10. SI and CI Emissions
•
•
•
•
•
•
•
•
Sources of Emissions
Nitrogen Oxides
Hydrocarbons
Carbon Monoxide
Particulate Matter
Emissions Control Strategies
Regulatory Efforts
Aftertreatment Devices
10-1
Primary Pollutants
•
Oxides of Nitrogen (NOx)
– Result of unwanted secondary reactions
– Affect respiratory tract, increase airway resistance and damage lung tissue
•
•
Carbon Monoxide (CO)
– Result from oxygen-deficient combustion of carbonaceous materials
– Toxic effects spring from the greater affinity that hemoglobin has for CO than O2
Unburned Hydrocarbons (HC)
– Fuel molecules that survive combustion or arise through partial oxidation
– Encompasses hundreds of compounds (alkynes, alkenes, alkanes, aromatics, etc…)
– Can be carcinogenic, irritates the mucous membranes
•
Particulate Matter (PM)
– Born in soot nuclei of oxygen-deficient core of fuel sprays and grow in size through
incorporating gas phase molecules
– Very important in C.I. engines, also pertinent in gasoline direct-injection engines
– Many particles small enough to bypass the respiratory system defense, can slow
ciliary function and cause bronchitis
•
Oxides of Sulfur (SOx)
– Sulfur in fuel converted to gas
– Also adsorbed by PM
10-2
Secondary Pollutants
• Acid Rain
– SOx and NOx are precursors to acid rain
– Acidification of lakes and soil damage,
forest die-back
Simplified Acid
Rain Scheme
• Smog and Troposphere Ozone
– NOx and HC will react in the presence of
sunlight to form
– Irritation of the eyes, nose and throat,
impairment of lung function, coughing,
nausea and chest discomfort
– Hagen-Schmidt (1950’s) - mechanism of
Los Angeles smog
– NO2 is the key factor that starts the
reactions for photochemical smog in the
atmosphere:
NO2 + hn → NO + O
O + O2 + HC → HC + O3
Smog and Ozone Reaction Scheme 10-3
Greenhouse Gases
• Carbon Dioxide (CO2)
– Inevitable result of burning fossil fuels,
can only be restricted by reducing fuel
consumption
• Nitrous Oxide (N2O)
– Result of unwanted secondary reactions
• Methane (CH4)
– Relatively stable hydrocarbon, so it does
not participate in atmospheric reactions
(hence not regulated with HC)
10-4
Sources of Emissions
Bulk of the pollutants in the exhaust are associated
with combustion, but there are other sources
Problem for
carbureted
engines
HC
CO
Nowadays, we are
typically left with
only the emissions
that result from
combustion and incylinder processes
HC
CO
Ring clearances have been reduced, any blow-by will be trapped
in a carbon canister and recycled into the intake system
Incomplete combustion,
rich combustion,
dissociation,
chemical kinetics
HC, CO
CO2, N2O
PM
NOx, SOx
10-5
Effect of F/A on Pollutant Concentration
• Faced with the realities of the
combustion process
• No matter how good a combustion
chamber you design and no
matter how well you mix the fuel
and air, there will be emissions
issues lean, stoichiometric and
rich
– Dissociation, chemical kinetics,
crevices, flame quenching, etc…
10-6
SI Pollutant Formation Mechanisms
Deposits
absorb HC
NOx formed in CO forms under high
temperatures
high temperature
and if fuel rich As burned gases cool,
burned gas
NOx chemistry freezes
first, then CO chemistry
Deposits
desorb HC
Oil layers
absorb HC
HC flow to crevices
Unburned mixture
forced into crevices
Oil layers
desorb HC
Outflow of HC
from
crevices (some
burns to form CO)
Piston scrapes HC
off walls
(expansion)
(exhaust)
End gas source of
HC if incomplete
combustion
(compression)
(combustion)
Boundary conditions and crevices are setup during the compression,
combustion and expansion processes which may create sources of emissions
10-7
NOx: Equilibrium vs. Kinetic Rate-Limited
• NOx (NO and NO2) forms primarily via slow chemical
reactions in the high temperature burned gases
• In engine combustion chambers, values of TB and p
change very rapidly, crank-angle by crank-angle, before
equilibrium can be reached for species that form via slow
chemical reactions (e.g. NO and N).
SI burned and
• Assume that O, OH, O2 and N2 equilibrate instantly.
unburned zones
– [O]e, [OH]e, [O2]e and [N2]e values are realistic.
• However, NO and N have relatively slow kinetics.
– [NO]e and [N]e values are reference, long-term equilibrium
concentrations not reflective of instantaneous [NO] and [N]
in engine chamber
– The latter should be determined from rate-controlled kinetic
schemes (e.g. the extended Zeldovich scheme)
– If [NO] < [NO]e then NO is being formed
– If [NO] > [NO]e then NO is being eliminated
10-8
Nitrogen Oxide
• Extended Zeldovich Mechanism for NO formation:
production and destruction of NO
d  NO
 k1 O N 2   k2  N O2   k3  N OH 
dt
(1)
 k1  NO N   k2  NOO  k3  NO H 
k1
Zeldovich
N2  O 
NO  N

k1
k2
N  O2 
NO  O

k2
Heywood
k3
where [ ] species concentration in mole/cm3
and k’s have strong temperature dependence
N  OH 
NO  H

k3
The k terms are often called Arrhenius
rate expressions and are an
exponential function of temperature:
production and destruction of N
d  N
 k1 O N 2   k2  N O2   k3  N OH 
dt
 k1  NO N   k2  NOO  k3  NO H 
Assuming [N] quasi-steady, set
Then substitute result in (1).
(2)
 E 
k  A exp  a 
 RuT 
d N
 0 and solve for [N] in (2).
dt
10-9
Nitrogen Dioxide
• NO formed in the flame zone can be rapidly converted to
NO2
NO  HO  NO  OH
2
2
• Reversion back to NO can occur
NO2  O  NO  O2
– Unless NO2 formed in flame is quenched by mixing with cooler
fluid
– Explains high NO2/NO ratios occurring at light loads in diesels
• NO2 can be 10 to 30% of total nitrogen oxide (NO)
emissions in diesel engines, especially at low load
10-10
Illustration of SI Engine NOx Formation Model
From Flame Traversing Charge not all
combustion happens at same time
compression
each element
that burns has
different temp
history
near spark plug
expansion
in end gas
unburned
Measured cylinder pressure
(p) and calculated mass
fraction burned (xb)
The first
elements that
burn form the
bulk of the
NO
emissions
because of
time and
temperature
Calculated temperature of unburned
gas (Tu) and burned gas (Tb) in earlyand late-burning elements
Once NO is
Calculated NO
concentrations in formed it needs
time and high
early- and latetemperatures to
burning elements for react back into
O2 and N2 –
rate-controlled
leads to frozen
model and at
NO
equilibrium
10-11
Summary of SI NOx Formation
• Nitric Oxides (NOx) form throughout the high-temperature burned
gases behind the flame front, through chemical reactions involving
nitrogen and oxygen atoms and molecules, which do not attain
chemical equilibrium
– Flame front is negligible with respect to mass that it does not
significantly contribute to NOx emissions
• The higher the burned gas temperature, the higher the rate of
formation of NOx
• The first mixture element to burn is compressed considerably and
thus reaches the highest burned gas temperature. Thus, the early
burned mixture contributes much more to NOx formation than the
later burned mixture
• As the burned gases cool during the expansion stroke (below
2000 K), the reactions involving NOx freeze and leave NOx
concentrations far in excess of levels corresponding to equilibrium
at exhaust conditions.
10-12
Effect of A/F and EGR on SI NOx Emissions
EGR acts as
a diluent and
reduces the
flame
temperatures
which leads
to lower NO
production
(Chapter 4)
Excess
oxygen and
high
temperatures
leads to NO
formation
(Chapter 4)
Variation of exhaust NO
concentration with A/F and
fuel/air equivalence ratio.
We have already discussed
the theory and what should
happen, now we see
experimental verification
Variation of exhaust NO
concentration with percent
recycled exhaust gas (EGR)
(1600 rev/min, v = 50 %, MBT timing)
10-13
Effects of Spark Timing, A/F, Coolant
Temperature and Deposits on SI NOx Emissions
Compressing early burned elements
more – will see higher pressures and
temperatures for spark advance
As coolant temperature increases, less
possibility for heat transfer to take energy away
from gas – leads to hotter chamber as a whole
deposits can act as insulation
Variation of exhaust NO
concentration spark retard.
(1600 rev/min, v = 50%; lefthand end of curve corresponds
to MBT timing for each A/F)
Increased coolant
temperature or presence of
deposits reduce heat losses,
both of which increase NOx
10-14
SI NOx Reduction Strategies
Brake specific HC and NOx
emissions (relative basis) versus
air-fuel ratio
Effect of exhaust gas recirculation and
NOx reducing catalyst on relative brake
specific NOx emissions
10-15
Direct-Injection NOx Formation
• SI analysis can be directly
applied to a diesel engine
– NOx still forms in burned gas
Lean flame-out
Volume: HC
Initial rapid
combustion:
noise
Fuel jet
mixing with
air: rich
mixture
Burned gas: NO
Premixed
Burned gas: NO
White/yellow flame:
soot oxidation
Rich zones in
fuel jet:
soot formation
Flame quench on
walls: HC
Fuel vapor
from nozzle
sac volume
Mixing controlled
10-16
Unburned Hydrocarbon Mechanisms
•
•
•
•
•
•
•
•
•
Crevice Flows
Oil Layer Absorption/Desorption
Wall Wetting/Fuel Puddles
Combustion Chamber Temperature/Deposits
Exhaust Gas Temperature
Partial Burn, Quenching and Misfire
Nozzle Sac Volume
Overleaning
Undermixing
10-17
HC Emissions from SI Engines- Sources
• Cartoon of
cylinder showing
origin of NO, HC
and CO
Exhaust port
burnup
Exhaust
Liquid fuel
cold start HC’s
HC’s desorbed
from oil layer
and crevice HC’s
Crevice outgassing
CO, HC
Bulk gas
Slow/partial burns NO and CO (rich)
CO, HC
 Wall effects
• Deposits
• Oil layers
• Liquid fuel during cold
start
 Crevice
• Trapping and release of
unburned mixture
 Bulk quenching
• Late and partial burns –
cold start/transient
• Can be minimized by
good fuel and air control.
10-18
Conceptual Sources of SI HC Emissions,
Including Crevice Volume Flows
From J. Eng, SAE 2005-01-3867
10-19
Crevice Flow Mechanism
•
•
•
•
•
Volumes 1 – 4 make up the crevice. The pressure in Volume 1
equals the cylinder pressure
Crevice gas temperature is approximately constant (from ~ 400 –
500 K), so to satisfy the ideal gas law, fuel and air fill volume 1 and 2
of the crevice as the cylinder pressure increases during compression.
There is some flow from 2 to 3 due to blowby during this time.
As peak cylinder pressure drops (post combustion ~ 730 deg), fuel
(HC) and air flow out of the crevice into the combustion chamber to
again satisfy the ideal gas law.
As the top ring lifts and seats on “B” at 760 deg, Volume 1 is isolated
from 2.
Beyond 760 deg here, the pressure in Volume 2 - 4 decreases due to
blowby (to Volume 5) and flow back into the cylinder via Volume 1
due to gas leakage across the top ring
Top Ring seats on “C”
(Isolates Volume 2 from 3)
Top Ring lifts,
seats on “B”
(Isolates Volume
1 from 2)
Top Ring seats on “C”
(Isolates Volume 2 from 3)
Mass flows into
the crevice from
the cylinder
Top Ring
(moves between
“B” and “C”)
Bottom Ring
(always
seated on “D”)
Top Ring lifts,
seats on “B”
(Isolates Volume
1 from 2)
10-20
Effect of Crevice Volume on HC Emissions
The larger the
crevice, the more
HC that can fit
into this area
Piston top-land
crevice volume
Eventually, it levels off.
Crevice becomes large
enough for flame to penetrate.
Effect of increasing top-land
clearance on exhaust
hydrocarbon emissions
Unthrottled SI engine (N = 6885 rev/min, A/F = 13, MBT timing)
10-21
Effect of Blow-by Flow on HC Emissions
• Effect of increasing crankcase
blow-by on exhaust
hydrocarbon emissions using
production pistons and rings.
• As blowby increases, the
exhaust gas HC emissions
decrease
• This results in part from flow out
of the crevice, into the
crankcase, instead of into the
combustion chamber
SI Engine (N = 1200 rev/min, A/F = 14.2, intake manifold pressure = 0.6 atm)
10-22
Fuel Puddle Formation (SI Engine)
• During port fuel injection, the
fuel injector spray is targeted
towards the intake valve
• Liquid films may form on the
intake valve and port as well
as inside the cylinder
• Fuel may vaporize too late for
combustion to burn completely
– Depends on temperatures of port
and cylinder walls
• In particular, this is a problem
for cold starting
10-23
SI Unburned HC Emissions due to
Deposit Growth for Various Fuels
• Deposits have been
shown to promote
unburned HC emissions
– Soak up HCs
– Release it late in expansion
when there is not a flame
• Experiment on right shows
that as deposits form over
time, the unburned HC
emissions increase
• Additives may help to
reduce deposit formation,
hence HC absorption
Doelling, et al. SI Engine, SAE Paper 720500 (1500 rpm,  = 1.1, S.T. = 15º BTC)
10-24
Effect of Combustion Chamber Surface
Temperature on SI HC Emissions
• The unburned HCs will go
down when operating with
hotter walls
– Flames will not get quenched
as easily
– Deposits may not form on hot
walls
– Clearances get smaller (more
expansion and less crevice
flows)
• However, may lead to
higher NOx emissions,
knock and loss in
volumetric efficiency (air
density)
Wentworth, SAE Paper 710587, SI Engine, SAE Transactions vol. 80
10-25
Effect of Spark Retard on HC Emissions
• Increased exhaust temperature because of lower
cycle efficiency
• Increased port burnup of the hydrocarbons
10-26
Partial Burn, Quenching and Misfire
Combustion Efficiency [%]
100
90
C.I. inherent combustion
efficiency advantage
80
Diesel
Spark Ignition
Simple Model
70
Fuel: Isooctane
p /p = 1.0
60
i
e
0% EGR
SI combustion slows with
dilution and may not be
complete by EVO for
high dilution levels
c,lean ~ 1
High levels of
CO and H2

c,rich ~ 1/
Lean
Rich
50
0.0
Figure 3-9 Heywood
0.2
0.4
0.6
0.8
1.0
Equivalence Ratio
1.2
1.4
1.6
10-27
Overleaning (CI Engine)
• Associated with Phase 0 and I
• Schematic of diesel engine fuel spray showing
equivalence ratio () contours at time of ignition.
– L = equivalence ratio at lean combustion limit (~ 0.3)
– Shaded Volume contains fuel mixed leaner than L
Air swirl
 > 1 (rich)
=0
 > L
Injection nozzle
Fuel jet boundary
Ignition location
=1
Overmixed HC
10-28
HC and Ignition Delay (CI Engines)
• Correlation of exhaust HC concentration with duration of
ignition delay for DI diesel engines.
• Various fuels, engine loads, injection timings and boost
pressures at 2800 rev/min
Too long of a delay,
there is too much
overleaning of the
mixture on the leading
edges. This will lead
to more flame-out and
higher HC emissions
Too short of a
delay, there is not
enough mixing
and more soot is
created.
10-29
Nozzle Sac Volume
• Effect of nozzle sac volume on exhaust HC
concentration, DI diesel engine at minimum ignition
delay
– Vd = 1 dm3/cylinder, N = 1700-2800 rev/min
10-30
Overfueling or Undermixing
• Effect of overfueling on exhaust HC concentration
– DI diesel engine, speed = 1700 rev/min, injection timing at full
load 15ºBTC
10-31
Carbon Monoxide
• Carbon monoxide forms during the combustion process
• With rich fuel-air mixtures, there is insufficient oxygen to
fully burn all the carbon in the fuel to CO2
• Partial oxidation of crevice and wall layer HC produce CO
• In high temperature products, even with lean mixtures,
dissociation ensures that there are significant CO levels
• Later in the expansion process (below 1500 K), CO
oxidation (to CO2) freezes as burned gas temperatures fall
• CO is fairly easy to oxidize in catalysts → not a big
problem in today’s engines as long as the catalyst is
warmed up
Drivers for CO Formation
* Equivalence Ratio
* Dissociation
* Reaction Kinetics
10-32
SI Engine Exhaust Composition as a
Function of Equivalence Ratio
Discussed
previously in
Chapter 4
10-33
Particulate Matter
• Mainly consists of organic compounds
that originate from unburned or partially
burned fuel and lubricant and sulphates
from the sulfur in the fuel.
• Formed predominantly during the
diffusion burning period, where within
the jet the local fuel-air ratios are rich.
• Rich combustion results in the
formation of carbon particles.
• Jacobs, T. J., Pd.D. Thesis,
• University of Michigan
(2005).
10-34
Particulates
• Any substance (other than H2O) that can be collected on
filter paper in the exhaust
• Consists of two parts:
– Solid Carbon (soot)
– Organic fraction (compounds that adsorb to soot)
 CO 
Soot Formation Model
first stage of combustion: C H   xO2
CO  O2  CO2
Cs   O 2  CO 2
1
2
second stage of
combustion:



H 2   x   O2
2
2

2 xCO    2 x  C s  

2
H2
Clean
2x  
Sooting
2x < 
H 2  12 O2  H 2O
Bosch Smoke Number
A scale that reflects reflectivity of filter paper
0.5 – no smoke
1 – clear
2 – light
4 – dark
8 – very-dark
9 – heavy smoke
10-35
Particulate Matter Conceptual Model
Flynn, P. F., et al. (1999) Diesel combustion: An integrated view combining laser diagnostics, chemical kinetics, and
empirical validation. SAE Paper No. 1999-01-0509
10-36
Particulate Matter Conceptual Model - 2
• HC and soot emissions are
formed within the diesel jet
during Phase II combustion
• Much of these species are
oxidized by the diffusion flame,
or during Phase III
• Those species that survive
oxidation are exhausted as soot,
HC and CO emissions
Image of Phase II Combustion
SAE Paper No. 1999-01-0509
10-37
Diesel Jet-Wall Interaction
• There is significant flame wall interaction during the
diesel combustion process
• We see OH in the diffusion flame (green) quenching as it
interacts with the piston bowl wall
• Soot (red), CO and hydrocarbons that are not oxidized
will be emitted as emissions
Piston bowl wall
SAE Paper No. 2001-01-1295
10-38
DI NOx and Soot Emissions Tradeoff
•
•
NOx is formed in high temperature
burned gases
Soot is formed in rich fuel zones within
the diesel jet
– Most, but not all will be oxidized in the
diffusion flame
• The NOx soot tradeoff is one of the
perennial challenges of diesel
combustion
Smoke (FSN)
– As peak temperature is reduced to
decrease NOx, smoke emissions
increase
Nishimura et al.
SAE 981929
10-39
Emissions Control Strategies
• Improved Mixture Preparation
• Improved Combustion Process
• Improved Engine Design
• Optimizing Engine Operating Parameters
• Improved Control Strategies and Algorithms
• After treatment Systems - if we do not succeed
in-cylinder
10-40
Tailpipe Regulations
CO: 3.4 g/mi
Tier 0 : 0.41 HC, 0.7 NO , 7.0 CO, 0.2 PM (1990)
x
Tier 1 : 0.25 HC, 0.4 NO , 3.4 CO, 0.08 PM
x
0.2
TLEV: 0.125 HC, 0.4 NO , 3.4 CO, 0.08 PM
x
LEV: 0.075 HC, 0.2 NO , 3.4 CO, 0.08 PM
CO: 3.4 g/mi
x
ULEV: 0.04 HC, 0.2 NO , 1.7 CO, 0.04 PM
x
0.1
CO: 3.4 g/mi
LEV II: 0.075 HC, 0.05 NO , 3.4 CO, 0.01 PM
x
ULEV II: 0.04 HC, 0.05 NO , 1.7 CO, 0.01 PM
x
0
CO: 1.0 g/mi
SULEV: 0.01 HC, 0.02 NO , 1.0 CO, 0.01 PM
CO: 1.7 g/mi
0
x
0.2
0.4
0.6
NOx Emissions [g/mi]
0.8
1
Emission Standards USA Heavy-Duty Diesel Truck Engines [g/bhp-hr]
12
0.7
10
NOx/NMHC+NOx
0.6
PM
0.5
8
0.4
6
4
2
0.2
0.1
0
1988
0
0.3
Particulate Matter
– SI and CI
0.3
LEV II
ULEV
ULEV II
SULEV
NMHC: 0.5
• Effectively zero in 2010
and beyond
Tier 0
Tier 1
TLEV
LEV
Nitrogen Oxides
2004: Non-methane hydrocarbons + NOx
– ZEV: Zero-emission
vehicle
CO: 7.0 g/mi
0.4
HC Emissions [g/mi]
• Emission regulations are
getting more and more
restrictive
• California leads the way
California Air Resources Board Emission Standards (Passenger Car)
0.5
0.0
1990
1
1991
2
1994
3
1998
4
2004a
5
2004b
6
2007
7 2010
10-41
Worldwide Emission Regulations
US07/10 NOx emissions are
typically more stringent
10-42
Modern Three-Way Catalyst
• Consists of a combination of Pt, Rh
and Pd noble metals
• Three-Way Reduction of HC, CO
and NOx
• Washcoat provides high surface
area of catalytic material
– Can also enhance reactions and
stabilize the noble metals
– Cerium often used
• Small channels cause laminar flow
– Increases residence time
– Negligible pressure drop
CO  12 O2  CO2
HC  O2  CO2  H 2O
CO  NO  CO2  12 N 2
Ce2O3  NO  2CeO2  12 N 2
10-43
Effect of A/F on SI Three-Way Catalyst
Conversion Efficiency
• Around stoichiometry
we have CO, HC and
NOx due to incomplete
combustion and
dissociation
• Any left over oxygen
can be used to convert
CO and HC
• NOx is also a great
oxidizer of CO and HC
The conversion efficiency of a catalyst is the ratio of
the rate of mass removal in the catalyst of the
particular constituent of interest to the mass flow
rate of that constituent into the catalyst: e.g. for HC
cat ,HC 
mHC,in  mHC,out
m
 1  HC,out
mHC,in
mHC,in
10-44
Fuel Flow Calculation: Closed Loop
• Requires EGO Sensor or UEGO
• Reasons
– Stoichiometric Fuel Delivery for Three-Way Catalyst Efficiency
– Adaptive Fuel Learning Based on Component Tolerances
While we are trying to
do the best job we can
initially (open loop)
there will always be
changes/errors in the
sensors and injectors
that require constant
adjustment
Exhaust Gas Oxygen
(EGO) sensor gives us a
catch-up opportunity to
look for imbalances in O2
due to under fueling or
over fueling events
Can adaptively learn
which cylinders might
have a problem (C-T-C
variability)
10-45
Direct-Injection Lean-Burn Engines?
– Cannot use TWC NOx
reduction reactions
lean
0.0140
0% EGR
10% EGR
20% EGR
30% EGR
0.0112
Mole Fraction NO
• Better fuel economy
• Reduction in CO2
emissions as a byproduct
• CO and HC emissions
nearly negligible
• Soot now becomes a
problem
• NOx issues
Increasing EGR
Effect
0.0084
0.0056
0.0028
Fuel: Isooctane
Initial Temp: 700 K
Initial Pres: 10 bars
0.0000
0.0
0.2
0.4
0.6
0.8

1.0
1.2
1.4
1.6 10-46
Controlling NOx Emissions – Fundamental
Difficulties of Diesel Engines
• High thermal efficiency and compression ratios of diesel
cycle cause high peak gas temperatures.
• NOx – Particulate trade-off
– Suppression of NO formation has a tendency to increase
particulates and vice-versa.
• Excess oxygen in exhaust prevents the use of standard
stoichiometric TWC technology for the reduction of NOx
10-47
Diesel Oxidation Catalyst
• Used excess O2 to reduce any CO and
HC that exits the engine
– SI: incorporated excess air injection
– CI: excess air is already present!
• Produces NO2 for use with LNT/SCR
devices
• Will reduce SOF of PM
Diesel Oxidation Catalyst
CO, HC and SOF Conversion
Clean-up of Regeneration Event
NO2 Production from NO for DPF
CO  12 O2  CO2
HC  O2  CO2  H 2O
NO  12 O2  NO2
10-48
Ammonia Selective Catalytic Reduction
Urea Injection
• One possible way to handle NOx
– NH3 great reducer for NOx
– Vanadium/Zeolite catalysts like to store
NH3
• Industrial use of NH3 SCR dates back
to 1970s
T
NH 2  CO  NH 2 
 NH 3  HNCO
HNCO  H2O  NH3  CO2
– Used homogeneous reactions because of
higher temperatures
• Automobiles use urea injection in
exhaust stream
• Relatively easy to control
– Optimize engine for performance
4NH3  4NO  O2  4N2  6H2O
4NH3  2NO  2NO2  4N2  6H2O
4NH3  3NO2  72 N2  6H2O
4NH3  6NO  5N 2  6H 2O
10-49
Ammonia SCR Issues
•
•
•
•
Packaging
Tampering Issues
Consumer Conception
Urea Infrastructure
– Europe “AdBlue” urea availability project
– Mercedes Bluetec with urea charge
• Cost not directly factored into mpg
– Urea takes energy to make
– Well-to-wheel efficiency
10-50
Lean NOx Trap
• Two Modes of Operation
– Lean: NOx storage on Barium or similar material
• NO2 is preferred storage species
– Rich: NOx regeneration through TWC type of
reactions
• Platinum Group Metal (PGM) often incorporated
• Rich
– To regenerate trap need CO, H2 or HC to
promote TWC reactions
– Two options:
• Late in-cylinder fuel-injection
• Fuel-injection in exhaust stream
NOx Adsorption Catalyst
– Take heavy HCs and break down into species
more suited for regeneration
NOx Storage During Lean Operation on
Alkali or Alkali Earth Metal
CO, HC and NOx Conversion over PGM
During Rich Operation (Regeneration)
Lean Operation
Rich Operation
NO  12 O2  NO2
Ba  NO3  2  13 C3H 6  BaCO3  H 2O  2NO
BaCO3  2NO 2  12 O 2  Ba  NO3  2  CO 2
CO  NO  CO2  12 N 2
10-51
Lean NOx Trap Issues
• Very Hard to Control
• SOx Likes to Store More Than NOx
– EPA legislated sulfur in fuel (15 ppm as of 2007)
• Regeneration Species
– Likes CO and H2 for regeneration
– Late in-cylinder injection has high temperatures and excess O2
• Breaks down fuel to proper components
• Will also oxidize and lower yield
– Post cylinder injection
• Temperatures lower and less oxidation possible
• More heavy HCs into LNT
• Direct Fuel Penalty
10-52
Particulate Filter
• Alternating Flow Channel Pattern
– Exhaust travels through a porous wall
• Stores PM on Inlet Channel Walls
• At high enough temperatures, soot
oxidizes and provides large exothermic
reaction
• NO2 reduces the light-off temperature of
soot
• Add catalytic material to wall to promote
NO2 generation and other reactions
• Nearly 100% Effective at Trapping Soot
• Issues:
– Increasing pressure drop with soot loading
– Possible runaway combustion reaction
– Ash accumulation
• In use for all 2007 Diesel engine
platforms
Diesel Particulate Filter
Deposit Collection and Oxidation
by O2 and NO2
10-53
Catalyzed DPFs Available