Most power-producing devices operate on cycles, and the study of power cy-cles is an exciting and important part of thermodynamics. The cycles encoun-tered in actual devices are difficult to analyze because of the presence of complicating effects, such as friction, and the absence of sufficient time for es-tablishment of the equilibrium conditions during the cycle. To make an analyt-ical study of a cycle feasible, we have to keep the complexities at a manageable level and utilize some idealizations (Fig. 8–1). When the actual cycle is stripped off all the internal irreversibilities 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(Fig. 8–2). A simple idealized model enables engineers to study the effects of the ma-jor parameters that dominate the cycle without getting bogged down in the de-tails. 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 ac-tual 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 val-ues obtained from the analysis of an ideal cycle, however, are not necessarily representative of the actual cycles, and care should be exercised in their inter-pretation (Fig. 8–3). The simplified analysis presented in this chapter for var-ious 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 other forms of en-ergy (usually in the form of heat) to work, and their performance is expressed in terms of the thermal efficiencyhth , which is the ratio of the net work pro-duced by the engine to the total heat input: hth or hth wnet qin Wnet Qin

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 this 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 encoun-tered in practice differ significantly from the Carnot cycle, which makes it un-suitable as a realistic model. Each ideal cycle discussed in this chapter is related to a specific work-producing device and is an idealizedversion 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 ir-reversibilities external to the system such as heat transfer through a finite tem-perature difference. Therefore, the thermal efficiency of an ideal cycle, in general, is less than that of a totally reversible cycle operating between the same temperature limits. However, it is still considerably higher than the thermal efficiency of an actual cycle because of the idealizations utilized (Fig. 8–4). 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 insulated, 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 powercycles. This is a reasonable assumption since in devices that involve shaft work, such as turbines, compressors, and pumps, the kinetic and potential en-ergy 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-υand T-sdia-grams have served as valuable aids in the analysis of thermodynamic processes. On both the P-υand T-sdiagrams, the area enclosed by the process curves of a cycle represents the net work produced during the cycle (Fig. 8–5), which is also equivalent to the net heat transfer for that cycle. The T-sdiagram is particularly useful as a visual aid in the analysis of ideal power cycles. An ideal power cycle does not involve any internal irreversibilities, and so the only effect that can change the entropy of the working fluid during a process is heat transfer. On a T-sdiagram, a heat-additionprocess proceeds in the direction of in-creasing entropy, a heat-rejectionprocess proceeds in the direction of de-creasing entropy, and an isentropic (internally reversible, adiabatic) process proceeds at constant entropy. The area under the process curve on a T-sdia-gram represents the heat transfer for that process. The area under the heat ad-dition process on a T-sdiagram is a geometric measure of the total heat supplied during the cycle qin, and the area under the heat rejection process is a measure of the total heat rejected qout. The difference between these two (the area enclosed by the cyclic curve) is the net heat transfer, which is also the net work produced during the cycle. Therefore, on a T-sdiagram, the ratio of the area enclosed by the cyclic curve to the area under the heat-addition process curve represents the thermal efficiency of the cycle. Any modification that will increase the ratio of these two areas will also improve the thermal efficiency of the cycle. Although the working fluid in an ideal power cycle operates on a closed loop, the type of individual processes that comprises the cycle depends on the individual devices used to execute the cycle. In the Rankine cycle, which is the ideal cycle for steam power plants, the working fluid flows through a se-ries of steady-flow devices such as the turbine and condenser, whereas in the Otto cycle, which is the ideal cycle for the spark-ignition automobile engine,the working fluid is alternately expanded and compressed in a piston-cylinder device. Therefore, equations pertaining to steady-flow systems should be used in the analysis of the Rankine cycle, and equations pertaining to closed sys-tems should be used in the analysis of the Otto cycle.