Viscosity index
Encyclopedia
Viscosity index is an arbitrary measure for the change of viscosity
with temperature. It is used to characterize lubricating
oil in the automotive industry.
The viscosity
of liquids decreases as temperature
increases. The viscosity of a lubricant
is closely related to its ability to reduce friction
. Generally, the least viscous lubricant which still forces the two moving surfaces apart is desired. If the lubricant
is too viscous, it will require a large amount of energy to move (as in honey
); if it is too thin, the surfaces will rub and friction will increase.
As stated above, the Viscosity Index highlights how a lubricant's viscosity
changes with variations in temperature. Many lubricant
applications require the lubricant
to perform across a wide range of conditions: for example, in an engine
. Automotive lubricants must reduce friction
between engine
components when it is started from cold (relative to engine operating temperature
s) as well as when it is running (up to 200 °C (392 °F)). The best oils (with the highest VI) will not vary much in viscosity over such a temperature range and therefore will perform well throughout.
The VI scale was set up by the Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are 100 and 210 °F (37.8 and 98.9 C). The original scale only stretched between VI=0 (worst oil, naphthalene
) and VI=100 (best oil, paraffin
) but since the conception of the scale better oils have also been produced, leading to VIs greater than 100 (see below).
VI improving additives
and higher quality base oils are widely used nowadays which increase the VIs attainable beyond the value of 100. The Viscosity Index of synthetic oil
s ranges from 80 to over 400.
vi=(L-U)/(L-H)×100
this is the formula used to find the viscosity index.
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
with temperature. It is used to characterize lubricating
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...
oil in the automotive industry.
The viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of liquids decreases as temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
increases. The viscosity of a lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
is closely related to its ability to reduce friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
. Generally, the least viscous lubricant which still forces the two moving surfaces apart is desired. If the lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
is too viscous, it will require a large amount of energy to move (as in honey
Honey
Honey is a sweet food made by bees using nectar from flowers. The variety produced by honey bees is the one most commonly referred to and is the type of honey collected by beekeepers and consumed by humans...
); if it is too thin, the surfaces will rub and friction will increase.
As stated above, the Viscosity Index highlights how a lubricant's viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
changes with variations in temperature. Many lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
applications require the lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
to perform across a wide range of conditions: for example, in an engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
. Automotive lubricants must reduce friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
between engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
components when it is started from cold (relative to engine operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
s) as well as when it is running (up to 200 °C (392 °F)). The best oils (with the highest VI) will not vary much in viscosity over such a temperature range and therefore will perform well throughout.
The VI scale was set up by the Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are 100 and 210 °F (37.8 and 98.9 C). The original scale only stretched between VI=0 (worst oil, naphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
) and VI=100 (best oil, paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
) but since the conception of the scale better oils have also been produced, leading to VIs greater than 100 (see below).
VI improving additives
Oil additive
Oil additives are chemical compounds that improve the lubricant performance of base oil . By utilizing the same base stock, many different oils can be manufactured, each with its distinctive properties. Additives comprise up to 5% by weight of some oils.Nearly all commercial motor oils contain...
and higher quality base oils are widely used nowadays which increase the VIs attainable beyond the value of 100. The Viscosity Index of synthetic oil
Synthetic oil
Synthetic oil is a lubricant consisting of chemical compounds that are artificially made . Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials...
s ranges from 80 to over 400.
vi=(L-U)/(L-H)×100
this is the formula used to find the viscosity index.