Carnot cycle theoretically has the maximum possible efficiency, but cannot be applied in actual practice. Its main importance is that it provides the designer of an engine with the ideal goal which he or she has to achieve. The figure below shows the four thermodynamic processes of a Carnot cycle.
Carnot cycle is made up of four thermodynamic processes, out of which two are isothermal and two are isentropic. It operates between two temperature limits T(h) and T(c), which are measured on an absolute temperature scale. The efficiency of the Carnot cycle is a function of these two temperatures only, and it is determined by-
The efficiency is called Carnot efficiency or ideal efficiency. The Carnot efficiency does not depend upon the working substance. It can be shown that no engine working between the same temperatures can have efficiency higher than the Carnot efficiency. For an actual heat engine to be more efficient, the difference between the two temperatures must be as high as possible. In case of a steam engine, the lowest temperature cannot be below 40°C and not below 400°C in case of an IC engine.
Watch video on next page to understand Carnot cycle in detail-
The major problem in achieving lower temperatures is the requirement of a very long exhaust stroke and very slow rate of heat transfer. Since the heat engines are required to work at high speeds, to obtain larger power, the exhaust temperature must be kept high. With these limitations, another way to improve the efficiency is to increase the upper temperature. Most IC engines go up to 2300°C However, there are limits to it due to limited strength of the material at higher temperatures.