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. Capacitive Discharge Ignition CDI Application Note. CDI, History, Principle, Advantage, Disadvantage and Reference.