A considerable number of articles have been written on ignition systems, with the object of explaining the operation of the magneto and battery systems. However, the writer will attempt to cover this subject in such a way as to enable the laymen to acquaint themselves with the action of the magnetos on commercial car motors without mastering electrical theories. The subject is a broad one, and even the following may seem unnecessarily long, but it should be remembered that a magneto is a complicated instrument and any description in few words must necessarily be superficial and can be of little value.
For ignition purposes the magneto is at present considered the most efficient device. There are three main types of magnetos on the market. First, the low-tension magneto, with step-up coil, furnishing a jump spark. Second, the low-tension magneto in connection with the make and break system. Third, the high-tension magneto, furnishing a jump spark.
The first system is largely used, having the advantage of low first cost, but necessitates considerable complications and in reality is a high-tension system; as the spark is made to jump across the air gap between the spark plug electrodes, with the use of the step-up coil. The air which lies between the two electrodes is a non-conductor and is an obstacle to the flow of electrical current, so that a very high electrical pressure is needed to send a spark through it; therefore, all different methods which involve a spark plug may be called high-tension systems. The ultimate object of all ignition systems is to get a spark between two conductors located in the engine cylinders.
The second system is very rarely used on commercial cars at present, being discarded owing to its mechanical complications and the large amount of attention required to keep it in working conditions. In this system the spark plugs are discarded and two conductors are so arranged that they actually touch each other in the cylinder. One conductor is fixed and insulated from the cylinder, while the other is arranged to be moved by a mechanical mechanism, so that a gap is created, through which the spark passes.
The third system is now the most popular, as the high tension is produced directly in the armature winding of the magneto, without the use of a coil or other auxiliary. The wiring is also the simplest, as it consists of high-tension wires to each spark plug and a low-tension wire to the switch on the dash.
The principal difference between the high-tension and the low-tension ignition systems is, that in the high-tension system there is an incomplete circuit and a spark must be driven across an air gap; whereas, in the low-tension system an electric current already flowing in a conducting circuit is driven by its own momentum across an air gap which is suddenly formed on it.
For high-tension, what is needed is a very hot spark of high frequency, positive and of a certain length to penetrate the air space between the electrodes of the spark plugs, which is generally from 1/64 to 1/32 in.
Magnetos are classified in two groups, according to the basic principles employed in the magnetic field to generate the initial electrical impulses. These two classes are known as the armature and conductor type.
In the former, electrical current is generated by revolving several thousand feet of fine copper wire, which is wound around a soft iron core, between the pole pieces of the magneto. As the winding rotates within its narrow confines, electrical impulses are set up within the winding.
The above types may again be divided into two classes. One is called the primary armature magneto and the other is called the compound armature type. The primary type has but a single winding in the magnetic field and generates a low voltage current. It requires an outside transformer coil to step up the current to the high potential required at the spark plug. The compound armature type incorporates a second winding, or secondary winding, also upon the armature shaft. In addition, a condensor must be incorporated in the magneto.
The inductor type consists of revolving a solid steel shaft, upon which are mounted two steel fan-shaped inductor wings, within a stationary winding in the magnetic field.
The high-tension magneto is not only a current generator, or a substitute for the battery, but combines all the elements of a complete ignition system except the spark plugs and the switch. It generates the current, transforms it to a high pressure and distributes the high-tension current to the cylinders.
The structural portion of the magneto consists of permanent magnets of inverted U-shape, sometimes referred to as horseshoe magnets. Two such magnets are generally used on the smaller magnetos, while four are used on the larger ones. The free ends of these magnets are termed the poles, one as the north and the other as the south pole. To these poles are secured soft iron blocks, known as pole pieces or pole shoes. The magnets and pole pieces are mounted upon a non-metallic base, while the pole pieces are bored out cylindrically to receive the armature, which is of substantially cylindrical form. This armature consists of a soft iron core of II section, and serves to form a bridge for the magnetic flux between the pole pieces and also to carry the winding in which the current is induced. After the core has been properly insulated, several layers of heavily insulated wire are wrapped around it. To the end of this heavy wire the beginning of a very fine silk-insulated wire is connected and the wire is wound on the core until the slot is almost filled. After the outer insulating cloth is in place, bands are put around the circumference of the armature to hold the winding in place under high armature speeds. The end plates which form the shafts are then attached.
The heavy or primary winding serves primarily to generate the current, and in connection with the fine or secondary winding, also serves to multiply the pressure or voltage to such an extent that it will produce a spark between the electrodes of the spark plug in the cylinder.