146 Lubricant Additives: Chemistry and Applications
have the ability to associate with soot. These can be additives or polar lubricant oxidation and deg-
radation products. Carbon deposits are lower in carbon content than soot and, in most cases, contain
oily material and ash. This makes knowledge of the ash-forming tendency of a lubricant important
to a formulator. This concern was addressed in Chapter 4.
When soot associates with resin, one gets either resin-coated soot particles or soot-coated resin
particles [16]. The fi rst type of particles results when resin is in excess, and the second type results
when soot is in excess. The amount of soot in resin determines the color of the deposits: the higher
the soot, the darker the deposits. Sludge results when resin, soot, oil, and water mix [9].
Deposit formation in gasoline engines is initiated by NOx and oxidation-derived hydroperoxides
that react with hydrocarbons in the fuel and the lubricant to form organic nitrates and oxygenates
[14,21]. Being thermally unstable, these species decompose and polymerize to form deposits. The
deposits typically include resin, varnish, and low-temperature sludge. In diesel engines, however,
soot is an important component of the deposits, which include lacquer, carbon deposits, and high-
temperature sludge [16]. Typically, carbon deposits are of high metal content, which is mainly due
to the presence of detergent additives in the lubricant [22,23].
Detailed mechanism of deposit formation in engines is described elsewhere [24,25]. The mecha-
nism is based on the premise that both the lubricant and the fuel contribute toward deposit forma-
tion. The role of the blowby, NOx, and high-temperature oxidative and thermal degradation of the
lubricant, described earlier, are substantiated [24]. The importance of oxygenated precursors—their
decomposition, condensation, and polymerization to form deposits—is also supported. The deposit
precursors consist of approximately 15–50 carbon atoms and contain multiple hydroxy and carboxy
functional groups. Because of the polyfunctionality, these molecules have the ability to thermally
polymerize to high-molecular-weight products [14,16]. As mentioned earlier, soot associates with
polar oxidation products in oil to cause a viscosity increase. Viscosity increase can also occur in
gasoline engine oils that have little or no soot. This happens when the oxygen content of the precur-
sors is low and the resulting polymer is of low molecular weight and of good oil solubility [14]. This
phenomenon is commonly referred to as oil thickening [6]. Conversely, if the oxygen content of the
precursors is high, the polymerization results in the formation of high-molecular-weight products of
low lubricant solubility. Such products constitute resin, which is of low oil solubility and separates on
surfaces. If the surfaces are hot, subsequent dehydration and polymerization lead to the formation of
varnish, lacquer, and carbon deposits. It is important to note that deposits are a consequence of lubri-
cant oxidation that accelerates once the oxidation inhibitor package in the lubricant is exhausted.
Three other internal combustion engine problems—oil consumption, ring sticking, and cor-
rosion and wear—are also related to lubricant degradation. Oil consumption is a measure of how
much lubricant travels past piston rings into the combustion chamber and burns. A certain minimum
amount of the lubricant is necessary in the vicinity of the piston rings to lubricate cylinder walls and
cylinder liners and hence facilitate piston movement and minimize scuffi ng. However, if too much
lubricant ends up in the combustion chamber, serious emission problems will result. Modern piston
designs, such as articulated pistons and pistons with low crevice volume, allow just enough lubricant
to minimize scuffi ng, but without adversely contributing to emissions [26,27]. Other parameters
that affect oil consumption include the integrity of pistons and cylinders and the viscosity, volatility,
and sealing characteristics of the lubricant. Pistons with stuck rings and out-of-square grooves and
cylinders with increased wear will result in a poor seal between the crankcase and the combustion
chamber [15]. As a consequence, a larger amount of blowby will enter the crankcase and increase the
rate of lubricant breakdown. This will complicate the situation further. Ring sticking occurs when
sticky deposits form in the grooves behind the piston rings. This is a serious problem because it not
only results in a poor seal but also leads to poor heat transfer from the cylinder to the wall. If not con-
trolled, this will result in nonuniform thermal expansion of the pistons, loss of compression, and ulti-
mately the failure of the engine [15]. The wear of pistons and the cylinders is undesired for the same
reasons. Wear of engine parts is either corrosive or abrasive. Corrosive wear arises from the attack
of fuel sulfur-derived products, such as sulfur oxides or sulfuric acid, or the acidic by-products of