2024 ‘Key Reflections’ on Sadi Carnot’s 1824 ‘Réflexions’ and 200 Year Legacy

This author is not a philosopher nor a historian of science, but an engineering thermodynamicist. In that regard, and in addition to various philosophical "<i>why and how</i>" treatises and existing historical analyses, the physical and logical "<i>what it is</i>" <i>reflections, as sequential Key Points</i>, where <i>a key Sadi Carnot reasoning infers the next one</i>, along with novel contributions and original generalizations, are presented. We need to keep in mind that in Sadi Carnot's time (early 1800s), steam engines were inefficient (below 5%, so the heat in and out was comparable within experimental uncertainty, as if <i>caloric</i> were conserved), the conservation of <i>caloric</i> flourished (might be a fortunate misconception leading to the critical analogy with the waterwheel), and many critical thermal concepts, including the conservation of energy (The <i>First Law</i>), were not even established. <i>If Clausius and Kelvin earned the title</i> "<i>Fathers of thermodynamics</i>", <i>then Sadi Carnot was 'the ingenious'</i> "<i>Forefather of thermodynamics-to-become</i>".