It's satisfying to see the mechanical equivalent of a motion application I would use servos or linear motors for, typically. This is grass roots organic engineering, where what I do is materialize a concept by choosing different parts and writing a program to control it. While the latter is more versatile, it is much easier to accomplish with less effort and overall understanding. It's akin to the difference between writing a song, and learning to play someone else's song and believing you wrote it just because you play it well. I play other people's songs well, but don't often write my own.
I doubt any of these mechanisms would ever be used in an engine. More likely in a machine tool like a shaper or a slotting head. Shapers often use the first type of mechanism with a variable crank throw to allow the stroke length to be controlled.
