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Beginner's Guide to Propulsion and EngineSim
Fuel and Air Relationships


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?


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 (

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?

Related Pages:
Propulsion Activity Index
Propulsion Index


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Thu, Jun 12 04:39:27 PM EDT 2014

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