Short Answer:

Crank effort is the turning or tangential effort applied on the crank of an engine to produce rotary motion. It is the component of the force acting along the connecting rod that is perpendicular to the crank. This effort is responsible for rotating the crankshaft, thereby converting the reciprocating motion of the piston into rotary motion.

In simple words, crank effort is the effective force that causes rotation in the crank mechanism of an internal combustion engine. It varies during the rotation of the crank depending on the piston position and the angle between the crank and connecting rod.

Detailed Explanation :

Crank Effort

Crank effort is the tangential or turning effort applied on the crank of an engine, which produces the rotary motion required to perform useful mechanical work. It is an important concept in engine mechanics because it helps in analyzing the forces acting on different engine components such as the crankshaft, connecting rod, and piston.

When an engine operates, the combustion of fuel in the cylinder pushes the piston downwards. This generates a gas pressure on the piston head, which produces a force along the connecting rod. The connecting rod transmits this force to the crank pin, which is connected to the crankshaft. However, not all of this force contributes to rotating the crankshaft. The component of the force that acts tangentially to the crank (perpendicular to it) is known as the crank effort.

This tangential force creates a torque or turning moment about the crankshaft axis, causing it to rotate continuously. The variation of this crank effort throughout the cycle gives rise to fluctuations in the turning moment, which can be analyzed using turning moment diagrams.

Mathematical Expression of Crank Effort

Let,

•  = Force on the piston due to gas pressure
•  = Tangential force on the crank (crank effort)
•  = Angle of crank with the line of stroke
•  = Angle of inclination of connecting rod with the line of stroke

The tangential force or crank effort is given by:

This equation shows that the crank effort depends on the gas pressure, crank angle, and connecting rod geometry. It varies for each position of the crank during the working cycle.

Working of Crank Effort in an Engine

During the power stroke, when the fuel-air mixture burns in the cylinder, the piston is forced downward due to high gas pressure. The connecting rod transmits this motion to the crankshaft through the crank pin. The direction of the force acting along the connecting rod changes continuously as the crank rotates.

• When the crank is near the top dead center (TDC) or bottom dead center (BDC), the tangential component of the force is small because the crank and connecting rod are almost in line.
• When the crank moves to an angle of about 90° from TDC, the tangential component (crank effort) becomes maximum.
• Hence, crank effort varies during the rotation and determines the smoothness and power output of the engine.

The value of crank effort at any instant helps in calculating torque on the crankshaft, given by:

where  is the radius of the crank.

Factors Affecting Crank Effort

1. Gas Pressure in Cylinder: Higher pressure on the piston head increases the force and thus crank effort.
2. Crank Angle: The direction of the connecting rod and crank angle influences the magnitude of the tangential component.
3. Connecting Rod Length: A longer connecting rod reduces the obliquity angle, making the effort more uniform.
4. Frictional Losses: Friction in bearings and joints decreases the effective crank effort.
5. Engine Speed: At higher speeds, inertia forces affect the total turning moment, modifying the crank effort.

Importance of Crank Effort

• Determines Engine Torque: Crank effort helps calculate the turning moment on the crankshaft, which directly affects torque output.
• Aids in Design: Engineers use crank effort calculations to design crankshafts, bearings, and flywheels.
• Helps in Balancing: Understanding crank effort variations is essential for engine balancing and vibration control.
• Indicates Power Transmission Efficiency: Higher effective crank effort means better conversion of combustion energy into rotary motion.

Variation of Crank Effort

The crank effort is not constant throughout one engine cycle. It increases and decreases depending on the piston’s position and gas pressure.

• During the Power Stroke: It is maximum.
• During Exhaust, Suction, and Compression Strokes: It becomes negative or small because the piston resists motion instead of producing it.

To maintain uniform motion of the crankshaft, a flywheel is used, which stores excess energy when crank effort is high and releases it when crank effort is low.

Conclusion:

Crank effort is the tangential component of the force acting on the crank pin that causes rotation of the crankshaft. It depends on gas pressure, crank angle, and the geometry of the connecting rod. The crank effort plays a vital role in determining the torque, power, and smooth operation of an engine. Understanding crank effort helps engineers design efficient and balanced engines for smooth power delivery and durability.