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This question is about a thought experiment described in this YouTube video: https://youtu.be/JzwfzgfJiJ4?t=1139

The video is attempting to explain how a jet engine works, specifically, the importance of having a high compression ratio. The thought experiment seems to consist of the following:

  1. Isothermal compression
  2. Isochoric heating
  3. Isothermal expansion (<-- work done on the environment here)
  4. ???

This isn't the Brayton cycle. Significantly, the process of adding heat in the Brayton cycle is isobaric, with the expansion of the gas happening immediately and continuously.

I don't fully understand the Brayton cycle, and I'm not trying to sneer at Tech Ingredients. But these processes seem sufficiently different that I'm not sure what correct intuition can be derived from them.

I am aware that the Brayton cycle can be analysed in temperature-entropy coordinates by using the Carnot cycle: "Ideal Brayton cycle as composed of many elementary Carnot cycles" https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node47.html That's obviously a lot more complicated. So what is going on in the Tech Ingredients explanation?

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2 Answers 2

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It's probably just plain wrong.

This is what happens when a nontechnical person gets stuck having to explain something technical. They usually get it wrong, but since the audience is also nontechnical they can't tell the difference.

But for the engineers in the audience, seeing junk like that make steam come out of our ears.

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  • $\begingroup$ I think it's not justified to say that the (anonymous) guy who presents Tech Ingredients is "nontechnical". But it is clear that he's playing fast and loose with the thermodynamic description of the Brayton cycle. A bit of research suggests that he might have the Otto cycle in mind (which has adiabatic expansions and compressions). en.wikipedia.org/wiki/Otto_cycle $\endgroup$
    – Theo H
    Commented Jun 24, 2023 at 18:43
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After doing some more research, I'm going to tentatively answer this with the following two points:

  1. The demonstration in the video is probably intended to represent the Otto cycle. Compression and expansion are, thus, adiabatic rather than isothermal, and combustion is isochoric. The intention is probably to illustrate internal combustion in general, rather than the specific workings of Brayton cycle engines.

  2. In terms of the possibility of a gas turbine cycle using isothermal compression and isothermal expansion, rather than adiabatic compression and adiabatic expansion: this gives the Ericsson cycle, which actually produces more work per stroke than the Brayton cycle. So, as I understand it, the temperature increase associated with adiabatic compression is in fact detrimental to power output (rather than, say, being essential to the operation of the engine). https://en.wikipedia.org/wiki/Ericsson_cycle#Comparison_with_the_Brayton_cycle

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