Short Answer:

A turning moment diagram is a graphical representation that shows how the torque (or turning moment) varies on the crankshaft of an engine during one complete cycle. It is also called a crank effort diagram. The horizontal axis of the diagram represents the crank angle, while the vertical axis shows the turning moment on the crankshaft.

This diagram helps to study the fluctuation of energy during engine operation. It shows when the engine produces more or less torque, allowing engineers to design flywheels and other components that balance energy variations for smooth and continuous rotation.

Detailed Explanation :

Turning Moment Diagram

A turning moment diagram is a graph that shows the variation of the turning moment (torque) on the crankshaft of an engine during one working cycle. It provides a clear idea of how the engine’s torque changes with the rotation of the crank. In simple words, it represents how much twisting force acts on the crankshaft at different crank positions due to the pressure of gases in the engine cylinder.

This diagram is also known as the crank effort diagram because it shows the effort produced by the gas pressure on the piston, which is then transmitted through the connecting rod and crank to produce a turning moment on the crankshaft.

In a four-stroke engine, the turning moment diagram is drawn for two revolutions of the crankshaft because the engine completes one cycle in four strokes. The shape of the diagram depends on the pressure inside the cylinder during each stroke.

Construction of Turning Moment Diagram

The turning moment diagram is constructed by following these basic steps:

Crank Angle (X-axis): The horizontal axis represents the crank angle, which is measured in degrees for one complete engine cycle.
Turning Moment (Y-axis): The vertical axis represents the turning moment or torque acting on the crankshaft.
Pressure Variation: The gas pressure on the piston changes during different strokes of the engine. These pressures are used to calculate the torque on the crankshaft at corresponding crank angles.
Plotting the Graph: The calculated turning moments are plotted for each crank angle and connected to form a continuous curve. The resulting curve is called the turning moment diagram.

The area under the curve represents the work done per cycle, while the fluctuations in the curve show the changes in torque and energy during the cycle.

Phases of Turning Moment Diagram in a Four-Stroke Engine

In a four-stroke engine, the diagram consists of four distinct parts corresponding to four strokes:

Suction Stroke:
The piston moves downward, and the air-fuel mixture enters the cylinder. The pressure inside the cylinder is slightly below atmospheric pressure, so the turning moment is small and slightly negative.
Compression Stroke:
The piston moves upward, compressing the mixture. Work is done on the gas by the piston, resulting in a negative turning moment.
Expansion (Power) Stroke:
The fuel-air mixture burns, producing high-pressure gases. This pressure pushes the piston down with high force, producing a large positive turning moment on the crankshaft. This is the only stroke where the engine delivers useful work.
Exhaust Stroke:
The piston again moves upward to expel exhaust gases from the cylinder. A small negative turning moment occurs because work is required to push out the gases.

After the exhaust stroke, the cycle repeats. The diagram’s shape repeats after every two revolutions of the crankshaft.

Mean Turning Moment

The mean turning moment is the average torque developed during one complete cycle. It is represented by a horizontal line that passes through the diagram in such a way that the areas above and below it are equal.

Mathematically, the mean turning moment is obtained by dividing the total work done per cycle by the angle turned by the crankshaft during one cycle.

This mean torque represents the uniform torque that would produce the same amount of work as the actual varying torque in the same time.

Fluctuation of Energy

As the engine torque varies throughout the cycle, sometimes it is greater than the mean torque and sometimes less. When the torque is greater, the engine accelerates and stores excess energy in the flywheel. When the torque is less, the flywheel releases stored energy to keep the crankshaft rotating at a nearly uniform speed.

The difference between the maximum and minimum energies in the flywheel during one cycle is called the fluctuation of energy. The turning moment diagram helps determine this energy fluctuation, which is used to design the size and mass of the flywheel.

Importance of Turning Moment Diagram

The turning moment diagram is very important in engine design and analysis for several reasons:

Design of Flywheel:
It helps in calculating the fluctuation of energy and the moment of inertia required for the flywheel to maintain steady rotation.
Engine Balancing:
It helps in understanding the unbalanced forces and torque variations in multi-cylinder engines.
Smooth Operation:
By studying the torque variations, engineers can ensure uniform crankshaft speed and reduce vibration.
Performance Study:
The diagram helps to study how pressure and torque vary during different engine strokes, improving the overall performance and efficiency of the engine.
Power Transmission Design:
It assists in designing couplings, shafts, and other parts that must handle variable torque.

Example

In a single-cylinder four-stroke engine, the turning moment diagram shows a large positive torque during the power stroke and small negative torques during suction, compression, and exhaust strokes. In a multi-cylinder engine, the combined turning moment diagram of all cylinders smooths out these variations, resulting in more uniform torque on the crankshaft.

Conclusion

The turning moment diagram is a vital tool in mechanical and automotive engineering. It graphically shows how the torque on the crankshaft varies during an engine cycle. By studying this diagram, engineers can determine energy fluctuations, calculate mean torque, and design flywheels to ensure smooth engine operation. Thus, it plays an essential role in understanding and improving the performance, balance, and efficiency of internal combustion engines.