Short Answer:

A flywheel is a heavy rotating mechanical device used to store energy in the form of rotational motion. It helps maintain uniform engine speed by balancing the power produced during the power stroke and supplying energy during the other strokes when no power is generated. Flywheels are mounted on the crankshaft of engines and play a key role in ensuring smooth and steady engine operation.

The flywheel absorbs excess energy when the engine speed increases and releases stored energy when the engine speed decreases. This balancing action helps prevent fluctuations in engine speed, provides smoother motion, and improves mechanical efficiency. It also assists in starting the engine and transmitting power to other parts.

Detailed Explanation :

Flywheel

A flywheel is a heavy circular disc or wheel attached to the crankshaft of an internal combustion engine. Its main function is to store rotational energy and maintain a constant speed of the crankshaft throughout the engine cycle. Since power in a reciprocating engine is produced only during the power stroke, the flywheel provides the required energy during the remaining three strokes — suction, compression, and exhaust — where no power is developed. This helps the engine run smoothly and efficiently.

The flywheel is made of cast iron, steel, or composite materials, and it has a large moment of inertia, which means it resists sudden changes in rotational speed. The amount of energy stored in a flywheel depends on its mass, shape, and rotational speed. The heavier the flywheel or the faster it spins, the more energy it can store.

When the engine runs, during the power stroke, the piston moves down due to the explosion of the air-fuel mixture, generating more energy than is needed at that instant. The flywheel absorbs this excess energy by speeding up slightly. During the next strokes — intake, compression, and exhaust — when the piston needs energy to continue its motion, the flywheel releases the stored energy, keeping the crankshaft moving at a steady rate. This exchange of energy ensures that the crankshaft rotates uniformly, avoiding jerky or uneven motion.

Flywheels are generally attached at the rear end of the crankshaft, between the engine and the clutch in vehicles. In stationary engines, it is mounted directly on the crankshaft. The flywheel rim is often fitted with gear teeth, which mesh with the pinion gear of the starter motor. When the engine is started, the starter motor rotates the flywheel, which in turn spins the crankshaft to begin the engine cycle. Once the engine starts running, the flywheel continues to spin freely.

The functions of a flywheel include:

1. Storing energy: It stores energy during the power stroke and releases it when needed.
2. Maintaining uniform speed: It minimizes speed fluctuations between strokes.
3. Reducing vibrations: It balances the rotating parts, reducing engine vibration.
4. Assisting in starting: The starter motor engages with the flywheel gear to start the engine.
5. Transmitting power: It transmits engine torque to the clutch and gearbox in vehicles.

The design of a flywheel depends on engine type, number of cylinders, and speed variation allowed. For example, in single-cylinder engines, large and heavy flywheels are required because power is produced only once every four strokes. In multi-cylinder engines, smaller flywheels are sufficient because the power strokes occur more frequently, providing smoother operation.

Flywheels can be classified based on their shape and application.

• Solid disc flywheel: Commonly used in engines and machines where compact design and smooth operation are needed.
• Rim-type flywheel: Has most of its mass concentrated near the rim for higher energy storage; used in large engines.
• Composite flywheel: Made from advanced materials like carbon fiber, used in high-speed modern systems for energy recovery and electric vehicles.

The energy stored in a flywheel can be mathematically expressed as:
Where,
= Energy stored in joules
= Moment of inertia of the flywheel
= Angular velocity in radians per second

This formula shows that energy stored increases with both rotational speed and moment of inertia. Therefore, high-speed lightweight flywheels are being developed in modern applications like hybrid vehicles, power recovery systems, and renewable energy systems.

In vehicles, the flywheel not only ensures smooth running but also helps in gear shifting by providing rotational inertia to match engine speed with transmission speed. During idle conditions, the flywheel keeps the engine rotating smoothly until the next power stroke occurs.

Without a flywheel, the engine would run irregularly, with sharp speed variations and vibrations. It would also be difficult to start or maintain stable engine operation. Hence, it is an essential component of every reciprocating engine and many mechanical systems involving rotation.

Conclusion:

A flywheel is an important mechanical device used to store rotational energy and maintain uniform speed in engines. It smooths out power fluctuations, reduces vibration, and helps in starting the engine. By storing energy during power strokes and releasing it during non-power strokes, the flywheel ensures steady and efficient operation. Its presence greatly enhances engine performance, stability, and durability.