Load Expansion of Stoichiometric HCCI Using Spark Assist and Hydraulic Valve Actuation

Jim Szybist, Eric Nafziger, and Adam WeallSeptember 29, 2010

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Purpose of advanced combustion strategies in gasoline engines: improve efficiency without increasing tailpipe-out emissions

• Tailpipe-out emissions are extremely low for gasoline engines due to mature 3-way catalyst exhaust aftertreatment at stoichiometric conditions

• Advanced combustion techniques must provide improved efficiency while not increasing tailpipe-out emissions– Either extremely low NOx or stoichiometric operation to maintain compatibility

with a 3-way catalyst

• To have largest impact, advanced combustion strategies must be applicable over portion of engine map where engine operates most

2007 Saab Biopowerefficiency contour

Vehicle speed trace for FTP driving cycle

Data markers from FTP driving cycle overlaid on efficiency contour

Previously unpublished data collected at ORNL’s chassis lab facility.Thanks to Brian West and others.

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Single cylinder research engine with Sturman hydraulic valve actuation (HVA)

• Modified 2.0L GM Ecotec engine with direct injection

• Cylinders 1-3 are disabled, cylinder 4 modified for Sturman HVA system

• Engine management performed with Drivven engine controller

• Custom pistons to increase compression ratio

• UTG-96 certification gasoline

9.20 CR 11.85 CR

Bore 86 mmStroke 86 mmConnecting Rod 145.5 mmFueling Direct InjectionCompression Ratio 11.85Valves per Cylinder 4

SI Combustion SA-HCCIFuel Rail Pressure (bar) 95 95Fuel Injection Timing (CA)* -280 -340Equivalence Ratio 1.0 1.0Intake Valve Opening (CA)* -344 -313 to -234Intake Valve Closing (CA)* -180 -180 to -124Intake Valve Lift (mm) 9 3 to 6Exhaust Valve Opening (CA)* 180 170Exhaust Valve Closing (CA)* 349 234 to 313Exhaust Valve Lift (mm) 9 2 to 3.5*0 CA refers to combustion TDC

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High load limit increased to 7.5 bar from 1000 to 3000 rpm with operating strategy

Attributes of the advanced combustion strategy

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Net

IMEP

(kPa

)

Engine Speed (rpm)

SI Combustion SA-HCCI

HCCI

• Stoichiometric A/F ratio

• Spark assist

• Negative valve overlap for internal

• No external EGR

• Unthrottled operation

• Variable intake valve closing angle to control effective compression ratio

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1000 2000 3000

BM

EP o

r IM

EP (k

Pa)

Engine Speed (rpm)

2002-01-0420(BMEP)

2002-01-0422(BMEP)

2005-01-0130(IMEP and spark assist)

Current Investigation Previous NVO Investigations

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Combustion analysis reveals role of spark assist2500 rpm, 2 bar IMEP

• By retarding spark timing and intake valve closing angle as load increases, pressure rise rate can be controlled to < 7bar/deg at all points

• Under light-load conditions, combustion event is dominated by volumetric combustion

• As engine load increases, a larger portion of the heat release occurs during the spark-ignited portion of combustion

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-360 -180 0 180 360

SI CombustionHCCI

Cyl

inde

r Pre

ssur

e (k

Pa)

Valve Lift (mm

)

Crank Angle

Intake Valves Exhaust Valves

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t Rel

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ssur

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Pa)

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Crank Angle

Intake Valves Exhaust Valves

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-20 -10 0 10 20 30 40 50 60

SI CombustionSA-HCCI Combustion

Hea

t Rel

ease

Rat

e (J

/deg

)

Crank Angle

Initial SI Portionof Combustion

2500 rpm, 6.5 bar IMEP

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Late intake valve closing allows control of maximum pressure rise rate

• Retarding intake valve closing reduces the effective compression ratio– Similar to technique using

late intake valve closing angle to mitigate knock demonstrated during ethanol optimization work*

– Effective CR varied from 11.85:1, which is the geometrical compression ratio, to 10:1

• Some tradeoff exists between spark timing and intake valve closing angle

Net

IME

P (k

Pa)

Engine Speed (rpm)

Maximum Pressure Rise (kPa/CA)

1000 1500 2000 2500 3000100

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t Rel

ease

Rat

e (J

/deg

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Crank Angle

Retarded IntakeValve Closing

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-158 -156 -154 -152 -150 -148

Max

Pre

ssur

e R

ise

Rat

e (k

Pa/C

A)

MFB

50 (CA

)

Intake Valve Closing Angle

Sweep of intake valve closing angle 2000 rpm, 5 bar IMEP

*For Example see SAE 2010-01-0619

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Improved combustion efficiency and NOx emissions for SA-HCCINOx emissions for SA-HCCI can still be substantial

• CO and HC emissions reduced by more than a factor of 2 at most operating points, resulting in very good combustion efficiency

– Low combustion efficiency for SI combustion can be attributed to un-optimized combustion and engine breathing differences caused by HVA valvetrain

• Near-zero NOx emissions at lowest loads, but sharp increase with increasing load– Due to a combination of larger fraction of SI combustion, less internal EGR, and more fuel consumed per

cycle– Stoichiometric A/F ratio and sufficiently high exhaust temperature maintains compatibility with 3-way catalyst

technology

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Ind

icat

ed

Sp

eci

fic

NO

x Em

issi

on

s (g

/kW

-h)

Net IMEP (kPa)

SA-HCCI Combustion

SI Combustion

Net

IME

P (k

Pa)

Engine Speed (rpm)

Combustion Efficiency (%)

1000 1500 2000 2500 3000100

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94

94.5

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95.5

96

96.5

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97.5

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98.5

99

Net

IME

P (k

Pa)

Engine Speed (rpm)

Combustion Efficiency (%)

1000 1500 2000 2500 3000100

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1200

94

94.5

95

95.5

96

96.5

97

97.5

98

98.5

99

SI Combustion

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SA-HCCI provides a substantial efficiency improvement at most operating conditions

• Efficiency benefits for SA-HCCI are greatest at low-load conditions– Thermal efficiency for SA-HCCI is nearly constant across entire operating range– Wide variations in efficiency in SI

• Increased efficiency for SA-HCCI is attributed to 3 factors1. Higher combustion efficiency2. Reduced pumping work3. Shorter combustion duration

Net

IME

P (k

Pa)

Engine Speed (rpm)

Indicated Thermal Efficiency (%)

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Net

IME

P (k

Pa)

Engine Speed (rpm)

Indicated Thermal Efficiency (%)

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SI CombustionSA-HCCI

Combustion

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Summary and conclusions• A mode of advanced combustion has been developed and base-lined

– Stoichiometric SA-HCCI with negative valve overlap– Unthrottled operation– Late intake valve closing angle and spark timing control pressure rise rate

• Capable of higher engine load than most advanced combustion strategies because of increased control over pressure rise rate

– Combustion event dominated by volumetric heat release at low loads– Larger fraction of fuel energy burned during SI combustion at higher loads

• Improvement in CO and HC emissions compared to SI combustion

• Improvement in NOx emissions, but sufficiently high that there is a need for NOxaftertreatment

– Stoichiometric A/F ratio maintains compatibility with conventional 3-way catalysts

• Efficiency benefit at most operating conditions– Efficiency benefit greatest at low load

• This study will be presented at the 2010 SAE Powertrains, Fuels and Lubricants Conference as SAE Technical Paper 2010-01-2172

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Paths to SA-HCCI implementation

• SA-HCCI combustion on cam valvetrain is the most likely route to commercial viability

– HVA valvetrains face numerous implementation barriers

• 2-step valvetrains have the potential to switch between conventional SI combustion and advanced combustion modes

– Conventional valve lifts and duration for SI combustion– Low lift, short duration valve events for advanced combustion

• Valve lift and duration of the advanced combustion cams will be fixed– Phase cams for desired NVO duration and intake valve closing angle– Fixed cam duration and lift will ultimately limit the operating range compared to HVA

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Future work

• This study is being done as part of the DOE fuels program

• A single, high octane gasoline was used in this initial study to baseline the operating strategy

• Continuing work is primarily investigating fuel effects– Fuels include low octane gasoline, ethanol blends (E10 and E85) and butanol

blends– Additional investigation and refinement of the operating strategy will continue as

part of fuel effects investigations

AcknowledgementThis research was supported by the US Department of Energy (DOE) Office of Vehicle Technology under the fuels technologies program with a DOE management team of Kevin Stork and Dennis Smith.

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Backup Slides

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Pressure rise is effectively controlled with a combination of spark timing and intake valve closing

• Retarding intake valve closing reduces effective compression ratio, retards combustion, and reduces pressure rise

• Retarding spark timing delays the start of combustion, retards combustion phasing, and reduces pressure rise

• The two control strategies are, to some extent, interchangeable– Tradeoffs between the two are a subject of an ongoing investigation

Net

IME

P (k

Pa)

Engine Speed (rpm)

Intake Valve Closing (CA)

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k Ti

min

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A)

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TDC

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k Ti

min

g (C

A)

Net IMEP (kPa)

TDC

Intake Valve Closing

Spark Timing

1000 rpm 3000 rpm

(♦) SI combustion and for (●) SA-HCCI combustion

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Efficiency comparison at constant engine speed

• At low engine speed, SA-HCCI provides an efficiency benefit for the entire operating range

• At high engine speed, SI combustion becomes more efficient at loads > 5 bar IMEP

• Discrepancy is due to higher efficiency for SI combustion rather than efficiency degradation for SA-HCCI

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0 200 400 600 800 1000 1200Indi

cate

d Th

erm

al E

ffici

ency

(%)

Net IMEP (kPa)

ISFC = 458 g/kW-h

ISFC = 235 g/kW-h

SA-HCCIISFC = 233 to 249 g/kW-h

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25

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35

40

0 200 400 600 800 1000 1200Indi

cate

d Th

erm

al E

ffici

ency

(%)

Net IMEP (kPa)

ISFC = 375 g/kW-h

ISFC = 212 g/kW-h

SA-HCCIISFC = 231

to 244 g/kW-h

1500 rpm 3000 rpm

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Pumping work and combustion duration differences

• Pumping work is reduced for SA-HCCI combustion strategy– Further reductions are likely possible with optimization of fuel injection timing and

valve events

• Shorter combustion duration allows for better utilization of expansion stroke– Very different trends in combustion duration for SI and SA-HCCI combustion– SA-HCCI combustion achieves much shorter duration

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SA-HCCI CombustionSI Combustion

PMEP

(kPa

)

Net IMEP (kPa)

Pumping Work

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SI Combustion: 1000 rpmSI Combustion: 1500 rpmSI Combustion: 2000 rpmSI Combustion: 2500 rpmSI Combustion: 3000 rpmSA-HCCI Combustion: 1000 rpmSA-HCCI Combustion: 1500 rpmSA-HCCI Combustion: 2000 rpmSA-HCCI Combustion: 2500 rpmSA-HCCI Combustion: 3000 rpmSA-HCCI Combustion Curve Fit

10-9

0% C

ombu

stio

n D

urat

ion

(CA

)

Net IMEP (kPa)

Increasing Engine Speed

Combustion Duration