Aviation gasoline is one of the most restrictive fuels produced in a refinery. Quality control parameters have been gradually added and refined from the early specifications of the past century to give a highly developed performance fuel where production must meet stringent aviation requirements to ensure a high level of quality control, cleanliness, and traceabil- ity from refinery to aircraft. Strict process control is required to ensure that the stringent (and sometimes conflicting) requirements are met for antiknock ratings, volatility, and calo- rific values. Careful handling is essential during storage and distribution to guard against various forms of contamination.
Aviation gasoline consists substantially of hydrocarbons.
Sulfur- and oxygen-containing impurities are strictly limited by the specifications, and only certain additives are permit- ted. (Refer to the section on “Aviation Fuel Additives.”) The main component of high-octane aviation gasoline is isooc- tane produced in the alkylation process by reacting refinery butanes with isobutene over acid catalysts. To meet mini- mum volatility requirements of the final blend, a small pro- portion of isopentane (obtained by the superfractionation of light straight-run gasoline) is added. The aromatic compo- nent required to improve the rich rating is usually a catalytic reformate consisting primarily of toluene. The amount of aromatic component is limited by the high gravimetric calo- rific value (specific energy) requirement and the distillation end point. The low freezing point is the specification param- eter that excludes benzene. All blending components must have high-octane values.
Only the low-octane grade can include a proportion of straight-run gasoline, because such gasolines that contain various amounts of paraffins, naphthenes, and aromatics lack the necessary branched paraffins (isoparaffins) required to produce a high-octane fuel.
Specifications CONTENT
Aviation gasoline specifications generally cover composition and chemical and physical tests. The composition section
1In preparation of this chapter, the contents of the sixth and seventh editions were drawn upon. The author acknowledges the authors of the sixth edition, Geoffrey J. Bishop of Shell International Petroleum Company, London, UK and Cyrus P. Henry, Jr., of DuPont Company, Deep- water, NJ and author of the seventh edition Kurt H. Strauss retired. The current edition will review and update the topics as addressed by the previous authors, introduce new technology that has been developed, and include up-to-date references.
2Air BP, Warrenville, IL 80
stipulates that the fuel must consist entirely of hydrocarbons, except trace amounts of specified additives including tetra- ethyl lead antiknock additive, oxidation inhibitors, and con- ductivity improvers. Nonhydrocarbon blending components, such as oxygenates, are not permitted. The chemical and phys- ical test section is the one most familiar to users, because it carefully defines the allowable limits for the properties as well as the test methods to measure and control these properties.
FUEL GRADES
As many as six grades were in use up to the end of World War II. In the following decade, the turbine engine became the pri- mary military engine with commercial aviation following shortly afterward. This increase in turbine engines caused decreased AVGAS demand, which led to the elimination of both the high- and low-octane grades. This was also facilitated by the fact that only the octane requirement and the permitted tetraethyl ethyl-lead (TEL) content differed between the vari- ous grades. Fewer grades allowed the reduction of manufactur- ing, storage, and handling costs with subsequent benefits to the consumer. (At many commercial airports, the AVGAS tanks were converted to jet storage.) Although three grades—80, 91, 100, and 100LL—are listed in the ASTM Specification for Avia- tion Gasoline (D910), only the 100LL grade is predominant in the United States and much of the rest of the world.
Various bodies have drawn up specifications covering the various grades. The most commonly quoted specifica- tions are issued by ASTM (D910) and the British Ministry of Defence (DefStan 91/90). Table 1 lists grades in former and in current use and indicates their identifying colors and present status.
Due to the international nature of aviation activities, the technical requirements of Western specifications are virtu- ally identical, and only differences of a minor nature exist between the specifications issued in the major countries.
Russian GOST (gosudarstvennyy standard or state standard later changed to Gosstandart) specifications differ in the grades covered and in respect to some of the limits applied, but in general the same properties are used and most test
methods are basically similar to their Western equivalents [ASTM and Institute of Petroleum (IP) standards]. Russian aviation gasoline grades are summarized in Table 2.
Table 3 provides the detailed requirements for aviation gasoline contained in the ASTM Specification for Aviation Gas- oline (D910). In general, the main technical requirements of all other Western specifications are virtually identical to those in Table 3, although differences can occur in the number of per- mitted grades and the amount of maximum permitted TEL content. Within the specification, the various grades differ only in certain vital respects such as color, antiknock rating, and TEL content. The two remaining grades in the GOST specifica- tion are subdivided into a regular and a premium grade with differing limits for aromatics, olefins, sulfur, and acidity.
The limits for Western aviation gasoline were, in most cases, originally dictated by military aircraft engine require- ments. Since then, the performance requirements for civil and military engines have changed very little. However, improved manufacturing techniques and the reduced demand for cer- tain grades have allowed fuel suppliers to produce modified fuel grades more suitable to the market. The primary result of this trend has been the 100LL grade, which is certified for both low- and high-output piston engines.
Characteristics and Requirements ANTIKNOCK PROPERTIES
The various grades are classified by their “antiknock” charac- teristics measured in special laboratory engines.
TABLE 1—Aviation Gasolines, Main International Specification Grades, and Current Specifications
Identifying Color
Nominal Antiknock Characteristics
Lean/Rich NATO Code Number
DefStan 91/90 British Ministry of
Defence ASTM D910 Use
Colorless 73 F13A . . . . . . Obsolete
Colorless 80 . . . . . . . . . Obsolete
Red 80/87 F-12A 80 80 Minor civil
Purple 82 . . . . . . 82ULB New engine fuel
Blue 91/96 F-15A . . . . . . Obsolete
Blue 100/130 F-18A 100LL 100LL Major civil
Green 100/130 . . . 100 100 Minor civil/military
Brown 91/98 . . . 91 91 Minor civil
Brown 108/135 . . . . . . . . . Obsolete
Purple 115/145 F-22A . . . . . . Military—obsolete
AObsolete designation.
BASTM Specification D6227.
TABLE 2—Russian Aviation Gasoline Grades
Specification Grade Color Use
Tu 38.10913-82 B70 Colorless Obsolete
GOST-1012 B91/115A Green Current
GOST-1012 95/130 Yellow Current
AOf regular quality.
TABLE 3—Detailed Requirements for Aviation Gasolines ASTM Specification D910A
Requirement Grade 80 Grade 91 Grade 100LL Grade 100
Knock value, lean mixture
Motor octane number min 80.7 90.8 99.6 99.6
Aviation lean rating min 80.0 91.0 100.0 100.0
Knock value, rich mixture
Octane number min 87 98 . . . . . .
Performance number min . . . . . . 130.0 130.0
Tetraethyl-lead, mL
TEL/L max 0.13 0.53 0.53 1.06
gPb/L max 0.14 0.56 0.56 1.12
Color dye content
Blue dye, mg/L max 0.2 3.1 2.7 2.7
Yellow dye, mg/L max None None None 2.8
Red dye, mg/L max 2.3 2.7 None None
Orange dye, mg/L max None 6.0 None None
Requirements for All Grades
Density at 15C, kg/m3 Report
Distillation
Initial boiling point,C Report
Fuel evaporated
10 volume percent atC max 75
40 volume percent atC min 75
50 volume percent atC max 105
90 volume percent atC max 135
Final boiling point,C max 170
Sum of 10 % þ 50 % evaporated temperatures,C min 135
Recovery volume percent min 97
Residue volume percent max 1.5
Loss volume percent max 1.5
Vapor pressure, 38C, kPa min 38.0
max 49.0
Freezing point,C, max –58
Sulfur, mass percent max 0.05
Net heat of combustion, MJ/kg min 43.5
Corrosion, copper strip, 2 h @ 100C max No. 1
Oxidation stability 5 h aging (16 h aging)
Potential gum, mg/100 mL max 6 (max 10)
Lead precipitate, mg/100 mL max 3 (max 4)
(Continued)
Knock, or detonation, is a form of abnormal combustion where the air/fuel charge in the cylinder ignites spontane- ously in a localized area instead of being consumed by the spark-initiated flame front. Knocking combustion can damage the engine and cause serious power loss if allowed to persist.
For a gasoline engine to work effectively, the fuel must ignite at the correct moment and burn smoothly delivering power to the piston. The various grades were designed to guarantee knock-free operation for engines ranging from those used in light aircraft to those in high-powered transports and military aircraft. The fact that higher-octane fuels than those required for an engine can be used without problems has been a major factor in the historical elimination of several grades.
The first aviation gasoline specification written by the U.S.
Navy on September 1, 1917, was to standardized the fuel. The fuel was highly volatile with a low distillation end point and lacked performance parameters. By 1922, it was recognized that performance parameters were necessary to regulate vapor lock and carburetor icing, and methods were needed to quan- tify fuel combustion quality to control knock. Various methods were developed to quantify fuel combustion quality such as the
“toluene scale,” but industry harmonized on the “octane scale”
to prevent disputes over quality, and by 1929 the Cooperative Fuels Research (CFR) engine was developed. Antiknock ratings of aviation gasolines are determined in single-cylinder ASTM laboratory engines by matching a fuel’s knock resistance against reference blends of pure isooctane (2,2,4-trimethylpen- tane), assigned an octane rating of 100, andn-heptane with a rating of 0. A fuel’s rating is given as an octane number (ON), which is the percentage of isooctane in the matching reference blend. Fuels of higher antiknock performance than pure isooc- tane are rated against isooctane containing various percentages of TEL additive. The ratings of such fuels are expressed as per- formance numbers (PN), which are defined as the percentage of maximum knock-free power output obtained from the fuel compared to the power obtained from unleaded isooctane.
Two different engine methods are used to rate a fuel. Early on, knock was detected under cruise conditions where the fuel portion of the mixture was decreased as much as possible to improve efficiency. This condition, known as the lean or weak mixture method, is measured by the ASTM Test for Knock Char- acteristics of Motor and Aviation Fuels by the Motor Method (D2700/IP 236). Knocking conditions are obtained by increasing engine compression ratio under constant conditions in the engine described by this method. At the beginning of World War II, newly designed, high-power-output, supercharged engines also were found to knock under engine takeoff conditions.
Here, mixture strength is increased (richened) with the addi- tional fuel acting as a coolant. This suppresses knocking com- bustion and results in higher power output, until ultimately knock occurs under these conditions also. To duplicate these conditions, a different single-cylinder engine was developed
from a Pegasus radial engine with supercharging and variable fuel/air ratio. ASTM Test for Knock Ratings of Aviation Fuels by the Supercharge Method (D909/IP 119) produces the resulting
“rich or supercharged” rating.
Until 1975, ASTM Specification D910 designated aviation gasoline grades with two ratings, such as 100/130, in which the first number was the lean and the second number the rich rating.
Although the specification now uses only one number (the lean rating) to designate a grade, some other specifications use both.
However, both ratings are required to meet the specification.
It is important to note that the operating conditions of both laboratory engines were developed to match the knock performance of full-scale engines in service during the World War II period. Since then, considerable engine devel- opment has taken place in the smaller in-line engines, so that the relationship between current full-scale and labora- tory engines may be different from that which paced the original laboratory engine development. As a result, the Fed- eral Aviation Administration (FAA) is conducting an exten- sive program of rating the knock resistance of current production engines to reestablish the relationship with the laboratory engines. Other work has also indicated that mod- ern, in-line piston engines are not knock-limited under take- off conditions, compared to the older, larger radial engines.
VOLATILITY
All internal combustion engine fuels must be convertible from the liquid phase in storage to the vapor phase in the engine to allow the formation of the combustible air/fuel vapor mixture, because liquid fuels must evaporate to burn.
If gasoline volatility is too low, liquid fuel enters the cylinders and washes the lubricating oil off the walls. This increases engine wear and causes dilution of the crankcase oil. Low vol- atility can also give rise to critical maldistribution of mixture strength between cylinders. Too high a volatility causes fuel to vaporize too early in the fuel compartments and distribution lines, giving undue venting losses and possible fuel starvation through “vapor lock” in the fuel lines. The cooling effect due to rapid evaporation of highly volatile material can also cause carburetor icing, which is due to moisture in the air freezing on the carburetor under certain conditions of humidity and temperature. Many modern engines, therefore, have anti-icing devices on the engines, including carburetor heating.
Volatility is measured and controlled by the gasoline dis- tillation and vapor pressure. Distillation characteristics are determined with a procedure (ASTM D86/IP 123) in which a fuel sample is distilled and the vapor temperature is recorded for the percentage of evaporated or distilled fuel throughout the boiling range. The following distillation points are selected to control volatility for the reasons indicated.
1. The percentage evaporated at 75C (167F) controls the most volatile components in the gasoline. Not less than
TABLE 3—Detailed Requirements for Aviation Gasolines ASTM Specification D910A (Continued)
Requirements for All Grades Water reaction
Volume change, mL max 62
Electrical conductivity, pS/m max 450
AFor additional requirements contained in specification footnotes, refer to Table l in ASTM D910.
10 % but no more than 40 % must evaporate at that temperature. The minimum value ensures that volatility is adequate for normal cold starting. The maximum value is intended to prevent vapor lock, fuel system vent losses, and carburetor icing.
2. The requirement that at least 50 % of the fuel be evapo- rated at 105C (221F) ensures that the fuel has even distillation properties and does not consist of only low boiling and high boiling components (“dumb-bell” fuel).
This provides control over the rate of engine warm-up and stabilization at slow running conditions.
3. The requirement that the sum of the 10 % plus the 50 % evaporated temperatures exceed 135C (307F) also con- trols the overall volatility and indirectly places a lower limit on the 50 % point. This clause is another safeguard against excessive fuel volatility.
4. The requirement that a minimum of 90 % of the fuel be evaporated at 135C (275F) controls the portion of less volatile fuel components and, therefore, the amount of unvaporized fuel passing through the engine manifold into the cylinders. The limit is a compromise between ideal fuel distribution characteristics and commercial considerations of fuel availability, which could be adversely affected by further restrictions on this limit.
5. The final distillation limit of 170C (338F) maximum limits undesirable heavy materials, which could cause maldistri- bution, crankcase oil dilution, and in some cases combus- tion chamber deposits.
All spark ignition fuels have a significant vapor pres- sure, which is another measure of the evaporation tendency of the more volatile fuel components. Additionally, when an aircraft climbs rapidly to high altitudes, the atmospheric pressure above the fuel is reduced and may become lower than the vapor pressure of the fuel at that temperature. In such cases, the fuel will boil and considerably more quanti- ties of fuel will escape through the tank vents.
Vapor pressure for aviation gasoline is controlled and determined by any of three methods, consisting of ASTM D323/
IP 69, Test for Vapor Pressure of Petroleum Products (Reid Method), ASTM D5190, Test for Vapor Pressure of Petroleum Products (Automatic Method), and D5191/IP 394, Test for Vapor Pressure of Petroleum Products (Mini Method). In case of disputes, D5191 is designated the referee method. Allowable limits are between 38 and 49 kPa (5.5–7.1 psi). The lower limit is an additional check on adequate volatility for engine starting, while the upper limit controls excessive vapor formation dur- ing high-altitude flight and “weathering” losses in storage.
A review of the aviation gasoline specification reveals that volatility, unlike that for motor gasoline, contains no adjustments for differing climatic condition but is uniform and unchanging wherever the product is used.
DENSITY AND SPECIFIC ENERGY
No great variation in either density or specific energy occurs in modern aviation gasolines because these properties depend on hydrocarbon composition, which is already controlled by other specification properties. However, the specific energy require- ment limits the aromatic content of the gasoline. Both proper- ties have greater importance for jet fuels, as discussed later.
FREEZING POINT
Maximum freezing point values are set for all aviation fuels as a guide to the lowest temperature at which the fuel can
be used without risking the separation of solidified hydrocar- bons. Such separation could lead to fuel starvation through clogged fuel lines or filters, or loss in available fuel load due to retention of solidified fuel in aircraft tanks. The low freez- ing point requirement also virtually precludes the presence of benzene, which, while a high-octane material, has a very high freezing point.
The standard freezing point test involves cooling the fuel until crystals form throughout the fuel and then rewarming the fuel and calling the temperature at which all crystals disappear the freezing point. The freezing point, therefore, is the lowest temperature at which the fuel exists as a single phase. Freezing points are determined by ASTM Test for Freezing Point of Aviation Fuels (D2386/IP 16).
STORAGE STABILITY
Aviation fuel must retain its required properties for long periods of storage in all kinds of climates. Unstable fuels oxi- dize and form polymeric oxidation products that remain as a resinous material or “gum” on induction manifolds, carbu- retors, valves, etc., when the fuel is evaporated. Formation of this undesirable gum must be strictly limited and is assessed by the existent and accelerated (or potential) gum tests.
The existent gum value is the amount of gum actually present in fuel at the time of the test. It is determined by the ASTM Test for Existent Gum in Fuels by Jet Evaporation (D381/IP 131). The potential gum test, ASTM Test for Oxida- tion Stability of Aviation Fuels (Potential Residue Method) (D873/IP 138), predicts the possibility of gum formation dur- ing protracted storage.
To ensure that the strict limits of the stability specifica- tion are met, aviation gasoline components are given special refinery treatments to remove the trace impurities responsi- ble for instability. In addition, controlled amounts of oxida- tion inhibitors are normally added. Currently, little trouble is experienced with gum formation or degradation of the anti- knock additive.
SULFUR CONTENT
Total sulfur content of aviation gasoline is limited to 0.05 % mass maximum, because most sulfur compounds have a del- eterious effect on the antiknock effect of alkyl lead com- pounds. If sulfur content were not limited, specified antiknock values would not be reached for highly leaded grades of aviation gasoline. The sulfur content is measured by ASTM Test for Sulfur in Petroleum Products (Lamp Method) (D1266/IP 107) or by ASTM Test for Sulfur in Petroleum Products by X-Ray Spectrometry (D2622/IP 447).
Some sulfur compounds can have a corroding action on the various metals in the engine system. Effects vary according to the chemical type of sulfur compound present.
Elemental sulfur and hydrogen sulfide are particularly impli- cated. Because copper is considered the most sensitive metal, fuel corrosivity toward copper is measured in ASTM Test for Detection of Copper Corrosion from Petroleum Products by the Copper Strip Test (D130/IP 154).
WATER REACTION
The original intent of the water reaction test was to prevent the addition of high-octane, water-soluble compounds, such as alcohol, to aviation gasoline. The test method involves shaking 80 mL of fuel with 20 mL of buffered water under standard conditions and observing phase volume changes.