PRINCIPLES OF NAVAL ENGINEERING 



WORKING 

 SUBSTANCE 

 (MIXTURE OF 

 ATMOSPHERIC 

 AIR AND FUEL) 



(LOW PRESSURE OR 



HIGH PRESSURE SIDE 



OF THE CYCLE, 



DEPENDING UPON " 



POSITION OF PISTON 



IN THE CYLINDER) 



HEAT RECEIVER 

 (ATMOSPHERE) 



HEAT SOURCE 

 (COMPRESSION OR SPARK) 



HEATED ENGINE 



(PISTON AND 



CYLINDER) 



147.63 

 Figure 8-9.— Essential elements of open, 

 heated-engine cycle. 



itself. When a shaft is rotating, we expect a tem- 

 perature rise in the bearings; when the shaft has 

 been stopped, we would be truly amazed to ob- 

 serve internal energy from the bearings flowing 

 to the shaft and causing it to start rotating again. 

 When we drag a block of wood across a rough 

 surface, we expect some of the mechanical energy 

 expended in this act to be converted into thermal 

 energy— that is, we expect a storage of internal 

 energy in the wooden block and the rough surface, 

 as evidenced by temperature rises in these ma- 

 terials. But if this stored internal energy should 

 suddenly turn to and move the wooden block back 

 to its original position, our incredulity would 

 know no bounds. 



All of which merely goes to show that we have 

 certain expectations, based on experience, as to 

 the direction in which processes will move. The 

 reasonableness of our expectations is attested by 

 the fact that in all recorded history there is no 

 report of water freezing instead of boiling when 

 heat is applied; there is no report of a lukewarm 

 fluid unmixing itself and separating into hot and 

 cold fluids; there is no report of a gas compress- 

 ing itself without the agency of some external 

 force; there is no report of the heat of friction 

 being spontaneously utilizedtoperform mechan- 

 ical work. 



Are these actions really impossible? The 

 first law of thermodynamics says that mechanical 



energy and thermal energy are mutually con- 

 vertible, but it says nothing about the direction 

 of such conversions. If we consider only the first 

 law, all the improbable actions just mentioned 

 are perfectly possible and all processes could be 

 thought of as being reversible. In an absolute 

 sense, perhaps, we cannot guarantee that water 

 will never freeze instead of boil when it is placed 

 on a hot stove; but we are certainly safe in saying 

 that this or any other completely reversible 

 thermodynamic process is at the outer limits of 

 probability. For all practical purposes, then, 

 we will say that there is no such thing as a com- 

 pletely reversible process. 



Nevertheless, the concept of reversibility is 

 extremely useful in evaluating real thermody- 

 namic processes. At this point, therefore, let us 

 define a reversible thermodynamic process as 

 one which would have the following characteris- 

 tics: (1) the process could be made to occur in 

 precisely reverse order, so that the energy sys- 

 tem and all associated systems would be re- 

 turned from their final condition to the conditions 

 that existed before the process started; and (2) 

 all energy that was transformed or redistributed 

 during the process would be returned from its 

 final to its original form, amount, and location. 



THE SECOND LAW OF THERMODYNAMICS 



Since the first law of thermodynamics does 

 not deal with the direction of thermodynamic 

 processes, and since experience indicates that 

 actual processes are not reversible, it is ap- 

 parent that the first law must be supplemented by 

 some statement of principle that will limit the 

 direction of thermodynamic processes. The 

 second law of thermodynamics is such a state- 

 ment. Although the second law is perhaps more 

 empirical than the first law, and perhaps some- 

 thing less of a ''law" in an absolute sense, it is 

 of enormous practical value in the study of 

 thermodynamics. 16 



The second law of thermodynamics may be 

 stated in various ways. One statement, known as 

 the Clausius statement, is that no process is 

 possible where the sole result is the removal of 

 heat from a low temperature reservoir and the 

 absorption of an equal amount of heat by a high 

 temperature reservoir. Amongother things, this 



The interested stu(dent will find an excellent (iiscus- 

 slon of the second law of thermodynamics in Max 

 Planck, Treatise on Thermodynamics . Dover Publi- 

 cations, New York, 1945. (A. Ogg. trans.) 



180 



