Viscosity is arguably the most fundamental property of lubricants. Almost all consumers have heard of 10W-30 oil (their heads may explode, though, when they first see an SAE 0W-12 oil). SAE J300, Engine Oil Viscosity Classification, is the standard for defining the viscosity grade of vehicle engine oils.
Dating back to 1911 and regularly updated, SAE J300 initially defined kinematic viscosity at low-shear conditions for two temperatures: 40 degrees C (KV40) and 100 C (KV100). Measurement was made by a capillary viscometer and reported in mm2/sec, although its more common to cite results in centiStokes (cSt) now.
Over time, more tests were added to address engine needs, such as shear rates and higher operating temperatures, always using standard ASTM test methods that are available industrywide. Table 1 on page 30 lists the current J300 grades – newly updated for 2015 – and shows their defined limits.
As the table shows, low-temperature viscosity is also defined in J300 using dynamic tests. One is the maximum cranking viscosity, which relates to the oils facilitation of starting the vehicle at low temperatures. This a high-shear viscosity that is measured by the cold-cranking simulator (ASTM method D5293). Another is the maximum low-temperature pumping viscosity, which relates to the oils ability to circulate throughout the engine; this is a low-shear viscosity that is measured by the Mini Rotary Viscometer (ASTM 4684). The temperature at which each low-temperature test is conducted is dependent on the SAE viscosity grade of the oil. Both parameters are reported in mPasec, or centipoise (cP).
Finally, high-temperature, high-shear viscosity is defined at 150 C (HTHS150) and measured by tapered bearing or tapered plug methods at a shear rate of 106 sec -1, or by a capillary viscometer at 1.4 x 106 sec -1. (It is difficult to explain 106 sec -1 in an easily understandable way. Lets just call it a hunka, hunka churning stuff.) HTHS is reported in mPasec or cP, and is believed to be related to oil film thickness in high shear areas of the engine.
Although not specifically covered in J300, viscosity index (V.I.) is another key physical property of engine oils. It is a measure of the variation in kinematic viscosity with temperature. A log-log plot of viscosity vs. temperature gives a straight line that is inversely proportional to V.I. (See Figure 1.)
V.I. is calculated from an oils KV40 and KV100. This formula was developed long ago; a poorly refined naphthenic base oil was somewhat arbitrarily assigned a V.I. of 0, and a highly refined paraffinic base oil was assigned a V.I. of 100. With todays hydrocracked and synthetic oils and viscosity index improvers, a V.I. far greater than 100 is now achievable.
Figure 1 shows the difference between high V.I. and low V.I. oils. The higher the V.I. of an oil, the lower the variation of its viscosity with temperature. A benefit of a high V.I. oil is that it can be blended to meet the same minimum HTHS150 as a low V.I. oil, but will have a lower viscosity over the rest of the (lower) temperature range – potentially resulting in a fuel economy improvement.
Currently most original equipment manufacturers specify a minimum HTHS150 to ensure sufficient oil film thickness in hydrodynamic lubrication conditions, such as youd see in their engines connecting rod and main bearings.
A very high V.I. oil could be formulated to meet a minimum HTHS150, but actually fall below the minimum KV100 requirement, as Figure 2 illustrates. Instead of a log-log plot, this graphic shows a linear plot to more clearly highlight the viscosity differences at lower temperatures. The 164 V.I. oil has an HTHS 150 of 3.0 cP, and the 216 V.I. oil an HTHS 150 of 2.9 cP; both meet the HTHS150 minimum of 2.9 cP for an SAE 5W-30 oil.
As Figure 2 shows, the KV100 of the 164 V.I. oil is 10.0 cSt, well above the SAE J300 minimum of 9.3 cSt. The KV100 of the 216 V.I. oil however is 8.5 cSt, below the minimum of 9.3 cSt, which would drop it to an SAE 5W-20 oil – despite the fact that it meets the HTHS150 minimum for an SAE 5W-30 oil.
Comparing the KV40 of the two oils in Figure 2 shows that the higher V.I. oil is 40 percent lower. For those who are not oil geeks, think of the difference between the two lines as fuel economy potential.
So, what is all the point of all this? The contention is that the current KV100 minimum may actually hinder the fuel economy achievable for a given viscosity grade with a very high V.I. oil.
Figure 3 illustrates the fuel economy obtained with a very high V.I. oil (which still meets the KV100 minimum) in four different fuel economy test cycles, the NEDC, FTP75, WLTC and JC08. These data are from tests conducted on a 2.0-liter Opel turbocharged engine on an engine dynamometer.
The reference oil in this test was a commercial oil meeting ILSAC GF-5 and Dexos 1 specifications. The very high V.I. test oil uses the same detergent/inhibitor additive package at the same concentration, the same base oils – with a different V.I. improver.
The V.I.s of the two oils are 164 for the reference oil and 242 for the test oil. The repeatability for three tests on each oil and each test cycle are excellent, as shown by the range of results; they varied less than 0.4 percent and usually less than 0.2 percent. (The repeatability shown for the reference oil was the worst repeatability over the four cycles.)
The fuel economy improvement with the very high V.I. oil over the commercial oil ranges from 0.8 percent to 1.6 percent; the variability from cycle to cycle is due to the different appetites of the test cycles, and is not unexpected. This level of fuel economy to the OEMs is like the latest cell-phone technology to a teenager.
Hopefully these (and yet to come) fuel economy data are compelling enough to provoke discussion on the need for consideration of revising the KV100 specification minimum in SAE J300. Possible options include lowering the KV100 limits for each grade, or changing the parameter to a report rather than being critical.
So what is the path forward? The SAE Engine Oil Viscosity Classification Panel would likely be the proper vehicle for discussion. I believe that, at a minimum, this panel would probably want to first collect more fuel economy data to prove that there is compelling incentive to proceed.
Next would be to generate a list of all potential issues associated with such a change. (This may not be too difficult, as people generally become more vocal when they feel threatened by a change!) For instance, will there be any durability issues not adequately addressed by HTHS150? Could oil-pump flow capacity be compromised? Could oil consumption be a problem? There are likely several others, as well.
The next step would be to sort out which are technical issues and which are commercial issues. For the technical issues, data or testing required to adequately address these issues should be defined, and a cooperative program designed. For the commercial issues – such as this change being problematic for a small oil market for which SAE J300 was never intended – possible solutions should be discussed. We should be clear, however, that commercial issues should not be a show stopper.
Many would say this
proposal is blasphemous
– and yet still,
Weve done much despite ourselves
when we decided we will.
Its naive to believe this change could be straight-
forward or quick,
But its time to get moving, the clock continues to tick.
Based in the Detroit area, Don Smolenski is Evonik Oil Additives OEM liaison manager for North America. He has been involved in developing engine oil tests and specifications, such as ILSAC and General Motors Dexos, for more than 30 years, and is the co-creator of GMs patented Oil Life Monitor. He is also the parent of 22 kids, including 19 foreign exchange students over the years. E-mail him at donald.
smolenski@evonik.com