Magneto Ignition System

A magneto ignition system is a unique ignition system that uses an electric generator to provide the required energy for the vehicle (automobile) system. It is mounted on the engine and replaces all components of the coil ignition system except the spark plug. When the motor rotates, a magneto can produce a very high voltage and doesn't need a battery as a source of external energy.

Figure 1 shows a schematic diagram of a high-tension magneto ignition system. This system incorporates the windings to generate the primary voltage and set it up, thus not requiring the spark plug to operate.

Figure 1: Schematic Diagram of Magneto Ignition System

Magneto ignition system can be either a rotating armature or a rotating magneto type.

1. Magneto ignition system can be either a rotating armature or a rotating magneto type.
2. In the first type, the armature consists of the primary and secondary windings, which all rotate between the poles of a stationary magnet.
3. In the second type, the magnet revolves, and windings are kept stationary.

The third type of magneto is called the polar inductor type. In this type of magneto, the magnet and the windings remain stationary. Still, the voltage is generated by reversing the flux field with the help of soft iron polar projections called inductors.

The working principle of the magnetic ignition system is the same as that of the coil ignition system. With the help of a cam, the primary circuit flux is changed, a high voltage is produced, and the cam changes.

The variation of the production with the speed fornication system and the magnetic ignition system is shown in Graph 1. Since the cranking speed at stat is low, the current generated by the magneto is relatively tiny. As the engine speed increases, the current flow also increases. Thus, with magneto, there is always a starting difficulty; sometimes, a separate battery is needed. The magneto ignition system is best suited at high speeds and is widely used in sports and racing cars, aircraft engines, etc. The variation of the breaker current with speed for the coil ignition system and the magnetic ignition system is shown in Graph 1. Since the cranking speed at stat is low, the current generated by the magneto is relatively tiny. As the engine speed increases, the current flow also increases. Thus, with magneto, there is always a starting difficulty; sometimes, a separate battery is needed. The magneto ignition system is best suited at high speeds and is widely used in sports and racing cars, aircraft engines, etc.

Graph 1: Breaker current vs speed in coil and magneto ignition system

In comparison, the battery ignition system is more expensive but highly reliable. Because of the magneto system's poor starting characteristics, the battery ignition system is invariably preferred to the magneto system in automobile engines. However, a magneto ignition system is favored in two-wheelers because it is lightweight and less maintenance.

The main advantage of the high-tension magneto ignition system is that the wirings carry a very high voltage. Thus, there is a strong possibility of causing engine misfire due to leakage. To avoid this, the high-tension wires must be suitably shielded. Developing the low-tension magneto system is an experiment to prevent this trouble. In the low-tension magneto ignition 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. All these changes limit the high voltage current in the small part of the ignition system wiring and thus avoid the possibility of leakage, etc.