Science and engineering long ago for the most part abandoned the search for highly efficient heat engines. Fuel has been cheap and convenient enough that the problem of producing substantial power without using fuel (or bright sunlight, high winds, tall dams, etc.) has not been solved. The problem involves no new scientific principles, just careful attention to real-world details like unwanted heat flow, friction, leaks, etc., plus large scale, a lot of money, and maybe some new insight. This question was asked on the wiki: Renewable Energy Design.