184 STEAM ENGINES 



Let e =real number of expansions; 



i = clearance, expressed as a per cent, of the stroke; 

 k =real cut-off; 

 ^1 = apparent cut-off; 



r = apparent number of expansions = . 



ki 



Then, --and* = - (1) 



k e 



EXAMPLE. The length of stroke is 36 in. ; the steam is cut off 

 when the piston has completed 16 in. of the stroke; the clear- 

 ance is 4%. Find the apparent cut-off, the real cut-off, and 

 the real number of expansions. 



SOLUTION. Apparent cut-off = f = | = .444. 



ki+i .444 + .04 .484 



Real cut-off = k = - = - = - = .465. 

 1+t 1 + .04 1.04 



Real number of expansions = e = - = - = 2.15. 



MEAN EFFECTIVE PRESSURE 



Finding the Mean Effective Pressure. In order to find the 

 horsepower of an engine, it is necessary to know the mean 

 effective pressure, abbreviated M. E. P., which is defined as 

 the average pressure urging the piston forwards during its 

 entire stroke in one direction, less the pressure that resists 

 its progress. The mean effective pressure is usually found 

 from the indicator diagram in one of two ways. 



1. The area of the diagram in square inches may be meas- 

 ured by an instrument called a planimeterj the M. E. P. is 

 then found by dividing the area of the diagram in square inches 

 by the length of the diagram in inches, and multiplying the 

 quotient by the scale of the spring. 



2. Where a planimeter is not available, the M. E. P. may 

 be found with a fair degree of accuracy by multiplying the 

 length of the mean ordinate by the scale of the spring. 



