A flywheel is a heavy rotating disk attached to an engine crankshaft. Its main job is to smooth out the uneven pulses produced by the pistons. Each power stroke gives the crankshaft a strong push, but the engine must keep rotating between those pushes.
The flywheel stores rotational energy so the engine turns more steadily instead of speeding up and slowing down sharply.
Key Facts
- Rotational kinetic energy stored in a flywheel is E = 1/2 Iω^2.
- Moment of inertia measures resistance to changes in rotation, so a larger I means steadier rotation.
- For a solid disk flywheel, I = 1/2 MR^2.
- Torque changes angular speed according to τ = Iα.
- The flywheel absorbs energy when the crankshaft speeds up and releases energy when it slows down.
- A heavier or larger-radius flywheel smooths engine pulses more, but it also makes the engine respond more slowly to throttle changes.
Vocabulary
- Flywheel
- A rotating disk attached to the crankshaft that stores energy and helps keep engine rotation smooth.
- Crankshaft
- The rotating shaft in an engine that converts the pistons' back-and-forth motion into rotation.
- Power stroke
- The part of the engine cycle when expanding gases push a piston and add energy to the crankshaft.
- Moment of inertia
- A measure of how strongly an object resists changes in its rotational motion.
- Angular velocity
- The rate at which an object rotates, usually measured in radians per second.
Common Mistakes to Avoid
- Thinking the flywheel creates extra energy is wrong because it only stores energy from the engine and returns some of it later.
- Confusing mass with moment of inertia is wrong because where the mass is located matters, and mass farther from the axis stores more rotational energy.
- Assuming a heavier flywheel always improves performance is wrong because it smooths rotation but can make acceleration and engine response slower.
- Ignoring engine pulses is wrong because piston engines do not deliver perfectly steady torque, and the flywheel helps fill the gaps between power strokes.
Practice Questions
- 1 A solid disk flywheel has mass 12 kg and radius 0.25 m. Find its moment of inertia using I = 1/2 MR^2.
- 2 A flywheel has moment of inertia 0.38 kg m^2 and angular speed 120 rad/s. Calculate its stored rotational kinetic energy using E = 1/2 Iω^2.
- 3 Explain why an engine with no flywheel would tend to shake, stall more easily at low speed, or rotate unevenly between power strokes.