Motor-car principles; the gasoline automobile

by Roger B. Whitman

LOW-TENSION IGNITION SYSTEM

In utilizing the current of a magneto for ignition purposes, the circuit is so arranged that it is first closed and then opened, the closing of the circuit permitting the current to flow, and the breaking of the circuit resulting in a spark at the break. The closing and opening of the circuit is due to the action of the igniter, which is a device located in the cylinder wall, one end projecting into the combustion space and the outside end bearing the parts by which it is operated. The two chief parts of an igniter are the stationary and movable electrodes, so arranged and connected that when the movable electrode comes into contact with the stationary electrode the circuit is closed, and when it moves out of contact the circuit is broken.

The stationary electrode is a steel pin or screw passing through the cylinder wall, and carefully insulated from it by a tube or bushing of mica or other insulant that is proof against the heat and pressure produced in the combustion space. The current developed by the magneto is led to this stationary electrode. The movable electrode rocks in a bearing in the cylinder wall, the inside end of its short shaft carrying a finger that touches or separates from the stationary electrode as the shaft is rocked. The outer end of the short shaft carries one or two short arms by which the shaft is moved. The operation of the igniter is due to the action of a cam on the half-time shaft, which moves a tappet rod that in turn acts on the arm controlling the movable electrode.

When the cam is not acting on the tappet, the movable electrode is held out of contact with the stationary electrode, and there is consequently no circuit for the current. As the tappet is moved by the revolution of the cam, the movable electrode is freed and is brought into contact with the stationary electrode by the action of a light spring. This closes the circuit, and the current flows. The further revolution of the cam separates the movable electrode from the stationary, and the sudden breaking of the circuit results in the production of a spark between the two.

Fig. 8.—Make-and-Break System.

There is a great variety in the construction of igniters, but the operation is the same in all. Once in its revolution the cam operates the movable arm of the igniter, permitting it to touch the stationary electrode, and this establishment of the circuit is followed by its rupture as the continued movement of the cam causes the tappet again to operate the movable electrode. A better spark for ignition purposes is obtained when the movable electrode is moved out of contact with the stationary electrode sharply than when it is moved slowly, and for this reason the parts are so arranged that the tappet strikes the movable electrode a blow instead of pushing or pressing it. This construction is shown in Fig. 8. When the cam is not acting on the tappet, the head on the tappet holds the movable electrode away from the stationary, against the action of a light spring. As the cam lifts the tappet, the tappet head releases the movable electrode, and the light spring draws it upward and against the stationary electrode, establishing the circuit. When the tappet drops off the cam, it is brought down sharply by the tappet spring, and its head strikes the movable electrode a sharp blow that results in the sudden breaking of the circuit.

In some types, the movable electrode comes into contact with the end of the stationary electrode, which is then a screw, and the distance between them may be adjusted by screwing it in or out. In other types, contact is made with the side of the stationary electrode, in which case the adjustment of the distance is provided for on the movable electrode. This distance should be about one sixteenth of an inch, for this is ample for the production of a satisfactory spark, and a greater distance will result in the more rapid burning of the contact points.

In multicylinder engines, the current from the live end of the armature winding is led to a bus-bar of conducting material that serves to distribute it to the igniters. The stationary and insulated electrode of each igniter is connected to this bus-bar by a short length of wire that either leads through a switch of ordinary construction, or is provided with a plug on its end that fits into a socket on the bus-bar and serves the purpose of a switch by which the igniters may be thrown out of circuit when it is desirable. As the magneto is grounded when it is placed on the engine, and as the movable electrode of the igniter is also grounded on the engine, a circuit for the current is provided only when a movable electrode is in contact with a stationary electrode. The construction of the cam shaft is such that only one igniter at a time makes contact, which occurs during the compression stroke, so that the rupturing of the circuit will occur at the point in the compression stroke when the spark is required for ignition.

To cut off the ignition circuit for the purpose of stopping the engine, it would be bad practice to break the contact between the magneto and the bus-bar, for while this would bring the desired result it might result in the injury of the magneto, as there would then be no circuit for the current that the magneto would still be developing. The best practice is to short-circuit the magneto, and this is provided for by means of a simple switch, one point of which is grounded, and the other connected to any insulated part of the ignition system; the magneto terminal, bus-bar, or other. When this switch is closed, a circuit is provided for the magneto current, which, in the arrangement shown in Fig. 8, flows from the magneto to the bus-bar, to the switch (when closed), to ground, and back to the magneto. If this circuit is provided, the current will follow it, abandoning the paths across the igniters as they operate, and the production of sparks ceasing, the engine will come to a stop.