Capacitor discharge ignition

Capacitor discharge ignition (CDI) is a type of automotive electronic ignition system which is widely used in motorcycles, lawn mowers, chain saws, small engines and recently in some cars. Capacitor discharge ignition uses capacitor discharge current output to fire the spark plugs.


The basic principle

Most of typical ignition system used in cars are inductive ignition system, which is solely relying on the electric inductance at the coil to produce high-voltage electricity to the spark plugs. In a CDI system, the system charges a capacitor by default, and during the ignition point the system stopps charging the capacitor, allowing the capacitor to discharge its output to the final coil before reaching the spark plug.

A typical CDI module may consist of a small transformer, a charging circuit, a triggering circuit and a main capacitor. Firstly, the system voltage is raised up to 400 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 from triggering devices such as Hall effect sensor or pulse generator during the ignition point, the triggering circuit stops the operation of the charging circuit, allowing the capacitor to discharge its output rapidly to the ignition coil. The rapid capacitor discharge then produces a very high voltage at about 40 kV to be fired at the spark plug. When there's no triggering signal, the charging circuit is re-connected to charge back the capacitor.


CDI modules can be generally divided into two:-

* 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.
* DC-CDI - The DC-CDI module is powered by the battery, and therefore an additional DC/DC inverter circuit is included in the CDI module to raise the 12 V DC to 400 V DC, making the CDI module slightly larger. However, the vehicle that uses DC-CDI system has more precise ignition timing and the engine can be started easier when cold.


Advantages and Disadvantages of CDI

CDI system produces higher ignition voltage (about 40 kV) compared with typical inductive ignition system (about 20 kV). The higher voltage produced by the CDI system produces a hotter spark, enabling the engine to be operated even with badly-fouled spark plugs.

The CDI system also has a faster voltage rise time (between 3 ~ 10 kV/μs) compared with typical inductive systems (300 ~ 500 V/μs). The higher voltage rise time results in a shorter spark duration with the CDI system (10 ~ 12 μs) and therefore the spark output is more accurate.

However, the shorter spark duration means the CDI system is not suitable for sharing between cylinders in multi-cylinder engines. It was not until the end of the 1990s that CDI system could be practically used in multi-cylinder engines, especially in cars, as a result of the development of the direct ignition system, where each cylinder has its own ignition coil.

CDI systems also have problems with lean air-fuel mixture and high compression engines as well as cold-starting problems. However, the problems can be solved by using waste-spark methods.


History

THe history of capacitor discharge ignition system can be traced back in 1950s together with the development of other electronic ignition systems. The first commercial motorcycle using the CDI system was manufactured by Kawasaki.

By the end of 1960s, the US government made new laws enforcing strict emission standards. As a result, more and more electronic ignition systems were developed, and starting from 1970s all smaller engines installed CDI system to replace the contact point system, including Honda Cub which began to use AC-CDI system.

By the end of 1990s, direct ignition system using capacitor discharge ignition system was developed and started to be installed on some newer car models.