Motor-car principles; the gasoline automobile

by Roger B. Whitman

L. T. MAGNETO WITH SECONDARY COIL

The advantages of a magneto as a current producer for the low-tension system pointed the way to its adoption for the jump spark system also, but it was at once recognized that it would be impossible to lead the current direct to the primary winding, as the current from a battery is used. The reason for this is that while the battery delivers a current of constant value, the current obtained from a magneto is fluctuating, its intensity depending on the positions of the armature as it revolves, and the speed at which it is driven. A current is induced in the secondary winding of the coil as the magnetic field set up by the core changes its strength, and as has been explained, the induced current is strongest when the greatest change in the strength of the field occurs in the shortest possible time. The effect of the flow of the magneto current through the primary winding of the coil would be to magnetize and demagnetize the core slowly, as the magneto current increased to its maximum and died away to the minimum, and these gradual changes in the magnetic field of the core would induce currents in the secondary that would be too feeble to produce ignition of the charge. When a battery is used with a coil, the operation of the vibrator produces rapid magnetizations and demagnetizations of the core, but as the field set up by the core dies away more rapidly than it is established, the greatest current is induced by the breaking of the circuit. In the best known method of applying a magneto to the operation of a secondary coil, the magnetization of the core is caused to occur with exceeding rapidity, and the current induced in the secondary winding as this occurs is sufficient for ignition.

This is known as the Eisemann system, and is in very general use. A diagram illustrating the connections is shown in Fig. 9.

Fig. 9.—Eisemann Ignition System.

The magneto is of the usual type, but the winding is so proportioned that the current is of lower voltage than is delivered by what has been described as the low-tension type of magneto. One end of the winding is grounded on the metal of the armature, and the live end is carried to a contact screw. The current flows to this screw, and from there two paths are presented by which it may flow back and complete its circuit. One of these paths leads through the primary winding of a secondary coil, and the other through a lever that for the greater part of the revolution of the armature touches the contact screw. The lever is grounded on the metal of the magneto, and when it touches the contact the circuit that is then completed is short and of low resistance, and the current follows it in preference to the circuit of higher resistance through the primary winding of the coil. The flow of current is thus from the armature winding to the contact screw, which is insulated from the metal of the magneto, to the lever, and by the ground back to the winding. Attached to the end of the armature shaft, and so placed that it operates the lever, is a cam with two projections, these projections being arranged to move the lever away from the contact screw. This arrangement is called the interruptor, and by its operation it closes and opens the low-resistance circuit by which the magneto current may flow.

When the armature is horizontal, and during the time that it is approaching the vertical, the circuit is closed through the interruptor, but when the armature reaches the position in which it gives its maximum current, the interrupted is opened by the cam, and the current, losing its low-resistance circuit, is required to flow through the primary winding of the coil, for that is the only path by which it can return to the armature winding. The sudden flow of this current through the primary winding results in the production of a powerful magnetic field around the core, and this rapid growth in the strength of its field induces a current in the secondary winding that is sufficient for the ignition of the charge. As the magneto current is at its maximum twice during each revolution of the armature, the cam is arranged to open the low-resistance circuit at such periods that each maximum is required to flow through the primary of the coil.

In applying this system to a multicylinder engine, it is necessary to distribute the secondary current to the different cylinders, so that a spark will be produced in each as it is required. This is accomplished by means of a secondary distributor, consisting of a revolving conductor to which the secondary current is led, and a stationary contact for each cylinder arranged in a circle, to be touched as it revolves. These stationary contacts are connected with the spark plugs, according to the firing order of the engine. During one revolution of the distributor, a spark is produced in each of the cylinders, and as two revolutions of the crank shaft are necessary in order to have power strokes occur in all of the cylinders, the distributor must revolve at half the speed of the crank shaft. The magneto produces a current that is sufficient for ignition twice during each revolution of the armature, and therefore, for four cylinder engines, must run at the speed of the crank shaft. For the sake of compactness, the distributor is built into the magneto, its shaft being carried in bearings that support it above the armature and under the arch of the field magnets. It is geared to the armature so that the two revolve in a fixed relation to each other, and in order that the distributor may run at half the speed of the armature, its gear has twice the number of teeth of the gear on the armature shaft that drives it.

The secondary induction coil used is similar in construction to the coils used for ignition by battery, except that it is heavier, and has no vibrator. The period during which the magneto is delivering its maximum current is so brief that there would hardly be time for a vibrator to get into action, and even if this were not the case, the making and breaking of the circuit by the vibrator is not necessary, as the single change in the strength of the magnetic field set up by the core is so abrupt that the current induced by it is sufficient for the purpose.

The strength of this secondary current is such that it is necessary to provide a circuit for it if by any accident or oversight the circuit through the distributor or spark plugs is interrupted, as would be the case should one of the secondary wires become disconnected. This is taken care of by what is known as the safety spark gap, which is a gap provided between the secondary terminals of the coil, or between the live end of the secondary winding and the ground. This safety spark gap, in the case of the coil furnished by the Eisemann company, takes the form of two flat pieces of brass, each in contact with one of the secondary terminals of the coil, their pointed ends being separated by a distance of about half an inch. When there is no interruption of the secondary circuit, and the parts of the secondary system do not present undue resistance to the flow of the current, the higher resistance of the air space between the points of the safety spark gap prevents the current from jumping between them, but should one of the secondary cables become disconnected, the pressure of the current will rise to a point that will enable it to jump the gap between the two points. If it were not for this safety gap, an interruption of the secondary circuit would make the pressure of the current rise to such an extent that there would be great danger of a spark passing inside of the coil, rupturing the insulation.

As the magneto is received from the makers, the parts will be in a fixed relation to each other; when the armature is vertical, and held in that position by the lines of force, the cam is in the act of opening the primary interruptor, and the revolving part of the secondary distributor has moved into contact with one of the stationary contacts about one thirty-second of an inch.

The interruptor of the magneto can be utilized as the timer for an ignition system by battery and coil, the secondary current being distributed to the cylinders by the distributor. A better arrangement, however, requires the use of two coils, and a timer which is carried on the rear end of the distributor shaft. The coil for the battery system is provided with a vibrator, and connections are made in the manner usual with a system composed of these parts. The coil is in a case that also contains the non-vibrator coil for the magneto system, and the two wirings are separate and distinct with the exception of the secondary connection between the coil box and the revolving parts of the distributor, which is common to both.

While little change in the position of the spark is required for this system, provisions are made by which it may be advanced or retarded. In one system, the interruptor is operated by a groove in the face of the cam disk, a pin on the end of the interruptor lever following the groove and being moved by the irregularities. In this case, the advance and retard of the spark is obtained by rotating a plate carrying the interruptor in one direction or the other, so that the lever is moved earlier or later in the revolution of the cam. In another type the gear driving the armature is not attached direct to the armature shaft, but to a sleeve surrounding the shaft, and the pulling out of a second sleeve between the two alters the position of the armature in relation to the driving gear, resulting in the advancing or delaying of the moment when the maximum current is produced.