what is carnot engine?

An ideal, frictionless engine which operates in a Carnot cycle.

In 1824, a French scientist N.L.Sadi Carnot, suggested an idealized engine which is called the Carnot engine, which has an intimate relation with the second law of thermodynamics. To understand the principle, let us consider an ideal gas taken in a cylinder. The bottom of the cylinder is diathermic, whereas the rest of it is adiabatic. An adiabatic piston is fitted into the cylinder. Also, suppose we have two large bodies, one at a constant high temperature T₁ and the other at a lower temperature T₂.

The figure (a) shows the basic process of a Carnot engine on a PV diagram. The other parts of the figure represent the process schematically. Consider a cylinder being kept in contact with the high-temperature body, at temperature T₁ in a compressed state. This state is represented by the point a in the PV diagram. The gas is isothermally expanded to a state b (figure b). Work is done by the gas and an amount of heat Q₁ is supplied to it by the body at temperature T₁. The cylinder is now kept on an adiabatic platform and the gas is allowed to expand further to the state c (figure c). This is an adiabatic expansion and the temperature falls from T₁ to T₂. Work is done by the gas. At this stage, the cylinder is put in contact with the lower-temperature body at temperature T₂. It is isothermally compressed to a state d (figure d). Work is done on the gas and the gas rejects an amount of heat Q₂ to the body at the lower temperature T₂. Finally, it is kept on the adiabatic platform and is further compressed to reach the state a, where the temperature is T₁ (figure e).

The process represented by abcda in figure(a) is a cyclic process. If the piston is frictionless and is moved very slowly, the process is a reversible cyclic process.

Efficiency of a carnot engine

The basic process of a Carnot engine, described above, is again shown in the above figure in a T-S (temperature-entropy) diagram. The points a, b, c and d represent the same states as in figure (a). Let the entropy in state a be S₁. An amount of heat Q₁ is supplied to the system in the isothermal process ab at the temperature T₁. The entropy increases in this part, as heat is supplied to the system. Also, by definition,

The entropy remains constant in the part bc as it describes an adiabatic process. Therefore, the entropy in state c is S₂. In the part cd, the system gives a heat Q₂ at the lower temperature T₂ and its entropy decreases. The part da represents an adiabatic process and the entropy remains constant. As the entropy in state a is S₁, the entropy in state d is also S₁. Using the definition of change in entropy for the process cd,

From (i) and (ii),

The efficiency of the engine is

Thus, the efficiency of the engine depends only on the temperatures of the hot and cold bodies between which the engine works.

Carnot's theorem

Carnot engine is a reversible engine. It can be proved from the second law of thermodynamics that:

All reversible engines operating between the same two temperatures have equal efficiency and no engine operating between the same two temperatures can have efficiency greater than this.

This theorem is called Carnot's theorem. It is a consequence of the second law and puts a theoretical limit    to the maximum  efficiency of heat engines