Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(101) Bosch Oscillating High Tension Magneto.

The oscillating type of magneto is used on slow speed heavy duty engines that move too slowly for the ordinary type of magneto. In the oscillating type the armature is given a short angular swing by the action of a tripping device operated by the engine which results in an intense spark at the lowest speeds.

Magneto type “29” is constructed with two powerful steel magnets, while magneto type “30” is provided with three; an armature of the shuttle type is arranged to oscillate between their pole-shoes.

The magneto is actuated by a rotating cam or other suitable device, which moves the armature 30° from its normal position whenever ignition is required. To permit this movement, a trip lever is mounted upon the tapered end of the armature shaft, this trip lever being held in a definite position by the tension of the spring or springs 1. The trip lever is only supplied when specially ordered, but each magneto is provided with the necessary springs and spring bolts.

When the trip lever is moved from its normal position by the operating mechanism, the springs are extended, and when the operating mechanism releases the trip lever, the later returns the trip lever and armature to their normal position, this movement resulting in the production of a sparking current in the armature winding.

The winding of the armature is composed of two parts, one being the primary winding, which consists of a few turns of heavy wire, and the other the secondary winding, which consists of many turns of fine wire.

The tension of the current produced by the oscillation of the armature is increased by closing the primary circuit at a certain position in its movement, and then interrupting it by means of the breaker. At the moment of the interruption, an arc-like spark is formed at the spark plug and ignition occurs.

Fig. 109. Elevation of Bosch Oscillating Magneto for Slow Speed Engines. High Tension Type.

On cam shaft (c) two cams are mounted side by side. One of these cams (a) is to be used for starting the motor, or for the retarded spark position, while the second (b) is to be used for operation, or for the full advance position. These cams are mounted on a sleeve, which may be moved longitudinally on the shaft, so that the trip lever may be operated by cam (a) or cam (b) as desired. The sleeve is caused to rotate with the shaft by a key. Between the cam (b) and a fixed collar (f) a spiral spring is arranged, which tends to maintain the sleeves in the position when the cam (b) is in operation. A stop collar is also provided to limit the movement of the sleeve beyond this full advance position. Over this collar is fitted a hand wheel, which, in the position illustrated in the diagram, acts together with the collar as a stop. Around the collar is a circular key-way, and a brass bolt is located in the hand wheel to lock into this key-way when the hand wheel is pushed into the position indicated by the dotted lines. This movement of the wheel forces the cam sleeve forward, and brings the retarded cam (a) into the operating position to permit the engine to be started.