According to Duke Engineering, its axial engine is the most efficient and lightest engine you can put in boats, light aircrafts and generators -- the mechanical engine of the (near) future! Maybe.
I don't know if its claims are true, and I don't really care. I just love watching it in action in this eternal GIF.
The final production model of the Duke engine is not finished yet, according to the company:
The design has come a long way since the Duke engine concept was first turned into hardware, yet optimisation of its features has barely begun. Despite this, the Duke engine is favourably comparable to modern conventional designs.
Mechanical and other main characteristics (Seal lubrication, combustion, performance, port timing, port shape, manifold geometry, reciprocating to rotary conversion mechanism etc) are performing satisfactorily at prototype stage, but all will no doubt benefit from concerted R&D effort.