Don't forget the Wankel (rotary) engine! Few people have even heard of them, and even fewer have any idea what they are or how they work. I love my dorito on a stick.

Down the street from where I lived in Stuttgart someone owned a NSU Ro 80.

I also remember seeing a few Mazda rotary pickups in my early years in the US - very cool but apparently not very reliable and very inefficient.

Relevant visual reference: http://www.animatedengines.com/wankel.html