flywheel
n. countable C1 Advanced US //ˈfɫaɪˌwiɫ// UK //flˈaɪwiːl// fly·wheel
n. a heavy wheel that spins inside a machine to keep it moving smoothly. It stores energy so the machine doesn't stop when the power goes out.
n. a mechanical device used to store rotational energy and provide a smooth, continuous output of power. It consists of a heavy wheel with a large moment of inertia, designed to maintain momentum during fluctuations in the power source.
The flywheel helps the engine run more smoothly.
In a steam engine, the flywheel ensures that the pistons continue moving even when the pressure drops between cycles.
Modern hybrid vehicles often use a flywheel to capture kinetic energy during braking, which is then released to assist the engine during acceleration, improving overall fuel efficiency.
From fly + wheel.