86 



Professor Alex. B. W. Kennedy 



[April 21 



Fig. 2. 



often about 30 per cent., in the case of a gas engine from 60 to 65 

 per cent. 



In any actually possible steam engine the actual process differs 

 so much from the ideal that not more than 20 or 25 per cent, (out of 

 the 30 per cent, just mentioned) could be attained even if the process 

 were carried out perfectly. But there are very great difficulties in 

 the way of carrying out even this imperfect process at all completely, 

 and so it comes about that, in the final results, only from 5 to perhaps 



15 per cent, of the whole heat of 

 the steam is ever turned into work, 

 sometimes a little more, more often 

 a little less. 



In Fig. 2 I have represented by 

 100 per cent., not the whole heat 

 given to the steam, but that fraction 

 of it (25 or 30 per cent.) which it 

 is physically conceivable that any 

 actual engine should turn into work. 

 The space between a and b shows 

 the losses due to the fact that the 

 engine works in a cycle far inferior 

 to the best possible cycle. The space 

 between b and c shows the further 

 losses which do actually occur in 

 fact, the area under c being all that 

 is utilised. 



What possibilities are there of 

 increasing the theoretical maximum 

 efficiency of an engine ? The whole 

 matter depends upon whether we 

 can increase the value of the fraction 



T - T 



— ^= — -. We can do this obviously 



by either making T] higher or T 2 

 lower, or both. It is not difficult 

 to see how this matter comes out 

 practically. In all heat engines 

 some fluid or other is used as a 

 vehicle for the transformation of 

 heat into work. It may be coal gas 

 or producer gas, steam, air alone, or 

 air mixed with the products of com- 

 bustion, or even ether or ammonia. With steam we work between 

 the temperatures of, say, 60° or 400° F., or thereabouts. Quite at 

 the other end of the scale come engines worked by gas of different 

 kinds, where combustion actually takes place inside the engine and 

 nt)t in a furnace, and where the highest temperature may, perhaps, be 



