Background Clark - NOx Fuel Tradeoff.pdf · Source: WVU chassis data – CRC Presentation •...

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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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

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NO

x (g

/s)

CO2 (g/s)

US FTP versus On-Road Operation

-80.00

-60.00

-40.00

-20.00

0.00

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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

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40.00

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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

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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”

US Measures to Insure Broader NOx Reduction

Source: Clark SAE Keynote

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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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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Pos

t SC

R E

xhau

st T

emp

(C)

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Cold Warm Hot #1 Hot #2 Hot #3 Hot #4

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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.