112 Lubricant Additives: Chemistry and Applications
the behavior of phosphorus compounds in wear and catalyst tests varies according to the
way in which phosphorus is incorporated into the molecule. Further work reports that
it is possible to achieve improvements in catalyst protection (and fuel economy) by
reducing the ZDDP content and then adding a metal-free phosphorus-containing AW addi-
tive [174].- In an attempt to increase fuel economy, the so-called fuel-effi cient lubricants are being
developed. These are usually lower-viscosity products (since energy losses decrease with
viscosity), sometimes complemented by the use of friction modifi ers. However, low-viscos-
ity oils may cause increased wear of some engine components, and the necessity for improv-
ing the wear protection is being studied. The ILSAC GF-5 specifi cation, for example, will
necessitate the use of some form of friction modifi er to guarantee the required level of
economy. Cur rently, molybdenum compounds or long-chain esters are under evaluation, but
other approaches (e.g., the use of functionalized viscosity modifi ers) are also being studied
as the ability of these additives to deliver reduced friction over long periods is uncertain
(Mainwaring, R., Shell Global Solutions, U.K.). ZDDP is linked to increased friction and
therefore reduced ZDDP levels may also be required. - To reduce the particulate (soot) levels in exhaust gas, the diesel engines in many
passenger cars and trucks need to use particulate fi lters. These fi lters can also remove the
ash-containing residue produced from metallic fuel and lubricant additives, and if they are
not occasionally cleaned, they will block causing a deterioration in engine performance.
The engine builders, however, are trying to preserve or even extend service intervals and
consequently are interested in reducing the ash content of the oil. Although ZDDP is not
the only source of metals in the oil, a reduction in zinc content will follow automatically
from any reduction in the phosphorus content (as long as ZDDP remains in formulations)
and will therefore help to reduce engine oil ash content (Mainwaring, R., Shell Global
Solutions, U.K.). - One of the techniques used to remove the soot from the particulate fi lters (and thereby
maintain an acceptable engine back pressure) has been to oxidize the deposited carbona-
ceous material by nitrogen dioxide (NO 2 ). This is obtained from the exhaust gas by catalyti-
cally oxidizing the NO component. The oxidation of the soot to carbon dioxide effectively
removes carbonaceous fi lter deposits, and the NO 2 is such a powerful oxidant that it enables
the process to be carried out at a relatively low temperature (∼250°C). Unfortunately, the
catalyst used to oxidize the NO preferentially oxidizes any SO 2 in the exhaust, thereby
reducing the effi ciency of the NO conversion. Additionally, the sulfur trioxide (SO 3 ) formed
passes through the trap in the gas phase and is converted there into sulfuric acid by the
water in the exhaust. The sulfuric acid (monitored as “sulfates”) contributes, as droplets,
to the overall level of particulate emissions and is clearly undesirable if the exhaust gas
is inhaled. Any reduction in sulfur content by lowering ZDDP levels in engine oils also
reduces the phosphorus content arising from this additive. Supplemental phosphorus may
therefore be needed. - Increased emphasis on fuel economy led some manufacturers to introduce direct injection
stratifi ed charge gasoline engines. Conventional catalysts cannot remove oxides of nitrogen
(NOx) in these “lean-burn” engines, and, as a result, NOx storage catalysts have been devel-
oped in which the oxides are stored as nitrates by reaction with barium sulfate contained in
the catalyst coating. When the barium-containing sites become saturated, the engine switches
to stoichiometric or slightly rich operation at which temperature the nitrates break down and
release the NOx, thus promoting its reduction through the conventional route of reaction
with hydrocarbons and carbon monoxide. Unfortunately, barium sites react preferentially
with any sulfur oxides present, reducing their ability to “store” NOx. As a consequence,
there is again pressure to reduce the fuel sulfur content. However, these levels are already
being lowered (see “Fuels” below), and at such levels, the engine oil begins to be a signifi cant