Thermodynamics

488 | ThermodynamicsPotatoWATER175ºCOVENACTUALIDEALFIGURE 9–1Modeling is a powerful engineeringtool that provides great insight andsimplicity at the expense of some lossin accuracy.PActual cycleIdeal cycleFIGURE 9–2The analysis of many complexprocesses can be reduced to amanageable level by utilizing someidealizations.FIGURE 9–3Care should be exercised in the interpretationof the results from ideal cycles.© Reprinted with special permission of KingFeatures Syndicate.v9–1 ■ BASIC CONSIDERATIONS IN THE ANALYSISOF POWER CYCLESMost power-producing devices operate on cycles, and the study of powercycles is an exciting and important part of thermodynamics. The cyclesencountered in actual devices are difficult to analyze because of the presenceof complicating effects, such as friction, and the absence of sufficienttime for establishment of the equilibrium conditions during the cycle. Tomake an analytical study of a cycle feasible, we have to keep the complexitiesat a manageable level and utilize some idealizations (Fig. 9–1). Whenthe actual cycle is stripped of all the internal irreversibilities and complexities,we end up with a cycle that resembles the actual cycle closely but ismade up totally of internally reversible processes. Such a cycle is called anideal cycle (Fig. 9–2).A simple idealized model enables engineers to study the effects of themajor parameters that dominate the cycle without getting bogged down in thedetails. The cycles discussed in this chapter are somewhat idealized, but theystill retain the general characteristics of the actual cycles they represent. Theconclusions reached from the analysis of ideal cycles are also applicable toactual cycles. The thermal efficiency of the Otto cycle, the ideal cycle forspark-ignition automobile engines, for example, increases with the compressionratio. This is also the case for actual automobile engines. The numericalvalues obtained from the analysis of an ideal cycle, however, are not necessarilyrepresentative of the actual cycles, and care should be exercised in theirinterpretation (Fig. 9–3). The simplified analysis presented in this chapter forvarious power cycles of practical interest may also serve as the starting pointfor a more in-depth study.Heat engines are designed for the purpose of converting thermal energy towork, and their performance is expressed in terms of the thermal efficiencyh th , which is the ratio of the net work produced by the engine to the totalheat input:h th W netQ inorh th w netq in(9–1)Recall that heat engines that operate on a totally reversible cycle, such asthe Carnot cycle, have the highest thermal efficiency of all heat enginesoperating between the same temperature levels. That is, nobody can developa cycle more efficient than the Carnot cycle. Then the following questionarises naturally: If the Carnot cycle is the best possible cycle, why do wenot use it as the model cycle for all the heat engines instead of botheringwith several so-called ideal cycles? The answer to this question is hardwarerelated.Most cycles encountered in practice differ significantly from theCarnot cycle, which makes it unsuitable as a realistic model. Each idealcycle discussed in this chapter is related to a specific work-producing deviceand 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 irreversibilitiesexternal to the system such as heat transfer through a finite temperaturedifference. Therefore, the thermal efficiency of an ideal cycle, ingeneral, is less than that of a totally reversible cycle operating between the