# Questions tagged [heat-engine]

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### Efficiency questions in a closed loop expander

So let's say I have a condenser, and both the inlet and outlet are connected to a pressure vessel that has hot gas fluid. Let's say there is a pneumatic pump creating a load on the connection to the ...
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### In this Stirling Engine process, how do we know that the fluid is heating up?

Since the volume of fluid in contact with the cold source and hot source is the same, how do we know the fluid is heating up instead of cooling down?
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### 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 ...
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### Why does the temperature of the working fluid have to be close to the temperature of the cold reservoir during a Carnot cycle?

I understand why the temperature of the hot reservoir has to be minimally higher than the temperature of the hot working fluid during the isothermal expansion phase of the Carnot cycle (to limit new ...
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### How would a heat engine with 3 bodies work?

Say we have 3 equal bodies in contact, and there is a heat engine that absorbs and emits heat to these 3 bodies and it does work to an external medium. How would that work? I imagine that it would get ...
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### In a Carnot Engine, how does the heat flow from the heat reservoir to the engine if both are at the same temperature?

From what I read about the Carnot Cycle, there are 4 major steps involved, viz. Isothermal Expansion, Adiabatic expansion, Isothermal compression and Adiabatic compression. During the Isothermal ...
34 views

### Approaching ideal efficiency of a thermal machine

Say we have a standard thermal machine which operates in cycles. It absorbs energy in form of heat from a hot source, does some work and then transmits the remaining energy to the cold sink also in ...
• 311
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### Efficiency of a cyclic processes [closed]

Is there a specific formula for solving the efficiency of a cyclic process in pV diagram? I have this diagram of a diatomic ideal gas cyclic process, where ${p} = 4{p_0}$ and ${V} = 4{V_0}$ The work ...
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### Calculation of specific heat experimentally and mechanical equivalent of heat

After reading some books and some google searches and reading some answers on stack exchange physics what I have understand is (correct me if I am wrong) to calculate heat supplied we need change in ...
19 views

### Efficiency of a refrigeration machine with inverted sign

I'm trying to calculate the efficiency of an ideal refrigeration machine ($η$) and I keep getting the right result with the wrong sign. My reasoning is as follows (I've attached a sketch for more ...
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### Why does adiabatic expansion occur in the Carnot cycle?

Several people have asked this question before and I haven't found any of the explanations even remotely satisfying. To begin, Let's imagine our system consists of an ideal gas enclosed in a cylinder ...
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### Efficiency of a Rankine cycle [closed]

So the efficiency of a Rankine cycle is given by the formula $$e=1-\frac{Q_{c}}{Q_{h}}=1-\frac{H_{4}-H_{1}}{H_{3}-H_{2}} \approx 1-\frac{H_{4}-H_{1}}{H_{3}-H_{1}}$$ I understand that the change in ...
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### Humphrey vs Brayton cycle efficiency

In practical engineering we are limited in the upper temperature of thermodynamic cycle due to the material properties. So, after the max temperature is fixed, we want to make our cycle as close as ...
1 vote
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### Is it possible for an absorption refrigerator to pump heat to a reservoir that is at a higher temperature than the heat source used to run it?

Judging by the wiki page I get the feeling that this type of heat pump would not function if it's radiator were hotter than the heat source, but I can't quite put into words why and I feel like there ...
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### Where is the heat loss occuring in the Carnot cycle, if compared to a ideal gas turbine

The most efficient heat engine cannot be more efficient than this (1 - tc/th) so let's assume a hypothetical gas turbine where there is no friction or conduction losses , say the ambient temp is 300k ,...
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### Apparently Spurious Argumentation in Justification of Clausius Inequality

In section 7-1 of Cengel and Boles text, "Thermodynamics - An Engineering Approach", the authors give a demonstration of the validity of the Clausius Inequality. In their justification, the ...
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### Would introducing water into a piston increase or decrease pressure?

Suppose you were to introduce a small amount of liquid water into the cylinder of an IC engine just after combustion. The timing and amount of water would be such that the water was entirely vaporized ...
3k views

### How can an airconditioning machine exist?

I work in an office where the airconditioning is not working, so I brought a small mobile airconditioning device: it's a small machine, emitting cooled air. It looks like this: However, according to ...
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### A violation of the Maximum Work Theorem?

Consider the PV diagram I've drawn at the end of this post. Consider the processes drawn thereon: (1-2-3) is an isobaric step followed by an isochoric step, while (1-3) is an adiabatic expansion (I ...
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### Can heat flow from colder to hotter body breaking the second law of thermodynamics? [closed]

In an isothermal process, the whole heat is converted to work but not in a cycle so it doesn't break the 2nd law of thermodynamics. $$dQ=dW$$ But let's imagine a scenario with it. If we connect an ...
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### Please explain why the Carnot cycle is reversible while the Otto cycle is irreversible using the kelvin or clausius statement of the second law [closed]

why is the Carnot cycle reversible while the Otto cycle irreversible according to Kelvin-planck statement
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### Calculation of efficiency of a heat engine [closed]

The efficiency of any heat engine doing work $W = |Q_{in}| - |Q_{out}|$ is given by: $N = |W|/|Q_{in}|$ The reasoning for this given in my textbook is along the lines of: The purpose of any heat ...
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### Why is the number of thermodynamic systems (that do not do work) in this scenario two?

The hardest thing for me about studying thermodynamics, is connecting the formal theory with actual problem instances. Here is an example: In Callen’s Thermodynamics and an Introduction to ...
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### Why does heat engine efficiency use absolute zero rather than thermal differential?

Carnot's theorem calculates efficiency of a heat engine versus absolute zero. I read the question Why can Carnot efficiency only be 100% at absolute zero? But I don't feel it addresses it precisely. ...
1 vote
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### Why can't a heat engine have 100% efficiency?

Using the idea of isotherms and adiabats, I came up with the following argument to convince myself. First imagine the working gas within the system being heated along an isochore (hence all heat ...
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### Distinction between "types of heat" in thermal efficiency

The definition of thermal efficiency I see in several sources is "total work" divided by "heat input". Wikipedia, for example, says: "For a heat engine, thermal efficiency is ...
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### Should I put my freezer in the middle of the living room? [duplicate]

There is a freezer in my kitchen. It has frozen fish fingers and spinach and potato waffles and leftovers. The food stays below freezing temperature using the principle of latent heat. The freezer ...
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### How can we say that work done by carnot engine in a cycle equals net heat released into it even when it is operated b/w 2 bodies and not 2 reservoir?

When a carnot engine is operated between 2 reservoir then after each cycle it return to its initial state so change in internal energy is zero and so work done by it equals net heat released into it. ...
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### Thermoacoustic mechanism for a thermoacoustic engine

How exactly does a thermoacoustic engine work? (I don't mean the engineering part) I understand the part on the Brayton cycle occurring and all, but how might we measure the numerical efficiency for ...
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### How does an internal combustion engine reduce the heat produced by burning fuel?

If I understand correctly, any heat produced by an internal combustion engine can be considered "waste heat"; energy from the fuel that did not get converted to mechanical energy. But if you ...
62 views

### The suction and thrust of an aircraft engine

Does only the air sucked in (not the air that is ejected at the back of the engine) by an airliner engine (turbofan engine) causes a thrust forward or in other words pull the engine forward? and why ? ...
901 views

### Does isothermal expansion not beat the efficiency of Carnot Heat Engine?

Ques: If I just take the 1st step of a Carnot Heat Engine (CHE) than it turns out to be more efficient than the CHE itself. Explanation: The 1st step of a CHE is the pure isothermal expansion of (...
1 vote
134 views

### Vapor Compression Refrigeration Cycle

My question here is, why do we need point 2 to be on the saturated liquid line, i.e. why do we need a saturated liquid for the throttling process? Why can we not use it in the liquid region or ...
1 vote
58 views

### Entropy of a non isolated system [closed]

I'm trying to self study thermodynamics. I've stumbled upon a problem on my textbook (which is italian by the way): a real thermal machine based on a Carnot's cycle works between two sources of ...
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### Why does adiabatic expansion occur in the carnot process?

(Spoiler: Why adiabatic expansion happens in Carnot cycle doesn't really answer the question for me.) In the Carno cycle, the open system is first brought into contact with the warm reservoir, which ...
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### Question on Thermodynamics and Heat Engines

Suppose we have a heat pump with 1 mol of some sort of fuel that goes through an isochoric process that takes the pressure of the gas from $P_2$ to $P_1$ while staying at $V_1$, an isobaric ...
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### Which law of thermodynamics is broken here? [closed]

This question came in the Dhaka university admission exam 18-19. A heat engine in each cycle does positive work and loses energy as heat with no heat energy input. Which law of thermodynamics does the ...
268 views

### What exactly is the limiting factor of the efficiency of a heat engine?

In an imagined scenario, $n$ moles of steam at a temperature $T_h$ and pressure $p$ enter an engine. The steam leaving the engine is at a temperature $T_c$ and pressure $p$. In this scenario, the heat ...
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### Why do internal combustion engines use adiabatic expansion and combustion?

In the gasoline engine, the gasoline mixture is heated at constant volume, followed by adiabatic expansion. In the diesel engine, the mixture is heated at constant pressure, and this is followed by ...
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### Will the heat generated from solar panel electricity eventually generate wind energy?

I'm curious about limits for extracting solar energy, but I never studied this stuff so I'm going to speculate a lot. My first question deals with the things like wind energy. I assume this energy is ...
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### How was adiabatic expansion conceived from the caloric theory perspective? [closed]

Nowadays we say that during the adiabatic expansion stage of the Carnot Cycle the internal energy due to the temperature of the gas gets transformed into work, but Carnot himself supported the caloric ...
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### How is a Carnot engine in quasi-equilibrium?

If the reversible isothermal expansion and adiabatic expansion both yield positive work (as the pv diagram indicates), it means that the gas inside the cylinder was compressed before those stages ...
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### How was Enrico Fermi able to predict a power output from his uranium and graphite Chicago Pile 1 (CP-1) reactor?

Uranium and graphite heat engine physics - early detection methods I've been inspired by the early developments in fission heat engines which happened even before uranium enrichment methods were ...
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### How to prove the efficiency of Carnot engine is bigger than Otto engine [closed]

I know the Carnot engine has maximum efficiency among all engines from Clausius statement, but how to prove the relationship of these two engines in equation? Thanks. More specifically, the ...
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### Does this system violate the second law of thermodynamics?

Consider a system (Blundell and Blundell 2nd edition, page 135) where a cycle consists of connecting each point to a reservoir at temperature $T_i$ and a heat amount $\delta Q_i$ enters that point. A ...
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### Origin of the thrust force in jet engine

I am reading a lot about the jet engine operation and I meet mostly explenation that the thrust is generated thanks to the exhaust nozzle that "accelerates" the air. However the nozzle ...
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### Comparing a gas turbine (Joule cycle) to the Carnot cycle?

My understanding of gas turbines is as follows: When you spin an impeller, you can compress air (this requires work) and when compressed air is expanded, it can be used to spin a turbine (this ...
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### Alternatives to Steam Engines [closed]

Since the 18th century, humankind relied on steam engines to convert heat into electricity on mass scale even in fission and fusion （in the future） power plants. However, this process involves several ...
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### Why are the pressures in the Joule-Thompson effect the same before and after?

I understand the Joule-Thompson effect and the mathematics behind it that results in the Enthalpy, $H$, remaining constant. However, one thing I conceptually fail to grasp is why the pressure on gases ...
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