Does Thermo Break Its Own Laws?

Factual Questions
1

Reviewing power cycles for different engines when I noticed something I never gave much thought to before now. Every power cycle when plotted on the
T-s plane will have to show a decrease in entropy (ds) to restart the cycle (i.e, the last leg that completes the cycle). Yet, we know this is not possible. I WAG the net ds for the actual cycle yields a positive value for ds whereas the box-like shape of the ideal cycle would yield a net ds of zero?

It’s been awhile since I’ve looked at these power cycles, but what am I forgetting? :confused: (Obviously, we cannot decrease entropy!)

  • Jinx
2

FWIW entropy can decrease in a non-isolated system. Somehow, some way entropy in the overall universe always increases but you can get a local decrease in entropy (hope I said that right).

3

That’s correct. The engine is an open system because it is constantly receiving energy. If it the energy source is removed and the engine allowed to run until it stops, you will not see entropy decrease.