MAGNETO, HIGH-TENSION. The magneto is a special application of the electric generator employed where the output of energy required is very small. It is used to furnish energy for ignition of compressed gases in various types of internal combustion engines. The ele ments of construction comprise a permanent magnetic field, an ar mature, rotated within that field, a circuit breaker and a distrib uting mechanism which serves to convey the generated voltage to a desired point. The low-tension magneto consists merely of a per manent magnet, of inverted U form, and, rotating between its poles, a shuttle-wound armature containing a number of turns of comparatively coarse wire, as shown in fig. I. The changes of flux in the armature core, due to its rotation in the permanent field, generate in the armature winding an electromotive force, the magnitude of which is pro portional to the field strength, the speed of rotation, and the number of turns of wire on the armature. As the armature core rotates into a position in a plane with the flux lines, the flux in the core increases to a maximum. As the rotation progresses, the flux in the core decreases until one-half cycle is completed. In the second half cycle, the coil direction is reversed with relation to the flux.
The generated electromotive force, consequently, is alternating in direction and magnitude, and follows the variation of flux in the core. The change of electromotive force with change of position of the armature is shown in fig. 2. The low-tension mag neto has been used in conjunction with an induction coil as an ignition source, the coil serving to transform the voltage to a value sufficient to cause a spark across the terminals of a spark plug.
Fig. 3 shows the construction of a shuttle-armature type of high tension magneto. The circuit breaker is operated by means of a cam, upon which the bumper or cam follower on the breaker bar, bears. Since the armature is directly connected to the engine, the frequency of the discharges through the secondary circuit is thus determined. (A shuttle-armature, having two poles, can generate two spark discharges per revolution.) The distributor serves to convey the voltage to each of the spark plug cables, in a desired sequence. Operating in synchronism with the armature, from
which it is driven by means of a gear and pinion, the brush or firing pin of the distributor arm makes contact with a segment in the distributor block each time the secondary discharge occurs. By this means, the high-tension energy is carried to each spark plug in order. For a four cylinder engine, the distributor block carries four segments, and since the shuttle-armature generates two sparks per revolution, the gear ratio between armature and distributor is 2 :I. The primary and secondary windings are com mon at one point and are there connected to the frame of the magneto. The return circuit of the secondary is thus completed through the body of the engine. The contact points of the cir cuit breaker, or interrupter, are made of an arc-resistant alloy.
A recent improvement in magneto design is offered in the induc tor type instrument in which the coil is stationary and the flux changes are brought about by means of magnetic shunt segments which rotate between the magnet poles and bridge between them and the coil core. A cross-sectional drawing of this type is shown in fig. 4. By the use of four rotor segments, spaced 9o° apart, four sparks per revolution may be generated. In addition, the flux direction through the coil core is reversed each quarter revo lution. The direction of current through the contact points of the interrupter is thus alternated, and the excessive point pitting, due to a unidirectional arc is eliminated.
In order to vary the time of occurrence of the spark in the engine cylinder, the magneto cam, or the circuit breaker, is made rotatable through part of a revolution, and is connected, by means of a linkage, to a convenient advance-retard lever. This makes it possible to alter the relationship between the engine crank shaft and the magneto, and to ignite the charge either before or after the piston has reached its dead centre position. Since the spark intensity of a magneto varies directly with engine speed, there is available an increase of energy at high speed when the require ments for ignition are most severe. At low speeds of rotation such as accompany the cranking or starting of automotive engines, the magneto will sometimes fail to produce a voltage sufficient to spark across the gap. There have been in use various impulse devices, such as "impulse couplings," which serve to couple the magneto to the engine and to accelerate its rotation in starting.
The magneto is employed on aircraft engines, where the highest engine efficiency must be maintained; on tractor engines, motor cycles, marine engines, and to a great extent on motor-buses and motor trucks. Stationary engines are also magneto-equipped. Minor commercial uses are found in the igniting of blasting charges (detonators) ; in igniters, for oil or gas burners; and, in conjunction with neon lamps, as stroboscopic indicating instru ments. The total world production of magnetos for all uses, in the year 1927, was in excess of 525,000. (See also INDUCTION