Firing Order

Every engine cylinder must fire once in every cycle. This requires that for a four-stroke four- cylinder engine the ignition system must fire for every 180 degrees of crank rotation. For a six- cylinder engine the time available is only 120 degrees of crank rotation.
The order in which various cylinders of a multi cylinder engine fire is called the firing order. The number of possibilities of firing order depends upon the number of cylinders and throws of the
crankshaft. It is desirable to have the power impulses equally spaced and from the point of view of
balancing this has led to certain conventional arrangements of crankshaft throws. Further, there are
three factors which must be considered before deciding the optimum firing order of an engine. These
(i) Engine vibrations
(ii) Engine cooling and
(iii) Development of back pressure.
Firing Order
Firing Order
For 4-Cylinder engines the possible firing orders are: 1-3-4-2 or 1-2-4-3
The former is more commonly used in the vertical configuration of cylinders

For a 6-Cylinder engine firing orders can be: 1-5-3-6-2-4 or 1-5-4-6-2-3 or 1-2-4-6-5-3 or 1-2-3-6-5-4
The first one is more commonly used.
Other Firing Orders

For 3 Cylinder engine 1-3-2
8 Cylinder in-line engine 1-6-2-5-8-3-7-4
8 Cylinder V shape engine 1-5-4-8-6-3-7-2, 1-8-4-3-6-5-7-2, 1-6-2-5-8-3-7-4, 1-8-7-3-6-5-4-2, 1-5-4-2-6-3-7-8
Simplistically speaking one can have any firing order based on the way we design the crank shaft. The idea is simply to maximize the power transferred to crank, based on one by one pushing of the piston. But things are not so simple here. Choice of order is a consideration for vibration, power, efficiency, fuel consumption etc. With a lot of research companies decide their specific firing order based on the size of engine they are designing for . 
In case of SI engines the distributor connects the spark plugs of different cylinders according to Engine Firing Order.
(a) A proper firing order reduces engine vibrations
(b) Maintains engine balancing.
(c) Secures an even flow of power. 
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