Low Temperature Aftertreatment for Future Engines – Challenges and Opportunities
1
Kushal Narayanaswamy Propulsion Systems Research Lab
General Motors Global R&D, Warren, MI
Acknowledgements – Paul Najt & Chang Kim
Is it needed?
Is high possible??
Is it affordable???
2
Drivers for Next Generation High-Efficiency Low-Temperature Aftertreatment
Global Fuel Economy Requirements
US 54.5mpg in 2025
EU 58mpg in 2020
CN 56mpg in 2020
Criteria Emission Regulations
Challenges
Exhaust Temperature
Fuel Economy Improvement
Enabling Cost-Effective
Aftertreatment Technology
3
Exhaust Oxygen N2O, Sulfur Content
PGM Management
Thermal Durability
(Robust Aging)
Particulates
The Exhaust Temperature Challenge*
4 * Excluding cold start
REDUCED EXHAUST TEMPERATURE
1.
Downsize Boost Lean: The use of lean-burn enhances efficiency, but dramatically reduces exhaust temperatures, impeding catalyst performance at light loads
Downsize Boost: Energy extracted by turbocharger lowers exhaust temperature
Downsize Boost w/EGR: Addition of high levels of EGR lower exhaust temperatures degrading catalyst performance at light loads
Manage peak temperature to minimize catalyst
performance loss & thermal degradation 2.
Downsize Boosted Lean Exhaust Temperatures
5
Ex
ha
us
t Te
mp
era
ture
(°C
)
100
200
300
400
500
600
0 NMEP (kPa) 85 85 134 267 296 329 382 414 433 676
800 1875 2625 2250 1900 3000 1312 2625 2250 2440 RPM
Exhaust Port
Turbo Out
Under Floor Min. T for HC
FTP Points
(max efficiency calibration)
Fuel Penalty for Raising Exhaust Temperature
~ 2 - 3% fuel penalty is estimated to raise the exhaust temperature by 50 °C
6
0
2
4
6
8
10
12
14
16
18F
ue
l P
en
alt
y (
%)
T after Cold Start (°C)
With Incidental Heat Loss of 20%
Without Heat Loss
Strategy Catalyst
Temperature
> 300 °C
Fuel Penalty
Stop/Start + -
Stop/Start with
Aggressive
DFCO
+ - -
Lambda/split ++ +
EHC during
idle
++++ ++
EHC during
deceleration
+++ +
EHC during
deceleration +
Stop-Start
+++ -
Exhaust Thermal Management
6/6/2013 7
.
CAT 1
CAT 3
CAT 3
CAT2
.
CAT 1 CAT 2 CAT 3
.
CAT 1
CAT 2
CAT 2
CAT 3
CAT 3
CAT 1
CAT 2 CAT 3
EHC = Electrically Heated Catalyst
Limitation of Low Temperature Performance
NO
x &
Te
mp
Tail Pipe NOx SCR In Temp
0 250 500 750 1000 1250
NO
x &
Te
mp
Time (sec)
Temperature Sensitivity of Ammonia-SCR for NOx Efficiency
Optimal Temperature for SCR
Optimal Temperature for SCR T ~ 50°C
FTP75
Oxidation Efficiency Limit over Conventional PGM-based TWC
A
ctivity
(GM-POSTECH, Chemical Engineering Journal 2012)
8
Can models help address the low temperature challenge?
PGM Content and Thermal Durability For Lean-Burn Exhaust Condition
C3H
6
9
0
0.1
0.2
0.3
0.4
0.5
150 175 200 225 250 275 300
Co
nve
rsio
n
Inlet Temperature (°C)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
100 125 150 175 200
Co
nve
rsio
n
Inlet Temperature (°C)
(GM-PNNL, Catal. Today 2012)
0%
20%
40%
60%
80%
100%
150 250 350 450 550
NO
to
NH
3co
nve
rsio
n
TWC inlet T (°C)
Pd Pd/Pd+Ce Pd+Ce LNTTWC formulation:
Pd-only
LNT-only
(ORNL, DEER 2012)
The Exhaust Oxygen Challenge
EXHAUST OXYGEN CONTENT
Conventional TWC - Poor NOx efficiency
with DFCO / Lean
Urea-SCR - Secondary urea tank
with injection system;
high urea consumption
for gasoline
- Urea Solution Freezing
Lean NOx Trap - High PGM Cost
- Sulfur Poisoning
- Desulfation Required
Passive Ammonia SCR System
PASS - How Does It Work?
TWC SCR
Use H2 and CO to generate NH3 over
TWC and store NH3 in multiple SCRs Use the stored NH3 for lean NOx
conversion
NOx + H2/CO NH3 + CO2 NOx + NH3 N2 + H2O
DURING RICH: DURING LEAN:
Urea-Free SCR System
NH3
NOx
RICH
LEAN
H2O
N2
FTP Cycle Results from Homogeneous Charge Application – with DFCO & Lean Idle
Post-TWC NOx
Tailpipe NOx
TWCConverter
WRAF
SCRConverter
TWC SCR
NH3
NOx
AFR Toggling
DFCO / Lean-idle
NH3 produced over TWC with
simple AFR toggling allowed SCR
to remove NOx effectively
Tier2
Bin3
50
100
NEDC Results from Stratified Charge Application – Extended Lean Operation
Post-TWC NOx
Tailpipe NOx
TWCConverter
WRAF
SCRConverter
TWC
Multiple SCRs
NH3/NOx ratio is always greater
than 1 over SCR for maximum
conversion efficiency
100
Veh
icle
Sp
eed
(km
/h)
Euro VI
Cumulative N2O Comparison: TWC vs LNT
14
0
150
300
450
600
750
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
0 200 400 600 800 1000 1200
Ve
hic
le S
pe
ed
(k
m)
Cu
m N
2O
(m
g/k
m)
Test Time (s)
TP N2O with TWC only
TP N2O with TWC+LNT
Vehicle Speed (km)
~40% lower N2O was produced over TWC only aftertreatment
architecture (e.g. PASS)
Particle Size Distribution – Gasoline vs Diesel
• SIDI particle size distributions are shifted to the smaller end compared to diesel
• SIDI soot aggregates tend to be less compact than diesel particles (relevant to filtration)
• SIDI soot resulting from all fuels tested had very high organic content which was tightly
integrated with the inorganic carbon (relevant to regeneration)
UW-PNNL-GM, 2012 DEER
Vehicle Telematics and Aftertreatment
6/6/2013 16
Summary and Future Research Needs • Available exhaust energy is going down as engine efficiency
improves
– Increased constraints on aftertreatment system
– Engineering solutions can take you only so far
• Fundamental experiments and modeling needed to address key knowledge gaps pertaining to low temperature aftertreatment
• Key areas of research include – Surface chemistry and physics for high-efficiency, low-temperature
catalysis and filtration – Interactions between reaction and diffusive transport phenomena to
enable low back-pressure, high filtration, and reduced gaseous emissions in a single device
– Robust models providing insights on performance and methods of optimizing configurations
6/6/2013 17
18
Thank You!
