DIEMER : THERMAL DIAGRAMS. 233 



by using a graduated feed- water supply tank, and obtaining the num- 

 ber of strokes jDer pound of cylinder feed, in case the boiler supplies 

 but the one engine, the necessary deduction being made for the feed 

 pump. In case there are other steam-consuming organs besides the 

 engine under test, the exhaust steam of the engine must be the gauge 

 of the weight of cylinder feed. 



A Rankine ideal engine cycle is next constructed for one pound of 

 steam. The average indicator card of the engine is next transformed 

 into a pv diagram for one pound of feed and superposed on the Ran- 

 kine diagram, the relative clearance volume being added to the dis- 

 placement volume shown by the indicator card. Or, the indicator 

 card itself may be surrounded by the Rankine ideal cycle for the 

 weight involved in the indicator diagram and a reduction factor em- 

 ployed in the transfer to the entropy chart. 



Superlieating effects are shown on the T<^ diagram by making use 

 of the fact that entropy of superheating must be equal to specific heat 

 of superheated steam X ^oge-ip—. By taking intermediate tempera- 

 tures between the temperature of the superheating and that of satu- 

 ration, the curve h'k (fig. 2) is obtained. If, in transferring the 

 indicator card to the entropy chart, it is found that the volume is 

 greater than that of saturated steam, use must be made of the rela- 

 tion, experimentally determined, that exists between the pressure, 

 volume and temperature of superheated steam, as expressed by Pea- 

 body, namely : /'^'sl,p = 98.5 Tgup — 911 p^'*, to determine the tempera- 

 ture of superheating. 



Thermal diagrams constructed in accordance with the above meth- 

 ods, added to the information given by the indicator card and other 

 data usually tc^ken during an engine test, disclose completely all heat 

 interchanges and heat relationships. The degree of refinement to 

 which calculations based u^Don them can safely be carried depends 

 necessarily upon the refinement and accuracy of the observational 

 data from which they have been built. 



In order to apply the thermal diagram to gas-engines, it is neces- 

 sary to know the specific heats of the mixture used. The relative 

 weights must be known of exhaust products, fuel charge, and air, also 

 the chemical composition of both fuel and exhaust. The specific 

 heats of each part multiplied by the percentage of weight it repre- 

 sents gives its relative specific heat, the total specific heat being the 

 sum of these individual relative heats. 



S-Bull. No. 6. 



