Twentieth Symposium (International) on Combustion/The Combustion Institute, 1984/pp. 133-140

INVESTIGATION ON IGNITION ABILITY OF COMPOSITE SPARKS IN FLOWING MIXTURES

M. KONO Department of Aeronautics, Faculty of Engineering

University of Tokyo, Bunkyo-ku, Tokyo, Japan

K. HATORI Ship Research Institute, Shinkawa, Mitaka, Tokyo, Japan

K. IINUMA Hosei University, Koganei, Tokyo, Japan

In order to clarify the mechanism of the ignition process of flowing gases by composite sparks and to pursue important factors influencing the process, the effects of the behavior of spark discharge path, spark duration, electrode size, discharge type and multiple spark on the minimum ignition energy were investigated by using a long-duration spark generator which can independently vary the spark duration and the discharge power. The results in- dicate that the behavior of the blown spark path in the flowing gaseous mixture is closely related to the ignition ability of the spark, and that the optimum spark duration for glow discharge does not appear clearly under flowing conditions. The effect of spark electrodes is suggested to be due to a loss of heat or radical species and a quenching of flame kernels by wake turbulence induced by the spark electrodes. Moreover, it is suggested that the multiple sp~k, appearing for long duration sparks under flowing conditions, considerably enhances the ignition ability due to its repetition effect.

1. Introduction 2: Experimental Apparatus

Recently, an electric spark of higher ignition ability is required for improvement of exhaust emissions and reduction in the fuel consumption of spark ignition engines. The electric spark mainly used in spark ignition engines is a long duration spark which is composed of capacitance and in- ductance components; therefore, the spark is re- ferred to as composite spark. Although ignition phenomena have been studied in quiescent gas mixtures 1'~ and under flowing conditions 3'4 by us- ing composite sparks, and also done by using long duration sparks in flowing gases, 5-7 many impor- tant factors to enhance the ignition ability are over- looked.

In the present work, for the purpose of obtaining the optimum condition for ignition of composite sparks, the effects of spark duration, spark path be- havior, electrode size, discharge type and multiple spark on the minimum ignition energy were inves- tigated. As regards the gap width, attention was paid to the smaller one than the so-called quenching distance s because the gap width used practically is relatively small, especially so when a lean mixture is expected, or on the basis of the flashover prob- lem of the spark plug.

The ignition unit used for long duration sparks consists of a capacitance spark and a following com- ponent generator; the breakdown of gases by the former triggers the latter discharge. A detailed de- scription of the unit can be seen elsewhere, 1 except for increased voltage of dc power source for the fol- lowing component spark from 1.35 to 5.6 kV and adoption of a triggering spark (0.5 p~s spark dura- tion) produced by a thyratron. 9 The power and du- ration of the spark produced by the ignition unit can independently be varied approximately from 2.5 to 100 W, and'from 0 to 70 ms, respectively.

Flowing gaseous mixture was basically generated by a convergent nozzle of 10 mm in diameter. Spark electrodes are located 2.9 mm downstream from the nozzle exit. A lean propane-air mixture from the nozzle is introduced into a cylindrical combustion chamber (20 mm in inner diameter and 95 mm in height), where the ignition is made at atmospheric pressure and room temperature. The mixture flow used is approximately laminar; the relative turbu- lent intensity is 0.79% at the flow velocity of 23 m/s.

Under a certain experimental condition, the spark discharges are repeated 30 times, and thus the ig-

133

134 AUTOMOTIVE ENGINE COMBUSTION

GLOW DISCHARGE ARC DISCHARGE

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VOLTAGE: 640V/div

CURRENT: 18mA/div POWER: 2,9W/div

SOURCE VOLTAGE: 1950V

VOLTAGE: 640V/div (64V/div:Om/s)

CURRENT: 89mA/div

POWER: 1,42W/div SOURCE VOLTAGE: 1950V

Fzc. 1. Voltage, current and power traces of spark discharges.

IGNITION ABILITY OF COMPOSITE SPARKS 135

nition probability can be obtained. The minimum 6 ignition energy is determined at 50% ignition prob- ability; the ignition energy is defined as the total s energy of the capacitance and the following cam- ~ ponents. In the present work, the effect of the ca- pacitance component energy was not examined be- ~ 3 cause the characteristic feature of the effect is much the same as that obtained for quiescent gaseous ~ 2 mixtures. 1 The energy of the capacitance compo- nent is 0.07 mJ~ 1

As spark electrodes, tungsten wires of 26 p,m-0.5 mm in diameter and steel wires of 0.8 and 1.0 mm in diameter were used. The wires of larger than 0.1 mm in diameter are ground to be tipped with a 30 ~ half angle cone.

3. Spark Discharge Characteristics

I F ame i _ ~ . - / K e r n e l /

' " W'~ SPark i -~POth / / /

, A ~.!.~.. r , �9 / , ~ V ~ ~ : "~

1 i I i

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- , (W) MIx

�9 AIF 35 ( A r c )

V A MIx. �9 A i r 39 (Glow]

D MIx. �9 �9 AIF lq (GIow] I I I

tps I00 200 300 ~00 =45 Vs Tim ps

FIG. 2. Spark path length determined from elec- trical measurements and schlieren photographs against time. Flow velocity: 23 m/s; gap width: 1 mm.

Figure 1 shows the typical example of the traces of voltage, current and power of the spark dis- charge for several air velocities. These traces ob- served in air are much the same as that in propane- air mixtures used. In Fig. 1, the breakdown voltage of about 3.5 kV is not shown in the voltage traces. The spark discharge in Fig. 1 shows two types of discharges; the glow discharge for less than about 0.1 A in current, and the arc discharge for larger values. It was found that, at the current of about 0.1 A, two types of discharges appear alternately at a random period in a continuing discharge, and that the critical value of the current, which is about 0.1 A in most cases, changes considerably according to the surface condition of the negative spark elec- trode.

As can be seen in Fig. 1, under flowing condi- tions the voltage and current vary with time due to a downstream movement of the spark path. As regards the power of the spark, however, it ap- proximately holds a constant value with time. Moreover, for glow discharge in a certain velocity range, it is found in Fig. 1 that the spark discharge shows well known repetition in a saw-toothed wave form (see e.g. Refs. 3 and 4); the spark in this case is referred to as multiple spark. For the present purpose of measuring the minimum ignition en- ergy, a fairly long duration spark such as saw-toothed or multiple spark was not used because of its quite larger spark energy to that required; however, the repetition effect of the multiple spark is examined in the discussion.

The spark path behavior in flowing mixtures is considered to be closely related to the ignition abil- ity of the spark. Figure 2 shows the typical example of the spark path length. The spark path length in Fig. 2 was determined from the voltage or current- gap width diagram measured in the quiescent gas, and also determined as the length of the leading edge of the schlieren image of the flame or the hot

air kernel. In Fig. 2, it is indicated that the true spark path (V-I) does not coincide with the leading edge of the flame kernel (Photo.); namely the spark path behavior is not affected by the existence of the flame kernel. Figure 2 shows that the spark path is blown downstream after a certain time, defined as tp~, from the spark discharge initiation, and thus the spark path behavior can approximately be as- sumed to be a model shown in Fig. 2. Therefore, the spark path length can be expressed as:

L = W for t < tp,, (1)

L = 2 U ( t - t p ~ ) + W fort=>tps, (2)

where L, U, t and W are the spark path length, gas velocity, time and gap width, respectively.

4. Ignition by Long Duration Spark

A typical result obtained by long duration sparks is shown in Fig. 3. These data are limited in the range from 2.5 to 100 W of power. It was impos- sible to perform the ignition experiment for above 100 W because of a severe melting of the electrode tip. For the power below 2.5 W, the spark dis- charge was impossible to continue after the capac- itance spark.

In Fig. 3, for glow discharge sparks, the mini- mum ignition energy of a quiescent mixture shows a lowest value at about 80 p,s spark duration (op- timum spark duration), but that of flowing mix- tures, by contrast, hardly shows a distinct lowest value. As is seen, the minimum ignition energies for arc discharge are half of those for glow dis- charge. The whole trend of the minimum ignition energy shown in Fig. 3 is much the same as that for other mixtures strengths.

136 AUTOMOTIVE ENGINE COMBUSTION

5 t_

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= 2

E

-=o,5 c

0.2 5

Flow Velocity v [ (m/s)

, ~--~-~-~ 23 Arc

Arc

10 20 50 100 200

Spork Durotlon ps

500

FIG. 3. Minimum ignition energy by long dura- tion sparks against spark duration. Propane-air mix- ture of 3.2 (vol.)%. Gap width: 1 mm; spark elec- trode diameter: 0.3 mm.

ity. With further increase in flow velocity, the min- imum ignition energy increases. When a spark path is blown downstream, such as the present experi- ment, the true ignition energy, referred to as ef- fective ignition energy, should be evaluated by the model offered by Swett. 5 Strictly speaking, the spark path is surely not blown downstream within the time of tp~. Under such condition, to estimate the min- imum ignition energy, the model of Ballal and Lefebvre 7 possibly needs to be employed, but it was not done because the flame kernel observed in the present work is not found to resemble that adopted by them. In the present work, the original model by Swett was reasonably changed so as to consider the spark path behavior as described by Eqs. (1) and (2).

The effective ignition power, Pw, which is as- sumed to be included in the flame kernel as shown in Fig. 2, can be expressed as:

Pw = Em/t~ for t < tn~, (3)

In order to obtain the effect of flow velocity on the minimum ignition energy, the data for glow discharge in Fig. 3 were arranged as shown in Fig. 4. Figure 4 shows that, with increasing flow veloc- ity, the minimum ignition energy decreases and reaches the minimum at about 1.5 m/s flow veloc-

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FIG. 4. Minimum ignition energy, effective igni- tion energy (calculated) and t,, against time, ex- perimental condition being same as Fig. 3.

Pw = (EMi/ts) (W/L) for t => tp,, (4)

where EMI and t~ are the minimum ignition energy and the spark duration, respectively. The effective ignition energy, Ee, can be expressed as follows:

Ee = L t' Pwdt = tpsEm/t~

+ WEre In 2U(t8 - tps) + W (5) 2tsU W

The value of tps in Eq. (5)can be obtained from the measured values shown in Fig. 4. The effective ignition energy, shown in Fig. 4, changes with the flow velocity and spark duration. The considerable change in the effective ignition energy is due to the suppression of the quenching effects on the flame kernel; these effects result in a loss of heat or rad- ical species to the spark electrodes and cause tur- bulence in the wake of spark electrodes. These quenching effects are examined in the following discussion.

5 . D i s c u s s i o n

Effect of Spark Electrodes

For the purpose of obtaining the detailed infor- mation on the quenching effects mentioned above, only capacitance sparks are used in order to ex- clude the complication induced by a downstream movement of the spark path. The results are shown in Fig. 5. Figure 5 shows that, for 1.5 mm gap width, with increasing flow velocity the minimum

IGNITION ABILITY OF COMPOSITE SPARKS 137

30 / GOD Width 1.5mm ~/~

I Electrode 7 | Dlam, (ram) I I o.8 o5 /

0.8

.3

0 1 0.05

,05 0,5 0.I

T "-,6o wi th3 , 0

0 5 10 15 20 25 Flow Velocity m/s

FIG. 5. Minimum ignition energy against flow ve- locity by using capacitance sparks. Propane-air mix- ture of 2.9 (voL)%.

ignition energy decreases in a complicated manner, and reaches the minimun 3-5 m/s flow velocity: This decrease in the minimum ignition energy by a factor of 3 is due to a decrease in quenching ef- fect of the spark electrode on the flame kernel be- cause of an increased distance between the flame kernel and the spark electrodes. With further in- crease in flow velocity, the minimum ignition en- ergy increases for all the electrode sizes. For 3 mm gap width, no change in the minimum ignition en- ergy can be observed at a low velocity region, and in the same region the minimum ignition energy is found to be constant irrespective of spark electrode sizes.

Figure 6 shows the effect of spark electrode size on the flame kernel configuration. As is seen, for a small size electrode (top), the flame kernel takes the form of a doughnut-like shape, so that the quench- ing of the flame kernel by heat loss to the spark electrode occurs at an upstream side of the dough- nut. For a large size electrode (middle), there exist wrinkles on the surface of the flame kernels, which may be caused by turbulence in the wake of the spark electrodes. This fact can be confirmed in the bottom photographs in Fig. 6; a pair of rods of 0.5 mm in diameter, located 1 mm upstream from the spark electrodes of 25 Ixm in diameter, produce the same wrinkles as those seen in the middle photo- graphs, and that the minimum ignition energies for both cases are much the same.

From the facts mentioned above, the spark elec- trodes affect the ignition process in two, so-called direct and indirect, ways; the quenching effect by a loss of heat or radical species to the spark elec- trodes, and the promotion of cooling of the flame kernel by turbulence in the wake of the spark elec- trodes. The above explanation is applied to the re- sults in Fig. 5 as follows; for a low flow velocity (less than 3-5 m/s), the increase in the minimum ignition energy with increasing spark electrode size is due to a loss of heat or radical species, and for a higher flow velocity the similar increase is due to a wake turbulence. Naturally, these quenching ef- fects are included in the behavior of the effective ignition energy shown in Fig. 4.

Effect of Multiple Spark In order to examine the effect of repetition of

sparks, two successive sparks were used. The in- dividual spark is produced by CDI (capacitor dis- charge ignition) unit; each unit is operated at a given time interval (spark interval). Typical example of the results, shown in Figs. 7 and 8, were obtained by using tungsten spark electrodes of 0.3 mm in di- ameter and the gap width of 1 mm.

Figure 7 shows that, with increasing spark in- terval, the ignition probability increases, reaches the maximum and asymptotically settles at a constant value. This constant value is well evaluated by the following equation:

P, = 1 - (1 - P1) n, (6)

where P,, P1 and n are the ignition probability for the repeating sparks of n times, the ignition prob- ability for a single spark (example data are shown in Fig. 7 (top)) and the number of sparks (n = 2 in the present case), respectively. The spark inter~ val at the maximum ignition probability is hereafter referred to as optimum spark interval. Figure 7 (top) shows that with changing mixture strength, the op- timum spark duration hardly varies.

In regard to effects of increased flow velocity, Fig. 7 shows that the optimum spark interval decreases from about 10 ms to about 150 ixs. The effect of spark duration on the individual spark is shown in Fig. 8 (top). A detailed examination of Fig. 8 (top) indicates that the optimum spark interval is always about 30 IZS larger than the spark duration; the close relation between them is well explained by over- lapping of the spark paths. As shown in Fig. 8 (bot- tom), for a small spark interval less than the total time of 30 p,s plus the spark duration, the first spark and the second one overlap each other along the same path. With further increase in the spark in- terval to exceed the above-mentioned total time, the second spark path is formed at the spark gap. Figure 8 shows that the maximum ignition proba-

138 AUTOMOTIVE ENGINE COMBUSTION

Time 5.1 (us) 17,5 43.5 83 107 250

Fro. 6. Schlieren photograph of flame kernels. Propane-air mixture of 4 (vol.)%. Flow velocity: 23 m/ s; gap width: 1 mm; diameter of spark electrodes: 25 I~m (top and bottom), 0.5 mm (middle).

bility appears at the maximum spark interval so far as the overlapping of the two sparks occurs; this suggests that surprisingly much heat or radical spe- cies are transferred from the flame kernel to the spark electrodes. This situation also holds for igni- tion by long duration sparks because the path of later discharge of them keeps off from the spark electrodes; as shown in Fig. 4, with increasing spark duration the effective ignition energy decreases for a flow velocity of more than 10 m/s.

The occurrence of the spark repetition for a long duration spark corresponds to the situation of two successive sparks, in which the individual spark separately occurs. Therefore, it is not expected that the multiple spark enhances the ignition ability markedly, but from the approximate coincidence of the spark intervals of both the multiple spark and the two successive sparksl it is suggested that the multiple spark increases the ignition probability considerably; such an increase is photographically ascertained to be due to a decrease in heat loss based on the thermal insulation by the adjacent flame kernel. Another advantage of multiple spark is to increase the ignition probability according to Eq.

(6); for example when P1 = 0.1, Pn is 0.41, 0.88 and 0.99 for n of 5, 20 and 50, respectively.

Superiority of Arc Discharge

It is clear from the results shown in Fig. 3 that the !gnition ability of arc discharge is superior to that of glow discharge. As well known, the cathode fall for glow discharge is remarkably larger than that for arc discharge. Therefore, a considerable pro- portion of the spark energy for glow discharge con- centrates in the vicinity of the surface of the neg- ative electrode, so that the amount of heat loss to the negative electrode is more than that for arc dis- charge. Whether discharge becomes glow or arc types depends on current density (current per unit area) at the surface of the negative electrode. Thus, it is easily expected that, by using a given ignition unit, if Spread on the negative glow along the sur- face of the negative electrode is suppressed by a certain method and thus the Current density is in- creased, it may be possible to realize an arc dis- charge and hence a spark of higher ignition ability. For example, the negative electrode (0.2 mm in di-

IGNITION ABILITY OF COMPOSITE SPARKS 139

lO0

50 o o _

5 l0 50 100 Spark Interval

\ Propane Concen, (Vol.%)

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500 1000 5000 Single ]JS Spark

~lOC

~ 50

~ o

- Flow ?eloclty / (m/S)

I I I ,

5 10 50 100 50 lO0 500 I000 Spark Interval ~S

~ r - -

\

5000 10000

FIG. 7. Effects of mixture strength (top) and flow velocity (bottom) on ignition probability of two suc- cessive sparks. Flow velocity: 20 m/s (top); spark duration: 150 IXS (top), 125 I~S (bottom); spark en- ergy: 3.5 mJ x 2 (top), 3 mJ x 2 (bottom); 2.55 (vol.)% propane (bottom)�9

"~ 100

~ 5O e~

e - 0

= 0

' ' ' ' " ' ~:J' Fl()w'V'eloclty 2om/s

/ I Propane Concen. . z I 2.ZVol.

/ z ( \ ~q Spark Durat ion-

\ ~ o 125 ~-~ \ --~ AI50

I 1 6 - ~ ,~ ' , , , , , [3200

50 100 200 500 Spark Interval ~S

o 100

50 r "

O .

c:x 0

o ~ 0 i I , i00 200 500

Spark Interval pS

FIG. 8. Effect of spark duration on optimum spark interval (top) and ratio of overlapping of two spark paths to total number of times of trials (bottom).

' Fi0W ~ VelocpltY/s

Propane C0ncen,- 2,7V01,1

Spark Duration- (NS) lO0 150 2OO

100

~ so

g ~ o g

�9 "0../N SPark Duration: 60 ps I Type Negotlve /Y~z~ Propane 3.1 (vol.) %1

-- Arc Electrode ~ /)k \ Gap Width: 1 mm --I

5 10 15 20 25

Flow Velocity m/s

FIG. 9. Comparison of ignition abilities between arc and glow type electrodes by using CDI system.

ameter) was surrounded by a porcelain tube (0.5 mm in outer diameter), the arc type shown in Fig. 9. The result obtained with this electrode is com- pared with that by the electrode of the same size, namely the glow type shown in Fig. 9. Ignition unit used is CDI type. With the same stored energies in the primary capacitors of the CDI units, about 80 and 30% of the total spark duration was found to be the duration of arc discharge for the arc and glow type electrodes, respectively. From Fig. 9, it is clear that the ignition ability of the arc type, in spite of its smaller value of the ignition energy, is superior to that of glow type.

Although from the practical point of view, the concept of the adoption of an arc type electrode mentioned above is in an undeveloped stage, an optimization of ignition units, for example, a re- duction in the output impedance of the ignition unit, may lead to an enhancement of the ignition ability of practical systems.

6. Suggestions for Enhancing the Ignition Ability of Practical Spark s

From the results obtained in the present work, the following are noted for the enhancement of ig- nition ability of practical ignition systems:

(1) For spark electrodes, it is not sufficient to en- large the gap width, but it is preferable to adopt the thinner spark electrodes or a special configu- ration, e.g. streamline, which produces a low in- tensity turbulence.

(2) Ignition ability is expected to be improved by using arc discharge in place of glow discharge; the latter is mainly used practically. As one of the methods for the improvement, increasing the cur- rent density at the negative electrode is suggested in the present work.

(3) With regard to the spark duration, the opti- mum spark duration does not need to be critically considered as far as the glow discharge is con- cerned.

(4) The multiple spark, usually observed in prac-

140 AUTOMOTIVE ENGINE COMBUSTION

tical spark discharges, is found to enhance the ig- nition ability considerably. Additionally, for long duration spark such as the multiple spark, the probability of the spark encountering a mixture close to the optimum is high compared with that for a short one; the mixing between fuel and air in prac- tical engines is usually incomplete so the supe- riority of the long duration spark appears at a far leaner mixture strength.

Acknowledgment

The authors gratefully acknowledge and thank Emeritus Professor S. Kumagai for his encourage- ment and discussion. Support for part of this work was provided by the Ministry of Education under Grant in Aid for Scientific Research (58116008) and is greatly appreciated.

REFERENCES

1. KONO, M., KUMAGAI, S. AND SAKAI, T.: Sixteenth Symposium (International) on Combustion, p.

757, The Combustion Institute, 1977. 2. KUMAGAI, S., SAKAI, T. AND KIMUBA, I.: J. Fac.

Engng. Univ. Tokyo, 24, 10 (1953). 3. KIMu~, I. AND KUMAGAI, S.: J. Phys. Soc. Ja-

pan, 11, 599 (1956). 4. HAarORk T., GOTO, K. AND OHIGASHI, S.: Inst.

Mech. Eng. C101/79 (1979). 5. SwEar JR., C. C.: Sixth Symposium (Interna-

tional) on Combustion, p. 523, Reinhold, New York, 1957.

6. DE SOETE, G. G.: Thirteenth Symposium (In- ternational) on Combustion, p. 735, The Com- bustion Institute, 1971.

7. BALLAL, D. R. AND LEFEBVRE, A. H.: Combus- tion and Flame, 24, 99 (1975).

8. LEWIS, B. AND VON ELBE, G.: Combustion, Flames and Explosions in Gases, 2nd Ed., p. 326, Ac- ademic Press, 1961.

9. KONO, M., KUMAGAI, S. AND SAKAI, T.: Com- bustion and Flame, 27, 85 (1976).

COMMENTS

G. F. W. Ziegler, Universitdt Stuttgart, W. Ger- many. Can you explain why the effective ignition energy is determined only by the discharge part perpendicular to the flow velocity, despite the fact that the discharge properties in the legs of the dis- charge is identical to the other part.

Authors" Reply. From the schlieren photographic

studies of the development of flame kernels after the termination of the .spark discharge, the image of the "legs" starts to disappear rapidly (see also Ref. 4). The spark energy contained in the legs of the flame kernel is originally reduced due to heat loss to the spark electrode. Therefore, the spark energy contained in the legs is smaller than that contained in the other part.