Lubricating oils

Lubricating oils are a product of the crude oil refining process. The

various properties required of the oil are obtained as a result of blending

and the introduction of additives. The physical and chemical properties

of an oil are changed by additives which may act as oxidation inhibitors,

wear reducers, dispersants, detergents, etc. The important lubricant

properties will now be examined.

Viscosity

also an important property of lubricating oils. Viscosity index is also

used, which is the rate of change of viscosity with temperature.

The

i.e. base, which is available in a lubricating oil to neutralise acids.

The

and neutralisation number is used as the unit of measurement.

The

neutralisation number. When excessively oxidised an oil must be

discarded.

The

particularly for oils exposed to heat. A carbon residue test is usually

performed to obtain a percentage value.

The

release the water in a centrifuge. This property is also related to the

tendency to form sludge.

has already been mentioned with respect to fuel oils, but it isTotal Base Number (TEN) is an indication of the quantity of alkali,acidity of an oil must be monitored to avoid machinery damageoxidation resistance of a lubricant can also be measured bycarbon-forming tendency of a lubricating oil must be known,demulsibility of an oil refers to its ability to mix with water and then

Corrosion inhibition

water is present in the oil. This is important where oils can be

contaminated by fresh or salt water leaks.

The modern lubricant must be capable of performing numerous

duties. This is achieved through blending and additives. It must prevent

metal-to-metal contact and reduce friction and wear at moving parts.

The oil must be stable and not break down or form carbon when

exposed to high temperatures, such as where oil cooling is used. Any

contaminants, such as acidic products of combustion, must be

neutralised by alkaline additives; any carbon build up on surfaces must

be washed away by detergent additives and held in suspension by a

dispersant additive. The oil must also be able to absorb water and then

release it during purification, but meanwhile still protect the metal parts
from corrosion.

The various types of engine and other equipment will have oils

developed to meet their particular duties.

Trunk piston engine lubricating oil must lubricate the cylinders as well

as the crankcase: some contamination from the products of combustion

will therefore occur, resulting in acidity and carbon deposits. The oil

must, in addition to lubricating, neutralise the acids and absorb the

deposits.

Turbine oil, while lubricating the moving parts, must also carry away

considerable quantities of heat from the bearings. This calls for a stable

oil which will not break down at high temperatures or form deposits.

Where gearbox lubrication is also required certain extreme pressure

(EP) additives will be needed to assist the lubricating film. Contact with

water in the form of steam will be inevitable so good demulsifying

properties will be essential.

Slow-speed diesel engines will have separate cylinder and crankcase

lubrication systems. The cylinder oil will have to neutralise the acidic

products of combustion and also have good detergent properties to keep

the metal, surfaces clean. Crankcase oils are either detergent type,

multi-purpose oils or rust and oxidation inhibited. Good demulsification

and anti-corrosive properties are required together with oxidation

resistance which is provided by the inhibited crankcase oil. The

detergent or multi-purpose oil is particularly useful where oil cooling of

pistons occurs or where contamination by combustion products is

possible.relates to the oil's ability to protect a surface when