In Part 1 of this article series, Understanding the Fundamentals of Viscosity Modifiers for Automotive Engine Oils, explained how an original objective of modifying viscosity was to overcome the effect of temperature changes on engine oil operating characteristics. With the introduction of viscosity modifiers (VMs) in the 1960s, it became possible to formulate engine oils which thin less at high temperatures and thicken less at low temperatures than conventional mono-grade oils. The result was that the oil’s viscosity could be maintained within acceptable limits in both hot and cold climates. This led to the advent of multi-grade oils for the convenience of the customer, who no longer had to change engine oils with the seasons.
As chemists and engineers continued research with viscosity modifying polymers, they discovered many additional benefits that positively impacted engine oil performance. Part 2 of this article series focuses on some of them, and explains how chemists and engineers measure viscosity improvement. In addition, we examine how Lubrizol has employed performance polymers to help improve operation of today’s passenger car engines, which are smaller than in the past, yet expected to provide better and longer operation than their predecessors. Finally, we look at how VMs improve performance of heavy duty diesel (HD) truck fleets and deliver value to transmission and gear oils.
Viscosity modifiers perform five main functions in engine oil lubricants.
- Reduce viscosity changes with temperature.
- Enable the engine to start (crank) at low temperatures, as measured by cold cranking simulator viscosity (CCS).
- Ensure engine durability during boundary layer lubrication regimes of valve-trains and rings/ liner, as measured by high temperature, high shear-rate (HTHS) viscosity.
- Provide important non-viscometric performance benefits such as improved piston cleanliness and deposit control, reduced soot-mediated viscosity increase and/or wear, and durability of seals and friction materials.
- Provide protection and better operation for a secondary usage of engine oil, hydraulics. In today’s engines, with variable valve timing, cylinder deactivation, and cam phasing for fuel efficiency and lower emissions, the engine oil doubles as a hydraulic fluid.
There are additional minor functions which viscosity modifiers provide, some of which will be detailed later in this article series.
How VMs Control Viscosity with Temperature Changes
There are two ways to explain how VM polymers control oil viscosity. First, it is important to understand that VM polymers are chain-like molecules that readily dissolve in mineral and synthetic base oils. These molecules are coiled chains resembling tiny spheres dispersed in the oil. When lubricating oil is pumped throughout the engine, the polymer coils provide a resistance to flow and thus a boost in viscosity. The amount of viscosity build is related to coil size and polymer concentration.
The thermal mechanism theory explains how polymers flow as they become warmer. The polymer coil expands at high temperature, thus increasing viscosity. At low temperatures the coils contract, taking up less space in the oil, thus reducing viscosity.
The solubility theory states that polymers contract and become more tightly coiled when dissolved in oils with less solubility. Therefore, for a certain concentration of VM polymer, the viscosity of a lubricant formulated with good solubility oil will be higher than one made with less soluble oil. Some say that polymer solubility becomes easier at higher temperatures and explains why polymer coils expand in hot oil and thus raising viscosity.
Both the thermal and the solubility views lead to the same result, that VM polymers thicken the oil at higher temperatures and allow the lubricant to flow more freely at low temperatures compared to base oil alone.
Viscosity Improver Properties
How do we measure viscosity improvement in base oils? What are the properties of viscosity modifiers, and how can we demonstrate them to OEMs and oil marketers?
There is an empirical method that measures the change in viscosity with temperature called – you guessed it – Viscosity Index (VI). This index was originally developed in 1929 by Dean and Davis to classify base oils. With the introduction of viscosity modifiers into engine oils in the 1960s, viscosity index became a valuable tool to evaluate the effectiveness of VMs to overcome the changes in engine oil viscosity due to temperature fluctuations. The index, calculated according to ASTM D2270, measures the relationship between the viscosity of the oil at temperatures of 40° and 100°C. The smaller the difference in viscosity between low and high temperatures, the higher the VI number or viscosity index obtained.
Typical API Group I or II paraffinic-based oils might score 95 to 105 on the Viscosity Index scale. The VI of a multi-grade oil containing VM polymers, such as SAE 15W-40, is around 140, and the VI of an SAE 5W- 30 is around 170. VMs with high scores temper the natural tendency of fluids to thin at higher temperatures and to thicken at lower temperatures.
Within an engine, there are many opportunities for VM polymer chains to be broken into smaller fragments. Linear polymers tend to break roughly half-way down the chain as shown in Figure 2. For example, as a lubricated piston ring moves up and down the cylinder wall, high cylinder temperatures and firing pressures can shear the VM polymer chains, lowering its molecular size. The ability of the VM polymer to resist shearing is called shear stability. Essentially, shear stability is the ability of the polymer to maintain its effectiveness in the oil over a long operating period. There is a measurement for this as well, called the Shear Stability Index (SSI).
A long-standing concern and an issue that has again come to the attention of the automotive industry is that some engine oils shear out of the original SAE viscosity grade, which may increase the incidents of piston and liner scuffing and/or bearing wear. For example, many oil marketers are requiring more shear-stable polymers with permanent shear stability ratings in the range of 25% (SSI of 25) as measured by the Kurt Orbahn 90-pass procedure (ASTM D7109). Oil in this range retains at least 75% of its VM viscosity contribution as measured by this method.
High Temperature High Shear-Rate (HTHS) Viscosity
HTHS viscosity of engine oil is a critical property that relates to fuel economy and engine durability. The drivers behind lowering HTHS viscosity relate to governmental regulations to improve fuel economy and lower greenhouse gases (GHG). Lower HTHS viscosity tends to improve fuel economy and lower GHG in newer vehicles designed to run on low HTHS engine oils, but lowering HTHS viscosity too much can lead to an increase in friction and wear resulting in cylinder liner scuffing as illustrated in figure 3. In general, high HTHS viscosity affords better wear protection. A careful balance must be found when formulating engine oils, and selecting the right VM polymer is an important consideration.
[caption id="attachment_127886" align="aligncenter" width="463"] Figure 3. On left, high performance engine oil protects engines from scuffing wear. Image on right shows a damaged cylinder liner from scuffing wear.[/caption]
Low Temperature Oil Pumpability
Control of engine oil viscosity at low temperature is important because it positively impacts engine operation and longevity. When an engine starts, it is critical that the engine oil can be pumped throughout the entire engine and all the way up to the valve-train as quickly as possible to minimize metal-to-metal contact and maximize wear protection. The correct VM polymer in combination with the right pour point depressant ensures that the oil will be delivered to all the moving parts of an engine. The mini-rotary viscometer (MRV) is used to measure low temperature oil pumpability according to ASTM D4684.
Low Temperature Startability
Engine startability is affected by the engine oil's low-temperature viscosity. Low-temperature cranking viscosity is measured by the Cold Cranking Simulator (CCS) viscometer, ASTM D5293. CCS viscosity relates to how much viscous drag the cold engine oil places on the crankshaft resting on its bearings. If the viscosity is too high, the starter motor and battery could be incapable of starting the engine. VM polymers play an important role in finding the optimal balance of engine oil viscosity to allow low temperature starting, oil pumping, and high temperature engine protection. SAE J300 Table 1 defines viscosity grades for engine oils in terms of viscosity alone. The CCS test temperature depends upon the "W" viscosity grade. The MRV test is run at -5°C lower to assure that if the engine starts then the oil will pump. Catastrophic engine failure could occur if the engine starts but no oil is pumping to provide lubrication to the moving parts.
Soot consists of sub-micrometer-scale particles of primarily elemental carbon. Soot loading in diesel engine oil can present wear and viscosity control problems. Diesel engines consume a carbon-rich fossil fuel that liberates soot as a byproduct of combustion. The soot migrates to the crankcase engine oil via blow-by past the piston rings and accumulates in the engine oil. Excessive levels of soot, if not properly controlled, can lead to both engine oil viscosity increase and soot-related wear.
Lubrizol has developed additive technologies to mitigate soot-related wear and excessive viscosity increase. For example, Lubrizol’s heavy duty diesel engine oil additive product, CV9601, in partnership with LubrizolTM 7418A styrene-butadiene (SBR) viscosity modifier, combines superior viscometrics and excellent pumpability, fuel economy, and durability. LubrizolTM 7418A viscosity modifier has proven advantages in controlling soot- related viscosity increase and soot-related wear.
Another benefit of high-performance VM polymers is cleanliness. Lubrizol offers market-leading SBR viscosity modifiers for use in multi-grade engine oil applications that require outstanding high-temperature cleanliness performance. Formulated with Lubrizol DI technology, SBR polymers meet the most demanding global OEM and industry specifications required of
premium lubricants. These formulations exhibit exceptional deposit control and piston cleanliness for improved durability and wear protection for both passenger car and commercial vehicle applications.
Additional VM Benefits
Viscosity modifiers are also at work in gear oils and transmission fluids of many passenger cars and HD trucks. Temperature variations and other environmental factors that harm engine oils also adversely affect the performance of these fluids.
Transmission fluids lubricate gears, shafts, and other moving parts that transmit power from an automobile or HD diesel engine to the driving wheels. Individual OEMs prescribe requirements for both low-temperature and high-temperature fluid viscosity. VMs help improve the performance and extend the drain interval of these fluids.
Last but not least, gear oils protect and lubricate the gears found in transmissions, differentials, and other types of gearboxes in passenger cars and HD truck engines. Gear oils function at a higher viscosity than engine oils. SAE International maintains a viscosity grade standard for automotive gear oils, J306, which is different than the SAE’s engine oil viscosity grade standard J300. Viscosity modifiers help meet and exceed the requirements for gear oils, helping maintain and improve the gears’ performance in the vehicle.
Next: Part 3 – Future Trends in High Performance Diesel Engine Oils
Key Terms Defined
SAE -- Formerly known as the Society of Automotive Engineers, SAE International is a US-based, globally active professional association and standards organization for engineering professionals.
SAE J300 - the global standard which defines engine oil viscosity grades. Its latest revision, January 2015, establishes ultra-low viscosity grades, SAE 16 (xW-8 and SAE xW-12).
KURT ORBHAN TEST – a diesel injector bench test for measuring shear stability of engine oils.
RHEOLOGY – the study of the flow and deformation of matter.
SSI – Shear Stability Index, which measures the ability of a polymer in oil to maintain its effectiveness over a period of time. Moving engine parts can shear a VM polymer and reduce its effectiveness.
VI – Viscosity Index
VM – Viscosity Modifier
CCS – Cold Cranking Simulator, a bench test to simulate engine cranking or ability to start at low temperatures.
HTHS – High Temperature High Shear is an indicator of oil's resistance to flow in the narrow confines between fast moving parts in an engine.
FILM STRENGTH – The amount of pressure required to force out a film of oil from between two metal surfaces.
VISCOSITY MODIFIER POLYMER –Synthetic molecules consisting of one or more monomer units strung together to form linear, branched or star molecules which dissolve in mineral and synthetic oils to provide multi-viscosity characteristics.
PUMPABILITY – How quickly the oil reaches all lubricated parts of the engine during cold start from the oil sump.
STARTABILITY – The ability of the starter motor and battery to turn over a cold engine against the viscous drag of the engine oil.