Why is the Carnot Cycle practically not possible?

Hint: Carnot cycle comprises two isothermal and two adiabatic processes. So, in order to solve the question, figure out the concept behind isothermal and adiabatic processes is the most important point. Moreover, knowledge of reversible and irreversible thermodynamic processes can be helpful.

Complete step by step solution:
Any machine which can convert heat into mechanical work is called a heat engine. The Carnot engine, a concept conceived by Sadi Carnot, is an ideal theoretical engine that is free from all the imperfectness of an actual machine.
Carnot engine comprises a source, sink, working substance, and an insulating stand. The working substance is subjected to four processes in the form of a cycle. It consists of processes like isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. These processes are being conducted at a very slow rate or quasi statically in order to have a reversible process. Ideally, the Carnot engine is taken to be reversible as it has the following properties:
1. A perfectly insulating frictionless piston and a cylinder with perfectly insulating walls are used as a working substance so that it doesn’t have any dissipative effects like friction, conduction of heat.
2. The working substance is always in thermal equilibrium with the source and sink.
3. The change in pressure and volume of the working substance must take place at a slow rate.
4. A Carnot Engine has 100 % efficiency.

But, in actual practice, there is friction and dissipation of heat whenever some work is done on a system. There is no such device that acts as a perfect insulator or perfect conductor. Also, such slow processes are difficult to carry out. In real engines, the heat transfers at a sudden change in temperature whereas in a Carnot engine, the temperature remains constant. In our day to day lives, reversible processes can’t be carried out and there is no such engine with 100 % efficiency. Thus, the Carnot cycle is practically not possible.


Note: Carnot Engine is an ideal situation which means that though it’s not possible to make it in real life, it sets a limiting factor. When an actual engine is made, the goal is to make it as perfect as possible so that it can have maximum efficiency. Just like Carnot engine another device refrigerator.