- Functions of oil
- Viscosity
- Oil additives
- Synthetic oils
Functions of oil
One function of a lubrication system is to reduce friction. Friction occurs between all surfaces in contact. When moving surfaces come together, friction tends to slow them down.
Sometimes that’s just what’s wanted. Friction can be useful.
But it can also cause serious damage. It can make metal parts so hot they melt and fuse together. When that happens, an engine is said to have seized.
How long an engine lasts depends to a large extent on how well it’s lubricated, especially at the points of extreme loading.
So lubrication reduces unwanted friction, and controls it where it is useful.
It reduces wear on moving parts. Clearances fill with oil so that engine parts move or float on layers of oil instead of directly on each other. Much less power is needed to move them and that’s a plus.
It helps cool an engine. It collects heat from the engine, then returns to the sump, where it cools.
It helps absorb shock loads. A power stroke can suddenly put as much as 2 tonnes force on main bearings. Layers of oil cushion this loading.
Oil is also a cleaning agent. It collects particles of metal and carbon and carries them back to the sump. Larger pieces fall to the bottom.
Viscosity
For oil to do all of the work that’s expected of it, it must have special properties.
Its viscosity is crucial. Viscosity is a measure of how easily a liquid flows. Low-viscosity liquid is thin and flows easily. High-viscosity liquid is thick and flows slowly.
Lubricating oil must be thin enough to circulate easily between moving parts, but not so thin that it will be forced out from between them.If it is forced out, parts will be left in direct contact and they’ll be damaged.
If it’s too viscous, it moves too slowly to protect parts, especially in a cold engine.
It was once normal practice for engines to need one grade of oil for summer, and another for winter. Oils are graded or classified by the American Society of Automotive Engineers, or SAE. An engine oil with an SAE number of 50 has a higher viscosity, or is thicker, than an SAE 20 oil. Oils with low viscosity ratings, say SAE 5W, 10W, and 20W were tested at a low temperature, around minus 18 degrees Celsius, and were used for cold conditions. Oils with high viscosity ratings, say, SAE 20, 30, 40 and 50, were tested at a high temperature, around 99 degrees Celsius, and were used for hotter conditions. Modern oils however are blends of oils which combine these properties. The oils are blended with additives, called viscosity index improvers, to form multi-grade, or multi-viscosity oils. They provide better lubrication, over a wider range of climatic conditions than monograde oils.
Oil is also classified by the American Petroleum Institute or API service classification. Oils for spark-ignition engines carry a prefix S, and diesel or compression-ignition engines use C. Some oils have additives that make them suitable for both. Manufacturers recommend which viscosity and API classification is ideal for a particular engine.
Oil additives
Special chemicals called additives are added to the base oil by the oil companies. Different combinations of these additives allow the oil to do different jobs in an engine.
Extreme-pressure additives coat parts with a protective layer so that the oil resists being forced out under heavy load.
Oxidation-inhibitors stop very hot oil combining with oxygen in air to produce a sticky material like tar, which clogs galleries.
Corrosion-inhibitors help stop acids forming that cause corrosion, especially of bearing surfaces.
Anti-foaming agents reduce the effect of oil churning in the crankcase and minimize foaming.
Detergents reduce carbon deposits on parts like piston rings and valves.
Dispersants collect particles that can block the system, separate them from each other and keep them moving. Then they will be removed when the oil is changed.
Synthetic oils
Raw crude oil is not a good lubricant. It contains high levels of nitrogen and sulphur compounds as well as wax crystals. Wax and sulphur cause base oil to be less stable, providing targets for heat and chemical degradation that shorten the oil’s useful lifespan.
Crude oil is constructed from a wide variety of long chain hydrocarbon molecules which allow oxygen to attack gaps in the molecular structure when the oil gets hot.
When crude oil is refined into base oil, molecules are separated according to their weight which has the effect of separating unwanted materials from desirable ones.
Three hydro-processing methods are used in ultra refining the oil to further enhance its desirable properties.
Hydrotreating is a process of adding hydrogen to the base oil at elevated temperatures in the presence of catalyst. They attach themselves to “gaps” in the oil’s molecular structure blocking oxygen molecules form oxidizing the oil, increasing the useful life of the base oil.
Hydrotreating removes some of the nitrogen and sulphur molecules from the base oil.
Hydro-cracking is a more severe form of hydro-processing. It is done by adding hydrogen to the base oil feed at even higher temperatures and pressures. Oil molecules are reshaped and cracked into smaller molecules leaving fewer gaps for oxygen molecules to attach themselves.
A great majority of the sulphur, nitrogen compounds, and aromatics are removed.
Wax hydro-isomerisation is a third step, which lowers the pour point of the base oil so that it flows well at low temperatures. Wax hydro-isomerisation also removes the majority of remaining sulphur and nitrogen making a base oil with exceptional purity and stability. The oil, typically, has no color.
Waxes are removed from the oil so it does not freeze in cold temperatures.
Aromatics are removed as they increase the tendency of the oil to oxidize and thicken at high temperatures reducing the oil’s viscosity index.
Lubricants are designed so their viscosity is low enough for good cold weather starting, yet high enough to provide adequate film thickness and lubricity at high engine temperatures. The oil’s viscosity has to remain consistent when the engine is hot or when operating in a cold environment. The oil’s ability to cope with this is the viscosity index. A high Viscosity index indicates a small change over the oil’s operating temperature range. A low viscosity index indicates the oil has a poor response to temperature change.
Group I base oil is characterized as having greater than 10% aromatics and greater than 300 parts per million sulphur content. The viscosity index of Group 1 base oil is less than 80.
Group II base oils contain significantly lower levels of impurities than Group I base oils - less than 10% aromatics and less than 300 parts per million Sulphur content, however the viscosity index of Group I1 base oil is also less than 80.
Group II oils are more inert than Group 1 base oil and forms less oxidation by-products. Improved purity means that the base oil and the additives in the finished product can last much longer. In some cases, lubricating oils formulated with Group II base oils can outlive more expensive synthetic oils.
Group III base oils are manufactured by essentially the same processing method as modern Group II base oils. They have a higher viscosity index, greater than 120, than Group II base oils which is achieved by increasing the temperature or time in the hydro-cracker.
Modern Group III base oils have properties which allow them to perform at a level that is significantly higher than Group I and Group II base oils. They can match the performance levels of synthetic oils.
Group IV or traditional “Synthetic” base oils are chemically engineered base oils. Man made Hydrocarbons called Poly-alpha-olefins or PAO's are a common example of a synthetic base stock.
Synthetic base oil can also be made by removing molecules from Ethylene gas, that accompanies crude as it comes from the ground, and reconstructing them to produce high quality base oil.
When combined with additives, synthetic base oils offer excellent lubrication properties. They are chemically stable and have uniform molecular chains.
Oxidation resistance and thermal stability are are also very high when compared to Group I, II and III base oils.
Better base oil stability means better additive stability and longer life with longer drain intervals.
In the past Group IV/PAO base oils have been superior to Group III base oils, particularly in regard to viscosity index, pour point, volatility, and oxidation stability. However, modern Group III oils can be designed and manufactured so that their performance closely matches Group IV/PAO base oils in most applications.
For this reason they may be legally labeled synthetic, as they have most of the performance characteristics of early synthetic base oils.
All the base oil groups: As defined by the American Petroleum Institute (API) Publication 1509.
Group Sulfur, Wt % Saturates V.I. I >0.03 and/or <90 80-119 II ≤0.03 and ≥90 80-119 III ≤0.03 and ≥90 ≥120 IV All Polyalphaolefins (PAOs)
