Research and Advanced Engineering

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The Influence of Fuel Cetane Number on Catalyst Light-Off Operation in a

Modern Diesel Engine

2nd CRC Advanced Fuel and Engine Efficiency Workshop Nov 3, 2016

Eric Kurtz, Ford Motor CompanyDiesel Combustion System Technical Expert

Research and Advanced EngineeringEmissions Challenge

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Class 2b Class 3

NOx [g/mi] NMOG [g/mi]

NOx + NMOG [g/mi]

NOx [g/mi] NMOG [g/mi]

NOx + NMOG [g/mi]

Tier 2 0.2 0.143 0.343 0.4 0.167 0.567Tier 3 0.170 0.230Difference -50% -60%

Heavy-Duty Pickups & Vans

Light-Duty Vehicles (Fleet Average)

NOx [g/mi] NMOG [g/mi] NOx + NMOG [g/mi]

Tier 2, Bin 5 0.07 0.09 0.160Tier 3, 2025 0.030Difference -81%

Future emissions standards require solutions that deliver substantial reductions in NOx + NMOG.

Research and Advanced EngineeringSources of Diesel Emissions

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Cumulative Tailpipe N

Ox + H

C*

SCR Tempe

rature

Bag 1 Bag 2 Bag 3

Shut down10 min soak

Catalyst Light-off temperature

*HC acts as a surrogate for NMOG

Increased exhaust temperature and enthalpy for faster light-off and lower emissions are needed during catalyst light-off operation.

• The majority of tailpipe emissions are emitted before the aftertreatment reaches operating temperature

• Most manufacturers operate in a catalyst light-off mode during that time to provide exhaust heat to sufficiently low emissions

Research and Advanced Engineering

-5 0 5 10 15260

280

300

320

340

360

Post Injection Retard [deg]

Exh

aust

Tem

pera

ture

[°C

]

Unconstrained HC

Engine Operation for Catalyst Light-Off

• Typical catalyst light-off strategy (retard combustion)– Retard SOIs– Shift fuel to post injections

• Temperature increases as combustion is retarded– Lower efficiency – shifting

energy to exhaust– Increase fuel quantity– Late phasing degrades stability

– Higher HC emissions

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PilotInjections Main Post

Injections

Poor ignition and combustion stability of late fuel injections can lead to excessive HC emissions.

-5 0 5 10 15260

280

300

320

340

360

Post Injection Retard [deg]

Exh

aust

Tem

pera

ture

[°C

]

Unconstrained HCConstrained HCNOx+HC < target

Maximum Potential

All injections burning

20°C

Texh penalty to control emissions

Research and Advanced Engineering

Exhaust Temperature vs. Emissions

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A tradeoff exists between exhaust temperature/enthalpy and emissions. Need to improve ignition and combustion stability of late fuel injections.

Various calibrations (one speed-load point):• Injection pressure• Injection timings (pilots, main, posts)• Injection quantities (pilots, posts)• EGR rate• Boost pressure

Curve of best emissions vs. exhaust enthalpy

+36%

+110%

Exhaust Enthalpy (to hasten catalyst light-off)

*HC acts as a surrogate for NMOG

*

Research and Advanced EngineeringFuel Cetane Number

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Does the relatively low cetane number with US fuel influence the ability to generate exhaust temperature and enthalpy while controlling emissions?

US: 40CN

EU: 48CN

=8CN

Min CN

Cetane Number: a measure of the ignition quality of a diesel fuel.

US: 45CN

EU: 54CN

=9CN

Avg CN

Minimum CN

Research and Advanced EngineeringTest Fuels

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US Certification Fuel

High Cetane Fuel

Cetane Number 46 53Lower Heating Value [MJ/kg] 42.9 42.9Density [kg/m3] 842 837H/C ratio 1.82 1.97O/C ratio 0 0Aromatics [wt%] 29.2 8.2Kinematic Viscosity [cSt] 2.9 2.3T90 [°C] 307 334

Tested two fuels with a cetane number difference similar to the gap between United States & European diesel fuel.

=7CN

Research and Advanced EngineeringSingle-Cylinder Study

• Single-cylinder version of the 6.7L PowerStroke®

• Four test conditions – Two speed-load points to

represent first 200s of FTP– 20° and 90°C coolant and oil

• 60-point experiment at each test condition to cover calibration space – Injection pressure– Injection timings– Injection quantities– EGR rate

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Research and Advanced Engineering

Vehicle Populations to Consider in Fuels Studies

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Modern vehicles

Future Fleet

Catalyst light-off calibration

Fixed engine calibration

Present

Fixed engine calibration

Older vehicles

Bulk of cetane effects studies

2007-2010

Legacy Fleet

Study focus – cetane effect on catalyst light-off operation

Catalyst light-off calibration

Research and Advanced Engineering

• A-B comparison at production calibration

• Pairwise statistical comparison across multiple calibrations, isolating fuel difference

• Comparison at optimum calibrations for each fuel– Use experimental data to optimize calibration for each fuel– Compare optimums

Data Analysis MethodsLegacy Fleet

Future Fleet

Research and Advanced Engineering

Example: Exhaust TemperatureFuel #1 Fuel #2 Difference

Calibration 1 X1 Y1 X1 – Y1

Calibration 2 X2 Y2 X2 – Y2

Calibration 3 X3 Y3 X3 – Y3

Calibration 4 X4 Y4 X4 – Y4

: : : :

Calibration 60 X60 Y60 X60 – Y60

Statistical comparison across pairwise data (calibrations) isolating difference between two fuels

Paired T-Test

0 inside CI = fuel has no effect (verify null hypothesis)0 outside CI = fuel has an effect

Null hypothesis: fuels have no effect

Research and Advanced Engineering

Effect on Legacy Fleet in a Catalyst Light-Off Mode

Comparison across identical calibrations (analysis of all data)• Little/no difference in exhaust T & H, noise, smoke• Higher NOx, but lower NOx + HC

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Increased cetane would have minimal impact on legacy fleet A/T function, but may reduce NOx + HC during catalyst light-off operation.

Analysis of data from all 4 points together

Difference (high CN – Low CN)

X production calibration

paired t-test 95% conf. interval

-60

-40

-20

0

20

40

60

Abs

. or R

el. D

iffer

ence

[°C

or %

]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B] -3

-2

-1

0

1

2

3

Abs

. Diff

eren

ce [d

B]

-3

-2

-1

0

1

2

3

-3

-2

-1

0

1

2

3

-3

-2

-1

0

1

2

3

-3

-2

-1

0

1

2

3

NO

x &

N

Ox

&

NO

x &

N

Ox

&

NO

x &

Research and Advanced Engineering

Effect on Future Fleet in a Catalyst Light-Off Mode

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Analysis of data from all 4 points together

Difference (high CN – Low CN)

X production calibration

paired t-test 95% conf. interval

optimized calibration

Optimize calibration for the fuel• 30% higher exhaust temperature and enthalpy for faster cast light-off• Higher NOx, but lower NOx+HC and smoke

Potential to more quickly light-off catalysts while controlling emissions with higher cetane fuel, thus reducing TP emissions.

-60

-40

-20

0

20

40

60

Abs

. or R

el. D

iffer

ence

[°C

or %

]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B]

Exh

. Tem

p. [°

C]

Exh

. Ent

halp

y [%

]

NO

x [%

]

THC

[%]

NO

x &

TH

C [%

]

Sm

oke

[%]

Noi

se [d

B] -3

-2

-1

0

1

2

3

Abs

. Diff

eren

ce [d

B]

-3

-2

-1

0

1

2

3

-3

-2

-1

0

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2

3

-3

-2

-1

0

1

2

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-3

-2

-1

0

1

2

3

-3

-2

-1

0

1

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3

NO

x &

N

Ox

&

NO

x &

N

Ox

&

NO

x &

Research and Advanced Engineering

-30 -15 0 15 30 45 60 75 90

0

10

20

30

40

Engine Postion [dATDC]

AH

HR

[J/d

eg]

Low Cetane @ Base Cal.High Cetane @ Base Cal.Fed Cert Fuel @ Base Cal

-30 -15 0 15 30 45 60 75 90

0

10

20

30

40

Engine Postion [CA ATDC]

AH

HR

[J/d

eg]

Low Cetane @ Base Cal.High Cetane @ Optimized Cal.Fed Cert Fuel @ Base Cal

Heat Release Analysis

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1200 rpm, 8% load, 90°C

-30 -15 0 15 30 45 60 75 900

20

40

60

80

100

Engine Postion [dATDC]

MFB

[%]

-30 -15 0 15 30 45 60 75 900

20

40

60

80

100

Engine Postion [CA ATDC]

MFB

[%]

Heat Release Rate Mass Burn Fraction

-30 -15 0 15 30 45 60 75 900

1

2

Engine Postion [CA ATDC]

Inj.

Sig

nal [

-]

-30 -15 0 15 30 45 60 75 900

1

2

Engine Postion [CA ATDC]

Inj.

Sig

nal [

-]

Higher cetane fuel enabled significantly later combustion while

controlling emissions.

Research and Advanced EngineeringConclusions

• This study suggests that:– increasing cetane would have minimal impact on catalyst light-off

and may slightly reduce tailpipe emissions in legacy vehicles.– the relatively low cetane fuel in the US may limit the ability to light-

off catalysts while controlling for future emissions standards.– further study is warranted.

• To fully understand the effect of changing fuels specifications, fuels testing should include:– evaluation with production calibrations to define effects on today’s

vehicles.– optimization of the calibration for the new fuel to estimate the

impact on tomorrow’s vehicles.

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Research and Advanced Engineering

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