Piston and Flywheel

This simple mechanism illustrates the relationship between the piston and the flywheel in a steam engine (or any other engine that converts back-and-forth reciprocal motion into rotary motion).

The piston pushes and pulls on the connecting rod, which in turn pushes and pulls on the crank arm (the point, offset from the center, at which the connecting rod is attached to the flywheel).

An important property of all such mechanisms is that there are two "dead spots". When the piston is at either end of its range of motion (at one end of the cylinder or the other), no amount of pushing or pulling will get the engine started. It is completely locked up, unless you have some other way of rotating the flywheel to a position where the piston has a chance to work.

In this model, the result of these dead spots is that if you want to make it spin smoothly, you have to push and pull the piston with just the right amount of force, at just the right speed, or the engine will lock up or even reverse direction. It can take a bit of practice to get it right, and if the model "stalls", you have to manually rotate the flywheel 1/4 turn before attempting to "restart" the engine. In real engines it's not so easy to rotate the flywheel (which may weigh many tons). One solution is to have two or more pistons connected at different points around the circle, so that at least one is always able to get the engine started no matter where the flywheel is. 

See the model in action: