1

Advanced Lean-BurnDI Spark Ignition Fuels Research

Magnus SjöbergSandia National Laboratories

May 10th, 2011

This presentation does not contain any proprietary, confidential, or otherwise restricted information

Project ID: FT006

2

Overview

• Project provides science to support industry to develop advanced lean/dilute-burn SI engines fornon-petroleum fuels.

• Project directions and continuation are reviewed annually.

• Project funded by DOE/VT.• FY10 - $630 K.• FY11 - $650 K.

Timeline Budget

Barriers• Goal is 45% peak efficiency.• Lack of fundamental knowledge of

advanced engine combustion regimes.• How to achieve both high combustion

robustness and fuel efficiency for SI engines using alternative fuels:

1. Lean, unthrottled DISI with spray-guided combustion.

2. Well-mixed charge and high boost.

Partners / Collaborators• PI: Sandia (M. Sjöberg)• 15 Industry partners in the Advanced

Engine Combustion MOU. • General Motors.• D.L. Reuss (formerly at GM).• HCCI Lab at Sandia.• LLNL & NUI Galway, Mechanisms.• UW Madison - KIVA modeling.• UNSW Australia – Multi-zone

Modeling.• Sandia – Biomass Conversion Team.

3

Objectives - RelevanceProject goals are to provide the science-base needed to understand:

• How emerging future fuels will impact the combustion systems of newhighly-efficient DISI light-duty engines currently being developed.

• How the fuels and combustion systems can be tailored to each other tomaximize thermal efficiency.

• Initial focus is on E85 and gasoline. Expand to other fuel blends (e.g. E20)and components (e.g. butanol and iso-pentanol) based on industry interest.

DISI with spray-guided stratified charge combustion system– Plagued by occasional misfires. – Depend highly on fuel-air mixture preparation/ignition/flame development.– These processes are strongly affected by fuel properties and required fuel mass.

• Study performance for both well-mixed stoichiometric and lean operation,and for lean stratified operation, and examine the effects of fuel properties.

• Develop high-speed optical diagnostics to be used to understand how to mitigate potential barriers (e.g. ensure robust combustion, and avoid superknock).

• Perform HCCI experiments to exploit the unique characteristics of ethanol.

4

• Combine metal- and optical-engine experiments and modeling to develop a broad understanding of the impact of fuel properties on DISI combustion processes.

• First, conduct performance testing with all-metal engine configuration over wide ranges of operating conditions and alternative fuel blends.

– Speed, load, intake pressure, EGR, and stratification level. Quantify engine operation and develop combustion statistics.

• Second, apply a combination of optical and conventional diagnostics to develop the understanding needed to mitigate barriers to high efficiency and robustness.

– Include full spectrum of phenomena; from intake flow to development of flame, and endgas autoignition (knock).

Supporting modeling:• Conduct chemical-kinetics modeling of flame-speed and autoignition for detailed

knowledge of governing fundamentals.– Perform validation experiment in HCCI lab and compare with literature.

• Collaborate with CFD modeling teams.

Approach

5

Technical Accomplishments• Commissioned engine and initialized all-metal performance testing.• Selected valve timings to provide low residuals and somewhat late IVC

(mild Miller cycle).• Performed an initial comparative study of E85 and gasoline for both well-

mixed stoichiometric and lean operation, and for lean stratified operation.• Characterized the robustness of the lean stratified spray-guided combustion

system for gasoline and E85.• Examined the direct effect of vaporization cooling on the thermal efficiency

for E85.• Optical engine experiments:

– Installed high-speed fuel-PLIF laser and set up laser-sheet forming optics.– Installed high-speed PIV laser and confirmed its performance.

• Used CHEMKIN to investigate the influence of in-cylinder conditions on the laminar flame speed for strong and weak cycles.

• Demonstrated the use of partial fuel stratification with ethanol to smooth HCCI HRR by vaporization-cooling-induced thermal stratification.(In HCCI lab at Sandia.)

6

• Piston bowl design is based on recommendations from GM.–Modified with cut-out for viewing into bowl.–Production-engine metal-piston rings.

Engine Configuration

High-speed 355 nm laser

for PLIF.

Oil and air jets.

Lower oil-control

cylinder.

Upper cooled

cylinder.

High-energyspark coil.

7

Engine Breathing• Bore = 86.0 mm, Stroke = 95.1 mm, 0.55 liter swept volume, CR = 12.• Selected valve timings to avoid valve overlap (not needed for low engine speeds.)• Provide low residual level (A) and somewhat late IVC (very mild Miller cycle).• Volumetric efficiency remains high even for low Pin.• Expanding exhaust-port/runner design provides low-amplitude pressure oscillation

during exhaust stroke (B), as predicted by GT-Power.• A and B minimize cyclic variability of residual mass.

– Residuals are now a relatively small factor when evaluating cyclic variations.

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140Intake Tank Pressure [kPa]

Volu

met

ric E

ffici

ency

[%]

1200 rpm1000 rpm

MotoredTin = 25°C

Tcoolant = 60°C

0.1

1

10

100

0.01 0.1 1Volume [liter]

Pres

sure

[bar

]

E85, φ = 1.0Pin = 44 kPa 1000 rpm

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Research Engine Layout• Two configurations of drop-down single-cylinder engine.• All-metal: Metal-ring pack and air/oil-jet cooling of piston (with lower cylinder for oil control).

Water-cooled exhaust for continuous operation.• Optical: Pent-roof windows, piston bowl window, 45° mirror, and quartz cylinder.• Identical combustion chamber geometry for both configurations, so no discrepancy between

performance testing and optical tests.• 8-hole injector with 60° included angle ⇒ 22° between each pair of spray center lines.

Spark gap is in between two sprays.

9

Parameter Space Parameter Current StudyCR 12Piston Bowl ∅ 46 mmIntake Flow TumbleValve Timings For Minimal

Residual LevelInjector &Spray Targeting

Bosch 8 x 60°C Straddling Spark

Pinj 170 barTcoolant 60°CTin 26°CEngine Speed 1000 rpmIMEPn 370 kPaPexhaust 100 kPaIntake Pressure 44 – 95 kPaEnd of Injection -294 to -25°CASpark Timing -36 to -14°CASpark Energy 6 – 116 mJEGR / [O2]in 21 – 17% O2

Fuel Type E85, Gasoline

• The parameter space is huge for performance testing.

– Grouped as hardware, static parameters& operating variables.

• Performed initial comparative study of E85 and gasoline.

– BMEP = 3 bar, so need to maintain IMEPn ≈370 kPa (all 4 strokes) for all comb. modes.

• Allows assessing the basic characteristics of combustion at one low load condition.

– One piece of the big picture.– Low load is relevant for stratified operation.– Study thermal efficiency and cyclic variability.

• Acquired data for 500 cycles per steady-state operating point.

– Cylinder, intake, exhaust, & fuel pressure.– Exhaust emissions and smoke.

• All presented well-mixed cases have spark timings for max IMEPn (≈ MBT-timing).

10

Gasoline Results• Thermal efficiency (TE) improves with lean

and lean-stratified operation.• Decreased pumping work is important factor.• Higher thermodynamic cycle efficiency for

compression/expansion is largest factor.– Lower in-cylinder heat transfer and less

exhaust heat (due to higher γ).• TE for stratified would ⇑ with later CA50, but

spark timing is not independent of EOI. (Examine in two slides.)

-10

-50

510

1520

2530

3540

45

Well-Mixed Phi = 1.0 Well-Mixed at LeanEfficiency Limit

Stratified, EOI =26°CAC

A50

[°C

A]

N

et In

dica

ted

Ther

mal

Eff.

[%]

Thermal EfficiencyCA50

+10.0%+24.0%

0

40

80

120

160

200

240

280

320

360

400

Well-Mixed Phi = 1.0 Well-Mixed at LeanEfficiency Limit

Stratified, EOI =26°CA

IMEP

n [k

Pa]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Equi

vale

nce

Rat

io [ φ

]

IMEPnPhi

-80

-60

-40

-20

0

20

40

60

80

100

Well-Mixed Phi = 1.0 Well-Mixed at LeanEfficiency Limit

Stratified, EOI =26°CA

PMEP

[kPa

]

Inta

ke P

ress

ure

[kPa

]

Intake PressurePMEP

11

Gasoline Cyclic Variability• Stoichiometric operation is very stable.

– Partly thanks to low residual level~5.7% by mass at this condition.

• Increased variability at lean efficiency limit.– Long burn duration,

with outlier cycles.

• Stratified combustion is fairly stable.– But 1 of 500 cycles misfires.– Not caused

by injectormalfunction.

– Need opticaldiagnostics tofind cause.

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a]

GasolineWell-mixed

φ = 1.0Pin = 44 kPaσ = 3.0 kPa

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a]

Gasoline, Well-mixedLean-Efficiency Limit

φ = 0.62Pin = 60 kPaσ = 8.6 kPa

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a]

Gasoline, StratifiedEOI = -26°CA

Spark = -22°CADwell = 4°CA

φ = 0.33, Pin = 96 kPaσ = 5.1 kPa, excl. misfire

150

155

160

165

170

175

-40 -30 -20 -10 0 10Crank Angle [CAD]

Inje

ctio

n Pr

essu

re [b

ar]

0

10

20

30

40

50

Cyl

inde

r Pre

ssur

e [b

ar]

Injection PressureCylinder Pressure

5 cycles spanning 1 misfire

320

330

340

350

360

370

380

390

5 10 15 20 25 30 35 4090% Burn Point [°CA]

IMEP

n [kP

a]

12

Stratified Spark-Timing Window• Stable stratified combustion requires careful

match of the spark timing to the injection event.

• Retarding the spark to phase CA50 closer to TDC does not work.

• Moving EOI and Spark in tandem improves TE, but higher cyclic variability and more misfires.

• Instead EGR can be used to phase CA50 later.–Study with E85.

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a]

Gasoline, StratifiedEOI = -26°CA

Spark = -22°CADwell = 4°CA

φ = 0.33, Pin = 96 kPaσ = 5.1 kPa, excl. misfire

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a]

Gasoline, StratifiedEOI = -26°CA

Spark = -21°CADwell = 5°CA

σ = 20 kPa

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a]

Gasoline, StratifiedEOI = -26°CA

Spark = -20°CADwell = 6°CA

σ = 39 kPa-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a] Gasoline, StratifiedEOI = -23°CA

Spark = -18°CADwell = 5°CA

σ = 29 kPa

13

E85 Results• TE improvement with well-mixed lean is nearly identical to gasoline.• Without EGR, TE improvement with stratified oper. is only +15%, vs. +24% for gasoline.• CA50 is very early, partly due to faster combustion.

– Fuel jets have 55% higher kinetic energy, and HRR is strongly influenced by mixing rates.• Apply EGR (N2 dilution) to phase CA50 later. Reduces NO as well. Find best trade-off.• Lower cyclic variability would improve thermal efficiency. (Examine in next slide).• TE improvement is still less than for gasoline. (Examine in three slides).

-15-10-505

1015202530354045

Well-MixedPhi = 1.0

Well-Mixedat Lean

EfficiencyLimit

Stratified,Most

Stable, NoEGR

Stratified,O2 =

18.0%,Best TE

Stratified,O2 =

17.5%,NO/TE

Trade-Off

Stratified,O2 =

17.5%, IfMadeStable

CA

50 [°

CA

], N

O [g

/kg

fuel

], N

et In

d. T

E [%

]

Thermal Eff.NOCA50

+9.9% +18.3%+15.0% +18.0% +19.8%

-10

0

10

20

30

40

50

-40 -30 -20 -10 0 10 20 30 40Crank Angle [°CA]

HR

R [J

/°CA

]

E85, 18.5mg fuelGasoline, 12.0mg fuel

StratifiedNo EGR

05

10152025303540

37.4 37.6 37.8 38 38.2 38.4 38.6Thermal Efficiency [%]

NO

[g/k

g fu

el]

E85Stratified

-15-10-505

1015202530354045

Well-MixedPhi = 1.0

Well-Mixedat Lean

EfficiencyLimit

Stratified,Most

Stable, NoEGR

Stratified,O2 =

18.0%,Best TE

Stratified,O2 =

17.5%,NO/TE

Trade-Off

Stratified,O2 =

17.5%, IfMadeStable

CA

50 [°

CA

], N

O [g

/kg

fuel

], N

et In

d. T

E [%

]

Thermal Eff.NOCA50

+9.9% +18.3%+15.0% +18.0% +19.8%

14

Stratified E85 Cyclic Variability• Cyclic variability increases with more EGR.• Higher TE would be realized if all cycles produce high IMEP.

– Need to understand cyclic variability before suggesting ways to achieve this.• Spark-energy sweep shows that the problem is not caused by failure to ignite,

as long as high spark energy is used.• For high spark energy, most low-IMEP

cycles have long burn durations.• Suggests that low IMEP is produced

by slow and incomplete flame propagation.• Examine if SL is relevant for explaining

two extreme cycles.

320

330

340

350

360

370

380

390

8 10 12 14 16 18 20 22 2410 - 90% Burn Duration [°CA]

IMEP

n [kP

a]

[O2] = 17.5% Spark Energy = 106 mJ0

0.10.20.30.40.50.60.70.80.9

1

310 320 330 340 350 360 370 380 390 400 410IMEPg [kPa]

Cum

ulat

ive

Prob

abili

ty [-

]

116 mJ106 mJ87 mJ67 mJ46 mJ

[O2] = 17.5%

Spark Energy

-500

50100150200250300350400

0 100 200 300 400 500Cycle number [-]

IMEP

n [kP

a] E85, Stratified[O2] = 17.5%EOI = -25°CA

Spark = -31°CADwell = -6°CA

σ = 9.5 kPa

15

Understanding Slow Burns• The laminar flame speed (SL) is one of the major

parameters that determine successful flame development.

• Flame modeling usingCHEMKIN-PRO atengine-relevant conditions.

• Use Ethanol torepresent E85.

• Flame speed increases rapidly with temperature.• Stoichiometric mixture has most robust flame.• CA-resolved SL shows that too-lean mixture

is a potential cause of slow burn.• Weak cycle has lower gas temperatures

ahead of the flame.– Contributes to slow burn rate of too-lean mixtures.

• Need to apply optical diagnostics for complete understanding of slow burns for these conditions.

-10

0

10

20

30

40

-40 -30 -20 -10 0 10 20 30 40Crank Angle [°CA]

HR

R [J

/°CA

]

StrongCycleWeak

E85, Stratified[O2] = 17.5%EOI = -25°CA

Spark = -31°CADwell = -6°CA

01020304050

Pres

sure

[bar

]

700

800

900

1000

Air

Tem

p. [K

]

0

20

40

60

80

100

120

500 600 700 800 900 1000Gas Temperature [K]

Lam

inar

Fla

me

Spee

d [c

m/s

]

Phi = 1.0Phi = 1.5Phi = 0.5

Pressure = 40 bar

01020304050607080

Lam

. Fla

me

Spee

d [c

m/s

] Including Vaporization

Cooling

φ = 0.5

φ = 1.0

Strong CycleWeak Cycle

16

TE Improvement Comparison1. Even with improved combustion stability, TE

improvement with stratified combustion would still be lower with E85 (19.8 vs. 24.0%).

2. Stoichiometric E85 operation has 3% higher TE.• Both explained by strong vap. cooling with E85.• For early injection, lower peak-combustion

temperatures provide higher work-extraction efficiency. (Higher γ.)

• E85 vaporization using valuable exergy near TDC hurts the thermal efficiency.

300

400

500

600

700

800

-60 -40 -20 0 20 40Crank Angle [°CA]

Tem

pera

ture

[K]

0

10

20

30

40

50

Pres

sure

[bar

]

E85, StratifiedNo Spark

SOI = -33°CAEOI = -25°CA

Motored w/o fuelwith fuel

0

5

10

15

20

25

30

35

40

45

Gasoline, Phi =1.0

Gasoline,Stratified

E85, Phi = 1.0 E85 Stratified,If Made Stable

Net

Indi

cate

d Th

erm

al E

ff. [%

]

+24% +19.8%

+3%

-505

101520253035

-30 -20 -10 0 10 20 30 40 50Crank Angle [°CA]

HR

R [J

/°CA

]

GasolineE85

600

900

1200

1500

1800

2100

2400

Tem

pera

ture

[K] Gasoline

E85

Phi = 1.0Well-Mixed

SOI = -300°CA

17

Ethanol HCCI Experiments• Ethanol vaporization cooling is 5x greater than for iso-octane.• Ethanol is a true single-stage ignition fuel.

– Autoignition timing is sensitiveto temperature.

• Vaporization cooling increases with φ,so CA10 retards strongly for DI.

• Use Partial Fuel Stratification (PFS) forstrong reduction of peak HRR and PRR.

-50

0

50

100

150

200

250

300

354 356 358 360 362 364 366 368 370 372 374Crank Angle [°CA]

Hea

t-Rel

ease

Rat

e [J

/°CA

] SOI = 40°CA, 20% DISOI = 280°CA, 20% DISOI = 280°CA, 30% DISOI = 280°CA, 40% DI

140145150155160165170175180185

0 20 40 60 80 100 120 140 160 180 200 220Start of Direct Injection [°CA]

BD

C T

empe

ratu

re [°

C]

95% Ethanol + 5% H2O, CR = 14iso-Octane, CR = 18

IVC

Pre

mix

ed

Pre

mix

ed

Theory

18

• General Motors.– Hardware, discussion partner of results, and for development of diagnostics.

• D.L. Reuss (formerly at GM, now at UM).– Development of optical diagnostics for high-speed PIV and PLIF.

• 15 Industry partners in the Advanced Engine Combustion MOU.– Biannual meetings with 10 OEMs and 5 energy companies.

• Sandia – Biomass Conversion Team.– Discussions of potential biofuels and compatibility with engine combustion.

• Sandia HCCI Lab (J.E. Dec).– Reference HCCI autoignition data and PFS operation with ethanol.

• UNSW – Australia (E. Hawkes).– Multi-zone modeling of ethanol SCCI.

• UW-M (J. Brakora, R. Reitz).– KIVA-CFD.

• LLNL (W. Pitz) & Univ. of Galway (H. Curran).– Chemical-kinetics mechanisms.

Collaborations / Interactions

19

Future Work FY 2011 – FY 2012• Make hardware alterations to allow deactivation of one valve to create swirl flow,

and contrast with current tumble-flow results. Add intake air heater.

• Expand operating range with gasoline and E85 to include higher speeds and boosted operation. Study mid-range blends (~E40) for selected operating points.

• Discuss results with industry partners and decide on most relevant operating points to study optically.

• Install quartz windows for piston-bowl and pent-roof access.

• Finish the installation of laser-sheet imaging for high-speed PLIF and PIV.

• Design and install full-quartz cylinder for better optical access.• Apply optical diagnostics to identify the in-cylinder processes that are

responsible for sporadic misfire cycles and partial burns.–Correlate variations in the flame growth with fuel concentration and flow field near

spark, and with large-scale intake and compression flow field.

• Continue using CHEMKIN to investigate combustion fundamentals.

• Study fuel effects on both regular knock and low-speed preignition/superknock under highly boosted conditions.

2012

20

11

20

Summary• The new lab is contributing to the science-base for the impact of alternative

fuel blends on advanced SI engine combustion.

• Using gasoline, lean stratified operation provides significant improvement of thermal efficiency.

• Improvements of TE are less with E85.• Strong vaporization cooling for fuel injection near TDC hurts efficiency.• Heat of vaporization is important factor that needs to be considered when

pursuing future fuels.• Cycle-to-cycle variations can be significant for low-NOx operation.

– More stable combustion would provide higher thermal efficiency.

• Development of high-speed optical diagnostics is nearly finished.– Apply to understand cyclic variability and propose ways to make more stable.

• Engine companies are encouraged to discuss the current project with us.– Welcome suggestions for both operating strategies and fuel blends.

• For HCCI, partial fuel stratification with ethanol offers potential to reduce peak HRR and lower PRR.

– Vaporization cooling enhances the naturally occurring thermal stratification.

21

Technical Back-Up Slides

22

• While DISI was being built, performed experiments in Dec’s HCCI lab to assess ethanol autoignition characteristics and compared with gasoline, iso-octane and other fuels.

• Covered wide range of conditions:– Engine speed.– Intake boost pressure.– Fuel/air equivalence ratio – φ.– Charge temperature.– EGR and constituents.– Vaporization cooling.– Partial fuel stratification using ethanol.

SAE Paper 2010-01-0338

Combustion Symposium 2010

HCCI Experiments

JSAE Paper for Kyoto meeting

• Ethanol is a true single-stage fuel with minimal early heat release.• Autoignition timing is sensitive to changes of temperature.• Ethanol has very strong vaporization cooling.• Partial-fuel stratification with ethanol therefore has potential to achieve

an extended burn duration.• Higher φ regions ignite last as those have more vaporization cooling.

23

Cummins B0.98 liter / cyl.

All-metal HCCI Engine

CR = 14

Combine Premixed and

DI Fueling

24

Vaporization Cooling• Vaporization-cooling effects can be particularly strong for ethanol.

• Test 190 proof ethanol (95% ethanol, 5% water). “Worst-case scenario”.• Potential gas cooling is >5x that of iso-Octane.• Observed cooling matches theory well.• Maximum (observable) cooling effect occurs for SOI ~2/3 of intake stroke.• Minimizes heat transfer from piston, so heat for vap. comes mostly from air.

0

5

10

15

20

25

Fuel per Cycle[mg]

Heat of Vap.(10x) [MJ/kg]

Heat of Vap.[J]

Potential GasCooling [K]

iso-OctaneEthanol

φ = 0.240.75 g air/cycle

+79%

+202%

+441%

+441%

140145150155160165170175180185

0 20 40 60 80 100 120 140 160 180 200 220Start of Direct Injection [°CA]

BD

C T

empe

ratu

re [°

C]

95% Ethanol + 5% H2O, CR = 14iso-Octane, CR = 18

IVC

Pre

mix

ed

Pre

mix

ed

Theory

25

φ – Sensitivity / PFS• Using a Fire-19/1 technique, both Tresiduals and Twall are held constant.• Quantify the combined effects of vap. cooling, γ and fuel-chemistry.• φ -sensitivity becomes stronger than for premixed operation.• SOI = 40 CA. Later SOI ⇒ more cooling

and higher φ -sensitivity.

• Partial Fuel Stratification (PFS) combines1. Premixing of most fuel with2. Late injection with remainder of fuelto achieve a staged combustion event.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Cycle #

Pres

sure

φ = variable

Acquired cycle

φ = 0.42Fire 19/1

1100

1050

1000

9509

1

1

1

Hwang and Dec, SAE 2007-01-4130

SOI = 305°CA

Iso-octane

26

PRR and HRR• Lowest PRR is observed for SOI = 270 – 280°CA.

– Decreases by 39%, from 9.8 to 6.0 bar/°CA (for 40%DI).• Peak HRR is reduced, and more early HR as leanest zones ignite first.• Shows that the thermal stratification has been enhanced.• Response of PRR to SOI is complex.• Fuel-vaporization cooling can

counteract natural thermalstratification due to heat transfer.

5

6

7

8

9

10

11

0 45 90 135 180 225 270 315Start of Injection [°CA aTDC]

Max

. PR

R [b

ar/°C

A]

20% DI30% DI40% DI

-50

0

50

100

150

200

250

300

354 356 358 360 362 364 366 368 370 372 374Crank Angle [°CA]

Hea

t-Rel

ease

Rat

e [J

/°CA

] SOI = 40°CA, 20% DISOI = 280°CA, 20% DISOI = 280°CA, 30% DISOI = 280°CA, 40% DI

KIVA-CFDby J. Brakora