At the heart of a CDI ignition is the capacitor, a small battery-like component that stores electrical energy for a short period of time. Unlike a battery, which requires long charging cycles to reach its maximum power output, a capacitor can store and releases its energy supply almost instantaneously. It is this ultra-fast operation that allows motorcycle ignition systems to operate at high rpm.
CDI stands for capacitive discharge ignition, a type of electronic ignition system used by most motorcycles built after 1980. A CDI ignition is typically more reliable and requires less maintenance than the older mechanical contact point-type ignitions used previously.
The Purpose of a Capacitor
A CDI ignition system is made up of a CDI box, which contains the capacitor, a diode and an electronic switch, and a pair of wire coils that are controlled by a magnet attached to the motorcycle's flywheel. As the magnet passes over the first coil, an electric current is sent of the CDI box, which is passed through the diode and stored in the capacitor. When the magnet passes the second coil, a second current is transmitted to the switch in the CDI box, releasing the capacitor's store of energy into the ignition coil. This cycle repeats at every revolution of the engine.
Benefits and Disadvantages
The nature of a CDI ignition allows for a nearly maintenance-free riding experience, especially when compared to the older points-type ignition systems. Points-type ignitions were affected by wear, requiring the rider to constantly adjust the points to maintain proper ignition timing. CDI ignitions rarely need adjustment, since the majority of the system is made from solid-state components that do not move. However, points-type ignitions were easily and inexpensively replaceable, unlike a costly, more complex CDI-based system.
Today, CDI ignitions are a common component on most modern motorcycles and scooters. While a CDI ignition is more than capable of handling almost any riding situation, many high-performance sport motorcycles have moved away from the basic CDI setup, opting instead to use computerized control modules. This is mostly brought about by the ever-increasing power outputs of these high-speed models, many of which require instantaneous changes to the engine's ignition timing.
- The Professional Motorcycle Repair Program: Charging and Ignition Systems, Volume 18; Professional Career Development Institute