>Two identical opposed pistons and valving machined in the cylinder walls
>as slits ? (see boxer motors and Jumo 205 WW2 era aircraft diesel engine).

It is sort of like this, but the two pistons in it are separate, one is low
pressure, and the other high pressure. They are also different diameters,
with the smaller one being made the same mass as the larger one to help
balance out vibration.

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