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HFRR (High Frequency Reciprocating Rig)
/ Research
Abstracts from papers.
Development of a laboratory test to predict lubricity properties of diesel fuels and its application to the development of highly refined diesel fuels.
Bovington C., Caprotti R., Meyer K., and Spikes H. A. 9th International colloquium, ecological and economic aspects of tribology, Esslingen, Germany 1994.
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.
Davenport J.N.
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. A considerable amount of additional pump testing, in both rigs and field trials, has demonstrated that the method is robust and reliable for use in a fuel specification.
The lubrication properties of gasoline fuels.
Wei D-P, Korcek S., and Spikes H. A. presented at Fuel Symposium, Technische Akademie Esslingen, Germany 1996.
1. Background
After considerable research it was realised that a prime cause of these problems was an increase in the severity of refinement of aviation kerosene by processes such as hydrotreatment which had been introduced in the preceding years to enhance fuel stability (2). This had reduced the inherent boundary lubricating properties of some aviation fuels to such an extent that they were no longer able to adequately lubricate the rubbing components in the piston and gear pumps used to deliver them.
A number of studies attempted to identify which components of an aviation fuel were responsible for providing its natural lubricity. Attention focused on small quantities of polar surfactants whose concentration was likely to be depleted by both hydrotreatment and clay treatment refining processes, but a number of alternatives were also suggested (3). The practical problem was eventually largely solved by a combination of changes to pump design, such as the introduction of low friction components and by the widespread addition of boundary lubricating agents to aviation fuels in order to enhance their depleted natural lubricity (4).
In the late 1980s and early 1990s environmental concern about the toxic and harmful emissions from diesel engines led to large reductions in the level of sulphur and aromatics in diesel fuels and the development of so-called "reformulated diesel fuels" (5)(6). Almost at once fuel pump failures began to occur in Sweden and parts of the United States and Canada where major reductions in sulphur levels for diesel fuel had been introduced (7). Unlike earlier aircraft fuel pump problems which appeared to be associated with high friction leading to scuffing (1) these failures were characterised by adhesive sliding wear and consequent rapid loss of acceptable fuel pump performance (8).
In response to these problems a series of studies were initiated to develop suitable diesel fuel lubricity test methods and also to identify effective lubricity additives for diesel fuels (7)-(12). In an anticipatory study carried out by the authors, a reciprocating wear test had been developed to measure the friction and wear properties of diesel fuels and this was employed to investigate the relationship between fuel composition and lubricity (13). A modified version of this test has recently become a CEC standard method for assessing diesel fuel lubricity (14). Studies showed that diesel fuel lubricity correlated with, but did not originate from sulphur content but was critically dependent upon small amounts of oxygenates or other polar materials. Polyaromatics and some nitrogen compounds were also shown to improve lubricity (13) whilst in low sulphur diesel fuels, viscosity and diaromatic content were also claimed to be important factors influencing fuel lubricity (12). Just as with aviation fuels the lubricity of highly refined diesel fuel proved to be responsive to the use of additives and therefore it is now common practice to include lubricity additives in reformulated diesel fuels. Thus with both aviation and diesel fuel there has been a similar cycle of events; a significant increase in severity of refinement followed by consequent service problems, the development of fuel lubricity test methods and, eventually solutions based largely on additive technology.
The work described in this paper is being carried out in anticipation of a similar cycle taking place with gasoline fuels. These fuels are also becoming subject to severe compositional constraints including reduced sulphur content, a principal concern being the effect of sulphur on catalyst life and performance. Currently the accepted European average level of sulphur in gasoline is 300 ppm but some city gasolines marketed for use in Sweden and Finland contain less than 100 ppm ant in the US the 1996 Californian Phase 2 reformulated gasoline (CaRF2) includes a 40 ppm flat limit. The lubricity requirements of gasoline are generally much lower than for diesel since gasoline fuel injection systems inject fuel upstream of the inlet valves and thus operate at much lower pressures than diesel fuel pumps. Even so there has been anecdotal evidence over the last few years of gasoline pump failures which have been ascribed to very low lubricity fuels (I5). These problems are likely to increase as injection systems become more sophisticated and fuels become more highly refined.
The topic of gasoline lubricity has recently become more urgent with the possible introductions of direct injection gasoline engines which will necessitate high pressure gasoline injection pumps (16)(17), a development which is most likely to place considerably more emphasis om the lubricating capability of gasoline.
Very little work on gasoline lubricity has been published. One recent paper describes a test method based upon an enhanced Falex apparatus with a fuel circulation system (15). This paper found that both aromatics and the combustion-improving oxygenate, methyl tertiary butyl ether (MTBE), enhanced lubricity.
The authors of this paper have also recently begun to systematically investigating the lubricity of gasoline and the relationship between composition and lubncity. The first stage of this was to modify the HFRR test to enable it to be used to study gasolines and to compare the overall levels of diesel fuel and gasoline fuel lubricity using the same test method (18). This showed that additive-free gasolines have significantly poorer lubricity than a highly refined Swedish Class I fuel whilst commercial, detergent additive-containing gasolines range from having wear properties slightly better to significantly poorer than a Swedish Class I fuel. The study also examined the response of gasolines to lubricity additives and showed that a commercial diesel fuel lubricity additive is equally effective in reducing wear in gasolines as in diesel.
It should be noted that the study of gasoline lubricity has wider benefits than simply of predicting and forestalling possible wear problems. One complication which has hindered understanding of aviation kerosene and diesel fuel lubricity is that in large proportion, the lubricity of these fuels appears to arise from very small quantities of polar, quite high boiling components. These are very difficult to analyse and probably chemically vary greatly depending upon the origin of the fuel. A minor but possibly significant proportion of the lubricity of aviation kerosene and diesel fuel appears to arise from the bulk hydrocarbon itself. On the other hand, gasoline, because of its low boiling range, has a simpler composition than aviation kerosene and diesel fuel and has very low naturally-occurring polar impurity content in most of its constituent refinery streams. This makes it a valuable vehicle for studying the contribution of the bulk, hydrocarbon components of fuels on lubricity, and possibly disentangling this from the polar constituent contribution. Such understanding might then be transferable to other, more complex fuels.
The aim of the current study of which preliminary stages are reported in this paper are thus:
1. to develop a simple test able to measure the lubricity of gasoline fuels.
2. to use this to assess the lubricity of current gasoline fuels.
3. to determine the extent to which bulk hydrocarbon components contribute to fuel lubricity.
4. to assess the contribution of additives to the lubricity of gasolines.
