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ACID - hydrogen containing compound that reacts with metals to form salts, and with metallic oxides and bases to form a salt and water. The strength of an acid depends on the extent to which its molecules ionize, or dissociate, in water, and on the resulting concentration of hydrogen ions (H+) in solution. Petroleum hydrocarbons, in the presence of oxygen and heat, can oxidize to form weak acids, which attack metals. See corrosion.

ACID NUMBER - See neutralization number.

ACID TREATING - refining process for improving the color, odor, and other properties of white oils or lube stocks, whereby the unfinished product is contacted with sulfuric acid to remove the less stable hydrocarbon molecules.

ADDITIVE - chemical substance added to petroleum product to impart or improve certain properties. Common petroleum product additives are: anti-foam agent, anti-icing additive, anti-wear additive, corrosion inhibitor, demulsifier, detergent, dispersant, emulsifier, EP additive, oiliness agents, oxidation inhibitor, pour point depressant, rust inhibitor, tackiness agent, viscosity improver.

ADHESIVE WEAR (OR GALLING) - Removal of material from a rubbing surface caused by seizing, scoring or scuffing

ADIABATIC COMPRESSION - compression of a gas without extraction of heat, resulting in increased temperature. The temperature developed in compression of a gas is an important factor in lubrication, since oil deteriorates more rapidly at elevated temperatures. Oxidation inhibitors help prevent rapid lubricant breakdown under these conditions.

AFTERCOOLING - the process of cooling compressed gases under constant pressure after the final stage of compression. See intercooling.

AFTERRUNNING - the continued running of a spark-ignited engine after the ignition is turned off; also known as dieseling. There are two basic causes of afterrunning: surface ignition and compression ignition. In surface ignition, the surfaces of the combustion chamber remain hot enough to provide a source of ignition after the spark ignition is terminated. In compression ignition, the conditions of temperature, pressure, fuel composition, and engine idle speed allow ignition to continue.

ALIPHATIC HYDROCARBON (HC) - hydrocarbon in which the carbon atoms are joined in open chains, rather then rings. See hydrocarbon, normal paraffin.

ALKYL - any of a series of monovalent radicals having the general formula CnH2n+1, derived from aliphatic hydrocarbons by the removal of a hydrogen atom: for example, CH3-(methyl radical, from methane).

ANTI-FOAM AGENT - one of two types of additives used to reduce foaming in petroleum products: silicone oil to break up large surface bubbles, and various kinds of polymers that decrease the amount of small bubbles entrained in the oils. See foaming.

ANTI-OXIDANT - See oxidation inhibitor.

ANTI-WEAR ADDITIVE - additive in a lubricant that reduces friction and excessive wear. See boundary lubrication.

APPARENT VISCOSITY - viscosity of a fluid that holds only for the shear rate (and temperature) at which the viscosity is determined. See shear stress, Brookfield viscosity.

AROMATIC - unsaturated hydrocarbon identified by one or more benzene rings or by chemical behavior similar to benzene. The benzene ring is characterized by three double bonds alternating with single bonds between carbon atoms (compare with olefins). Because of these multiple bonds, aromatics are usually more reactive and have higher solvency than paraffins and naphthenes. Aromatics readily undergo electrophylic substitution; that is, they react to add other active molecular groups, such as nitrates, sulfonates, etc. Aromatics are used extensively as petrochemical building blocks in the manufacture of pharmaceuticals, dyes, plastics, and many other chemicals.

ARYL - any organic group derived from an aromatic hydrocarbon by the removal of a hydrogen atom, for example (C6H5- {phenyl radical, from benzene}).

ASH - inorganic residue of combustion. Lubricating oil detergent additives contain metallic derivatives, such as barium, calcium, and magnesium sulfonates, that are common sources of ash. Ash deposits can impair engine efficiency and power. See detergent, sulfated ash.

ASPERITIES - microscopic projections on metal surfaces resulting from normal surface-finishing processes. Interference between opposing asperities in sliding or rolling applications is a source of friction, and can lead to metal welding and scoring. Ideally, the lubricating film between two moving surfaces should be thicker than the combined height of the opposing asperities. See boundary lubrications, EP additive.