The Carnot cycle is composed of four totally reversible processes: isothermal

heat addition, isentropic expansion, isothermal heat rejection, and isentropic

compression. The P-υand T-sdiagrams of a Carnot cycle are replotted in Fig.

8–6. The Carnot cycle can be executed in a closed system (a piston-cylinder

device) or a steady-flow system (utilizing two turbines and two compressors,

as shown in Fig. 8–7), and either a gas or a vapor can be utilized as the work-ing fluid. The Carnot cycle is the most efficient cycle that can be executed be-tween a heat source at temperature THand a sink at temperature TL

, and its

thermal efficiency is expressed as: n=1-TL/TH

Reversible isothermal heat transfer is very difficult to achieve in reality be-cause it would require very large heat exchangers and it would take a very

long time (a power cycle in a typical engine is completed in a fraction of a

second). Therefore, it is not practical to build an engine that would operate on

a cycle that closely approximates the Carnot cycle.

The real value of the Carnot cycle comes from its being a standard against

which the actual or the ideal cycles can be compared. The thermal efficiency

of the Carnot cycle is a function of the sink and source temperatures only, and

the thermal efficiency relation for the Carnot cycle (Eq. 8–2) conveys an im-portant message that is equally applicable to both ideal and actual cycles:

Thermal efficiency increases with an increase in the average temperature at

which heat is supplied to the system or with a decrease in the average tem-perature at which heat is rejected from the system.

The source and sink temperatures that can be used in practice are not with-out limits, however. The highest temperature in the cycle is limited by themaximum temperature that the components of the heat engine, such as the pis-ton or the turbine blades, can withstand. The lowest temperature is limited by

the temperature of the cooling medium utilized in the cycle such as a lake, a

river, or the atmospheric air.