A magneto provides pulses of electrical power to the spark plugs in some petrol-powered internal combustion engines, most commonly those in 2-stroke engines used in small motorcycles, lawnmowers and chainsaws, as well as in most small aircraft. In aircraft, typically each cylinder has two spark plugs, each driven from a separate magneto. This arrangement provides redundancy in the event of a failure of one of the magnetos, and two sparks burn more efficiently than one. As such, the magneto provides a similar function to the coil system found in automobiles.
Magnetos combine the functions of a dynamo, contact breaker points and coil into one unit. The engine turns a permanent magnet inside a coil of wire to provide a basic source of electrical energy. On each revolution, a cam opens the contact breaker one or more times, interrupting the current, causing the voltage in the secondary winding of the coil to reach a very high value, enough to arc across the electrodes of the spark plug. Because no battery or other source of energy is required, the magneto is a rugged, reliable and self-contained solution to providing ignition of the fuel.