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Question: Degradation of an airport’s aging fuel tanks is suspected, resulting in both rust and water contamination of the aviation fuel. Discuss the impacts of this scenario on the operation of aircraft fueled at this airport.
Hint: Review “Chemistry of Fuel” in the Fuel Contaminants section
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Research/references material/information Provided:
Chemistry of Fuel:
Aviation fuel is an application of organic chemistry, which is a sub-branch of chemistry devoted to the study of the properties and reactions of substances containing carbon. Aviation fuel is composed of hydrocarbons, molecules that contain only carbon and hydrogen.
The two major types of aviation fuel, jet fuel and avgas, are petroleum-based. There are 3 common commercial jet fuel types used in turbine engines: A, A-1 (unleaded kerosene), and B (naptha-kerosene blend). Jet B is ideal for cold-weather performance. The primary difference between Jet A and Jet A-1 is the addition of an antistatic additive to Jet A-1.
Avgas is a very pure gasoline used in piston engines. The most common Avgas is 100LL. Engine knock is minimized by using the additive tetraethyl lead. This is the same additive that is banned from automotive fuels in the U.S., China, and the EU. We will talk about leaded aviation fuel in detail in Module 9.
So the two commercial aviation fuel categories are gasoline and kerosene. Where do they come from? Aviation fuel is produced from crude oil at oil refineries. Crude oil is refined through fractional distillation. This process uses the different boiling points of the components of the mixture to separate the mixture. The lighter the molecules, the more volatile they are and, thus, they have a lower boiling point.
Operational Properties of Fuels
Understanding the chemical and physical properties of the fuel and fuel additives is critical to performance, safety, and environmental impact. Physical properties of matter can be observed without changing the nature of the substance. Conversely, observation of chemical properties fundamentally alters the substance being observed.
Physical Properties of Fuels
The chemical composition of aviation fuel is quite complex, containing hundreds or thousands of chemicals. The primary component is the hydrocarbon fuel fraction. Jet fuels contain four major categories of hydrocarbons: paraffin, olefin, naphthene, and aromatic. Fuel additive composition varies by purpose. Kerosene fuels (Jet A and A-1) have between 4 and 19 carbon atoms per molecule while Naptha fuels (Jet B) have between 5-15 carbon atoms per molecule. Gasoline fuels typically contain hydrocarbons with 4 to 10 carbon atoms per molecule.
Paraffin is a single-bonded saturated carbon chain (straight chain or branched). Naphthene (cycloparaffin) is a saturated ring. The ring structure decreases the H:C ratio but increases the density and lowers the freezing point. Olefins are unsaturated rings so the H:C is even lower. These compounds are very reactive and can react to form gums. Aromatics are fully unsaturated ring compounds. They tend to generate smoke when burned.
Density is a derived unit expressed as mass over volume. When discussing fuels, density is commonly reported as fuel density with the units kg/m3. Fuel density typically increases with the increasing molecular weight of the fuel molecules.
Fuel density is critical to making it from Airport A to Airport B. Density is temperature-dependent, which is why values are reported at a given temperature. As temperature increases, the volume of a liquid expands. What does this do to the density value? Pilots must use a volume correction factor.
At 15°C, Avgas density is 0.69 kg/L while Jet A is 0.82 kg/L and Jet A-1 is 0.80 kg/L. The standard density of jet fuel used in calculations is 0.8 kg/L.
Volatility is a liquid’s ability to enter the gas phase, quantified as vapor pressure. This is a critical property for fuel. Fuel must vaporize and mix in the appropriate ratio with air in order to combust. The more volatile a fuel, the easier the engine will start. This is important at low temperatures because temperature affects the evaporation rate. Two drawbacks to high volatility, though, are the tendency to vapor lock and the increased fire risk during a crash.
Vapor density is measured by comparing the relative weight of a gas to air. Air has been assigned an arbitrary value of 1. A gas with a vapor density above 1 will sink in air. Jet fuel has a vapor density of 5.7. Avgas also has a vapor density above 1.
This high vapor density is a health and safety risk. Because vapors sink in air, they can accumulate with the risk of ignition and – in certain scenarios – explosion. Exposure to vapors can result in asphyxiation or negative health effects from inhalation including a headache, dizziness, vomiting, and loss of consciousness. Avgas and Jet A/A-1 are potential carcinogens OSHA’s Safety Data Sheet (PDF). (Links to an external site.)
Viscosity is the resistance of a liquid to flow. The fuel should be able to adequately flow through the fuel system at the lowest temperatures the engine will experience. This characteristic significantly influences fuel injection nozzle performance, influencing fuel drop size of aspirated fuel, spray range, and spray angle.
The freezing point of a fuel is the temperature at which solid particles start to form in the fuel. Because fuels are a mixture of multiple hydrocarbons, there is not a single temperature at which the entire mixture will freeze. When solid particles form, they are typically held in suspension in the fuel. In other words, they do not quickly settle to the bottom of the fuel tank due to gravity. These solid particles are highly problematic for fuel systems as they can block filters and injectors.
Jet A freezes at -47°C while Jet A-1 freezes at -40°C. Avgas freezes at -58°C.
Chemical Properties of Fuels
Aviation fuels are flammable – they ignite easily and readily burn. Jet fuel has a higher flash point than avgas, requiring a higher ignition temperature.
The flashpoint is defined as the temperature at which a sufficient quantity of fuel has vaporized, allowing ignition from an outside heat source. The fire point is the temperature at which the vapor will continue to burn without an outside heat source. The autoignition temperature is the point at which the vapor will ignite without an outside heat source.
Jet A and A-1 have a flashpoint of approximately 38°C, while the flashpoint of Avgas is approximately -37°C. The auto-ignition temperature of jet fuel is 210°C while Avgas is 440°C. Which fuel has a higher fire risk from spark or flame at normal temperatures? Which fuel has a higher fire risk from high temperatures in the absence of a spark or flame? The flashpoint of jet fuel is controlled by the amount of naphtha in the mixture.
Smoke Point and Combustion Products
Recall that a higher aromatic fraction of the fuel will result in a smokier flame. A fuel that tends to produce a lot of smoke will have a low smoke point value. Low smoke point fuels have a lower quality of combustion. Incomplete combustion can result in solid products (e.g., soot) that can deposit in the engine. A buildup of these combustion products reduces engine efficiency.
Heat Energy Content
The energy content of a fuel is the amount of heat produced by burning a given amount of the substance. A common derived unit for reporting the energy content of aviation fuels is MJ/kg (megajoules per kilogram). The energy content of Avgas is ~44.5 MJ/kg while Jet A/A-1 are ~43 MJ/kg.
A high heat energy content is ideal for aviation fuels. The energy content and density are critical considerations for aircraft range or cargo weight (payload) considerations. The heat energy content is dependent upon the hydrocarbon composition. Recall that paraffin is a single-bonded saturated carbon chain. The high H:C ratio results in a high heat release per unit weight.
Aviation fuel must be very pure. Contaminants can significantly impact the efficiency and safety of aviation fuels. Water is a common contaminant as free water, dissolved water, or water-in-fuel emulsions. Because water will freeze before the hydrocarbons in the fuel, water contamination is problematic in cold temperatures.
When water is present, biological growth can occur. Microbes include algae, bacteria, and fungi that feed on the fuel through chemosynthesis. Microbial growth can foul the fuel quantity probes, with understandably significant consequences. The microbes can also increase fuel viscosity, making fuel controls sluggish. Some microbes release byproducts that are corrosive and can damage the fuel system in aircraft.
Particulate matter can also contaminate aviation fuel. Particles can include sediments like sand, dust, or metal compounds including rust. Particulates can increase fuel viscosity and block fuel filters and nozzles. Metals can undergo oxidation-reduction reactions and contribute to gum formation.
Sulfur is corrosive to some metals. Fuel quality strictly limits sulfur content.
Gums can form when fuel is oxidized or polymerized upon exposure to air. light, or high temperatures. Gums can clog the fuel system, reducing engine performance.
A fuel additive has a maximum concentration of 5% of volume. Additives serve many purposes, many of which are aimed at the primary fuel contaminants.
Metal deactivators prevent metals from contaminating the fuel from oxidizing with the fuel hydrocarbons and other compounds, reducing the formation of gum and other precipitates. Detergents prevent gum buildup and deposits in fuel tanks and engines.
Fuel system icing inhibitors (FSIIs) prevent ice crystal formation during cold temperatures commonly occurring at high altitudes. Some FSIIs are also biocides. Biocides kill microbes. Static electricity reducers (static dissipators) increase the conductivity of fuel, reducing the buildup of a static electric charge. Corrosion inhibitors prevent corrosion of fuel infrastructure (e.g. storage tanks).
Oxygenates are hydrocarbons with oxygen, which increases complete combustion. This boosts the octane rating of the fuel, reduces emissions, and reduces engine knock.
Jet fuel is subjected to very high temperatures. Thermal stability additives prevent degradation of the fuel that could foul jet engine parts like fuel nozzles and manifolds, possibly causing damage or flameouts (extinction of flame in the engine due to fuel starvation, stall, incorrect oxygen ratio, etc.).
In summary, additives can improve fuel efficiency, lower maintenance costs, improve engine safety, reduce aircraft emissions, and reduce fuel impurities.
Emerging Fuel Technologies
Biofuels are produced using the biomass-to-liquid method. These fuels can often be used without any modifications to the aircraft and are cleaner than conventional fuels, releasing fewer greenhouse gases and other pollutants. We will discuss biofuels and aircraft emissions in detail in Module 9. Unfortunately, there are significant barriers to widespread use, including politics, economics, and infrastructure.
Liquid natural gas (LNG) and compressed natural gas (CNG) are promising fuels but they cannot be used in aircraft without modification. Boeing theorized an aircraft able to run on LNG in their article How sweet the future of aviation (Boeing). (Links to an external site.)
Now that we have a basic understanding of the chemistry of fuel, we can explore how fuel is burned to generate the energy required for flight. Different aircraft use different propulsion devices, whether it is an internal combustion (piston engine) or a jet engine.
Watch the following:
The Combustion Reaction
The burning of fuel is a combustion reaction. Combustion is a unique type of oxidation-reduction reaction where a fuel reacts with oxygen. Aviation fuels are almost always liquid and the oxygen source (oxidizer) is usually a gas (atmospheric oxygen) for airplanes.
Converting Chemical Potential Energy to Kinetic Energy
Chemical bonds store chemical potential energy. Chemical reactions involve the breaking and forming of chemical bonds. In combustion reactions, the energy stored in the bonds of the products is much lower than the energy that was stored in the bonds of the fuel. According to the Law of Conservation of Energy, this energy is not destroyed. This excess energy is released as heat, referred to as the heat of combustion.
Gasoline contains 43.75 megajoules per kg. Jet fuel contains 43.90 megajoules per kg. This value is determined using calorimetry where a specific amount of fuel undergoes complete combustion in well-defined conditions of temperature and pressure. Specific energy is the energy per unit mass of fuel, while energy density is the energy per unit volume of fuel. Are the values reported above for gasoline and jet fuel-specific energy or energy density?
Heat is released in the combustion reaction, but heat is also needed to start the reaction. This is why fuel does not undergo combustion while stored in a fuel tank.
Once the combustion process is started, it provides enough heat to continue the process as long as adequate fuel and oxygen are present.
Combustion Basics Self-Check:
Physics of Propulsion
So we’ve learned the chemistry of fuels and fuel combustion. Now how do we take that kinetic energy released from fuel and make a plane go? Engines!
Before we dig in, it is helpful to have a basic understanding of the gas laws.
So What is Thrust?
The four forces that act on an airplane in flight are thrust, drag, lift, and weight (gravity). Thrust is what makes airplanes go. An aircraft’s thrust must overcome drag, a mechanical force due to the difference in velocity between a solid object and a liquid or gaseous fluid, in order to accelerate. Both thrust and drag are mechanical forces and thus a vector quantity (having both a magnitude and direction).
Read the following:
- What is thrust? (NASA) (Links to an external site.)
- General Thrust Equation (NASA) (Links to an external site.)
If you want to brush up on the basics of Newton’s Laws, watch this video:
Internal Combustion Engine & Propellers
Internal combustion engines operate by the movement of a piston back and forth (reciprocating) inside of a cylinder. Valves open and close to draw in a fuel-air mixture and release exhaust. The piston is connected to a shaft with a rod that converts the motion of the piston to a rotational motion. Internal combustion engines work on the principle of the ideal gas law.
During the intake stroke of the piston, the fuel-air mixture is drawn into the engine as the piston moves down, increasing volume. At the bottom of the intake stroke, the intake valve closes. The compression stroke begins when the piston moves upward, decreasing the volume of the gas through compression to achieve high pressure. At the top of the compression stroke, a spark ignites the fuel-air mixture. Recall that combustion releases heat energy. Both pressure and volume increase with increased temperature (which gas laws are these?) The increased pressure pushes the piston back down on the power stroke, thus reducing the temperature by the increase in volume and decrease in pressure. This is called the power stroke because this stroke produces the mechanical work that rotates the crankshaft. At the bottom of the power stroke, the exhaust valve opens, allowing the pressure to equalize between the piston and exhaust manifold. The piston moves upward in this exhaust stroke, pushing exhaust out of the cylinder to return to intake conditions.
One of the problems with this design is that power is only generated a fraction of the time (one out of four steps in the engine cycle). The power generated by a piston is correlated to the size of the cylinder and the distance the piston can move. This limits the aircraft performance, including speed and lift.
Now that the internal combustion engine has generated power to turn a crankshaft, how does the airplane go? Just like aircraft wings, propellers are airfoils that generate an aerodynamic force. While wings generate lift, propellers generate thrust. You can get into some truly interesting physics when discussing flow velocity relative to the propeller. We’ll keep it simple for this course!
Propellers have radial airfoil blades assembled to rotate on a longitudinal axis. A small blade pitch performs well against resistance but does not generate much thrust. For this reason, many propellers have a variable pitch that can be adjusted according to changes in aircraft speed and engine performance.
Keep in mind that the tip of the propeller is moving faster than at the hub. The propeller acts like a rotating wing so we can apply the same concepts from airfoil theory for wings. When in motion, the pressure behind the propeller is higher than the free stream of the fluid and pressure ahead of the propeller is lower than the free stream of fluid. The propeller does work on the fluid particles, increasing their velocity. Bernoulli’s equation can be applied to the air in front and behind the propeller.
The number and shape of blades is also an important consideration. The thrust generated is dependent upon the blade area. Propellers typically have 2 to 6 blades, but a propeller with more blades is not necessarily better; blades too close to each other can cause interference effects. The thrust (F) generated is equal to the pressure change (ΔP) times the propeller area (A).
Jet Turbine Engine
The 1-sentence summary of a jet engine is that it takes in air at a given velocity, combusts fuel inside the duct, and then ejects the air and exhaust at an even higher velocity. A jet engine combines all four steps from internal combustion engines into a simultaneous process, resulting in maximum power.
Air is sucked into the engine through a fan. This is no small amount of air. A commercial jet engine takes in 1.2 tons of air per second during takeoff. Some air bypasses the engine core and some air is directed through the core where it enters the compressor.
In some engine models, the compressor also serves as the fan. In the compressor, the air is squeezed to increase its pressure, which also increases its temperature to ~200-500°C. Some engines can compress the air to achieve pressures 40 times that of the air initially entering the compressor! To achieve this, compressor blades must spin fast – 1,000 mph fast. To achieve the pressurization, successive fans get gradually smaller, as does the cavity. Recall what happens to pressure when volume decreases.
The pressurized air then enters the combustion chamber where jet fuel is aspirated in the presence of an electric spark to ignite the fuel-air mixture. Recall that combustion releases gases and heat energy. Heat-resistant materials are critical to this engine component as temperatures can exceed 1700∘C. The temperature of the gases in the combustion chamber will ultimately determine thrust.
As the gases leave the combustion chamber, they spin turbine blades. As the turbine gains energy, the gases lose some energy by decreasing temperature and pressure. The turbine blades are attached to the compressor shaft. This means that some of the energy from the gases exiting the engine is used to compress the gases entering the engine. Exhaust gases at the turbine are still between 850 – 1700°C.
The high temperature (and thus high energy) exhaust gases are then released through a tapering nozzle on the back of the engine. Think of how water moves through a pipe of decreasing diameter accelerates – like a water gun. The same concept applies here. The tapered design accelerates the gas speed to over 2100 km/h! The speed of the air leaving the engine is double the speed of the air entering the engine. The jet engine changes the momentum of gas particles by increasing its velocity. The force of the gases shooting out of the engine generates thrust according to Newton’s 3rd law.
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