458 



FAEMER8' INSTITUTES. 



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Figs. 187 and 188. 



as early as one-eighth of the stroke of the piston ; bat in small engines, 

 designed to run at high speed, it is closed much later. In very small engines 

 live steam is usually made to follow the piston nearly through its stroke; in 

 this way more power is developed from a smaller engine, although at a 

 slightly greater expense for fuel. 

 Figures 187 and 188 on the diagram will, no doubt, serve to make the 



foregoing statements clearer. Let the 

 horizontal line represent tlie stroke of the 

 piston, and suppose it, for convenience, 

 divided into 12 parts. Through each 

 one of these parts draw a vertical line, 

 ^ and make its length jiroportional to the 

 pressure acting on each square inch of 

 the piston, when in that position. Join 

 the ends of these pressure lines and the 

 result will be a diagram like that shown 

 in Fig. 1 and Fig. 2, in both of which 

 FG represents line of atmospheric or no 

 pressure. The area of these diagrams 

 will represent the work done at each 

 .^stroke of the piston. The diagram in 

 Fig. 187 represents the case when the 

 steam is admitted only for one-third of 

 the stroke of the piston ; that at Fig. 

 188 is the case in which steam is admitted for three-fourths the stroke. \f^ 

 each diagram the piston is at 1, moving toward when steam is first 

 admitted, and the pressure is nothing. The pressure rises to AB, stopping 

 the piston at 0, and starting it in the reverse direction. These pressure 

 lines remain of the same length until the piston gets at B, at which time the 

 steam port is closed and the pressure falls at the successive positions of the 

 piston, as shown by the shortening of the lines between CD and ED. At 

 the port opens from the exhaust side and allows the steam to escape, so that 

 the pressure falls to the line ED, which is that of the air in non-condensing 

 engines, if there is no back pressure. The reverse operation takes place on 

 the other side of the piston, and would be represented by a diagram of similar 

 form, but turned the other side to. 



The length of the line C D represents the pressure not used, and conse- 

 quent loss. This is greater in Fig. 188 than in Fig. 187. In very large engines 

 this loss is an important matter, and every means is taken to get all the 

 power possible out of steam. Any derangement of the valve motion affects 

 very materially the form of these diagrams, and consequently the power of 

 the engine. Sometimes a certain amount of steam gets on the wrong side 

 of the piston and opposes its motion, so as to decrease the power very materi- 

 ally. Now, the nominal horse power of an engine can be realized only when 

 the engine is in perfect order and running without friction, as has been 

 shown by the foregoing explanation. The actual horse-power should, how- 

 ever, be greater in case of an engine in good order than the same number 

 of horses of average strength. By running the engine at higher speed than 

 rated, or carrying a higher steam pressure, the actual horse-power developed 

 may be made to exceed the nominal horse-power. 

 From the fact that the power of an engine varies within such wide limits,. 



