Viscosity index

Last updated

The viscosity index (VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature. The higher the VI, the more stable the viscosity remains over some temperature range. The VI was originally measured on a scale from 0 to 100; however, advancements in lubrication science have led to the development of oils with much higher VIs. [1]

Contents

The viscosity of a lubricant is closely related to its ability to reduce friction in solid body contacts. Generally, the least viscous lubricant which still forces the two moving surfaces apart to achieve "fluid bearing" conditions 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 come in contact and friction will increase. [2]

Relevance

Many lubricant applications require the lubricant to perform across a wide range of conditions, for example, automotive lubricants are required to reduce friction between engine components when the engine is started from cold (relative to the engine's operating temperatures) up to 200 °C (392 °F) when it is running. The best oils with the highest VI will remain stable and not vary much in viscosity over the temperature range. This provides consistent engine performance within the normal working conditions. Historically, there were two different oil types recommended for usage in different weather conditions. As an example, with winter oils and cold starting the engines, and with temperature ranges from, say, −30 °C to 0 °C, a 5 weight oil would be pumpable at the very low temperatures and the generally cooler engine operating temperatures. However, in hot climates, where temperatures range from 30 °C to 45 °C, a 50 weight oil would be necessary, so it would remain thick enough to hold up an oil film between the moving hot parts.

Thus the issue of multigrade oils came into being, where with variable temperatures of, say, −10 °C during the cold nights and 20 °C during the days, a 5 weight oil would be good as the oil would be pumpable in a cold engine and as the engine came up to running temperature, and the day warmed up, the characteristics of a 30 weight oil would be ideal. Thus the 5W-30 oils were introduced, rather than the fixed and temperature limiting grades  where the thin oils became too thin when hot and the thicker oils became too thick when cold.

The effects of temperature on a single-viscosity oil can be demonstrated by pouring a small amount of vegetable oil into a pot or pan and then either cooling it in a freezer or heating it on a cooking stove. When oils get cold enough in a deep freezer, they will solidify into a block of "wax"-like oil that cannot be pumped around inside an engine's lubrication system. However, when a spoonful of very cold oil is put into a pan on a stove and it is slowly heated and swirled around, the oil will gradually warm up, and there is a definite temperature range where the oil is warm and traditionally "oily". However, as the oil is heated further, the oil becomes thinner and thinner, until it is nearly smoking and is almost as thin as water  and thus it has almost no capacity to keep moving parts separated, resulting in metal-to-metal contact and damage of the components that are supposed to be kept apart with a thin film of oil.

Thus the multigrade oils are recommended for use based on the ambient temperature ranges of the season or environment.

Additionally, there are the issues of oil temperature maintenance, such as oil or engine heaters that enable easy starting and shorter warm-up period in very cold climates, and oil coolers to dump enough heat from the oil, and thus the engine, gearbox, or hydraulic oil circuit, so as to keep the oil's upper temperature to within a specified upper working limit.

Classification

The VI scale was set up by the Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are 100 and 210 °F (38 and 99 °C). The scale was originally interpolated between 0 for a naphthenic Texas Gulf crude and 100 for a paraffinic Pennsylvania crude. Since the inception of the scale, better oils have also been produced, leading to VIs greater than 100 (see below). [3]

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 oils ranges from 80 to over 400.[ citation needed ]

Viscosity indexClassification
Under 35Low
35 to 80Medium
80 to 110High
Above 110Very high

Calculation

The viscosity index can be calculated using the following formula: [4]

where U is the oil's kinematic viscosity at 40 °C (104 °F), Y is the oil's kinematic viscosity at 100 °C (212 °F), and L and H are the viscosities at 40 °C for two hypothetical oils of VI 0 and 100 respectively, having the same viscosity at 100 °C as the oil whose VI we are trying to determine. That is, the two oils with viscosity Y at 100 °C and a VI of 0 and 100 would have at 40 °C the viscosities of L and H respectively. These L and H values can be found in tables in ASTM D2270 [4] and are incorporated in online calculators. [5]

Related Research Articles

A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.

<span class="mw-page-title-main">Ball bearing</span> Type of rolling-element bearing

A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.

<span class="mw-page-title-main">Motor oil</span> Lubricant used for lubrication of internal combustion engines

Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergents, dispersants, and, for multi-grade oils, viscosity index improvers. The main function of motor oil is to reduce friction and wear on moving parts and to clean the engine from sludge and varnish (detergents). It also neutralizes acids that originate from fuel and from oxidation of the lubricant (detergents), improves the sealing of piston rings, and cools the engine by carrying heat away from moving parts.

<span class="mw-page-title-main">Hydraulic fluid</span> Medium to transfer power in hydraulic machinery

A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoes, hydraulic brakes, power steering systems, automatic transmissions, garbage trucks, aircraft flight control systems, lifts, and industrial machinery.

Tribology is the science and engineering of understanding friction, lubrication and wear phenomena for interacting surfaces in relative motion. It is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. The fundamental objects of study in tribology are tribosystems, which are physical systems of contacting surfaces. Subfields of tribology include biotribology, nanotribology and space tribology. It is also related to other areas such as the coupling of corrosion and tribology in tribocorrosion and the contact mechanics of how surfaces in contact deform. Approximately 20% of the total energy expenditure of the world is due to the impact of friction and wear in the transportation, manufacturing, power generation, and residential sectors.

<span class="mw-page-title-main">Synthetic oil</span> Lubricant consisting of artificially made chemical compounds

Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers.

The NLGI consistency number expresses a measure of the relative hardness of a grease used for lubrication, as specified by the standard classification of lubricating grease established by the National Lubricating Grease Institute (NLGI). Reproduced in standards ASTM D4950(“standard classification and specification of automotive service greases”) and SAE J310(“automotive lubricating greases”), NLGI's classification is widely used. The NLGI consistency number is also a component of the code specified in standard ISO 6743-9“lubricants, industrial oils and related products — classification — part 9: family X (greases)”.

<span class="mw-page-title-main">Gear oil</span> Lubricant used in vehicles and machinery

Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It has high viscosity and usually contains organosulfur compounds. Some modern automatic transaxles do not use a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission. Gear oils account for about 20% of the lubricant market.

Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil.

Oil analysis (OA) is the laboratory analysis of a lubricant's properties, suspended contaminants, and wear debris. OA is performed during routine predictive maintenance to provide meaningful and accurate information on lubricant and machine condition. By tracking oil analysis sample results over the life of a particular machine, trends can be established which can help eliminate costly repairs. The study of wear in machinery is called tribology. Tribologists often perform or interpret oil analysis data.

The Noack volatility test, named after Kurt Noack, determines the evaporation loss of lubricants in high-temperature service. This test is standardized as ASTM D5800.

Friction modifiers are added to lubricants in order to reduce friction and wear in machine components. They are particularly important in the boundary lubrication regime, where they can prevent solid surfaces from coming into direct contact, substantially reducing friction and wear.

<span class="mw-page-title-main">Shell Rotella</span> Brand of heavy-duty engine lubricant

Shell Rotella is a line of heavy-duty engine lubrication products produced by Shell plc. The line includes engine oils, gear oils and coolants. The oil carries both the American Petroleum Institute (API) diesel "C" rating as well as the API gasoline engine "S" rating. Ratings differ based on the oil. Rotella oils, like the T3 15W-40, meet both the API CJ-4 and SM specifications, and may be used in both gasoline and diesel engines. However, it is formulated specifically for vehicles without catalytic converters, containing phosphorus levels beyond the 600–800 ppm range. Therefore, Rotella is not recommended for gasoline vehicles with catalytic converters due to the higher risk of damaging these emission controls. Newer formulations of Rotella T6 however are API SM rated as safe for pre-2011 gasoline vehicles.

Oil additives are chemical compounds that improve the lubricant performance of base oil. The manufacturer of many oils can use the same base stock for each formulation and can choose different additives for each use. Additives comprise up to 5% by weight of some oils.

Crude oil is extracted from the bedrock before being processed in several stages, removing natural contaminants and undesirable hydrocarbons. This separation process produces mineral oil, which can in turn be denoted as paraffinic, naphthenic or aromatic. The differences between these different types of oils are not clear-cut, but mainly depend on the predominant hydrocarbon types in the oil. Paraffinic oil, for example, contains primarily higher alkanes, whereas naphthenic oils have a high share of cyclic alkanes in the mixture.

<span class="mw-page-title-main">Automotive oil recycling</span> The process of recycling used engine and motor oils

Automotive oil recycling involves the recycling of used oils and the creation of new products from the recycled oils, and includes the recycling of motor oil and hydraulic oil. Oil recycling also benefits the environment: increased opportunities for consumers to recycle oil lessens the likelihood of used oil being dumped on lands and in waterways. For example, one gallon of motor oil dumped into waterways has the potential to pollute one million gallons of water.

Base oils are used to manufacture products including lubricating greases, motor oil and metal processing fluids. Different products require different compositions and properties in the oil. One of the most important factors is the liquid’s viscosity at various temperatures. Whether or not a crude oil is suitable to be made into a base oil is determined by the concentration of base oil molecules as well as how easily these can be extracted.

Pour point depressants are used to allow the use of petroleum based mineral oils at lower temperatures. The lowest temperature at which a fuel or oil will pour is called a pour point. Wax crystals, which form at lower temperatures, may interfere with lubrication of mechanical equipment. High-quality pour point depressants can lower a pour point of an oil additive by as much as 40°C.

The mini-rotary viscometer is a device used to measure the pumpability of an engine oil at low temperatures. Engine oils must meet viscometric standards including those determined by the MRV in order to be classifiable within SAE J300 viscosity grades. The requirement was added after the winters of 1980-81 and 1981-82 caused major pumpability field problems in the US and Europe.

<span class="mw-page-title-main">SAE J300</span> Standard for engine oil

SAE J300 is a standard that defines the viscometric properties of mono- and multigrade engine oils, maintained by SAE International. Key parameters for engine oil viscometrics are the oil's kinematic viscosity, its high temperature-high shear viscosity measured by the tapered bearing simulator, and low temperature properties measured by the cold-cranking simulator and mini-rotary viscometer. This standard is commonly used throughout the world, and standards organizations that do so include API and ILSAC, and ACEA.

References

https://xenum.com/en/engine-oil-viscosity-index/

https://www.lubricants.total.com/what-motor-oil-vi

  1. "Viscosity Index Tables" (PDF). US: ASTM. Retrieved 26 March 2020.
  2. "Don't Ignore Viscosity Index When Selecting a Lubricant". US: Noria. Retrieved 26 March 2020.
  3. "Viscosity Index". UK: Anton Paar. Retrieved 29 August 2018.
  4. 1 2 "ASTM D2270-10(2016)". ASTM . Retrieved 2020-11-29.
  5. "Calculate Viscosity Index". Oil Viscosity Chart. Retrieved 2 February 2021.