Mariner 30 JET 40 (4 CYL), Руководство по обслуживанию

Страница: 20 / 532

1-14 - IMPORTANT INFORMATION 90-814676R1 DECEMBER 1996
How Weather Affects Engine
Performance
It is a known fact that weather conditions exert a pro-
found effect on power output of internal combustion
engines. Therefore, established horsepower ratings
refer to the power that the engine will produce at its
rated RPM under a specific combination of weather
conditions.
Corporations internationally have settled on adop-
tion of I.S.O. (International Standards Organization)
engine test standards, as set forth in I.S.O. 3046
standardizing the computation of horsepower from
data obtained on the dynamometer, correcting all val-
ues to the power that the engine will produce at sea
level, at 30% relative humidity at 77
F (25 C) tem-
perature and a barometric pressure of 29.61 inches
of mercury.
Summer Conditions of high temperature, low baro-
metric pressure and high humidity all combine to re-
duce the engine power. This, in turn, is reflected in
decreased boat speeds--as much as 2 or 3 miles-
per-hour in some cases. (Refer to previous chart.)
Nothing will regain this speed for the boater, but the
coming of cool, dry weather.
In pointing out the practical consequences of weath-
er effects, an engine--running on a hot, humid sum-
mer day--may encounter a loss of as much as 14%
of the horsepower it would produce on a dry, brisk
spring or fall day. The horsepower, that any internal
combustion engine produces, depends upon the
density of the air that it consumes and, in turn, this
density is dependent upon the temperature of the air,
its barometric pressure and water vapor (or humidity)
content.
Accompanying this weather-inspired loss of power is
a second but more subtle loss. At rigging time in early
spring, the engine was equipped with a propeller that
allowed the engine to turn within its recommended
RPM range at full throttle. With the coming of the
summer weather and the consequent drop in avail-
able horsepower, this propeller will, in effect, become
too large. Consequently, the engine operates at less
than its recommended RPM.
Due to the horsepower/RPM characteristics of an en-
gine, this will result in further loss of horsepower at
the propeller with another decrease in boat speed.
This secondary loss, however, can be regained by
switching to a smaller pitch propeller that allows the
engine to again run at recommended RPM.
For boaters to realize optimum engine performance
under changing weather conditions, it is essential
that the engine be propped to allow it to operate at or
near the top end of the recommended maximum
RPM range at wide-open-throttle with a normal boat
load.
Not only does this allow the engine to develop full
power, but equally important is the fact that the en-
gine also will be operating in an RPM range that dis-
courages damaging detonation. This, of course, en-
hances overall reliability and durability of the engine.