STEAM ENGINES 183 



the compression were carried up to the boiler pressure, there 

 would be very little, if any, loss, since the steam would then 

 fill the entire clearance space at boiler pressure, and the amount 

 of fresh steam needed would be the volume displaced by the 

 piston up to the point of cut-off, the same as if there were 

 no clearance. In practice, however, the compression is made 

 only sufficiently great to cushion the reciprocating parts and 

 bring them to rest quietly. 



It is not practicable to build an engine without any clear- 

 ance, on account of the formation of water in the cylinder due 

 to the condensation of steam, particularly when starting the 

 engine. Automatic cut-off high-speed engines of the best 

 design, with shaft governors, usually compress to about half 

 the boiler pressure, and have a clearance of from 7 to 14%. 

 Corliss engines require but very little compression, owing to 

 their low rotative speeds; they also have very little clearance, 

 since the ports are short and direct. 



Apparent Cut-Off . The apparent cut-off is the ratio between 

 the portion of the stroke completed by the piston at the point 

 of cut-off, and the total length of the stroke. For example, if 

 the length of stroke is 48 in., and the steam is cut off from the 

 cylinder just as the piston has completed 15 in. of the stroke, 

 the apparent cut-off is if = tV 



Real Cut-Off. The real cut-off is the ratio between the volume 

 of steam in the cylinder at the point of cut-off and the volume 

 at the end of the stroke, both volumes including the clearance 

 of the end of the cylinder in question. If the volume of 

 steam in the cylinder, including the clearance, at the point of 

 cut-off is 4 cu. ft., and the volume, including the clearance, 

 at the end of the stroke is 6 cu. ft., the real cut-off is $=?. 



Ratio of Expansion. The ratio of expansion, also called the 

 real number of expansions, is the ratio between the volume of 

 steam, including the steam in the clearance space, at the end 

 of the stroke, and the volume, including the clearance, at the 

 point of cut-off. It is the reciprocal of the real cut-off. For 

 example, if the volume at the end of the stroke is 8 cu. ft., 

 and the cut-off is 5 cu. ft., the ratio of expansion is 8-4-5 = 1.6; 

 in other words, the steam would be said to have one and six- 

 tenths expansions. The corresponding real cut-off would be f . 



