Question

Describe working of capacitive discharge ignition (CDI) system

Answer 1

Working of CDI system: It mainly consists of 6-12 V battery, ignition switch, DC to DC convertor, charging resistance, tank capacitor, Silicon Controlled Rectifier (SCR), SCR-triggering device; step up transformer, spark plugs. A 6-12 volt battery is connected to DC to DC converter i.e. power circuit through the ignition switch, which is designed to give or increase the voltage to 250-350 volts. This high voltage is used to charge the tank capacitor (or condenser) to this voltage through the charging resistance. The charging resistance is also so designed that it controls the required current in the SCR. Depending upon the engine firing order, whenever the SCR triggering device, sends a pulse, then the current flowing through the primary winding is stopped. And the magnetic field begins to collapse. This collapsing magnetic field will induce or step up high voltage current in the  secondary, which while jumping the spark plug gap produces the spark, and the charge of air fuel mixture is ignited.

OR

CDI system consists of primary circuit and secondary circuit The primary circuit consists of following components: i) Primary winding of pulse transformer ii) Condenser iii) Resistance iv) SCR v) Pulse generator. vi) Battery vii) DC to AC convertor/charging device

The secondary circuit consists of following components: i) Secondary winding of pulse transformer ii) Spark plug iii) Spark plug HT coil

Working: • CDI system uses charge of capacitor for generating spark- using pulse transformer • Thyrister/ silicon controlled rectifier is used as switch- for primary circuit current through capacitor. • It also uses a pulse generator to trigger SCR through Gate circuit. • Pulse transformer has low inductance, so the change in flux across primary and secondary windings is very rapid. • This provides high voltage spark (about 30,000V) during the entire speed range of the engine. • The electronic circuitry uses conversion of AC to DC charging device, signal conditioning and amplifying unit and control circuit.