Like all other types of high tension magnetos, the Bosch Type ZR. Ed. 16 explained in this chapter, generates its own current and is usually employed as sole ignition on an engine.

The timer and distributor are integral; and the rotation of the armature, between the poles of strong permanent field magnets, sets up or induces a current in the armature primary circuit, which is farther augmented at every one hundred and eighty degrees of revolution of the armature shaft, by the abrupt interruption of the primary circuit by means of the magneto interruptor. At the opening of the primary circuit the resulting discharge of current from this circuit induces a current of high voltage in the armature secondary circuit. The high tension current thus created is collected by a slip ring on the armature and passes to the slip ring brush then to the various magneto distributor terminals each of which is connected to a spark plug in its respective cylinder.

The operation of the instrument will be more clearly understood from a study of the complete circuits, primary and secondary, which follows.

The Primary or Low Tension Circuit.—The beginning of the armature primary circuit is in metallic contact with the armature core, and the end of the primary circuit is connected by means of the interruptor fastening screw to the insulated contact block supporting the long platinum contact on the magneto interruptor. The interruptor lever carrying a short platinum contact, shown in Fig. 60 at C is mounted on the interruptor disc, which in turn, is connected to the armature core. The primary circuit is completed whenever the two platinum contacts of the interruptor are brought together, and separated whenever these contacts are separated.

From the latter point the high tension current passes to the distributor brush (shown at D) which is held in a brush holder on the distributor gear, and consequently rotates with the distributor gear. Metal segments are imbedded in the distributor plate and as the distributor brush rotates it makes successive contacts with the segments, passing the current onto the spark plug gaps through the high tension cables which are attached to the segment terminal posts.

Fig. 61 shows a circuit diagram of the Type ZR. Ed. 16. Bosch Magneto. Note that the spark plugs must be connected up in accordance with the firing order of the engine. The metal segments imbedded in the distributor plate are connected with the terminal studs on the face of the plate, and the latter are connected by cable to the spark plugs in the various cylinders. In the cylinders the high tension current produces a spark which produces ignition, and then returns through the ground and the engine to the magneto armature, thus completing the circuit.

Timing the Magneto.—With the average four cycle engine the proper operating results are obtained by timing the magneto as follows: The crank shaft is rotated to bring the piston in No. 1 cylinder (in automobile practice this is the cylinder nearest the radiator) exactly on top dead center of the compression stroke. The timing control lever on the housing is then placed in the fully retarded position. With this done, the magneto distributor plate should be removed by withdrawing the two holding screws, or by releasing the two holding springs as the case may be.

The operation of the platinum contact points is controlled by the action of the interruptor lever as it bears against the two steel segments secured to the inner surface of the interruptor housing.

In Fig. 60, A shows the distributor with the face plate removed to show the position of the distributor segments which are connected to the terminal posts on the back of the plate. B shows the interruptor housing and cover removed from its position on the magneto. C shows the complete assembly of the distributor and interruptor. Note that the face plate of the distributor is fastened on with a set of screws while the interruptor cover is held in position with a latch.

The Secondary or High Tension Current.—The high tension current is generated in the secondary circuit only when there is an interruption of the primary circuit, the spark being produced at the instant the platinum interruptor contact points separate. The armature secondary circuit is a continuation of the armature primary circuit, the beginning of the secondary circuit being connected to the primary, while the end of the secondary is connected to the insulated current collector ring mounted on the armature just inside the driving shaft end plate of the magneto. The slip ring brush is held in contact with the slip ring by a brush holder at the shaft end of the magneto which receives the high tension current collected by the slip ring by means of a connecting bar which passes under the arch of the magnets, and passes the current to the center of the distributor plate, thus exposing the distributor brush and gear. The cover of the interruptor housing is also to be removed to permit observation of the interruptor points.

The armature should then be rotated by means of the exposed distributor gear in the direction in which it is driven until the platinum contact points are about to separate, which occurs when the interruptor lever begins to bear against one of the steel segments of the interruptor housing. Timing or installation is completed by replacing the interruptor housing cover and distributor plate, and connecting the cables between the magneto and the spark plugs.

Exact Magneto Timing.—The foregoing will establish the desired relationship between the magneto armature shaft and the engine crank shaft. It should be noted, however, that while these instructions cover the average engine, the exact magneto timing for individual engines is best determined by trial.

When specific instructions for magneto timing are given by the engine manufacturer, it is recommended that such instructions be followed in preference to those herein given.

It must always be borne in mind that while making connections the distributor brush travels in the opposite direction to the rotation of the armature shaft.

The Condenser.—The condenser consists of a set of metal discs, insulated from one another with tin foil. It is carried at the interruptor end of the magneto. It is connected in the primary current and forms a shunt connection with the interruptor contact points, and through resistance to the returning ground current prevents excessive sparking at the interruptor contact points which would soon burn the points and ruin the coils.

The Safety Spark Cap.—A safety spark cap is provided to protect the armature and other current carrying parts. Under normal conditions the current will follow its path to the spark plug, but if for any reason the resistance in the secondary wire is increased to a high point, as when a cable becomes disconnected, or a spark gap too wide, the high tension current will discharge across the safety spark gap.

Caution.—The current should never be allowed to pass over the safety spark gap for any length of time, and if the engine is operated on a second or auxiliary ignition system, the magneto must be grounded in order to prevent the production of high tension current. The snapping sound by which the passage of current across the safety spark gap may be noted should always lead to an immediate search for the cause of the difficulty.

The Safety Spark Gap.—The safety spark gap consists of a pointed metal electrode projecting from the mounting flange of the slip ring holder, inside the shaft end hood. The tip of the electrode extends to within a short distance of the connecting bar, extending from the brush holder to a magneto distributor plate center post.

Timing Range.—The magneto interruptor housing is arranged so that it may be rotated through an angle of thirty-four to thirty-seven degrees with respect to the armature shaft. The movement of this housing in one direction or another causes the interruptor lever to strike the steel segments earlier or later in the revolution of the armature, the spark occurring correspondingly earlier or later in the cylinder. The spark can be advanced by means of moving the interruptor housing which is connected to the spark lever on the steering gear, in the direction opposite the rotation of the armature. The armature rotation is usually indicated by an arrow on the cover at the driving end of the magneto.

Cutting Out Ignition.—Since a high tension current is generated only on the interruption of the primary circuit, it is evident that in order to cut out the ignition, it is merely necessary to divert the primary current to a path that is not affected by the action of the magneto interruptor. This is accomplished as follows: An insulated grounding terminal is provided on the cover of the magneto interruptor housing with its inner end consisting of a spring with carbon contact pressing against the head of an interruptor fastening screw. The outer end of the grounding screw is connected by low tension cable to one side of the switch, and the other side of the switch is grounded by connecting a cable between it and the engine or frame. When the switch is open the primary current follows its normal path across the interruptor points, and is interrupted at each separation of these contact points. However, when the switch is closed, the primary current passes from the head of the interruptor fastening screw to the carbon contacts of the grounding terminal, thence through the switch to the engine and back to the magneto, and as the primary current remains uninterrupted when following this path, no ignition current is produced.

Care and Maintenance.—Aside from keeping the magneto clean externally, practically the only care required is the oiling of the bearings. Of these there are two sets supporting the armature, and a single plain bearing supporting the shaft of the distributor gear. Any good light oil may be used for this purpose (never cylinder oil), and each of the bearings should receive not more than two or three drops about every thousand miles. Apply the oil through the oil ducts at each end of the armature shaft. The interruptor is intended to operate without oil, as oil on the interruptor platinum points prevents good contact, and causes sparking, burning, and misfiring. Care should be taken to prevent oil entering these parts.