Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

be utilized as the working fluid. The Carnot cycle is the most efficient cycle
that can be executed between a heat source at temperature THand a sink at
temperature TL, and its thermal efficiency is expressed as

(9–2)

Reversible isothermal heat transfer is very difficult to achieve in reality
because 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 temper-
atures only, and the thermal efficiency relation for the Carnot cycle
(Eq. 9–2) conveys an important 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 temperature at which heat is rejected from
the system.
The source and sink temperatures that can be used in practice are not
without limits, however. The highest temperature in the cycle is limited by
the maximum temperature that the components of the heat engine, such as
the piston 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.

hth,Carnot
 1 

TL

TH

Chapter 9 | 491

qin

qout

Isothermal

compressor

Isentropic

compressor wnet

Isentropic

turbine

Isothermal

turbine

1

2

3

4

FIGURE 9–7

A steady-flow Carnot engine.

EXAMPLE 9–1 Derivation of the Efficiency of the Carnot Cycle

Show that the thermal efficiency of a Carnot cycle operating between the

temperature limits of THand TLis solely a function of these two tempera-

tures and is given by Eq. 9–2.

Solution It is to be shown that the efficiency of a Carnot cycle depends on

the source and sink temperatures alone.