Motor-car principles; the gasoline automobile

by Roger B. Whitman

FOUR-SPARK MAGNETO

All of the magnetos described have been of the type in which the armature revolves, and two ignitions are secured per revolution. These are known as two-spark magnetos, or crank-shaft speed magnetos, from the fact that for four cylinder engines the magneto runs at the speed of the crank shaft. This is in distinction to the Bosch four-spark, or cam-shaft speed, magnetos, in which the armature as well as the field is stationary. This magneto for its simplicity, freedom from trouble, and workmanship, has few equals for the high-tension ignition of an internal combustion engine.

Fig. 13.—Diagrams showing Position of Shield Revolving about Armature.

The armature in this magneto is of the usual type, and is stationary, with the armature neck in a vertical position. Around the armature, and between it and the pole pieces, revolves a soft iron shield in two sections, these being the length of the armature and the same width as the heads. As it revolves, it forms bridges between the pole pieces and the core of the armature, the lines of force flowing through it as well as through the armature. The diagrams of Fig. 13 show the positions of the shield as it revolves about the armature. In the first position, one segment of the shield forms a bridge between the pole piece and the upper armature head, while the other segment is bridging the space between the lower head and the other pole piece. When in this position, the lines of force flow through the neck of the armature and magnetize it so that it sets up its own field. When the shield revolves so that it covers the heads, the lines of force abandon the neck and flow between the pole pieces by the segments, the field established by the neck dying out. In the third position, the lines of force again flow through the neck, while in the fourth position, with the shield completing a half revolution, the lines of force again pass directly across. In this half revolution there are therefore two periods when a magnetic field forms around the armature neck and dies out, which will result in the induction of two currents in the winding of the armature. If the revolution is continued it will be seen that the same conditions are repeated, the positions of the two segments of the shield being reversed, and that one revolution of the shield about the armature will produce four waves of current in the winding. The magneto may therefore be driven at cam shaft speed for a four cylinder engine, and in addition has the advantage of the wire windings being stationary. The action of the magneto is the same as that of the two-spark type already described, but the construction is simplified because the slow speed of the shield will permit the secondary distributor to be attached directly to the armature shaft, the secondary shaft being done away with.

A diagram of the wiring is shown in Fig. 14. The primary wiring is grounded, and the live end brought out to the stationary contact point of the interruptor. The moving part of the interruptor is pivoted in the center, and the lower end, a polished steel knob, bears against the face of a disk that has four ribs running from center to edge. When the knob is on the space between two of the ribs, the interruptor closes the circuit, but as the disk revolves and a rib touches the knob, the arm moves on its pivot, and the circuit is broken.

Fig. 14.—Wiring Diagram, Four-Spark Magneto.

The grounded end of the secondary winding is attached to the live end of the primary winding, and its other terminal passes to a contact point carried on a hard rubber disk placed on the revolving shaft immediately in front of the interruptor. The diagram shows a face view of this distributor, as well as a side view. A ring-shaped plate is set on one side of this disk, and is in connection with a contact piece carried on the edge of the disk. A carbon brush to which the live end of the secondary winding is attached is kept pressed against the ring-shaped plate, and the secondary current is thus led to the contact piece on the edge of the disk. As the disk revolves with the shaft of the shield the contact piece comes into successive contact with four carbon brushes, which are connected to the spark plugs.

When the shield is in such a position that the lines of force flow through the neck of the armature, the interruptor is closed, but when the intensity of the current increases to the maximum the interruptor opens, breaking the primary circuit, and producing a rapid demagnetization of the armature core. This dying out of the field induces a powerful current in the secondary winding, which is led by the distributor to one of the spark plugs, where it jumps the gap and returns to the winding by the ground connection.