Petroleum Fuels, I
Petroleum fuels have a number of characteristics and properties of interest to the user. Easy ignition is one of them and they all have a common composition, they consist of hydrocarbon (hydrogen and carbon) molecules and some small amounts of additives.
A basic understanding of aviation fuels is required as the pilot in command is responsible for the safe operation of the aircraft and this includes the refueling process and safe handling of these volatile products.
Petroleum fuel characteristics
Petroleum fuels should ignite and burn easily and release their energy completely. Which is a great deal compared to their weight. These fuels are made of hydrocarbons and refined from crude oil through fractional distillation and characterized by their boiling points and molecular weights. Natural gas consists of light hydrocarbon methane C1, ethane C2, propane C3, and butane C4.
Gasoline contains hydrocarbons in the range of C4-C10, kerosene and aviation fuels in C4-C19 range and diesel fuels can be found in the C8-C21 range. Other more heavier hydrocarbons are heating oils and diesel fuel #2 (C15-C22) and lubrication and hydraulic fluids are in the range of C20-C60.
Our well know fuels are combined of a number of different hydrocarbon compounds formulated to meet specific properties for an application. Volatility is one such property. It affects its ability to vaporize and form a combustible mixture with air in the engine. Lighter fuels as gasoline and AVgas are more volatile than JET or diesel fuels at the same temperature and pressure.
Refined fuels contain a number of impurities like gums, metals, microbial growth, sediment, sulfur and water through condensation. Gums are formed by oxidization or polymerization of hydrocarbon molecules when exposed to air or sunlight. Metals are introduced during refining and can contribute to fuel filter clogging. A metal deactivator additive is used to combat this.
As fuel is sterilized when it leaves the refinery, microbial growth occurs after fuel comes into contact with air and water. The microbes feed on fuel and when given the change and time they produce odorous slime clogging fuel filters. Minimizing water content and biocide additives are needed. Sediment is just rust, dirt, minerals, sand and such impurities. The use of fuel filters should address this.
Sulfur compounds can act corrosive to metals in the fuel systems of aircraft. Limiting the sulfur contents in fuels like diesel and JET will help alleviate this.
Gasoline fuels are rated with a octane number and this is an indication of their resistance against detonation (knocking). The higher the rating the more resistant. A high rating does not imply more power but that the fuel can be used in higher compression or turbo supercharged engines.
Relates to a liquids boiling point and its tendency to evaporate at normal ambient temperatures and pressures. Of all fuels used in aviation Mogas is highly volatile followed by AVgas and JET. This property is desirable in carburettors as we need an combustible air/fuel mixture to run the engine with. Mogas and AVgas vapors are some three times heavier than air and sink to lower areas.
Indicates how readily a fuel will start to burn, very helpful in an engine combustion chamber after the spark plug creates a spark.