3. the function of the is to convert the up-and-down motion of the piston into rotary motion that can be used to power the vehicle.

The function of the crankshaft is to convert the up-and-dwon motion of the piston into rotary motion that can be used to power the vehicle.

I attache an image of a crankshaft.

The pistons are connected to the crank pins ans when the pistons move up an down the crankshaft rotate, and so the recoprating movement of the pistons is converted into rotary motion.