Before the advent of ULSD (Ultra Low Sulfur Diesel), defined as diesel fuel having a maximum 15 ppm sulfur content, the characteristic of fuel lubricity was rarely questioned or reported. In fact, diesel specifications provided by refiners to diesel wholesalers and resellers often omitted lubricity values entirely.
Even the American Society of Testing Materials (ASTM), the organization responsible for establishing standardized industry guidelines and test procedures, did not establish a standard lubricity requirement until 2005. However, since model year 2007 and the EPA federal mandating of 15 ppm sulfur content, the conversation concerning lubricity has become not only increasingly prevalent but also, unfortunately, quite confused.
Many questions revolve around this subject of lubricity including its meaning and its determination, its correlation to sulfur content, its minimal and optimal limits for good engine life, and its specifications as per major suppliers and terminals. Collectively, resolution of these questions will lead to the answer to one final question’when and if there is ever a need for the ultimate retail diesel supplier to treat the fuel with additional lubricity enhancers? This article will attempt to remove the mystery about lubricity by detailing these many aspects of this very critical property of diesel fuel.
WHAT IS LUBRICITY?
Lubricity is most commonly defined as the ability of a fluid to minimize the degree of friction between surfaces in relative motion under load conditions. When the lubricity is not at a satisfactory level, then many internal engine components including fuel pumps and injectors are prone to excessive wear and metal damage. The results of such wear are inefficient performance, shortened service life and high replacement costs.
Notably, as the emission standards for diesel fuels have been decreased, the importance for adequate fuel lubricity has increased since now critical engine parts must perform at evermore demanding operating pressures and temperatures and, consequently, with greater degrees of metal to metal contact. Obviously, these factors explain why good lubricity quality is of prime importance to engine manufacturers and users alike.
WHAT IS THE CONNECTION BETWEEN SULFUR AND LUBRICITY?
There is a misconception that sulfur is what provides the lubricity to fuel oil, but that idea is only indirectly correct. Although the sulfur does contribute somewhat to lubricity, the lower lubricity level of low sulfur fuel is more a by-product of the refinery processes used for desulfurization. In its purest state, crude oil would consist of various bonds of hydrogen (H) and carbon (C ) atoms linked together in chains of various lengths.
However, no crude oil is entirely pure, but always contains trace amounts of various impurities. One such common impurity is sulfur; depending upon the crude source, the sulfur content can range from as low as 500 ppm (sweet crude) up to as much as 10 fold (sour crude) that amount.
During the desulfurization process via hydrotreating, hydrogen gas is introduced to the crude under extreme temperatures and pressures. The hydrogen combines with the sulfur to form hydrogen sulfides that are then removed and ultimately converted into elemental sulfur for resale.
Unfortunately, during desulfurization, critical polar and organic aromatic compounds innate to the fuel and identified as responsible for imparting significant lubricity quality, are destroyed under the necessarily intense operating conditions. The resulting yield is a satisfactory low sulfur diesel fuel, but also one that is unsatisfactorily low in lubricity.
WHY THE FUSS ABOUT LUBRICITY NOW?
For many decades, it was taken for granted that all distillate fuels, with the possible exception of kerosene, contained enough inherent lubricity to protect against engine wear. There was no reason to define lubricity or to establish a test method to quantify a lubricity value. For the most part, it was assumed that any fuel had the ability to provide adequate lubricity as one of its innate and irreversible properties.
However, everything changed once the EPA began to look into diesel fuel exhaust emissions and, early on, identified sulfur as one of the main sources for objectionable smog-causing air pollutants, oxides hazardous to health and long term corrosive acid rain damage. By the late ’90s, the EPA was exploring regulations to reduce the sulfur content of distillate fuels, especially on-road diesel.
As the likelihood of new regulations became more imminent and oil refiners investigated available processes available for crude oil desulfurization, one critical piece of information became obvious’that any significant reduction in sulfur content would result in a significant reduction in fuel lubricity as well.
Realizing this correlation, ASTM launched a task force to include lubricity requirements as part of its existing ASTM D975 Standard Specification for Diesel Fuels. Concurrently, another task force was assigned to determine an effective laboratory test method to measure lubricity values.
HOW ARE LUBRICITY VALUES DETERMINED?
Following exhaustive field and laboratory test data, in 2005, a lubricity standard was introduced into the existing ASTM D975 calling for a maximum wear scar of 520 microns as determined by the newly established ASTM D6751 HFRR test method.
The HFRR, high-frequency reciprocating rig test, involves a steel ball scraping over a steel disc that is immersed in the fuel under study.
This abrasion causes a wear scar both on the ball and on the disc, and such scars are evaluated and a definitive measurement is determined. Based on test results, coupled with studies under actual field conditions, it was ultimately determined that diesel fuel should yield a maximum wear scar of 520 microns per HFRR test. Any results exceeding that limit posed a strong potential for severe and excessive metal deterioration and accelerated damage to engine components.
HOW DO ULSD FUELS COMPLY WITH LUBRICITY REQUIREMNTS?
As explained previously, the desulfurization process significantly alters the chemistry of the final ULSD product and results in a fuel with a significant loss of lubricity. As a result, many fuel-regulating agencies now mandate that any No. 2 ULSD diesel fuel must comply with the 520 microns limit set forth in ASTM D275.
Furthermore, most engine manufacturers will void the warranty if the fuel used is not compliant with this lubricity limit. In order to ensure a proper lubricity, lubricity additives are often injected to the fuel at the distribution sites of the key ULSD refiners and producers and prior to sale to the reseller or wholesaler.
Refiners would probably have preferred to add the lubricity additives prior to pipeline transport, but are not allowed since there is a question of compatibility with other fuels, especially jet fuels that may also utilize the same pipeline channels. Many major refiners build the added cost for the additive directly into the fuel’s final marketing price, while others chose to tack on a type of surcharge to cover the additive cost.
The latter option seems a bit incongruous and similar to purchasing a new automobile, but being told that one has to spend extra money for the engine’s emission control system that is necessary to drive the car efficiently. Be that as it may, the end result is a ULSD fuel that satisfies ASTM D275 and can be sold as a legitimate diesel fuel having less than 15 ppm sulfur and a lubricity rating below a 520 microns wear scar.
SO WHAT ABOUT EXTRA LUBRICITY TREATMENT?
As explained, lubricity value should be a number less than 520 microns; if not, then the fuel is both suspect and also indicative of one that will likely cause engine damage and premature engine failure. Therefore, it is essential that any reseller determine from their refinery source exactly what is the lubricity value of the ULSD product being sold.
However, one should be aware that because this specification is not always mandatory and often depends upon local state and municipal regulations, determining this data might prove difficult.
Assuming that the level is satisfactory, then the next step is to determine just how much of a premium fuel is desirable for your marketing strategy. As a basis to make this decision, it must be noted that although the ASTM sets 520 microns as a minimum compliance standard, the influential Engine Manufacturers Association, as well as many local, state and federal agencies, now require a maximum wear scar of 460 microns.
In order to reach this next level lubricity, additives are available commercially that can be added directly into ULSD fuel well downstream of the original distribution terminal site. These products work by supplying a non-metallic, non-sulfurous, non-acidic, synthetic product that enhances boundary lubrication between moving parts and yields wear scars below the more stringent EMA requirements.
Because of the very minimal added cost for this additive, especially when added by the fuel reseller, it has received widespread industry acceptance and implementation. The benefit derived can and should be used to promote the offering of a truly premium high-quality high-performing ULSD fuel.
Of course, lubricity extenders can also be used but at higher treatment ratios, if it should be determined that the initial ULSD product does not even satisfy the 520 microns value. In such a situation, the decision of whether to add the additive at the terminal level or at the reseller’s site is a question of cost and convenience.
An interesting side note to this question of lubricity extenders pertains to biodiesels.
According to reports made by biodiesel agencies, a fuel with a 2 percent soy (B2) content will yield a diesel fuel that, as is, meets the premium lubricity criteria.
However, it should be noted that the rising cost for soy has forced many bioproducers to seek alternative feedstock including animal and vegetable fats and greases. Compared to soy production, these alternatives are far more difficult to manage with reliable quality control and base standards. As a result, their ability to consistently provide the necessary added lubricity tends to be uncertain and unpredictable without first knowing the actual composition and history of the bio-components involved. Obviously then, one certain way to ensure optimum lubricity even with biodiesel fuels at the B2 level is to incorporate into the product an effective lubricity supplement.
The property of lubricity helps to determine the fuel’s ability to minimize engine wear and to maximize engine life. The HFRR test D6751 is typically used to measure lubricity, and with a 520 microns wear scar now set by ASTMD27 as the maximum wear scar acceptable for diesel fuel. However, the Engine Manufacturers Association and many state and local agencies require the more demanding 460 microns as the maximum acceptable wear scar.
Since the requirements are not universal, the diesel reseller must determine the actual lubricity value of the fuels being supplied at the terminals. Then, an informed judgment can be made as to whether extra lubricity additive is needed and whether to have it additized at the terminal site or more economically at the reseller’s site.
In addition to the obvious considerations of cost and convenience, this decision should also be made in terms of marketing strategy and whether to promote a truly premium diesel fuel that satisfies the most demanding of lubricity requirements.
Jerome P. Sava is a ChemE and partner at C&S Scientific Corp., Box 1056, Hightstown, NJ 08520; a company identified with R&D and marketing of new fuel technology and chemical treatment. He can be contacted at (877) 448-7037 or JSava@CSScientific.com.