In the past, some shearing tended to be offset by oil thickening, but modern lubricants successfully resist oxidation- and soot-induced viscosity increase extremely well. This means any shearing of the oil could cause it to fall out of grade if it is not properly formulated, potentially causing wear problems.

Engines need to have a proper level of oil film thickness to protect the moving parts, and this film thickness is a function of viscosity. If oil loses viscosity by shearing in the engine, then general wear protection from the oil film will decline over time, explained David Morgan, Chevron Oronites team leader for viscosity modifiers.

The Sequence VIII engine test has been used since the introduction of ILSAC GF-3-the passenger car engine oil specification that debuted in 2001-to measure viscous shear stability, in addition to evaluating lubricant performance in preventing corrosion in copper, lead and tin bearings. But as with any engine test, it is costly, especially when measuring the shear stability of multiple formulations. The test has flexible base oil interchange and viscosity grade read-across rules for bearing corrosion but not for shear stability.

The diesel injector shear stability test (ASTM D6278), also known as the Kurt Orbahn test, has been used as a surrogate bench test for many years. While it is an inexpensive test, rules for this alternative do not encompass emerging viscosity grades such as SAE 0W-16. Early this year, Chevron Oronite initiated an effort to extend the use of the Kurt Orbahn test.

Laura Birnbaumer, product qualification program manager with Chevron Oronite, explained the background for the initiative:

In 2000, ASTM Internationals Passenger Car Engine Oil Classification Panel formed a team to correlate the 30-cycle ASTM D6278 bench test to the Sequence VIII test, which hinges on the viscosity of the oil after running in the engine test for 10 hours-referred to as the oils stripped viscosity. In addition to the fact that bench tests are more cost effective than engine tests, she noted, ASTM may have been motivated to save Sequence VIII engine parts in order to extend the engine tests life.

The team generated 158 data points across numerous additive and viscosity modifier technologies. They recommended test limits for SAE XW-20, XW-30, XW-40 and XW-50 viscosities, set so that an oil passing the ASTM D6278 test will also pass the Sequence VIII test. Its important to note, however, that while results from ASTM D6278 can steer formulators in the right direction, bench testing is not a substitute for engine testing and oils must ultimately pass the Sequence VIII test in order to receive ILSAC or API certification.

The ballot to adopt the recommended bench test limits into ASTM D4485 (the specification that covers engine oils for light- and heavy-duty engines, including the API S, C and Resource Conserving categories) failed. However, ASTM sent the proposal to the American Petroleum Institute, which accepted the recommended limits into its Engine Oil Licensing and Certification System (API 1509).

Oronite proposed extending the use of ASTM D6278 as an alternative to the Sequence VIII stripped viscosity test to SAE XW-16 and XW-60 viscosity grades, Birnbaumer said. (SAE XW-60 oils are sometimes marketed as racing oils but not commonly used.) In January, the company presented its data to the API BOI/VGRA Task Force, and asked other members to contribute data. Four other member companies obliged, and a total of 136 new data points along the viscosity grade spectrum were added. Oronites initiative would not be successful without these other companies providing data and historical documents, she stated.

The additive maker has statistically analyzed all 294 data points and will make a proposal for new limits for XW-16 and XW-60 oils in the ASTM D6278 test, as well as changing the existing XW-20 limit to reflect a 2010 change in SAE J300 limits for that viscosity, according to Birnbaumer.

A version of the Kurt Orbahn test, ASTM D7109, is used for heavy-duty engine oil specifications such as API FA-4 and CK-4. However, ASTM D7109 is run for 90 cycles, making it an even more severe test.

At a late June meeting of the BOI/VGRA Task Force in Phoenix, the group reached consensus on a final proposal. Birnbaumer and David Brass, lubricants formulation engineer at Infineum, were tasked with preparing a final recommendation so a proposal can be submitted to the API Lubricants Group to expand the use of the Kurt Orbahn test for SAE XW-16 and XW-60 oils.

Shearing of engine oils has a large impact on engine oil design, and that impact is growing as specifications impose tighter test limits and introduce new tests. SAE J300 has a fairly wide range of allowed viscosities at 100 degrees Celsius. For example, SAE 10W-30 ranges from 9.3 centistokes to less than 12.5 cSt, and 15W-40 ranges from 12.5 to less than 16.3 cSt.

But for API CK-4 and FA-4, the latest heavy-duty engine oil specifications, formulators need to be concerned with high-temperature high-shear viscosities, measured at 150 C, as well as HTHS measurements after shear. Alongside the dangers of shear stress, high temperatures can also cause breakdown of the oil film that protects moving parts from wear. Taking HTHS measurements into account significantly narrows the formulation window and potentially restricts the components that can be used to formulate oils.

For example, an SAE 10W-30 oil needs to be formulated at the high end of the SAE viscosity specification to meet a minimum HTHS of 3.5 cSt, and most API CK-4 engine oils will have viscosities greater than 11.5 cSt-well above the 9.3 cSt minimum. API FA-4 oils, which are formulated specifically for fuel efficiency, are at the low end of the specification, closer to 10 cSt at 100 C to meet a minimum HTHS of 2.9 cSt and a maximum of 3.2 cSt.

Viscosity modifiers have a dominant impact on shear stability and related testing. Viscosity modifiers can permanently shear, especially as they pass through engine oil pumps or gears, said Oronites Morgan. So it is important to select the appropriate type of VM to meet the shear stability requirements of relevant industry and OEM specifications so as to ensure extended durability.

In general, shear stability is inversely related to the thickening efficiency of a given viscosity modifier, he continued. Use of a less shear stable VM can be more economical by providing a greater level of thickening for a given amount of polymer. The relatively lower amount of polymer can also lessen the tendency for deposit formation. He noted the additional benefit that some viscosity modifiers with high thickening efficiency provide better fuel economy performance because of temporary shear thinning of the oil.

A more shear stable VM will help to maintain the oil viscosity through its useful life, said Morgan. Lubricants must maintain viscosity to protect the engine from wear over a wide range of operating conditions. VMs are selected to balance performance of the lubricant in terms of shear stability, deposit tendency, low-temperature viscosity, fuel consumption and of course cost.

Oil viscosity is a critical parameter for oil health and performance in the engine, asserted Hind M. Abi-Akar, fluids technical expert with heavy-duty OEM Caterpillar. Most Cat engines allow the use of 10W-30 oils; hence, it is critical that shearing of the engine oil is known, repeatable and predictable so that oils will not fall out of grade and go below our recommended viscosity limits. Too much shear, she explained, could necessitate more frequent oil changes or lead to wear problems over the long term.

This may be part of the reason that Caterpillar is approaching API FA-4 oils with caution. We are currently investigating the use of FA-4 oils, Abi-Akar said. Plans are being defined, and for the immediate future our primary recommendation is still API CK-4 SAE 15W-40. She pointed out, however, that Caterpillar allows SAE 10W-30 viscosity API CK-4 oils for cold-temperature applications in most of its engines.

Oil shearing [permanent shear thinning] and oil film robustness will be of even greater importance as we investigate lower viscosity grade oils, she went on. Engine temperature and pressure are key considerations as trends advance towards lower fuel consumption, increased brake mean effective pressure and improved engine performance.

Morgan of Oronite also affirmed that future SAE viscosity grades will be impacted by shear stability. It is possible to formulate SAE 0W-8 and SAE 0W-12 oils with varying amounts of viscosity modifier, so testing is necessary to ensure shear stability, he pointed out. Cost-effective tools will be needed for this testing, like the Kurt Orbahn test.

There was a proposal made at the June ASTM Auto Oil Advisory Panel meeting that the KRL test should be used to measure shear stability for ILSAC GF-6B, as the Sequence VIII test was determined to be unnecessary for the very-low-viscosity specification because it is not backwards compatible. The KRL tapered roller bearing shear stability test (DIN 51350-6 or CEC L45-A-99) is a severe test that is mainly used for hydraulic fluids and gear oils. Angela Willis, chair of the ASTM Passenger Car Engine Oil Classification Panel, noted that the International Lubricant Specification and Advisory Committee would also consider the Kurt Orbahn test for this purpose.

Engine oil viscosity will remain critical to oil formulation and engine operation. As heavy-duty OEMs move toward recommending API FA-4 oils and light-duty automakers specify SAE 0W-16 or lighter oils, the safety margins for shear thinning continue to be reduced.

Steve Haffner is president of SGH Consulting LLC. He has over 40 years of experience in the chemical industry, primarily with Exxon Chemicals Paramins and Infineum USA. He specializes in engine oil formulation and marketing. Contact him at sghaffn2015@gmail.com or 908-672-8012.