![]() ![]() Low shear kinematic viscosity (KV100) contributes to the definition of monograde oils and provides a viscosity range for high-temperature viscosity grades in multigrade oils. HTHS dynamic viscosity contributes to the SAE viscosity grade definition (for example, the 40 in 15W-40). Low-temperature pumping and cranking viscosities define the winter grade (for example, the 15 in 15W-40). SAE oil viscosity grades are defined by four different tests. Therefore, you could have two separate 10W-30 engine oils produce two different results in fuel economy. Engine oils closer to 9.30 cST will provide better fuel economy benefits than oils closer to 12.49 cST. For example, 10W-30 engine oils can fit anywhere in the range of 9.30 cST to 12.49 cST. “W’ grades, or the number before the “W” in the SAE viscosity, are measured at 40☌. SAE engine oil viscosity is measured in centistokes (cST) at 100☌. The stated viscosity of an oil does not represent an exact viscosity, but rather a viscosity within a range on the Society of Automotive Engineers (SAE) viscosity scale. HTHS is seen as a more accurate method for measuring viscosity. One such method is called the high-temperature, high-shear (HTHS) viscosity (defined as an oil’s resistance to flow under high-stress conditions at operating temperatures) that distinguishes CK-4 oils from FA-4 oils for modern engines. There are different types of viscosity measurements that can be used to describe and measure the viscosity. Lower viscosity oils pump more easily and help to reduce engine friction, which can improve fuel economy. truck increasing its fuel economy by just 1% would see an annual reduction of over four million tons of carbon dioxide emissions, which is the equivalent of removing more than 20,000 trucks from the road. It is concluded that to adequately reflect the effects of oil viscosity on engine performance in the field, the classification system must be revised to include a measure of low-temperature oil fluidity and a high-temperature, high-shear measure of viscosity which correlates with engine performance - a measure which has not yet been developed.Even the smallest increases in fuel economy can result in significant reductions in fuel consumption and carbon dioxide emissions. Several suggestions for improving the system are offered, although no one particular approach is advocated. Moreover, the high-temperature viscosity grades are based on an unrealistically low temperature of 98.9☌ (210☏) and an unrealistically lowshear (kinematic) viscosity compared with engine operating conditions. It is pointed out that the system has become quite complicated with four low-temperature and four high-temperature grades, five footnotes, and an appendix. ![]() A critical assessment of the present classification is made from opinions offered at an SAE Open Forum last year. Reasons for both high- and low-temperature viscosity grades, multigrading, and footnotes to the viscosity table are discussed, as well as other classification systems which have been discarded over the years. To understand how the present system evolved, the history of the classification is traced, from the original version first published in 1911 to the present 1976 version. An extensive effort is under way to revise the SAE Engine Oil Viscosity Classification System to reflect user needs more realistically. ![]()
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