Questions tagged [heat-engine]
370 questions
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Why is the efficiency of human cells less than the efficiency of an Otto engine?
I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest efficiency among all practical engines. But I…
lucas
- 3,199
17
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Why is the Carnot engine the most efficient?
It seems that the only condition used in proving that the Carnot engine is the most efficient is that it is reversible. More specifically, the Carnot engine can be run in reverse as a refrigerator. Furthermore, it is asserted that all reversible…
curiousgeorge
- 865
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Maximum theoretical efficiency of internal combustion engine
This Wikipedia article states that:
Most steel engines have a thermodynamic limit of 37%.
Is it correct? If so then it is not clear where this number comes from. Is it derived somehow from Carnot's theorem? I tried to google it but found nothing…
user40492
13
votes
4 answers
Human as a heat engine
humans are heat engines. heat engines can't operate when the source and the sink are at the same temperatures and also if the temperature of the sink is higher than that of the source. in humans' case, the source is our body itself (technically the…
newera
- 1,394
11
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5 answers
Why do we have to dump the extra entropy created in a heat engine?
In his introduction to thermophysics, Daniel Schroeder writes the following about the process in a heat engine:
Only part of the energy absorbed as heat can be converted to work. The reason is, that the heat, as it flows in, brings along entropy,…
PhylomatX
- 167
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1 answer
How efficient is the Crookes radiometer?
I remember many years ago, I think at 8th grade, seeing the teacher show us a Crookes radiometer. I remember it being very fascinating. Today I read the wiki article on it, after looking up what it was called, but the article wasn't very clear in my…
Valtteri
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How heat flow through finite temperature drop is an irreversible process?
Is it necessary that the drop/rise in temperature by flow of heat is irreversible?
Bibek_G
- 223
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10 answers
Why can't we make Carnot heat engine in real life?
Question is obvious: Why can't we make Carnot heat engine in real life?
I read Wikipedia and Fundamentals of Physics (Halliday) but I haven't found anything on my question. There are explanations about formulas and how it works but no obvious answer…
titansarus
- 221
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2 answers
Can the efficiency of a thermodynamic cycle be worked out from a pV diagram?
Is it true that a pV diagram of a thermodynamic cycle with a specified scale on both axes is enough to calculate its efficiency (such as the one below)? Is it the area within the shape divided by the area under the top curve?
mrk1357
- 158
9
votes
4 answers
Why cant we save the heat rejected in a heat engine?
Suppose the working fluid is heated in a boiler by combustion of fuel outside the heat engine. If we assume that this a steady flow process with no change in time, neglect the kinetic and potential energies and heat losses from the boiler. The…
Kakashi
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Why is steam a good working fluid for heat engines?
Steam (gas-phase H2O) is very commonly used as the working fluid for external-combustion heat engines. It seems to me that it ought to be a poor choice, because:
Unless I am seriously confused about something, you have no choice but to throw the…
zwol
- 1,857
8
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2 answers
What's the efficiency of real heat engines?
Real heat engines always have lower efficiency than the Carnot efficiency.
I wonder how efficient real engines can be? Can their efficiency get anywhere near the Carnot-limit?
Calmarius
- 8,428
7
votes
5 answers
Why is it that a Carnot heat engine will reject no heat to a zero temperature sink?
Looking at the equation for Carnot efficiency, I notice that as the temperature of the heat sink approaches zero, the efficiency approaches unity:
$$
\eta_{rev} = 1 - \frac{0}{T_H} = 1
$$
Seeing as the efficiency of a heat engine is the ratio…
pxq
- 741
7
votes
2 answers
Do thermodynamic cycles occur only in human-made machines?
Intuitively, it seems like heat engines and refrigerators require a good bit of technology to make a working substance pass through different thermodynamic states and then finally return to the initial state. Usually, at the very least, some tubing…
ether
- 325
7
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4 answers
How to derive the Clausius Inequality?
In my thermodynamics class, we've seen the Clausius inequality derived for a Carnot cycle, and then extended to any cycle. For the Carnot Cycle, we have that it's the most efficient possible cycle between two heat reservoirs, with an efficiency of 1…
joshuaronis
- 3,145