485
OF ENGINEERING FORMULÆ.
Horse-power (Indicated).
A = Area of pigton in square inches.
D = Diameter of piston in inches.
P = Average pressure of steam in lbs. per square inch in
cylinder.
S = Length of stroke in feet.
R = Number of revolutions per minute.
r = Number of revolutions per second.
I = Indicated horse-power.
2 APRS
1 — ----= '000047 61)2 PBS.
33000
2 A P r 8
I = ———- = -002856 DaPrS.
550 '
Nominal Horse-power.
“ Nominal ” horse-power means very little; the term is
arbitrary and varying. The actual or indicated horse-power
varying in stationary engines from 2i to 3 times the nominal
horse-power, and in marine engines it is sometimes 6 or I
times the nominal horse-power. It is now becoming obsolete
for marine engines.
ORDINARY RULES TOR NOMINAL HORSE-POWER.
■ = Mean velocity of piston in feet per minute.
D = Diameter of cylinder in inches.
S = Stroke of engine in feet.
H == Nominal horse-power of engine.
■ Da^S . ... ■
■ H = —, for high pressure.
la* 6
V = 128
H = for condensing engines.
V - 129 £/S.