Requirement of the ignition system and ignition/firing order in the IC engine

The ignition system is the most important part of the SI engine (spark ignition engine). Different types of ignition systems are used in the Internal Combustion SI engine. Ignition system used to ignite air-fuel mixture at the proper time and in a well suitable manner with the wild range of engine speed.

A requirement of the ignition system

Ignition system mainly needed in an engine due to the following reasons.

It should provide a good spark between the electrodes of the plug at the correct timing.
The ignition system should function efficiently over the entire range of engine speed continuously.
Also, it should be light, effective, and reliable in service.
It should be compact and easy to maintain.
It should be cheap and convenient to handle.
The interference from the high voltage source should not affect the functioning of the radio and television receivers inside an automobile.

Firing order in the IC engine

The order in which various cylinders of multiple cylinder engine fire is called firing order.

The main factor considered deciding the order of firing the engine cylinder are the balancing of the reciprocating engine to reduce vibration, engine cooling, and the development of backpressure.
By balancing reciprocating and rotary parts, the unbalanced force and couple transmitted to the bearing of the engine shaft are reduced.
Considering 4 cylinders inline engine, if the firing order is kept in line as 1-2-3-4, the exhaust of the cylinder will overlap. It would need a larger size of the exhaust pipe. Otherwise, the backpressure on the engine cylinder will increase.
Thus the firing order of the engine is decided based on the above consideration for 4 cylinder engines. The firing order is usually kept as 1-3-4-2 or 1-2-4-3. The firing timing order 1-3-4-2 is more commonly used.
Firing order for six-cylinder inline engine usually are 1-5-3-6-2-4, 1-5-4-6-2-3 and 1-2-3-6-5-4. The firing timing order 1-5-3-6-2-4 is most commonly used.

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