initiation and development in the experiments described here. Only the potassium compound deposits on the electrodes lead to a reduction in the glow voltage, primarily because of a reduction in the cathode fall, though reductions in the column voltage gradient are also observed. Such deposits, viz potassium sulfate, greatly enhance the ignition ability of a glow discharge, especially as the conditions for ignition become more difficult. In some cases, however, when the reduction in the glow voltage because of such deposits is too large, the ignition ability of the glow discharge can worsen. The results can be partially explained in terms of the presence of low electron work function material on the cathode and the consequent improvement in the energy transfer efficiency of the glow discharge. However, other questions about the details of the effect of such deposits on the physics of the glow discharge and on flame initiation and development remain unresolved.