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

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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. They are also common on aviation engines due to light weight, simplicity and non dependence on an electrical source. The ignition magneto combines the functions of a dynamo, contact breaker points, "condenser" (capacitor), and ignition coil.

Invented by Briton Frederick Simms, the magneto was perfected by Bosch and introduced on the 1899 Daimler Phönix. This was followed by Benz, Mors, Turcat-Mery, and Nesseldorf,^[1] and soon were used on most cars up until about 1918 in both low voltage (voltage for secondary coils to fire the spark plugs) and high voltage magnetos (to fire the spark plug directly, similar to coil ignitions, introduced by Bosch in 1903).^[2] In these cases the magneto advantage is in its compact nature and simple reliable function, which is why they are preferred in most small aircraft, some racing automobiles, and older tractors.

Magneto-equipped aircraft engines are typically dual-plugged, that is, each cylinder has two spark plugs, with each plug having its own separate magneto. This arrangement provides redundancy in the event of a failure of one of the magnetos. Two sparks provide for a more complete and efficient burn of the fuel mixture, and if placed far apart in the combustion chamber, decreases burn time, allowing higher engine speeds. For this reason, double-plugging is also used in high-performance automobiles and motorcycles.

In the type known as a shuttle magneto, the engine rotates a coil of wire between the poles of a magnet. In the inductor magneto, the magnet is rotated and the coil remains stationary.

On each revolution, a cam opens the contact breaker one or more times, interrupting the current, which causes the electromagnetic field in the primary coil to collapse. As the field collapses there is a voltage induced (as described by Faraday's Law) across the primary coil. As the points open, point spacing is such that the voltage across the primary coil will arc across the points. A capacitor is placed across the points to suppress the arc, set the amount of voltage across the primary coil, and to control the rate at which the electrical energy dissipates in the primary coil.

A second coil, with many more turns than the primary, is wound on the same The ratio of the number turns in the secondary winding to the number of turns in the primary winding, called the turns ratio,. When the voltage is distributed and dissipates across the primary coil, a proportional voltage is induced across the secondary winding of the coil. The turns ratio between the primary and secondary coil is selected so that the voltage across the secondary reaches a very high value, enough to arc across the gap 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 electrical 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.

Magnetos were also used to ring the bell in old telephones.

• Ignition system