Flame initiation by
nanosecond plasma discharges:
Putting some new spark into ignition
Paul D. Ronney
University of Southern California, USA
National Central University
Jhong-Li, Taiwan, October 3, 2005
Research supported by U.S. AFOSR, ONR & DOE
Travel supported by the Combustion Institute
Faculty collaborator: Martin Gundersen (USC-EE)
Research Associates: Nathan Theiss, Jian-Bang Liu
Graduate students: Jason Levin, Fei Wang,
Jun Zhao, Tsutomu Shimizu
Undergraduate students: Brad Tallon, Matthew Beck
Jennifer Colgrove, Merritt Johnson, Gary Norris
University of Southern California
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Paul Ronney
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
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B.S. Mechanical Engineering, UC Berkeley
M.S. Aeronautics, Caltech
Ph.D. in Aeronautics & Astronautics, MIT
Postdocs: NASA Glenn, Cleveland; US Naval Research Lab,
Washington DC
 Assistant Professor, Princeton University
 Associate/Full Professor, USC
 Research interests
 Microscale combustion and power generation
(10/4, INER; 10/5 NCKU)
 Microgravity combustion and fluid mechanics (10/4, NCU)
 Turbulent combustion (10/7, NTHU)
 Internal combustion engines
 Ignition, flammability, extinction limits of flames (10/3, NCU)
 Flame spread over solid fuel beds
 Biophysics and biofilms (10/6, NCKU)
Paul Ronney
Transient plasma ignition - motivation
 Multi-point ignition of flames has potential to increase
burning rates in many types of combustion engines, e.g.
 Pulse Detonation Engines
 Reciprocating Internal Combustion Engines
» (Simplest approach) Leaner mixtures (lower NOx)
» (More difficult) Redesign intake port and combustion chamber for
lower turbulence since the same burn rate is possible with lower
turbulence (reduced heat loss to walls, higher efficiency)
 High altitude restart of gas turbines
 Lasers, multi-point sparks challenging
 Lasers: energy efficiency, windows, fiber optics
 Multi-point sparks: multiple intrusive electrodes
 How to obtain multi-point, energy efficient ignition?
Transient plasma (“pulsed corona”) discharges
 Not to be confused with “plasma torch”
 Initial phase of spark discharge (< 100
ns) - highly conductive (arc) channel
not yet formed
 Characteristics
 Multiple streamers of electrons
 High energy (10s of eV) electrons compared
to sparks (~1 eV)
 Electrons not at thermal equilibrium with
ions/neutrals
 Ions stationary - no hydrodynamics
 Low anode & cathode drops, little radiation
& shock formation - more efficient use of
energy deposited into gas
Corona vs. arc discharge
Corona phase (0 - 100 ns)
Arc channel
High voltage
pulse
Arc phase (> 100 ns)
Images of corona discharge & flame
Axial (left) and radial (right) views of discharge
with rod electrode
Axial view of discharge & flame
(6.5% CH4-air, 33 ms between images)
Characteristics of corona discharges
 For short durations (1’s to 100’s of ns depending on
pressure, geometry, gas, etc.) DC breakdown threshold of
gas can be exceeded without breakdown if high voltage
pulse can be created and stopped quickly enough
Breakdown strength (kV/cm)
100
90
Transient
Steady
80
70
60
50
40
30
20
0
50
100
Time (ns)
150
200
Characteristics of corona discharges
Voltage (KV)
Energy
80
15
60
10
40
5
20
Current
0
-5
-50
0
50
100 150
Time (ns)
0
200
250
-20
300
20
150
Voltage
100
Energy
15
Current
10
50
5
0
0
-5
-50
Start
of arc
0
50
100 150
Time (ns)
200
250
-50
300
Corona + arc
Corona only
If arc forms, current increases some but voltage drops more,
thus higher consumption of capacitor energy with little
increase in energy deposited in gas (still have corona, but
followed by (relatively ineffective) arc)
Current (amps) or Energy (mJ)
Voltage
Current (amps) or Energy (mJ)
20
25
100
Voltage (KV)
25
Corona discharges are energy-efficient
 Discharge efficiency d ≈ 10x higher for corona than
conventional sparks
P  Volume
Energy deposited in gas
 1
d 

Electrical discharge energy
 IVdt

Discharge efficiency
1
0.1
0.01
10
Corona, 1 pin, Cylindrical combustion chamber
Corona, ring electrode
IC engine like chamber
Corona, Threaded rod electrode
Cylindrical combustion chamber
Spark, plain wire electrodes, gap = 1 mm
Cylindrical combustion chamber
Spark, Car spark plug
IC engine like chamber
100
Energy (mJ)
1000
Objectives
 Compare combustion duration and ignition energy
requirements of spark-ignited and corona-ignited flames in
constant-volume vessel
 Determine effect of corona electrode geometry and ignition
energy on combustion duration
 Determine if reduced combustion duration observed for
corona ignition in quiescent, constant-volume experiments
also applies to turbulent flames
 Integrate pulsed corona discharge ignition system into
premixed-charge IC engines
 Compare performance of corona-ignited and spark-ignited
engines
 Efficiency
 Emissions
Experimental apparatus (constant volume)
 Pulsed corona discharges generated using thyratron or
“pseudospark” gas switch + Blumlein transmission line
 2.5” (63.5 mm) diameter chamber, 6” (152 mm) long
 Rod electrode (shown below) or single-needle
 Energy release (stoich. CH4-air, 1 atm) ≈ 1650 J energy release ≈
 Discharge energy input for ignition is trivial fraction of heat release!
Definitions
 Delay time: 0 - 10% of peak pressure
 Rise time: 10% - 90% of peak pressure
12
10
8
6
90% of total
pressure rise
Delay
Time
14
Discharge trigger
Pressure (atm., abs)
16
Rise Time
10% of total pressure rise
4
2
-0.02
0
0.02
0.04
Time (s)
0.06
0.08
0.1
Electrode configurations
Rod electrode
1 ring with multi-pins
(only 4 pins case is shown)
Single pin electrode
Multi-rings with 2 pins/ring
(Only 4 rings case is shown)
Insulation is indicated with shaded patern
Pulsed corona discharges in IC engine-like geometry
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Top view
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Side view
Minimum ignition energy vs. mixture
 1 pin corona discharge vs. spark - ≈ same geometry
 MIE significantly higher (≈ 100x) for corona - more distributed
energy deposition in streamers?
 Minimum spark kernel diameter ≈ 0.2 mm for stoich. CH4-air
1000
CH /Air
Energy (mJ)
4
1 pin electrode
1 atm
100
Pulsed corona
10
Spark (Lewis and von Elbe)
1
0.1
0.6
0.7
0.8
0.9
1
1.1
Equivalence ratio
1.2
1.3
Pressure effects on MIE
Energy (mJ)
 MIE for pulsed corona does NOT follow Emin ~ P-2 as spark
ignition does; more like P-1 at low P, P0 at higher P
 Smaller chamber diameter enables ignition at higher P higher voltage gradient
Ignited (2.5")
Not Ignited (2.5")
Ignited (1.1")
Not Ignited (1.1")
MIE (2.5")
MIE (1.1")
100
CH -air,  = 1
4
Single-pin electrode
10
0.1
1
P (atm)
10
Effect of geometry on delay time
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)
100
CH /Air
4
10
0.65
P = 1 atm
0.7
0.75
0.8
0.85
0.9
Equivalence ratio
0.95
1
1.05
Effect of geometry on delay time
 Delay time of spark larger (≈ 1.5 - 2x) than 1-pin corona (≈
same geometry)
 Consistent with computations by Dixon-Lewis, Sloane that
suggest point radical sources improve ignition delay ≈ 2x
compared to thermal sources
 More streamer locations (more pins, rod) yield lower delay
time (≈ 3.5x lower for rod than spark)
 Suggests benefit of corona is both chemical (1.5 - 2x) and
geometrical (≈ 2x)
Effect of geometry on rise time
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)
100
CH /Air
4
P = 1 atm
10
0.65
0.7
0.75
0.8
0.85
0.9
Equivalence ratio
0.95
1
1.05
Effect of geometry on rise time
 Rise time of spark larger ≈ same as 1-pin corona (≈ same
flame propagation geometry)
 More streamer locations (more pins, rod) yield lower rise
time (≈ 3 - 4x lower for rod than spark), but multi-pin almost
as good with less energy
Peak pressures
6
CH /Air
Peak P/P
o
5.5
4
1 atm
5
4.5
4
corona, 1 pin, 75 mJ
spark at center, 75 mJ
corona rod, 710 mJ
corona, 2 ring x 2 pin, 170 mJ
corona, 4 ring x 2 pin, 170 mJ
3.5
3
0.65
0.7
0.75
0.8
0.85
0.9
0.95
Equivalence ratio
1
1.05
Peak pressures
 Peak pressures significantly higher for multi-point corona
that one-pin corona or spark
 Improvement (for rod) nearly independent of mixture
 Probably due to change in flame propagation geometry, not
heat losses
 Radial propagation (corona) vs. axial propagation (arc)
 Corona: more combustion occurs at higher pressure (smaller
quenching distance)
 Corona: lower fraction of unburned fuel
 Consistent with preliminary measurements of residual fuel
Energy & geometry effects on delay time
 What is optimal electrode configuration to minimize
delay/rise time for a given energy?
 Delay time: 2-ring, 4-ring & plain rod similar (all are much
better than spark)
35
corona, 1 ring x 2 pin
corona, 2 ring x 2 pin
corona, 4 ring x 2 pin
corona, 3.9 mm dia. rod
Spark
Delay Time (ms)
30
25
20
CH /Air
15
 = 1.0
P = 1 atm
4
10
5
00
100
200
300
400
500
Discharge energy (mJ)
600
700
Energy & geometry effects on rise time
 Rise time: 2-ring or 4-ring best
 Note “step” behavior for multi-point ignition at low energies
- not all sites ignite
 Delay time doesn’t show “step” behavior
70
corona,
corona,
corona,
corona,
Spark
Rise Time (ms)
60
50
1 ring x 2 pin
2 ring x 2 pin
4 ring x 2 pin
3.9 mm dia. rod
40
CH /Air
30
 = 1.0
P = 1 atm
4
20
10
0
0
100
200
300
400
500
Discharge energy (mJ)
600
700
Energy & geometry effects (lean mixture)
 Delay time: same conclusion as stoichiometric mixture
Delay Time (ms)
120
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
100
80
60
40
CH /Air
4
= 0.7
P = 1 atm
20
0
0
100
200
300
400
Discharge Energy (mJ)
500
600
Energy & geometry effects (lean mixture)
 Rise time: 4-ring stands out
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
350
Rise Time (ms)
300
250
CH /Air
4
= 0.7
P = 1 atm
200
150
100
50
0
0
100
200
300
400
Discharge Energy (mJ)
500
600
Rod diameter effects
File:030820
 Plain rod: optimal diameter
exists (≈ 0.15”), drod/dcyl ≈ 0.06
 Large d: low field
concentration, few streamers?
 Small d: Too many streamers,
too much energy deposition?
Delay or Rise Time (ms)
70
Delay T ime (ms)
60
Rise T ime (ms)
CH4/Air
Equivalence ratio: 1.0
P=1 atm.
Rod-cylinder electrode
Rod diameter: 0.09"
50
40
30
20
10
0
0
200
File:030813
Delay Time (ms)
Rise Time (ms)
50
CH4/Air
Equiv alence ratio: 1.0
P=1 atm.
Rod-cy linder electrode
Rod diameter: 0.155"
40
30
20
10
0
0
100
200
300
400
500
Energy (mJ/pulse)
600
600
800
700
1000
File:030818
100
Delay or Rise Time (ms)
Delay or Rise Time (ms)
70
60
400
Energy (mJ/pulse)
Delay Time (ms)
Rise Time (ms)
80
CH4/Air
Equiv alence ratio: 1.0
P=1 atm.
Rod-cy linder electrode
Rod diameter: 0.375"
60
40
20
0
0
100
200
300
400
Energy (mJ/pulse)
500
600
CH4/Air
Equivalence ratio: 0.7
Delay Time (ms)
P=1 atm.
Rise T ime (ms)
1 ring x 1 pin electrode
180
Delay T ime (ms)
File:030416
Rise T ime (ms )
Delay or Rise Time (ms)
Delay or Rise Time (ms)
Effect of number of pins on 1 ring
160
140
120
100
80
180
File:030509
160
140
120
100
80
60
60
80
90
100
110
120
130
140
40
150
60
Delay Time (ms)
Rise Time (ms)
Delay Time (ms)
File:030515a
Rise T ime (ms)
Delay or Rise Time (ms)
200
CH4/Air
Equiv alence ratio: 0.7
P=1 atm.
1 ring x 4 pin electrode
150
100
250
50
60
70
80
Energy (mJ)
90
100
110
100
120
140
CH4/Air
Equivalence ratio: 0.7
P=1 atm.
1 ring x 8 pin electrode
File:030516
200
150
100
50
0
120
50
40
80
Energy (mJ)
Energy (mJ)
Delay or Rise Time (ms)
CH4/Air
Equivalence ratio: 0.7
P=1 atm.
1 ring x 2 pin electrode
140
160
180
200
Energy (mJ)
220
240
Effect of number of pins on 1 ring
 MIE lower (!!) with more pins, optimal 4
 More pins: Slightly beneficial effect on delay time, slightly
adverse effect (!) on rise time
 More is not necessarily better!
Energy (mJ) or time (ms)
250
Minimum ignition energy
Maximum energy without arcing
Average delay time
Average rise time
200
CH /Air
4
 = 0.7
P = 1 atm
150
100
50
0
0
2
4
6
Number of pins
8
10
Thyratron vs. pseudospark generator
 Little effect of discharge generator type (pseudospark: ≈ 1/2
discharge duration compared to thyratron)
20
20
Pseudo-spark generator
15
Thyratron-switched generator
10
5
0
400
Rise Time (ms)
Delay Time (ms)
Pseudo-spark generator
CH /Air
4
600
700
Energy (mJ/pulse)
800
900
Thyratron-switched generator
10
5
 = 1.0
1 atm
Threaded electrode
500
15
0
400
CH /Air
4
 = 1.0
1 atm
Threaded electrode
500
600
700
800
Discharge energy (mJ)
900
Turbulent test chamber
HV Anode
Fan
Grounded Cathode
Turbulence effects
 Simple turbulence generator (fan + grid) integrated into coaxial
combustion chamber, rod electrode
 Turbulence intensity ≈ 1 m/s, u’/SL ≈ 3 (stoichiometric)
 Benefit of corona ignition ≈ same in turbulent flames - shorter rise
& delay times, higher peak P
 Note quiescent corona faster than turbulent spark! (Faster burn
with less heat loss)
4
CH /Air
4
Pressure (atm)
3.5
 = 1.0
1 atm
3
Quiescent, spark
2.5
2
Turbulent, spark
Quiescent, corona
1.5
Turbulent, corona
1
-0.02
0
0.02
0.04
Time (s)
0.06
0.08
0.1
Turbulence effects
 Similar results for lean mixture but benefit of turbulence
more dramatic - higher u’/SL (≈ 8)
0.6
CH /Air
0.55
Pressure (V)
0.5
4
 = 0.7
1 atm
Quiescent, corona
0.45
0.4
Quiescent, spark
0.35
Turbulent, spark
0.3
Turbulent, corona
0.25
0.2
-0.05
0
0.05
0.1
0.15
Time (s)
0.2
0.25
0.3
Engine experiments
2000 Ford Ranger I-4 engine with dual-plug head to test
corona & spark at same time, same operating conditions
National Instruments / Labview data acquisition & control
Horiba emissions bench, samples extracted from corona equipped cylinder
Pressure / volume measurements
 Optical Encoder mounted to crankshaft
 Spark plug mounted Kistler piezoelectric pressure transducer
Electrode configuration
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
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
Macor machinable ceramic used for insulator
Coaxial shielded cable used to reduce EMI
Simple single-point electrode tip, replaceable
“Point to plane” geometry first step - by no means optimal
On-engine corona ignition system
 Corona electrode and spark plug with pressure transducer
in #1 cylinder
 Wired for quick change between spark and corona ignition
under identical operating conditions
 ≈ 500 mJ/pulse (equivalent “wall plug” energy requirement
of ≈ 50 mJ spark)
 Range of ignition timings for both spark & corona
 3 modes tested
 Corona only
 Single conventional plug
 Two conventional plugs (results very similar to single plug)
On-engine corona ignition system
On-engine results
 Corona ignition shows increase in peak pressure under
all conditions tested
On-engine results
 Corona ignition shows increase in IMEP under all
conditions tested
IMEP at various air / fuel ratios
 Indicated mean effective pressure (IMEP) higher for corona
than spark, especially for lean mixtures (nearly 30%)
 Coefficient of variance (COV) comparable
40
0.1
35
IMEP (psi)
30
IMEP (spark)
IMEP (corona)
25
0.06
20
0.04
15
10
5
0
0.65
0.02
COV (spark)
COV (corona)
0.7
0.75
0.8
0.85
0.9
Equivalence ratio
0.95
1
0
1.05
Coefficient of Variance
0.08
IMEP at various loads
 Corona showed an average increase in IMEP of 16%
over a range of engine loads
40
0.4
3000 RPM, Phi = 0.7
0.35
30
0.3
25
0.25
Spark
Corona
Spark COV
Corona COV
20
15
0.2
COV
IMEP (psia)
35
0.15
10
0.1
5
0.05
0
0
0
5
10
15
Torque (ft-lb)
20
25
Burn rate
 Integrated heat release shows faster burning with corona
leads to greater effective heat release
2900 RPM,  = 0.7
Burn rates
 Corona ignition shows substantially faster burn rates at
same conditions compared to 2-plug conventional ignition
2900 RPM,  = 0.7
Emissions data - NOx
 Improved NOx performance vs. indicated efficiency tradeoff
compared to spark ignition by using leaner mixtures with
sufficiently rapid burning
Emissions data - hydrocarbons
 Hydrocarbons emissions similar, corona vs. spark
100
BSHC (g/hp-hr)
spark
corona
10
1
0
0.1
0.2
Indicated Efficiency
0.3
0.4
Emissions data - CO
 CO emissions similar, corona vs. spark
1000
BSCO (g/hp-hr)
spark
corona
100
10
1
0
0.1
0.2
Indicated Efficiency
0.3
0.4
Conclusions
 Flame ignition by transient plasma or pulsed corona
discharges is a promising technology for ignition delay &
rise time reduction
 More energy-efficient than spark discharges
 Shorter ignition delay and rise times
 Rise time more significant issue
» Longer than delay time
» Unlike delay time, can’t be compensated by “spark advance”
 Higher peak pressures
 Benefits apply to turbulent flames also
 Demonstrated in engines too
 Higher IMEP for same conditions with same or better BSNOx
 Shorter burn times and faster heat release
 Improvements due to
 Chemical effects (delay time) - radicals vs. thermal energy
 Geometrical effects - (delay & rise time) - more distributed
ignition sites
Future work




Improved electrode designs
Solid-state discharge generators
Multi-cylinder corona ignition
Corona-ignited, low turbulence (thus low heat loss)
engines???
 Transient plasma discharges for fuel electrospray
dispersion?
Thanks to…




National Central University
Prof. Shenqyang Shy
Combustion Institute (Bernard Lewis Lectureship)
AFOSR, ONR, DOE (research support)