Additives

Positive performance additives are the sophistications of gasoline, designed to enhance the characteristics of the basic components. Additives are a subject in themselves, ranging from necessary elements to others that are little more than refinements. For instance, one additive to motor gasoline in New Zealand is a dye—a different dye for each grade of petrol.

This provides grade identification and also fulfils an international requirement that all "leaded" products be dyed.

Gasolines are "leaded" by the introduction of another additive, organic lead compounds. These chemicals, usually in the form of tetra-ethyl-lead or tetra-methyl-lead, raise the octane rating of straight-run refinery gasoline, and increase power and economy. Tetra-methyl-lead has the advantage of being a light and volatile lead alkyl, and improves the anti-knock quality of the base gasoline over the whole range of temperatures at which the different components of the gasoline boil.

Generally speaking, the addition of lead compounds gives gasoline improved performance characteristics, and a greater degree of flexibility in operation.

The only drawback to using lead compounds to improve gasolines is the fact that they combine with oxygen to form ash residues on combustion chamber surfaces. Under normal conditions, operating temperatures never get hot enough to burn these deposits off, so another additive is put into leaded gasolines to prevent ash deposits from accumulating.

These additives are called "scavengers." which react with the lead deposits to form chemical compounds which can be burnt off within the heat range of the combustion chamber. The obvious benefits include improved engine operation and longer service life.

These are the most important additives. Others include antioxidants, anti-rust compounds, carburettor detergents, pre-ignition and plug-fouling entrols, and the anti-icing compounds mentioned earlier.

Gasoline, the bread and butter product of the oil industry, is remarkable stuff. Without it, the transport revolution that has taken only 60 short years to reach its present form of sophistication would never have been possible.