globi

I thought about the compound concept again and I know now why the rotors have to be placed on top of each other. The channel/pipe that connects the exhaust ports of the 2 stages has to have a very small volume, otherwise you'll lose valuable pressure.
Because of the timing of the ports, the rotors (even if they had equal mass) can't be balanced. So in order to balance them you need 2 + 2 rotors. I did a patent search and found this picture. That's basically the way you'd would need to build it. This way all the rotors are perfectly balanced (even if they don't have equal mass).
In order for this concept to work as a gasoline engine, you'd need to place powerful intercoolers between the stages (intake side).
Obviously with 4 rotors it's going to be more complicated, so it would be an engine concept that can compete against large displacement gasoline piston engines. This concept should beat any other gasoline piston engine effiency wise and maybe even power to weight ratio wise.
But it looks like there's not really a market for large displacement and efficient gasoline engines. Let's face it who cares whether some Ferrari or Rolls makes 10 mpg or 16 mpg?

But building this engine with 4 rotors (as described earlier) you'd end up with an engine that compares to a 6.0l displacement gasoline piston engine with a compression ratio of 16. Wouldn't this be fun?