Lubricants And Oils - Essential Fluids That Keep Your Car Alive
Lubricants And Oils – Essential Fluids That Keep Your Car Alive

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Engines run smoothly, thanks to sophisticated lubricants and oils. Lubricants and oils are remarkable fluids. During winter, the same engine oil has to operate reliably over temperatures ranging from -40 °C to above 250 °C. It also has to cope with contaminants including metal particles, sludge and soot. The final straw is that this fluid must deal reliably with these conditions, every day for up to two years. Sometimes, the recommended time between oil changes, according to some vehicle manufacturers.

Surprisingly, one of the major driving forces behind the development of lubricants and oils is the environment.

Carbon dioxide is a natural by-product from the combustion of fuel. Indeed, vehicles that have a high fuel consumption emit large amounts of carbon dioxide. Clearly manufacturers are making a number of engineering changes to their vehicles, to try to improve fuel economy.

Less well known, however, is the fact that the fuel consumption can be significantly improved just by changing the viscosity.

For example, it is possible to decrease the amount of fuel consumed by modern cars by up to 5%. Simply, by switching from, a typical multigrade oil, to a “friction-modified” lubricant with a lower viscosity.

So, What Are Lubricant And Oils ?

Engine lubricants and oils play four major roles:

  1. Control friction and wear in the engine.
  2. Protect the engine from rusting.
  3. Cool the pistons.
  4. Protect the engine oil stored in the sump from combustion gases.

Some 75%-95% of a typical engine oil is made up of a base oil. The remainder of the oil comprises a variety of additives, which are used to improve performance. Typically these include anti-wear additives, corrosion inhibitors, antioxidants, detergents, dispersants, antifoam additives, and large polymer molecules known as viscosity modifiers. Which, are added to improve the viscosity variation of the lubricant with temperature. Indeed, the viscosity is the most significant physical property of a lubricant.

The way in which it varies with temperature, shear rate and pressure determines, how the lubricant performs in an engine.

But, the chemistry of the lubricant is also important. Consequently, it must be resistant to oxidation and be able to “lay down” a protective film to combat wear. The behaviour of an oil film, trapped between two moving surfaces is quantified by the dynamic viscosity. More accurately, the dynamic viscosity relates to the shear stress.

Lubricants fall into two broad categories:

  1. Monograde
  2. Multigrade

Depending on whether their viscosity changes significantly with temperature or not.

As Devised, by the Society of Automotive Engineers (SAE):

  • The first number (10W) refers to the dynamic viscosity measured at low temperatures.
  • While the second (30) describes the kinematic viscosity at 100 oC.

Lower numbers describe runnier lubricants. The viscosity of an SAE-5W/30 multigrade, for example, is five times lower than that of SAE-20W/50 at -20 °C.

Roughly speaking, the energy lost due to friction varies with the square root of the viscosity. So, at -20 °C the friction losses of the low-viscosity oil, will be approximately half those of the thicker oil. Consequently, allowing the engine to start more easily. The viscosity grade of a multigrade oil is different at high and low temperatures. Due, to additives known as viscosity modifiers.

For example, SAE-10W/30 has a similar viscosity to the monograde lubricant SAE-30 at 100 °C. At lower temperatures, however, SAE-10W/30 is much thinner than the monograde oil. This means that the multigrade oil provides protection at high temperatures. And, is runny enough at low temperatures to enable engines to start easily. In contrast, the thick monograde oil would simply be unsuitable in winter.

There Are Four Distinct Types Of Lubrication:

  1. Hydrodynamic lubrication
  2. Mixed lubrication
  3. Boundary lubrication
  4. Elastohydrodynamic lubrication

Despite, the different lubrication requirements of these components. In fact, the thickness of the oil determines its coefficient of friction and defines four distinct regions of lubrication.

Hydrodynamic lubrication

Engine bearings and the piston mostly operate in the “hydrodynamic lubrication” region. Where, a thick film separates the moving metal surfaces so that there is no chance of them coming into contact.

Mixed lubrication

When the pistons are momentarily stationary, however, the layer of oil can be similar in thickness to the surface roughness. In this “mixed lubrication” region, the metal surfaces intermittently come into direct contact.

Boundary lubrication

If the thickness of the oil film is smaller than the surface roughness then the metal surfaces rub together repeatedly. Contact between the cams and the tappets in the valve train span the mixed and boundary regions.

Elastohydrodynamic lubrication

“Elastohydrodynamic lubrication” occurs under high loads. Here the pressure developed in the lubricant is sufficiently high to elastically deform the metal surfaces. This happens because the viscosity of these fluids increases significantly as the pressure rises. The valves and the piston rings occasionally operate in this region.

Finally, It’s the additives added to an engine oil, that give it its qualities to help with its performance.

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