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.
Perhaps because the problem involves no new scientific principles, or has been unsolved for so long that people have assumed it cannot be solved, the problem has not attracted the kind of funding devoted to nuclear fusion or even to cold fusion in its heyday.
Or perhaps people have forgotten that an efficient heat engine is more than just an incremental improvement over an inefficient heat engine. It is like the proverbial retailer trying to “make it up on volume” when selling at a loss. Because low margin heat engines must handle vastly more heat than the work they produce, anything less than 90 or 95% efficiency represents a machine which simply fails to operate, absorbing instead of producing work.