Research and
Advanced Engineering
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 Company
Diesel Combustion System Technical Expert
1
Research and
Advanced Engineering
Emissions Challenge
Light-Duty Vehicles (Fleet Average)
NOx [g/mi]
NMOG [g/mi]
NOx + NMOG
[g/mi]
0.07
0.09
0.160
Tier 2, Bin 5
Tier 3, 2025
0.030
Difference
-81%
Heavy-Duty Pickups & Vans
Class 2b
Tier 2
Class 3
NOx [g/mi]
NMOG
[g/mi]
NOx +
NMOG
[g/mi]
0.2
0.143
0.343
Tier 3
NOx [g/mi]
NMOG
[g/mi]
NOx +
NMOG
[g/mi]
0.4
0.167
0.567
0.170
Difference
-50%
0.230
-60%
Future emissions standards require solutions that deliver
substantial reductions in NOx + NMOG.
2
Research and
Advanced Engineering
Cumulative Tailpipe NOx + HC*
SCR Temperature
Sources of Diesel Emissions
Catalyst Light-off
temperature
Shut down
10 min soak
Bag 1
Bag 2
Bag 3
*HC acts as a surrogate for NMOG
• 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
Increased exhaust temperature and enthalpy for faster light-off and lower
emissions are needed during catalyst light-off operation.
3
Engine Operation for Catalyst
Light-Off
Research and
Advanced Engineering
• Typical catalyst light-off
strategy (retard combustion)
Pilot
Injections
– Retard SOIs
– Shift fuel to post injections
– Lower efficiency – shifting
energy to exhaust
– Increase fuel quantity
– Late phasing degrades stability
– Higher HC emissions
Post
Injections
All injections burning
360
Exhaust Temperature [°C]
• Temperature increases as
combustion is retarded
Main
20°C
340
Maximum
Potential
Unconstrained
HC
NOx+HC
< target
Constrained
HC
320
Texh penalty to
control emissions
300
280
260
-5
0
5
10
15
Post Injection Retard [deg]
Poor ignition and combustion stability of late fuel injections can lead to
excessive HC emissions.
4
Exhaust Temperature vs.
Emissions
*
*HC acts as a
surrogate for
NMOG
Research and
Advanced Engineering
Various calibrations (one speed-load point):
• Injection pressure
• Injection timings (pilots, main, posts)
• Injection quantities (pilots, posts)
• EGR rate
• Boost pressure
+110%
Curve of best
emissions vs.
exhaust enthalpy
+36%
Exhaust Enthalpy (to hasten catalyst light-off)
A tradeoff exists between exhaust temperature/enthalpy and emissions.
Need to improve ignition and combustion stability of late fuel injections.
5
Fuel Cetane Number
Research and
Advanced Engineering
Cetane Number: a measure of the ignition quality of a diesel fuel.
Avg CN
EU: 54CN
Min CN
EU: 48CN
US: 45CN
=9CN
US: 40CN
=8CN
Minimum CN
Does the relatively low cetane number with US fuel influence the ability to
generate exhaust temperature and enthalpy while controlling emissions?
6
Research and
Advanced Engineering
Test Fuels
US Certification
Fuel
Cetane Number
46
=7CN
High Cetane
Fuel
53
Lower Heating Value [MJ/kg]
42.9
42.9
Density [kg/m3]
842
837
H/C ratio
1.82
1.97
O/C ratio
0
0
Aromatics [wt%]
29.2
8.2
Kinematic Viscosity [cSt]
2.9
2.3
T90 [°C]
307
334
Tested two fuels with a cetane number difference similar to the gap
between United States & European diesel fuel.
7
Research and
Advanced Engineering
Single-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
8
Vehicle Populations to
Consider in Fuels Studies
Research and
Advanced Engineering
2007-2010
Present
Future Fleet
Legacy Fleet
Older vehicles
Modern vehicles

Fixed engine
calibration

Catalyst light-off
calibration
Bulk of cetane effects
studies

Fixed engine
calibration


Catalyst light-off
calibration

Study focus – cetane effect on
catalyst light-off operation
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Data Analysis Methods
Research and
Advanced Engineering
• 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
Legacy Fleet Future Fleet
• A-B comparison at production calibration
Research and
Advanced Engineering
Paired T-Test
Statistical comparison across pairwise data (calibrations)
isolating difference between two fuels
Example:
Exhaust Temperature
Fuel #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
Null hypothesis: fuels have
no effect
0 inside CI = fuel has no effect (verify null hypothesis)
0 outside CI = fuel has an effect
Effect on Legacy Fleet in a
Catalyst Light-Off Mode
Research and
Advanced Engineering
3
40
2
20
1
0
0
Analysis of data from all 4 points
together
Difference (high CN – Low CN)
X
production calibration
paired t-test 95% conf. interval
Noise [dB]
-3
Smoke [%]
-60
NOx
NOx&&THC [%]
-2
THC [%]
-40
NOx [%]
-1
Exh. Enthalpy [%]
-20
Abs. Difference [dB]
60
Exh. Temp. [°C]
Abs. or Rel. Difference [°C or %]
Comparison across identical calibrations (analysis of all data)
• Little/no difference in exhaust T & H, noise, smoke
• Higher NOx, but lower NOx + HC
Increased cetane would have minimal impact on legacy fleet A/T function,
but may reduce NOx + HC during catalyst light-off operation.
12
Effect on Future Fleet in a
Catalyst Light-Off Mode
Research and
Advanced Engineering
3
40
2
20
1
0
0
Analysis of data from all 4 points
together
Difference (high CN – Low CN)
X
production calibration
paired t-test 95% conf. interval
optimized calibration
Noise [dB]
-3
Smoke [%]
-60
NOx
NOx&&THC [%]
-2
THC [%]
-40
NOx [%]
-1
Exh. Enthalpy [%]
-20
Abs. Difference [dB]
60
Exh. Temp. [°C]
Abs. or Rel. Difference [°C or %]
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.
13
Research and
Advanced Engineering
Heat Release Analysis
1200 rpm, 8% load, 90°C
Heat Release Rate
Mass Burn Fraction
40
100
30
80
MFB [%]
AHHR [J/deg]
Low
Cetane
@@
Base
Cal.
Fed
Cert
Fuel
Base
Cal
Low
Cetane
@
Base
Cal.
Fed
Cert
Fuel
@
Base
Cal
High
Cetane
@@
Optimized
Cal.
High
Cetane
Base Cal.
20
10
60
40
20
0
-30
-15
0
15
30
45
60
75
90
Inj. Signal [-]
Engine
ATDC]
EnginePostion
Postion[CA
[dATDC]
0
-30
-15
0
15
30
45
60
75
90
Engine
ATDC]
EnginePostion
Postion[CA
[dATDC]
2
Higher cetane fuel enabled
significantly later combustion while
controlling emissions.
1
0
-30
-15
0
15
30
45
60
Engine Postion [CA ATDC]
75
90
14
Conclusions
Research and
Advanced Engineering
•
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 lightoff 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.
15
Research and
Advanced Engineering
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Cooperation and collaboration
to find high volume solutions
for today’s and tomorrow’s
challenges
16