After some study and discussion, members of the European Automobile Manufacturers Association (ACEA) came to the conclusion that the cause was used oils falling below the low-temperature pumpability requirements of ACEA Sequence A/B specifications for gasoline- and light-duty diesel-powered vehicles without emissions aftertreatment equipment. In addition, contamination with biodiesel was believed to increase the severity of the problem.

Since the field failures in Europe, original equipment manufacturers and oil formulators have been working to develop a suitable test to measure the susceptibility of engine oils to low-temperature pumpability failure. CEC (the Coordinating European Council for the Development of Performance Tests for Fuels, Lubricants and Other Fluids) Working Group TDG-L-105 has been charged with this task.

A decision was made by TDG-L-105 to develop a low-temperature pumpability test using an aging apparatus proposed by Daimler Benz. At this point, the proposed test consists of two parts: aging the oil in a modified Daimler Oxidation Test; then measuring viscosity according to ASTM D6896 (MRV/ TP-1) at temperatures specified in SAE J 300 for 49 hours. Each oil is tested three times and checked for the formation of gel structure.

Another Viewpoint

A U.S. veteran of pumpability problems believes that there is a more basic cause of the gelation problems in Europe — the engine oil itself. In an interview with LubesnGreases, Ted Selby, president of the Savant Group, said, Based on experience gained from the more rugged weather patterns of North America, aged oil usually is no worse than fresh oil in its tendency to gel.

Although particular biofuels may increase some oils tendency to gelate, it is important to use sufficiently sensitive bench tests to compare the gelation tendencies of the fresh oil to the bio-fuel contaminated oil. This will avoid obscuring the fact that the problem may initially reside in the engine oil itself.

Engine oil formulations in North America, which are largely based on sensitive pumpability bench tests, have essentially eliminated field gelation problems even with the use of biofueled diesel engines.

Starting automobiles in severe cold became a concern with the advent of electric starters. By the early 1960s, technological improvements had made cold-weather starting much more reliable. However, because engines were then more capable of starting at lower temperatures, in the 1960s and 1970s, OEMs became concerned about engine oil pumpability.

As a result, researchers conducted a large engine cold-room dynamometer study of pumpability that showed both expected and, to some, surprising results. Two forms of pumpability failure were identified. One was the easily predictable slow flow rate of engine oil at lower temperatures, called flow-limited behavior. This behavior is solely dependent on the engine oil viscosity increase with decreasing temperature.

The second form of pumpability failure – called air binding – was a surprise in that, depending on cooling conditions, an oil might become semi-solid at temperatures well above the flow-limited response temperature. Such a response was essentially unpredictable and brought calls for bench tests that would reveal such tendencies.

Air binding has been explained as the tendency under particular cooling conditions for oil molecules to form a structured network – a gel – that restrains the bulk of the oil from flowing. Under the influence of atmospheric pressure, the oil pump draws a vortex in the gelated oil, permitting air to enter the pump and oil supply lines and seriously reduce or entirely eliminate the critical lubricant supply to the engine.

Cold-room engine studies of over a dozen oils were completed by 1970 and provided both flow-limited and air binding pumpability reference oils. By 1979, a bench test called the Mini-Rotary Viscometer (MRV) was shown to correlate with these reference engine oils in their resistance to flow at certain temperatures, either by flow-limited or air-binding behavior.

The MRV instrument and procedure became ASTM D3928 in 1979 and required measuring oil flow resistance at two or three low temperatures to determine a so-called Borderline Pumping Temperature (BPT) of the engine oil. On the basis of its correlation with cold-room engine tests, the method quickly became a part of the SAE Engine Oil Viscosity Classification System.

Failures in Sioux Falls

Unfortunately, during the winter of 1980-1981, a number of engines were seriously damaged in Sioux Falls, South Dakota, U.S., by the pumpability failure of a widely used engine oil. Engines containing this oil had been exposed overnight to temperatures that hovered at minus 9 degrees C for about eight hours and then dropped to about minus 15 degrees for a period before morning. When the engines were started and the vehicles put into normal use, they failed within a few minutes on the road.

Again, these gelation-induced field failures were a surprise to OEMs and engine oil and additive manufacturers, particularly because ASTM D3928 was thought to have addressed pumpability problems. These field failures (and others that also occurred during the same period in Europe) made it apparent that the MRV-BPT bench test procedure did not provide protection against the wide variety of cooling sequences possible in nature.

These failures spurred work to modify the MRV technique, and, by 1985, a protocol called the MRV TP-1 was published that responded to the Sioux Falls field-failing oil as well as earlier reference oils. It became ASTM D4684 and was later required by the SAE Viscosity Classification System.

In response to the Sioux Falls problem, a new gelation-sensitive pumpability bench test was developed in Selbys Savant Laboratories in 1982. It applied a different approach, using a very slow-moving rotor affixed to a special Brookfield viscometer head. Called the Scanning Brookfield Technique (SBT), it measured oil viscosity continuously as the oil was slowly cooled at 1 degree per hour from minus 5 to minus 40 C.

This technique allows both the viscosity and any gel-forming tendency to be monitored continuously. Data is collected throughout the cooling range and analyzed to determine whether gelation occurs and to what degree. According to Selby, the method was shown to predict the field-failing gelation of the oil associated with the problems in Sioux Falls.

In analyzing an oils cold-temperature pumpability, the SBT not only continuously measured the viscosity of the oil but also generated a value called a gelation index, a rating indicating the oils tendency to form a gelated structure at any temperature.

For example, the Sioux Falls field-failing oil had a gelation index of 16 at minus 9 C. Accordingly, specifications set by ILSAC have established that the gelation index should be 12 or less when evaluated over the temperature range of minus 5 to minus 40 C. At present, ACEA sequences do not specify a gelation index limit.

A sister company of Midland, Michigan-based Savant, Tannas Co., markets SBT test machines. Selby said the SBT method is extremely sensitive to the formation of gel and that some in the industry have complained that it is too sensitive. However, over time, the SBT became ASTM Test Method D5133 for fresh engine oils and D7110 for used oils. Since its development in 1982, the SBT has been used in OEM specifications to ensure the formulation of gelation-resistant engine oils. Most recently, it became a required test for engine oils in the API Energy-Conserving SN category and ILSAC (International Lubricant Standardization and Approval Committee) GF-5 specification.

Testing Gel Susceptibility

Selby contended that Europes attention should first be given to fresh oils. To support this contention, he pointed to gelation index data obtained from another Savant sister, the Institute of Materials. In each year since 1982, IOM collected and tested 200 oils purchased from retail sites in Europe. Over that period, the number of samples with gelation indices of 12 or higher has fluctuated but not shown a clear downward trend, although the portion of oils with very high indices – above 36 – has decreased.

In contrast, results of tests performed on oils from the North American market – where specifications require both the MRV TP-1 and SBT tests – show a steady decline and significantly lower portions of products with indices above 12.

Selby said IOMs data indicates that the tendency of North American engine oils to gelate has steadily declined because the bench tests included in engine oil specifications have virtually eliminated gelation and adverse viscous effects. Until recently, he said, Europe has had little reason to be concerned about oil gelation. However, as shown by the test data, engine oil gelation is avoidable no matter what the weather pattern.

Selby went on to say, I believe that the first line of defense in protecting engines against engine oil gelation is to measure and control the gelation index level. With this basic information on gelation, other factors such as biofuel dilution or oxidation effects can then be more clearly identified. With little question, the level of gelation found in fresh European engine oils alone could be expected to produce the engine failures observed during the cold spell of 2008-2009.

Selby contends that cold-temperature performance of North American oils has improved because North American specifications require a rigorous test that checks for gelation throughout the cooling process instead of at just a few temperature points. Pumpability of European oils hasnt improved as much, he said, because they dont use such a test.

Several attempts were made to obtain reactions from European OEMs and oil suppliers. Most did not address Selbys contentions, but a representative of Chevron Oronite said Europeans working on the problem have no plans to adopt a test like the SBT.

Chevron Oronite supports the development of the CEC test, said Emmanuelle Faure-Birchem, product line manager for automotive engine oils in Europe. The objective of the test is to allow selection of oils that will not cause field issues. As a consequence, field correlation is a critical parameter of the test development. We also feel that the proposed development is appropriate to address the new impact of biofuel dilution. There is no evidence at this stage that the Scanning Brookfield test would provide better discrimination of oils, especially because this test is performed on fresh oils only.

CECs TDG-L-105 continues its work to develop a test for low-temperature pumpability. The group has identified four reference oils and is now in the midst of tests meant to assess the procedure. Adoption appears to be at least a few months away.