23 March 2018

What is capacitive discharge ignition system | Construction | Working | Advantages

The capacitor is used to store the ignition energy and the capacitance and the charging voltage of the capacitor determine the amount of stored energy. An induction coil is used to store the ignition energy in the battery ignition system. 

Construction of the CDI system:

The CDI trigger box includes the capacitor, thyristor power switch, charging device that is used to convert battery voltage to change the voltage of 300 to 500 V by pulse through a voltage transformer and pulse shaping unit and control unit.

Capacitive discharge ignition system

Working of CDI system :

This system works by passing an electrical current over a capacitor. This type of ignition builds up a charge very quickly and starts by generating a charge and storing it up before sending it out to the spark plug in order to ignite the engine. 

The power passes through a capacitor and is transferred to an ignition coil that helps to boost the power by acting as a transformer and allowing the energy to pass through it instead of catching.

This system allows the engine to keep running as long as till charge is available in the power source.

Advantages of the CDI system:

  • Insensitive to electrical shunts resulting from spark plug fouling.
  • This system is suited to an application where insufficient dwell time is available because the capacitor can be fully charged in a very short time. 
  • Short transient response.
  • A fast voltage rise and shorter spark duration. 

Disadvantages of the CDI system: 

  • The spark is strong but short in order to 0.1 to 0.3 ms which leads to ignition failure during lean mixture operating conditions because of fast capacitive discharge.
  • This system generates a huge electro-magnetic noise so CDI is rarely used by automobile manufacturers.