Diesel exhaust is a major source of particulate air pollution that poses health risks. It consists mostly of soot, heavy hydrocarbons, and sulfates. While engine modifications have reduced emissions, additional control technologies are needed. Diesel particulate filters can trap over 90% of particles but require regeneration. Regeneration can be passive using catalyst coatings to lower the ignition temperature, or active through external heating. Catalytic fuel additives also help control particulate by promoting soot oxidation.

1. Vehicular Pollution Management:
Diesel Exhaust Emission Control
Dr. Nitin K. Labhsetwar
Scientist
Air Pollution Control Division, NEERI, Nagpur - 440 020
The increased use of diesel driven vehicles for all categories of
commercial automobiles and even for private cars is the major trend observed
worldwide over the last two decades in transportation field. While the energy
advantages of the diesel engines are unquestioned, lower cost of diesel fuel is
also responsible for its increasing popularity, particularly with respect to the less
developed countries. Although, cleaner than gasoline engines from the
standpoint of view of carbon monoxide (CO) and hydrocarbons (HCs), diesel
engines produce more aldehydes, sulfur oxides (because of the higher sulfur
content in diesel fuel) and nitrogen oxides. Smoke and odor emissions are also a
problem of great concern, most importantly, however, uncontrolled diesel
engines emit significant amounts of particulate. These particulate emissions are
a direct health concern as well as a serious source of overall environmental
degradation.
Diesel particulate matter consists mostly of three components: soot
formed during combustion, heavy hydrocarbons condensed or adsorbed on the
soot, and inorganic compounds, mainly sulfates. In order, diesel engines soot
was typically 40 to 80 percent of the total particulate mass, but soot
emissions from modern emission-controlled engines are much lower. Most of the
remaining particulate mass consists of heavy hydrocarbons adsorbed or
condensed on the soot. This is referred to as the soluble organic fraction (SOF)
of the particulate matter. The SOF is derived partly from the lubricating oil, partly
from unburned fuel, and partly from compounds formed during combustion. Most
of the soot formed during diesel combustion is subsequently burned during the
later stages of the expansion stroke. Typically, less than 10 percent of the soot

2. formed in the cylinder should be emitted into the atmosphere; in modern
emission-controlled diesel engines, even lesser amount of soot may be emitted.
Black smoke from diesel engines is due to the soot component of diesel
particulate matter. Under some conditions, diesel engines may also emit white,
blue, or gray smoke. These are due to the presence of condensed hydrocarbon
droplets in the exhaust. Unlike soot, these droplets scatter light, thus giving the
smoke a bluish or whitish cast. Blue or gray smoke is generally due to vaporized
lubricating oil and indicates an oil leak into the cylinder or exhaust system. White
smoke is common when engines are first started in cold weather and usually
goes away when the engine warms up.
Environmental and Health Impacts of Diesel Particulate
Uncontrolled diesels emit approximately 30 to 70 times more particulate
than gasoline-fueled engines equipped with catalytic converters and burning
unleaded fuel. These particles are a concern from several standpoints:
• Many areas already experience unhealthy air quality levels for total
suspended particulate (TSP) matter. TSP comes from many sources but
diesels also contribute considerably. These particles in urban air are of
concern because a strong correlation between suspended particulate and
variations in infant mortality and total mortality rates has been established.
Further, clear evidence emerges from the epidemiological literature that
implicates particles in aggravating disease among bronchitis, asthmatics,
cardiovascular patients and people with influenza. Any significant increase
in diesel particulate emissions would add to the difficulty of solving this
problem.
• Beyond the overall impact on TSP, diesel particles raise a special health
concern because they are very small (averaging about 0.2 microns in
size). Small particles, which are much more likely to be deposited in the
deepest recesses of the lung (alveolar region) and which require much
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3. longer periods of time to be cleared from the respiratory tract, have a
greater potential to adversely affect human health than larger particles. In
addition, when emitted, they remain suspended in the air near the
breathing zones of people for long periods of time.
• In addition, diesel particulate has also been recognized as especially
hazardous and toxic because of its composition. The U.S. EPA has noted
that up 10,000 chemicals may be adsorbed on the surface of diesel
particles and drawn deep into the lung with them. Many of these chemical
compounds are known to be mutagenic in short term bioassays, and to be
capable of causing cancer in laboratory animals.
• While health issues have been the cause of most concern, diesel and
other particles can also become a nuisance, degrade aesthetics and
material usage through soiling and may contribute directly, or in
conjunction with other pollutants, to structural damage by means of
corrosion or erosion.
• Impairment of visibility has been widely noted as an adverse effect of
increased particulate. Diesel particles because of their composition
(primarily carbon based) and size (in the size range of 0.2 microns) are
very high light absorbers and scatterer and therefore, have the potential to
be especially harmful to visibility.
Diesel Particulate Emission Control
Some major efforts have been made towards the improvement in diesel
engine technology, which resulted in considerable reduction of both NOx and
particulate emissions from the modern engines. The cost of these engine
modifications has been broadly compensated by the improved fuel efficiency,
which in addition also resulted in environmental benefits. However, it is now
widely accepted that the future emission norms for diesel vehicles are not
achievable only through these engine modifications and some kind of emission
control technology is indispensable.

4. As regards to the diesel particulate emission control, following control
options have found success, at least towards partly controlling the emissions:
• Diesel Particulate Filters (DPF)
• Diesel Catalytic Converter
• Fuel Additives
Diesel Particulate Filter
A DPF system has a particulate filter media in the engine exhaust stream
and some means of burning (oxidizing) of collected particulate matter from the
filter. Manufacturing a filter capable of collecting soot and other particulate matter
from the exhaust stream is straightforward, and effective trapping media have
been developed and demonstrated. The shallow wall type wall-flow particulate
filter is today the most efficient soot collection device, attaining filtration
efficiencies of the order of 90% at nominal operation conditions. The most
successful trap-oxidizer systems use either the ceramic monolith or the ceramic -
fiber coil traps. Recently SiC based shallow wall type DPF are becoming most
popular due to their better thermal stability and chemically stable nature. The
main problem of trap-oxidizer system development is how to remove the soot
effectively and regenerate the filter. Diesel particulate matter consists of solid
carbon coated with heavy hydrocarbons. This mixture ignites at 550 to 600°C,
well above the normal range of diesel engine exhaust temperatures (150-400°C).
Special means are therefore, needed to ensure ignition. The regeneration
behavior of DPF depends on many factors including inlet gas temperature,
oxygen concentration, particulate loading, gas flow rate etc.
Many techniques for regenerating particulate trap-oxidizers have been
proposed, and much development effort has been invested. Regeneration
techniques can be divided into passive and active approaches. Passive systems
attain the conditions required for regeneration as a result of normal vehicle
operation. This requires a catalyst (as either a coating on the trap or a fuel
9R0

5. additive) to reduce the ignition temperature of the collected particulate matter.
Regeneration temperature of lower than 400°C has been reported with catalytic
coatings, and further lower temperatures can be achieved with fuel additives.
Active systems monitor particulate matter in the trap and trigger specific actions
to regenerate it when needed. A variety of approaches to trigger regression have
been proposed, including diesel-fuel burners, electric heaters, and catalyst
injection systems.
DPF regeneration can be broadly achieved by one of the following
means:
• Thermal regeneration by use of engine measures or by applying external
heating source
• Catalytic regeneration, which eventually reduce the regeneration
temperature to the exhaust temperature range. This may also includes
fuel additives.
• Aerodynamic cleaning using compressed air.
Applications of Catalysts
Catalyst can be applied in the following ways for the particulate
emission control :
• Catalyst fuel additives, mixed with the fuel
• Continuous oxidation of SOF through an oxidative catalytic converter
• Catalytic oxidation of soot for regeneration of DPF.
Catalyst Fuel Additives
Catalyst fuel additives have found useful applications in diesel engines.
Most of these are organo-metallic compounds of various metals and are mixed
with the fuel in a very small quantity. The additive is oxidized in the combustion
chamber and its oxides form the kernels of particulates, which are incorporated
with the soot on their way to the exhaust stream. Minor secondary affects that

6. may arise by the use of catalytic fuel additives include incomplete filter cleaning
and filter back-pressure increase due to the retaining of fuel additive ash. New
generation ash-less additives offer the remedy to this problem however.
Catalytic fuel additives are nevertheless, found as a useful catalytic option.
Oxidation Catalytic Converter
In cylinder diesel PM control has greatly reduced PM emission levels.
Progress has been most effective in reducing the soot portion of PM emissions,
so the soluble organic fraction (SOF) of particulate matter now accounts for a
much larger share. Depending on engine and operating conditions, the soluble
organic fraction may account for 30 to 70 percent of PM emissions. Oxidation
catalysts of a diesel catalytic converter oxidizes a large portion of the
hydrocarbons present in the soluble organic fraction of PM emissions, as well as
gaseous hydrocarbons, carbon monoxide, odor (from organic compounds such
as aldehydes), and mutagenic emissions. Oxidation catalytic converters have
been used in light-duty vehicles and demonstrated to be effective for heavy-duty
applications as well. They have little effect on nitrogen oxide emissions, but can
reduce volatile organic compound and carbon monoxide emissions by up to 80
percent. The durability of oxidation catalytic converters on heavy-duty engines
has yet to be determined, but it is likely to be acceptable. The main difficulty with
using oxidation catalytic converters on heavy-duty diesel engines is that they can
cause the formation of sulfuric acid and sulfates from sulfur dioxide in the
exhaust. If fuel sulfur levels are significant, these compounds can add
considerably to particulate mass.
DPF Regeneration Catalysts
Diesel Particulate Filter remains by far the most effective option to collect
the particulate matter of diesel vehicle exhaust. Therefore, intensive efforts are
under way to overcome the problems associated with this technical option. The
most important being efficient regeneration. Catalyst is mostly applied as a fine
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7. coating on the wall-flow filters. Although a large number of catalyst compositions
have been studied, however, very effective lowering of regeneration
temperatures could not be attained. Even under the most favorable conditions,
less than 100 °C decrease in regeneration temperature is attained, this is one of
the areas, which is gaining increased attention, and development of improved
regeneration catalyst could possibly make the DPF technology an efficient and
practically feasible option for particulate emission control. Many reports suggest
the favorable participation of S02 and NOx in the soot oxidation reactions,
however, development of an active and stable soot oxidation catalyst without
involving these components would be more feasible on practical fronts.
Table-1
Estimates of Emission Control Technology Costs for Diesel-Fueled
Vehicles (percent)
Technology Engine cost increase
Baseline engine, no emission control
equipment
0
Injection timing retard 0
Low sac volume and valve covering nozzle Minimal
Turbocharging 3-5
Charge cooling 5-7
Improved fuel injection 13-15
High-pressure fuel injection with electronic
control
14-16
Variable geometry turbocharging 1-3
Particulate trap 4-25
Particulate emission control from diesel vehicle exhaust is a major
environmental concern and deserves immediate attention. However, no
straightforward and techno-economically feasible option is so far available, and a
rather comprehensive approach will be required for diesel exhaust emission

8. management. Apart from the development of suitable after treatment technology,
it is equally important to improve the fuel quality, vehicle maintenance and other
related issues. The experience of developed countries will again be useful in this
regard.
References:
1. A. Faiz, C.S. Weaver and M.P. Walsh "Air Pollution from Motor ehicles;,
Standards and Technologies for controlling Emissions" World Bank Pub
No. -TL214 P6F35.
2. J.H. Johnson, T.M. Baines & J.C. Clerc, "Diesel Particulate Emissions;
Measurement Techniques, Fuel Effects & Control Technology", Pub by
SAE International, Pub No - PT 42 (1992).
3. Howitt J.S., and Montierth M. R., " Cellular Ceramic Diesel Particulate
Filters", SAE Paper 810114 (1981).
4. Abthoff J., Schuster H.D., Langer H.J. and Loose G., "The regenerable
trap oxidiser-an emission control technique for diesel engines" SAE
850015(1985).
5. Johnson J.H., Bagley S.T., Gratz L.G., and Leddy D.G. "A review of diesel
particulate control technology and emission effects" SAE Paper 940233
(1994).
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