"Brian Gaff" wrote in message
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I'd have thought it was pretty obvious really.
The oones I never understood properly were the Wankel engines with their
rounded sided triangle in a round hole with a gear on the inside and three
spark plugs.

Yes: how does the increase in pressure in one "cylinder" space translate
into rotational movement of the "piston" around the central shaft which then
rotates? Definitely needs a moving model to explain how it works.

The "round hole" is not actually round but more like two semi-circles that
have been split apart and a straight section inserted, to form a shape
called an epitrochoid.