Question

Proof that no engine can beat Carnot
I understand the logic behind the proof, that if there was a better engine, you could make carnot engine a refrigerator and then make the system transfer heat from lower temp to higher temp.

But what I don't get is, we could use that logic for any pathetic engine (more pathetic than Carnot) and use the same logic and prove, nothing can beat the efficiency of that engine too (which is ofcourse wrong, because Carnot engine beats it)

So whats going on?

Answer 1

@Vincent-Lyon.Fr

Answer 2

Do you have a link? I do not know the proof you are talking about.

Answer 3

yes I do .. wait 1 min.. Ll give the youtube link with exact time..

Answer 4

https://youtu.be/ouSLRgkPzbI?t=6m25s

Answer 5

You cannot do it with any engine, because in your proof, you must somewhere use the fact that the Carnot engine is reversible, whereas any engine is not.

Answer 6

oh yes
that was the only logical conclusion

So why is it that any engine is not reversible?
and that carnot engine is??
(without the math)

We can ignore the link now..

Answer 7

I understand the logical argument.. but I don't get the intuitive idea behind a reversible and irreversible engines!

Answer 8

Carnot engine is a model in which you postulate reversible transformations (no creation of entropy) and it is still consistent with the laws of thermodynamics.

Answer 9

I don't get it :-/

Answer 10

I always had difficulty understanding reversible processes

I mean why can't any cyclic process be reversible?

Answer 11

Because there are stages where heat is received from a hotter source than the body and this cannot work the other way round.

Answer 12

The (non)reversibility depends not only on the body or engine but also on the heat sources you are using.

Answer 13

wait wait


so are you saying this can't be a reversible process?

Answer 14

The x axis is V and y axis is P

Answer 15

I think it can, but only if the heat sources are the right temperatures all along the cycle.

If you have only a hot and a cold heat source of fixed temperatures, then this cannot be a reversible engine.

Answer 16

a) I don't understand how you are able to intuitively state the first point

b) Even if you say it is true only for those right temperatures, then does that mean even that cycle will have the exact same efficiency as Carno?

Answer 17

I think the initial statement should read:
No engine working between two fixed" heat sources T1 and T2 can beat the Carnot engine."

Answer 18

Yes yes.. We are going to consider the same tempearture sources while comparing the engine with carno!

Answer 19

For your point a), remember any time there is a heat exchange in the cycle, in order to be reversible, the heat source must be the same temperature as the system.

Answer 20

Along your horizontal line, all temperatures are different.