Flywheel is a heavy disc attached to an engine or motor that helps maintain a stable speed. It is used where the forces driving the engine shaft are not constant. The driving forces in a gasoline engine, for example, occur in cycles in which a piston moves back and forth in the engine’s cylinder. The driving forces produce the power needed by the engine’s load. Sometimes, the driving forces grow momentarily larger than necessary for the engine’s load, and the engine speed increases. Then the flywheel absorbs the excess energy and prevents the speed from increasing rapidly. At other times, the driving forces from the piston become momentarily smaller than necessary. Then the flywheel’s inertia—that is, its tendency to stay in motion—keeps the speed from decreasing quickly. The action of a flywheel decreases as the number of cylinders of a gasoline engine increases. For example, less of the engine cycle energy must be temporarily stored by the flywheel with an eight-cylinder engine than with a four-cylinder engine.
