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NOTES ON STEAM. 459
To Find the Mean Effective Pressure by the
Preceding Table.
Find the constant in the column corresponding to tr e con-
ditions of expansion, and to the given cut-off. Multiply this by
the absolute initial pressure, and the product is the average
pressure. Subtract the back pressure and the remainder is the
mean effective pressure.
Example.
Find the mean effective pressure for isothermal expansion
when the engine is cutting off at one-quarter stroke. The initial
pressure is 90 pounds absolute. The absolute back pressure is
18 pounds.
Solution :
M. E. P. — 90 X 0.596 — 18 = 53.64 — 18 = 35.64 pounds.
Note.—All such calculations must be made from absolute
pressure (not gage pressure), and when determining the cut-off
the clearance must be considered.
Clearance.
The clearance of an engine is usually expressed as a per-
centage of the piston displacement. The space between the
piston and the cylinder head at the end of the stroke, also the
cavities due to the steam ports, must be included in considering
clearance.
In high-class Corliss engines the clearance does not exceed
2>£ to 5 per cent., but in common slide-valve engines the clear-
ance may go as high as 5 to 15 per cent. When clearance is
taken into account the actual ratio of expansion is
r = Actual ratio of expansion.
R = Nominal ratio of expansion.
c = Clearance, expressed as a fractional part of the length
of the stroke.
Example.
The nominal ratio of expansion is 4, and the clearance is
5 per cent. What is the actual ratio of expansion ?
Note. —5 per cent, is %oo = M20 = 0.05 of the stroke
Solution
:
r _ 1+0.05
4- + 0.05
1 05
r = ’
= 3.5 = Actual ratio of expansion.
0. O
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