333 



large, the loss due to the weight and mass of the parts is at this 

 speed comparatively insignificant. The total inevitable loss due 

 to friction in the machine alone, even excluding the accidental 

 friction due to tightness of piston and glands, and the power 

 required to work the valve gear, is nearly 9 per cent, cf the 

 whole indicated horse-power. This example shows the complete 

 fallacy of the experimental method sometimes adopted with the 

 object of testing the efficiency of an engine. The engine is run 

 with no resistance, and the indicated H.P. observed. This is 

 assumed approximately to represent the loss due to the engine 

 itself when doing useful work against a large resistance ; but 

 from Example A we see that the power required to overcome the 

 friction in the engine when a small resistance was being over- 

 come was only 3,210-2.974 inch Ibs., or 236 inch Ibs. ; whereas 

 in Example E the power required is 1,190 inch Ibs., or nearly 

 five times as much. This ratio would be somewhat diminished 

 by the constant accidental resistances in glands, etc., and by the 

 power required for valves and pumps. It must, however, always 

 remain very large. 



In Ej and E 2 the discontinuity of the curves at positions 

 and 12 is very marked. There must also be a slight break near 

 positions 6 and 18, due to the change in the bearing-points on 

 the crosshead pin ; but the frictional loss at this point is so in- 

 significant that the break in the curve is not sensible. There 

 is very little difference in general character between curves E t 

 E 2 and E 3 E 4 . Mass and weight play a very small part in the 

 general result. 



38. Example F, Fig. 52. Example F is important and 

 instructive, showing the result of running the engine at 4 

 revolutions per second instead of 1 revolution per second, but 

 maintaining the same indicator diagram as in Example E, 

 the mean effective pressure being 18-905 Ibs. as before. We 

 might expect this high speed to diminish the efficiency, as in 

 Examples B and D, but this is not the case. Curves F, and F 2 

 are identical with E, and E 2 , and are therefore omitted ; curves 

 F 3 and F 4 are, however, very different in character from E 3 and 

 E 4 . The resistance to acceleration causes the effort to be much 

 more uniformly distributed. It greatly diminishes the maximum 

 pressure, and largely increases the pressure during the second 



