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Hydrocarbon molecules

Petroleum Fuels, Properties

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 fuels.


Petroleum fuel characteristics

Crude Oil

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.

Hydrocarbons

Gasoline contains hydrocarbons in the range of C4-C10, kerosine 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 combustable 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.

Impurities

Refined fuels contain a number of impurities like gums, metals, microbial growth, sendiment, 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.

Fuel microbes

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. Sendiment is just rust, dirt, minerals, sand and such impurities. 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.

Octane

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.

Volatility

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 desireable in carburettors as we need an combustible air-fuel mixture to run the engine with. Mogas and AVgas vapours are some three times heavier than air and sink to lower areas.

Flammability

Indicates how readily a fuel will start to burn, very handy in an engine combustion chamber after the spark plug creates a spark.

Fuel additives

Additives are used to reduce fuel consumption, lower the cost of maintenance, reduce emissions and deposits. Fuels are formulated with an additive package depending on the application. The precise content of the package is usual proprietary to the manufacturer, think Shell V-Power or BP Ultimate. Additives are usually approved by the American Society of Testing and Materials (ASTM) or military standards (MIL).

Fuel Additives

Some commonly used fuel additives are:

  • Alkyl lead is used in gasoline to rase its octane rating for use in higher compression engines to reduce the risk of knock. Mogas used to contain about 1 gram per liter of alkyl lead (1000 ppm). Unleaded mogas now contains only a few ppm lead. AVgas still contains lead.
  • Anti-oxidants are used to prevent gum in gasolines and aviatin fuels.
  • Biocide additives are used to control microbial growth in the fuel system.
  • Conductivity additives are used to increase the conductivity of the fuels reducing the buildup of static electricity during transfer.
  • Corrosion inhibitors will gaurd against corrosion during transfer and storage of the fuel. Some improve lubricity which reduces the friction of the movings parts in a fuel pump or injector.
  • Detergent additives will like anti-oxidants help against gum, keeping fuel filter clean. Usually found in mogas and diesel fuels.
  • Icing inhibitors mainly for aviation fuels preventing ice crystal formation during flight at high altitudes. Diethylene glycol monomethyl ether is used for this.
  • Metal deactivators will prevent metal contaminants from oxidizing with hydrocarbons and clogging the system.
  • Oxygenates boost the octane rating of mogas, therby reducing the smog forming tendencies and supresses knock. Also, the increased oxygen in the fuel provides for a more complete combusting. Ethanol is used for this.
  • Thermal stability additives mainly used in JET fuel to stabilize the fuel in hot environments preventing carbon build up in engine nozzles, burners etc.

Some Terms

Below some commonly used terms and their explanations as used in the fuel and petroleum industry.

  • Alcohol - Organic compound with an oxygen hydrogen (OH) group bonded to a hydrocarbon group.
  • Biodegration - The reduction in quality of the petroleum by naturally residing microorganisms.
  • Boiling Point - Temperature at which a liquid changes state to a vapor or gas.
  • Crude Oil - The oil as found in the crust of the earth.
  • Gasoline - Refined product for use in spark ignited engines.
  • Hydrocarbons - Organic compounds composed of hydrogen and carbon. Petroleum products are complex combinations of hydrocarbons.
  • Kerosine - Colorless, low sulfur, low smoke producing petroleum product mainly used in JET fuels.
  • Petroleum - Naturally occuring, oily, flamable liquid made of hydrocarbons and found with gas in underground reservoirs, oil wells.
  • Vapor Pressure - The pressure at which a liquid and its vapor are in equilibrium at a given temperature. The more volatile a liquid and the lower its boiling point, the higher its vapor pressure is.
  • Viscosity - The resistance of a fluid to flow. The more viscous a liquid, the more it will resist to flow. It usually decreases with increasing temperature of the liquid.
  • Volatility - Indicates how quickly a substance forms a vapor at a defined temperature and pressure.



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