Questions tagged [heat-engine]

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2answers
54 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
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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2answers
364 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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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
146 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
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-...
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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 ...
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1answer
473 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
46 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
120 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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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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1answer
3k views

Derive the Carnot Efficiency without using the Carnot Cycle? [closed]

How can you derive the Carnot efficiency using only properties of reversible cycles?
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3answers
234 views

Early atmospheric Watt steam engine - how does the steam move from the cylinder to the condenser?

Below is a picture of one of the earliest designs of Watt's steam engine. The basic principle of operation is this: The weight which is attached to the beam $E$ pulls the piston $P$ up, sucking ...
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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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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
746 views

Does Piston moves faster when we accelerate the car?

I am learning about car basics. I found that engine is nothing but a system where heat energy is converted into up/down motion of the piston in the cylinder. Finally, this up/down motion is converted ...
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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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2answers
210 views

Why does a heat engine convert only some of the heat into work

I am aware that no heat engine can ever be 100% efficient due to the limit imposed by the 2$^{\mathrm{nd}}$ law of thermodynamics; and that the best we can do is reach the Carnot limit. When heat ...
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0answers
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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2answers
353 views

Carnot engine with draining reservoir? [closed]

Two identical bodies of constant heat capacity $C$ at temperatures $T_1$ and $T_2$ respectively are used as reservoirs for a heat engine. If the bodies remain at constant pressure, show that the ...
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1answer
91 views

Reverse the combustion engine

All you know what a combustion engine is doing: Transform chemical energy into kinetic energy. What about the opposite? I know when I park a car at a slope and lose the breaks it will drive ...
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1answer
204 views

Efficiency of a 3 stage engine

A possible ideal-gas cycle operates as follows: From an initial state ($p_1$, $V_1$) the gas is cooled at constant pressure to ($p_1$, $V_2$); Lets call the start and end temperature $T_1$ and $T_2$ ...
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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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3answers
370 views

Muliple heat sources, same object - final temperature reasoning

What happens when two or more independent heat sources heat the same object? For example, where one heat source on its own creates an equilibrium temperature with the object of 35C, and the other ...
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0answers
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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1answer
2k views

How to draw $T$-$S$ diagrams?

I am having a bit of trouble visualising how to go, say, from a $p$-$V$ diagram for the cycle of a gas, into a $T$-$S$ diagram for the same process. In class, we studied four main types of ...
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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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1answer
2k views

Power output of a heat engine

The thermodynamic cycle of a heat engine is shown. The shape of the curve is elliptical with one axis parallel to the S-axis. This cycle is executed 20 cycles/s. The power output of the engine is ...
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2answers
69 views

Can a Rocket be Propelled by Steam and a Cold Sink instead of Exhaust Nozzle

Is it possible to have a “rocket” which uses steam flowing into the chamber as the pressure source and which uses a cold sink at one end to condense the steam instead of discharging it outside? A ...
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1answer
115 views

In an adiabatic process, how can you get work without applying heat?

Suppose you have a cylinder filled with an ideal gas and a piston on top which can slide up and down. If you want to get work done using isobaric, isothermic or any other process (other than adiabatic)...
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1answer
203 views

Why doesn't the first law of thermodynamics show the work done in this example?

Let's consider this example We have an isolated cylinder, inside of it is an ideal gas. The pressure is constant (note that the thing where the constant force is acting upon can move up and down). ...
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2answers
666 views

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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1answer
162 views

Most efficient mechanism for heating air

I was read about small scale ion treatment which uses heats to high temperatures, would this be scalable and if so would there be any more efficient methods. In summary, if you wanted to heat large ...
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2answers
1k views

Why is steam a good working fluid for heat engines?

Steam (gas-phase H2O) is very commonly used as the working fluid for external-combustion heat engines. It seems to me that it ought to be a poor choice, because: Unless I am seriously confused about ...
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3answers
596 views

How heat flow through finite temperature drop is an irreversible process?

Is it necessary that the drop/rise in temperature by flow of heat is irreversible?
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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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3answers
222 views

How is the $2^\mathrm{nd}$ law of thermodynamics valid?

For example, if we have an infinite cylinder fitted with a piston having adiabatic curved walls and a diathermal base. Now, bring the base in contact with some higher temperature body and let the ...
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1answer
70 views

Heat Engine - Is it possible to reroute lost energy to the engine again?

Recently, I have studied thermodynamics in my physics class and have gone over the topic of the heat engine and how it works. In my understanding, some of the energy is used for work (to move the car ...
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1answer
183 views

How can I make the heat engine efficiency become 100%? [closed]

Is it possible to make it 100%? Or in what kind of situation the heat engine efficiency could be 100%?
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1answer
676 views

Proof of a Carnot engine being the most efficient heat engine

Most of the physics textbooks I’ve read try to prove that the Carnot engine is the most efficient engine possible by showing that if you coupled a superefficient engine to a Carnot refrigerator, you’d ...
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1answer
38 views

Is it correct to find ideal engine's efficiency using celsius scale?

I am solving a problem from my book which asks to calculate the maximum efficiency of an engine whose hot body is at temperature 300°C and cold body is at 25°C. Using 1- Tc/Th, I got efficiency equals ...
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3answers
2k views

Need of the cold body in heat engine

Purpose of heat engine is to transfer heat into the useful work, basically. Working object either gas or liquid takes heat from a thermal energy reservoir as a result of which their internal energy ...
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1answer
1k views

Carnot engine (theorem) Proof

I am facing issues in the proof. some links to the proof of carnot engine: https://en.wikipedia.org/wiki/Carnot%27s_theorem_%28thermodynamics%29 2. https://www.khanacademy.org/science/physics/...
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4answers
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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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1answer
143 views

Question about air-conditioning cycle

All explanations of the air-conditioning cycle are somewhat like this: https://www.swtc.edu/ag_power/air_conditioning/lecture/basic_cycle.htm My question is... how do we ensure the refrigerant is in ...
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1answer
428 views

What is the sign of the work done by a gas with below atmospheric pressure?

In thermodynamics, given a piston containing a fixed quantity of gas, when the gas is heated the piston expands and does work on the environment. Using the convention that $$\Delta U_{in} = Q_{in} -...
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3answers
10k views

Does the heat transfer rate depend on the temperature difference between the objects? .

If I have a device that gets to temperatures of 2000-3000 F, and another object that's temp is 100-200 F Will the hotter object transfer the great faster since the other object is cooler and Since ...