How close are we to the theoretical limit of gas engine design?

I'm asking this question because of the Bentley W12 engine.Are there any new technologies that ha vent been perfected yet, which will revolutionize the engine?Are we at a point where the gas engine is pretty much as good as it can get, with future breakthroughs coming in hybrid and electric technology?Is there a limit to how much energy can be used from a gallon of gasoline?I'm not talking about regenerative breaking, and things like that, but purely engine technology. via /r/cars http://ift.tt/2tIRbSa