Motor-car principles; the gasoline automobile

by Roger B. Whitman

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.

SETTING UP THE L. T. SYSTEM

To obtain successful results, the spark must be produced at the correct point in the compression stroke, and this requires such adjustment of the igniters that the movable electrode breaks the circuit at this point. Because the magneto delivers a fluctuating current, it is clear that the most satisfactory spark will be produced when it is so set that it will be delivering its maximum current at the instant that the igniters separate. As most of the running of a car is done on an advanced spark, the point of maximum advance must be determined in order to set or adjust a low-tension ignition system. This point varies with different engines; in a large proportion the piston has a “lead” of one half inch, while in a smaller number the “lead” varies from this to three quarters of an inch. By this it is meant that when advanced, the spark occurs when the piston still has one half inch to travel to reach top dead center of the compression stroke, or anything up to three quarters of an inch, as the case may be. Inquiry of the manufacturers regarding this point will secure the information, but if this is not possible, the point may be found by experiment.

The first step is to bring one of the pistons to within one half inch of top dead center of the compression stroke. This position may be determined by dropping a stiff wire through a compression relief cock or other opening in the cylinder head, so that it rests on the piston and is moved by it. The crank shaft of a four-cycle engine must make two revolutions to complete the cycle in one of the cylinders, and during these two revolutions the piston will twice pass top dead center, as will be shown by the movements of the wire. It is necessary, however, to distinguish between top dead center of the compression stroke and top dead center of the exhaust stroke, and this may be accomplished by watching the stem of the exhaust valve. When the wire shows that the piston is moving toward top dead center, and the exhaust valve stem shows the valve to be open, the piston is known to be making the exhaust stroke, and to get it to top dead center of the compression stroke it is necessary to continue the revolution of the crank shaft while the piston moves downward on the inlet stroke, and again upward on the compression stroke. During this second upward stroke the exhaust valve stem will not move, showing the valve to be closed. Cranking must stop when the lack of movement of the wire shows the piston to be at top dead center, and then the crank shaft must be turned backward by means of the flywheel until the piston has moved down one half inch. This may be determined by making a mark on the wire and holding a rule firmly in such a position that as the wire follows the piston the mark will move along the graduations. When the mark has moved down one half inch below top dead center, the movement of the crank shaft must be stopped so that the piston is held in that position.

Place the spark control lever in the advanced position, when the nose of the cam should be just ceasing to act on the tappet. The movable electrode must then be adjusted so that it is in the act of separating from the stationary electrode, and this is performed by shortening or lengthening the tappet, or otherwise setting the parts so that the tappet head is in contact with the outside arm of the movable electrode and beginning to move it away from the stationary electrode. This point may be accurately determined by the use of an electric bell circuit. Connect a bell with three or four dry cells, grounding the free terminal of the battery on the engine. Then breaking any connection between the igniter terminal and the ignition system, connect the free terminal of the bell to the igniter. The circuit will then consist of battery, bell, igniter, and ground return, and the bell will ring when the movable electrode is making contact with the stationary. Having adjusted the movable electrode so that it is approximately correct, it may be tested by cranking the engine. The bell should begin to ring during the compression stroke, and should stop ringing when the marks of the wire show the piston to be one half inch from top dead center. The igniter of each cylinder must be adjusted separately, and when tests have shown them to be correct, the lock nuts must be set up to hold them in position.

In attaching the magneto to the engine, means must be provided for driving it at the proper speed. In a four-cylinder engine, two revolutions of the crank shaft are necessary for the production of a cycle in each of the cylinders, two power strokes occurring in each revolution. As the magneto delivers its maximum current twice in each revolution of the armature, it must be driven at the speed of the crank shaft, and as the drive is usually taken from the cam shaft, the gear on the magneto must have half as many teeth as the gear on the half-time shaft with which it meshes. To set the magneto, the crank shaft should be revolved until one of the pistons (it makes no difference which) is in the firing position; that is, the position for which the igniter is set to open. The armature should then be revolved by hand until it is just past the vertical position of Fig. 6, with about one sixty-fourth of an inch of space showing between its rear edge and the edge of the pole piece from which it is moving. This can most easily be accomplished by removing the dust cover and reaching under the arch of the field with the fingers. Holding the armature in this position, its driving gear should be slipped into mesh with the gear on the half-time shaft.

In some makes of magnetos the gear is keyed to the armature shaft, which determines its position in relation to the armature. In this case, the gears must be meshed as close to the indicated position as possible. In other designs the magneto is driven by a positive clutch, so that it is impossible to mesh it in any but the correct position. In Bosch magnetos there is no key, but the shaft is tapered to fit a tapered hole in the gear, so that a very exact setting of the magneto is possible.

When the gear is keyed to the armature shaft, the closeness of the setting depends on the thickness of the teeth; the thinner the teeth, the closer it is possible to adjust the magneto. If the armature gear has an uneven number of teeth, however, it is possible to make the setting by one half the thickness of a tooth. If it is found that the thickness of the teeth is such that the armature cannot be set correctly, give the armature and its gear a half turn, when, if the number of teeth is odd, it will be found that a tooth on one side corresponds to the space between two teeth on the opposite side.

Having set the magneto and drawn up the nut securing the gear to the armature shaft, the system may be wired by running a wire from the terminal of the magneto to the bus-bar, and other wires from the bus-bar to each of the igniter terminals. The short circuiting switch should also be connected up, the running of its wires depending on convenience.

The system installed, it should be tested. With the system adjusted as indicated, the engine should start, and a very short run will show if it is correct. If it does not respond to advancing the spark, or if its operation is not satisfactory, a readjustment should be made, giving the piston a lead of one eighth of an inch more, or five eighths of an inch in all, with the spark advanced. If this shows an improvement, but the engine still does not develop its full power, the operation may be repeated, the piston being given more lead by one sixteenth of an inch.