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Renault 190HP magneto

A magneto is an electrical generator that uses permanent magnets to produce pulses of high voltage alternating current. At one time the magneto was used in the ignition system of most gasoline-powered internal combustion engines to provide power to the spark plugs.[1] The magneto is now confined mainly to engines where there is no available electrical supply, for example in lawnmowers and chainsaws. It is also universally used in aviation engines even though an electrical supply is usually available. This is because a magneto ignition system is more reliable than a battery-coil system. People discussing magnetos and coils used in early internal-combustion engines generally used the term "tension" instead of the more modern term "voltage."



The first person to develop the idea of a high-tension magneto was Andre Boudeville, but his design omitted a condenser (capacitor); Frederick Richard Simms in partnership with Robert Bosch were the first to develop a practicable high-tension magneto.[2]

The magneto was introduced on the 1899 Daimler Phönix. This was followed by Benz, Mors, Turcat-Mery, and Nesseldorf,[3] and soon was 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).[3]

The magneto also had a medical use on some mind illness in the beginnings of electromedicine. In earliest 1850, Duchenne, a French medic, developed and manufactured a magneto with a variable outer voltage and frequency, through varying revolutions by hand or varying the inductance of the two coils, putting out or putting in both ferromagnetic cores.

One popular and common use of magnetos of today is for powering lights on bicycles. A small magneto is mounted on the wheel of the bicycle and generates power as the wheel turns.


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 iron core to form an electrical transformer. The ratio of turns in the secondary winding to the number of turns in the primary winding, is called the turns ratio. Voltage across the primary coil results in a proportional voltage being 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.

In a modern installation, the magneto only has a single low tension winding which is connected to an external ignition coil which not only has a low tension winding, but also a secondary winding of many thousands of turns to deliver the high voltage required for the spark plug(s). Such a system is known as an "enegy transfer" ignition system. Initially this was done because it was easier to provide good insulation for the secondary winding of an external coil than it was in a coil buried in the construction of the magneto (early magnetos had the coil assembly externally to the rotating parts to make them easier to insulate - at the expense of efficiency). In more modern times, insulation materials have improved to the point where constructing self contained magnetos is relatively easy, but energy transfer systems are still used where the ultimate in reliability is required such as in aviation engines.


Because it requires no battery or other source of energy, the magneto is a compact and reliable self-contained ignition system, which is why it remains in use in many general aviation applications.

Magneto-equipped aircraft engines are typically dual-plugged; each cylinder has two spark plugs, with each plug having a separate magneto system. This provides better engine performance and also redundancy in the event of a failure of one of the magnetos. Two sparks provide two flame fronts within the cylinder. The two flame fronts decrease the time needed for the complete fuel charge to start burning, and therefore most of the fuel is already burning at a lower temperature and pressure. As the combustion pressure rises within a single plug cylinder, lower octane portions of the fuel mixture far from the original flame front can explode, igniting another flame front elsewhere in the cylinder. This leads to engine knock. Two flame fronts can help to decrease the octane requirement for an engine. This was important for the large bore size of most aircraft engines around World War II.


Some aviation engines as well as some older luxury cars have had dual-plugged systems with one set of plugs fired by a magneto, and the other set wired to a coil, dynamo, and battery circuit. This was done to improve engine efficiency without sacrificing reliability. Magnetos were once considered a more reliable ignition source, but have the disadvantage of having fixed timing. This means that the timing must be a compromise setting, which is neither the best for low RPM nor the best for high RPM. On the other hand, battery ignition systems have almost always had a timing advance system which can set the timing to the best setting for the speed the engine is turning, improving power output and fuel efficiency. As the reliability of battery ignition systems improved, the magneto fell out of favor for automotive use.

Modern engines have much smaller combustion chambers and less opportunity for knock given their modern combustion chamber design, which attempts to eliminate "hot" areas of the cylinder. These modern chambers decrease octane demand by their engineered shape and swirl/tumble of incoming charge. Therefore, they no longer require double plugs to decrease octane demand. Harley Davidson motorcycle engines, being single-pin, air-cooled, and having a large "antique" combustion chamber design, can still benefit from double plugging. The same holds true for most drag racing engines where the volume and type of fuel (nitromethane) and engine design (mostly based on the 1960s Chrysler Hemi engine) requires additional flame centers in order to use the massive amount of fuel injected during the intake cycle.

1896 Telephone, hand cranked magneto on right (Sweden)


Many early manual telephones had a hand cranked "magneto" generator to produce a (relatively) high voltage alternating signal to ring the bells of other telephones on the same (party) line and to alert the operator. These were usually on long rural lines served by small manual exchanges, which were not "common battery". The telephone instrument was "local battery", containing two large "No. 6" carbon-zinc dry cells.

See also


  1. ^ Selimo Romeo Bottone (1907). Magnetos for Automobilists, how Made and how Used: A Handbook of Practical Instruction in the Manufacture and Adaptation of the Magneto to the Needs of the Motorist. C. Lockwood and son.  
  2. ^ Kohli, P.L. (1993). Automotive Electrical Equipment. Tata McGraw-Hill. ISBN 0074602160.  
  3. ^ a b Georgano, G.N. Cars: Early and Vintage, 1886-1930. (London: Grange-Universal, 1985).


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