Fig. 47.—Outside View of a High Tension Magneto.
Fig. 48.—End View of a High Tension Magneto, showing High Tension Distributor and Low Tension Contact Breaker.
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.
Fig. 49.—End View of a High Tension Magneto,
showing the Earthing Terminal (P).
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.