I recently took a class on VM's and thought that some of the information that was covered would be useful for BITOGer's who want to know more about how VII/VM's work and why:
RHEOLOGY
The branch of physics that deals with the deformation and flow of matter, especially the non-Newtonian flow of liquids and the plastic flow of solids.
Every multigrade engine oil today contains some kind of viscosity modifier to adjust the rate of flow of the fluid in and around engine components. The concentration and type of viscosity modifier (also called a viscosity index improver) varies greatly depending on the desired attributes. In a practical sense, polymers are used to thicken base oil mixtures to raise the finished fluid viscosity to the desired grade. Where it gets especially interesting is how these polymers impart non-newtonian flow characteristics to the base oil mixture, which would otherwise operate in a newtonian manner. The resulting fluid - base oil plus polymer also adapts for various tests which make up a Grade of oil as it relates to the cold (CCS, MRV, Brookfield viscosity etc) and heat (KV and the viscosity index relationship, HTHS, etc). For example a low molecular weight polymer may be chosen to alter the fluid characteristics at cold temperatures, but have minimal effect on the KV or HTHS. Conversely a viscosity modifier can be used to improve HTHS and Viscosity index, but make no change to the cold temperature performance. Or you could have a polymer with great "thickening power" which raises the KV to the target window in operation, allowing the use of thinner base oils for improved low temperature performance.
There are many factors which impact the flow (or resistance to flow) of a fluid. The temperature, the pressures or load under which the fluid is operating, and in the case of a lubricant, the speed at which moving parts interact with each other. Each of these factors plays a role in selecting the type and concentration of viscosity modifier used, and this selection is also done in tandem with selecting the mixture of base oils to reach the desired viscosity performance and grade for the application.
NEWTONIAN VS NON-NEWTONIAN
Consider this example:
Quote:
Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature. Although this viscosity will change with temperature, it does not change with the strain rate. Only a small group of fluids exhibit such constant viscosity. The large class of fluids whose viscosity changes with the strain rate (the relative flow velocity) are called non-Newtonian fluids.
Rheology generally accounts for the behavior of non-Newtonian fluids, by characterizing the minimum number of functions that are needed to relate stresses with rate of change of strain or strain rates. For example, ketchup can have its viscosity reduced by shaking (or other forms of mechanical agitation, where the relative movement of different layers in the material actually causes the reduction in viscosity) but water cannot. Ketchup is a "shear thinning material", like yoghurt and emulsion paint (also called latex paint or acrylic paint), exhibiting thixotropy, where an increase in relative flow velocity will cause a reduction in viscosity, for example, by stirring. Some other non-Newtonian materials show the opposite behavior, rheopecty: viscosity going up with relative deformation, and are called "shear thickening" or dilatant materials. Since Sir Isaac Newton originated the concept of viscosity, the study of liquids with strain rate dependent viscosity is also often called Non-Newtonian fluid mechanics.
So non-Newtonian fluids can have exhibit 2 major characteristics: Either Thinning when pressure and strain are applied OR Thickening when pressure and strain are applied. Both can be very useful in the control of the flow rate of a lubricant within a given systems.
SHEAR, COMPRESSION, TRACTION
In physics shear, compression and tension all describe the forces that are acting in a given system. For lubricants, each of these forces have significant impact on the flow and effectiveness of lubricant elasticity.
SHEAR - a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material. Shear strain is distinguished from volumetric strain, the change in a material's volume in response to stress. (ie as two parallel plates move past one another the fluid flowing between them is subject to a shear rate).
COMPRESSION - the application of balanced inward ("pushing") forces to different points on a material or structure
TRACTION - the application of balanced outward ("pulling") forces to different points on a material or structure
Take for example a journal bearing - here you have shearing forces acting between the stationary surface (the bearing) and the moving surface (the crank shaft), you also have compression where the fluid is carrying the load to keep the crank and bearing surfaces apart. In each case the shearing properties and the compressive strength of the lubricant are essential to reducing friction, limiting wear and improving energy efficiency.
TEMPORARY SHEAR & SHEAR STRESS
When shear is discussed (specifically when a lubricant "shears-down"), some people erroneously think that you are talking about a breaking or cutting of the molecules in a lubricant. What this actually refers to is the property of the lubricant where the viscosity changes when a shearing force (parallel internal surfaces sliding past one another) is applied to the lubricant. In most cases "shearing down" is a desirable trait because the fluid is altering it's resistence to flowing through the moving parts. This helps to properly circulate the fluid, deliver surface active chemistry, dissipate heat and provide a sliding film between the materials which eliminates friction. On the molecular level a shear-thinning molecule responds to the applied force by realigning itself to allow other molecules to pass by more easily. Conversely a shear-thickening molecule (like cornstarch in water) responds to the applied force by becoming more solid and greatly resisting flow - or increasing in viscosity.
Because shearing and temperature can both affect the lubricant viscosity, one of the required attributes for the SAE J300 is sufficient High-Temperature, High-Shear Viscosity. This HTHS is measured using a Tapered Bearing Simulator Viscosmeter and is correlated with the flow characteristic required for engine bearings.
It is possible that the shearing forces applied to a fluid may be beyond the ability of the molecule to handle (by it's shear-stability or shear strength). In this case the fluid may demonstrate permanent viscosity loss due to extreme shearing forces. In these cases the molecules may in-fact be breaking and loosing their ability to provide the desired characteristics. This is a separate phenomenon from the desired shear-thinning that is discussed above. Viscosity modifying polymers are often classified by their shear stability or their resistance to permanent viscosity loss.
LOW VISCOSITY LUBRICANTS AND SHEARING
It has been abundantly demonstrated that lower HTHS viscosity provides improved fuel economy benefits. This is the primary driver behind recent changes to the J300 introducing XW8, XW12, XW16 viscosity grades as well as the introduction of new engine oil categories for diesel (FA-4) and gasoline (GF-6B). New advancements in viscosity modifier technology, improved performance and availability of high VI base stocks has all led the development of advanced flow characteristics allowing for efficiency and protection in systems where it was never previously thought possible.
By reducing the HTHS viscosity you minimize parasitic losses in the system from using a fluid that is thicker than necessary. If you are able to do this an still maintin the minimum oil film thickness, while providing addtional support through effective surface active chemistry - you can directly contribute to fuel economy savings in an industry. When this method is also paired with developments in engine design and the selection of bearing materials you can also enable addtional technologies that also contribute to fuel economy and energy efficiency. An effective Ultra Low Viscosity lubricant will employ shear-thinning responses to the directional shear forces applied to it to arrive at the perfect viscosity window that keeps asperities separated and parts moving with the least amount of viscous drag.
COMPRESSIVE STRENGTH
Now you may be thinking to yourself - what happens when the load increases drastically and I'm using an ULV lubricant... surely it's so thin that I am compromising durability. There is an answer to that as well. Not only do fluids have to adapt to shear forces; compressive forces are also at work within an engine. When developing a finished lubricant, the formulator must also ensure that it has enough compressive strength to withstand increases in compressive load. The molecules of the viscosity modifiers and some friction modifiers also respond to compressive forces by realigning themselves to resist compression. This creates a sort of "thickening" or "film strength" at the interacting surfaces to keep them separated. A fluid layer may have low shear resistance(meaning it allows movement in the direction of the sliding surfaces) and high compressive resistance (meaning it stops the surfaces from being pushed together). In this way, the films may be thinner - but are much stronger - this is demonstrated in the current FA-4 diesel engine oil developments where the fluids are required to pass the exact same durability tests, but at a lower viscosity.
The Rheology of Viscosity Modifiers and an understanding of the way lubricants flow under various forces helps us develop lubricants that are directly contributing to and enabling advancements in technology that allow us to more efficiently use energy in transportation and industrial application. These advancements are akin to to comparing an iphone to a rotary dial telephone on a party line. Unfortunately many end users do not understand the advancements and are still applying rotary dial thinking to touch screen mobile technology. It's time to upgrade!
RHEOLOGY
The branch of physics that deals with the deformation and flow of matter, especially the non-Newtonian flow of liquids and the plastic flow of solids.
Every multigrade engine oil today contains some kind of viscosity modifier to adjust the rate of flow of the fluid in and around engine components. The concentration and type of viscosity modifier (also called a viscosity index improver) varies greatly depending on the desired attributes. In a practical sense, polymers are used to thicken base oil mixtures to raise the finished fluid viscosity to the desired grade. Where it gets especially interesting is how these polymers impart non-newtonian flow characteristics to the base oil mixture, which would otherwise operate in a newtonian manner. The resulting fluid - base oil plus polymer also adapts for various tests which make up a Grade of oil as it relates to the cold (CCS, MRV, Brookfield viscosity etc) and heat (KV and the viscosity index relationship, HTHS, etc). For example a low molecular weight polymer may be chosen to alter the fluid characteristics at cold temperatures, but have minimal effect on the KV or HTHS. Conversely a viscosity modifier can be used to improve HTHS and Viscosity index, but make no change to the cold temperature performance. Or you could have a polymer with great "thickening power" which raises the KV to the target window in operation, allowing the use of thinner base oils for improved low temperature performance.
There are many factors which impact the flow (or resistance to flow) of a fluid. The temperature, the pressures or load under which the fluid is operating, and in the case of a lubricant, the speed at which moving parts interact with each other. Each of these factors plays a role in selecting the type and concentration of viscosity modifier used, and this selection is also done in tandem with selecting the mixture of base oils to reach the desired viscosity performance and grade for the application.
NEWTONIAN VS NON-NEWTONIAN
Consider this example:
Quote:
Newtonian fluids can be characterized by a single coefficient of viscosity for a specific temperature. Although this viscosity will change with temperature, it does not change with the strain rate. Only a small group of fluids exhibit such constant viscosity. The large class of fluids whose viscosity changes with the strain rate (the relative flow velocity) are called non-Newtonian fluids.
Rheology generally accounts for the behavior of non-Newtonian fluids, by characterizing the minimum number of functions that are needed to relate stresses with rate of change of strain or strain rates. For example, ketchup can have its viscosity reduced by shaking (or other forms of mechanical agitation, where the relative movement of different layers in the material actually causes the reduction in viscosity) but water cannot. Ketchup is a "shear thinning material", like yoghurt and emulsion paint (also called latex paint or acrylic paint), exhibiting thixotropy, where an increase in relative flow velocity will cause a reduction in viscosity, for example, by stirring. Some other non-Newtonian materials show the opposite behavior, rheopecty: viscosity going up with relative deformation, and are called "shear thickening" or dilatant materials. Since Sir Isaac Newton originated the concept of viscosity, the study of liquids with strain rate dependent viscosity is also often called Non-Newtonian fluid mechanics.
So non-Newtonian fluids can have exhibit 2 major characteristics: Either Thinning when pressure and strain are applied OR Thickening when pressure and strain are applied. Both can be very useful in the control of the flow rate of a lubricant within a given systems.
SHEAR, COMPRESSION, TRACTION
In physics shear, compression and tension all describe the forces that are acting in a given system. For lubricants, each of these forces have significant impact on the flow and effectiveness of lubricant elasticity.
SHEAR - a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material. Shear strain is distinguished from volumetric strain, the change in a material's volume in response to stress. (ie as two parallel plates move past one another the fluid flowing between them is subject to a shear rate).
COMPRESSION - the application of balanced inward ("pushing") forces to different points on a material or structure
TRACTION - the application of balanced outward ("pulling") forces to different points on a material or structure
Take for example a journal bearing - here you have shearing forces acting between the stationary surface (the bearing) and the moving surface (the crank shaft), you also have compression where the fluid is carrying the load to keep the crank and bearing surfaces apart. In each case the shearing properties and the compressive strength of the lubricant are essential to reducing friction, limiting wear and improving energy efficiency.
TEMPORARY SHEAR & SHEAR STRESS
When shear is discussed (specifically when a lubricant "shears-down"), some people erroneously think that you are talking about a breaking or cutting of the molecules in a lubricant. What this actually refers to is the property of the lubricant where the viscosity changes when a shearing force (parallel internal surfaces sliding past one another) is applied to the lubricant. In most cases "shearing down" is a desirable trait because the fluid is altering it's resistence to flowing through the moving parts. This helps to properly circulate the fluid, deliver surface active chemistry, dissipate heat and provide a sliding film between the materials which eliminates friction. On the molecular level a shear-thinning molecule responds to the applied force by realigning itself to allow other molecules to pass by more easily. Conversely a shear-thickening molecule (like cornstarch in water) responds to the applied force by becoming more solid and greatly resisting flow - or increasing in viscosity.
Because shearing and temperature can both affect the lubricant viscosity, one of the required attributes for the SAE J300 is sufficient High-Temperature, High-Shear Viscosity. This HTHS is measured using a Tapered Bearing Simulator Viscosmeter and is correlated with the flow characteristic required for engine bearings.
It is possible that the shearing forces applied to a fluid may be beyond the ability of the molecule to handle (by it's shear-stability or shear strength). In this case the fluid may demonstrate permanent viscosity loss due to extreme shearing forces. In these cases the molecules may in-fact be breaking and loosing their ability to provide the desired characteristics. This is a separate phenomenon from the desired shear-thinning that is discussed above. Viscosity modifying polymers are often classified by their shear stability or their resistance to permanent viscosity loss.
LOW VISCOSITY LUBRICANTS AND SHEARING
It has been abundantly demonstrated that lower HTHS viscosity provides improved fuel economy benefits. This is the primary driver behind recent changes to the J300 introducing XW8, XW12, XW16 viscosity grades as well as the introduction of new engine oil categories for diesel (FA-4) and gasoline (GF-6B). New advancements in viscosity modifier technology, improved performance and availability of high VI base stocks has all led the development of advanced flow characteristics allowing for efficiency and protection in systems where it was never previously thought possible.
By reducing the HTHS viscosity you minimize parasitic losses in the system from using a fluid that is thicker than necessary. If you are able to do this an still maintin the minimum oil film thickness, while providing addtional support through effective surface active chemistry - you can directly contribute to fuel economy savings in an industry. When this method is also paired with developments in engine design and the selection of bearing materials you can also enable addtional technologies that also contribute to fuel economy and energy efficiency. An effective Ultra Low Viscosity lubricant will employ shear-thinning responses to the directional shear forces applied to it to arrive at the perfect viscosity window that keeps asperities separated and parts moving with the least amount of viscous drag.
COMPRESSIVE STRENGTH
Now you may be thinking to yourself - what happens when the load increases drastically and I'm using an ULV lubricant... surely it's so thin that I am compromising durability. There is an answer to that as well. Not only do fluids have to adapt to shear forces; compressive forces are also at work within an engine. When developing a finished lubricant, the formulator must also ensure that it has enough compressive strength to withstand increases in compressive load. The molecules of the viscosity modifiers and some friction modifiers also respond to compressive forces by realigning themselves to resist compression. This creates a sort of "thickening" or "film strength" at the interacting surfaces to keep them separated. A fluid layer may have low shear resistance(meaning it allows movement in the direction of the sliding surfaces) and high compressive resistance (meaning it stops the surfaces from being pushed together). In this way, the films may be thinner - but are much stronger - this is demonstrated in the current FA-4 diesel engine oil developments where the fluids are required to pass the exact same durability tests, but at a lower viscosity.
The Rheology of Viscosity Modifiers and an understanding of the way lubricants flow under various forces helps us develop lubricants that are directly contributing to and enabling advancements in technology that allow us to more efficiently use energy in transportation and industrial application. These advancements are akin to to comparing an iphone to a rotary dial telephone on a party line. Unfortunately many end users do not understand the advancements and are still applying rotary dial thinking to touch screen mobile technology. It's time to upgrade!
