Fuel Consumption and NOx Trade-offs on a Port-Fuel-Injected SI Gasoline Engine Equipped with a Lean-NOx Trap J.A. Lymburner, R.W. McCabe, and J.R. Theis Ford Motor Company 2009 DEER Conference Dearborn, MI August 4, 2009 Contact: [email protected]
Fuel Consumption and NOx Trade-offs on a Port-Fuel-Injected SI Gasoline
Engine Equipped with a Lean-NOx Trap
J.A. Lymburner, R.W. McCabe, and J.R. Theis
Ford Motor Company
2009 DEER Conference
Dearborn, MI
August 4, 2009
Contact: [email protected]
Focus of Presentation
Three Main Aspects:
1. Effects of Lean-burn, EGR, and deVCT on BSFC
3 combustion features
and BSNOx at 1200 rpm, 4 bar BMEP
2. NOx trap purge fuel requirements at several of the best BSFC/BSNOx points
3. Trade-offs between tailpipe NOx emissions and “effective” BSFC* (and implications for lean-burn engine operation in general)
= Cycle-weighted BSFC for lean operation (NOx storage) *BSFCeffective
and rich operation (NOx trap purge)
Engine: 2005 5.4L 3V V-8* with: • dual equal VCT (deVCT)
• non-production external EGR system (see below)
*Stein, et al., The Combustion System of the Ford 5.4L 3-Valve Engine Proc.
2003 Global Powertrain Congress, Vol. 24, Ann Arbor, MI Sept. 23-25, 2003
What’s new? • lean operation
• EGR system (non-production)
• aftertreatment (TWC+LNT)
But…… • 1200 rpm, 4 bar BMEP only
• 250 combinations of lean AF, cam
retard, and % EGR
250
0
2
4
6
8
10
12
14
16
BS
NO
x (
g/k
W-h
r)
245
255
260
265
270
275
BS
FC
(g
/kW
-hr)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
RM
S C
OV
of
IME
P (
%)
Combustion
becomes
unstable
NOx & BSFC
both too high
NOx
Fuel
Consumption
Combustion
Stability
5.4L PFI V-8
Engine @1200
rpm/4 bar BMEP
lean rich
~8%
Lean PFI has narrow
operating range
compared to stratified
gasoline and diesel
Gasoline lean-burn
can default to stoich
or rich conditions
The Lean
Challenge
14 15 16 17 18 19 20 21 22
AFR
Series1 289.1 270.3 273.4 262.0 262.9 251.4 259.6 253.5
0 0 0 0 0 0 0 0
BSNOx 13.50 7.76 4.90 2.36 13.31 8.14 3.99 4.20
BSFC vs BSNOx comparison for various combustion features (1200 rpm/4 bar)
230
240
250
260
270
280
290
300
BS
FC
(g
/kW
-h)
Base VCT EGR EGR +
VCT Base VCT EGR
EGR +
VCT
Stoichiometric Lean
AF R 14.7 14.7 14.7 14.7 20.1 19.1 20.2 17.1
CAM 0 45 0 40 0 40 0 30
EG R 0 0 15 17 0 0 10 17 COV 0.45 0.73 0.64 1.07 0.83 1.40 1.66 1.61
16
14
12
10
8
6
4
2
0
BS
NO
x (
g/k
W-h
)
Fuel breakdown analysis
St oich
Stoich
w/
deVCT
Stoich
w/ EGR
Stoi ch
w/
EGR
w/
deVCT
Lean
(20:1)
Lea n +
deVCT
17 AFR
30 CAM
16.7EGR
BSNOx (g/kW-h) 19.14 10.34 5 .80 2.93 13.30 11.14 4.28
BSFC (g /kW-h) 287.3 273.0 274.2 2 62.1 261.1 253.2 253.9
% decrease vs. stoich
( measured) 5 .00 4 .58 8.79 9.12 1 1.89 11.64
cal cu lated % decrease in
BSFC due to:
y PMEP 4 .54 1 .89 6.37 1.69 5.88 6.28
y Dilut ion 0 .45 2 .58 2.84 6.90 5.62 5.05
y HC 0 .45 -0 .35 -0.29 -0.66 -0.43 -0.68
y CO -0 .93 0 .35 -0.01 1.34 1.37 1.39
y Comb ustion
Phasing 0 .13 0 .09 0.13 0.08 0.13 0.12
y FMEP 0 .36 0 .02 -0.25 -0.23 -0.67 -0.52
~ 3% BSFC benefit for best lean case vs best stoich case
How lean-burn helps fuel economy
• Decreases pumping losses (i.e. PMEP), but no incremental benefit over VCT + EGR
• Improves fuel conversion efficiency via the
dilution (or “gamma”) effect on the burned gas
expansion process
– = 1-(1/r γ-1) [where γ = C /C , and the greater mass ηf c p v
of gas results in lower burned gas T and Cv and thus
greater γ; also less heat transfer and dissociation]
– Partial overlap with EGR and slight overlap with VCT
• Decreases CO/H2 emissions (lost fuel)
Engine-Out Fuel Consumption & NOx Optimization
RMS COV
AFR CAM EGR BSNOx BSFC IMEP
Stoich at Base Cam 14.66 0 0 13.50 289.1 0.45
Stoich at 45 CAM 14.69 45 0 7.76 270.3 0.73
Stoich + VCT + EGR 14.70 40 17 2.36 262.0 1.07
Lean only 20.05 0 0 13.31 0.83
Lean + VCT 19.12 40 0 8.14 1.40
Lean + EGR 20.19 0 10 3.99
262.9
251.4
259.6 1.66
Lean + EGR + VCT 17.05 30 17 4.20 253.5 1.61
• Best BSFC is obtained with Lean & VCT only.
• Adding EGR greatly reduces NOx but at the expense of
higher BSFC than with best Lean+VCT case.
• Best engine operating condition can only be determined based
on tailpipe emissions and fuel consumption.
Part 2: LNT Impact on NOx
Tailpipe Emissions and BSFCeffective
• NOx tailpipe emissions:
– LNT needs low engine-out NOx to operate
with high efficiency
• BSFCeffective
:
– cycle-weighted BSFC for lean storage and
rich purge
• Trade-off between low BSFC and low TP NOx
Lean-rich duty cycle for LNT operation
0 50 100 150 200 250 300
Time (sec.)
0
50
100
150
200
250
300
350
400
BS
FC
(g
/kW
h)
250 g/kWh during
Lean operation (17:1)
330 g/kWh during
Rich operation (12:1)
17 AFR, 16 EGR, 30 CAM
20 lean 10 rich
The Modern IC Engine with Lean-Burn Aftertreatment System –
a rich integration of chemical, mechanical and controls engineering
Gasoline internal combustion engine
Three-way catalyst NOx
storage/reduction
catalyst (LNT)
optional heat exchanger
sensors & strategy for
control and diagnostics
Lean NO x Traps
Flow
Ceramic substrates (cordierite)
400 cells/sq inch
Washcoat – a thin layer
coated onto substrate
Like TWC, washcoat of LNT
contains - PGM (Pt, Rh, optional Pd)
- Al2O3, ceria for OSC, stabilizers
LNT also contains high levels
of NO x storage materials -Alkaline-earth metals (Ba, Mg)
-Alkali metals (K, Cs, Na)
Lean NOx Traps • Convert HC, CO, & NOx at λ=1
• at λ>1 convert HC & CO, store NOx
• Reduce stored NO x to N2 at λ < 1 • NO x storage performance not
as durable as TWC (800oC max)
Automotive Catalytic Converter
(TWC or LNT)
Effect of NO x Concentration & Flow Rate (SV) – Lab Data
LNT Aged 50 hrs 900 C Max
Eval on 60/5 Test at 450 C N
Ox
Bre
akth
rou
gh
(p
pm
)
1200 25k hr
-1 10k hr
-1 25k hr
-1 10k hr
-1
1000 1500 ppm 1500 ppm 500 ppm 500 ppm
Inlet Concentrations
600
800 Larger LNT
Lower FG NOx
500 or 1500 ppm NO,
10% CO2 & H2O,
5% O2 (lean),
5.0% CO+1.7% H2 (rich)
400 Lower FG NOx & larger LNT
200
0
0 500 1000 1500 2000
Time (sec)
Low feedgas NOx and large LNT volumes will be vital for achieving very low NOx emissions
Engine-Out Fuel Consumption & NOx Optimization
RMS COV
AFR CAM EGR BSNOx BSFC IMEP
Stoich at Base Cam 14.66 0 0 13.50 289.1 0.45
Stoich at 45 CAM 14.69 45 0 7.76 270.3 0.73
Stoich + VCT + EGR 14.70 40 17 2.36 262.0 1.07
Lean only 20.05 0 0 13.31
251.4
262.9 0.83
Lean + VCT 19.12 40 0 8.14 1.40
Lean + EGR 20.19 0 10 3.99 259.6 1.66
Lean + EGR + VCT 17.05 30 17 4.20 253.5 1.61
Tailpipe Fuel Consumption & NOx Optimization Lean Purge (Matching Effective BSFC)
BSFC TP Nox
20 sec lean (effective) (ppm)
Lean + VCT 19.12 40 0 8.69 262.00 523.49
Lean + EGR + VCT 17.05 30 17 3.66 262.44 46.47
30 sec lean
Lean + VCT 19.12 40 0 9.49 257.28 709.52
Lean + EGR + VCT 17.05 30 17 3.99 257.08 68.32
4s R
3s R
3s R 2s R
The Effect of Engine Conditions on NOx Conversion Efficiency
Tailpipe Comparison: Best NOx & Best BSFC Cases
0
500
1000
1500
2000
2500
3000
TP
NO
x (
pp
m)
— 20 lean 4 rich
73.9% Conversion Eff.
BSFC = 262 g/kW-h
19:1 AFR, 40 CAM, 0% EGR
— 20 lean 3 rich
94.9% Conversion Eff.
BSFC = 262.4 g/kW-h
17:1 AFR, 30 CAM, 16% EGR Equivalent BSFC
Lean FG NOx = 2200 ppm Lean FG NOx = 970 ppm
Comparison carried out at equivalent effective BSFC
0 100 200
Time (sec.)
Effect of NO x Concentration & Flow Rate (SV) – Lab Data
LNT Aged 50 hrs 900 C Max
Eval on 60/5 Test at 450 C N
Ox
Bre
akth
rou
gh
(p
pm
)
1200 25k hr
-1 10k hr
-1 25k hr
-1 10k hr
-1
1000 1500 ppm 1500 ppm 500 ppm 500 ppm
Inlet Concentrations
600
800 Larger LNT
Lower FG NOx
500 or 1500 ppm NO,
10% CO2 & H2O,
5% O2 (lean),
5.0% CO+1.7% H2 (rich)
400 Lower FG NOx & larger LNT
200
0
0 500 1000 1500 2000
Time (sec)
Low feedgas NOx and large LNT volumes will be vital for achieving very low NOx emissions
Engine-Out Fuel Consumption & NOx Optimization
RMS COV
AFR CAM EGR BSNOx BSFC IMEP
Stoich at Base Cam 14.66 0 0 13.50 289.1 0.45
Stoich at 45 CAM 14.69 45 0 7.76 270.3 0.73
Stoich + VCT + EGR 14.70 40 17 2.36 262.0 1.07
Lean only 20.05 0 0 13.31
251.4
262.9 0.83
Lean + VCT 19.12 40 0 8.14 1.40
Lean + EGR 20.19 0 10 3.99 259.6 1.66
Lean + EGR + VCT 17.05 30 17 4.20 253.5 1.61
Tailpipe Fuel Consumption & NOx Optimization Lean Purge (Matching Effective BSFC)
BSFC TP Nox
20 sec lean (effective) (ppm)
Lean + VCT 19.12 40 0 8.69 262.00 523.49
Lean + EGR + VCT 17.05 30 17 3.66 262.44 46.47
30 sec lean
Lean + VCT 19.12 40 0 9.49 257.28 709.52
Lean + EGR + VCT 17.05 30 17 3.99 257.08 68.32
4s R
3s R
3s R 2s R
Purge fuel breakdown analysis
• Purge not efficient
• Not optimized (L→R step change)
• Richer purge λ best at higher T (Theis, et
al. SAE 2003-011159)
Summary
• Lean-burn improves PFI fuel economy by ~3% relative to best stoichiometric VCT/EGR conditions, when used in combination with VCT&EGR.
• The benefit of lean-burn is largely due to improved fuel combustion efficiency owing to dilution.
• Both VCT and (especially) EGR reduce BSNOX, but the extra fuel required to purge a NOx trap gives back virtually all of the BSFC benefit of lean-burn.
• Successful implementation of lean-burn at low emission standards may require: – Engine-out NOx reduction with VCT and EGR
– Alternative to LNT for NOx control (urea-SCR; LNT+in-situSCR)
– New combustion modes (DI stratified or homogeneous lean, HCCI, PCCI) for ultra-low BSNOx
– Engine operation in a fixed or narrow speed-torque range
Acknowledgments
• Robert Wiley and Shawn Bogedain (cell technicians)
• Tom Leone, Bob Stein and Frank Wong (general advice and help with fuel breakdown analysis)