DEER Conference 2009

Passive Ammonia SCR

For Lean Burn SIDI Engines

Wei Li, Kevin Perry, Kushal Narayanaswamy, Chang Kim, and Paul Najt

GM R&D Center

1

DEER Conference 2009

Outline

Background and Concept

NEDC Test Results

NH3

Formation

NOx Conversions

Issues and Summary

2

DEER Conference 2009

Lean NOx Aftertreatment

3

LNT

TWCMB CLS 350 CGI•

High PGM Cost

•

Narrow Operating Temperature Window

•

Sulfur Poisoning and Desulfation

•

Secondary urea tank with injection system

•

High urea consumption

•

Urea solution freezing

•

Need customer intervention

Lean NOx Trap (NOx Absorber Catalyst)

Urea SCR

Optional

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New Concept: Passive NH3

SCR

Use rich pulses to generate NH3

on the TWC and store it on the SCRRich: NOx + CO/H2

NH3

Use the stored NH3

for subsequent lean NOx

conversionLean: NOx + NH3

N2

Rich AFR NOx must at least balance Lean AFR NOx

LSF

LSULSU

LSF

Flex

TWC

SCR

No PGM required

DEER Conference 2009

Goal of the Work

Proof of Concept

Ability to make NH3

Ability to store NH3

and convert NOx

Catalyst Durability

High temperature redox aging

Aftertreatment System Architecture

Thermal management

5

Engine

2.2L stratified-charge direct-injection engine

Controller: d-SPACE with micro-Autobox

Transient Dynamometer : simulated NEDC cycle

Experimental

DEER Conference 2009

NH3

Formation over TWC

6

NO + 1.5H2

+ CO NH3

+ CO2

NH3

TWC OutNOx

EO

H2

EO

NOx limitedCO/H2

limited

H2

Eng. OutNOX

Eng. Out

NH3

TWC Out

12.012.212.412.612.813.013.213.413.613.814.014.214.414.614.815.0

Engine Air/Fuel Ratio

ppm

H2

0

ppm

NO

x, pp

m N

H3

DEER Conference 2009

Maximize Lean Operations During NEDC

7

Lean 

Operation

s

Stoich /rich 

Operations 

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NH3

Formation on the TWC Catalysts

Rich operations (AFR =14-14.2) during accelerations lead to NH3

formation; minimum FE penalty (within test variability) based on test cell data

NH3 Co

ncen

tration

DEER Conference 20099

Typical Thermal Operating “Windows”

with Aged Catalysts

LNT & SCR NOx

Conversion Capability

0

20

40

60

80

100

0 100 200 300 400 500 600

Catalyst Temp, Deg C

NOx Eff, %

NEDCCatalystTemps

LNT Range

SCR Range (w/Urea)

Passive SCR Range

Passive SCR conversion window about as broad as LNT, but at lower temperatures

Lacking steady NH3

supply, passive SCR NOx

conversion limited at high temperature

Effective NOX

conversion only possible in reduced window of operation

DEER Conference 200910

NH3

Storage Capacity Limitations –

Why passive SCR has smaller conversion window

Passive SCR is a discontinuous approach –

must store NH3

for use in lean periods

Storage capacity of known catalysts is negligible beyond 400 C

Effective NOX

conversion limited by intrinsic storage capacity

Cu 

SCR

Fe SCR

~Zero Storage 

capacity beyond 

~400C

100 200 300 500400

NH3Storage capacity

Catalyst Temperature / C

DEER Conference 2009

Multiple SCR Architecture

11

Operating temperature window can be expanded with a multiple-SCR architecture

SCR1

SCR1SCR2

SCR2

TWC

DEER Conference 2009

NOx

Conversion Efficiencies over SCR

12

TWC Out NOx

SCR Out NOx SCR1 Bed T

Passive SCR 

Conversion 

Window

DEER Conference 2009

NOx Conversion During EUDC

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Multiple‐SCR architecture required to reduce NOx emissions during high speed 

(>100km/h) lean cruise during EUDC

Post TWC

Post SCR1

Post 

SCR2

NO

x C

once

ntra

tion

DEER Conference 200914

NEDC Test Results with NH3 SCR

Passive NH3

SCR has potential to meet Euro6 emission targets with aged converters

Minimum fuel economy penalty for NH3

formation during NEDC (within test variability)

No significant slip of secondary emissions (NH3

, N2

O)4‐1 exhaust manifoldPUP Converter (aged RAT H 50h)4L SCR (aged RAT 750 50 h)

NH3

formation and OBD control methodSulfur impact: sulfur is known to greatly inhibit NH3

formation on TWCHC emission reduction is challenging due to the low exhaust temperaturesFurther improvements in SCR catalyst technologies are required for high speed (≥100 km/h) lean operations.

Remaining Issues

DEER Conference 2009

Summary

Passive NH3

SCR has been demonstrated as a high efficiency and low cost alternative lean NOx

aftertreatment

technology for stratified gasoline engines.

Very high NOx

conversion efficiencies were achieved during NEDC transient cycles.

Multiple SCR architecture expands the operating temperature window of passive SCR

Plan to further develop the system for US applications (SULEV)

15

Passive NHPassive NH

33

SCR: key enabler for lean gasoline enginesSCR: key enabler for lean gasoline engines