The Petrol Engine / A Text-book dealing with the Principles of Design and Construction, with a Special Chapter on the Two-stroke Engine

by Francis John Kean

Fig. 52.—Low Tension Contact
Breaker for Single Cylinder Coil
Ignition System (Wipe Contact).

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 But this has been largely superseded by the use of electric motors for starting the engine, although the magneto is still relied upon for the ignition of the charge in the cylinders.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.

Fig. 53.—Wiring Diagram for Four Cylinder
Engine with High Tension Magneto Ignition.

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.