For more than a decade, phosphorus in engine oil has been attacked as poison to catalytic converters on vehicles. Can the engine oil industry continue to offer the proven antiwear and antioxidant value of phosphorus, while blunting its negative impact on emissions systems? For answers, LubesnGreases turned to researchers, testing labs, additive manufacturers and the new Emissions Systems Compatibility Improvement Team headed by DaimlerChryslers Chris Engel.

Last month: The volatile issue of phosphorus

This month: Additive companies test the limits

One of the sharpest tools in the engine oil formulators kit is zinc diorgano dithiophosphate (ZDDP), a popular antiwear and antioxidant additive. This more than 50-year-old chemistry is reliable, relatively inexpensive and available worldwide.

It also contains phosphorus, which auto companies want to see curtailed in engine oils. Their research shows that phosphorus from ZDDP volatilizes in the exhaust stream, leaving a gummy coating on the catalyst and shortening its useful life, which EPA has mandated to be 120,000 miles.

In 1995, automakers put engine oils on a low-phosphorus diet, but they havent weaned themselves from it entirely. For todays ILSAC GF-4 engine oils, phosphorus content is limited to 0.06 percent minimum and 0.08 percent maximum percent mass. Even lower limits may be in store for the next generation of gasoline-fueled engine oils, GF-5, which are due for introduction in 2009.

A new group is also on the scene, the interindustry Emissions Systems Compatibility Improvement Team (ESCIT), chaired by Christopher Engel of DaimlerChrysler. Among other ideas, ESCIT is probing a theory by Ted Selby of Savant that phosphorus woes are caused by ZDDP volatility, not its mere presence. Selby proposed a Phosphorus Emissions Index or PEI, to quantify these deleterious effects (see last months issue).

Recently, five additive companies – Afton Chemical, Chevron Oronite, Ciba Specialty Chemical, Infineum and Lubrizol – offered their own research and views on phosphorus to ESCIT. Here are their thoughts on how this volatile issue is shaping up.

Afton: A Measured Approach

Afton Chemicals approach to measuring the phosphorus volatility-related effects of engine oil formulations on catalyst poisoning is heavily data based, by means of a proprietary engine test it calls the Afton Catalyst Test (ACT). Its primary goal is to determine the relevance of Selbys Phosphorus Emissions Index at 250 degrees C (PEI-250) on catalyst poisoning. In Version II of this test, in use since January, engine conditions such as speed, oil and coolant temperature and oil charge, have all been increased in order to exacerbate any impact that phosphorus volatility may have on catalyst poisoning.

Employing ACT, Afton tested two pairs of oils: the first pair was a GF-4-style chemistry with dramatically different PEI-250 values; the second pair had GF-2 chemistry from an Afton/Ford/Delphi 2001 field test. Both oils in the second pair had the same ZDDP additive package, but one had no detergent. The specific goal of this test was to determine if the correlation between PEI-250 and the oils in the above field test was the result of an artifact (the lack of detergent in one field oil.)

Aftons Greg Guinther pointed to the results of the test. Phosphorus throughput to the catalyst is a complicated mechanism not easily simulated. Analysis of blowby condensate suggests at least three mechanisms for phosphorus depletion – volatilization of phosphorus from the bulk oil, as well as from the cylinder wall oil film, and from consumption of oil mist. Moreover, blowby contains additive elements other than phosphorus, and alkaline-earth metal detergents are carried out with blowby and perhaps have an impact on deposition in the catalyst.

Guinther added, There is no strong evidence that PEI-250 relates to increased phosphorus throughput and the resulting catalyst poisoning. Alkaline-earth metal detergents are carried out with the blowby in high PEI-250 oils – it is well established that alkaline-earth metal detergent chemistry has an impact on catalyst poisoning. Observations in the field test about elevated PEI-250 leading to increased catalyst poisoning is most likely related to the lack of detergent.

Guinther summarized, It appears that a bench volatilization test cannot generalize the complicated blowby mechanism.

Chevron Oronite: Use the Right Tool

Chevron Oronite presented data from an analysis of four engine tests – the IIIG, VG, IVA and VIB Sequence tests – using a prototype GF-4 SAE 5W-30 oil with a phosphorus level of 0.75 percent mass (750 parts per million). It illustrated its findings with graphs showing the relevance of factors such as absolute phosphorus levels, base oil volatiles, and temperature. (see left)

Phosphorus volatility can be determined by using conventional used oil analysis, and the Sequence VIB test appears to show reasonable scale, Oronites Eric Olsen noted. The possibilities of multitasking a GF-5 engine test to also yield measures of phosphorus volatility should be investigated. However, the goal is the efficient utilization of data, and not an extra pass/fail criterion for the test.

Olsen also pointed out, There are substantial differences among ZDDP additives both for phosphorus volatility and for antiwear performances. Recent OEM data has compared a conventional ZDDP versus a low impact ZDDP; the meaning of these terms is unclear and may be relevant to only a single additive supplier.

Ciba: How Phosphorus Behaves

Ciba Specialty Chemical acknowledged the increasing pressure to limit the amount of ash-forming (metal) additives and phosphorus in automotive engine oil in order to minimize the contamination of exhaust gas aftertreatment devices.

Previous Ciba publications reported on several methods for measuring phosphorus volatility of engine oils. But according to Cibas Mouhcine Kanouni, we concluded that bench test conditions for measuring phosphorus volatility of engine oils should be similar to actual engine conditions, for example, bulk oil in crankcase (about 150 degrees C) for hours and/or thin oil film on piston ring/liner (250 C) for seconds/minutes.

Kanouni presented two procedures to ESCIT for evaluation of phosphorus losses due to evaporation from formulated oils. The first procedure, he stated, involved vapor analysis from a thin film of oil heated at 150 degrees C and then at 250 C for 10 minutes. For the second procedure, phosphorus emission was measured from bulk oils heated at 160 C using Viscosity Increase Test type of experiment. To validate the methods, the Volkswagen PV 5106 Rig Test and the Daimler Benz OM611 engine test were used, with ashless antiwear compounds in a 5W-40 Euro IV oil. The results clearly showed that appropriate ashless antiwear components pass this long duration, soot-induced wear test.

He continued, The positive performance of the ashless additives prompted an investigation into the effect of different phosphorus chemistries on exhaust gas catalysts. A deactivation study of a three-way catalyst, as a function of oil ash accumulation, was performed [as described by Jens Franz et al, in SAE publication 2005-01-1097].

Hydrocarbon, carbon monoxide and nitrogen oxide emissions increased with increasing exposure of the catalysts to ZDDP formulated oil. However, certain ashless-antiwear-based oil caused a much lower increase in emissions upon aging. Thus it appears that not all phosphorus behaves similarly, and appropriate ashless phosphorus chemistries may have an advantage over ZDDP in terms of minimizing catalyst poisoning.

Infineum: Show Me the Science

Infineum noted that ZDDP is the principal source of phosphorus in modern lubricants. While ZDDPs are multifunctional, their main function is the provision of wear protection. But a very important secondary function is to prevent oil loss from oxidation so that the oil does not become too thick to do its job as a lubricant and coolant. ZDDPs are not only highly effective as additives but they are, by far, the most cost-effective solution available.

Lubricants must protect the engine first and foremost, urged Infineums Andy Ritchie, but must not have a negative impact on other aspects of vehicle system performance, particularly the catalyst system. Unfortunately, the phosphorus from ZDDP, and likely from most other sources, can at certain levels have a negative impact upon the performance of catalyst systems, causing evaporation – often called phosphorus volatility or poisoning from consumption-burn.

Ritchie continued, Reduction of the level of phosphorus in a lubricant may reduce the risk of poisoning the emissions control catalyst. What is not clear is whether, at the current level of 0.08 max percent phosphorus in GF-4 lubricants, we have met the law of diminishing returns. Or to put it another way, do we begin to compromise the performance and general cost-effectiveness of the lubricant by further phosphorus reduction for what may be limited or even no return on catalyst durability?

At the moment all we know with absolute certainty is that additive costs will go up very significantly, as will, of course, the marketplace price of a finished lubricant, as phosphorus levels come down. We would like greater assurance of the value before going down this route.

For the previous three ILSAC GF categories – GF-2, GF-3 and GF-4 – the maximum phosphorus levels were reduced from 0.12 to 0.08 percent maximum, a reduction of more than 30 percent, Infineum pointed out. However, a limit on the maximum amount of phosphorus in an oil does not control the type of phosphorus used. And a low level of one volatile phosphorus component may poison a catalyst more than a higher level of a less-volatile phosphorus component.

Ritchie concluded, A further reduction in the phosphorus maximum level has been proposed by some OEMs for GF-5. As noted above, Infineum questions the value of further tightening of the phosphorus limit in the absence of clear and unambiguous science based on bench, engine and field testing to justify the benefit. We strongly support development of a meaningful catalyst performance test through the ESCIT activity as the best opportunity to meet the needs of equipment manufacturers to reduce the deleterious effects of phosphorus, while at the same time maintaining current ZDDP levels that provide protection for the engine.

Lubrizol: Zipping Toward GF-5

Lubrizol, the worlds largest lubricant additives company, has been actively doing research on lubricant formulations and emissions system durability, and feels it has developed a fundamental understanding of the issue. The companys Lew Williams provided ESCIT some insights based on a huge database drawn from tests over a wide range of driving conditions and across many engine types.

Weve looked at used oil phosphorus levels in field-test results, he noted, and these range from 6 to 16 grams of phosphorus exiting the engine over 90,000 to 150,000 miles. Most of this phosphorus ends up on the catalyst.

Further, weve looked at our data on phosphorus loss from four engine tests – the Sequence IIIF which measures oxidation, piston deposits and wear; the IVA which measures valve train wear; the VG which measures sludge and varnish; and the VIB which measures fuel economy – and have found that the VIB and VG tests correlate to our field test results best. Since the Sequence VIB may soon be replaced by the Sequence VID, we recommend that the Sequence VID Consortium which is currently developing a new fuel economy test look at the used oil phosphorus levels versus new oil, to determine how the Sequence VID compares to our field experience.

Williams added, Our data indicates that phosphorus loss is a concern and should be addressed in GF-5. Whether this is done through measurements of phosphorus loss in engines tests, or through elemental limits, or both, will be subject for further investigation and debate.