Corona Discharge Ignition Corona Discharge Ignition for Advanced Stationary for Advanced Stationary

Natural Gas EnginesNatural Gas EnginesASME Internal Combustion Engine DivisionASME Internal Combustion Engine DivisionFall Technical Conference, Long Beach, CAFall Technical Conference, Long Beach, CA

October 25, 2004October 25, 2004

Supported by DOE-UREPSupported by DOE-UREP

Principal Investigator: Prof. Paul D. RonneyCo-Principal Investigator: Prof. Martin GundersenResearch Associates: Nathan Theiss, Dr. Jian-Bang LiuGraduate students: Fei Wang, Jun Zhao Undergraduate students: Brad Tallon, Matthew Beck

Jennifer Colgrove, Merritt Johnson, Gary Norris

ASME Paper # ICEF2004-891

MotivationMotivation• Multi-point ignition has the potential to increase Multi-point ignition has the potential to increase

burning rates in internal combustion enginesburning rates in internal combustion engines• (Simplest approach) Leaner mixtures (lower NO(Simplest approach) Leaner mixtures (lower NOxx))• (More difficult) Higher compression ratios + water injection (More difficult) Higher compression ratios + water injection

(higher efficiency with same NO(higher efficiency with same NOxx))• (Most difficult) Redesign intake port and combustion (Most difficult) Redesign intake port and combustion

chamber for lower turbulence since the same burn rate is chamber for lower turbulence since the same burn rate is possible with lower turbulence (reduced heat loss to walls, possible with lower turbulence (reduced heat loss to walls, higher efficiency)higher efficiency)

• Lasers, multi-point sparks challengingLasers, multi-point sparks challenging• Lasers: energy efficiency, windows, fiber optics…Lasers: energy efficiency, windows, fiber optics…• Multi-point sparks: multiple intrusive electrodesMulti-point sparks: multiple intrusive electrodes

• How to obtain multi-point, energy efficient ignition?How to obtain multi-point, energy efficient ignition?

Transient plasma (“pulsed corona”) dischargesTransient plasma (“pulsed corona”) discharges

• Not to be confused with “plasma torch”Not to be confused with “plasma torch”• Initial phase of spark discharge (< 100 ns) - highly Initial phase of spark discharge (< 100 ns) - highly

conductive (arc) channel not yet formedconductive (arc) channel not yet formed• CharacteristicsCharacteristics

• Multiple streamers of electrons - possible multiple ignition sitesMultiple streamers of electrons - possible multiple ignition sites• High energy (10s of eV) electrons compared to sparks (~1 eV)High energy (10s of eV) electrons compared to sparks (~1 eV)• Electrons not at thermal equilibrium with ions/neutralsElectrons not at thermal equilibrium with ions/neutrals• Low anode & cathode drops, little radiation & shock formation - Low anode & cathode drops, little radiation & shock formation -

more efficient use of energy deposited into gas more efficient use of energy deposited into gas • Enabling technology: USC-built discharge generators (Prof. Enabling technology: USC-built discharge generators (Prof.

Martin Gundersen)Martin Gundersen)

Corona vs. arc dischargeCorona vs. arc discharge

Arc channel

High voltage pulse

Corona StreamersPlasma Zone

Corona dies out in pulsed mode

Coaxial ground electrode - no dielectric barrier needed

High voltage pulse

Corona phase (0 - 100 ns)Corona phase (0 - 100 ns)

Arc phase (> 500 ns)Arc phase (> 500 ns)

Images of corona discharge & flameImages of corona discharge & flame

Axial (left) and radial (right) views of discharge Axial (left) and radial (right) views of discharge with rod electrodewith rod electrode

Axial view of discharge & flame Axial view of discharge & flame (6.5% CH(6.5% CH44-air, 33 ms between images)-air, 33 ms between images)

 

Characteristics of corona dischargesCharacteristics of corona discharges

• If arc forms, current increases some but voltage drops more, If arc forms, current increases some but voltage drops more, thus higher consumption of capacitor energy with little thus higher consumption of capacitor energy with little increase in energy deposited in gas (still have corona, but increase in energy deposited in gas (still have corona, but followed by (almost useless) arc)followed by (almost useless) arc)

Corona only Corona + arc

-5

0

5

10

15

20

25

-50

0

50

100

150

-50 0 50 100 150 200 250 300

Time (ns)

Energy

Voltage

Current

Startof arc

-5

0

5

10

15

20

25

-20

0

20

40

60

80

100

-50 0 50 100 150 200 250 300

Time (ns)

EnergyVoltage

Current

Corona discharges are energy-efficientCorona discharges are energy-efficient• Discharge efficiency Discharge efficiency dd ≈ 10x higher for corona than for ≈ 10x higher for corona than for

conventional sparksconventional sparks

€

d =Energy deposited in gas

Electrical discharge energy=

ΔP ⋅Volumeγ −1

IVdt∫

0.01

0.1

1

10 100 1000

Energy (mJ)

Corona, Threaded rod electrodeCylindrical combustion chamber

Spark, plain wire electrodes, gap = 1 mmCylindrical combustion chamber

Spark, Car spark plugIC engine like chamber

Corona, ring electrodeIC engine like chamber

Corona, 1 pin, Cylindrical combustion chamber

Program objectivesProgram objectives• Characterize advantages of pulsed corona discharges Characterize advantages of pulsed corona discharges

for NG ignition in static combustion chambersfor NG ignition in static combustion chambers• Integrate pulsed corona discharge ignition system into Integrate pulsed corona discharge ignition system into

stationary natural gas enginesstationary natural gas engines• 1998-2002 Ford Ranger, 2.5L SOHC 4-cylinder engine, 2 plugs 1998-2002 Ford Ranger, 2.5L SOHC 4-cylinder engine, 2 plugs

per cylinder (1 conventional plug, 1 corona ignition port)per cylinder (1 conventional plug, 1 corona ignition port)• Large-bore stationary natural gas engineLarge-bore stationary natural gas engine

• Determine if the ≈ 3x shorter burn times found with Determine if the ≈ 3x shorter burn times found with pulsed corona discharges apply to NG engines alsopulsed corona discharges apply to NG engines also

• If so, exploit the shorter burn timesIf so, exploit the shorter burn times• Assess the possibility for NOAssess the possibility for NOxx reduction using reduction using

additional corona discharges during the exhaust additional corona discharges during the exhaust strokestroke

Progress to dateProgress to date• Installed new engine in laboratory with two spark plug ports Installed new engine in laboratory with two spark plug ports

per cylinder (2000 Ford Ranger 2.5L I-4) and converted to NGper cylinder (2000 Ford Ranger 2.5L I-4) and converted to NG• Updated lab engine data acquisition & control system Updated lab engine data acquisition & control system

hardware and software (National Instruments / LabView)hardware and software (National Instruments / LabView)• Interfaced emissions analyzer with LabView systemInterfaced emissions analyzer with LabView system• Implemented student-designed in-cylinder pressure Implemented student-designed in-cylinder pressure

monitoring system on enginemonitoring system on engine• Built static test chamber that simulates engine geometry for Built static test chamber that simulates engine geometry for

electrode testing electrode testing • Constructed turbulent test chamber and conducted bench Constructed turbulent test chamber and conducted bench

tests to characterize effects of turbulence on corona ignition & tests to characterize effects of turbulence on corona ignition & combustioncombustion

• Studied and characterized minimum ignition energies of Studied and characterized minimum ignition energies of corona dischargescorona discharges

• Developed electrode for engine combustion chamber using Developed electrode for engine combustion chamber using machinable ceramicsmachinable ceramics

• Developed trigger system for firing corona generator on Developed trigger system for firing corona generator on engineengine

• Performed on-engine testing with pulsed corona discharge Performed on-engine testing with pulsed corona discharge firing on one cylinder over a range of air/fuel ratios, engine firing on one cylinder over a range of air/fuel ratios, engine loads and ignition timingloads and ignition timing

Oscilloscope Trigger Pulse

Generator

HV DCPowerSupply

Transformer

Pressure Transducer

AirFuel

Vacuum

Central Electrode

Cylindrical Combustion Chamber

Laboratory test apparatus (constant volume)Laboratory test apparatus (constant volume)• 2.5” (63.5 mm) diameter chamber, 6” (152 mm) long2.5” (63.5 mm) diameter chamber, 6” (152 mm) long• Energy release (stoich. CHEnergy release (stoich. CH44-air, 1 atm) ≈ 1650 J energy -air, 1 atm) ≈ 1650 J energy

release ≈ 60,000x minimum ignition energyrelease ≈ 60,000x minimum ignition energy• Energy input for ignition is trivial fraction of heat release!Energy input for ignition is trivial fraction of heat release!

• Delay time: 0 - 10% of peak pressure (can be Delay time: 0 - 10% of peak pressure (can be compensated for by adjusting “spark advance”)compensated for by adjusting “spark advance”)

• Rise time: 10% - 90% of peak pressure (can’t be fixed Rise time: 10% - 90% of peak pressure (can’t be fixed with spark advance!)with spark advance!)

DefinitionsDefinitions

2

4

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12

14

16

-0.02 0 0.02 0.04 0.06 0.08 0.1Time (s)

Rise Time

10% of total pressure rise

90% of total pressure rise

Rod electrode Single pin electrode

1 ring with multi-pins(only 4 pins case is shown)

Multi-rings with 2 pins/ring(Only 4 rings case is shown)

Insulation is indicated with shaded patern

Electrode configurationsElectrode configurations

10

100

0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05

corona, 1 pin, 75 mJ

spark, 75 mJ

corona, 3.9 mm dia rod, 710 mJ

corona, 2 ring x 2 pin, 170 mJ

corona, 4 ring x 2 pin 170 mJ

Delay Time (ms)

Equivalence ratio

CH

4

/Air

P = 1 atm

Effect of geometry on delay timeEffect of geometry on delay time• Spark delay time ≈ 2x larger than 1-pin corona (≈ same geometry)Spark delay time ≈ 2x larger than 1-pin corona (≈ same geometry)• Consistent with computations by Dixon-Lewis, Sloane suggesting point Consistent with computations by Dixon-Lewis, Sloane suggesting point

radical sources improve ignition delay ≈ 2x compared to thermal sourcesradical sources improve ignition delay ≈ 2x compared to thermal sources• More streamer locations (more pins, rod) yield lower delay time (≈ 3.5x More streamer locations (more pins, rod) yield lower delay time (≈ 3.5x

lower for rod than spark)lower for rod than spark)• Benefit of corona on delay time both chemical (≈ 1.5x) & geometrical (≈ 2x)Benefit of corona on delay time both chemical (≈ 1.5x) & geometrical (≈ 2x)

10

100

0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05

corona, 1 pin, 75 mJ

spark, 75 mJ

corona, 3.9 mm dia. rod, 710 mJ

corona, 2 ring x 2 pin, 170 mJ

corona, 4 ring x 2 pin, 170 mJ

Rise Time (ms)

Equivalence ratio

CH

4

/Air

P = 1 atm

Effect of geometry on rise timeEffect of geometry on rise time• Rise time of spark larger ≈ same as 1-pin corona (≈ same flame Rise time of spark larger ≈ same as 1-pin corona (≈ same flame

propagation geometry)propagation geometry)• More streamer locations (more pins, rod) yield lower rise time More streamer locations (more pins, rod) yield lower rise time

(≈ 3 - 4x lower for rod than spark), but multi-pin almost as good (≈ 3 - 4x lower for rod than spark), but multi-pin almost as good with much less energywith much less energy

0

20

40

60

80

100

120

0 100 200 300 400 500 600

corona, 1 ring x 2 pin

corona, 2 ring x 2 pin

corona, 4 ring x 2 pin

spark

corona, 3.9 mm dia rod

Delay Time (ms)

Discharge Energy (mJ)

CH

4

/Air

φ = 0.7

= 1 P atm

Energy & geometry effects (lean mixture)Energy & geometry effects (lean mixture)• What is optimal electrode configuration to minimize delay/rise What is optimal electrode configuration to minimize delay/rise

time for a given energy?time for a given energy?• Delay time: 2-ring, 4-ring & plain rod similar (all are much Delay time: 2-ring, 4-ring & plain rod similar (all are much

better than spark)better than spark)

Energy & geometry effects (lean mixture)Energy & geometry effects (lean mixture)• Rise time: 2-ring or 4-ring bestRise time: 2-ring or 4-ring best• Note “step” behavior for multi-point ignition at low energies - Note “step” behavior for multi-point ignition at low energies -

not all sites ignitenot all sites ignite• (Delay time doesn’t show “step” behavior)(Delay time doesn’t show “step” behavior)

0

50

100

150

200

250

300

350

0 100 200 300 400 500 600

corona, 1 ring x 2 pin

corona, 2 ring x 2 pin

corona, 4 ring x 2 pin

spark

corona, 3.9 mm dia rod

Rise Time (ms)

Discharge Energy (mJ)

CH

4

/Air

φ = 0.7

= 1 P atm

Simulated engine chamberSimulated engine chamber• Test fixture built to same dimensions as engine cylinder and Test fixture built to same dimensions as engine cylinder and

piston crown at TDC to test corona in this geometrypiston crown at TDC to test corona in this geometry• Enables initial testing of electrode geometries and visualization Enables initial testing of electrode geometries and visualization

of coronaof corona• Allows optimization of electrode geometries and discharge Allows optimization of electrode geometries and discharge

conditions before conducting on-engine testingconditions before conducting on-engine testing

Ignition in simulated engine chamberIgnition in simulated engine chamber• Delay time actually longer with corona in this geometry (but Delay time actually longer with corona in this geometry (but

can be compensated by ignition advance)can be compensated by ignition advance)• Rise time 2x faster with corona, with far lower energy inputRise time 2x faster with corona, with far lower energy input• Have ignited with corona only (no arc) up to 10 atmHave ignited with corona only (no arc) up to 10 atm

2

4

6

8

10

12

14

16

0 0.02 0.04 0.06 0.08 0.1

Spark (39 mJ)

Corona (14 mJ)

Corona + arc (46 mJ)

Time (s)

Pinitial

=3.5 atm.

CH4/Air

Equivalence ratio=1.0Ring electrode

Each curve is an average of 3 runs

Discharge Discharge typetype

Delay Delay time time (ms)(ms)

Rise Rise time time (ms)(ms)

CoronaCorona 2020 1010

Corona + Corona + arcarc 99 1919

SparkSpark 13.213.2 1919

Turbulent test chamberTurbulent test chamber

Fan

HV Anode

Grounded Cathode

Turbulence effectsTurbulence effects• Simple turbulence generator (CPU cooling fan + grid) Simple turbulence generator (CPU cooling fan + grid)

integrated into coaxial combustion chamber, rod electrodeintegrated into coaxial combustion chamber, rod electrode• Mean flow ≈ 11 m/s + turbulence intensity ≈ 1 m/s, u’/SMean flow ≈ 11 m/s + turbulence intensity ≈ 1 m/s, u’/SLL ≈ 3 ≈ 3

(stoichiometric) (stoichiometric) • Benefit of corona ignition ≈ same in turbulent flames - shorter Benefit of corona ignition ≈ same in turbulent flames - shorter

rise & delay times, higher peak Prise & delay times, higher peak P

1

1.5

2

2.5

3

3.5

4

-0.02 0 0.02 0.04 0.06 0.08 0.1Time (s)

CH4/Air

φ = 1.01 atm

, Quiescent spark

, Turbulent spark

, Turbulent corona

, Quiescent corona

Turbulence effectsTurbulence effects• Similar results for lean mixture but benefit of turbulence more Similar results for lean mixture but benefit of turbulence more

dramatic - higher u’/Sdramatic - higher u’/SLL (≈ 8) (≈ 8)

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3

Pressure (V)

Time (s)

CH4/Air

φ = 0.71 atm

, Quiescent spark

, Turbulent spark

, Turbulent corona

, Quiescent corona

Engine experiments at USCEngine experiments at USC• 2000 Ford Ranger I-4 engine with dual-plug head to test corona 2000 Ford Ranger I-4 engine with dual-plug head to test corona

& spark & spark at same time, same operating conditionsat same time, same operating conditions• National Instruments / Labview data acquisition & controlNational Instruments / Labview data acquisition & control• Horiba emissions bench, samples extracted from corona - Horiba emissions bench, samples extracted from corona -

equipped cylinderequipped cylinder• Pressure / volume measurementsPressure / volume measurements

• Optical Encoder mounted to crankshaftOptical Encoder mounted to crankshaft• Spark plug mounted Kistler piezoelectric pressure transducerSpark plug mounted Kistler piezoelectric pressure transducer

Electrode configuration Electrode configuration • Macor machinable ceramic used for insulatorMacor machinable ceramic used for insulator• Coaxial shielded cable used to reduce EMICoaxial shielded cable used to reduce EMI• Simple single-point electrode tip, replaceableSimple single-point electrode tip, replaceable

On-engine pulsed corona discharge ignition systemOn-engine pulsed corona discharge ignition system• Pulsed corona discharges generated using “pseudospark” switch + Blumlein transmission line, triggered from camshaftPulsed corona discharges generated using “pseudospark” switch + Blumlein transmission line, triggered from camshaft• ≈ ≈ 500 mJ/pulse (equivalent “wall plug” energy requirement of ≈ 50 mJ spark)500 mJ/pulse (equivalent “wall plug” energy requirement of ≈ 50 mJ spark)• Corona electrode and spark plug with pressure transducer in #1 cylinderCorona electrode and spark plug with pressure transducer in #1 cylinder• Switch wired for quick change between spark and corona ignition under identical operating conditionsSwitch wired for quick change between spark and corona ignition under identical operating conditions• Stock timing for spark ignition, variable timing for coronaStock timing for spark ignition, variable timing for corona• 3 modes tested3 modes tested

• Corona onlyCorona only• Single conventional plugSingle conventional plug• Two conventional plugs (results very similar to single plug)Two conventional plugs (results very similar to single plug)

On-engine pulsed corona discharge ignition systemOn-engine pulsed corona discharge ignition system

On-engine resultsOn-engine results• Corona ignition shows increase in peak pressure under Corona ignition shows increase in peak pressure under

all conditions testedall conditions tested

2000 rpm, 25 Ft-lb torque, Phi = 1

0

20

40

60

80

100

120

140

160

180

0 1000 2000 3000 4000 5000

Crank angle (deg)

spark

corona

On-engine resultsOn-engine results• Corona ignition shows increase in IMEP under all Corona ignition shows increase in IMEP under all

conditions testedconditions tested

3000 RPM, 25 ft-lb, phi = 0.7

0

50

100

150

200

250

0 10 20 30 40 50

Cyl Volume (cu in)

corona

spark

IMEP at various loadsIMEP at various loads• Corona showed an average increase in IMEP of 16% over a range of Corona showed an average increase in IMEP of 16% over a range of

engine loads, A/F ratios, ignition timingsengine loads, A/F ratios, ignition timings• Slight decrease in COV with coronaSlight decrease in COV with corona• Stronger ceramic is needed for electrode to test at higher loads - need Stronger ceramic is needed for electrode to test at higher loads - need

collaboration with plug manufacturercollaboration with plug manufacturer

3000 RPM, Phi = 0.7

0

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25

30

35

40

0 5 10 15 20 25

Torque (ft-lb)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

SparkCoronaSpark COVCorona COV

IMEP at various air / fuel ratiosIMEP at various air / fuel ratios

3000 RPM

20

25

30

35

40

45

50

0.5 0.7 0.9 1.1 1.3 1.5

Phi

IMEP (psia)

Spark

Corona

Burn ratesBurn rates• Corona ignition shows Corona ignition shows substantiallysubstantially faster burn rates at faster burn rates at

same conditions compared to 2-plug conventional ignitionsame conditions compared to 2-plug conventional ignition

2900 RPM, = 0.7, Pintake = 5.9 psia

0.01

0.1

1

10

0 0.1 0.2 0.3 0.4

Indicated Efficiency

spark

corona

Emissions data - NOxEmissions data - NOx• Improved NOImproved NOxx performance vs. indicated efficiency performance vs. indicated efficiency

tradeoff compared to spark ignition by using leaner tradeoff compared to spark ignition by using leaner mixtures with sufficiently rapid burningmixtures with sufficiently rapid burning

Emissions data - hydrocarbonsEmissions data - hydrocarbons

• Hydrocarbons emissions similar, corona vs. sparkHydrocarbons emissions similar, corona vs. spark

1

10

100

0 0.1 0.2 0.3 0.4

Indicated Efficiency

spark

corona

Emissions data - COEmissions data - CO

• CO emissions similar, corona vs. sparkCO emissions similar, corona vs. spark

1

10

100

1000

0 0.1 0.2 0.3 0.4

Indicated Efficiency

spark

corona

ConclusionsConclusions• Flame ignition by transient plasma (“pulsed corona”) Flame ignition by transient plasma (“pulsed corona”)

discharges is a promising technology for ignition delay & rise discharges is a promising technology for ignition delay & rise time reductiontime reduction

• More energy efficient than spark dischargesMore energy efficient than spark discharges• Shorter ignition delay and rise timesShorter ignition delay and rise times

» Rise time more significant issueRise time more significant issue• Longer than delay timeLonger than delay time• Unlike delay time, can’t be compensated by “spark Unlike delay time, can’t be compensated by “spark

advance”advance”• Higher peak pressuresHigher peak pressures• Benefits apply to turbulent flames alsoBenefits apply to turbulent flames also• Demonstrated in enginesDemonstrated in engines

» Higher IMEP (15% - 20%) for same conditions with same or Higher IMEP (15% - 20%) for same conditions with same or better BSNObetter BSNOxx

» Shorter burn times and faster heat releaseShorter burn times and faster heat release» Higher peak pressuresHigher peak pressures

• Improvements due to Improvements due to • Chemical effects (delay time) - radicals vs. thermal energyChemical effects (delay time) - radicals vs. thermal energy• Geometrical effects - (delay & rise time) - more distributed ignition Geometrical effects - (delay & rise time) - more distributed ignition

sitessites

Future WorkFuture Work

• Install corona ignition on all 4 cylindersInstall corona ignition on all 4 cylinders• Construct corona electrode from ceramic that can Construct corona electrode from ceramic that can

withstand higher engine loads - need collaboration with withstand higher engine loads - need collaboration with plug manufacturerplug manufacturer

• Test effectiveness of corona for NOTest effectiveness of corona for NOXX reduction in exhaust reduction in exhaust• Implement corona ignition on large bore stationary Implement corona ignition on large bore stationary

engineengine