Flywheel, heavy wheel attached to a rotating shaft so as to smooth out delivery of power from a motor to a machine. The inertia of the flywheel opposes and moderates fluctuations in the speed of the engine and stores the excess energy for intermittent use.
This is a flywheel for the Isuzu FTR model truck. The flywheel provides mass for rotational inertia to keep your engine running. Otherwise, the engine will stall when you let your foot off the accelerator. It balances the engine. A flywheel is specifically weighted to the crankshaft to smooth out the rough feeling caused by even a slight imbalance.