LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

1. Purpose and Function of the Lubrication System

Purpose and Function of the Lubrication System

Lubrication systems in a diesel engine are built primarily to circulate pressurized engine oil to supply moving parts with a film of lubricating oil Figure 4. Lubricating oil stored in an oil pan is picked up by an oil pump and circulated through the engine by way of oil galleries and other drilled passages. Some oil is used to cool pistons by spraying the underside of piston crowns with pressurized oil. To remove this extra heat picked up by the oil in diesel engines, the lubricating system uses an oil cooler. A variety of valves regulate oil pressure, and filters remove contaminants trapped by engine oil. The lubrication system’s design, and the construction and operation of its components, is also organized to perform the following tasks:

1. Reduce friction between moving parts to minimize engine wear and the creation of heat.
2. Cool a variety of internal engine parts and remove some heat from the engine.
3. Clean contaminants from inside the engine and remove dirt and abrasives from lubricating oil.
4. Assist in the sealing of the combustion chamber by forming a film between the piston rings and the cylinder wall. (Most seals require or perform better with a film of lubricant on rotating surfaces.)
5. Absorb shock loads between bearings and gears, thus cushioning and protecting engine parts while minimizing engine noise production.
6. Store an adequate supply of oil for lubricating internal engine parts.
7. Minimize corrosion of internal engine components.
8. Neutralize combustion contaminants.

FIGURE 10-4: A simplified diagram of a diesel engine lubrication system.

FIGURE 10-5: A screen from a Caterpillar Electronic Technician (Cat ET), where programable engine protection system monitoring conditions are adjusted.

Throughout this chapter, each of the lubrication system components will be examined to understand how they function to achieve the intended purpose of the lubrication system overall. The lubrication system in today’s engines also performs each of its functions by incorporating a variety of physical and virtual sensor technologies to protect the engine from damage. Data from many of the sensors is integrated into the operation of the engine protection system, which is one of many functions regulated by software within the engine control unit (ECU). The engine protection system alerts the driver to engine conditions that can potentially cause catastrophic engine failure. Note that the engine protection system is configured to either only alert the driver, alert and shut down, or alert and derate the engine power Figure 5. Derating the engine power has two purposes: The first is to provide an operator with the incentive to have the condition diagnosed and corrected; the other is to change the operating conditions that could result in engine failure. For example, high oil temperature could be caused by low oil volume combined with high engine load. Reducing engine power in turn lowers the amount of heat absorbed by the oil. Typically, the engine requires an ignition cycle to reset the engine protection system alert.

Virtual sensors are not actually real, like physical sensors. Instead, software makes the sensor data appear real. Virtual sensors typically collect data from other sensors to predict or calculate an unknown and difficult to measure physical quantity. An engine oil life monitor is an example of a virtual sensor. Data from the engine, such as the amount of fuel burned, engine speed, oil temperature, and so on, are plugged into an algorithm used to predict how much service life remains for the engine oil before it requires draining. On the other hand, real, physical sensors in the lubrication systems include:

• Low oil pressure warning switches
• Oil pressure sensors
• Oil level sensors to warn if inadequate oil volume is present before a low-pressure condition develops
• Oil temperature sensors
• Oil quality sensors

1.1. Video: Lubrication System

1.2. Anim: Crankshaft Lubrication

1.3. Friction and Lubrication

Friction and Lubrication

Understanding how lubricant works is a starting point for understanding the purpose and function of a lubricating system. A typical lubricant molecule consists of a long chain of carbon and hydrogen atoms. Because even the most highly polished metal surfaces are rough and irregular at a microscopic level, moving surfaces need to be separated to avoid wear and friction. Oil works by forming a separating film between parts to minimize direct contact between moving parts. Because oil is slippery, it reduces friction and heat buildup caused by friction. Oil is capable of separating moving parts because its molecules behave like billions of tiny ball bearings that physically separate moving metal surfaces Figure 6. At a molecular level, the adhesive forces attracting oil molecules to metal surfaces are greater than the cohesive forces causing oil molecules to stick to one another. This permits lubricants to adhere to the irregular surface of metal yet easily slide over other lubricant molecules. When a supply of oil is available between a rotating shaft and a plain bearing surface, shaft rotation generates oil pressure to form what is called a hydrodynamic wedge . A hydrodynamic wedge actually lifts a rotating shaft journal from direct contact with a plain bearing surface, much like a water ski is pushed up by the water pressure beneath the ski. Like a water ski sliding over a water surface, a shaft glides over a bearing surface with enough lubricant between the two parts Figure 7. When the shaft’s rotational speed increases, more oil is forced into the wedge and the pressure separating the shaft and bearing journal increases Figure 8. The leading rotational edge of the shaft drags sticky oil into the clearance between the shaft and the bearing surface to increase the amount of oil supplied to the hydrodynamic wedge.

FIGURE 10-6: Oil lubrication behaves like ball bearings, enabling metal surfaces to slide over one another without metal-to-metal contact.

FIGURE 10-7: A hydrodynamic wedge is formed between a rotating shaft and a bearing surface. The leading edge of the rotating shaft drags sticky oil between the surfaces.

FIGURE 10-8: The faster a shaft rotates, the greater the pressure formed by a hydrodynamic wedge.

1.4. Bearing Clearance and the Hydrodynamic Wedge

Bearing Clearance and the Hydrodynamic Wedge
It is important to remember a few points about the hydrodynamic wedge and bearing clearances on plain bearings. First, the faster the shaft rotates as it is supplied oil, the greater the pressure separating the surfaces Figure 9. Next, the thickness of the oil film matters. Thicker, more viscous oil forms stronger hydrodynamic wedges than thinner oil, but thicker, more viscous oil requires more space to fit between a bearing and a shaft to form a strong wedge compared to thinner oils. There is also an optimal bearing oil clearance that matches the viscosity of an oil. Thicker, more viscous oil requires a larger oil clearance than thinner, less viscous oil. Too large a bearing clearance combined with oil that is too thin will not form adequate pressure for an oil wedge to support a heavy load. The result can be shaft-to-bearing contact. Last, if bearing-to-shaft clearances are made larger, a greater volume of oil passes through the bearing clearance and eventually leaks off at the edges of the bearing Figure 10. If more oil flows through a larger clearance, more energy is required to push that oil through a lubricating system to maintain oil pressure Figure 11. Note too that ball and roller bearings operate much the same way, except that they do not have as much bearing surface area to form as large a wedge as a crankshaft or camshaft plain bearing.

FIGURE 10-9: Thinner, less viscous oils require a smaller clearance between the bearing and the rotating shaft to provide adequate load support. An optimal bearing oil clearance exists for each viscosity of engine oil.

FIGURE 10-10: As the bearing clearance increases for this viscosity of oil, more oil volume is required to maintain pressure due to oil leak-off.

FIGURE 10-11: Thicker oils require larger bearing clearances and consume more engine power to pump. Power loss is due to higher oil pump volume and the effect of shearing chains of oil molecules at the bearing surface.

1.5. Lubricating Oil Properties and Oil Classification

Lubricating Oil Properties and Oil Classification

Oil is commonly classified by its application, such as whether it is lubricating an engine or gears in a transmission or power divider or whether it is operating in a hydraulic system. Depending on its use, oil may be derived from biological sources, such as fish, vegetables, or animal fat. More often, lubricating oil, even synthetic oil, is primarily a product of petroleum refined from crude oil, also called mineral oil , which originates from the ground. Base stock is the term used for raw mineral oil processed from crude petroleum oil. It is interesting to point out that synthetic oil for lubricating engines is commonly made from selected molecules extracted from petroleum-based oil and are not artificially created. Polyalphaolefin (PAO) oil molecules, which are used to make synthetic engine lubricating oils, are chemically derived from petroleum and have the best characteristics of any hydrocarbon oil molecule used for lubrication compared to any other molecules found in mineral oils.

Conventional mineral oils contain a variety of different hydrocarbon molecules that have various sizes, shapes, and lubricating qualities. This means they will perform and respond differently to heat, pressure, bearing loads, and other engine operating factors. To be approved for the operating conditions and the unique engine technologies of various manufacturers, engine oils are refined and classified for use by the American Petroleum Institute (API) in North America and by the European Automobile Manufacturers (ACEA) in Europe. Before any engine oil is used, a technician must be certain that is has labeling or certification by the ACEA or API indicating its suitability for the types of technology affected by lubricating oil in a particular engine application. Given the variations of engine technology plus potentially different oil characteristics and qualities, the ACEA and API have developed a variety of specifications to define engine oil performance standards. For example, engines using high-speed turbochargers and recirculating exhaust gases into the combustion chamber place different demands on oil compared to lubricant demands in a naturally aspirated engine with little or no emissions controls. Length of drain intervals and the severity of operating conditions are some other factors that are important determinants for the selection of an appropriate lubricating oil.

1.6. S- and C-Series American Petroleum Institute Oil Classification

S- and C-Series American Petroleum Institute Oil Classification
Two fundamental classifications of oils are recognized by API performance specifications for either spark or compression ignition engines. S-series oils, where the S designates spark ignition systems, are designed to meet performance standards for spark ignition systems in engines burning gasoline, propane, or natural gas fuels. C-series oils meet compression ignition engine performance requirements for diesels. S- and C-series oils are each uniquely formulated and use a different additive package to adapt to contamination from combustion byproducts of gasoline and diesel fuel. For the same reason, natural gas engines require a unique oil and additive formulation, but oils for natural gas engines are a subset of S-series oil.

These API S and C standards have progressively evolved since they were first introduced in the 1940s to meet demands for advances in engine technology and engine manufacturers’ performance requirements. The S and C are followed by another letter to indicate what level of revision the oil is certified to meet. For example, the earliest API classification was C followed by the letter A. This classification is now obsolete but was followed by successive standards indicated by progressive alphabetic designations: CB, CC, CD, and so on.

Today’s oils are highly refined petroleum products with a package of chemicals mixed into the oil called additives that enable lubricating oil to meet unique technological operating requirements of engines (Figure 12 and Figure 13) . In response to a request from a group of manufacturers, the API develops any new specification for oils from what is called a temporary proposed category (PC) . In January 2016, the specification that was called PC-11 became CK-4, replacing the older CJ-4 specification needed for engines built in 2007 and later. Since 2007, diesel engines that use exhaust particulate filters required CJ-4 oil that did not contain additives that could harm the catalysts in the aftertreatment system. The next PC-12 will improve oil performance qualities beyond the current API CK-4 engine oils that are compatible with new biofuels and have properties to reduce engine wear for enhanced engine durability.

FIGURE 10-12: API oil performance standards have evolved to help meet emissions standards and changes to fuel quality. Earlier oils had to tolerate 5% sulfur content in fuel, plus high amounts of combustion particulate and acid-forming byproducts of oxides of nitrogen (NOx).

FIGURE 10-13: Changes in technology and emissions control systems have driven changes to oil performance standards.

The number 4 in the API classification indicates that the oil has high levels of detergent. This feature was introduced in the late 1990s to help cope with additional soot loading of engine oil caused by retarded injection timing and to extend oil drain intervals. Typically, the engine manufacturer makes the recommendation to the API for any new classification of oil based on upcoming technology. Engine manufacturers also have their own specifications for oil appearing alongside an API of ACEA classification. This information can be found in the vehicle’s operator manual or manufacturer’s service manual.

1.7. Oil Viscosity

Oil Viscosity Oil viscosity is an important oil property and a term used to describe the difference between how well liquids flow or do not flow. For example, to compare the flow characteristics of maple syrup and water, the property of viscosity is most useful. A commonly used oil viscosity rating is the Society of Automotive Engineers (SAE) viscosity rating, established by the SAE and not the API. Until API standards were first developed in 1947, viscosity was the only way oil was rated. The term viscosity refers to a liquid’s resistance to flow Figure 14. A numerical designation and test standard developed earlier by the SAE is still used to measure oil viscosity. The numbers basically indicate whether a particular oil flows quickly or slowly (i.e., thicker or thinner viscosity). Engine lubricating oils having a viscosity with low numbers, such as SAE 0, 5, or 10, flow more easily than viscosities of SAE 20, 30, or 50. In other words, the higher the number, the thicker the viscosity and the more resistant the oil is to flow.

FIGURE 10-14: Viscosity is a liquid’s resistance to flow. Low-viscosity oil flows easier than high-viscosity oil.

Viscosity is measured using two methods. The simplest and oldest method is kinematic viscosity, which is a measurement of a fluid’s resistance to flow while acted upon by only the forces of gravity TABLE 10-1. It is determined by measuring the time, in seconds, required for a specific volume of fluid to flow a standard distance through a calibrated orifice at a closely controlled temperature. The second method, now used for the latest oils, is dynamic viscosity, which is a measurement of a fluid’s resistance to flow when acted upon by different bearing loads. Under varying pressure conditions, such as between a rotating shaft and a bearing, the viscosity of fluids can change. Dynamic viscosity measurement is considered the best predictor of oil viscosity in real-world operating conditions.

1.8. Oil Temperature and Viscosity

Oil Temperature and Viscosity When it is hot, oil thins and becomes less viscous. Likewise, oil thickens and the viscosity increases as the temperature decreases Figure 15. When an oil’s kinematic viscosity is measured at 0° F (−18° C), it is given the suffix W, indicating that it has a winter viscosity rating. This scale for measuring viscosity is also known as the absolute viscosity rating . Oil is also tested at 212° F (100° C), which is the expected operating range of oil inside an engine. When measured at this temperature, the SAE kinematic viscosity is referred to as a hot or summer viscosity and no letter accompanies the SAE viscosity grade. Multigrade Oils Oils with only a single viscosity number are designated single-grade or single-weight oils. Single-grade oils are obsolete for use in any modern diesel because of the introduction of multigrade or multi-weight oils. Multigrade oils can be a blend of several different oils with different viscosities or oils containing viscosity- changing additives Figure 16. Multigrade oil viscosity today is indicated by two numbers (e.g., 15W-40), which means they have their viscosity measured under both hot and cold conditions. The numbers provide viscosity information about the oil’s performance under a wide range of operating conditions. The first number (e.g., 15W in 15W-40) is the low-temperature viscosity of the base oil, and it helps predict how well an engine might crank in cold temperatures when the oil is thick and resistance to cranking is high. Generally, a lower number means the oil is thinner and less resistant to flowing between moving parts, allowing the engine to start easier in cold weather. The low-temperature or winter number also indicates how well the oil will flow to lubricate critical engine parts during severe cold-starting conditions Figure 17. Colder temperatures require oil with a lower or thinner viscosity to properly lubricate an engine. Pour point depressants—additives that lower the oil’s cold temperature viscosity—may be added to give the oil this property. The use of an engine block and an oil pan heater can also help warm oil to prevent it from thickening too much when the engine is shut down. The second number (e.g., 40 in 15W-40) is the high-temperature viscosity of the oil when it contains an additive to slow down the thinning of the oil’s viscosity as its temperature increases. The danger of hot oil is that its viscosity becomes too low, and it cannot develop a proper hydrodynamic oil wedge. The use of additives to thicken hot oil or mixing oils with different viscosities prevents the oil from becoming too thin and unable to protect the engine. The oil’s second viscosity number does not have a W suffix and is measured at 212° F (100° C).

FIGURE 10-15: Temperature changes an oil’s viscosity. Multigrade oils have a lower viscosity index than single-weight oils. This means multigrade oil viscosity does not change as dramatically with temperature.

FIGURE 10-16: Temperature changes an oil’s viscosity. Additives and blends of oil with different viscosities can alter the rate at which oils change viscosity with temperature.

FIGURE 10-16: Selection of oil viscosity for a diesel engine is dependent upon the operating temperatures expected. Most heavy-duty engines in North America built before 2010 operated using SAE 15W-40-grade engine oil.

1.9. 15W-40 Oil in Diesels

15W-40 Oil in Diesels

SAE 15W-40 has traditionally been almost universally used in heavy-duty (HD) diesels for a long time. The reason is that, generally, thick, viscous oil compared to thinner, less viscous oil provides a good hydrodynamic wedge and better load-bearing characteristics at normal engine operating temperatures. The use of 15W-40 with a hot viscosity of SAE 40 in HD diesels is needed to support heavy bearing loads, a characteristic of HD, high power output diesels operating with very high cylinder pressures. 15W-40 consists of a base oil of SAE 15W that has a viscosity of SAE 40 at operating temperature.

The main advantage of multigrade oil is improved cold-start characteristics with less engine drag. At operating temperature, a 5W-40 oil and a 15W-40 oil will have the same viscosity. When cold, the thinner 5W oil will have less cranking resistance, but it may not provide the protection to bearing surfaces that a 15W oil can. Many HD diesel engines today are capable of operating with 10W-30 viscosity, whereas 15W-40 was mainly used up until 2010. Now, other grades and classifications of oil are replacing 15W-40 to reduce fuel consumption and to meet GHG targets. Enabling the use of thinner, lower-viscosity oil are newer specialized anti-wear additives that can prevent metal-to-metal contact and new bearing technology with harder shell surfaces and scuff-resistant coatings.

1.10. The American Petroleum Institute Service Symbol

The American Petroleum Institute Service Symbol
The API service symbol and certification mark, which resembles a donut, identifies the performance standards of engine oils used in gasoline- and diesel-powered vehicles Figure 18. Engine oil carrying API designations can only be certified after specialized testing to determine whether they meet minimum API standards. Only certified oils meeting minimum API criteria are permitted to carry the API service symbol.

The API donut symbol is divided into three parts: The top half designates the oil’s performance standard set by the API test criteria. The center identifies the oil’s SAE viscosity rating. The bottom half is the API certification mark. Gasoline engines may carry a starburst symbol, whereas diesels do not. In gasoline engines, the mark or starburst symbol indicates whether the oil has demonstrated energy-conserving properties in a standard test in comparison to reference oil. The use of friction-reducing additives, such as graphite, molybdenum disulfide, or other suspended oil additives, provides energy-saving enhancements to the oils.

FIGURE 10-18: The API service symbol and certification mark. It identifies the performance standard (1), the SAE viscosity rating (2), and the energy-conserving or other special properties (3).

1.11. The American Petroleum Institute Certification Mark for Diesels

The American Petroleum Institute Certification Mark for Diesels

Intended for 2017 and later diesel engines to help meet GHG emissions targets, oils that are tested for and meet the API CK-4 specifications are permitted to carry the certification mark. CK-4 was introduced in the 2017 model year for GHG engines using exhaust gas recirculation (EGR), diesel particulate filters (DPF), and selective catalytic reduction (SCR) systems. The PLUS-4 suffix (e.g., CI-4 and CJ-4) indicates that the oil provides a higher level of protection against changes to viscosity related to soot loading and viscosity loss due to oil molecule shear in diesel engines. This 2017-PLUS oil standard is built on the CJ-4 specification for compatibility with diesel engine applications burning ultra-low–sulfur diesel (ULSD) fuel and with the intent to prevent poisoning as well as the plugging of the exhaust catalyst. Because combustion blowby products entering the oil around the rings is different in engines burning ULSD and using EGR, these oils must be capable of holding more soot while neutralizing less acidic oil than engines burning fuel with more sulfur. Starting in 2007, because oil consumed in the cylinders ends up in the particulate filter where oil additives leave ash residue that must be physically cleaned out of the particulate filter, CJ-4 oils were formulated to produce minimal ash residue that could block exhaust gas flow through the DPF. This ash cannot be removed by regeneration. When the DPF is loaded with ash, the filter requires removal and cleaning. The primary differences between CJ-4 and the latest CK-4 are GHG-reducing technologies that require engines to run hotter and consume less fuel. By lowering internal engine friction and energy losses caused by pumping oil, the new oil contributes to the fuel savings. There are several advantages to using the CK-4 oil:

• Improved shear stability: It will not lose its viscosity as quickly due to cutting of oil and viscosity improver molecules when compared to earlier oils.
• Improved oxidation resistance: Oil can combine with oxygen when hot and begin to thicken.
• Aeration resistance: Small air bubbles will not remain dissolved in the oil as easily compared to earlier oils.
• Improved resistance to scuffing: Improved anti-wear additives will lubricate metal surfaces when inadequate oil film is momentarily missing.

1.12. Global Oil Standards

Global Oil Standards
ACEA, a French acronym for the European Automobile Manufacturers Association, has developed standards for diesel engine lubricants using performance specifications similar to the API called the European Oil Sequences . Each set of oil sequences is designated by a two-part code consisting of a letter to define the class of vehicle and a number to identify the vehicle category or application. For example, HD diesels are designated as class E with the additional categories 4 through 9 that are differentiated by severity of service. Together the ACEA classes include:

• A/B = Gasoline and light-duty (LD) diesel engines
• C = Catalyst-compatible oils for gasoline and LD diesel engines with aftertreatment devices
• E = HD diesel engines

The year for introducing the designated category may follow the code. For example, B4-16 means the oil is compatible with LD diesels with exhaust catalysts operating in severe service conditions as designated beginning in the year 2016.

E9 is the latest recommended oil for high-power-output diesel engines meeting Euro I, Euro II, Euro III, Euro IV, Euro V, and Euro VI emissions requirements and operating under severe conditions. It is suitable for engines with or without particulate filters, most EGR engines, and engines with SCR NOx aftertreatment systems. A global oil standard also exists that includes collaboration with vehicle manufacturers in Asia and Japan. These standards, designated DL-1, DL-2, and DL-3, are global standard designations. DLD-1 is intended to provide a basic level of performance while DLD-3 provides the highest level of performance. The International Lubricant Standardization and Approval Committee (ILSAC) only approves oils for gasoline-fueled engines.

1.13. Viscosity Change and Viscosity Index Improver

Viscosity Change and Viscosity Index Improver
Looking at the viscosity numbers of multigrade oil 15W-40, it appears that the oil is thicker when hot and thinner when cold. The explanation for this is that multigrade oils are formulated with an additive to control viscosity change. The additive not only prevents the oil from thinning out when hot but also can actually thicken the oil. Viscosity additives are necessary because of the nature of petroleum oil. Because petroleum oil is made of a variety of hydrocarbon molecules, some of which are light and thin while others are thick and tar-like, changes to viscosity with temperature can occur more dramatically with some oils compared to others. This change is measured by the viscosity index (VI), which refers to the change in oil viscosity with temperature Figure 19. Oil with a low VI will change viscosity faster than oil with a high VI. Oil with a low VI thickens and thins too much with temperature change, giving it an undesirable, uneven lubrication performance. An additive ingredient called viscosity index (VI) improver can be used to control the VI properties of lubrication oil. The VI improver is a special hydrocarbon polymer or, more simply, a chain of oil molecules that changes shape with temperature. The additive minimizes the change in oil viscosity by thickening oil when it is hot and thinning oil viscosity when it is cold.

FIGURE 10-19: The viscosity index of oil refers to how much its viscosity changes with temperature. A high viscosity index changes little while a low viscosity index changes considerably. Ideal oils have a high viscosity index.

FIGURE 10-20: The volume of a viscosity index improver molecule expands with temperature to thicken engine oil as its temperature increases.

The unique property of the VI improver polymer molecule is that when cold, it coils into a smaller ball-like shape. This means that when cold, the oil tends to be less viscous compared to oil without the VI improver additive Figure 20. When heated, the polymer molecule changes shape again, stretching into a bigger ball or a long rod, which thickens the oil. When added to oil, VI improver will counteract the oil’s natural tendency to thin when hot and thicken when cold. Unfortunately, one problem with VI improver additives is that they wear out as the polymer chains are broken through the mechanical action of shearing. Shearing takes place when moving metal edges and lubricated parts actually cut or shear the VI improver molecules, making them much smaller and limiting their ability to control temperature-related changes to oil viscosity.

1.14. Viscosity Index and Shear

Viscosity Index and Shear
Moving oil through pumps and between rotating bearings and shafts at high pressure and scraping oil from cylinder walls literally chops or shears molecules of the oil and its VI improver. When sheared, the smaller oil and VI improver molecules allow a faster temperature-related change in viscosity and loss in viscosity as the oil warms up. The problem with oil when it becomes too thin is that it cannot properly form a hydrodynamic wedge in the oil clearance, which means it cannot properly separate parts or support a load. Higher viscosity enables the oil to support heavy loads without damage. Again, thinner, lower- viscosity oil can leak past piston rings and valve guides and burn, leading to increased oil consumption. It may make sense to use thicker, more viscous oils, but higher-viscosity oils operating with larger oil clearances increase the energy required from the engine to pump oil. To reduce fuel consumption, it is no longer acceptable to use thicker-viscosity oils pushed through large bearing clearances. Newer engine oils with lower viscosity contain additives that prevent metal-to-metal contact, which in turn permit the use of energy-saving, lower-viscosity oil.

1.15. High-Temperature, High-Shear Oil

High-Temperature, High-Shear Oil
Since 2017, recommendations for engine oil used in new diesel engines have changed. The introduction of the new categories of diesel engine oil, FA-4 and CK-4, has signalled that change. The primary difference is that the traditional SAE kinematic measurement method of oil viscosity is no longer considered the best predictor of oil viscosity. Instead, the new standards focus on an oil’s dynamic viscosity measurement, which measures a hot oil’s resistance to flow in narrow areas, such as between a highly loaded bearing and a shaft journal. In high temperatures, oil operating in high shear conditions, such as in the oil clearance between a crankshaft and its bearings, is under extreme pressure, resulting in a density change. Dynamic viscosity, which measures oil’s performance under these conditions, is the current industry standard that best predicts this type of oil behavior in an engine. Dynamic viscosity measurement is like stirring a can of liquid oil. When stirring the oil slowly at low speed, the stir stick may move easily. However, resistance increases when the oil is stirred more quickly. Oils having the same kinematic viscosity will not always have the same resistance when stirred. Instead, oil can have different dynamic viscosity when moved under pressure and between narrow operating clearances TABLE 10-2.

The focus on what is called high-temperature, high-shear (HTHS) oil is in response to the requirement to use thinner, less viscous oils that can operate effectively when hot in narrower oil clearances yet provide superior lubricating properties. As already mentioned, oil viscosity must be thick enough to maintain separation of the critical moving parts but thin enough to match narrower oil clearances. It is the narrower oil clearances that minimize the energy required to pump it through an engine. HTHS oils are evaluated using a measurement of its dynamic viscosity. The viscosity rating of HTHS oil measures an oil’s resistance to flow at operating temperatures under constant shear conditions and high speeds between narrow clearances of moving parts. Using oil with a lower dynamically measured resistance to flow results in increased engine efficiency because less engine power is redirected to pump oil. Measurement units for dynamic HTHS viscosity are in millipascal-seconds (mPa-s) but are also commonly reported in centipoise (cP). Centipoise is the SI unit for measuring dynamic viscosity; 1 cP is equivalent to 1 gram/second/centimetre. Units for the measurement of kinematic viscosity are centistoke (cSt); 1 centistoke (cSt) is equivalent to 1 mm/second of oil movement through a calibrated orifice.

Typically, HD diesel engine oil like 15W-40 has had a minimum HTHS viscosity of 3.5 cP. Lower HTHS viscosity of between 2.9–3.2 cP are now only used in engines with hardened sputter bearings that have coatings capable of resisting scuffing when there is a momentary loss of lubricant film Figure 21.   Technicians Tip     It is important for technicians to understand that engine oils can have the same SAE viscosity grade but different HTHS viscosity since the 2017 recommendation update. Using low-viscosity HTHS oil in an engine designed for high-viscosity HTHS oil or vice versa can cause severe engine damage. This means it is vital to use the correct original equipment manufacturer (OEM)-recommended lubricant to provide the necessary engine protection and fuel efficiency benefits. Observing HTHS ratings on oil containers is one of the best methods to verify that the correct oil is used for an engine application. Knowing which manufacturers’ engines and models require FA-4 oils is another method.

FIGURE 10-21: A polymer coating on this sputter bearing allows low-viscosity, HTHS oils to flow through narrower bearing clearances.