Fuel lubricity and its laboratory evaluation

Abstract

This literature review paper discusses the subject of lubricating properties of liquid hydrocarbon-based fuels and laboratory bench tests applied in lubricity evaluation. The analysis was made in order to highlight the importance of fuel lubricity evaluation, especially application of relatively rapid laboratory tests. Inadequate lubricity may lead to an excessive wear of fuel injection system components and in some cases ? even to catastrophic failure what, in turn, manifests itself in higher replacement costs, shortened service life, inefficient engine performance and increased tailpipe emissions. Nowadays, when more and more rigorous emissions standards for transportation fuels are continuously established, the satisfactory fuel lubricity is of great importance. Lubricity determines the antiwear behaviour of the lubricant over the regime of boundary lubrication when the moving surfaces are separated only by a very thin fluid film adhering to them. The most important role in forming such films is played by polar compounds and aromatic hydrocarbons that are naturally present in crude oil derived fuels. However, the refinery processes applied in fuel production remove them, thus reducing the lubricity. Fuel lubricity problems were first defined in the mid-1960s and resulted from more severe refining and treatment processes applied in the production of aviation kerosene. In those days, injection equipment failures in aircraft turbine engines were reported. Then, in the late 1980s, similar problems were revealed after the implementation by US and NATO forces of ?The Single Fuel Forward? policy which mandated that all military vehicles must be operable with kerosene-based fuel. Lubricity problems regarding diesel fuel emerged in the late 1990s when some countries set limits on the sulphur and aromatic hydrocarbon content in this fuel. The paraffinic diesel fuel produced by the Fischer-Tropsch synthesis or hydrotreatment process that is more commonly applied nowadays also possesses very low lubricating properties. Generally, to provide good fuel lubricity, various additives are applied and bench tests are mostly employed to estimate their effectiveness. Since 1960 many test rigs have been developed. Several inter-laboratory test programs were carried out to select the best bench tests that would show good correlation with field experience. Among them, only BOCLE, HFFR, and SLBOCLE test methods become industry standards.