What is a Flywheel? Its Working, Functions, Selection Criteria & FAQs (with PDF)

flywheel feature

Last Updated on March 16, 2023 by Noaman Adenwala

What is a Flywheel?

A flywheel is a heavy disk-like structure used in the machinery which acts as a storage device to store energy when energy input exceeds demand and releases energy when energy demand exceeds supply. In steam engines, internal combustion engines, reciprocating compressors, and pumps, energy is produced during one stroke, and the engine is designed to run the entire cycle with the energy produced during that one stroke.


For example, in an Internal Combustion engine, energy is produced only during the power stroke, which is much higher than the engine load. For a 4-stroke engine, no energy is produced during the intake, compression, and exhaust strokes, and for a 2-stroke engine, during compression. The extra energy generated during the power stroke is absorbed by the flywheel and released to the crankshaft during other strokes where no energy is generated, causing the crankshaft to rotate at a steady speed.

How does a Flywheel Work?

The basic working principle of a flywheel is to absorb rotational energy during the power stroke and release that energy during the other strokes (Suction stroke, Compression stroke, Exhaust Stroke). First of all, power is given to the flywheel in the early stages. It moves the crankshaft & thereby moves the piston in the cylinder. Fuel burns out in the combustion chamber during the compression stroke. When the Power stroke is activated, the flywheel draws power from the Power stroke, which is used in three other strokes. In this way, it helps stabilize the rotary motion of the transmission system.

Energy equation

The energy equation for the flywheel depends on the angular velocity and moment of inertia of the flywheel. It is therefore clear that the energy stored in the flywheel increases with increasing weight, size, and angular velocity.

How is Energy Stored in a Flywheel?

Well, you can compare it with the mechanics of a mechanical battery. A battery stores energy in chemical form, whereas a flywheel stores power in the form of motion or kinetic energy, to be exact. A little consideration shows that when the flywheel absorbs energy it speeds up and when it releases it slows down. So instead of maintaining a constant speed, the flywheel only reduces speed fluctuations.


A flywheel can store more energy if it spins faster or has a higher moment of inertia. But spinning faster than adding mass always works best. In machines that operate intermittently, such as punch presses, shears, riveters, and crushers, flywheels store energy from the power source for most of the operating cycle and release energy only for short periods of the cycle. Thus, energy from the power source is supplied to the machine throughout the operation at a constant rate.

What is the Function of a Flywheel?

A flywheel helps the vehicle run smoothly. A flywheel makes it easier to overcome short-term overloads, such as when starting the engine from the rest condition. It stores large amounts of energy and releases it again when needed. Flywheels are sometimes used to deliver intermittent pulses of energy. By absorbing energy into the flywheel over time and releasing it quickly, it delivers energy beyond the capabilities of the machine. Example: riveting machine.

  • It suppresses torque fluctuations and makes the rotation of the crankshaft smoother.
  • It balances the crankshaft of the engine.
  • A flywheel allows the mechanism to continue past the dead center.
  • It also helps charge the battery.

How to Choose the Correct Flywheel for Your Engine?

Flywheel selection depends on many factors, including engine torque, vehicle weight, transmission, and intended use. Choosing the right weight is critical to enhance your driving experience. Weight depends on flywheel diameter. Therefore, the fewer cylinders an engine has, the lighter the flywheel will be. A standard weight flywheel can store more energy. As a result, it has proven to be suitable for moving stock transmission vehicles as well as heavier vehicles such as trucks, drag cars, and rock crawlers.

In addition, heavy flywheels slow throttle response and reduce engine RPM when decelerating. A lighter flywheel can provide the inertia of a heavier flywheel. However, as the RPM increases, a slightly modified driving style is required to accommodate the clutch slipping to keep the vehicle moving. In general, the lighter the flywheel, the heavier the vehicle will feel. Of course, there are exceptions that depend on the model of the flywheel.

Frequently asked questions (FAQs)

Why does the flywheel rotate clockwise in an engine?

The flywheel is of little help in determining the direction of rotation of the engine. It’s easy to see which direction the engine is spinning by looking at the engine flywheel. This is because the flywheel does not have its own direction of rotation. However, please follow the direction of the installed engine and see it from the flywheel side. If it rotates clockwise, the engine is clockwise. Similarly, if the engine’s flywheel rotates counterclockwise, it is of the left-handed type.

Heavy multi-cylinder engines such as industrial and marine. These flywheels can also be used to determine the piston position within the cylinder. You can see which cylinder has the piston at TDC by looking at the cylinder markings on the flywheel. This can be used to determine the firing order of the engine. If the engine has been stopped for a long time and needs flushing. These flywheels are really useful. To bleed air from the engine, you must turn the flywheel through the engine either manually or with the indicator valve open.

Engines can rotate both clockwise and counterclockwise, but in general, most engines rotate counterclockwise. However, it is not a hard and fast rule as which one to choose is a matter of the designer’s decision. Therefore, it is always necessary to know the correct direction of rotation of the engine. Either by the engine manual or by visual inspection of the flywheel.

Why are flywheels having teeth?

Not all flywheels have teeth, but most flywheels on cars, motorcycles, heavy generators, and ships do. but why? As you know, flywheels help maintain constant power output. But how can you get that power if you don’t start the engine? If you own or have a vehicle, you should know that you have to press the pedal or start it yourself to keep the engine running. It is to rotate a flywheel through a small gear attached to the teeth of a large diesel engine. These flywheel teeth are used for high-pressure fuel in large diesel engines where compressed air is used to start the engine.

Is flywheel & governor the same?

Both the flywheel and the governor are used to regulate engine speed but in another way. On the one hand, the flywheel regulates the speed of the engine with different strokes. This is to keep the average speed constant.

However, governors are used to regulating the average speed of an engine with variable loads. This is achieved by regulating fuel injection to meet the power demand. If the load increases sharply; the engine begins to slow down. The governor increases the fuel injected into the cylinder to produce more torque, thereby increasing its RPM to keep the speed constant. Although flywheels & governors appear to do the same thing in actuality, they are different.

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