Kansas IIni?ersity Science Bulletin. 



Vol. II, No. 6. NOVEMBER, 1903. Iv^rxuX'^B 



THERMAL DIAGRAMS AND THEIR PRACTICAL USE. 



BY HUGO DIEMER. 



'T^HE subject of this discussion is not a new one, but it is believed 

 -*- that the manner of presentation may be found advantageous. 

 The indicator diagram taken from piston engines reveals the rela- 

 tions between pressure and volume at all points of the cycle. The 

 area which the diagram encloses measures, when properly scaled, the 

 amount of work done. The indicator card, in connection with a 

 dynamometer oj* brake test, affords complete data as to the mechanical 

 efficiency of a heat engine. It does not reveal prima facie anything 

 with regard to the thermodynamic efficiency of an engine, or of the 

 heat interchanges taking place during the cycle. The total area 

 underneath any line in an indicator diagram represents foot-pounds 

 of work done by or on the medium during the change of condition 

 represented by that line. It would be desirable to have an instru- 

 ment which might record in a similar way, by areas underneath lines, 

 the total heat rejected or taken in by the medium during the same 

 cycle of operations shown by the inaicator diagram. The areas in 

 this case would measure thermal units, whereas in the indicator dia- 

 gram they measure foot-pounds. 



In a system representing work by areas, the two factors whose prod- 

 uct make up the area are units of force and of distance — that is, units 

 of intensity and of extent. In a system representing heat units by 

 areas, we will at once agree that the factor of intensity must be tem- 

 perature. To the factor of extent has been given the name entropy. 



The plotting of thermal diagrams with temperature and entropy as 

 coordinates is of great practical value in investigations of thermal effi- 

 ciency of heat engines. It is not only briefer than analytic methods, 

 but it is more completely exact, since it gives the conditions existing 

 at all points, whereas analytic methods can only establish an average 

 condition during a certain period. 



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