Understanding Diesel Lubricity
Diesel fuel injection pumps are lubricated primarily by the fuel itself. Traditionally, fuel viscosity was used as a rough indicator of a fuel’s ability to provide wear protection, but since the advent of low sulphur diesel, even some fuels of higher viscosity have been found capable of producing wear. This paper provides further insights into the main contributors to diesel fuel lubricity, their source and the impact of refinery processing. The most effective way to monitor lubricity is also considered. We have found that diesel lubricity is largely provided by trace levels of naturally occurring polar compounds which form a protective layer on the metal surface. Typical sulphur compounds do not confer this wear protection themselves rather it is the nitrogen and oxygen containing hetero-compounds that are most important. A complex mixture of polar compounds is found in diesel and some are more active than others. The process of hydrotreating to reduce sulphur levels also destroys some of these natural lubricants. Other refinery processes also influence the concentration of the lubricity agents in the final fuel blend. Lubricity additives have been developed to compensate for the deterioration in natural lubricity observed in low S diesels. The interaction between natural polars and lubricity additive has been investigated and the findings may explain why some poor lubricity fuels are more responsive to lubricity additive than other. Difficulties are encountered when using knowledge of refinery streams to predict the lubricity of a diesel blend. The most effective way to monitor lubricity performance is by making measurements on the finished fuels. Vehicle tests have shown that the High Frequency Reciprocating Rig is a good indicator of diesel lubricity performance.
Study on the lubrication properties of biodiesel as fuel lubricity enhancers
Unrefined biodiesels containing small quantities of monoglycerides, diglycerides, and triglycerides, and refined biodiesels not containing
these glycerides were added to diesel fuel and the resulting lubricity was measured using the High Frequency Reciprocating Rig (HFRR)
method. The unrefined biodiesels showed higher lubricity properties than refined biodiesels. The chemical factors influencing the lubricity
properties of biodiesels were investigated. Methyl esters and monoglycerides are the main compositions that determine the lubricity of
biodiesels that meet international standards. Free fatty acids and diglycerides can also affect the lubricity of biodiesel, but not so much as
monoglycerides. Triglycerides almost have no effects on the lubricity of biodiesel.
Improving the Precision of the HFRR Lubricity Test
Researchers and cooperative groups worldwide conducted research and developed several test methods to gauge the lubricity of diesel fuel. This was necessary because the more recent fuel specifications require a higher level of hydrotreating which in turn can result in a reduction of diesel fuel lubricity. An appropriate test method was needed to enable measures to restore fuel lubricity and to enable fuel suppliers to comply with the newly adopted diesel fuel lubricity specification.The current test method that is used by most regions of the world is the High Frequency Reciprocating Rig, HFRR. Although this test method was developed and selected by the ISO group originally, and is fundamentally the same, it has been adopted in slightly different forms by regions of the world. One common element in all forms of the test method is its poor precision when compared to test methods for other fuel properties.In an effort to identify sources of variability in one version, ASTM Test Method D 6079 (1),* a workshop was conducted in which seasoned operators observed, discussed, tested, and provided recommendations to improve the existing version of the test method.This document will provide details of this project along with statistical analysis of the test results. It identifies the relevance of several parameters and practices and their effects in reducing test method variability.
Fuel Lubricity Reviewed
Many components on both aircraft and ground vehicles rely on fuel for lubrication and cooling of sliding contacts. Reliable performance of these power sources depends on the fuel providing sufficient lubrication to protect each of the many contact types within the pump and injection system. This characteristic of fuel has come to be known as lubricity. The subject of fuel lubricity has gone through a number of phases, most of which resulted from changing the composition of the fuel, which historically has been driven by fuel stability and by environmental regulations. This paper reviews these phases in chronological order. Beginning with the fuel system failures reported in aviation equipment in the 1960s and 70s, through the Military experience with low lubricity kerosene fuels in compression ignition engines in the 80s and 90s, to the ongoing introduction of more severely refined diesel fuel in progress in many developed countries around the world. The paper tracks the principal wear mechanisms observed in each instance, along with the laboratory-scale wear test procedures that were developed to simulate the condition. A complete description of this rapidly developing subject is beyond the scope of the present publication. However, it is hoped that the more salient issues are addressed, with numerous references also given to allow the reader to easily obtain additional information if needed.
Evaluation of the Wear Mechanisms Present in the HFRR Fuel Lubricity Test
Development of a Laboratory Test to Predict Lubricity Properties of Diesel Fuels and Its Application to the Development of Highly Refined Diesel Fuels
In the last few years there has been an increasing requirement for the provision of environmentally benign diesel fuels. However, the introduction of such fuels into service has been associated with high levels of field failure of rotary distribution fuel pumps due to wear. This is because the refining processes necessary to produce ecologically acceptable fuels result in greatly reduced levels of sulphur compounds, aromatics, and polar material, many of which are potential lubricity agents.
This paper describes the development of bench test methods to evaluate diesel fuel lubricity and thus enable the identification of appropriate ‘solutions’.
It has been found that the key to obtaining good correlation between field experience and bench tests is (1) to reproduce the thermal conditions present in operating pump contacts and (2) to ensure that the same mechanisms of wear operate in the bench test as in the pump environment. The physical and chemical processes involved in the lubrication of fuel pumps and the influence of temperature on these processes are outlined.
As a result of the work described in this paper, effective additive solutions have been discovered for controlling the failure of diesel fuel pumps in the field and a provisional ISO (ISO/TC 22 / SC 7 M595: ‘Diesel engines – diesel fuel – performance requirement and test method for assessing fuel lubricity’) and CEC test method for assessing diesel fuel lubricity has also been developed.
Development and Verification of the HFRR test for automotive diesel fuels
The drive towards lower sulphur levels in automotive gasoils (AGOs) has led to a reduction in fuel lubricity, and many low sulphur (<500 mg/kg) base fuels require additive treatment for them to afford adequate protection to the moving surfaces of more sensitive fuel injection equipment.
A collaborative ISO/CEC work programme, carried out in 1994, identified the HFRR (high frequency reciprocating rig) test as a suitable means of assessing AGO lubricity, but it was then found that the test had to be modified for fuels of intermediate lubricity. Following the modifications, an extensive round robin was carried out to derive the precision of the method and to examine the correlations between the HFRR results and wear in fuel injection equipment.
The test method, which is available as CEC F-06-A-96, shows a significant precision improvement over previous HFRR procedures, and a strong linear correlation with critical rotary distributor pump wear ratings. A correction factor allows the test to be carried out over a wide range of ambient conditions.
Biodiesel as a lubricity additive for ultra low sulfur diesel
With the worldwide trend to reduce emission from diesel engines, ultra low sulfur diesel has been introduced with the
sulfur concentration of less than 10 ppm. Unfortunately, the desulfurization process inevitably reduces the lubricity of diesel
fuel significantly. Alternatively, biodiesel, with almost zero sulfur content, has been added to enhance lubricity in an ultra
low sulfur diesel. This work has evaluated the effectiveness of the biodiesel amount, sourced from palm and jatropha oil,
and origin in ultra low sulfur diesel locally available in the market. Wear scar from a high-frequency reciprocating rig is
benchmarked to the standard value (460 m) of diesel fuel lubricity. It was found that very small amount (less than 1%) of
biodiesel from either source significantly improves the lubricity in ultra low sulfur diesel, and the biodiesel from jatropha oil
is a superior lubricity enhancer.
The need to reduce the exhaust emissions of diesel engines has driven the development of new
diesel engine technology. These innovations have focused on the development of: 1) diesel
fuel injection technology, 2) exhaust after-treatment technology, and 3) diesel fuel that has been
refined to higher standards. The diesel fuel injection technology of a modern diesel engine
operates at higher pressures than its counterparts (1). This new technology has demanded
better lubrication from the diesel fuel that has traditionally lubricated the fuel injection system of
the diesel engine.
Prior to October 1993, the diesel fuel that was sold in the US had a sulfur level of approximately
5000 ppm. In 1993, the Environmental Protection Agency (EPA) mandated that all diesel fuel
sold in the US contain 500 ppm or less sulfur. The petroleum refineries, largely due to special
hydrotreating of the diesel fuel, produced a cleaner diesel fuel that met this requirement.
On June 1, 2006 the EPA will again lower the level of sulfur in petroleum diesel fuel. The new
standard will be 15 ppm. This reduction of sulfur is projected to reduce diesel engine exhaust
emissions by as much as 90% when compared to the 500 ppm low sulfur diesel fuel era. The
reduction in engine exhaust emissions is projected for new diesel engines that are equipped
with diesel engine exhaust catalytic converters.
Research has demonstrated that catalytic converters last longer, aromatic hydrocarbon
emissions are lower, and oxides of nitrogen emissions are lower when cleaner fuels are burned
in diesel engines. Unfortunately, the hydrotreating that was used to reduce the sulfur produced
a fuel that sometimes failed to provide adequate lubrication for the fuel injection system of the
diesel engine (1) (2) (3) (4).
Lubricity analysis using the SLBOCLE and HFRR test procedures have indicated that the new
15 ppm low sulfur diesel fuel will exhibit lower lubricity than found in 500 ppm diesel fuel (5).
Engine manufacturers have proven that a single tankful of diesel fuel with extremely low lubricity
can cause the diesel fuel injection pump to fail catastrophically.
Research conducted using 1-2 percent blends of biodiesel mixed with petroleum diesel fuel
revealed an increase in lubricity (6). HFRR test procedures using a two percent blend of
biodiesel reduced the wear scar diameter by nearly 60 percent (from 513 to 200 microns).
The Lubrication Properties of Gasoline Fuels
Liquid hydrocarbon- based fuels must possess a modicum of lubricating ability to be able to protect pumps and related fuel supply equipment from wear. The lubricating ability of fuels, because of their very low viscosity, stems largely from boundary film forming properties. This paper reviews the lubricity of gasoline and describes the service problems that may result from increasing constrains on gasoline composition and from the solutions devised to meet them.
Comparison of the Lubricity of Gasoline and Diesel Fuels
The High Frequency Reciprocating Rig (HFRR) commonly used to measure the friction and wear properties of diesel fuels has been modified to study gasoline lubricity.Wear tests have been carried out on a range of gasoline and diesel fuels. The non-additised gasolines tested all give higher wear than severely-refined Class I diesel fuels. The effect of relative humidity on the wear properties of both gasoline and diesel fuels has been compared. Both types of fuel give wear behavior which is almost independent of water vapour pressure down to 0 8 kPa, but show a reduction of wear below this humidity level.In practice most gasoline fuels contain detergent additives. The influence of two commercial gasoline detergent additives of different structure on gasoline lubricity has been studied. Both additives reduce wear, to an extent which is dependent upon additive concentration and also upon the base fuel. Wear tends to remain high up to a detergent additive concentration of about 200-300 ppm but then falls above this level. This concentration is of the same order as that used in practice in commercial gasolines.Wear tests have been carried out using a commercial diesel fuel lubricity additive. This has been found to be equally effective in reducing wear and friction coefficient in gasolines as in diesel fuel
The Lubricity of Gasoline
A study has been made of the lubricating properties of gasoline fuel. A conventional HFRR diesel fuel lubricity tester has been modified to measure gasoline wear. Using this test equipment, a number of features of gasoline lubricity have been investigated, including the comparative lubricating behavior of gasoline, the influence of detergent additives and oxygenates on wear and the wear behavior of a series of refinery streams employed in gasoline blending.The lubricity of a range of pure organic chemicals known to be present in gasoline has also been studied. From these measurements it has been shown that, except for components such as dienes and diaromatics, the HFRR lubricating properties of most gasoline hydrocarbon constituents are broadly independent of chemical structure bur depend significantly on viscosity. Using these measurements, predictive wear equations based on gasoline group analysis have been developed.Because it has been found that viscosity plays a role in determining the wear properties of gasoline, the elastohydrodynamic (EHD) film-forming and friction properties of gasoline have been measured and compared to those of diesel fuels. This shows that the combination of gasoline’s very low viscosity and low pressure-viscosity coefficient results in very thin EHD film thickness generation and also very low friction in full-film EHD conditions
Pushing the Boundaries of the HFRR: Impact of Increased Test Severity on Wear
The high frequency reciprocating rig (HFRR) was developed in the early 1990s as a test method to assess diesel fuel lubricity in order to provide wear protection for fuel injection pumps. This was necessary in response to the many field failures that occurred following the introduction of ultra-low sulphur diesel in Sweden. The prevalent fuel injection equipment (FIE) technology at this time utilised rotary pumps capable of reaching maximum fuel pressures of ∼650 bar in systems for direct injection engines.
The continued drive for efficiency led to many changes in FIE technologies, materials and pressures. Modern high pressure common rail pumps reach significantly higher pressures, with 2200 bar available today and pressures up to 3000 bar discussed in the industry. Alongside these hardware changes there have been significant changes in the diesel fuel, with a continued move towards more highly refined fuels, and the introduction of highly paraffinic sources from Fischer-Tropsch processes and hydrogenated vegetable oils.
Despite these changes there have been no widespread field issues since the introduction of HFRR specifications. The objective of this work was to understand the flexibility of the HFRR test by investigating the impact of increased test severity on wear. The main variable investigated was the effect of load, which was varied from 2-10N. Other variables investigated were frequency, test duration and stroke length. Additionally, preliminary studies were conducted using phosphate coated specimens more in line with those expected in current and future FIE systems.
HFRR tests utilising a more highly loaded contact exhibit an increased mean wear scar. Trends observed are similar to those under standard conditions (2N), with differentiation between untreated fuels with poor lubricity and those that are additised. Certain fuels exhibit unusual friction coefficient behaviour, with temporary periods of significantly elevated friction. This is attributed to micro-seizure; however under additised conditions it was not observed. One interesting observation was a correlation between wear scar pattern and additive technology. Ester based lubricity additives give a striated wear scar, whereas acid technologies produce a stippled pattern, likely due to low level corrosion, a known possibility with this type of chemistry. The relatively soft nature of the phosphate coatings used meant that under the highly loaded contact conditions of the HFRR they quickly wore through; however the presence of additives minimised this wear and prolonged coating lifetimes. Further work indicated that other fuel additives such as cetane improver can impact lubricity performance, and may require different lubricity improver treat rates in order to meet specifications.
In summary, increasing contact severity within the HFRR increases the level of wear in the system, with some evidence of micro-seizure. However, the use of additives under these conditions can reduce the wear to acceptable levels and prevents micro-seizure events. The trends and discrimination between conventional and higher load conditions remains similar. This indicates that modifications to the HFRR test method are unlikely to provide a means of defining how to achieve additional protection for future more severe conditions and the current test method remains suitable.
Screening vegetable oil alcohol esters as fuel lubricity enhancers
Methyl and ethyl monoalkyl esters of various vegetable oils were produced for determining the effects of type of alcohol and fatty acid profile of the vegetable oil on the lubricity of the ester. Four methyl esters and six ethyl esters were analyzed for wear properties using the American Society for Testing and Materials method D 6079, Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig. Ethyl esters showed noticeable improvement compared to methyl esters in the wear properties of each ester tested. No correlation was found between lubricity improvement and fatty acid profile of the ester, except that esters of castor oil had improved lubricity over other oils with similar carbon chain-length (C18) fatty acids.