What part converts reciprocating piston motion into rotational motion?

The part that converts reciprocating piston motion into rotational motion is the crankshaft.

Pistons move up and down inside their cylinders, but a vehicle needs rotary motion to turn its transmission and wheels. The connecting rods link the pistons to offset crankpins on the crankshaft, causing the shaft to rotate as the pistons travel through their strokes. The crankshaft therefore converts the engine’s back-and-forth motion into continuous rotation.

Combustion pushes a piston down during the power stroke, while the crankshaft’s rotation drives the piston upward during the other strokes. Main bearings support the shaft in the engine block, and counterweights help balance moving masses and reduce vibration. A flywheel or flexplate attached to the crankshaft helps smooth and transmit the output.

The connecting rod is essential to the mechanism, but it mainly transfers force between piston and crankshaft rather than serving as the rotating output shaft. The camshaft controls valve operation, while the flywheel stores rotational energy and smooths speed fluctuations. In electric motors, by contrast, there is no piston-and-crankshaft conversion because the motor produces rotation directly.