| Magneto (electrical) |
Article Index for Magneto |
Information AboutMagneto (electrical) |
|
A magneto provides pulses of electrical power to the Spark Plug s in some gasoline-powered Internal Combustion Engine s where batteries are not available, most commonly those in 2-stroke and 4-stroke engines used in small Motorcycles , Lawnmower s and Chainsaw s. Magnetos were also used on most cars up until about 1918 in both low voltage (that would create voltage for secondary coils to fire the spark plugs) and high voltage magnetos that fire the spark plug directly 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; their simplicity and self-contained nature means greater reliability as well as lighter weight in the absence of a battery and generator or alternator. Magnetos combine the functions of a Dynamo , Contact Breaker points, capacitor and Coil in one unit. The engine rotates a coil of wire between the poles of a permanent Magnet to provide a basic source of electrical energy, called a shuttle type magneto (though in some variants the Permanent Magnet is rotated and the coil remains stationary) this type is called an inductor type of magneto. 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 generated across the primary coil. As the points open for a moment the point spacing is such that the voltage across the primary coil will arc across the points. A capacitor is placed across the points to stop the arcing, set the amount of voltage across the primary coil and to control the rate at which the electrical energy disapates across the primary coil. A second coil is wound so that it is as close as practical to the primary coil and the ratio of the number turns in the secondary coil is several times the number of turns in the primary coil. 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 potential, 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 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. SEE ALSO |
|
|