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

(102) The Mea High Tension Magneto.

Fig. 110. Diagram of Oscillating Magneto, Showing Cam and Trigger Arrangement.

The low tension winding of the ordinary type of magneto is short-circuited by a breaker which opens at certain points of each revolution with the result that a high voltage is generated across the high tension winding at the moment of the break, and a spark produced across the spark gap in the cylinder to which it is connected. The quality of this spark, or in other words the heat value, depends among other factors upon the particular position of the armature in relation to the magnetic field at the moment the spark is produced. As the armature in this type of magneto is in a favorable position for obtaining a spark twice every revolution, two sparks can be obtained per revolution. The timing of the spark is accomplished by opening the breaker earlier or later, by shifting the breaker housing naturally with the unavoidable result that if the position of the magnetic field remains stationary, the relative position between armature and field at the moment of the break must vary. Since, however, as explained above, the quality of the spark depends upon this relative position, it is apparent that a good spark, can, with a stationary magnetic field, be produced only at one particular timing.

Fig. 111. Side Elevation of “Mea” Magneto, Showing the Magnets, and Cradle in Which the Magneto Swings When Advanced and Retarded.

Fig. 112. Longitudinal Section of “Mea” High Tension Magneto.

The result of these conditions are known to everybody familiar with automobiles. They are the difficulty of cranking a motor on one of the average high tension magnetos, if the spark is fully retarded, and of operating the motor on the magneto at very low speed, particularly when it is overloaded, as for example, in hill climbing. Attempts have therefore been made to obtain the spark, independent of the timing, always at the same favorable position of the armature.

The distinct innovation and improvement incorporated in the Mea magneto consists in bell shaped magnets (Fig. 111) placed horizontally and in the same axis with the armature, instead of the customary horse-shoe magnets placed at right angle to the armature.

This at once makes possible and practicable the simultaneous advance and retard of magnets and breaker instead of the advance and retard of the breaker alone as the magnets may be moved to and fro with the breaker housing. It will be seen that as a result of this new departure the relative position of armature and field at the moment of sparking is absolutely maintained, and the same quality of spark is therefore produced, no matter what the timing may be. Furthermore, the range of timing, which with the horse-shoe type of magneto is limited to 10° or 15° at low speeds (i. e. at speeds at which a retarded spark is of value) becomes limited only by the necessity of supplying a suitable support for the magnets. With the standard types of Mea magnetos described in the following, this range varies from about 45° to 70°, but if necessary this range can be increased to any amount desired.

The bell-shaped magnets are fixed to the casing which is mounted on a base supplied with the magneto. The timing is altered by turning the casing and magnets together on the base.

Fig. 112 shows a longitudinal section of a four cylinder Mea magneto. The armature F with the ball bearings 17–18 rotates in the bell-shaped magnets 100, the poles of the magnets being on a horizontal line opposite the armature 1. The armature is of the ordinary H type iron core wound with a double winding of heavy primary and fine secondary wire. On the armature are mounted the condenser 12, the high tension collector ring 4, and the low tension circuit breaker 26–39.

The circuit breaker consists of a disc 27 on which are mounted the short platinum 33, the other contact point 34 is movable and is supported by a spring 30 which is fastened to the insulated plate 28 mounted on disc 27. Fiber roller 31 in connection with cam disc 40 which is provided with two cams is located inside the breaker. Revolving with the armature the roller presses against the spring supported part of the breaker whenever it rolls over the two cams which of course is twice per revolution.

Magneto of Roberts Motor in Advanced Position.

112-a. Advance and Retard Mechanism Used in the Roberts Motors. The Magneto is Driven by a Helical Gear from the Small Pinion. By Shifting the Gear Back and Forth on the Pinion, the Armature of the Magneto is Advanced or Retarded in Regard to the Piston Position. The Reason for this Change Will be Seen from the Cuts by Noting the Position of the Lower Helix.

Inspection of the breaker points is made easy by an opening in the side of the breaker box. The box is closed by a cover 74 supporting at its centre the carbon holder 47 by means of which the carbon 46 is pressed against screw 24. This latter screw connects with one end of the low tension winding while the other end is connected to the core of the armature. It will, therefore, be seen that the breaker ordinarily short-circuits the low tension winding and that this short-circuit is broken only when the breaker opens; it will also be apparent that when the screw 24 is grounded through terminal 50 and the low-tension switch to which it is connected, the low-tension winding remains permanently short-circuited, so that the magneto will not spark. The entire breaker can be removed by loosening screw 24.

The high tension current is collected from collector ring 4 by means of brush 77 and brush holder 76, which are supported by a removable cover 91 which also supports the low tension grounding brush 78 provided to relieve the ball bearings of all current which might be injurious. Cover 91 also carries the safety gap 89 which protects the armature from excessive voltages in case the magneto becomes disconnected from the spark plugs.

The distributor consists of the stationary part 70 and the rotating part 60 which is driven from the armature shaft through steel and bronze gears 7 and 72. The current reaches this distributor from carbon 77 through bridge 84 and carbon 69. It is conducted to brushes 68 placed at right angles to each other and making contact alternately with four contact plates embedded in part 70. These plates are connected to contact holes in the top of the distributor, into which the terminals of cables leading to the different cylinders are placed.

In the front plate of the magneto is provided a small window, behind which appear numbers engraved on the distributor gear which correspond to the number of the cylinder the magneto is firing. This indicator is of great value as it allows a setting or resetting after taking out, without the necessity of opening up the magneto to find out where the distributor makes contact.

The magneto proper is mounted in the base 53 which is bolted to the motor frame and the arrangement is such that the magneto can be removed from its base by removing the top parts 60a and 60b of the two bearings. The variation in timing is affected by turning the magneto proper in the stationary base which is accomplished by the spark lever connections attached to one of the side lugs 88. The spark is advanced by turning the magneto opposite to rotation and is retarded by turning it with rotation. One cylinder magnetos are similar to the four cylinder except that the distributor and gears are omitted.