The speed of rotation of the crankshaft increases each time the shaft receives a power impulse from one of the pistons. The speed then gradually decreases until another power impulse is received. If permitted to continue unchecked, these fluctuations in speed (their number depending upon the number of cylinders firing on one crankshaft revolution) would result in an undesirable situation with respect to the driven mechanism as well as to the engine. Therefore, some means must be provided so that shaft rotation can be stabilized. In most engines, this is accomplished by installation of a flywheel on the crankshaft. In other engines, the motion of such engine parts as the connecting rod journals, webs and lower ends of the connecting rods, and such driven units as the clutch and generator serves the purpose. The need for a flywheel decreases as the number of cylinders firing in one revolution of the crankshaft and the mass of moving parts attached to the crankshaft increase.

A flywheel stores up energy during the power event and releases it during the remaining events of the operating cycle. In other words, when the speed of the shaft tends to increase, the flywheel absorbs energy. When the speed tends to decrease, the flywheel gives up energy to the shaft in an effort to keep shaft rotation uniform. In doing this, a flywheel (1) keeps variations in speed within desired limits at all loads; (2) limits the increase or decrease in speed during sudden changes of load; (3) aids in forcing the piston through the compression event when an engine is running at low or idling speed; and (4) provides leverage or mechanical advantage for a starting motor.

Flywheels are generally made of cast iron, cast steel, or rolled steel. Strength of the material from which the flywheel is made is of prime importance because of the stresses created in the metal of the flywheel when the engine is operating at maximum designed speed.

In some engines, a flywheel is the point of attachment for items such as a starting ring gear or a turning ring gear. (See fig. 4-27.) The rim of a flywheel may be marked in degrees. With a stationary pointer attached to the engine, the degree markings can be used for a determination of the position of the crankshaft when the engine is being timed.