The well-known discussion on an engine consisting of a ratchet and a pawl in [R.P. Feynman,R.B. Leighton, and M. Sands, The Feynman Lectures on Physics, vol. 1 (AddisonWesley, Reading, Massachusetts, 1963), pp. 46.1-46.9] is shown to contain some misguidedaspects: since the engine is simultaneously in contact with reservoirs at different temperatures,it can never work in a reversible way. As a consequence, the engine can never achievethe efficiency of a Carnot cycle, not even in the...