Carnot engine

[skulldude]

They say it is impossible to manufacture a ideal engine.

First what we require is a source with a infinite thermal capacity.So to keep its temperature constant we can add external heat to it. By that way though we take heat from it its temperature will remain constant.

 

Similarly for the sink we can use a coolant constantly to keep its temperature very low.

 

Is it possible to do that.

 

I am just 15 years old. So please dont mistake me if my question is ridiculous

[swansont]

Infinite thermal capacity is impossible.

[Klaynos]

The first problem I see with this is the coolant and heating mechanisms would not be 100% efficient and you'd have to take that into account when working out your engines efficiency....

[ydoaPs]

They say it is impossible to manufacture a ideal engine.

First what we require is a source with a infinite thermal capacity.So to keep its temperature constant we can add external heat to it. By that way though we take heat from it its temperature will remain constant.

 

Similarly for the sink we can use a coolant constantly to keep its temperature very low.

 

Is it possible to do that.

 

I am just 15 years old. So please dont mistake me if my question is ridiculous

 

In a carnot engine, the working fluid is at the temperature of the heat source when the fluid is at the heat source, and it is at the temperature of the heat sink when it is at the heat sink.Heat transfer, however, is impossible without a difference in temperature([math]Q=mc(T_{hot}-T_{cold})[/math])

 

This is a temperature-entropy diagram of an ideal carnot cycle:

The line from A to B represents the heat being transferred into the system.The line from B to C represents an Ideal work process performed by the system. Notice how the line is straight. In all real work processes, friction exists, and thus entropy increases. The line from C to D represents heat being transferred out of the system. The line from D to A represents an Ideal work process performed on the system. Note this line is straight as well. In real heat engines, the work lines are curved, because they both increase entropy.

 

edit:gah, I play with a heat engine every day....I should be able to explain it better than that.

[skulldude]

Thank you but it was too technical for me to understand

[Jez]

I'm interested in this question and it's unclear to me if/why there are few/no examples of a Carnot engine.

I don't mean a 'perfect' Carnot engine, just something that aims to be isentropic-isothermal-isentropic-isothermal.

I understand there has to be a temperature difference in the hot/cold sources to the working fluid else no thermal energy will be transferred. So the transfer can never be ideally isothermal.

What I mean is that, for example, an Otto cycle is described as isentropic-isochoric-isentropic-isochoric, but there's no way a petrol ICE works 'exactly' that way, it's just a 'general' way to describe the aim of the 4 strokes, and any heat engine has rounded off corners and bits that don't match up perfectly to thermodynamic processes.

So, likewise, a Carnot engine is one that is isentropic-isothermal-isentropic-isothermal, are there/have there been any examples of actual engines that 'aim' to work that way generally (accepting that they are 'imperfect', just like an ICE petrol engine is not a perfect Otto cycle but aims to be one)?

 

 

 

Edited May 30, 2023 by Jez

[exchemist]

38 minutes ago, Jez said:

I'm interested in this question and it's unclear to me if/why there are few/no examples of a Carnot engine.

I don't mean a 'perfect' Carnot engine, just something that aims to be isentropic-isothermal-isentropic-isothermal.

I understand there has to be a temperature difference in the hot/cold sources to the working fluid else no thermal energy will be transferred. So the transfer can never be ideally isothermal.

What I mean is that, for example, an Otto cycle is described as isentropic-isochoric-isentropic-isochoric, but there's no way a petrol ICE works 'exactly' that way, it's just a 'general' way to describe the aim of the 4 strokes, and any heat engine has rounded off corners and bits that don't match up perfectly to thermodynamic processes.

So, likewise, a Carnot engine is one that is isentropic-isothermal-isentropic-isothermal, are there/have there been any examples of actual engines that 'aim' to work that way generally (accepting that they are 'imperfect', just like an ICE petrol engine is not a perfect Otto cycle but aims to be one)?

 

 

 

I don't know but I imagine it is that such an engine might be rather impractical for doing useful amounts of work. A Stirling cycle has pseudo-isothermal heat addition and removal, but then there's the thermodynamically complex issue of the heat regenerator.  

[studiot]

6 hours ago, Jez said:

I'm interested in this question and it's unclear to me if/why there are few/no examples of a Carnot engine.

I don't mean a 'perfect' Carnot engine, just something that aims to be isentropic-isothermal-isentropic-isothermal.

I understand there has to be a temperature difference in the hot/cold sources to the working fluid else no thermal energy will be transferred. So the transfer can never be ideally isothermal.

What I mean is that, for example, an Otto cycle is described as isentropic-isochoric-isentropic-isochoric, but there's no way a petrol ICE works 'exactly' that way, it's just a 'general' way to describe the aim of the 4 strokes, and any heat engine has rounded off corners and bits that don't match up perfectly to thermodynamic processes.

So, likewise, a Carnot engine is one that is isentropic-isothermal-isentropic-isothermal, are there/have there been any examples of actual engines that 'aim' to work that way generally (accepting that they are 'imperfect', just like an ICE petrol engine is not a perfect Otto cycle but aims to be one)?

 

 

 

Hello newcomer and welcome.

 

It would be better to have started your own thread concentrating on what you want get out of the discussion, rather than reviving a short question from many years ago.

 

Yes there is a big difference between practical engineering and thermodynamics, just as there is a big difference between practical applications and theoretical considerations and simplifications in almost any subject you care to imagine.

 

[Endy0816]

Reactor is probably closest.

 

In more typical settings, you'll want to exhaust waste heat containing fluids for improved efficiency and lowered cost.

 

Edited May 30, 2023 by Endy0816

[Jez]

19 hours ago, studiot said:

Hello newcomer and welcome.

 

It would be better to have started your own thread concentrating on what you want get out of the discussion, rather than reviving a short question from many years ago.

 

Yes there is a big difference between practical engineering and thermodynamics, just as there is a big difference between practical applications and theoretical considerations and simplifications in almost any subject you care to imagine.

 

OK, will do that.