Prepare to lose a few hours of your life. Clear your schedule for a bit, and head over to AnimatedEngines.com for some wonderfully explanitory animated diagrams of different engines.
I think I just spent an hour there after [Static] tipped us off in a comment. This is a perfect resource for explaning how an engine works to someone. I know it would have saved a bunch of time when my friend was attempting to describe a Wankel engine to me.
The site is very basic right now. There aren’t a TON of engines, but each one has some extra information when available.