Experimental Investigation of Combined LNT + SCR DieselExhaust Aftertreatment

Thomas Wittka & Bastian Holderbaum & Peter Dittmann &

Stefan Pischinger

Received: 30 September 2014 /Accepted: 26 January 2015 /Published online: 13 February 2015# Springer SIP, AG 2015

Abstract In the present work, an exhaust aftertreatment sys-tem combining lean NOX trap (LNT) plus exhaust bypass,passive selective catalytic reduction catalyst (SCR) andengine-independent LNT reductant supply by onboard ex-haust fuel reforming was developed. Further, it was experi-mentally investigated in steady-state operation on an enginetest bench and on road in a demonstrator vehicle. The intrinsicNH3 formation during the LNT regeneration was intensivelystudied as one key function for passive SCR without activeurea dosing. LNT regeneration duration and temperature arethe most significant parameters for NH3 emission. In steady-state operation, the passive SCR could be forced to a contri-bution to the total NOX conversion of up to 35 %. At 170 °C,50 % steady-state total NOX conversion was achieved, and at250 °C and a fuel consumption penalty of <2 %, completeNOX conversion could be demonstrated. Compared to con-ventional LNT operation with frequent engine enrichment,the engine-independent LNT regeneration with reformategas is energetically more efficient and advantageous regardingCO2 emissions. Additionally, by shifting NOX reduction fromLNT to SCR, fuel consumption penalty due to LNT enrich-ment can be reduced. Finally, the combination system wasdemonstrated in transient test cycles and by real driving oper-ation on road.

Keywords NOX.LNT .SCR .LNT+SCR .NH3formation .

Diesel reformer

1 Introduction

Worldwide, more stringent pollutant and CO2 emission limi-tation in upcoming legislation norms for diesel passenger carapplications require the introduction of exhaust aftertreatmentsystems with increased purification and energetic efficiency.In addition to more restrictive emission limits, new Europeanemission evaluation test procedures, like Worldwide harmo-nized Light vehicles Test Procedure (WLTP), real drivingemissions (RDE), and Portable Emissions Measurement Sys-tem (PEMS) result in an enlarged emission relevant engineoperation area. Beside the increased coverage of engine loadand speed, also the emission testing including cold start andhigh temperature cycles demands an extended active temper-ature range of aftertreatment systems. Both state-of-the-arttechnologies for mobile NOX reduction, lean NOX trap(LNT), and selective catalytic reduction catalyst (SCR) showchallenges for robust performance under strongly extendedemission relevant operation conditions. One technical solutionon the aftertreatment side is the combination of LNTand SCRfunctionalities. To avoid urea necessity in active SCR systems,the combination of LNT and passive SCR (without urea dos-ing) was favored in this study over the two alternative combi-nation systems based on active SCR (LNT + active SCR;passive NOX adsorber (PNA)+active SCR).

The LNT + passive SCR combination were the center ofresearch of several previous studies. A schematic overview ofgeneral layout concepts is shown in Fig. 1. Moreover, generalpositions of the fuel reformer functionality are integrated. Inprinciple, the combination can be realized by a parallel or

T. Wittka : P. Dittmann (*) : S. PischingerInstitute for Combustion Engines, RWTH Aachen University,Aachen, Germanye-mail: dittmann@vka.rwth-aachen.de

B. HolderbaumFEV GmbH, Aachen, Germany

Emiss. Control Sci. Technol. (2015) 1:167–182DOI 10.1007/s40825-015-0012-0

quasi-parallel layout or by a sequential layout. The intentionof a parallel layout in a multilayer design [1–5] or of a quasi-parallel layout in a multi in-line design [6] is the in situ utili-zation by storage and conversion of the axial NH3 formationduring the LNT regeneration. In this layout, the intermediateNH3 does not get re-oxidized but stored on the SCR layer orslice. Thereby, high NH3 amounts might be used for NOX

conversion on the SCR. Nevertheless, the impact of the acidSCR on the alkaline LNT surface and its storage performanceof acid NOX as well as the SCR poisoning by hydrocarbons(HC) and the absence of O2 during the rich operation is stillopen. Additionally, flexibility in shifting of NOX conversionbetween LNT and SCR by application is not possible. Duringthe LNTNOX adsorption period, as the LNTNOX load axiallyincreases, the SCR NH3 load gets axially consumed.

In the sequential layout, the passive SCR is only operatedby the NH3, which is emitted from the LNT at the end of theLNT regeneration. The design can be set up as a single-path[7–20] bypass [21–23] or dual-path [24, 25] concept. Thesingle-path concept requests in any case a specific engineoperating mode for the LNT regeneration. Comparable to con-ventional LNT application, the LNT regeneration can be con-ducted by engine internal enrichment. Optionally, an in-linereformer catalyst might be installed upstream of the LNT forincreasing H2 and CO and decreasing HC concentrations andthereby improving especially the regeneration efficiency atlow temperature. However, due to the necessity of high

reformer operation temperatures usually in the range of 800–950 °C for sufficient reforming activity, the capability of an in-line reformer is limited. In case of reductant breakthrough, theSCR is operated at an air-to-fuel ratio A/F (λ) below 1 with therisk of HC poisoning. Alternatively, an out-line reformermight be controlled in optimum operation range, resulting inenhanced reformate composition regarding H2 and CO con-centration. Nevertheless, the engine still has to be operated atleast at λ=1 to minimize excess residual O2 in the exhaust gasand high exothermic reactions on the LNT during the regen-eration. In contrast to the single-path concept, both bypass anddual-path concepts do not require any engine internal opera-tion mode change. However, in any case, these concepts needan out-line fuel reformer. The engine can be operated indepen-dently at λ>1 due to local enrichment of solely the LNT. In thedual-path concept, the two LNTs are operated alternatively byswitching the exhaust gas flaps. The LNT bypass in the bypassconcept is only activated during the LNT regeneration.

The intrinsic ammonia (NH3) formation on the LNT duringthe rich LNT regeneration is one key functionality for com-bined concepts with passive SCR. A number of recent studiesfocused on the mechanism and the influence of regenerationconditions and catalyst formulation. The discontinuous under-stoichiometric (λ<1, also called “rich”) reduction of theadsorbed NOX on the LNT during the LNT regeneration byexcessive feeding of unburned HC, carbon monoxide (CO),and hydrogen (H2) is typically realized by engine internal rich

λλ>1

LNT

SCR

LNT

Ref.

λ>1

λ>1

λ<1

LNT SCR

Ref.λ>1λ<1 λ>1

Sequential Layout

Mu

ltila

yer

Co

nce

pt

Parallel / Quasi-Parallel Layout

SCR Layer

LNT Layer

Al2O3

Exhaust Gas

Sin

gle

-Pat

h

Co

nce

pt

Byp

ass

Co

nce

pt

Du

al-P

ath

C

on

cep

t

LNT SCR

λ<1 λ<1

Ref.

optional

λ<1

LNT SCR

λ=1λ<1 λ<1Ref.

Mu

lti I

n-L

ine

Co

nce

pt

LNT

SC

RLN

TS

CR

LNT

SC

RLN

TS

CR

LNT + passive SCR Combination Concepts

Fig. 1 Schematic overview of general layout concepts for LNT and passive SCR combination systems

168 Emiss. Control Sci. Technol. (2015) 1:167–182

operation (λexhaust=λengine<1). Alternatively, the reductantspecies can be dosed engine externally by fuel or reformategas injection directly into the exhaust gas upstream of theLNT (λexhaust<1, λengine≥1). Under this reducing exhaustgas conditions, various elementary reactions take place onthe LNT surface. However, the surface reaction system is notfully selective toward the desired product nitrogen (N2) butalso results in products NH3 and nitrous oxide (N2O). Figure 2gives a simplified and schematic overview of the main ele-mentary surface N-species during the under-stoichiometricLNT regeneration and possible gaseous products. Startingpoint are the adsorption of NO and NO2 and subsequent dis-sociation steps. The NH3 formation is mainly described bymultiple elementary reactions with H(s) to NH3(s) and finaldesorption [26–29]. Alternatively (not displayed in Fig. 2),reaction pathways over isocyanate species NCO(s) and sub-sequent hydrolysis to NH3 are in discussion [28, 30–32].

In principle, a distinction has to be made between the dis-continuous NH3 formation inside the reaction front from previ-ously stored NOX and the continuous NH3 formation behindthe reaction front from gaseous NOX feed into the catalyst.According to the state of knowledge, inside the reaction front,which axially propagates through the LNT during the rich re-generation [33–36], NH3 is intermediately [37–39] formed byover-reduction of NOX. The so-formed NH3 behaves as reduc-tant like HC, CO, andH2. It re-oxidizes, e.g., to nitrogen [40] byreaction with surface oxygen sources (oxygen storage capacity(OSC), nitrate (NOX)) downstream the reaction front. At theend of the LNT regeneration, when the rich reaction frontreaches the catalyst outlet, the intermediate NH3 cannot be re-oxidized anymore and typically gets emitted as NH3 peak. The-se principle NH3 formation processes are strongly influencedby the catalyst formulation [7, 8, 41–50] (load, type, and dis-persion of platinum group metals (PGMs), NOX adsorbenttype, OSC load, aging influence), reaction conditions [33–35,37, 44, 51–55] (temperature, reductant species), and LNT re-generation strategy [26, 27, 51, 56, 57] (LNT NOX load, regen-eration duration, concentration of reductant species).

The main targets of the investigation results discussed inthis study are the demonstration of the combination of LNTand passive SCR aswell as a detailed insight into the influence

of the LNT regeneration strategy on the NH3 formation.Therefore, commercial and full size catalyst formulations withonboard generated H2 and CO reductant species were inves-tigated under realistic boundary conditions on test bench andin a demonstrator vehicle.

2 Description of Aftertreatment Concept

The investigated LNT-based aftertreatment system comprisesan LNT plus exhaust bypass, a downstream positioned passiveSCR (without active urea dosing), and an onboard exhaust gasfuel reformer. In contrast to the current technical trend of po-sitioning the NOX purification systems (LNT, SCR-coateddiesel particulate filter (DPF)) as close as possible to engineout due to cold start reasons, the investigated DeNOX system(LNT+passive SCR) is placed underfloor. So, it is down-stream of the serial close-coupled diesel oxidation catalyst(DOC)+cDPF unit and low-pressure exhaust gas recirculation(EGR) branch (LP-EGR). Beside cold start operation, thestudy focuses as well on RDE with high-temperature opera-tion. Under these conditions, a close-coupled LNT positionmight be disadvantageous due to limited high-temperatureNOX adsorption capacity as well as limited NH3 formationtendency. Additionally, the position upstream of the LP-EGR branch could be unfavorable. The increased exhaustand hence NOX mass flow would result in fast consumptionof the LNT NOX storage capacity as well as SCR NH3 load.The final concept layout and the simplified method of opera-tion are shown schematically in Fig. 3.

The system is discontinuously operated in two operationmodes. During NOX adsorption mode, the total system is op-erated under lean condition. The LNTexhaust path is opened;the LNT bypass is closed. NOX treatment takes place either byNOX adsorption on the LNT or continuous reduction on theSCR by consuming previously stored NH3. The integratedonboard exhaust gas fuel reformer is inactive. Opening thebypass path and closing the LNT path leads to a feed of H2

and CO containing reformate gas at low space velocity via theactivated fuel reformer. Consequently, the LNT is locallyenriched for the frequently necessary LNT regeneration. The

NO2 NO2(s)

NH(s)

NH2(s)

catalytic surface

+(s)-O(s)

+H(s)

+NO(s)

+N(s)

+H(s)

+H(s)

NO NO(s) N(s)+(s) -O(s)

NH3(s) NH3-(s)

N2O(s) N2O-(s)

N2(s) N2-(s)

Fig. 2 Simplified and schematicoverview of the main elementarysurface N-species during theunder-stoichiometric LNTregeneration (educt adsorption,product desorption, elementarysurface reactions for O(s)consumption, and back reactionsare not displayed; (s) symbolizesadsorbed species)

Emiss. Control Sci. Technol. (2015) 1:167–182 169

temperature of the reformate gas is below 200 °C due to notinsulated reformate pipes. By pipe insulation, the residual re-formate heat could be used for LNT heating. The exhaustpaths are actuated by two exhaust flaps, well known fromserial application. The exhaust mass flow slip over the closedflap is below 5%. The LNT regeneration is controlled in termsof high NH3 formation and emission. The NH3 emitted duringthe LNT regeneration adsorbs on the SCR and increases theSCR NH3 load for the next NOX adsorption mode. After theend of the NOX regeneration mode, the exhaust flaps areswitched back into NOX adsorption mode for the nextadsorption-regeneration cycle. The integrated onboard fuelreformer is predominantly operated by exhaust gas, which istaken downstream of the DPF. Diesel is injected by an air-assisted, low-pressure injector at a significant under-stoichiometric air/fuel ratio (λRef=0.35–0.80) and is catalyzedto a H2- and CO-rich synthesis gas. The reaction enthalpy,which is released a ring-shaped reformer catalyst, supportsfuel vaporization via internal heat transfer. The reforming re-action process can be described by superposition of exother-mic partial oxidation (POx) by the residual oxygen in theexhaust gas as well as the secondary air and endothermicsteam (SR) and CO2 reforming (DR) which occurs via theH2O and CO2 introduced by the exhaust gas. Furthermore,the reforming products are temperature-dependent in chemicalequilibrium, among others mainly according to the water-gasshift (WGS) and methanation reaction. In comparison to purePOx, exhaust gas reforming offers favorable thermal behavioras well as benefits regarding H2 and CO yield, due to theendothermic reforming reactions. The reformer operation isstrongly dependent on the exhaust gas composition. The re-former itself is controlled in terms of maximizing H2 and COas well as minimizing HC concentration in the reformate,while complying with temperature limits.

3 Experimental

3.1 Catalyst Preparation

The LNT and SCR catalysts for full-scale investigation wereprepared based on metallic substrates with platinum (Pt) andrhodium (Rh) as catalytically active components. Two differ-ent LNT formulations (called “LNT A” and “LNT B”) weretested, which basically consisted of different sets of NOX ad-sorbents. Additionally, the PGM load of the LNT was varied(“PGM low”: 85 g/ft3 and “PGM high”: 130 g/ft3). The SCRcatalyst is copper (Cu) zeolite based and close to conventionalcommercial formulation. In all investigations shown here, thevolume of the LNT is 1.4 L and of the SCR 2.9 L. Only in thevehicle experiments, a DOC-like slip catalyst of 0.8 L wasinstalled directly downstream of the SCRmainly for oxidizingCO slip from the LNT regeneration. The catalysts were notfull lifetime aged but thermally conditioned by several hoursof DPF regeneration operation in real exhaust gas (hydrother-mally aged). The reformer catalyst is ring-shaped and basedon ceramic substrate with a PGM load of 60 g/ft3 and a vol-ume of 0.5 L. The exhaust bypass is operated by serial exhaustflaps (application in LP-EGR systems). Diesel is injected intothe reformer mixing chamber by a serial air-assisted, low-pressure (<5 bar) injector.

3.2 Engine Test Bench Experiments

The full-scale experiments for investigating the NOX purifica-tion behavior of the combination system as well as the perfor-mance of the fuel reformer were conducted with a high effi-cient 1.4 L three-cylinder diesel engine on a transient enginetest bench. The new developed control logic of theaftertreatment system was running on a rapid control system

NOX Adsorption Mode

Engine: > 1LNT: > 1

NOX-AdsorptionSCR: NOX-ConversionReformer: inactiveBypass: inactive

NOX Regeneration Mode

Engine: > 1LNT: < 1

NOX-ReductionNH3-Formation

SCR: NH3-AdsorptionNOX-Conversion

Reformer: activeBypass: active

LNTDOC SCR

Ref.

DPF

Fuel + Air

Exhaust gas

LP-EGR

LNTDOC SCRDPF

Ref.

LP-EGR

Fig. 3 Simplified operationmethod of combinedaftertreatment system with close-coupled DOC + cDPF, underfloorLNT plus exhaust bypass, passiveSCR, and integrated exhaust gasfuel reformer

170 Emiss. Control Sci. Technol. (2015) 1:167–182

(ES1000 Fa. ETAS) independently from the engine. The nec-essary information was transferred between engine andaftertreatment control via ECU bypass. In steady-state inves-tigations, the engine operation (speed, load, operation mode)was set in order to adjust target conditions at the LNT (tem-perature, NOX concentration, exhaust mass flow, A/F ratio).Additionally, transient test cycles (New European Driving Cy-cle (NEDC), Worldwide harmonized Light duty driving TestCycle (WLTC), FTP75, CADC, US06, ADAC) were conduct-ed in order to evaluate and calibrate the system performanceand operation strategy. The emissions were measured by dif-ferent analyzers (CLD: NOX, NO; NDIR: CO, CO2; Magnos:O2; FID: THC; FTIR: NO, NO2, CO, CO2, H2O, NH3, N2O,CH4, SO2; LDS: in situ NH3; mass spectrometer: H2, H2S,SO2, COS, CS2) and sensors (NOX, λ) at various positions(engine out, downstream of the DPF, upstream and down-stream of the SCR). The process mass spectrometer (PrMS)is an analytical unit constructed by V&F including the TWIN-MS (CIMS with soft ionization by source gases: Xe, Kr, Hg)and the H-Sense (EI with ionization by ion beam), whichallows to detect all data parallel in lifetime with a very highdynamic (30 ms for each substance). The gas composition of

the out-line exhaust gas fuel reformer was calculated based onemission measurements in the full exhaust gas flow and theaccording dilution factor. The total aftertreatment system wasset up very modularly by the possibility of replacing eachcatalyst unit by an empty exhaust pipe. Additionally to theonboard fuel reforming, fundamental investigations of theLNT regeneration were performed by bottled synthetic refor-mate gas, which was dosed by a mass flow controller (MFC).Based on the achieved onboard reformate gas composition,the bottled reformate substitute gas contains molar fractionsof 20 % H2, 20 % CO, and 60 % N2. A schematic overview ofthe experimental setup on the engine test bench is given inFig. 4.

3.3 Vehicle Experiments

The exhaust aftertreatment system was investigated in aVolkswagen Golf Variant, basically with the same enginespecification and aftertreatment layout as on the engine testbench (Fig. 5). In contrast to the engine test bench investiga-tions, a DOC-like slip catalyst was positioned downstream ofthe SCR mainly for oxidizing CO slip from the LNT

DOC DPF

LNT

SCR

LP-EGR

+

-

compressor

Diesel

(5 bar)

air

injector

H2

CO

N2

MFC

HFM

Heating

device optional:

WGS

Mixer

NOX

NOX

NOX

λ λλλ

p,T p

p

12xT

7xT

T

AMA2FTIR H-Sense LDSAMA1

AMA2FTIR PrMS LDSAMA1

ES

100

0

ES

11

35

ES

123

2

ES

122

2

ES

130

3

ES

131

0

ES

133

0

ECUTest bench control

and data logging

Power Stages

*)

*)

Fig. 4 Schematic experimental setup on engine test bench: overview of exhaust gas analyzers, sensors, actuators, and rapid prototyping control (AMA:exhaust analyzer system comprising chemiluminescence detection, non-dispersive infrared spectrometry, flame ionization detection and O2 detection)

Emiss. Control Sci. Technol. (2015) 1:167–182 171

regeneration. The LNT regeneration was performed by bottledreformate substitute gas, of the same composition as on theengine test bench. The system was investigated by transienttesting in NEDC andWLTC on a chassis roller test bench withan inertia mass of 1590 kg and on road in real driving (RDE)with “soft”, “normal”, and “aggressive” driving style. Due tothe additional control equipment and passengers, the real ve-hicle mass on road was 1780 kg without PEMS and 1970 kgwith PEMS. The RDE cycle had a distance of approx. 90 kmwith a share of approx. 1/3 urban driving, 1/3 rural driving,and 1/3 highway driving.

4 Results and Discussion

4.1 Exhaust Fuel Reformer

The fuel reformer was intensively studied but is not partof this paper. However, for better understanding andranking of the fuel economy penalty caused by LNTregeneration, which is discussed later, the reductant for-mation yield (YRed) is an important indicator in terms ofenergetic efficiency. YRed is defined as ratio of oxygenconsumption potential of the reducing agent available inthe exhaust gas relative to the additional fuel introducedto reach rich operation. It can generally be calculated byEq. 1 or for engine operation in case of non-availablemeasurement of all exhaust gas species also by Eq. 2.The theoretical optimum reductant yield is representedby the direct use of fuel for the LNT regeneration, e.g.,by fuel injection upstream of the LNT with closed ex-haust flap. In this case, the reductant yield is 100 %.

However, injecting liquid or even pre-vaporized fuel in-to almost zero exhaust mass flow is hardly practicable.

YRed ¼nH2 þnco þ 2xþ y

2

� �⋅nCx−2⋅no2−nNO−2⋅nNO2

2xþ y

2

� �⋅nCxHy;Diesel

ð1Þ

YRed;engine ¼mengine fuel;richmode⋅ 1−λengine;rich mode

� �

mengine fuel;richmode−mengine fuel;leanmode

ð2Þ

The conventional LNT regeneration done by engine inter-nal enrichment is mainly achieved by throttling and henceintake oxygen reduction as well as applying early and late postinjections for excess HC, CO, and H2 concentrations. How-ever, the main portion of the additionally injected fuel is con-sumed for compensating combustion efficiency losses causedby engine measures to reduce the typical lean diesel air-to-fuelratio toward λ=1. As shown in Fig. 6, only 10–45 % of theadditionally expended fuel is available as reducing agent inthe exhaust gas. The scatter band in Fig. 6 includes variouscurrent diesel engine calibrations. The reductant yield isslightly increasing with increasing BMEP or decreasing A/Fratio during normal lean operation mode. Besides drivabilityand acoustic reasons, engine internal enrichment should beconducted during vehicle acceleration for lowering the fueleconomy penalty (FEP) for LNT regeneration. However, themaximum achievable reductant yield is limited by frequencyand length of high engine load periods. In contrast to the dieselengine enrichment, the reductant yield of the fuel reformerreaches a significantly higher level of about 50–60 %, as

Dosing of reformatesubstitute gas (20 % H2+ 20 % CO + 60 % N2)

LNT SCR ASCExhaust

flaps

Fig. 5 Final integration of thecombined aftertreatment systeminto the demonstrator vehicle(viewing direction underfloor,left: front, right: rear)

172 Emiss. Control Sci. Technol. (2015) 1:167–182

shown in Fig. 6. The A/F ratio and temperature of theexhaust gas fuel reformer were optimized by the reformercontrol algorithm for each single operation point with re-gard to low HC emission and high energetic efficiency.The reformer A/F ratio is in the range of 0.35 to 0.80.The reformer temperature reaches 750 and 950 °C. The

reductant loss in the reformer is caused by the partialoxidation of the fuel with oxygen from lean exhaust gasor secondary air. Nevertheless, the reformer enables todecrease the fuel economy penalty and CO2 emissionsfor the LNT regeneration in comparison to the convention-al approach by engine internal rich operation.

Engine BMEP [bar]

0 3 6 9 12 15 18

Re

du

cta

nt

form

atio

n y

ield

of

ad

ditio

na

l fu

el in

jectio

n [

%]

0

20

40

60

80

100

λEngine

in lean mode [-]

1 2 3 4 5 6 7 8

fuel reformer (λreformer

variable)

engine internal rich operation (λengine

= 0.95)

(b)(a)

Fig. 6 Reductant formation yield of additional fuel for LNTenrichment by fuel reforming in comparison to diesel engine enrichment of various enginecalibrations a as function of A/F ratio of the engine during lean operation and b as function of the engine’s brake mean effective pressure (BMEP)

(a) LNT NOX load: 1.0 g (b) LNT NO

X load: 2.5 g

ψH

2,

ψC

O [

pp

m]

0

5000

10000

15000

20000

T [

°C

]

250

275

300

325

LN

T e

xh

au

st

fla

p

open

closed

λ [-

]

0.6

0.8

1.0

1.2

Time [s]

0 10 20 30 40 50

λupstr. LNT

λdownstr. LNT

TLNT inlet

TLNT mid

TLNT outlet

ψN

H3,

ψN

2O [

pp

m]

0

500

1000

1500

NH3

N2O

Control LNT exhaust flap

H2

CO

Σ(H2 carrier species)*

Σ(reductant)**

H2, CO calculated

H2 + CO calculated

Time [s]

0 10 20 30 40 50

Fig. 7 LNT regeneration as function of time at 250 °C: concentrations ofthe reductants H2 and CO as well as of the secondary emissions NH3 undN2O downstream of the LNT in the full exhaust gas, A/F ratio upstreamand downstream of the LNT, temperatures inside LNT, a 1.0 g NOX load,

b 2.5 g NOX load (reformate substitute dosing=45 g/min, SVLNT,Reg=3900 h−1, LNT A “PGM high”), *Σ(H2 carrier species)=ψH2+3,5ψNH3+3 ψCH4, **Σ(reductants)=ψH2+3,5 ψNH3+4 ψCH4+ψCO

Emiss. Control Sci. Technol. (2015) 1:167–182 173

4.2 LNT NH3 Formation

The LNT regeneration strategy has a high impact on NH3

formation and emission. Due to the key function of the NH3

formation for a passive SCR system, the influence of the LNTtemperature, of the LNT NOX load, and of the regenerationduration on the NH3 emission are discussed in detail.

Two LNT regenerations showing gas concentrations, A/Fratios, and temperatures as function of time at 250 °C for 1.0and 2.5 g NOX load are reported in Fig. 7. Basically, theregeneration period can be divided into two phases. In phase1, the LNT reduction takes place (typical λdownstream LNT=1).The completion of enrichment is identifiable by λ-breakthrough (λdownstream LNT<0.95). Phase 2 defines the re-generation duration after λ-breakthrough. In line with currentliterature, the NH3 emission starts at the end of phase 1. TheLNT enrichment in conventional application is stopped at theend of phase 1 in order to avoid reductant and NH3 slip.However, the lean exhaust gas downstream of the SCR causedby the bypass system offers reductant slip oxidation and there-fore significantly elongated rich duration, which enables en-larged NH3 emission. The longNH3 emission profile indicatesthat the NOX load is still not completely reduced even in thecase of λ-breakthrough and high reductant slip. In the givenexample at 250 °C and 1.0 g NOX load, the profile of the sumof all H2 carrier reductant species (H2, NH3, CH4) equals theCO profile downstream of the LNT. This suggests that NH3 isonly formed by the H2 feed at this temperature (CH4 not ev-ident). At higher temperatures (not shown), the profile com-parison suggests that also CO contributes to the NH3 forma-tion. However, it is not possible to distinguish between a directCO path and CO consumption by WGS and subsequent NH3

formation by the newly formed H2. The final NH3 emissionpeak during opening the LNT exhaust flap is caused by sud-den purging and hence less dilution of the formed NH3 in thefull exhaust gas upstream of the SCR. N2O emission is evidentmostly during the beginning of the LNT reduction in phase 1as well as after the regeneration during the switch back fromrich to lean conditions. Possibly, the adsorbed N-species getoxidized during the O2-flush.

The NH3 formation yield (molar ratio of cumulated NH3

amount during LNT regeneration to initial LNT NOX load atLNT regeneration start, Eq. 3) or the NH3 formation selectiv-ity (molar ratio of cumulated NH3 amount during LNT regen-eration to converted NOX, Eq. 4) are important parameters foroperating the combined system. Figure 8 shows the NH3 se-lectivity as function of LNT temperature for various regener-ation durations after λ-breakthrough in comparison for LNTA(low and high PGM) and LNT B.

YNH3¼ nNH3

nNOX loadLNT

ð3Þ

SNH3¼ nNH3

nNOX convertedð4Þ

Independent of the LNT type, the NH3 selectivity stronglyincreases with prolonged LNT enrichment after λ-breakthrough and saturates at higher durations. Nevertheless,it must be mentioned that the NH3 emission occurs roughly inparallel to reductant breakthrough. Therefore, high NH3 for-mation causes increased reductant slip. For all three LNT, theNH3 selectivity shows a maximum at medium temperature(200–250 °C). This is in line with current literature. The

0.7

5

0.4

0

NOX Adsorption:

[gNO

X/l

LNT] 0

.25

0.7

5

0.4

0

0.2

5

0.4

0

0.7

5

LNT Temperature [°C]

100 200 300 400

(b)LNT A "PGM high"

NH

3 S

ele

ctivity [

%]

0

10

20

30

40

50

60

70

80

LNT Temperature [°C]

100 200 300 400

(a)LNT A "PGM low"

LNT Temperature [°C]

100 200 300 400

Regeneration duration after

λ-breakthrough downstr. LNT:

0 s 10 s

5 s 15 s

(c)LNT B "PGM high"

Fig. 8 NH3 selectivity of LNT regeneration as function of LNT temperature for various regeneration durations after λ-breakthrough downstream of theLNT: a LNTA “PGM low”, b LNTA “PGM high”, c LNT B “PGM high” (reformate substitute dosing = 45 g/min, SVLNT,Reg=3900 h

−1)

174 Emiss. Control Sci. Technol. (2015) 1:167–182

NH3 emission results basically from superposition of the in-trinsic NOX reduction to NH3 and the in-catalyst NH3 re-ox-idation. The reaction rate of both reactions increases with in-creasing temperature but forms a maximum profile due todifferent light-off temperatures. LNT A shows significantlyhigher NH3 selectivity compared to LNT B.

Finally, the influence of the NOX load on the NH3 forma-tion and related parameters for various regeneration durationsafter λ-breakthrough are presented in at Fig. 9 at 250 °C.

The cumulated emitted NH3 quantity (Fig. 9a) steadily in-creases with increasing NOX load. A saturation of the NH3

emission is earlier reached for lower NOX load than at higherNOX load. This suggests a limited rate of nitrate decomposi-tion and subsequent reduction to NH3. In contrast, except forshort regeneration duration, the NH3 yield decreases with in-creasing NOX load. Either, more NH3 gets re-oxidized athigher NOX load or the increasing substrate temperature bythe exothermic reductant conversion decreases the NH3 for-mation tendency. The increasing NH3 yield at short regenera-tion duration might be explained by the axial NOX load pro-file. With ongoing NOX adsorption, especially the local NOX

load at the substrate outlet increases. According to the NH3

formation principle, with increasing NOX load at the rear partof the substrate, more NOX gets reduced to NH3 and less NH3

gets re-oxidized. However, the cyclic NH3/NOX ratio up-stream of the SCR (αcyclic so-called feed ratio, Eq. 5) is more

relevant for the passive SCR operation than the single NH3

formation parameter. The NOX feed to the SCR increases withincreasing NOX load due to the decreasing NOX adsorptionefficiency. Moreover, uniquely for an LNT bypass concept,the NOX feed to the SCR is also related to the NOX slip due tothe open LNT bypass. The latter can be limited by temporarilylowering the NOX engine out emission during the open bypassphase. Summarizing, the superposition of NH3 emission andNOX slip results in a maximum formation for αcyclic upstreamof the SCR. By relating the αcyclic to the fuel economy penaltyFEP (Eq. 6), a parameter can be defined, which provides anoverall efficiency indication. The FEP declines with increas-ing adsorption duration but increases with increasing regener-ation duration. Hence, the LNT regeneration strategy can becontrolled either in terms of high NH3 emissions for fast SCRfilling (maximum αcyclic) or in terms of best overall efficiency(maximum αcyclic/FEP).

Acyclic ¼∫ nNH3

� �SCR upstream

∫ nNOX

� �SCR upstream

264

375adsorptionþregereneration

ð5Þ

FEP ¼ ∫ mDiesel fuel; LNT Regerneration

∫ mDiesel fuel; engine lean operation

ð6Þ

α cyclic u

pstr

. S

CR

[-]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

NOX Load [g

NOX/l

LNT]

0.0 0.5 1.0 1.5 2.0

(c)

[1/%

]

Fu

el E

co

no

my P

en

alty

1.0

1.5

2.0

2.5

3.0

3.5

4.0

NOX Load [g

NOX/l

LNT]

0.0 0.5 1.0 1.5 2.0

(d)

α cyclic u

pstr

. S

CR

NH

3 Y

ield

[%

]

0

10

20

30

40

50

60(b)

NH

3 F

orm

atio

n [

mm

ol]

0

5

10

15

20

25

30

(a)

Regeneration duration after

λ-breakthrough downstr. LNT:

0 s 10 s

5 s 15 s

LNT A "PGM low"

Fig. 9 NH3 formation as function of NOX load and regeneration duration(displayed as duration after λ-breakthrough downstream of the LNT): acumulated NH3 quantity, b NH3 yield, c cyclic NH3/NOX ratio (αcyclic)

upstream of the SCR (usually called feed ratio), d ratio of αcyclic to thefuel economy penalty (reformate substitute dosing=45 g/min,SVLNT,Reg=3900 h

−1, TLNT=250 °C, LNTA “PGM low”)

Emiss. Control Sci. Technol. (2015) 1:167–182 175

The fuel economy penalty (FEP, Eq. 6) is defined bythe ratio of additional diesel fuel injected into the re-former during LNT regeneration and the diesel fuel con-sumption of the engine. To minimize the NOX slipthrough the open bypass during the LNT regeneration,the engine is operated in a low NOX raw emissionmode. In the applied extent, this engine mode switchhad only negligible influence on the engine fuel con-sumption and hence on the fuel economy penalty.

4.3 Steady-State LNT + SCR Investigation on Engine TestBench

An example of the NOX conversion behavior of the combinedLNT + SCR system during cyclic adsorption-regeneration op-eration is shown in Fig. 10 for an LNT temperature of 250 °C.As shown in the upper diagram, after a short period with highLNT NOX adsorption efficiency, the NOX concentrationdownstream of the LNT is continuously increasing due tothe increasing LNT NOX load and hence lower NOX adsorp-tion efficiency. However, the NOX tailpipe concentration iskept close to zero due to continuous NOX conversion by thepassive SCR catalyst. The cumulated NH3-NOX ratio up-stream of the SCR (αcyclic) over the total NOX adsorptionand regeneration cycle is slightly above 1. For instance, theLNT and bypass NOX slip are more than equaled by the on-board NH3 formation during LNT regeneration. To minimizethe NOX slip through the open bypass during the LNT regen-eration, the engine is operated at lower NOX raw emission.The continuous α can reach temporarily high values (here≫50), which demands high NH3 adsorption efficiency of thepassive SCR. In the given example, no NH3 slip downstream

of the SCR was observed. Contrary to a conventional LNTapplication, the LNT NOX slip does not automatically dropthe system NOX conversion but can be converted by the pas-sive SCR. Therefore, the LNT NOX adsorption duration canbe elongated, and the LNT can be operated at higher NOX

storage levels resulting in a higher degree of material utiliza-tion. As shown in the bottom diagram, only the LNT isenriched locally during the rich LNT regeneration. Both theengine and the tailpipe flow path downstream of the recombi-nation of main and bypass line are continuously operated atλ>1. The engine-independent LNTenrichment avoids the riskof engine oil dilution, which might be caused by engine inter-nal late post injection. Additionally, drivability and acousticdrawbacks due to switch into rich engine operation can beavoided. Furthermore, the lean tailpipe exhaust gas composi-tion offers the possibility to oxidize the occasional reductantslip during the LNT regeneration. Usually, the higher thespace velocity (SV), the lower is the catalyst performancebecause of kinetic limitations especially at low temperature.As shown in the bottom diagram, the closed LNT flow pathduring LNT regeneration results in a significant drop of LNTspace velocity, which offers the potential for increasing regen-eration efficiency.

The dependency of the total NOX conversion and αcyclic onthe fuel economy penalty for various temperatures isdiscussed in Fig. 11. As shown before, with elongated regen-eration duration and hence increasing fuel economy penalty,αcyclic can be increased. The total NOX conversion also in-creases with increasing FEP, on the one hand due to the in-creased αcyclic and on the other hand due to a higher LNTefficiency. The latter could be reached by either higher com-pletion of LNT regeneration or higher NOX adsorption

SV

LN

T [

h-1]

0

25000

50000

75000

100000

Time [s]

7900 7950 8000 8050 8100 8150 8200 8250

α co

ntin

uo

us [

-]

0

20

40

60

80

100

120

ψN

OX [

pp

m]

0

100

200

300

400

500

engine out

upstr. SCR

downstr. SCR

by LNT

NOX conversion

by SCR

NOX conversion

NO

2/N

OX r

atio

[-]

α cyclic [

-]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

αcontinuous

αcyclic

NO2/NO

X ratio upstr. SCR

λ [-

]

0.0

0.5

1.0

1.5

2.0

SVLNT

λdownstr. turbine

λdownstr. LNT

λdownstr. SCR

Fig. 10 NOX conversionbehavior of LNT+ SCR system incyclic adsorption-regenerationoperation as function of time(reformate substitute dosing = 45g/min, SVLNT,Reg=3900 h

−1,TLNT=250 °C, TSCR=225 °C,FEP=1.9 %, SVSCR=24,000 h

−1,LNTA “PGM high”)

176 Emiss. Control Sci. Technol. (2015) 1:167–182

efficiency by operation at lower NOX load. Thus, NOX con-version and fuel consumption are directly linked. The alreadydiscussed strong temperature dependency of the NH3 forma-tion results in large variation of maximum αcyclic and hencealso of the maximum total NOX conversion. As expected, theNOX conversion of the passive SCR solely depends on thefeed ratio (αcyclic).

A significant influence on the total NOX performance andespecially on the conversion share between LNT and SCR isgiven by the LNT NOX adsorption duration and therefore theLNT NOX load. In Fig. 12, the NOX conversion efficiencies,NOX conversion shares of LNTwith bypass and passive SCR,and the related FEP are shown as function of LNT NOX load.Considering the boundary conditions of the discussed

NO

X c

on

ve

rsio

n L

NT

+S

CR

[%

]

0

20

40

60

80

100

150 °C / 125 ppm / 135 °C

185 °C / 135 ppm / 155 °C

200 °C / 225 ppm / 170 °C

250 °C / 200 ppm / 225 °C

350 °C / 320 ppm / 310 °C

385 °C / 320 ppm / 350 °C

420 °C / 320 ppm / 375 °C

TLNT

/ ψNO

X upstr. LNT / T

SCR:(a)

LNT A "PGM low"

α cyclic u

pstr

. S

CR

[-]

0.0

0.5

1.0

1.5

2.0

2.5

Fuel Economy Penalty [%]

0 1 2 3 4 5

(b)

NO

X c

on

ve

rsio

n S

CR

[%

]0

20

40

60

80

100

αcyclic

upstr. SCR [-]

0.0 0.5 1.0 1.5 2.0 2.5

(c)

maximum SCR

NOX conversion

at NO2/NO

X ratio ≤ 0.5

Fig. 11 NOX conversion of the LNT + SCR system as function of fueleconomy penalty at various temperatures in cyclic adsorption-regeneration operation: a cyclic total NOX conversion (LNT+SCR), b

αcyclic upstream of the SCR, c cyclic NOX conversion of the passive SCRas function ofαcyclic (reformate substitute dosing = 45 g/min, SVLNT,Reg=3900 h−1, LNTA “PGM low”)

NO

X c

on

ve

rsio

n

Sh

are

[%

]

0

20

40

60

80

100 U

NOx total

UNOx

LNT + Bypass

UNOx

SCR

share LNT + Bypass

share SCR

Fuel Economy Penalty

LNT A "PGM low"

NO

X c

on

ve

rsio

n U

NO

X [

%]

0

20

40

60

80

100

LNT NOX load [g

NOX]

0.0 0.5 1.0 1.5 2.0 2.5

Fu

el E

co

no

my P

en

alty [

%]

0.0

0.5

1.0

1.5

2.0

2.5

Fig. 12 NOX conversionefficiencies, related fuel economypenalty (FEP), and NOX

conversion shares of LNTwithbypass and passive SCR asfunction of LNT NOX load(reformate substitute dosing = 45g/min, SVLNT,Reg=3900 h

−1,TLNT=250 °C, LNTA “PGMlow”)

Emiss. Control Sci. Technol. (2015) 1:167–182 177

example in Fig. 12, the total NOX conversion is almost con-stant for the NOX load of 0.4–1.4 g. However, with elongatingNOX adsorption duration and therefore increasing LNT NOX

load, the NOX conversion distribution is significantly shiftingfrom LNT to SCR. As shown in the bottom diagram, the lossof LNT conversion is almost compensated by the increasingSCR conversion, which achieves a maximum of 40 %. TheNOX conversion share between LNT and SCR is changingfrom 75/25 % at 0.4 g NOX load to 60/40 % at 1.4 g NOX

load. The increasing NOX slip through LNT and bypass forNOX adsorption durations up to the NOX load level of 1.4 gcan be compensated by the NH3 formation on the LNT. αcyclic

upstream of the SCR is ≥1 (not displayed) for LNT NOX loadbelow 1.4 g. In case of LNT NOX loads above 1.4 g, αcyclic isdropping below the stoichiometry of 1, which results in cor-responding decline of SCR NOX conversion and thereforealso of the total NOX conversion.

A great potential of the conversion shifting between LNTand SCR is the impact on the fuel economy penalty, shown inthe bottom diagram of Fig. 12. By expanding the NOX ad-sorption duration toward higher LNT NOX loads, the FEP canbe lowered from 1.5 % at 0.4 g to 0.75 % at 1.4 g. The higherthe NOX load, the more reducing agent is consumed for NOX

reduction. Reasons are the lower regeneration frequency aswell as the higher NHX selectivity of the LNT regeneration(not displayed) for higher NOX loads. A further reason for thebenefit of high NOX loads is caused by the OSC of the LNTwhich has to be reduced in parallel to the NOX reduction. Thereducing agent amount, which is consumed by the OSC dur-ing the LNT regeneration, increases the fuel economy penaltywithout any NOX reduction. Assuming constant OSC, by in-creasing the NOX load, relatively more reducing species willbe consumed by NOX than by OSC. The higher the NOX load,the more beneficial becomes the ratio of oxygen molaramount stored in barium nitrate to the sum of oxygen inbarium nitrate and cerium oxide (OSC), resulting in de-creasing FEP.

Figure 13 depicts the correlation of NOX conversion, fueleconomy penalty, and NOX raw emission level for LNT-onlyand LNT+SCR at 230 °C. Generally, increasing NOX conver-sion progressively causes the drawback of increasing fuel con-sumption penalty. The NOX conversion converges asymptot-ically against a maximum. Furthermore, higher NOX rawemission shows significantly negative impact on NOX conver-sion for certain FEP and maximum achievable NOX conver-sion. Hence, for achieving high NOX conversion at reasonablefuel economy penalty, the NOX raw emission must be limited.In comparison to the LNT-only, the combined LNT+passiveSCR system shows high benefits regarding FEP and NOX

conversion. In the given example, high maximum total NOX

conversion of up to >95 % is achievable at low FEP of ≈ 1 %for reasonable NOX raw emissions. At high NOX raw emis-sion level, the total NOX conversion decreases but is signifi-cantly higher than for LNT-only. At a certain NOX conversion,significant lower FEP is attainable by the LNT + SCR.

Figure 14 shows the steady-state NOX conversion and fueleconomy penalty of the combined LNT + passive SCR systemfor two system variants with onboard generated reformate andreformate substitute gas as function of LNT temperature. Theattained NOX conversion is comparable for the operation withthe integrated onboard reformer as well as for the operationwith the bottled reformate substitute gas. The system with thehigher PGM-loaded LNTachieves a slight increase in conver-sion at T<250 °C. At 250 °C, almost complete conversion isachieved at FEP of ≈ 1.5 %. The operation strategy of NOX

adsorption and regeneration is optimized with regard to max-imum total NOX conversion and suitable fuel consumptionpenalty. The SCR significantly contributes to the total NOX

conversion. At T<250 °C, the total steady-state NOX conver-sion drops caused by reduced LNT activity as well as NH3

yield. The FEP increases due to higher LNT regeneration fre-quency as well as lower engine fuel consumption due to lowerengine load. Nevertheless, 65–70% steady-state NOX conver-sion at 185 °C and 30 % steady-state NOX conversion at

NO

X c

on

ve

rsio

n [

%]

0

20

40

60

80

100

Fuel Economy Penalty [%]

0 1 2 3 4 5

1000

100

200

580

(a)

ψNO

X upstr. LNT [ppm]

LNT A "PGM high"

LNT (without bypass

consideration)

Fuel Economy Penalty [%]

0 1 2 3 4 5

1000

100

200

580

(b)

ψNO

X upstr. LNT [ppm]

LNT + SCR

Fig. 13 Correlation betweenNOX conversion and fueleconomy penalty as function ofNOX raw emission level: a LNT-only, b LNT + passive SCR(reformate substitute dosing = 45g/min, SVLNT,Reg=3900 h

−1,TLNT=230 °C, LNTA “PGMhigh”)

178 Emiss. Control Sci. Technol. (2015) 1:167–182

150 °C are still achieved. In transient operation, higher conver-sion rates can be achieved due to pure NOX adsorption at theLNT as well as SCR conversion with previously stored NH3. AtT>350 °C, the steady-state NOX conversion drops because ofdecreasing (for thermodynamic reasons) NOX adsorption capac-ity of the LNT as well as lower NH3 yield of the LNT regener-ation. The decreased LNT NH3 yield causes limited steady-stateSCR conversion in contrast to urea-based SCR operation. How-ever, during transient operation, complete conversion can still bemaintained by the SCR for short time, e.g., during DPF regener-ation, by consuming previously stored NH3 depending on SCRNH3 load and NOX mass flow. This represents high NOX per-formance benefits compared to conventional LNT application.

4.4 Transient LNT + SCR Investigation in DemonstratorVehicle

Finally, Fig. 15 shows the final results for NOX conversionand fuel economy penalty (separated in FEP for heating andFEP for rich LNT regeneration) in all transient cycles mea-sured in the demonstrator vehicle. The underfloor-positionedaftertreatment system requires significant fuel economy pen-alty by heating but only minor by LNT enrichment in the coldstart cycles NEDC and WLTC. However, in real driving withlonger warm operation periods, the system performanceshows high NOX conversion and realizes low NOX emissionat low fuel economy penalty of less than 2 %. The fuel

NO

X C

on

ve

rsio

n [

%]

0

20

40

60

80

100

LNT Temperature [°C]

100 150 200 250 300 350 400 450

LNT + Bypass

SCR

NOX Conversion:

System 1, LNT + Bypass (Reformate Substitute)

System 1, total (Reformate Substitute)

System 2, total (Reformate)

System 2, total (Reformate Substitute)

Fuel Economy Penalty:

System 1 (Reformate Substitute)

System 1 (Reformate)

System 2 (Reformate Substitute)

Fu

el E

co

no

my P

en

alty [

%]

0

2

4

6

8

10

Fig. 14 Steady-state NOX

conversion and fuel economypenalty of combined LNT +passive SCR system withreformate and reformatesubstitute gas as function of LNTtemperature; EU4 NOX rawemission level (system 1: LNTA“PGM low”+SCR, system 2:LNTA “PGM high” + SCR)

NEDC

1590 kg

WLTC

1590 kg

RDE

"soft"

1780 kg

RDE

"normal"

1970 kg

RDE

"aggressive"

1970 kg

ADAC

Cycle

1590 kg

RDE

"soft"

1970 kg

NO

X E

mis

sio

n [

mg

/km

]

0

100

200

300

400

500

600

700

800

900

Fu

el E

co

no

my

Pe

na

lty [

%]

0

1

2

3

4

5 LNT-Regeneration

Heating

Fuel Penalty Limit

- 65 %

- 76 %

- 83 %

- 91 %

- 81 %

- 65 %

Engine Out

SCR upstream

SCR downstream

NOX Target (72 mg/km)

- 87 %

Fig. 15 Summary of transientNOX conversion and fueleconomy penalty of combinedLNT + passive SCR system(operation with reformatesubstitute gas, LNTA “PGMhigh”, vehicle weight differing toinertia weight class due toadditional equipment andpassengers)

Emiss. Control Sci. Technol. (2015) 1:167–182 179

economy penalty is generally low compared to conventionalengine internal enrichment, due to higher reductant formationefficiency of the reforming approach. The passive SCR can sig-nificantly contribute to the total NOX conversion. The high NOX

raw emissions and exhaust temperature during aggressive drivingare challenging for the LNT-based systemwith only passive SCRdue to the LNT NOX conversion drop at high temperature. Ad-ditionally, high space velocity caused by high engine speed re-sults on one hand in limited NOX adsorption efficiency on theLNT. On the other hand, it causes fast consumption of SCRNH3

load due to LNT NOX breakthrough. However, especially underthese conditions, the underfloor position is beneficial comparedto a close-coupled position and enables maximum NOX conver-sion for operating without active urea dosing. Summarizing, aclear trade-off between short cold cycles and long hot driving isobvious.

5 Summary and Conclusions

The upcoming emission legislation requires emission controlfor the complete exhaust temperature range from cold start upto high temperature operation in real dynamic testing proce-dures. This is challenging especially for the NOX treatment.Both currently used NOX reduction systems, LNT and SCR,provide optimum reduction efficiency at a specific tempera-ture range only. A combination of both systems with activeapplication (LNT with frequent rich regeneration, SCR withurea dosing) is promising but will probably request urearefilling by the customer as well as CO2 emission drawbackdue to fuel economy penalty caused by the LNT regeneration.As an alternative approach, the combination of LNT and pas-sive SCR using NH3 produced by NOX reduction on the LNTwas investigated. Furthermore, the LNT reduction was per-formed in LNT bypass operation with onboard fuel reformategas (H2+CO) at low space velocity in order to enhance theNOX conversion efficiency especially at low temperature andminimize the fuel economy penalty of the reduction process.

The NH3 formation by the LNT NOX reduction causesdiscontinuous NH3 supply to the SCR. Due to the NH3 for-mation characteristic of the LNT, the optimum temperaturewindow for steady-state operation for the passive SCR ap-proach is in the range of 200–300 °C. At lower temperature,the LNTadsorption and regeneration efficiencies decrease dueto kinetic reasons. At higher temperature, the NOX adsorptioncapacity of the LNT decreases due to thermodynamic reasons.Additionally, the NH3 formation becomes negligible so thatthe passive SCR does not reach the high efficiency of an activeSCR. The combined LNT and passive SCR system achievesnearly 100 % NOX conversion efficiency at 250 °C with fueleconomy penalty of 1.5 % for Euro 4 NOX raw emission level.Caused by high NH3 selectivity of up to approx. 50 % regard-ing the LNT NOX load, the passive SCR contributes with up

to 35 % significantly to the total NOX conversion. At an ex-haust temperature of 175 °C, the NOX conversion efficiencyof approx. 40 % by the LNT can still be increased up to 60 %by the combined passive SCR.

The reduction of the fuel penalty was caused primarily bytwo different measures. The NH3 production on the LNT of-fers an LNT operation at higher NOX load due to the conver-sion of LNT NOX breakthrough by the passive SCR. Thehigher NOX load leads to a reduced regeneration frequency.At high NOX load, relatively more fuel reacts with NOX in-stead of OSC. Hence, the fuel efficiency could be increasedcompared to a conventional LNT application. Additional im-provement of fuel economy might be achieved by using LNTwith lower OSC due to no necessity of NH3 formation pre-vention on the LNT. Further, the fuel economy penalty wasreduced significantly by the use of an engine-independentLNT regeneration with reformate gas in bypass operation.Compared to diesel engine enrichment, the reducing agentgeneration by fuel reforming is more efficient.

Regarding the vehicle integration of the aftertreatment sys-tem, especially the LNT position in a passive SCR combina-tion is crucial. In short cold start cycles, a close-coupled po-sition is favorable because of faster heat-up. However, in con-trast to the current technical trend, in longer warm operation inreal driving, an underfloor position becomes favorable due totaking the full potential of the passive SCR approach.

Summarizing, in comparison to conventional LNTapplica-tion, a combined system of LNT in bypass operation withpassive SCR improves the NOX performance at lower fueleconomy penalty and represents an attractive approach forfuture emission aftertreatment. While LNT and passive SCRcombinations already reached commercial maturity, relevantpractical topics like OBD and complexity need more investi-gations for the reformer itself.

Acknowledgments The research leading to these results has receivedfunding from the European Union 7th Framework Program [FP7/2007–2011] under grant agreement no. 234032. The authors are grateful to thefunding of EU in the POWERFUL research project and all persons, whoknow to have their contribution in this study.

The authors would also like to thank Volkswagen AG for the vehiclemeasurements and Dinex Ecocat for the catalyst supply.

Furthermore, the authors would like to thank theDFG for enabling thiswork by funding the mass spectrometer measurement devices.

Parts of this work are the result of the successful cooperation withRWTH’s Center for Automotive Catalytic Systems Aachen (ACA).

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