CDI

A capacitor discharge ignition system is an electronic ignition that uses a charged capacitor to generate a high-voltage spark at the spark plug. Unlike conventional points-and-condenser ignition, CDI has no mechanical breaker points to wear or adjust. Instead, a transistor or thyristor switches the discharge of a capacitor through the primary coil, producing a fast, hot spark regardless of engine speed or load. Most modern two-stroke and four-stroke small engines, motorcycles, and outboards rely on CDI rather than points-based systems.

The core principle is simple: a capacitor charges to several hundred volts through a rectifier circuit powered by the alternator or magneto. When the piston nears top dead center, a trigger signal (usually from a Hall sensor, points, or a magnetic pickup coil) tells a switching transistor to dump the capacitor's charge into the ignition coil primary. This rapid discharge induces a very high voltage in the secondary coil, sending spark to the plug. The resulting spark is sharp and short, typically 1 to 2 milliseconds, but intense enough to ignite fuel reliably across a wide RPM range.

Advantages and limitations

CDI systems excel where conventional ignition struggles. They require no maintenance, deliver consistent spark from idle to redline, and tolerate moisture and vibration better than points. They are cheaper to manufacture and more compact. The downsides are real: they demand a functioning alternator or magneto to charge the capacitor, so a dead battery or bad coil grounds the engine. Repair requires replacement of the entire CDI unit or individual modules; field repair of a failed capacitor is not practical on modern sealed units. Electromagnetic interference can sometimes cause misfires in environments with heavy radio transmission.

CDI systems are ubiquitous in small engines because of cost and reliability. Chainsaws, generators, and lawn mowers almost always use CDI. Motorcycles built after the 1980s moved to CDI or digital fuel injection ignition. Marine outboards switched to CDI because the system tolerates salt spray and vibration. Even some racing engines use CDI variants because the spark quality is superior to points, though top-tier motorsport often uses programmable digital ignition systems for finer control.

The term 'capacitor discharge' distinguishes it from other electronic ignition types. Some systems use an energy store (the capacitor) to fire the spark, while others use the ignition coil itself as the energy store and simply switch it on and off electronically. Both are electronic, but only CDI uses a capacitor as the primary energy reservoir. In service, a weak or aged capacitor may charge slowly, causing hard starting or weak spark at low RPM. Testing the capacitor voltage with a multimeter under load is the standard diagnostic step when a CDI engine misfires.