Transcript PowerPoint Presentation - Corona discharge ignition in engines

Corona Discharge Ignition
for Advanced Stationary
Natural Gas Engines
ASME Internal Combustion Engine Division
Fall Technical Conference, Long Beach, CA
October 25, 2004
Supported by DOE-UREP
Principal Investigator: Prof. Paul D. Ronney
Co-Principal Investigator: Prof. Martin Gundersen
Research Associates: Nathan Theiss, Dr. Jian-Bang Liu
Graduate students: Fei Wang, Jun Zhao
Undergraduate students: Brad Tallon, Matthew Beck
Jennifer Colgrove, Merritt Johnson, Gary Norris
ASME Paper # ICEF2004-891
Motivation
• Multi-point ignition has the potential to increase
burning rates in internal combustion engines
• (Simplest approach) Leaner mixtures (lower NOx)
• (More difficult) Higher compression ratios + water injection
(higher efficiency with same NOx)
• (Most 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)
• 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 - possible multiple ignition sites
High energy (10s of eV) electrons compared to sparks (~1 eV)
Electrons not at thermal equilibrium with ions/neutrals
Low anode & cathode drops, little radiation & shock formation more efficient use of energy deposited into gas
• Enabling technology: USC-built discharge generators (Prof.
Martin Gundersen)
Corona vs. arc discharge
Plasma Zone
High voltage
pulse
Corona Streamers
Corona dies out
in pulsed mode
Coaxial ground electrode no dielectric barrier needed
Corona phase (0 - 100 ns)
Arc channel
High voltage
pulse
Arc phase (> 500 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
Voltage (KV)
Energy
80
15
60
10
40
5
20
Current
0
-5
-50
0
50
100 150
Tim e (ns)
0
200
Corona only
250
-20
300
20
150
Voltage
Energy
100
Current
50
15
10
5
0
0
-5
-50
Start
of arc
0
50
100 150
Tim e (ns)
200
250
-50
300
Corona + 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 corona, but
followed by (almost useless) 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 for
conventional sparks
P  Volum e
Energy deposited in gas
 1
d 

Electrical discharge energy
 IVdt

Discharge efficiency
1
0.1
0.01
10
Cor ona, 1 pin, Cylindrical combustion chamber
Cor ona, ring electrode
IC engine like chamber
Cor ona,Threaded rod electrode
Cylindrical combustion chamber
Spark, plain w ire electrodes, gap = 1 mm
Cylindrical combustion chamber
Spark, Car spark plug
IC engine like chamber
100
Energy (mJ)
1000
Program objectives
• Characterize advantages of pulsed corona discharges
for NG ignition in static combustion chambers
• Integrate pulsed corona discharge ignition system into
stationary natural gas engines
• 1998-2002 Ford Ranger, 2.5L SOHC 4-cylinder engine, 2 plugs
per cylinder (1 conventional plug, 1 corona ignition port)
• Large-bore stationary natural gas engine
• Determine if the ≈ 3x shorter burn times found with
pulsed corona discharges apply to NG engines also
• If so, exploit the shorter burn times
• Assess the possibility for NOx reduction using
additional corona discharges during the exhaust
stroke
Progress to date
• Installed new engine in laboratory with two spark plug ports
per cylinder (2000 Ford Ranger 2.5L I-4) and converted to NG
• Updated lab engine data acquisition & control system
hardware and software (National Instruments / LabView)
• Interfaced emissions analyzer with LabView system
• Implemented student-designed in-cylinder pressure monitoring
system on engine
• Built static test chamber that simulates engine geometry for
electrode testing
• Constructed turbulent test chamber and conducted bench
tests to characterize effects of turbulence on corona ignition &
combustion
• Studied and characterized minimum ignition energies of
corona discharges
• Developed electrode for engine combustion chamber using
machinable ceramics
• Developed trigger system for firing corona generator on engine
• Performed on-engine testing with pulsed corona discharge
firing on one cylinder over a range of air/fuel ratios, engine
loads and ignition timing
Laboratory test apparatus (constant volume)
• 2.5” (63.5 mm) diameter chamber, 6” (152 mm) long
• Energy release (stoich. CH4-air, 1 atm) ≈ 1650 J energy
release ≈ 60,000x minimum ignition energy
• Energy input for ignition is trivial fraction of heat release!
Definitions
• Delay time: 0 - 10% of peak pressure (can be
compensated for by adjusting “spark advance”)
• Rise time: 10% - 90% of peak pressure (can’t be fixed
with spark advance!)
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
Effect of geometry on delay time
• Spark delay time ≈ 2x larger than 1-pin corona (≈ same geometry)
• Consistent with computations by Dixon-Lewis, Sloane suggesting 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)
• Benefit of corona on delay time both chemical (≈ 1.5x) & geometrical (≈ 2x)
corona, 1 pi n, 75 m J
spark, 75 mJ
corona, 3.9 mm dia rod, 710 m J
corona, 2 ring x 2 pin, 170 m J
corona, 4 ring x 2 pin 170 m J
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 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 much less energy
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
Energy & geometry effects (lean mixture)
• 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)
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: 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)
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
Simulated engine chamber
• Test fixture built to same dimensions as engine cylinder and
piston crown at TDC to test corona in this geometry
• Enables initial testing of electrode geometries and visualization
of corona
• Allows optimization of electrode geometries and discharge
conditions before conducting on-engine testing
Ignition in simulated engine chamber
• Delay time actually longer with corona in this geometry (but can
be compensated by ignition advance)
• Rise time 2x faster with corona, with far lower energy input
• Have ignited with corona only (no arc) up to 10 atm
16
P
=3.5 atm.
Absolute pressure (atm.)
initi al
14
CH /Air
12
Equivalence ratio=1.0
Ring electrode
4
10
8
6
Spark (39 mJ)
Corona (14 mJ)
Corona + arc (46 mJ)
4
Each curve is an average of 3 runs
2
0
0.02
0.04
Time (s)
0.06
0.08
0.1
Discharge
type
Delay
time
(ms)
Rise
time
(ms)
Corona
20
10
Corona +
arc
9
19
Spark
13.2
19
Turbulent test chamber
HV Anode
Fan
Grounded Cathode
Turbulence effects
• Simple turbulence generator (CPU cooling fan + grid) integrated
into coaxial combustion chamber, rod electrode
• Mean flow ≈ 11 m/s + turbulence intensity ≈ 1 m/s, u’/SL ≈ 3
(stoichiometric)
• Benefit of corona ignition ≈ same in turbulent flames - shorter
rise & delay times, higher peak P
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 at USC
• 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
• Macor machinable ceramic used for insulator
• Coaxial shielded cable used to reduce EMI
• Simple single-point electrode tip, replaceable
On-engine pulsed corona discharge ignition system
• Pulsed corona discharges generated using “pseudospark”
switch + Blumlein transmission line, triggered from camshaft
• ≈ 500 mJ/pulse (equivalent “wall plug” energy requirement of ≈
50 mJ spark)
• Corona electrode and spark plug with pressure transducer in
#1 cylinder
• Switch wired for quick change between spark and corona
ignition under identical operating conditions
• Stock timing for spark ignition, variable timing for corona
• 3 modes tested
• Corona only
• Single conventional plug
• Two conventional plugs (results very similar to single plug)
On-engine pulsed corona discharge ignition system
On-engine results
• Corona ignition shows increase in peak pressure under
all conditions tested
180
2000 rpm, 25 Ft-lb torque, Phi = 1
160
spark
corona
Pressure (psia)
140
120
100
80
60
40
20
0
0
1000
2000
3000
Crank angle (deg)
4000
5000
On-engine results
• Corona ignition shows increase in IMEP under all
conditions tested
250
3000 RPM, 25 ft-lb, phi = 0.7
Pressure (psia)
200
150
corona
spark
100
50
0
0
10
20
30
Cyl Volume (cu in)
40
50
IMEP at various loads
• Corona showed an average increase in IMEP of 16% over a
range of engine loads, A/F ratios, ignition timings
• Slight decrease in COV with corona
• Stronger ceramic is needed for electrode to test at higher
loads - need collaboration with plug manufacturer
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
IMEP at various air / fuel ratios
50
3000 RPM
45
Spark
Corona
IMEP (psia)
40
35
30
25
20
0.5
0.7
0.9
1.1
Phi
1.3
1.5
Burn rates
• Corona ignition shows substantially faster burn rates at
same conditions compared to 2-plug conventional ignition
4 .0 E +0 5
3 .5 E +0 5
Heat release rate (W)
3 .0 E +0 5
Spark
C orona
2 .5 E +0 5
2 .0 E +0 5
1 .5 E +0 5
1 .0 E +0 5
5 .0 E +0 4
0 .0 E +0 0
- 5 .0 E +0 4
- 1 .0 E +0 5
0
0 .0 0 5
0 .0 1
0 .0 1 5
Time (seconds)
2900 RPM,  = 0.7, Pintake = 5.9 psia
0 .0 2
Emissions data - NOx
• Improved NOx performance vs. indicated efficiency
tradeoff compared to spark ignition by using leaner
mixtures with sufficiently rapid burning
10
BSNOx (g/hp-hr)
spark
corona
1
0.1
0.01
0
0.1
0.2
Indicated Efficiency
0.3
0.4
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 (“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
» Higher IMEP (15% - 20%) for same conditions with same or
better BSNOx
» Shorter burn times and faster heat release
» Higher peak pressures
• Improvements due to
• Chemical effects (delay time) - radicals vs. thermal energy
• Geometrical effects - (delay & rise time) - more distributed ignition
sites
Future Work
• Install corona ignition on all 4 cylinders
• Construct corona electrode from ceramic that can
withstand higher engine loads - need collaboration with
plug manufacturer
• Test effectiveness of corona for NOX reduction in exhaust
• Implement corona ignition on large bore stationary
engine