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The purpose of engine oils is mainly to reduce friction between two surfaces, to remove it at best - therefore to lubricate.
Friction produces heat.
For this reason, an engine oil has to absorb the heat and to cool it.
This is carried out via the transport from the oil cooler and to the sump;
Sealing of the annular clearance between piston and cylinder;
Protect the motor against corrosion;
Transfer the combustion pressure from the piston to the connecting rod, to the crankshaft;
Control the viscosity-temperature behaviour in order to achieve an optimum functioning in hot and cold state;
It has to be gasket-compatible in order to prevent the engine gaskets (elastomers) from drying out, becoming brittle or softening as well.
It has to be age-resistant in order to guarantee long oil change intervals.
These are only the most important aspects. There are approximately 50 others that are included in the service instructions of automobile manufacturers.

All basic oils for lubricating and hydraulic oils and fuels, heating oil, bitumen and also many plastic materials are produced on the basis of - in addition to vegetable oils like for example rapeseed oil - crude oil. Regarding their composition, the above-mentioned products are all combinations of the elements carbon ( C = carboneum ) and hydrogen ( H = hydrogenum ) that differ mainly due to the extremely varying size of the hydrocarbon molecules. The simplest hydrocarbon is methane CH4, it is for example also the main component of natural gas. In practice, an almost unlimited variety of molecules with different sizes is possible. On the basis of the C atoms in the molecule, they can be classified as follows:

Natural gas: methane C1
Petroleum gas: Ethane C2, propane C3, butane C4
Petrol / four-star petrol C5 to C12
Diesel / heating oil EL C10 to C20
Lubricating oils C20 to C35
Vacuum gas oil / heating oil over C35
Bitumen over C80
Plastic materials (plastic)

We classify them as follows:

Starting product: crude oil. Oldest production procedure for lubricating and hydraulic oils; many standard products today are still raffinates.
Production run:
Distilling: Heating/evaporating/condensing of crude oil and extraction
of petrol and middle distillate (diesel, heating oil EL) in the process.
Vacuum distilling: Residues from distillation are distilled under vacuum, oils of varying viscosities are extracted.
Refining: Removing of undesirable components, thus improvement of e.g. ageing stability.
Deparaffininising: Freezing out of paraffin, thus improvement of the low-temperature behaviour.
Final products in the pure extraction form: Mineral lubricating oils.

Hydrocrack oils (HC synthesis).
Starting product: Paraffin slack wax resulting from deparaffinisation of raffinate or vacuum gas oil from vacuum distillation.
Production run:
Cracking + hydrating (hydrocracking): Cracking of the very long hydrocarbon molecules, e.g. of vacuum gas oil, to the size of lubricating molecules. Unsaturated cracks are saturated (hydrated) with hydrogen.
Vacuum distilling: Extracting of several oils with varying viscosities.
Deparaffininising: Freezing out of paraffin, thus improvement of the cold procedure.
Final products in the pure extraction form: Partially but also fully synthetic lubricants.

Synthetic hydrocarbons, starting product: Petrol or similar hydrocarbons. Extraction procedure for maximum quality as fully synthetic.
Production run:
Cracking: Petrol molecules are cracked up into even smaller gas molecules, either ethane or butane.
Synthesis: Combining of ethane to poly-alpha-olefin ( PAO ) or butane to poly-iso-butene (PIB).
Vacuum distilling: Extracting of several oils with varying viscosities.
Hydrating (only with PAO): Still existing unsaturated cracks are saturated with hydrogen.
Final products in the pure extraction form: Fully synthetic lubricating oils.

Practial behaviour of the oils:
Raffinates have a normal ageing stability and can be produced at a relatively low cost. The lower viscosity, the higher the evaporation loss at high temperatures and thus the engine oil consumption conditioned by the oil. Their low-temperature behaviour (pour point) is medium and has to be ameliorated using additives in most cases. Their viscosity index ( VI ) is approx. 90 to 100, i.e. they are single-grade oils. If a multigrade oil is to be produced on this basis, this is only possible when viscosity index improvers are used.
Hydrocrack oils and synthetic hydrocarbons:
The specific advantage of these basic oil types compared to conventional raffinates is their considerably more uniform composition. As a result, they have a much higher ageing stability and a reduced evaporation loss. Their viscosity index ( VI ) is 130 to 150, they are multigrade oils. The low-temperature behaviour is particularly good with PAO, the pour point is below -50 °C. On account of more expensive production procedures, however, the basic oil price of hydrocrack oils as well as synthetic hydrocarbons (e.g. PAO) is clearly higher than the one for raffinates.

The exacting requirements for lubricants in motor vehicles can only be fulfilled through the use of oils with specific, oil-soluble additives. The type and the quantity of the additives have to be adjusted exactly to the relevant case of application. The additive share may be less than 1% up to 25%. The performance of the ready-formulated lubricants has to be proven in comprehensive, standardised and practically oriented tests.

Additives have the following properties and/or tasks:
Prevent/reduce/wash off paint and coal-like deposits on hot components (in particular pistons). Neutralise acid combustion products.
Prevent/reduce formation and depositing of sludge at low operating temperatures. Neutralise acid combustion products.
Wear protection:
Protect metal surfaces against wear.
Corrosion protection:
Protect metal surfaces against corrosion.
Friction coefficient modifying agent:
Reduce/adjust friction between surfaces
Viscosity index improver:
Sufficient viscosity at high oil temperatures.
Pour point reducers:
Reduce the pour point
Elastomere swellers:
Prevent shrinking (drying out) of rotary shaft seals when specific synthetic hydrocarbons are used.
Age-protecting agents:
Prevent/reduce formation of resin, paintwork, sludge, acid, polymer-like oil ageing products.
Metal deactivating agents:
Prevent/reduce the catalytic influence of finest metal particles on oil ageing.
Antifoaming agents:
Prevent/reduce formation of foam.

ACEA - see separate explanation in this encyclopaedia

API - see separate explanation in this encyclopaedia

SAE - see separate explanation in this encyclopaedia

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