Chapter 5—ENGINE PERFORMANCE AND EFFICIENCY
The ratio of length of stroke to cylinder bore
Using the factors which influence the engine’s
is fixed in engine design; in most slow speed
capacity to develop power, the general or
engines, the stroke is greater than the bore.
standard formula for calculating ihp is as follows:
Revolutions Per Minute
Revolutions per minute (rpm) is the speed at
which the crankshaft rotates. Since the piston is
connected to the shaft, the rpm, along with the
length of the stroke, determine piston speed.
Since, during each revolution, the piston com-
pletes one up-stroke and one down-stroke, piston
speed is obtained by multiplying the rpm by twice
the length of the stroke. This speed is usually ex-
pressed in feet per minute (fpm). If the stroke is
10.5 inches (or 10.5/12 of a foot), and the speed
of rotation is 720 rpm, the piston speed is com-
puted as follows:
HORSEPOWER COMPUTATION
The power developed by an engine depends
upon the type of engine as well as the speed of
the engine. A cylinder of a single-acting, 4-stroke
cycle engine will produce one power stroke for
every two crankshaft revolutions, while a single-
acting, 2-stroke cycle engine produces one power
stroke for each revolution.
Indicated Horsepower
The power developed within a cylinder can be
calculated by measuring the imep and the engine
speed. (The rpm of the engine is converted to the
number of power strokes per minute.) With the
bore and stroke known (available in engine
manufacturers’ technical manuals), the
horsepower (hp) can be computed. This power is
called indicated horsepower (ihp) because it is ob-
tained from the pressure measured with an engine
indicator. Power loss due to friction is not con-
sidered in computing ihp.
ihp =
P × L × A × N
33,000
where
P = Mean indicated pressure, in psi
L = Length of stroke, in feet
A = Effective area of the piston, in
square inches
N = Number of power strokes per minute
33,000 = Unit of power (one horsepower), or
footpounds per minute.
To illustrate the use of this formula, assume
that a 12-cylinder, 2-stroke cycle, single-acting
engine has a bore of 8.5 inches and a stroke of
10 inches. Its rated speed is 744 rpm. With the
engine running at full load and speed, the imep
is measured and found to be 105 psi. What is the
ihp developed by the engine?
In this case
Substituting these amounts in the formula,
you have
This amount represents the horsepower
developed in only one cylinder; since there are 12
cylinders in this engine, total horsepower for the
engine will equal 12 times 111.9, or approximately
1343.
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