Market Topics

A Volatile Situation


Engine oil volatility is commanding considerable attention as engine oil viscosity creeps lower and drain intervals extend further. Accordingly, considerable effort is being made to improve the precision and consistency of instruments used to measure volatility, and the flaws in ASTM D5800 are becoming glaring.

In September, the ASTM Test Monitoring Center issued a bulletin on the D5800 bench test procedure identifying a monthly severity adjustment for test equipment. TMCs Frank Farber described it as an adjustment to be applied based on an instruments demonstrated severity level using approved Lubricant Test Monitoring System criteria, similar to the way engine tests are adjusted for lab severity bias. The level of adjustment is based on the specific stands reference data, varying from instrument to instrument. This is unprecedented, as ASTM repeatability and reproducibility data are normally the only requirement for bench tests.

With questions about the reliability of volatility tests and the necessity of monthly severity corrections, the industry has begun an intensive effort to determine whether D5800 will continue to be the preferred method for volatility testing, or whether other procedures can give equivalent results on a more reliable basis.

In the oil industry, volatility describes how readily a petroleum product vaporizes. For lubricants, this can have a powerful impact on engine operation and longevity, which is why volatility requirements are a key part of engine oil specifications.

Ted Selby, Savant Group founder and vice president of technical development, notes that in the last decade concern about engine oil volatility has increased as a result of three factors: the trend toward longer drain intervals, the use of lower viscosity engine oils to improve fuel efficiency, and the phosphorus that is independently volatilized from some antiwear and antioxidant additives.

Engine oil volatility ties strongly into drain intervals. Low volatility means less oil evaporates during operation. Higher volatility oils will lose more of their volume to the environment, which also results in greater emissions of air pollutants.

Increased oil loss poses a threat to engines, especially if motorists check oil levels too infrequently. If an engine becomes starved for oil, it can suffer premature wear and even failure. Fast oil change outlets have reported servicing vehicles with upwards of 10,000 miles since the last oil change that are two to three quarts low on oil. Oil that is used significantly beyond its prescribed drain interval can become heavily oxidized and too viscous to drain from the engine, let alone lubricate it effectively.

The heavy duty engine market is pushing toward longer drain intervals for two reasons. First, less frequent maintenance would reduce downtime for commercial operations vehicle fleets. When vehicles are down, they are not producing revenue.

Second, improvements that formulators have achieved with the latest engine oil categories for heavy duty diesel engines (API CK-4 and FA-4) have made it possible for original equipment manufacturers to increase their oil drain intervals-a goal incentivized by proposed U.S. government regulations on fuel economy and carbon dioxide emissions. Engine manufacturers also demanded improved protection from higher engine temperatures, improved shear stability and reduced engine oil aeration.

Passenger car motor oils face similar pushes for longer drain intervals and lower viscosity. In fact, the PCMO market has seen considerably lower viscosities than the heavy duty market, due primarily to stricter fuel economy and emissions requirements. Moreover, both passenger car and diesel engine oils have the additional challenge of phosphorus volatility and its negative impact on emissions system catalysts. With these challenges in mind, volatility limits have grown tighter from ILSAC GF-1 to todays GF-5 passenger car engine oil specification.

A reliable measuring tool is the first step to control volatility. ASTM D5800 Standard Test Method for Evaporation Loss of Lubricating 7Oils by the Noack Method, commonly known as the Noack test, is the current industry standard testing method for volatility and reflects the environment of the engine much better than other methods proposed over time.

Noack was originally a European test (DIN 51 581, replaced by CEC L-40-T-87 in 1987) but was introduced into the North American market at the request of passenger car OEMs, who were concerned about the impact of volatility on oil quality and emissions. At the time Noack was the only accepted method for measuring volatility, and the OEMs requested its inclusion in the ILSAC GF-1 passenger car engine oil specification as well as ASTM test methods. Since then, it has been the accepted method for determining formulated oil volatility.

According to Josh Frederick of Valvoline, who is chairman of the ASTM D02.B.07 surveillance panel for D5800, the method has three codified procedures. The original, Procedure A, used a low-melting solid alloy called Woods metal to heat the sample cup. While the method was perfectly acceptable, Woods metal (an alloy of bismuth, lead, tin and cadmium) is considered hazardous. Today, there are only a few units heated by Woods metal still in use.

In the mid-1990s, the health hazard of Woods metal led to the development of Tannas Co.s Selby-Noack device, which replaced Woods metal with electric heating. The instrument is used in Procedure C of ASTM D5800. The machine also collects volatiles for subsequent analysis. Tannas Co., which is part of the Savant Group, began selling the Selby-Noack II in 2016. The company says the new instrument is faster, more precise and easier to use.

Petroleum Analyzer Company (PAC) developed an electrically heated Noack testing unit in the late 1990s to replace the Woods metal unit the company had been producing. This testing unit is used in Procedure B of ASTM D5800.

In the Noack test, as in an engine, volatility is highly dependent on the rate at which the volatilized material is carried away by the flow of air through the space above the oil. The Noack cup, with inlet venting and outlet tubing, simulates conditions in an engine. The rate of air flow is controlled by both a slight, measured vacuum and the precise inlet vent dimensions. In PACs ISL NCK2-5G Noack units, the vent dimensions for each cup and cap are fixed, while in Tannas Selby-Noack instruments, inlet vent dimensions are adjustable.

Valvolines Frederick points to reliability concerns with the Noack test that have been raised based on data collected from laboratories participating an ASTM Test Monitoring Center study. It is unclear whether the decrease in reliability stems from a problem with the test method itself or with one or both of the two Noack instruments used by study participants. Eighty-five percent of participating labs used PAC units; the remaining used Selby-Noack units.

In the PAC unit, the cup used to hold the oil sample has a screw cap that is tightened down to prevent oil from escaping except through the outlet hole in the top of the cap. In labs where there are several of these cup-and-cap sets, keeping each set together can be a problem. As would be expected, the caps and cups are occasionally interchanged, which may affect results.

Any change in the diameters of the three inlet vents or the outlet vent in the cap will also have an effect. It appears that small variations in the diameters of these holes, which may produce considerable variation in results, have occurred in some sets of equipment. The changes could be due to cleaning procedures or erosion within the vents. The variability of the test from sample set to sample set is such that it could be possible to pass an oil through an easy sample set.

Frederick related that the surveillance panel has discussed many other potential contributors to the reliability problem, such as improper cleaning of cups and lids, the possibility of oil splashing on the lid and skewingmass measurements, pump calibration and maintenance, firmware revisions, control ofdelta-P instead of mass flow rate, and others. While the relationship of test variability toany of these factors has yet tobe rigorously and conclusivelydefined, Frederick observed, the panel does not believe long-term storage of reference oils is a factor. Even after establishing brand new reference oils in 2013, the panel quickly saw a significant severity shift.

This spring, the American Petroleum Institutes Auto-Oil Advisory Panel will sponsor a round robin of tests. Several labs will run the same oils in D5800, high performance liquid chromatography (HPLC), thermogravimetric analysis (TGA) and Selby-Noack II test methods. The results could be an important gauge of which test offers the best precision as well as the best representation of actual engine conditions.

Base Oils Determine Volatility

Base oils are almost entirely responsible for the volatility of a finished engine oil.

When base oils are distilled, viscosity grade stems from the boiling range of a given cut, explains Amy Claxton of My Energy Consulting & Training. Lighter viscosity oils have a lower boiling point. The volatility of the cut results from the amount of lighter material that will vaporize easily.

Base oils that contain a higher percentage of molecules more resistant to vaporizing will be less volatile. A process called hydro-isomerization used in some base oil plants boosts the number of isoparaffins-a stable molecule that does not evaporate easily. The process also results in higher base oil yields with higher viscosity index.

Viscosity index and volatility go hand-in-hand. An oils viscosity index is related to how much its viscosity changes with temperature. Higher viscosity index means less change in viscosity over a range of temperatures, and is a desirable property for engine oils. For example, according to Claxton, a typical 4 centiStoke viscosity oil with a V.I. of 100 has volatility of 20 to 30 percent. Another 4 cSt oil with a V.I. of 120 would have about 13 to 16 percent volatility.

-Caitlin Jacobs

Related Topics

Market Topics