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4. Basic Considerations in the Analysis of Power Cycles

Most power-producing devices operate on cycles, and the study of power cycles is
an exciting and important part of thermodynamics. The cycles encountered in actual
devices are difficult to analyze because of the presence of complicating effects, such as
friction, and the absence of sufficient time for establishment of the equilibrium
conditions during the cycle. To make an analytical study of a cycle feasible, we have to
keep the complexities at a manageable level and utilize some idealizations. When the
actual cycle is stripped of all the internal irreversibility’s and complexities, we end up
with a cycle that resembles the actual cycle closely but is made up totally of internally
reversible processes. Such a cycle is called an ideal cycle.
A simple idealized model enables engineers to study the effects of the major
parameters that dominate the cycle without getting bogged down in the details. The
cycles discussed in this chapter are somewhat idealized, but they still retain the general
characteristics of the actual cycles they represent. The conclusions reached from the
analysis of ideal cycles are also applicable to actual cycles. The thermal efficiency of
the Otto cycle, the ideal cycle for spark-ignition automobile engines, for example,
increases with the compression ratio. This is also the case for actual automobile engines.
The numerical values obtained from the analysis of an ideal cycle, however, are not
necessarily representative of the actual cycles, and care should be exercised in their
interpretation (Fig. 3). The simplified analysis presented in this chapter for various
power cycles of practical interest may also serve as the starting point for a more in-
depth study.
Heat engines are designed for the purpose of converting thermal energy to work,
and their performance is expressed in terms of the thermal efficiency η th, which is the
ratio of the net work produced by the engine to the total heat input:

W
 th  net (4.1)
Q
in

Recall that heat engines that operate on a totally reversible cycle, such as the Carnot
cycle, have the highest thermal efficiency of all heat engines operating between the
same temperature levels. That is, nobody can develop a cycle more efficient than the
Carnot cycle. Then the following question arises naturally: If the Carnot cycle is the
best possible cycle, why do we not use it as the model cycle for all the heat engines
instead of bothering with several so-called ideal cycles? The answer to this question is
hardware related. Most cycles encountered in practice differ significantly from the
Carnot cycle, which makes it unsuitable as a realistic model. Each ideal cycle discussed
in this chapter is related to a specific work-producing device and is an idealized version
of the actual cycle.
The ideal cycles are internally reversible, but, unlike the Carnot cycle, they are not
necessarily externally reversible. That is, they may involve irreversibility external to the
system such as heat transfer through a finite temperature difference. Therefore, the
thermal efficiency of an ideal cycle, in general, is less than that of a totally reversible

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