Common to piston engine airplanes, the magnetos are self-contained high voltage generators. The main purpose of the magneto is to provide electrical current to the spark plug so it can ignite the fuel and air mixture at the right time in the engine cycle. Electrical current is generated when a magnet moves through a coil of copper wire. The magnetic field of the magnet generates an electrical current in the coil. This electrical current makes a light bulb light up. If there is enough current generated, it can even create a spark in the spark plug inside an engine cylinder.

The spark ignites the fuel and air mixture, causing it to burn rapidly, expand, and force the piston down to produce engine power. In reality, the spark plug needs a very high voltage electrical current to create the spark. In a magneto, the magnet is very strong. It's driven by the engine through a gear system and rotates inside an iron core. In a four-cylinder engine, the magnet rotates at crankshaft speed. As the magnet rotates, its magnetic field flows up and down the core and through the copper wire coils. The magneto has two copper wire coils: the primary and secondary coil. The primary coil is smaller than the secondary coil.

Electrical current is generated in the primary coil and it flows around its circuit. There is not enough voltage in the primary coil, however, to create the spark in the spark plug. But the current in the primary coil creates its own magnetic field around the iron core. In a magneto, the breaker point is used to suddenly stop the current in the primary coil. The breaker point is driven by a cam connected to the gear system. The breaker point opens and stops the current twice per crankshaft revolution. When the breaker point opens, it suddenly stops the current in the primary coil. This makes the magnetic field in the core collapse very quickly.

The collapse of the magnetic field in the core induces a voltage spike in the primary coil. This voltage spike is transformed into the secondary coil. The secondary coil has many more wire coil windings, and because of this, it generates a much higher voltage for the spark plugs to create the spark. The best spark is created when the voltage spike is strong, but when the breaker point opens, electricity arcs across the points, and the magnetic field in the core does not collapse as quickly if this happens. So the voltage spike is not as strong. The solution to this problem is the capacitor. It sits across both sides of the breaker point. When the breaker point opens, it doesn't arc anymore because the current flows into the capacitor instead.

Thanks to the capacitor, the magnetic field in the core collapses very quickly, thus the voltage spike is high and the spark is strong. This illustration represents a four-cylinder engine, but only one cylinder is shown here. The distributor is a rotating switch that is mechanically driven by the engine at half crankshaft speed. The distributor routes the high voltage current from the secondary coil to each spark plug at the correct time and in the correct firing order. The magneto is a simple and reliable device. Over time, however, the plastic gears may fracture or the capacitor may break, and the magneto may fail to create the spark. For this reason, airplanes have two magnetos for backup and improved performance. Thanks for watching, and please be sure to check out these related videos about the impulse coupling and the Otto cycle.