The pulleys driven by engine belts are not pulleys at all. Simple machines are basic objects or combinations of objects that are able to transfer a force from one place to another, change the direction of a force, or cause a change in the magnitude or speed of a force. Wheels, axles and pulleys are simple machines that are similar in appearance but perform very different functions.
In the simplest terms, a wheel is a circular object able to turn around an axis that passes through its center. Typically, a shaft called an axle is attached to the wheel at the point where that axis meets the wheel, and the wheel either may spin freely around the axle while the axle remains stationary, or the axle may turn along with the wheel. If the wheel is able to spin freely on the axle, it is not considered to be part of a wheel-and-axle simple machine because force cannot be transferred back and forth between the wheel and the axle.