Although their exact functions vary by application, flywheels provide engine balance to a vast range of small and large engine applications. These seemingly simple devices come in a variety of shapes and sizes, and ensure consistent power throughout an engine's four cycles, using stored energy in the form of inertia.
Perhaps the most basic kind of flywheel, but equally as important for proper engine balance, is the type found in cars with automatic transmissions. Attached directly to the engine's crankshaft, a flywheel's heavy, 12 to 15-inch steel discs spin along within the engine. This balancing system smooths out the engine's power-strokes -- which can occur at a rate as low as twice per rotation of the crankshaft in a four-cylinder engine -- to provide consistent rotation. The disc's mass and size means that it puts a load on the engine, resisting acceleration during the power-stroke and resisting deceleration between power-strokes.
In addition to balancing the engine's power-strokes, flywheels in manual transmission cars also provide friction for the clutch system. Designed with polished and friction-bearing surfaces, these toothed flywheels ensure proper clutch performance.
In small engine applications, flywheels function in a variety of ways, in addition to balancing the engine. Often these smaller flywheels (compared to automobile flywheels) are finned to provide a primitive cooling system. Additionally, small engine flywheels may contain magnets that produce an electrical charge as the flywheel spins, charging batteries or other components. Since 1982, small engine flywheel brakes have also been required in order to safely stop lawnmower blades within three seconds.
