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25 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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1answer
35 views

Application of Carnot's theorem to calculate efficiency of thermal engine

I understood Carnot's theorem in theory, but I still have some doubt on the practical use of it. In particular I would like to know: if I have any reversible cycle followed by a gas, then the thermal ...
3
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1answer
44 views

Heat pumps: can $COP\leq 1$ and when does $COP=\frac{1}{\eta}$ hold?

Heat pump COP is defined as $$\xi=\frac{|Q_{HOT}|}{|W|}$$ Where $Q_{HOT}$ is the heat given to the hot source. I have two linked doubts about it Under what conditions I can say that $\xi=\frac{1}{\...
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1answer
44 views

Prove all reversible heat engines that work between just two temperatures have the same efficiency

Can someone explain this explanation to me: A completely different argument focuses on the requirement that the device only move heat at one or other of the two temperatures. Then, provided ...
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1answer
34 views

How would a Stirling Engine's throttle work?

I recently stumbled upon this video of a Stirling engine which includes a throttle mechanism. Unfortunately, there was no information about how the throttle worked. I know that an internal ...
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3answers
64 views

Heat Pumps and COP?

Heat Pump COP (Coefficient of Performance) $$COP=1-\frac{T_c}{T_H}$$ With Heat Pumps is the efficiency/COP more dependent on the hot or the cold reservoir and why?
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0answers
19 views

Efficiency of engine in the case non-boundary work done on the system

The engine efficiency is defined as $$\eta = \frac{W_{\mathrm{produced}}}{Q_{\mathrm{absorbed}}}\tag{1}$$ But suppose that, in one of the processes that takes place in the cycle of an engine that ...
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4answers
5k views

Why is the efficiency of human cells less than the efficiency of an Otto engine?

I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest ...
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4answers
166 views

Does a reversible heat engine exchanging heat with an ideal gas that does a transformation imply that transformation is reversible?

Consider a reversible heat engine working between two sources. Suppose that one of the sources is a thermostat, while the other is an ideal gas which follows a transformation and exchanges some heat ...
5
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1answer
72 views

Can the efficiency of a thermodynamic cycle be worked out from a pV diagram?

Is it true that a pV diagram of a thermodynamic cycle with a specified scale on both axes is enough to calculate its efficiency (such as the one below)? Is it the area within the shape divided by the ...
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1answer
39 views

Thermodynamics: efficiency of a heat engine [closed]

How can we calculate efficiency of a real heat engine? Do we have to consider volume of an object while calculating efficiency? Like in this question If so, how we have to proceed? Thanks in ...
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3answers
59 views

Clausius Inequality with three reservoirs

Deriving Clausius inequality in the case of two reservoirs is quite simple. We assume that there is a thermal machine operating between two reservoirs, the first at temperature $T_1$ and the second at ...
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1answer
31 views

How is conversion inside heat engines of heat (random motion) to work (organized motion) explained in quantum physics?

When heat engines convert heat into work, they change the random motion of millions of particles into motion in a single direction. What is the phenomenon responsible for this alignment of ...
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1answer
36 views

How can I get 100% fuel efficiency from an IC Engine in a car?

I understand that the current technology of internal combustion (IC) engines gives about 20% for a normal car. I want to understand what I can do to achieve 100% or near 100% efficiency in the same ...
0
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1answer
49 views

Question about reversible heat engine efficiency

I have a question regarding heat engines that cropped up whilst I was doing a practice question. I will summarise the results I obtained for the previous parts of the question so as to save your time. ...
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3answers
108 views

Why does heat engine need to reject energy to environment?

The most efficient thermodynamics cycle is the Carnot cycle, it involves 1/ Receiving heat from hot source, expanding and doing work on the surround 2/ Receiving work from environment, being ...
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3answers
63 views

Using thermal expansion for mechanical work

Assume I could heat up a bar with zero heat loss to the environment. I could then use the thermal expansion to do some work, for example compress a gas. Where does the energy needed for the ...
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1answer
47 views

Expressing thermal efficiency for a closed Brayton cycle in terms of static enthalpies

I have a Brayton cycle made of a compressor (stage 2 to 3), a heat exchanger (stage 3 to 4) and a turbine (stage 4 to 5). The turbine and compressor are connected through a shaft. The cycle is closed ...
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0answers
26 views

Energy potential of a cold sink inside the sun

An answer in this question discusses using a fusion engine in a theoretical spaceship within a star. http://worldbuilding.stackexchange.com/questions/37589/what-are-the-consequences-of-literal-...
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0answers
48 views

Calculating car engine RPM from inputs [closed]

So I am willing to calculate engine RPM of car. The usual process for doing this in games is: $\omega_\mathrm{e} = \omega_\mathrm{w} * D_\mathrm{k} * g_\mathrm{k} * 60 / 2\pi$ However, this ...
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2answers
76 views

Are there alternatives to steam in thermal power stations?

'A thermal power station is a power plant in which heat energy is converted to electric power. In most of the world the prime mover is steam driven. Water is heated, turns into steam and spins a steam ...
3
votes
1answer
44 views

Exhaust liquefaction in internal combustion engines

Does an internal combustion engine provide enough efficiency to still deliver mechanical power, if it is forced to liquify it's own exhaust gases by compression? If gas containers would be added to ...
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0answers
41 views

Self-powered heat exchanger w/energy profit? [closed]

It is possible, considering the laws of physics, to build a heat exchanger that runs on the power it gathers from the environment? A typical heat exchanger uses less power to harvest heat for a ...
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2answers
90 views

Heat diffusion in a house

I was wondering about this problem (which is NOT homework, it's one of my mental problems due to what surrounds me): supposing I'm in a room, and it's winter. Heating is on, the boiler works and the ...
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0answers
77 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 ...
2
votes
1answer
94 views

Why isn't Hydrogen used as fuel in Internal combustion engine? [closed]

I was looking at hydrogen cars and found out that hydrogen is mainly used in hydrogen cells to produce electricity instead of combusting it like gasoline in a cylinder to produce heat energy. Why isn'...
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4answers
90 views

Why do coal power stations not directly use combustion to turn the turbines?

Why do coal power stations run by creating steam rather than from the expansion due to combustion? I understand that in order to generate electricity in a coal power plant, steam is usually produced ...
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0answers
86 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
12 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 ...
2
votes
1answer
55 views

Why must the total change in entropy be 0 for a heat cycle to be as efficient as possible?

If $\Delta S_{total}$ were not zero, then heat flow would take place at different temperatures. It is easy to see why this is true by examining the opposite: that when heat flow takes place at the ...
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1answer
543 views

Ways to increase efficiency of carnot refrigerator and a four stroke engine? [closed]

What are ways to increase the efficiency of a Carnot refrigerator. I remember something about decreasing the temperature but also about something about the flow rate, and fins? Also, for four stroke ...
2
votes
3answers
1k views

How is the efficiency of a heat engine related to the entropy produced during the process?

I'm reading Schroeder's An Introduction to Thermal Physics. Regarding heat engines, it is stated: Unfortunately, only part of the energy absorbed as heat can be converted to work by a heat engine. ...
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1answer
159 views

Efficiency in a thermodynamic cycle

Question The area on a PV diagram within one thermodynamic cycle of a heat engine is 610J. THe total thermal energy transferred out of the gas in one cycle is 1300J. Determine the efficiency of ...
2
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1answer
86 views

Heating up water with a heat pump

Suppose we wish to heat 1 litre of 0°C water up to 50°C during wintertime by using an ideal heat pump (Carnot cycle in "reverse"). We will place this water amount in an initally empty container, in ...
2
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2answers
357 views

Why can't the Carnot cycle be used to operate automobiles, if its the most efficient?

Since the cycle proposed by Carnot (consisting of two isothermal and two adiabatic processes) is the most efficient one on which to run a heat engine, why do we use other, less efficient ones (like ...
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0answers
145 views

Ericsson cycle: what happens if you add more mass?

We recreated the Ericsson cycle using a heat engine apparatus, with air as the working fluid (assumed an ideal gas). During the isothermal compression and isobaric expansion processes, a mass of ...
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2answers
180 views

More torque or more horsepower? Which engine is more powerful [closed]

Two internal combustion engines : $250\,\mathrm{hp}$ and $900\,\mathrm{Nm}$ torque $770\,\mathrm{hp}$ and $790\,\mathrm{Nm}$ torque Which of the above two engines will be more powerful? P.S. ...
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3answers
2k views

Questions on Carnot's theorem

This article on Carnot's theorem states that All heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoirs. However, it only ...
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0answers
125 views

Efficiency of a compressed air engine

I am doing some personal research on the efficiency of compressed air cars. I am currently trying to come up with an 'ideal' cycle to use as a reference for efficiency (c.f. Otto Cycle for petrol cars)...
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1answer
56 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
115 views

COP of Stirling engine

If we consider the theoretical maximum of COP for a heat pump, we will see that it is possible to make it infinite by approaching the temperatures of each source to each other. Yet, in a real engine ...
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0answers
41 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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1answer
769 views

Carnot vs. Otto

Otto cycle is reversible and according to Carnot theorem should have the same efficiency as Carnot $\eta_{Carnot}=1-T_C/T_H$. The efficiency of Otto is $\eta_{Otto}=1-\frac{T_4-T_1}{T_3-T_2}$ (see ...
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1answer
149 views

Efficiency of a real engine

Can anyone tell me what is wrong with the following argument? Consider a Carnot engine and an engine $X$ whose efficiencies are $\eta_c$ and $\eta_x$ respectively. Let us assume that $\eta_c>\...
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5answers
604 views

What is the heat sink in an internal combustion engine?

I believe that the heat source is the spark plug because it heats up the fuel-air mixture and that the working substance is the fuel-gas mixture. Then, I know that the explosion created by the fuel ...
2
votes
1answer
101 views

How to heat water at very high temepratures?

I am working on a project and recently came through a situation where I'll be working with water evaporation. On searching the internet I found that water heated at 350Celsius would generate almost ...
2
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1answer
217 views

How to move liquid uphill against a thermal gradient, without moving parts

I am looking for a conceptual way of dealing with the following. I am not looking for a set of blueprints. Consider a 5 meter x 10 cm vertical closed tube filled with a substance in the gas and ...
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1answer
845 views

Efficiency larger than one?

The efficiency of a heat engine is the work we can do divided by the heat we take out of the hot reservoir. This quantity is always $ \le 1$. The efficiency of a heat pump is the heat we can release ...
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3answers
2k views

How much heat from a fire actually warms your home?

A fire in a hearth disperses heat to, I guess, three places: the bricks of the chimney out the hearth (where the person tending the fire is standing) out the chimney, above the house How would you ...
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2answers
82 views

Create air stream from heat

We know that heating the air inside a bounded box raises its pressure. And if there is a hole in the box, then air flows out until the pressure inside and outside balances. So in this case we would ...