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Magneto
This article is about the engine component. For other uses of the term, see Magneto (disambiguation).
A magneto is a device used in the ignition system of gasoline-powered internal combustion engines to provide pulses of high tension current to the spark plugs. Once found in every gasoline-powered automobile, it is now confined mainly to lawnmowers, chainsaws, and internal-combustion aviation engines. It should be noted that magnetos and coils used in early internal-combustion engines used the term "tension" in lieu of the more modern term "voltage." The term "high tension" is equivalent to "high voltage."
History
The first person to develop the idea of a high-tension magneto was Andre Boudeville, but his design omitted a condenser (capacitor); Frederick Simms in partnership with Robert Bosch were the first to develop a practicable high-tension magneto. [1]
The magneto was introduced on the 1899 Daimler Phönix. This was followed by Benz, Mors, Turcat-Mery, and Nesseldorf,[2] 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]
Operation
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 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.
Aviation
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 is still in use in many general aviation applications. 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.
Telephone
Many early manual telephones had a "Magneto" or hand cranked 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" dry cells.
See also
Notes
de:Zündung (Verbrennungsmotor) he:מגנטו pl:Iskrownik ru:??????? fi:Magneetto sv:Magnettändning uk:Магнето Bold text
| This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Magneto". |
