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Capacitor discharge ignition

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Capacitor discharge ignition (CDI) or thyristor ignition is a type of automotive electronic ignition system which is widely used in motorcycles, lawn mowers, chain saws, small engines, Turbine powered aircraft, and some cars. It was originally developed to overcome the long charging times associated with high inductance coils used in inductive ignition systems, making the ignition system more suitable for high engine speeds (for small engines, racing engines and rotary piston engines). Capacitor discharge ignition uses capacitor discharge current output to fire the spark plugs.

1 History
2 The basic principle
3 Advantages and Disadvantages of CDI
4 References

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.

The basic principle

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. 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 dependant on the voltage and capacitance of the capacitors used, but usually it's around 50 mJ. 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/AC inverter circuit is included in the CDI module to raise the 12 V DC to 400-600 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

A CDI system has a short charging time, a fast voltage rise (between 3 ~ 10 kV/μs) compared to typical inductive systems (300 ~ 500 V/μs) and a short spark duration limited to about 50-80 µs. The fast voltage rise makes CDI systems insensitive to shunt resistance, but the limited spark duration can for many applications be too short to provide a reliable ignition. The insensitivity to shunt resistance and the possibility to fire multiple sparks can provide improved cold starting ability. Since the CDI system only provides a short spark, it's also possible to combine this ignition system with ionization measurement. This is done by connecting a low voltage (about 80 V) to the spark plug, except when fired. The current flow over the spark plug can then be used to calculate the temperature and pressure inside the cylinder.