Published: Jan 2008
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HISTORY—UNTIL THE START OF WORLD WAR II, there were as many as 12 grades of aviation gasoline on the market, including three military and a number of proprietary specifications. However, distillation characteristics differed only slightly between the various grades, the main differences being in the octane requirements. To meet new performance requirements and to rationalize supplies during WW II, a single set of distillation requirements was adopted by the military specification AN-F-28, later MIL-G-5572. These same requirements were accepted for civil fuels in D910 in 1945 and are listed in Table 1. They have remained unchanged until the present. Because aviation gasoline is in very limited use in the U.S. military, Specification MIL-G-5572 was dropped in March 1988 and any military purchases of aviation gasoline are to specification D910. Performance—Aviation gasoline volatility is controlled by a combination of distillation and vapor pressure limits. (Vapor pressure limits are discussed later on in this chapter.) Because of the extreme operating requirements of aviation gasoline, the volatility range has to allow for fuel evaporation, i.e., engine starting, at very low temperatures, but at the same time protect against vapor lock at elevated fuel system temperatures as well as help prevent carburetor icing at low temperatures. Additional limitations can be placed on the fuel by high altitude conditions where very low ambient pressures in unpressurized aircraft tankage could lead to high evaporation losses. Because of major differences in the performance of specific engines and aircraft, an aircraft designer is forced to adjust the design to accommodate existing fuel specifications. Any new aircraft/engine system has to be tested over a wide range of operating conditions to assure safe operation. Ultimately, the combination of fuel volatility and specific aircraft/engine designs becomes a rather inflexible operating system with little permissible variability. To simplify world-wide usage, other international specifications have incorporated the same volatility limits. As in automotive engines, the front end distillation limits define the evaporability of the gasoline, with lower distillation limits representing more volatile fuel. However, because the same distillation limits apply to systems ranging from simple carburetors to complex injection systems in supercharged engines, clear-cut relationships between front end volatility and aircraft performance exist only for specific systems and do not apply across the aircraft/engine spectrum.
Strauss, Kurt H.
Consultant, Portland, ME