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Questions tagged [heat-engine]

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Efficiency of irreversible engine in absence of dissipative process

My book states that a heat engine based on idealised reversible processes (Carnot engine) achieves highest efficiency possible. It further states that irreversibility is associated with dissipative ...
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1answer
26 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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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 to calculate the mechanical work done by gas when it is absorbing heat and it's temperature rising, volume increasing and so it's pressure?

How do I calculate the mechanical work done by gas when it is absorbing heat and it's temperature rising, volume increasing and so it's pressure? I know $Q = \Delta U + \int pdV$ Here $\int pdV$ is ...
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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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Brayton Cycle PV diagram when regeneration is not possible

https://www.ecourses.ou.edu/ebook/thermodynamics/ch09/sec092/media/th090203p.gif Above is the case of ideal Brayton cycle when the temperature at exit of turbine ( state 4 ) is greater than the ...
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Brayton cycle PV diagram with isentropic efficiencies introduced

https://imgur.com/4FKdiEN "PV diagram edited with Isentropic efficiencies for Brayton Cycle" I think that when isentropic efficiencies for compressor and turbine are introduced , the compressor takes ...
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1answer
38 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
38 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
168 views

How I can calculate a coefficient of performance (COP) from this cycle?

I had to solve a simplistic inverse and reversible triangular cycle with an ideal and diatomic gas transport fluid. Here's my solution attempt: NOTE: Evolution 2 to 3 is really an isothermic process,...
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1answer
44 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
44 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
105 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
35 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
70 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 ...
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1answer
132 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
147 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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0answers
10 views

Calculating heat transferred to re generator and work of the Stirling engine cycle when only given a few variables

I am asked to basically find the work per cycle and heat transferred to the regenerator. My only issue is that only few things are given in the problem that I can utilize: //I am assuming the problem ...
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1answer
56 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
37 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
72 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
91 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
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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
18 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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46 views

Coupling two Stirling engines for cold generation

I am working on the coupling of two Stirling engines for cold generation, and I can't find any related subject on this matter on the internet. I would be glad if you guys help me find papers, etc. on ...
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2answers
72 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
22 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
29 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
72 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
405 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
100 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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19 views

Adiabatic expansion in flow processes

In the brayton cycle, my book says the turbine work is change in enthalpy during initial and final states of the turbine which is an adiabatic expansion process. It is because by first law $q = w_f + ...
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1answer
38 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
46 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
47 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
35 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
200 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
50 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
29 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
37 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
70 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 ...
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1answer
47 views

What will happen if I exceed the documented temperature gradient on my thermoelectric generator?

For fun, I am designing a electrical generator powered from natural gas. I have ordered 20 count of these devices: https://www.ebay.co.uk/itm/Thermoelectric-Power-Generator-Peltier-Module-TEG-...
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1answer
41 views

Heat transfer area across a surface

This is the formula I use for heat transfer across a surface - Q= U x A x LMTD where , U , A and LMTD are Overall heat transfer coefficient, Heat transfer area and log mean temperature difference ...
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1answer
248 views

Final equilibrium temperature of two reservoirs connected together via a heat engine (thermal efficiency of 50%)

I am considering a thermally isolated system of constant pressure. Where 10kg of air at 1000K (assume $Cp_{air}=0.98$ kJ/kgK, denote this as the hot reservoir by H) is connected to 10kg of water at ...
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1answer
31 views

Pneumatic Motor vs Heat Engine

I noticed Wikipedia has separate articles for Pneumatic Motors and Heat Engines, however, given the relationship between temperature and pressure, aren't Pneumatic Motors just a subset of Heat Engines?...
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1answer
101 views

$\sum{\frac{Q}{T}}$ is zero

I was going through a proof in my book and couldn't understand the last part of the proof. I mean I understand $$\frac{Q_1^{\prime}}{T_1}-\frac{Q_2^{\prime}}{T_2}\leq 0$$ But after that how did ...
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150 views

Entropy and Work Output from a system

When Heat is transferred to a system, the molecules become so disorganized( Entropy increases), but how are we able to extract work from a system which is so disorganized?