Difference between revisions of "Capacitor discharge ignition"

Most ignition systems used in cars are inductive ignition systems, which are solely relying on the electric inductance at the coil to produce high-voltage electricity to the spark plugs as the magnetic field breaks down when the current to the primary coil winding is disconnected (disruptive discharge). In a CDI system, a charging circuit charges a high voltage capacitor, and during the ignition point the system stops charging the capacitor, allowing the capacitor to discharge its output to the ignition coil before reaching the spark plug.  

A typical CDI module consists of a small transformer, a charging circuit, a triggering circuit and a main capacitor. First, the system voltage is raised up to 400-600 V by a transformer inside the CDI module. Then, the electric current flows to the charging circuit and charges the capacitor. The rectifier inside the charging circuit prevents capacitor discharge before the ignition point. When the triggering circuit receives triggering signals, the triggering circuit stops the operation of the charging circuit, allowing the capacitor to discharge its output rapidly to the low inductance ignition coil, which increase the 400-600 V capacitor discharge to up to 40 kV at the secondary winding at the spark plug. When there's no triggering signal, the charging circuit is re-connected to charge back the capacitor.

The amount of energy the CDI system can store for the generation of a spark is dependent on the voltage and capacitance of the capacitors used, but usually it's around 50 mJ, or more. The standard points, coil, distributor ignition, more properly called the Kettering ignition system, produces 25mJ at low speed and drops off quickly as speed increases.  

* '''AC-CDI''' - The AC-CDI module obtains its electricity source solely from the alternating current produced by the alternator. The AC-CDI system is the most basic CDI system which is widely used in small engines.