Characteristics of a plasma generated by combustion in a spark ignition engineW. G. Rado Citation: Journal of Applied Physics 46, 2468 (1975); doi: 10.1063/1.322231 View online: http://dx.doi.org/10.1063/1.322231 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/46/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of premixed ratio on combustion characteristics of a homogeneous charge compression ignition-directinjection engine fueled with dimethyl ether J. Renewable Sustainable Energy 6, 013106 (2014); 10.1063/1.4861783 Dynamic instabilities in sparkignited combustion engines with high exhaust gas recirculation AIP Conf. Proc. 1339, 158 (2011); 10.1063/1.3574854 Combustion of CNG in Charged Spark Ignition Engines AIP Conf. Proc. 1190, 98 (2009); 10.1063/1.3290172 Combustion process in a spark ignition engine: Dynamics and noise level estimation Chaos 14, 461 (2004); 10.1063/1.1739011 Controlling Cyclic Combustion Variations in LeanFueled SparkIgnition Engines AIP Conf. Proc. 622, 265 (2002); 10.1063/1.1487542

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

Characteristics of a plasma generated by combustion in a spark ignition engine

w. G. Rado

Scientific Research Staff, Ford Motor Company, Dearborn, Michigan 48121 (Received 19 August 1974; in final form 24 January 1975)

Plasma probe measurements at dc and bulk absorption measurements at microwave frequencies have been made on a plasma generated by the combustion in a spark ignition engine. From these measurements, estimates of the charge concentration and characteristics of the space-charge region formed around the plasma probe under the condition of positive-ion collection have been deduced. The degree of ionization is estimated to be as high as I : 10' in the burned gases, depending on engine load. The charge species are concluded to consist primarily of electrons and positively charged ions. In addition, the applicability of various plasma probe theories predicting I -V relationships has been investigated. The available data strongly indicate that the probe theory including the effect of flow of the ionized gases is the applicable one.

PACS numbers: 52.70.G, 52.80.M

I. INTRODUCTION

Ionization probes have been used! extensively in the past 40 years in developing new and improving the per­formance of existing spark ignition engines. Among the ionization probe's many uses are the indication of the occurrence of knock and preignition and the mea­surement of the flame front propagation velocity. Re­cently, correlations2 have been established between the peak values of ionization and simultaneously recorded cylinder pressure traces and engine performance. A variation in the resistance of the plasma with changes in the timing of the spark ignition signal has also been observed. 2 In general, when the peak value of the cyl­inder pressure increased, the ionization probe signal showed a corresponding increase for a constant applied probe voltage.

Although the above uses of ionization probes have been known for some time, the electrical characteris­tics of plasmas generated by combustion in spark and compression ignition engines have not been heretofore investigated. The plasma characteristics of flames have been researched extensively, 3-5 but it was only recently that microwave measurements were made on spark ignition engines for the purposes of timing and piston position diagnostics. 6

The purpose of this investigation was to characterize electrically, for the first time, Le., I-V relationships, charge concentration, size of the sheath thickness, and its variation with probe bias, etc., the plasma gene rat -ed by the combustion in the cylinder of a spark ignition engine. Two types of experiments were carried out: measurements of the bulk absorption of microwave energy by the plasma, and measurements with plasma probes of various geometry and material composition under various dc biasing conditions.

II. EXPERIMENTAL

All of the experiments to be discussed in this investi­gation were carried out on a Sears 2. 5-hp 122-cm3

displacement (5. 87-cm bore, 4. 7-cm stroke) four­stroke utility engine with an approximate compression ratio of 7 to 1. The cylinder head of the engine was

2468 Journal of Applied Physics, Vol. 46, No.6, June 1975

modified to accommodate independently both a pressure transducer pick up to monitor the cylinder pressure variations due to combustion, and ionization and micro­wave coupling probes of various design. A photograph of a modified cylinder head is shown in Fig. 1. The spark plug used for ignition is located in the center of the right-hand side of the head. The probe located in the left upper region is a O. 32-cm -diam spherical ionization probe made of Pt, while a pressure trans­ducer pick up is located in the lower left region.

A. Microwave measurements and results

The measurement circuit used in the investigation of the absorption of microwave energy by the combustion­created plasma is shown in Fig. 2. The coupling probe used was a spark plug with its center conductor pene­trating into the plasma and its ground tip machined off. General Radio rigid 50-Q coaxial cable components were modified to provide a coaxial connection from the spark plug microwave coupler to the coaxial Tee.

The fact that the mOving piston in the engine cylinder

FIG. 1. Photograph of a modified engine cylinder head with a spherical probe and a pressure transducer.

Copyright © 1975 American I nstitute of Physics 2468

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

HP 8690 A

MICROWAVE

HP423 A

CRYSTAL DETECTOR

CHANNEL I TEKTRONIX

547 IAI PLUG IN

COUPLE~R~~~~zz~~~~~~

FROM TDC MAGNETIC PICK UP

CHANNEL 2 TEKTRONIX

547 IAI PLUG IN

FIG. 2. Circuit used for measuring the absorption of micro­wave energy by the combustion-created plasma.

formed a continuously varying length microwave cavity to which microwave energy could be coupled preferen­tially at a specific frequency and piston position formed the basis of the microwave measurements.

First, while repetitively sweeping through the fre­quency range afforded by the HP8790A-HP8694B signal generator, the engine was cranked manually with the ignition system disconnected. In the process of crank­ing, several frequencies were found at which energy could be coupled into the engine cylinder resonantly. EffiCient coupling was realized, in the 8-12-GHz re­gion, to the TMll1> TMo12, and the TMll2 modes. It was found, however, that coupling to each mode had a maximum efficiency at a Single specific frequency corresponding to a specific piston position. The largest value of Q, of about 3000, was observed for coupling into the TMo12 mode at 8.36 GHz corresponding to a piston position of ± 133 crank angle degrees with re­spect to TOe (top dead center).

The change in the impedance of the engine cylinder at microwave frequencies due to the ionized gases was investigated by monitoring the decrease in the Q asso­ciated with the coupling into the TMOl2 mode at 8.36 GHz during the power stroke (at a piston position of 133 crank angle degrees after TOe) compared to the com­pression stroke (133 crank angle degrees before TOe). The coupling of microwave energy into the engine cyl­inder was indicated by a decrease in the amount of microwave energy received by the detector diode. The decrease was always less during the power stroke than during any other stoke. This observation was inter­preted to mean that a decrease in the Q of the engine cylinder cavity took place due to the ionized species in the burned gases.

Operating the engine at a speed of 3000 rpm without

2469 J. Appl. Phys., Vol. 46, No.6, June 1975

load, the Q during the power stroke decreased on the average by a factor of 4 compared to the compression stroke. That is, it changed from 3000 to about 750. The fluctuation in the value of Q observed during the power stroke was about a factor of 2.5 on a spark plug firing to spark plug firing basis.

Although the 133 crank angle pOSition of the piston corresponded to the elapse of more than two-thirds of the power stroke, the microwave absorption measure­ments were considered meaningful as long as ionization probes still indicated the existence of a plasma.

Simultaneously recorded traces of the TOe marker signal, the cylinder pressure signal, and an ionization Signal obtained through the spark plug used for ignition are shown in Fig. 3. The TDe marker signal was generated in a magnetic pick-up coil by a steel screw tip, embedded in the flywheel, moving by the coil at the time corresponding to TDe. The cylinder pressure signal was obtained using a Kistler Model PZ 14 pres­sure transducer and a Kistler 504 D charge amplifier. The ionization signal was generated by applying a bias voltage to the spark plug's center electrode from a battery through a resistive voltage dividing network. A Zener diode was used across the load resistor to limit the high-voltage ignition signal. The time at which the microwave absorption measurements were carried out during the power stroke is indicated by the vertical line drawn through all three traces. As seen from Fig. 3, the ionization signal amplitude at 133 crank angle de­grees after TDe is about 0.1 times its maximum value.

From the observed decrease in the value of Q during the power stroke, the concentration of the charged species in the combustion generated plasma was calcu-1ated from a Simplified form of the expression 7

1 1 .2Aw 1 - - - -} -= - (f v u(w)E2dV)(f vE2dV)-l, (1) Q1 Qo Wo EoWO

where Qo is the Q during the compression stroke, Q1 is the Q during the power stroke, Wo is the compres-

POWER EXHAUST STROKE STROKE

TOC BoC ToC ToC

MARKER SIGNAL

CYLINDER PRESSURE

SIGNAL

:::.--.: VACUUM LEVEL

.J.------, r----- BACKGROUND

TIME OF MICROWAVE

ABSORPTION MEASUREMENT

IONIZATION PROBE

SIGNAL

FIG. 3. Simultaneously recorded traces of the TDC marker signal, cyUnder pressure signal, and ionization signal obtained through the spark plug used for ignition.

W.G. Rado 2469

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

sion stroke resonant frequency, Eo is the free-space dielectric constant, Aw is the shift in resonant frequen­cy due to plasma, a(w) is the complex conductivity of the plasma, and E is the microwave frequency electric field strength in the resonant cavity. Carrying out the inte­gration over the volume of the resonant cavity formed in the engine cylinder assuming that the complex conduc­tivity of the plasma could be taken to be independent of the spatial coordinates, the following expression was obtained for No, the concentration of the charged species in the plasma,

No= A( ~) €o;a~ (1 + w~ T~), (2)

where M is the mass of the charged carrier, T c is the time between collisions between neutrals and the speci­fic charged carrier, and e is the charge of an electron, 1. 6xlO-19 C.

Values of Tc were calculated tusing the expression Tc

= (Nac Vth)-~~ where N is the density of neutrals in the plasma, V tb is thermal velocity, and Uc is the collision cross section of the charged carrier with neutrals}. For example, using an estimated value for the gas tempera­ture of 2000 oK and a valueS for ac = 4± 3xlO-15 cm2, Tc

for electrons was calculated to be 2, 5x10-12 sec at 1 atm gas pressure.

Using the expression in Eq, (2) for No, two extreme values of No were calculated. It could either be assumed that all of the negatively charged species were electrons or that they were all negatively charged ions. Taking a value of 2 atm for the average cylinder pressure at the time of the microwave absorption measurement, the two extreme values of No were found to be 1. 4 x1010

cm-3 for electrons and 3. 5x1014 cm-3 for negative ions. For the latter value an average ion mass number of 20 was assumed with a value of ac from Ref. 8. The iden­tity of the ionized species is currently unknown and Significant contributions to the total could be due to alkali compounds in the lubricants and lead compounds in the fuel.

The idea that the electron concentration may be small compared to the negative-ion concentration is based on flame plasma measurements. In hydrocarbon-oxygen flames at 1 atm gas pressure the concentrations of the positive and negative ions have been observed9 to be very nearly equal. The increased pressures encountered in spark ignition engines should therefore further in­crease the probability for electron attachment.

Due to the assumptions and inaccuracies of the values for the various parameters, the above values of No can be assumed correct only as an order-of-magnitude indication of the charge concentration in the burned gases,

B. de measurements and results

The same engine, previously used for the microwave measurements, was also used for plasma probe mea­surements under dc bias conditions. The purpose of these measurements was to determine the value of the charge concentration and its type, i. e" electrons ver­sus ions, in the plasma and to investigate the applica-

2470 J. Appl. Phys., Vol. 46, No.6, June 1975

bility of various theories describing probe character­istics under certain biasing conditions.

1. de /- V measurements

First, I-V measurements were carried out followed by an investigation of the variation of the sheath thick­ness with voltage for large negative probe potentials. These data were all analyzed relying on the following calculations of I-V relationships and sheath thicknesses applicable to collision dominated plasmas:

I. Spherical probe, stationary plasma10:

Ip= (47T€0)0.25 f-I.I2r~·5(47TNokT)0. 75~.5,

r = I. s 7TNokT f-I.l

n, Spherical probe, flowing plasmall (rs»rp):

Ip"" (3€of-l.l)0.4(7TNoeV,)0.6(r pVp)o.s,

r _(~)0.5. s- 7TNoeV,

m. Cylindrical probe, stationary plasma12:

I "" 47TNokT !liZ, p In (7Tl/4r p)

(3)

(4)

(5)

(6)

(7)

Iprp (8) r"" , s No f-I.lkT27Tl

IV, Cylindrical probe, flowing plasma13 (rs - r p):

Ip"" 5. 3 (€of-l.lrp)0.25(eV,No)0.75l~·5, (9)

r s =I/2NoeV,l (10)

where Ip is the probe current, Vp is the probe voltage, rp is the probe radius, 1 is the probe length, No is the concentration of the changed species in the plasma, e is the charge of an electron, 1. 6xlO-19 C. rs is the sheath radius, k is Boltzmann's constant, 1.38xlO-23

J/oK, T is the gas temperature (assumed to be the same for neutrals as well as for all charged species), f-I.l is the mobility of the positive ions, and V, is the gas flow velocity by the probe.

All of the I - V measurements were made by maintain­ing a dc bias on the probe at all times (independent of the particular stroke of the engine) and recording the peak value of the probe current during combustion, The monitoring circuit is shown in Fig. 4, The probe cur­rent was determined from the voltage variation ob­served across the 1-kO load resistance. The 20-kO series resistance was chosen so as to make the probe

CIRCUIT FOR D.C. MEASUREMENTS

20K

HP 6515A DC POWER SUPPLY 0- 1600 Volts

ION IZATION GAP IN CYLINDER HEAD

1 k

FIG. 4. Circuit used for dc plasma probe measurements.

W.G. Rado 2470

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

o Pt PROBE. POSITIVE POTENTIAL • Pt PROBE. NEGATIVE POTENTIAL '" Ni PROBE. POSITiVE POTENTIAL ... Ni PROBE. NEGATIVE POTENTIAL

~~~ (fl G:JS> 0; Q.

~ >l « 1000 & 0

H rol>< Q: u ~"O~ cA ~ I ,. ,\'l-

I- orjj, .. ~~. Z •• lI'- O<? LLJ

og~ • t ...... ~v Q: • Q: t'" :::> 0'" • ... u 100 '" t

0 ... LLJ '" ... III 0 •• 0 • Q: '" • .t Q. • ...

• •

10 L-__ L-__ ~ __ ~ __ ~ ____ -L __ -L __ ~~

10 100 1000 PROBE POTENTIAL - VOLTS

FIG. 5. Typical 1- V curves obtained for identical size Pt and Ni spherical probes.

voltage very nearly equal to the power supply voltage even for the highest plasma conductivities encountered.

I-V curves obtained for a Pt and a Ni 0.32-cm-diam spherical probe located 0.25 cm from the cylinder wall for both positive and negative probe potentials are shown in Fig. 5. For these data as well as for all those to be reported subsequently, the engine operating con­dition was always the same. The engine was set to operate at 6000 rpm without load. Then it was loaded until the speed dropped to 3000 rpm. Running in this condition the peak values of the probe current as a function of voltage were recorded. The values of probe current like those reported in Fig. 5 are average values. For positive probe potentials, the peak value of the current was found to be substantially more stable than for negative probe potentials. For positive probe poten­tials, the fluctuation in the peak value of probe current was about ± 20% on a spark plug firing to spark plug firing basis. Typical traces of the probe currents for an unloaded engine and the TDC marker signal are shown in Fig. 6. A series resistance of about 20 M!1 and a load resistance of about 1 M!1 were used in the circuit of Fig. 4. The easily noticeable difference in signal character for the collection of positive compared to negative species remained unchanged as the engine was loaded down. However, both signals became shorter in time duration and the signal for the collection of the negati ve species lost most of its flat top character.

The I-V cu rves of Fig. 5 indicate that negatively charged species are collected more efficiently than positively charged ones, For an absolute value of probe bias of a few hundred volts the Pt probe collected four while the Ni probe collected about seven times more negatively than positively charged species. Comparing all of the different materials used for probes, other

2471 J. Appl. Phys., Vol. 46, No.6, June 1975

Source Potential: - 300 V

t LLJ <.!l TIME--<I: I­....J

~ Source Potential: + 300V

I 15ms TIME --

IV

10V

FIG. 6. Typical ionization probe traces for positive and nega­tive probe potentia Is .

parameters being equal, Pt probes always collected the largest amount of positive-ion current for a specific bias. This perhaps suggests a possible contribution to the probe current due to thermionic emission or adsorp­tion on the probe surface. Such effects were not investi­gated and comparisons between theory and experiment were made without regard to such influences.

Additional I - V curves as a function of probe geometry and time of observation during combustion are given in Figs. 7 and 8, respectively.

For reference, the data of Fig. 5 corresponding to the Pt sphere probe are replotted in Fig. 7. The modi­fied spark plug probe was a J -9 Champion spark plug with the ground tip machined off. The Pt-Rh wire probe made use of a 0.075-cm-diam wire penetrating 0.4 cm into the plasma.

Some of the immediate observations that can be made based on Figs. 5, 7, and 8 are as follows:

(i) The collection of negatively charged species is more efficient than that of positive ions but only by a relati vely small factor, not by the factor of 200 expect-

PROBE POTENTIAL - VOLTS

FIG. 7. Typical 1-V curves obtained for probes of various geometry.

W.G. Rado 2471

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

1000 0

~ 0 o~~· ~\; ..

00 •• •• 0

0 • • (f) 0 • Q. 0

~ 0 • A,t~ <I 100 f.-0 0 ... CJ:: AAtI>

~ •• A

• ~ • A A

I • ~ Z

A

!oJ CJ:: CJ:: ::;)

10 f-U A .. .. " !oJ

.. .. m 0 .. CJ:: Q. o Pt PROBE + POTENTIAL .. • Pt PROBE - POTENTIAL

6 Pt PROBE + POTENTIAL 1.6ms .. Pt PROBE - POTENTIAL 1.6ms I I

100 1000 PROBE POTENTlAL- VOLTS

FIG. 8. A comparison between the 1-V curves taken based on the peak value of the probe current and taken at a time 1. 6 msec later.

ed from the ratio of electron to ion mobilities.

(ii) The unity slope character of the 1- V curves for the collection of negatively charged species suggests an Ohmic behavior.

(iii) Although there are differences in the actual values of the poSitive-ion current, drawn at a specific bias, with probe geometry, probe matelial, and time of observation, they all indicate I-V relationships with the same slope of 0.64 approximately. This value does not agree with any of the theories. However, the lack of a saturation predicted for a cylindrical probe in a stationary plasma strongly suggests the importance of the gas flow on the collection of charged species in the engine cylinder during combustion.

Calculations of No and rp were also carried out using the expressions for spherical probes in stationary and in flowing plasmas. The data in Fig. 5 corresponding to the Pt sphere probe were used and calculations were made for a value of probe bias of -140 and -750 V. A value of 1 m/sec was assumed for the velocity of the burned gases in the engine cylinder. It was estimated that the actual value could have been anywhere between 0.2 and 4 m/sec based on anemometer measurements of the gas velocities in engines of different designs. As before, a temperature of 2000 OK was assumed and cylinder pressure measurements indicated a value of 14 atm at the time of peak plasma probe currents. Also, both possibilities, the negatively charged species being electrons or ions, were conSidered. The results are given in Table I.

2. Comparison between microwave and dc /- V measurements

Before a quantitative comparison can be made be­tween calculations based on the dc I - V measurements

2472 J. Appl. Phys., Vol. 46, No.6, June 1975

TABLE I. Calculation of plasma probe parameters using the data in Fig. 5.

Experimental values Calculated values

{j..tA) (cm)

Spherical probe, stationary plasma

100 -140 3X1015 0.24 300 -750 5X1015 0.43

Spherical probe, flowing plasma

100 -140 4x1015 0.02 300 - 750 3 x1015 0.04

and those based on the microwave measurements, the relationship between the experimental conditions for the two measurements has to be established •

The cylinder pressure variation was monitored during the dc I - V measurements also in a manner similar to that during the microwave measurements. For the microwave measurements, the engine was unloaded. The plasma probe current signals indicated that at the time of the microwave measurement the probe current was generally ten times less than the peak value during the power stroke. Correspondingly, the value of the cylinder pressure was 3.5 times less than the peak value during the power stroke.

All of the dc I-V measurements, on the other hand, were made under loaded engine conditions with the data based on the peak value of the probe current. Experi­mentally, the peak value of the probe current increased by a factor of 15 when the engine was loaded for the dc I - V measurements. Correspondingly, the peak value of the cylinder pressure increased by a factor of 2.

Based on these observations, then, the probe currents corresponding to the dc I-V mea'surements were 150 times, while the cylinder pressure signals were seven times those corresponding to the microwave measurements.

USing Eqs. (3) and (5) in combination with the above increases in the values of probe current and cylinder pressure, values of No were calculated from those corresponding to the microwave measurements. The results are given in Table II.

The values of No given in Table II, compared to the values of No given in Table I, indicate that electrons are the dominant species and better agreement is found

TABLE II. Calculation of No from the microwave measurement corresponding to the ionization current level of the dc probe measurements.

Spherical probe, statlonary plasma

No=1.4 x1014 cm-3

No = 3.5 X1018 cm-3 Electrons dominant Negative ions dominant

Spherical probe, flowing plasma

NO=5x1014 cm-3 No=1.1 X1019 cm-3

Electrons dominant Negative ions dominant

W.G. Rado 2472

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

TABLE m. Results of the sheath thickness measurements.

Probe diameter (em)

0.025 0.075 0.025 0.025

Separation (em)

0.5 0.22 0.11 0.06

Ratio of c-urrents 2 probes/lprobe

+lon coIl. electron colI.

1.8 1.8

1. 08 1.15

1.1

between measurements if the spherical-probe -with­flowing-plasma theory is assumed to be applicable.

Using the appropriate values of No the resistance of the plasma for electron collection was also calculated and compared to measured values, both for the case of the microwave measurements using the information in Fig. 6 and for the case of the dc I-V measurements using the data of Fig. 5. The experimentally measured values of resistance were found to be 300 kO for the dc I-V and 10 MO for the microwave measurements. The corresponding values of the plasma resistance calculated based on the appropriate values of No were found to be 13000 for dc I-V and 20 k 0 for the micro­wave measurements.

This comparison indicates that the observed linear I-V curve for electron collection does not necessarily correspond to OhmiC behavior and, in fact, the electron collection appears to be limited by the poSitive-ion flow to the cylinder walls of the engine. This is suggested since the calculated value of plasma resistance is found to be much smaller than the observed value.

3. Sheath thickness measurements

To further verify the applicability of the spherical­probe-with-flow theory, additional information was ob­tained from sheath thickness measurements as a func­tion of geometry and bias voltage.

Although most of the dc I-V measurements were carried out using spherical probes, the sheath thick­ness measurements were made using cylindrical probes because of the ease of predicting the effect of over­lapping sheaths.

Measurements were carried out by noting the in­crease in probe current when a second identical probe was switched into the circuit. The size and the separa­tion of the probes were varied as well as the polarity and the magnitude of the probe voltage. The typical voltage range over which a second probe was switched in was from about ± 100 to ± 1000 V.

Measurements which showed no variation in the ratio of current through two probes versus one for the com­plete probe bias voltage range are summarized in Table m.

However, data corresponding to poSitive-ion collec­tion with probe separations of less than 0.1 cm showed variations in the current ratios with probe voltage. Fig­ure 9 shows the variation of the current ratio with volt­age for the same probe separation running the curves over and over again. As seen from the figure, the data show a substantial scatter. but nonetheless indicate a

2473 J. Appl. Phys., Vol. 46, No.6, June 1975

W III 0 a::

PROBE SEPARATION - 0.084 em a.. -..... ~ 2.0 0 a:: a.. N 1.8 I en I-z w

1.6 a:: a:: ::> u w 1.4 III 0 a:: a.. 0 RUN I IL. 1.2 A RUN 2 0 • RUN 3 0 l-e:[ 1.0 0.£ -100

PROBE VOLTAGE- VOLTS

FIG. 9. Variation of the current ratio for two probes versus one probe as a function of probe voltage. Probe separation was the same for each run.

trend. The current ratio decreases for increasing probe voltage, suggesting the overlap of sheaths. The data in Fig. 10 shows the variation of the current ratio with probe voltage for several probe separations. The data for a probe separation of 0.02 cm suggests that the sheaths around the two identical probes actually over­lapped for the whole voltage range. Therefore, it is concluded that the sheath thickness in the -100 to - 1000 V probe bias range is of the order of 0.02 cm. This is in agreement with the values calculated in Table I from the I-V data of Fig. 5 using the spherical-probe-with­flow theory.

W III o a:: a..

;;; 2.0 w III

~ ~ 1.8 I

en I-

~ 1.6 a:: a:: ::> u 1.4 w III o a:: a.. 1.2 IL. o o

PROBE SEPARATION

o .065 em A .04 em • .02 em

- 1.0~--L----L ____ L-____ -L ____ L-__ ~~

~ a:: -100 -1000

PROBE VOLTAGE - VOLTS

FIG. 10. Variation of the current ratio for two versus one probe as a function of probe voltage and probe separation.

W.G. Rado 2473

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14

TABLE IV. Characteristic parameters of the plasma tested with a spherical probe (data from Fig. 5).

Parameters with assumed values:

T=20000K Vf =200 cm/sec o (viscous boundary layer) =1 x1o-3 cm Ue (electron-neutral collision cross section at 1 atm)

=4 x1o-15 cm2

M (mass of average ion) =20me

Parameters with measured values:

rprd>.= 0.16 cm Vprob.=-140 V ill'Ob.=100 iJ.A P =14 atm

Parameters calculated based on the above:

A (mean free path) = 7 x1o-5 cm Ao IDebye length) =4 x10-5 cm Us (ion sheath thickness) = O. 02 cm iJ.j = 1. 6 cm2 IV sec D j (ion diffusion coefficient) = O. 27 cm2/sec Vc (ion-neutral collision frequency at 1 atm) =3 x1010 sec-I Vd (drift velocity through sheath) = 103 cm/sec VD (diffusion velocity) = Dj/os =13 cm/sec No=4 x1015 cm-3

In addition, the data in Table III again imply that the collection of electrons is limited by the poSitive-ion flow through the walls of the engine cylinder. It was expected that if the two probes were independent, as supposed for electron collection, the doubling of the collection surface should have doubled the probe cur­rent. If electron collection is limited by the area for poSitive-ion collection (engine cylinder wall surface), however, then of course increasing the electron collect­ing probe's area does nothing.

III. CONCLUSIONS Characteristics of a plasma generated by combustion

in a spark ignition internal combustion engine have been investigated. Measurement of the absorption of micro­wave energy by the plasma as well as plasma probe measurements under various dc bias conditions have been carried out and reported for the first time. These measurements were found to be consistent with each other and lead to the conclusion that the dominant charged species in the plasma are electrons and posi­tive ions. This is a significant conclusion, since some measurements on atmospheric flames indicated the existence of nearly identical positive- and negative-ion concentrations.

Based on the microwave frequency and the dc plasma probe measurements and the use of certain assumed values for some parameters, the plasma existing in the engine cylinder at the time of peak probe current can be characterized by the parameters given in Table IV.

2474 J. Appl. Phys., Vol. 46, No.6, June 1975

The value of No=4x101S cm-3 corresponds to a frac­tional ionization of about one part in 1014

• Such a high degree of ionization cannot be produced by thermal ionization of the normally abundant species. It is there­fore postulated that either chemi -ionization produces such high concentrations for which the mechanisms are not understood or that these are impurities with low ionization potentials like alkali compounds which under­go thermal ionization. In addition to the question con­cerning the origin of such a high degree of ionization, additional questions remain with respect to the time duration (well into the power stroke) of the ionization signals. Why does the probe current signal last as long as it does, well past the time of a flame front passing by the probe?

In addition, the dc I-V curve and sheath thickness measurements indicate that a probe theory developed for collision -limited dense plasmas including the effects of the flow of the ionized species is applicable. However, questions still remain about the apparent limiting in­fluence of the pOSitive-ion flow through the cylinder wall on the electron current collected by a probe.

ACKNOWLEDGMENT

The author wishes to thank Dr. D. F. Winterstein currently of Chevron Field Research Corporation for his assistance in making the plasma probe measure­ments and Dr. A.D. Brailsford of Ford Motor Company and Professor G. W. Ford of the University of Michigan for their many stimulating discussions, comments, and help with the calculations.

IG. A. Harrow, 1968 National Fuels & Lubricants Meeting (Society of Automotive Engineers, Warrendale, Pa., 1968), SAE Paper No. 680765.

2V. Arrigoni, F. Calvi, G. M. Cornetti, and U. Pozzi, 1973 Automotive Engineering Congress and Exposition (SOCiety of Automotive Engineers, Warrendale, Pa., 1973), SAE Paper No. 730088.

3R.M. Clements and P.R. Smy, J. Appl. Phys. 40, 4553 (1969) •

4R.M. Clements and P.R. Smy, J. Phys. D 2, 1731 (1969). 5G. Cox and P.B. Curtis, J. Phys. D 3, 1255 (1970); 3, 1263

(1970). GA.L. Merlo, IEEE Trans. Ind. Electr. Cont. Industr. 17, 60

(1970). TC. B. Warton, Plasma Diagnostic Techniques, edited by R. Hudd1estone and S. Leonard (Academic, New York, 1965), Chap. 11, p. 449.

8M. F. Calcote, 9th Symposium (International) on Combustion (Academic, New York, 1963), p. 622.

9A. Feugier and A. van Tiggelin, 10th Symposium (Interna­tional) on Combustion tthe Combustion Institute, Pittsburgh, 1965), p. 621.

IOC.M. Su and S.M. Lam, Phys. Fluids 6, 1479 (1963). II A. D. Brailsford (private communication). 12C.M. SuandR.E.Kiel, J. Appl. Phys. 37,4907 (1966). 1~.M.Clements and P.R. Smy, J. Appl. Phys. 41, 3745

(1970).

W.G. Rado 2474

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

128.252.67.66 On: Sun, 21 Dec 2014 03:02:14