Questions tagged [heat-engine]

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Can a heat pump have efficiency greater than 100%?

A heat pump has an efficiency greater than 100%. Does this violate the laws of thermodynamics? My answer: I'm assuming that "heat pump" means heat engine. $e>1$ $\displaystyle\implies\frac{W}{...
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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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Third Law of Thermodynamics and Thermal Pollution

Unlike coal power plants which produce CO2 and intensify the greenhouse effect, every engine produces thermal pollution because of the constraints on engine efficiency. No engine can convert 100% of ...
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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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Reversible engines

As far as my understanding goes, the only reversible heat engine is the Carnot engine, described by the Carnot cycle. On the other hand, in all of the resources I've found the change in entropy for ...
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Work done in Brayton cycle

I am studying Brayton cycle. In Brayton cycle, heat addition and exhaust is carried out at constant pressure. Now, the efficiency is $$\eta=\frac {w_{net}}{q_{in}}$$ My book, while calculating $w_{net}...
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Why does intercooling improve the efficiency of a Brayton Cycle?

I've read in numerous sources that intercooling improves the efficiency of a Brayton cycle. The explanation given is that it decreases the compression work by minimizing the temperature during the ...
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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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Brayton Cycle Efficiency

Hi, I'm trying to calculate the efficiency for the Brayton Cycle. The end result is supposed to be: $$ \eta = \frac{C_p(T_3-T_4) - C_p(T_2-T_1)}{C_p(T_3-T_2)}$$ Where the denominator is from $Q_h$. ...
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Work done by a heat engine given thermodynamic cycle

A heat engine uses an ideal gas as its working substance, and is described by the following thermodynamic cycle; Isobaric expansion at $4 \text{ atm}$ from a volume of $4~\text L$ to $8~\text L$. ...
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Frequency of refrigeration cycle

How often does the refrigeration cycle repeat itself? I have read that the compressor runs at about 1000 rpm. Does that mean that the frequency of the cycle is 1000/60=17 Hz?
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Formula for the heat input in a heat engine

There's one situation relating to heat engines where there's one equation I never understood completely. The situation is something like: there's a heat engine operating between two systems $A$ and $B$...
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About the 2nd Carnot Corollary

I have a question about the 2nd Carnot Corollary. According to Moran, Fundamentals of Engineering Thermodynamics, the 2nd Carnot Corollary stated that "all reversible power cycles operating between ...
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Is a Curie pendulum an example of a heat engine?

By definition, a heat engine absorbs energy and uses that energy to do work. A Curie pendulum doesn't seem to do work when absorbing that energy. Instead, the magnet is heated and loses it's magnetic ...
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Concerning heat engines and entropy

In deriving the inequality $$e \leqslant 1 - \frac{T_c}{T_h},$$ Schroeder, in An Introduction to Thermal Physics, uses the fact that for one complete cycle of a heat engine, $$\frac{Q_c}{T_c} - \frac{...
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Thermodynamic costs of implementing real heat engines and how to analyze them [closed]

There are several well-studied cycles, like the Carnot, Otto, etc., among infinitely many. But, what happens when we also consider the costs of driving the system through a particular cycle? How does ...
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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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How can Carnot's theorem and the second law of thermodynamics logically equivalent?

Can anyone please explain how Carnot's theorem which states that "working between two same temperature limits,a reversible engine has maximum efficiency " and the second law of thermodynamics ? By ...
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Can a heat engine be made with vacuum as the working material?

Sounds absolutely ridiculous, for vacuum is no 'material' in strict sense. But special relativity tells me that length may be contracted or expanded. I find no problem in extending this idea, that ...
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Better understanding the Clausius Inequality

My conceptually understanding draws on the section on the Clausius Inequality from Finn's Thermal Physics. Attached is the graphic used for the derivation in the textbook. If I'm to understand ...
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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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Why is there a cold reservoir in a Carnot engine?

I'm trying to wrap my head around the idea behind a Carnot engine. The main question I'm trying to figure out is how a Carnot engine actually works in producing work. I'll provide a picture for ...
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Is spark from spark plug in SI engine (IC engine) not counted as a hot reservoir in Heat Engine?

Are the Heat Engine (HE) and Internal Combustion Engine (ICE) not the same because ICE only exchanges the heat to the surrounding[only one reservoir]? And I have a question what if it's an SI engine ...
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Why do we use heat engines if they are so inefficient?

Let's say I wish to build an engine which will launch a box vertically upwards. I propose two such engine designs: Engine 1 (finger, spring, box) Step 1: Orient uncompressed spring vertically. ...
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What kind of ideal gas process a positively sloped line in a $pV$-diagram represents?

I've come across an ideal gas process which can be represented by a positively sloped line in the $pV$ diagram. I found something about this on KhanAcademy, but it's simply a worked example on how to ...
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Calculate efficiency of thermodynamic cycle [closed]

A monatomic gas undergoes a cycle consisting of two isothermal and two isobars.If the minimum and maximum temperature of the gas during the cycle are T 1 = 400 K and T 2 =800K, respectively, and the ...
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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 ...
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Entropy generation in an endoreversible engine

An endoreversible engine is one that is internally reversible (i.e., a perfect Carnot engine) but the heat transfers occur across imperfect conductors (resistances), thus rendering the engine ...
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Does real gas's entropy increase in $T$-$S$ diagram of Brayton Cycle efficiency?

I got T-S diagram for efficiency of Brayton Cycle, which includes ideal cycle (black line), real cycle (red line) and GE's cycle (blue line), can't understand the entropy, whether it increases or ...
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About *Carnot's cycle* and *engine*

we know the Carnot's cycle diagram, so A, B, C, D are points regarding the state of the system. Consider a piston with weight on it. -----AB is just making the piston go up Q = W by providing heat no ...
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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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Source and sink in an ideal reversible otto cyle?

what the temperature of source and sink is in an otto cycle? Since the temperature of the surroundings continuously change during the isochoric parts, we cannot have a single source and sink ...
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Efficiency of reversible carnot engine must be 0?

In order for heat to transfer in a reversible process, the temperature difference must be approximately zero. Therefore at the isotherms in a carnot engine, there is negligible temperature difference. ...
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How is the work done by Carnot Engine obtained or converted to usable form?

I read about the Carnot Cycle and Carnot engine where it was said that the work done by this engine is equal to the area enclosed in the PV diagram. I understood the proof and everything but I am not ...
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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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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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Can a heat engine cool its heat source?

Another way to put my question would be: Can a heat engine act as a negative thermal resistance? Though that wording makes me expect the answer is “no”. Suppose we have: a heat source (of constant ...
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Why is there a constant pressure when lifting a weight with expanding gas?

A while ago at university we did this experiment: http://dev.physicslab.org/Document.aspx?doctype=2&filename=Thermodynamics_HeatEngineLab.xml In the experiment we had a piston with air inside, ...
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Replacing Qh with Ql in Etropy equation in Heat Pump

In the heat pump below, the $T_H$ is not constant. It is assumes that the process is steady and reversible. So when constructing an entropy equation over the CV (the purple-colored area): $$\dot ms_1+\...
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Carnot's Engine Proof flawed?

https://www.youtube.com/watch?v=LUoUb4hGMH8 I was looking at the reason as to why a Carnot Engine and watched this video on KhanAcademy. He used a very clever method of proving this by using the ...
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Thermodynamic cycles

Consider a system that takes excess air (assume it to be an ideal gas, $Cp = \frac{7}{2} R$) from a power plant stack at $370 \rm K$ (typo in figure) and $2$ bar and compresses it to 20 bar ...
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Clarification on Clausius Inequality

Consider a heat reservoir which gains heat $Q$ irreversibly at temperature $T$ from the surroundings which is at temperature $T_0$. The entropy change of reservoir is then given by $\frac{Q}{T}$, ...
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Work obtained from Rankine cycle vs Carnot cycle

In this diagram, We can say that the work obtained from Carnot cycle is greater than the work obtained from the Rankine cycle because the area enclosed by the curve is greater. However, if we try ...
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Seebeck effect or Peltier effect for optimal thermoelectric effect?

I am building an experimental thermopile, and I'm getting confused with the definitions for the Seebeck effect and Pettier effects. From Wikipedia: The Seebeck effect: The Seebeck effect is the ...
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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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Lowering of temperature in expansion valve of refrigerator

We know that the high pressure liquid refrigerant (comming from condenser) passes through the expansion valve and lowers the temperature (due to decrease in pressure). My Question: From the formula ...
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Why does taking work from a system not reduce its entropy?

In Daniel Schroeder's book Thermal Physics, he makes the point that as energy is put into a system it's entropy goes up (there are some exceptions to this generally, but for this question we can ...
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Carnot cycle with varying reservoirs

I was thinking what will happen to the standard Carnot cycle, if instead of - say - the usual cold reservoir, we adopt a smaller refrigerator, whose temperature in fact changes after the heat release ...
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Internal energy of ideal gas in adiabatic process

In above diagram it can be seen that the curve for adiabatic compression is more steeper than curve for adiabatic expansion. The difference between p1 and p4 is more than p2 and p3 - it means that the ...
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Could a Carnot engine linked to a Carnot “refrigerator” create a perpetual heat flow between two reservoirs?

Here is a diagram of the system in question (adapted from the diagram on this page): All of the work produced by the Carnot engine is used to drive the Carnot refrigerator, which is simply a Carnot ...