Introduction
to Capacitive Discharge (CDI) Ignition
Electronic capacitive discharge ignition (CDI) systems have been common on large industrial engines because the technology has been in use since the 1960's.
An advantage of the capacitive discharge ignition system is that the energy storage and the voltage ‘step up' functions are accomplished by separate circuit elements allowing each one to be optimised for its job.
Capacitive discharge ignition systems work by storing energy in an external capacitor, which is then discharged into the ignition coil primary winding when required. This rate of discharge is much higher than that found in inductive systems, and causes a corresponding increase in the rate of voltage rise in the secondary coil winding. This faster voltage rise in the secondary winding creates a spark that can allow combustion in an engine that has excess oil or an over rich fuel air mixture in the combustion chamber. The high initial spark voltage avoids leakage across the spark plug insulator and electrodes caused by fouling, but leaves much less energy available for a sufficiently long spark duration; this may not be sufficient for complete combustion in a “lean burn” turbocharged engine resulting in misfiring and high exhaust emissions.
The high voltage power supply required for a capacitive discharge system can be a disadvantage, as this supply provides the power for all ignition firings and is liable to failure.
Ignition in lean fuel mixtures by capacitive discharge systems can sometimes only be accomplished by the use of multi-spark ignition, where the ignition system duplicates the prolonged spark of inductive spark systems by sparking a number of times during the cycle. This adds greater stress onto the high-tension leads and can cause considerable spark plug wear and possible failure.
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