Carnot’s Cycle and Carnot’s Heat Engine [with Diagrams]

By | March 3, 2019

Carnot’s Cycle

Carnot’s Heat engine was a concept developed by Nicolas Leonard Sadi Carnot (1796-1832, a French Military Engineer and Physicist) so that one can visualize a reversible heat engine in practice.

Carnot’s engine is a reversible heat engine which works on Carnot’s cycle.

Carnot’s Cycle

P-V Diagram

Carnot’s cycle comprises of four processes.

  1. Reversible isothermal process of heat addition (Process A-B)
  2. Reversible adiabatic process of expansion (Process B-C)
  3. Reversible isothermal process of heat rejection (Process C-D)
  4. Reversible adiabatic process of compression (Process D-A)
Carnot’s engine

T-S Diagram

Carnot’s Heat Engine

Now we understand Carnot’s engine physically in the following way.

Process A-B

In this process heat is released from the hot reservoir and is absorbed by the ideal gas particles within the system. Thus, the temperature of the system rises. The high temperature causes the gas particles to expand hence pushing the piston upwards and doing work on the surroundings.

Process B-C

In this process expansion continuous, however there is no heat exchange between the system and the surroundings. Thus, the system is undergoing adiabatic expansion. The expansion allows the ideal gas particles to cool, decreasing the temperature of the system.

Process C-D

In this process surroundings do work on the system which causes heat to be released. The temperature within the system remains the same. Thus, isothermal expansion occurs.

Process D-A

No heat exchange occurs in this process however, the surroundings continue to do work on the system. Adiabatic compression occurs which raises the temperature of the system and puts the piston back to its original state (Prior to process A-B).

Also read:

What is Coefficient of Performance?

What is Carnot’s Theorem?

What is a irreversible process?

What are basic laws of thermodynamics?

Image attributions:

T-S Diagram: By Original:PAR~commonswikiVector:Smieh – This file was derived from: CarnotCycle1.png:, CC0,

Images of Carnot’s Engine: By BlyumJ – Own work, CC BY-SA 4.0,

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