>It provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work
The word 'efficiency' here is misleading... If one reverses the process, converting work back into heat, you get back all the original energy in an ideal machine (and real machines, for example gas turbines, are within ~15% of ideal for the temperature & pressure change between input and output)
Thanks for the clarification. I still have issues understanding the intermediate stage, where the world is entirely and reversibly converted into a temperature differential. I might have to go back to drawing T-S diagrams.
>It provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work
The word 'efficiency' here is misleading... If one reverses the process, converting work back into heat, you get back all the original energy in an ideal machine (and real machines, for example gas turbines, are within ~15% of ideal for the temperature & pressure change between input and output)