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
2
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
3
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
4
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
5
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
6
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
9
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
13
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
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
-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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