How Does Capacitor Discharge Ignition Work?

Capacitor Discharge Ignition Defined

A capacitor discharge ignition is a type of ignition system used in both small and high-speed motors. They power motorcycles, lawnmowers, and some cars and aircraft. A capacitor discharge ignition uses a condenser, or capacitor, to discharge a quick jolt of electricity, which powers the spark plug and ignites the fuel in the cylinder.

The Transformer

Electric power flows from a car's alternator into a step-up transformer. A transformer has two coils next to each other. When a moving current is applied to one coil, it creates a magnetic field that induces a current in the second coil. In a step-up transformer, the secondary coil has more turns than the primary. This increases the voltage. In the case of a capacitor ignition system, the transformer increases the voltage from about 12 volts to between 400 and 600 volts.

Charging the Capacitor

Electricity flows into a charging circuit, which charges a capacitor. A capacitor works like a very small battery. It holds an electric current until it is discharged. The capacitor stays charged until it gets a signal from the ignition circuit. When that happens, the charging circuit turns off, and the capacitor discharges through the ignition coil.

Ignition Coil and Spark Plug

The ignition coil is a very powerful step-up transformer. The capacitor charges the ignition coil. When the capacitor runs out of energy, the current in the ignition coil drops rapidly. This change in current creates a pulse of extremely high-voltage electricity from the ignition coil to the spark plug. Up to 40,000 volts of electricity flow out of the ignition coil into the spark plug. This voltage is powerful enough to make a spark, which ignites the fuel in the cylinder.