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Transient Plasma Discharge Ignition for Internal Combustion Engines: Putting some new spark into an old flame Paul D. Ronney University of Southern California, USA 23 rd National Conference on I. C. Engines and Combustion SVNIT , Surat , India , December 13-16, 2013.
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Transient Plasma Discharge Ignition for Internal Combustion Engines:Putting some new spark into an old flamePaul D. RonneyUniversity of Southern California, USA23rd National Conference on I. C. Engines and CombustionSVNIT, 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 • 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 • 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 • 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 ICEsfor 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 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
Characteristics of TPDs 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) Transient plasma only Transient plasma + arc
TPDs are energy-efficient! • Discharge efficiency d ≈ 10x higher for TPD than conventional sparks
Today’s talk • Compare combustion duration and ignition energy of spark-vs 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 thyratrongas 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
TPDs in IC engine-like geometry Top view Side view
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 • 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
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)
Turbulence effects • Similar results for lean mixture but benefit of turbulence more dramatic - higher u’/SL (≈ 8)
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
IMEP at various loads • Average increase in IMEP ≈ 16% with TPD
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 • Improved Brake Specific NOxperformance vs. indicated efficiency tradeoff compared to spark ignition by using leaner mixtures with sufficiently rapid burning BSNOx (g/hp-hr)
Emissions data - hydrocarbons • Hydrocarbons emissions similar, TPD vs. spark
Emissions data - CO • CO emissions similar, TPD vs. spark
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, so same const.-v curve but less T due to heat loss • Major effect on efficiency th - 0.281 vs. 0.177for case shown Red solid: adiabatic Blue dashed: with heat loss Wall T
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!
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!
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 Modified – smooth, bends reduced, valve guide boss removed Stock intake port
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 • 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, KanchanaGunasekera, Si Shen, Parth Merchant • Undergraduate students: Many! • AFOSR, ONR, DOE (research support)