Why didn’t rotary engines replace piston engines?
May he rest in piece. The rotary engine was a brilliant concept.
Unfortunately, sealing the rotors’ apexes turned out to be pretty much beyond modern technology, leaving the engines with great power-to-weight ratios, smooth operation, and an almost linear power curve but using too much fuel and oil to accomplish it.
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+1
Mr. Yamamoto surely improved on Dr. Wankel’s original design, but what he did was still VERY far from perfecting it. In addition to high fuel and oil consumption, those early Mazda rotary engines also needed to have the rotor seals replaced VERY frequently, which required a tear-down of the engine.
I had a coworker that had a late model Mazda with a rotary engine bite the dust. I’m not sure why or exactly what was wrong, he traded it with the engine bad. It was one of those sporty looking jobs with the rear suicide doors, can’t recall the model. It sort of soured me on the rotary engine, although I’m not in the sports car market stage of my life right now anyway.
That was undoubtedly an RX-8:
I believe that’s right. Was a pretty slick looking little car.
Mostly because of how terrible piston engines were not.
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In addition to the short rotor seal life and oil and gas guzzling the engine had little torque.
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At the bottom end, but once you wind it up… YAHOOOOOO!!! 
Sometimes the best theory becomes the biggest challenge when one tries to manufacture it.