Expansion of burning gas drives the piston down. Exhaust valve opens. 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. 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. Firing order is the sequence in which the spark-plugs ignites a series of single sparks per cylinder, in the cylinders .