Close-Coupled SCR: An Approach to Meet Ultra-Low NOx Requirements for Heavy-Duty Diesel Engines

Abstract

The California Air Resources Board and US Environmental Protection Agency have defined their new ultra-low NOx standards for heavy-duty diesel engines (HDDE) over the last two years. In parallel, AVL has continued its research on how to meet these new limits and to address the continuing challenge of cold start emissions from HDDE, including incorporating
• Advanced thermal management strategies to accelerate exhaust aftertreatment system (EAS) warm-up
• Close-coupled selective catalytic reduction (ccSCR) system upstream of the current Euro VI or EPA 2013 EAS

This paper will continue from the presentation at the 2019 CLEERS Workshop and present further developments in the research program including experimental results. These results include the following:
• Measured effects of thermal management strategies on EAS warm-up
• Test results using the ccSCR and “underfloor” SCR in combination
• Strategies for successful ultra-low NOx compliance
• Interactions between ccSCR system and on-board diagnostics (OBD) requirements

AVL recommendations and next steps will also be discussed.

[See https://cleers.org/cleers-workshops/workshop-abstracts/?entry_id=3199]

Full text

Public
AVL Powertrain Engineering Inc.
13 September 2021
John J. Kasab, Klaus Hadl, Bernhard Raser, Thomas Sacher, Gernot Graf
An approach to meet ultra-low NOx requirements for
heavy-duty diesel engines
Close-Coupled SCR

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3
2
Introduction1
Overview
Emissions Concepts
ULNOx Demonstrator
Conclusion

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Legislation Trends
Transition towards Zero-Impact Emissions
2020 20302025
-75%
-90%
-80%
EGR & SCR / SCR-only Dual Stage SCR & EGR
Relative NOxReduction

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Legislation
Challenges for Future Worldwide Emission Legislations
Significant increase in development and validation effort compared to previous legislations
Diagnosis & Controls
Lifetime and Durability
Emission Compliance
Real Driving Emission
Legislative Boundaries
New Aftertreatment
Concepts
Sensor Technology
and Accuracy
On Board Monitoring
Time to Market

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CARB Ultra-Low NOx Requirements Pose Challenges
in Parallel with HD GHG Requirements
NOxreduction of up to 90% compared to current standard proposed by CARB
•Limit change of carve out NTE-regions and HDIUT with MAW method
MY 2022
MY 2023
MY 2026
•HDIUT: MAW method with 300s window size divided into 3 bins (cold start excluded)
•Pilot program on Real Emission Assessment Logging
•NOx:0.02 g/bhp-hr FTP & RMC SET; 0.02 g/bhphr LLC; 5 g/hr idling
•Lengthen useful life and Warranty for all engine classes
•HDIUT: MAW method with 300s window size divided into 3 bins (cold start included)
MY 2024
STEP 1
STEP 2
STEP 3
*: Nationwide certification of standard for complete fleet mandatory incl. compliance with all applicable existing and proposed requirements for the MY (i.e., warranty, OBD, HDIUT,…)
Source: Heavy-Duty Low NOxProgram, “Proposed Heavy-Duty Engine Standards”, CARB, Public Workshop, Diamond Bar, CA, 26/09/2019
Titles 13 and 17. California Air Resources Board, “; Notice of public hearing to consider the proposed heavy-duty engine and vehicle omnibus regulation and associated Amendments”, CARB, Letter, 23/06/2020
Proposed Optional 50-State Standard*:
•NOx: 0.10 g/bhp-hr FTP & RMC SET
0.30 g/bhp-hr Low Load Cycle
10 g/hr idling standard
California Standards:
•NOx: 0.05 g/bhp-hr FTP & RMC SET
0.20 g/bhp-hr Low Load Cycle
10 g/hr idling standard
MY 2027 and subsequent

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Other considerations
There is much work to be done on the engine side for both NOx and GHG.
Manufacturers still need to improve engine fuel consumption or GHG emissions
50% BTE seems feasible from other AVL work, but will need
Advanced combustion
Higher peak firing pressure (PFP)
Improved air and EGR flows
On-board diagnostics (OBD) monitoring requirements are changing to a MAW method
More operation will be included in monitoring
OBD thresholds are expected to change when EPA implements its ultra-low NOx standard
Sensors are a key challenge

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Introduction1
Overview
Emissions Concepts
ULNOx Demonstrator
Conclusion

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Vehicle class N2, M2, M3
Intercity Bus
Distribution Truck
Vehicle class N3
Haulage
Tipper
Euro VII
CLOVE
“HD2”
(350mg/kWh NOx
in cold conditions;
90mg/kWh in hot)
•2-stage SCR system with optional Ecat
•optional DOC required in case of not using vanadium SCR technology
Euro VII
CLOVE
“HD3”
(175mg/kWh NOx
in cold conditions;
90mg/kWh in hot)
•2-stage SCR system with Ecat
•optional DOC required in case of not using vanadium SCR technology
Aftertreatment Concepts
Potential Emission Concepts for Euro VII (MD, HD)
DPF SCR
HC
TC
DEF
SCR
D
O
C
DEF
A
S
C
A
S
C
E
c
a
t
D
O
C
DPF SCR
HC
TC
DEF
SCR
D
O
C
DEF
A
S
C
A
S
C
E
c
a
t
D
O
C
DOC optional HC Doser optional
Dual Stage SCR enables
•Highest NOxReduction
•Real World Emissions Control
•Highest Emission Durability
•NOx/NH3/N2O Control
•OBD/OBM Ability
*: Euro VI no NH3and N2O limit in ISC
E-Cat optional
2-stage SCR and additional heat-up measures as key enabler for lowest emissions.

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Concept 1
US 1-stage SCR & Euro VII 2-stage SCR
Concept 2
Global usage of 2-stage SCR
US CARB MY27
Single-stage SCR for CARB MY27
1 global concept
Vanadium-SCR to be certified in US
(no ccDOC required)
Optional ccDOC and Cu/zeolite ccSCR
N2O emissions in case Cu/zeolite used
Potential alternatives to Ecat:
Early Exhaust Valve Opening
Cylinder Deactivation
Burner
Euro VII
Extension of system via close coupled Vanadium-SCR
to master stringent N2O, NH3and PN requirements via
2-stage SCR
Alternative to Ecat: Burner upstream DOC
Aftertreatment Concepts
Possible Global Emission Concepts for Euro VII and ULNOx
Vanadium
Fe/Cu-zeolite*
Fe/Cu-zeolite*
Box type muffler
Box type muffler
Extension of system by
a close coupled SCR
Vanadium or
Cu-zeolite
Fe/Cu-zeolite*
Box type muffler
Optional
*: Optional usage of Vanadium SCR if usage in US feasible
Optional
Optional
Optional
Sophisticated aftertreatment system required for future emission rules.

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Aftertreatment strategies for Ultra-Low NOx
Need fast heat up with lower EO NOx to meet tailpipe NOx emissions targets.
Need “keep warm” strategy to maintain high NOx conversions at low load.
Lower engine-out (EO) NOx
during warm-up gives EAS
more time to get to 100%
NOx conversion.
3.5 g/bhp-hr: <50 s
1.5 g/bhp-hr: 240 s
0
25
50
75
100
125
150
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 50 100 150 200 250 300 350 400 450 500
Engine Speed in %
NO
x
Tailpipe* in g/bhp-hr
Time [s]
0.75g/bhp-hr NOₓ EO 1.5g/bhp-hr NOₓ EO 2.0g/bhp-hr NOₓ EO
3.5g/bhp-hr NOₓ EO Engine speed
<50sec: 100% DeNO
x
170sec: 100% DeNO
x
240sec: 100% DeNO
x
* Assumption: 100% NO
x
conversion in warm US HDDTC
NO
x
in cold US HDDTC weighted (1/7) 440sec: 100%
Sources: de Monte, et al., 2018.

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AVL has continued work on the close-coupled SCR concept
•Use close-coupled SCR (ccSCR) for NOx control
•Improves cold start performance (for HDDTC a.k.a. HD FTP)
•Cheaper and more robust than close-coupled LNT
•Use E-Cat upstream of ccSCR
•Close-coupled DOC is optional—trade-off between thermal mass and oxicat performance
•Use separate downstream DOC, DPF, SCR, and ASC for NOx and PM control
Box type muffler
DPF SCR
HC
TC
DEF
SCR
DEF
A
S
C
A
S
C
E-CAT
D
O
C
Source: de Monte, et al., 2018; Kasab, et al., 2019; Hadl, et al., 2021.

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AVL Solution
Different diagnosis functions available
e.g., using the reduced storage capacity, efficiency based
Use control strategy in combination with OBD strategy to robustly detect 2 stage SCR systems
References
Single dosing application with AVL controls and OBD functionality for CN VI in Series
Internal and customer projects using AVL dual-dosing software including OBD functionality
Challenges for OBD
detection:
•Robust detection of SCR systems
with high engine out NOx
•Complex detection strategies
needed for 2 stage SCR systems
2-Stage SCR Software
OBD Challenges and Solutions

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3
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Introduction1
Overview
Emissions Concepts
ULNOx Demonstrator
Conclusion

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AVL Demonstrator: Ultra-low NOxemissions demonstrated on testbed under laboratory conditions
•NOxTailpipe 18 mg/bhp-hr in HDDTC (FTP) cold/hot
•NOxTailpipe 34 mg/kW∙h in WHTC cold/hot
AVL Demonstrator
AVL R&D Ultra-low NOx& Euro VII Demonstrator
Target:
Definition of engine and EAS layout & calibration to fulfill ultra-low
NOxand Euro VII emission targets
Key System Features:
2L/cyl heavy duty engine
2-Stage SCR concept using AVL controls and catalyst technology
Electrical heated catalyst as system heat-up measure
Uncooled HP-EGR as NOxreduction measure in cold conditions
Smart DeNOxbalancing for passive DPF regeneration and low N2O
Favorable Trade-off between costs & efficiency Results partly published in Hadl, et al., 2021
The data included herein were collected in a laboratory which has not been certified by the relevant
authorities/agencies to perform emissions testing. These are indicative data and do not represent a
guarantee that the tested ca talyst or emissions system will pass the relevant emissions legislation.

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-3000 -2000 -1000 0 1000 2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
0
25
50
75
100
125
150
175
200
0 250 500 750 1000 1250
NOxhot [mg/bhp-hr]
NO
x
cold [mg/bhp-hr]
NOxEmissions in US HDDTC (FTP)
AVL Demonstrator
Performance of a 2-Stage EAS – FTP c/h
97.1% DeNOxcold // 99.9% DeNOx hot // 99.6% DeNOxcombined
NOxEO cold: ~4g/bhp-hr
NOxEO hot: ~4.5g/bhp-hr
0
20
40
60
80
100
120
Cold Hot Combined
NOxand N2O in mg/bhp-hr
NOx
Tailpipe
N2O
Tailpipe
5000
3500
2000
1500 4000
Limit N2O
V/Cu-zeolite Cu-zeolite
Engine Out
Limit NOx
Ultra-low NOxEmissions achieved on Demonstrator using 2-stage SCR.

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0 100 200 300 400 500 600 700 800 900 1000 1100 1200
Time [s]
NOx ds ccSCR
[ppm]
0
500
1000
n
[1/min]
0
1000
2000
Engine Mode
[-]
0
1
2
T Us ufSCR
[°C]
50
100
150
200
250
300
ccSCR_Mode
[-]
0
1
Ecat
[kW]
0
5
10
Cooler Bypass
[-]
0
1
AVL Demonstrator
2-Stage EAS – FTP Cold and Hot
FTP Cold and Hot
FTP Cold FTP Hot
NOxEO cold: ~4g/bhp-hr
NOxEO hot: ~4.5g/bhp-hr
0
20
40
60
80
100
120
Cold Hot Combined
NOxand N2O in mg/bhp-hr
NOx Tailpipe
N2O Tailpipe
Limit
N2O
Limit
NOx
Rapid Heat-up
Best Fuel Consumption
Keep Warm
ccSCR Loading Based
ccSCR Efficiency Based
V/Cu-zeolite Cu-zeolite
3 Mode Engine Strategy for lowest CO2, 2-stage SCR and additional heat-up measures as key enabler for
lowest emissions, ccSCR efficiency/loading based control for high degree of passive regeneration.

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0
50
100
150
200
250
300
350
400
450
500
NOx TP N2O TP
Emissions [mg/kWh]
Cold Hot Combined
0 200 400 600 800 1000 1200 140 0 1600 1800
Time [s]
NOx ds ccSCR
[ppm]
0
500
1000
n
[1/min]
0
1000
2000
Engine Mode
[-]
0
1
2
T Us ufSCR
[°C]
50
100
150
200
250
300
ccSCR_Mode
[-]
0
1
Ecat
[kW]
0
5
10
Cooler Bypass
[-]
0
1
AVL Demonstrator
2-Stage EAS – WHTC Cold and Hot
WHTC Cold and Hot
WHTC Cold WHTC Hot
NOxEO cold: ~7g/kWh
NOxEO hot: ~8g/kWh
Rapid Heat-up
Best Fuel Consumption
Keep Warm
ccSCR Loading Based
ccSCR Efficiency Based
CLOVE „HD2“
(350mg/kWh)
CLOVE 90%
hot MAW
CLOVE „HD3“
(=175mg/kWh)
N2O
from
ccSCR
Further N2O emission
reduction potential
by usage of different
catalyst technology
(e.g., Vanadium)
V/Cu-zeolite Cu-zeolite
3 Mode Engine Strategy for lowest CO2, 2-stage SCR and additional heat-up measures as key enabler for
lowest emissions, ccSCR efficiency/loading based control for high degree of passive regeneration.

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4
3
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Introduction1
Overview
Emissions Concepts
ULNOx Demonstrator
Conclusion

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ccSCR
AVL Active DPF Soot
Management
HP EGR
Dual Stage
DEF dosing
AVL Model-Based
EAS control
E-Cat
2-Stage SCR for future worldwide legislation
Dual-stage SCR EAS for highest NOx reduction with maximum durability
Dual Stage SCR will enable
Highest NOx reduction
Real world emissions control
Highest emission durability
NOx, NH3, & N2O control
OBD / OBM ability

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Conclusions
The future standards are possible to meet but will not be easy to meet.
CARB published the ultra-low NOx standards for MY2024 and MY2027
0.05 (g NOx/bhp∙h) for MY2024; 0.02 (g NOx/bhp∙h) for MY2027
Addition of Low Load Cycle and extended Not-to-Exceed zone
EPA Cleaner Trucks Initiative is still in process for MY2027
Thermal management of the EAS will be critical to meet NOx conversion targets
Need active measures for EAS warm-up and keep warm
AVL has demonstrated 18 (mg NOx/bhp-hr) using a ccSCR system with Ecat and aged parts
Ensuring durability over the extended FUL periods remains a challenge
Supporting future OBD requirements remains a challenge

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References
de Monte, M., Hadl, K., Noll, H., and Mannsberger, S., “SCR Control Strategies with Multiple
Reduction Devices for Lowest NOx Emissions,” SAE Heavy-Duty Diesel Emissions Control
Symposium, Gothenburg, 2018.
Kasab, J.J., de Monte, M., Hadl, K., Noll, H., Mannsberger, S., Graf, G., Theissl, H., and
Arnberger, A., “Using Close-Coupled SCR to Meet Ultra-Low NOx Requirements,” 2019
CLEERS Workshop, Ann Arbor, MI, 2019. (at https://cleers.org/wp-
content/uploads/formidable/3/2019CLEERS_Kasab_Web.pdf)
Edvardsson J., Hadl K., Hein, E., Kraus, G., Noll, H., Schwarz, C., Tamm, S., and Theissl H.,
“HD Diesel Engine – Exhaust Gas Temperature Management and Advanced Exhaust Gas
Aftertreatment Technology for Ultra-Low NOx Emission Legislation,” In Liebl J., Beidl C., Maus
W. (eds) Internationaler Motorenkongress 2020. Proceedings. Springer Vieweg: Wiesbaden,
2020. https://doi.org/10.1007/978-3-658-30500-0_11.
Hadl, K., Sacher, T., Raser, B., and Graf, G., “Solutions for Ultra Low NOx and Lowest CO2on
Heavy Duty Engines,” MTZ-Motortechnische Zeitschrift 82, no. 3 (2021): 16–23,
https://doi.org/10.1007/s38313-020-0613-z.

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