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Aviation Gasoline - Content Specifications

The density and heat of combustion of aviation gasoline is specified. However, other specification, such as distillation range, greatly limit any variability in these measurements. The freezing point of the gasoline is specified to prevent the formation of solid hydrocarbons during prolonged cold soak at altitude. The formation of solids would of course jeopardize fuel flow and prevent full fuel availability. Avgas must also be stable in storage and under a variety of conditions. A primary form of deterioration is the formation of "gum" through oxidation and polymerization of fuel molecules. These can deposit on fuel system components and cause serious problems. Therefore gum formation is strictly limited in the specifications. The addition of anti-oxidants is required and results in good to excellent storage stability. The sulfur content of avgas is limited to a very small amount. This is needed because sulfur can cause a deterioration in the anti-knock performance of the lead additive. Sulfur also contributes to corrosion of fuel system and engine components. Fuel corrosiveness is tested by a cooper strip corrosion test. Avgas is also tested for water reaction. In this test, samples of fuel and water are mixed and resulting changes are noted. This is used to detect the presence of high octane, and water soluble components such as alcohols in the fuel. These are not permitted and can be detected by both volume change and phase separation in the test sample.

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