29
V. EFFECTS OF EMISSION-REDUCTION
TECHNOLOGIES ON FRICTION AND WEAR
The two components of diesel exhaust receiving the most attention are nitrogen oxides (NO
x
)
and particulate matter (PM). Industry has already made substantial progress in reducing both
constituents, but additional efforts are required, especially for NO
constituents, but additional efforts are required, especially for NO
x
.
Between 1978 and 1998, NO
x
emissions have been reduced by a factor of almost 4, from 15
g/hp-hr to 4 g/hp-hr. However, another factor of 2 reduction to 2 g/hp-hr will be required by
October 2002. The four most likely technologies for meeting the new requirements are exhaust
gas recirculation (EGR), higher fuel pressures, catalytic after-treatments, and alternative or
improved fuels. In addition to causing an anticipated 5 to 10% decrease in fuel efficiency, each
of these technologies also introduces new problems related to friction and wear. As explained
earlier in this document, increased soot loading due to EGR will place new demands on the
lubricant and may require wear-resistant coatings on critical parts. Higher fuel pressures will
require tighter tolerances and produce higher stresses in fuel injectors, which probably will
necessitate stronger materials and low-friction, wear-resistant coatings. Catalytic aftertreatment
will require low-sulfur fuels and low-sulfur, low-phosphorus lubricants. It will be necessary to
find a substitute for the current sulfur- and phosphorus-containing additive in order to maintain
satisfactory life of components, such as parts of the valve train, that are subjected to high contact
stresses. Alternative fuels, as well as higher-purity diesel fuels, will have lower lubricity, which
will require development of an additive or new coatings for critical engine components.
October 2002. The four most likely technologies for meeting the new requirements are exhaust
gas recirculation (EGR), higher fuel pressures, catalytic after-treatments, and alternative or
improved fuels. In addition to causing an anticipated 5 to 10% decrease in fuel efficiency, each
of these technologies also introduces new problems related to friction and wear. As explained
earlier in this document, increased soot loading due to EGR will place new demands on the
lubricant and may require wear-resistant coatings on critical parts. Higher fuel pressures will
require tighter tolerances and produce higher stresses in fuel injectors, which probably will
necessitate stronger materials and low-friction, wear-resistant coatings. Catalytic aftertreatment
will require low-sulfur fuels and low-sulfur, low-phosphorus lubricants. It will be necessary to
find a substitute for the current sulfur- and phosphorus-containing additive in order to maintain
satisfactory life of components, such as parts of the valve train, that are subjected to high contact
stresses. Alternative fuels, as well as higher-purity diesel fuels, will have lower lubricity, which
will require development of an additive or new coatings for critical engine components.
Particulates have been reduced from about 1.5 g/hp-hr in 1978 to 0.1 g/hp-hr in 1994. That level
meets the requirements for 2002. However, the health effects of PM are being investigated in
many ongoing research programs. Recent evidence suggests that the mass emission rate may not
be the most appropriate standard. Current standards deal with particles that are less than 10
micrometers (
meets the requirements for 2002. However, the health effects of PM are being investigated in
many ongoing research programs. Recent evidence suggests that the mass emission rate may not
be the most appropriate standard. Current standards deal with particles that are less than 10
micrometers (
µ
m) in diameter. However, recent research has shown that much smaller particles
less than 50 nanometers (0.05
µ
m) in diameter might be much more dangerous than particles
of 0.1 to 10
µ
m. As is shown in Figure 9, these very small nanoparticles can represent a large
fraction of the number of particles even though they account for a small fraction of the mass of
the PM.
the PM.
6
Furthermore, efforts to reduce the mass of the emissions sometimes can cause an
increase in the number of nanoparticles. For example, soot in the exhaust will tend to suppress
the formation and growth of nanoparticles by adsorbing sulfuric acid and hydrocarbons and by
acting as substrates for the coagulation of nuclei.
the formation and growth of nanoparticles by adsorbing sulfuric acid and hydrocarbons and by
acting as substrates for the coagulation of nuclei.
6
While there is much evidence to suggest that PM is not good, research to date has not defined
how itshealth effects are related to factors such as particle number, density, surface area, shape,
and chemical composition. In epidemiological studies, it has been difficult to separate the effects
of particulate air pollution from the effects of other air pollutants.
how itshealth effects are related to factors such as particle number, density, surface area, shape,
and chemical composition. In epidemiological studies, it has been difficult to separate the effects
of particulate air pollution from the effects of other air pollutants.
7
Few of the epidemiological
studies to date have isolated the effects of ultra-fine particles, and a consistent pattern is not
apparent.
apparent.
8
The anticipated high inflammatory potential from nanoparticles has not been
demonstrated in traditional experimental animal studies unless very high concentrations,
approaching 1 mg/m
approaching 1 mg/m
3
and higher, were used. "Such studies with high doses have no
environmental relevance and results should be interpreted with caution."
9
There is general
agreement that considerably more research must be done to clarify what kinds of particulate
pollutants are harmful and what concentrations are dangerous.
pollutants are harmful and what concentrations are dangerous.