Additives are the chemical compounds added just to enhance, maintain and preserve the properties of fuel.
 

7.1 ADDITIVES FOR AVIATION GASOLINE:

Small amounts of certain chemicals reduce the tendency of gasoline to knock i.e improves octane rating of the gasolines. The action of these compounds is to react with those activated oxygenated intermediate compounds whose decomposition products lead to knocking combustion. Of the large number of additive experimented with the most effective in this field has been tetraethyl lead (TEL) and about 2ml per gallon are currently added to gasolines. Larger amounts upto 4ml / gallon, are used in the American Continent and in parts of Europe. The increase in octane number resulting from the addition of a given quantity of TEL, turned the lead susceptibility, is not uniform but depends on the composition of the gasoline. Sulphur compounds should be reduced to a minimum since these reduce the lead susceptibility.

As the quantity of TEL/TML added to a gasoline is increased, its effect on the octane number decreases and addition of more than 6ml/gallon has very little effect. Moreover, on combustion these lead compounds form a non volatile oxides which is deposited in the combustion chamber, on the exhaust valves and sparking plugs, and contributtors to the generally undesirable effects of combustion chamber deposits. To counteract this, "Scavenger" components such as ethylene dibromide and ethylene dichloride are added to the TEL/TML.

These compounds convert the lead, on combustion, in to the relatively volatile lead bromide and lead chloride which pass out with the exhaust gas. The TEL/TML together with the scavengers and a dye to indicate the poisonous nature of the compounds are known as ethyl or methyl fluids. Other additives both metallic and non metallic are some times use in conjunction with the lead compounds as "Co-anti knock agents"

The efficiency of TEL/TML depends on the composition of the fuel to which they are added. Increasing octane number with out lead by the above mentioned refining methods is an expensive business, and it becomes a question of economics to what extent high octane compounds or lead compounds are used to obtain the required quality.

7.2 ADDITIVES FOR JET FUELS.

• Anti-static Additives:

 
The fuelling of air-craft gives rise to a fire or exclusion hazard from ignition by sparks due to static electrical charges. Static electricity is generated by liquids in motion at the interface between the liquid and another phase which may be solid, liquid or even gaseous. The faster the liquid flows the more it become electrified. The hazard has thus increased as the fuelling rates of air craft have increase and consequently increasing attention has been given to the fuelling of air-craft, especially large turbine powered craft, which have much greater fuel requirement yet demand shorter, run-round times. In addition to the usual precautions of coupling all equipment and hoses to provide a continuous electrical conductor, it is now possible to reduce the hazard by incorporating "anti static additives" in the fuel, which even at one part per million or less are very effective in preventing the buildup of static electricity by increasing the conductivity of the fuel.
 
• Corrosion Additives:

 

Fuel for Jet engine must be free from acidic materials and corrosive sulphur compounds to prevent corrosion of metal equipment and there is a rigid specification to ensure this. Although combustion of the fuel inevitably produces sulphur gases little or no effect on the combustion chamber or turbine blades is experience provide the sulphur content of the fuel is with in 0.4% by weight. Corrosion inhibitor/additives are therefore added to jet fuels in an amount of 3mg/lit of fuel to supress corrosion.
 
 

• ANTIOXIDANT:

Ionol, ditertiary butyl methyl phenol and alkylation – Carboxylation additives are extensively used as antioxidant. These additives are added in an amount 20 mg/lit of the fuel.
 

• ANTI-ICING ADDITIVES:

Another potential hazard of low temperature operation is the freezing of entrained water to form ice particles which could clog filters and reduce fuel flow. Jet fuel does not normally come into contact with water, but if this should happen by some miss-chance it is desirable that the water should separate cleanly from the fuel, and not become emulsified in it. It is therefore a specification requirement that the Jet fuel should be free from water. Anti-icing additives are therefore added to jet fuels in order to prevent icing of the fuel.
 

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