The opoc (opposing cylinders) engine. There are two pistons per cylinder, and is a mutant boxer motor. Check out this animation, it’s just down right nutso.
It looks like it works similar to a two stroke engine. They claim it has tremendous power and fuel economy. Also, because the stroke of each piston is so short, they claim they can run it at twice the normal speed. If this is a boxer-type motor I can rev up to 16,000 rpm, I want one, with a T88 turbo attached.