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Diesel Lubricity Requirements of Future Fuel Injection Equipment
Abstract:
This paper looks at the underlying fundamentals of diesel fuel system lubrication for the highly-loaded contacts found in fuel injection equipment like high-pressure pumps. These types of contacts are already occurring in modern systems and their severity is likely to increase in future applications due to the requirement for increased fuel pressure.
The aim of the work was to characterise the tribological behavior of these contacts when lubricated with diesel fuel and diesel fuel treated with lubricity additives and model nitrogen and sulphur compounds of different chemical composition. It is essential to understand the role of diesel fuel and of lubricity additives to ensure that future, more severely-loaded systems, will be free of any wear problem in the field.
The lubricity with the High-Frequency Reciprocating Rig (HFRR) and the critical load of incipient scuffing (load-carrying capacity) in the High-Temperature Oscillating Machine (HiTOM) using real components of a Common Rail (CR) pump as test samples depends on the composition of the base fuels. A content of 5 % rapeseed-methyl ester (RME) in fuel increases the load-carrying capacity and increases the lubricity. Hydrodesulphurization decreases the lubricity of gas oil by 80 - 200 μm and the load-carrying capacity by 1500 N. Model sulphur compounds benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene cannot restore the lubricity of a hydrotreated diesel fuel, but can increase the load-carrying capacity. The model nitrogen compound 8-hydroxyquinoline improves the lubricity at very low concentrations (20 ppm nitrogen) whereas quinoline and acridine need a higher concentration (100 ppm nitrogen) for some lubricity improvement.
The outcome of this work has confirmed that specific lubricity additive chemistries can stretch the mixed lubrication area where highly-loaded contacts can operate safely. Specific lubricity additives with carboxylic acid-, ester- and amide based chemistries can increase the lubricity in a variety of base fuels at a concentration of 200 ppm to meet the lubricity requirement according to EN 590. These compounds can provide protection against adhesive wear by increasing the incipient scuffing load. Higher than market-typical additive concentrations can further increase lubricity, but are levelling at concentrations between 1000 ppm and 2000 ppm. The ester-based additives level at a lower value than the acid and amide based additives and so can provide further protection to FIEs. The load-carrying capacity of test fuel - ULSD 1 - can be increased by 4500 N by an ester content of 1000 ppm. The acid can only provide an increase of 2500 N. The ester-based additive at 200 ppm can already provide an improvement of the load-carrying capacity by 3000 N at a fuel temperature of 90 °C.
Tribological investigations with the HiTOM showed an excellent correlation between the results of this test and the HFRR results.
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