Transient Plasma Discharge Ignition
for Internal Combustion Engines:
Putting some new spark
into an old flame
Paul D. Ronney
University of Southern California, USA
23rd National Conference on I. C. Engines
and Combustion
SVNIT, Surat, India,
December 13-16, 2013
University of Southern California
 Established 130 years ago
 …jointly by a Catholic, a Protestant and a Jew - USC has
always been a multi-ethnic, multi-cultural, coeducational
university
 Today: 32,000 students, 3000 faculty
 2 main campuses, both near downtown Los Angeles:
University Park and Health Sciences
USC Viterbi School of Engineering
 Naming gift by Andrew & Erma Viterbi
 Andrew Viterbi: co-founder of Qualcomm, co-inventor of CDMA
 1800 undergraduates, 3500 graduate students, 180 faculty, 30
degree options
 >$200 million external research funding
 Distance Education Network (DEN): 900 students in 28 M.S.
degree programs
 More info: http://viterbi.usc.edu
Paul Ronney
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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 (since 1993)
 Research interests
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Microscale combustion and power generation
Microgravity combustion and fluid mechanics
Turbulent combustion
Internal combustion engines
Ignition, flammability, extinction limits of flames
Flame spread over solid fuel beds
Biophysics and biofilms
My first time in India…
… and you can see how sad my children are that I’m gone.
Some objectives for my visit
 Learn about combustion research activities in India
 Look for possible collaborations (U.S. National Science
Foundation and other agencies)
 Provide educational experiences to Indian students
 Combustion science
 Writing compelling journal papers
 Introduce Indian researchers & students to USC
 Summer internships
 Graduate student assistantships
 Have fun!
Introduction
 Hydrocarbon-fueled ICEs are the power plant of choice for
vehicles in the power range from 5 Watts to 100,000,000
Watts, and have been for over 100 years
 > 80% of world energy production results from combustion
of fossil fuels
 Our continuing habit of burning things and our quest to find
more things to burn has resulted in
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Economic booms and busts
Political and military conflicts
Global warming (or the need to deny its existence)
Human health issues
 Hydrocarbon-fueled ICEs are dirty, noisy, unreliable and
use fuel that is too expensive, so there MUST be something
better than ICEs for transportation
 … or can we do better with the ICEs we have?
Transient plasma ignition – why?
 Multi-point ignition of flames has potential to increase
burning rates in many types of combustion engines, e.g.
 Reciprocating Internal Combustion Engines
» (Simplest approach) Leaner mixtures (lower NOx)
» (More difficult) Low turbulence, low heat loss engine
 Pulse Detonation Engines
 High altitude restart of gas turbines
 Lasers, multi-point sparks challenging
 Lasers: energy efficiency, windows, fiber optics
 Multi-point sparks: multiple intrusive electrodes –
sites for heat loss, autoignition
 How to obtain multi-point, energy efficient ignition?
Transient plasma discharges (TPDs)
 Also known as “pulsed corona”
discharges
 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
TPI vs. arc discharge
Transient plasma phase (0 - 100 ns)
Arc channel
High voltage
pulse
Arc phase (> 100 ns)
Images of TPDs & flames
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 TPDs
 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 TPDs
80
15
60
10
40
5
20
Current
0
-5
-50
0
50
100 150
Time (ns)
0
200
250
Transient plasma only
-20
300
20
150
Voltage
Energy
100
Current
50
15
10
5
0
0
-5
-50
Start
of arc
0
50
100 150
Time (ns)
200
250
-50
300
Transient plasma + arc
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 TPD, but
followed by (relatively ineffective) arc)
Current (amps) or Energy (mJ)
Energy
Current (amps) or Energy (mJ)
Voltage
20
Voltage (KV)
25
100
Voltage (KV)
25
TPDs are energy-efficient!
 Discharge efficiency d ≈ 10x higher for TPD than
conventional sparks
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
Today’s talk
 Compare combustion duration and ignition energy of sparkvs TPD-ignited flames in constant-volume vessel
 Determine effect of TPD electrode geometry
 Determine effect of turbulence on combustion duration with
TPD
 Compare TPD-ignited and spark-ignited engines
 Efficiency
 Emissions
 Assess ways to exploit benefits of TPI in engines
Experimental apparatus (constant volume)
 TPDs generated using thyratron gas switch + Blumlein
transmission line (recently all-solid state systems)
 Coaxial chamber, 63.5 mm diameter chamber, 152 mm long
 Rod electrode (shown below) or single-needle
 Energy release (stoich. CH4-air, 1 atm) ≈ 1650 J
 Ignition energy << 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
TPDs in IC engine-like geometry
Top view
Side view
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 TPD (≈
same geometry)
 Consistent with computations by Dixon-Lewis (1978),
Sloane (1990) 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 TPD on delay time 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 TPD (≈ 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
 Suggests benefit of TPD on rise time is mostly geometrical,
not chemical
 Rise time a more significant benefit for IC engines
 Spark ignition has longer delay time, but is compensated by
advancing ignition timing
 Spark ignition has longer rise time, cannot be compensated by
ignition timing, inherently lower efficiency with spark than TPD
Turbulent test chamber
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 TPD ≈ same in turbulent flames - shorter rise &
delay times, higher peak P
 Quiescent/TPD faster than turbulent/spark! (faster burn with
less heat loss)
4
CH /Air
4
Pressure (atm)
3.5
f = 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
f = 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
TPD & spark at same time, same operating conditions
 National Instruments / Labview data acquisition & control
 Horiba emissions bench, sampled from TPD cylinder
 Pressure / volume measurements
Electrode configurations
 Simple single-point electrode tip (left) - “Point to plane”
geometry
 Spark-plug compatible disc electrode (right) – circular
pattern
 First steps – neither geometry optimized yet
On-engine TPD ignition system
 TPD electrode and spark plug with pressure transducer in
#1 cylinder
 ≈ 500 mJ/pulse (equivalent “wall plug” energy requirement
of ≈ 50 mJ spark)
 Range of ignition timings for both spark & TPD
 3 modes tested
 TPD only
 Single conventional plug
 Two conventional plugs (results very similar to single plug)
On-engine results
 TPD ignition shows increase in peak pressure under all
conditions tested
On-engine results – spike electrode
 TPD ignition shows increase in IMEP under all conditions
tested
Spike electrode, 2900 RPM,  = 0.7
On-engine results – disc electrode
 TPD ignition shows increase in IMEP under all conditions
tested
Disc electrode 1900 RPM,  = 1
IMEP at various air / fuel ratios
 Indicated mean effective pressure (IMEP) higher for TPD
than spark, especially for lean mixtures
 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
 Average increase in IMEP ≈ 16% with TPD
40
0.4
3000 RPM, Phi = 0.7
0.35
30
0.3
25
0.25
Spark
Corona
Spark COV
Corona COV
20
15
COV
IMEP (psia)
35
0.2
0.15
10
0.1
5
0.05
0
0
0
5
10
15
Torque (ft-lb)
20
25
Burn rate
 From P vs. t & V vs. t plots, heat release can be calculated faster burning with TPD, greater net heat release
2900 RPM,  = 0.7
Burn rate
 Integrated heat release shows faster burning with TPD leads
to greater effective heat release
Disc electrode 1900 RPM,  = 1
Burn rates – spike electrode
 TPD ignition shows substantially faster burn rates at same
conditions compared to 2-plug conventional ignition
Spike electrode, 2900 RPM,  = 0.7
Burn rates – disc electrode
 TPD ignition shows substantially faster burn rates at same
conditions compared to 2-plug conventional ignition
Disc electrode 1900 RPM,  = 1
Emissions data - NOx
BSNOx (g/hp-hr)
 Improved Brake Specific NOx performance vs. indicated
efficiency tradeoff compared to spark ignition by using
leaner mixtures with sufficiently rapid burning
Emissions data - hydrocarbons
 Hydrocarbons emissions similar, TPD 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, TPD vs. spark
1000
BSCO (g/hp-hr)
spark
corona
100
10
1
0
0.1
0.2
Indicated Efficiency
0.3
0.4
New idea – low heat loss engines
 Using TPI in conventional engines is advantageous, but still
have tradeoff between efficiency & NOx
 Faster burn, higher T, more NOx
 Alternative idea – low turbulence, low heat loss engine
 1970s: “adiabatic engines” – high wall T, less heat loss, higher
efficiency, right?
 Need high-T materials (e.g. ceramics)
 Must run without lubricant
 But idea failed – efficiency not improved – why?
 Can explain this using simple spreadsheet-type model:
http://ronney.usc.edu/spreadsheets/aircycles4recips.xls
New idea – low heat loss engines
 Heat transfer during intake, higher T & s than adiabatic case
 Heat addition during 1st part of compression (ds > 0), heat loss
(ds < 0) during 2nd part of compression & rest of cycle
 Nearly constant-volume
combustion,
const.-v curve
Compression
Combustion so same
Expansion
Compression
Combustion
Expansion
Blowdown
Intake
Exhaust
Intake
Exhaust
but less T Blowdown
due
to
heat
loss
Close T-s
T-s cycle
cycle
Close
11
22
4 - 0.281 vs. 0.177
55 for case shown
 Major effect33on efficiency 4th
Temperature (K)
1000
66
77
T-s diagram
900
Red solid: adiabatic
Blue dashed: with heat loss
800
700
600
500
Wall T
400
300
0
200
400
Entropy (J/kg-K)
600
800
New idea – low heat loss engines
 With higher wall temperature but same heat loss coefficient
 More heat transfer to fuel-air mixture during compression – higher
T at start of compression
 More work input during compression
 Higher work output during expansion almost exactly cancelled by
higher compression work – no change in cycle efficiency!
3000
400K wall
2500
700K wall
Temperature (K)
2000
1500
1000
500
0
0
500
1000
1500
Entropy (J/kg-K)
2000
2500
New idea – low heat loss engines
 Even simple spreadsheet model shows that what matters is
not wall T but heat transfer coefficient (h)
 How to decrease h??? Need to decrease turbulence!
 But decreased turbulence means lower burning rates!
 How to burn fast with less turbulence – TPI!
Brake thermal efficiency
0.4
h*
Dimensionless h =
rCP Nd
Also called Sherwood #
0.3
Tw = 400K, varying h
h = 0.01, varying Tw
0.2
h* = dimensional heat transfer coeff.
d = cylinder diameter
N = rotation speed
0.1
0
0
0.05
0.1
0.15
0.2
Heat transfer coefficient (h) or Twall/10,000
New idea – low heat loss engines
 How to reduce turbulence in engines?
 Intake port – smooth port, minimize tumble and swirl
 Piston – use dome-shape (anti-squish) instead of dish-shape
(squish)
 Cylinder head – grooved to laminarize flow
 Additional benefit – smaller radiator, lower aerodynamic
drag!
Traditional piston
crown
Dome-shaped piston
crown
New idea – low heat loss engines
Stock intake
port
Modified – smooth, bends reduced,
valve guide boss removed
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
 Benefits apply to turbulent flames also
 Improvements due to
 Chemical effects (delay time) - radicals vs. thermal energy
 Geometrical effects - (delay & rise time) - more distributed
ignition sites
 Demonstrated in engines
 Higher IMEP for same conditions with same or better BSNOx
 Shorter burn times and faster heat release
 Potential for low-turbulence, low heat loss engines
» Engine efficiency gains
» Reduction in aerodynamic drag (reduced radiator size)
» “Fuel agnostic” – gasoline, natural gas, ethanol, biofuels, hydrogen…
» Easily retrofit to existing engines
Future work
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Test low-turbulence engine
Improved electrode designs
Multi-cylinder corona ignition
Transient plasma discharges for fuel electrospray
dispersion?
Thanks to…
 Indian Section, Combustion Institute
 Prof. S. A. Channiwala
 Collaborators
 Faculty collaborator: Martin Gundersen (USC-EE)
 Research Associates: Nathan Theiss, Jian-Bang Liu
 Graduate students: Cody Ives, Kanchana Gunasekera, Si Shen,
Parth Merchant
 Undergraduate students: Many!
 AFOSR, ONR, DOE (research support)