By Charles T. Hare and Magdi K. Khair
`
`T he October 1998 consent
`
`decree involving the major
`heavy-duty diesel engine
`manufacturers, the
`Environmental Protection Agency (EPA),
`and the u.s. Department of Justice has
`forced an accelerated timeline to achieve
`the tightest emission goals in history for
`on-road diesel vehicles.
`As recently as early 1998, diesel
`engine companies producing for the U.S.
`market were carrying out the long-term
`research needed to meet new emissions
`requirements that were anticipated for
`the 2004 model year, some five years
`away. Overnight, however, long-term
`
`research became short-term development
`as the decree called for the new standards
`to become effective in 2002 -
`a brief
`three years over the horizon. * Meeting
`these standards will only be possible with
`a concerted effort to finalize some of the
`diesel emissions reduction technology
`currently under development.
`The diesel engine is the most effi(cid:173)
`cient of the internal combustion power(cid:173)
`plants. Highway trucks, urban buses, and
`industrial equipment are powered almost
`exclusively by diesel engines. However,
`heavy-duty diesel emissions have been
`increasingly identified as a major source
`of smog and acid rain precursors in the
`northeastern United States and in
`California. Estimates of the contribution
`of on-highway diesel engines to total
`
`oxides of nitrogen (NOx) emissions
`nationally range from 12 to 18 percent.
`The EPA indicates that this fraction is
`increasing as commercial traffic activity
`rises and emissions attributable to other
`sources, such as passenger cars, decline.
`As for particulate matter (PM), highway
`diesels are estimated to contribute
`between 20 and 30 percent of the national
`loading. Diesel trucks and buses make up
`four to five percent of all vehicles on the
`road, but they are estimated to account
`for up to 40 percent of all vehicle miles
`
`* The 1998 consent decree stipulates that, by the
`year 2002, heavy-duty diesel engines can emit
`no more than 2.5 grams of oxides of nitrogen
`(NOx) + hydrocarbons (He) and 0.10 grams
`of particulate matter (PM) per horsepower
`hour (g/hp-hr).
`
`2
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`Technology Today· Summer 1999
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`

`
`Staff Engineer Magdi Khair (left) and Director Charles
`Hare utilize equipment for the control and
`monitoring of diesel engine test cell operation in their
`investigations within the SwRI Emissions Research
`Department. Khair specializes in reducing emissions
`from diesels using engine controls and aftertreatment
`concepts. He also provides an educational seminar on
`diesel engine technology through the Society of
`Automotive Engineers. Hare oversees a broad range of
`emissions research and development programs cover(cid:173)
`ing engines used in all types of mobile equipment, from
`lawnmowers to locomotives.
`
`and research into the
`effects of fuels on engine
`performance. As a result of
`the accelerated schedule
`for lower diesel emission
`standards, the Institute's
`work has taken on an
`increased sense of urgency.
`In diesel engines such
`as those used in trucks and
`buses, only NOx and PM
`emissions (among the crite-
`ria pollutants) are consid(cid:173)
`ered problematic. The levels of carbon
`monoxide (CO) and nonmethane hydro(cid:173)
`carbons (NMHC) these engines produce
`are not thought to be major contributors to
`national air pollution levels. The inclusion
`of HC emissions in the combined
`NMHC + NOx standards for the industry
`only serves to place a cap on such emis(cid:173)
`sions, to ensure that HC levels would not
`markedly increase as a result of any new
`technologies that might be implemented.
`Emissions of the greenhouse gas carbon
`dioxide (C02 ) are somewhat lower from
`diesels than from most spark-ignited
`engines - perhaps by as much as
`20 percent for equivalent power output.
`Achieving the mandated EPA
`requirements within this abbreviated
`time frame will require combining techni(cid:173)
`cal solutions. No single technology may
`be capable of achieving the desired goals.
`A combination of fuel improvement,
`enhancements of in-engine performance,
`and exhaust aftertreatment will probably
`be necessary.
`
`Traditional solutions
`under investigation
`
`Fortunately, the Institute already has
`considerable experience with many of
`these technologies and their effectiveness
`in reducing emissions. A grouping of past
`and present research projects includes
`design changes to the engine itself, com(cid:173)
`bined with improvements in the charac(cid:173)
`teristics of the fuel and lubricants now in
`use. Researchers can, for example:
`Improve fuel injection systems,
`including the placement, hole size, and
`design of the injector, as well as the tim(cid:173)
`ing and rate shaping of injection and
`metering, aided by electronic controls.
`These techniques promote fuel! air mix(cid:173)
`ing, control the rate of combustion and
`pressure rise, decrease noise, and reduce
`the "white smoke" emitted by cold
`engines at startup.
`Redesign the combustion chamber
`and combine it with an improved intake
`port design to allow better air flow and
`fuel mixing, and to provide more com(cid:173)
`plete combustion. More complete com(cid:173)
`bustion reduces HC and CO emissions
`and improves engine efficiency.
`Augment air boost and motion,
`and control charge air temperature
`together with better fuel! air mixture
`preparation to reduce black smoke or
`"soot," as well as CO and the total mass
`of PM emissions.
`Manipulate fuel characteristics,
`such as sulfur content, viscosity, lubricity,
`and molecular structure, as well as inves-
`
`Technology Today· Summer 1999
`
`3
`
`traveled, and operate at comparatively
`high power levels, proportional to the
`heavy loads they transport.
`The Emissions Research Department
`at Southwest Research Institute (SwRI)
`has developed long-standing technical
`relationships with the diesel manufac(cid:173)
`turers affected by the EPA consent
`decree, either through dedicated project
`work or through a number of coopera(cid:173)
`tive industry research projects (see
`related discussion, page 16). In the effort
`to control and reduce diesel emissions,
`Institute staff have contributed to engine
`design and development, tuning of
`emission control systems, quality audits,
`emissions certification, development and
`comparative study of testing methods,
`lubricant evaluation and qualification,
`
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`

`
`Changes in Heavy-Duty Diesel Engine Regulations
`
`Consent Decree
`7 ,----------------------+-------,----,---,-,
`Old Lead Time
`
`New Lead
`Time
`
`0.7
`
`0.6
`
`0.5
`
`Possible
`New Limit
`0 L-~-L-L~ __ L-~-L~~ __ L-~-L~ __ L-~-L-L~ __ ~
`0
`2002 2004 2006
`1994
`1998
`1991
`1988
`Year
`
`6
`
`.t:.
`
`5
`...
`I ,g.4
`--C)
`>< 3 o z
`
`2
`
`1
`
`tigate the use of syn(cid:173)
`thetic fuels. Reducing
`sulfur content and opti(cid:173)
`mizing the molecular
`structure of a fuel can
`lead to less sulfate for(cid:173)
`mation, lower emis-
`sions of "toxics" such
`aspolynucleararo-
`matic hydrocarbons,
`and to some extent,
`lower NOx' Viscosity
`and lubricity control
`are important in improving fuel spray
`characteristics to lower He emissions,
`maintain fuel injector and fuel pump life,
`and ensure good cold-weather operation.
`Finally, alter oil properties that
`can affect volatility, soot tolerance, and
`dispersion, and take advantage of
`performance improvements available
`using the latest additive packages.
`Improved oil characteristics can reduce
`the soluble fraction of PM, keep deposits
`to a minimum, and protect against wear
`caused by the presence of soot derived
`from the combustion process.
`Using these five areas of technology
`in various combinations, the in-engine
`solutions have, so far, generally been ade(cid:173)
`quate to meet emission standards through
`the current model year. The advantages of
`this approach include the ability to keep
`technology completely under the control
`of the manufacturer and to minimize
`complexity and potential impact on the
`consumer. In addition, manufacturers
`
`"tJ
`0.4 :i:
`(Q --~
`
`0.3 "C
`
`I
`~
`~
`
`0.2
`
`0.1
`
`than fresh air, but also
`has more water and car(cid:173)
`bon dioxide, all of
`which reduce or absorb
`heat from the combus(cid:173)
`tion process, lowering
`the temperature and
`reducing NOx forma(cid:173)
`tion. With or without
`the use of EGR, diesels
`produce a certain
`amount of solid PM.
`One technique to control
`this is to use a diesel particulate filter
`that, with either a passive or active
`regeneration system, can continuously
`remove soot from the exhaust without
`becoming clogged.
`Oxidation catalysts, which can
`reduce PM by helping to control the
`volatile organic fraction. They adsorb
`organic material, including gaseous
`hydrocarbons, on their high surface areas
`where the precious metal catalysts they
`contain can facilitate oxidation.
`Lean NOx catalysts operate in a
`different way, bringing NOx molecules
`into contact with hydrocarbon reduc(cid:173)
`tants to cause a reduction reaction. The
`
`retain the sole responSibility for certifying
`and tracking the in-use performance of
`their engines.
`
`Exhaust emission control using
`aftertreatment technologies
`
`In-engine solutions are still being
`developed and improved by manufacturers
`and engineers at SwRI in the effort to meet
`future standards. A variety of other tech(cid:173)
`nologies may also be needed and are con(cid:173)
`sidered increasingly likely to meet the new
`2002 standards. These methods are exhaust
`aftertreahnent techniques and include:
`Exhaust gas recirculation (EGR),
`which pipes a small
`fraction of the
`exhaust back to the
`inlet duct, replacing
`some of the intake
`air with exhaust
`gas. This gas con(cid:173)
`tains less oxygen
`
`Equipment for monitoring gas concentrations
`in three or four streams simultaneously is
`needed during the development of diesel
`exhaust aftertreatment schemes. Taken in
`order, sample locations include the fresh air
`intake, the inlet manifold (when EGR is used),
`the exhaust manifold, and the tailpipe
`(following the aftertreatment device).
`
`o
`
`'" ~
`
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`hydrocarbon reductants are generally in
`the form of fuel injected into the exhaust,
`upstream from the catalyst itself.
`Yet another option, called the lean
`NOx trap, is a two-stage device that con(cid:173)
`verts exhaust nitric oxide (NO) to nitro(cid:173)
`gen dioxide (N02) in a first stage. The
`second stage acts as storage for the N02,
`and also acts as a reducing element when
`a hydrocarbon reductant (again, generally
`fuel) is injected upstream from the device.
`The operation of catalytically regen(cid:173)
`erated particulate filters bears some
`resemblance to that of lean NOx traps, in
`that they contain a first-stage precious
`metal catalyst to generate N02 from NO.
`From that point on, however, they differ
`because the N02 is used to oxidize the
`trapped carbon particulate directly, and
`no reducing agent is added.
`Another particulate control method
`is the catalytically assisted passively
`regenerated diesel particulate filter,
`which uses a catalytic coating to reduce
`volatiles and provide some heat assist.
`Regeneration is further enhanced by the
`presence of a fuel-borne
`catalyst (sometimes used
`independently to control
`emissions), which reduces
`the ignition temperature of
`the collected particulate to
`the point that it burns off
`more frequently and keeps
`the filter flowing freely.
`Plasma reactors and
`plasma-assisted catalysis
`form a relatively new field
`of interest for diesel engi(cid:173)
`neers. This is a technology
`in which a single reactor
`has been observed to con(cid:173)
`trol both the NOx and PM
`simultaneously. The high
`potential payoff of plasma
`technology is tempered,
`however, by the unknown
`developmental risks and
`
`Diesel Emission Reduction Technologies
`
`Lean NOx Catalyst
`
`Selective CatalytiC
`Reduction
`
`Lean NOx Trap
`
`This can produce an NOx conver(cid:173)
`sion efficiency of 80 percent or more.
`However, while the com(cid:173)
`plications this technology
`introduces are understood,
`the engineering solutions
`needed to minimize the
`problems have not
`been completed.
`
`Diesel particulate
`filter cores made of
`specialty ceramics such as
`cordierite and silicon
`carbide (shown) are studied
`for particle collection
`efficiency, resistance to
`exhaust heat, buildup of
`backpressure, and
`completeness of
`regeneration (cleaning).
`
`Finally, selective catalytic reduction,
`using urea or ammonia reagents, is
`another technique with high potential for
`diesel engine NOx control. As the name
`implies, this method selectively promotes
`the reduction of NOx to N2 over propri(cid:173)
`etary catalysts when specific reducing
`agents are mixed with the exhaust stream.
`The basic forms of these reactions are:
`
`4NO + 4NH3 (ammonia) + 02 -+ 4N2 + 6H20
`or 6NO + 2(NH2)2CO (urea) -+ 5N2 + 4H20 + 2C02
`
`costs of the device. The system is also
`fairly complex as currently designed.
`Plasma reactors generally consist of
`a packed bed or a central-wire chamber
`in which a nonthermal plasma is electri(cid:173)
`cally generated, producing a stream of
`ions that cause reactions to accelerate in
`the exhaust passing through the device.
`Plasma-assisted catalysts add another
`element, a lean-NOx catalyst. A synergy
`has been observed between the actions of
`the plasma and the catalyst that may
`prove to enhance conversion of pollu(cid:173)
`tants, reduce plasma energy require(cid:173)
`ments, or both.
`
`Added
`
`Urea
`
`Exhaust NOx
`
`Exhaust NOx
`
`Oxidation Catalyst
`
`Diesel Particulate Filter
`
`Exhaust ---",.;.~
`PM
`
`PM
`
`Combining emissions
`reduction technologies
`
`An important question
`that remains is, which tech(cid:173)
`nologies can logically be
`combined? In general,
`improving fuel quality
`helps all the other tech(cid:173)
`niques to function as
`intended, so progressing
`from high sulfur to low sul(cid:173)
`fur to essentially zero sul(cid:173)
`fur fuel would be helpful.
`
`Less
`PM
`
`Technology Today. Summer 1999
`
`5
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`

`
`SwRI engineers are investigating
`a variety of aftertreatment
`technologies to meet 2002
`heavy-duty diesel emission
`standards. The diagram shows
`NOx and PM emissions levels
`using technologies singly and in
`combination. Note that combined
`device performance and test
`variability for all technologies is
`approximately ± 0. 15 for NOx
`and ± 0.002 for PM.
`
`NOx and PM Emissions Reduction
`NOx Emissions (g/hp-hr)
`2
`3
`4
`
`5
`
`o
`
`Baseline
`Engine
`
`With Diesel
`Oxidation Catalyst
`
`With Lean NOx Catalyst
`and Oxidation Catalyst
`
`1111111111111 EGR
`111111111111111 With EGR and Diesel
`
`With
`
`Particulate Filter
`
`With Selective
`Catalytic Reduction
`
`note that results achieved so far
`in the laboratory have met the
`demonstration goals of 1.0 gram
`per horsepower-hour (g/hp-hr)
`for NOx and 0.01 g/hp-hr for
`PM that the program stipulated.
`Another technology,
`plasma-assisted catalysis, is not
`fully evaluated yet. Combined
`reductions in emission levels
`may be pOSSible through this
`single technology, but many
`uncertainties concerning power
`requirements, efficiency. and
`by-product generation remain to
`be studied.
`
`0.25
`
`Conclusions
`
`For more than 30 years,
`the Institute has been instru(cid:173)
`mental in reducing emission
`problems associated with diesel engines,
`starting with odor and smoke control,
`and progressively working on PM, toxic
`substances, and gaseous pollutants in
`response to ever more demanding stan(cid:173)
`dards. Development projects using in(cid:173)
`engine and aftertreatment technologies
`are continuing, and within a few years,
`the most successful combinations will be
`appearing on production engines.
`Providing crucial assistance to the diesel
`engine industry as it strives to overcome
`emissions challenges is a rewarding role
`for SwRI, and one that will yield envi(cid:173)
`ronmental benefits as well as help pre(cid:173)
`serve the position of the diesel as the
`most efficient motive power for a range
`of essential applications . • :.
`
`Comments about this article? Contact Hare at
`(210) 522-2646 or chare@swri.org, or Khair at
`(210) 522-5311 or mkhair@swri.org.
`
`However, these changes
`involve costs to the refiner,
`which would be passed
`along to the consumer.
`Likewise, progress from
`today's fuel composition to a
`low-aromatic fuel, and pos(cid:173)
`sibly progressing to syn(cid:173)
`thetic fuels containing no
`aromatic compounds at all,
`would be advantageous.
`However, this would also
`entail substantial additional
`costs for NOx and PM reduc-
`tions of approximately 10-20
`percent for advanced
`engines. Reductions of this
`magnitude may be helpful,
`but they are not even close
`to the 50-75 percent control
`efficiencies needed for the
`year 2002 and beyond.
`Several SwRI programs have been in
`progress throughout the 1990s to develop
`low-emission diesel systems using such
`combinations of technologies. Because the
`Institute has equal interest in the success
`of all candidate technologies, it is in a
`good position to evaluate and develop a
`variety of combinations. Doubtless some
`systems may have particular advantages
`for individual engine companies and
`engine applications, and results so far
`show a large variation in the attributes
`that will ultimately determine which
`combinations of technologies will be most
`widely used on future engines. These
`attributes include cost, size and weight,
`efficiency of control for NOx and PM, the
`amount of power consumption and sub(cid:173)
`sequent fuel penalty, difficulty of collat(cid:173)
`eral problems, the reliability of the
`technology, and its current development
`status and future potential.
`
`"11111111 With Selective Catalytic Reduction
`
`and Diesel Particulate Filter
`
`With EGR, Selective Catalytic Reduction,
`and Diesel Particulate Filter
`
`? With Plasma-Assisted
`Catalyst
`
`o
`
`0.05
`
`0.20
`0.15
`0.10
`PM Emissions (g/hp-hr)
`
`Recent efforts in the Emissions
`Research Department have focused on
`EGR adaptation and improvements, intake
`air flow augmentation, and a variety of
`aftertreatment technologies. Generalized
`NOx and PM results obtained from
`research carried out in 1998 and early 1999
`are shown in an accompanying graphic for
`some of these systems. Although the
`attributes of these systems have not been
`completely assessed, the graphic shows
`that there is a real potential to control the
`targeted pollutants.
`Using the technologies in combina(cid:173)
`tion has proved the best way to achieve a
`reduction in both NOx and PM levels.
`However, the selection of technology is
`also related to system complexity and
`cost. Improving and refining the design
`process to reduce potential disadvantages
`is one of the serious challenges currently
`facing the industry. It is encouraging to
`
`6
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`Technology Today· Summer 1999
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