- #1
electric_head_uk
- 5
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I have a question on Why the Carnot cycle is not used with real engines? I need to refer to real cycles in my explanation.
Why the Carnot cycle is not used with real engines?
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SUMMARY
The Carnot cycle is not utilized in real engines due to its theoretical nature, which assumes no increase in entropy and requires isothermal heat transfer, conditions that are physically impossible in practical applications. While the Carnot cycle serves as a benchmark for maximum efficiency between two temperature reservoirs, real engines like the Stirling engine and Otto cycle are preferred for their practicality and implementability. The Stirling cycle can achieve maximum theoretical efficiency akin to the Carnot cycle but is more feasible for certain applications, such as cryocoolers. The limitations of the Carnot cycle stem from the necessity of a temperature gradient for effective heat transfer, which cannot be achieved in real-world scenarios.
PREREQUISITES- Understanding of thermodynamic cycles, specifically the Carnot cycle
- Knowledge of entropy and its implications in thermodynamics
- Familiarity with heat transfer mechanisms, including isothermal and adiabatic processes
- Basic principles of Stirling and Otto cycles in engine design
- Research the practical applications of Stirling engines in cryogenic systems
- Study the principles of heat transfer in thermodynamic systems
- Explore the efficiency calculations for the Carnot cycle and its real-world implications
- Investigate the differences between isothermal and adiabatic processes in engine cycles
Engineers, thermodynamics students, and professionals in mechanical engineering who are interested in the efficiency and practical applications of thermodynamic cycles.
- #2
jaap de vries
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Carnot cycle uses the same temperature for heat transfer something that is physically impossible(there needs to be a temperature gradient for heat transfer).
- #3
h2oski1326
- 81
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That isn't really true. The carnot cycle does operate between two temperature sinks.
The carnot cycle is merely a cycle which assumes that there is no increase in entropy due to imperfections. It is useful because it is the pinnacle of thermodynamic cycles and it can tell you thinks like the maximum efficiency any cycle can reach when operating between two given temperatures. It isn't used because it cannot be achieved, it just isn't possible.
However, the stirling cycle is implementable and has a maximum theoretical efficiency which matches the carnot cycle. Stirling engines are used in a few applications, like a stirling cryocooler, successfully. However, it is generally more practical to use mechanical devices which run on other cycles like an Otto cycle (automobile) for example.
The carnot cycle is merely a cycle which assumes that there is no increase in entropy due to imperfections. It is useful because it is the pinnacle of thermodynamic cycles and it can tell you thinks like the maximum efficiency any cycle can reach when operating between two given temperatures. It isn't used because it cannot be achieved, it just isn't possible.
However, the stirling cycle is implementable and has a maximum theoretical efficiency which matches the carnot cycle. Stirling engines are used in a few applications, like a stirling cryocooler, successfully. However, it is generally more practical to use mechanical devices which run on other cycles like an Otto cycle (automobile) for example.
- #4
jaap de vries
- 166
- 0
I know there are two different temperatures but the heat transfer takes place isothermally which is physically impossible.
So that is different than isentropic because no imperfections which off course is a requirement that also needs to be met.
There is isothermal expansion, adiabatic expansion, isothermal compression, adiabatic compression. I should have stated more clearly that there IS a heat source and sink which are at different temperatures.
So that is different than isentropic because no imperfections which off course is a requirement that also needs to be met.
There is isothermal expansion, adiabatic expansion, isothermal compression, adiabatic compression. I should have stated more clearly that there IS a heat source and sink which are at different temperatures.
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- #5
h2oski1326
- 81
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The heat transfer can take place at one temperature if it is a saturated liquid vapor mixture. But you are correct, if the cycle is for a single phase fluid it also cannot happen.
- #6
jaap de vries
- 166
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These are always interesting discussions. However, even do a saturated liquid can absorb heat isothermally, there still needs to be some kind of gradient at its boundary to get the energy into the saturated system, whether heats enters through conduction, convention or through some radiative means. Holding a source against the body of interest at equal temperature simply will not produce any flow of heat. Even in theory this would happen at an infinitesimally small pace. That is why Carnot cycles cannot operate in real life.
- #7
russ_watters
Mentor
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As stated in post 3, the defining characteristic of the Carnot cycle is the lack of a change in entropy through the different processes. This isn't just a matter of friction in parts, but there is entropy lost due to the behavior of the working fluid itself.
- #8
engware
- 62
- 2
Hi there:
Here are a few plots regarding the Carnot Cycle operation and usefulness.
Thanks,
Gordan
Here are a few plots regarding the Carnot Cycle operation and usefulness.
Thanks,
Gordan
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