What you need to know to select the proper oil for your diesel engine

Nov. 1, 2017
Diesel engine oil has been taken to a higher level of performance with the development of new American Petroleum Institute (API) specifications known as API CK-4 and FA-4, which were officially released for licensing on Dec. 1, 2016.

There is no shortage of challenges for those who design the vehicles we drive. Ever-tightening safety and emissions regulations, combined with increasing customer demands, keep automotive engineers continually on their toes. The moment one problem is conquered, they are sent back to the drawing board to deal with the next pressing issue. This has led to vehicles being designed in a more holistic manner, because the “low-hanging fruit” has already been harvested. For any given problem that needs solving, progressively more vehicle systems become candidates for redesign.

Lubricant engineers are not exempt from this dynamic. Engine lubrication is an example of a system that has sometimes been viewed in narrow terms. No one would deny that oil is the lifeblood of the internal combustion engine, however, it is now widely recognized that it plays a critical role in fuel economy as well. Fuel economy has always been important to the diesel engine manufacturers, but their hand is now being forced with the advent of greenhouse gas (GHG) emissions regulations. This issue has gained importance in recent years because fuel economy is directly related to the production of CO2, which is considered to be a greenhouse gas. And now, like never before, engine oil is playing an increasingly important role in meeting those regulations, as well as lowering the total cost of ownership to the vehicle owner.

(Photo courtesy of Shell Lubricants) Diesel engine oil has undergone another transformation with the development of the PC-11 category, which includes the CK-4 and FA-4 specifications.

Diesel engine oil has been taken to a higher level of performance with the development of new American Petroleum Institute (API) specifications known as API CK-4 and FA-4, which were officially released for licensing on Dec. 1, 2016. There are some important changes that the diesel engine owner should be aware of regarding this new category of oils, and this information can help them choose the correct engine oil for their specific application.

An historical perspective

Big changes were afoot for on-road diesel engines leading up to the 2007 model year. The diesel particulate filter (DPF) was being widely adopted as a solution in order to comply with new EPA regulations regarding particulate matter (PM) emissions. The DPF wasn’t a bolt-on fix; there were numerous changes that had to take place in fuel and lubricant technology to make it all work. First, sulfur had to be virtually eliminated from diesel fuel because it disabled the catalyst coating found on most DPFs. The allowable sulfur limit had been 500 parts per million (ppm), now it was being lowered to 15 ppm. Diesel engine oil additive formulations would also have to change. For decades, engine oil blenders had made extensive use of an additive known as ZDDP, which contains zinc and phosphorous for anti-wear protection. Sulfated ash is also used to increase the total base number (TBN) of the oil. These metals formed part of the PM emissions of the engine and would collect as ash in a DPF, thereby shortening the DPF service life.

To address these and other issues, API developed the PC-10 category for diesel engine oils, which led to the release of the CJ-4 specification in 2006. Among other improvements, CJ-4 placed chemical limits on the use of phosphorous and sulfated ash in diesel engine oil in order to extend the service life of DPFs. CJ-4 was backwards compatible with previous diesel engine oils, and met the requirements that had been established with CH-4, CI-4 and CI-4+. All 2007 and newer on-road diesel engines required the use of CJ-4, which became the standard for more than 10 years.

(Photo courtesy of Shell Lubricants) CK-4/FA-4 oils are formulated for better aeration control, increased oxidation resistance, and improved shear stability.

The advent of CK-4 and FA-4

While discussions concerning a new diesel engine oil category started shortly after CJ-4 was introduced, work on the new diesel engine oil specification began in December 2011 with the formation of the New Category Development Team (NCDT), chaired by Dan Arcy, Global OEM Technical Manager for Shell Lubricants. The NCDT committee consisted of members from the API Lubricants Group, Truck & Engine Manufacturers Association (EMA), American Chemistry Council (ACC), and representatives from the major engine manufacturers. The NCDT’s main objectives were to improve specific aspects of diesel engine oil performance, as well as to provide a tool for engine manufacturers to meet the 2017 EPA regulations regarding greenhouse gas emissions.

The NCDT’s work on the new category, initially identified as PC-11, evolved into the development of two sub-categories, PC-11A and PC-11B, which then became known as CK-4 and FA-4. CK-4 is a direct replacement for CJ-4, and while it has improved performance, it is also engineered to be backwards compatible. CK-4 is available in multiple viscosities, including XXW-40 and XXW-30 (by way of explanation, XXW-30 includes 0W-30, 5W-30, and 10W-30). FA-4, on the other hand, is specifically formulated as an oil with lower viscosity and is only available as a XXW-30 grade. FA-4 is not backwards compatible and would only be used if specifically recommended by the diesel engine manufacturer.

(Photo courtesy of Shell Lubricants) The difference between CK-4 and FA-4 is viscosity.  FA-4 engine oil has a lower High-Temperature, High-Shear viscosity and is able to achieve increased fuel economy and lower GHG emissions.

The process of developing a new engine oil category includes creating tests to prove out the performance of the oil. In the case of CJ-4, there were seven tests that were used, but some of these had become obsolete/unavailable over the years, so new ones had to be created for CK-4/FA-4. A total of nine tests are being used to assess the performance of CK-4 and FA-4 engine oils. The following are three main areas where CK-4/FA-4 was improved over CJ-4:

Oxidation resistance. The Mack T-13 oxidation test was developed by Volvo and ExxonMobil to assess the oxidation resistance of CK-4 and FA-4 engine oils. The newest generation of diesels is designed to operate at higher temperatures in order to increase their efficiency. Engine oil reacts with oxygen in atmospheric air, an effect that accelerates as the oil temperature increases. The rule of thumb is that oil life is cut in half for every 18o F (10o C) increase in oil temperature. Oil oxidation is a breakdown in the base oil itself and results in the accumulation of acids, varnish and sludge. Oxidation will also cause increases in oil viscosity and corrosive wear. Additives can be used to reduce the effects of oxidation on engine oil, prolonging its life and extending the drain interval. The net effect is a reduction in operating expenses by not having to change the engine oil as often.

Aeration control. Modern diesel engines are more suspect to oil aeration issues due to higher operating temperatures and pressures. It is inevitable that air will become trapped in the oil; higher performing oils will release the air more quickly. To assess aeration dissipation of CK-4/FA-4 engine oil, the Caterpillar Oil Aeration Test (COAT) is used. The COAT test measures how quickly an oil can release entrained air, and of course, faster is considered to be better. Improved aeration control can help limit oil oxidation, as well as improving its cooling and lubrication characteristics. This is particularly important to HEUI (Hydraulically-actuated Electronically-controlled Unit Injector) diesel engines built by Caterpillar and Navistar, which suffer from major drivability issues (including no-starts) if their engine oil is aerated.

(Photo courtesy of Shell Lubricants) CK-4 was formulated to be backwards-compatible and thus be a direct replacement for CJ-4.

Shear stability. Viscosity (resistance to flow) is the most important property of an engine oil. Multigrade engine oils use viscosity index (VI) improvers to increase their viscosity at higher temperatures. For example, a 10W-30 engine oil starts out as a 10W base oil, then has polymers added that make it act like an SAE 30 oil at engine operating temperature. VI improver polymers are chains of molecules that curl up into a ball at low temperatures, then unwind at high temperatures to increase the oil’s viscosity. A common problem with these polymers is that they are often vulnerable to “shear,” which is being physically cut apart when under severe stress. This can take place when the oil passes through high stress areas such as between helical gears or a cam lobe and its follower.  Shearing is a serious problem because it permanently lowers the viscosity of the oil, which can lead to wear of critical engine parts. There was a time when soot thickening could compensate for shear, but modern engines don’t produce nearly as much soot as they once did. Polymer shear can also lead to misdiagnosis of engine issues based on oil sample results. According to Dan Arcy, “Sheared polymers can make a 15W-40 engine oil test as a 15W-30, which could cause a technician to conclude that they are dealing with a fuel dilution issue.” Arcy continues by explaining that “the cure for this is higher quality polymers that are less suspect to shear.”

More on FA-4

The FA-4 category was designed as lower viscosity to give diesel engine manufacturers another tool to help them meet the latest GHG emissions regulations. Lower viscosity engine oils pump more easily and lower engine-rotating friction, which translates into increased fuel economy and, therefore, reduced carbon dioxide production.

There are four different tests that are used to assess the SAE viscosity of engine oil. These include:

  1. Low-Temperature Pumping Viscosity
  2. Low-Temperature Cranking Viscosity
  3. Low Shear Rate Kinematic Viscosity
  4. High-Temperature High-Shear (HTHS) Viscosity

Of these four, the tests that define the SAE viscosity grade (for example, the 30 in 10W-30) are Kinematic viscosity and HTHS. The HTHS test is a defining characteristic of CK-4 and FA-4 engine oils. It is important to note that the only difference between a CK-4 and FA-4 oil of the same grade is that the FA-4 oil will have a lower High-Temperature, High-Shear (HTHS) viscosity. For example, a 10W-30 CK-4 oil is spec’d at the high end of the HTHS viscosity range for 10W-30 oils, where a 10W-30 FA-4 oil is spec’d at the low end of the viscosity range.

(Photo courtesy of Shell Lubricants) The API donut symbol for FA-4 is unique in order to help prevent incorrect purchases.  FA-4 currently has limited diesel engine manufacturer approval for use in 2017 diesel engines.

FA-4 currently has limited endorsement for 2017 diesels. This includes GM, FCA and Ford, none of whom recommend FA-4 for use in the diesel engines available in their heavy-duty pickups. The initial licensing date for CK-4 and FA-4 was originally slated for April 2016, however, various delays in development pushed this date back to December 2016. According to Kevin Ferrick, API Engine Oil Program Senior Manager, “The late release of the CK-4 and FA-4 categories might not have given the diesel engine manufacturers enough time to assess the use of FA-4 oils in their latest engine designs. For this reason, and perhaps others, we have seen limited adoption of FA-4 as a recommended engine oil.”

While FA-4 is currently awaiting approval from almost all of the diesel engine manufacturers, not everyone has endorsed CK-4 as a replacement for CJ-4. Shortly after API introduced the CK-4 and FA-4 standards, Ford Motor Company issued a position statement that said, “Ford will not be recommending the use of CK-4 motor oils in any Ford diesel engines, new or old. Ford testing has shown that some CK-4 type formulations have shown inadequate wear protection compared to CJ-4 formulations developed and licensed before 2016.” Despite what appears to be a prohibition of CK-4 oils, Ford goes on to state that they have developed a diesel engine oil standard of their own known as WSS-M2C171-F1, and that their customers should use an oil that meets this specification. At the time the position statement was published, there weren’t any oils available that had been approved for WSS-M2C171-F1 and PowerStroke owners were being told to use a CJ-4 engine oil in the interim. Since that time, Ford has accumulated a long list of approved CK-4 oils; the current list can be viewed at MotorAge.com/FordCK4.

The difficulties Ford identified with the use of CK-4 engine oil in PowerStroke engines are most likely related to chemical limits on the use of the anti-wear additive phosphorous in some formulations of CK-4. Ferrick explains; “If an engine oil blender wants to have their 10W-30 diesel engine oil approved for both the CK-4 and SN categories, they cannot use more than 800 ppm phosphorous. In contrast, an XXW-40 engine oil that is approved for CK-4 can have up to 1200 ppm phosphorous.” In their position statement, Ford says that a 10W-30 oil that is approved for both CK-4 and SN cannot meet WSS-M2C171-F1 and should be avoided.

Ford Motor Company has expressed concerns on the use of CK-4 in PowerStroke diesels.  To get Ford’s approval, a CK-4 engine oil must meet their WSS-M2C171-F1 specification.

The bottom line? You can use CK-4 motor oil in a PowerStroke diesel, provided it meets the Ford Material Engineering Specification WSS-M2C171-F1. It should be listed right on the bottle, otherwise check the link above to see if your engine oil has Ford approval. For any other brand of diesel, do your research and only install the oil that is recommended by the engine manufacturer. You don’t want to make a mistake that could shorten the life of your very expensive diesel, and could jeopardize your warranty as well.

The List is Growing

Shell Lubricants has recently announced that they have a total of six CK-4 approved engine oils that also meet the Ford WSS-M2C171-F1 specification. Other oil blenders are making similar announcements as they continue to develop their products to meet the needs of the marketplace. While the list of Ford-approved oils is growing, you must continue to do your research and make absolutely sure that you purchase and install the correct engine oil for your application. The days of “one size fits all” are long gone, and every diesel engine owner must stay informed about the specific engine oil requirements for their vehicle.

The future

CK-4 and FA-4 were a long time in coming, and the hope is that it will be around for some time to come. In the meantime, API has established a work group to start the discussions on how the next category of diesel engine oil will be developed. To be sure, the drivers will be the same as they were for CK-4 and FA-4; a new phase of regulations known as HD-EMA GHG Reduction Phase 2 kicks in 2021 to 2027. According to Shell’s Arcy, “look for a move towards 0W-20 and 5W-20 oils and increased drain intervals for HD fleets. However, so far no change is being talked about for pickup trucks.”

There are two things we know for sure: requirements for engine oil will increase and lubricant engineers will be working hard for the money. And, at the end of the day, we will have diesel engines that produce more and pollute less.

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