Firing order in IC engine

The order in which the ignition take place in various cylinders of a multi cylinder engine is called firing order. Every engine cylinder must fire once in every cycle. Three factors must be considered before deciding the firing order in IC engine. These are
Engine Vibrations: Load on the bearing should not be imbalanced due to firing order of cylinder so firing order should be kept in such an order that the load is equally distributed on bearings. The imbalance load on the two bearings would result in severe engine vibrations.
Engine cooling: The firing should be in such an order that the cooling system should work in an effective manner. The cooling position should not change its position with time otherwise the task of the cooling system becomes very difficult.
Development of back pressure: There should be sufficient time and space for the exhaust gases to travel in the exhaust pipe otherwise the danger of back flow will rise.
The commonly used firing orders for engines are:
Three cylinder inline engine: 1-3-2
Four cylinder inline engine: 1-2-4-3, 1-3-4-2
Six cylinder inline engine: 1-5-3-6-2-4, 1-5-4-6-2-3, 1-2-4-6-5-3, 1-2-3-6-5-4
Eight cylinder inline engine: 1-6-2-5-8-3-7-4
Eight 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