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Questions tagged [heat-engine]

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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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5answers
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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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1answer
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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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1answer
37 views

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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0answers
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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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2answers
303 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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1answer
118 views

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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2answers
70 views

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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4answers
133 views

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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1answer
358 views

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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2answers
65 views

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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0answers
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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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1answer
49 views

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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1answer
69 views

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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1answer
45 views

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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2answers
60 views

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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2answers
99 views

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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1answer
67 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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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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1answer
55 views

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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1answer
66 views

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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1answer
71 views

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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2answers
57 views

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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1answer
292 views

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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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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1answer
126 views

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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1answer
113 views

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 ...
0
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1answer
269 views

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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2answers
283 views

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 ...
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1answer
71 views

Clausius statement — Reservoir vs Body

Consider the two statements below: (C1) It is impossible for a cyclic engine/heat-pump to transfer heat from a colder body to a hotter body without any third work/heat channel. (C2) It is ...
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1answer
49 views

Phase of refrigerant when entering evaporator?

In the diagrams for refrigeration cycle(vapour-compression), the refrigerant at the inlet to the evaporator lies under the vapour dome i.e partly vapour and partly liquid. Now i have been studying ...
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1answer
170 views

Actual sequence of refrigeration cycle

Whenever i have studied refrigeration cycle on books, i have found that the diagrams are always explained starting from the compressor*, then condenser then throttle and finally evaporator. I find it ...
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1answer
197 views

Can you plot an irreversible cycle on a PV diagram?

You can only plot reversible processes in PV diagrams. And I suppose a cycle made of reversible processes should be reversible (am I right?). So every cycle plotted in a PV diagram should be ...
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2answers
85 views

Brayton cycle: heat calculation for isobaric processes

I have a brayton cycle and want to calculate the efficiency and the back work ratio. In my textbook it is stated that the heat added at process 3-4 happens at constant pressure (isobaric) and ...
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1answer
19 views

Kelvins alternative to direct heating

There is this method proposed by kelvin, which is an alternative to direct heating. In this method, the fuel instead of direct heating is first fed to a heat engine and the work obtained is used to ...
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2answers
157 views

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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1answer
25 views

Replacing an expansion valve with a piston to improve efficiency?

Could you replace an expansion valve in a refrigeration system with a piston to get useful work out of the expansion of the fluid between the high side and the low side? If so, why does this not seem ...
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1answer
32 views

End expansion point on overheated Rankine cycle

In an overheated Rankine cycle, the end point of the adiabatic expansion could end inside the dual-phase bell, or on the right. In the first case, the end expansion point falls of course on the ...
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0answers
131 views

Rankine cycle overheated VS Carnot cycle

In an overheated Rankine cycle, it is known efficiency is better than normal, non-overheated Rankine, as we add a new subcycle with heat absorb at an higher mean T, and so with better efficiency. But,...
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2answers
934 views

Does entropy increase with a decrease or with an increase in a system’s temperature?

I was trying to understand why in a heat engine there is an increase in entropy. If heat (lower) quality energy is converted to work (higher quality energy), even if not all, why wouldn’t entropy ...
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3answers
129 views

Is there an implemented example of an almost reversible heat engine?

I'm reading Feynman's discussion of Reversible engines (Vol I 44.3) in The Feynman Lectures on Physics. His discussion is very abstract, and leaves a lot of practical questions unanswered. Rather ...
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1answer
66 views

Clausius Inequality application to engine

So I am aware of the Clausius inequality which states the following: ∮δQ/T≤0 around a cycle. Now consider the following question: Consider a real heat pump which in one cycle extracts heat Qc from ...
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1answer
156 views

How Much Heat Is Absorbed/Rejected By The Heat Engine?

If a heat engine undergoes a reversible cycle consisting of two isobaric processes and two adiabatic processes, what methodology is used to determine how much heat is absorbed or rejected by the heat ...
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2answers
49 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 ...
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1answer
37 views

Burning atomic elements

when someone says "the carbon was completely burnt up" eg. in a car's engine. what does that mean?, Where does it go? I know no new elements are formed and of course it really didn't disappeared. so ...
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2answers
301 views

Is the efficiency of a reversible heat engine independent of the processes involved?

Does the following question conflict that fact that efficiency of a reversible engine is independent of processes involved?
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1answer
54 views

Why does brushless motors heat up?

I would like to know what makes a brushless motor heat up. I'm so aware of the problems generating the high temperetature such as wires contact problems or over loaded motors. I want to know what ...
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0answers
30 views

Finding the Efficiency of this Reversible Process

I am unable to find out the efficiency of this reversible process: An ideal monatomic gas undergoes the following four steps in this order; 1) Compressed adiabatically from volume 8 m³ to 1 m³. 2) ...
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1answer
38 views

Is there any relationship between the heat going into the system and the work output of a gas turbine

I am doing a bit of research on Brayton Cycle and find this T-S diagram for the cycle in https://www.grc.nasa.gov/www/k-12/airplane/brayton.html and have learned that the area encloses by the curve ...
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1answer
128 views

Calculating work from enthaply and internal energy

When do we use enthalpy to calculate work done and when do we use internal energy? For e.x in ideal rankine cycle (working substance steam) the work done by the turbine(3-4) is calculated from the ...