Animated Engines is a site by Matt Keveney that has some great animated illustrations of a variety of engines that shows how they work. In addition to the animation, there are step by step explanations of how the the engine works and even some history of that type of engine.
Engines include 2 and 4 stroke internal combustion engines, diesel engines, Wankel, Atkinson and Gnome engines, Jet Engines include rockets, turbojet, turbofan and turboprop, and a variety of steam engines.
The animations are nicely done and do a great job of explaining how the engines work.
This is a great site for pre-engineering, engineering, and science classes.