Capacitor Discharge Ignition (CDI): What is it?

RX-Z CDI Unit Assy

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.

History

The history of capacitor discharge ignition system can be traced back to the 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 (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.

Most CDI modules are generally of two types:

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.

Note that not all small engine ignition systems are CDI. Some older engines, and engines like older Briggs and Stratton use magneto ignition. The entire ignition system, coil and points, are under the magnetized flywheel.

Another sort of ignition system commonly used on small off-road motorcycles in the 1960s and 1970’s was called Energy Transfer. A coil under the flywheel generated a strong DC current pulse as the flywheel magnet moved over it. This DC current flowed through a wire to an ignition coil mounted outside of the engine. The points sometimes were under the flywheel for two-stroke engines, and commonly on the camshaft for four-stroke engines. This system worked like all Kettering (points/coil) ignition systems… the opening points trigger the collapse of the magnetic field in the ignition coil, producing a high voltage pulse which flows through the spark plug wire to the spark plug.

If the engine was rotated while examining the wave-form output of the coil with an oscilloscope, it would appear to be AC. But you must consider that since the charge-time of the coil corresponds to much less than a full revoltion of the crank, the coil really ‘sees’ only DC current for charging the external ignition coil.

There exist some electronic ignition systems that are not CDI. Some systems use a transistor to switch the charging current to the coil off and on at the appropriate times. This eliminated the problem of burned and worn points, and provided a hotter spark because of the faster voltage rise and collapse time in the ignition coil.

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, vehicles that use DC-CDI systems have more precise ignition timing and the engine can be started more easily 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 some applications be too short to provide reliable ignition. The insensitivity to shunt resistance and the ability 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.