All diesel fuel injection equipment has some reliance on diesel fuel as a lubricant. The lubricating properties of diesel fuel are important, especially for rotary and distributor type fuel injection pumps. In these pumps, moving parts are lubricated by the fuel itself as it moves through the pump—not by the engine oil. Other diesel fuel systems—which include unit injectors, injectors, unit pumps, and in-line pumps are partially fuel lubricated. In these systems the mechanism typically consists of a plunger or needle operating in a sleeve or bore, where the fuel is used to lubricate the walls between the reciprocating piece and its container. The lubricity of the fuel is an indication of the amount of wear or scarring that occurs between two metal parts covered with the fuel as they come in contact with each other. Low lubricity fuel may cause high wear and scarring and high lubricity fuel may provide reduced wear and longer component life.
Lubricity has sometimes been mistakenly compared to the viscosity, or thickness of a fuel. The following statement from Lucas1 (the leading fuel injection equipment manufacturer in England, that was recently purchased by Delphi) explains it well:
“The lubrication of the fuel is not directly provided by the viscosity of the fuel, but by other components in the fuel which prevent wear on contacting metal surfaces.”
For many years, the lubricity of diesel fuel was sufficient to provide the protection needed to maintain adequate performance. Recent changes (1993 and beyond) in the composition of diesel fuel, primarily the need to reduce fuel sulfur and aromatic levels, and the common chemical process used to accomplish these changes (called hydro-treating) have inadvertently caused the removal of some of the compounds that provide lubricity to the fuel. According to Mr. Paul Henderson, Quality Management Systems Manager for Stanadyne Automotive Corp. (the leading independent US manufacturer of diesel fuel injection equipment) in comments provided to the Chairman of the Kansas House Environment Committee March 8, 2000:
“There have been numerous examples from the field where lack of lubricity in the fuel has caused premature equipment breakdowns and in some cases, catastrophic failures. This problem will be more dramatic as EPA moves to further reduce the sulfur levels in petrodiesel fuel.”
The lubricity of diesel fuel can vary dramatically. It is dependent on a wide variety of factors, which include the crude oil source from which the fuel was produced, the refining processes used to produce the fuel, how the fuel has been handled throughout the distribution chain, and the inclusion of lubricity enhancing additives whether alone or in a package with other performance enhancing additives. Typically, Number 1 diesel fuel (commonly referred to as kerosene), which is used in colder climates, has poorer lubricity than Number 2 diesel fuel.
A 1998 review paper on fuel lubricity worldwide2 showed that diesel fuel in the US and Canada is some of the poorest lubricity fuel found in the entire world (see Figure 1 attached). Of the 27 countries surveyed, only Canada, Switzerland, Poland and Taiwan had poorer lubricity fuel than the US. With a mean fuel lubricity of just under the recommended specification of an HFRR wear scar diameter of 460 microns, fully 50% of the US fuel was found to be above that recommended by equipment manufacturers.
These US data are with diesel fuel refined to meet the current EPA restriction of 500 ppm maximum sulfur specification. The severe hyrdrotreating required to reduce fuel sulfur to the new EPA 2006 specification of 15 ppm sulfur maximum will cause a further reduction in fuel lubricity compared to today’s diesel fuel, and is of concern to engine and fuel injection equipment manufacturers.
Lubricity Benefits Provided by Biodiesel
The addition of biodiesel, even in very small quantities, has been shown to provide increases in fuel lubricity using a variety of bench scale test methods. A diagram of the various testing apparatus can be seen in chart provided by Lucas (attached). The two most popular bench test methods for lubricity are the Ball on Cylinder Lubricity Evaluator (BOCLE), and the High Frequency Reciprocating Rig (HFRR). The BOCLE is commonly used to evaluate the lubricity of fuels or fuel blends but does a poor job of characterizing the lubricity of fuels containing lubricity additives, while the HFRR is commonly used for both the neat fuels and with fuels containing small amounts of lubricity enhancing additives.
The Fuel Injection Equipment (FIE) manufacturers have adopted the use of the HFRR (ISO 12156-2:1998), and recommend that all diesel fuel meet a limit of 460 micron maximum Wear Scar Diameter (WSD)3. For the HFRR, a lower wear scar indicates better lubricity.
Biodiesel has been tested, at varying concentrations, with poor lubricity Number 2 and Number 1 diesel fuels representative of that on the market after 1993 (i.e. fuel refined to meet a 500 ppm maximum sulfur content). The results are illustrated in the table on the next page.
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