Short Answer:

Torque on a crankshaft is the turning or twisting force that acts on the crankshaft of an engine due to the pressure of gases inside the cylinder. It is produced when the force on the piston is transmitted through the connecting rod to the crankshaft, causing it to rotate.

In simple words, torque on a crankshaft is the measure of how much force the engine can apply to produce rotation. This torque determines the engine’s ability to perform work, such as moving a vehicle or driving a machine, and is an important indicator of engine performance.

Detailed Explanation :

Torque on a Crankshaft

The torque on a crankshaft is a very important concept in mechanical and automotive engineering. It represents the turning effect produced by the gas pressure acting on the piston, which is transmitted through the connecting rod to the crankshaft. The crankshaft converts the reciprocating motion of the piston into rotational motion.

When combustion occurs in the cylinder, gases expand and push the piston downward with a certain force. This force acts at a particular distance (called the crank radius) from the center of the crankshaft. The product of this force and the perpendicular distance from the crank center to the line of action of the force gives the torque on the crankshaft.

Mathematically, torque can be expressed as:
T = F × r
Where,
T = Torque on the crankshaft (N·m)
F = Force acting on the crankpin (N)
r = Crank radius (m)

This torque causes the crankshaft to rotate, and its value changes continuously during the engine cycle due to varying gas pressure and crank angles.

Formation of Torque on the Crankshaft

The torque on a crankshaft is developed during different strokes of an internal combustion engine. The key stages of this process are as follows:

1. During Power Stroke:
   The combustion of fuel generates high pressure on the piston. This pressure creates a large downward force, transmitted to the connecting rod and then to the crankshaft. The torque is maximum during this stroke because the gas pressure is highest.
2. During Compression Stroke:
   The piston moves upward to compress the air-fuel mixture. The torque is negative in this phase because the engine has to work against the gas pressure.
3. During Exhaust and Suction Strokes:
   The torque on the crankshaft is either very small or negative since no useful work is done. The flywheel helps maintain motion during these strokes by providing stored energy.

Thus, the torque on a crankshaft is not constant; it varies throughout the cycle. The average value of this varying torque is known as the mean torque, which determines the steady output of the engine.

Factors Affecting Torque on Crankshaft

1. Gas Pressure in Cylinder:
   The higher the combustion pressure, the greater the force on the piston and hence, the higher the torque.
2. Crank Radius:
   The torque is directly proportional to the crank radius. A larger crank radius increases the torque for the same force.
3. Connecting Rod Length:
   The length of the connecting rod affects the angle at which force is transmitted. Longer rods give smoother torque variation.
4. Engine Speed:
   At higher speeds, combustion may not be complete, which can reduce effective pressure and torque output.
5. Mechanical Efficiency:
   Frictional losses and mechanical inefficiencies reduce the effective torque available at the crankshaft.

Variation of Torque During Engine Cycle

The torque on the crankshaft does not remain uniform because the force on the piston changes at different crank angles. During the expansion (power) stroke, the torque rises sharply and reaches a peak. In other strokes, such as compression or exhaust, the torque decreases or even becomes negative.

This variation of torque can be represented graphically by a turning moment diagram, which shows how torque changes with crank angle. The area under this curve over one cycle represents the total work done, and the mean torque can be obtained from this area.

To keep the crankshaft rotating uniformly despite torque variation, a flywheel is used. The flywheel stores excess energy when torque is high and releases it when torque is low, thus maintaining steady motion.

Importance of Torque on Crankshaft

1. Measurement of Engine Power:
   Torque is directly related to engine power. The power produced by the crankshaft is given by:
   Power (P) = (2πNT)/60,
   where N is the engine speed in revolutions per minute (rpm).
2. Smooth Engine Operation:
   Uniform torque distribution ensures smooth operation and reduces vibration in mechanical systems.
3. Vehicle Performance:
   In vehicles, higher torque means greater pulling power and better acceleration.
4. Design and Testing:
   Knowing the torque on the crankshaft helps engineers design strong shafts and bearings that can withstand maximum stresses.
5. Energy Transmission:
   The torque transmitted through the crankshaft drives the gearbox and other connected components, ensuring efficient power delivery.

Conclusion :

Torque on a crankshaft is the twisting effect produced by the gas pressure acting on the piston and transmitted through the connecting rod. It is responsible for the rotational motion of the crankshaft, which drives the machine or vehicle. The torque varies during each cycle depending on engine conditions and load. Understanding this torque is essential for designing efficient engines, ensuring smooth performance, and achieving the desired power output.