Volatility & Vapour Pressure

Aviation gasoline must be a fuel that is easily converted from its liquid form to a vapor to allow the formation of a combustible air/fuel mixture. If it is not volatile enough, liquid fuel will wash cylinder walls and pistons causing increased wear and crankcase oil dilution. The fuel is also not distributed well amongst the cylinders in carbureted engines. On the other hand, too light of a fuel can cause vapor lock, increased carburetor icing and excessive venting losses. In specifications for aviation gasoline, distillation ranges are given. These specify at what temperature a certain percentage of a sample is evaporated. Initially, between 10 and 40 percent of the fuel must evaporate by 167 F. The 10% requirement ensures sufficient volatility for cold weather starting while the 40% maximum restrains problems with vapor lock and carburetor icing. A 50%, or mid-point, is specified to ensure the fuel consists in an even mixture of components and not combinations of light and heavy materials only. At the upper end of distillation, 90% of the sample must evaporate by 275 F. This helps ensure that lower volatility components are held in proportion. Lower temperatures could be specified, however, a too restrictive specification could result in lower product availability. The sample must be fully evaporated by 338 F. This precludes the inclusion of very heavy materials that would adversely affect performance and contribute to motor oil dilution. This is also used to check fad samples for contamination by distillate fuel such as diesel or jet fuels. The vapor pressure measures the fuels tendency to form vapors over the liquid fuel. The vapor pressure must be high enough to allow adequate vapor formation for starting However, when an aircraft proceeds to higher altitudes, it is possible to lower the pressure over the liquid fuel to less than the vapor pressure and cause the fuel to vapor off. Avgas must have a vapor pressure of between 5.5 to 7.0 psi. This provides the fuel with adequate vapor pressure for starting while maintaining suitability for high altitude flight.

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