Anything that needs to rev to 9000+ rpm to produce power isn't going to be fuel efficient.
Efficiency wasn't an issue, was meeting unburn fuel and carbon oxides at low revs
Radial engines are just plain cool. Wankel is fine by me too, but as you stated they've not been fully developed.
Sort of a non issue anyway as converting energy into motion is much better done with coils of wire and magnetic fields.
@lynx started
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Anything that needs to rev to 9000+ rpm to produce power isn't going to be fuel efficient.