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Flame initiation and development from glow discharges—Effect of electrode deposits
321
COMBUSTION A N D F L A M E 77: 321-336 (1989)
Flame Initiation and Development from Glow Discharges--Effect of -Electrode Deposits GAUTAM T. KALGHATGI Shell Research Ltd., Thornton Research Centre, P.O. Box 1, Chester CH1 3SH, England
The predominant discharge mode in the spark from a conventional automobile coil ignition system is a glow discharge. It is also known that certain gasoline additives--alkali and alkaline earth metal compounds known as "spark-alders"--enhance the ignition process in spark ignition engines by forming deposits on the spark plug electrodes. This work describes experiments performed in a combustion bomb to study flame initiation and development from glow discharges, and the effects of electrode deposits on these phenomena. The two phenomena, which are intimately linked, are shown to be profoundly influenced by the presence of electrodes and to depend on electrode gap, material, shape and size as well as spark power, energy, mixture characteristics, and pressure. Marginal conditions for flame initiation can result from a suitable combination of any of these parameters. Crystalline deposits on the electrodes make breakdown of the gap easier and more repeatable, but breakdown itself plays a relatively unimportant role in flame initiation and development in the experiments described here. Only the potassium compound deposits on the electrodes lead to a reduction in the glow voltage, primarily because of a reduction in the cathode fall, though reductions in the column voltage gradient are also observed. Such deposits, viz potassium sulfate, greatly enhance the ignition ability of a glow discharge, especially as the conditions for ignition become more difficult. In some cases, however, when the reduction in the glow voltage because of such deposits is too large, the ignition ability of the glow discharge can worsen. The results can be partially explained in terms of the presence of low electron work function material on the cathode and the consequent improvement in the energy transfer efficiency of the glow discharge. However, other questions about the details of the effect of such deposits on the physics of the glow discharge and on flame initiation and development remain unresolved.
1. INTRODUCTION Flame initiation and early flame development, which greatly affects cyclic variation in engines, are both significantly influenced by the spark discharge process in a spark ignition engine. In general, problems associated with ignition become noticeable in engine operation when the burning velocity of the mixture is low, for example, with lean mixtures. In vehicle operation terms, such problems appear in drivability criteria such as idling stability and acceleration performance [1]. If engine designs move towards lean-burn and/or high-exhanst gas recirculation, these problems are likely to become more pressing, Hence, there has been considerable interest both in understanding and in finding ways of enhancing the ignition process in spark ignition engines [1, 2]. Copyright © 1989 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010
Recent work has shown that certain fuel additives-alkali and alkaline earth metal compounds, collectively termed "spark aiders"--can significantly increase early flame development rates and reduce cyclic variations [3, 4]. Dynamometer tests have shown that a potassium-based additive improves idling and acceleration performance [5]. We have also shown that a potassium-based spark alder extends the lean limit and improves misfire characteristics [6]. Spark-aiders deposit material on spark electrodes [3], and it is postulated that this leads to an improvement in the efficiency of the transfer of spark electrical energy to the gas, because of the low electron work function of the deposits. TMs in turn, leads to observed improvements in early flame development. If a current i, has been established in the coil
0010-2180/89/$03.50
322
G. T. KALGHATGI
before switching the primary circuit, the energy stored m the coil is ~ L t , where L is the inductance. This energy, minus losses in the circuit, is transferred through the secondary circuit and dissipated in the spark [7, 8]. The discharge is a complex phenomenon and has been extensively studied by Maly, Ziegler and co-workers [9-11]. It can be split sequentially into three phases-breakdown, arc, and glow. Of these, the glow phase contains over 90 % of the total spark energy. In glow discharges, the current is low and there is a substantial voltage drop, known as the cathode fall, close to the cathode surface. The energy dissipated here is largely lost to the electrodes and constitutes at least 70% of the glow discharge energy [10, 11]. Low electron work function cathodic materials reduce the cathode fall [12]. They also promote transition from the glow to the arc mode, which is more efficient at transferring energy to the gas [10]. Thus, in a coil ignition system, loss of spark energy to the electrodes is reduced by low electron work function cathode deposits and more spark energy is transferred to the gas. This article describes experimental studies of low electron work function metal compound deposits, on the sparking and the ignition ability of glow discharges. The results demonstrate that flames can be initiated more easily by glow discharges if the cathode is coated with potassium salts. •
•
•
1
"2
2. EXPERIMENTAL DETAILS The experiments were in a cylindrical combustion bomb of stainless steel with an internal diameter of 70 mm and length of 90 mm. The bomb had two quartz windows to allow full optical access along the axis of the cylinder. The electrodes had easily interchangeable tips and were located in PTFE inserts normal to the cylinder axis. The electrode gap could be accurately adjusted by a micrometer. The spark circuit has been described in Ref. 13. The gap breaks down at a gap voltage Vb and the spark consists of a very short duration ( - 40 ns) breakdown phase followed by a self-sustaining discharge that is constrained to be a glow dis-
charge by limiting the current. The current, Ig, and the duration, r, of the glow discharge could be varied. The gap voltage was measured with a Tektronix high-voltage probe and the current with a curreng transformer. The current measurements were also checked using measured voltage drops across known resistances. A schlieren system using a continuous light source in conjunction with a high-speed cine camera (Hadland Hycam) was used to study flame development. The camera generated a signal when the film speed reached the required value of 5000 frames per second, and this initiated the spark. The developed films were processed through an image analyzer (MOP Video Plan) to yield quantitative information on flame development. The flammable mixture was prepared in a separate vessel by the method of partial pressures, and stored at pressures up to 37 bar. The bomb was evacuated to at least 10 torr and then filled with mixture to a pressure up to 1 bar higher than that required. The pressure was relieved to the required value. 3. RESULTS
3.1 Spark Discharge Characteristics The effect of different electrode deposits, viz sulfate, nitrite, hydroxide, chloride and nitrate of potassium, nickel sulfate, and sugar (which has no metallic content), on sparking characteristics have been described in detail in Ref. 13. Potassium has a low electron work function (2.3 V) compared to nickel (5.04 V) or carbon (5.0 V). A summary of these findings is given below.
3.1.1 Breakdown Characteristics Breakdown is always preceded by a statistical or initiatory time lag, tl [14]. When the gap voltage increases rapidly with time as in the present experiments, breakdown occurs at a time tl after the gap voltage has reached the static breakdown voltage Vs, at a voltage Vb, which is higher than Vs. When the electrodes are clean, the mean and standard deviation for tl and hence Vb are large. However, when the electrodes were coated with
323
FLAME INITIATION FROM GLOW DISCHARGES any of the crystalline materials considered, the mean and standard deviation of tl and Vb were reduced [13]. Indeed, with potassium salts and nickel sulfate, breakdown always occurred below ,-,X~e static breakdown voltage for clean electrodes [13]. Under a microscope, crystalline structures that were large for the potassium salts and nickel sulfate, but small for sugar could be observed on the electrode surface. These phenomena could perhaps be explained in terms of the enhancement A of electric field strengths at the tips of microprotrusions on coated electrodes and the consequent increase in the field emission of electrons so that the initiatory time lag is reduced [15].
3.1.2 Glow Discharge Characteristics The glow voltage Vg can be expressed as Vg = Vy + Bd + V,,, where V/is the cathode fall, Va is the anode fall, B is the voltage gradient, and d is the gap width. In general, Va is much smaller (a few percent of Vg) than the other two terms [11, 12] and is neglected in the following discussion. The cathode fall was reduced substantially by potassium compounds on the electrodes (primarily the cathode). However, coatings of sugar, nickel sulfate [13], and strontium chloride led to an increase in the cathode fall. Potassium compounds also reduced the voltage gradient, B, substantially (Fig. 2 in Ref. 13). When the glow voltage is reduced for the coated electrodes there should be a very slight--less than 2.0%--increase in the glow current to account for this voltage reduction across the gap. However, the instruments used were not sensitive enough to measure this increase. The effect lasted for several hundred firings after the electrodes were coated-especially for potassium sulfate. It is likely that a very thin layer of deposit is sufficient to give the observed effect. The glow voltage increases with pressure-primarily because of an increase in the voltage gradient, B--and coating the electrodes with potassium sulfate reduced the glow voltage even at high pressure (Fig. 3, Ref. 13). Thus all crystalline deposits tested made breakdown easier and more repeatable. However, of the coatings tried, only the potassium salts, which are expected to be
of low electron work function, led to a reduction in the glow voltage.
3.2 Ignition Ability of Glow Discharges In these experiments brass electrodes with 1.5-mm tungsten carbide hemispherical tips were used. If Cg is the capacitance of the gap, the spark electrical energy, Es, is given by 1 E~=~-G Vo2+I~ • v g . ~ .
Cg was measured to be very low ( < 2 pf). Hence the breakdown component of the spark energy is less than 0.1 mJ in our tests and thus much smaller than the glow discharge energy. For coated electrodes, as Vb is smaller, the breakdown component is even smaller. The glow current, Ig, and the glow duration, r, were first set. The probability P, of successful flame initiation in which the whole of the mixture in the bomb is burned was then established based on at least 20 spark events. After establishing the variation of P with r or Es, with clean electrodes, the electrodes were coated in situ with a solution of the appropriate salt (potassium sulfate in most experiments) in distilled water, dried, and the whole procedure repeated after "conditioning" the electrodes by firing them for upto 100 firings (Section 3.2.6). The ignition ability of the spark for a given set of conditions is determined by Eig,, the value of the spark energy Es at which P is 0.5 [11]. For a given set of conditions, the spark energy to achieve a given probability of ignition can be considerbly reduced by coating the electrodes with potassium sulfate (e.g., Figs. 4b and 5b, Ref. 13). This improvement is the more marked, the less favorable the conditions for ignition, that is, the higher the value of Eign with clean electrodes. In some cases, with clean electrodes, Eign is not definable because P never reaches 0.5 with increasing spark energy. In one such case considered in Fig. 5b of Ref. 13, Eign was reduced to 10.5 mJ with a coating of potassium sulfate but with nickel sulfate, which slightly increases the cathode fall [13], the ignition ability of the spark demonstrably worsened compared to the clean
324
G . T . KALGHATGI 100 Not definable 20
~\ x ~'\.
10
g LU
2.0 1.0
n
0.5
I
I
•
1.0
1.5
2.0
SPARK GAP, d, mm
Fig. 1. Effect of changing spark gap on Eign. Lean methaneair mixture (4 = 0.67) at atmospheric pressure. Tungsten carbide hemispherical tip electrodes. A, glow current lg = 15 mA; © , I g = 3 0 m A ; [Z,18 = 58 m A ; x f r o m Ref. 11. Filled symbols are for electrodes coated with potassium sulfate.
cathode and ignition was completely suppressed [131. In some cases the improvement due to the coating could be simply attributed to the reduction in the cathode fall (Fig. 4 in Ref. 13). However, when ignition was more difficult with clean electrodes, for a given set of conditions, a higher ignition probability could be obtained for the same glow duration when electrodes were coated with potassium sulfate (Fig. 5a in Ref. 13), suggesting an increase in the energy deposited in the mixture in spite of the reduction in the glow voltage and hence the spark energy.
trodes. These electrodes had blunt ends with a diameter between 0.5 and 0.64 mm, with a glow current between 5 and 70 mA. The results look closer to the present 15-mA case, but the differences in electrode material and size (see also Section 3.2.5) make detailed comparisons difficult. In all cases, coating the electrodes with potassium sulfate reduces Eig, and this reduction is most marked in the limiting conditions. For instance, there were two cases with this mixture, d = 1 mm, Ig = 15 m A a n d d = 0.8 mm, Ig = 30mA, when Eign was not definable, but with coated electrodes, Eign was 6.3 and 15 mJ, respectively. 3.2.2 Effect of Changing the Glow Current When results in Fig. 1 are replotted in terms e(~ Eig, against the glow current Ig, it is seen that for a given spark gap, with clean electrodes, increasing the glow current facilitates ignition, as is shown by the decrease in Eign. Coating the electrodes with potassium sulfate reduces Eign, as already discussed.
100 60
Not definable at 1 arm
\
\ \
10 _
\
.9 Ltl
3.2.1 Effect of Changing the Spark Gap In Fig. 1, Eign has been plotted against the spark gap, d, for a weak methane-air (¢ = volume fraction of the fuel/stoichiometric volume fraction of fuel = 0.67) mixture at atmospheric pressure for both clean and coated electrodes and for different glow currents. As the gap increases, Eign decreases. This is to be expected because the quenching effect of the electrodes on the flame decreases with increasing gap [16]. Also shown in Fig. 1 are results from Ziegler et al. [11] for this mixture for glow discharges using copper elec-
1 0.5
I 2
I 3
PRESSURE OF UNBURNT MIXTURE, p/atm Fig. 2. Effect o f changing pressure on Ei~. Rich methane-ah" mixture (4 = 1.29). Tungsten carbide hemispherical tip electrode. Spark gap d = 1 rnm, glow current Ig = 30 mA. Filled symbols are for electrodes coated with potassium sulfate.
325
FLAME INITIATION FROM GLOW DISCHARGES TABLE 1
3.2.3 Effect of Changing the Pressure In Fig. 2, Eig n has been plotted against the pressure in the vessel prior to the spark, for a rich methaneair mixture (q~ = 1.29); the spark gap was 1 mm ~nd the glow current 30 mA. It can be seen that Eign was not definable at atmospheric pressure but decreased dramatically on increasing the pressure. Such an effect is well established in the literature-in Ref. 16 it is shown that for a stoichiometric mixture of methane and air "minimum ignition energy" decreases by a factor of more than 20 if the pressure is increased from 0.1 to 1 atm. Coating the electrodes with potassium sulfate again reduces Eig,, even at the higher pressures.
3.2.4 Results for Propane-Air Mixtures • ~ Table la values of Eign for a lean propane-air mixture (q~ = 0.8) at atmospheric pressure are listed for clean electrodes and electrodes coated with potassium sulfate for two values of the glow current. In both cases, the coating leads to a reduction in Eign. Similar results are shown in Table lb for propane-air mixtures at a pressure of 3 atm, at two leaner stoichiorfietries of ~b = 0.7 and q~ = 0.65. On comparing the results for clean electrodes in Table la with those in Table lb, it can be seen that if the pressure is increased, the mixture can be leaner and still have similar values of Eig,. It can also be seen, for example, from the results for Ig = 58 mA, in Table lb, that making the mixture leaner near the lean limit sharply increases Eign for clean electrodes. Potassium sulfate deposits again lead to a reduction in E~gn. These results serve to illustrate that the effects of potassium sulfate deposits are not peculiar to methane-air mixtures and also that even at higher pressures, if the stoichiometry is right, ignition probabilities are greatly reduced, and, in such cases, the potassium deposits could again lead to large improvements.
3.2.5 Effect of Electrode Shape and Size The quenching effect of the electrodes on the flame is known to depend on their shape and size [16, 17]. The ignition probabilities for different spark electrical energies were established for a
Results for Lean Propane-Air Mixtures la. Spark gap, d = 1 mm Pressure, p = 1 atm, ~b = 0.8 E, gn (mJ) Current Ig (mA)
Clean Electrodes
Potassiffm Sulfate Coated Electrodes
58 30
3.4 5.7
2.5 1.8
lb. Spark gap, d = I mm Pressure, p = 3 atm E ~ (mJ) Current I, (mA) 58 30
4' = 0.7 Clean Coated
4.1 5.4
3.0 3.4
~ = 0.65 Clean Coated
144 --
37 --
rich methane-air mixture (~ = 1.29) at atmospheric pressure for a spark gap of 1 mm and a glow current of 58 mA for three different pairs of electrodes: 1. Those tipped with 1.5-mm tungsten carbide balls 2. Flat-faced brass (1.5-ram-diameter) electrodes 3. Pointed brass electrodes (tip diameter = 0.15
mm) It was found that the increase in ignition probability with increasing spark energy is much sharper and Eign much lower, indicating that the quenching effect is less marked, for the pointed brass electrodes compared to the other electrodes. In Table 2, Eign for the three types of electrode are tabulated for both clean and coated electrodes for two currents. Minimum ignition energy increases with the boiling point of the electrode material [ 17, 18] for arc discharges because of increasing losses to the electrodes in forming a cathode spot. Ziegler et al. [11] found a similar result for glow discharges, which they explain in terms of the cathode fall for different materials. Hence Eign might have been expected to be larger for the tungsten carbidetipped electrodes than for the brass electrodes. As
326
G. T. KALGHATGI TABLE 2 Effect of Electrode Size and Shape on Etsna lg (mA)
58 30
Tungsten Carbide Tips 1.5 mm Diameter
Flat-Faced Brass 1.5 mm Diameter
Pointed Brass 0.15 nun Diameter
Clean
Coated
Clean
Coated
Clean
Coated
42.0 Not definable
5.0
61.0
9.5
6.4
2.6
10.5
--
--
48
7.2
° Ei~ (mJ) for methane-air, 4~ = 1.29, d = 1 mm, atmospheric pressure.
this is not the case, it appears that the difference in the shape of the electrode tips, for these two cases overrides the difference in electrode material. The hemispherical tips give higher ignition probability than the fiat-faced tips, the pointed tips being the best of the three. Again, coating the electrodes with potassium sulfate led to a reduction in Eign shown in Table 2.
3.2.6 Effect of Glow Voltage in Coating Experiments It was recognized early that there were cases, when the glow voltage was reduced too much, where the ignition ability of the glow was reduced by the potassium sulfate coating.--hence the need to "condition" the electrodes before the second ignition experiments, as described in Section 3.2. To investigate this phenomenon further, we considered the ignition probability from a glow discharge of 2 ms duration at different glow voltages, but with the glow current set at 58 mA. These experiments were in a rich methane-air mixture (~ = 1.29) at atmospheric pressure with fiat-faced brass electrodes with a gap of 1 mm for which the glow voltage, Vg, was around 460 V. The electrodes were coated with a saturated solution of potassium sulfate, and dried, whereupon Vg was reduced to very low values--around 75 V. Higher values of Vg were obtained by "washing" the electrodes with a few drops of distilled water (presumably removing some potassium sulfate) and drying the electrodes. At these different stages, ignition probabilities were established and are plotted in Fig. 3. During these
determinations, each of which required at least twenty firings, Vg did not remain constant, and the spread of some of the symbols in Fig. 3 reflectsthis. It can be seen that the coating could increase the ignition probability from 0.35 for clean electrodes to 1.0, but P could also be reduced to zero if the glow voltage was reduced too much. A possible explanation is considered in Section 4.
3.3 Flame Development At first electrodes with tungsten carbide tips were used in these studies of flame development. However, the repeatability of the results was poor. The early flame development was often unsymmetric, and this precluded any meaningful study of ' the effects of glow characteristics on flame development. The films highlighted the profound effect of electrode quenching on early flame development. For instance, depending on where the spark channel occurred on the electrodes, one half of the flame would be more free of quenching and develop faster than the other half, which, in many cases, would be extinguished. With the pointed brass electrodes, however, flame development was repeatable and symmetric about the electrode axes, and the results described in this section are all for these electrodes. During the early flame development (kernel diameter less than 40 mm), the pressure rise was negligible. The experiments were all with a rich methane-air mixture (4~ = 1.29) at atmospheric pressure and a spark gap of 1 mm. The ignition probability for
327
FLAME INITIATION FROM GLOW DISCHARGES % 1.0 Z
-
9
I.-¢C
V5 0.8
u.
~ 0.6 >...I
~ 0.4 on,-
CLEAN
Q.
0.2
100
I I 200 300 GLOW VOLTAGE, Vg/V
1 400
Fig. 3. Effect of glow voltage on the probability of flame initiation in coating experiments. Rich methane-air ($ = 1.29) mixture at atmospheric pressure. Hat-faced brass electrodes; spark gap d = 1 mm; glow current Ig = 58 mA, glow duration ¢ = 2.0 ms.
this case is plotted against glow duration in Fig. 4 for these conditions. On each frame of a developed film, the maximum distance between the flame front and the mid point of the spark gap was measured on either side of the electrodes and an average flame front distance estimated, using the image analyzer. This distance can be taken to be the flame kernel radius once the kernel grows larger than about 10 mm in diameter, since it then assumes a near spherical shape. In Fig. 5 the flame front distance is plotted against time from breakdown with clean electrodes for two sparks, both of 1 ms duration, but with the glow current of 58 mA for spark A and 30 mA for spark B. The repeatability of flame development was confirmed in four separate experiments with spark A. From Fig. 4 it can be seen that the ignition probability for spark A is 1.0 and for spark B is zero. This is reflected in Fig. 5: the kernel from spark B is seen not to grow beyond 2.1 mm in radius and eventually die out, whereas spark A leads to successful flame initiation and a continuously growing, self-sustained flame. These results are in line with the theoretical as well as the experimental findings of Joulin and co-
workers [19-21], that below a critical size, spherical flames have a tendency to collapse because of the straining of the flame front and they can be driven beyond this critical size only if the energy/ power input from the spark is sufficiently high. In Fig. 6a, the flame front distance is plotted against time after breakdown for three discharges of 0.5, 1, and 2 ms duration from clean electrodes; the spark current was fixed at 58 mA. It can be confirmed from Fig. 4 that for all three sparks, the ignition probability was 1. Figure 6a shows that continuing to supply electrical energy in the spark, beyond 0.5 ms at a fixed power level, leads to faster early flame development, though increasing the glow duration from 1 to 2 ms does not appear to have a large effect. This is better illustrated in Fig. 6b, where the flame speed, calculated from Fig. 6a, is plotted against time after breakdown. Figure 6b also well illustrates the dynamics of the flame front, as described by Champion et al. [21]. When the spark energy is slightly larger than that required to successfully initiate the flame, flame speed decreases as the flame kernel radius approaches the critical radius and then increases as it grows beyond that radius, eventually to reach a
328 G. T. KALGHATGI
i~
1.0
. . . . . .
0.8
u
~ 0.6
0.4
0.2
I 1.0
2.0 3.0 GLOW DURATION, T/ms
I
J
4.0
5.0
Fig. 4. Probability of flame initiation with pointed brass electrodes. Rich methane-air mixture (¢ = 1.29) at atmospheric pressure. Spark gap, d = 1 ram; © , glow current/~ = 58 mA; E3, lg = 30 mA. Filled symbols are for electrodes coated with potassium sulfate.
15.0
-
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J
t
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9.0
l 12.0
TIME AFTER BREAKDOWN, t/ms
Fig. 5. Comparison of flame development from two sparks with pointed brass electrodes. Flame front distance versus time. Rich m e t h a n e - a i r mixture ( ~ = 1.29) at atmospheric pressure. Spark gap = 1 ram, duration = 1 ms. Glow current 18 = 58 m A for spark A and 30 m A for spark B. (Es = 25.5 rrd for A and 13.2 mJ for B.)
FLAME INITIATION FROM GLOW DISCHARGES
329
20.0 4- O
E
~O
n,-
4-0 4-O
(D Z I-tO B E3 t-Z O
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A
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O
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~
f
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.,.
1 I 6.0 9.0 TIME AFTER BREAKDOWN, t/ms
1
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1 15.0
(a) 2.0 +
A
+ O ,O ÷
+o
.o + o . ~
O
A z~
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a tl.i ku
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-
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+
++ ++
+++.o
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a
i,
I 3.0
I 1 1 6.0 9.0 12.0 TIME AFTER BREAKDOWN, t/ms
I 15.0
(b) Fig. 6. Effect of glow duration on flame development, with pointed brass electrodes. (a) flame front distance vs. time. (b) flame front velocity vs. time. Rich methane-air mixture (~ = 1.29) at atmospheric pressure. Spark gap = 1 ram, g l o w current I z = 58 m A , g l o w p o w e r = 25.5 W; A , g l o w duration r = 0.5 ms; O , r = 1.0 ms; + , r = 2 . 0 ms.
normal propagation speed (=1.37 m/s in this case), characteristic of the mixture. Increasing the initial spark power or for a fixed power, the spark duration can increase the flame speed and shorten this middle phase of flame development [22, 23] before normal propagation. Indeed, at very high
initial spark powers, the initial flame speed, before normal propagation, can be much higher than the normal propagation speed [10, 24]. However, for electrodes coated with potassium sulfate, simple considerations of measured spark characteristics can be misleading, as is illustrated
330
G. T. K A L G H A T G I 20.0
E o COATED
rr-
tu"
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L) Z < I-
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o
C~ 10.0 I-Z 0 n.It. LU :E <
o z~
o Z~
o &
:I I
I
I
6.0
3.0
I
1
9.0
15.0
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T I M E A F T E R BREAKDOWN, t/ms
(a) t/ms
0.1
0.2
0.3
1
I
I .............
\
400
>
0.5
0.4
--
L,
[
2' ~
0.6
I
130 V
COAIED "~""
I~--.,~1----440V C L EAN
800
(b) Fig. 7. Comparison of flame development from clean and coated (with potassium sulfate) pointed brass electrodes. (a) flame front distance vs. time. (b) spark voltage vs. time. Rich methane-air mixture (q~= 1.29)at atmospheric pressure. Spark gap d = 1 mm, glow current
I~= 58mA.
in Fig. 7a, which compares flame development for clean and coated electrodes. Figure 7b shows the voltage traces of the sparks that led to the flames considered in Fig. 7a: the spark current was 58 mA for both cases. For the coated electrodes the glow duration, r, is lower--350 as compared to 500 #s--and the glow voltage, and hence the apparent electrical power and energy of the spark, are also lower. On both counts, the clean electrodes could be expected to lead to faster flame development, but the contrary is the case (Fig. 7a). Thus the flame initiation ability of a spark
depends on the power and energy available to the mixture, rather than the apparent electrical power and energy. This point is further illustrated by Fig. 8 where the kernel radius in air at atmospheric pressure, is plotted against time in Fig. 8a and the glow voltage for the two sparks (r = 1 ms, I s = 58 mA) producing the two kernels, is shown in Fig. 8b. It can be seen that the spark from electrodes coated with potassium sulfate gives rise to a bigger kernel, indicating more efficient energy input into the air, even though the apparent spark voltage and
FLAME INITIATION FROM GLOW DISCHARGES
331
6.0
COATED
E 4.0
O
00000
tt~
O
0 o°
<
A zx &
0
rr
o o: 2.0
-& &
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& p , tX & A
I
J
1
6.0
2.0 4.0 T I M E AFTER BREAKDOWN. t/ms
(a)
0.2 .
I
0.4
0.6
0.8
I
I
I
1.0 t/ms ~.. ~ - - " ' - - C O A T E D
4OO
-
t
>
J ~
CLEAN ELECTRODE
(b)
Fig. 8. Comparison of kernel development in air at atmospheric pressure form clean and coated (with potassium sulfate), pointed brass electrodes. (a) kernel radius vs. time. (b) spark voltage vs. time. Spark gap d = 1 ram, glow current I~ = 58 mA.
hence power and energy are lower, compared to the clean electrodes. The breakdown voltages for the clean and coated cases were 4.6 and 4.53 kV, respectively. Thus there is little difference in the breakdown components of the energy for these two sparks. 4. DISCUSSION Flame initiation and early flame development in a premixed combustible mixture are very closely
connected. As discussed by Joulin [20], two separate criteria have to be met to successfully initiate a flame by a local (point) input of energy, such as by a small electric spark--an ignition criterion that is to raise the local temperature enough to cause thermal runaway and start the chemical reaction and a more demanding initiation criterion that requires that the power and the duration of the energy input be sufficient to overcome the straining effects in the incipient flame front and drive the flame kernel beyond a
G . T . KALGHATGI
332 critical size. Thus, three sequential regimes of flame development can be identified--localized ignition, spark-assisted flame propagation, and normal flame propagation [25]. Of these, localized ignition is always ensured by a spark because during the first, breakdown, phase of any spark discharge, extremely high temperatures--up to 60,000 K--are achieved in the spark column [10]. Normal flame propagation depends only on the mixture characteristics--mixture, stoichiometry, pressure, temperature, and turbulence characteristics if propagation is in a turbulent medium. Thus it is only the middle phase, during which the flame could be "strained out," that can be influenced by changes in the spark characteristics following breakdown. It is also during this phase that in a turbulent field the effects of turbulence on flame propagation, such as additional straining of the flame front, set in [25, 26]. However turbulence effects are not considered further since the experiments were all conducted in quiescent mixtures. 4.1 Effect of the Presence of Electrodes The presence of electrodes has a profound effect on flame initiation and development because of the quenching effect of the electrodes on the flame and spark energy loss to the electrodes.
coupled with the fact that the spark discharge can take place at different physical sites on the electrodes, leads to greater variability in flame development and a greater spread in the ignition probability versus spark energy curves for thelarger electrodes compared with the pointed brass electrodes (Section 3.3). 4.1.2 Losses of Spark Energy to the Electrodes It has been appreciated previously [17, 18] that not all the spark energy is deposited in the mixture. However, it is most notably the work of Maly, Ziegler, and co-workers [9-11] that has highlighted the effect that the loss of spark energy to the electrodes and the consequent deterioration of spark efficiency has on the ignition ability of the spark. This is especially marked for glow discharges, which can have efficiencies of the order of 10% [27, 28]. Ignoring the anode fall, the power in the glow discharge can be written as Vg.lg = Vy.Ig + (Bd)lg, where Vyis the cathode fall, B is the voltage gradient, and d is the spark gap. At the simplest level it is believed that only the power described by the second of these terms is not lost to the electrodes and hence is available to the gas, so that the efficiency )7 of the glow discharge can be simply expressed as
4.1.1 Flame Quenching by the Electrodes Lewis and von Elbe [16] defined the quenching distance as the critical distance between two heat sinks (such as the electrodes) below which flame propagation is suppressed. The quenching distances for methane-air and propane-air mixtures at atmospheric pressure are listed in Table 6 of Ref. 16. In all the experiments (at atmospheric pressure) considered in Section 3, the actual spark gap (_<2 mm) was less than the corresponding quenching distance. In such circumstances, as the spark gap is reduced, the increasing quenching effect of the electrodes has to be compensated for by an increasing spark energy to obtain successful flame initiation. As the pressure and temperature of the charge increase, the quenching distance decreases [16]. The quenching effect is also larger, the larger the physical size of the electrodes [16, 17, 24], as discussed in Section 3.2.5. This,
Bd 'I-
v/+ B-------d"
(1)
As the spark gap increases, ~/increases because
111 remains constant. Also with increases in pressure, B increases (see Section 3.1.2) and hence ,/increases. Thus increasing the spark gap or the pressure leads to a reduction of the quenching effect of the electrodes on the incipient flame as well as an increase in the efficiency of the glow discharge in transferring electrical energy to the gas. This results in a reduction in Eig~, as demonstrated in Figs. 1 and 2. 4.2 The Effect of Spark Power The importance of high energy input in the "early stages" of the spark, on ignition and flame development has been well recognized in the
FLAME INITIATION FROM GLOW DISCHARGES literature. Such a strategy produces sharp temperature gradients and the right plasma geometry to enhance early flame development [10] and its effects will be imprinted on the flame front and continue to be felt after sparking. It also happens that the spark discharge modes with the higher currents have smaller energy losses to the electrodes [10J--both current and energy transfer efficiency increasing in the order of glow, arc, and breakdown. From these considerations, Maly and co-workers have argued that for best results, the energy of the spark should be concentrated in the breakdown phase, and indeed experiments in quiescent mixtures do show that short-duration, high-power sparks lead to higher early flame development rates. The wor k of Joulin and coworkers [19-21] puts this in perspective by demonstrating the importance of supplying spark energy throughout the early flame development period. In our experiments, as in many practical cases with automobile coil ignition systems, the breakdown phase, efficient though it may be in transferring energy to the mixture, is too weak to bring about successful flame initiation. In such cases increasing the spark power, after breakdown, should and does lead to improved ignition, as seen by the fact that Eign decreases as the glow current increases in Fig. 1. Indeed if the glow power is too low, successful flame initiation may not occur (Fig. 5). 4.3 Effect of Deposits of Potassium Sulfate on the Electrodes
The results clearly show that, as expected, electrode coatings of potassium sulfate lead to significant reductions in the cathode fall, and also to dramatic improvements in the ignition ability of glow discharges, though in certain extreme cases, there might be a deterioration in ignition ability. There are results where the actual energy deposited in the mixture does not appear to change significantly upon coating the electrodes (e.g., Fig. 4a in Ref. 13). The improvement in the spark efficiency in these cases could be simply explained by a reduction in the cathode fall Vy, in Eq. 1. However, the voltage gradient B can also be reduced by a coating of potassium sulfate [13]. In
333
that case, the power in the positive colunm-B" d'/g--will be reduced, and if it is reduced too much, flame initiation will be adversely affected, regardless of the fact that Vf might have been reduced and the efficiency improved. However, it is more difficult to explain results such as those in Figs. 7 and 8 (also Fig. 5a in Ref. 13), where it appears that upon coating the electrodes, the energy deposited in the mixture apparently increases even though Vg is reduced, while Ig remains constant. This cannot be because of any changes in the breakdown properties because, in fact, for coated electrodes, the energy in the breakdown phase will be less than for the clean electrodes because the breakdown voltage will be lower (Section 3.1.1). Moreover, nickel sulfate, which affects breakdown characteristics in the same manner as potassium sulfate (Section 3.1.1), has quite the opposite effect on the ignition ability of the spark (Section 3.2). A probable explanation could lie in the fact that the current density is increased by the low electron work function deposit [12]. Bradley and Lung [25] have presented a model that demonstrates that the spark kernel growth rate, and, by implication, the ignition ability of the spark, should increase with the volumetric electrical power, that is, the current density of the discharge. It is also possible that in these cases the reduction in the cathode fall is accompanied by an increase in its physical width, so that some of the energy dissipation in the cathode fall region occurs far enough away from the cathode for it not to be lost to the electrodes; this would lead to additional energy deposition in the mixture when the electrodes are coated. In any case the phenomenon discussed in this paragraph is really noticeable when conditions for ignition are especially severe, namely when Ei~ with clean electrodes is large. It is likely that in such cases, only small improvements in the quantity or quality of energy deposition in the mixture are needed to give large improvements in flame initiation and development. Thus, a simple explanation based on relating the losses to the electrode entirely to the cathode fall and then invoking the reduction of the cathode fall by the potassium sulfate deposit, cannot fully account for all our observations. It appears that
334 depending, perhaps, on the nature and thickness of the coating, its effect on flame initiation and development is different. These matters can satisfactorily be resolved only by a much more detailed and careful study of the effect of such deposits on the physics of the glow discharge, using a reliable technique to produce repeatable coatings and independently measuring the losses to the electrodes.
4.4 Significance for Engine Operation In engines, the voltage across the spark gap increases rapidly with time, as in the present experiments, but the statistical time lag is likely to be very small because of high pressures and temperatures and the presence of water vapor [ 14]. Moreover, deposits would form on spark plug electrodes even in the absence of fuel additives. Hence any improvements in breakdown characteristics because of fuel additives are likely to be much less marked than in the present experiments. Spark gaps are fixed and are of the order of 0.61.0 ram. In normal circumstances, because of the higher pressures and temperatures in the engine this gap should be larger than the quenching distance for the mixture, which depends on mixture strength, pressure, and temperature. However, in many regimes of engine operation quenching distances could be large so that the quenching effects of the electrodes become important. The laminar burning velocity is also low under those conditions that can occur, for example, during idling when the pressure and also the temperature of the charge is low; during part throttle acceleration, when the mixture strength becomes momentarily very lean because the fuel flow does not increase as quickly as the air flow; and during cold starting when the mixture is lean because of inadequate fuel vaporization. Thus all engines operate in regimes where flame initiation is difficult and the misfire probability is high. The fact that flame development takes place in turbulent flow fields also contributes to these difficulties, which can be expected to become more important if engine design trends move towards lean-burn and/or high-exhaust gas recirculation concepts.
G. T. KALGHATGI In a conventional automobile coil ignition system, the spark is not terminated as in the experiments described here--it continues until the voltage to sustain it drops below a threshold level as the stored coil energy is dissipated. Low electron work function deposits on the electrodes do not change this total energy of the spark but make a larger proportion of it available to the gas because they reduce the losses to the electrodes, as shown in this work, and also extend the spark duration [6]. In engines, an extension of the spark duration is in itself beneficial because it compensates for the cyclic variations of mixture strength in the spark gap and reduces the probability of misfire [29, 30]. More importantly, as demonstrated in this work, such deposits bring about a dramatic improvement in the ability of the glow discharges, the predominant discharge mode in a coil ignition system, to ignite difficult mixtures. Thus sparkaider fuel additives, which lead to deposits of low electron work function on the electrodes, bring about improvements in engine misfire characteristics [6] and early flame development and, consequently, reduce cyclic variation [3]. This in turn leads to the observed improvements in drivability characteristics [5]. Even with spark-aider concentrations in the fuel far greater than any likely to be used (e.g. 80 ppmK), the glow voltage is never reduced to the extent that the ignition ability of the spark worsens.
o
CONCLUSIONS
1. All the crystalline electrode deposits tested (potassium sulfate, nickel sulfate, and sugar) reduce the mean as well as the standard deviation of the breakdown lag time and hence the breakdown voltage when the gap voltage increases with time. . Of the deposits tested, only the potassium compound deposits on the electrodes (primarily the cathode) lead to a reduction in the glow voltage, mostly because of a reduction in the cathode fall though reductions in the column voltage gradient were also observed. (The detailed results to support these first two conclusions can be found in Ref. 13.)
335
FLAME INITIATION FROM GLOW DISCHARGES
3. The presence of electrodes has a profound effect on, and leads to variability in, flame initiation and development because of quenching effects of the flame front as well as losses of the spark energy to the electrodes. 4. The ignition ability of a spark--a glow discharge in the present experiments--depends on a combination of spark gap, power, and energy, electrode material, size, and shape, and mixture strength and pressure. For a given mixture, in general, the ignition ability of the glow increases as the spark gap, spark power, and mixture pressure are increased. Pointed electrodes are better than flat-face or spherical electrodes for ignition. Marginal conditions for flame initiation can result because of any suitable combination of these factors--for instance if the pressure is high, by weakening of the mixture strength. 5. Flame initiation and early flame development are intimately linked and in quiescent mixtures, factors that facilitate flame initiation also enhance the early flame development rate. 6. The ignition ability of a glow discharge can be greatly enhanced by deposits of potassium sulfate on the electrodes. This improvement becomes especially marked as the conditions for ignition become more difficult. Under some conditions, however, when the fall in the glow voltage because of such deposits is too large, the ignition ability can worsen. Thus potassium sulfate deposits bring about changes in the electrical characteristics of the spark that are expected of a low electron work function material. However, the mechanism for the improvement in the energy transfer efficiency of the spark, as reflected in its enhanced ignition ability, due to such deposits is not fully explained, as the detailed effects of such deposits on the physics of the glow discharge remain unresolved. Thanks are due to Mr. D. A. J. Jones of Thornton Research Centre, Shell Research Ltd. for conducting the experiments.
2. 3. 4. 5.
6. 7. 8. 9.
10.
11.
12.
13.
14.
15.
16.
17. 18. 19. 20. 21. 22.
REFERENCES 1.
Kalghatgi, G. T. SAE Paper No. 870163, 1987.
Dale, J. D., and Oppenheim, A. K., SAE Paper No. 810146, 1981. Kalghatgi, G. T., Combust. Sci. TechnoL 52:427-446 (1987). Muller, H., and Schaperkotter, H., Motortech. Z. 48:429-434 (1987). Blackmore, D. R., Gralff, L. B., Harrow, G. A., Jones, J. M., Kalghatgi, G. T., and Miles, R., International Conference on Petroleum Based Fuels and Automotive Applications, Proceedings o f the Institution o f Mechanical Engineers, Institution of Mechanical Engineers, London, 1986, p. 47. Kalghatgi, G. T., Combust. Sci. Technol. 62:1-19 (1988). Hurtley, D., Auto. Eng., 148-152 (1969). Weber, H., SAE Paper No. 740021, 1974. Maly, R., and Vogel, M., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1979, p. 821. Maly, R. R., in Fuel Economy: Road Vehicles Powered by Spark Ignition Engines (J. C. Hilliard and G. S. Springer, Eds.), Plenum Press, New York, 1984, Chap. 3. Ziegler, G. F. W., Wagner E. P., and Maly R. R., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 1817. Cobine, J. D., Gaseous Conductors Theory and Engineering Applications, Dover, New York, 1958, Chap. 8. Kalghatgi, G. T., International Conference on Combustion in Engines Technology and Applications, Proceedings o f the Institution o f Mechanical Engineers, Institution of Mechanical Engineers, London, 1988, p. 9. Dutton, J. D., in Electrical Breakdown o f Gases (J. M. Meek and J. D. Craggs, Eds.), Wiley, New York, 1979, Chap. 3. Grey Morgan, C., in Electrical Breakdown o f Gases (J. M. Meek and J. D. Craggs, Eds.), Wiley, New York, 1979, Chap. 7. Lewis, B., and Von Elbe, G., Combustion Flames and Explosion o f Gases, Academic, New York, 1961, Chap. 5. Rose, H. E., and Priede, T., Seventh Symposium on Combustion, Butterworths, London, 1959, p. 436. Ballal, D. R., and Lefebvre, A. H., Combust. Flame 24:99-108 (1975). Deshaies, D., and Joulin, G., Combust. Sci. Technol. 37:99-116 (1984). Joulin, G., Combust. Sci. Technol. 43:99-113 (1985). Champion, M., Deshaies, B., Joulin, G., and Kinoshita, K., Combust. Flame 65:319-337 (1987). Anderson, R. W., and Lim, T. M., Eighth International Conference on Gas Discharges and Their Applications, GD85 Organizing Committee, Leeds University Press, 1985, p. 511.
336 23.
Karpov, V. N., Malov, V. V., and Severin, E. S., Combust. Expl. Shock Waves 22:707-712 (1987). 24. DeSoete, G. G., International Conference on Combustion in Engineering, Institution of Mechanical Engineers, London, 1983, Voi. 1, p. 93. 25. Bradley, D., and Lung, F. K. K., Combust. Flame 69:71-93 (1987). 26. Kalghatgi, G. T., Combust. Flame 60:299-308 (1985). 27. Saggau, B., Arch. Elektotech. 64:229 (1981).
G. T. K A L G H A T G I 28.
Teets, R. E., and Sell, J. A., SAE Paper No. 880204, 1988. 29. Aiman, W. R., Combust. Sci. Technol. 15:129-136 (1977). 30. Nakai, M., Nakagawa, Y., Hamai, K., and Sone, M . , SAE Paper No. 850075, 1985.
Received 23 June 1988; revised 10 October 1988
COMBUSTION A N D F L A M E 77: 321-336 (1989)
Flame Initiation and Development from Glow Discharges--Effect of -Electrode Deposits GAUTAM T. KALGHATGI Shell Research Ltd., Thornton Research Centre, P.O. Box 1, Chester CH1 3SH, England
The predominant discharge mode in the spark from a conventional automobile coil ignition system is a glow discharge. It is also known that certain gasoline additives--alkali and alkaline earth metal compounds known as "spark-alders"--enhance the ignition process in spark ignition engines by forming deposits on the spark plug electrodes. This work describes experiments performed in a combustion bomb to study flame initiation and development from glow discharges, and the effects of electrode deposits on these phenomena. The two phenomena, which are intimately linked, are shown to be profoundly influenced by the presence of electrodes and to depend on electrode gap, material, shape and size as well as spark power, energy, mixture characteristics, and pressure. Marginal conditions for flame initiation can result from a suitable combination of any of these parameters. Crystalline deposits on the electrodes make breakdown of the gap easier and more repeatable, but breakdown itself plays a relatively unimportant role in flame initiation and development in the experiments described here. Only the potassium compound deposits on the electrodes lead to a reduction in the glow voltage, primarily because of a reduction in the cathode fall, though reductions in the column voltage gradient are also observed. Such deposits, viz potassium sulfate, greatly enhance the ignition ability of a glow discharge, especially as the conditions for ignition become more difficult. In some cases, however, when the reduction in the glow voltage because of such deposits is too large, the ignition ability of the glow discharge can worsen. The results can be partially explained in terms of the presence of low electron work function material on the cathode and the consequent improvement in the energy transfer efficiency of the glow discharge. However, other questions about the details of the effect of such deposits on the physics of the glow discharge and on flame initiation and development remain unresolved.
1. INTRODUCTION Flame initiation and early flame development, which greatly affects cyclic variation in engines, are both significantly influenced by the spark discharge process in a spark ignition engine. In general, problems associated with ignition become noticeable in engine operation when the burning velocity of the mixture is low, for example, with lean mixtures. In vehicle operation terms, such problems appear in drivability criteria such as idling stability and acceleration performance [1]. If engine designs move towards lean-burn and/or high-exhanst gas recirculation, these problems are likely to become more pressing, Hence, there has been considerable interest both in understanding and in finding ways of enhancing the ignition process in spark ignition engines [1, 2]. Copyright © 1989 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010
Recent work has shown that certain fuel additives-alkali and alkaline earth metal compounds, collectively termed "spark aiders"--can significantly increase early flame development rates and reduce cyclic variations [3, 4]. Dynamometer tests have shown that a potassium-based additive improves idling and acceleration performance [5]. We have also shown that a potassium-based spark alder extends the lean limit and improves misfire characteristics [6]. Spark-aiders deposit material on spark electrodes [3], and it is postulated that this leads to an improvement in the efficiency of the transfer of spark electrical energy to the gas, because of the low electron work function of the deposits. TMs in turn, leads to observed improvements in early flame development. If a current i, has been established in the coil
0010-2180/89/$03.50
322
G. T. KALGHATGI
before switching the primary circuit, the energy stored m the coil is ~ L t , where L is the inductance. This energy, minus losses in the circuit, is transferred through the secondary circuit and dissipated in the spark [7, 8]. The discharge is a complex phenomenon and has been extensively studied by Maly, Ziegler and co-workers [9-11]. It can be split sequentially into three phases-breakdown, arc, and glow. Of these, the glow phase contains over 90 % of the total spark energy. In glow discharges, the current is low and there is a substantial voltage drop, known as the cathode fall, close to the cathode surface. The energy dissipated here is largely lost to the electrodes and constitutes at least 70% of the glow discharge energy [10, 11]. Low electron work function cathodic materials reduce the cathode fall [12]. They also promote transition from the glow to the arc mode, which is more efficient at transferring energy to the gas [10]. Thus, in a coil ignition system, loss of spark energy to the electrodes is reduced by low electron work function cathode deposits and more spark energy is transferred to the gas. This article describes experimental studies of low electron work function metal compound deposits, on the sparking and the ignition ability of glow discharges. The results demonstrate that flames can be initiated more easily by glow discharges if the cathode is coated with potassium salts. •
•
•
1
"2
2. EXPERIMENTAL DETAILS The experiments were in a cylindrical combustion bomb of stainless steel with an internal diameter of 70 mm and length of 90 mm. The bomb had two quartz windows to allow full optical access along the axis of the cylinder. The electrodes had easily interchangeable tips and were located in PTFE inserts normal to the cylinder axis. The electrode gap could be accurately adjusted by a micrometer. The spark circuit has been described in Ref. 13. The gap breaks down at a gap voltage Vb and the spark consists of a very short duration ( - 40 ns) breakdown phase followed by a self-sustaining discharge that is constrained to be a glow dis-
charge by limiting the current. The current, Ig, and the duration, r, of the glow discharge could be varied. The gap voltage was measured with a Tektronix high-voltage probe and the current with a curreng transformer. The current measurements were also checked using measured voltage drops across known resistances. A schlieren system using a continuous light source in conjunction with a high-speed cine camera (Hadland Hycam) was used to study flame development. The camera generated a signal when the film speed reached the required value of 5000 frames per second, and this initiated the spark. The developed films were processed through an image analyzer (MOP Video Plan) to yield quantitative information on flame development. The flammable mixture was prepared in a separate vessel by the method of partial pressures, and stored at pressures up to 37 bar. The bomb was evacuated to at least 10 torr and then filled with mixture to a pressure up to 1 bar higher than that required. The pressure was relieved to the required value. 3. RESULTS
3.1 Spark Discharge Characteristics The effect of different electrode deposits, viz sulfate, nitrite, hydroxide, chloride and nitrate of potassium, nickel sulfate, and sugar (which has no metallic content), on sparking characteristics have been described in detail in Ref. 13. Potassium has a low electron work function (2.3 V) compared to nickel (5.04 V) or carbon (5.0 V). A summary of these findings is given below.
3.1.1 Breakdown Characteristics Breakdown is always preceded by a statistical or initiatory time lag, tl [14]. When the gap voltage increases rapidly with time as in the present experiments, breakdown occurs at a time tl after the gap voltage has reached the static breakdown voltage Vs, at a voltage Vb, which is higher than Vs. When the electrodes are clean, the mean and standard deviation for tl and hence Vb are large. However, when the electrodes were coated with
323
FLAME INITIATION FROM GLOW DISCHARGES any of the crystalline materials considered, the mean and standard deviation of tl and Vb were reduced [13]. Indeed, with potassium salts and nickel sulfate, breakdown always occurred below ,-,X~e static breakdown voltage for clean electrodes [13]. Under a microscope, crystalline structures that were large for the potassium salts and nickel sulfate, but small for sugar could be observed on the electrode surface. These phenomena could perhaps be explained in terms of the enhancement A of electric field strengths at the tips of microprotrusions on coated electrodes and the consequent increase in the field emission of electrons so that the initiatory time lag is reduced [15].
3.1.2 Glow Discharge Characteristics The glow voltage Vg can be expressed as Vg = Vy + Bd + V,,, where V/is the cathode fall, Va is the anode fall, B is the voltage gradient, and d is the gap width. In general, Va is much smaller (a few percent of Vg) than the other two terms [11, 12] and is neglected in the following discussion. The cathode fall was reduced substantially by potassium compounds on the electrodes (primarily the cathode). However, coatings of sugar, nickel sulfate [13], and strontium chloride led to an increase in the cathode fall. Potassium compounds also reduced the voltage gradient, B, substantially (Fig. 2 in Ref. 13). When the glow voltage is reduced for the coated electrodes there should be a very slight--less than 2.0%--increase in the glow current to account for this voltage reduction across the gap. However, the instruments used were not sensitive enough to measure this increase. The effect lasted for several hundred firings after the electrodes were coated-especially for potassium sulfate. It is likely that a very thin layer of deposit is sufficient to give the observed effect. The glow voltage increases with pressure-primarily because of an increase in the voltage gradient, B--and coating the electrodes with potassium sulfate reduced the glow voltage even at high pressure (Fig. 3, Ref. 13). Thus all crystalline deposits tested made breakdown easier and more repeatable. However, of the coatings tried, only the potassium salts, which are expected to be
of low electron work function, led to a reduction in the glow voltage.
3.2 Ignition Ability of Glow Discharges In these experiments brass electrodes with 1.5-mm tungsten carbide hemispherical tips were used. If Cg is the capacitance of the gap, the spark electrical energy, Es, is given by 1 E~=~-G Vo2+I~ • v g . ~ .
Cg was measured to be very low ( < 2 pf). Hence the breakdown component of the spark energy is less than 0.1 mJ in our tests and thus much smaller than the glow discharge energy. For coated electrodes, as Vb is smaller, the breakdown component is even smaller. The glow current, Ig, and the glow duration, r, were first set. The probability P, of successful flame initiation in which the whole of the mixture in the bomb is burned was then established based on at least 20 spark events. After establishing the variation of P with r or Es, with clean electrodes, the electrodes were coated in situ with a solution of the appropriate salt (potassium sulfate in most experiments) in distilled water, dried, and the whole procedure repeated after "conditioning" the electrodes by firing them for upto 100 firings (Section 3.2.6). The ignition ability of the spark for a given set of conditions is determined by Eig,, the value of the spark energy Es at which P is 0.5 [11]. For a given set of conditions, the spark energy to achieve a given probability of ignition can be considerbly reduced by coating the electrodes with potassium sulfate (e.g., Figs. 4b and 5b, Ref. 13). This improvement is the more marked, the less favorable the conditions for ignition, that is, the higher the value of Eign with clean electrodes. In some cases, with clean electrodes, Eign is not definable because P never reaches 0.5 with increasing spark energy. In one such case considered in Fig. 5b of Ref. 13, Eign was reduced to 10.5 mJ with a coating of potassium sulfate but with nickel sulfate, which slightly increases the cathode fall [13], the ignition ability of the spark demonstrably worsened compared to the clean
324
G . T . KALGHATGI 100 Not definable 20
~\ x ~'\.
10
g LU
2.0 1.0
n
0.5
I
I
•
1.0
1.5
2.0
SPARK GAP, d, mm
Fig. 1. Effect of changing spark gap on Eign. Lean methaneair mixture (4 = 0.67) at atmospheric pressure. Tungsten carbide hemispherical tip electrodes. A, glow current lg = 15 mA; © , I g = 3 0 m A ; [Z,18 = 58 m A ; x f r o m Ref. 11. Filled symbols are for electrodes coated with potassium sulfate.
cathode and ignition was completely suppressed [131. In some cases the improvement due to the coating could be simply attributed to the reduction in the cathode fall (Fig. 4 in Ref. 13). However, when ignition was more difficult with clean electrodes, for a given set of conditions, a higher ignition probability could be obtained for the same glow duration when electrodes were coated with potassium sulfate (Fig. 5a in Ref. 13), suggesting an increase in the energy deposited in the mixture in spite of the reduction in the glow voltage and hence the spark energy.
trodes. These electrodes had blunt ends with a diameter between 0.5 and 0.64 mm, with a glow current between 5 and 70 mA. The results look closer to the present 15-mA case, but the differences in electrode material and size (see also Section 3.2.5) make detailed comparisons difficult. In all cases, coating the electrodes with potassium sulfate reduces Eig, and this reduction is most marked in the limiting conditions. For instance, there were two cases with this mixture, d = 1 mm, Ig = 15 m A a n d d = 0.8 mm, Ig = 30mA, when Eign was not definable, but with coated electrodes, Eign was 6.3 and 15 mJ, respectively. 3.2.2 Effect of Changing the Glow Current When results in Fig. 1 are replotted in terms e(~ Eig, against the glow current Ig, it is seen that for a given spark gap, with clean electrodes, increasing the glow current facilitates ignition, as is shown by the decrease in Eign. Coating the electrodes with potassium sulfate reduces Eign, as already discussed.
100 60
Not definable at 1 arm
\
\ \
10 _
\
.9 Ltl
3.2.1 Effect of Changing the Spark Gap In Fig. 1, Eign has been plotted against the spark gap, d, for a weak methane-air (¢ = volume fraction of the fuel/stoichiometric volume fraction of fuel = 0.67) mixture at atmospheric pressure for both clean and coated electrodes and for different glow currents. As the gap increases, Eign decreases. This is to be expected because the quenching effect of the electrodes on the flame decreases with increasing gap [16]. Also shown in Fig. 1 are results from Ziegler et al. [11] for this mixture for glow discharges using copper elec-
1 0.5
I 2
I 3
PRESSURE OF UNBURNT MIXTURE, p/atm Fig. 2. Effect o f changing pressure on Ei~. Rich methane-ah" mixture (4 = 1.29). Tungsten carbide hemispherical tip electrode. Spark gap d = 1 rnm, glow current Ig = 30 mA. Filled symbols are for electrodes coated with potassium sulfate.
325
FLAME INITIATION FROM GLOW DISCHARGES TABLE 1
3.2.3 Effect of Changing the Pressure In Fig. 2, Eig n has been plotted against the pressure in the vessel prior to the spark, for a rich methaneair mixture (q~ = 1.29); the spark gap was 1 mm ~nd the glow current 30 mA. It can be seen that Eign was not definable at atmospheric pressure but decreased dramatically on increasing the pressure. Such an effect is well established in the literature-in Ref. 16 it is shown that for a stoichiometric mixture of methane and air "minimum ignition energy" decreases by a factor of more than 20 if the pressure is increased from 0.1 to 1 atm. Coating the electrodes with potassium sulfate again reduces Eig,, even at the higher pressures.
3.2.4 Results for Propane-Air Mixtures • ~ Table la values of Eign for a lean propane-air mixture (q~ = 0.8) at atmospheric pressure are listed for clean electrodes and electrodes coated with potassium sulfate for two values of the glow current. In both cases, the coating leads to a reduction in Eign. Similar results are shown in Table lb for propane-air mixtures at a pressure of 3 atm, at two leaner stoichiorfietries of ~b = 0.7 and q~ = 0.65. On comparing the results for clean electrodes in Table la with those in Table lb, it can be seen that if the pressure is increased, the mixture can be leaner and still have similar values of Eig,. It can also be seen, for example, from the results for Ig = 58 mA, in Table lb, that making the mixture leaner near the lean limit sharply increases Eign for clean electrodes. Potassium sulfate deposits again lead to a reduction in E~gn. These results serve to illustrate that the effects of potassium sulfate deposits are not peculiar to methane-air mixtures and also that even at higher pressures, if the stoichiometry is right, ignition probabilities are greatly reduced, and, in such cases, the potassium deposits could again lead to large improvements.
3.2.5 Effect of Electrode Shape and Size The quenching effect of the electrodes on the flame is known to depend on their shape and size [16, 17]. The ignition probabilities for different spark electrical energies were established for a
Results for Lean Propane-Air Mixtures la. Spark gap, d = 1 mm Pressure, p = 1 atm, ~b = 0.8 E, gn (mJ) Current Ig (mA)
Clean Electrodes
Potassiffm Sulfate Coated Electrodes
58 30
3.4 5.7
2.5 1.8
lb. Spark gap, d = I mm Pressure, p = 3 atm E ~ (mJ) Current I, (mA) 58 30
4' = 0.7 Clean Coated
4.1 5.4
3.0 3.4
~ = 0.65 Clean Coated
144 --
37 --
rich methane-air mixture (~ = 1.29) at atmospheric pressure for a spark gap of 1 mm and a glow current of 58 mA for three different pairs of electrodes: 1. Those tipped with 1.5-mm tungsten carbide balls 2. Flat-faced brass (1.5-ram-diameter) electrodes 3. Pointed brass electrodes (tip diameter = 0.15
mm) It was found that the increase in ignition probability with increasing spark energy is much sharper and Eign much lower, indicating that the quenching effect is less marked, for the pointed brass electrodes compared to the other electrodes. In Table 2, Eign for the three types of electrode are tabulated for both clean and coated electrodes for two currents. Minimum ignition energy increases with the boiling point of the electrode material [ 17, 18] for arc discharges because of increasing losses to the electrodes in forming a cathode spot. Ziegler et al. [11] found a similar result for glow discharges, which they explain in terms of the cathode fall for different materials. Hence Eign might have been expected to be larger for the tungsten carbidetipped electrodes than for the brass electrodes. As
326
G. T. KALGHATGI TABLE 2 Effect of Electrode Size and Shape on Etsna lg (mA)
58 30
Tungsten Carbide Tips 1.5 mm Diameter
Flat-Faced Brass 1.5 mm Diameter
Pointed Brass 0.15 nun Diameter
Clean
Coated
Clean
Coated
Clean
Coated
42.0 Not definable
5.0
61.0
9.5
6.4
2.6
10.5
--
--
48
7.2
° Ei~ (mJ) for methane-air, 4~ = 1.29, d = 1 mm, atmospheric pressure.
this is not the case, it appears that the difference in the shape of the electrode tips, for these two cases overrides the difference in electrode material. The hemispherical tips give higher ignition probability than the fiat-faced tips, the pointed tips being the best of the three. Again, coating the electrodes with potassium sulfate led to a reduction in Eign shown in Table 2.
3.2.6 Effect of Glow Voltage in Coating Experiments It was recognized early that there were cases, when the glow voltage was reduced too much, where the ignition ability of the glow was reduced by the potassium sulfate coating.--hence the need to "condition" the electrodes before the second ignition experiments, as described in Section 3.2. To investigate this phenomenon further, we considered the ignition probability from a glow discharge of 2 ms duration at different glow voltages, but with the glow current set at 58 mA. These experiments were in a rich methane-air mixture (~ = 1.29) at atmospheric pressure with fiat-faced brass electrodes with a gap of 1 mm for which the glow voltage, Vg, was around 460 V. The electrodes were coated with a saturated solution of potassium sulfate, and dried, whereupon Vg was reduced to very low values--around 75 V. Higher values of Vg were obtained by "washing" the electrodes with a few drops of distilled water (presumably removing some potassium sulfate) and drying the electrodes. At these different stages, ignition probabilities were established and are plotted in Fig. 3. During these
determinations, each of which required at least twenty firings, Vg did not remain constant, and the spread of some of the symbols in Fig. 3 reflectsthis. It can be seen that the coating could increase the ignition probability from 0.35 for clean electrodes to 1.0, but P could also be reduced to zero if the glow voltage was reduced too much. A possible explanation is considered in Section 4.
3.3 Flame Development At first electrodes with tungsten carbide tips were used in these studies of flame development. However, the repeatability of the results was poor. The early flame development was often unsymmetric, and this precluded any meaningful study of ' the effects of glow characteristics on flame development. The films highlighted the profound effect of electrode quenching on early flame development. For instance, depending on where the spark channel occurred on the electrodes, one half of the flame would be more free of quenching and develop faster than the other half, which, in many cases, would be extinguished. With the pointed brass electrodes, however, flame development was repeatable and symmetric about the electrode axes, and the results described in this section are all for these electrodes. During the early flame development (kernel diameter less than 40 mm), the pressure rise was negligible. The experiments were all with a rich methane-air mixture (4~ = 1.29) at atmospheric pressure and a spark gap of 1 mm. The ignition probability for
327
FLAME INITIATION FROM GLOW DISCHARGES % 1.0 Z
-
9
I.-¢C
V5 0.8
u.
~ 0.6 >...I
~ 0.4 on,-
CLEAN
Q.
0.2
100
I I 200 300 GLOW VOLTAGE, Vg/V
1 400
Fig. 3. Effect of glow voltage on the probability of flame initiation in coating experiments. Rich methane-air ($ = 1.29) mixture at atmospheric pressure. Hat-faced brass electrodes; spark gap d = 1 mm; glow current Ig = 58 mA, glow duration ¢ = 2.0 ms.
this case is plotted against glow duration in Fig. 4 for these conditions. On each frame of a developed film, the maximum distance between the flame front and the mid point of the spark gap was measured on either side of the electrodes and an average flame front distance estimated, using the image analyzer. This distance can be taken to be the flame kernel radius once the kernel grows larger than about 10 mm in diameter, since it then assumes a near spherical shape. In Fig. 5 the flame front distance is plotted against time from breakdown with clean electrodes for two sparks, both of 1 ms duration, but with the glow current of 58 mA for spark A and 30 mA for spark B. The repeatability of flame development was confirmed in four separate experiments with spark A. From Fig. 4 it can be seen that the ignition probability for spark A is 1.0 and for spark B is zero. This is reflected in Fig. 5: the kernel from spark B is seen not to grow beyond 2.1 mm in radius and eventually die out, whereas spark A leads to successful flame initiation and a continuously growing, self-sustained flame. These results are in line with the theoretical as well as the experimental findings of Joulin and co-
workers [19-21], that below a critical size, spherical flames have a tendency to collapse because of the straining of the flame front and they can be driven beyond this critical size only if the energy/ power input from the spark is sufficiently high. In Fig. 6a, the flame front distance is plotted against time after breakdown for three discharges of 0.5, 1, and 2 ms duration from clean electrodes; the spark current was fixed at 58 mA. It can be confirmed from Fig. 4 that for all three sparks, the ignition probability was 1. Figure 6a shows that continuing to supply electrical energy in the spark, beyond 0.5 ms at a fixed power level, leads to faster early flame development, though increasing the glow duration from 1 to 2 ms does not appear to have a large effect. This is better illustrated in Fig. 6b, where the flame speed, calculated from Fig. 6a, is plotted against time after breakdown. Figure 6b also well illustrates the dynamics of the flame front, as described by Champion et al. [21]. When the spark energy is slightly larger than that required to successfully initiate the flame, flame speed decreases as the flame kernel radius approaches the critical radius and then increases as it grows beyond that radius, eventually to reach a
328 G. T. KALGHATGI
i~
1.0
. . . . . .
0.8
u
~ 0.6
0.4
0.2
I 1.0
2.0 3.0 GLOW DURATION, T/ms
I
J
4.0
5.0
Fig. 4. Probability of flame initiation with pointed brass electrodes. Rich methane-air mixture (¢ = 1.29) at atmospheric pressure. Spark gap, d = 1 ram; © , glow current/~ = 58 mA; E3, lg = 30 mA. Filled symbols are for electrodes coated with potassium sulfate.
15.0
-
A
E E 12.0 u J"
O Z
Ira a 9.0 IZ O kl. lit 6 . 0 tl--I
3.0
. . l # '°
End of Spark
J
t
J
3.0
6.0
9.0
l 12.0
TIME AFTER BREAKDOWN, t/ms
Fig. 5. Comparison of flame development from two sparks with pointed brass electrodes. Flame front distance versus time. Rich m e t h a n e - a i r mixture ( ~ = 1.29) at atmospheric pressure. Spark gap = 1 ram, duration = 1 ms. Glow current 18 = 58 m A for spark A and 30 m A for spark B. (Es = 25.5 rrd for A and 13.2 mJ for B.)
FLAME INITIATION FROM GLOW DISCHARGES
329
20.0 4- O
E
~O
n,-
4-0 4-O
(D Z I-tO B E3 t-Z O
4-
O
D, D,
A
+
10.0
O
+ 0
+
nr" kl.
~
f
u.
.,.
1 I 6.0 9.0 TIME AFTER BREAKDOWN, t/ms
1
3.0
l 12.0
1 15.0
(a) 2.0 +
A
+ O ,O ÷
+o
.o + o . ~
O
A z~
-H-
+o 1.0
a tl.i ku
w
-
o
A
-t~
~ A .t.o o
+
++ ++
+++.o
~_oO~
;xW+ 0 0 2 z~
a
i,
I 3.0
I 1 1 6.0 9.0 12.0 TIME AFTER BREAKDOWN, t/ms
I 15.0
(b) Fig. 6. Effect of glow duration on flame development, with pointed brass electrodes. (a) flame front distance vs. time. (b) flame front velocity vs. time. Rich methane-air mixture (~ = 1.29) at atmospheric pressure. Spark gap = 1 ram, g l o w current I z = 58 m A , g l o w p o w e r = 25.5 W; A , g l o w duration r = 0.5 ms; O , r = 1.0 ms; + , r = 2 . 0 ms.
normal propagation speed (=1.37 m/s in this case), characteristic of the mixture. Increasing the initial spark power or for a fixed power, the spark duration can increase the flame speed and shorten this middle phase of flame development [22, 23] before normal propagation. Indeed, at very high
initial spark powers, the initial flame speed, before normal propagation, can be much higher than the normal propagation speed [10, 24]. However, for electrodes coated with potassium sulfate, simple considerations of measured spark characteristics can be misleading, as is illustrated
330
G. T. K A L G H A T G I 20.0
E o COATED
rr-
tu"
O &
L) Z < I-
&CLEAN
o
C~ 10.0 I-Z 0 n.It. LU :E <
o z~
o Z~
o &
:I I
I
I
6.0
3.0
I
1
9.0
15.0
12.0
T I M E A F T E R BREAKDOWN, t/ms
(a) t/ms
0.1
0.2
0.3
1
I
I .............
\
400
>
0.5
0.4
--
L,
[
2' ~
0.6
I
130 V
COAIED "~""
I~--.,~1----440V C L EAN
800
(b) Fig. 7. Comparison of flame development from clean and coated (with potassium sulfate) pointed brass electrodes. (a) flame front distance vs. time. (b) spark voltage vs. time. Rich methane-air mixture (q~= 1.29)at atmospheric pressure. Spark gap d = 1 mm, glow current
I~= 58mA.
in Fig. 7a, which compares flame development for clean and coated electrodes. Figure 7b shows the voltage traces of the sparks that led to the flames considered in Fig. 7a: the spark current was 58 mA for both cases. For the coated electrodes the glow duration, r, is lower--350 as compared to 500 #s--and the glow voltage, and hence the apparent electrical power and energy of the spark, are also lower. On both counts, the clean electrodes could be expected to lead to faster flame development, but the contrary is the case (Fig. 7a). Thus the flame initiation ability of a spark
depends on the power and energy available to the mixture, rather than the apparent electrical power and energy. This point is further illustrated by Fig. 8 where the kernel radius in air at atmospheric pressure, is plotted against time in Fig. 8a and the glow voltage for the two sparks (r = 1 ms, I s = 58 mA) producing the two kernels, is shown in Fig. 8b. It can be seen that the spark from electrodes coated with potassium sulfate gives rise to a bigger kernel, indicating more efficient energy input into the air, even though the apparent spark voltage and
FLAME INITIATION FROM GLOW DISCHARGES
331
6.0
COATED
E 4.0
O
00000
tt~
O
0 o°
<
A zx &
0
rr
o o: 2.0
-& &
CLEAN ELECTRODE
& p , tX & A
I
J
1
6.0
2.0 4.0 T I M E AFTER BREAKDOWN. t/ms
(a)
0.2 .
I
0.4
0.6
0.8
I
I
I
1.0 t/ms ~.. ~ - - " ' - - C O A T E D
4OO
-
t
>
J ~
CLEAN ELECTRODE
(b)
Fig. 8. Comparison of kernel development in air at atmospheric pressure form clean and coated (with potassium sulfate), pointed brass electrodes. (a) kernel radius vs. time. (b) spark voltage vs. time. Spark gap d = 1 ram, glow current I~ = 58 mA.
hence power and energy are lower, compared to the clean electrodes. The breakdown voltages for the clean and coated cases were 4.6 and 4.53 kV, respectively. Thus there is little difference in the breakdown components of the energy for these two sparks. 4. DISCUSSION Flame initiation and early flame development in a premixed combustible mixture are very closely
connected. As discussed by Joulin [20], two separate criteria have to be met to successfully initiate a flame by a local (point) input of energy, such as by a small electric spark--an ignition criterion that is to raise the local temperature enough to cause thermal runaway and start the chemical reaction and a more demanding initiation criterion that requires that the power and the duration of the energy input be sufficient to overcome the straining effects in the incipient flame front and drive the flame kernel beyond a
G . T . KALGHATGI
332 critical size. Thus, three sequential regimes of flame development can be identified--localized ignition, spark-assisted flame propagation, and normal flame propagation [25]. Of these, localized ignition is always ensured by a spark because during the first, breakdown, phase of any spark discharge, extremely high temperatures--up to 60,000 K--are achieved in the spark column [10]. Normal flame propagation depends only on the mixture characteristics--mixture, stoichiometry, pressure, temperature, and turbulence characteristics if propagation is in a turbulent medium. Thus it is only the middle phase, during which the flame could be "strained out," that can be influenced by changes in the spark characteristics following breakdown. It is also during this phase that in a turbulent field the effects of turbulence on flame propagation, such as additional straining of the flame front, set in [25, 26]. However turbulence effects are not considered further since the experiments were all conducted in quiescent mixtures. 4.1 Effect of the Presence of Electrodes The presence of electrodes has a profound effect on flame initiation and development because of the quenching effect of the electrodes on the flame and spark energy loss to the electrodes.
coupled with the fact that the spark discharge can take place at different physical sites on the electrodes, leads to greater variability in flame development and a greater spread in the ignition probability versus spark energy curves for thelarger electrodes compared with the pointed brass electrodes (Section 3.3). 4.1.2 Losses of Spark Energy to the Electrodes It has been appreciated previously [17, 18] that not all the spark energy is deposited in the mixture. However, it is most notably the work of Maly, Ziegler, and co-workers [9-11] that has highlighted the effect that the loss of spark energy to the electrodes and the consequent deterioration of spark efficiency has on the ignition ability of the spark. This is especially marked for glow discharges, which can have efficiencies of the order of 10% [27, 28]. Ignoring the anode fall, the power in the glow discharge can be written as Vg.lg = Vy.Ig + (Bd)lg, where Vyis the cathode fall, B is the voltage gradient, and d is the spark gap. At the simplest level it is believed that only the power described by the second of these terms is not lost to the electrodes and hence is available to the gas, so that the efficiency )7 of the glow discharge can be simply expressed as
4.1.1 Flame Quenching by the Electrodes Lewis and von Elbe [16] defined the quenching distance as the critical distance between two heat sinks (such as the electrodes) below which flame propagation is suppressed. The quenching distances for methane-air and propane-air mixtures at atmospheric pressure are listed in Table 6 of Ref. 16. In all the experiments (at atmospheric pressure) considered in Section 3, the actual spark gap (_<2 mm) was less than the corresponding quenching distance. In such circumstances, as the spark gap is reduced, the increasing quenching effect of the electrodes has to be compensated for by an increasing spark energy to obtain successful flame initiation. As the pressure and temperature of the charge increase, the quenching distance decreases [16]. The quenching effect is also larger, the larger the physical size of the electrodes [16, 17, 24], as discussed in Section 3.2.5. This,
Bd 'I-
v/+ B-------d"
(1)
As the spark gap increases, ~/increases because
111 remains constant. Also with increases in pressure, B increases (see Section 3.1.2) and hence ,/increases. Thus increasing the spark gap or the pressure leads to a reduction of the quenching effect of the electrodes on the incipient flame as well as an increase in the efficiency of the glow discharge in transferring electrical energy to the gas. This results in a reduction in Eig~, as demonstrated in Figs. 1 and 2. 4.2 The Effect of Spark Power The importance of high energy input in the "early stages" of the spark, on ignition and flame development has been well recognized in the
FLAME INITIATION FROM GLOW DISCHARGES literature. Such a strategy produces sharp temperature gradients and the right plasma geometry to enhance early flame development [10] and its effects will be imprinted on the flame front and continue to be felt after sparking. It also happens that the spark discharge modes with the higher currents have smaller energy losses to the electrodes [10J--both current and energy transfer efficiency increasing in the order of glow, arc, and breakdown. From these considerations, Maly and co-workers have argued that for best results, the energy of the spark should be concentrated in the breakdown phase, and indeed experiments in quiescent mixtures do show that short-duration, high-power sparks lead to higher early flame development rates. The wor k of Joulin and coworkers [19-21] puts this in perspective by demonstrating the importance of supplying spark energy throughout the early flame development period. In our experiments, as in many practical cases with automobile coil ignition systems, the breakdown phase, efficient though it may be in transferring energy to the mixture, is too weak to bring about successful flame initiation. In such cases increasing the spark power, after breakdown, should and does lead to improved ignition, as seen by the fact that Eign decreases as the glow current increases in Fig. 1. Indeed if the glow power is too low, successful flame initiation may not occur (Fig. 5). 4.3 Effect of Deposits of Potassium Sulfate on the Electrodes
The results clearly show that, as expected, electrode coatings of potassium sulfate lead to significant reductions in the cathode fall, and also to dramatic improvements in the ignition ability of glow discharges, though in certain extreme cases, there might be a deterioration in ignition ability. There are results where the actual energy deposited in the mixture does not appear to change significantly upon coating the electrodes (e.g., Fig. 4a in Ref. 13). The improvement in the spark efficiency in these cases could be simply explained by a reduction in the cathode fall Vy, in Eq. 1. However, the voltage gradient B can also be reduced by a coating of potassium sulfate [13]. In
333
that case, the power in the positive colunm-B" d'/g--will be reduced, and if it is reduced too much, flame initiation will be adversely affected, regardless of the fact that Vf might have been reduced and the efficiency improved. However, it is more difficult to explain results such as those in Figs. 7 and 8 (also Fig. 5a in Ref. 13), where it appears that upon coating the electrodes, the energy deposited in the mixture apparently increases even though Vg is reduced, while Ig remains constant. This cannot be because of any changes in the breakdown properties because, in fact, for coated electrodes, the energy in the breakdown phase will be less than for the clean electrodes because the breakdown voltage will be lower (Section 3.1.1). Moreover, nickel sulfate, which affects breakdown characteristics in the same manner as potassium sulfate (Section 3.1.1), has quite the opposite effect on the ignition ability of the spark (Section 3.2). A probable explanation could lie in the fact that the current density is increased by the low electron work function deposit [12]. Bradley and Lung [25] have presented a model that demonstrates that the spark kernel growth rate, and, by implication, the ignition ability of the spark, should increase with the volumetric electrical power, that is, the current density of the discharge. It is also possible that in these cases the reduction in the cathode fall is accompanied by an increase in its physical width, so that some of the energy dissipation in the cathode fall region occurs far enough away from the cathode for it not to be lost to the electrodes; this would lead to additional energy deposition in the mixture when the electrodes are coated. In any case the phenomenon discussed in this paragraph is really noticeable when conditions for ignition are especially severe, namely when Ei~ with clean electrodes is large. It is likely that in such cases, only small improvements in the quantity or quality of energy deposition in the mixture are needed to give large improvements in flame initiation and development. Thus, a simple explanation based on relating the losses to the electrode entirely to the cathode fall and then invoking the reduction of the cathode fall by the potassium sulfate deposit, cannot fully account for all our observations. It appears that
334 depending, perhaps, on the nature and thickness of the coating, its effect on flame initiation and development is different. These matters can satisfactorily be resolved only by a much more detailed and careful study of the effect of such deposits on the physics of the glow discharge, using a reliable technique to produce repeatable coatings and independently measuring the losses to the electrodes.
4.4 Significance for Engine Operation In engines, the voltage across the spark gap increases rapidly with time, as in the present experiments, but the statistical time lag is likely to be very small because of high pressures and temperatures and the presence of water vapor [ 14]. Moreover, deposits would form on spark plug electrodes even in the absence of fuel additives. Hence any improvements in breakdown characteristics because of fuel additives are likely to be much less marked than in the present experiments. Spark gaps are fixed and are of the order of 0.61.0 ram. In normal circumstances, because of the higher pressures and temperatures in the engine this gap should be larger than the quenching distance for the mixture, which depends on mixture strength, pressure, and temperature. However, in many regimes of engine operation quenching distances could be large so that the quenching effects of the electrodes become important. The laminar burning velocity is also low under those conditions that can occur, for example, during idling when the pressure and also the temperature of the charge is low; during part throttle acceleration, when the mixture strength becomes momentarily very lean because the fuel flow does not increase as quickly as the air flow; and during cold starting when the mixture is lean because of inadequate fuel vaporization. Thus all engines operate in regimes where flame initiation is difficult and the misfire probability is high. The fact that flame development takes place in turbulent flow fields also contributes to these difficulties, which can be expected to become more important if engine design trends move towards lean-burn and/or high-exhaust gas recirculation concepts.
G. T. KALGHATGI In a conventional automobile coil ignition system, the spark is not terminated as in the experiments described here--it continues until the voltage to sustain it drops below a threshold level as the stored coil energy is dissipated. Low electron work function deposits on the electrodes do not change this total energy of the spark but make a larger proportion of it available to the gas because they reduce the losses to the electrodes, as shown in this work, and also extend the spark duration [6]. In engines, an extension of the spark duration is in itself beneficial because it compensates for the cyclic variations of mixture strength in the spark gap and reduces the probability of misfire [29, 30]. More importantly, as demonstrated in this work, such deposits bring about a dramatic improvement in the ability of the glow discharges, the predominant discharge mode in a coil ignition system, to ignite difficult mixtures. Thus sparkaider fuel additives, which lead to deposits of low electron work function on the electrodes, bring about improvements in engine misfire characteristics [6] and early flame development and, consequently, reduce cyclic variation [3]. This in turn leads to the observed improvements in drivability characteristics [5]. Even with spark-aider concentrations in the fuel far greater than any likely to be used (e.g. 80 ppmK), the glow voltage is never reduced to the extent that the ignition ability of the spark worsens.
o
CONCLUSIONS
1. All the crystalline electrode deposits tested (potassium sulfate, nickel sulfate, and sugar) reduce the mean as well as the standard deviation of the breakdown lag time and hence the breakdown voltage when the gap voltage increases with time. . Of the deposits tested, only the potassium compound deposits on the electrodes (primarily the cathode) lead to a reduction in the glow voltage, mostly because of a reduction in the cathode fall though reductions in the column voltage gradient were also observed. (The detailed results to support these first two conclusions can be found in Ref. 13.)
335
FLAME INITIATION FROM GLOW DISCHARGES
3. The presence of electrodes has a profound effect on, and leads to variability in, flame initiation and development because of quenching effects of the flame front as well as losses of the spark energy to the electrodes. 4. The ignition ability of a spark--a glow discharge in the present experiments--depends on a combination of spark gap, power, and energy, electrode material, size, and shape, and mixture strength and pressure. For a given mixture, in general, the ignition ability of the glow increases as the spark gap, spark power, and mixture pressure are increased. Pointed electrodes are better than flat-face or spherical electrodes for ignition. Marginal conditions for flame initiation can result because of any suitable combination of these factors--for instance if the pressure is high, by weakening of the mixture strength. 5. Flame initiation and early flame development are intimately linked and in quiescent mixtures, factors that facilitate flame initiation also enhance the early flame development rate. 6. The ignition ability of a glow discharge can be greatly enhanced by deposits of potassium sulfate on the electrodes. This improvement becomes especially marked as the conditions for ignition become more difficult. Under some conditions, however, when the fall in the glow voltage because of such deposits is too large, the ignition ability can worsen. Thus potassium sulfate deposits bring about changes in the electrical characteristics of the spark that are expected of a low electron work function material. However, the mechanism for the improvement in the energy transfer efficiency of the spark, as reflected in its enhanced ignition ability, due to such deposits is not fully explained, as the detailed effects of such deposits on the physics of the glow discharge remain unresolved. Thanks are due to Mr. D. A. J. Jones of Thornton Research Centre, Shell Research Ltd. for conducting the experiments.
2. 3. 4. 5.
6. 7. 8. 9.
10.
11.
12.
13.
14.
15.
16.
17. 18. 19. 20. 21. 22.
REFERENCES 1.
Kalghatgi, G. T. SAE Paper No. 870163, 1987.
Dale, J. D., and Oppenheim, A. K., SAE Paper No. 810146, 1981. Kalghatgi, G. T., Combust. Sci. TechnoL 52:427-446 (1987). Muller, H., and Schaperkotter, H., Motortech. Z. 48:429-434 (1987). Blackmore, D. R., Gralff, L. B., Harrow, G. A., Jones, J. M., Kalghatgi, G. T., and Miles, R., International Conference on Petroleum Based Fuels and Automotive Applications, Proceedings o f the Institution o f Mechanical Engineers, Institution of Mechanical Engineers, London, 1986, p. 47. Kalghatgi, G. T., Combust. Sci. Technol. 62:1-19 (1988). Hurtley, D., Auto. Eng., 148-152 (1969). Weber, H., SAE Paper No. 740021, 1974. Maly, R., and Vogel, M., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1979, p. 821. Maly, R. R., in Fuel Economy: Road Vehicles Powered by Spark Ignition Engines (J. C. Hilliard and G. S. Springer, Eds.), Plenum Press, New York, 1984, Chap. 3. Ziegler, G. F. W., Wagner E. P., and Maly R. R., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 1817. Cobine, J. D., Gaseous Conductors Theory and Engineering Applications, Dover, New York, 1958, Chap. 8. Kalghatgi, G. T., International Conference on Combustion in Engines Technology and Applications, Proceedings o f the Institution o f Mechanical Engineers, Institution of Mechanical Engineers, London, 1988, p. 9. Dutton, J. D., in Electrical Breakdown o f Gases (J. M. Meek and J. D. Craggs, Eds.), Wiley, New York, 1979, Chap. 3. Grey Morgan, C., in Electrical Breakdown o f Gases (J. M. Meek and J. D. Craggs, Eds.), Wiley, New York, 1979, Chap. 7. Lewis, B., and Von Elbe, G., Combustion Flames and Explosion o f Gases, Academic, New York, 1961, Chap. 5. Rose, H. E., and Priede, T., Seventh Symposium on Combustion, Butterworths, London, 1959, p. 436. Ballal, D. R., and Lefebvre, A. H., Combust. Flame 24:99-108 (1975). Deshaies, D., and Joulin, G., Combust. Sci. Technol. 37:99-116 (1984). Joulin, G., Combust. Sci. Technol. 43:99-113 (1985). Champion, M., Deshaies, B., Joulin, G., and Kinoshita, K., Combust. Flame 65:319-337 (1987). Anderson, R. W., and Lim, T. M., Eighth International Conference on Gas Discharges and Their Applications, GD85 Organizing Committee, Leeds University Press, 1985, p. 511.
336 23.
Karpov, V. N., Malov, V. V., and Severin, E. S., Combust. Expl. Shock Waves 22:707-712 (1987). 24. DeSoete, G. G., International Conference on Combustion in Engineering, Institution of Mechanical Engineers, London, 1983, Voi. 1, p. 93. 25. Bradley, D., and Lung, F. K. K., Combust. Flame 69:71-93 (1987). 26. Kalghatgi, G. T., Combust. Flame 60:299-308 (1985). 27. Saggau, B., Arch. Elektotech. 64:229 (1981).
G. T. K A L G H A T G I 28.
Teets, R. E., and Sell, J. A., SAE Paper No. 880204, 1988. 29. Aiman, W. R., Combust. Sci. Technol. 15:129-136 (1977). 30. Nakai, M., Nakagawa, Y., Hamai, K., and Sone, M . , SAE Paper No. 850075, 1985.
Received 23 June 1988; revised 10 October 1988