Modern engine oils are based on different base oils or the resulting base oil mixtures depending on their type and performance. Additives are also used that perform different tasks. Only a balanced formulation (base oil and additive components) produces a high-performance engine oil.
STRUCTURE OF A TYPICAL MULTI-GRADE ENGINE OIL
- 78% base oil
- 10% viscosity index improvers
- 3% detergents
- 5% dispersants
- 1% wear protection
- 3% other components
BASE OILS
The base oils used give the lubricants basic specific properties that are clearly noticeable in the performance of the finished products.
- Mineral oils: hydrocarbon compounds of different shapes, structures, types and sizes (VI: 80-95)
- Hydrocrack oils: refined mineral oils with a higher degree of purity and improved molecular structure (VI: 130-140)
- Polyalphaolefins (PAO's): synthetic products from the petrochemical industry - chemically constructed linear hydrocarbon compounds (VI: 130-145)
- Synthetic esters: chemically produced compounds of organic acids with alcohols, consisting of molecules with a defined shape, structure, type and size (VI: 140-180)
MINERAL OIL BASIC BASE OIL
The crude oil is first treated in an atmospheric distillation. This produces ethylene, the basic building block for the production of polyalphaolefins (PAO). The next processing step is a vacuum distillation, followed by refining. In the next step, unwanted paraffins are removed. In special cases, a final hydrogen treatment is carried out before the mineral oil is obtained as the end product. Please click on the graphic for an enlarged view.
HYDROCRACK OILS
The starting product is the long-chain (solid) normal paraffins from the dewaxing of raffinates. The paraffin molecules are broken (cracked) into shorter lubricant molecules in special cracking plants in a hydrogen atmosphere in the presence of special catalysts. Due to the process, this mainly produces isoparaffins (branched hydrocarbon chains). In a subsequent vacuum distillation, they are separated according to viscosity and the remaining normal paraffins (unbranched hydrocarbon chains) are removed in a subsequent dewaxing process. The oils produced in this way have a high isoparaffin content and have clearly uniform molecular structures.
POLYALPHAOLEFINS
Polyalphaolefins, or PAOs for short, are synthesized from ethylene as the basic building block in a chemical process. The hydrocarbon compounds resulting from this process have a defined molecular structure.
SYNTHETIC ESTERS
Synthetic esters are chemically produced compounds from organic acids and alcohols. Depending on the desired properties of the ester, defined molecular structures can be synthesized. Here you can see the general chemical formula for the reaction of acid and alcohol to form ester and water and vice versa.
ADDITIVES
Additives are oil-soluble additives or active ingredients that are added to the base oils mentioned. They change or improve the properties of the lubricants through chemical and/or physical effects.
Chemically active additives:
- Detergents
- Dispersants
- Antioxidants
- Antiwear additives
- Corrosion inhibitors
Physically active additives:
- VI improvers
- Antifoam additives
- Pour point improvers
- Friction modifiers
Detergents
Detergents are washing-active substances that counteract the formation of deposits on thermally stressed components. They keep the engine clean. In addition, they form the alkaline reserve in the engine oil, i.e. acidic reaction products from combustion are neutralized.
Dispersants
The task of dispersants is to envelop solid and liquid contaminants that are introduced into the oil during engine operation and to keep them finely distributed and suspended. This prevents deposits in the engine. A distinction is made between the following active processes:
Peptization
This means enveloping and keeping solid contaminants in the oil in suspension, such as dust, reaction products from combustion or aging products of the oil.
Solubilization
Solubilization is the coating and suspension of liquid impurities in the oil, such as condensation water or acids that are produced during engine combustion.
Lubricating oils tend to oxidize (age) under the influence of heat and oxygen. This decomposition process is accelerated by acidic reaction products from combustion and traces of metals that have a catalytic effect (abrasive or corrosive wear). The addition of antioxidants provides significantly improved protection against aging. They cannot prevent the aging process, but they can slow it down. Viscous and dark used oil
Oxidation
When oil ages, acids form as well as varnish, resin and sludge-like deposits that are mostly insoluble in oil, such as oil carbon. Anti-aging agents can work in three ways:
Radical scavengers (primary anti-aging agents): Radicals are hydrocarbon chains in which free valences have been created by chain breakage or the removal of hydrogen atoms. Oxygen immediately accumulates here (oxidation). Radical scavengers saturate (repair) the "gap" by transferring hydrogen from the additive to the free valence.
Peroxide decomposers (secondary anti-aging agents): These only work when anti-aging agents (oxygen compounds) have already formed. They have an "oxygen-removing" effect and form harmless compounds.
Passivators / metal ion deactivators: They passivate iron and copper particles and thus end or weaken the catalytic effects of these metals on the aging process. They "cling" to the metal ions in the oil so that they practically no longer have any catalytic activity.
Wear protection additives
Using suitable additives, extremely thin layers can be built up on sliding surfaces, the shear strength of which is significantly lower than that of metals. It is solid under normal conditions, but slippery under wear conditions (pressure, temperature). This prevents excessive wear (seizure or welding). If necessary (metal/metal contact), the layers are constantly reformed through a chemical reaction.
Extreme Pressure and Antiwear (EP / AW) Additives
The oldest EP additive is pure sulfur. EP/AW additives are surface-active substances and can contain the elements zinc, phosphorus and sulfur in various combinations in the polar group. The best-known representative of this type is zinc dithiophosphate - ZDDP-, which also acts as an anti-aging and anti-corrosion additive. Schematic representation of friction processes in the engine
Effect of anti-wear additive - ZDDP
During the start-up phase of the engine, the state of mixed friction exists (transition between sliding and static friction). Heat is generated wherever there is metal/metal contact. The zinc/phosphorus compound reacts on the surface and forms an additional layer that protects against wear.
Anti-corrosion additive
Corrosion is generally the chemical or electrochemical attack on metal surfaces. Surface-active additives that can be ash-free or ash-producing are particularly suitable for corrosion protection. The polar group attaches itself to metal surfaces, the alkyl residue forms dense, fur-like, hydrophobic (water-hostile) barriers. Due to their polar structure, corrosion protection additives compete with EP/AW additives, i.e. they can impair their effectiveness.
VI improvers
The use of VI improvers (VI = viscosity index [/]) enables the production of multigrade engine oils. VI improvers increase or extend the viscosity of an oil and thus improve the viscosity-temperature behavior. Figuratively speaking, they are very long, fibrous molecules that are clumped together in the oil when cold and offer relatively little resistance to the movement of the oil molecules. As the temperature increases, they unravel, take up a larger volume and form a network of meshes that slows down the movement of the oil molecules and delays the oil from "thinning out" too quickly. VI improver before shearing (long, left) and after shearing (short, right)
VI - improver / shearing Under load, VI improvers can be sheared, i.e. the long molecules are literally torn apart. This is associated with a loss of viscosity. The loss of viscosity is irreversible and in this context one speaks of permanent shearing. The torn molecules take up a smaller volume and therefore have a lower thickening effect. The shear stability of a lubricant is essentially determined by the quality of the VI improver. High shear loads occur, for example, in the piston ring area (high speeds, sliding speeds, pressures and temperatures).
Antifoam additives
Polysilicones (silicone polymers), polyethylene glycol ethers and others reduce the tendency of an oil to foam. This is achieved by generally trapping fewer gases (air and combustion gases) in the oil. Trapped gases can also escape from the oil more quickly. Foam formation significantly impairs the lubricant properties (oxidation, pressure behavior) of a lubricant.
A lubricant with poor foam behavior can lead to significantly higher oil temperatures, wear and hydraulic tappet rattling.
Pour point improvers
The pour point is the low temperature in degrees Celsius at which the oil just about flows. The "curdling" of an oil is determined by the crystallization of the paraffins present in the base oil at low temperatures. By adding pour point reducers, the crystallization of the paraffins is delayed and the low-temperature behavior of the oils is improved.
Friction modifiers
Friction-reducing additives, so-called friction modifiers, can only work in the area of mixed friction. These active ingredients form fur-like films on the surfaces (physical process) that can separate metal surfaces from each other. FM are very polar, i.e. there is a high affinity to the surface combined with friction-reducing properties.