1. Introduction
The continuous advances in diesel engine technology
and the more stringent emissions standards have led to
higher injection pressure and tomodifications in some fuel
properties. Among the latter ones, diesel fuels need to
increase their lubricity to protect the fuel injection system
and other engine components. A higher boiling point and
aromatic, nitrogen, and sulfur contents appear to improve
diesel fuel lubricity.1 Several studies have reported that the
key agents are the highly polar compounds (especially
those containing oxygen and nitrogen) present in diesel
fuels which derive in forming a protective layer on the
metal surface.2 However, many of these surface-active
polar compounds are eliminated during the desulfurization
processes causing loss of lubricity.3,4 To meet the wear
scar limits established in diesel fuel standards (460 and 520
μm in European and U.S. regulations, respectively (EN
590:2009,5 ASTM D 9756)), a variety of lubricity additives
can be used,which have a high affinity to metallic surfaces
forming a thin protective metal-metal contact layer. This
lubricant film is formed by the adsorption of the polar
molecules of the additives on the metal surface, which is
negatively charged.4