The effect of shunted resistance, or plug leakage, on the sparking performance of an electrical ignition system

A description is given of a series of tests made to determine the effect of shunted resistance on the spark voltage of most of the leading types of electrical ignition systems in use. The effect of the alteration of some of the electrical constants is considered. It is shown that the quicker the rate of rise of the secondary potential the less is the fall of voltage with leakage. The order of merit of the various types is as follows:- (1) Rotating armature types with movable pole-shoes. (2) Ordinary rotating-armature types. (3) Sleeve inductor types. (4) Polar inductor types. (5) Battery and coil systems. A theoretical discussion is given in which formulae are derived for the secondary voltage in the case of an ignition system shunted with a resistance of moderately low value. A number of curves are calculated for various cases. The actual measured values of the maximum voltage are shown to be in substantial agreement with these formulae. It is also shown that to get the best possible results, the number of secondary turns should be made as small as possible. The effect of the coupling is considered and it is found that there is in each case a particular value of this factor which gives the maximum potential. An appendix is given showing some tests of the leakage at high temperatures, and with high-tension current, of actual plugs that have missfired on engines. It is found that the average resistance of a spark plug that has run a moderately long time is about 200,000 ohms.