Principle: The four cylinder internal combustion engine, during the four stroke cycle, converts the chemical energy stored in their fuels into heat energy when fuel is burnt. The fuel energy is converted into mechanical energy by the expansion of gases against pistons. The pistons as a result undergo a forward and backward motion. This forward and backward motion moves a connecting rod which in turn makes the crankshaft rotate. It is this rotatory motion which passes through gears and finally makes the wheels rotate. The four stroke cycle
Induction stroke: The inlet valve is open, the exhaust valve closed. The piston descends, inducing a flow of mixture. Soon after this stroke, the inlet valve is closed.	Compression stroke : Both inlet and exhaust valves are closed. The rising piston compresses the mixture in the combustion chamber and compression heat vaporises the mixture.	Power stroke: Both valves remain closed. The compressed gas is ignited by a spark from the spark-plug. Expansion of burning gas drives the piston down. Exhaust valve opens.	Exhaust stroke: The inlet valve is closed, and exhaust valve open. The piston rises to expel burnt gases, inlet valve opens, exhaust valve closes. Then the cycle restarts.
		Arrows indicate strokes of piston and related coloured lines show period valves are open.

Pressure on the piston causes it and the connecting rod to move downwards, and rotate the crankshaft. Four cranks together complete the crankshaft; each crank has a piston and connecting rod to turn it.

Arrangement of the Cranks
A - Front end for fitting pully or vibration damper B - Web extension serves as balance weight C - Journal of crankshaft rotates in a main bearing D - Crankpin carries big-end of connecting rod linked to piston E - Flange to which flywheel is bolted
Fig (A)	With a four-cylinder engine, a 180-degree crank-shaft, arranged as shown in Fig(A) always is used. The crank arms for No.1 and 4 cylinders project in the same direction, and the crank arms for No.2 and 3 cylinders project from the opposite side of the crankshaft. In the four-cylinder engine, No. 1 and 4 piston are always moving in the opposite direction from pistons No.2 and 3 . This arrangement tends to neutralize the primary inertia forces as, if the pistons are equal in weight, they will balance each other and give good primary mechanical balance. (Hence the angle between the pairs of throws is 180 degrees.)
Firing Order
Table 1 (American Standard) Cylinder 1 2 3 4 1st revolution P E C I E I P C 2nd revolution I C E P C P I E   Table 2 (Regular - The moving image above) Cylinder 1 2 3 4 1st revolution P C E I E P I C 2nd revolution I E C P C I P E	Also the Fig.(A) will help to explain the firing order of a four cylinder engine. Firing order is the sequence in which the spark-plugs ignites a series of single sparks per cylinder, in the cylinders. As No 1 piston moves downward on power, No 4 piston must move downward on induction; No 2 piston can be moving upward on exhaust or compression and No 3 piston will be moving upward on compression or exhaust. Table 1 shows the power balance with No 2 piston on exhaust and No 3 piston on compression. Table 2 shows the power balance rusulting from No 2 piston moving upward on compression and No 3 on exhaust. With either arrangement the power impulses are evenly distributed, that is, they are 180 degrees apart. Each arrangement gives a different firing order. That of table 1 gives a firing order of 1-3-4-2 and that of table 2, a firing order of 1-2-4-3. American four-cylinder passenger cars have standardized on a firing order of 1-3-4-2. If the firing order were to be 1-2-3-4, the crankshaft and engine mountings would be subjected to considerable stress and vibration.
I - Induction Stroke, C - Compression Stroke, P - Power Stroke, E - Exhaust Stroke