In magneto ignition system, the battery is replaced with a magneto. Fig. shows the wiring diagram of a magneto ignition system. It consists of a switch, magneto, contact breaker, condenser, distributor and spark plugs. This system is used in two wheelers like motor cycles, scooters.
The magneto ignition system consists of a rotating magnet assembly driven by a engine and a fixed armature. The armature consists of primary and secondary windings. The primary circuit consists of a primary winding condenser and contact breaker. The secondary circuit consists of a secondary windings, distributor and spark plugs.
When the contact breaker points are closed:
- The current flows in the primary circuit.
- This produces a magnetic field in the primary winding.
- When the primary current is at the highest peak, the contact breaker points will be opened by the cam.
When the contact breaker points are opened:
- There is a break in the primary circuit.
- The magnetic field in the primary winding is suddenly collapsed.
- A high voltage (15000 volts) is generated in the secondary winding.
- This high voltage is distributed to the respective spark plugs through the rotor of the distributor.
- The high voltage tries to cross the spark plug gap and a spark is produced in the gap. This spark ignites the fuel-air mixture in the engine cylinder.
Advantages and disadvantages of Magneto ignition system:
- It has no maintenance problem like coil ignition (i.e. for battery). So, it is more reliable.
- When the speed increases, it provides better intensity of spark and thus provides better combustion as compared to battery coil ignition system.
- Less space is required as compared to battery ignition system.
- It is very light in weight and compact in size.
- Initial cost is very high as compared to coil ignition system.
- Minimum 75rpm necessary to start the engine.
- For higher power engines, some other devices are necessary to start ignition.
Comparison of Magneto and Coil Ignition System
Simplicity: A coil ignition system requires greater attention against possible defects because the wiring is quite complicated. On the other hand, the wiring of a magneto ignition system is comparatively simple and it forms a compact unit.
Cost: Due to less precision work, the manufacturing cost in coil ignition system is less than that in magneto ignition.
Starting and low speed operation: At the time of starting and at low speeds, the strength of the spark in magneto ignition is low while a good spark is given by the coil ignition.
Strength of spark at high speeds: With the increase in speed, the strength of spark, given by the magneto ignition system increases but it decreases in the case of coil system as shown in fig. Therefore, coil system ignition is unsuitable for high speed racing cars and aeroplanes.
Dependence on battery and charging dynamo: The operation of the coil ignition system is greatly influenced by the
condition of the battery and charging dynamo. If a car stands for a few weeks and its battery gets discharged, it becomes difficult to start the engine even by hand cranking. But there is no such difficulty experienced in magneto ignition system which is more reliable.