At this point, the focus of the discussion is, though the caloric theory is recognized as obsolete, there is a persistent remnant stated in a somewhat vague way in courses on thermodynamics that some heat must always be rejected to the sink in a heat engine. My insulation is not perfect, so indeed, it is within the realm of possibility that "some" heat is still getting through, but if the functionality of a heat engine depends in any way on such "flowing through" of heat, then insulating th