Page 247 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

Working

As the magneto rotates, the voltage and current are generated in the primary circuit, and circuit is completed
passing the current through the contact breaking point and through the ground.
As the current passes through the primary coil through the contact breaker, the circuit is completed through
ground.
As the camshaft rotate, the camp opens the contact breaker and interrupt the flow of current in the primary.
This causes the decay in the magnetic field lines and cuts the line of magnetic field in the secondary, and a high
voltage is generated in the secondary.
The process of generating the high voltage in the secondary is known as an induction phenomenon.
The voltage generated in the secondary depends upon the ratio of number of turns in the secondary and primary
winding
Magneto ignition system is similar to the battery ignition system.
Main difference is that it has own current generating device instead of battery.
It is mostly used in moped, motorcycle, scooter and racing car.

As most of the parts of both types are same except battery, working of magnetic ignition system is similar to the
battery ignition system.
Application of magneto ignition system
It is used in racing car and aero plane engines since the magneto system provides a strong spark at high speeds.

It is preferred in two wheelers because of its low weight and low maintenance.
Advantages of Magneto System
1 Less maintenance
2 Light in weight and occupies less space
3 Provides high intensity spark at high speeds.
4 System is reliable.
Disadvantages of Magneto System
1 Since wirings carry high voltage current, there is a strong possibility of leakage which may cause misfiring of
engine.
2 The system requires extensive shielding to prevent leakage of high voltage current.
3 At low speeds it develops poor quality of spark at the time of starting.
3 Electronic ignition system
Principle of electronic ignition system
A timer is used in the distributor of electronic ignition system. It sends electrical pulses to an electronic control unit
(ECU) which switches off the flow of cur- rent to the primary winding as a result, a high volt- age is induced in the
secondary winding which is then distributed to the spark plugs as in the case of the breaker point ignition system.
The electronic control unit later switches on the flow of current to the primary circuit so that the primary circuit
current can be built up for the next cycle.
The timer may be a pulse generator or a hall effect sensor.
A pulse generator consists of a permanent magnet a timer coil and reluctor . The permanent magnet and the
time coil remain stationary. The reluctor is fitted to the distributor shaft and is in the shape of a wheel with teeth.
When the distributor shaft revolves, the reluctor wheel rotates, its teeth moving very near to the pole plates of the
permanent magnet.
The reluctance of the air gap between the other inductor tooth and the timer coil is reduced. At the same time,
the reluctance of the air gap between the reluctor tooth and the magnet is also reduced. This results in a strong
magnetic field around the timer coil.

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CITS : Automotive - Mechanic Diesel - Lesson 71 - 75