Why is the Carnot Cycle The most efficient heat engine?

[Kayra]

Could anyone explain to me in non-math terms why a heat engine can not reach (or approach) 100%?

 

I have a difficult time visualizing this limitation.

[Erasmus00]

Could anyone explain to me in non-math terms why a heat engine can not reach (or approach) 100%?

 

I have a difficult time visualizing this limitation.

 

Not just Carnot cycles, but any reversible engine has the maximum efficiency. The reason has to do with conservation of energy.

 

First, what is a reversible engine? It takes X amount of energy as input and does y amount of work. Or in the reverse, you can do Y amount of work, and it moves X amount of energy. (I'm being vague on the type of energy. A carnot cycle is a heat engine, but theoretically engines could work by moving gravitational potential energy, electrical potential, etc)

 

So why must a reversible engine have the maximum efficiency? Well, lets say it didn't, and there was a more efficient engine out there. This engine takes X amount of energy, and does Y+Z work. The work is higher then Y, becuase our engine is more effecient then the reversible engine.

 

So if we start with X energy and run it through our efficient engine, we get Y+Z work, and then we take the Y work and run it backwards through our reversible engine and we move X energy. So now we have the same energy we started with, X, and we have still managed to do work. This means we have violated conservation of energy (and created energy). Since this can't be, we must assume that our efficient engine is impossible.

Hope this helped.

-Will

[arkain101]

Visualize this.

 

Heat is always trying to get away. Aka, Put on some thermal goggles on your heat machine and you will see heat. Why is this bad? Because, that means millions of infrared electromagnetic radiation (light we cant see) is being expelled from the machine. In other words, you cant insulate it enough because the heat will just pass from thing to thing and if its not where it should be to do work, then its lost.

 

Heat is moving particles remember.. so think of it as energy.. they lose it all the time, and it is the state all matter strives for.. low energy/very cold, but the suns energy and gravity and all that keep earth warm.

[biotech7]

Greetings, I am new to this forum. For the looks of it there is a buch of well educated individuals in here. Its about time something this interactive and informative was put up.

 

Anyways. About the answer that was provided. Could not this heat dispersion be significally reduced by creating a magnetic field? I do know however, that maximization is not achievable rarely aproached. Meaning perfection can not be attained.

[arkain101]

Yah, I dont know about magnetic fields and control of photons..

 

Atoms also arent purely elastic.. when they bump momentum is lost. It is lost because energy can neiter be created or destroyed just changed. So they bump and give off energy, and slow down.

So thermodynamics is just the way she goes.. and it would be really difficult as far as I undersatnd -and I am just a grade 12 graduate, who self educates .. lol... - to redirect energy back into itself.

[Kayra]

Not just Carnot cycles, but any reversible engine has the maximum efficiency. The reason has to do with conservation of energy.

 

First, what is a reversible engine? It takes X amount of energy as input and does y amount of work. Or in the reverse, you can do Y amount of work, and it moves X amount of energy. (I'm being vague on the type of energy. A carnot cycle is a heat engine, but theoretically engines could work by moving gravitational potential energy, electrical potential, etc)

 

So why must a reversible engine have the maximum efficiency? Well, lets say it didn't, and there was a more efficient engine out there. This engine takes X amount of energy, and does Y+Z work. The work is higher then Y, becuase our engine is more effecient then the reversible engine.

 

So if we start with X energy and run it through our efficient engine, we get Y+Z work, and then we take the Y work and run it backwards through our reversible engine and we move X energy. So now we have the same energy we started with, X, and we have still managed to do work. This means we have violated conservation of energy (and created energy). Since this can't be, we must assume that our efficient engine is impossible.

Hope this helped.

-Will

 

Well Drat

Still not quite there B)

 

If we had a reversible engine that was 100% efficient, then with X energy we could still do Y work, and reversing the engine Y work should still produce X energy, No?

 

In reference to the other replies, I think I will need to keep this simple and not involve any losses (friction, radiation, convection, conduction, etc) until I have a handle on how a perfect engine might work, thanks B)

 

And welcome aboard biotech7, this is a pretty good bunch of folks here (so says the lurker)

[Erasmus00]

Well Drat

Still not quite there B)

 

If we had a reversible engine that was 100% efficient, then with X energy we could still do Y work, and reversing the engine Y work should still produce X energy, No?

 

The question is, could you design a reversible engine that was 100% efficient? Restricting our attention to heat engines, it turns out all reversible engines that operate between the same two temperatures are equally efficient.

 

Why? If they weren't, you could still set it up in such a way to violate conservation of energy as per the argument above. If you have access to a university library try looking through Volume 1 of the Feynman lectures on physics. Chapter 4 develops gravitational potential energy using Carnot's argument.

-Will

[InfiniteNow]

About the answer that was provided. Could not this heat dispersion be significally reduced by creating a magnetic field?

Perhaps some, but this would still not bring efficiency to 100%. Further, it would likely require energy to generate said magnetic field, thus causing additional losses with E0 (starting energy).

[arkain101]

Building a perfect engine could be possible. I have been working on such an idea for a hell of a long time. Even if my design was to work, the engine would only of had may just enough power to rotate itself, but it would have no extra energy to power anything else.

 

Every action has an equal and opposite reaction.. so the engine will generally just barely work if you set it up just right.

 

I have however been working a new concept of multi-free energy absorbtion techniques to allow a system to operate in self powering manners.

It may not be 100% efficient, but as a car design, which it is, it will allow a car to nearly drive forever, in our standards.

There is a topic here about it.

http://hypography.com/forums/strange-claims-forum/4451-exciting-news.html

[Qfwfq]

The reason has to do with conservation of energy.

:rainbow: that's a principle of mechanics, not of thermodynamics.

 

The 2nd of thermodynamics is a statistical principle, it really isn't the conservation of a quantity.

[Kayra]

And unfortunately does not help me visualize why a heat engine is limited to the Carnot cyle of efficiency.

I guess I have difficulty seeing why this limitation exists, although this link I found does help a bit.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html#c1

 

Sorry for being so thick guys :rainbow:

[Erasmus00]

:rainbow: that's a principle of mechanics, not of thermodynamics.

 

The 2nd of thermodynamics is a statistical principle, it really isn't the conservation of a quantity.

 

The carnot cycle argument is entirely dependant on the first law of thermodynamics, which is conservation of energy. This isn't a statistical principle at all. See volume 1 of the Feynman lectures, in the 4th chapter he develops gravitational potential energy from a Carnot-like argument.

-Will

[Kayra]

Hmm.

 

I thought the second law was statistically based, and further, affected the scope of the first law. (IE, limited it even further). I guess that means it is based on it.

 

Statistically based meaning that while it is probable that heat will flow from a hot area to a cool one, it is not impossible for it to spontaneously flow in the other direction... just incredibly improbable. At least, that is how i understood it.

 

Could someone tell me.. Does the carnot efficiency tell me the maximum energy I can harness from the total energy in a closed system, or useable energy in a closed system?

 

Note: found this to be an excelent reference:

http://acnet.pratt.edu/~arch543p/readings/thermodynamics.html

 

 

 

The answer may resolve my visualization dimlema :rainbow:

[Erasmus00]

Could someone tell me.. Does the carnot efficiency tell me the maximum energy I can harness from the total energy in a closed system, or useable energy in a closed system?

 

Generally, a carnot cycle is set up with two large heat reservoirs, one at some hot temperature, one at some cold temperature. This implies your engine is not a closed system. If you swapped out the hot reservoir (as an example) and replaced it with a hot brick (just something of finite size) then to find the maximum work you could do as you cooled the brick you need to look at a quantity called the free energy.

-Will

[Kayra]

<blink blink>

 

Oh my.. I did say closed system, didn't I :rainbow: Twice even.

 

Let my try my question again.

 

Given Th (Temp Hot) and Tc (Temp cold) , does the carnot formula of (Th-Tc) / (Th) tell me how efficiently I can harness the available energy (Th-Tc) or the total Energy Th.

[arkain101]

I've been hearing about this zero point energy.

 

Some "claim" (like Nikola Tesla) that there is a frequency or some kind of pressure that exists in space-time and using high enough ocillators and basically off the shelf like equiptment you can literally absorb the universe and create quite alot of electricity. This is 100% free energy, and we're talking high speed space travel stuff. Although it sounds mostly like Sci-Fi as of now but I think this is going to be for real soon enough.

 

This is one person who is working on it.

 

http://pesn.com/2005/06/17/9600113_MagneticPowerInc_Pre-Production/

 

This is their company website I am assuming:

http://www.magneticpowerinc.com/

 

and this is some mathamatical theory that is also looking into such a thing.

 

http://www.science-spirit.org/article_detail.php?article_id=126

 

It is cool to say the least!

[arkain101]

and uh this guy is planning to build a never ending tornado to power a huge wind generator.. It sounds like acutally quite a smart idea!

http://pesn.com/2005/12/14/9600210_Atmosphere_Vortex_Engine/

 

 

-YEA, us canadians get some pretty wild ideas lol..