LiquidPiston’s X Engine is a non-Wankel rotary embodiment of the company’s innovative High Efficiency Hybrid Cycle (HEHC). The X Engine has few parts and three combustion events per rotor revolution, resulting in tremendous power density.
   • xTechSearch 3 Finalist Technology Ove...  
Inventor Alexander Shkolnik
They are claiming 50% thermal Efficiency.
BEAT THAT STIRLING ENGINES !
http://liquidpiston.com/
Let me see your design for getting 2 HP from a 5" diameter Engine on pump gas and 4 stroke.
https://vimeo.com/99002635
https://wefunder.com/liquidpiston
see also
   • See Thru Liquid Piston Rotary Engine ...  
And Watch Intgeza Visit the factory !
   • THE LIQUID PISTON ENGINE  

Recommended Study to understand the principals here :
Carnot efficiency (Thermal Efficiency)
   • Physics 29  Efficiency Of Heat Engine...  
PV Diagrams
   • PV Diagrams  

Wankel Engine differences:
1./ Centre "piston" is ovoid not tricloid.
2./ Housing is Tricloid not Ovoid.
3./ Apex seals are stationary in the housing.
4./ Expansion ratio is larger than compression ratio - see Atkinson Cycle.