![Page 1: Background Clark - NOx Fuel Tradeoff.pdf · Source: WVU chassis data – CRC Presentation • Chassis dynamometer test data from 2007 Cummins ISL 320 over OCTA driving schedule •](https://reader035.vdocuments.us/reader035/viewer/2022071212/602569fdd6660a4c3136c204/html5/thumbnails/1.jpg)
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Background × NOx and PM Standards have driven diesel engine
design for two decades × Test methods have evolved over that time
× Manufacturers have adopted efficiency initiatives where customer return on investment demands would be satisfied
× Climate change and energy concerns have now initiated vehicle efficiency standards
× The emissions and efficiency requirements are not fully aligned
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Progression of Complexity in US Diesel Engine Controls
(NOx & PM Reduction) × Mechanical Injection
× Electronic Injection (Injection Timing Authority)
× Boost Management (Wastegate, Electronic Wastegate)
× Cooled Exhaust Gas Recirculation
× Multiple Injections
× Oxidation Catalysts (some buses)
× Diesel Particulate Filtration
× Urea Selective Catalytic Reduction
× Increasingly Sophisticated Control From Clark 2011 Fall ASME ICE Keynote
Progression differed in Europe – parallel SCR & non-SCR (EGR) tracks for low NOx
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History of NOx – Efficiency Tradeoff
× Unregulated on-road mechanically injected diesel engines typically produced 10-15 g/kW hour NOx × Optimal engine efficiency × Manageable component
temperatures × Simple injection systems
× Modest NOx reductions were possible simply by retarding injection timing × Loss of efficiency × Approach used to reach about 5 g/
kW hour NOx
NOx = 0.0165 CO2 - 0.0558
R² = 0.7448
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 20 40 60
NO
x (g
/s)
CO2 (g/s)
Data Source: WVU chassis dynamometer data
Advancing timing on mechanical Mack Engine CO2 (g/bhphr) -- -15% -12% -7% 11% NOx (g/bhphr) -- 61% 131% 183% 270%
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NOx-CO2 Relationship: Electronic Injection and EGR
× Chassis dynamometer testing of OTR tractor
× Early EGR management
× NTE not yet enforced
× High scatter of NOx relative to CO2: linear relationship is lost
× Data Source: CRC E-55/59 Program
5
NOx = 0.0046 CO2 + 0.0082 R² = 0.6578
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 20 40 60 80
NO
x (g
/s)
CO2 (g/s)
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US FTP versus On-Road Operation
-80.00
-60.00
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
-40.00 -20.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00
Speed
Torque
-80.00
-60.00
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
-40.00 -20.00 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00
Speed
TorqueSource: Thesis research
Radermacher, WVU
FTP emphasizes operation near rated speed and torque
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“Off-Cycle” Data Regain efficiency and protect engines
y = 0.0043x + 0.1906R2 = 0.7104
y = 0.0012x + 0.1049R2 = 0.3387
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 50 100 150 200 250 300 350
Dispersed and Time Aligned Axle Horsepower (ahp)
NOx
Emiss
ions
(g/s)
Cruise
Transient
Source: WVU data – Clark SAE Keynote
Steady-State “Post Holes”
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US Measures to Insure Broader NOx Reduction
Source: Clark SAE Keynote
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NOx – Fuel Interactions × Cases where both NOx and CO2 are reduced together
× Reduced friction / Better lubricants × Reduced load (e.g. lighter vehicle)
× Cases where both NOx and CO2 increase together × DPF Regeneration × Exhaust back pressure
× Cases where NOx and CO2 trade off × Retarded timing × Exhaust gas recirculation
× Indicated efficiency × Pumping work
× Complex cases (e.g. enable reduction but demand power) × EGR cooling demand × Multiple injections & rate shaping × High pressure injection × Downspeeding / managing powertrains / hybrid technology
× Upstream implications of urea × “Driving to find urea!”
× Choice of units / engine & vehicle / engine sizing
Krishnamurthy et al. (Atmos. Environ 2007) show ~10% fuel use increase for NOx reduction from 5 to 2.5 g/bhp-hr standard (US 1995 to 2002)
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Causes of Measurement Variability
× Sensitivity of EGR & timing strategy to transient operation
× Effects of changing exhaust backpressure with DPF
× DPF regeneration fuel use
× Cold start strategies
× SCR thermal effects & control effects
× Increasing difficulty in quantifying very low levels
× Hybrid operation brings additional complexities × See SAE J2711 × Powertrain controls to a lesser degree
× Vehicle-based efficiency measurements and modeling results cannot characterize small efficiency differences accurately
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DPF Regeneration raising NOx and CO2
NOx = 0.0027 CO2 - 0.0017 R² = 0.6549 (normal)
NOx = 0.0028 CO2 + 0.0028 R² = 0.6317 (regen)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 10 20 30 40 50
NO
x (g
/s)
CO2 (g/s)
Normal Regen
Source: WVU chassis data – CRC Presentation
• Chassis dynamometer test data from 2007 Cummins ISL 320 over OCTA driving schedule
• REGENERATION DOES
NOT INFLUENCE NOx-CO2 relationship substantially, but both have highest values during regeneration
• Data show that relationships between NOx and power and CO2 and shaft power are affected.
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OCTA Driving Schedule NOx over Three Repeat Runs per Bus
(Data from CAFEE Database, DOE, DOT & LYNX data)
0 5
10 15 20 25 30 35 40 45
NO
X (g
/mil
e)
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OCTA Driving Schedule NOx Variability over Three Repeat Runs per Bus
(Data from CAFEE Database, DOE, DOT & LYNX data)
0% 2% 4% 6% 8%
10% 12% 14% 16% 18%
NO
X M
easu
rmen
t Coe
ffic
ien
t V
aria
nce
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EGR and SCR × To meet 2002-2010 2.5 g/bhp-hr and 1.2 g/bhp-hr NOx
heavy-duty on-road levels, the US manufacturers opted for in-cylinder and injection controls, and for cooled EGR.
× Euro IV levels were met by using either SCR or EGR in Europe. [Erkkila & Nylund report shows SCR offers better efficiency in most cases].
× Post-2010 US and Euro VI employ SCR, which may be used with or without EGR.
× SCR accommodates higher engine-out emissions, usually offering an efficiency gain, but SCR must be active to reduce NOx. [US studies of school bus efficiency favor SCR].
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Test-to-test Variability: Urea SCR
15
0
50
100
150
200
250
300
0 200 400 600 800 1000 1200
Pos
t SC
R E
xhau
st T
emp
(C)
Time (s)
Cold Warm Hot #1 Hot #2 Hot #3 Hot #4
0
2
4
6
8
10
12
14
16
0 200 400 600 800 1000 1200
Inte
grat
ed N
Ox
(gra
ms)
Time (s)
Cold Warm Hot #1 Hot #2 Hot #3 Hot #4
• 2012 OTR Tractor • 2011 Diesel engine (Mack MP8)
• Urea-SCR exhaust aftertreatment
WVU Data – CRC Presentation
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Hot- and Cold-start NOx with SCR
Hot Start Emissions Cold Start Emissions
Source: SAE 2011-01-2469 Clark, McKain, Wayne, Carder & Gautam, WVU
2010 30 foot Transit Bus – Paris Cycle
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NOx below 0.2 g/bhp-hr in the US
× California has funded a study aiming at 0.02 g/bhp-hr NOx from heavy-duty engines (additional 90% reduction) × Diesel × Natural Gas
× Different possible pathways for diesel × More intensive cooled EGR with SCR (reduced engine-out) × Increasingly sophisticated SCR (sensors and models) × Aggressive injection of reductant with cleanup
× Ammonia concerns
× Roberts (2011) and Johnson (2012) have discussed engine-out vs. aftertreatment tradeoffs
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Summary & Observations × NOx reduction through retarded timing and cooled EGR reduces
engine efficiency. × Some EGR cooling burdens are not measured in a test cell
× SCR has offered a pathway to recovering efficiency, but urea is now required. × Urea has an upstream footprint and cost × Emissions will be high if the catalyst is inactive
× Present pathways suggest that further NOx reduction will imply engine efficiency loss and/or higher urea usage.
× Regulatory tools are not fully aligned with on-road use and are about to face an information and control onslaught.
× Low emissions levels are hard to measure. Small changes in efficiency are hard to measure.