Corona%20Discharge%20Ignition%20for%20Advanced%20Stationary%20Natural%20Gas%20Engines%20ASME%20Internal%20Combustion%20Engine%20Division%20Fall%20Technical%20Conference,%20Long%20Beach,%20CA%20October%2025,%202004%20Supported%20by%20DOE-UREP

1
Corona Discharge Ignition for Advanced Stationary
Natural Gas EnginesASME Internal Combustion
Engine DivisionFall Technical Conference, Long
Beach, CAOctober 25, 2004Supported 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

2
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?

3
Transient plasma (pulsed corona) discharges

• Not to be confused with plasma torch
• Initial phase of spark discharge (lt 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)

4
Corona vs. arc discharge
Corona phase (0 - 100 ns) Arc phase (gt
500 ns)
5
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)

6
Characteristics of corona discharges
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
  (almost useless) arc)

 
7
Corona discharges are energy-efficient

• Discharge efficiency ?d 10x higher for corona
  than for conventional sparks

8
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

9
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

10
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!

11
Definitions

• Delay time 0 - 10 of peak pressure (can be
  compensated for by adjusting spark advance)
• Rise time 10 - 90 of peak pressure (cant be
  fixed with spark advance!)

12
Electrode configurations
13
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)

14
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

15
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)

16
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 doesnt show step behavior)

17
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)

18
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 doesnt show step behavior)

19
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

20
Test Chamber Constructed from Engine Components

• Allows quicker testing of insulation and
  electrode configurations without the need to
  repeatedly remove cylinder head from engine

21
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

Discharge type Delay time (ms) Rise time (ms)
Corona 20 10
Corona arc 9 19
Spark 13.2 19
22
Turbulent test chamber
23
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

24
Turbulence effects

• Similar results for lean mixture but benefit of
  turbulence more dramatic - higher u/SL ( 8)

25
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

26
Electrode configuration

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

27
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)

28
On-engine pulsed corona discharge ignition system
29
On-engine results

• Corona ignition shows increase in peak pressure
  under all conditions tested

30
On-engine results

• Corona ignition shows increase in IMEP under all
  conditions tested

31
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

32
IMEP at various air / fuel ratios
33
Burn rates

• Corona ignition shows substantially faster burn
  rates at same conditions compared to 2-plug
  conventional ignition

2900 RPM, ? 0.7, Pintake 5.9 psia
34
Emissions data - NOx

• Improved NOx performance vs. indicated efficiency
  tradeoff compared to spark ignition by using
  leaner mixtures with sufficiently rapid burning

35
Emissions data - hydrocarbons

• Hydrocarbons emissions similar, corona vs. spark

36
Emissions data - CO

• CO emissions similar, corona vs. spark

37
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, cant 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

38
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