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]

Originally Posted by 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.

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]

Originally Posted by Erasmus00

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

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

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

[Erasmus00]

Originally Posted by Kayra

Well Drat
Still not quite there

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]

Originally Posted by biotech7

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.
Exciting news.

[Qfwfq]

Originally Posted by Erasmus00

The reason has to do with conservation of energy.

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.