Steam Engines. 503 



that of confined steam, and if it were only possible to util- 

 ize 80 or 90 per cent, of this power the steam engine would 

 be a very inexpensive motor. 



627. Efficiency of the Steam Engine. It is unfortunately 

 true of the steam engine as a source of power, that in prac- 

 tical experience it is only able to render available from 2.5 

 to 20 per cent, of the full heat value of the fuel burned in 

 the fire box, and it is still more unfortunate that there 

 seems to be little hope that its efficiency can ever be made 

 to much exceed 31.5 per cent. The reason this is so is be- 

 cause it has not been found practicable to use steam at very 

 high temperatures nor to cool it much below that of the 

 ordinary air conditions. To enable a water wheel to util- 

 ize the highest per cent, of the power of a falling stream it 

 must be so arranged as to be able to take the water at the 

 highest possible level and not to release it until it has 

 reached the lowest possible level, and the principle is the 

 same with the steam engine. If the steam could be taken 

 into the cylinder at a temperature of 1,000 F. and re- 

 leased from it only after its temperature had fallen to 60 

 F. it is clear that much more work could be performed 

 than when the temperature is only permitted to fall be- 

 tween 300 F. and 212 F. 



Where heat is converted into work the efficiency is al- 

 ways equal to the quantity of heat taken into the engine 

 minus the quantity given out divided by the quantity 

 taken in; thus, if the steam entering the cylinder carries 

 100 heat units and it escapes from the cylinder with .90 

 heat units after moving the piston the efficiency of the en- 

 gine has been only 



100 - 90 



1QO = 10 per cent. 



So, too, if steam enters a cylinder at a temperature of 300 

 F. and escapes at 212 F., the maximum efficiency would 

 be only 



(461 -I- 300) - (461 + 212) 

 461 + 300 - = H.5 per cent 



