Dispersants 145
engines and gasoline engines that are subjected to severe service, such as long-distance driving for
extended periods. The NOx formation initiates when the temperature reaches 137°C [10,11]. Zinc
dialkyldithiophosphates are commonly used as oxidation inhibitors in engine oils [12,13]. All these
acids are neutralized by basic detergents to form inorganic metal salts and metal carboxylates. These
compounds are of low hydrocarbon solubility and are likely to fall out of solution.
The aldehydes and ketones undergo aldol-type condensation in the presence of bases or acids to
form oligomeric or polymeric compounds. These can further oxidize to highly oxygenated hydrocar-
bons, commonly referred to as oxygenates. The oxygenates are usually of sticky consistency, and the
term resin is often used to describe them [14]. Resin is either the basic component in or the precur-
sor to all types of deposits. Common types of deposits include varnish, lacquer, carbon, and sludge
[15,16]. Varnish, lacquer, and carbon occur when resin separates on hot surfaces and dehydrates or
polymerizes to make tenacious fi lms. The quantity and the nature of deposits depend on the proxim-
ity of the engine parts to the combustion chamber. The parts closer to the combustion chamber, such
as exhaust valve head and stem that experience approximate temperatures of 630–730°C [17,18], will
develop carbon deposits. The same is true of the combustion chamber wall, piston crown, top land,
and top groove, which are exposed to approximate temperatures of 200–300°C. Carbon deposits are
more common in diesel engines than in gasoline engines and result from the burning of the liquid
lubricating oil and the high-boiling fractions of the fuel that adhere to hot surfaces [19].
As we move away from these regions to the low-temperature regions, such as the piston skirt,
the deposits are not heavy and form only a thin fi lm. For diesel engine pistons, this type of deposit
is referred to as lacquer; for gasoline engine pistons, this type of deposit is called varnish. The
difference between lacquer and varnish is that lacquer is lubricant-derived and varnish is largely
fuel-derived. In addition, the two differ in their solubility characteristics. That is, lacquer is water-
soluble and varnish is acetone-soluble [15]. Lacquer usually occurs on piston skirts, on cylinder
walls, and in the combustion chamber, whereas varnish occurs on valve lifters, piston rings, piston
skirts, valve covers, and positive crankcase ventilation (PCV) valves.
The coolest parts of the engine, such as rocker arm covers, oil screen, and oil pan, that are
exposed to temperatures of ≤200°C experience sludge deposits. Sludge can be watery or hard in
consistency, depending on the severity of service. If the service is extremely mild and of short
duration, as in the case of stop-and-go gasoline engine operation, the sludge is likely to be watery
or mayonnaiselike [15]. This type of sludge is called low-temperature sludge, which occurs when
the ambient temperature is <95°C. The high-temperature sludge is more common in diesel engines
and gasoline engines with long, continuous operation. This type of sludge occurs when the ambient
temperature is >120°C and is hard in consistency. In the former case, the engine does not get hot
enough to expel combustion water, which stays mixed with oil, imparting sludge, a mayonnaiselike
appearance. In the latter case, however, the ambient temperature is high enough to expel water,
thereby resulting in hard sludge. Sludge is common in areas that experience low oil fl ow, such as
crankcase bottoms and rocker boxes.
Another component of the combustion effl uent that must be considered is soot. Soot not only
contributes toward some types of deposits such as carbon and sludge, but it also leads to a viscosity
increase. These factors can cause poor lubricant circulation and lubricating fi lm formation, both of
which will result in wear and catastrophic failure. Soot is particulate in nature and results from the
incomplete combustion of the fuel and of the lubricating oil from the crankcase that might enter the
combustion chamber by traveling past the piston rings [20]. Fuel-derived soot is a chronic problem
in the case of diesel engines because diesel fuel contains high-boiling components that do not burn
easily. In addition, diesel engine combustion is largely heterogeneous, with poor air–fuel mixing,
hence poor combustion [20]. Soot is made of hydrocarbon fragments with some of the hydrogen
atoms removed. The particles are charged and hence have the tendency to form aggregates. When
aggregates occur on surfaces, such as those of the combustion chamber, soot deposits result. These
deposits are soft and fl aky in texture. If these occur in oil, lubricant experiences an increase in vis-
cosity. A soot-related viscosity increase usually requires the presence of polar materials in oil that