Theory

This is a single-acting steam engine. That means that the steam only pushes the piston one way. The flywheel pushes it back again.

1. The valve lets steam into the cylinder. 
2. The piston moves in the cylinder and makes the flywheel spin.
3. When the piston gets to the end the valve opens.
4. The piston pushes the steam out through the open valve.
5. When the piston gets to the other end the valve closes and the cycle starts all over again.

Video:

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