Chapter 8- INTRODUCTION TO THERMODYNAMICS 



the exhaust products are returned to the atmos- 

 phere. In fact, another way to describe an open 

 cycle is to say that it is one which includes the 

 atmosphere at some point. 



The closed cycle is exemplified by the con- 

 densing steam power plant used for ship pro- 

 pulsion on many naval ships. In such a cycle, the 

 working substance (water) is changed to steam in 

 the boilers. The steam performs work as it ex- 

 pands through the turbines, and is then condensed 

 to water again in the condenser. The water is 

 returned to the boilers as boiler feed, and is 

 thus used over and over again. 



Thermodynamic cycles are also classified 

 as heated-engine cycles or as unheated-engine 

 cycles, depending upon the point in the cycle at 

 which heat is added to the working substance. 1^ 

 In a heated-engine cycle, heat is added to the 

 working substance in the engine itself. An in- 

 ternal combustion engine has a heated-engine 

 cycle. In an unheated-engine cycle, the working 

 substance receives heat in some device which 

 is separate from the engine. The condensing 

 steam power plant has an unheated-engine cycle, 

 since the working substance is heated separately 

 in the boilers and then piped to the engines (steam 

 turbines). 



There are five basic elements in any thermo- 

 dynamic cycle: (1) the working substance, (2) the 

 engine, (3) a heat source, or high-temperature 

 region, (4) a heat receiver, or low-temperature 

 region, and (5) a pump. 



The working substance is the medium by 

 which energy is carried through the cycle. The 

 engine is the device which converts the thermal 

 energy of the working substance into useful me- 

 chanical energy in the form of work. The heat 

 source supplies heat to the working substance. 

 The heat receiver absorbs heat from the working 

 substance. The pump moves the working sub- 

 stance from the low pressure side of the cycle 

 to the high pressure side. 



The essential elements of a closed, unheated- 

 engine cycle are shown in figure 8-8. This is 

 the basic plan of the typical condensing steam 

 power plant. 



15 



The terms heated ensine and unheated engine should 



not be confused with the term heat engine. Any ma- 

 chine which is designed to convert thermal energy to 

 mechanical energy in the form of work is known as a 

 heat engine. Thus, both internal combustion engines 

 and steam turbines are heat engines; but the first has 

 a heated-engine cycle and the second has an unheated- 

 engine cycle. 



• WORKING SUBSTANCE 



HEAT 

 SOURCE 



I- 



UNHEATED 

 ENGINE 



PUMP fmmmmmM 



I 



HEAT 

 RECEIVER 



147.62 

 Figure 8-8. — Essential elements of closed, 

 unheated-engine cycle. 



In an open, heated-engine cycle such as that 

 of an internal combustion engine, the essential 

 elements are all present but are arranged in a 

 somewhat different order. In this type of cycle, 

 atmospheric air and fuel are both drawn into the 

 cylinder of the engine. Combustion takes place 

 in the cylinder, either by compression or by 

 spark, and the resulting internal energy of the 

 working substance is transformed into work by 

 which the piston is moved. Since the space above 

 the piston is a high pressure area when the piston 

 is near the top of its stroke and a low pressure 

 area when the piston is near the bottom, the 

 piston may be thought of as a pump in the sense 

 that it "pumps" the working fluid from the low 

 pressure to the highpressure side of the system. 

 Thus, in terms of function, the piston-and- 

 cylinder arrangement may be thought of as in- 

 cluding the heat source, the engine, and the pump. 

 An open, heated-engine cycle might therefore be 

 represented as shown in figure 8-9. 



THE CONCEPT OF REVERSIBILITY 



When we put a pan of water on the stove and 

 turn on the heat, we expect the water to boil 

 rather than to freeze. After we have mixed hot 

 and cold water, we do not expect the resulting 

 mixture to resolve itself into two separate 

 batches of water at two different temperatures. 

 When we open the valve on a cylinder of com- 

 pressed air, we expect compressed air to rush 

 out; we would be quite surprised if atmospheric 

 air rushed into the cylinder and compressed 



179 



