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Most of the automotive engines have an electronic ignition system to start it off when the engine runs. The capacitor discharge ignition is generally used in lawn mowers, motorcycles, outboard mowers, chainsaws, in some cars and turbine-powered aircraft. However, most of the cars use inductive discharge ignition that is different from CDI systems.

A typical CDI consists of a charging circuit, small transformer, a main capacitor, and a triggering circuit. The system raises a voltage of 400-600 volts by the transformer. This current is then channeled to the charging circuit and to the capacitor. The charging unit has a rectifier to prevent discharging before the ignition is executed.

On triggering the ignition, the triggering system stops the charging and makes the capacitor to discharge rapidly to up to 40kv at the winding of the spark plug.

Types of CDIs


The AC-CDI ignition system is the most common CDI in small engines. The module gets its power solely from the alternator in form of alternating current.


This kind of CDI is powered by a battery. The system comes with a DC/AC inverter circuit to raise the voltage from the 12V DC up to 400-600 V DC. This DCI is slightly larger than the AC-CDI ignition systems. DC-CDI modules enable precise ignition timing and make it easy to start the engine even when it is very cold

There is also another form of ignition system that closely looks like the DC-CDI system but was used by the older engines and was called Energy Transfer. It included a coil under the flywheel which generated a very strong DC current pulse when the flywheel magnet moved close to it. The direct current was then channeled through a wire to an ignition coil that was located outside the engine. The system was triggered by opening points near the flywheel thereby leading to collapse of the magnetic system.

Which is the best CDI system?

There is no CDI system that can be said to be better than the other but each is good in different situations. The DC-CDI works well in areas where there is extreme cold temperatures and is very precise during ignition. The AC-CDI on the other hand, is simpler and rarely runs into problems. It is smaller and convenient too.

The CDI system is insensitive to shunt resistance and can fire multiple sparks at once and thereby great to use in various applications without any delay once the system is started.

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