Capacitor discharge ignition (CDI) or thyristor ignition systems work by storing energy in an external capacitor, which is then discharged into the ignition coil primary winding when required. The spark necessary to ignite the air/petrol mixture in the combustion chamber is produced by the CDI module. This system consists of 7 stages, voltage supply, capacitor, switch, sensor, conditioning, ignition coil, and spark plug. This device is widely used in motorcycles, lawn mowers, chain saws, small engines, Turbine powered aircraft, and some cars.
Most CDI modules are generally of two types: AC-CDI, this module obtains its electricity source solely from the alternating current produced by the alternator; and DC-CDI – this module gains its power from the battery, therefore an additional DC/AC inverter circuit is included in the CDI module.
Capacitive Discharge Ignition CDI Circuit Diagram:
Capacitive Discharge Ignition CDI Application Note – Link (source: www.transmic.net)
CDI, History, Principle, Advantage, Disadvantage and Reference – Link (wikipedia.org)