You don't necessarily need such valves to make engines easier to start. I remember my father-in-law's Lister diesel generator, from over forty years ago, that would start automatically when you turned on a light or appliance. It had a tiny starter motor and also a starting handle in case the battery was flat. There was a switch or valve that you would turn manually, or if the battery was not flat automatically, that made it possible to turn the motor. It was pretty much impossible to turn the engine over manually without doing that; with it, it was easy.

I'm not a mechanical engineer and I have no idea how it worked beyond knowing that it reduced the degree of compression.