Maybe everyone knows about this engine but me, but if not, this is a very cool little piece of machinery worth looking into. It's the rotary-valve4-stroke used in the Honeywell hovering drone that's being prepared for field use by the US Army: It's built by a British company, RCV Engines Ltd., and it's amazingly simple. They've made available a 7-year-old SAE paper about it, with lots of detail about the seals (for anyone who's followed rotary-valve engines over the years, that's where most of the action is): It must be pretty darned reliable to be used in the drone, so this is more than just another weird engine curiosity.
Here's RCV's home page. They make model engines, too:
-- Ed Huntress