How long is the stroke of an engine compared to the length of a crankshaft throw?

How long is the stroke of an engine compared to the length of a crankshaft throw?

• A. Same length
• B. Twice the length
• C. Half the length
• D. Three times the length

Answer: (B)

The key idea is how the crank converts circular motion into linear motion. The crank throw is the offset from the crank center to the crank pin, i.e., the radius of the circle the crank pin describes. As the crank makes one full revolution, the piston travels from top dead center to bottom dead center, which is the full diameter of that circle. That distance is 2 times the throw. So the stroke length is twice the crankshaft throw. If you imagined the travel as the same as the offset, you’d miss the full up-and-down movement. If you thought it were half, you’d be underestimating the actual piston travel. The standard relationship in a typical crank-and-rod engine is stroke = 2 × throw.

The key idea is how the crank converts circular motion into linear motion. The crank throw is the offset from the crank center to the crank pin, i.e., the radius of the circle the crank pin describes. As the crank makes one full revolution, the piston travels from top dead center to bottom dead center, which is the full diameter of that circle. That distance is 2 times the throw. So the stroke length is twice the crankshaft throw.

If you imagined the travel as the same as the offset, you’d miss the full up-and-down movement. If you thought it were half, you’d be underestimating the actual piston travel. The standard relationship in a typical crank-and-rod engine is stroke = 2 × throw.