An Atkinson model engine. Atkinson cycle engines are quite similar to otto (gasoline) engines. They are widely used in hybrid cars like the Toyota Prius.

The timing of the valves is not correct. Also I do not include a spark plug.

All credit for the design of the parts goes to Jan Ridders. I just did the 3D modeling.
You can visit his website here:
http://ridders.nu/
and request the drawings for any of his engines free of charge (I would recomend a small donation though because the quality of the drawings is very good)