Short Answer:

Turning moment is the torque or twisting effect produced on the crankshaft of an engine due to the force of gas pressure acting on the piston. When the piston moves during the power stroke, it exerts a force on the connecting rod, which then applies a tangential force on the crank. This tangential force multiplied by the crank radius gives the turning moment.

In simple words, the turning moment represents the rotating effort that drives the crankshaft and keeps the engine running. It varies during the engine cycle because the gas pressure in the cylinder changes continuously as the piston moves.

Detailed Explanation :

Turning Moment

Turning moment, also known as torque, is one of the most important parameters in the study of internal combustion engines and reciprocating machines. It can be defined as the product of the force acting on the crank pin and the perpendicular distance of this force from the crankshaft axis. It represents the rotational effect that tends to turn or rotate the crankshaft.

When the combustion gases expand inside the cylinder during the power stroke, they apply a high pressure on the piston head. This gas pressure creates a force that pushes the piston downward. The piston is connected to the crankshaft through a connecting rod. As the piston moves, the connecting rod transmits this force to the crank pin, creating a tangential force. The tangential force acting at a distance from the crank center produces the turning moment or torque on the crankshaft.

Mathematically,
Turning Moment (T) = Force × Perpendicular Distance (r)

Where,

• Force = Tangential component of the force transmitted through the connecting rod
• r = Radius of the crank

Thus, the turning moment depends on the gas pressure, connecting rod geometry, and crank angle.

Variation of Turning Moment

The turning moment is not constant throughout the engine cycle. It varies with the crank angle because the pressure inside the cylinder and the direction of the connecting rod continuously change.
During one complete revolution of the crankshaft, there are four strokes in a four-stroke engine — suction, compression, power, and exhaust strokes. The gas pressure and thus the force on the piston vary in each of these strokes.

1. Suction Stroke:
   During this stroke, the inlet valve is open, and the piston moves downward drawing in the air-fuel mixture. The pressure inside the cylinder is slightly less than the atmospheric pressure, so the turning moment is very small and slightly negative.
2. Compression Stroke:
   In this stroke, the piston moves upward with both valves closed. The air-fuel mixture is compressed, and pressure rises, but the piston movement is opposite to the direction of the gas force. Hence, the turning moment is negative during this stroke.
3. Power Stroke:
   This is the most important stroke where combustion occurs, and the high-pressure gases push the piston downward. The force on the piston is maximum, and therefore, the turning moment reaches its highest positive value during this stroke. It is the only stroke that produces useful work.
4. Exhaust Stroke:
   The exhaust valve opens, and the piston moves upward to expel the burnt gases. The gas pressure is slightly above atmospheric, so the turning moment becomes small and negative again.

Because of these changes, the turning moment fluctuates during each revolution. The shape of this variation can be represented by a turning moment diagram or crank effort diagram, which shows the variation of torque with crank angle.

Turning Moment Diagram

The turning moment diagram is a graphical representation that helps to understand how the torque changes throughout the engine cycle.

• The x-axis represents the crank angle, and the y-axis represents the turning moment.
• The area under the curve gives the work done per revolution of the crankshaft.
• The mean turning moment represents the average torque produced during one revolution.

If the mean turning moment is known, the power developed by the engine can be calculated using the formula:
Power (P) = (2πN × Mean Turning Moment) / 60
where N is the crankshaft speed in revolutions per minute (rpm).

This diagram also helps in determining the energy fluctuation in the flywheel. The flywheel stores energy when the turning moment is higher than the mean value and releases it when it is lower, maintaining a uniform speed of rotation.

Importance of Turning Moment

1. Design of Flywheel:
   The size of a flywheel depends on the fluctuation of the turning moment. Greater fluctuation requires a larger flywheel to maintain speed stability.
2. Power Calculation:
   Turning moment is used to calculate the power output of an engine.
3. Dynamic Balancing:
   It helps in understanding the dynamic behavior of engine parts and maintaining balance.
4. Efficiency Estimation:
   The variation in turning moment helps in identifying the effective power stroke and overall efficiency.
5. Machine Design:
   It is used in the design of shafts, couplings, and other rotating components subjected to torque.

Factors Affecting Turning Moment

• Gas Pressure: Higher cylinder pressure increases the turning moment.
• Crank Radius: Larger crank radius gives greater torque for the same force.
• Connecting Rod Length: The angular position of the connecting rod affects the tangential force.
• Speed of Engine: Variation in speed affects the magnitude and smoothness of the turning moment.
• Frictional Losses: Friction between moving parts reduces the effective turning moment.

Conclusion

Turning moment is the torque or rotational force produced on the crankshaft due to the gas pressure acting on the piston. It is a vital concept in the analysis of engine performance, as it directly relates to power generation and smooth running of the engine. Since the pressure changes during each stroke, the turning moment also varies continuously. Understanding this variation helps in designing engine components, flywheels, and mechanical systems that operate efficiently and reliably.