Chapter VIII: Ignition. High Tension

This system is used to the largest extent, so that we ought to have a full explanation of the devices which are required to do the work. While magnetos are used with the low tension system, for the reasons stated, they are especially necessary with the _Jump Spark_ method.

Magnetos.--The most important element in this system is the magneto, so we shall try and make the subject as explicit as possible. As stated, a magneto is a special type of dynamo which will now be explained. For this purpose it will be necessary to show the elementary operation of an alternating current dynamo.

Alternating Current.--In Fig. 42 A is a bar of soft iron, around which is a coil of wire B, the wire being insulated, so that it will not touch the bar. There is no magnetism in this bar, and this simple form of structure is shown, merely to represent what is called the _field_ of a dynamo.

The object of the coil of wire is to make a magnet of the bar, for the moment a current is sent over the wire, a magnet is formed, and the magnetism leaves the bar the moment the current ceases to flow. If this bar should be of hard steel it would retain the magnetism.

Now, the primary difference between the magneto and the dynamo, is that this field bar is a permanent magnet in the magneto, whereas the field is only a temporary magnet in the dynamo. This should always be kept in mind.

The end of a magnet, whether it is a temporary one, or permanent, has a magnetic field of force at the ends as well as at all parts of it, exterior to the surface of the bar. Such a field is indicated, and in the dynamo, no such field exists unless a current is passing over the wire B, which is called the _field winding_.

The U-shaped piece of metal C represents the armature. It is shown hinged to the top of two posts, for clearness in understanding, and is adapted to turn to the right, and in turning the loop passes the end of the field bar B, and passes through the magnetic field which is indicated by the dotted lines D.

Now, if the loop is simply permitted to remain in the position shown in Fig. 42, a current would flow through the loop, this transference of the current being called induction, and this characteristic of the flow of electricity will be explained and its utility explained.

Cutting Lines of Force.--The loop will now be turned to the right so that it passes the magnetic field and goes beyond it in its revolution. This motion of passing the armature through the magnetic field is called _cutting_ the _lines of force_. While the loop was lying within the magnetic field, and also when it was moving through the field, the current set up in the loop flowed in the direction of the darts F, or to the right, through the pivots D.

In Fig. 43 the loop is shown as having made a quarter turn, and it is now vertical, or at right angles to its former position. The loop in thus passing away loses its force, until it reaches the position shown in Fig. 44, when there is a surging back of the current to the opposite direction, as indicated by the arrows.

When the loop reaches the lowest position, shown in Fig. 45, it again begins to get the influence of the magnetic field, and a reversal back to its former direction takes place, this surging movement back and forth being due to the reversal of the polarity in the coil brought about by the position in which it is placed relative to the magnetic field.

It is now an easy matter to connect the ends of the loop with wire conductors. This is shown in Fig. 46, where a small metal wheel G is placed on each end of the spindle, and in having a strip of metal bearing H on the wheel. These are not commutator brushes, but are merely wiping brushes to take the current from the turning parts. Wires I connect with these wiping bars, and through them the current is transmitted to perform the work.

Plurality of Loops.--The dynamo may have a plurality of loops, which are called _coils_, and there may be a single magnet or any number of magnets. Instead of driving these coils past the face of the magnet, or magnets, the latter may be driven past the coils. In fact with most of the alternating current machines the fields are the rotating parts and the armatures, or the coils, are fixed.

The voltage is increased if the coils have a large number of turns on the armature, and also if the armature, or the turning part, is speeded up. Voltage will also be higher if larger or more powerful magnets are used in the magnetos.

The Electro-Magnet.--The permanent magnet, such as is used in the magneto, is distinguished by the fact that it contains a permanent charge of magnetism, but this is not an _electro-magnet_. This is a magnet made of soft iron, so it will be readily demagnetized. While not shown in the diagrams, an iron core may be placed within the loop or coil, and this is done in all dynamos, because the iron core acts as a carrier of the magnetism, concentrating it at the center, because it is a much better conductor than air.

The Dynamo Form.--Consult the diagram, Fig. 47. The iron heads A represent the bar in the previous diagrams, and B the wire around the bar. C is the armature, which in this case represents a number of loops, or coils, and D is the commutator, which is used in the direct current machine to correct the alternations referred to in the previous diagrams, so as to send the current in one direction only, the commutator brushes E being used to carry off the current for use.

The Magneto Form.--The metal loop F, in Fig. 48, being a permanent magnet, the armature, G, formed of a plurality of loops, has no field wires to connect with it, as in the case of the dynamo.

Advantage of the Magneto.--The magneto has a pronounced advantage over the dynamo, as a source of power for ignition purposes, in the particular that the strength of the magnetic field is constant. In a dynamo this varies with the output, because when used on an automobile where the speed is irregular, the voltage will vary. The voltage of the magneto is a constant one, and is thus better adapted to meet the needs of ignition.

Induction Coil.--The induction coil is a device which is designed to produce a very high voltage from a low tension, so that a current from it will leap across a gap and make a hot spark.

We stated in a previous section that a current leaps across from one conductor to another, so that electricity can be transferred from a wire to another not touching it, by means of induction.

Look at Fig. 49, which represents two wires side by side. The current is flowing over one wire A, and by bringing wire B close to A, but not touching it, a current will be induced to leap across the gap and the wire B will be charged. If the ends of the wire B are brought together, so as to form a circuit, and a current detector is placed in the circuit it will be found that a current is actually flowing through it, but it is now moving in a direction opposite to the current flowing through A.

Changing the Current.--But we have still another thing to learn. If the two wires are not of the same thickness it would not prevent the current from leaping across, but another astonishing thing would result.

First, we shall use a wire B double the thickness of wire A. If now, we had an instrument to test the voltage and the amperage, it would be found that the voltage in B is less than that in A, and also that the amperage is greater.

Second, if the conditions are reversed, and the wire A is thicker than B, the latter will have an increase of voltage, but a lower ampere flow than in A.

Now this latter condition is just what is necessary to give a high tension. Voltage is necessary to make a current leap across a gap. By this simple illustration we have made an induction coil which may be used for making a high tension jump spark.

Construction of a Coil.--Two wires side by side do not have the appearance of a coil, and even though such an arrangement might make a high tension current, it would be difficult to apply. To put the device in such a shape that it can be utilized, a spool is made, as shown in Fig. 50.

This spool A has a number of layers of thick, insulated wire B first wound around it, the layers being well insulated from each other, and the opposite ends brought out at one end or at the opposite ends, as shown at C, D. On this is a layer of finer wire, also insulated, this wire E having its terminals also brought out at the ends of the spool, and after the whole is thus wound, the outside of the coil is covered with a moisture proof material.

The Primary Coil.--The winding of thick wire is called the _primary_ coil. The current from the battery or the electric generator is led to this inner coil.

The Secondary Coil.--The fine wire wrapping represents the secondary coil, which is raised to a high voltage, and this actuates the sparking mechanism.

In the art it is customary to illustrate the various contrivances by certain conventional forms. Fig. 51 shows the manner of designating an induction coil in a diagram, in which the heavy zig-zag line indicates the primary, and the lighter zig-zag lines the secondary coil.

Contact Maker.--A simple little device used in the primary circuit of an induction coil, is known as a _contact maker_. This, as shown in Fig. 52, is merely a case A, through which is a shaft B that carries within the shell a cam C. A spring finger D has its free end normally bearing against the cam, and when the nose on the cam moves out the spring finger, the latter is moved outwardly so it contacts with a plug E in the side wall of the case, although it is insulated therefrom. This contact establishes a current through the plug, spring finger and case.

The diagram, Fig. 53, illustrates the principles of construction and arrangement of a high tension jump spark ignition, in which the electrical source is a battery actuating an induction coil.

High Tension With Battery and Coil.--The battery A has one side connected up by wire B with one terminal of the primary C in the induction coil, and the other side of the battery has a wire D leading to the contact maker. A switch E is placed in the line of this wire.

The other terminal of the primary has a wire F leading to the insulated contact plug G of the contact maker. This completes the generating circuit. The cam H is on a shaft I, which travels one half the speed of the engine shaft.

One side of the secondary coil J has a wire K leading to the spark plug, while the other terminal of the secondary has a wire L which is grounded on the engine M.

When the nose of the cam pushes over the spring finger and closes the cam, the circuit through the finger flows through the primary coil and excites the secondary. When the cam again immediately breaks the circuit a high tension current is momentarily induced in the secondary, so that the current leaps the gap in the spark plug and makes the spark.

Metallic Core for Induction Coil.--In the previous description of the induction coil it was stated that the spool might be made of wood. These coils are also provided with metal cores, which can be used to make what is called a vibratory coil.

The Condenser.--A necessary addition to the circuiting provided by an induction coil, is a _condenser_. This is used in the primary circuit to absorb the self-induced current of the primary and thus cause it to oppose the rapid fall of the primary current.

The condenser is constructed of a number of tinfoil sheets, of suitable size, each sheet having a wing at one end, and these sheets are laid on top of each other, with the wings of the alternate sheets at opposite ends. Very thin sheets of waxed paper are placed between the tin foil sheets so that they are thus insulated from each other.

The wings at the ends are used to make connections for the conducting wires. The device is not designed to conduct electricity, but to act as a sort of absorbent, if it might so be termed. The large surface affords a means where more or less of the current moves from the conductor at one end to the conductor at the other end, and as it is designed to absorb a portion of the current in the line it is merely bridged across from one side of the circuit to the other.

The diagram, Fig. 55, represents the conventional form of illustrating it in sketching electrical devices.

Operation of a Vibrator Coil.--The illustration, Fig. 56, shows the manner in which a vibrator coil is constructed and operated. The coil comprises a metal core A, the primary winding B being connected at one terminal, by a wire C, with a post D, and the other terminal by a wire E with one side of a battery F. A switch G is in the line of this conductor.

The post D holds the end of a vibrating spring H, which has a hammer H´ on its free end, which is adapted to contact with the end of the metal core A, but is normally held out of contact, so that it rests against the end of an adjusting screw I which passes through a post J.

The post J is connected up with the battery by a wire K, and a wire L also runs from the wire K to the conductor C, through a condenser M.

The secondary coil N, has the outlet wires O, P, which run to the spark plug Q on the engine.

The operation is as follows: When the switch G closes the circuit, the battery thus thrown in the primary coil magnetizes the core A, and the hammer H´ is attracted to the end of the core, thus breaking the circuit at the contact screw I. The result is that the core is immediately demagnetized, and the spring H draws the hammer back to be again attracted by the core which is again magnetized, so that the hammer on the vibrator arm H goes back and forth with great rapidity.

From the foregoing explanations it will be understood how the primary induces a high tension current in the secondary, and in order that the spark may occur at the right time, a _timer_ for closing and opening the primary circuit must be provided. By this means an induced high tension current is caused to flow at the time the spark is needed in the cycle of the engine operation.

_The Distributer._--The distributer is a timing device which controls both the primary and the secondary currents, and it also has reference to the revolving switch on the shaft of a magneto whereby the current is distributed to the various cylinders in regular order.

Fig. 57 shows a form of distributer which will illustrate the construction. A is the shaft which is driven at one half the engine speed. It is usually run by suitable gearing direct from the shaft of the magneto.

Its outer end rests in a bearing plate B, of insulating material, which plate serves as the disk to hold the contact plates, 1, 2, 3, 4, to correspond with the four cylinders to which the current is to be distributed.

Wires 5, 6, 7, and 8, run to the respective spark plugs C from these contact plates. The projecting end of the shaft A carries thereon a contact finger D, which is designed to contact with the respective plates, and an insulating ring E is interposed between the shaft and finger so as to prevent short circuiting of the high tension current.

On the side of the finger is a hub F, integral therewith, and a wiper attached to a post bears against the hub so as to form continuous contact. A wire leads from the post to one terminal of the secondary coil.

Circuiting With Distributer.--The diagram Fig. 58 shows the complete connections of a system which comprises a magneto, induction coil, condenser, and a distributer. The magneto A has on its armature shaft B two revolving disks C, D, one of which must be insulated from the shaft, and one end of the coil E of the armature is connected with one of these disks, and the other end of the coil is attached to the other disk.

Alongside of these disks is another disk F which has projecting points G to engage with and make temporary contact with a spring finger which actuates the interrupter I, this being a contact breaker which breaks the primary current at the time a spark is required.

One terminal of this interrupter is connected by a wire J with one end of the primary winding K, of the induction coil, and the other end of the primary has a wire L which runs to the disk C.

The other terminal of the interrupter has a wire M leading to a condenser N, and from the other side of the condenser is a wire O leading to the wire J before described. The wiper of the other disk D has a wire connection with the wire M.

The distributer shaft P is so mounted that it may receive its motion from the shaft of the magneto, and for this purpose the latter shaft has a gear Q one half the diameter of the gear R on the distributer shaft.

The distributer S has been described with sufficient clearness in a preceding diagram, to show how the wires T lead therefrom and connect up with the spark plugs U. One terminal of the secondary coil V is connected by a wire W with the wiper X which contacts with the hub of the distributer finger X´, and the other terminal of the primary is grounded at Y, which represents the metal of the engine.