This month: The volatile issue of phosphorus

Next month: Oil formulators experience and recent work

Last November, the Washington Post editorialized that fertilizer containing phosphorus is one of the most dangerous pollutants in the Chesapeake Bay – a stark reminder of how beneficial and negative impacts can exist side-by-side in one high-profile chemical. A vital ingredient needed by farmers, phosphorus was blamed for leading to greater environmental pollution.

In the same vein, engine oil formulators face a similar conundrum with phosphorus. This singular element is a key antiwear agent used to boost performance and protect engines – and also is blamed for contributing to pollution, by shortening the useful life of catalytic converters on vehicle exhaust systems. For more than a decade the oil industry has struggled to balance these contradictory needs, to maximize the major benefits of engine oil phosphorus while minimizing its negative aspects.

Beneficially, phosphorus is a major component of ZDDP, zinc diorgano dithiophosphate, which for over 50 years has been the most dependable, very widely used antioxidant and anti-wear agent available. This relatively inexpensive chemical additive – particularly in its antiwear effectiveness – has contributed to the development of more highly efficient, powerful and longer-lasting engines. However, vehicle manufacturers, supported by data from lubricant additive companies, have presented convincing evidence that phosphorus volatilizes from the ZDDP in the exhaust stream, where it can deposit a gummy coating on the catalyst surface that reduces catalyst effectiveness. This, in turn, can compromise emission systems, which today must maintain an EPA-mandated useful life of 120,000 miles.

To address this problem, the auto industry worked through its lubricants group, the International Lubricant Standardization and Approval Committee (ILSAC), to cap the phosphorus content of engine oil additives. Beginning in January 1995, phosphorus was capped at 0.12 percent mass – 1,200 parts per million – for engine oils meeting the ILSAC GF-1 specification. In the most recent update of that specification, GF-4, which went into effect in January 2004, the maximum phosphorus level was set at 0.08 percent, and a minimum level of 0.06 percent was established to assure engine protection. Even lower limits are under serious consideration for GF-5, the next generation of gasoline-fueled engine oils which are expected to debut in 2009.

For the first time now on the heavy-duty diesel engine side too, responding to dramatically tightened EPA emissions requirements for 2007 model year engines, a heavy-duty quality upgrade includes a phosphorus limit of 0.12 percent. The new CJ-4 diesel oils will be available for first API licensing on Oct. 26, 2006, and are already being marketed by companies including Chevron, Petro-Canada and others.

Throughout this struggle to hammer down phosphorus content, engine oil formulators have remained opposed to chemical limits because they put boundaries around their formulating scope and ability to be innovative. Instead, formulators say, theyd prefer an engine or bench test to evaluate a given oils effects on emissions systems. Then, different levels and concentrations of components could be evaluated in prototype formulations – not simply limited or banned outright.

Enter ESCIT

For about a year the Emission System Compatibility Improvement Team, an interindustry committee initiated by ILSAC, has been meeting to evaluate potential methods for determining the impact of GF-5 engine oil formulations on emission system function and durability. Its focus is on the impact of phosphorus and sulfur on catalysts and oxygen sensors, while considering physical, bench, field and engine tests as an alternative to chemical limits.

ESCIT is chaired by Christopher Engel, product development engineer at DaimlerChrysler in Auburn Hills, Mich. Engel emphasized that elemental limits for GF-5 for both phosphorus and sulfur are not under discussion in the team. Thats a decision reserved for the ILSAC/Oil committee, the group that is developing the GF-5 specification.

Nor will the ESCIT members try to create an engine sequence test to measure the impact of phosphorus on emissions systems; thats a complicated, time-consuming and doubt-ridden undertaking. Instead, Engel earlier this year narrowed the scope of his team to focusing on recommending a phosphorus volatility test.

This focus on phosphorus volatility has substantial support from vehicle manufacturers and the chemical additives industry. In May, for example, additive company Infineum noted that, ESCIT activities breathe new life into this area, highlighting the options for moving forward.

The four major additive companies, both independent U.S. engine test laboratories, another independent lab, one specialized additive maker and one major oil company have already contributed data and technical analyses to the effort. ESCITs goal is to submit a report to ILSAC by Jan. 1, 2007 – a tight deadline as GF-5 oils are scheduled to debut in mid-2009.

What is Volatility?

Volatility is defined as the vaporization of material during usage. For the engine and emissions system, two kinds of volatility are of concern: the volatility of the oil itself, and the volatility of phosphorus, which occurs with but independent of oil volatility.

Is phosphorus volatility the missing link in evaluating and controlling its impact on the catalyst?

Until relatively recently, there has been no formal recognition in engine oil specifications that phosphorus volatility might have a measurable and manageable impact on catalyst coating. Understanding and controlling phosphorus emission holds out the possibility of reevaluating the phosphorus cap, looking toward maintaining the approved concentration of phosphorus permitted in the oil from ZDDP, and keeping its anti-wear and antioxidation benefits available for future oil categories.

The Noack volatility test was developed in the 1930s and has been formalized over the years by a number of technical societies, including as ASTM method D5800. The Noack test heats the oil and then measures the percentage of oil lost through volatility over one hour at 250 degrees C. It is a requirement for all API licensed oils; for todays GF-4 engine oils, the maximum oil loss allowed is 15 percent.

One drawback to the original Noack test device was its use of Woods metal, a heat transfer medium based on a toxic amalgam of tin, bismuth, cadmium and lead. In the early 1990s, test designers at Savant Laboratories Inc., an independent laboratory in Midland, Mich., developed a replacement for the Noack instrument which eliminated the Woods metal and substituted a resistive heater made of plated platinum. This new apparatus was relatively compact and simple to use, and showed good precision, correlation with the Noack device and repeatability. Ted Selby, head of Savant, conceived the technique in the new instrument, which was dubbed the Selby-Noack device. This new apparatus became designated as Procedure B in ASTM D5800.

The crucial addition is that the Selby-Noack also collects all the volatile material produced during the 1-hour test – including phosphorus.

The Emission Index

Having developed an apparatus to collect volatilized engine oil and phosphorus, the next logical step was to compare the amount of phosphorus volatilized to the amount of phosphorus initially in the oil. Was this volatility the same across all engine oils?

To compare the phosphorus volatility of one engine oil to another, Selby conceived the Phosphorus Emission Index (PEI), which was defined as the mass of phosphorus volatilized and captured by the Selby-Noack apparatus, in milligrams of phosphorus volatilized per liter of engine oil. (Or, for the scientifically inclined, PEI = Mgvol/Loil.)

In a 2002 technical paper exploring the PEI concept, Selby suggested that phosphorus volatility was related to differences in the decomposition mechanism of the phosphorus-containing additives, as well as to the presence and influence of other additives in the formulated oil. Follow-up research showed that this was in fact the case, he indicated in a paper the next year.

Much more important was Selbys assertion that there is essentially no correlation between the amount of ZDDP in the engine oil and the amount of phosphorus volatilized. That is, the volume of phosphorus in an oil does not determine how much may cross over to the vehicles exhaust; rather, some forms of ZDDP appear more likely to give up their phosphorus than others. Selby came to this conclusion after comparing the PEIs of hundreds of engine oils from a proprietary database maintained by Savants affiliate, the Institute of Materials.

In Selbys words, low initial levels of ZDDP could produce high phosphorus volatility, and relatively high levels of ZDDP could produce relatively low levels of volatile phosphorus. Clearly, the data showed, different formulations of ZDDP behave differently.

This finding was presented in 2004 and 2005 to conferences in Europe and Asia, to automobile manufacturers, and to the ILSAC/Oil Committee. Along with other technical input, it led to the formation of ESCIT to study phosphorus volatility and appropriate means of measuring its level.

With the availability of the Selby-Noack instrument to capture and measure engine oil phosphorus volatiles, and a simple technique for comparing volatiles across oils (the PEI), the stage was set to move toward, as Engel had requested, a phosphorus volatility test.

Engine Test Blues

In addition to the work at Savant, another major effort was under way in the mid-90s, to develop a fired engine sequence test called the Oil Protection of Emission Systems Test (OPEST). This work was funded by an industry consortium and was performed at Southwest Research Institute in San Antonio, the countrys largest independent fuels and lubricants test laboratory. Its goal was to measure the effects of engine oil formulation on catalyst conversion efficiency and light-off temperature. Although progress was made, OPEST was not ready when the GF-3 oil category was approved, and thus chemical limits of phosphorus were specified.

In the late 90s work resumed, this time on OPEST II with an eye to potential inclusion in GF-4. A second engine test attempt was made by Ford, with Southwest Research Institute performing a parallel study using a proprietary burner system named FOCAS instead of a fired engine. Engine oils with field history were tested in OPEST II and the results were presented to industry in May 2002. OPEST II was more successful in that it did produce oil poisoning of catalysts that ranked the oils similarly to field data.

However, as SwRIs Gordon Bartley noted, the differences in performance between the oils over a reasonable test length were considered too small for a useful test. The work performed demonstrated only the bulk phosphorus effect, and it did not take into account phosphorus volatility, which only came to light as a potential issue towards the end of this work.

Although OPEST II showed some promise that it could be developed into a viable test, there was insufficient time to produce an acceptable procedure for inclusion into the oil category, GF-4.

In line with ESCITs focus on phosphorus volatility, SwRI, using internal funding, is currently working on the development of a fired engine test, utilizing knowledge gained during OPEST and OPEST II. This test involves aging a catalyst under engine operating conditions that were selected to maximize the transport of volatilized phosphorus into the exhaust system, explained Southwests Scott Ellis. The program will compare re-blends of two of the same oils that were used in OPEST II, in terms of their effect on catalyst conversion efficiencies over time. We anticipate that preliminary test results will be available to the ESCIT by the end of third-quarter 2006.

Prospects for a Test

Aside from the potential technical difficulties in developing a phosphorus volatility test, two issues are overarching.

The first is the opportunity to have a meaningful measure of phosphorus volatility related directly to its effect on the exhaust catalyst and then to apply an appropriate chemical limit.

The second major issue is the extremely short time remaining to develop a test for inclusion in the GF-5 specification. Central to this effort is the need to develop field correlation between the phosphorus volatility test and catalyst deposition.

There is, and has been for at least a decade, significant concern about reducing the level of such an important engine oil additive as ZDDP. Japanese auto manufacturers have clearly voiced this concern in communications within ILSAC; thats one reason that todays GF-4 oils must contain a minimum of 0.6 percent phosphorus.

Perhaps an even higher hurdle, however, is EPAs mandate that emission systems retain their useful life through 120,000 miles. To ensure they meet the mandate, as well as have a 10 to 20 percent cushion, vehicle manufacturers would have to be absolutely convinced, beyond any doubt whatsoever, that a phosphorus volatility test would be effective before they would agree to even modestly adjust the limit on phosphorus content in engine oils.

Reducing engine oil phosphorus by means of a cap, a limit, may be a blunt instrument and a formulating constraint – but it has provided vehicle manufacturers with a high comfort level that the EPA mandate will be met.