MAGNETO IGNITION SYSTEM
Magneto is a special type of ignition system with its own electric generator to provide the necessary energy for the system. It is mounted on the engine and replaces all the components of the coil ignition system except the spark plug. A magneto when rotated by the engine is capable of producing a very high voltage and it does not need a battery as a source of external energy.
The high tension magneto incorporates the windings to generate the primary voltage as well as to step up the voltage and thus does not require a separate coil to boost up the voltage required to operate the spark plug.
Magneto can be either rotating armature type or rotating magnet type. In the first type, the armature consisting of the primary and secondary windings all rotate between the poles of a stationary magnet. whilst, in the second type the magnet revolves and the windings are kept stationary.
A third type of magnet to called the polar inductor type is also in use. In the polar inductor type magneto both the magnet and the windings remain stationary but the voltage is generated by reversing the flux field with the help of soft iron polar projections called inductors. The working principle of the magneto ignition system is exactly the same as that of the coil ignition system. With the help of a cam, the primary circuit flux is changed and a high voltage is produced in the secondary circuit.
since the cranking speed at start is low the current generated by the magneto is quite small. As the engine speed increases the flow of current also increases. Thus, with magneto there is always a starting difficulty and sometimes a separate battery is needed for starting. The magneto is best at high speeds and therefore is widely used for sports and racing cars, aircraft engines etc.
In comparison, the battery ignition system is more expensive but highly reliable. Because of the poor starting characteristics of the magneto system invariably the battery ignition system is preferred to the magneto system in automobile engines. However, in two wheelers magneto ignition system is favered due to light weight and less maintenance.
The main disadvantage of the high tension magneto ignition system lies in the fact that the wirings carry a very high voltage and thus there is a strong possibility of causing engine misfire due to leakage. To avoid this the high tension wires must be suitably shielded. The development of the low tension magneto system is an attempt to avoid this trouble. In the low tension magneto system the secondary winding is changed to limit the secondary voltage to a value of about 400 volts and the distributor is replaced by a brush contact. The high voltage is obtained with the help of a step-up transformer.
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