1Glenn Research Center at Lewis Field

Problems in Minimum Ignition Energy Determination

Dr. Martin J. Rabinowitz

Research and Technology DirectorateNASA Glenn Research Center

Brookpark, Ohio 44122

Collaborators:Robert R. Bickford, Gregory S. Calhoun, Christopher L. Hegedus,

Bryan E. Knepper, Yves C. Lamothe, James E. Sexton

2Glenn Research Center at Lewis Field

This Seems Well Understood

Well, not really.

The determination of minimum ignition energy curves for fuel/airmixtures have been problematical with widely varying results fordifferent groups using different fuels, fuel treatment regimes and ignition sources.

The focus of attention on the root cause of the variations has been in the fuel and fuel treatment. While there is considerable variation in refinery products and handling such variations are also observed for chemically pure fuels.

Perhaps there are other concerns.

3Glenn Research Center at Lewis Field

Successes and Failure for Ignition

0.1

1.0

10.0

100.0

1000.0

10000.0

5 10 15 20 25 30 35 40 45

Temperature ( 0C)

MIE

(mJ)

IgnitionNon-Ignition

Methanol MIE

0 5 10 15 20 25 30 35 40

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Type of Ignition Source

Hot Wire

Spark

Optical

Du

rati

on

Physical Chemistry Chemical Engineering Mechanical Engineering

Initiation

ns

µs

ms

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Combustion Vessel

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GRC MIE ExperimentationCombustion Vessel Apparatus

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Sparks Move Around

30 µs later

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“Dancing” Spark30 µs Between Frames

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GRC MIE ExperimentationCombustion Vessel Apparatus ~ Internal Configuration

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Ωµ

Ω

Ω

Ω

µ

Ω

Ω

Ω

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Typical Ionization Profile70µJ Spark Energy

-2

-1

0

1

2

3

4

5

6

7

8

0 500 1000 1500 2000

Time (ns)

Volta

ge (k

V)

Breakdown = 7000 VVoltage Rise = 1.3e10 V/s

Ionization Spark Duration 100 ns

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Typical Energy MeasurementEnergy = ∫ (Voltage x Current) dt

Current

Voltage

Power

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Oscillating Sparks

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Nice Looking Spark

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Nice Looking Spark -- Animation

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Reaction Wave

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Carbon Build Up

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Carbon Build Up --Animation

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Methanol

0

200

400

600

800

1000

0.0 0.5 1.0 1.5 2.0 2.5 3.0Equivalence Ratio

TIm

e to

Pea

k Pr

essu

re (m

s)

0

10

20

30

40

50

60

Pres

sure

Ris

e (p

si)

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Effect of Vapor Composition on Minimum Ignition

-1.5-1.0-0.50.00.51.01.52.02.53.03.5

0.0 1.0 2.0 3.0 4.0 5.0 6.0

(τ/ms )

E ig/mJ

OLD Methanol @ 27.5C

50/50 Isopropanol/Methanol@ 27.5C90/10 Isopropanol/Methanol@ 27.5CMinimum Spark Line ~ NoFuel

Methanol

Methanol/Isopropanol

Isopropanol

Minimum Spark in Air

21Glenn Research Center at Lewis Field

-1.5-1.0-0.50.00.51.01.52.02.53.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Equivalence Ratio (Φ )

log

(MIE

/mJ)

Series1Series2Series3

Methanol Minimum Ignition EnergyEffect of Electrical Isolation

Teflon

No Teflon

22Glenn Research Center at Lewis Field

22.5 oC 25.0 oC 27.5 oC 32.5oC

Methanol MIE vs Spark Duration

-1.5

-0.5

0.5

1.5

2.5

3.5

0 1 2 3 4 5 6

log (τ/µs)

log

(Eig

/ m

J)

12.5 oC 14.5 oC 17.5 oC 20.0 oC

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-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

0.0 1.0 2.0 3.0

Equivalence Ratio (Φ )

log

(MIE

/mJ)

Poly.

Methanol Minimum Ignition EnergyEffect of Varying Spark Duration

10 µs

100 µs

1000 µs

Actual Minima

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Iso-Octane MIE @ 210CEffect of Varying Spark Duration

10

100

1000

10000

10 100 1000 10000 100000

Spark Duration (µs)

Min

imum

Igni

tion

Ener

gy (m

J)

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More Than Enough Energy

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MIE Curves for Methanol and Iso-Octane

0.01

0.1

1

10

100

1000

0 1 2 3 4 5

Equivalence Ratio

Min

imum

Ign

ition

Ene

rgy

Iso-Octane Methanol All Spark Durations

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Conclusions

MIE Depends Upon Ignition Source and Duration

Shape of MIE vs Temperature Curves Varies Strongly with Spark Properties

Not So Easy – Need To Find Spark Duration for MIE at Each Condition