What component converts the pistons' movements into rotary motion?

The crankshaft is the component responsible for converting the linear motion of the pistons into rotary motion within an internal combustion engine. As the pistons move up and down within the cylinders due to the combustion process, their movement is directly connected to the crankshaft via connecting rods. The design of the crankshaft, with its offset bearings, allows it to rotate as the pistons complete their cycles. This rotational movement is essential for powering the vehicle, as it allows the engine's power to be transmitted to the transmission system, which ultimately drives the wheels.

The camshaft, while also critical to engine function, serves a different purpose. It regulates the opening and closing of the engine's valves but does not participate in converting piston motion to rotary motion. The flywheel acts as a stabilizing mass that helps maintain engine speed and smoothness but does not directly perform the conversion process. Meanwhile, the transmission deals with managing and delivering the power generated by the crankshaft to the vehicle’s wheels but doesn't convert piston motion itself. Thus, the crankshaft is indeed the correct answer for this process.