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A component of an internal combustion engine that is relatively simple. It's a heavy wheel that is attached to the output shaft of the engine. When you get right down to it; it's there to store energy and to ensure that the motor runs smoothly. It basically does this by storing energy in its angular momentum. This means that when, for example, you hit a temporary heavy load, the engine keeps going because it draws on the energy from the flywheel to overcome the heavy load. For example, take a lawnmower. Say you hit a patch of weeds or something. If there was no flywheel, the engine would probably stop dead; but with a flywheel, the flywheel's angular momentum will reduce the slowing down that happens.

Similarly, when the engine, for some reason, speeds up, the flywheel makes sure that the engine speeds up in a regular, smooth manner, instead of quickly jumping to a new high speed, which would damage the engine.

Recently, flywheels have been used as a means of storing energy outside of internal combustion engines. So, you attach a motor/generator to it. When you want to put energy into it, you use the motor to make it spin faster. When you want to take energy out, you connect a load across the motor/generator and it acts as a generator.