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

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.