Flywheel is a disk with a large moment of inertia. Its main function is to store part of the kinetic energy input to the crankshaft in the power stroke, so as to overcome the resistance in other strokes, drive the crank connecting rod mechanism over the top dead center and bottom dead center, ensure the rotation angular speed and output torque of the crankshaft as uniform as possible, and make the engine overcome the overload in a short time as possible.

In addition, in structure, flywheel is often used as the driving part of friction clutch in automobile transmission system. Flywheels are mostly made of gray cast iron. When the linear speed of the wheel flange exceeds 50M / s, the high strength ductile iron or cast steel should be used.