University of Southern California

Transient Plasma Discharge Ignition for Internal Combustion

Engines:Putting some new spark

into an old flamePaul 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 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 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 EnginesHigh altitude restart of gas turbines

Lasers, multi-point sparks challengingLasers: energy efficiency, windows, fiber opticsMulti-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 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

Electrode configurations

TPDs in IC engine-like geometry

Top view Side view

Effect of geometry on delay time

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

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)

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 NOx performance vs. indicated

efficiency tradeoff compared to spark ignition by using leaner mixtures with sufficiently rapid burning

BSN

Ox (

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. 0.177 for case shown

Red solid: adiabaticBlue 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!

0 500 1000 1500 2000 25000

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3000400K wall

Entropy (J/kg-K)

Tem

pera

ture

(K)

0 500 1000 1500 2000 25000

500

1000

1500

2000

2500

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700K wall

Entropy (J/kg-K)

Tem

pera

ture

(K)

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

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 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)