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TRANSCRIPT
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
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
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
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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%
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
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0.15
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0.25
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0.35
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0 20 40 60 80
NO
x (g
/s)
CO2 (g/s)
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
“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
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0.6
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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”
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)
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
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)
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0.02
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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.
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)
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
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].
Test-to-test Variability: Urea SCR
15
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50
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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
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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
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
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
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.