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Why oil is so slick

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Reducing friction is only half the story… oil has an amazing amount of other properties. Jake Venter explains

There’s a lot more to lubrication than just pumping oil to all parts of an engine. Some components are happy to get just a sniff, others want a spray and the hardest working parts want a supply of oil under pressure.

Oil also helps to cool an engine, to combat rust and to increase the sealing power of compression rings. This implies that its delivery volume and pressure has to be carefully controlled to avoid starvation in some parts and ooding in others.

Lubrication of parts proceeds in stages and it’s interesting to note that it’s not suf cient to have enough oil in the sump to cover demand – provision must also be made for certain types of relative movement.

Lubrication theory

Straight line motion: Moving a heavy weight over a fairly smooth surface is not easy when the contact patch is dry, but the force required diminishes substantially if oil is introduced to lubricate the path.

If you push the weight fast enough there will come a time when it suddenly seems to move with almost no effort – experiment for yourself:

• In the dry state there is a complete lack of lubrication, so that frictional forces impede efforts to move the weight.

• When oil is added, minute cavities that are present on the contact surface – even if it has been machined – ll with the liquid, but friction remains due to the interlocking of prominent high spots.

• As the motion speeds up, a layer of oil begins to form a wedge shape, and thevpushing action on the weight funnels it backwards. Accordingly, the weight begins to ride over the contact patch at a slight angle, rather like a speedboat riding the water.

This has the effect of increasing the pressure in the oil at the rear, with the result that the weight begins to lift away from the contact surface to travel mainly on a lm of oil. Most – but not all – surface contact is eliminated.

In the case described, the nature of lubrication is referred to as a blend of boundary and hydrodynamic – often called mixed lubrication – and explains why the reduction in effort suddenly comes about.

Incidentally, if the pushing movement is very fast, the oil lm gets thicker and increases in pressure, so that there is no more physical contact between the weight and the surface. This is known as full hydrodynamic lubrication.

Pistons inside an engine experience both mixed and hydrodynamic lubrication. Near the top and bottom dead centres, where their speed is close to zero, lubrication is mixed, but for travel down the bore – where speeds often exceed 50km/h – the lubrication process is hydrodynamic.

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Rotation

A similar action occurs inside a bearing shell surrounding a shaft:
• Initially, when there is no movement, the shaft rests on the bottom of the bearing cavity, resulting in contact between it and the bearing shell.

• If the shell is completely dry, any movement of the shaft will tend to roll it a small distance up the slope due to dry friction.

• When oil is introduced, the shaft will slide back to the bottom of the shell but, if the shaft is rotated, the oil layer will get thicker and will be dragged along by the outer surface of the journal to form a wedge

With a further increase in speed, the wedge will be strong enough to lift the shaft off the bottom of the bearing shell, giving rise to mixed lubrication.

• At high speeds, the oil lm increases in thickness and pressure, displacing the shaft centre and signalling the onset of full hydrodynamic lubrication.

When this happens, the pressure in the oil is many times the delivery pressure of the oil pump. If speed diminishes, lubrication will revert to being mixed in nature so, in town driving with changing loads, the shaft centre oscillates between two positions as the engine speed uctuates.

Oil’s duties

In many older engine designs, the high temperature of the oil – which arises from the combustion process as well as from heat generated by friction – was seen as inevitable and little thought was given to trying to control it.

When oil gets too hot its pressure drops, and it starts to degrade in such a way that it loses its properties to lubricate. The result could be bearing failure. This is the reason why many engines in veteran cars are not able to sustain high speeds on a freeway.

Today, this is no longer a problem. Not only has oil formulation been improved beyond all recognition but, in a well-designed engine, oil temperatures can be controlled to the extent that the liquid can be used to help cool the unit.

Oil is also used as a rust preventative, for instance in the humid atmosphere of a crankcase, where it clings to the compression rings, also helping the sealing process. However, the more it degrades over time, the less ef cient it becomes, necessitating an oil change.

Oil chemistry

To cope with all these forms of lubrication and protection, oil needs to cling to metal surfaces so that the relative motion actually takes place by shearing the layer of oil molecules that form the wedge. The oil can only do this if some of its molecules have a chemical af nity with metals.

However, we require the oil to last a long time, which means it should be chemically stable – and therein lies a conundrum, for the majority of molecules should NOT have an af nity with anything. Accordingly, one of the main tasks of an oil chemist is to achieve a balance between opposing requirements.

How does the oil manage to perform all these functions under conditions of extreme temperature and pressure? The answer lies in the complex re ning and blending process, which is evaluated constantly, and improved by continuous engine testing.

Crude oil is a complex mixture of hydrocarbons, some more stable than others, and requires a great deal of re ning to produce the base oil. The end-product is produced by adding substances to change, or enhance, the various qualities the oil should have.

Base oils have to be carefully selected because not all of them are compatible with various additives.

The first quality that springs to mind is oiliness. It is not related to viscosity, but is the quality that is responsible for the boundary layer of molecules that can cling to a metal surface and provide lubrication after the bulk of the oil has been displaced.

This quality can be enhanced by adding certain agents. Other additives include viscosity index improvers, detergents, dispersants to minimise sludge build-up, corrosion inhibitors and alkaline additives to neutralise the acid that forms during combustion.

A zinc compound is often added to reduce wear in the case of metal-to-metal contact but it is done sparingly to minimise negative effects on catalytic converters.

Synthetic oil

While regular oil is refined from crude oil and contains some impurities that cannot easily be removed, the best synthetic oil consists of molecules that are synthesized from simpler chemical compounds.

The process allows chemical engineers choose characteristics – producing oils that ow more freely at low temperatures and which don’t break down easily at high temperatures.

However, the word synthetic is also applied to mixtures of regular and synthetic oil so that it pays to study the small print on a can of oil. Conventional oil is good enough for most normal applications but the manufacturers of high performance and turbocharged engines usually specify high- grade synthetic oil for use in their units.

Viscosity

Viscosity is a measure of the ability of oil molecules to cling together, dragging adjacent layers of oil with them when relative motion exists between a pair of solid surfaces separated by oil.

In a common engineering sense, one can say viscosity is a measure of oil’s resistance to ow. However, the higher the temperature gets in an engine, the lower the resistance, so any measurement of viscosity is meaningless unless the temperature is speci ed.

Properly measuring viscosity is a fairly complicated process, which is why the Society of Automotive Engineers (SAE) developed the SAE numbering system, which is an arbitrary number indicating the viscosity range of a lubricant.

Oils are rated in two ways, depending on intended use. Oils whose viscosity number does not include the letter W are intended for use at higher temperatures, and their grading is based on a minimum viscosity measured at 100 ̊ C only.

Oils that have the letter W included are intended for use at lower temperatures, and the grading is not only based on the minimum viscosity at 100 ̊ C, but on a maximum borderline pumping temperature, as well as a maximum low- temperature viscosity.

Multi-grade oils meet the requirements of more than one SAE viscosity classi cation and may therefore be suitable for use over a wider temperature range than single grade oil. The two classi cation numbers given indicate their low temperature and their high temperature parameters respectively.

For example, a 20W-50 oil is graded for low temperature use (letter W) and is fairly thin (number 20) at low temperatures, but is fairly thick at high temperatures (number 50).

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