182 STEAM ENGINES 



Finally, the cord is unhooked, and the card is removed from 

 the drum. 



If but one indicator and a three-way cock are used, as 

 shown in Fig. 3, the cock is opened to admit steam from one 

 end of the cylinder, and the diagram from that end is taken; 

 then the cock is turned to admit steam from the other end, 

 and another diagram is taken; finally, the steam is shut off 

 -entirely, and the atmospheric line is drawn. 



CLEARANCE AND CUT-OFF 



Clearance. The term clearance is used in two senses in 

 connection with the steam engine. It may be the distance 

 between the piston and the cylinder head when the piston is 

 at the end of its stroke, or it may represent the volume between 

 the piston and the valve when the engine is on dead center. 

 To avoid confusion, the former is called piston clearance, and 

 the latter is termed simply clearance. Piston clearance is 

 always a measurement, expressed in parts of an inch. Clear- 

 ance, however, is a volume. 



The clearance of an engine may be found by putting the 

 engine on a dead center and pouring in water until the space 

 between the piston and the cylinder head, and the steam port 

 leading into it, is filled. The volume of the water poured it 

 is the clearance. The clearance may be expressed in cut 

 feet or cubic inches, but it is more convenient to express it 

 as a percentage of the volume swept through by the piston. 

 For example, suppose that the clearance volume of a 12"X18" 

 is found to be 128 cu. in. The volume swept through 

 by the piston per stroke is 1 2-' X .785-1X18 = 2,035.8 cu. in. 

 Then, the clearance is 128 -=-2,035.8= .063 = 6.3%. The cl 

 ance may be as low as \% in Corliss engines, and as high 

 It', in high-speed engines. 



Effects of Clearance. Theoretically, there should be 

 clearance, since the steam that fills the clearance space does 

 no work except during expansion; it is exhausted from the 

 cylinder during the return stroke, and represents so much dead 

 loss. This is remedied, to some extent, by compression. If 



