The Wankel made it all the way to full production but has still never displaced the piston engine.
And, as far as I’m concerned from a motorcycle perspective the four-stroke still is unproven in the highest echelons of racing.
:-) .. on that last comment.
And what of the Stirling?
Yep, agree. Actually, the Wankel was in production for several years before EPA reg’s killed it. Also, just by looking at the drawing, I would say that this engine's genesis is in the Wankel. It's certainly, not a revolutionary idea. The latest idea that I think has potential as far as the gasoline engine is concerned is a high compression engine, about 20:1 compression ratio, much the same as a diesel engine. From what I've read, it has great performance and great mpg. I think Ford is tinkering with it.