Introduction of Carnot Cycle

The best and ideal cycle of thermodynamics is Carnot cycle. Carnot cycle describes the different events through which a substance pass when expose to heat or high temperature. According to Carnot cycle the substances pass through four different events while interacting with heat. The first event is known as isotherm expansion of a substance. And the second event is adiabatic expansion of the substance. Adiabatic is the process in which the heat is not transfer to the environment. The third event through which substances pass while interacting with heat is known as isothermal compression. And in the fourth and last event is the adiabatic compression of the substance. In the last stage of the Carnot cycle the substance returns to its original state as it was before the start of the reaction. Basically the Carnot cycle is the description of the reversible reactions. The Carnot cycle describes the heath exchange between the two substances participating in the reaction and for a power cycle is:

Coefficient of performance for the refrigerator is;

If we talk about the mechanism of action of freezer then it cannot perform its function without having power from any other source of power. Same is the case with heat engine. It cannot perform its function while receiving energy from a single source. Because Carnot cycle describes all the aspects of the reversible cycle that is major reason of the widely spreading use of this cycle. Carnot cycle is the diagrammatic representation of the heat engine which has two compression and two expansion processes. (Jacoby, 2014)

The principle of the power cycle is the basic principle of most of the electric power and motor cars of the world. The power cycles have two types including ideal cycles and real cycles. Because of the presence of the factors like friction and insufficient time to get the state of equilibrium, the real cycles cannot be studied easily. (Ami fault, 2018)

Figure 1: Carnot Engine Diagram of Carnot Cycle

Concept of Cycle

The below figure 2 explains the whole concept of the Carnot cycle;

Source:http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnotcon.html#c1

Figure 2: Concept of Cycle

Principle of Carnot Cycle

Basically the reversible reaction phenomenon is an ideal and unrealistic phenomenon. Naturally the reversible reactions do not exist. If the heat engine works on the reversible reaction then there will be no need of refilling of the fuel on the engine. But in real life once the heat engine uses the fuel. It cannot be reversed again.

The Principles of the Carnot cycle is a very good description of one of the laws of thermodynamics. These principles states:

Reversible engines are always more efficient than irreversible engines while functioning with the same two resources.

And all the reversible engines have same efficiencies while working with same energy resources.

The efficiencies of all the reversible reactions are same while working with same energies resources. (Toppr.com)And the below figure 3 explains the principle of the Carnot;

Figure 3: principle of the Carnot Cycle

Thermodynamics Theory of Carnot Cycle

There are two main processes;

Reversible Process

Irreversible Process

Any reaction or procedure, which can be reversed at any stage it, is known as reversible reaction. And after the reaction is finished then all the components and the surroundings can be restored in their original state. As they were before the reaction was started. But if the components and surroundings do not regain their original state, then thus reaction is called irreversible reaction (Logan, 1999).

Following are the factors that make the irreversible process to happen;

A finite temperature makes the heat to transfer

Gas expands in unrestrained manner

Two gases mix

Friction takes place

Chemical reactions take place

Inelastic deformation happens

Electricity passes through the resistance

Figure 4: Example of reversible and irreversible

Externally and Externally Reversible Processes

When a system takes place then its irreversibility could be measured within a specific system. In case if a system has the ability to be restored by maintaining its equilibrium states that, it is called the internally reversible characteristics. Moreover, there is need to understand that on the boundaries of the system irreversibility cannot take place because it has to go through a sequential process and this type of process is named as externally reversible (Ecourses.ou.edu, 2018). Moreover, a process, which is completely reversible in internal and external system then it, is called totally reversible process.

Carnot Cycle Process of Carnot Cycle

The heat engine works within a cycle and so its efficiency depends on the base of the processes of the system. Reversible cycles are regarded as comparatively more efficient by addressing its capability of in this system the processes support the cycle reversibility.

Following this, the in practices it is not possible to acquire the reversible cycles whereas on the real cycle performance, it provides upper limits. Carnot cycle is known with its effectiveness regarding its cycle’s reversibility as it consists on the 4 reversible processes. In the example of a piston cylinder containing gas, the four reversible processes could be seen. Following are the four reversible process of the Carnot cycle;

Process 1-2: Reversible Isothermal Expansion

In figure 5 it can be seen that the is transferring to cylinder from hear source and in this process, the temperate difference plays vital role. In this process, the heat is transferring in a reversible way. Slow expansion of the gas moves towards the surroundings while maintaining a constant temperature, which is denoted as TH. Following this, the Q H here representing the total amount of the heat that is transferred. 

Source: http://www.ecourses.ou.edu/cgi-bin/eBook.cgi?topic=th&chap_sec=05.3&page=theory

Figure 5: Carnot Cycle 1-2

Process 2-3: Reversible adiabatic expansion of Carnot Cycle

In the following figure it can be seen that after removing the heat source that gas is expanding in the adiabatic way in the cylinder. It can be seen that in the cylinder that gas is expanding and this expansion of gas takes place slowly and at the same time, it is working until the drop of the gas temperature to TL from TH. In this process, there is reversibility and at the same time, movement of the piston is assumed friction less (Www.shmoop.com, 2018)

Source: http://www.ecourses.ou.edu/cgi-bin/eBook.cgi?topic=th&chap_sec=05.3&page=theory

Figure 6: Carnot Cycle 2-3

Process 3-4: Reversible isothermal compression of Carnot Cycle

In the following figure it can be observed that at the temperature TL, the cylinder is connected with the heat sink. Here an external force push the piston and it effects on the gas inside the cylinder and compression starts. Gar temperature maintains at TL even in the compression and the heat transfer in reversible manners. During this process the Q L is the total amount of the heat that which is rejected by the heat sink. 

Source: http://www.ecourses.ou.edu/cgi-bin/eBook.cgi?topic=th&chap_sec=05.3&page=theory

Figure 7: Carnot Cycle 3-4

Process 4-1: Reversible adiabatic compression of Carnot Cycle

On the following stage of the process, the gas started to compress in adiabatic way as the heat sink is removed. At this stage, gas started to compress in slow speed as it receive force from the surroundings and it results in the increase of the temperature to TH from TL. With this process, the gas again comes to its initial state and completes the process (Chem.libretexts.org, 2017).

Figure 8: Carnot Cycle 4-1

Importance of Carnot Cycle

The heat engines efficiency works on the base of the intensity of the temperature, therefore, it efficiency directly depends on the minimum and maximum temperature. In the Carnot, cycle the air or steam works as a medium and it the heat it receives depends on the temperature and in case if the temperature it lower then it rejects the heat therefore, the efficiency of the cycle depends on the temperature intensity. Before discussing the Carnot Cycle, a general perception is that the engine’s efficiency depends on the fluid that is used in the engine cycle. Contrasting this, the Carnot cycle’s show that the engine does not depends on the fluid type in terms of its efficiency but it receive effects from the temperature level which plays vital in generating engine cycle’ heat.

Second law of Thermodynamics; According to the Carnot cycle it is found that the high temperature reservoirs absorb the heat whereas the low temperature reservoirs reject the heat. It is very important findings as it provides basic notion for the thermodynamics second law. Furthermore, according to the second law of thermodynamics it is believed that the heat’s natural is linked with the reservoirs high temperature to low temperature, and accordingly work done is based on it (Khemani, 2018). Following this, the Carnot Cycle engine is found as one of the most efficient heat engine because it is based on the two processes of isothermal and it also consist two processes of adiabatic. Subsequently, based on the physics laws, the Carnot cycle founds as one of the most efficient engine. According to the second law of the thermodynamics that all the heat produced in the heat engine cannot be converted into the work but the on the other hand, in Carnot cycle fraction of heat set value that can also be used.

Efficiency of Carnot Cycle

The Carnot cycle puts limit on the engine cycle’s efficiency because of its reversible properties. On the other hand, contrasting to the Carnot cycle engine, the other engines have irreversible characteristics that are why they give low efficiency while working on the same temperature with the Carnot cycle based engine. In addition, there is another factor that effects the working efficiency of the Carnot cycle and that is the working of the fluid in the cycle. Moreover, in the Carnot cycle, at maximum temperature the heat reinforces the fluid working therefore, it acquire maximum efficiency ( Esposito & et.al, 2010).

The process must be reversible in order to work with the Carnot efficiency. With this characteristic, the process can work with high efficiency even in high temperature and reflects no effect on the entropy. Currently, no real engine works with irreversible properties, therefore, it shows that the Carnot cycle is idealization.

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html

On the base o the assumptions the Carnot Cycle is regarded as highly efficient engine in the absence of the incident wasteful processes and at different temperature of the different parts there is no assumption of heat exist. The ratio between the energy output and input are used to define the efficiency of the Carnot engine  (Lutz & et.al, (2002).)

 Conclusion of Carnot Cycle

Esposito, M., & et.al. (2010, October 8). Lindenberg, K., & Van den Broeck, C. (2010). Efficiency at Maximum Power of Low-Dissipation Carnot Engines. Physical Review Letters,, 105((15)).

Amirault , S. (2018). Carnot Cycle & Ideal Cycles. Retrieved from https://sbainvent.com/thermodynamics/carnot-cycle-ideal-cycles/

Chem.libretexts.org. (2017, September 3). Carnot Cycle. Retrieved from https://chem.libretexts.org/Textbook_Maps/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Thermodynamics/Thermodynamic_Cycles/Carnot_Cycle

Ecourses.ou.edu. (2018). THERMODYNAMICS - THEORY Reversible and Irreversible Process. Retrieved from http://www.ecourses.ou.edu/cgi-bin/eBook.cgi?topic=th&chap_sec=05.3&page=theory

Jacoby, J. (2014). The Most Efficient Engine: The New Carnot Cycle. Createspace Independent Pub.

Khemani, H. (2018). Carnot Cycle and Carnot Theorem: Working and Relation to Second Law of Thermodynamics – Part 1. Retrieved from https://www.brighthubengineering.com/thermodynamics/3882-the-carnot-cycle-and-the-second-law-of-thermodynamics-part-one/

l Horsley, M., & et.al. (1996). Thermofluids (illustrated ed.). CRC press.

Logan, E. (1999). Thermodynamics: Processes and Applications. CRC Press.

Lutz, A., & et.al. ((2002).). Thermodynamic comparison of fuel cells to the Carnot cycle. International Journal of Hydrogen Energy, 27((10)), 1103–1111.

Toppr.com. (n.d.). Thermodynamics Carnot Cycle . Retrieved from https://www.toppr.com/guides/physics/thermodynamics/carnot-engine/

Www.shmoop.com. (2018). The Carnot Cycle. Retrieved from https://www.shmoop.com/thermodynamics/carnot-cycle.html