A flywheel is an engine component mounted over the crankshaft. It functions to maintain the constant speed of the reciprocating motion of the piston. It helps to store energy during the power stroke and release it during the other strokes.

• Flywheels are energy-storing devices used in internal combustion engines, compressors, mechanical power presses, and machine tools.
• In an internal combustion engine, if a flywheel is not fitted, the crankshaft will accelerate on the combustion stroke and retard on the exhaust, induction, and compression strokes; thus the crankshaft will speed up and slow down during each revolution.
• The flywheel, when fitted, absorbs the excess energy produced during the combustion stroke and releases it as required to maintain nearly uniform motion during the remaining parts of the cycle.
• In a mechanical power press, if a flywheel were not fitted, the electric motor would need to be very large to provide all the energy necessary during the working stroke.
• The flywheel, when fitted, slowly absorbs kinetic energy from the constant-speed constant-torque motor during the non-working part of the cycle and then releases it very quickly to assist the motor during the working stroke.
• During the working stroke, the speed of the flywheel is decreased, while during the non-working cycle its speed is increased.