Short Answer:

Torque fluctuation refers to the variation of torque produced by an engine during one complete cycle of operation. It occurs because the power output of the engine is not uniform throughout the cycle. During the power stroke, the torque increases, while during suction, compression, and exhaust strokes, the torque decreases. These variations cause irregular motion of the crankshaft.

The fluctuation of torque mainly depends on the number of cylinders, firing order, and engine speed. To maintain smooth operation, a flywheel is used to absorb the excess energy during high torque and release it during low torque. This helps to minimize speed variations and ensure uniform motion.

Detailed Explanation:

Torque Fluctuation

Torque fluctuation is a common mechanical phenomenon that occurs in reciprocating engines due to the periodic and uneven generation of power by the cylinders. In a single-cylinder engine, the torque produced by the crankshaft is not constant throughout one revolution because the engine produces useful work only during the power stroke. The other strokes — suction, compression, and exhaust — consume energy, causing variations in torque.

In multi-cylinder engines, torque fluctuation still occurs, but it is less severe because the power strokes of different cylinders overlap. The total torque on the crankshaft is the sum of individual cylinder torques, which helps reduce overall fluctuation. However, complete elimination of torque variation is not possible due to differences in the timing and duration of each cylinder’s power delivery.

Causes of Torque Fluctuation

1. Intermittent Power Strokes:
   The main reason for torque fluctuation is that the engine power is produced only during the power stroke. During the other three strokes, energy is consumed to compress the mixture or expel exhaust gases.
2. Inertia Effects of Moving Parts:
   The piston and connecting rod move with alternating acceleration and deceleration. Their inertia forces act alternately in opposite directions, contributing to variations in torque.
3. Unequal Cylinder Pressure:
   The pressure in the combustion chamber changes during the engine cycle. The maximum torque is generated at the midpoint of the power stroke, and it decreases before and after this point.
4. Irregular Combustion or Misfiring:
   If the combustion process is not uniform in every cycle, it can lead to sudden increases or decreases in torque.

Effect of Torque Fluctuation

Torque fluctuation directly affects the uniformity of engine speed. During the power stroke, the crankshaft tends to accelerate because the torque exceeds the resistance torque. During the other strokes, it decelerates due to insufficient torque. This irregular motion can lead to mechanical vibrations, noise, and reduced engine efficiency.

If these fluctuations are not controlled, they can also cause fatigue failure in engine components, reduce the life of the crankshaft, and increase fuel consumption.

Role of Flywheel in Reducing Torque Fluctuation

A flywheel is used to minimize the effect of torque fluctuation. It stores kinetic energy during the period when the torque is more than the mean torque and releases it when the torque is less than the mean torque. This helps in maintaining a nearly constant angular speed of the crankshaft.

The flywheel effect smoothens the motion of the crankshaft by balancing the energy difference between high and low torque periods. As a result, engine operation becomes more stable, power transmission becomes uniform, and mechanical stress on engine parts is reduced.

Mathematical Explanation

Let:

•  = instantaneous torque
•  = mean torque

The torque fluctuation  can be expressed as:

The fluctuation of energy stored in the flywheel corresponds to the area between the torque curve and mean torque line in the turning moment diagram. The greater the difference between maximum and minimum torque, the higher the required flywheel capacity to maintain speed uniformity.

Torque Fluctuation in Multi-cylinder Engines

In multi-cylinder engines, the firing order and phase angle between cylinders help distribute power strokes more evenly. For example, in a four-cylinder engine, each cylinder fires once every 180° of crank rotation, which reduces torque variation compared to a single-cylinder engine. However, a flywheel is still necessary for smooth running and to absorb small fluctuations that remain.

Advantages of Controlling Torque Fluctuation

1. Ensures smooth engine operation.
2. Reduces mechanical vibrations.
3. Increases component life and reliability.
4. Improves fuel efficiency.
5. Provides uniform power transmission to the vehicle or machinery.

Conclusion:

Torque fluctuation is the variation of torque produced during different strokes of the engine cycle. It occurs due to non-uniform power generation and inertia effects of moving parts. These fluctuations cause speed variation and mechanical stress on the crankshaft. The flywheel plays a crucial role in controlling torque fluctuation by storing and releasing energy, ensuring steady engine operation, and reducing vibration.