*       Tetraethyl Lead

     This was first introduced during WW1 in order to improve the octane rating of cheaper grades, however, the lead clogs modern catalytic converters within a few minutes, in addition, lead is toxic and so it was subsequently banned although it is still utilised today by aircraft.

Tetraethyl lead

*       Methyl Tertiary Butyl (MTBE)

MTBE is a volatile, flammable and colourless liquid at room temperature and dissolves easily in water. MTBE began to be introduced in gasoline after 1979 and more significantly after the Clean Air Act of 1990. In addition to raising the octane level (by reducing 'knocking'), MTBE is an oxygenate which adds oxygen during combustion thus reducing the amount of unburnt hydrocarbons therefore improving the efficiency of the reaction and reducing harmful emisions. The extra oxygen released by MTBE displaces other components of gasoline including aromatics and sulfur in addition to optimising the oxidation during combustion.

 

However, MTBE is thought to be potentially carcinogenic and so can present problems with the possible contamination of groundwater, petrochemical companies have therefore begun the implementation of ethanol rather than MTBE since this is not a cancer threat despite being more expensive.

*       Polymer Additives:   

Recently (August 2000), General Technology Applications, the developer of this research, have claimed that the addition of the polymer polyisobutylene is more effective than current oxygenates with 20% increases in gas mileage and 70% reductions in pollutants such as carbon monoxide and nitrogen oxide. Polyisobutylene is formed by the cationic vinyl polymerisation using isobutylene starting material as can be seen above.

 

Scientists found that as gasoline enters the combustion cylinders, the smaller hydrocarbons begin to separate from the larger ones and so they burn faster, leaving the heavier, longer chains potentially unburnt. However, the addition of the polymer has been claimed to change the physical properties of the gasoline thus reducing the separation of the hydrocarbons

A monomer of isobutylene                       Polyisobutylene  

*       Other additives:
Corrosion Inhibitors Protect ferrous metals from corrosion
Demulsifiers Improves water shedding
Dyes Allows identification of octane level
Metal deactivators Prevents combustion catalysation by fuel system metals such as copper
Oxidation inhibitors Prevents formation of gum upon oxidation of reactive compounds
Anti-Icing Prevents formation of ice in the fuel system
Deposit control Limits deposits of by-products/pollutants in combustion chamber

              Infomation from Chemistry In Context, 5th ed, Hill & Holman