What is the optimal angular relationship between the connecting rod and the crank-web for power transmission?

• A. 0 degrees
• B. 45 degrees
• C. 90 degrees
• D. 135 degrees

Answer: (C)

The key idea is that power transfer to the crankshaft comes from the tangential component of the force acting at the crank pin. The torque the crank can develop is the tangential force times the crank radius. This tangential component is largest when the force from the piston is perpendicular to the crank radius, which corresponds to the connecting rod being at 90 degrees to the crank web. At this orientation, you get the greatest turning moment for a given piston force, so power transfer is maximized. If the angle is closer to 0 degrees, the force acts more along the radius and produces little torque; at intermediate angles the torque is less than at 90 degrees. In actual operation the angle changes through the stroke, so torque and power vary, but the 90-degree relationship yields the maximum instantaneous power transmission.