Ignition of gases by high-energy sparks

Abstract

For an ultra-high-speed spark-ignition engine, with a maximum speed approaching 20,000 rpm, it becomes very important to reduce the prepropagation period and the initial, or accelerating, period of flame development that follows the passage of a spark in a combustible mixture. This means producing as large an initial flame or flame kernel as possible. Even with the spark energy released by an ordinary ignition coil or magneto, which is only about 0.1 J, the time required for the initial flame to develop can be reduced considerably by using the multi-gap spark plug designed for an ultra-high-speed spark-ignition engine, because with this arrangement the original flame kernels merge into one flame in the ignition process. Two ignition sources for producing larger flame kernels have been investigated: (1) metal wires exploded by a condenser discharge, and (2) triggered capacity sparks, which can release energies that are a few to several orders of magnitude higher than the minimum ignition energy. High-speed schlieren motion pictures of the developing flame reveal that the pre-propagation period is reduced and subsequent flame propagation is promoted by the turbulence of the combustible mixture generated by the rapid ejection of the fine particles of the exploded material, and by the hot gas produced primarily by a triggered capacity spark which acts as a massive ignition source. A triggered capacity spark with 0.5 to 1.0 joule energy is sufficient for engine applications in the ultra-high-speed range.