Carnot efficiency. 30% is pretty typical in land-based motive systems. Shipboard combustion engines and coal-fired power plants (of all things) are rated as high as 45% efficiency.
This is based on input and output temperatures, more than anything.
Good eye, though it's not immediately clear that that's strictly edging in on Carnot efficiencies or if other aspects (two-stroke power cycle, valveless port-based design, eliminating cylinder-head heat loss -- on which, doesn't that eventually reach equilibrium temp anyway?, etc.) are to credit.
I've seen a few other radical reciprocating energy designs, there are some interesting YouTube vids I should track down. I think "radial engine" might reveal something. New Zealand, possibly Australian engineering firm, IIRC.
This is based on input and output temperatures, more than anything.