The function of the Magneto

This article is about an engine component. For other uses of the term, see Magneto (disambiguation).

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, as well as in most small aircraft and some racing automobiles, serving a similar function to the coil-type ignition system found in automobiles. 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.

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 (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.