Magneto
Coil ignition system
is a special type of ignition system with its own electric generator to provide
the required necessary 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.
A
magneto, when rotated by the engine, it is capable of producing a very high
voltage and doesn’t need a battery as source of external energy.
A
schematic diagram of a high tension magneto ignition system is shown in the
below figure.
The
high tension magneto ignition system incorporates the windings to generate the
primary voltage as well as to set up the voltage and thus does not require to operate
the spark plug.
Magneto
ignition system can be either rotating armature type or rotating
magneto type.
- In the first type, the armature consisting of the primary and secondary windings all rotate between the poles of a stationary magnet.
- In the second type, the magnet revolves and windings are kept stationary.
- The third type of magneto called the polar inductor type. 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.
Working Principle
The
working principle of the magnetic ignition system is 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.
The
variation of the breaker current with speed for the coil ignition system and
the magnetic ignition system is shown in the graph below. It can be seen that
since the cranking speed at stat is low the current generated by the magneto is
quite small.
*As the engine speed increases the current flow also increases.
Thus, with magneto, there is always a starting difficulty and sometimes a
separate battery is needed for starting.
*The magneto ignition system is
best suitable at high speeds and is widely used in automobiles like sports and
racing cars, aircraft’s engines etc.
In comparison,
the battery ignition system is more expensive but highly reliable.
Because of the poor
starting characteristics of the magneto system, mostly the battery ignition
system is preferred than the magneto system in automobile engines.
However,
in two wheelers magneto ignition system is favored due to light weight and less
maintenance.
The main advantage 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 experiment to avoid 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 distributor is replaced by
a brush contact.
The high voltage is obtained with the help of a step-up
transformer. All these changes have the effect of limiting the high
voltage current in the small part of the ignition system wiring and thus avoid
the possibilities of leakage etc.