[jj]

The flywheel stores up rotation energy during the power impulses of the engine. It releases this energy between power impulses, thus assuring less fluctuation in engine speed and smoother engine operation. The size of the flywheel will vary with the number of cylinders and the general construction of the engine. With the large number of cylinders and the consequent overlapping of power impulses, there is less need for a flywheel; consequently, the flywheel can be relatively small. The flywheel rim carries a ring gear, either integral with or shrunk on the flywheel, that meshes with the starter driving gear for cranking the engine.

...the above was pasted from some engine geek (no, not I) site