The performance of coil ignition systems, with particular reference to double contact-breakers and the effects of variation of the period of open circuit

The paper deals with experimental results obtained on a modern coil ignition system operated by a double contact breaker with a small angle of open circuit. At high speeds the contact breaker closes before the spark discharge ceases. Measurements of the effect of the persistence of the spark dicharge upon the coil performance for various secondary loading conditions are described. It is shown that the increase in the just-regularly-sparking output which is obtained is approximately independent of secondary loading due to normal values of shunted capacitance and/or resistance. A theoretical investigation, based on the results, is carried out to determine the approximate increase in sparking frequency with a given output, due to the effect of the persistence of the spark dischafge, for various periods of open circuit. Under certafa conditions, the estimated increase is 35 per cent for 0¿4 millisecond and 63 per cent for 0-1 millisecond. The maximum sparking frequency of 12-volt coil systems, capable of meeting the output requirements of the Service specification for ignition apparatus, is estimated for various values of the open-circuit period. It is shown that the probable limit for reliable operation with a double contactbreaker is about 300 sparks per sec. per coil.