An engine requires a rotating mass to build _________ to keep it running between power pulses.

An engine requires a rotating mass to build _________ to keep it running between power pulses.

• A. Pressure
• B. Speed
• C. Friction
• D. Inertia

Answer: (D)

The rotating mass provides inertia that stores kinetic energy to smooth out the intermittent torque from each power pulse. In an engine, each cylinder fires at intervals, so without a reservoir of energy the crankshaft would slow between firings and the engine would stumble or stall. A heavy rotating mass (the flywheel) resists changes in speed, keeping the crankshaft turning and delivering a steadier rotation until the next pulse. The energy stored in this rotating inertia depends on the moment of inertia and how fast it’s spinning. This is why inertia is the right concept here. Pressure, friction, and speed don’t capture that smoothing energy storage in the rotating system.