In order that a gas engine may run properly, the mixture must be set on fire, or ignited, at exactly the right time; if ignition occurs too early or too late, there will be a loss of power.

The greatest pressure will be obtained at the instant when all of the mixture is burning, and this should take place just as the piston begins to move outward on the power stroke. A little time is required for the mixture to burn; there is a brief interval between the instant when it is set on fire and the instant when it is all in flame. Thus it is clear that if the mixture is all to be burning as the piston starts the power stroke, it must be set on fire before that time, or, in other words, toward the end of the compression stroke.

The point at which ignition should occur depends on the speed of the engine and should change when the speed changes. The time required for the flame to spread throughout the mixture does not change; let us say that, with the engine running at 1200 revolutions a minute, the mixture can be ignited when the piston is ¼ inch from the end of the compression stroke, and will all be in flame by the time the piston starts on the power stroke. If the engine is slowed down to 600 revolutions a minute and no change is made in the ignition, the mixture will all be in flame before the piston reaches the end of the compression stroke; pressure will then be produced before the piston is in position to perform the power stroke. The pressure will try to make the engine run backwards; it will sometimes be sufficient to make the engine stop. If the momentum of the flywheel is sufficient to force the piston to the end of the stroke against the pressure, this condition will cause a loss of power. This is called preignition, or ignition that occurs too soon. One effect of it is to produce a hard, metallic knocking, due to the oil being squeezed out of the bearings by the great pressure, which permits the bearing and shaft to strike. The remedy is to make ignition occur later in the stroke.

If the engine is speeded up above 1200 revolutions, the piston will have had time to move some distance on the power stroke before the mixture is all in flame; the combustion space will then be too large to permit the mixture to produce its greatest pressure, and again there will be a loss of power. The remedy in this case is to make ignition occur earlier in the compression stroke.

When ignition is made to occur early in the compression stroke, it is said to be advanced; when it is made to occur late in the stroke, it is said to be retarded.

To get the best results, the engine should be run with ignition advanced as far as is possible without causing knocking.

The charge of mixture is always set on fire by an electric spark, and the parts that produce and control this spark are called the ignition system.

An ignition system consists of: First, the apparatus that produces the electric current, which is usually a magneto; second, a timer, which controls the instant at which the spark occurs; third, the spark plugs, which project into the cylinders, and at which the sparks take place; fourth, a switch, by which the sparking current can be turned on or off, and fifth, the wires, or cables, by which the parts are connected.

The electric current that gives the spark is always produced by magnetism. In a magneto, magnetism is obtained from the heavy steel magnets that are part of it; there is a constant flow of magnetism from one end of these to the other. To obtain an electric current, a coil of wire is placed in the magnetism, and the strength of the magnetism is made to change; it alternately becomes weak and strong. Whenever a change in strength takes place, an electric current flows in the wire, and it continues to flow as long as the magnetism continues to change in strength. When the change in strength is very great, that is, when the magnetism changes from very weak to very strong, or from very strong to very weak, the electric current is more powerful than when there is only a little change in strength. A more powerful current is also produced by a change that takes place suddenly than by a change that takes place slowly.

The electrical principle that produces a current in this manner is called induction; the current produced is known as an induced current.

A magneto has two or more magnets, and between their ends, or poles, there revolves a piece of iron called the armature. A piece of iron placed between the poles of a magnet becomes a magnet itself; the armature is so shaped that, as it revolves, its magnetism continually changes in strength, and it is the changes in the strength of the magnetism of the armature that produce the sparking current.

The iron armature of the Bosch magneto, which is the best known type, is shown in Figure 42. It has a central bar with two heads, the wire being wound around the central bar, or core. The shafts on which it revolves are attached to the ends of the heads.

Figure 43 shows different positions of the armature between the poles of the magnet, and illustrates the changes in the magnetism of the central bar. There is a continual flow of magnetism from one pole of a magnet to the other; if a piece of iron lies between them the magnetism will use it as a bridge, but often its easiest path will be through the air. In A, Figure 43, the armature lies crossways, and its central bar or core forms a perfect bridge for the magnetism. Practically all of the magnetism flows through it, and it then becomes a powerful magnet itself. It sets up its own flow of magnetism, which flows through the core to one head, through the air to the other head, and so back to the core.

In B, the armature has revolved a little. Most of the magnetism is still flowing through the core, but some of it is finding an easier path by flowing through the heads and across the air space to the other pole. The magnetism of the core is, therefore, a little weaker than it is in A.

In C, the heads alone form bridges between the poles, and none of the magnetism flows by the core because that no longer forms a path. The core is no longer producing magnetism; in moving from A to C there has thus been a complete change in the strength of the magnetism of the core, for from full strength it has died away to nothing.

By a further movement, as in D, the core again acts as a bridge, and another change in strength occurs, this time from nothing to full strength again. In moving from D to B, there are slight changes in strength, but not enough to produce a sparking current; it is only in passing from B to D that a sparking current can be produced.

In this type of magneto the space between the heads is wound full of wire, which of course revolves with the armature; the more turns of wire there are, the more intense will be the current, so very fine wire is used to get the greatest possible number of turns.

In the Bosch magneto the first few layers are of coarse wire, and are the primary winding. The remainder, called the secondary winding, is very fine wire, and the two are connected so that one forms an extension of the other.

It has been explained that it is most important to have the spark occur at exactly the right instant in the stroke. On a magneto the instant of sparking is controlled by a timer, or circuit breaker, which is a switch that is automatically operated at the time when the magneto is producing a current sufficiently intense to form a spark.

Figure 44 illustrates one complete revolution of the armature, and it will be seen that it passes twice from position B to position D. This shows that it gives a sparking current twice during each revolution. The circuit breaker must therefore operate twice during each revolution. It is placed at the end of the magneto; in some makes it revolves with the armature and is operated by stationary cams, while in others it is stationary, and is operated by a cam on the armature shaft. In either case the effect is the same.

Figure 45 shows the way in which the winding on the armature of a Bosch magneto is connected with the circuit breaker and with the armature. The circuit breaker shown is not the kind used on the Bosch, and serves only to illustrate the principle. It consists of a lever pivoted at one end, with the other end resting against the tip of a screw. A cam bears against the lever and can move it to break the contact with the screw. The cam is so set that it moves the lever at the time when the current is most intense.

The coarse wire, or primary winding, on the armature is connected with the lever and with the screw of the circuit breaker; when the lever is touching the screw, any current produced in the primary winding has a complete path, or circuit, in which to flow.

The fine wire, or secondary winding, is wound on top of the primary, and its inmost end is connected to the outmost end of the primary so that one forms a continuation of the other. The outmost end of the secondary leads to the spark plug; any current produced in the secondary winding flows to the spark plug, and, if intense enough, will jump across the small gap in the plug, and return to the secondary by way of the primary.

Referring to Figure 43, a weak current is produced in the primary while the armature revolves from D to B; at that time the circuit breaker is closed, so the current can flow in the path thus provided for it. A current also tries to flow in the secondary, but is too weak to jump across the gap in the spark plug. As the armature comes closer to the point C, Figure 43, the primary current becomes more intense, and the electricity in the secondary increases its endeavor to jump the gap in the spark plug, but is still unable to do so.

As the armature passes over the point C, the circuit breaker opens. The primary current, which is then most intense, finds its path taken away from it, and it seeks another, which it finds by flowing into the secondary winding. This flow of primary current, added to the pressure already existing in the secondary, forms a current sufficiently intense to jump across the gap in the spark plug, and in so jumping it produces the ignition spark.

As the armature passes to position D, Figure 43, the circuit breaker closes, and the action is repeated.

A magneto of this type is thus seen to give two sparks to every revolution of the armature.

K-W and Dixie magnetos operate on the same general principle as the Bosch, with the difference that the wire windings are separate from the armature, and do not revolve. The revolving part, which is called an inductor, consists of blocks of iron, so shaped that, as they revolve, they alternately lead the magnetism to the core of the winding and then away from it. The result is that the core gains magnetism and then loses it, and these continual changes in strength produce sparking currents in the winding.

The inductor of a K-W magneto is shown in Figure 46. It consists of a shaft on which are mounted two blocks of iron at right angles. The section of shaft that joins them is the core of the winding; the wire is wound on it just as thread is wound on a spool, but with a space between, so that the shaft may revolve inside of the coil.

Figure 47 shows the inductor in three positions of its revolution between the poles of the magnet. When it is in the first position, magnetism can flow from one pole of the magnet to the other by going into one end, A, of one block, through the core, and out of one end, C, of the other block. This makes a magnet of the core and it forms magnetism of its own. When the inductor turns to the second position magnetism can get across without flowing through the core, for the blocks now give it a path. As the flow through the core ceases, the core’s magnetism dies away, which gives the change in strength that is needed to produce a sparking current.

When the inductor is in the third position, the core again becomes the path for the magnetism and is magnetized; these changes continue as long as the inductor turns.

While an armature type of magneto, like the Bosch, produces two sparks to every revolution, the K-W produces four, for there are four periods during every revolution when there is sufficient change in the strength of the magnetism of the core to produce a sparking current.

In these magnetos the revolving shaft is parallel to the ends of the magnets, but in the Dixie magneto it is at a right angle, as shown in Figure 48. The shaft is of some metal, such as brass or bronze, through which magnetism will not flow; otherwise the shaft would form a continuous path. The inductor blocks are mounted on the shaft, and act as extensions of the poles of the magnet. The core on which the wire is wound is a separate piece, placed under the arch of the magnets, with ends that extend down and form a tunnel in which the inductor revolves.

Figure 49 shows an end view of the inductor, the magnets being cut away so that the core may be seen. As inductor block A is an extension of one pole of the magnet, magnetism tries to flow from it to block B, which is an extension of the other pole of the magnet. When the inductor is in position 1, Figure 49, magnetism can flow from block A through the core to block B, the core then being magnetized. In position 2, magnetism can flow from one block to the other by going through the ends of the core instead of through the core itself; the core then loses its magnetism, but regains it when the inductor moves to position 3.

In practically all makes of magnetos the circuit breaker is at the end of the armature or inductor shaft, and is operated by it. The Bosch circuit breaker is illustrated in Figure 50, the parts being mounted on a plate attached to the shaft and revolving with it. The lever is L-shaped, pivoted at the angle, with one end resting on the tip of a screw. When the shaft revolves, the other end of the lever is dragged over a block of metal that acts as a cam; this makes it move on its pivot and separates it from the screw. By turning the screw the distance of separation may be adjusted.

In the circuit breaker of the K-W magneto it is the cam that revolves, while the lever is stationary, as shown in Figure 51. It will be noticed that the cam will move the lever only twice during each revolution; the magneto can produce four sparks during a revolution, but with this arrangement of the cam only two of them are used.

It has been said that an intense sparking current is produced when there is a great change in the strength of the magnetism, and when the change in strength occurs suddenly. There cannot be any alteration in the change in strength, for the greatest magnetic strength of the core is what is given it by the magnet, and changing from this to nothing is the greatest change possible. The suddenness with which the change takes place, however, depends on the speed at which the magneto runs. A 4-cylinder engine requires two sparks to each revolution of the crank shaft; the armature of a Bosch magneto for this engine will therefore run at the same speed as the crank shaft.

The K-W magneto, giving four sparks to the revolution, could run at half of the speed of the crank shaft, but then the change in the strength of the magnetism would take place slowly, and the sparking current would not be sufficiently intense. By using only two of the sparks the magneto is run at the same speed as the crank shaft; the change in strength then takes place more suddenly, and a more intense sparking current is produced.

The circuit breaker of a magneto for a 1-cylinder engine has only one cam, so that a single spark is produced during each revolution of the armature; the armature makes one revolution to every two revolutions of the crank shaft.

However many cylinders an engine may have, the magneto must be revolved from one point of sparking to the next in the interval between ignition in one cylinder and ignition in the next cylinder to fire. A magneto is driven by the crank shaft through gears or by a chain, which are so proportioned and set that the magneto is at a point of sparking at the instant when a piston is in position for ignition.

A magneto for an engine with more than one cylinder is provided with a distributor, which passes the sparking current to the particular cylinder that is ready for ignition. A distributor is a revolving switch built into the magneto, with as many points, or contacts, as the engine has cylinders. At the instant when the magneto produces a sparking current, the revolving distributor arm is in position to pass the current to one of the contacts, and the current flows to the spark plug with which it is connected.

An electric current must have a complete path, or circuit, in order to be able to flow. In a magneto ignition system this path is partly of wire and partly of the metal of the engine. The diagram in Figure 45 indicates that the current returns to the magneto from the circuit breaker lever and the spark plug by wire, but in actual construction it returns by the metal of the engine. This is called a ground return; the circuit is said to be grounded.

Figure 52 is a side view of a Bosch magneto, partly broken away to show the interior. As can be seen, one end of the primary winding is screwed to the armature, and is thereby connected with the metal of the magneto; as the magneto is attached to the engine the primary winding is thus in contact with that also. The other end of the primary winding leads to the insulated block of the circuit breaker, Figure 50. This block is insulated from the disk; that is, while it is attached to the disk, it is kept from touching it by means of pieces of hard rubber or mica. Through these an electric current cannot pass.

The lever is grounded; that is, it is in contact with the metal of the magneto. When the lever touches the screw of the insulated block, current can flow; when they are separated, the circuit is broken.

One end of the secondary winding, Figure 52, is attached to the outer end of the primary. The other end leads to the slip ring, which is a metal rim on a hard rubber wheel attached to the armature and revolving with it. Sparking current flowing to the slip ring is led off by a carbon brush and passed to the distributor.

Should a spark plug wire fall off while the engine is running, the current would lose its path and would seek another; it is quite powerful enough to make a path for itself by breaking through the windings. As this would injure the magneto, such a thing is prevented by providing a safety spark gap, which acts like a safety valve in giving the current a path when the regular path is interrupted. It consists of two points of metal, one attached to the metal of the magneto and the other connected with the slip ring brush; it is a more difficult path than the one through the spark plug, but easier than breaking down the windings.

Figure 53 is a section of the K-W magneto. As the coil does not revolve, no slip ring is necessary; the sparking current flows directly to the distributor.

To start an engine, the crank shaft must be turned at sufficient speed to drive the magneto fast enough to produce a spark. With large engines this is often a difficult matter, so it is very usual to equip a magneto with an impulse starter. One part of this is attached to the magneto shaft and the other to the engine shaft that drives the magneto; the two are connected by a spring. When starting, a catch holds the armature and prevents it from turning. The drive shaft turns, however, and in so doing winds up the spring. At a certain point the catch is automatically released, and the spring then throws the armature over at a speed that gives a good spark. A spark is thus assured, even though the engine is being cranked very slowly.