Aviation Fuels
`
`Technical Review
`
`Aviation Fuels Technical Review | Chevron Products Company
`
`Chevron Products Company
`6001 Bollinger Canyon Road
`San Ramon, CA 94583
`
`Chevron Products Company is a division of a wholly
`owned subsidiary of Chevron Corporation.
`
`http://www.chevron.com/productsservices/aviation/
`
`© 2007 Chevron U.S.A. Inc. All rights reserved.
`Chevron and the Caltex, Chevron and Texaco hallmarks are federally
`registered trademarks of Chevron Intellectual Property LLC.
`
`IDC 1114-099612
`
`Recycled/Recyclable paper
`Recycled/recyclable paper
`MS-9891 (11/14)
`
`GE-1024.001
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`

`
`Table of Contents Notes
`
`General Introduction ............................................................i
`
`1 • Aviation Turbine Fuel Introduction ........................... 1
`Types of Fuel
`Fuel Consumption
`
`2 • Aviation Turbine Fuel Performance ......................... 3
`Performance Properties
`Cleanliness
`Safety Properties
`Emissions
`
`3 • Aviation Turbine Fuel
`Specifications and Test Method ...............................14
`Specifications
`Test Methods
`
`4 • Aviation Turbine Fuel Composition ........................24
`Base Fuel
`Property/Composition Relationships
`Chemistry of Jet Fuel Instability
`Water in Jet Fuel
`Additives
`
`5 • Aviation Turbine Fuel Refining ................................33
`Raw Material
`Refining Processes
`The Modern Refinery
`
`6 • Aviation Turbine Engines .........................................38
`Principle of Operation
`Engine Types
`Combustion in the Engine
`
`7 • Aviation Gasoline Introduction .............................. 43
`Grades of Fuel
`Fuel Consumption
`
`8 • Aviation Gasoline Performance ............................ 45
`Performance Properties
`Cleanliness
`Safety Properties
`
`9 • Aviation Gasoline
`Specifications and Test Methods .......................... 54
`Specifications
`Future Fuels
`Test Methods
`
`10 • Aviation Gasoline Composition ............................. 63
`Composition
`Property/Composition Relationships
`Additives
`
`11 • Aviation Gasoline Refining ..................................... 66
`Alkylation
`Avgas Blending
`
`12 • Aircraft Piston Engines .......................................... 68
`Internal Combustion Engines
`Engine Development
`
`A • Aviation Fuel Distribution and Handling ............... 74
`Fuel Distribution System
`Receiving Fuel at Airports
`Contamination and Cleanup
`Quality Control
`
`B • About Hydrocarbons................................................. 85
`Hydrocarbon Chemistry
`
`Sources of More Information ....................................... 89
`
`Abbreviations ................................................................... 90
`
`Please note: This information is accurate as of fall 2004. It may be superseded by new regulations, specifications,
`or advances in fuel or engine technologies.
`
`Written, edited, and designed by employees and contractors of Chevron: Greg Hemighaus, Tracy Boval, John Bacha,
`Fred Barnes, Matt Franklin, Lew Gibbs, Nancy Hogue, Jacqueline Jones, David Lesnini, John Lind and Jack Morris.
`
`The authors would like to express their sincere thanks to Steve Casper (United Airlines), Cesar Gonzalez (consultant),
`Oren Hadaller (Boeing), Rick Moffett (Textron Lycoming), Roger Organ (Caltex), Jerry Scott (UVair), Stan Seto (GE Aircraft
`Engines), Rick Waite (Velcon Filters), and Ron Wilkinson (Electrosystems) for reviewing a draft version of this publication
`and making many helpful suggestions. Any remaining errors or omissions are the sole responsibility of the authors.
`
`Aviation Fuels Technical Review (FTR-3)
`
`© 2007 Chevron Corporation. All rights reserved.
`
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`General Introduction
`
`Man has long been captivated by the possibility of flight. From Icarus’ wings to DaVinci’s flying
`machines to lighter-than-air balloons, inventive minds worked to turn the dream into a reality.
`But what was lacking was a means of propulsion. This finally became available in the early years
`of the 20th century with the development of the internal combustion engine. This engine provided
`a compact and portable source of power that allowed man to overcome the pull of gravity.
`
`The early aircraft engines were similar to those used in automobiles and burned the same fuel.
`The need for increased power led to the development of specialized engines and aviation gasolines
`(avgas) tailored to their requirements. In the 1940s, the turbine engine emerged as the answer to
`the quest for still more power. In a replay of avgas development, kerosine – the fuel used in the
`first aircraft turbine engines – was eventually replaced by specialized aviation turbine fuels (jet fuels).
`
`In the last 90 years, aviation has grown from a novelty to an essential, even defining, element of
`modern society. It enables people and goods to move around the globe in hours, rather than the
`weeks or months that used to be required.
`
`Aviation is powered by petroleum fuels. This is not an accident; the choice is based on petro-
`leum’s recognized advantages. Liquid fuels have higher energy contents per unit volume than
`gases, and are easier to handle and distribute than solids. Among liquids, liquid hydrocarbons
`offer the best combination of energy content, availability, and price.
`
`This Review covers the performance, properties, specifications, composition, and manufacture of
`aviation fuels, both turbine fuel and aviation gasoline. Since engine and fuel are interdependent
`components of a single system, it also touches on engine basics. And it addresses the special
`precautions incorporated in the distribution system to ensure quality and cleanliness as fuel is
`moved from refinery to aircraft.
`
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`1 • Aviation Turbine Fuel Introduction
`
`Jet propulsion can be traced back to the 1st century B.C. when an Egyptian, Hero, is credited with
`inventing a toy that used jets of steam to spin a sphere. Sixteen centuries later, Leonardo da Vinci
`sketched a device that used a flux of hot gas to do mechanical work. By the 17th century, inventors
`were beginning to develop simple turbine systems to operate machinery.
`
`Figure 1.1
`Hero’s Toy
`
`Water contained in the sphere is heated and
`the steam escaping through the jets causes
`the sphere to turn in the opposite direction.
`
`The development of a turbine engine for aircraft began independently in Germany and Britain
`in the 1930s. In Germany, Hans von Ohain designed the engine that powered the first jet flight in
`1939. Germany deployed the jet-powered Messerschmitt 262 late in World War II.
`
`In Britain, Frank Whittle obtained a patent for a turbine engine in 1930. An aircraft powered by
`an engine he designed first flew in 1941. The first British jet fighter, the Gloster Meteor, also flew
`late in World War II.
`
`Types of Fuel
`
`Illuminating kerosine, produced for wick lamps, was used to fuel the first turbine engines. Since
`the engines were thought to be relatively insensitive to fuel properties, kerosine was chosen
`mainly because of availability; the war effort required every drop of gasoline.
`
`After World War II, the U.S. Air Force started using “wide-cut” fuel, which, essentially, is a hydro-
`carbon mixture spanning the gasoline and kerosine boiling ranges. Again, the choice was driven
`by considerations of availability: It was assumed that a wide-cut fuel would be available in larger
`volumes than either gasoline or kerosine alone, especially in time of war.
`
`However, compared to a kerosine-type fuel, wide-cut jet fuel was found to have operational
`disadvantages due to its higher volatility:
`
`• Greater losses due to evaporation at high altitudes.
`
`• Greater risk of fire during handling on the ground.
`
`• Crashes of planes fueled with wide-cut fuel were less survivable.
`
`So the Air Force started to change back to kerosine-type fuel in the 1970s and has essentially
`completed the process of converting from wide-cut (JP-4) to kerosine-type (JP-8) system-wide.
`The U.S. Navy has used a high flashpoint kerosine-type fuel (JP-5) on aircraft carriers because
`of safety considerations since the early 1950s. See Figure 3.1 for a list of U.S. military jet fuels.
`
`When the commercial jet industry was developing in the 1950s, kerosine-type fuel was chosen
`as having the best combinations of properties. Wide-cut jet fuel (Jet B) still is used in some parts
`of Canada and Alaska because it is suited to cold climates. But kerosine-type fuels – Jet A and
`Jet A-1 – predominate in the rest of the world.1
`
`1 The CIS and parts of Eastern Europe use a Russian fuel, TS-1, which is a light kerosine-type fuel.
`
`1
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`Jet A is used in the United States while most of the rest of the world uses Jet A-1. The important
`difference between the two fuels is that Jet A-1 has a lower maximum freezing point than Jet A
`(Jet A: –40°C, Jet A-1: –47°C). The lower freezing point makes Jet A-1 more suitable for long
`international flights, especially on polar routes during the winter.
`
`However, the lower freezing point comes at a price. Other variables being constant, a refinery can
`produce a few percent more Jet A than Jet A-1 because the higher freezing point allows the incorpo-
`ration of more higher boiling components, which in turn, permits the use of a broader distillation
`cut. The choice of Jet A for use in the United States is driven by concerns about fuel price and
`availability. Many years of experience have shown that Jet A is suitable for use in the United States.
`
`Fuel Consumption
`
`The consumption of jet fuel has more than doubled in the United States over the past 25 years,
`growing from 32 million gallons per day in 1974 to 72 million gallons per day in 2000, declining
`to 68 million gallons per day in 2002. Most of this growth has occurred since 1984.2
`
`Data for the worldwide use of jet fuel are available only for years after 1989 (see Figure 1.2). In
`2001, the most recent year for which data are available, consumption was 189 million gallons per
`day, up 20 percent from 1990. Figure 1.2 shows how this total is distributed around the globe.
`The United States is the largest single market, consuming about 37 percent of the worldwide total.
`
`Figure 1.2
`World Apparent Consumption of Jet Fuel
`
`Region
`
`North America
`
`Central and South America
`
`Western Europe
`
`1990
`
`69.2
`
`6.3
`
`26.7
`
`1991
`
`66.8
`
`6.3
`
`26.3
`
`Consumption, Million US Gallons/Day
`1992
`1993
`1994
`1995
`
`66.3
`
`6.0
`
`28.1
`
`66.7
`
`6.0
`
`29.1
`
`69.4
`
`7.0
`
`30.8
`
`69.0
`
`6.1
`
`32.4
`
`1996
`
`72.2
`
`7.4
`
`34.0
`
`11.4
`
`1997
`
`73.5
`
`7.7
`
`35.5
`
`10.1
`
`1998
`
`74.8
`
`7.7
`
`37.7
`
`11.8
`
`1999
`
`77.4
`
`5.3
`
`40.7
`
`11.0
`
`2000
`
`2001
`
`79.6
`
`8.5
`
`42.2
`
`11.3
`
`76.1
`
`9.5
`
`40.9
`
`11.4
`
`Eastern Europe and FSU
`
`24.1
`
`20.4
`
`18.4
`
`15.2
`
`12.4
`
`12.3
`
`Middle East
`
`Africa
`
`6.6
`
`4.6
`
`6.5
`
`4.8
`
`6.2
`
`5.1
`
`6.7
`
`5.0
`
`7.9
`
`5.2
`
`6.5
`
`5.2
`
`7.8
`
`4.9
`
`5.5
`
`5.7
`
`6.3
`
`5.0
`
`6.1
`
`5.7
`
`6.7
`
`5.6
`
`6.9
`
`6.2
`
`Far East and Oceania
`
`20.2
`
`21.1
`
`25.6
`
`28.9
`
`30.7
`
`36.0
`
`36.1
`
`38.7
`
`35.2
`
`36.6
`
`37.1
`
`38.2
`
`Total
`
`157.6
`
`152.2
`
`155.6
`
`157.5
`
`163.4
`
`167.5
`
`173.7
`
`176.7
`
`178.4
`
`182.8
`
`191.1
`
`189.1
`
`Source: U.S. Energy Information Administration
`
`2
`
`2 Energy Information Administration, U.S. Department of Energy.
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`2 • Aviation Turbine Fuel Performance
`
`Performance Properties
`
`Since the primary function of aviation turbine fuel (jet fuel) is to power an aircraft, energy
`content and combustion quality are key fuel performance properties. Other significant perform-
`ance properties are stability, lubricity, fluidity, volatility, non-corrosivity, and cleanliness. Besides
`providing a source of energy, fuel is also used as a hydraulic fluid in engine control systems and
`as a coolant for certain fuel system components.
`
`Energy Content
`An aircraft turbine engine generates power by converting chemical energy stored in the fuel into
`a combination of mechanical energy and heat (see Chapter 6). Since space is at a premium in most
`aircraft, the amount of energy contained in a given quantity of fuel is important.
`
`The energy content of jet fuel can be measured: It is the heat released (also called the heat of
`combustion) when a known quantity of fuel is burned under specific conditions. The amount
`of heat released depends on whether the water formed during combustion remains in the vapor
`phase or is condensed to a liquid. If the water is condensed to the liquid phase, giving up its heat
`of vaporization in the process, the energy released is called the gross energy content. The net
`energy content is lower because the water remains in the gaseous phase (water vapor). Since
`engines exhaust water as vapor, net energy content is the appropriate value for comparing fuels.
`
`Energy content can be expressed either gravimetrically (energy per unit mass of fuel) or volumet-
`rically (energy per unit volume of fuel). The International Metric (SI) units are megajoules per
`kilogram (MJ/kg) and megajoules per liter (MJ/L). In the United States, the gravimetric unit is
`British thermal units per pound (Btu/lb), and the volumetric unit is British thermal units per
`gallon (Btu/gal).
`
`Because the energy contents of individual hydrocarbons can differ, jet fuel composition has some
`effect on energy content (see page 25). The effect is usually predicted by fuel density, which is
`also a function of composition. Generally, less dense jet fuels have a higher gravimetric energy
`content, and more dense jet fuels have a higher volumetric energy content. This effect is more
`pronounced when different types of fuel are compared (see Figure 2.1).
`
`Figure 2.1
`Fuel Energy Content vs. Density
`
`Fuel
`
`Typical Density
`at 15°C (60°F)
`g/mL
`lb/U.S. gal
`
`Typical Energy Content
`Gravimetric
`Volumetric
`MJ/kg
`Btu/lb
`Btu/gal
`MJ/L
`
`Aviation Gasoline
`
`0.715
`
`5.97
`
`43.71
`
`18,800
`
`31.00
`
`112,500
`
`Jet Fuel:
`Wide-cut
`Kerosine
`
`0.762
`0.810
`
`6.36
`6.76
`
`43.54
`43.28
`
`18,720
`18,610
`
`33.18
`35.06
`
`119,000
`125,800
`
`Fuels differ in density, and therefore, in
`energy content per unit weight or unit
`volume. Less dense fuels, such as avgas,
`have a higher energy content per unit
`weight and a lower energy content per
`unit volume. The relationships are reversed
`for more dense fuels.
`
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`Which is preferred for aviation fuel, a higher density fuel with a higher volumetric energy content?
`Or a lower density fuel with a higher gravimetric energy content? The answer is obvious for
`aircraft that take off with their fuel tanks full, as most military aircraft do. A fuel with a high
`volumetric energy content maximizes the energy that can be stored in a fixed volume and thus
`provides the longest flight range.
`
`The answer is not so obvious for commercial airliners, most of which don’t fill their fuel tanks
`before each flight. Instead, they take on enough fuel to reach their intended destination, plus an
`adequate safety margin. In this situation, is it more advantageous to use a less dense fuel with a
`high gravimetric energy content to minimize fuel weight? Or does the increased range provided
`by the same volume of a more dense fuel with a high volumetric energy content offset the added
`weight? The relationship among these variables is complex, and beyond the scope of this Review.
`But, in most cases, it turns out that the answer is the same as for the “full tank” case: A more
`dense fuel with a high volumetric energy content is preferred.
`
`Jet fuel is a commodity product that is typically bought and sold by volume, with no price
`adjustment for density or energy content.
`
`Combustion Characteristics
`The principal difference between piston and jet engines is that combustion is intermittent in a
`piston engine and continuous in a jet engine. As a result, the engines have different fuel combus-
`tion quality requirements. In piston engines, combustion timing is critical to good performance.
`When combustion is continuous, combustion timing is no longer important.
`
`In a jet engine, small carbonaceous particles are formed early in the combustion process. These
`particles continue to burn as they pass through the flame and are completely consumed under
`suitable conditions. But these particles become incandescent under the high temperature and
`pressure conditions of the combustion section. Absorption of this infrared radiation by the
`combustor walls augments the normal heat received by heat transfer from the combustion gases.
`High combustor wall temperatures or hot spots can lead to cracks and premature engine failures.
`
`If these carbonaceous particles are not completely consumed by the flame, they can also be
`harmful if they impinge on turbine blades and stators, causing erosion. Carbon deposits can
`also plug the holes in the combustor wall that supply dilution air to the combustion section,
`disrupting the flow pattern of the combustion products.
`
`Fuels with high aromatics content, and especially fuels with high naphthalenes content, form
`more of these carbonaceous particles. Since these carbonaceous particles are potentially harmful,
`both the total aromatic content and the total naphthalenes content of jet fuel are controlled.
`
`Carbon particles that are not completely consumed are responsible for the visible smoke that
`some engines emit. Smoke formation is determined mainly by engine design and operating
`conditions, although for a given design, fuel composition can influence emissions. Better mixing
`
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`Chapter 2
`
`Aviation Turbine
`Fuel Performance
`
`of fuel and air results in more complete combustion and, thus, less carbon formation. Newer
`engines emit much less smoke because of design changes that improve mixing.
`
`Stability
`A stable fuel is one whose properties remain unchanged. Factors that can lead to deleterious
`changes in fuel properties include time (storage stability) and exposure to high temperatures
`in the engine (thermal stability).
`
`Jet fuel instability involves multi-step chemical reactions, some of which are oxidation reactions.
`Hydroperoxides and peroxides are the initial reaction products. These products remain dissolved
`in the fuel, but may attack and shorten the life of some fuel system elastomers. Additional
`reactions result in the formation of soluble gums and insoluble particulates. These products may
`clog fuel filters and deposit on the surfaces of aircraft fuel systems, restricting flow in small-
`diameter passageways.
`
`Storage Stability Instability of jet fuel during storage is generally not a problem because most fuel
`is used within weeks or months of its manufacture. Storage stability is an issue for the military,
`which often stores fuel for emergency use. And it can be an issue at small airports that don’t use a
`lot of fuel. Jet fuel that has been properly manufactured, stored, and handled should remain stable
`for at least one year. Jet fuel subjected to longer storage or to improper storage or handling should
`be tested to be sure it meets all applicable specification requirements before use.
`
`Because it is the more reactive fuel components that cause instability, storage stability is influ-
`enced by fuel composition. It is also influenced by storage conditions; instability reactions occur
`faster and to a greater extent at higher ambient temperatures. Antioxidants may be added to fuel
`to improve its storage stability (see page 31).
`
`Thermal Stability Thermal stability is one of the most important jet fuel properties because the
`fuel serves as a heat exchange medium in the engine and airframe. Jet fuel is used to remove heat
`from engine oil, hydraulic fluid, and air conditioning equipment. As noted above, the resulting
`heating of the fuel accelerates the reactions that lead to gum and particulate formation. These
`gums and particles may deposit:
`
`• On fuel filters, increasing the pressure drop across the filter and reducing fuel flow.
`
`• In fuel injector nozzles, disrupting the spray pattern, which may lead to hot spots in the
`combustion chamber.
`
`• In the main engine control, interfering with fuel flow and engine system control.
`
`• On heat exchangers, reducing heat transfer efficiency and fuel flow.
`
`These deposits may lead to operational problems and increased maintenance. Antioxidants that
`are used to improve fuel storage stability are not generally effective in improving thermal stability.
`
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`Engine problems related to inadequate fuel thermal stability typically become evident only after
`hundreds or thousands of hours of operation. The long time and the large volume of fuel
`consumed make it impractical to test fuel thermal stability under conditions identical to those
`that exist in engines. Instead, the fuel is subjected to more severe conditions in a bench test in
`order to be able to see a measurable effect in a reasonable period of time.
`
`Test equipment has been developed to pump fuel over a heated aluminum surface and then
`through a filter to collect any solid decomposition products. The equipment is intended to model
`two sensitive areas of an engine: the surface of a fuel-oil heat exchanger and a fuel injection nozzle.
`The first standardized apparatus (ASTM D 1660) was called the Coker. It has now been replaced
`by the Jet Fuel Thermal Oxidation Tester (JFTOT; pronounced jef´tot) (ASTM D 3241).
`
`Lubricity
`Lubricity is the ability to reduce friction between solid surfaces in relative motion, so it is a
`measure of a material’s effectiveness as a lubricant. Jet fuel must possess a certain degree of
`lubricity because jet engines rely on the fuel to lubricate some moving parts in fuel pumps and
`flow control units.
`
`The lubrication mechanism is a combination of hydrodynamic lubrication and boundary lubrica-
`tion. In hydrodynamic lubrication, a layer of the liquid lubricant prevents the opposing moving
`surfaces from contacting each other. Higher viscosity liquids provide more hydrodynamic lubrica-
`tion than lower viscosity liquids. While jet fuel specifications do not include an explicit lower
`limit on viscosity, the distillation specification serves as a surrogate limit. Jet engines are designed
`to work with jet fuels within the normal viscosity range, and therefore, typical jet fuels provide
`adequate hydrodynamic lubrication.
`
`When close tolerances squeeze out most of the liquid layer that provides hydrodynamic lubrica-
`tion, boundary lubrication becomes important. Now, small areas of the opposing surfaces are in
`contact. Boundary lubricants are compounds that form a protective anti-wear layer by adhering
`to the metal surfaces.
`
`Straight-run jet fuels (see page 34) are good boundary lubricants. This is not due to the hydrocar-
`bons that constitute the bulk of the fuel, but is attributed to trace amounts of certain oxygen-,
`nitrogen-, and sulfur-containing compounds. Evidence for the role of trace quantities is the fact
`that adding as little as 10 ppm of a lubricity enhancing additive to a poor lubricity fuel can make
`it acceptable.
`
`The naturally occurring compounds that provide jet fuel with its natural lubricity can be removed
`by hydrotreating – the refining process used to reduce sulfur and aromatic content (see page 35).
`However, low sulfur or aromatics levels in jet fuel are not, per se, signs of inadequate lubricity.
`The boundary lubricity of jet fuel cannot be predicted from bulk physical or chemical properties,
`it can only be measured in a specially designed test apparatus (see page 23). Fuels with similar
`sulfur and aromatics content can have different lubricity.
`
`6
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`Chapter 2
`
`Aviation Turbine
`Fuel Performance
`
`Coefficient of Thermal Expansion
`Liquids increase in volume as their temper-
`atures increase. The coefficient of thermal
`expansion is a measure of the rate of volume
`increase with temperature. A typical value
`for the coefficient of thermal expansion of
`kerosine-type jet fuel is 0.00099 per degree
`Celsius [(°C)–1] [(0.00055°F)–1]. At this rate,
`one gallon of jet fuel will expand 4.0 percent
`for an increase in temperature of 40°C [1.000
`gallon at 0°C (32°F): 1.040 gallon at 40°C
`(104°F)].
`
`Of course, the relationship is reversible. For
`example, as jet fuel cools in the aircraft’s
`tanks during flight, it will occupy a smaller
`volume than it did on the ground. The coef-
`ficient of thermal expansion can be used to
`calculate the volume decrease.
`
`Fluidity
`Obviously, jet fuel must be able to flow freely from fuel tanks in the wings to the engine through
`an aircraft’s fuel system. Fluidity is a general term that deals with the ability of a substance to flow,
`but it is not a defined physical property. Viscosity and freezing point are the physical properties
`used to quantitatively characterize the fluidity of jet fuel.
`
`Jet fuel is exposed to very low temperatures both at altitude – especially on polar routes in winter-
`time – and on the ground at locations subject to cold weather extremes. The fuel must retain its
`fluidity at these low temperatures or fuel flow to the engines will be reduced or even stop.
`
`Viscosity Viscosity is a measure of a liquid’s resistance to flow under pressure, generated either by
`gravity or a mechanical source. “Thin” liquids, like water or gasoline, have low viscosities; “thick”
`liquids, like maple syrup or motor oil, have higher viscosities. The viscosity of a liquid increases
`as its temperature decreases.
`
`Jet fuel at high pressure is injected into the combustion section of the turbine engine through
`nozzles. This system is designed to produce a fine spray of fuel droplets that evaporate quickly as
`they mix with air. The spray pattern and droplet size are influenced by fuel viscosity. If it is too
`high, an engine can be difficult to relight in flight. For this reason, jet fuel specifications place an
`upper limit on viscosity.
`
`Fuel viscosity influences the pressure drop in the fuel system lines. Higher viscosities result in
`higher line pressure drops, requiring the fuel pump to work harder to maintain a constant fuel
`flow rate. Fuel viscosity also influences the performance of the fuel system control unit.
`
`Freezing Point Because it is a mixture of many hundreds of individual hydrocarbons, each with
`its own freezing point, jet fuel does not become solid at one temperature the way water does. As
`the fuel is cooled, the hydrocarbon components with the highest freezing points solidify first,
`forming wax crystals. Further cooling causes hydrocarbons with lower freezing points to solidify.
`Thus, the fuel changes from a homogenous liquid, to a liquid containing a few hydrocarbon (wax)
`crystals, to a slush of fuel and hydrocarbon crystals, and, finally, to a near-solid block of hydro-
`carbons. The freezing point of jet fuel is defined as the temperature at which the last wax crystal
`melts, when warming a fuel that has previously been cooled until wax crystals form (see page 21).
`Thus the freezing point of fuel is well above the temperature at which it completely solidifies.
`
`The primary criterion for fuel system performance is pumpability – the ability to move fuel from
`the fuel tank to the engine. Pumpability is influenced both by fuel fluidity and fuel system design.
`In lieu of a fuel system flow simulation test, the industry uses freezing point as an indicator of a
`fuel’s low-temperature pumpability. Jet fuel typically remains pumpable approximately 4°C to
`15°C (8°F to 27°F) below its freezing point.1
`
`1 Aviation Fuels, Maxwell Smith.
`
`7
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`The U.S. Air Force is evaluating the use of additives that may prevent the formation of large wax
`crystals that are responsible for reduced fuel flow.
`
`Volatility
`Volatility is a fuel’s tendency to vaporize. Two physical properties are used to characterize fuel
`volatility: vapor pressure and distillation profile. A more volatile fuel has a higher vapor pressure
`and lower initial distillation temperatures.
`
`Volatility is important because a fuel must vaporize before it can burn. However, too high a
`volatility can result in evaporative losses or fuel system vapor lock.
`
`Volatility is one of the major differences between kerosine-type and wide-cut jet fuel. Kerosine-
`type jet fuel is relatively non-volatile. It has a Reid vapor pressure2 of about 1 kiloPascal (kPa)[0.14
`pound per square inch (psi)]. Wide-cut jet fuel has a Reid vapor pressure as high as 21 kPa (3 psi).
`
`Wide-cut jet fuel is better suited for cold weather applications because it has a lower viscosity
`and freezing point than kerosine-type jet fuel. In such applications, evaporative losses are less
`of a concern.
`
`Non-corrosivity
`Jet fuel contacts a variety of materials during distribution and use. It is essential that the fuel not
`corrode any of these materials, especially those in aircraft fuel systems. Typically, fuel tanks are
`aluminum, but fuel systems also contain steel and other metals. Fuel tanks may also have sealants
`or coatings, and elastomers are used in other sections of the fuel system. Engine and airframe
`manufacturers conduct extensive fuel compatibility testing before approving a material for fuel
`system use.
`
`Corrosive compounds potentially present in jet fuel include organic acids and mercaptans. The
`specifications limit these classes of compounds. By-products of microbial growth also can be
`corrosive (see Microbial Growth, page 9).
`
`Contamination from trace amounts of sodium, potassium, and other alkali metals in the fuel can
`cause corrosion in the turbine section of the engine.
`
`Cleanliness
`
`Fuel cleanliness means the absence of solid particulates and free water. Particulates – rust, dirt,
`etc. – can plug fuel filters and increase fuel pump wear. Water, in addition to not burning in an
`engine, will freeze at the low temperatures encountered in high altitude flights. The resulting ice
`may plug fuel filters and otherwise impede fuel flow. Water in the fuel also may facilitate the
`corrosion of some metals and the growth of microorganisms (see page 27 for a more detailed
`discussion of water in fuel).
`
`8
`
`2 Reid vapor pressure (RVP) is measured at 38°C (100°F).
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`Chapter 2
`
`Aviation Turbine
`Fuel Performance
`
`In addition to being clean, fuel delivered to aircraft must also be free from contaminants.
`The most common sources of contamination encountered with aviation fuels are discussed in
`Appendix A. These include product mixes, surfactants, microbes, and dyes.
`
`Microbial Growth
`Jet fuel is sterile when it is first produced because of the high refinery processing temperatures.
`But it quickly becomes contaminated with microorganisms that are always present in air and
`water. Microorganisms found in fuels include bacteria and fungi (yeasts and molds). The solids
`formed by microbial growth are very effective at plugging fuel filters. Some microorganisms also
`generate acidic by-products that can accelerate metal corrosion.
`
`Since most microorganisms need free water to grow, microbial growth is usually concentrated at
`the fuel-water interface, when one exists. Some organisms need air to grow (aerobic organisms),
`while others grow only in the absence of air (anaerobic organisms). In addition to food (fuel) and
`water, microorganisms also need certain elemental nutrients. Jet fuel can supply most of these;
`phosphorus is the only one whose concentration might be low enough to limit microbial growth.
`Higher ambient temperatures also favor microbial growth. Other factors affecting microbial
`growth and its control are discussed in ASTM D 6469, Standard Guide for Microbial Contamination
`in Fuel and Fuel Systems.3
`
`The best approach to microbial contamination is prevention. And the most important preventive
`step is keeping the amount of free water in fuel storage tanks and aircraft fuel tanks as low as possible.
`
`When microorganisms reach problem levels in aircraft fuel tanks, approved biocides may be used
`under controlled conditions. But biocides have their limits. A biocide may not work if a heavy
`biofilm has accumulated on the surface of the tank or other equipment, because then it doesn’t
`reach the organisms living deep within the biofilm. In such cases, the tank must be drained and
`mechanically cleaned.
`
`And even if the biocide effectively stops microbial growth, it still may be necessary to remove the
`accumulated biomass to avoid filter plugging. Since biocides are toxic, any water bottoms that
`contain biocides must be disposed of appropriately.
`
`Safety Properties
`
`Jet fuel can be hazardous if not handled properly. First, and foremost, it is easy to ignite and it burns
`rapidly. Second, exposure to jet fuel liquid or vapor should be limited. Anyone planning to handle
`jet fuel should obtain and