Jet Fuel and Jet Engines

Intake the Excellence!

D.J. Mares, Brinton Mitchell, and Scott Symonds

Research Question

What are the different components that make up engines and the fuel those engines use and how do they

contribute to efficiency? In investigating this question we plan to research different engines. We will be

investigating the internal combustion engine, the turbojet, the scramjet. We will research how each engine works, to include: intake methods, compression methods, and

thrust. The other important aspect of our question is the fuel used. We must understand how fuels vary and how that affects performance. Different additives are used in

fuel to enhance the fuel’s properties. We will be examining Jet A-1/JP 8 fuel, the standard fuel for civilian

and military jets, and diesel fuel as a comparison.

Hypothesis

Since an engine requires a certain intake of air we predict that research will show that efficiency will change according to the speed of incoming air and the method

the engine employs to capture the air and compress it to the desired pressure. The other factor that we expect to affect efficiency is the fuel used. Fuel efficiency will be highly dependent on the additives used. The different

properties of fuel will show that only in certain conditions is one fuel better than another. We expect to find that

anti-icing agents, anti-corrosion agents, antioxidants, and other additives are mix with fuel to improve the performance of the fuel depending on the use.

Basic Fuel Properties

Jet A-1/JP-8 Fuel Diesel Fuel

43.15mJ/kgEnergy Density

.8075kg/LDensity at 15°C

-47°C (-52.6°F)Freezing Point

210°C (410°F)

Auto-ignition (Temperature at which substance ignites without external aid)

38°C (100.4°F)

Flash Point (Temperature at

which liquid form ignitable mixture

with air)

46.94mJ/kgEnergy Density

.85kg/LDensity at 15°C

Between -19 °C (-2.2 °F) and -15 °C (5 °F)

Freezing Point

210°C (410°F)

Auto-ignition (Temperature at which substance ignites without external aid)

Between 52°C and 96°C (126°F-204°F)

Flash Point (Temperature at

which liquid form ignitable mixture

with air)

Jet A-1/JP-8 Additives

Include:• Antioxidants

• Antistatics

• Corrosion inhibitors

• Icing Inhibitors

Antioxidants

Antioxidants prevent the fuel from gumming. Gumming occurs when a mixture of hydrocarbons start to solidify.

Since fuel is a mixture of varies hydrocarbons certain compounds solidify at different temperatures causing

globules to form clogging fuel systems. They do this by preventing the hydrocarbon oxidation by free radical chains. Alkylated phenols are used. The common

alkylated phenols used in fuel include; 2, 4-Dimethyl-6-tert-butylphenol, 2, 6-di-tert-butyl-4-methylphenol, and N,

N’-Di-sec-butyl-p-phenylenediamine.

Antistatics

These are used to lower static electricity buildup by making the fuel more conductive or absorbing

moisture. A product called Stadis 450 or 425 is used. It contains Toluene, Solvent Naphtha, and

Dinonylnaphthylsulfonic acid. These work because static is built-up when two different surfaces rub

against each other. These compounds have a hydrophobic and a hydrophilic side. The molecule

aligns so that the hydrophobic side touches the fuel and the hydrophilic side faces the air and absorbs

water. Prevent true contact of two different materials.

Corrosion inhibitors

These are added to prevent the fuel from corroding the metal inside a jet engine,

which must be kept very clean and free of debris. Jet fuel uses what is called an

anodic inhibitor, usually a chromate that creates a protective layer over the metal

and repels water molecules.

Icing Inhibitors

Since jet fuel has a small amount of H2O in it as a plane increases altitude those water

molecules freeze, and clog the fuel system. Ethylene glycol monomethyl

ether is mixed into the fuel as it is pumped into the plane, so as to assure even

distribution. This molecule is able mix with water to form a solution that doesn’t freeze

till about –43 degrees Celsius.

Diesel Fuel AdditivesAfter distillation of crude oil very little additives are mixed with diesel.

The conditions that diesel fuel is used in are much less extreme than that of aviation fuel. So additives are for limited. Antioxidants are not added in the making of diesel fuel, but may be bought at the discretion of the user. As for antistatics the amount of static built-up during fueling of a diesel-powerful vehicle is negligible. Also, diesel

engines are typically coated with corrosion-resistant materials. Aviation engines cannot be coated with significant corrosion-

resistant because they need to be more aerodynamic. Since diesel-powered vehicles usually don’t need to be aerodynamic diesel fuel

doesn’t have anti-corrosion additives. Since a ground vehicle is less likely to experience large changes in temperature freezing of fuel is

minimal. So the necessity for anti-icing agents is not small.

ConclusionAfter our intensive investigation we found that the type of engine and fuel used

are the primary factors determining efficiency. We found that the efficiency of an engine really is related to its function. After comparing the internal

combustion engine, turbojet engine, and scramjet engine we found that their efficiencies are highly dependent on the speed it was built to go. For

example, a scramjet is more efficient than a turbojet when going greater than Mach 6, but is less efficient than a turbojet at less than Mach 6. So we have concluded that the efficiency of an engine is almost solely based on its intended function. When researching fuel we came to a similar conclusion.

While diesel fuel has a greater energy density at 46.94mJ/kg than Jet A-1/JP-8 fuel, at 43.15mJ/kg, it is much more flammable and there for more

dangerous. Also, we discovered that the additives used are completely dependent on the function of fuel. Jet A-1/JP-8 fuel has many additives,

including; antioxidants, anti-corrosive agents, and anti-icing agents. That is because aviation fuel and aviation fuel systems are relatively fragile and work under extreme, diverse and constantly changing conditions. On the

other hand, diesel fuel is used in far less extreme conditions. In conclusion, jet engine components, jet fuel and its additives are specially designed for a

certain purpose. As a result, the efficiencies are tailored to the intended function.

Turbojet Engine• The turbojet engine is the oldest kind of jet engine. It works by first allowing

air into the engine through the front of the engine. Then it is compressed using turbines rotating at a high speed, squeezing the air into a smaller place, thus placing it at a higher pressure and temperature. Then fuel is injected into the air making an air-fuel mixture and is combusted, a highly exothermic reaction. The burning mixture then is allowed to go through the turbine, creating energy to run the compressor. Finally, the gasses are allowed to exit the engine out of the nozzle, creating thrust.

De Laval Nozzle

• The turbojet is able to expel gasses at such a high velocity partially because of the De Laval nozzle. The De Laval nozzle is a tube that is pinched in the middle to create an hourglass shape. The gas flow through a De Laval nozzle is isentropic (the entropy is constant). This nozzle allows for the engine’s efficiency at supersonic speeds.

Scramjet Engine• The scramjet, while still in development, could blur the line between airplanes and

spaceships. The scramjet (supersonic combustion ramjet) has very few or no moving parts. It has an extremely simple design. First, air is forced into the intake, where it is compressed, thus increasing temperature and pressure. At the peak pressure, fuel is injected and it spontaneously combusts since it is so pressurized and so hot. The fuel then expands out the back of the engine, creating thrust. The top speed of the scramjet is estimated as anywhere in between Mach 12 to Mach 24. The advantages for using a scramjet are that there are few moving parts and it is efficient at very high speeds. The disadvantages are that it is still in development, it cannot function under Mach 6, and it has a very poor thrust to weight ratio.

Internal Combustion Engine• The standard automobile four stroke internal combustion engine is included

for comparative purposes. The four stroke internal combustion engine (ICE) has four basic steps: Intake, compression, combustion, and exhaust. In the intake phase, a fuel-air mixture is put in the combustion chamber. Next it is compressed so the pressure and temperature increase. Next, the compressed mixture is combusted so the gasses rapidly expand, pushing down a piston, which moves the automobile. Finally, the combusted mixture is expelled out of the exhaust pipe.

Engine Efficiency

• These engines are only useful if the velocity of the gas from the engine is greater than the airplane velocity. An approximate equation for calculating the net thrust of a jet engine is:

• Where: Intake mass flow rate Fully expanded jet velocity (in the exhaust plume) Aircraft flight velocityWhile the term represents the gross thrust of the nozzle, the term represents the ram drag of the intake.The efficiency of an engine also depends on the speed at which the aircraft is traveling. The diagram on the next slide shows that although the ramjet, scramjet and rocket are able to operate at higher speeds, they are not very efficient, whereas the turbofan (turbojet) is much more efficient, but at lower speeds.

Engine Efficiency (cont.)

• Similarly, an engine can only be efficient when the exhaust is emitted at the same speed at the velocity of the jet itself. The formula for this propulsive efficiency is:

• Where c is the velocity of the exhaust gas and v is the velocity of the plane. The diagram on the next slideshows that as the ratio of c to v increases, the propulsive efficiency of the engine decreases.

Crude Oil Distillation

In Greek, petroleum (also known as crude oil) literally means 'rock oil'. It is a flammable liquid formed by the heating and compression of prehistoric algae and

zoo plankton. Crude oil is one of our main sources of energy. It is a mixture of organic compounds (especially hydrocarbons) of various weights. Petroleum can be separated into many useful products, including the main component of jet fuel,

kerosene, through fractional distillation. Fractional distillation is utilized to separate the different components of crude oil, including methane (which

contains only 1 carbon atom) to larger hydrocarbons (25+) used to make asphalt and tar. The principle behind refining oil is that molecules with longer carbon chains will have higher boiling points than molecules with very short carbon

chains. Below is a diagram of how oil is distilled:

Crude Oil Distillation

Crude Oil Distillation

Step 1: After being extracted from the earth, crude oil is sent to a refining plant, where it is heated to a very high temperature (often 600 degrees Celcius) and

injected into a vertical fractionating coulmn.

Step 2: The various components of crude oil form a vapor and rise in the column. The very largest molecules with very high boiling points remain at the bottom of

the coulmn and are removed with a pipe.

Step 3: As the different components of oil rise through the distillation coulmn, they cool until they are below their boiling points and become liquid. At this point,

they are directed away from the distillation coulmn. As you can see in the diagram above, the tower has various levels for collecting compounds that have boiling points anywhere from > 600 to < 20 degrees Celcius. Each different class

of hydrocarbon has different applications. Kerosene, the main component of many jet fuels, is generally made up of molecules with 10 to 16 carbons.