Questions tagged [heat-engine]

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4answers
8k views

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 ...
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3answers
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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'...
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5answers
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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, ...
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4answers
7k views

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,...
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3answers
12k views

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. ...
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3answers
618 views

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?
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2answers
6k views

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 ...
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2answers
6k views

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?
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5answers
2k views

Does chaining Carnot heat engines make them more efficient?

Since the efficiency of a Carnot cycle depends on the difference between the hot and cold sides of the engine, could you put multiple heat engines in a series to maximize overall efficiency? A $3000 \,...
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3answers
8k views

How much heat from a fire actually warms your home?

A fire in a hearth disperses heat to, I guess, three places: the bricks of the chimney out the hearth (where the person tending the fire is standing) out the chimney, above the house How would you ...
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2answers
514 views

Power generation by extracting heat from the environment?

It is chilly outside now and I am kept warm by a heat pump with COP 5.5: eating only 0.6 kW, it steals 2.6 kW from the outside air (making it even colder) and delivers the resulting 3.2 kW into my ...
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4answers
4k views

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 didn't found anything about my question. there we're explanations ...
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2answers
1k views

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 ...
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5answers
4k views

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 ...
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2answers
651 views

Heat pump intuition

What is an intuitive explanation for the concept of heat pumps? I know that it is basically a reversed Carnot process. We can for example take an amount of heat $Q_1$ out of a warmer system and ...
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0answers
386 views

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 ...
4
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2answers
379 views

Why does there need to be an isothermal compression in a Carnot cycle?

This is the part that allows the Carnot engine to not violate the 2nd law of thermodynamics, but, hypothetically, why can't we just adiabatically compress the working substance to get it back to State ...
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5answers
2k views

What is the heat sink in an internal combustion engine?

I believe that the heat source is the spark plug because it heats up the fuel-air mixture and that the working substance is the fuel-gas mixture. Then, I know that the explosion created by the fuel ...
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3answers
1k views

Why does a cooling car engine crackle?

Why does a cooling car engine crackle? What determines the frequency of the crackles?
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2answers
7k views

Efficiency of reversible engines

I'm a little confused about something. All reversible engines have the same efficiency, or one could drive the other to move more heat in the reverse direction. Also, no engine has an efficiency ...
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6answers
2k views

Extracting heat energy without a heat engine

Is it possible to extract the molecular kinetic energy from a system directly (without the use of a heat engine / temperature gradient) and convert that to another form of energy, such as electricity, ...
4
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1answer
356 views

Understanding heat engines and its relationship with the second law of thermodynamics

I am having trouble with understanding the so called Second Law of thermodynamics. There are a million different definitions. All of them get to the essence: "Disorder increases, stuff gets wild ...
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3answers
1k views

Heat Pumps and COP?

Heat Pump COP (Coefficient of Performance) $$COP=1-\frac{T_c}{T_H}$$ With Heat Pumps is the efficiency/COP more dependent on the hot or the cold reservoir and why?
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0answers
112 views

Are there actual technical realizations of the Carnot cycle?

The Carnot cycle is composed of four steps: isothermal expansion, adiabatic expansion, isothermal compression and adiabatic compression. In the case of combustion engines, we have the idealized Otto ...
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3answers
222 views

How is the $2^\mathrm{nd}$ law of thermodynamics valid?

For example, if we have an infinite cylinder fitted with a piston having adiabatic curved walls and a diathermal base. Now, bring the base in contact with some higher temperature body and let the ...
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4answers
515 views

Does a reversible heat engine exchanging heat with an ideal gas that does a transformation imply that transformation is reversible?

Consider a reversible heat engine working between two sources. Suppose that one of the sources is a thermostat, while the other is an ideal gas which follows a transformation and exchanges some heat ...
3
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1answer
96 views

Can exhaust gases be diverted to other cylinders during engine operation?

Here's a description of new combustion engine improvement by Mazda, called SkyActive-G. They claim that in a "generic" engine... when the exhaust manifold is short, the high pressure wave from the ...
3
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1answer
164 views

Most efficient mechanism for heating air

I was read about small scale ion treatment which uses heats to high temperatures, would this be scalable and if so would there be any more efficient methods. In summary, if you wanted to heat large ...
3
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1answer
112 views

Heat pumps: can $COP\leq 1$ and when does $COP=\frac{1}{\eta}$ hold?

Heat pump COP is defined as $$\xi=\frac{|Q_{HOT}|}{|W|}$$ Where $Q_{HOT}$ is the heat given to the hot source. I have two linked doubts about it Under what conditions I can say that $\xi=\frac{1}{\...
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1answer
650 views

Why must the total change in entropy be 0 for a heat cycle to be as efficient as possible?

If $\Delta S_{total}$ were not zero, then heat flow would take place at different temperatures. It is easy to see why this is true by examining the opposite: that when heat flow takes place at the ...
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1answer
3k views

Carnot vs. Otto

Otto cycle is reversible and according to Carnot theorem should have the same efficiency as Carnot $\eta_{Carnot}=1-T_C/T_H$. The efficiency of Otto is $\eta_{Otto}=1-\frac{T_4-T_1}{T_3-T_2}$ (see ...
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0answers
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What is the problem of this thermodynamic fallacy?

Consider the following (alleged) cycle: Quasi-static adiabatic expansion of a vapor in such a condition that it results in the condensation of part of the mols*; Out of equilibrium adiabatic ...
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1answer
179 views

Exhaust liquefaction in internal combustion engines

Does an internal combustion engine provide enough efficiency to still deliver mechanical power, if it is forced to liquify it's own exhaust gases by compression? If gas containers would be added to ...
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4answers
3k views

Why does heat engine need to reject energy to environment?

The most efficient thermodynamics cycle is the Carnot cycle, it involves 1/ Receiving heat from hot source, expanding and doing work on the surround 2/ Receiving work from environment, being ...
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3answers
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Will this type of engine produce thrust?

I was wondering that if we replace the combustion chambers in a jet engine with heaters heating the compressed air up to temperature near to plasma then what will be the case, will it produce any kind ...
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3answers
5k views

Questions on Carnot's theorem

This article on Carnot's theorem states that All heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs. However, it only ...
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3answers
709 views

Can the vibrational energy of a engine be used to increase efficiency?

In whole mechanics, we never talk about wasted vibrational energy! As we see in our daily life, a lot of energy is wasted as vibrational energy in every engine, for example as we fire our car's engine,...
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2answers
60 views

Why aren't there any air conditioning systems that compress, cool, then expand air directly?

I realize there would not be any phase change in this system, but as long as the compressed air get hot enough above the outside ambient temp it seems like you could extract heat (and even some ...
2
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2answers
240 views

Why does a heat engine convert only some of the heat into work

I am aware that no heat engine can ever be 100% efficient due to the limit imposed by the 2$^{\mathrm{nd}}$ law of thermodynamics; and that the best we can do is reach the Carnot limit. When heat ...
2
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4answers
19k views

How is the efficiency of a heat engine related to the entropy produced during the process?

I'm reading Schroeder's An Introduction to Thermal Physics. Regarding heat engines, it is stated: Unfortunately, only part of the energy absorbed as heat can be converted to work by a heat engine. ...
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2answers
3k views

Why can't the Carnot cycle be used to operate automobiles, if its the most efficient?

Since the cycle proposed by Carnot (consisting of two isothermal and two adiabatic processes) is the most efficient one on which to run a heat engine, why do we use other, less efficient ones (like ...
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3answers
2k views

Intuitive explaination for why higher engine compression ratio is more efficient?

Intuitively to me it seems likes increasing compression ratio would require more work to compress the gasses before ignition, so you'd just end up getting back what you put in - like a spring. What am ...
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2answers
440 views

Carnot cycle: What happens to the rest of the energy?

In a Carnot cycle, the work potential is $$W =Q_\text{in} \left(1 -\frac{T_0}{T_\text{R}}\right)$$ where $T_0$ is the temperature of the sink, $T_\text{R}$ is the temperature of the source heat ...
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2answers
94 views

Could a refrigeration unit be used to provide useful heating?

I was thinking about refrigerators and air conditioners. They remove the heat from somewhere and put it into somewhere else. But why are we use this usually for cooling, and not so much for heating (I ...
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1answer
411 views

Prove all reversible heat engines that work between just two temperatures have the same efficiency

Can someone explain this explanation to me: A completely different argument focuses on the requirement that the device only move heat at one or other of the two temperatures. Then, provided we ...
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3answers
300 views

Using thermal expansion for mechanical work

Assume I could heat up a bar with zero heat loss to the environment. I could then use the thermal expansion to do some work, for example compress a gas. Where does the energy needed for the ...
2
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1answer
203 views

How to heat water at very high temepratures?

I am working on a project and recently came through a situation where I'll be working with water evaporation. On searching the internet I found that water heated at 350Celsius would generate almost ...
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1answer
75 views

How does the 2nd Law of Thermodynamics demand the existence of at least two thermal reservoirs in a cycle?

In my studies, I've come across Kelvin's statement of the 2nd law: It is impossible to undergo a cyclic process whose only effect is removing heat from a reservoir and generating an equivalent ...
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3answers
829 views

How would a Stirling Engine's throttle work?

I recently stumbled upon this video of a Stirling engine which includes a throttle mechanism. Unfortunately, there was no information about how the throttle worked. I know that an internal ...
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3answers
1k views

Clausius Inequality with three reservoirs

Deriving Clausius inequality in the case of two reservoirs is quite simple. We assume that there is a thermal machine operating between two reservoirs, the first at temperature $T_1$ and the second at ...