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SAE TECHNICALPAPER SERIES 1999-01-0108

Diesel Exhaust Treatment - New Approaches toUltra Low Emission Diesel Vehicles

Hartmut Lders and Peter StommelFEV Motorentechnik, Aachen

Sam GecklerFEV Engine Technology, Auburn Hills

Reprinted From: Diesel Exhaust Aftertreatment 1999(SP-1414)

International Congress and ExpositionDetroit, Michigan

March 1-4, 1999

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1 1999-01-0108

Diesel Exhaust Treatment - New Approaches toUltra Low Emission Diesel Vehicles

Hartmut Lders and Peter StommelFEV Motorentechnik, Aachen

Sam GecklerFEV Engine Technology, Auburn Hills

Copyright 1999 Society of Automotive Engineers, Inc.

ABSTRACT

Currently, throughout the world combustion engine devel-opment is influenced by two primary concerns. First isthe increasing concern for global warming, and second isthe concern over particulate and oxides of nitrogen emis-sions, each of which affect the environment and humanhealth because of the particles' toxicity and ground levelozone production, respectively.To address the global warming issue, in late 1997, vari-ous nations approved the Kyoto Protocol to reduce CO2emissions because of its identified contribution to thegreenhouse effect. The Diesel engine is the most efficientpower plant for mobile and stationary purposes and,thus, Diesel engines are considered to be one alternativeto gasoline engines to reduce fuel consumption and,thus, CO2 emissions.To address the emission concerns, the European Com-munity and the U.S. Environmental Protection Agency(EPA) proposed emissions standards prescribing sub-stantial reductions of NOX and PM emissions [1,2]1. As aresult of these proposed standards, reductions in particu-late and NOX emissions have become major challengesin Diesel engine development. Unfortunately, in the para-digm of Diesel engine development, ultra low emissionsand very low fuel consumption are two opposing objec-tives. Moreover, NOX reduction from lean exhaust is fun-damentally difficult because of the excess oxygenpresent. Particulate mass emissions from modern Dieselengines are already on very low levels; however, recentlyparticulate numbers rather than particulate mass haveattracted much attention and, thus, further reductions arenecessary.This paper describes new approaches to ultra low emis-sion Diesel vehicles that comply with the proposed emis-sion standards. Advanced exhaust gas aftertreatment

technology, including Diesel particulate filters and NOXcatalysts, have previously been subjected to detailedresearch. In this paper, some new approaches aredescribed, and their potential to large-scale vehicle appli-cation is assessed.

1 INTRODUCTION

Because of environmental concerns associated with Die-sel exhaust emissions, emission limits for Diesel-powered vehicles have been reduced throughout theworld. Figure 1 shows the emission standards currentlyin force in the European Community and those to beimplemented in the near future [1].

Figure 1. European Emission Limits

All regulated pollutants will be subjected to substantialreductions. Worth particular scrutiny are the proposedstandards for oxides of nitrogen (NOX) and particulatematter (PM) which might be on a critical level even formodern Diesel engines. Emission limits for both thesepollutants are reduced by as much as 50 %.Apart from the European Community, the United Statesof America, lead by the state of California, have adopted

1 Numbers in parenthesis correspond to the refer-ences at the end of the paper.

CO NOx NOx + HC PM0,0

0,5

1,0

1,5

Currently no Limitation ofNO

X as a Single Pollutant

Emis

sion

[g/k

m]

European Emission Limits for PassengerDiesel Vehicles

EURO 2 (currently in force) EURO 3 (to be implemented in 2000) EURO 4 (to be implemented in 2005)

2the most severe emission standards. In 1997, the Califor-nia Air Resources Board (CARB) announced proposedlight-duty LEV II emission standards listed in Table 1 [2].

Under the CARB proposal, NOX and PM standards for allemission categories are significantly tightened. Begin-ning with the 2004 model year, all light-duty LEV andULEV vehicles must meet a 0.07 g/mi NOX standard tobe phased-in over a three year period. A full useful lifePM standard as low as 0.01 g/mi is proposed for light-duty Diesel vehicles and trucks less than 7,000 lbs curbweight certifying to LEV, ULEV, and SULEV standards,also beginning in 2004. Certainly, the SULEV proposalwill be a real challenge for Diesel vehicles.Having identified these challenges, it is likely that mostpassenger cars will achieve EURO 3 standards withoutadvanced aftertreatment devices such as NOX adsorbercatalysts (NAC) or Diesel particulate filters (DPF). EURO4 and SULEV standards, however, are supposed to beserious challenges especially for heavy-weighted vehi-cles, and from today's point of view many vehicles mayrequire aftertreatment to remove either NOX or particu-lates or perhaps both.

2 ULTRA-LOW EMISSION TECHNOLOGIES

It is now clear that both engine and aftertreatment tech-nology must make a significant contribution towardsachieving either EURO 4 or LEV II compliance.Furthermore, it is now widely understood that during thedevelopment of a Diesel engine combustion system, theengine control and aftertreatment system technologymust be considered jointly, as depicted in Figure 2, sincemodern, highly flexible engine control systems make itpossible to consider the requirements of the aftertreat-ment system. To comply with future emission standards itis more important than ever to consider the typical needsof the aftertreatment system from early in the enginedevelopment process. If, for example, the engine is cali-brated for ultra-low HC emissions, some NOX-reducingtechnologies may not be effective.

Figure 2. Simultaneous Emissions Reduction Process

2.1 DIESEL ENGINE TECHNOLOGY Modern Dieselengine technology has drastically changed the image ofthe Diesel engine that was in the past badly reputed as anoisy, smoky, and sluggish power plant. Today's powerfulDiesel engines combine low fuel consumption, excellentdriving performance, and superior driving comfort withlow emission characteristics. For these reasons, Dieselengines have garnered a significant European marketshare even in the automobile luxury class. Because of itsexcellent driving performance, the Diesel engine hasmade its arrival even in racing cars [3] - something unbe-lievable some years ago. Recently, a Diesel-poweredvehicle won a German 24 hour race [4].Figure 3 provides an overview of the exhaust emissionreductions that have been achieved in the last few yearsin Europe. From model year '96 to model year '98 com-bined HC and NOX emissions, as well as particulateemissions, were reduced by 60 % on average. Thesereductions resulted primarily from careful combustionsystem development together with Diesel oxidation cata-lyst improvements.Ultra low emissions and minimum fuel consumption, how-ever, are targets which readily oppose each other, andtherefore, exhaust aftertreatment might play an importantrole to overcome the dilemma. Emissions of oxides ofnitrogen and particulate matter are the subject of particu-lar concern, hence advanced aftertreatment systemsfocus on the reduction of these pollutants.

2.2 DeNOx AFTERTREATMENT TECHNOLOGY Anumber of DeNOx technologies have been developed.Table 2 provides a survey of current DeNOx technologiesand typical NOX reduction rates achieved over the NewEuropean Driving Cycle (NEDC). Maximum NOX reduc-tion largely depends on the catalyst, the vehicle, and thecalibration; therefore, individual results may significantlydiffer from those given below.

Table 1. Proposed LEV II Standards*)

Category NOX [g/mi] PM [g/mi]TLEV 0.6 0.04LEV 0.07 0.01

ULEV 0.07 0.01SULEV 0.02 0.01

*) CARB proposal for a 120,000 miles (11 years) useful life

Exhaust GasAftertreatment

CombustionSystem

Steady-State Application

Vehicle Application

3Figure 3. Current Emission Status of European DI Diesel Passenger Cars; Emissions Data from [5]

The designation Lean NOX refers to technologies rely-ing on hydrocarbons as the reductant. Passive Lean NOXsystems use the hydrocarbons present in the raw engineexhaust. Unfortunately, the hydrocarbon concentration inDiesel exhaust is inherently low and, thus, maximumNOX conversion with passive systems is currently limitedto approximately 15 %. By supplying additional hydrocar-bons, e.g., by means of post-injection, maximum NOXconversion rates can be increased to about 30 % (suchsystems are referred to as active systems).An evolving technology for NOX removal is the non-ther-mal plasma device [10]. It is, however, not clear whethernon-thermal plasma technologies are suitable for vehicleapplication, and research on this technology is ongoing.The results of these investigations shall be presented in afuture paper on the technology.When NOX conversion rates of more than 60 % arerequired, only two (2) promising technologies are cur-rently available:

Selective Catalytic Reduction (SCR) technology rely-ing on Ammonia or Urea, and

NOX adsorber catalysts.Each these technologies is discussed in the followingsections.

SCR Technology SCR technologies utilizing Ammoniaor Urea to control NOX are well known from stationarypower plants. Applying these technologies to transientoperating conditions appeared exceedingly difficult. Yet,for the first time, in 1995, a SCR system based on anaqueous Urea/water solution was demonstrated for vehi-cle application [7]. NOX reduction rates as high as 65 %over the New European Driving Cycle were achieved withthis system. Over the US FTP cycle NOX reduction ratesof more than 75 % were achieved.

Figure 4. Urea SCR Injection & Control System

Urea SCR is a favorable technology, and its feasibility hasbeen successfully demonstrated. For vehicles operatedin fleets under well defined conditions, accurate mainte-nance and proper infrastructure (reductant availability),an Urea SCR system relying on an aqueous Urea/watersolution might be a good solution. For passenger carapplication, however, such a SCR system can not bepractical because of the weight and the volume of theUrea/water tank required to cover practical refilling inter-vals.

After reflecting on these results it seems prudent to envi-sion a system that does not require water to be storedon-board. A reductant - probably one other than Urea -could be stored in a canister or a cartridge. Without theneed for additional on-board water, as much as 60 % inreductant volume and about 70 % in reductant mass canbe saved compared to a conventional Urea/water SCRsystem. With such a system, refilling intervals of about15,000 to 30,000 km appear achievable, hence the car-tridge could easily be exchanged during maintenance orservice intervals. There are at least two (2) persuasivearguments supporting this technology:

this technology does not require a special fuel quality,and

Table 2. DeNOx Technologies

Technology Reductant Maximum NOX Reduction

Lean NOx (pas-sive)

HC 15 % 1)

Lean NOx (active)

HC supplement 30 % 1)

SCR Urea, Ammonia 65 % 2)

NOX Adsorber CatalystsHC, CO, H2 54 % 3)

Plasma Technolo-gies

- / Ammonia 10 / 50 % 4)

1) over the New European Driving Cycle, Ref.: [6]

2) over the New European Driving Cycle, Ref.: [7]

3) over the New European Driving Cycle, Ref.: [8]

4) Steady-State Testing, Ref.: [9]

Parti

culat

es [g

/km]

Prototypes

0.15

HC+NOx [g/km] 0.00 0.25 0.50 0.75 1.00 1.25 1.50

0.00

0.05

0.10 Euro II

Euro III

Euro IV

MY 97 MY 96

MY 98

4 it is suitable for retrofitting since direct interaction withthe engine controller is not necessary.

For these reasons, there is ongoing work focusing on asolid reductant technology.

NOX Adsorber Catalysts (NAC) An emerging technol-ogy is the NOX adsorber catalyst which relies on chemi-cal adsorption of NOX in lean exhaust and periodicregeneration under rich conditions. Principles of thistechnology are described in [12,13,14]. For lean burngasoline engines, the NOX adsorber catalyst is currentlythe most promising aftertreatment technology, and someproduction direct injection gasoline vehicles already usethese catalysts.The primary technical concerns with this technology, inapplication to Diesel engines, are its sulfur tolerance -actually the lack of sulfur tolerance - and the regenerationunder rich exhaust conditions.On Diesel engines regeneration under rich conditions isfundamentally difficult since Diesel engines readily pro-duce black smoke under rich conditions. Steady-statetests, however, have shown that rich Diesel engine oper-ation is feasible without unacceptable penalties concern-ing smoke number as long as the combustion process iscarefully calibrated. With this method for regeneration,HC concentrations up to 7,500 ppm and CO concentra-tions as high as 4 % were achieved, Figure 5, which issufficiently high for NOX adsorber catalyst regeneration. Under rich conditions, fuel consumption increases, asshown in Figure 6. For the purpose of this initialapproach, smoke numbers of about 2.8 Bosch wereaccepted as shown in Figure 6. Considering, however,that regenerating conditions will be applied infrequentlyand even then only for a few seconds, the total fuel con-sumption penalty and particulate emissions increase lessthan 2 % for the conditions shown in Figures 5 and 6.Further testing has indicated that the increase in particu-late emissions can be reduced to levels much lower thanthose shown in Figure 6. From the operating conditions shown above a test cyclewas derived. Testing was conducted on a steady-statetest bench with cyclic changes from lean operation(adsorption) to rich operation (regeneration). To providesufficient regeneration conditions an adsorption/regener-ation duration ratio of 50 sec to 2 sec was selected. Figure 7 shows that the regenerating conditions appliedwere sufficient to properly regenerate the NOX adsorbercatalyst. Over the cycle shown a NOX reduction effi-ciency of more than 90 % was achieved with a new cata-lyst.While regeneration of NOX adsorber catalysts is feasibleunder steady-state conditions, regeneration under tran-sient operating conditions - as is the case during actualvehicle operation - requires a highly sophisticated controlstrategy. Since the driver must not be able to detect the

regeneration, significant application effort will be neces-sary.Not all manufacturers will move to injection and controlsystems with the flexibility required to properly controlthis technology. Thus, retrofit systems are under develop-ment, i.e. systems that can be installed in the exhaustsystem of indeed any Diesel engine.In addition to the question of how to regenerate a NOXadsorber catalyst on a Diesel engine, sulfur poisoning ofthese catalysts is a major concern. Even with very lowfuel sulfur levels, rapid aging has been observed. It isbelieved that sulfur tolerance is one of the most importantissues to resolve to make NOX adsorber catalyst technol-ogy feasible for mass production.

Figure 5. Steady-State Lean and Rich Engine Operation of a 2 liter Common Rail Diesel Engine at n = 2,000 rpm and BMEP = 1, 2, and 4 bar

1 bar 2 bar 4 bar0,0

0,5

1,0

1,5

2,0

2,5

3,0 Lean Rich

Air /

Fue

l Rat

io [1

]

1 bar 2 bar 4 bar0

250

2500

5000

7500

10000

HC

[ppm

]

1 bar 2 bar 4 bar0,000,050,100,15

2

4

6

Break Mean Eff. Pressure

CO

[%]

5Figure 6. Steady-State Lean and Rich Engine Operation of a 2 liter Common Rail Diesel Engine at n = 2,000 rpm and BMEP = 1, 2, and 4 bar

Figure 7. Adsorption/Regeneration Test Cycle at n = 2,000 rpm, BMEP = 2 bar, Exhaust Gas Temperature = 300 C (NOX Adsorber Catalyst provided by Engelhard Technologies)

Which DeNOx-Technology to Use? Table 3 provides anoverview of the preferred applications for various DeNOxtechnologies, but Table 3 does not cover all possibleapplications.For light- and medium-weight vehicles, NOX reduction ofless than 30 % should be sufficient to meet the EURO 4

standards making either an active or a passive Lean NOXcatalyst appropriate for European vehicles in these cate-gories. For special applications, even light-weight vehi-cles may require very high NOX reduction rates, e.g. if thecombustion process is calibrated for very low particulateemissions.

2.3 DIESEL PARTICULATE FILTER TECHNOLOGY For many years Diesel particulate filters (DPF) were notconsidered for large scale vehicle application primarilybecause there was no technical need. A short-livedapproach was introduced to the U.S.-market by a Ger-man manufacturer in the mid 80's. Recently a Frenchmanufacturer announced that it would bring DPFs intomass production in the year 2000 [11]. Clearly, the DPFhas seen rising interest since the proposal of ultra lowparticulate standards.In recent years, particle number emissions rather thanparticulate mass emissions have become the subject ofcontroversial discussions. Recent results from healthstudies imply that it is possible that particulate mass doesnot properly correlated with the variety of health effectsattributed to Diesel Exhaust. Concern is instead nowfocusing on nano-sized particles. Since Diesel particulatefilters have proven to be the sole technology capable ofreducing emissions of nano-sized particles by more thantwo orders of magnitude, shown in Figure 8, a highdemand for DPF technology has arisen.Filtration of particulate matter as such is not a problemsince a number of filtering materials are available. Themajor problem associated with DPF technology remainsthe removal of particulates from the filter (regeneration).Though DPF regeneration has been subjected to exten-sive development work for more than 15 years, a fullyconvincing solution for passenger car application is notyet available.

Active DPF Regeneration Systems Active systems relyon a secondary energy supply, by means of electricalheaters or fuel burners, to heat the exhaust gas to tem-peratures at which soot burns. Most of the active systemsavailable today suffer from high costs and high technicaland application effort. For these reasons active systemsare not a preferred choice for passenger car application.

1 bar 2 bar 4 bar200

400

600

800

1000

1200

BSFC

[g/k

Wh]

1 bar 2 bar 4 bar0

1

2

3

4

5

6 Lean Rich

Break Mean Eff. Pressure

Smok

e N

umbe

r [Bo

sch]

0

100

200

300 Adorption/ Regeneration Ratio = 25/1 sec

downstream NOx Adsorber Cat.upstream NOx Adsorber Cat.

NO

x [pp

m]

Time

1234

Rel. Air-Fuel Ratio [1]

Table 3. Applications of DeNOx TechnologiesTechnology Preferred Application

Lean NOx (passive)

light- and medium weight vehicles

Lean NOx (active)

light- and medium weight vehicles

SCR medium- and heavy-weight vehiclesNOX Adsorber Catalysts

medium- and heavy-weight vehicles with flexible injection Systems

Plasma Tech-nologies

to be determined

6Figure 8. Effect of a Diesel Particulate Filter on Particle Number Emission

Passive DPF Regeneration Systems Passive systemsfully rely on catalytic effects, e.g. from additives blendedinto the fuel, additives injected into the DPF, or catalyticelements on the DPF surface (coated DPF). With thesesystems, DPF regeneration at temperatures as low as350 C was observed. There are, however, indicationsthat it might not succeed in covering the low temperatureoperating range of today's modern Diesel engines.Another approach to passively regenerating DPFs is toinstall a catalyst upstream of the DPF whose productsare capable of regenerating the DPF. One such system,known as a continuously regenerating Diesel particulatefilter, is commercially available. With this system, a Pt-based catalyst is utilized to convert NO to NO2 whichthen is utilized to oxidize the soot trapped in the DPF [15].If ultra-low sulfur Diesel fuel (

7Figure 12 shows again typical backpressure and exhaustgas temperature traces from a road test but recordedafter approximately 5,000 km. The average backpressurelevel is still very low indicating the excellent soot oxidationperformance of the system.

Figure 12. Typical Backpressure and Exhaust Gas Temperature Traces, Mileage: 5,000 km

For vehicles operating under well defined conditions thistype of system may very well be an acceptable technicalsolution. It is especially true that light-weight vehicles,which normally have exhaust gas temperatures signifi-cantly lower than those of the light-duty truck described,may need another solution since low temperature opera-tion is not favorable for this technology.DPF regeneration by means of reactive species asdescribed previously appears to be a promisingapproach. Since current technologies require sulfur freeDiesel fuel and exhaust gas temperatures sufficientlyhigh to form the reactive species, new approaches thatdo not require special fuel qualities were investigatedwith regard to the feasibility of DPF regeneration.

Non-Thermal Plasma Technologies A new approach toexhaust gas aftertreatment is available in non-thermalplasma technologies. In a non thermal plasma device, ahigh local electrical field is applied to generate microdis-charges in the exhaust gas which directly produce highlyenergetic electrons. Because of the short duration of themicrodischarges (< 100 nsec) the electrons and the bulkexhaust gas do not thermally equilibrate and, thus, thetemperature of the bulk exhaust gas remains essentiallyconstant. By electron-impact dissociation and ionizationof exhaust gas molecules, reactive species, e.g. O3,O(2)*, OH and NO2 are produced which either promoteoxidation or reduction reactions.As described before, this technology was originally devel-oped to reduce NOX in a lean exhaust environment. Itwas, however, discovered that the process can bedesigned to enhance the oxidizing pathway rather thanthe reducing pathway, and thereby, form oxidizing specieslike NO2, O3, O*, and OH. Using these species soot

trapped in a DPF can be easily oxidized, e.g. by one ofthe following reactions:

Figure 13 depicts test results from a 2.5 l DI Dieselengine (n = 1,500 rpm, BMEP = 2 bar; T = 230 C).Energy consumption of the prototype plasma device was1 kW. The results identify the successful regeneration ofthe DPF when the exhaust gas was treated with theplasma.

Figure 13. Backpressure Traces with and w/o Plasma-Treatment (2.5 l DI Diesel Engine, n = 1,500 rpm, BMEP = 2 bar, T = 230 C)

Though the results appear quite promising there remainssignificant work to be done in the development of thistechnology. Today's systems must be reduced in bothsize and energy consumption.

Engine-Controlled DPF Regeneration Systems Whendescribing NOX adsorber technology, it was mentionedthat today's Diesel engine provides highly flexible injec-tion and control systems. Using these systems, the com-bustion process can be tuned to obtain the properregenerating conditions as occasionally required byeither a NOX adsorber catalysts or a DPF. For DPFregeneration, the combustion process can be occasion-ally adjusted to achieve exhaust gas temperatures highenough to oxidize the soot trapped in the DPF, e.g. byvarying injection timing. Of course this must be doneunder both steady-state and transient operation withoutany impact on driveability. Thus, similar to NOX adsorbercatalyst regeneration, some application effort is neces-sary.Although, in contrast to NOX adsorber catalyst regenera-tion, DPF regeneration appears to be easier. With com-mercially available pressure sensors a closed-loopcontrol strategy can be realized. Furthermore, if the

0 5 10 15 200

200

400

600

Exhaust Gas Temperature

Backpressure

Back

pres

sure

[mba

r] / T

empe

ratu

re [

C]

Time [h]

C + NO2 CO + NO

C + O3 CO + O2

C + 2 OH CO + H2O

0,0

0,2

0,4

0,6

0,8 untreated exhaust gas plasma-treated exhaust gas

Time

Back

pres

sure

[bar

]

8engine produces additional smoke during regeneration,the DPF will trap the smoke and reduces it to invisible lev-els.

Significant fuel consumption penalties are not anticipatedwith the application of DPF's because modern Dieselengine DPF regeneration will be rarely required and thenonly for a very short duration.Engine-controlled DPF regeneration appears to be one ofthe most promising regeneration technologies now avail-able for development. However, up to this point in time,this approach is only applicable to common rail Dieselengines.

Combined DPF Regeneration Systems Another prom-ising approach to DPF regeneration may be the combina-tion of a passive system with an active one. In this case,the fuel additive or the coated DPF covers the mediumand high temperature operating range and an additionalactive system functions as a stopgap measure only if theengine is operated at low load conditions resulting inexcessive backpressure.

3 SUMMARY AND CONCLUSIONS

The EURO 4 and LEV II standards demand a significantreduction in NOX and particulate emissions from Dieselengines for compliance. Furthermore particle numberemissions might become subject to future emission regu-lations.

Some new exhaust gas aftertreatment approaches forultra low emission Diesel engines were investigated withregard to suitability for large-scale vehicle application.Though feasibility of NOX adsorber catalysts and engine-controlled DPF regeneration has been demonstrated inprinciple there remains much work to resolve the drive-ability issue when using these systems. Moreover, thedurability of these approaches has not been addressedsufficiently with significant concern remaining for thedurability of NOX adsorber catalysts.For NOX reduction, a SCR system relying on a solidreductant is a promising approach when NOX reductionrates of more than 50% are required. This technologysinsensitivity toward fuel sulfur and the lack of control sys-tem interaction are the most convincing arguments todevelop this approach.Diesel combustion system development, in the form ofstate-of-art engine technologies, and Diesel exhaustaftertreatment can independently and substantiallyreduce both particulate emissions and NOX emissions.Together, however, they can reduce Diesel engines emis-sions to very low levels as shown schematically in Figure14.

Figure 14. Emissions Reduction Potentials

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Engelhard Technolo-gies

for supporting the work on NOX adsorber catalystregeneration.The authors also thank Johnson Matthey Catalytic Sys-tems Division

for supporting the work included on light-duty Diesel truck particulate regeneration.

REFERENCES

1. EC Press Release 98/230 (July 07, 1998)2. California Air Resources Board Press Release3. n.n. Motorsport Information; Auto News - The Internet Auto

Magazin (December 1998)4. Backhaus, R. Diesel-Rennmotor mit Direkteinspritzung von

BMW, MTZ Motortechnische Zeitschrift 59, Issue 9, Page578 - 579 (1998)

5. Type Approval Data, Kraftfahrtbundesamt Flensburg(March 1998)

6. Diesel Lean NOX Catalyst Technologies, SAE SpecialPaper No. 1211 (October 1996)

7. Lders, H.; Backes, R.; Hthwohl, G.; Ketcher, D.A.; Hor-rocks, R.W., Hurley, R.G.; Hammerle, R.H. An Urea LeanNOX Catalyst System for Light Duty Diesel vehicles, SAE-Paper No. 952493 (1995)

8. Krmer, M.; Abthoff, J.; Duvinage, F.; Krutzsch, B.; Lieb-scher, Th. Chancen von Abgasreinigungskonzepten frden Pkw-Dieselmotor mit schwefelfreiem Kraftstoff, 19.Internationales Wiener Motorensymposium (1998)

9. Non-Themal Plasma Session on 1998 SAE InternationalFall Fuels & Lubricants Meeting and Exposition

10. Hentschel, K.; Wolters, P.; Lepperhoff, G. Lean-CombustionSpark-Ignition Engine Exhaust Aftertreatment Using NonThermal Plasma, SAE-Paper No. 982512 (1998)

0 20 40 60 80 1000

20

40

60

80

100100 % = Today's Emission Levels

Lean NOxTechnology

NOx Adsorber or SCRTechnology

DPF Technology

Engine Technology

Parti

cula

te E

mis

sion

Lev

el [%

]

NOx Emission Level [%]

911. Engine Technology International, Issue 11 (1998)12. Burk, P.; Punke, A.; Dahle, U. Future Aftertreatment Strate-

gies for Gasoline Lean Burn Engines, Graz ConferenceEngine and Environment (1997)

13. Strehlau, W.; Hhne, J.; Gbel, U.; Trillaart, J.A.A.; Mller,W.; Lox, E. New Developments in the Catalytic ExhaustGas Aftertreatment of Lean Burn Engines, Graz Confer-ence Engine and Environment (1997)

14. Brogan, M.S.; Clark, A.D.; Brisley, R.J. Recent Progress inNOX Trap Technology, SAE-Paper No. 980933 (1998)

15. Warren, J.J.; Allanson, R.; Hawker, P.N.; Wilkins, A.J.J.Effects of Aftertreatment on Particulate Matter when usingthe CRT, Global Emission Management, Issue 7 (1998)