This is an accurate representation of a compressed-air engine. It has all the inner tubing also. Modeled from blueprints in Blender 2.73.

Detailed polygon counts:
SubD level 0 = vertices 7049, faces 7240, triangles 14122
SubD level 1 = vertices 28602, faces 28602, triangles 57204
SubD level 2 = vertices 114432, faces 114408, triangles 228816

All the renderings were done on subdivision level 1, which is enough to make level 0 square holes round. There are some triangles, but these are located so that they won't affect subdivision by causing pinching problems.

Piston is rigged so that you only have to key the rotation of the flywheel if you want to change the animation speed. Out of the box, each revolution takes 36 frames.

Every single object in the .blend has two modifiers; subdivision surface and edge split. These modifiers have not been applied, so you can easily up the quality of the meshes.

Engine block and groundplate is UV-unwrapped so that you can easily add your logo or similar if you so choose.

This model is ideal for engineering teachers that want to demonstrate how these types of engines work.

File contains both the standalone and the entire render scene, that has both the material and wireframe versions the engine. It also has three-point lighting with a camera fully set up, so you only have to click the render button and see great results.