# Carnot efficiency

the carnot engine does not obey any law of thermodynamics,because it has
100% efficiency,now do we have any engine that has come close to this hypothetical
engine?(in terms of efficiency ) can this process of carnot be used for a 6 stroke engine?(hypothetically)

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the carnot engine does not obey any law of thermodynamics,because it has
100% efficiency,now do we have any engine that has come close to this hypothetical
engine?(in terms of efficiency ) can this process of carnot be used for a 6 stroke engine?(hypothetically)
Which thermodynamic law did Carnot engine break?? And where did you read that its efficiency is 100%?

Here's a http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html" [Broken] where you practice some calculations.

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well my engineering book of thermodynamics says that carnot engine(imaginary)
is supposed to have 100% efficiency,the perfect engine.
thus it disobeys the 2nd law,which states that heat cannot be totally converted
to work?

It's also a fact that it's impossible to construct a Carnot cycle engine in real life for that very reason. Carnot only really demonstrates the 1st law, its a purely paper concept and so doesnt have to obey the 2nd law as its idealised. It's also why other cycles are compared to Carnot efficieny.

Also a Carnot engine isnt actually 100% efficient, its just the best possible efficiency (which is why it says 100%) between two heat reservoirs. The Carnot efficieny is 1- Tc/Th.

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russ_watters
Mentor
well my engineering book of thermodynamics says that carnot engine(imaginary)
is supposed to have 100% efficiency...
Could you quote the passage for us? You clearly misread it.

russ_watters
Mentor
Also a Carnot engine isnt actually 100% efficient, its just the best possible efficiency (which is why it says 100%) between two heat reservoirs. The Carnot efficieny is 1- Tc/Th.
That doesn't make sense - why would the book say that an engine with 65% efficiency (for example) is 100% efficient? I think what we have here is a complete misread.

That doesn't make sense - why would the book say that an engine with 65% efficiency (for example) is 100% efficient? I think what we have here is a complete misread.
As between the two temperatues, a Carnot Cycle will extract the maximum possible efficiency compared to a Rankine cycle for example. Thats the only reason why I can think that a book would say its 100% efficient. The book chose a poor (and misleading) choice of words if it does indeed say that.

Like you said though, its probably a misread.

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stewartcs
well my engineering book of thermodynamics says that carnot engine(imaginary)
is supposed to have 100% efficiency,the perfect engine.
thus it disobeys the 2nd law,which states that heat cannot be totally converted
to work?
The thermal efficiency is always less than unity. Carnot is just the best theoretical efficiency (not practical though). It's typically used in thermodynamics to help understand the general concept of the second law.

It doesn't break any laws of thermodynamics, but it can not exist practically since there is no such thing as a truly reversible process.

CS

the lines go like this
" is it that we cannot achieve 100%efficiency due to practical difficulties or is there
concept of an ideal engine in which no energy loss takes place"

so do you mean to say even theoritically ,100%efficiency is not possible,but they
have not listed any loss in case of carnot engine.

russ_watters
Mentor
Yeah, you misread that, though it may be a little poorly worded. It is saying that Carnot envisioned how the perfect thermodynamic enegine could work. "No losses" doesn't mean no heat of rejection (just no friction, heat transfer, etc.) and isn't meant to imply 100% efficiency.

Anyway, yes, it means 100% efficiency is not possible: Carnot efficiency is the maximum that is theoretically possible.

Maybe think of it this way: If you could reject heat to a sink at absolute zero, then your Carnot engine would be 100% efficient.

With normal everyday heat sinks at something like normal environmental temperatures (500 to 550 R), the efficiency is going to be alot less.