CA1056242A - Spark-plug for automobile internal combustion engine - Google Patents

Abstract

Title of the Invention Spark-plug for automobile internal combustion engine Abstract of the Invention Improved spark-plugs for automobile internal combustion engines are made in a manner that thermal conductance (G) from flame nucleus to the electrodes is made as small as possible through decreasing of fluid resistance with respect to minute flame nucleus gas flow in the discharge, the fluid resistance being made small by making the discharge electrodes thin or making at least one of its discharge electrodes streamlined.
The spark-plugs are capable of igniting and burning such lean gas mixture of 1.25 or more in excess air ratio (F).

Description

S6;~42 ,.: .
~ackground of the Invention This invention relates to an automobile internal ,.
combustion engines and particularly, to electric spark-~ plugs for automobile internal combustion engines. An ; object of this invention is to provide an antipollution internal combustion engine which is superior in exhaust gas properties.
To enable the prior art to be described with the aidof drawings, the accompanying drawings will first be listed.
Fig. 1 is a graph s~owing the relationships of the NOX concentration (by parts per million, i.e., ppm, graduated on the left ordinate) in the exhaust gas, the ,/ HC concentraticn (by hexane equivalent ppm, graduated on the left ordinate) therein anu the CO concentration (by ~, graduated on the right ordinate) therein, respectively, with respect to the excess air ratio F (abscissa) of the gas mixture in the internal combustion engine.
Fig. 2 is a graph showing the relationship, in one example, among the left and right sides of the ignition theoretical inequality (1) of the inventors and the excess air ratio F, with V as a parameter.
Fig. 3 is a graph showing the relationship, in one example, between the ignition limit excess air ratio Fc and the electrode gap distance Ls, with G of the inequality (1) as a parameter, being based on the calcula-tions from Fig. 2, the shaded region being the ignitable region.
Fig. 4, (a) and (b) are schematic views showing a : thermal boundary layer and a hydrodynamic boundary layer formed on the electrode surface by electro-flame-wind, respectively.

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~ Fig. 5 and Fig. 6 together show one example of the ; conventional thin electrode type spark-plugs, the respective ; parts (a)s and (b)s of Fig. 5 and Fig. 6 showing the sectional side views and the bottom views, Fig. 6(c) showing the sectional front view.
; Fig. 7 shows one embodiment of the two-electrode spark-plug of the present invention, the part (a) of Fis. 7 being the sectional side view of the two-electrode spark-plug, the part (b) thereof being the sectional side view of ~i 10 essential parts and the part (c) thereof being the bottom . .
. view.
Fig. 8 to Fig. 14, Fig. 17 to Fig. 26, Fig. 29, Fig.

32, Fig. 34 to Fig. 36 show the other embodiments of the .
, two-electrode spark-plug of the present invention, the respective parts (a) and (c) thereof showing the sectional side view of the essential parts and the sectional front .,~ , .
`~ view, the part (b) thereof being the bottom view.
Fig. 15 is a side enlarged view of the electrode dischargins faces of the spark-plug, the Eo of Fig. 15 showing the conventional spark-plug electrodes, the El, E2 and E3 showing the spark-plug electrodes of the present invention.
Fig. 27, Fig. 28, Figs. 30, 31 and 33 show the enlarged views of the electrode essential parts of the spark-plugs of the present invention, respectively, the respective parts (a) and (b) of Fig. 27 and Fig. 28 being the sectional side view and the top view, the part (c) of Fig. 28 being the sectional front view, and the respective parts (a), (b) and (c) of Fig. 30, Fig. 31, and Fig. 33 being the sectional side view, the top view, and the sectional front view.

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;' Fig. 16 shows one embodiment of three-electrode spark-plugs of the present invention, the part (a) thereof being -~ the sectional side view and the part (D) thereof being the bottom view.
Fig. 37 shows one example of the conventional spark- -:,.
; plugs of surface creeping discharge type, and Fig. 38 to Fig. 40 show one embodiment of the spark-plugs of surface creeping discharge type of this invention, respectively, tne parts (a) thereof being the sectional side views, ; 10 the parts (b) thereof being the bottom views.
...
Fig. 41, (a) and (a') show the streamlines (in section) ~ of the electro-flame-wind in the conventional thin electrode -; type spark-plug, and the structure (in section) of the boundary layers with respect to the electro-flame-wind, respectively, the parts (b) to (-g) thereof showing the streamlines (in section) of the electro-flame-wind in the various spark-plugs of the present lnvention, the parts (b') to (g') thereof showing the constructions (in section) of the boundary layers with resepct to the electro-flame-winds corresponding to the parts (b) to (g), respectively, any of electrode saps in these spark-plugs being the same value Ls.
Fig. 42 to Fig. 45 show the experimentally measured characteristics of the ignition limit excess air ratio FL
of the spark-plugs (ordinate on the left side) and the electrode gap distance L (abscissa); in any graphs, the ordinate on the right side showing the ignition limit air fuel ratio, a horizontal dotted line Xl showing an equivalent air fuel ratio (F = 1), curves W, W' and Eo showing the FL-LS characteristic curves of the conventional spark-plugs, th other curves A, D, El to E6, G2, G3, Kl, 1 3 4 6 - 4 _ 1~6242 `; and N7 showing the FL-L5 characteristic curves of the ' spark-plugs of this invention, X2 showing each level of ',~t the air fuel ratio = 19.25, (F = 1.25), the region below j/~
" the each curve being the ignitable region.
Fig. 46 shows the ignition limit excess air ratio FL
(ordinate on the left side), with the spark gap Ls as a parameter,in the spark-plug shown in Fig. 7, or shows ' experimentally measured characteristics of dependence of `~ ignition limit air fuel ratio (ordinate on the right side) ` 10 upon the first electrode projection height Hl (abscissa).
Fig. 47 snows the ignition limit excess air ratio '~ (ordinate on the left side), with the spark gap Ls as a parameter in the spark-plug shown in Fig. 12, or shows experimentally measured characteristics of dependence of ignition limit air fuel ratio (ordinate on the risht side) upon the curvature v (abscissa) of the electrode discharging face.
~ An internal combustion engine provides power through ; the burning operation of a gas mixture of air and hydro-20 carbon fuels such as gasoline, petroleum gas, etc. As shown in Fig. 1, the respective concentrations of exhaust ` gas constituents such as nitrogen oxides (NOX), carbon monoxide (CO) and hydrocarbons (HC),' etc. which are discharged from the internal combustion engine, vary in accordance with an excess air ratio F (which means a ratio of an air fuel ratio to an equivalent air fuel ratio, the air fuel ratio indicating a ratio of air mass and fuel mass, the equivalent air fuel ratio indicating an air fuel ratio provided that H2O and CO2 have been produced sto-ichiometri-30 cally through reaction of the fuel and the oxygen. Accordingly, the smaller the value of the excess air ratio F is, the ;..
higher the fuel concentration is. The greater the value of the excess air ratio F is, then the lower the fuel ':'`, ' :
concentration is). For example, the gas mixture in the range of 0.9<F<1.25 produces a large quantity of IIOx due to high combustion temperatures (approximately 2,000 K to 3,000 ~). The gas mixture in the range of F<0.9 produces relatively small quantity of NOx, but an extremely large amount of CO and ~C. Accordingly, in order to improve the exhaust gas properties to develop antipollution internal combustion engines, it is required to burn lean gas mixture in the range of F>1.25, or to effect exhaust gas recirculation (EGR) to make fuel concentration lean to lower the combustion temperature of the gas mixture down to approximately 1,500 K or less.
In the conventional thin electrode type spark-plug, for example, a spark-plug shown in Fig. 5 and by Table 1, line W, which comprises a grounded electrode 1 composed of wide and long plate and with a high-tension electrode

2 of thin cylinder with flat end face, has, as shown in Fig. 42, extremely narrow ignitable region (a region below the curve). Fig. 42 shows the relationship of the ignition-limit excess air ratio FL of the gas mixture under 1 atmospheric pressure and room temperature vs. the electrode gap distance, namely, spark gap Ls. As seen from Fig. 42, in the conventional spark-plug, the ignition of the lean gas mixture in the range of the F>1.25 can be ; realized only when the spark gap is greater than Ls = 2.28mm (the Ls shows a measured ignitable spark gap for an excess air ratio of ignition-limit of FL = 1.25).

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i Table 1 (Prior art) . .. .
Marks of FL-Ls Grounded High-Tension Ls ~ Characteristic Types of Electrode Electrode :~ Curves Electrodes Dimensions Dimension (mm) (mm) (mm) ~ W Fig. 5 2.7 width1.0 diameter 2.28 ;~` 1.3 thickness 5 length ;
W' Fig. 6 2.7 width1.0 diameter 2.04 1.3 thickness ` 10 5 length Also, as in the other conventional spark-plug of Fig. 6 and Table 1, line W', according to a thin electrode type spark-plug which comprises the same high-tension electrode -2 as that of Fig. 5, and a grounded electrode 1 with U-shaped ~ groove 1' along the lengthwise direction of the discharging Pj`-` plane on the wide and long plate, the characteristic curves -between the excess air ratio FL of ignition-limit and the electrode gap distance Ls are improved as compared with :.
the curve W, as apparent from the curve W' of Fig. 42.
- 20 However, the ignitable region is so narrow that the ignition of the lean gas mixture in the range of the F>1.25 can be ` realized only when the electrode gap distance is greater than Ls = 2.04 mm.

In using an ignition power supply on the market, dischargeable-limit gap distance Ls for discharge with two-electrode type plug is about 2 mm for a gas mixture (gas mixture on heavy loading of the internal combustion engine with a compression ratio of approximately 10), which is compressed into molar density as eight times high as that under one atmospheric pressure. Accordingly, in the conventional spark-plugs, it has been difficult to directly ignite the lean gas mixture where F>1.25. In 1 , ;
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order to eli~inate such difficulty, the following three methods have been proposed.
A torch igniting means has been considered as a first method, according to which an antipollution internal com-bustion engine is realized by use of spark-plugs of the prior art. The torch makes it possible to ignite such lean gas mixture that has an air fuel ratio at the explosion limit. Accordingly, by making the gas mixture ; only near the spark-plug high in fuel concentration to effect the igniting operation, the excessively lean gas mixture (on the whole) can be burned through the formation of the torch. However, the disadvantages with the above torch ignition means are that a combustion subchamber, and subcarburetor, or extra fuel injection system, etc. are required to realize the above-described burning operation , . .
of the lean gas mixture.
Secondly, it is also possible to take measures for individual operation modes. During starting and warming-up operations, idling (no-load~ operation and engine-braking (negative load operation), less NOX are produced independently of the value of the excess air ratio F, because of the low tem-perature and low pressure of the gas mixture. However, in such modes an F value of nearly equal to 1 or smaller is required to assure ignition operation. Thus, the number of revolutions of the engine in idling mode has to be high to ensure F~l, ahd hence, the fuel consumption increases adversely. In the heavy loading operation mode and the others than the above mentioned modes, temperature and pressure of the gas mixture is high, and therefore, the lean gas mixture in the range of approximately F = 1.2 or smaller can be spark-ignited. Accordingly, the less NOX is produced with F ~ 1.2 and simultaneously better heat transfer rate through the cylinder wall of the engine. The .~

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disadvantage of such individual-operation-mode measure is that the internal combustion engines should provide a system that the value of the excess air ratio F is to be adjusted and controlled in the extremely narrow allowable value range for every opera-tion mode.
Thirdly, it is possible to provide an internal combustion engine wherein an excessively rich gas mixture (on the whole) is ignited to burn at relatively low temperature for preventing the NOX production and C0 and HC are rear-treated through the use of -~10 catalyst or thermal reactor. However, in order to realize the ~, .
above requirements, rear-treating system including catalyst, - thermal reactor, air pump, catalyst antioverheat equipment, etc.
are required, thus resulting in power loss in exhaust system increased fuel consumption due to excessively rich fuel, sul-furic acid mist and scattering heavy metal.
The internal combustion engines used in the above-- mentioned flrst, second and third methods have such G~ disadvantages as increased weight and cost of the engines, complicated adjustment of the optimum operation conditions of the engines, inferior stability of the optimum operation, difficult mass production controlling, and complicated, difficult adjustment of the manner of use.
Summary of the Invention This invention consists of an electric spark-plug for automobile internal combustion engine comprising: a metallic screw portion to be engaged with an internal combustion engine, an electrode terminal part, an electric insulator for supporting said screw part and said electrode terminal part in a coaxial relation, keeping them insulated from each other, and at least a pair of electrodes, the pair including a first electrode and a second electrode, said first electrode being supported by a supporter formed g ~

10562~Z, on the metallic screw part, said second electrode being rod-shaped and electrically connected to said electrode . terminal pa~t by means of a central conductor disposed on the axis of said screw part and in said insulator, said first electrode and said second electrode being supported insulatedly from each other by means of said insulator, ; keeping a given ignition gap between their discharging `` faces, characterized in that: fluid resistance against -gas flow of flame nucleus of said electrodes is decreased in a manner to have an ability of ignition that is defined by capability to ignite and burn a lean gas mixture of .: isobutane and air under the conditions of normal temperature :
- (T) of about 20C, the pressure of lean gas mixture equals 1 atmospheric pressure (density index ~1), excess air ~:. ratio (F) of more than 1.25 and electrode gap distance (Ls) . of less than 2mm, so that the following ignition conditional .~ inequality applies thereby to decrease the thermal con- :
ductance G from the flame nucleus to the electrodes:
. G ~exp(Eb/RT (n + 4.773 mF)n+m*
~ V l(d - l)B~ n+m* 3.773 n m (Xt - Xi) m F J
: 20 . . . . . . . . . . (1), ~ where J, E and m* are given by J = ~ Eb (Ts - To) + RTs ] / ~ RTs, E = [ (760 -p ) / 760] Rc, m* = (1 + y)m, wherein V is the nucleating volume size and is represented by an equation of V=9Ls wherein 9 is a proper constant and Ls is the electrode gap distance, T is a temperature of the nucleating volume, To is the temperature of the electrodes, :-Xi is a molar density in 1 atmospheric pressure of incom-bustible gases other than nitrogen, F, E, p and Rc are .
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1~562~2 excess air ratio, density index of molecules, absolute value of negative pressure (mm Hg~ in intake pipe and compression ratio at the time of ignition, respectively, in the gas mixture, J being practically a constant when the dependence of the singular point temperature T upon -. the variable quantities V, G, Xi, F and E are sufficiently : small, Eb is an activation energy in Arrhenius' equation, and is a constant inherent to individual fuel, n and m are the molecularities of reaction of fuel and oxygen, respectively, y is a parameter showing the participation degree of nitrogen molecules in reaction process, ~ is a multiplication factor of chain carriers, R is a gas constant, B, Xt and ~ are constants, respectively.
The invention also consists of a method of igniting an automobile internal combustion engine of lean gas mixture combustion type using the lean gas mixture having excess air ratio F of F> l in operation modes including ~- idling, engine-braking,-constant speed, acceleration and deceleration, by means of at least one sprak-plug com-prising: a metallic screw portion to be engaged with said internal combustion engine, an electrode terminal part, an electric insulator for supporting said screw part and said electrode terminal part in a coaxial relation, keeping them insulated from each other, and at least a pair of electrodes, the pair including a first electrode and a second electrode, said first electrode being supported by a supporter formed on the metallic screw part, said second electrode being rod-shaped and electrically connected to said electrode terminal part by means of a central conductor disposed on the axis of said screw part and in said insulator, said first electrode and said second electrode being supported insulatedly from each other by ;:
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10~ 42 means of said insulator, keeping a given ignition gap between their discharging faces, the method being char-acterized in that: a lean mixture of gas is ignited with a decreasing fluid resistance against gas flow of flame nucleus of said electrodes in a manner to have an ability of ignition that is defined by capability to ignite and burn a lean gas mixture of isobutane and air under con-, . . .
ditions of normal temperature (T) of about 20C, the ; pressure of lean gas mixture equalling 1 atmospheric : 10 pressure (density index of about 1), excess air ratio ~. (F) of more than 1.25 and electrode gap distance (Ls) of ; less than 2mm, so that the following ignition conditional :
inequality applies thereby to decrease the thermal conductance (G) from the flame nucleus to the electrodes:
G ~exp(Eb/RT) (n + 4.773 mF) V l(d - l)B 3 773rm nnmm (x - Xi)n m F J
. . . . . . . . . . '~1), ~ , wherein J, and m* is given by J = [Eb (Ts ~ To) + RTs ] / ~ RTs .` = [(760 - p ) / 760~RC, . 20 m*= (1 + y)m, wherein V is the nucleating volume size and is represented by an equation of V=~LS wherein 0 is a proper constant and Ls is the electrode gap distance, T is a temperature of the nucleating volume, To is the temperature of the electrodes, Xi is a molar density in 1 atmospheric pressure of incom-bustible gases other than nitrogen, F, ~, p and Rc are ; excess air ratio, density index of molecules, absolute value of negative pressure (mm hg) in intake pipe and compression ratio at the time of ignition, respectively, , . .

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;' :. l~t~6'Z42 in the gas mixture, J being practically a constant when .~ the dependences of the singular point temperature Ts upon ~- the variable quantities V, G, Xi, F and E are sufficiently . small, Eb is an activation energy in Arrhenius' equation, i -and is a constant inherent to individual fuel, n and m : are the molecularities of reaction of fuel and oxygen, respectively, y is a parameter showing the participation : degree of nitrogen molecules in reaction process, ~ is a multiplication factor of chain carriers, R is a gas .' lO constant, B, Xt and ~ are constants, respectively.
-. The invention also consists of an automobile internal combustion engine of lean gas mixture combustion type including a combustion chamber, a lean gas mixture producing device for producing lean gas mixture having . excess air ratio of F of F> l in operation modes including ; idling, engine-braking, constant speed, acceleration and decleration, a compressing means which compresses said lean gas mixture containing air and fuel by varying volume of ` said combustion chamber, at least one electric spark-plug for igniting said compressed lean gas mixture, said spark-plug comprising a metallic screw portion to be engaged with an internal combustion engine, an electrode terminal part, an electric insulator for supporting said screw .. part and said electrode terminal part in a coaxial . relation, keeping them insulated from each other, and at least a pair of electrodes, the pair including a first electrode and a second electrode, said first electrode :- being supported by a supporter formed on the metallic screw part, said second electrode being rod-shaped and 30 electrically connected to said electrode terminal part by r means of a central conductor disposed on the axis of said :' ; -` 1056242 screw part and in said insulator, said first electrode and said second electrode being supported insulatedly from each other by means of said insulator, keeping a given ignition gap between their discharging faces, char~
acterized in that: fluid resistance against gas flow of flame nucleus of said electrodes is decreased in a manner to have an ability of ignition that is defined by ~-.
capability to ignite and burn a lean gas mixture of . .
isobutane and air under the conditions of normal tem-- 10 perature tT) of about 20C, the pressure of lean gas mixture equals 1 atmospheric pressure (density index ~
approximately equal to 1), excess air ratio (F) of more than 1.25 and electrode gap distance (Ls) of less than 2mm, so that the following ignition conditional inequality applies thereby to decrease the thermal conductance G
from the flame nucleus to the electrodes:
G ~exp(Eb/RT) (n + 4.773 mF) V l(d - l)B~n+m 3.773~m nnmm (X - X )n+m* Fm*J

; . . . . . . . . . .(1), ~
. wherein J, and m* are given by -J = ~Eb (Ts ~ To) + RTs ] / ~ RT 2, ~ - [760 - p ) / 760] Rc, m* = (1 + ~)m, wherein V is the nucleating volume size and is represented by an equation of V=~LS wherein ~ is a proper constant and Ls is the electrode gap distance, T is a temperature of the nucleating volume, To is the temperature of the electrodes, Xi is a molar density in 1 atmospheric pressure of incombustible gases other than nitrogen, F,~ , p and Rc are excess air ratio, density index of molecules, ., ~ . .

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i absolute value of negati~e pressure (mm Hg) in intake pipe ;~ and compression ratio at the time of ignition, respectively, . .
`; in the gas mixture, J being practically a constant when ....
~;~ the dependences of the singular point temperature Ts upon .; ,................................................................... .
the variable quantities V, G, Xi, F and ~ are sufficiently ~- small, Eb is an activation energy in Arrhenius' equation, . "
; and is a constant inherent to individual fuel, n and m are the molecularities of reaction of fuel and oxygen, respectively, y is a parameter showing the participation degree nitrogen molecules in reaction process, ~ is a multiplication factor of chain carriers, R is a gas constant, B, Xt and ~ are constants, respectively.
Detailed Disclosure of the Embodiments A new theory concerning growth or decay of the initial flame nucleus, which determines the success or failure of the electric spark ignition of the combustible gas mixture, is as follows: Namely, the model of micro chain reaction has been established to analyze the model mathematically.
Not only elements governing the ignition limit, such as ' 20 gas mixture temperature, density index of molecules, excess air ratio F, incombustible gas density, flame t nucleus volume, and thermal conductance G to electrodes, but also the relationships among them have been clarified.
A new concept has been introduced wherein high speed fluid motion named "electro-flame-wind" is caused at the time of the spark discharging. As a result, the fluid resis-tance of the electrodes play a principal part for the thermal conductance G, and the electrodes, which are adapted to have small fluid resistances, can ignite the 30 lean gas mixture. This invention is embodied, based on this theory.

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516'~42 It is an object of this invention to provide electric spark-plug for automobile internal combustion engines, based on the above new theory, which is capable of directly igniting the lean gas mixture, and antipollution automobile internal combustion engines of lean gas mixture combustion type which are superior in exhaust gas characteristics.
It is also an object of this invention to provide a method of igniting an automobile internal combustion engine of lean gas mixture combustion type, based on the above new theory, which is the method of directly igniting the lean gas mixture, thereby easily to realize the anti-pollution automobile internal combustion engines which are superior in exhaust gas characteristics.
This invention will be described hereinafter in detail with reference to the theory and embodiments.
(A) Theory: In the following, the new theory concerning the electric spark isnition of the combustible gas mixture will be described in five steps. First, five steps are summarized:
(1) first step (model of micro chain reaction) First, th~e model of reaction scheme of the chain combustion in the initial flame nucleus is made to establish the equations which determine chain carrier molar density X in the flame nucleus and the behaviour of the temperature T.
(2) second step (derivation of ignition conditional inequality) The equations obtained in the above step is analyzed by use of a known Liapunov's stability theorem to derive the ignition conditional inequality.

(3) third step (derivation of ignition conditional .. : .

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inequality expressed in the excess air ratio F) The dependence of ignition conditional inequality on the mixing ratio obtained in the second step is examined - about the gas mixture composed of air, fuel, inert gases except nitrogen to rewrite the ignition conditional inequality in the expression of the excess air ratio F.

(4) fourth step (examination of dependence of excess air ratio of ignition limit on each parameter) In accordance with the ignition conditional inequality obtained in the third step, the dependence of the ignition limit excess air ratio, upon the gas mixture temperature T, upon the density index ~ of molecules thereof, upon the incombustible gas molar density Xi thereof, upon the flame ? nucleus volume V thereof, and upon the thermal conductance ; G to the electrodes, particularly the dependence upon the flame nucleus volume V and the thermal conductance G to the electrodes is examined in detail.
`~ (5) fifth step (relationship between thermal conductance G and fluid resistance of electrode(s)) Finally, the proportional relationship between the thermal conductance G and the fluid resistance ~ of the electrodes is clarified hydrodynamically. As a result, since the thermal conductance G becomes small proportionally as the fluid resistance ~ becomes small, accordingly, the lean gas mixture ignition can be possible. At the same time, the reasons why the conventional spark-plugs can not ignite the lean gas mixture are described. Now, the steps are described in detail.
First Step (model of micro chàin reaction) There exists a nucleating volume-(volume V, temperature T), that is, a minute space, in which the enlargement of .
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the pre-explosion flame (flame nucleus) formed around dis-charge channel is limited. In this or smaller volume, no significant local heating can take place and the molar density of the chain carrier does not increase so fast .:
as to cause an explosion. Assume that in the volume, the molar densities ~mol/cm3) of the chain carrier, fuel, oxygen and nitrogen are X, ~Xf, ~XOX and ~Xni, respectively.
~; Therein, ~ is a density index of molecules expressed in the following equation:

; 760 - p ~ . Rc ...................................... (2), -~ 760 wherein p is an absolute value (mm Hg) of negative pressure in intake pipe, P~c being a compression ratio at the time .
; of ignition.
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The reaction scheme of the branched-chain combustion in the nucleating volume is as follows.

n (fuel) + m (ox) + ym (ni) + (chain carrier) h >
(inactive products) + ~ . (chain carrier) .. (3), (chain carrier) t > (inactive products) ......... .(4), wherein ox is oxygen, ni is nitrogen in the air, kb and kt are the rate constants for branching and termination, respectively, ~ is a multiplication factor of chain carrier, n and m are the molecularities of reaction of fuel and oxygen, respectively (the power indices of the rate terms of fuel molecule and of oxygen molecule, respectively, in the rate equation of reaction). The incombustible molecules such as nitrogen, etc. are also constituents of the conservation system of kinetic energy and momentum produced through the collision reaction between the fuel molecules and the oxygen molecules. Participation of the incombustible molecules in this conservation system is not always :

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effected after the reaction of the fuel and the oxygen has been terminated. Therefore, it is reasonable to consider that the participation is performed even in the way to the reaction. The direct participation of the nitrogen molecules in the combustion process should be : considered particularly under conditions where NOX, even if very small in amount, may be produced. The number of the nitrogen molecules participated in the reaction is assumed to be above-described ym (ni). Generally, y is 3.773 (a molar concentration ratio of the nitrogen in the atmosphere to the oxygen therein) or less.
From the reaction formulas (3) and (4), the kinetic equations concerning the molar density of chain carrier and the energy conservation equation are expressed in the following equations (5) and (6), respectively:
dt = ~(~ - 1)( Xt) (EXoX)m(Xni)Ymk-kt~ X .......... (5), dt pc (d-l)(Xf) (xox) (Xni) Y kaHb+ktaHt} - Vpc (T-To) .......... (6), wherein t is time, p is the density of the gas mixture, CS is average sp~cific heat of the gas mixture, AHb and aHt are the reaction enthalpy of the chain branching and chain termination, respectively. The thermal conductance from the nucleating volume, namely, the flame nucleus to the spark-plug electrodes (temperature To) iS assumed to be G. Also, the temperature dependence of k is assumed to follow the usual Arrhenius' equation k = B exp (- Eb/RT), (wherein B is a constant, Eb is activation energy for branching, R is a gas constant), but kt is assumed to be independent on the temperature.

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In the actual com~ustion reaction, the chain reaction shown in the eq. (5) and the thermal reaction shown in the eq. (6) proceeds simultaneously. Accordingly, we deal simultaneously with the two equations (5) and (6) to obtain the conditions of ignition, namely, the conditions where flame nucleus will not go out. If all of Xf, XOX and Xni are sufficiently larger than X, and the changes of ' all of Xf, XOX, and Xni are negligible as compared with the change of X, the reaction becomes a pseudo-first order reaction. Therefore, in this case by writing kb = (~-l)B( ~Xf ) ( ~XOX ) ( ~Xni )Y eXP(-Eb/RT) ~ (7)~
~Hb/pCS= hb~ l~Ht/PCs ht the equations (5) and (6) can be simplified as follows:

= (kb - kt)X ...................................... .(9), ' ldT ( bhb + ktht)X ~ VpG (T - T ) Second Step (derivation of ignition conditional inequality) ~; Since the equations (9) and (10) contain no independent ~, variable explicity on the right side, a phase ( T - X ) plane .- method can be used to examine the behavior in a phase plane.
, 20 The following equation can be obtained by writing the left side of the equations (9) and (10) as f and g, respectively, g = dT = ( b b t t) VPCs ( T To) (11) The singular points of this equation are obtained by solving the following simultaneous equations:

~(kb ~ kt)X = 0 ....................................... (12~, l (kbhb + ktht) X - VpC (T - To) = ~ .,,,,,,,, (13) -We denote the coordinates of the singular points by (Xs, Ts) ', '~`' i~sl624~:

and X - X and T - T are used as new variables, respectively, to enable the eq. (11) to be linearized in the neighborhood of the singular points. So, we transform X - Xs and T - Ts to X and T, respectively, as f and g to become both O in the origin of the coordinates (O, O). If the f and g have second derivatives at the origin, according to the Taylor's theorem, the following equation should hold:
g(X, T) = g(, ) + gx(~ O)X + gT(O, O)T +

2 f xx gXT ( ~ ~ ~ ) XT + gTT ( ~ ) T 2,3 ........... ( 14 J
wherein gX = ag/aX. gT = ag~aT~ gXX = a2g/aX2. gXT = a2g/aXaT, gTT = a g/a T , ~, = ~X, ~ T ( O <(~ <1 ) .
Accordingly, in the neighborhood of the origin, namely, in a location where ¦X¦ and ¦T¦ are small, g(X, T) are approximated as follows:
f(X, T) = rix + r2T~ wherein rl = gx(O, O), r2 = gT(O' ) .......... (15).
Likewise, f(X, T) is also approximated as follows:
f(X, T) = r3X + r~T, wherein r3 = fx(~ )~ r4 = fT(O, O) .......... (16).
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20 Accordingly, the eq. (11) is expressed as follows:

dX = 1 r2T RTS (ktht + kbhb)x + (X kbEbh - ~)T
3 + 4T RTS (kb - kt) X + (XSkbEb) .......... (17) wherein ~ = RTS (aT{vpc (T - To)¦~ (T = Ts) The nature of the signularity is found by examining the characteristic equation associated with the eq. (17), i.e., ~ (r2 + r3)~ + r2r3 - rlr4 = O -------............ (18), .~ -`" lOS624;~
, ~ (r +r ) +~(r - r )2 + 4r r the roots of which are ~ + = 2 3 2 3 1 4 .. , (r? + r3) --/(r - r )2- +~4r r : 2 If ~+ and ~- are both real and differing in sign (i.e. r2r3 -rlr4<0), that particular signularity is a "saddle point" and . .
unstable. At this time, the T and X diverge. Physically, the flame nucleus does not go out and the combustion spreads to the entire gas mixture. I~amely, it means that the ignition (explosion) occurs. The condition for this is rlr4>r2r3.
Namely, the following equation is given:
(k h + ktht)XSkbEb ~ (kb ~ kt)(xskbEb b ~5i! This is an inequality expressing the ignition (explosion) s condition. ~ = RTS2vpGc is substituted into the above equation and the both sides of the inequality are divided ,.
by RTS to obtain the following inequality:
kbkt(hb + ht)Xs b2 > t b G .......................... (21).

The eq. (7) is substituted into the inequality (21) and the both sides of the inequality are multiplied by Vpcs to obtain ~- the following inequality.

b t b Ht)XSRT 2 V ~ (kt - kb) G .................... (22) The inequality (22) can be expressed as (~Hb + ~Ht)XSRT ~ ~ V-( kb ~ kt) ---................ (23) One of the singularities which simultaneously satisfy the equations (12) and (13) is obviously (Xs, Ts) = (0, To).
This is a stable nodal point, but is not a saddle point. The relationship between Xs and Ts is obtained for another 1~56242 singularity (saddle point). From Xs ~ and the eq. (12) the following equation is obtained: t ~,...
b t ~ (24) The equation (24) is substituted into the eq. (13) to hold:

s V kt(~Hb + ~H ) ~ ................................ (25).

~: On the other hand, the termination of the chain carrier is :
mainly due to collision deactivation against the electrode face. Chain carriers (particles) reach the electrode face ` and are annihilated, and simultaneously the carried heat energies are transferred to the electrode. Since the mass flow and the heat flow are performed through the same .; molecular mechanism, the rate constant kt of the termination is proportional to the thermal conductance G. Since the flame ~, nucleus volume V is considered to increase in proportion to . ~
the spark gap Ls and the electrode annihilation of the chain carriers grows out of the end of the nucleus volume V near the electrode face, the decreasing rate of the average density of chain carrier over the nucleating volume is in - inverse proportion to Ls, according to V.
The kt is given by k _ ~ G ............................................. (26) t V ~-wherein ~ is a constant.
The equations (25) and (26) are substituted into the inequality (23) to obtain the following equation:

Eb(rS - rO) + RTS > G
~ RTs V kb -----~ (2 On the other hand, from the equations (7), (24) and (26) and under the condition of T = Ts, the following equation is obtained:

.' .

.

:... ::

l~S6Z4~

~ .

exp(Hmb )=V ......................................... (28) . s (~ - l)B(~Xf) (~XOX) (~ ni~

Although Ts depends upon V or [(~Xf)n(~XoX)m(~Xni)Ym~ 1, the dependency of the T on them is extremely small. For example, as typical Eb and Ts in the actual combustion reaction, in . - the neighborhood of Eb = 50 Kcal/mol, T = 500K, if V or Xf) (sXc ~m(~X i)Ym~ 1 increases twice, T increases only by 1.4%. Namely, the left side in the inequality (27) can be considered to be a constant independent from V or a mixing ratio, and hence, by writing it as J, the ignition condition is given by J V kb (29) . Considering Xni = 3.773 XOX in the atmosphere, the eq. (7) is substituted into the inequality (29) to obtain the following ` inequality:

G exp (Eb/RT) (d-l)B3 773Ym~n+(l+y)mxn X (l+y)m ---- (30)-ox Third Step (derivation of the ignition conditional inequality of the excess air ratio representation) The gas mixture which consists of air, fuel and incom-bustible gases other than nitrogen is examined. The entire molar density of the gas mixture is constant, independently of the mixing ratio. From this condition, the following equation is give~
Xf + XOX + Xni + Xi = Xf + 4.773 XOX + Xi Xt Co .......... (31), wherein Xi is the molar density of incombustible gases, other than nitrogen, contained in the nucleating volume when E = 1. Now, the excess air ratio F is defined as shown in the . 105624Z
following equation:

F ~A/Mf (MA/~)st ...................................... (32).

wherein MA/Mf is an air fuel ratio (a mass ratio of air to fuel), (MA/Mf.)st is an air fuel ratio of an equivalent gas mixture. The air fuel ratio MA/Mf can be expressed in the following equation:

. A ox~ox ni~ni _ ox (~ox ~ni) ox ~a Mf Xf~f Xf~f Xf ' ~f .......... (33), wherein the molecular weight of the fuel, oxygen, and nitrogen are ~f~ ~ox' ~ni' respectively, and ~a = ~ox + 3 773 ~ni On the other hand, in the equivalent gas mixture, X x/Xf = m/n is provided, and thus the air fuel ratio of the equivalent gas mixture is given from the eq. (33) by the following equation:

MA m ~a (Mf)st n ~f -------.................................. .......... (34 Resultantly, from the.equations (32), (33) and (34), F is given by the following equation:

F n ox . ............................................ .......... (35).

Xf and X x are given, from the equations (31) and .(35), -as the following equation:

n ( t - Xi) m(Xt - Xi)F
Xf n + 4.773 mF' Xox n + 4.773 mF - --- (36) The eq. (36) is substituted into the inequality (30) to express the ignition condition by the following inequality:

Gr exp Eb/RT~ (n + 4.773 mF)n+(l+Y.)m ¦(d-l)BE ( Y) 3-773~m nnm(l+l~)m (X -x )n*(l+~)mr(l+~y)mJ
.......... (1').

6;Z4Z
.'.,' .
This ignition conditional inequality means the easier the ignition becomes as the smaller the right-hand side becomes.
Now, by defining Z(T, E, Y) and Y (F, Xi) as follows:

Z(T ~ Y) = exp ( b/RT) y (37) Y(F, X ) = (n + 4.773m ~)n+(l+y)m .773ym nnm(l+y)m(x -X )n+(l+Y)M F(l~Y)m .......... (38), the condition of ignition can be simplified as follows:

J > V Z (T, E ~ Y) .................................. (39).

^~ Accordingly, the ignition limit condition given by J = V Z (T,~ , Y) ................................... (40).
;, Fourth Step (examination of the dependence of ignition limit excess air ratio on each parameter) (a) With regard to parameters G and V of ignition limit conditional equation (40).
Dependence of F of VZ in the right side of the eq. (40) is expressed through the Y (F, Xi) in the eq. (38) and the Z ~T, ~, Y) of the eq. (37). In Fig. 2, curves I to IV show one example of VZ ~ FL characteristic curve group with various numerical values of G/V which is optionally selected to be listed in Table 2. The left side of the eq. (40) is independent of the value F, and therefore can be expressed by a horizontal line J. A curve I does not cross the hori-; zontal line J. In this V = 1.000 case, the ignition is not effected in any mixing ratio. VZ curve moves downwardly when V is made small to 0.867, and the curve II and the horizontal ' line J comes to contact with each other at one point, thus allowing the ignition to be effected only at the excess air ratio of F = FR2 = FL2. If the V is made smaller to 0.743, 1~56~4Z
.
the V-Z curve moves further downwardly so that the curve may cross the horizontal line J at two points, F = FR3 and FL3.
Thus, the ignitable region is FR3<F<FL3. In the case where V = 0.629 (curve IV), the ignitable region is further enlarged to FR4<F~FL4. Thus, if the parameter V is reduced under a given limit, the ignitable region can be enlarged to the lean mixture (and the rich mixture).
Table 2 (relative values of V for various curves) ~ ~.
~ urves in a b c d \ Fig.3 \ (G=1.000Gl) (G=0.867G ) (G=0.743G ) (G=0.629G ) Curves in \ 1 1 1 _ ._ .
I (-V = 1.000) 1.000 0.867 0.743 0.629 II(V = 0.867) 1.154 1.000 0.857 0.726 - III(V- = 0.743) 1.346 1.167 1.000 0.847 IV(y = 0.629) 1.591 1.379 1.182 1.000 Accordingly, under a condition where G is constant, if the V is made larger, the excess air ratio FL of the lean mixture side ignition limit can be made larger and the excess air ratio FR of the rich mixture side ignition limit can be made smaller. On the o her hand, under a condition where the V is constant, if the G is made smaller, the FL
can be made larger and the FR can be made smaller. This description will be given in detail in the belowmentioned fifth step.
Then, dependence of the FR and FL upon the V, hence, dependence upon the electrode gap distance Ls, will be examined hereinafter. Assume that the nucleating volume size V is increased in proportion to the electrode gap distance Ls, namely, : - . . , -s ....................................... (41), wherein ~ is a proper constant. The eq. (41), t37~ and (38) are substituted into the ignition limit conditional equation . ~ .
~ (40) to rewrite the equation explicitly. Thus, eq. (40) is ~ . ~
rewritten:

:- J_Gz_ G exp( b/RT) (n + 4.773m FC)n+(l+y)m V ~Ls (d l)B~n+(l+y)m 3.773Y nnm(1+~)m(Xt-Xi) Fc .:
~ .......... (42), I
wherein Fc is an ignition limit excess air ratio, and hence Fc is equal to FR or FL.
Each numeral in the columns under a, b, c and d of Table 2, shows the relative values of V calculated, respec-tlvely, corresponding to the curves I to IV of Fig. 2 in the ~
respective cases of G = Gl, G = 0.867 Gl, G = 0.743 Gl, ;
G = 0.629 Gl. The curves a, b, c and d in Fig. 3 are ` obtainable by converting the V values of Table 2 to the electrode gap distance Ls on the basis of the eq. (41) and then re-plotting the ignition limit excess air ratios Fc of Fig. 2 with respect to Ls. Such curves a, b, c and d in Fig. 3 correspond to the a, b, c and d of Table 2.
These curves apparently shows that either of leaner mixture gas and richer mixture gas becomes ignitable as the electrode ;~ gap distance Ls, namely nucleating volume V, increases.
Furthermore, if G is made small, the same lean gas mixture ,.:
becomes ignitable even in a narrower electrode gap distance Ls, and also in case of the same electrode gap distance Ls, the excess air ratio FL of the lean mixture side ignition limit can be increased. The ignitable ~egion in the value of each G is a shaded portion in Fig. 3. When F-0.614 (corresponding to an minimum point of the curve II in Fig. 2) ; is established in each curve a, b, c and d, the shortest gap distance of ignition limit Lq is provided. The Lq can be derived as follows. The excess air ratio F* corresponding ` to the minimum point of the curve II in Fig. 2 is given, through solving dY=O by the following equation:

F* = 4l7~3 .......................................... t43).

Referring to the eq. (42), since Ls = Lq is established in F*=(l+y)/4.773, the Lq is expressed by the following equation:

Lq=G ~ exp( b/RT) ~4.773 l(l+y)m b~( l)B~n+(l+Y)m3 773 ym nnl(l+y)mJ
~ n+(l+Y)m~ ........................................ - (44) This is the electrode gap distance corresponding to a so-called quenching distance, which has been known experimentally.
It is found out from the eq. (44) and Fig. 3 that the gap distance of ignition limit Lq becomes smaller as the G
becomes smaller.
- It is obvious from Fig. 3 that the ignition limit curve ~ with respect to FL portion moves leftwards and upwards, and ` the ignition limit curve with respect to FR portion moves leftwards and downwards when G becomes small. Namely, the ignition of the same lean gas mixture can be realized with a narrow electrode gap distance Ls if the G is made small.
Also, in the case of the same electrode gap distance Ls, the excess air ratio FL of the lean mixture ignition limit can be increased.
As described hereinabove, the lean mixture ignition is possible when the V is made small. It also is effective for the lean mixture ignition operation to control the other 1~56Z42 , .
parameters T, and Y in the right side of the eq. (40~, thereby to make Z (T, E, Y ) small.
(b) with regard to parameters T, E and Y of ignition limit conditional equation (40)..

.
As apparent from the eq. (37), the higher the ~ of ` the gas mixture is (absolute value p of the negative pressure in intake pipe is smaller and compression ratio Rc is larger), or the higher the gas mixture temperature is, then the smaller Z becomes.
Also, apparent from the eq. (38), Y becomes small as the incombustible gas amount Xi is small. Accordingly, the Z becomes small as is clear from the eq. (37). In these cases, as'the characteristic curve VZ mves downwards in Fig. 2, a point of right side intersection FL moves right-wards so that the leaner gas mixture can be ianited.
However, as described hereinafter, the E, T and Xi are subject to limits, depending upon the actual operating ^ modes of the internal combustion engine. During the engine brakins operation or negative load, the absolute value of the negati~e pressure in intake pipe becomes approximately p = 600 mm Hg, and thus the density index E of molecules of the gas mixture in the engine of a compression ratio ; RC=6 reaches only E = 1. 26 even at the top dead center position of the piston. Since the compression ratio of the internal combustion engine is normally Rc = 6 to 10, the limit in low density indexes of molecules can be considered E~l. Since the pressure rise accompanied by adiabatic process is hardly produced under the condition of providing E ~1, the limit in the gas mixture pressure, which is disadvantageous to the ignition of the actual internal combustion engines, is considered to be equal to 1 atmospheric - lOS6~2 pressure. Since the temperature of the gas mixture is approximately equal to atmospheric temperature during the starting and warming-up operations of the internal combus-tion engine, the atmospheric temperatures in very cold districts during the winter are severe temperature conditions to the igniting operation. However, it is difficult to limit the conditions numerically. Since the pressure and the mixing ratio compensate for each other, actually the approximately 20C (room temperature or normal temperature) can be considered as a reference temperature. ~lso, as -.
: described hereinbefore, when the negative pressure in intake pipe is larger, one portion of the combustion exhaust gas flows backward (formlng automatic recirculation of exhaust gases, self EGR) during the overlapping operation of the exhaust and intake valves. Thus, the molar density . ~ , of the incombustible gases Xi increases. Needless to say, more Xi is increased when the EGR is used through an : . :
external recirculation circuit. ~s described hereinbefore, it is found out that the ~, T and Xi are limited by the actual conditions of the internal combustion engine and the condition of the ignition becomes disadvantageous particu-larly during light load. -In order to control the production of the NO during acceleration or heavy load in the driving operation of the internal combustion engine, namely, under the high tem-perature of the gas mixture and/or under the high pressure conditions, the value of F mustIbe larger than around 1.25.
It is sufficient for the antipollution combustion that the electric spark can ignite the gas mixture of F ~ 1.25 under 30 the severest igniting condition, 1 atmospheric pressure and ;~
normal temperature. The reason why are that the combustion ~, . ..

s~z~;z temperature of the lean gas mixture (F > 1.25) is low ; regardless of the operation modes of the internal com-bustion engine and therefore is not suitable for the condition of the NOX production, and that the excess of oxygen in the lean gas mixture is not suitable for the condition of production of CO and hydrocarbons.
Fifth Step (relationship between thermal conductance G and electrode fluid resistance) ; As described in this step, the high speed fluid motion which we called "electro-flame-wind" is caused during the electric spark ignition in the flammable gas mixture. Accord-ingly, the thermal conductance G from the flame nucleus of temperature T to the electrodes of temperature To has to be determined through hydrodynamical process.
(a) with respect to electro-flame-wind Combustion of the gas mixture certainly accompanies the fluid motion of gas. The combustion process is not only a chemical phenomenon, but also a hydrodynamical phenomenon.
The thermodynamical quantities (temperature, pressure, density, enthalpy, entropy, etc.), which are caused through the gas fluid velocity and the combustion reaction, are subject to laws of conservation of mass, momentum and energy.
; In the electric spark ignition, the flame nucleus is made within the electrode gap, and co-exists with discharge during the duration time of discharge. Thus, during this period, the flow of the flame nucleus gases is subject to the influence from not only the thermodynamical quantities, but also electric fields. Namely, it has to be kept in 3~ mind that the flow of the flame nucleus gases has influences from the electro-flame-wind in the form, size and arrangement :

105~'4;~
.
of the electrode and the cation drag thereof, instead of simply having normal vector of the flame surface. According to the experiments of the inventors, voltage across the spark, obtained from oscillogram of the discharge current in case of the electrode gap distance Ls = 0.84 mm, is 960 V
at its initial value. Accordingly, the strength of the electric field is estimated to be 1.27 x 10 V/cm. Using the value of mobility 1.9 cm /V.s of N2 ion in N2 gas of 1 atmospheric pressure and 20C temperature, considering the highest partial pressure of N2 in the gas mixture, then, the drift velocity of N2 ion moving from positive electrode to negative electrode becomes 217 m per second. Other ions i have approximately the same drift velocities as the N2 ion has. Since this drift velocity diffuses even to the neutral molecules through viscosity, the high speed flow of gases moving from the positive electrode to the negative electrode is caused. This flow can not stay within the spark region due to the continuity and viscosity of the flow.` Thus the flow of gas phase is caused in a region around the electrode and the spark.
This is a phenomenon known as electric wind, in incombustible gas. In the inflammable gas mixture, the combustion flame itself is weak plasma, and the cation in the flame before the discharge is terminated also, acts to strengthen the electric wind. This flow has a vector of axial direction going from the positive electrode to the ne~ative electrode. But the flame has a vector in the normal direction of the flame surface, and hence, has a radial vector which is perpendicular to a line connecting the positive electrode and the negative electrode. Hence, the flow which is expressed in a resultant vector of the .

~OS6Z4'~
''.
axial vector and the radial vector is considered to ha~e been produced in the initial stage of the electric spark ignition, namely, within a period (3.7 ms in the above experiment) of the current flowing. The inventors call this flow the "electro-flame-wind". The electro-flame-wind caused in the inflammable mixture has larger expansion than the simple electric wind caused in incombustible mixture has.

,;.
The electro-flame-wind is considered as a unique high speed flow, which is different from the simple electric ~' ; wind, the torch which is flame without the electric dis-charge, and a simple overlapping of the simple electric wind and the torch. According to the calculations which are based on the experimental data obtained by the inventors, the flow speed reaches u ~ 33 m per second.
The Nusselt number Nu concerning a plate of width w = I and length y is given by Nu = 0.686 (Pr)3 ~ .................................. (45), wherein Pr is the Prandtl number, ~ being kinematic viscosity.
Considering that the abovementioned equation is applicable ; even to a cylindrical electrode having flat discharge plane of radius r = 0.05 cm, and assuming that the average tem-perature of the flame nucleus is 1,000K, then by substituting the Prandtl number of the air, Pr = 0.702 (at 1,000K), the kinematic viscosity, ~ = 1.201 cm2/s (at 1,000K), and the above-described electro-flame-wind velocity u ~ 3.3x103 cm/s into the eq. (45), Nu ~ 7.0 is obtained. Since the physical meaning of the Nusselt number is a ratio of a heat transfer rate in fluid to the heat transfer rate in still fluid, the heat transport in such high speed flow has .

: - lOS6iZ4Z
:
to be handled hydrodynamically, (b) with regard to relationship between thermal conductance G and electrode fluid resistance (i.e., fluid dra~) The thermal conductance G mainly depends upon the fluid drag of the electrodes, namely, the fluid resistance which is determined by the form, size and arrangement of , the electrodes. The reason therefor will be described hereinafter. Since the success or failure of the ignition is determined by the growth or decay of the flame nucleus of the nucleating volume V, the words "fluid drag of the electrodes" used herein is for the flow of the flame nucleus gas, namely, the electro-flame-wind inside the nucleating volume.
Generally, fluid drag working upon an object in viscous fluid is the sum of friction drag ~f and pressure drag p.
= f + p ......................................... (46) The pressure drag p is caused through a decrease of surface pressure of object downstream of the point of separation, due to boundary layer separation caused on the object surface. The pressure distribution upstream of the point of separation can be analyzed theoretically, while no method of correctly analyzing the pressure distributions is provided downstream of the point of separation, except that actual measurement is effected.
First, a case where the electro-flame-wind is a laminar flow and boundary layer is not separated will be described about symmetrically opposed electrodes with flat plane. In this case, the fluid drag of the electrodes is only the friction drag f and can be handled analytically. As shown -. . ~ .

:
: 1056'~4Z
. . .
- in Fig. 4, (a) and (b), an x axis (abscissa) is set in parallel to the electrode face and a y axis (ordinate~ is set in a direction vertical to the electrode face (Fig. 4 shows the sectional view of one electrode). For brevity's sake, assume that the thermal conductance to one electrode - and the fluid drag due to one electrode are equal for positive electrode and negative electrode respectively and are 1/2 of the entire thermal conductance and the entire fluid drag, respectively (actually, the position of nucleating volume is not at the center of the negative electrode and the positive electrode, but is closer to the negative electrode due to the electro-flame-wind. Accordingly, the thermal conductance to the negative electrode is greater than that to the positive electrode). As shown in Fig. 4, (b), the fluid velocity u = O is given, due to the bonding force - between the molecule and the solid surface in y = 0 (electrode surface). Hence, heat flow to one electrode face, namely, heat flux density q (x), per unit area and per unit time, is proportional to the temperature gradient (~T/~y)y=0 on the electrode surface. Assume that k is the thermal conductivity of the flame nucleus, then the heat flux density q(x) is given by q(x) = - K (ay)y=o ............................ ,.,, (47)~ ~
and (aT/ay)y=0 is approximated as follows:

(aay)y=0 X ~ .................................. ......(48), wherein T is temperature of the flame nucleus gas in the offing of the electrodes, To is electrode temperature, and ~T(X) is the thickness of a thermal boundary layer (region in which a temperature gradient exists) as shown in Fig. 4(a).
The ~T(x) is given by the following equation, wherein in the ~, , ' , ' .:

:

~, - lOS6~,4Z
.... . .
laminar flow, the temperature conductivity of the fluid is x, the fluid viscosity at the offing being u ; Accordingly, from the equations (47), (48) and (49), the total heat flow Q which flows, per unit time, to the symmetrically opposed electrodes with flat plane of width w = 1, and length x = Q is given, by the following equation:
. . .
. Q
. Q = 2 IO q(x)dx ~ -2~ (T-To)lO ~ dx ................. (50).

. On the other hand, the friction drag per unit area 10 a (x) of one~electrode is the flow of momentum per unit area i and per unit time, and therefore is proportional to the velocity gradient (au/ay)y 0 on the electrode surface. The fractional drag a(x) is given by the following equation, wherein . ~ is the viscosity:

~: ~(x) = -t (a~)y=o --------------.-.................. (51).
. The (a u/ay)y=o is approximated as follows:
' au (a Y)y=o x ~ - - - - - - - - - .. - ................................... (52).

~ The ~v(x) is the thickness of hydrodynamic boundary layer (region in which velocity gradient exists) as shown in Fig. 4, (b) and is given by:

Accordingly, from the equations (51), (52) and (53), the friction drag f which works upon the opposed electrodes .` with flat plane of width w = 1, length x = Q is given by the following equation:

Q Q
= 2lo ~(x)dx ~ -2~ulo ~ ( ) dx ....................................... (54)-.

. . , . ~
- . . ~ . .

~0~6;~4Z

From the equations (49) and (53), the following equation is given:

~T(x) ~ ~v(x) = - ~ (x) ............................ (55), wherein Pr is the Prandtl number. Accordingly, from the equations (50), (54) and (55), the following equation is given:
Q ~ r (T To)~ ....................................... (56).

On the other hand, the thermal conductance G iB defined by Q = G (T-To) (57) From the equations (56) and (57), the relationship between the thermal conductance G and the friction drag ~f is given by:

~ u i ........................................... (58).

Even when the electro-flame-wind on the flat plane electrode is originally turbulent flow, the following relation is given between the q(x),~and a(x) due to Reynolds analogy, t q(x) ~ P( )a(x) ..................................... (59), wherein T and u are the average temperature and average speed of the turbulent flow, respectively, and Cp is specific heat at constant pressure. Therefore, even in the turbulent flow as in the laminar flow, the relationship between the thermal conductance G and the fraction drag ~f is given by G ~ Cp ~j ........................................... (60) Independently of the laminar flow or the turbulent flow, the thermal conductance G is proportional to the friction :
drag ~f of the electrodes, when the flow velocities u & u are under a given constant condition and the fluid drag consists only of the friction drag. By substituting the .

-- lOti6~

- eq. (53) into the e~. (54) and effecting the integration, the following equation is giyen:

.' ~ ~ 4~u2Q2 v 2 It is found out from the above equation that, in the electrodes with flat discharging plane, as the length Q
decreases, the friction drag ~f with respect to the flow :
of the flame nucleus gas decreases in proportion to ~-.
.,~
~ As a result, the thermal conductance G is in proportion -to ~ and thus the condition of the ignition is improved ,10 as can be understood from the inequality (1). Generally speaking, in the electrode with flat discharging plane, ;~f and G decrease as dimensions, which govern the fluid resistance with respect to the flow of the flame nucleus gas, decrease, and therefore, the condition of the ignition is improved, thus allowing the lean gas mixture to be ignited and burned effectively. It is too complex precisely and perfectly to indicate the abovementioned "dimensions".
But, for the electrode arrangement that end face is the discharging face~ the "dimensions" can be indicated or represented by diameter for circular cylinder, by length of diagonal for square section cylinder or by length of the longest diagonal for polygonal section cylinder.

.
The above description is given only about the frictional drag ~f in the fluid drag ~ of the electrodes. In the case of the electrode having flat discharging plane with edges on the ends of discharge plane of the electrode, the separation of the boundary layer occurs at the edges.

Thus, wakes of the turbulent flow type are likely to be caused around the separated boundary layer. Viscosity, , ~OS~i~4Z
.-diffusion and thermal conduction are phenomena wherein momentum, constituent molecules and kinetic energy are transported by molecular motion, respectively, but the turbvlence transport, the momentum, the constituent molecules and the kinetic energy in extremely large scale (eddy transport). Accordingly, in the turbulence, apparent coefficient of viscosity, apparent diffusion coefficient and apparent thermal conductivity which are called as eddy viscosity, eddy mass diffusivity and eddy thermal diffusivity, respectively, become extremely large in value.
Thus, when the turbulent flow is caused, the pressure drag } ~p is produced and is added to the friction drag ~f. There- t fore, the total fluid drag ~ _ ~f + ~p increases, thus .~
`~ resulting in remarkable increase of the thermal conductance G, which remarkably deteriorates the condition of the ignition.
On the other hand, even in a thick electrode, if the discharging plane is made convex (streamline shape, without discontinuous edge such boundary layer separation as described hereinabove, namely, the turbulent flow occurrence can be prevented. Since the pressure drag ~p is hardly -caused, the total fluid drag ~ decreases by the amount of ~p(~=~p+~f~f)/ thereby effectively decreasing the thermal conductance G. As the thermal conductance G becomes also small, effectively, the condition of the ignition is improved remarkably. If the elèctrode is made streamline and thin in shape, the ~f is further decreased, thus resulting in further improving igniting conditions.
(c) Reasons why the conventional spark-plug can not ignite lean gas- mixture./
The streamline of the electro-flame-wind in the .

.: -- . :
.

- 105~6,'1;:4Z

; conventional type of spark-plug as shown in Fig. 5 is shown in section, in Fig. 41, (a~ and the structure of the boundary layers accompanied in this flow is shown in section, in Fig. 41, (a'). The boundary layer is supposed to be in ;~ such a shape, as in Bl, in the front face of the positive electrode 21 and in such a shape, as in B2, in the front face of the negative electrode 22, in accordance with the equations (49) or (53). As apparent from the above description, the friction drag ~f increases when the streamline passes through the boundary layer. As a wide plate, namely, of large in dimensions Q, is used for the positive electrode 21, the boundary layer Bl is thick and the ~f is large. Besides, the streamline with a mark * in Fig.
41, (a) shows the wake of turbulent flow type. The turbulent flow caused on the side face and the rear side of ; the positive electrode 21 is large on scale and the pressure drag~p also increases remarkably. As a result, the fluid drag ~=~f+~p~ hence, the thermal conductance G
; from the flame nucleus to the electrode is large, and the establishment of the ignition limit conditional inequality (1) becomes hard. Thus, such a conventional type of spark- -plug as shown in Fig. 5 can not sufficiently ignite the lean gas mixture as apparent from the curve W of Fig. 42 showing the ignition limit excess air ratio FL and electrode gap distance Ls characteristics.
On the other hand, as shown in Fig. 6, the FL ~ Ls characteristic curve W' of the conventional type of spark plug lies on the left hand of the curve W, thus resulting in some improvements. However, W' does not go upwardly, being different in quality from the behavior of Fig. 3, wherein the ignition limit characteristic curves displace .:
~, . . . :
i '' 10~6Z4Z
;'`''' ' ' leftwardly and simultaneously upwardly when G is made small as shown in Fig. 3. This fact will be described hereinafter.
Namely, the spark-plug of the curves W' is provlded with uneven grounded-electrode having U-shaped groove on a rectangular parallelepiped plate hence having many edges as shown in Fig. 6. Accordingly, these edges are likely to , .
'-; separate the boundary layer to cause the turbulent flow, ; and therefore the percentage of the pressure drag ~ in the fluid drag ~ is large. The pressure drag ~ is given by the following equation, wherein p is fluid density, u is flow velocity, Q is the "dimensions" of the object and ~ is a factor related to the shape of the object.
u2Q2 ......................................... (62).
As described hereinbefore the pressure drag ~p increases in proportion to the square of the flow velocity u (Newton's i~ law of resistanceJ.
Next, the dependency on spark gap Ls f the electro-flame-wind velocity u will be described. Discharge current is shows exponential decay concerning time t as follows:
i = io exp (~T / T), (io: discharge;current in t=O,T:decay time constant for discharge). T = 2.27 ms for LS=0.84 mm, and T = 1. 93 ms for IS=1.56 mm have been obtained from the oscillogram analysis of the is under a given condition of discharge energy. Namely, as the electrode gap distance Ls is made larger, the discharging ceases in a shorter time, and thus the energy consumption rate (speed) increases.
Since one portion of the discharge energy is converted, into the kinetic energy of the electro-flame-wind, which is greatly subject to the influences from the cation drag, an increase in the energy consumption rate causes the increase in the electro-flame-wind velocity u. Accordingly, an . ~ ' , .
~ .

` ~os~jz4z increase of the electrode gap distance Ls causes an increase of the electric flame fluid velocity u, and therefore in accordance with the eq. (62) the pressu-e drag ~p, hence, the thermal conductance G increases. In the conventional type of spark-plug as shown in Fig. 6, the characteristic curves of the excess air ratio FL of lean mixture side ignition limit vs, electrode gap distance Ls of Fig. 3 are plotted for various G as parameter. Therefore, as Ls increases, the operation point moves from curve to curve as d -~ c -~ b -~ a in Fig. 3. As a result, the upward movement of the FL ~ Ls characteristic curves is prevented remarkably. Accordingly, the characteristic shown in , the curve r~ of Fig. 42 are provided, thus making easy ignition of the lean gas mixture hard. The result of the abovementioned theoretical analysïs is as follows. When the electrodes of the spark-plug has the form, size and arrangement for decreasing the fluid resistance against the ; electro-flame-wind contained inside the nucleating volume, then the thermal conductance G can be made small, thus increasing the excess air ratio FL of the lean mixture ignition limit in accordance with the relationship of the inequality (1). In the conventional spark-plug, the fluid resistance of the electrode with respect to the electro-flame-wind has not been taken into consideration and the thermal conductance G has been large, and hence the spark-plug itself has had a flame-arrestive characteristic so that the flame nucleus once made has been liable for the annihilation.
From the theoretical analysis of the above description, the requirements on the form, size and arrangement of the electrodes which allow the spark-plug to ignite the'lean gas :`
" 1~5;6Z42 t mixture are found as follows;
(1) The positive and negative electrodes shall be thin and be disposed in coaxial relation so that the axes of both electrodes may be aligned in line with each other.
For the electrodes of the present invention, thin electrodes, for instance, rod-shaped or cylindrical electrodes can be used as elucidated hereinafter referring to the examples. Hereinafter, the word "rod" is used to imply a cylinder or analogous one which includes any types of cylinder, for instance, circular cylinder, square section cylinder (square prism), polygonal section cylinder, etc.
or slightly tapered cylinders wherein one bottom is slightly larger than the other.
(2) At least one of the electrodes shall have discharging ~- face made convex surface (in streamline shape) and be disposed in coaxial.
- (B) Embodiment: ~
: ' First, the condition for experiments to obtain the excess air ratio Fc of ignition limit (especially, to obtain the excess air ratio FL of the ignition limit for the lean mixture side) in each embodiment of the present invention will be described.
In order to precisely obtain the excess air ratio FL of ignition limit (degree of leanness of fuel), isobutane of 100% in degree of vaporization was used. The isobutane is a component of liquefied petroleum gas (LPG) having the ignition temperature 673K which is above the ignition temperature about 523K for gasoline. Accordingly, the isobutane is usable as sample fuel in this experiment.
For power supply, an apparatus which generates peak voltage of 35KV at no-load by operating an ignition coil .
(rating: primary coil for 12 V, 4.1A, inductance of 8.58 mH, - stored energy 72 mJ) by means of a transistor switch was used.
., The power supply was connected to the spark-plug from the positive terminal to the grounded electrode (a first electrode) , of the spark-plug and from the negative terminal to the : high-tension electrode (a second electrode).

It is desirable that electrode gap distance is 0.8 mm .~,.
or less when the ordinary ignition power supply of the internal combustion engine is used. From measurements and calculations, the "dimensions" of the electrodes of the spark-plug, which allows the characteristic curves of the excess air ratio of ignition limit FL vs. the spark gap s to cross an equivalent air fuel ratio straight line X
~ when Ls-0.8mm, are obtained as described hereinafter.
i` Namely, for the electrode of which front end is a discharging face, 1.7 mm of diameter when the cross-sectional view of the electrode was circular, and 1.7mm of diagonal line when `~ the cross-sectional view thereof is rectangular are to be used as the dimensions. For the electrode, of which side face is the discharging face, 1.2mm width and 2mm of thickness are to be used as the dimensions. The width used here is the size of the electrode face measured in the direction having right angle to the discharging direction and in the widthwise direction of the electrode, while the thickness used here is the size of the rod-shaped electrode along the discharging direction.

Therefore, according to the present invention, in a rod-shaped electrode, in which the front end is of discharging face, the diameter should be 1.7mm or less if the cross-sectional plane of the electrode is circular, and thediagonal line should be 1.7mm or less if the cross-sectional 10~i6Z4Z
.~

plane thereof is rectangular, in the rod-shaped electrode, in which a side face is of discharging plane, the width should be 1.2mm or less, and the thickness should be 2mm or less.
The automobile internal combustion engines of lean gas mixture combustion type according to this invention comprises a combustion chamber, a lean gas mixture producing device for producing the lean gas mixture having F>l in operation modes including idling, engine-braking, constant speed, acceleration and deceleration, a compressing means for compressing lean gas mixture containing air and fuel by varying volume of the combustion chamber and at least one electric spark-plug for igniting the compressed lean gas mixture.
The abovementioned spark-plug can be constructed as described in the following.
The following examples 1 to 4 relate to spark-plugs comprising a pair of rod-shaped electrodes each with flat discharging face, for first and second electro`des.
Example 1 (Fig. 7) As shown in Fig. 7, (a), (b), (c) and Table 3, the spark-plug 200 of the present invention comprises a metallic screw part 29 to be engaged with the internal combustion engine, an electrode terminal part 27, an insulator 24 for supporting the above-mentioned screw part and the electrode terminal part in given relative positions, keeping them insulated from each other, and a pair of electrodes, namely a first electrode 21 and a second electrode 22. The first electrode is installed on a supporter 23 which is mounted on the metal screw part 29.
The second electrode 22 is a rod-shaped member, which is electrically-connected to the electrode terminal part 27 by ``:
)5~4Z
~' .
means of a central conductor 26, and is arranged in parallel ; with the central axis of the metallic screw part 29 and ~- is supported by means of the insulator 24, so that a given ignition gap distance is provided between the given part on the first electrode 21 and the rod-shaped member 22.
' Numeral 28 designate a gasket.
The first electrode 21 is a rod-shaped electrode, which ':
is projected, by a given height hi, e.g. lmm, from the end of supporter 23 made of heat-resisting nickel alloy, etc., towards the second electrode 22, in the direction of the central axis of the metallic screw part 29 having at its front end, ` a discharging plane normal to the central axis. The first electrode is installed on the supporter 23 through methods of welding, driving, forced insertion, or caulking after insertion, etc. In order to simplify the machining operation, ; it is recommended that the first electrode 21 and the supporter 23 should be composed of a continuous member.
The above-mentioned structure and machining can be applied even to the spark-plugs of Examples 2 to 17 given herein-after. The second electrode 22 is a rod-shaped electrode, which is projected, by a given height h2, e.g. lmm, from the end of insulator 24 and has a discharging plane normal to the central axis. The first and second electrodes 21 and 22 are disposed coaxially with spark gap of, for example, 1.28mm inbetween.
In the present embodiment, when a rod-shaped electrode of from 1.7mm to 0.3mm in diameter and preferably, about ; lmm in diameter are used, it is desirable for the sake of durability, to use noble metals, which are superior in heat resisting and electro-corrosion resisting properties, such as Pt, Pd, Au, or alloy thereof.

: ~05~iZ4Z
In the spark-plug in this e~bodiment, both electrodes have small diameters, respectively. As shown in ~ig. 41, (b'), both boundary layers Bl and B2 are thin and the friction drag ~f is small as apparent from the eq. (61).
Therefore, from the equations (58) and (60), the thermal conductance G from the flame nucleus to the electrodes becomes small. As a result, the condition of ignition is improved as is indicated by the inequality (1). The FL-L
characteristic curve A of Fig. 42 (The curve A corresponds to a case where the projection height of grounded electrode hl = 1 mm and the raised height of high-tension electrode h2 = lmm, as shown in the line A of Table 3) of the spark-plug of this embodiment lies remarkably on the left-upward position as compared with the characteristic curves W & W' of the conventional spark-plug.
As described hereinbefore, the condition of ignition is remarkably improved when both electrodes 21 and 22 are ; projected and raised from the supporter 23 and the insulator 24, resepctively. The degree of the improvements depend upon the projection height hl and~or h2 as described hereinafter.
The speed (u) of the electro-flame-wind produced in a position in the electrode gap decreases as the position departs farther from the gap. Accordingly, the thickness ~3 of a boundary layer which is formed on the surface of the supporter 23 is considerably large as compared with the respective thickness ~1 and ~2~of boundary layers Bl and B2.
The influences of ~ at least, on viscosity frictional loss can be neglected under the condition of hl + ~1 > ~3. There-fore, only the electrode thickness effect Q should be considered. Accordingly, the ignition limit air fuel ratio FL must be almost saturated for above a given critical , - 4~ -: ~5624Z

projection height hl. Inversely, in the projection height hl under the condition of hl + ~ 3, the effect of the viscosity frictional loss due to the thick boundary layer of ~3 thickness along the supporter 23 exists, and thus the ignition limit air fuel ratio FL depends upon the projection height hl. The results of the experiments by which the above description is proved are shown in Fig.
46 and in Table 3, lines Al to A6. Fig. 46 shows the experi-mental results measured concerning the relation between the projection height hl of the first electrode and the ignition ; limit air fuel ratio FL of the spark-plug shown in Fig. 7, with the spark gap Ls as a parameter. In the experiments, a metal-plate supporter 2.7mm wide, about 5mm long is used as the supporter 23, while the cylindrical electrodes of lmm in diameter are used as the first electrode 21 and the second electrode 22, respectively. The raised height h2 of the second electrode is lmm. Also, in Fig. 46, curve Ul shows the measured results of Ls = 0.85 mm; curve U2, Ls=0.9mm;
curve U3, Ls=l.Omm; curve U4, Ls=1.25mm; curve U5, Ls=l.Smm;
and curve U6, Ls=2.0mm, respectively. As apparent from Fig~
46, in each characteristic curve, the ignition limit excess air ratio FL i5 almost saturated for the range of hl > 0.25mm.
Namely, the critical value of hl is hl=0.25mm. Thus, it is desirable for the projection height hl to be larger than 0.25mm to ignite the lean mixture. Since the end of the supporter 23 (Fig. 41(b')) is retreated from the electrode gap by hl, the turbulent flow is difficult to be produced on the side face and the rear side. Accordingly, the pressure drag ~p, hence thermal conductance G decreases further and the curve A of Fig. 42 goes further leftwards and upwards with respect ` ~ ~ lOS6~4Z
to the curves W and Wl of Fig 42. Accordingly, the leaner ~as mixture can be ignited.
The effect of projection of the first electrode 21 from the supporter 23 as described hereinbefore is similarly realized, if the second electrode 22 is raised from the end of insulator 24.
Examples 2 to 17 which will be described hereinafter relate to spark-plug, with first electxode 21 and the second electrode 22 both raised from the supporter 23 and the insulator 24 by 0.25mm or more, respectively. Accordingly, in the examples 2 to 17, given hereinafter, the forms, sizes and arrangements of the electrodes only and effects thereof will be described, omltting descriptions on common items.
. . .
~ Referring to the drawings of the spark-plug in all .:.
examples, the same reference marks and numerals as those - for Example 1 will be given to all the corresponding ~ components.
.; .
Example 2 (Fig. 8) ~~ 20 The spark-plug in this embodiment shown in Fig. 8, (a) and (b) has two pairs of electrodes. A pair of first electrodes 21, and 21 are rod-shaped electrodes, which are - -projected from the supporters 23 and 23 by a given height .
~-~ hl, respectively, towards a second electrode 22 and 22, in the direction normal to the central axis of the metallic screw part 29. The electrodes have in their front end, discharging faces in parallel with the axis, respectively.
The second electrodes 22 and 22 are rod-shaped electrodes, which are raised by a given height h2, from the central axis ` 30 26 in the direction vertical to the axis and have a dis-charging planes in parallel to the axis, respectively. The , , ~OS624Z

first electrodes 21 and 21 and the second electrodes 22 and 22 are disposed to allow their discharging planes to face each other, respectively, with the electrode gap distance Ls between each pair of the electrodes 21 and 22.
By providing two pairs of electrodes, the spark-plug of this embodiment has an advantage, in terms of durability (service life), of having longer life ~han the spark-plug of Fig. 7 (Example 1). It is also recommendable that three pairs of electrodes or more should be provided to obtain further increased service life.
Example 3 (Fig. 9) . .
In the spark-plug of this embodiment shown in Fig. 9, a first electrode 21 is a rod-shaped electrode, which is projected by a given height from a supporter 23 towards below the second electrode 22 in the direction vertical to the central axis of the metallic screw part 29, and has ~ ,.
discharging face on its upper side face, while the second electrode 22 is a rod-shaped electrode, which is raised, by a given height, from an insulator 24, and has, at its front end, a discharging face normal to the axis. The lengthwise axis of the first and the second rod-shaped electrodes 21 and 22 are disposed at right angle with each other. As apparent from the value in line B, column Ls* of Table 3, the effects similar to those of Example 1 are obtained by this spark-plug.
Example 4 (Fig. 10) As shown in Fig. 10 and Table 3, C line, in the spark-plug of this embodiment, a first electrode 21 is a rod-shaped electrode, which is projected, by a given height, from a supporter 23, towards a second electrode 22, in the direction parallel to the central axis of the metallic screw ; :

~ lVS6Z4Z
part 29, and has on its side face, a discharging plane which is parallel to the axis,while the second electrode 22 is a rod-shaped electrode, which is pro,ected, by a given height, from an insulator 24 and has, on its side face, a discharging ` plane parallel to the axis. The first and second rod-shaped electrodes 21 and 22 are disposed in parallel to each other with their discharging faces of respective given lengths QQ (for example lmm) opposing to each other. The opposing lengths QQ of both electrodes are determined through con-sideration of electro-corrosion-resisting property, namely, service life. As apparent from the value of Table 3, line C, column Ls*, the effects similar to those of the Example ~ 1 are obtainable also by this example. In this spark-plug, -~ area of discharging face increases in proportion to the opposing lengths QQ of both electrodes. Accordingly, the spark-plug in this Example 4 is superior, in electro-corrosion resistivity.
In the rectangular type spark-plug of the Example 3 (Fig. 9), one of the electrodes, or a first electrode is shaped so as to have its axis having right angle with sparking lines. Accordingly, the boundary layer Bl shown in Fig. 41, (b'), spreads, also to the axial direction of the electrode. Also, in the spark-plug of Example 4 (Fig. 10), both electrodes have their axis at right angles with the sparking lines, and thus the boundary layers Bl and B2 spread also in the axial direction of the electrode.
Therefore, the friction drag ~f increases.
Accordingly, from the equations (61), (58), and -inequality (1) and from the experimental results, it is generally obvious that establishment of the ignition condition is more advantageous for the right-angle-arranged ~ .

S6'~,4Z
, rod electrodes type than that of a parallel rod electrodes type, and further advantageous for coaxial-arranged rod ;, .
electrodes type. The effect of this electrodes arrangements are also obtainable in the following examples 5 to 17.
.
Table 3 (Examples 1, 3 and 4) Dimensions Dimensions Marks of (mm) of (mm) of ; F -Ls Electrodes Grounded High-Tension C~aracter- Types of Arrange- Electrode Electrode L *
10 istic ~lectrodes ment (First (Second s Curves Electrode 21) Electrode 22)(mm) . . _ A Fig. 7coaxial 1.0 diameter 1.0 diameter 1.28 1.0 hl 1.0 h2 (Al) Fig. 7 - 2.7 width 1.0 diameter 1.3 thickness 1.0 h 2.28

5 length 2 :~ O hl (A2) Fig. 7coaxial 1.0 diameter 1.0 diameter 2.06 0.07 hl 1.0 h2 ~:i 20 ( 3) Fig. 7coaxial 1.0 diameter 1.0 diameter ; 0.14 hl 1.0 h2 1.81 (A4) Fig. 7 coaxial 1.0 diameter 1.0 diameter 0.25 hl 1.0 h2 1.59 (A5) Fig. 7 coaxial 1.0 diameter 1.0 diameter 0.5 hl 1.0 h2 1.45 (A8) Fig. 7 coaxial 1.0 diameter 1.0 diameter 2.0 hl 1.0 h2 1.28 (B5) Fig. 9 with right 1.0 width angle 0.62 thick- 1.0 diameter 1.52 ness 5 length (C)Fig. 10 parallel 1.0 width 1.0 width (with 0.62 thick- 1.0 thick-opposing ness nesS 1.52 lengths 5 length 5 length QQ=l.Omm) Note: Curves for (Al) to (A6), (B5) and (C) are almost analogous to that of A, and therefore, are not shown in the graphs.
The belowmentioned examples 5 to 11 relate to spark-plugs ,-each comprising the first and second electrodes, at least one :: ~

: - 1056Z4J2 of which has convex discharging face.
Example 5 (Figs. 11 to 13 and 15~
As shown in Fig. 11, in the spark-plug of this embodiment, a first electrode 21 is an electrode, which is projected by ", a given height hl towards below a second electrode 22 in the axial direciion, and has at its front end a flat discharging face with right angle with the axis, while a second electrode 22 is an electrode, which is projected, by a given height h2 from an insulator 24 and has at its front end a convex discharging face. And the first and second electrodes 21 and 22 are disposed coaxially with each other.
In order to provide further imrpoved ignition char-'~ acteristics, it is recommended that both electrodes should have convex discharging faces at respective front end as shown in Fig. 12, and Fig. 13.
As shown in Fig. 41, (c) and (c'), the boundary layers Bl and B2 concerning the electro-flame-wind on the electrodes of the spark-plug of, for instance, Fig. 12 are both retreated with respect to and from the flame nucleus space, the dimensions Q effectively become small and friction drag ` ~f decreases. Simultaneously, both electrodes are stream-lined, thus resulting in smaller occurrence of the turbulent flow, and the pressure drag ~p is extremely reduced. Accord-ingly, the fluid drag ~ = ~f + ~p and the thermal conductance G becomes small, and the FL-LS characteristic curve consider-ably moves leftward and upward with respect to the curve W as shown in Fig. 42, curves E3 and E6. Therefore, suf-ficiently lean gas mixture can be ignited. The above-mentioned effect of retreating the boundary layer increases as the curvature v of the discharging plane increases.
The above conclusion is apparent from the F~-LS characteristic ; ~

05;6Z4Z

curves Eo to E3 of Fig. 43 and Fi~. 4 7, which indic~tes that in the practical range of an electrode gap Ls of less - than 2mm, the excess air ratio of ignition limit rapidly increases with the curvature V and saturates in the region ;
~.`' around the numerical value 0.46mm 1 of curvature. Curves :
Eo to E3 are for the cases of various curvatures Eo to E3 shown by Fig. 15 and in Table 4 lines Eo to E3, column C.
Fig. 43 shows measured characteristic curves indicating the relationship between the ignition limit excess air ratio 10 FL and the electrode gas distance L , in a case where the electrodes have diameters fixed at 2. 55 mm, and the curvature ; v only on the discharging face of the electrode front end is changed variably as shown in Table 4. Fig. 47 shows measured characteristic curves of dependence the excess air ratio FL of ignition limit upon the curvature v of the electrode discharging face with the electrode gap distance ~ Ls for the curves Eo to E3 of Eig. 43 as a parameter. In ; Fig. 43, only the curve E3 lles above the curves Eo~ El and E2. In the electrode of the curve E3, the front end 20 discharging face is of a hemispherical type and connects with cylindrical side face smoothly without edge, accordingly no turbulent flow is produced. On the other hand, as shown in Fig. 15, in the electrodes of the curves Eo, El and E2, the convex discharging faces and the electrode side faces t make edges inbetween, so that a turbulent flow are caused.
Needless to say, it is understood from Fig. 42, curve D, that the ignition characteristics of the aforementioned spark-plug of a type shown Fig. 11 and in Table 4, line D
is between those of the spark-plugs represented by the above-30 mentioned curves E3 and Eo~
From comparison of curves E3 to E6 it is apparent that - 10~24Z
.: , .
the condition of the ignition is further improved when the electrode diameter is made small and the curvature v on the discharging face is made great.
` In this embodiment of Figs. 11 to 13 the first electrode 21 is projectedly secured to the supporter 23 by such a method described in Example 1.
;:.j Example 6 (Fig. 14) As shown in Fig. 14, the spark-plug of this embodiment is analogous to the spark-plug in Fig. 12 in the Example 5.
That is, a first electrode 21 is an electrode, which is projected by a given height hl towards below a second electrode 22 in the axial direction, and has at its front end, a . .
convex discharging face, while a second electrode 22 is an electrode, which is projected, by a given height h2 from an . . .
insulator 24 and has at its front end a convex discharging face. And the first and second electrodes 21 and 22 are " disposed coaxially with each other.
In this embodiment, the first electrode 21 is projected, on supporter 23, to form a convex discharging face through a knock-out method. Accordingly, the present example has an advantage of easy manufacture.

,:
Example 7 (Figs. 17 to 21) As shown in Figs. 17 to 21, a first electrode 21 is projected from a supporter 23 by a given height, towards below a second electrode 22 vertically-with respect to the axis, and has on its side face, a convex discharging face, while a second electrode 22 is projected from an electric insulator 24 by a given height, and has at its front end, a convex discharging face. Thus, a tip side face of the first electrode faces the end of the second electrode and axes of both electrodes are disposed at right angle with respect ; l~S624Z

to each other. As apparent from ~alues in Table 4, Gl to - G3 lines, column LS , similar effects to those of Fig. 12 and Fig. 13 (Example 5 with both electrodes with convex discharging faces) are obtainable. As described hereinbefore, the spread of the electro-flame-wind is large enough to reach the rear face of the discharging plane. Accordingly, it is advantageous to use a streamline-shaped electrode for preventing occurrence of turbulence and resulting in effective improvements of the ignition condition. And the streamline-shaped electrode should have smooth curved convex face, wherein the discharging face is smoothly connected with the circumferential side part of the electrode. Similarly, it is desirable that the front end part and adjacent part of the electric insulator 24 and screw portion 29, where the electro-flame-wind or the flame nucleus during its growing stage contact, should be formed with gently-curved surfaces as shown in Figs. 13, 20 and 21. The first electrode 21 and the supporter 23 are formed in such continuous shape as in Fig. 19, and therefore are manufactured easily through 20 a drawing using a drawing die.
Example 8 (Fig. 22) As shown in Fig. 22, in the spark-plug of this embodi- t ment, a first electrode 21 is projected, by a given height, towards a second electrode 22 in a direction parallel to the central axis of the metallic screw part 29, and has, on its side face, a convex discharging face, while a second electrode is projected, by a given height, from an electric insulator 24, and has, on its side face, a convex discharging face. The first and second electrodes 21 and 22 are dis-30 posed in parallel relation with given opposing lengths QQ
(for example, 2mm excepting hemispherical end parts) between -: : - ~ '' ' . :
- ~

~OS624Z
,.
the electrodes. As apparent from the Ls* value of lines Hl, H2 of Table 4, the effects similar to those of Figs.

12 and 13 (Example 5) are obtainable. The opposing lengths QQ of electrodes 21 and 22 should be determined through consideration of electro-corrosion resisting properties, i.e., service life.

Example 9 (Fig. 23) . . .
As shown in Fig. 23, in the spark-plug of this embodi-,; ment, a first electrode 21 is projected from a supporter 23 .
by a given height, in a direction vertical to the axis towards below a second electrode 22 and has on its side ` face (actually, upper face), a convex discharging face, while the second electrode 22 is a rod-shaped electrode projected from an electric insulator 24 by a given height, and has at . .
its front end, a flat discharging face, and the axes of the first and the second electrodes 21 and 22 have right angle .,: -~, between each other. The spark-plug of this example is formed by a combination of a rod-shaped second electrode 22 with small friction drag ~f, and a streamlined first electrode 21 with small friction drag ~f and pressure drag ~p. As shown in the Ls value of lines Il, I2 of Table 4, ,.
the condition of ignition is improved through combined operation of the both electrodes.
Example 10 (Fig. 24) As shown in Fig. 24, in the spark-plug of the embodiment, a first electrode 21 is a rod-shaped electrode, which is projected vertically, by a given height,-towards below a second electrode 22 in a direction vertical to the central axis of the metallic screw part 29 and has, on its side face, a flat discharging face having right angle to the axis, while a second electrode is an electrode, which is :

. .

~ ;

-` 10~6Z4Z
.: , projected, by a given height, from an electric insulator 24, and has at its front end, a convex discharging face.
The first and second electrodes axe disposed at right angle with each other. In the spark-plug in this embodiment has a combination of thin first electrode with small friction ; drag ~f and streamlined second electrode with small friction drag ~f and pressure drag ~p. As shown in the Ls line of Jl~ J2 of Table 4, the condition of ignition is improved through the combined operation of the both electrodes.
Example 11 (Fig. 20) Both electrodes 21 and 22 of the spark-plug in this embodiment, shown in Eig. 20 and Table 4, line G3, are both streamlined. The dimensions, such as thickness, of the positive electrode 21 as a first electrode are smaller than those of the negative electrode 22 as a second electrode.
The dimensions should be the size measured in the direction having right angle to the discharging direction, for example, diameter for circular cylindrical electrode, diagonal line for square section cylinder electrode. Such structure of the electrode was determined, based on the new knowledge of the inventors that a positive electrode is not directly hit by the electro-flame-wind, and no defacement of the electrode due to the direct hit is made, and accordingly, the dimensions of the positive electrode 21 can be smaller or thinner than those of the negative electrode 22, without any sacrifice of the spark-plug service life.
As shown in Fig. 41, (d) and (d'), space occupied by the boundary layer Bl on the positive electrode 21 is small ,:

:, .

~S6Z4Z

and the turbulence is hard to occur~ In addition, the boundary layer B2 on the negative electrode 22 is retreated from the flame nucleus space. Accordingly, the fluid drag ~ = ~f 1 ~p and the thermal conductance G are extremely small. Further-more, expansion waves due to the electro-flame-wind are hard to occur in the space ~ on the front face of the positive electrode 21 and the low density space of molecules are hard to form. Thus, the establishment of chain combustion is easy from the inequality (1). Through the above-described actions, the FL-L characteristic curve G3 of the spark-plug of a type shown in Fig. 20 lies on the left and above part of Fig. 44, and therefore the lean gas mixture can be ignited sufficiently.
In the service life tests, the Ni alloy negative electrode (2.55 mm in diameter) of the spark-plug of this embodiment was less in defacement than the conventional negative electrode (lmm in diameter) of expensive heat-and-defacement resisting Au-Pd alloy (Fig. 5). Accordingly, the spark-plug of this embodiment has advantages that the low-priced Ni alloy can be used and the longer service life is provided.
In order to further improve ignition characteristics, it is recommended that the negative electrode diameter for -spark-plug of this embodiment should be as thin as, for example, 1 mm.
Also, in order to provide increased mechanical firmness of the thin positive electrode 21, it is recommended that ;
both ends of the positive electrode 21 should be secured to the screw part 29 as shown in Fig. 21.

.

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The next Example 12 relates to a spark-plug, wherein at least one of said electrodes circumferential portion connected to the discharging face is formed streamlined.
Example 12 (Figs. 25 to 28) ; As shown in Fig. 25 and 27 and in the line Kl of Table 5, in the spark-plug of this Example, in at least one (having diameter of d) of a first and a second electrodes 21 and 22, the end faces are made approximately flat circular (having radius r) discharging faces Sl- Sl, and the circumferential part which is connected to the discharging face Sl-Sl has convex face Sl-S2 connected to the discharging face Sl-Sl, without edges between its circumferential part and the discharging face Sl-Sl. Such structure leads to an advantage that, as shown by Fig. 44, curves Kl for this Example 12 and A for Example 1, respectively, the ignition characteristics of this Example 12 are improved in region for such wide electrode gap Ls>lmm, wherein a turbulent flow is likely to be produced because of speed-increasing '~ electro-flame-wind.
Also, the electrode of the spark-plug of this Example ;- can be made by slicing the front end part of the hemispherical end face shown in Fig. 12, thereby to form circle plane of, for example, 1 mm in diameter. Thus, in terms of the igniting characteristics, as shown in Fig. 44, the char-acteristic curve Kl of this spark-plug is slightly poorer in the range of Ls qmm than the characteristic curve E3 of the spark-plug of Fig. 12. However, this E~ample 12 is remarkably superior in terms of durability, since the dis-charging spots are not concentrated at one point.
As described hereinbefore, such spark-plugs having the pair of an electrode with almost flat discharging face of small area and turbulence-preventing stream-lined (convex face) circumferential part connected to the discharging face, both facing each other, has almost superior features of the FL-LS characteristics of the spark-plugs wherein each ` electrode has turbulence-preventing streamlined discharging face, and furthermore is superior in durability due to the construction to prevent the discharging spots from concen-~` trating in one point.
As apparent from the values of line K2 or K3, column LS* of Table 5, the effects of the above Example 12 are obtainable even in a spark-plug shown by Fig. 26 and Table 5, line K2 where both electrodes are disposed at right angle with each other, and even in a spark-plug of Table 5, line K3 where both electrodes are disposed in parallel to each other. Fig. 28 shows a magnified sectional view of the posi$ive electrode 21 of Fig. 26. Referring to the drawing, w designates the width of the discharging face Sl-Sl, Sl-S2 designates streamlined convex side face for preventing the occurring of the turbulence, d designates an electrode diameter , - 64 - ~

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The following Examples 13 to 16 relate to spark-plugs with electrodes, at least one of the electrodes having a recess on a part of its fac~ that is facing the other electrode.
Example 13 (Figs. 29 to 33) ~' In the spark-plug of this embodiment, as shown in Fig. 29 and 30, a first electrode (diameter d) 21 is projected by a given height from a supporter 23 vertically towards below a second electrode 22 in a direction vertical to the central axis of the metallic screw part 29, while the second electrode ' 22 is projected by a given height from an electric insulator . .
24, at least one, for example, the first electrode 21, of , ., the first and second electrodes 21 and 22 having a recess on a part of its face that is facing the other electrode 22.
The recess c (depth h, length 1) is formed not in a part directly facing the discharging face s of the second electrode :: ~
22, but in another part adjacent to the abovementioned direct-, facing part. The discharging face of the first electrode 21 and a circumferential part coupling to this discharging face ~,:
form a convex face (streamline type).

j~ The recess serves to prevent the occurrence of turbulence, '~.' namely, the pressure drag ~p and to reduce the thickness of the boundary layer. The prevention of the occurrence of turbulence is made by inhaling such part of the boundary layer on the long electrode, that is close to the electrode wall and hence is liable to separation through losing kinetic energy due to shearing stress on wall plane. The reducing . .
of the thickness of the boundary layer is made by retreating the boundary layer from the flame nucleus space. In addition, the first electrode has the convex discharging face, and therefore, the friction drag ~f and the pressure drag ~p ., are both satisfactorily small. Therefore, the fluid drag becomes small and the thermal conductance G becomes small to improve the condition o~ ignition remarkably.
Also, the effect given by the recess c in this spark-plug electrode is the same as that given by the projection of electrode described in Example 1. Accordingly, it is desirable to make the depth of the recess c as deep as t approximately 0.25mm or more, like the projection height h and/or h2 of Example 1 being so.
Fig. 31 shows a spark-plug electrode, having a small flat discharging face s (width w) made by slicing a part of the convex discharging face of the spark-plug electrode of Figs. 29 and 30. A spark-plug using this electrode has similar superior ignition characteristics to those of Figs.
- 29 and 30. Besides, the Example of Fig. 31 is advantageous in terms of durability, since the discharging spots are not concentrated in narrow part.
Also, in order remarkably to simplify manufacturing, as shown in Figs. 32 & 33, and Table 6, line M, a spark-plug may also be made by using a s~uare rod electrode of width w, ~ thickness z with a recess c (depth h, length i). In Fig.
-~ 33, s is discharging plane). The recess is not in a part ; of a face that is facing the other electrode but in another part ad~acent to the direct-facing part. This spark-plug is also improved in the condition of the ignition due to small pressure drag ~p and friction drag ~f as shown in the Ls* value of the line M of Table 6.
Example 14 (Figs. 34 and 35) As shown in Fig. 34 and Table 6, line Nl, in the spark-plug of this embodiment, a first electrode 21 is projected by a given height from a supporter 23 vertically towards ~ ~S6Z~Z
below a second electrode 22 in a direction vertical to the ' central axis of the metallic screw part 29. And the dis-charging face and the circumferential part are forming -continuous convex face (stream-line shape). The second electrode 22 is projected by a given height from an i insulator 24. And the discharging face and the circum-ferential part are forming continuous convex face (stream-line shape). At least one electrode of the first and second electrodes 21 and 22 has a recess c in the dis-charging face, of the one electrode (for example, first electrode 21), which is directly facing the discharging ;~ face of the other electrode (for example, the second ` electrode 22).
As shown in Fig. 41, (f) and (f'), in the spark-plug of this Example, the expansion waves, which caused in the space ~ of the discharging face of the first electrode due to the electro-flame-wind are offset by gas molecules supplied from the recess c as reservoir of gas. Accordingly, the discharging face of the first electrode 21 is not liable to become the space of low molecular density. Thus, the E
in the inequality (1) does not become small, and accordingly, ` the establishment of chain combustion becomes easy.
` Furthermore, as shown in Fig. 41, (f'), this spark-plug has an effect of reducing ~f, since as a result of forming the recess c the boundary layer Bl formed along the first electrode 21 becomes thinner than the boundary layer of the first electrode (for example, Fig. 41 (e')) not equipped with recess c. Furthermore, since both electrodes are formed streamlined, the turbulences are hard to form, and therefore, the ~p becomes small.
Such FL-LS characteristics of the spark-plug of this , ~

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Example as shown in the curve Ni of Fig. 45 are obtainable, through the compound efects, of preventing the formation of the space of low density of molecules, reducing the boundary layer Bl in thickness and preventing the occurrence ; of the turbulent flow due to streamlined shape. Namely, not only in a relatively long electrode gap region, but also in a relatively short electrode gap region, ignition of lean mixture can be made sufficiently.
Also, in order to provide further improved ignition characteristics, as shown in Fig. 35 and Table 6, line N3, . it is recommended coaxially to arrange the first electrode ~; 21 with gas reservoir recess c and the second electrode 22.
~ Through this arrangement, the dimensions Q of the first -.
~ electrode 21 can be made effectively small and the friction ', drag ~f and the thermal conductance G decrease. Accordingly, as shown in the curve N3 of Fig. 45, easy lean gas ignition ' is further made possible. The same operating effect as described hereinbefore is obtainable even with a through hole as the gas reservoir recess c of the first electrode 21.
~;s sh^wn in the Ls* value of line N2 of Table 6, the lean mixture ignition can be effected sufficiently with such through hole. This spark-plug has advantages in that the gas reservoir hole hardly be clogged with dusts, and easier in cleaning when the reservoir hole is closged with dusts.
It is also recommended to arrange the electrodes 21 and 22, at least one of which electrodes has said recess, in parallel to each other with their discharging faces of respective given lengths opposing to each other, in order to improve the electro-corrosion resistivity.
30 Example 15 tFig. 36) -As shown in Fig. 36 and in Table 6, line N4, a second :, : . ., : . ~ , , - lOS62~'Z

, , electrode 22 as a negative electrode has such gas reservoir recess c.
. .
In a spark plug of this Example shown in Fig. 41, (g), ; (g), electric field is negligibly small inside the recess c.
.
-` Accordingly, when the electro-flame-wind, which has been accelerated in the electric field in the electrode gap Ls, rushes into the recess c and the electro-flame-wind is not ,; .
accelerated any more. Accordingly, the positive ions in the ~ electro-flame-wind lose the momentum, the kinetic energy, ?~
- 10 etc. while colliding against the neutral molecules, and are sufficiently decelerated before positive ions reach the wall of the recess c. In other words, the electro-flame-wind ' can effectively deliver its own thermodynamic quantity to the unburned mixture inside the recess c, and on the other hand ` the collision of the electro-flame-wind against the negative :, j~ electrode wall ~ is relieved. Thus, the direct loss of the :.
~; thermodynamic quantity caused through the collision of the electro-flame-wind to the negative electrode is reduced remarkably. Furthermore, this spark-plug has more effect of thinning boundary layer B2 formed along the negative electrode 22 (as shown in Fig. 41(g)) than that of the negative electrode that has no recess (e.g. Fig. 40, (e')), and hence, the f is also reduced. sesides, the turbulent flow haxdly occurs because of the streamline shape of both electrodes 21 and 22, and therefore, p becomes small. As is seen from the FL-LS characteristics (Fig. 45, curve N4) i of the spark-plug of this embodiment, through the compound effects of preventing the direct collision of the electro-flame-wind against the negative electrode wall ~ by means of recess c, reducing the boundary layer B2 in thickness, and preventing the occurrence of the turbulent flow by means of ., ~ .
.. ..

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streamline shape, the lean mixture ignition can be suf-ficiently effected not only in a relatively long electrode gap region, but also in a relatively short electrode gap region.
Also, the ignition characteristics are further improved as shown in the curve N6 of Fig. 45 if the positive electrode 21 and the negative electrode 22 with the recess c are disposed coaxially each other as shown in Table 6, line N6.
Even when the gas reservoir recess c in the negative ~, .
~; 10 electrode 22 is made as a through hole, the same operating - effect as described hereinbefore is provided and thus the ~ lean mixture ignition can be sufficiently effected as shown -~ in the Ls* value of the line N5 of Table 6. This spark-plug - has advantages in that the gas reservoir is hardly clogged - with dusts, etc. and easier in cleaning even if clogged with the dusts.
Example 16 The spark-plug of this embodiment has a gas reservoir recess, on both electrodes as shown in Table 6, line N7.
As is seen from the ignition characteristics in Fig. 45, curve N7, the lean gas mixture ignition can be sufficiently effected by the spark-plug, of this embodiment by means of - combined operation between the operating effect of the gas reservoir recess of the positive electrode 21 and the ; operating effect of the gas reservoir recess of the negative electrode 22.

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The following Example 17 relates to three-electrode type spark-plug wherein an electrode for trigger discharge ~ . .
is added to the spark-plugs of all the Examples 1 to 16.
Example 17 (Fig. 16¦
A spark-plug of this Example, as shown in Fig. 16, is formed by adding a trigger electrode 33 (projection height h3), which has the form, size and arrangement of small fluid resistance with respect to the electro-flame-wind, in a position of distance L13 from the positive electrode 21, ! 10 to any of the preceding type of spark-plug, for instance, in Fig. 12 wherein two-electrodes are provided. The main electrodes 21 and 22 are disposed to face each other with the main electrode gap distance L12 inbetween.
In the spark-plug of this construction, the fluid resistance of the electrodes is small, and therefore the thermal conductance G is small. The main discharge limit gap with respect to a given applied voltage pulse can be enlarged up to approximately 1.75 times the discharge limit - gap of the above-mentioned two-electrode spark-plugs, so that the main electrode gap distance Ls, and hencej the flame nucleus volume V can be selected large. Accordingly, the condition of the ignition of an inequality (1) is improved and accordingly very lean mixture can be ignited.
; In case of this three-electrode spark-plug, the spark-plug can be driven by means of an ordinary ignition power source for two-electrode spark-plugs without using other power source, by connecting terminal 34 to the metallic screw part 29 or to a high tension electrode terminal through a resistor or a capacitor.
The following Example 18 relates to a spark-plug of surface creeping discharge type wherein a surface creeping . . - , -; `

discharge path is on the face of an electric insulator 24 between a first electrode 21 and a second electrode 22.
Example 18 (Figs. 38 to 40) :.
, As shown in Fig. 38, in the spark-plug of this Example ` 18, the first and second electrodes 21 and 22 are projected . .
by a given heights hl and h2, respectively, above the face of the surface creeping discharge path 44. Both of these projection heights hl and h2 are as low as, for example, 0.3mm or less.
The FL-LS characteristics of this spark-plug are remarkably enough improved to ignite the sufficiently lean mixture, as compared with the FL-L characteristics of the conventional spark-plug of surface creeping discharge type (Fig. 37) with the first electrode 1 and the second electrode 2 both projected by approximately 0.5mm above the surface creeping discharging path 4. This effect is obtained through -the improved condition of the ignition as can be understood from the inequality (l), since the respective projection heights hl, h2 of the electrodes 21 and 22 of this spark-plug have been made small, and thus the dimensions Q of the .:
discharging faces of the opposing electrodes become effectively small to reduce the friction drag ~f, hence, the thermal conductance G.
In order to provide easier ignition of the lean mixture, it is recommended that as shown in Fig. 39 and Fig. 40, the electrode projection heights from the surface creeping path 44 should be 0.3mm or less alike the spark-plug of the Fig. 38 !

the shape of the electrodes should be of gentle convex faces, and the shape of the surface creeping discharge path 44 of the electric insulator 24 should be, also, of gentle convex face.

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In addition, in order to widen the main electrode gap distance, it is also xecommended that a trigger discharge electrode be added to the surf~ce creeping discharge type spark-plug of this embodiment to use as a three-electrode type spark-plug.
: As described elucidating, the abovementioned eighteen examples, the spark-plugs and engines of this invention have superior ignition characteristics which ensure positive and reliable ignition under conditions where combinations of temperature, pressure and concentration of the gas mixture - are hard for ignition. The spark-plug and engine make it possible to ignite and burn the lean gas mixture of 1.25 or more in excess air ratio F, under the normal temperature, - 1 atmospheric pressure, and with an electrode gap narrower than Ls=2mm with which ignition has been impossible with ~ conventional spark-plugs and engines.

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Claims (37)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electric spark-plug for automobile internal combustion engine comprising:
a metallic screw portion to be engaged with an internal combustion engine, an electrode terminal part, an electric insulator for supporting said screw part and said electrode terminal part in a coaxial relation, keeping them insulated from each other, and at least a pair of electrodes, the pair in-cluding a first electrode and a second electrode, said first electrode being supported by a supporter formed on the metallic screw part, said second electrode being rod-shaped and electrically connected to said electrode terminal part by means of a central conductor disposed on the axis of said screw part and in said insulator, said first electrode and said second elec-trode being supported insulatedly from each other by means of said insulator, keeping a given ignition gap between their discharging faces, characterized in that:
fluid resistance against gas flow of flame nucleus of said electrodes is decreased in a manner to have an ability of ignition that is defined by capability to ignite and burn a lean gas mixture of isobutane and air under the conditions of normal temperature (T) of about 20°C, the pressure of lean gas mixture equals 1 atmospheric pressure (density index .epsilon. ? 1), excess air ratio (F) of more than 1.25 and electrode gap distance (Ls) of less than 2mm, so that the following ignition condi-tional inequality applies thereby to decrease the thermal conductance G from the flame nucleus to the electrodes:

............ (1) wherein J, .epsilon. and m* are given by J = [ E? (Ts - To) + RTs2] / .pi. RTs2, .epsilon. = [ (760 - p-) / 760] Rc, m* = (1 + ?)m, wherein V is the nucleating volume size and is represented by an equation of V=.theta.Ls wherein .theta. is a proper constant and Ls is the electrode gap distance, T is a temperature of the nucleating volume, To is the temperature of the electrodes, X; is a molar density in 1 atmospheric pressure of incombusti-ble gases other than nitrogen, F, .epsilon. p- and Rc are excess air ratio, density index of molecules, absolute value of nega-tive pressure ( mm Hg) in intake pipe and compression ratio at the time of ignition, respectively, in the gas mixture, J
being practically a constant when the dependences of the singular point temperature Ts upon the variable quantities V, G, Xi, F and .epsilon. are sufficiently small, E? is an activation energy in Arrhenius' equation, and is a constant inherent to individual fuel, n and m are the molecularities of reaction of fuel and oxygen, respectively, ? is a parameter showing the participation degree of nitrogen molecules in reaction process, .alpha. is a multiplication factor of chain carriers, R is a gas constant, B, Xt and .pi. are constants, respectively.

2. An electric spark-plug for internal combustion engine according to claim 1 wherein the first and second electrodes are rod-shaped electrodes, respectively, the rod-shaped electrode having discharging face at its front end and being 1.7mm or less in diameter in case the cross-sectional view of the electrode is circular, and in the longest diagonal line length in case the cross-sectional view thereof is polygonal, and the other rod-shaped electrode having discharging face at its side face and being 1.2mm or less in width and 2mm or less in thickness.

3. An electric spark-plug for internal combustion engine according to claim 2 wherein said first electrode is a rod-shaped electrode, which is projected in the axial direction towards a second electrode from the supporter by a given height and has at its front end, a flat discharging plane perpendicular to the axis, while said second electrode is a rod-shaped electrode, which is projected by a given height from said electric insulator, and has n flat dis-charging plane having right angle with said axis, the first and second rod-shaped electrodes being disposed in a coaxial relation with respect to each other.

4. An electric spark-plug for internal combustion engine according to claim 2 wherein said first electrode is a rod-shaped electrode, which is protected by a given height from the supporter towards a second electrode in a direction perpendicular to the axis, and has at its front end a discharging plane parallel to the axis, while said second electrode is a rod-shaped electrode which is protected by a given height, from said central conductor in a direction perpendicular to said central conductor, and has a discharging plane parallel to the axis and facing the discharging plane of said first electrode.

5. An electric spark-plug for internal combustion engine according to claim 2 wherein said first electrode is a rod-shaped electrode which is projected in the direction perpendicular to the axis towards a second electrode from the supporter by a given height and is at, on its side face, a discharging plane which has right angle with the axis, while said second electrode is a rod-shaped electrode which is projected by a given height from the electric insulator and is at, on its front end, a dis-charging plane which has right angles with the axis,. the axes of said first and second rod-shaped electrodes being disposed at right angle with each other.

6. An electric spark-plug for internal combustion engine according to claim 2 wherein said first electrode is a rod-shaped electrode which is protected by a given height from the supporter towards a second electrode in a direction parallel to the axis, and has, on its side face, a discharging face which is parallel to the axis while said second electrode is a rod-shaped electrode which is protected by a given height from the electric insulator and has on its side face, a discharging face parallel to said axis, the axes of said first and second rod-shaped electrodes being disposed in parallel to each other.

7. An electric spark-plug for internal combustion engine according to claim 2 which further comprises a trigger electrode.

8. An electric spark-plug for internal combustion engine according to claim 1 wherein at least one of said first and second electrodes has a convex discharging face.

9. An electric spark-plug for internal combus-tion engine according to claim 8 wherein said first elec-trode is an electrode which is projected by a given height from the supporter toward said second electrode in the axial direction and has, on its front end, a discharging plane which is at right angle to the axis, while said second electrode is an electrode which is projected by a given height from the electric insulator and has, on its front end, a convex discharging face with a curvature in the range of 0.46mm-1, said discharging face and its circumferential part forming a continuous convex face, said first and second electrodes being disposed in a co-axial relation with each other.

10. An electric spark-plug for internal combus-tion engine according to claim 8 wherein said first elec-trode is an electrode which is projected by a given height from the supporter toward said second electrode in the axial direction, and has on its front end, a convex dis-charging face, while said second electrode is an electrode which is projected by a given height from the electric insulator and has on its front end, a convex discharging face with a curvature in the range of 0.46mm-1, said discharging face and its circumferential part forming a continuous convex face, said first and second electrode being disposed in coaxial relation with respect to each other.

11. An electric spark-plug for internal combustion engine according to claim 8 wherein said first electrode is an electrode which is projected by a given height towards said second electrode in a direction perpendicular to the axis and has on its side face, a convex discharging face with a curvature of at least 0.46mm-1, said discharging face and the circumferential part forming a continuous convex face, while said second electrode is an electrode which is projected by a given height from the electric insulator, and has on its front end, a convex discharging face with a curvature of at least 0.46mm-1, said discharging face and the circumferential part forming a continuous convex face, said first and second electrodes being disposed at right angle with each other.

12. An electric spark-plug for internal combustion engine according to claim 8 wherein said first electrode is an electrode which is protected by a given height from the supporter towards said second electrode in a direction parallel to the axis and has,on its side face, a convex discharging face, while said second electrode is an electrode which is protected by a given height from the electric insulator and has,on its side face, a convex discharging face, said first and second electrodes being disposed in parallel to each other.

13. An electric spark-plug for internal combustion engine according to claim 8 wherein said first electrode is an electrode which is protected by a given height from the supporter vertically towards below said second electrode in a direction vertical to the axis, and has on its side face, a convex discharging face, while said second electrode is a rod-shaped electrode which is protected by a given height from the electric insulator, and has on its front end, a discharging plane which has right angle with said axis, axes of first and second electrodes being disposed at right angle with each other.

14. An electric spark-plug for internal combustion engine according to claim 8 wherein said first electrode is a rod-shaped electrode which is protected by a given height from the supporter perpendicularly towards said second electrode in a direction perpendicularly to said axis, and has on its side face, a discharg-ing plane which is at a right angle with said axis, while said second electrode is an electrode which is projected by a given height from the electric insulator, and has on its front end, a convex discharging face, said discharging face and its circum-ferential part forming a continuous convex face, said first and second electrodes being disposed at right angle with each other.

15. An electric spark-plug for internal combustion engine according to claim 8 wherein at least a negative electrode has a convex discharging face, said convex dis-charging face and its circumferential part forming a con-tinuous convex face, and said positive electrode is made smaller in size measured in the direction extending at a right angle to the discharging direction from said nega-tive electrode.

16. An electric spark-plug for internal combustion engine according to claim 8 wherein at least one of said electrodes has on its side face, a convex discharging face, and a curved circumferential part smoothly connected to said discharging face.

17. An electric spark-plug for internal combustion engine according to claim 8 which further comprises an electrode for trigger discharge.

18. An electric spark-plug for internal combustion engine according to claim 1 wherein at least one of said electrodes has a recess on an extended part of its discharging face, said discharg-ing face and its circumferential part forming a continuous convex face.

19. An electric spark-plug for internal combustion engine according to claim 18 wherein said first electrode is projected by a given height from the supporter perpendicularly towards said second electrode in a direction perpendicular to the axis, said second electrode is protected by a given height from the electric insulator, at least one of said first and second electrodes having a recess on a position which is adjacent to and on a race continuous to said discharging face.

20. An electric spark-plug for internal combustion engine according to claim 18 wherein said first electrode is projected by a given height from the supporter towards said second electrode in a direction perpendicular to the axis, said second electrode is projected by a given height from the electric insulator, at least one of said first and second electrodes having said recess on the discharging face at a position correctly facing the discharging face of the other electrode.

21. An electric spark-plug for internal combus-tion engine according to claim 18 wherein said first electrode is projected by a given height from the sup-portor towards below a second electrode in a direction parallel to the axis, said second electrode is projected by a given height from the electric insulator, said first and second electrodes being disposed coaxially with each other at least one of said first and second electrodes having said recess on the discharging face at a position correctly facing the discharging face of the other electrode said discharging face and its circumferential part forming a continuous convex face.

22. An electric spark-plug for internal combustion engine according to claim 18 wherein said first electrode is projected by a given height from the supportor towards said second electrode in a direction parallel to the axis, said second electrode 18 projected by a given height from the electric insulator, said first and second electrode being disposed in parallel to each other, and at least one of said first and second electrodes having said recess on the discharging face at a position correctly facing the discharging face of the other electrode.

23. An electric spark-plug for internal combustion engine according to claim 18 which further comprises an electrode for trigger discharging.

24. An electric spark-plug for internal combustion engine according to claim 1 wherein a surface creeping discharge path between said first and second electrodes is formed on the surface of said insulator.

25. An electric spark-plug for internal combustion engine according to claim 24 wherein said first and second electrodes are projected, respectively, by 0.3 mm or less above the surface of said surface creeping discharge path.

26. An electric spark-plug for internal combustion engine according to claim 24 wherein the surface creeping discharge path is on a convey face and both of said first and second electrodes are projected, by 0.3mm or less over said surface creeping discharge path.

27. An electric spark-plug for internal combustion engine according to claim 24 which further comprises an electrode for trigger discharging.

28. An electric spark-plug for internal combustion engine according to claim 1 wherein at least one of said first and second electrodes has a substantially flate discharging plane and a circumferential portion connected to said flat discharging plane and formed with a convex face connected without ridge line to said discharging plane.

29. An electric spark-plug for internal combustion engine according to claim 28 which further comprises an electrode for trigger discharging.

30. Method of igniting an automobile internal com-bustion engine of lean gas mixture combustion type using the lean gas mixture having excess air ratio F of F ? 1 in operation modes including idling, engine-braking, constant speed, acceleration and deceleration, by means of at least one spark-plug comprising:
a metallic screw portion to be engaged with said internal combustion engine, an electrode terminal part, an electric insulator for supporting said screw part and said electrode terminal part in a coaxial relation, keeping them insulated from each other, and at least a pair of electrodes, the pair including a first electrode and a second electrode, said first electrode being supported by a supporter formed on the metallic screw part, said second electrode being rod-shaped and electrically connected to said electrode terminal part by means of a central conduc-tor disposed on the axis of said screw part and in said insulator, said first electrode and said second elec-trode being supported insulatedly from each other by means of said insulator, keeping a given ignition gap between their discharging faces, the method being characterized in that:

a lean mixture of gas is ignited with a decreasing fluid resistance against gas flow of flame nucleus of said electrodes in a manner to have an ability of ignition that is defined by capability to ignite and burn a lean gas mixture of isobutane and air under con-ditions of normal temperature (T) of about 20°C, the pressure of lean gas mixture equalling 1 atmospheric pressure (density index .epsilon. of about 1), excess air ratio (F) of more than 1.25 and electrode gap distance (Ls) of less than 2mm, so that the following ignition conditional inequality applies thereby to decrease the thermal conduc-tance (G) from the flame nucleus to the electrodes:

.........(1), wherein J, .epsilon. and m* is given by J = [Eb (Ts - To) + RTs2] / .pi. RTs2, .epsilon. _ [(760 - p-) / 760]Rc, m* = (1+?)m, wherein V is the nucleating volume size and is represented by an equation of V=.theta.Ls wherein .theta. is a proper constant and Ls is the electrode gap distance, T is a temperature of the nucleating volume, To is the temperature of the electrodes, Xi is a molar density in 1 atmospheric pressure of incom-bustible gases other than nitrogen, F, .epsilon., p- and Rc are excess air ratio, density index of molecules, absolute value of negative pressure (mm hg) in intake pipe and compression ratio at the time of ignition, respectively, in the gas mixture, J being practically a constant when the dependences of the singular point temperature Ts upon the variable quantities V, G, Xi, F and .epsilon. are sufficiently small, Eb is an activation energy in Arrhenius' equation, and is a constant inherent to individual fuel, n and m are the molecularities of reaction of fuel and oxygen, respectively, ? is a parameter showing the participation degree of nitrogen molecules in reaction process, .alpha. is a multiplication factor of chain carriers, R is a gas con-stant, B, Xt and .pi. are constants, respectively.

31. Method of igniting an internal combustion engine with a spark-plug of claim 30 characterized in that at least a negative electrode has a convex discharging face, said convex discharging face and its circumferential part forming a continuous convex face, and one of said electrodes is made smaller in size measured in the direction having right angle to the discharging direction than the other electrode, said electrode with smaller size is used as a positive electrode and said other electrode is used as a negative electrode.

32. An automobile internal combustion engine of lean gas mixture combustion type including a combustion chamber, a lean gas mixture producing device for pro-ducing lean gas mixture having excess air ratio F of F ? 1 in operation modes including idling, engine-braking, con-stant speed, acceleration and decleration, a compressing means which compresses said lean gas mixture containing air and fuel by varying volume of said combustion chamber, at least one electric spark-plug for igniting said compressed lean gas mixture, said spark-plug comprising a metallic screw portion to be engaged with an internal combustion engine, an electrode terminal part, an electric insulator for supporting said screw part and said electrode terminal part in a coaxial relation, keeping them insulated from each other, and at least a pair of electrodes, the pair including a first electrode and a second electrode, said first electrode being supported by a supporter formed on the metallic screw part, said second electrode being rod-shaped and electrically connected to said electrode terminal part by means of a central conductor disposed on the axis of said screw part and in said insulator, said first electrode and said second elec-trode being supported insulatedly from each other by means of said insulator, keeping a given ignition gap between their discharging faces, characterized in that:
fluid resistance against fas flow of flame nucleus of said electrodes is decreased in a manner to have an ability of ignition that is defined by capability to ignite and burn a lean gas mixture of isobutane and air under the conditions of normal temperature (T) of about 20°C, the pressure of lean gas mixture equals 1 atmospheric pressure (density index .epsilon. approximately equal to 1), excess air ratio (F) of more than 1.25 and electrode gap distance (Ls) of less than 2mm, so that the following ignition conditional inequality applies thereby to decrease the thermal conductance G from the flame nucleus to the electrodes:
.......(1), wherein J, .epsilon. and m* are given by J = [Eb (Ts - To) + RTs2] / .pi. RTs2, .epsilon. = 1760 - p-) / 760] Rc, m*= (1 + ?)m, Wherein V is the nucleating volume size and is represented by an equation of V=.theta.Ls wherein .theta. is a proper constant and Ls is the electrode gap distance, T is a temperature of the nucleating volume, To is the temperature of the electrodes, .lambda.; is a molar density in 1 atmospheric pressure of incom-bustible gases other than nitrogen, F,.epsilon., p- and Rc are excess air ratio, density index of molecules, absolute value of negative pressure (mm Hg) in intake pipe and com-pression ratio at the time of ignition, respectively, in the gas mixture, J being practically a constant when the dependences of the singular point temperature Ts upon the variable quantities V, G, Xi, F and .epsilon. are sufficiently small, Eb is an activation energy in Arrhenius' equation, and is a constant inherent to individual fuel, n and m are the molecularities of reaction of fuel and oxygen, respectively,.delta. is a parameter showing the participation degree nitrogen molecules in reaction process,.alpha. is a multiplication factor of chain carriers, R is a gas con-stant, B, Xt and .pi. are constants, respectively.

33. An automobile internal combustion engine of lean gas mixture combustion type according to claim 32 wherein in said electric spark-plug the first and second electrodes are rod-shaped electrodes, respectively, the rod-shaped electrode having discharging face at its front end and being 1.7mm or less in diameter where the cross-sectional view of the electrode is circular, and in the longest diagonal line length where the cross-sectional view thereof is polygonal, and the other rod-shaped electrode having a discharging face at its side face and being 1.2mm or less in width and 2mm or less in thickness.

34. An automobile internal combustion engine of lean gas mixture combustion type according to claim 32 wherein in said electric spark-plug at least one of said first and second electrodes has a convex discharging face with a curvature of at least 0.46mm-1, said discharging face and its circumferential part forming a continuous convex face.

35. An automobile internal combustion engine of lean gas mixture combustion type according to claim 32 wherein in said electric spark-plug at least one of said electrodes has a recess on or extended part of its discharging face.

36. An automobile internal combustion engine of lean gas mixture combustion type according to claim 32 wherein in said electric spark-plug a surface creeping discharge path between said first and second electrodes is formed on surface of said insulator.

37. An automobile internal combustion engine of lean gas mixture combustion type according to claim 32 wherein in said electric spark-plug at least one of said first and second electrodes has a substantially flat discharging plane and a circumferential portion connected to said flat discharging plane and formed in a convex face connected without ridge line to said discharging plane.