POSSIBLE EFFICIENCY OF HEAT-ENGINES. 839 



sible efficiency of an engine working between these two temperatures 



,, , , 671-4-491-4 



would therefore be > 27 per cent, nearly. 



671-4 1 J 



Heat-engines are often spoken of as very inefficient machines, be- 

 cause they transform into mechanical effect but a small proportion 

 of the heat used. The inefficiency is not so much the fault of the ma- 

 chine as of the conditions under which it is worked. Consider the case 

 of a condensing engine with a boiler pressure of 45 pounds and a vac- 

 uum of 25 inches of mercury. The temperature of the source is here 

 about 294 and of the refrigerator 140 Fahr. The possible efficiency 

 under these conditions is about 20 per cent., that is, a perfect engine 

 working between those temperatures could give in mechanical effect 

 no more than one fifth the energy of the heat. The best steam-en- 

 gines would, under these circumstances, give one-horse power for some- 

 thing less than two pounds of coal per hour. This is an efficiency of 

 10 or 12 per cent., or more than half the possible efficiency. The en- 

 gine, as a machine, is not so very imperfect. In speaking of the engine, 

 I of course include the boiler as a part of the machine. Any great 

 improvement must come from an increased range of temperature be- 

 tween the source and refrigerator. The temperature of the refriger- 

 ator can not well be lower than the general temperature of surround- 

 ing objects, and there are great practical difficulties in the way of a 

 very high temperature of the source. Suppose an engine could be 

 worked with a source at a temperatui-e of 1250 of the absolute scale, 

 or nearly 800 Fahr., and a refrigerator at 500 of the absolute scale, 

 or nearly 40 Fahr., the possible efficiency would be 1 T 5 25o> or only 

 60 per cent. It appears, then, that there is not much hope that any 

 large percentage of the energy of heat can, by any practical means, 

 be converted into mechanical effect. But are we, for this reason, 

 to continue wasting the energy of fuel as it is wasted now ? Is 

 there no other way in which the energy of chemical separation of car- 

 bon from oxygen can be converted into mechanical effect except by 

 first converting it into heat ? Why may not the union of carbon with 

 oxygen be made to generate electric currents instead of heat ? Elec- 

 tric energies have been made that convert into mechanical effect 60 to 

 70 per cent, of the energy of the electric current, and a much higher 

 efficiency might, no doubt, be obtained. Already something has been 

 done toward the generation of electric currents by the union of carbon 

 and oxygen ; but, so far, no means has been discovered by which such 

 a union can be effected, except at a high temperature, and this involves 

 a great waste of energy in the form of heat. A discovery that would 

 enable us to convert the energy of fuel into electric currents directly 

 and completely would revolutionize, not only the methods of obtaining 

 power, but the methods of obtaining light and distributing heat as 

 well. I have shown elsewhere that, if a Brayton oil-engine is used to 



