same temperature limits. However, it is still considerably higher than the
thermal efficiency of an actual cycle because of the idealizations utilized
(Fig. 9–4).
The idealizations and simplifications commonly employed in the analysis
of power cycles can be summarized as follows:
1.The cycle does not involve any friction.Therefore, the working fluid
does not experience any pressure drop as it flows in pipes or devices
such as heat exchangers.
2.All expansion and compression processes take place in a quasi-
equilibriummanner.
3.The pipes connecting the various components of a system are well insu-
lated, and heat transferthrough them is negligible.Neglecting the changes in kineticand potential energiesof the working
fluid is another commonly utilized simplification in the analysis of power
cycles. This is a reasonable assumption since in devices that involve shaft
work, such as turbines, compressors, and pumps, the kinetic and potential
energy terms are usually very small relative to the other terms in the energy
equation. Fluid velocities encountered in devices such as condensers, boilers,
and mixing chambers are typically low, and the fluid streams experience little
change in their velocities, again making kinetic energy changes negligible.
The only devices where the changes in kinetic energy are significant are the
nozzles and diffusers, which are specifically designed to create large changes
in velocity.
In the preceding chapters,property diagramssuch as the P-vand T-sdia-
grams have served as valuable aids in the analysis of thermodynamic
processes. On both the P-vand T-sdiagrams, the area enclosed by the
process curves of a cycle represents the net work produced during the cycle
(Fig. 9–5), which is also equivalent to the net heat transfer for that cycle.
Chapter 9 | 489FIGURE 9–4
An automotive engine with the
combustion chamber exposed.
Courtesy of General Motors