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Requirements

An engine fuel must be tailored to an engine and vice versa since there must be enough quantities of fuel available to the engine. Some significant properties of aviation fuels are discussed below.

Heat Energy Content or Net Heating Value. The energy content or heating value of a fuel is expressed in heat units (British thermal units [BTUs]). A fuel satisfactory for aircraft engines must have a high heat energy content per unit weight. A high heat energy content causes the weight of fuel carried to be lower than a low heat energy content. Then more of the load-carrying capacity is available for the payload Aviation gasoline and JP fuels are very desirable from this standpoint. The heat energy content for aviation gasoline is about 18,700 BTUs/pound, and for JP fuels about 18,200 BTUs/pound. The various alcohols, which have maximum energy content of about 12,000 BTUs/pound, do possess some other desirable characteristics as an internal combustion engine fuel.

Volatility. A volatile liquid is one capable of readily changing from a liquid to a vapor when heated or when contacting a gas into which it can evaporate. Since liquid fuels must be in a vaporous state to burn volatility is an important property to consider when choosing a suitable fuel for an aircraft engine. Volatility determines the starting accelerating vapor-locking and distribution characteristics of the fuel. Gasoline and JP fuels are very satisfactory because they can be blended during the refining process to give the desired characteristics. Because of the nature of constant pressure combustion in gas turbine engines a highly volatile fuel is not necessary. JP fuels are of rather low volatility while aviation gasoline is highly volatile. Comparing a highly volatile fuel like aviation gasoline to a less volatile one like JP fuel the following effects become apparent. The highly volatile fuel--
Starts easier in cold temperatures.
Has a slightly better combustion efficiency.
Leaves less deposit in the combustion chamber and on the turbine blades.
Is a greater fire hazard.
Creates a greater danger of vapor lock of the fuel system.
Has high evaporation losses through the breather of the fuel tank at high altitudes.

NOTE: The last two difficulties are practically nonexistent with fuels having low volatility.

Stability. The fuels used in aircraft engines must be stable. Because aviation fuels are sometimes stored for long periods, they must not deposit sediment. The gums that are normally formed are insoluble in gasoline and JP fuels and may cause restrictions in fuel strainers and liners. Aviation fuel must also retain its original properties during storage.

Purity. Aviation fuel must be free from water, dirt, and sulfur. Small amounts of water will not usually cause any difficulty because water can be removed from the fuel system by draining. Large amounts of this impurity, however, can cause complete engine failure. It is very important that corrosive sulfur be eliminated from fuel. The sulfur content of fuel may form corrosive acids when brought in contact with the water vapor formed in the combustion process.

Flash Point. The flash point is the lowest temperature at which fuel will vaporize enough to form a combustible mixture of fuel vapor and air above the fuel. It is found by heating a quantity of fuel in a special container while passing a flame above the liquid to ignite the vapor. A distinct hash of flame occurs when the flash point temperature has been reached.

Fire Point. The fire point is the temperature which must be reached before enough vapors will rise to produce a continuous flame above the liquid fuel. It is obtained in much the same manner as the flash point.

Reid Vapor Pressure. Reid vapor pressure is the approximate vapor pressure exerted by a fuel when heated to 100%. This is important because it is used to determine when a fuel will create a vapor lock.

Specific Gravity. Specific gravity is the ratio of the density (weight) of a substance (fuel) compared to that of an equal amount of water at 60o F. Specific gravity is expressed in terms of degrees API. Pure water has a specific gravity of 10. Liquids heavier than water have a number less than 10. Liquids lighter than water have a number greater than 10. An example is JP-4, whose specific gravity in degrees API is 57. The American Petroleum Institute (API) has chosen pure water by which to measure the specific gravity of fuels.

NOTE: Both flash and fire points give a relative measure of the safety properties of fuel a high flash point denotes that a high temperature must be reached before dangerous handling conditions are encountered. The minimum flash point permitted in a fuel is usually written into the specifications.
Grades

Turbine fuels are high-quality fuels covering the general heavy gasoline and kerosene boding range. They do not contain dyes or tetraethyl lead.

One of the major differences between the wideboiling and kerosene types is the fuel volatility. JP-4 fuels have a wider boding range with an initial boiling point considerably below that of kerosene. As a group these fuels have lower specific gravities than kerosene types. Wide-boil-range fuels have Reid vapor pressures of 2 to 3 pounds and flash points below room temperature. Kerosene-type fuels have Reid vapor pressures of less than 0.5 pound and flash points higher than l00o F (38o C). Wide-boiling-range fuels generally have lower freezing points than kerosene fuels.

The fuel authorized for Army aircraft gas turbine engines is JP-4. The letters "JP" stand for jet propulsion; the number 4 indicates fuel grade.

Military specification MIL-T-5624 covers JP-4, JP-5, and JP-8 fuels. Jet A, Jet Al, and Jet B are commercial fuels which conform to the American Society for Tinting Materials specification ASTM-D-1655.

Jet B is a JP-4 type fuel; its freezing point is -56° F (-49o C) instead of -72o F (-58o C) for JP-4.

JP-5, Jet 4 and Jet A-1 are kerosene-type fuels. ASTM Jet A and A-1 differ primarily in their fuel freezing points. Jet A is considered suitable down to fuel temperatures of -36o F (-38o C); Jet A-1, to -54o F (48o C).

JP-4 is a fuel consisting of approximately 65 percent gasoline and 35 percent light petroleum distillate, with rigidly specified properties. JP-4 is currently the Army standard fuel for turbine engines.

JP-5 is a specially refined kerosene having a minimum flash point of 140o F and a freezing point of -51o F (-46° C).

JP-8 is a specially refined kerosene with a minimum flash point of 110o F and a freezing point of -54° F (-48° C). This fuel is being classified as a total replacement fuel for all of NATO. It will replace all fuels currently used in military equipment from generators to tanks to aircraft and even to trucks. This classification will ease logistics in combat. Having only one fuel for all equipment prevents accidentally mixing or using the wrong fuels. To date testing of JP-8 is proceeding well with the total single-fuel concept on its way to full fielding.

JP fuels vary from water white to light yellow; color coding, however, does not apply to these fuels.

Additives in JP fuels include oxidation and corrosion inhibitors, metal deactivators, and icing inhibitors. Icing inhibitors also function as biocides to kill microbes in aircraft fuel systems.

Should mixing of JP fuels become necessary, there is no need to drain the aircraft fuel system before adding the new fuel. Due to the different specific gravities of these fuels, mixing them will affect the turbine engine's performance. Be sure to consult appropriate technical manuals for additional information and procedures.

When changing to a fuel with a different specific gravity, externally adjusted fuel controls and fuel flow dividers on some engines may require retrimming or readjustment for optimum performance.

Regards
Fernando

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