hcci combustion: the sources of emissions at low loads and the effects of gdi fuel ... · 2014. 3....
TRANSCRIPT
John E. Decand
Magnus SjöbergSandia National Laboratories
Sponsor: U.S. Dept. of Energy, OTT, OAAT and OHVTProgram Managers: Kathi Epping and Gurpreet Singh
HCCI Combustion: the Sources of Emissions atLow Loads and the Effects of GDI Fuel Injection
8th Diesel Engine Emissions Reduction WorkshopAugust 25-29, 2002
Introduction
HCCI engines are a low-emissions alternative to diesel engines.
– Provide diesel-like or higher efficiencies.
– Very low engine-out NOX and PM emissions.
Research is required in many areas to resolve technical barriers tothe development of HCCI engines by industry.
– The objective of our work is to help provide this understanding.
Establish a laboratory to investigate HCCI combustion fundamentals.
– All-metal engine: fully operational – result are subject of presentation.
– Optically accessible engine: examine in-cylinder processes (end of 02).
CHEMKIN kinetic-rate computations
– Guide experiments
– Assist in data analysis
– Show limiting behavior
Engine and Operating Conditions
Six-cylinder diesel engine converted forbalanced, single-cyl., HCCI operation.
Versatile facility to investigate variousoperational and control strategies.– Compression ratios from 13 - 21 (18)*
– Swirl ratios from 0.9 - 3.2; 7.2 (0.9)*– Speeds to 3600 rpm (600 - 1800 rpm)*
– Multiple fueling systems.> Fully premixed (curr.)*> Port fuel injection (PFI)> Direct injection, gasoline-type (curr.)*> Direct injection, diesel-type
– Liquid or gas-phase fuels (iso-octane)*
Complete intake charge conditioning.– Intake temperatures to 180° C. (varies)*
– Intake pressures to 4 bars. (varies)*
– Simulated or real EGR. (none)*
HCCI All-Metal Engine
* Values in (red) are used for current work.Based on Cummins B, 0.98 ltr./cyl.
Engine Appears to be Working Well
88
90
92
94
96
98
100
90 100 110 120 130 140 150Intake Temperature [°C]
Co
mb
ust
ion
effi
cien
cy[%
]
36
38
40
42
44
46
48
Th
erm
alef
fici
ency
[%]
Combustion efficiency
Thermal efficiency
CR = 18; φφφφ = 0.24; Pin = 120 kPa;1200 rpm; Well-Mixed Charge
– Large squish clearance.– Ring-land crevice 1% of TDC vol.– Various compression ratios
0
25
50
75
100
125
150
175
200
90 100 110 120 130 140 150Intake Temperature [°C]
CO
&H
C[g
/kg
fuel
]
0
100
200
300
400
500
600
700
800
NO
x[m
g/k
gfu
el]
CO
HC
NOx
0
50
100
150
200
250
300
350
400
90 100 110 120 130 140 150Intake Temperature [°C]
IME
Pg
[kP
a]
0
0.5
1
1.5
2
2.5
3
3.5
4S
td.D
ev.o
fIM
EP
g[%
]
IMEPg
Std. Dev. IMEPg
Computational Approach
Senkin application of the CHEMKIN-III kinetics rate code.
– Single-zone model with uniform properties and no heat transfer.
– Allows compression and expansion with slider-crank relationship.
– Full chemistry for iso-octane (Westbrook et al., LLNL).
Great oversimplification of a real engine. Model cannot reproduceall real-engine behavior.
Model is well suited for investigating certain fundamental aspects ofHCCI combustion.
– Allows the effects of kinetics and thermodynamics to be isolated andevaluated without complexities of walls, crevices, and inhomogeneities.> Assists in analysis of experimental data by separating chemical-kinetic
and physical effects.
– Represents the adiabatic limit for bulk-gas behavior in real engines.
– Guide experiments by showing approximate trends in ignition timing &temperature compensation with changes in operating conditions.
0.0 0.1 0.2 0.30
20
40
60
80
100
120
Equivalence Ratio (Phi)
Com
bust
ion
Effi
cien
cy(%
)
0.0 0.1 0.2 0.3 0.4 0.51e-8
1e-7
1e-6
1e-5
1e-4
1e-3
1e-2
1e-1
Equivalence Ratio (Phi)
Mol
eF
ract
ion/
Phi
HCOHCHC + OHCCO
100 ppm for φφφφ = 1.0
Below φ = 0.2, emissions rise followed by a drop in combustion efficiency.– Temperatures are too low to complete reactions, especially CO → CO2.
Indicates high emissions of OHC as well as CO and HC.– OHC not well-detected by standard FID HC detector, and they can be harmful.
Results for bulk-gas alone, in the absence of heat transfer.– Occurs in range of interest (typical diesel idle conditions are φ = 0.10 - 0.12).
– In real engine, heat transfer will shift onset of incomplete reactions to higher φ.
– Walls & crevices also add to emissions.
CHEMKIN predicts incomplete bulk-gas reactions at low loads.Iso-Octane; 1800 rpm; CR = 21; Tin = 380 K; Pin = 1 atm.
SAE Paper 2002-01-1309
Vary φφφφ: Experiment and CHEMKIN
Iso-Octane ; CR = 18; 1200 rpm;Pin = 120 kPa; Pre-Mixed
Intake temperature adjusted for50% burn at TDC for φ = 0.14.
– Experiment: Tin = 140° C
– CHEMKIN: Tin = 117° C
Experiment shows greatervariation in combustion phasing.
– Heat transfer and residuals.
– Advanced timing at higherloads has little effect onemissions.
– Timing retard at low loads issmall and has little effect onemissions.
40
60
80
100
120
Pre
ssur
e(B
ars)
MotoredPhi = 0.06Phi = 0.10Phi = 0.14Phi = 0.18Phi = 0.22Phi = 0.26
CHEMKINTin = 117° C
340 350 360 370 380 390 40020
30
40
50
60
70
80
90
100
Crank Angle (Degrees)
Pre
ssur
e(B
ars)
MotoredPhi = 0.06Phi = 0.10Phi = 0.14Phi = 0.18Phi = 0.22Phi = 0.26
ExperimentTin = 140° C
Emissions: Experiment and CHEMKIN
1200 rpm; CR = 18; Pin = 120 kPa;Pre-Mixed Fueling
Experimental Tin was increasedto maintain near-TDC ignition.– Compensate for heat transfer.
Experimental emissions matchmodel closely.
– CO levels match closely ~65%> Bulk-gas source.
– HC, rise for φ < 0.2 is similar tomodel indicates bulk-gassource at low φ.> Near-constant baseline
level for φ > 0.2 suggestcrevice source.
– “Missing carbon” in experimentindicates presence of OHCs.> Similar to model, but lower
due to FID response.
0
10
20
30
40
50
60
70
80
90
100
0.02 0.06 0.1 0.14 0.18 0.22 0.26
Equivalence Ratio [φφφφ]
Fu
elC
arb
on
into
Em
iss
ion
s[%
]
CO
CO2
HC
OHC
CHEMKINTin = 117° C
0
10
20
30
40
50
60
70
80
90
100
0.02 0.06 0.1 0.14 0.18 0.22 0.26
Equivalence Ratio [φφφφ]
Fu
el
Ca
rbo
nin
toE
mis
sio
ns
[%]
CO
CO2
HC
OHC
ExperimentTin = 140° C
0
10
20
30
40
50
60
70
0.02 0.06 0.1 0.14 0.18 0.22 0.26 0.3Equivalence Ratio [φφφφ]
Fu
elC
arb
on
into
CO
[%]
CHEMKIN, Tin = 117° CExperiment, Tin = 118° CExperiment, Tin = 131°CExperiment, Tin = 142° CExperiment, Tin = 159° CInterpolated 50% HR@TDC
Comparison of CO Emissions with φφφφ at Various Tin
Magnitude of increase in CO with decreasing φ agrees well with theCHEMKIN results. Shows incomplete bulk-gas reactions are the cause.
– Onset of rise in CO levels is shifted to higher φ in engine due to heat transfer.
– Rise in CO shifts to progressively higher φ as Tin is reduced (lower Tcombustion.).
– Engine data also show a large drop in combustion efficiency at low loads,corresponding to the increase in CO.
-50
0
50
100
150
200
250
300
350
0.02 0.06 0.1 0.14 0.18 0.22 0.26Equivalence Ratio [φφφφ]
IME
Pg
[kP
a]
0
2
4
6
8
10
12
14
16
Std
.Dev
.IM
EP
g[k
Pa
&%
]IMEPg [kPa]
Std. Dev. IMEPg [kPa]
Std. Dev. IMEPg [%]
Efficiencies and Combustion Stability
Combustion efficiency dropsfrom 95% to 60% as fuel isreduced to low-idle, φ = 0.1.– Similar drop in pressure-
indicated thermal efficiency.
– Commensurate with therapid rise in CO.
Std. Dev. of IMEP is 2 – 4kPa for all fueling rates (φ).– Increase at φ = 0.16 due this
being in the middle of therapid rise in CO.
Normalized σIMEPincreases below φ = 0.1because IMEP is near zero.– Std. Dev. of IMEPg ≤ 2.6%
from φ = 0.1 to φ = 0.26.
0
10
20
30
40
50
60
70
80
90
100
0.02 0.06 0.1 0.14 0.18 0.22 0.26Equivalence Ratio [φφφφ]
Co
mb
ust
ion
effi
cien
cy[%
]
0
5
10
15
20
25
30
35
40
45
50
Th
erm
alE
ffic
ien
cy[%
]
Combustion efficiency [%]
Thermal efficiency [%]
CR = 18; Pin = 120 kPa;Tin = 140° C; Pre-Mixed
Effect of Intake Pressure
1200 rpm; CR = 18; Pre-Mixed
Changing Pin from 101 to 120kPa has little effect on onset ofincomplete bulk-gas reactionswhen combustion phasing ismaintained.
– Experiment and CHEMKINboth show slightly lower COvalues during rapid rise.
Tin adjusted for 50% burn atTDC for φ = 0.14.
– 101 kPa: Tin = 157° C
– 120 kPa: Tin = 140° C
0
10
20
30
40
50
60
70
0.02 0.06 0.1 0.14 0.18 0.22 0.26 0.3
Equivalence Ratio [φφφφ]
Fu
elC
arb
on
into
CO
[%]
1.0 bar, Expr. Tin = 157°C
1.2 bar, Expr. Tin = 140°C
1.0 bar, CHEMKIN, Tin = 121° C
1.2 bar, CHEMKIN, Tin = 117° C
0
10
20
30
40
50
60
70
80
90
100
0.02 0.06 0.1 0.14 0.18 0.22 0.26 0.3Equivalence Ratio [φφφφ]
Fu
elC
arb
on
into
Em
issi
on
s[%
]
CO2 100 kPa
CO2 120 kPa
CO 100 kPa
CO 120 kPa
HC 100 kPa
HC 120 kPa
OHC 100 kPa
OHC 120 kPa
0
10
20
30
40
50
60
70
0.02 0.06 0.1 0.14 0.18 0.22 0.26Equivalence Ratio [φφφφ]
Fu
elC
into
CO
&H
C[%
]
30
40
50
60
70
80
90
100
Co
mb
ust
ion
effi
cien
cy[%
]
CO 600 rpmCO 1200 rpmCO 1800 rpmHC 600 rpmHC 1200 rpmHC 1800 rpmCE 600 rpmCE 1200 rpmCE 1800 rpm
Effect of Engine Speed on Bulk-Gas Reactions
Tin adjusted to maintain combustion phasing at TDC for φ = 0.14.– Higher compression temperatures compensate for reduced time for reactions.
Engine speed has little effect on the fueling rate at which the onset ofincomplete bulk-gas reactions occurs – for iso-octane.– In agreement with CHEMKIN computations.
Results suggest that special combustion strategies will be required forlow-load operation.
GDI Fueling: Vary Injection Timing
Early InjectionProvides a fairly uniform mixture.
– Can lead to incomplete bulk-gasreactions at low loads, aspredicted by CHEMKIN.
Late InjectionCan provide partial chargestratification.
– Mixture locally richer for thesame fueling rate.
– Offers the potential to mitigateincomplete bulk-gas reactions atlight loads.
Also, could prevent fuel fromreaching ring-land crevice.
– Reduce baseline emissions.
Variation in Injection Timing: φφφφ = 0.1
Tin = 142° C; Pin = 120 kPa;1200 rpm; GDI fueling
Early injection (0-90° aTDCintake) provides a well-mixedcharge.
– High CO and low combustionefficiency for φ = 0.1.
Retarding injection improvescombustion and emissions forlow-load operation.
– Injection at 290° reduces COand HC emission substantiallywith only about 1g/kg-fuel NOX(4 ppm).
– Combustion efficiencyincreases from 59% to 82%.
Further improvements possiblewith optimized stratification.
0
10
20
30
40
50
60
70
80
90
100
0 45 90 135 180 225 270 315 360Injection Timing [deg. CA]
IME
Pg
[kP
a]&
Co
mb
ust
ion
effi
cien
cy[%
]
0
2
4
6
8
10
12
14
16
18
20
Std
.Dev
.of
IME
Pg
[%]
IMEPg [kPa]
Combustion efficiency [%]
Std. Dev. IMEPg [%]
0
200
400
600
800
1000
1200
1400
0 45 90 135 180 225 270 315 360Injection Timing [deg. CA]
CO
&H
C[g
/kg
fuel
]
0
5
10
15
20
25
30
35
NO
x,50
xS
oo
t[g
/kg
fuel
]
CO HC
Soot NOx
Summary and Conclusions - 1
Metal HCCI research engine appears to be functioning well.
– At fully combusting conditions: ηthermal ~ 46%, σIMEP < 1%, CO < 65g/kg (1000 ppm), HC < 35 g/kg (1200 ppm), NOX ~ 0.06 g/kg (1 ppm).
CHEMKIN results show that for fuel loads below φ ~ 0.16, bulk-gasreactions are incomplete, even for an idealized adiabatic engine.
– Significant combustion inefficiencies, very high CO, and increased HC.> Temperatures are too low to complete reactions, mainly CO → CO2.
– Indicate that significant OHC emissions should occur. (OHC is ~2x HC).
Experimental data show a very similar trend to the changes inemissions and combustion efficiency as fuel loading is reduced.
– CO levels match very closely with those of the model (~65% of fuel C).> Bulk-gas must be the source.
– Onset of incomplete bulk-gas reactions occurs at higher φ ~ 0.2 due toheat transfer cooling the charge.
Summary and Conclusions - 2
The “missing carbon” in the emissions measurements matches theexpected OHC trends.– HC detector (FID) has low sensitivity to OHC.
Combustion stability was good even at idle loads, σIMEP ≤ 2.6%.
Increasing Pin from 1.0 to 1.2 bars had little effect on the onset ormagnitude of incomplete bulk-gas reactions when ignition timingwas maintained.
Changing speed from 600 to 1800 rpm has almost no effect on theonset or magnitude of incomplete bulk-gas reactions for iso-octane.
– Increased compression temperatures required to maintain ignitiontiming compensate for reduced time to complete reactions.
Partial charge stratification by late-cycle fuel injection appears tohave a strong potential for mitigating the difficulties of low-loadoperation.