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Fig. 97:—Longitudinal Cross Section of K. W. Four-Cylinder Magneto
Using Stationary Windings and Rotary Inductor Members In
stead of Revolving Windings. safety valve of a steam boiler. If currents of sufficiently high voltage are produced by the armature to burn out or short circuit the winding it can escape to the ground by leaping across the air gap of the safety device.
Q. What is the difference between a magneto for a single cylinder engine and one intended for a multiple cylinder engine ?
A. The construction of a single cylinder magneto is considerably simpler than that intended for a multiple cylinder engine because the secondary distributor and its driving gears are eliminated and the high tension current is delivered directly from the collector brush that bears on the ring carried by the ebonite, fiber or hard rubber spool on the armature shaft. A multiple cylinder magneto
will have a distributor to which the high tension current is delivered which in turn is commutated to the various segments which deliver the current to the spark plug. The contact breaker of a one cylinder magneto employs a single point cam, whereas that of a multiple cylinder employs a two point cam.
Q. How fast are magnetos driven on two- and four-cycle engines?
A. The armature speeds are just twice as fast as the following values for a two-cycle engine having the same number of cylinders as the four-cycle types that follow. A single cylinder magneto turns at camshaft speed, a two cylinder magneto will be driven at either camshaft or crankshaft speed depending upon the arrangement of the contact breaker box and whether or not it is fitted with a distributor. A magneto intended for a three cylinder engine is driven
Fig. 99.—Longitudinal Cross Section and End View of Mea Magneto Which Employs Bell-Shaped Mag
nets and in Which the Spark is Advanced by Displacing Magnetic Field by Rocking the Magnets.
at three-quarters the speed of the crankshaft, while a four cylinder device is driven at the same speed as the crankshaft. Owing to the greater frequency in explosions in a six cylinder motor, it is necessary to turn the magneto armature at 1.5 times the crankshaft speed.
Q. How are distributor contacts spaced?
A. The contact segments in a secondary distributor are spaced justithe same as those of a primary timer when used on the same type of engine.
Q. Why must magneto speed and distributor arrangement be changed for the different number of cylinders?
A. As the number of cylinders are increased the number of explosions needed each revolution of the crankshaft augments proportionately, so it is necessary to drive the magneto faster because a spark is obtained but twice each revolution of the magneto armature. On a four cylinder engine, which has two explosions per revolution of the crankshaft, the magneto armature is driven at the same speed as the crankshaft, because it is capable of supplying a spark each half revolution of the armature. On a six cylinder engine where the crankshaft receives three power impulses per revolution it is necessary to drive the magneto at a speed that will enable it to deliver three sparks for each revolution of the crankshaft, which means that the magneto armature must turn half again as fast as the motor shaft. Obviously, the distributor contacts must be changed in number on each size of magneto because one contact segment is used for each cylinder to be fired.
Q. How are magnetos driven?
A. Magnetos of the high tension type used for ignition purposes are always positively driven by chain or gear connections with the engine crankshaft or camshaft.
Q. Why are positive driving means essential?
A. The magneto armature must be driven positively because the current generation must be timed to occur at the proper time for ignition in the engine cylinders and this makes it imperative that the drive be by some non-yielding positive means in order to