Flywheel is a heavy steel wheel attached to the rear end of the crankshaft. The size of the flywheel depends upon the number of cylinders and the general construction of the engine. 

The flow of power from the engine cylinder is not smooth although the power impulses in a multi-cylinder engine overlap or follow each other to provide a fairly even flow of power, however additional leveling off of power impulse is required. This is done by a flywheel. To understand the function of a flywheel in a better way, take the example of a four stroke, single cylinder engine. There are times when more power is being delivered than at other times. This tends to make the crankshaft speed up and then slow down. The engine delivers power during one stroke only - the power stroke and it absorbs power during the other three strokes - to push out the exhaust gases, to intake fresh charge in the cylinder and to compress this charge. Thus during power stroke, the engine tends to speed up and during the other three strokes, it tends to slow down. The inertia of the flywheel tends to keep it running at constant speed. When the engine tends to speed up, the flywheel resits it. When the engine tends to slow down, the flywheel resists it. Thus the flywheel absorbs energy as the engine tries to speed up and gives back energy when the engine tries to slow down, keeping the engine speed almost constant.

Flywheel is also used as a part of clutch mechanism and fluid drive unit. The flywheel also has a teeth on its outer edge to mesh with the electric cranking motor driven pinion when the engine is being cranked to start it.