Has anyone built a Carnot engine before, are there any examples?

[Jez]

I don't mean the 'perfect ideal' Carnot of theory lessons.

 

I just mean one that uses isentropic and isothermal phases.

 

Like an Otto engine uses 2x isentropic/isochoric, and Stirling uses 2x isochoric/isothermal, so why are there so few examples, if any(?) of a 2x isentropic/isothermal machine?

[studiot]

17 hours ago, Jez said:

I don't mean the 'perfect ideal' Carnot of theory lessons.

 

I just mean one that uses isentropic and isothermal phases.

 

Like an Otto engine uses 2x isentropic/isochoric, and Stirling uses 2x isochoric/isothermal, so why are there so few examples, if any(?) of a 2x isentropic/isothermal machine?

 

Practicality as mechanical engineering developed probably played a large part in this.

 

Early machines were generally reciprocating, curiously with the exception of the first steam engine by Hero.

Not only that but they were simple in that they only used one or at the most two cylinders.

 

As machines developed, particularly the internal combustion engine, more was required than a simple machine output shaft to drive auxiliary equipment such as pumps, fans, generators, camshafts compressors and so on.

 

All this has to work against that other mechanical beast - friction.
Furthermore as engines became more sophisticated the Carnot 'hot' and 'cold' reservoirs only exist for a small part of the machine's cycle.

 

Does this help ?

[exchemist]

18 hours ago, Jez said:

I don't mean the 'perfect ideal' Carnot of theory lessons.

 

I just mean one that uses isentropic and isothermal phases.

 

Like an Otto engine uses 2x isentropic/isochoric, and Stirling uses 2x isochoric/isothermal, so why are there so few examples, if any(?) of a 2x isentropic/isothermal machine?

I can’t see offhand why someone couldn’t make an engine with such a cycle for demonstration purposes, though like you I can’t find any examples of it having been done. It does seem, though, that such an engine would probably work rather slowly and so might not be a very practical source of mechanical work.

One would need insulating shutters to alternately expose and shut off the working fluid from hot and cold sinks (assuming a cylinder whose walls had negligible heat capacity) and then let pseudo-adiabatic expansion and compression occur in between. Or something.

You’d have thought some enterprising soul at Imperial College or MIT - or perhaps at one of the Grandes Ecoles in Paris - might have given it a go.

[sethoflagos]

On 6/3/2023 at 8:13 PM, Jez said:

I don't mean the 'perfect ideal' Carnot of theory lessons.

 

I just mean one that uses isentropic and isothermal phases.

Fast processes tend towards the isentropic. Efficiency and overall economics work hand in hand.

Isothermal processes in extremis call for infinite cycle time and/or infinite heat transfer area. The economics just don't work as a practical proposition so we have to compromise. 

There is really no such thing as a Carnot design as it is an abstract theoretical concept: not a tangible object like an Otto or Stirling engine. Some designs can approach the Carnot limit more closely than others and that's all there is to it. Closer approaches need bigger budgets with ever-declining potential Return On Investment.

 

[Jez]

14 hours ago, sethoflagos said:

Fast processes tend towards the isentropic. Efficiency and overall economics work hand in hand.

I've heard that said and sort of drawn the same conclusion. Is there some more formal explanation/definition of that principle?

Of course, that does not necessarily mean the isothermal phase would need to be fast too. If we take those two things together, the question would be how to construct a mechanism with fast isentropic phases and slow isothermal.

On 6/4/2023 at 3:09 PM, exchemist said:

You’d have thought some enterprising soul at Imperial College or MIT - or perhaps at one of the Grandes Ecoles in Paris - might have given it a go.

I think guys out of academic situations tend to try to work with new tech things, applying something new.

I mean, sure, there might be someting new tech to bring to that table, they like coming up with new solutions using new tech.

I'm just thinking that a solution would be more about putting the right lumps of existing known engineering together in a new way, and I think most of the time it's guys tinkering in their garage that tend to come up with new solutions using old tech.

[sethoflagos]

11 minutes ago, Jez said:

I've heard that said and sort of drawn the same conclusion. Is there some more formal explanation/definition of that principle?

The 2nd Law formally states that the primary prerequisite for an isentropic process is that there is no nett exchange of heat with the surroundings. Since heat transfer (and other dissipative processes such as friction) are relatively slow processes then fast processes lean toward the isentropic.

24 minutes ago, Jez said:

Of course, that does not necessarily mean the isothermal phase would need to be fast too. If we take those two things together, the question would be how to construct a mechanism with fast isentropic phases and slow isothermal.

Like a power station?

Near isentropic compression and expansion through the boiler feedwater pumps and steam turbines respectively are extremely rapid. But the working fluid spends far more residence time exposed to the heat transfer surfaces. The steam condenser is intrinsically near isothermal in operation, and although the radiatively heated water wall is not, the ancillary surfaces: deaerator, economiser, superheater and reheater bundles; are sized and positioned so as to maximise overall station thermal efficiency. Which is equivalent to saying that they are designed to fit the station cycle to the Carnot optimum theoretical cycle as closely as possible. 

I guess this is not what you envisage when you write 'engine'. A better word may be 'machine' which has a precise engineering meaning: a device that transforms one form of energy into another. Heat engines, electrical transformers and power stations are all machines by this definition.