Thermodynamics

have long been of only theoretical interest. However, there is renewed interestin engines that operate on these cycles because of their potential forhigher efficiency and better emission control. The Ford Motor Company,General Motors Corporation, and the Phillips Research Laboratories of theNetherlands have successfully developed Stirling engines suitable for trucks,buses, and even automobiles. More research and development are neededbefore these engines can compete with the gasoline or diesel engines.Both the Stirling and the Ericsson engines are external combustion engines.That is, the fuel in these engines is burned outside the cylinder, as opposed togasoline or diesel engines, where the fuel is burned inside the cylinder.External combustion offers several advantages. First, a variety of fuels canbe used as a source of thermal energy. Second, there is more time for combustion,and thus the combustion process is more complete, which meansless air pollution and more energy extraction from the fuel. Third, theseengines operate on closed cycles, and thus a working fluid that has the mostdesirable characteristics (stable, chemically inert, high thermal conductivity)can be utilized as the working fluid. Hydrogen and helium are two gasescommonly employed in these engines.Despite the physical limitations and impracticalities associated with them,both the Stirling and Ericsson cycles give a strong message to design engineers:Regeneration can increase efficiency. It is no coincidence that moderngas-turbine and steam power plants make extensive use of regeneration. Infact, the Brayton cycle with intercooling, reheating, and regeneration, which isutilized in large gas-turbine power plants and discussed later in this chapter,closely resembles the Ericsson cycle.9–8 ■ BRAYTON CYCLE: THE IDEAL CYCLEFOR GAS-TURBINE ENGINESThe Brayton cycle was first proposed by George Brayton for use in the reciprocatingoil-burning engine that he developed around 1870. Today, it is usedfor gas turbines only where both the compression and expansion processestake place in rotating machinery. Gas turbines usually operate on an opencycle, as shown in Fig. 9–29. Fresh air at ambient conditions is drawn intothe compressor, where its temperature and pressure are raised. The highpressureair proceeds into the combustion chamber, where the fuel is burnedat constant pressure. The resulting high-temperature gases then enter the turbine,where they expand to the atmospheric pressure while producingpower. The exhaust gases leaving the turbine are thrown out (not recirculated),causing the cycle to be classified as an open cycle.The open gas-turbine cycle described above can be modeled as a closedcycle, as shown in Fig. 9–30, by utilizing the air-standard assumptions. Herethe compression and expansion processes remain the same, but the combustionprocess is replaced by a constant-pressure heat-addition process froman external source, and the exhaust process is replaced by a constantpressureheat-rejection process to the ambient air. The ideal cycle that theworking fluid undergoes in this closed loop is the Brayton cycle, which ismade up of four internally reversible processes:1-2 Isentropic compression (in a compressor)2-3 Constant-pressure heat additionChapter 9 | 507INTERACTIVETUTORIALSEE TUTORIAL CH. 9, SEC. 4 ON THE DVD.