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

(100) High Tension Magnetos.

The “true” high tension type magneto is complete in itself, requiring no jump spark coil nor timer, the high tension current being generated directly in the coils carried by the armature. This arrangement reduces the wiring problem to a minimum, as the only wires required are those leading directly to the spark plugs, and one low tension wire connecting the cutout switch used for stopping the engine.

Fig. 105. Single Cylinder High Tension Bosch Magneto.

The armature of this type of magneto carries two independent windings, one of a few turns of coarse wire called the primary coil, and the other consisting of thousands of turns of extremely fine wire called the secondary coil. It is in the latter coil that the high tension current is generated. The timer is connected directly to the armature shaft, and is an integral part of the magneto. All primary connections are therefore made within the magneto.

Belts or friction drives cannot be used with this type of magneto.

As there are no vibrators or independent coils used, the spark occurs exactly at the instant that the timer operates or breaks the primary circuit. It will be noted that the spark is produced with this magneto when the primary circuit is broken by the timer, instead of made as is the case with battery coils, or coils used with low tension magnetos. There is no lag and consequently the time of ignition is not affected by variations in the engine speed, which requires an advance and retard of the spark with batteries and vibrator coils.

When used with multiple cylinder engines the high tension magneto is provided with a distributor, which connects the high tension current with the different cylinders in their proper firing order. The timer determines the time at which the spark is to occur and the distributor determines the cylinder in which the spark is to take place.

Fig. 106. Connecticut High Tension Magneto.

The sparks delivered by the high tension magneto are true flames or arcs of intense heat, and exist in the spark gap for an appreciable length of time. It is evident that such flames possess a much greater igniting value than instantaneous static spark delivered by the high tension spark coil used with the battery or operated by the low tension magneto, and are capable of firing much weaker mixtures.

Like low tension magnetos, the true high tension type may be of either the inductor or shuttle wound class. All high tension magnetos are positively connected or geared to the engine in such a manner that there is a fixed relation between time of the current impulse produced by the magneto and the firing position of the engine piston.

The current is generated on the same principle as in the low tension shuttle type; that is, by a coil of wire revolving in the magnetic field established by permanent magnets.

During each revolution of the armature, two sparks are produced at an angle of 180° from each other.

The advance and retard of the spark is obtained by means of the timing lever which shifts the timer housing back and forth which results in the primary current being interrupted earlier or later in the revolution of the armature.

Fig. 107. Longitudinal Section Through Bosch High Tension Magneto.

The timing lever can turn through an angle of 40° measured on the armature spindle, and the angle of advance for multiple engines is as follows:

Advance for 1 cylinder	40°
Advance for 2 cylinders	40°
Advance for 3 cylinders	50°
Advance for 4 cylinders	40°
Advance for 6 cylinders	27°

A timer is used with the magneto on a “jump spark” system in the same way as with a battery, providing a vibrating coil is used.

In one type of magneto the Connecticut, the coil is part of the magneto, and is fastened to the magneto frame. This type of magneto uses a non-vibrating coil, and produces but a single spark each time the primary circuit is broken by the magneto timer. As the timer on this type is driven by the magneto shaft, it is evident that the magneto must be “timed” with the engine, or must have its armature shaft connected to the shaft of the engine in such a manner that the timer contact is broken, and the single spark produced at the instant that ignition is required in the cylinder.

Unlike the dynamo, the alternating current magneto cannot be used with a storage battery, the alternating current producing no chemical change in the electrodes of the battery.

Four Cylinder “D4” High Tension Bosch Magneto Showing Distributor.

The Bosch high tension magneto is a typical high tension magneto having the primary and secondary windings wound directly on the armature shaft, there being no external secondary coil. The end of the primary winding is connected to the plate (1) Fig. 107, which conducts the current to the platinum screw of the circuit breaker (3). Parts (2) and (3) are insulated from the breaker disc (4), which is in electrical contact with the armature core and frame. When the circuit breaker contacts are together the primary winding is short circuited, and when they are separated the current is broken and the spark occurs. The breaker contacts are simply two platinum pointed levers that are separated and brought together by the action of a cam as they revolve. A condenser (8) is provided for the circuit breaker to suppress the spark and to increase the rapidity of the “break.”

The secondary winding of fine wire is a continuation of the primary winding, and the secondary is wound directly over the primary. The outer end of the secondary connects with the slip ring (9) on which slides the carbon brush (10), which conducts the high tension current from the armature. This brush is insulated from the frame by the insulation (11). From (10) the current is led through the bridge (12) through the carbon brush (13) to the distributor brush (15). Metal segments are imbedded in the distributor (16), the number of which corresponds to the number of cylinders. As the brush rotates, it makes consecutive contact with each of the segments in turn and therefore leads the current to the cylinders in their firing order. Wires from the cylinders are connected to sockets that in turn connect with the segments. The disc driving the distributor brush (15) is geared from the armature shaft in such a way that the armature turns twice for every revolution of the distributor, when four cylinders are fired, and three times for the distributors once when six cylinders are fired.

Fig. 108. Bosch High Tension Circuit.

The voltage of the current generated in the secondary coil by the rotation of the armature is increased by the interruption of the primary circuit caused by the opening of the contact breaker.

At the instant of interruption of the primary circuit the high tension spark is produced at the spark plug.

As the spark must occur in the cylinder of the engine at a certain position of the piston, it is necessary that the interrupter act at a point corresponding to a definite position of the piston, consequently this type of magneto must be driven positively from the motor by means of gears, or directly from the shaft.

These magnetos run in only one direction. This running direction should be given when magneto is ordered, as being “clockwise” or “counter-clockwise” when looking at the driving end of the magneto.

The magneto for the single and double cylinder engines has no distributor, the high tension current being led directly from the armature.

The circuit diagram of the Bosch four cylinder magneto is shown by Fig. 108, the winding and plug connections being clearly shown. When connecting the magneto care should be taken to have the distributor and plug connections arranged so that the cylinders will fire in the proper order.