Glossary

Acid

Any substance containing hydrogen in combination with a non-metallic element(s) and capable of producing hydrogen ions in solution. An acid is capable of neutralising or being neutralised by a base.

Acidity

In lubricants, acidity denotes the presence of acidic constituents whose concentration is usually defined in terms of an acid number.

Additives

A chemical compound or compounds added to a lubricating oil for the purpose of imparting new properties or enhancing existing properties.

Aniline point

The lowest temperature at which a hydrocarbon fluid is completely miscible with an equal quantity of aniline. The higher the reading, the lower the aromatic content, and hence the smaller the effect on rubber.

antifoam agent

An additive used to suppress the foaming tendency of petroleum products in service. May be a silicone oil to break up surface bubbles or a polymer to decrease the number of small entrained bubbles.

antiwear agent

Additives or their reaction products which form thin, tenacious films on highly loaded parts to prevent metal-to-metal contact.

API gravity

An arbitrary scale adopted by the American Petroleum Institute for expressing the relative density of an oil.

Degrees API =(141.5/rel.density@60°F) - 131.5

aromatics

Hydrocarbons of ring structure having the smallest hydrogen to carbon ratio.

ash

Some additives, particularly conventional detergent additives, leave behind a powdery residue after combustion. This residue is known as ash and can cause engine malfunction if allowed to build up in the combustion chamber, cylinder liner ports and turbochargers.

ash (sulphated)

The ash content of an oil, determined by charring the oil and breaking the residue with sulphuric acid and evaporating to dryness. Expressed as % by mass.

asphaltenes

Components of asphalt which are insoluble in petroleum naphtha but are soluble in aromatic solvents. They are hard and brittle and made up largely of high molecular weight polynuclear hydrocarbon derivatives containing carbon, hydrogen, sulphur, nitrogen, oxygen and usually nickel, iron and vanadium.

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