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Altitude Versus Power Output
The altitude at which an engine operates has a
dramatic effect on power output. Since atmospheric
pressure (14.7psi at sea level) drops as altitude
increases air becomes less dense and lighter.
Therefore it has the same effect on horsepower
output as air temperature described in the previous
section. The rate of horsepower decrease is 3% for
each 1000 feet increase in altitude. As an example
an engine that makes 100 horsepower in
Washington, D.C. (elevation 30 feet) will only make
about 85 horsepower in Denver Colorado
(elevation 5280 feet).
An advantage to using gaseous fuels (LPG, CNG)
over liquid fuel (gasoline) is that when altitude
increases the density of air and gaseous fuels
change at about the same rate, therefore the air
fuel ratio remains unchanged. However with a
liquid fuel, as altitude increases, air becomes less
dense but the liquid fuel does not change therefore
the air fuel mixture becomes richer (less pounds of
air to a fixed amount of gasoline) and engine
horsepower output decreases more than the 3%
reduction (per 1000-feet) caused by the decrease
in atmospheric pressure.
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