This is a book he purchased recently entitled How I Built a 5 HP Stirling Engine. He's been enjoying poring over it, so it's worth a mention (we don't get any $$...just including the link if anyone's interested, it's a little pricey).
Here's a link to a youtube vid showing an animation of the Stirling engine configuration. If I understand it correctly, it operates from the expansion and contraction of gases (heated and cooled) to create a vacuum that operates a piston (maybe I didn't get that right, but I'm new to this) :)
Here are a couple more just for fun...Jack's intent on exploring different options for energy.