We have already stated that the charge of explosive mixture is ignited in the cylinder at the end of the compression stroke by means of an electric spark. The electric spark takes place as the result of an electric discharge across the gap between the electrodes of the sparking plug.

The Sparking Plug.—Two views of a typical sparking plug are shown in Figs. 45 and 46, in which A is the high tension electrode which is periodically charged with electricity at high voltage (or electrical pressure) from a high tension magneto or a high tension coil, and B₁, B₂ are electrodes which, being in metallic contact with the cylinders and framework of the engine, are thus at zero potential. The electric discharge occurs across the gap C₁, C₂ in the form of a spark or flash. The electrode A is heavily insulated from the metal casing D of the sparking plug by porcelain insulators E and F. The locknuts G and H serve to keep the plug gas-tight and hold the several portions together mechanically. The terminal K is used for clamping the wire (or lead) which brings the supply of high tension electricity. The high tension electric current may be supplied either by (1) a magneto machine or (2), a coil and accumulator ignition system.

The High Tension Magneto.—In Figs. 47, 48 and 49 we show a modern high tension magneto suitable for a four-cylinder engine. It consists of the stationary magnets A, the driving spindle B, the high tension electrode D, the high tension distributor C, and the low tension contact breaker E. The armature, condenser, and distributor gear wheels are not shown in the drawings, but are situated inside the machine in the space between the high tension electrode D and the low tension contact breaker E. As the spindle B is rotated by gearing driven from the engine crankshaft the armature attached to it generates a high tension current and a low tension current. The high tension current passes to the high tension electrode D and thence across the machine to the carbon brush H of the high tension distributor C. The low tension current passes through the platinum-tipped contact screws F₁, F₂ of the low tension contact breaker. Twice during each revolution of the armature these contacts are separated owing to the fibre block attached to the bell crank lever G passing over the stationary cams T₁, T₂; this constitutes the make-and-break device for interrupting the primary current. The momentary interruption of the primary current in this way causes a very great increase in the electrical pressure (or voltage) of the secondary or high tension current which is sufficient to bring about the spark discharge across the gap between the electrodes of the sparking plug. Since there are two of these cams on the low tension contact breaker it will be understood that the armature can supply current for two sparks in every revolution it makes. If we bear this fact in mind we will have no difficulty in determining the relative speeds of the magneto armature and the engine crankshaft for any type of engine. A four-stroke engine requires one spark in every two revolutions made by the crankshaft, so that a four-cylinder engine of this type requires two sparks per revolution, and the magneto armature must run at crankshaft speed. A six-cylinder engine working on the four-stroke cycle would require three sparks per revolution, but the armature of the magneto only supplies two, therefore it must be driven at one-and-a-half times the crankshaft speed.

The high tension distributor consists of the carbon brush H driven by gearing from the magneto armature and the metal segments M₁, M₂, M₃, M₄, which are mounted in a block of insulating fibre. There must be as many segments on the distributor as there are cylinders on the engine, one segment for each sparking plug; but the armature cannot supply more than two sparks per revolution, and therefore if the distributor has four segments it must be driven at half the armature speed, and if it has six segments it must be driven at one-third of the armature speed. Each metal segment is electrically connected to a sparking plug lead such as L₁, L₂, L₃, L₄. The high tension electrode D is attached to a light carbon brush which presses on a gunmetal collector ring at the high tension end of the armature winding. A special terminal is provided at P, so that when a wire is attached to it and connected to the frame of the engine (usually through a switch) the low tension windings are short-circuited or closed on themselves, and the make-and-break has no effect, because there is always the path through the switch until it is opened again. Under these circumstances the voltage of the high tension circuit is not sufficient to cause the spark discharge, and the ignition is then said to be switched off. The instant at which the spark occurs may be advanced or made earlier by moving the rocker arm K, which carries the stationary cams T₁, T₂ backwards, whereas if it is moved forward the ignition is retarded and occurs later in the stroke. Normal ignition occurs when the lever is midway in its range of movement and corresponds to the position of the piston when the crank is on the top dead-centre, whereas advanced ignition occurs just before the piston has completed the compression stroke, and retarded ignition will take place after the crank has passed the dead-centre and when the piston has moved down a little on the power (or explosion) stroke. Advancing the ignition increases the speed, and retarding the ignition reduces the speed, except when the engine is overloaded, and then it may pick up speed a little or run better if the ignition is slightly retarded—but the exact behaviour will depend on the temperature of the metal walls and piston within the cylinder.

We have mentioned that normal ignition occurs when the crank is exactly on the dead-centre and the piston at the top of its stroke. If we set the magneto when the engine is at rest so that ignition ought to occur on dead-centre when the arm K is in its mid position the actual sparking will be late on account of the time lag of the electric current. The current takes time to flow and in that brief element of time the crank has moved a few degrees off the dead-centre, at high speeds. Hence the ignition must be advanced if the charge is to be correctly fired when the engine is running fast. If the ignition is too far advanced it will cause the engine to “knock,” especially under heavy loads. If the ignition is retarded the charge is not fired at the commencement of the stroke so that a portion of the power theoretically available in the fuel is lost to exhaust at the end of the stroke. Retarded ignition always causes overheating of the exhaust system.

If the arm K is fixed mechanically in its mid position so that the ignition can neither be advanced nor retarded, we have what is known as fixed ignition.

An Ignition Coil suitable for a single cylinder engine is shown in Figs. 50 and 51, in which A and B are the low tension terminals and C is the high tension terminal. The trembler blade is shown at D, with the adjusting screw F and the platinum-tipped contacts G₁, G₂. The iron core of the coil projects a little above the case, as shown at E in Fig. 50. The strength and character of the spark may be varied considerably by slightly screwing F up or down. When current is supplied to the low tension terminals of the coil it flows through the primary winding and magnetizes the iron core, completing its circuit by passing across the platinum contacts. When the trembler blade is attracted to the iron core the primary circuit is broken by the temporary separation of the platinum contacts, and therefore the magnetism ceases, the trembler is released, and the circuit is completed again. Thus the trembler blade is set rapidly vibrating and making and breaking the primary circuit as long as the roller attached to the rotating arm H of the low tension contact breaker shown in Fig. 52 is in contact with the metal segment K, and this results in the production of a succession of sparks at the sparking plug which is connected to the terminal C of the high tension winding. This is very useful especially when starting an engine, but with modern high-speed engines the trembler has only time to give one spark at high engine speeds, and therefore the magneto has the advantage except for easy starting. This has led to the introduction of dual ignition systems, and in particular to that system in which the main ignition is by magneto, but there is a supplementary coil fitted to supply high tension current to the ordinary high tension magneto distributor when the engine is at rest, the coil being cut out after the engine has got up speed. But this has been largely superseded by the use of electric motors for starting the engine, although the magneto is still relied upon for the ignition of the charge in the cylinders. The contact breaker and coil just described would be very suitable for a single cylinder petrol engine, or a non-trembler coil might be used in conjunction with a contact breaker of the quick break type used on magnetos and illustrated in Fig. 48. In the case of a multi-cylinder engine having coil ignition we may use separate coils without a high tension distributor, or a single coil and a high tension distributor having as many segments as there are engine cylinders and arranged similarly to the magneto distributor of Fig. 48. When no high tension distributor is fitted there must be a separate coil for each cylinder, and the high tension wire runs direct from the coil to the sparking plug, so that the character of the spark as well as the exact instant at which it occurs may not be the same in each of the cylinders. If there is a high tension distributor it should be mounted on the same driving spindle as the low tension contact breaker, in order that the ignition may be synchronized, i.e., the spark will occur at the same point in the piston’s stroke for all the cylinders. The ignition may be advanced or retarded by moving the casing of the low tension contact breaker relative to the roller arm, thus causing it to make contact either earlier or later in the revolution.

At one time it was thought that two-point ignition gave increased power and efficiency. Two-point ignition means simultaneous firing of the charge from more than one plug. Sometimes two high tension leads were led from each distributor segment and connected to the two plugs in the corresponding cylinder—this constituted the parallel system. Another system employed a special plug with both electrodes insulated from the engine frame; this was coupled in series with an ordinary plug so that the spark jumped the gaps in succession. It is quite evident, however, that if the gas is thoroughly mixed up and in a state of violent agitation as the result of rapid compression, a single well-placed spark will fire it successfully and so no gain results from simultaneous ignition at another and less favoured point.

Wiring Diagram for Magneto Ignition System.—The electrical connexions are extremely simple in the case of a high tension magneto ignition system. In Fig. 53 we show a four-cylinder engine fitted with high tension magneto. The only wires required are the four high tension cables from the high tension distributor to the sparking plugs and the earthing wire leading from the short circuiting terminal to the frame of the engine through a switch as indicated. The firing order of the cylinders may be either 1, 3, 4, 2 or 1, 2, 4, 3, as desired (provided the cranks are arranged in the usual manner, that is, in the order shown in Fig. 21). In determining the order of firing of the respective cylinders the engine should be turned round very slowly by hand and careful note made of the order in which the firing strokes occur. To determine the firing stroke the piston should be moving downwards and the position of the valves noted; if both valves are shut then this is the firing stroke, but if the inlet valve is opening it is the suction stroke.

Wiring Diagram for a Coil Ignition System.—The electrical connexions for a coil ignition system are slightly more difficult to follow out; they are shown in Fig. 54 for the same engine illustrated in Fig. 53. In the diagram we show four separate trembler pattern coils, each of which can give a succession of sparks as long as contact is being made on any one segment of the low tension contact breaker connected to it. All the low tension terminals of the coils are connected together to a common busbar, which is supplied with current from the accumulator direct. The current flows from the busbar through the low tension windings of each of the coils in turn, as it comes into operation through the engine-driven contact breaker, and returns to the battery through the frame of the engine. High tension cables lead from the high tension terminal of each coil direct to the sparking plugs, and therefore the ignition is not necessarily synchronized.

When the switch in the low tension circuit is opened the ignition is off, because the current is then permanently interrupted; when the switch is closed the ignition is on. To economize current a quick make-and-break device should be used instead of the wipe form of contact breaker illustrated, and a non-trembler coil used. It is very important to fully retard the ignition lever when starting an engine having coil ignition, because it is very liable to backfire and injure the operator’s wrist; with magneto ignition this is less liable to happen.

Timing the Ignition.—Various instructions are given from time to time for correctly timing magneto ignition, but the following will be found to give satisfactory results. First ascertain the firing order of the cylinders as explained above, and then bring No. 1 piston on to the top dead-centre. Rotate the driving spindle of the magneto until the carbon brush H of the high tension distributor makes contact on the segment connected to the lead marked (1). If the leads are not marked it will be necessary to determine which is No. 1 by observing the direction of rotation of the brush. Next adjust the position of the driving spindle very carefully by turning it to and fro, so that when the ignition lever K (see Fig. 48) is in its mid position the platinum contacts F_l, F₂ are fully separated, the brush H still being on segment No. 1. Then push the magneto gear wheel into mesh with the engine gear wheel which is to drive it, and firmly bolt down the magneto to its bracket. Similar instructions may be followed out for the coil ignition system.