Fuel Type

Two types of fuels are in general use worldwide in aircraft gas turbine engines. They are referred to as "wide cut fuels" and "kerosene-based fuels."

Both types of fuel are composed of a complex mixture of a range of individual hydrocarbon compounds. As the name suggests, the wide cut fuels have a wider range of hydrocarbons than kerosene-based fuels. The composition of the fuel is controlled, rather loosely, by the fuel specifications through defining limits on such factors as distillation range, density, flash temperature, heat of combustion, vapor pressure, freezing point, additives, and limits on certain compounds. Wide cut fuels are characterized by relatively low density, low freezing point (—65°F/— 53.9°C), high vapor pressure and low flash point (—45°F/—42.8°C). Kerosene-based fuels on the other hand are characterized by a relatively high density, higher freezing point (—40°F/—40°C), low vapor pressure, and high flash point (140°F/60°C). The United States designations for wide cut fuels for commercial and military uses are Jet B and JP-4 respectively. The designations for kerosene-based fuels are Jet A1 for commercial uses and JP-5 and JP-8 for military uses. Jet A1 predominates in production because of the huge demands of the airlines. Jet A1 is used in commercial service worldwide primarily for safety reasons because of its high flash point. Jet B is only used when its low freezing point is required; for example, in northern Canada in the winter. The U.S. Air Force previously used JP-4 as their standard fuel, but for availability and safety reasons have switched to JP-8, which is a military version of Jet A1. The navy uses JP-5 because of its high flash point for safety considerations on board aircraft carriers.

The knowledge of the true vapor pressure of the fuel used is necessary in the design and test evaluation of the centrifugal pump element. Because the fuels are a mixture of a range of hydrocarbons, the direct determination of the true vapor pressure is difficult. It is determined indirectly, for a desired temperature, through the Reid Vapor Pressure (RVP). The RVP is an average vapor pressure determined under a defined set of conditions, established by test at 100°F (38°C). The True Vapor Pressure (TVP) at this temperature is slightly higher than the RVP. Moreover, the TVP-versus-temperature relationship is determined by the RVP (see Reference 1).

Another key property of the fuel that directly influences the design and performance of airframe boost pumps is the solubility of air in the fuel. This is defined by Henry's law through a solubility coefficient. The ullage volume above the fuel in the aircraft tanks is generally vented to the ambient pressure. The maximum amount of dissolved air in the fuel will occur on the ground. The fuel will be in an air-saturated condition.

Any reduction in the pressure of the fuel will result in the release of this air in accordance with Henry's law and expansion of this air to a volume in accordance with Dalton's law of partial pressures. In addition, as the pressure is reduced, the vapor pressure of the light end hydrocarbon constituents of the fuel is reached. They too will vaporize, adding to the volume of vapor evolved. Reference 1 provides a detailed review of all of the properties of aircraft gas turbine engine fuels.

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