Research supported by U.S. AFOSR, ONR & DOE Travel supported by the Combustion Institute 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

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Paul Ronney  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 ( ns) 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% CH 4 -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

Characteristics of corona discharges 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) Corona only Corona + arc

Corona discharges are energy-efficient  Discharge efficiency  d ≈ 10x higher for corona than conventional sparks

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. CH 4 -air, 1 atm) ≈ 1650 J energy release ≈  Discharge energy input for ignition is trivial fraction of heat release!

Definitions  Delay time: % of peak pressure  Rise time: 10% - 90% of peak pressure

Electrode configurations

Pulsed corona discharges in IC engine-like geometry Top viewSide 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

Pressure effects on MIE  MIE for pulsed corona does NOT follow E min ~ P -2 as spark ignition does; more like P -1 at low P, P 0 at higher P  Smaller chamber diameter enables ignition at higher P - higher voltage gradient

Effect of geometry on delay time

 Delay time of spark larger (≈ x) 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 ( x) and geometrical (≈ 2x)

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

 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)

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

Energy & geometry effects (lean mixture)  Delay time: same conclusion as stoichiometric mixture

Energy & geometry effects (lean mixture)  Rise time: 4-ring stands out

Rod diameter effects  Plain rod: optimal diameter exists (≈ 0.15”), d rod /d cyl ≈ 0.06  Large d: low field concentration, few streamers?  Small d: Too many streamers, too much energy deposition?

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!

Thyratron vs. pseudospark generator  Little effect of discharge generator type (pseudospark: ≈ 1/2 discharge duration compared to thyratron)

Turbulent test chamber

Turbulence effects  Simple turbulence generator (fan + grid) integrated into coaxial combustion chamber, rod electrode  Turbulence intensity ≈ 1 m/s, u’/S L ≈ 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)

Turbulence effects  Similar results for lean mixture but benefit of turbulence more dramatic - higher u’/S L (≈ 8)

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  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

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

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

Emissions data - CO  CO emissions similar, corona vs. spark

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)