[TigersEye]

You should watch the video of the one they have up and running on propane. It’s not hand-cranked or a video simulation.

[count-your-change]

I did watch video and then went to the engine home page. There are apparently some parts not shown in the drawings nor fully explained.

It sounds like there is a small piston or set of seals that does not leave the cylinder and the larger piston simply pushes against it.