Past success can make future progress harder for heavy-duty lubricants. But even then, there’s been headway made in developing tests for Proposed Category 11 (PC-11), the North American heavy-duty diesel engine oil specification about to debut in late 2016.
Today’s heavy-duty diesel engines show very low wear levels and carbon deposit build-up. Recent advances in engine hardware and oil quality mean that both wear and build-up are close to the level of random noise, which makes it difficult to measure differences between oils.
“Options moving forward would be to either increase the test length or develop a more precise measurement technique. Both wear and deposits can be dramatically affected by oil condition, so, future tests may be required to ‘break’ the oil and then measure the wear and deposits,” said James McCord, manager of the engine lubricants research department at Southwest Research Institute (SwRI) based in San Antonio, Texas, U.S.A., which is actively involved in the PC-11 test development.
McCord spoke in May at the annual meeting of the Society of Tribologists and Lubrication Engineers (STLE) in Dallas, Texas.
The key drivers behind PC-11 are a desire for increased fuel economy, along with greater engine durability and longer drain intervals. For instance, oxidation and nitration stability is vital to achieving longer drain intervals, as greater stability means that the oil can be used longer before it must be changed.
To measure stability, the industry has developed a Volvo T13 test to check oxidation control. Future tests will likely
combine an evaluation of oxidation and nitration, possibly along with an end-of-test measurement of deposits and wear, McCord said.
PC-11 drivers may work against each other to a degree. Hence, there must be a balancing act, according to Stede Granger, Shell’s OEM technical services manager. To see the trade-offs involved, consider one important parameter: viscosity. Lowering viscosity would increase fuel economy by reducing engine friction. That, however, would do little good if the oil did not protect an engine against wear. The challenge is made greater because future engines are expected to run at elevated cylinder pressure, combustion temperature and overall sump temperature, compared to present engines. These changes are a consequence of attempts to improve fuel economy.
“We started looking very early on at the ability to lower the friction of the engine oil and still properly lubricate the engine,” Granger said. “How low can you go and still maintain engine durability?”
Shell has worked with OEMs in making this evaluation, he said. For instance, the company ran a 0W-20 candidate oil in an engine that previously had been running a 15W- 40 product, conducting the test for 1,000 hours at very high horsepower. The results were good enough to give confidence that things were moving in the right direction with low viscosity PC-11 oils, Granger said.
The new candidate oils have to match the current ones in terms of wear and carbon deposit build-up, as well as other areas. The industry has no desire to go backward from the current excellent results, he said.
Gary Parsons, global OEM and industry liaison manager at Chevron Oronite, pointed out that heavy-duty engines are changing. “There is a general trend towards smaller displacement, high output engines, for example 13 liters, which have higher power density and operate at elevated cylinder pressures or BMEP. New piston and ring designs are often required in these high output, smaller displacement engines,” he said.
At the same time, when PC-11 is launched, it will go into new as well as old engines. PC-11 has two types. PC-11A oils will be backward compatible. PC-11B oils, which will be offered in lower viscosity grades, will have limited backward compatibility. That is one reason why wear performance will continue to be assessed as it is in the current API CJ-4 spec, to help ensure backward compatibility.
The evaluation will be done using the Cummins ISM test for cross head and injector adjusting screw wear, Cummins ISB test for cam and tappet wear, GM 6.5L RFWT for roller follower wear, and the Mack T-12 test for ring and liner wear. These tests are run at high operating loads and relatively high engine oil soot levels, with the Cummins ISM test being run on 500 parts-per-million sulphur diesel fuel to increase severity, Parsons said.
The development of PC-11 is complicated because of these two types. There is the potential for market confusion between these two types of oils, and so there is a need for end-user education, Parsons said. PC-11A will be designated API CK-4 when it is released in the marketplace and PC-11B will be designated API FA-4.
Not all OEMs have exactly the same goals in the development of PC-11. For instance, Caterpillar, the heavy equipment maker, is not looking to the lubricant as a way to increase fuel economy, said Hind Abi-Akar, technical expert in Caterpillar’s fluids group. Instead, in off-road applications, fuel economy comes from the design of the engine and machine systems integration.
“Oils are desired to provide optimal components’ protection and to allow the durability designed into the engines and components to be achieved. Low viscosity oils must be validated to prove that they can protect HD engines, in particular under the high loads that are experienced in these [off-road] applications,” Abi-Akar said.
Data from field use indicate that improved aeration testing in PC-11 is needed, she added. One issue is that the current test engine won’t be available. Another is that the current aeration test results do not match what is seen in the field. So Caterpillar is developing a new aeration test.
Past and future requirements present challenges to new oil specifications and test development. New oils, for the most part, must be backward compatible with older engines. That means that some of the old tests must be kept, which can be difficult, particularly if the engine or parts are no longer available.
As for the future, there are increasing numbers of engines that are less than five-liter displacements. At this size, such engines fall between traditional heavy-duty and light-duty diesel engines. Currently, North American engine makers primarily depend on industry specifications for heavy-duty oils, together with their own proprietary requirements. But they may someday require – and have the volume to support – their own specifications.
At present, this is not a critical need, said Abi-Akar. She said that Europe has a significant population of small diesel engines, and specifications developed there give insight into the need for such specifications elsewhere. The difference between such specifications and those of heavy-duty lubricants is one reason why she deemed the development of a new spec and tests to not be critical.
Test development for a new oil category or to update an existing one takes money and time. McCord estimated that the test development costs for PC-11 are somewhere between USD 5 and 7 million.