The so-called elbow engine is a popular model among engine modelers...
The engine consists of two, free to rotate, cylinder blocks with three equally-spaced cylinders mounted at 90 deg to each other. One block is pivoted to the base and the other, carrying the flywheel is pivoted on an upright. The pistons connect these two blocks. Each piston actually consists of two orthogonal pistons, each fitted into a cylinder in one of the blocks. Valving is passive and consists of recesses carved into the base and upright behind each block. Holes drilled through the base and upright conduct air/steam to the valves.
Air is supplied simultaneously to each piston of a pair. This forces them out of their respective blocks and, by this action, generates a torque on the blocks and this leads to rotation of the blocks.
At first blush it looks like it should be mechanically locked up and unable to rotate. Yet it does, as the video below proves.
Two remarks about this engine...
It's a hideous air hog, perhaps the least efficient engine I've ever built. There's really no way to seal it properly and it needs about 20 psi to get going. Thankfully, it's only a novelty and no one would ever build one for a practical application.
Building these engines is no easy task. Maintaining orthogonality between the cylinders and bending the piston assemblies to exact orthogonality is absolutely essential. Don't try it as your first engine modelling attempt.
https://www.youtube.com/watch?v=3sJtkKLWHsk
