The LiquidPiston X engine is a non-Wankel rotary embodiment of the HEHC (High Efficiency Hybrid Cycle). The engine has only a few parts and 3 combustion events per rotor revolution. This results in quite a lot of power density.
It was the dream and Darpa made it reality. They succeeded in manufacturing and engine that weighs 1.8 kilograms and has the dimensions of a 5-inch smartphone. The most attractive thing about it, however, is the fact that it needs no oil nor pistons for operation.
The creators of this marvelous piece of machinery, Nikolai Shkolnik and Alexander Shkolnik, promise a 50% thermal efficiency. Check it out!