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So, 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. But, 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.
So, 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. So, 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:
- Control friction and wear in the engine.
- Protect the engine from rusting.
- Cool the pistons.
- Protect the engine oil stored; in the sump from combustion gases.
So, Some 75%-95% of a typical engine oil is made up of a base oil. As a result, 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:
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. But, 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:
- Hydrodynamic lubrication
- Mixed lubrication
- Boundary lubrication
- 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.
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.
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.
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” 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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