Bryan Jarvis, age 15, of Barre, Vt., for his question:

WHAT DOES A FLYWHEEL DO?

A flywheel is a heavy wheel attached to the shaft of an engine to keep its speed nearly constant. It is used where the forces driving the engine shaft are not constant.

The driving forces in a gasoline engine come from a aeries of explosions in the engine cylinder. The driving forces produce the power needed by the engine's load. Sometimes, the driving forces become 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 cylinder become momentarily smaller than necessary. Then the flywheel's inertia keeps the speed from decreasing quickly.

These speed fluctuations are small, usually less than one percent. They may occur several times in a single revolution.

The flywheel permits a nearly steady speed.