*flywheel*

n. A heavy wheel or disk which stores kinetic energy by rotating on a shaft, and by its momentum smoothes the operation of a reciprocating engine by reducing fluctuations of speed. It is used in certain types of machinery, such as automobiles. [ WordNet 1.5 +PJC ]

☞ Flywheels rotating at high speed have also been proposed as a means to store kinetic energy for use as a low-polluting source of energy in vehicles. [ PJC ]