Comments about "Carnot cycle" in Wikipedia

This document contains comments about the article Carnot cycle in Wikipedia
In the last paragraph I explain my own opinion.




The article starts with the following sentence.
In a Carnot cycle, a system or engine transfers energy in the form of heat between two thermal reservoirs at temperatures TH and TC (referred to as the hot and cold reservoirs, respectively), and a part of this transferred energy is converted to the work done by the system.
The cycle is reversible, and there is no generation of entropy. (In other words, entropy is conserved; entropy is only transferred between the thermal reservoirs and the system without gain or loss of it.)
The cycle is maybe reversible. The problem is that the whole process, of converting heat into work is irreversible.
When work is applied to the system, heat moves from the cold to hot reservoir (heat pump or refrigeration).
This process cannot directly be reversed.
When heat moves from the hot to the cold reservoir, the system applies work to the environment.
This process cannot directly be reversed.

1. Stages

A Carnot cycle as an idealized thermodynamic cycle performed by a heat engine (Carnot heat engine) consists of the following steps.
1. Isothermal expansion. Heat (as an energy) is transferred reversibly from hot temperature reservoir at constant temperature TH to the gas at temperature infinitesimally less than TH (to allow heat transfer to the gas without practically changing the gas temperature so isothermal heat addition or absorption).
The question is if this a reversible operation?
2. Sentropic (reversible adiabatic) expansion of the gas (isentropic work output). For this step (2 to 3 on Figure 1, B to C in Figure 2) the gas in the engine is thermally insulated from both the hot and cold reservoirs, thus they neither gain nor lose heat, an 'adiabatic' process.
3. Isothermal compression. Heat transferred reversibly to low temperature reservoir at constant temperature TC (isothermal heat rejection).
4. Isentropic compression. (4 to 1 on Figure 1, D to A on Figure 2) Once again the gas in the engine is thermally insulated from the hot and cold reservoirs, and the engine is assumed to be frictionless and the process is slow enough, hence reversible.
This is based on assumptions, which are in reality not valid

1.1 The pressure–volume graph

2. Properties and significance

2.1 The temperature–entropy diagram

2.2 The Carnot cycle

2.3 Reversed Carnot cycle

2.4 Carnot's theorem

In other words, the maximum efficiency is achieved if and only if entropy does not change per cycle.
An entropy change per cycle is made, for example, if there is friction leading to dissipation of work into heat.
In that case, the cycle is not reversible and the Clausius theorem becomes an inequality rather than an equality.
This sentence amplifies that the carnot cycle is irreversible.

2.5 Efficiency of real heat engines

Carnot realized that, in reality, it is not possible to build a thermodynamically reversible engine.
Carnot was a wise man.
Although Carnot's cycle is an idealization, Equation 3 as the expression of the Carnot efficiency is still useful.
Carnot was a wise man.

3. Carnot heat engine as an impractical macroscopic construct

These (and other) "infinitesimal" requirements make the Carnot cycle take an infinite amount of time. Other practical requirements that make the Carnot cycle hard to realize (e.g., fine control of the gas, thermal contact with the surroundings including high and low temperature reservoirs), so the Carnot engine should be thought as the theoretical limit of macroscopic scale heat engines rather than a practical device that could ever be built.
This final sentence says it all.

4. See also

Following is a list with "Comments in Wikipedia" about related subjects

Reflection 1 - General

The most important information from the side is that the "Carnot" cycle describes an irreversible process.
This is what can be expected, because the use any motor requires energy. A motor always performs forward in time. This process cannot be reversed. It is impossible to recreate all the energy used, by turning the motor backwards with your hands.
This depends on the type of energy used. For a steam engine it is completely impossible when wood is burned.
An electric motor is a different story. The motor consists of a magnet which rotates in rotating electric field caused by an alternating current. When this alternating current is caused by a waterwheel which turns under a waterfall, the backward chain is broken.

Reflection 2

Reflection 3


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Created: 1 November 2022

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