Questions tagged [heat-engine]

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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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0answers
375 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 ...
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0answers
109 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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350 views

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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0answers
66 views

What is the potential energy of a Carnot engine?

Say you've got a closed (but very large) system with a hot and cold reservoir a la the Carnot cycle. You can add up the heat in the two reservoirs and find that the total amount never changes (because ...
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0answers
522 views

What variables affect a Heat engine's efficiency? (and more)

I'm writing a Minecraft mod and I'm trying to create the equations for the engines players can build. I'm a highschool sophomore, so my knowledge of physics and mathematics is limited. I asked first a ...
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0answers
893 views

What is the best material to use in a solar heater box to convert solar energy into heat energy?

Suppose you want to design a solar heater box with the dimensions: 1 sq meter x 30.48 cm H. 5 sides of the box's interior will be insulated and the interior will be totally black. Ideal glass will be ...
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83 views

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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0answers
32 views

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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0answers
109 views

Does negative absolute temperature imply heat engine efficiency greater than one?

As it is shown in Prove that negative absolute temperatures are actually hotter than positive absolute temperatures, a body with absolute negative temperature is hotter than a body with any absolute ...
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179 views

Confusion about Clausius inequality

I am trying to learn the derivation of Clausius inequality in the following manner. Consider the following diagram "Reversible cyclic device" is working in Carnot's cycle. The combined system gets ...
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4answers
6k views

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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1answer
68 views

What does the ratio of reservoir temperatures have to do with the efficiency of a heat engine?

I am having a conceptual difficulty with heat engine efficiency. I do not understand why having a larger difference in temperature between the hot and cold reservoirs have an effect on the thermal ...
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234 views

Carnot cycle's paradox

Carnot cycle is a reversible cycle. By definition a reversible cycle must have an infinitesimal temperature difference. That means ($T_{high}-T_{low}$) is very very small. Any finite temperature ...
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0answers
23 views

microturbine heat loss

Would the hot combustion gasses in the millimeter-wide turbine in http://thefutureofthings.com/3063-engine-on-a-chip/ lose heat so quickly that the engine efficiency would be low? The velocity of ...
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0answers
60 views

Maximum temperature of an elliptical thermodynamic process

Given a heat engine made from the curve $$V(t)=V_0 +A \cos(\omega t); \qquad P(t)=P_0-B \sin(\omega t).$$ What are the hottest and coldest points of this cycle, and what are the temperatures at those ...
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1answer
55 views

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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1answer
15 views

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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2answers
34 views

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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1answer
50 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 ...
0
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1answer
73 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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2answers
190 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
86 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
41 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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1answer
39 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
76 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-...
0
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1answer
54 views

How can I calculate the efficiency of an air-powered car?

An air-powered car is powered using high pressure compressed air stored in a tank. The pressurized tank is connected to an air-powered engine. The volume of the tank is 0.3 m3, and the initial ...
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1answer
53 views

Thermodynamic temperature

In Feynman's lectures an argument for efficiency of engines goes as: If an engine absorbs heat $Q_1$ at temperature $T_1$ and heat $Q_2$ at temperature $T_2$, and both times give the same heat $Q$s at ...
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2answers
139 views

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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1answer
188 views

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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1answer
102 views

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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1answer
226 views

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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39 views

Mass engine efficiency limit

Let's hypothesise a machine that draws useful work out of mass in accordance with $E=mc^2$. Its efficiency can be defined as $\eta=\frac{W}{m}$ where $m$ is input mass and $W$ is output work. Does ...
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68 views

Water as heat source in a heat pump

Consider we have water flowing in a pipe, which is working as a hot source for a heat pump. We are able to know the temperature of the water in both ends of the pipe, yet we don't have information ...
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257 views

diesel steam engine

Let's imagine a diesel engine's cylinder head would be heated by external means, to some hundred degree celsius, and the fuel would be replaced by water. We've made modifications so that water would'...
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1answer
206 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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1answer
61 views

How long will pressure exist in this case?

I'm currently fabricating my project, which currently exists only on paper, and there's one question to which I have stumbled today. Suppose, that I have a pressure vessel capable of holding upto ...
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1answer
53 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 ...