Ignition of gases by two successive sparks with reference to frequency effect of capacitance sparks

Abstract

The fact that the igniting ability of a capacitance spark increases as the frequency of an oscillatory discharge decreases has been previously ascertained down to about 260 kHz. The present study was undertaken to investigate how matters stand at lower frequencies. In the experiments using lean propaneair mixtures, the igniting ability of a capacitance spark is found to reach a maximum at a frequency of about 100 kHz. In order to elucidate the mechanism of such a frequency effect, two successive sparks of very short duration, which pass across the gap at a controllable time interval, are used as substitutes for halves of the first cycle of an oscillatory discharge. The experimental results show that the igniting ability of two successive sparks is highest at an interval of 10 to 50 μsec, which corresponds to a frequency of 50 to 10 kHz for the oseillatory discharge. This fact suggests that the discharge frequency rather than the spark duration is the predominant factor in ignition by a capacitance spark. A simplified thermal theory taking into consideration the shape and behavior of a spark kernel is given to explain the effect of frequency on the igniting ability of capacitance sparks.