FR2796767A1 - SURFACE CANDLE - Google Patents

Abstract

A surface effect spark plug for an internal combustion engine of the positive ignition type comprising a central electrode (106), supplied with high voltage through an ignition system, and a base (103), connected to ground, said electrode (106) being separated from the end of the base (103), set back from said electrode (106), by an insulator (100), with a dielectric coefficient greater than one, allowing the progression of 'a spark on its surface between the central electrode (106) and the base (103), said base (103) comprising a body (108) and an end in the form of a collar (101), frustoconical or cylindrical, characterized in that that the collar (101) is connected to the body (108) of the base (103) by a connecting piece (107) shaped to regulate the heat flows between the collar (101) and the rest of the base (103).

Description

i 2796767 Surface effect candle The present invention relates to

  technical field of surface effect spark plugs for internal combustion engines, intended in particular to equip a motor vehicle. To meet pollution abatement standards and to reduce the consumption of internal combustion engines, car manufacturers have developed new positive-ignition engines capable of operating with so-called lean burn fuel mixtures, that is to say having an excess of air in relation to the quantity of fuel injected. In particular, manufacturers have developed direct injection engines for which fuel is supplied directly to the

combustion chambers.

  The spark plugs fitted to these positive-ignition engines operating in lean mixture are currently identical to those fitted to traditional engines operating with stoichiometric fuel mixtures. These ignition devices conventionally consist of a spark plug whose electrodes are more or less projected in the combustion chamber and of a high voltage electrical supply circuit, of the static or semi-static type, controlled by the computer. engine control electronics to trigger the spark at a predetermined instant in the

  engine cycle depending on operating conditions.

  Such ignition devices prove to be very difficult to develop when they equip lean-mix engines and more particularly when they equip

direct injection engines.

  Indeed, if a flame front can propagate in a very lean mixture (richness lower than 0.3), the initiation of combustion as for it can occur only for appreciably higher richnesses (richness higher than 0, 7) and preferably close to stoichiometry (richness 1). Thus, even if the fuel mixture present in a combustion chamber is generally lean, it turns out

2 2796767

  necessary to maintain sufficient richness at the level of the inter-electrode space

  high to allow good initiation of combustion.

  In contrast to a homogeneous mixture where the richness is the same at all points in the combustion chamber, this is called a stratified mixture, which means that the richness decreases as one moves away from the candle . Usually, the stratification of the mixture in the chamber is obtained by guiding the jet of fuel so that the latter meets the spark plug when it produces a spark. The guidance of the jet can be obtained in different ways by combining aerodynamic phenomena with a shape

particular piston.

  In all cases, we are faced with the delicate problem of making the spark instant coincide with the presence in the vicinity of the inter-electrode space of a cloud of air-fuel mixture at a richness close to 1, cloud in movement in a

generally poor environment.

  Furthermore, during laminate operation, the fuel jet can directly strike the spark plug and therefore cause more or less pronounced fouling of the insulation. This fouling promotes the appearance of leakage currents between the central electrode and the ground and therefore affects the generation of sparks: the spark being short-circuited by a carbon path of low impedance which limits the difference

  potential between the spark plug electrodes.

  In this context, new candle geometries have appeared. Their operation is based on the propagation of a spark along an insulating surface, generally ceramic, hence their name of surface effect candles or

surface spark.

  Patent application FR 9904733, not yet published, describes the general shape of a surface effect candle. This is conventionally composed of a low voltage cylindrical electrode, in contact with the engine ground, which serves as a metal base. It surrounds a central electrode, called high voltage

  terminated by a cylindrical washer in the shape of a mushroom.

3 2796767

  The central electrode, intended to be connected to a high voltage source supplied by an automobile ignition, is separated from the base by means of an insulating sleeve and possibly by an annular recess. The spark is

  propagates between the two electrodes along the insulating sleeve.

  By using the effects of amplification of the electric field in the air in the vicinity of the insulator which surrounds the central electrode, the surface effect candle makes it possible to increase the spatial length traversed by the spark compared to a candle classic without increasing the breakdown voltage. The

  probability of encountering the fuel cloud is therefore increased.

  At the same time, the passage of the spark over the insulating sleeve allows

  clean it of any carbon deposits linked to fouling.

  The surface spark plug is therefore a better solution

  with direct injection operation than a conventional candle.

  The differences between these two spark plug concepts do not stop with the simple absence of one or more protruding ground electrodes in the chamber and the addition of a cap at the end of the central electrode. Indeed, in the case of a surface candle, the amplification of the field on the surface of the insulating sleeve during the rise in tension which precedes the breakdown is maximum at the end of the base which has a shape at this point flange folded at a close distance (a few tenths of a millimeter) or even in contact with the insulation. This is indeed where the equipotentials of the electrostatic field are most tight. The electronic avalanche phenomena which govern the physics of the spark will therefore always appear in this zone where the electrostatic field is more than 10 times greater than the field at the end of the central electrode. This remains true regardless of the polarity of the

  high voltage applied to the central electrode.

  The breakdown voltage of the surface spark plugs will then be all the more

  weak that the electric field will be strong at this location.

  Although the shape, length and thickness of the insulating sleeve play a role

4 2796767

  very important role, as described in document FR 9904733, it is therefore important not to neglect the geometry of the flange located at the end of the base since it is its presence in the vicinity of the insulating sleeve which generates the local tightening of the electrostatic field lines. At the same time, its dimensions will have an immediate effect on its temperature and will therefore also act on the value of the breakdown voltage as we will see later. Finally, the type of high voltage applied to the spark plug also plays a fundamental role since the phenomena of electronic avalanches will be very different

according to the polarity.

  The present invention therefore proposes to define a base geometry

  for surface effect candles to minimize voltage requirements.

  To this end, it offers a surface effect spark plug for an internal combustion engine of the spark ignition type comprising a central electrode, supplied with high voltage by means of an ignition system, and a base, connected grounded, said electrode being separated from the end of the base, set back from said electrode, by an insulator, with a dielectric coefficient greater than one, allowing the progression of a spark on its surface between the central electrode and the base, said base comprising a body and a frustoconical or cylindrical end, characterized in that the collar is connected to the body of the base by a connecting piece

  shaped to regulate the heat flows between the flange and the rest of the base.

  According to another characteristic of the invention, the connecting piece is substantially annular and is in contact, on at least part of one of its faces,

with the body of the base.

  According to another characteristic of the invention, the collar defines with the surface

  of the insulation a radial clearance of a few tenths of a millimeter.

  According to another characteristic of the invention, the collar extends along the insulating sleeve, defining with it a radial clearance of the order of a few tenths of

millimeters over its entire length.

2796767

  According to another characteristic of the invention, the collar has a thickness

  substantially constant and less than 1 mm.

  According to another characteristic of the invention, the collar has a

  length, L, of the order of 2 to 3 mm.

  According to another characteristic of the invention, the end of the collar 101

  has a beveled shape, the angle formed by the bevel being less than 45.

  According to another characteristic of the invention, the end of the collar is in

  linear contact with the surface of the insulation.

  According to another characteristic of the invention, at least part of the collar is in contact with the insulation, thus forming a planar connection with said insulation

  over a determined length Lco ,, c.

  According to another characteristic of the invention, the central electrode is supplied

in negative high voltage.

  The aims, aspects and advantages of the present invention will be better understood,

  according to the description given below of different embodiments of the invention,

  presented by way of nonlimiting examples, with reference to the accompanying drawings, in which: - Figure 1 is a cross-sectional view of the lower end of a surface effect candle according to a first embodiment of the present invention, - Figure 2 is a cross-sectional half view of the spark plug of Figure 1, - Figure 3 is a cross-sectional half view of the previous candle on which have been shown schematically the distribution of equipotentials when 'a high voltage is applied to the central electrode, - Figure 4 shows the evolution of the module of the electrostatic field along the surface of the insulating sleeve of the spark plug shown in Figure 2, - Figure 5 shows schematically the law from Paschen,

6 2796767

  - Figure 6 is a view similar to Figure 2 describing a second embodiment of the present invention, - Figure 7 is a view similar to Figure 2 describing a third embodiment of the present invention, - Figure 8 is a view similar to FIG. 2 describing a fourth embodiment of the present invention, FIG. 9 is a view similar to FIG. 2 describing a fifth embodiment of

  realization of the present invention.

  Referring to Figure 1, there has been schematically described a surface effect candle 110 object of the present invention. The head of the spark plug 110 intended to open into the combustion chamber formed in the lower wall of the cylinder head of the engine is shaped to have a symmetry of revolution around

the longitudinal axis D of the spark plug.

  The spark plug 110 includes a low-voltage cylindrical electrode which serves as a metal base 103 intended to be screwed into a recess produced in the cylinder head of the engine, and opening out inside the combustion chamber. The base 103 is therefore intended to come into contact with the cylinder head of the engine and therefore to be connected

electrically grounded.

  This base 103 surrounds a cylindrical high voltage electrode 106 disposed in the central position, intended to be connected to the high voltage generator ignition system. The central electrode 106 is isolated from the base 103 by means of an insulating sleeve 100, made of a material whose dielectric coefficient is greater than one, for example of ceramic, and possibly of an obviously annular 102 called commonly dead volume. This traditional 102 obviously on classic candles is not at all necessary on surface effect candles, it

  nevertheless makes it possible to reduce the thermal stresses in the insulating sleeve 100.

  The central electrode 106 and its insulating sleeve 100 protrudes at

  the exterior of the base 103 from a height 1.

7 2796767

  This height I corresponds substantially to the length of the spark generated when a high voltage is applied to the central electrode 106 of the spark plug 110 and when the phenomenon of breakdown occurs between the central electrode 106 and the base 103 to ground. . The insulating sleeve 100 is of substantially cylindrical shape when it is inside the base 103, then it is of substantially converging conical shape when it projects out from the base 103. The base 103 has a body 108 and an axial end shaped so as to form a connecting piece 107 supporting a folded flange 101 having a beveled edge extending in the immediate vicinity of

the surface of insulation 100.

  The body 108 of the base may have at its end in contact with the connecting piece 107 a substantially cylindrical shape, of axis D, and of determined interior and exterior radius. The connecting piece 107 is preferably substantially annular and coaxial with the body 108 of the base 103. It has internal and external radii which may be different from those of the end of the body 108 of the base 103, so that the area of the contact surface in the connecting piece 107 and the body 108 of the base 103 can be chosen precisely. The connecting piece 107 supports on its face opposite to the face in contact with the body 108 of the base 103 the

flange 101.

  The presence of the dielectric 100 creates an amplification of the electrostatic field in the air in its vicinity, so that the spark generated between the beveled edge of the flange 101 of the base 103 and the free end 104 of the central electrode 106 goes is

  propagate on the surface of insulation 100, where the field in the air is strongest.

  With breakdown voltages between 5 and 25 kV, and for a base 103, a sleeve 100 and a central electrode 106 of respective diameters 14 mm, 5.5 mm and

  2.8 mm, it is thus possible to obtain values of 1 of the order of 50 mm.

  The free end 104 of the central electrode 106 is shaped to have a flared head of hemispherical shape against which the sleeve bears.

insulator 100, which she oversees.

8 2796767

  Finally, the variations in thickness given by the conical shape of the insulator 100 between the base at the outlet of the base 103 and its flange 101 and the axial end 104 of the central electrode 106, are intended to allow a good evacuation of the heat from the end as well as a reduction of the electrostatic stresses in the insulation at the level of the flange 101, without greatly penalizing the amplification of the field on the surface,

  which is all the greater the smaller the thickness of the insulating sleeve 100.

  The formation of a spark is always initiated by the setting in motion of some electrons subjected to an intense electric field. During the rise of the potential on the spark plug electrode, this electric field increases and, when it is sufficient, the electrons, accelerated by electrostatic forces will strike molecules of the air. These will then release an electron and a photon capable in turn of ionizing other molecules. These chain reactions will

  finally ionize the inter electrode space and thus create a conductive channel.

  FIG. 3 represents the half-view in section of the surface effect spark plug 110 of FIG. 2. The typical distribution of the equipotentials of the electrostatic field can be seen when a voltage is applied between the central electrode 106 and

the base 103.

  The electrostatic field vectors are perpendicular to the lines

  equipotentials and their modules are proportional to their density.

  In other words, in the case which is shown in FIG. 3, the electrostatic field is very large at the end of the flange 101 since this is where the equipotentials are most tightened. The first avalanche

will therefore leave from this place.

  FIG. 4 represents the evolution of the module of the electrostatic field vector along the insulating sleeve 100, between the points A and B represented

  in Figure 2. The voltage applied here to the central electrode 106 is 25 kV.

  This representation makes it possible to highlight the effects of amplification of the field by the flange 101. The field module at the end 104 of the central electrode 106 is 10 times smaller than at the end

9 2796767

  of the base 103. We then understand better why the first avalanche will be

  always initiated in this region.

  Subsequently, this avalanche will make it possible to ionize the air upstream and to create a space charge which has a potential close to that of the flange 101 and which will therefore behave as an extension of this. Thus, as the avalanche front propagates, the electric field is amplified upstream of the latter, and promotes the creation of new avalanches. The phenomenon is self-sustaining until a conductive ionized channel is created up to the end 104 of the central electrode 106. The critical point of the process is therefore at the level of the creation of the first

  avalanche, that is to say the departure of the avalanche front.

  It is therefore essential to pay particular attention to the design of the flange 101 in order to obtain the strongest possible amplification of the field, and thus minimize the voltage requirements. In particular, a thin flange thickness 101 of less than a millimeter located at a distance of a few tenths at most from the ceramic will produce a significant amplification of the electric field by promoting "peak effects" on the one hand, and in accentuating on the other hand the

  deflection of the equipotentials by the insulating sleeve 100.

  Another point to bear in mind when designing the flange is its

  temperature, mainly for two reasons.

  Pyrolysis of carbon deposits due to fouling can only be done when the temperature is above about 450 C. Since the base 103 is relatively cold in operation, it is imperative to limit the heat exchanges with the latter in order to prevent the collar 101 from becoming excessively clogged during operation. In addition, the temperature of the collar 101 can play a significant role on the ability of the metal to release or absorb electrons. Indeed, depending on the polarity of the high voltage on the flange

  101, the electrons will be emitted or on the contrary absorbed by the metal.

  The second reason involves the concept of boundary layer

2796767

  thermal in the vicinity of the flange 101. When the latter is cold relative to the temperature of the inlet gases at the end of compression, the molecular density will tend to increase in its vicinity. For a fixed pressure, the local density will therefore be greater the lower the temperature. The average free path between two molecules will therefore decrease in high density areas and the electric field will have to be greater in order to sufficiently accelerate the electrons between two consecutive shocks. That is to say that the greater the distance between two molecules, the less the field necessary for the creation of an avalanche will be important, because the electrons will have "more

place "to accelerate.

  This remark obviously remains true provided that it does not descend to a density that is too low below which the number of shocks is no longer sufficient to create an avalanche. This condition, called Paschen minimum is shown in Figure 5. This highlights, for a given length on which we want to create an avalanche, the existence of a pressure (and therefore a density) for which the homogeneous statistical field necessary for the

  creation of an avalanche is minimal.

  This result can be interpreted as follows: the homogeneous statistical field necessary for the creation of an avalanche over a given distance is higher the higher the density, which is explained by the reduction in free path way between two shocks. However, there is a minimum density below which the field to be applied must increase in order to

  to compensate for the insufficient number of molecules over the distance considered.

  Since the breakdown voltage is proportional to the distance, it is then possible to standardize the expression of the minimum breakdown voltage in

  expressing it according to the product (Pressure x Distance).

  Another approach then consists in considering that for a given pressure, the Paschen minimum makes it possible to deduce the minimum distance over which it is necessary to maintain a constant field to create an avalanche. This

  distance, is called the mean free ionization path.

  l 2796767 Thus, this notion highlights the need for an amplified field over a length at least equal to the mean free path of ionization. The latter, the value of which varies as a function of the pressure, is of the order of a hundred microns. s To achieve optimal amplification, the end of the flange must be placed as close as possible to the ceramic, and tangent to it. In this case, in fact, the orientation of the maximum electric field vector is almost parallel to the surface of the insulator. The amplification of the field is therefore maximum

in this direction.

  The temperature effects also make it possible to understand why it will be preferable to use a negative high voltage on the central electrode 106 to reduce the voltage requirements in the transient operating points of the motor. In this case, in fact, the first electrons which will be accelerated by the electrostatic field towards the flange can be taken outside the thermal boundary layer in a less dense medium. In addition, electron emission will then occur from the end of the central electrode 106 which will be much hotter than the flange 101, and therefore more suitable for

provide electrons.

  During sudden transients of the motor from a cold operating point to a hot operating point, it is therefore important, in addition to using a high voltage of negative polarity on the central electrode 106, to minimize the thermal inertia of the collar 101 so that it rises as quickly as possible in temperature and does not create in its vicinity gradients of

too high density.

  In FIG. 2, which represents the first embodiment of the present invention, we have shown the different dimensions characteristic of the flange 101, of the connecting piece 107 and of the body 108 of the base 103, and their

  respective position relative to the insulating sleeve 100 and the central electrode 106.

  The collar 101 has, typically a frustoconical or cylindrical shape, and its

  free end, closest to the insulating sleeve 100, has a bevel.

12 2796767

  The most important geometrical parameters are: * The length projected on the axis D of the flange 101: L. This length corresponds to the height of the truncated cone, or of the cylinder formed by the

flange 101.

  * The thickness of the collar 101: e.

  * The angle of the bevel at the free end of the flange 101: (.

  * The contact height between the connecting piece 107 and the body 108 of the base 103: h. * The distance between the end point of the flange 101 and the sleeve

insulator 100: d.

  * The distance between the point located at the base of the flange 101 and the sleeve

insulator 100: d '.

  * The thickness at the end of the body 108 of the base 103: S.

  The temperature of the flange 101 will strongly depend on L, S / h and e.

  If we want to give the flange 101 a low thermal inertia as well as an operating temperature on the engine higher than that of the rest of the base 103, we can play on the value of these four parameters by increasing for example the projected length L of the collar 101 and by reducing its thickness e and the heat exchange surface with the body 108 of the base 103 which

depends on the S / h ratio.

  The amplification and orientation of the electric field depend mainly on the angle a, the length L, the thickness e and the

  distances d and d between the collar 101 and the insulating sleeve 100.

  In order to favor the peak effects which make it possible to locally increase the intensity of the field, it is important that the end of the bevel of the collar 101 does not have too large a radius of curvature. You should not

  nor does the distance d exceed a few tenths of a millimeter.

13 2796767

  At the same time, the orientation of the equipotentials, shown in FIG. 3, which exit from the insulating sleeve 100 condition the orientation of the field relative to the surface of the insulating sleeve 100. However, the refraction in the air of the equipotentials is also very sensitive at the projection L of the collar, at its thickness e, at the value of the angle a (, and at the air thicknesses between the collar 101 and the insulating sleeve 100, d and d. For example, a length L less than 1 mm, thicknesses e, d, or too high or an angle cc close to 90 would have the effect of "laying down" the equipotentials emerging from the insulating sleeve 100. However, this would go against the desired goal which is to obtain an orientation of the field tangent to the surface of the insulating sleeve 100. Preferably, the angle a is chosen less than 45, the distance L of the order of 2 mm to 3 mm, and the thickness e less to 1 mm. Preferably, the flange 101 is shaped in. ur along the insulating sleeve 100 over the entire length L so that d and d are of

  the order of a few tenths of a millimeter.

  Given the influence of the dimensions of the flange 101 on both the electrostatic and thermal parameters, it is difficult to act

  independently on one of these two groups of parameters.

  FIG. 6, representing a second embodiment of a candle 210 according to the invention, is however an example of a solution making it possible to modify the temperature of the flange 201 without, however, significantly modifying the field distribution at its end. The idea is indeed to vary the contact section between the connecting piece 207 and the body 208 of the base 203 by playing on the S / h ratio. This in order to increase or decrease the heat exchange by conduction, materialized by an arrow, between the flange 201 and the rest of the base 203. FIG. 6 represents an extreme case where the connecting piece 207 and the end body 208 of base 203 or the same outer radius, this of

maximize heat flux.

  FIG. 7 represents a third embodiment of a candle 310 from the base 303 which allows, while keeping the same geometry of the collar 301 as in the case of FIG. 2, to decrease the value of the electrostatic stresses

14 2796767

  in the insulating sleeve 300. Indeed, taking into account the particular geometry of the flange 301 which brings the potential of the mass to a distance very close to the surface of the insulating sleeve 300, the latter is subjected, during the rise in tension of the spark plug 310, at electrostatic stresses much higher than those of a conventional spark plug. Thus, if the thickness of the insulating sleeve 300 is insufficient or if the breakdown voltage is high, the spark risks passing through the insulating sleeve 300 by piercing it. It can no longer play its role of insulator and the spark plug 310 becomes unusable. Current ceramics, which are examples of materials which can constitute the insulating sleeve 310, can withstand a homogeneous electric field of the order of 12 kV for one millimeter

thick and 20 kV for 2 mm.

  To reduce the electrostatic stresses, there are two solutions. The first consists in increasing the thicknesses d and of air between the flange 301 and the insulating sleeve 300. The second consists in increasing the thickness of the insulating sleeve 300. These two solutions have been shown in FIG. that it is preferable to play on the thickness of the insulating sleeve 300 rather than on the distances d and d 'between the insulating sleeve 300 and the flange 301 taking into account the fact that the latter must not exceed a few tenths

  as we saw earlier.

  Indeed, ceramic has a dielectric permittivity 8 times higher than that of air. In terms of increasing the breakdown voltage, adding 0.1 mm of air is equivalent to adding 0.8 mm of ceramic thickness to the radius. In addition, the direction of the field on the surface depends mainly on the spaces d and between the flange 301 and the insulating sleeve 300. In order not to modify the orientation of the field on the surface too much, it is therefore preferable to increase

  the thickness of the insulating sleeve 300 while keeping distances d and small.

  FIG. 8 represents a fourth embodiment of a candle 410 according to the present invention in which the space d between the insulating sleeve 400 and

  the flange 401 has been reduced to 0.

  Assuming that the insulating sleeve 400 has sufficient dielectric permittivity to withstand the electrostatic stresses generated by a linear contact with the end of the flange 401, this solution makes it possible, for a flange geometry 401 identical to FIG. 5, to straighten the equipotentials and therefore allow the maximum field vector to tangent a little more the surface of the insulator 400. This embodiment of a spark plug 410 is therefore the optimal solution in terms of orientation of the field on the surface of the insulating sleeve 400, because, as we saw previously, the direction of the maximum electrostatic field is all the more tangent to the surface of the insulating sleeve I0 400 as the spaces d and d 'between the flange 401 and the insulating sleeve 400

are weak.

  FIG. 9 proposes a fourth embodiment of a candle 410 according to the invention for which the flange 501 at the end of the base 502 has a circular internal section which is in contact over a more or less long distance Lcoc with the insulator 500 which is cylindrical in shape. This solution makes it possible to maintain physical contact between the flange 501 and the insulation 500 (and therefore an optimal orientation of the field), even in the case of wear of

the end of the flange 501.

  Of course, the invention is in no way limited to the embodiments

  described and illustrated which have been given only as examples.

  On the contrary, the invention includes all the technical equivalents of the means described as well as their combinations if these are carried out according to

his mind.

  Thus, the connecting piece, connecting the flange and the body of the base can it have a more complex geometric shape than an annular shape. It can be, for example, a frustoconical element whose base rests on the body of the

  base and the top of which supports the collar.

16 2796767

Claims (9)

CLAIMS:   1.Surface effect spark plug for an internal combustion engine of the spark ignition type comprising a central electrode (106), supplied with high voltage by means of an ignition system, and a base (103) , connected to ground, said electrode (106) being separated from the end of the base (103), set back from said electrode (106), by an insulator (100), with a dielectric coefficient greater than one, allowing the progression of a spark on its surface between the central electrode (106) and the base (103), said base (103) comprising a body (108) and a frustoconical or cylindrical end (101), characterized in that the flange (101) I0 is connected to the body (108) of the base (103) by a connecting piece (107) shaped to   regulate the heat flows between the collar (101) and the rest of the base (103).   2. Spark plug according to claim 1, characterized in that the connecting piece (107) is substantially annular and is in contact, on at least part of   one of its faces, with the body (108) of the base (103).   3. Spark plug according to any one of claims 1 or 2,   characterized in that the end of the collar (101) defines with the surface of   the insulator (100) a radial clearance, d, of a few tenths of a millimeter.   4. Spark plug according to one of claims 1 to 3, characterized in that   the collar (101) extends along the insulating sleeve (100) defining with it a radial clearance of the order of a few tenths of a millimeter over its entire length, L.   5. Spark plug according to one of claims 1 to 4, characterized in that   the collar (101) has a thickness, e, substantially constant and less than 1 mm.   6. Spark plug according to one of claims 1 to 5, characterized in that   the collar (101) has a length, L, of the order of 2 to 3 mm.   7. Spark plug according to any one of claims 1 to 6, characterized   in that the end of the flange 101 has a bevelled shape, the angle formed by the bevel being less than 45. 17 2796767   8. Spark plug according to any one of claims 1 to 7, characterized   in that the end of the flange (401) is in linear contact with the surface of the insulator (400).   9. Spark plug according to any one of claims 1 to 8, characterized   in that at least part of the collar (501) is in contact with the insulator (500),   thus forming a planar connection with said insulator over a determined length Lcontact.   1 0. Spark plug according to any one of claims 1 to 9,   characterized in that the central electrode (106) is supplied with negative high voltage.