What is carnot cycle and How it works?

 A Carnot cycle is a type of ideal thermodynamic cycle that was first postulated by Sadi Carnot, a French physicist, in 1824 and developed f...

 A Carnot cycle is a type of ideal thermodynamic cycle that was first postulated by Sadi Carnot, a French physicist, in 1824 and developed further by others in the 1830s and 1840s. According to Carnot's Theorem, there is a maximum efficiency for any classical thermodynamic engine when converting heat into work or, in the opposite case, for a refrigeration system when producing a temperature difference by applying work to the system.

In a Carnot cycle, a system or engine transfers energy in the form of heat between two thermal reservoirs at temperatures displaystyle T H T H and displaystyle T C T C, respectively. Part of this transferred energy is then converted into the work that the system performs. There is no entropy generation, and the cycle is reversible. (In other words, entropy is conserved; it is neither gained nor lost; it is only transported between the thermal reservoirs and the system.) Heat transfers from the cold to the hot reservoir when the system is put to action (heat pump or refrigeration). The mechanism exerts energy on the environment when heat is transferred from the hot to the cold reservoir. 

The following steps make up a Carnot cycle, which is an idealised thermodynamic cycle carried out by a heat engine (Carnot heat engine)

1. Isothermal expansion:- Isothermal growth Heat (as an energy) is reversibly transferred from the hot temperature reservoir to the gas at a temperature that is infinitesimally lower than TH (to allow heat transfer to the gas without practically changing the gas temperature so isothermal heat addition or absorption). The gas is permitted to expand during this step (1 to 2 on Figure 1, A to B in Figure 2), performing work on the surroundings by gas pushing up the piston while being thermally in contact with the hot temperature reservoir (stage 1 figure, right).Although the pressure decreases from point 1 to point 2 (figure 1), the temperature of the gas remains constant because the heat transferred from the hot temperature reservoir to the gas is precisely utilised by the gas to perform work on the surroundings, resulting in no changes to the gas internal energy (no gas temperature change for an ideal gas). Heat QH > 0 is absorbed from the hot temperature reservoir, increasing the gas's entropy by the amount that Delta S H=Q H/T Delta S H=Q H/T H. 

   

   
   

   
   
2. Gas isentropic expansion (reversible adiabatic expansion) (isentropic work output). The gas in the engine is thermally isolated from the hot and cold reservoirs for this step (2 to 3 on Figure 1, B to C in Figure 2), so they neither gain nor lose heat, a process known as adiabatic. The gas keeps expanding as its pressure is reduced, working on its surroundings (rising the piston; stage 2 picture, right), and expending internal energy equal to the work that is being done. When a gas expands without receiving heat from an external source, it loses internal energy and cools to a temperature that is infinitesimally higher than the temperature of a cold reservoir, or TC. Since the system (the gas) and its surroundings do not exchange any heat (Q = 0), the entropy does not change during the operation, making it an isentropic one. 
   
   
   
3. Isothermal compression. At constant temperature TC, heat transported reversibly to a low temperature reservoir (isothermal heat rejection). The gas in the engine is in thermal contact with the cold reservoir at temperature TC (but thermally insulated from the hot temperature reservoir) in this stage (3 to 4 on Figure 1, C to D on Figure 2), and the gas temperature is infinitesimally higher than this temperature (to allow heat transfer from the gas to the cold reservoir without practically changing the gas temperature). The environment exerts pressure on the gas, driving the piston downward (stage 3 figure, right).According to the universal convention in thermodynamics, a portion of the energy the gas gains from this work precisely transfers as a heat energy QC 0 (negative as leaving the system) to the cold reservoir. As a result, the system's entropy decreases by a portion equal to the amount ${\displaystyle \Delta S_{C}=Q_{C}/T_{C}}$, ${\displaystyle \Delta S_{C}<0}$ because isothermal compression reduces the gas's multiplicity
4. Isentropic compression. (Figure 1: 4 to 1, Figure 2: D to A) Again, it is assumed that the engine is frictionless and that the operation is slow enough to be reversible because the gas in the engine is thermally isolated from the hot and cold reservoirs. As the gas moves through this stage, the environment exerts pressure on it, pushing the piston lower (stage 4 figure, right), increasing its internal energy, compressing it, and causing its temperature to rise back to the temperature that is infinitesimally lower than TH. However, the entropy of the system is unaffected. The gas is currently in the same condition as it was at the beginning of step 1. 
   
   
   

Carnot Theorem 

Comparing real ideal engines to the Carnot cycle (right). A actual material's entropy varies with temperature. The curve on a T-S diagram shows this transition. The curve for this illustration shows an equilibrium between vapour and liquid (See Rankine cycle). The ideal is not possible at every stage due to irreversible systems and heat losses, such as those caused by friction.

No engine working between two heat reservoirs can be more effective than a Carnot engine operating between the identical reservoirs, according to Carnot's theorem.

${\displaystyle \eta _{I}={\frac {W}{Q_{\mathrm {H} }}}=1-{\frac {T_{\mathrm {C} }}{T_{\mathrm {H} }}}}$