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.
Understanding Automotive Technology: How a Flywheel Works
An engine does not produce turning force continuously. In a four stroke engine, each cylinder makes useful power during only one part of its cycle. The other strokes draw in air, compress the mixture, or push exhaust gases out.
During these parts, the crankshaft must use energy already stored in the rotating parts. A flywheel helps carry the crankshaft through them.
Engines with more cylinders have power strokes closer together, so their rotation is naturally smoother. A single cylinder engine has long gaps between power strokes, so it relies much more heavily on a flywheel.
The important feature is not only how much metal the flywheel contains. It is where that metal sits. Material near the outer rim has a much stronger effect than material near the center because it moves faster around the axis.
This is why many flywheels are shaped with a thick outer section. Moving mass outward increases the resistance to changes in rotational speed without making the whole part equally thick. Engineers choose the size carefully.
Too little rotational inertia can make idle speed rough and make stalling easier. Too much can make the engine feel slow when the driver presses or releases the accelerator.
In a manual transmission vehicle, the flywheel has another major job. Its flat face provides the surface that the clutch disc presses against. When the clutch pedal is released, friction locks the engine to the transmission and sends torque to the wheels.
When the pedal is pressed, the clutch separates so gears can be changed. Flywheels can become damaged by overheating, wear, or an uneven clutch surface. A mechanic may machine a worn flywheel flat again if it remains within the maker's thickness limit.
Many automatic vehicles use a thinner flexplate instead of a heavy clutch flywheel. The flexplate connects the crankshaft to the torque converter and often carries the starter ring gear.
The starter system shows another everyday use. Teeth around the flywheel edge mesh with a small gear on the starter motor. The starter turns the flywheel, which turns the crankshaft until combustion begins.
Once the engine runs, the starter gear pulls away. Students should separate rotational speed from torque when studying this system. Torque is the twisting effect that can change rotation.
A large flywheel does not create extra engine power. It temporarily stores energy from earlier power strokes and returns it later. Its smoothing effect is most noticeable at low engine speeds, during idling, pulling away from a stop, and when an engine is under a changing load.
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.