A magneto provides pulses of electrical power to the spark plugs in some gasoline-powered internal combustion engines where batteries are not available, most commonly those in 2-stroke and 4-stroke engines used in small motorcycles, lawnmowers and chainsaws, serving a similar function as the coil-type ignition system found in automobiles. In these cases the magneto advantage is in its compact nature and simple reliable function. They are used in most small aircraft, some racing automobiles and in older tractors. 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 provide for a more complete and efficient burn of the fuel mixture.
Magnetos combine the functions of a dynamo, contact breaker points and coil into one unit. The engine rotates a coil of wire between the poles of a permanent magnet to provide a basic source of electrical energy (though in some variants the permanent magnet is rotated and the coil remains stationary). 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. In some modern magneto designs, an electronic switch replaces the contact breaker.
Since the magneto is a self-contained integrated ignition unit providing its own elctrical power generation, a vehicle with other electrical equipment (lights, radios, etc.) also requires an alternate means to provide electrical power for that equipment, such as an external dynamo or alternator. Most importantly, the magneto has not any provision for initally causing the rotation of the engine or itself, so that an alternative starting means will be required for a machine using a magneto. Such means may be a starter motor, kickstart, starting crank or pull cord, starting charge, or the like.