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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 indicate 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 inductance components; therefore, the spark is referred to as composite spark. Although ignition
phenomena have been studied in quiescent gas
mixtures 1'~ and under flowing conditions3'4 by using composite sparks, and also done by using long
duration sparks in flowing gases, 5-7 many important factors to enhance the ignition ability are overlooked.
In the present work, for the purpose of obtaining
the optimum condition for ignition of composite
sparks, the effects of spark duration, spark path behavior, electrode size, discharge type and multiple
spark on the minimum ignition energy were investigated. 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 problem of the spark plug.
The ignition unit used for long duration sparks
consists of a capacitance spark and a following component generator; the breakdown of gases by the
former triggers the latter discharge. A detailed description of the unit can be seen elsewhere, 1 except
for increased voltage of dc power source for the following component spark from 1.35 to 5.6 kV and
adoption of a triggering spark (0.5 p~s spark duration) produced by a thyratron. 9 The power and duration o f t h e 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 turbulent 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 E N G I N E COMBUSTION
GLOW DISCHARGE
AIR
VELOCITY
ARC DISCHARGE
-'I
~'0
0
r
t...)
>0
Om/s
,..,,0
0
0
r'~
/
C)
.__-
>
JJJ
'
0
>
7,3m/s
<._)
0
..~0
0
r',,
_J
O
>
13m/s
,4
<..._)
I,I
0
t
0
23m/s
,J
rY"
0
O
O,4msldiv,
VOLTAGE: 640V/div
CURRENT: 18mA/div
POWER: 2,9W/div
SOURCE VOLTAGE: 1950V
O,4ms/div.
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
nition probability can be obtained. The minimum
ignition energy is determined at 50% ignition probability; the ignition energy is defined as the total
energy of the capacitance and the following camponents. In the present work, the effect of the capacitance component energy was not examined because the characteristic feature of the effect is much
the same as that obtained for quiescent gaseous
mixtures. 1 The energy of the capacitance component is 0.07 mJ~
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
Figure 1 shows the typical example of the traces
of voltage, current and power of the spark discharge for several air velocities. These traces observed in air are much the same as that in propaneair 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 electrode.
As can be seen in Fig. 1, under flowing conditions 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 approximately 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 energy, 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 ability 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 currentgap 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
6
I F ame
s
~
~3
i
_ ~.-/Kernel
/
W'~ SPark
~.!.~..
~~
1
/
-~POth
'"
135
r,
,A
:"~
i
II
i
~L=2U(t-tps)+W
/
i
/
9
/
V~
,
-
~2
V
,
9
A
9
1
D
9
I
tps
=45 Vs
I00
Tim
200
ps
9
I
300
(W)
MIx
AIF 35 ( A r c )
MIx.
Air
39 (Glow]
MIx.
AIF lq (GIow]
I
~00
FIG. 2. Spark path length determined from electrical measurements and schlieren photographs
against time. Flow velocity: 23 m/s; gap width: 1
mm.
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 assumed to be a model shown in Fig. 2. Therefore,
the spark path length can be expressed as:
L = W for t < tp,,
L=2U(t-tp~)+W
fort=>tps,
(1)
(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 impossible 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 discharge was impossible to continue after the capacitance spark.
In Fig. 3, for glow discharge sparks, the minimum ignition energy of a quiescent mixture shows
a lowest value at about 80 p,s spark duration (optimum spark duration), but that of flowing mixtures, by contrast, hardly shows a distinct lowest
value. As is seen, the minimum ignition energies
for arc discharge are half of those for glow discharge. 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
Flow Velocity
v [ (m/s)
5
, ~--~-~-~ 23
Arc
t_
kd
=2
E
-=o,5
Arc
c
0.2
5
10
20
50
100
500
200
Spork Durotlon
ps
FIG. 3. Minimum ignition energy by long duration sparks against spark duration. Propane-air mixture of 3.2 (vol.)%. Gap width: 1 mm; spark electrode diameter: 0.3 mm.
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 velocity, the minimum ignition energy decreases and
reaches the minimum at about 1.5 m/s flow veloc-
ity. With further increase in flow velocity, the minimum ignition energy increases. When a spark path
is blown downstream, such as the present experiment, the true ignition energy, referred to as effective 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 minimum ignition energy, the model of Ballal and
Lefebvre7 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 assumed to be included in the flame kernel as shown
in Fig. 2, can be expressed as:
Pw = Em/t~ for t < tn~,
(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~
200!100 I
I
+ WEre In 2U(t8 - tps) + W
2tsU
W
I
I I
I
I
,
I //O'-"-
0
t~
2
--
d
1
9:
.~9
ps
O 50
A 100
5" /
O
Energy
6J
/ / / i n
~
~
I
I
I
I
5
10
15
20
Flow Velocity
--
_
1
o.~
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 radical species to the spark electrodes and cause turbulence in the wake of spark electrodes. These
quenching effects are examined in the following
discussion.
250
2 - Effective Ignttlon
g
(5)
25
m/s
FIG. 4. Minimum ignition energy, effective ignition energy (calculated) and t,, against time, experimental condition being same as Fig. 3.
5.
Discussion
Effect of Spark Electrodes
For the purpose of obtaining the detailed information on the quenching effects mentioned above,
only capacitance sparks are used in order to exclude 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
30 /
I
|
GODWidth 1.5mm
I
o.8
From the facts mentioned above, the spark electrodes 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 electrodes, and the promotion of cooling of the flame
kernel by turbulence in the wake of the spark electrodes. The above explanation is applied to the results 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 effects are included in the behavior of the effective
ignition energy shown in Fig. 4.
~/~
Electrode
Dlam, (ram)
7
I
/
o5
0.8
.3
0 1
0.05
,05
0,5
137
0.I
Effect of Multiple Spark
0
T "-,6o wi th3 ,
0
5
10
Flow Velocity
15
m/s
20
25
FIG. 5. Minimum ignition energy against flow velocity by using capacitance sparks. Propane-air mixture 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 effect of the spark electrode on the flame kernel because of an increased distance between the flame
kernel and the spark electrodes. With further increase in flow velocity, the minimum ignition energy increases for all the electrode sizes. For 3 mm
gap width, no change in the minimum ignition energy 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 quenching of the flame kernel by heat loss to the spark
electrode occurs at an upstream side of the doughnut. 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 photographs, and that the minimum ignition energies for
both cases are much the same.
In order to examine the effect of repetition of
sparks, two successive sparks were used. The individual spark is produced by CDI (capacitor discharge 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 diameter and the gap width of 1 mm.
Figure 7 shows that, with increasing spark interval, 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 probability 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 optimum 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 overlapping of the spark paths. As shown in Fig. 8 (bottom), 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 interval 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
Time
(us)
AUTOMOTIVE ENGINE COMBUSTION
5.1
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 species are transferred from the flame kernel to the
spark electrodes. This situation also holds for ignition 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 proportion of the spark energy for glow discharge concentrates in the vicinity of the surface of the negative electrode, so that the amount of heat loss to
the negative electrode is more than that for arc discharge. 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 surface of the negative electrode is suppressed by a
certain method and thus the Current density is increased, it may be possible to realize an arc discharge and hence a spark of higher ignition ability.
For example, the negative electrode (0.2 mm in di-
IGNITION ABILITY OF COMPOSITE SPARKS
lO0
139
100
\
Propane
Concen,
(Vol.%)
o 50
~ so
9
"0../N SPark Duration: 60 psI
Type Negotlve
/Y~z~ Propane 3.1 (vol.) %1
-- Arc Electrode ~ /)k \ Gap Width: 1 mm --I
o_
n2,85
5
l0
50 100
500 1000
5000 Single
Spark
Spark Interval ]JS
~
~lOC
g
~
g
o
5
10
15
Flow Velocity
20
25
m/s
FIG. 9. Comparison of ignition abilities between
arc and glow type electrodes by using CDI system.
r--
\
~ 50
- Flow ?eloclty
/
(m/S)
~o
I
I
5
10
I
,
50 100
lO0
500 I000
Spark Interval ~S
5000 10000
FIG. 7. Effects of mixture strength (top) and flow
velocity (bottom) on ignition probability of two successive sparks. Flow velocity: 20 m/s (top); spark
duration: 150 IXS (top), 125 I~S (bottom); spark energy: 3.5 mJ x 2 (top), 3 mJ x 2 (bottom); 2.55
(vol.)% propane (bottom)9
"~ 100
~
' ~:J' Fl()w'V'eloclty
' ' ''"
2om/s
.z
5O
I Propane Concen.
/
/
I
z(\
2.ZVol.
~q Spark Duration-
e~
e0
~-~
=
o
r"
1 6 - ~
I
0
50
100
\
\
--~
,~', , , , ,
100
200
Spark Interval
500
~S
Propane C0ncen,2,7V01,1
Spark Duration-
O.
c:x
o~
o 125
AI50
[3200
~
' Fi0W~ VelocpltY/s
50
0
0
i00
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 compared 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 reduction in the output impedance of the ignition unit,
may lead to an enhancement of the ignition ability
of practical systems.
i
I ,
200
500
Spark Interval pS
(NS)
lO0
150
2OO
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).
6. Suggestions for Enhancing the Ignition Ability
of Practical Sparks
From the results obtained in the present work,
the following are noted for the enhancement of ignition ability of practical ignition systems:
(1) For spark electrodes, it is not sufficient to enlarge the gap width, but it is preferable to adopt
the thinner spark electrodes or a special configuration, e.g. streamline, which produces a low intensity 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 current density at the negative electrode is suggested
in the present work.
(3) With regard to the spark duration, the optimum spark duration does not need to be critically
considered as far as the glow discharge is concerned.
(4) The multiple spark, usually observed in prac-
140
AUTOMOTIVE ENGINE COMBUSTION
tical spark discharges, is found to enhance the ignition 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 practical engines is usually incomplete so the superiority 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 encouragement 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.
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. Japan, 11, 599 (1956).
4. HAarORk T., GOTO, K. AND OHIGASHI, S.: Inst.
Mech. Eng. C101/79 (1979).
5. SwEar JR., C. C.: Sixth Symposium (International) on Combustion, p. 523, Reinhold, New
York, 1957.
6. DE SOETE, G. G.: Thirteenth Symposium (International) on Combustion, p. 735, The Combustion Institute, 1971.
7. BALLAL, D. R. AND LEFEBVRE, A. H.: Combustion and Flame, 24, 99 (1975).
8. LEWIS, B. ANDVON ELBE, G.: Combustion, Flames
and Explosions in Gases, 2nd Ed., p. 326, Academic Press, 1961.
9. KONO, M., KUMAGAI, S. AND SAKAI, T.: Combustion and Flame, 27, 85 (1976).
REFERENCES
1. KONO, M., KUMAGAI,S. AND SAKAI,T.: Sixteenth
Symposium (International) on Combustion, p.
COMMENTS
G. F. W. Ziegler, Universitdt Stuttgart, W. Germany. 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 discharge 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.