The problem with rotary engine is that its max power efficiency is at a very narrow rpm range and loses power at most other rpm ranges. This is due to the cons

The problem with rotary engine is that its max power efficiency is at a very narrow rpm range and loses power at most other rpm ranges. This is due to the constant air intake inherent in the design. Piston engine has variable valve air intake to ensure optimal air oil mixture compression to deliver max power efficiency at a long range of rpm’s. That’s why rotary design is not popular for cars. Mazda is reviving small single cylinder rotary engine design for HEV by running it at a constant speed with max power efficiency to charge the battery.