This 3D model was made with Autodesk Inventor, it has about 72 pieces (without the screws) and was modelled after an old modeller steam engine blueprint, the drawings were all in English dimensions and afterwards converted in to metric dimensions.
In this blog you can find the original Blueprints of the project and i the following pictures the explanation how it works.
But to sum it up here is a shorter one.
The mechanics are very simple the piston runs up and down with the help with air or steam and it comes in from two different holes that are opened from the vale block. The block runs up and down and this is programmed from two off centred wheels that you see below. The lever system that is attached to the two off centred, works as an inverter for the engine. When the lever on the front is put to the right or the left that makes a change in the rotation of the flywheel.