Beginner's
Guide to Propulsion and EngineSim
Fuel and Air Relationships
Activity
Background
For a jet engine
to produce thrust, fuel and air must be combined in the combustor
and ignited. When we mix different substances, such as paints of different
colors or ingredients in a recipe, we can produce different results
depending on the amounts of each substance used. We can express the
amount of each substance as a ratio, or percent, of the sum of
all the other substances. Or we can create a ratio of the amount of
one substance to any other substance (25% red to 75% blue or 1 part
red for every 3 parts of blue). For jet engine combustion, the important
parameter is the
fuel/air ratio
(f/a) which is the ratio of the mass
of fuel to the mass of air being burned in the engine. (For additional
information, click on burner
thermodynamics and specific
fuel consumption.)
The fuel/air ratio
of a jet engine is the ratio of the masses of fuel and air. For
example, at sea level an airliner taking off at 375 mph with the throttle
at 100% has an f/a of .017. This means that 17 ounces of jet fuel are
required to be mixed with 1000 ounces of air in order for the jet engine
to operate. Open EngineSim
and set the "Output Display" to Numerical to see the f/a ratio
for the masses of fuel/air.
You will notice a change in the f/a ratio as you change engine
operating conditions (throttle, speed, or altitude).
In this exercise
we will investigate the ratio of the volume of fuel and air combined
in an engine. This problem will be a little more complicated, because
fuel and air normally exist as different states of matter. Fuel is normally
a liquid, while air is a gas. Most of you probably have at least a vague
idea of the size of container needed to hold 17 ounces of water. However,
do you have any idea of the size a container would be if it were large
enough to hold 1000 ounces of air?
Activity
Your problem is
to compare the fluid volumes of jet fuel (a liquid) and air (a
gas). To solve the problem, use the density of both air and jet fuel.
Density
is the ratio of the mass to the volume of any substance. The
density of many liquids can be found in textbooks and is usually referenced
to the density of water. Jet fuel weighs approximately 62.5% of the
weight of water. The density of the air changes with altitude. At sea
level, the density of air is 1.222 kilograms per cubic meter. For additional
information, click on air
properties definitions.
You should have
a basic understanding of metric/English conversions; standard relationships
are given in the table shown below. Additional information on length,
volume, and area conversions can be found at Online Conversions
(http://www.onlineconversion.com/).
|
One cubic
foot
|
=
|
62.4
pounds
|
(Density
of Water)
|
|
One gallon
|
=
|
8
pounds
|
(Weight
of Water)
|
|
One gallon
|
=
|
128
ounces
|
|
|
One inch
|
=
|
2.54
centimeters
|
|
|
One cubic
meter
|
=
|
1,000,000
cubic centimeters
|
|
|
One cubic
foot
|
=
|
1728
cubic inches
|
|
As stated earlier, it takes 17 ounces of jet engine fuel mixed with 1000
ounces of air for the proper mix to operate a jet engine. Your problem
is to determine how many more times the VOLUME of air is to the
fuel. Show both answers for volume in cubic meters, then compare them
to determine the ratio.
1. Find the volume
of the air.
2. Find the volume
of the jet fuel.
3. What is the ratio
of the volume of fuel/volume of air?
