EP1662626A1 - New mounting method of a spark plug and bobbin assembly using a transmission of the tightning couple through the bobbin housing - Google Patents

Abstract

Spark plug (1) for the internal combustion engine of a motor vehicle, of generally cylindrical general shape, of the type mainly comprising a substantially capacitive part (C), the closest to the cylinder head of said engine, and a part essentially inductive (I), furthest from said cylinder head of said motor, said essentially capacitive (C) and essentially inductive (I) portions being connected in series to form an LC resonator, and having a common longitudinal axis substantially coincident with the longitudinal axis (Z) of said spark plug (1); wherein said substantially inductive portion (I) is surrounded by an envelope (6) whose dimensions and materials are, in particular, suitable for producing an electromagnetic shielding of said essentially inductive portion (I), characterized in that said essentially capacitive part (C) and said essentially inductive part (I) of said spark plug (1) are mechanically and electrically interconnected by a single rigid device.

Description

The imperatives of increasing efficiency and reducing pollution generate increasingly significant constraints on the architecture of the engines. These constraints are reflected in particular, for the ignition system, spark plug dimensions increasingly reduced, as well as a reduction in the radial dimensions of the access well to said candle, in order to be able, in particular, increase the space available in the cylinder head to allow the passage of new cooling channels: the reduction of the space allocated to the candle and its access well led, among other things, to the realization of coils of the "pencil" type "placed directly in said access shaft to the candle.

However, due, on the one hand, to the difference in service life between the actual spark plug and its coil, as well as, on the other hand, to the standardization objectives of the components (each engine type requires a candle specifically adapted, while a type of coil of said spark plug may be common to several types of engines), the spark plug and its coil are, in most cases, separated, and the only link existing between them is then of the electric type, the mechanical maintenance of each of these two elements being provided independently, including in the case of "pencil" coils previously mentioned.

Furthermore, according to a mounting method well known in the state of the art, the tightening torque of the spark plug is generally applied to the base of said spark plug, said cap comprising, conventionally, a male thread intended to cooperate with a thread. complementary female made in the cylinder head, and an element for the controlled tightening of said threaded assembly. Such a mounting method then requires the use of a specific tool that is slid around said coil and said spark plug, in said access well to said spark plug: such a configuration therefore imposes, in particular, that there is, around said coil and in said access shaft to said candle, an unused free space outside the assembly operations and disassembly of the candle. However, the existence of such a space goes against the dimensional constraints previously mentioned.

In what follows, the part of a spark plug intended to be the closest to the combustion chamber of an internal combustion engine will be referred to as "lower" and the "opposite" part of candle.

The term "conductive thin layer" of a material having a high electrical conductivity will also denote a metal layer of non-magnetic material having a thickness at least equal to or greater than the skin thickness in the frequency domain under consideration. that is, between 1 Megahertz and 10 Megahertz, ensuring an electromagnetic shielding function. The high electrical conductivity of the materials will be similar to that of copper or silver, reference materials in this field (the value of the electrical conductivity of copper is 5.9 x 10 ⁷ Siemens per meter and that of silver is greater than that of copper).

Various methods of mounting a spark plug and its coil are known in which the space reserved around the assembly for the passage of a tool intended for mechanical mounting of said spark plug on the cylinder head of the engine is reduced to a minimum.

The document EP1249907 thus presents, for example, a method of mounting a candle and its coil in which the candle access well consists of a certain number of smooth, substantially cylindrical, concentric holes, and whose respective diameters are adapted to form at least one seat substantially perpendicular to the axis of said well, a bearing surface of said candle perpendicular to the axis common to said candle and said access well bearing on said seat , the positioning of said spark plug being produced by means of a projecting element situated on the lower part of said spark plug and intended to cooperate with a complementary machining performed in the walls of said access well. In the device presented in EP1249907, a cap consisting of a substantially cylindrical tubular portion terminated by a substantially planar flange whose outer diameter is greater than the diameter of said access well and whose surface is substantially perpendicular to the axis. said tubular portion and said well access, is then placed on the upper part of the candle. The dimensions of said cap are adjusted so that, when said substantially flat flange bears against the upper face of said candle access well, the end of the tubular portion of said cap bears against a portion of said spark plug, thus ensuring a compression hold of said spark plug in its access well: the tightening of the assembly is achieved by screwing said substantially flat flange on said upper face of said combustion chamber. In the device presented in document EP1249907, the coil is further integrated in said cap, the electrical contact between said coil and said spark plug being made by means of a helical spring placed in compression during the positioning of said cap.

Such a device, if it eliminates the need to provide a space specifically for the passage of a clamping tool in the access well to the candle, however requires the achievement of a large number of parts precision.

It is, moreover, only imperfectly adapted to radiofrequency plasma candles, in particular because of the mode of connection between the candle and its retained coil.

Similarly, the publication EP0969575 proposes a candle associated with a coil. The candle in this publication is a classic spark plug model and does not have any capacitive parts, such as radiofrequency plasma candles.

Indeed, a radiofrequency plasma spark plug generally comprises an upper part, essentially inductive, consisting mainly of the coil of said spark plug and a shielding jacket of said essentially inductive portion, and a substantially capacitive lower portion composed mainly of a structure coaxial, the whole behaving like an LC resonator. However, it is known that the continuity and quality of the electrical connection between the different components of such a resonator play a vital role in optimizing its performance. Moreover, the dimensions of said elements of said resonator must be defined, as a function of the radiofrequency operating domain, so as to optimizing the performance of said resonator: in particular, the length of said substantially capacitive portion should be as small as possible, so that the value of the capacitance remains low; in addition, the diameter of said substantially inductive portion should be as high as possible, so that the value of the overvoltage coefficient of the resonator thus formed is optimal. These constraints then involve reviewing the embodiment and assembly of the elements of the assembly constituting the previously mentioned resonator.

The object of the present invention is to propose an assembly device in which the connection between the coil and the other elements constituting the spark plug ensures both electrical continuity and mechanical coupling, while allowing the assembly to be put in place. thus constituted without having to slide a tool around said coil to come to bear on the base of said candle.

Preferably, the invention applies to a radiofrequency plasma spark plug.

The invention achieves its goal by means of a spark plug for the internal combustion engine of a motor vehicle, of substantially cylindrical general shape, of the type comprising mainly a substantially capacitive part, the closest to the cylinder head of said engine, and an essentially inductive part, furthest from said cylinder head of said engine, said essentially capacitive and essentially inductive parts being connected in series and having a common longitudinal axis substantially coinciding with the longitudinal axis of said spark plug; wherein said substantially inductive portion is surrounded by an envelope whose dimensions and materials are, in particular, suitable for performing electromagnetic shielding of said substantially inductive portion, characterized in that said substantially capacitive portion and said substantially inductive portion of said spark plug are interconnected mechanically and electrically by a single rigid device.

In a preferred embodiment of the invention, said essentially capacitive part of said spark plug comprises, in particular, a base surrounding a substantially cylindrical central electrode and acting as a high voltage electrode, and said essentially inductive part of said candle is made of one or more materials and has a configuration for transmitting a torque to said base of said candle.

Advantageously, said base has, on the outer face of its part closest to the cylinder head of said internal combustion engine equipped with said spark plug, a suitable shape for the introduction and tightening of said spark plug on said cylinder head; said spark plug further comprising, at its end opposite its nearest end of said yoke, a connection element integral with a connecting device, said connecting element having a suitable shape enabling, by virtue of said transmission of said substantially inductive portion to said base, a tightening torque, the use of a simple and compact tool for the introduction and tightening of said spark plug on said cylinder head of said engine.

• ladite enveloppe de ladite partie essentiellement inductive est une feuille métallique mince, soudée audit culot de ladite bougie d'allumage, réalisée dans un matériau présentant une forte conductivité électrique ou revêtue, sur sa face intérieure, d'une couche mince conductrice d'un matériau présentant une forte conductivité électrique,
• ladite enveloppe de ladite partie essentiellement inductive et ledit culot de ladite bougie sont réalisés en une unique pièce métallique, dans un matériau présentant une forte conductivité électrique ou revêtue, sur la face intérieure, d'une couche mince conductrice d'un matériau présentant une forte conductivité électrique,
• ladite enveloppe de ladite partie essentiellement inductive est un manchon réalisé en matériau polymère, revêtu, sur sa face extérieure, d'une couche mince conductrice d'un matériau présentant une forte conductivité électrique,
• ladite enveloppe est réalisée en matériau polymère, et la partie dudit culot destinée à être en contact avec ladite enveloppe comporte une empreinte complémentaire de l'empreinte réalisée sur la section de l'enveloppe ; dans cette configuration, la couche mince conductrice s'étend jusqu'à l'empreinte réalisée sur la section de l'enveloppe, de manière à assurer un contact électrique entre les deux empreintes.
• Alternativement, ledit couple de serrage est transmis par l'âme centrale de ladite partie essentiellement inductive de ladite bougie, et l'invention peut alors présenter une ou plusieurs des caractéristiques suivantes :

• ladite âme centrale de ladite partie essentiellement inductive a la forme d'un mandrin présentant un diamètre élargi dans sa partie proche du culot de ladite bougie,
• ledit mandrin est réalisé dans un matériau isolant,
• ledit mandrin présente, dans sa partie proche dudit culot de ladite bougie, une forme sensiblement polygonale, et la forme complémentaire de cette forme sensiblement polygonale est réalisée dans le culot de ladite bougie,
• une troisième pièce est insérée radialement dans un logement commun audit mandrin et audit culot de ladite bougie,
• ladite troisième pièce est un pion inséré dans ledit logement commun selon une direction sensiblement perpendiculaire audit axe commun à ladite âme centrale de ladite partie essentiellement inductive et audit culot de ladite bougie,
• ladite troisième pièce est une clavette inséré dans ledit logement commun selon une direction sensiblement parallèle audit axe commun à ladite âme centrale de ladite partie essentiellement inductive et audit culot de ladite bougie,
• un manchon isolant est placé autour de ladite âme centrale de ladite partie essentiellement inductive, et la face extérieure dudit manchon isolant est revêtue d'une couche mince conductrice d'un matériau présentant une forte conductivité électrique,
• ladite couche mince conductrice d'un matériau à forte conductivité électrique, déposée sur ledit manchon isolant, est protégée par une couche mince conductrice d'un matériau barrière, ou intégrée à un film polymère appliqué sous vide sur ladite face extérieure dudit manchon isolant.

Advantageously, said tightening torque is transmitted by the perimeter of said essentially inductive portion of said spark plug and the invention may then have one or more of the following characteristics:

• said envelope of said substantially inductive part is a thin metal sheet, welded to said base of said spark plug, made of a material having a high electrical conductivity or coated on its inner face with a conductive thin layer of a material having a high electrical conductivity,
• said envelope of said essentially inductive part and said base of said spark plug are made of a single metal part, of a material having a high electrical conductivity or coated, on the inner face, a conductive thin layer of a material having a high electrical conductivity,
• said envelope of said essentially inductive part is a sleeve made of polymer material, coated on its outer face with a conductive thin layer of a material having a high electrical conductivity,
• said envelope is made of polymeric material, and the portion of said base intended to be in contact with said envelope has a complementary imprint of the impression made on the section of the envelope; in this configuration, the conductive thin layer extends to the imprint made on the section of the envelope, so as to ensure electrical contact between the two cavities.
• Alternatively, said tightening torque is transmitted by the central core of said essentially inductive portion of said spark plug, and the invention may then have one or more of the following characteristics:

• said central core of said essentially inductive portion is in the form of a mandrel having an enlarged diameter in its part near the base of said spark plug,
• said mandrel is made of an insulating material,
• said mandrel has, in its near part of said cap of said candle, a substantially polygonal shape, and the complementary shape of this substantially polygonal shape is formed in the base of said candle,
• a third piece is inserted radially into a housing common to said mandrel and to said base of said candle,
• said third piece is a pin inserted into said common housing in a direction substantially perpendicular to said axis common to said central core of said substantially inductive portion and said base of said candle,
• said third piece is a key inserted into said common housing in a substantially parallel direction said axis common to said central core of said substantially inductive portion and said base of said candle,
• an insulating sleeve is placed around said central core of said essentially inductive part, and the outer face of said insulating sleeve is coated with a conductive thin layer of a material having a high electrical conductivity,
• said conductive thin layer of a material with high electrical conductivity, deposited on said insulating sleeve, is protected by a conductive thin layer of a barrier material, or integrated in a vacuum-applied polymer film on said outer face of said insulating sleeve.

• la figure 1 est une vue en coupe schématique selon son axe longitudinal de ladite bougie à plasma radiofréquence,
• les figures 2A et 2B sont des vues schématiques en coupe selon son axe longitudinal d'un mode de réalisation préféré de l'invention dans lequel le couple de serrage est transmis par le périmètre de ladite partie essentiellement inductive,
• les figures 3 et 4 sont des vues en perspective du culot de la bougie et de l'enveloppe de la partie essentiellement inductive selon deux variantes du mode de réalisation présenté par la figure 2B,
• la figure 5 est une vue schématiques en coupe selon son axe longitudinal d'un mode de réalisation alternatif de l'invention dans lequel le couple de serrage est transmis par l'âme centrale de ladite partie essentiellement inductive,
• les figures 6A et 6B sont des vues schématiques en coupe selon son axe longitudinal de deux variantes du mode de réalisation alternatif de l'invention présenté par la figure 5.

Other characteristics and advantages of the invention will emerge on reading the description which follows, with reference to the appended drawings in which the invention is described in its application to a radiofrequency plasma candle and in which:

• FIG. 1 is a diagrammatic sectional view along its longitudinal axis of said radiofrequency plasma candle,
• FIGS. 2A and 2B are diagrammatic views in section along its longitudinal axis of a preferred embodiment of the invention in which the tightening torque is transmitted by the perimeter of said essentially inductive portion,
• FIGS. 3 and 4 are perspective views of the base of the spark plug and the casing of the essentially inductive portion according to two variants of the embodiment shown in FIG. 2B,
• FIG. 5 is a schematic sectional view along its longitudinal axis of an alternative embodiment of the invention in which the tightening torque is transmitted by the central core of said essentially inductive part,
• FIGS. 6A and 6B are schematic views in section along its longitudinal axis of two variants of the alternative embodiment of the invention presented in FIG. 5.

In what follows, the same element will be marked by the same notation on all the figures.

As shown in FIG. 1 and as previously described, a radiofrequency plasma plug 1 of substantially cylindrical general shape mainly comprises a substantially capacitive lower part C and an essentially inductive upper part I, said parts C and I being of shape. substantially elongated, connected in series and having a common longitudinal axis Z.

Said substantially capacitive portion C comprises, in particular, a base 2 intended to be connected to the ground and surrounding a central electrode 3, substantially cylindrical, axis Z, acting as a high voltage electrode. An insulator 4 is placed between the base 2 and the central electrode 3, said insulator 4 being configured to guide the sparks between the electrodes 2 and 3. In a manner well known in the state of the art, said base 2 has, on the outer face of its lower part closest to the cylinder head of the internal combustion engine equipped with said spark plug 1, a shape suitable for placing, holding and tightening said spark plug 1 on said cylinder head ( for example and without limitation, as shown in Figure 1: a thread).

Said essentially inductive part I of said spark plug 1 comprises, for its part, a winding 5 whose axis is advantageously substantially coincident with the axis Z of the central electrode 3 and the spark plug 1. Said winding 5 is, moreover surrounded by an envelope 6, preferably made of a nonmagnetic conductive material, intended to reduce the electromagnetic emissions of said spark plug 1 and to act as a shield, the dimensions of said envelope 6 being calculated so as to minimize the risks of breakdown between said winding 5 and said envelope 6. Advantageously, in order to further reduce the risks of breakdown, sources of parasitic energy dissipation, an insulator 7 is placed between said winding 5 and said envelope 6, and said winding 5 is made around a solid element 8 made of a non-magnetic insulating material. In the embodiment of the radiofrequency plasma candle shown in FIG. 1, a complementary insulator 9 is furthermore placed between said insulator 7 and said insulator 4 separating said base 2 and said central electrode 3, a part 10 electrically connected to said base 2 ensuring the connection between the holding of said insulators 4, 7, and 9.

The materials and embodiments and assembly of these various elements are defined and implemented so as to minimize any inclusion of air at the interfaces, also sources of breakdowns and losses.

A current measurement winding 11 is also generally made around said winding 5, and a connection device 13, integral with a connector 12 (not detailed in the figures), is placed at the upper end of said candle 1. A end of said coil 5 is connected to said connector 12, the other end of said coil 5 being connected by appropriate means 14 (not detailed in the figures) to an inner end of said central electrode 3.

As mentioned above, such a candle 1 behaves like an LC resonator, in which the capacitive part and the inductive part are placed in series, and in which the capacitive part C is composed by the base 2, the central electrode 3, the insulator 4 placed between said central electrode 3 and said base 2, and by the internal parasitic capacitances related to the geometry of said spark plug 1, the essentially inductive part I consisting mainly of the winding 5, the insulator 7 and the envelope 6.

In order to reduce these parasitic capacitances as much as possible (as mentioned above, the performances of the resonator thus constituted are all the better as the capacitance value of said capacitive part is small), the length of the connection between said substantially capacitive portion C and said substantially inductive portion I should be reduced as much as possible. Similarly, the overvoltage coefficient of the resonator LC constituted by said substantially inductive portion I and said substantially capacitive portion C placed in series is even better than the diameter of said substantially inductive portion I is high. Finally, the electrical quality of said connection between said substantially capacitive C and inductive I portions (continuity electrical, in particular) must also be maximum in order to optimize the performance of said resonator.

The device according to the invention proposes to introduce a rigid connection mode between some of the elements of each of the essentially capacitive parts C and essentially inductive I, and, by the implementation of materials and forms allowing the transmission of a couple clamping said substantially inductive portion I to said substantially capacitive portion C to allow the connection between said portions C and I of said spark plug 1 to be optimally performed both mechanically and electrically.

As will now be described, said tightening torque is, more particularly, transmitted to said base 2 of said spark plug 1, and this transmission of said tightening torque can be achieved either by the perimeter of said substantially inductive portion I (first embodiment of the device according to the invention, shown in FIGS. 2A, 2B, 3, and 4), or by the central core 8 of said essentially inductive part I (second embodiment of the device according to the invention shown in Figures 5, 6A and 6B). In addition, depending on the nature of the materials chosen, and in particular, depending on the thermomechanical properties including said insulation 7, several variants of each of the embodiments of the invention are possible.

FIGS. 2A and 2B are diagrammatic views in section, along the longitudinal axis Z of a radiofrequency plasma candle 1, variants of the first embodiment of the device according to the invention, in which the tightening torque for the embodiment the connection between said essentially capacitive portion C and said essentially inductive portion I of said spark plug 1 is transmitted by the perimeter of said substantially inductive portion I. The main advantage of this type of transmission is that it reduces the mechanical stresses on the larger radius available, where the leverage effect is optimal, thus minimizing mechanical stress on the actual materials. In this embodiment of the invention, said transmission of said pair clamping said substantially inductive portion I to said substantially capacitive portion C then returns to transmitting a torque of said casing 6 to said cap 2.

FIG. 2A shows an embodiment of the invention in which said envelope 6 is metallic, thus also ensuring the aforementioned shielding role. According to known industrial processes, said envelope 6 can be made of different metallic materials and by various appropriate techniques; it may, in particular, by way of example and in a non-exhaustive manner, be carried out at the same time as the base 2 (by stamping or stamping), or be welded to said base 2 after separate production of the two parts (stamping of said base 2 and drawing / rolling of said envelope 6).

Two variants are possible, depending on the thickness of said envelope 6.

In a first variant, the thickness of said envelope 6 is small (as an indication, it is substantially less than or equal to 0.5 mm): in this case, said insulator 7 placed between said coil 5 and said envelope 6 is chosen from way that it contributes to the mechanical strength of the whole being in solid and incompressible form. In this case also, a connection by welding of said envelope 6 to said base 2 will be preferred.

In a second variant, the thickness of said envelope 6 is greater (typically it is substantially greater than or equal to 0.5 mm): in this case, the intrinsic mechanical strength of said envelope 6 is sufficient to mechanically ensure the transmission of a couple torsion to said base 2, and said insulator 7 can then be in the form of a pressurized gas or an insulating liquid with a low coefficient of expansion. In this case then, the realization of said envelope 6 and said base 2 in the same room will be preferred.

In these two variants, the inner face of said envelope 6 must be coated with a layer of a few tens of microns thick (typically 30 to 50 μm, for example) of a material with high electrical conductivity (for example non-restrictive way: silver or copper), in order to improve the radiofrequency conductivity of said envelope 6 and to limit losses by skin effect within it.

Whatever the variant chosen, in this embodiment of the invention, the transmission of the tightening torque (torque here) of said substantially inductive portion I to said substantially capacitive portion C of said spark plug is obtained by the metal bond rigid made, during manufacture, between said casing 6 and said base 2, by one of the means described above (simultaneous manufacture of said casing and said base by stamping or stamping, or separate fabrications, then rigid assembly of said casing 6 and said base 2, for example by welding).

FIG. 2B shows an embodiment in which said envelope 6 is made of an insulating material of polymer type. These materials having a lower mechanical rigidity than metal materials, said envelope 6 must then have a relatively large thickness (substantially, and for example, of the order of a few millimeters) so as to allow the transmission of said pair Moreover, the material selected for producing said envelope 6 must, in this case, present a dielectric loss factor of the same order as that of the material of said insulator 7. In this embodiment of According to the invention, said insulator 7 can equally well consist of a pressurized gas or an insulating liquid with a low coefficient of expansion.

It should be noted that, in order to ensure the shielding function of said essentially inductive part I, the outer face of said envelope 6 must, in this embodiment of the invention, be coated with a layer of a few tens of microns thickness (typically and by way of non-limiting example: 30 to 50 μm) of a material with high electrical conductivity (for example and without limitation: silver or copper): the application of this coating on the outer face, and not on the inner face, of said envelope 6 makes it possible to increase slightly the outside diameter of said essentially inductive part I, which improves the overvoltage coefficient of the resonator consisting of said parts C and I as previously described.

In this embodiment of the invention, the transmission of the tightening torque between said essentially inductive part I and said essentially capacitive part C of said spark plug 1 is obtained by appropriate shaping of the interface between said base 2 and said envelope 6, two variants of which are shown in Figures 3 and 4.

FIG. 3 shows a mode of shaping said base 2 and said envelope 6 in which the part of said envelope 6 intended to come into contact with said base 2 has the shape of a male cavity 15, and a complementary female cavity 16 said male cavity 15 is made on the portion of said base 2 intended to come into contact with said envelope 6.

Alternatively, and as shown in FIG. 4, said male cavity 15 is made on the part of said base 2 intended to come into contact with said envelope 6, and said female cavity 16 complementary to said male cavity 15 is formed on the part of said envelope 6 intended to come into contact with said base 2.

It should be noted that the geometry of said male and female fingerprints 16 shown in FIGS. 3 and 4 is indicated by way of nonlimiting example, and that the shape of said fingerprints 15 and 16 is, in general, to be adjusted. in particular, depending on the materials used and the costs of production.

It should also be noted that the principle of producing a male footprint 15 and a complementary female footprint 16 that has just been mentioned can, in the embodiment of the invention presented in FIG. transposed to the realization of the connection between said envelope 6 and the connecting element 13 for receiving the connector 12 of electrical connection of said candle 1 to its supply device.

FIGS. 5, 6A and 6B are diagrammatic views in section along the longitudinal axis Z of a radiofrequency plasma candle 1 according to the invention, variants of the second embodiment of the device according to the invention, in which the tightening torque for the embodiment of the connection between said substantially capacitive portion C and said essentially inductive portion I of said spark plug 1 is transmitted by the central core of said substantially inductive portion I. An important advantage of this mode of transmission is that it makes it possible to release the all of the outer space of said winding 5 of said substantially inductive portion I, and that it allows the use, for said insulator 7, of materials having significant coefficients of expansion with temperature.

For this transmission mode of said tightening torque, and as shown in FIG. 5, said central element 8, around which said winding 5 is made, is produced in the form of a mandrel having, in its part that is closest to said base 2, an enlarged portion 17 of complementary shape and dimension of the inner shape of the base 2, so as to be able to mechanically couple with said base 2. For example and without limitation, the outer shape of said enlarged portion 17 may to be substantially polygonal, a cavity complementary to said substantially polygonal shape being formed in said base 2. Alternatively and in a nonlimiting manner, a connecting element (not shown in FIG. 5) can also be inserted into a housing that is common to said base 2 and to said enlarged portion 17 of said central element 8: this element may, for example and non-exhaustively, be a key inserted substantially parallel to the longitudinal axis Z of said spark plug 1 according to the invention, or a pin inserted substantially perpendicularly to said longitudinal axis Z of said spark plug 1.

Advantageously, a rounded shape will be preferred at a sharp angle at the level of the widening 17 of said central element 8, so as to limit the mechanical stresses and the risks of electromagnetic breakdown in this zone.

In this embodiment, the shielding function previously mentioned, and provided by said envelope 6, can be obtained by various means, illustrated by FIGS. 6A and 6B.

FIG. 6A shows an embodiment in which said envelope 6 surrounding said insulator 7 placed around said winding 5 is a sheet of a few tens of microns thick (typically and not limited to 75 to 150 μm) made of a material having a high electrical conductivity, or covered, on its face in contact with said insulator 7, with a conductive thin layer of a material having a high electrical conductivity (for example and without limitation: silver or copper) , so as to preserve, as already described above, the electromagnetic quality of the assembly. In the case where said insulator 7 has a high coefficient of thermal expansion, said envelope 6 is preferably wound in a polygonal shape around said insulator 7: said insulator 7 can thus expand by converging towards a substantially cylindrical external shape without inducing mechanical stress additional on the perimeter of said envelope 6.

FIG. 6B shows an alternative embodiment, in which said envelope 6 is replaced by the deposition, on the outer surface of said insulator 7, of a conductive thin layer 18 (for example and without limitation, a layer of 30 to 50 μm thick) of a material having a high electrical conductivity (silver or copper, for example and without limitation).

According to a variant of the embodiment shown in FIG. 6B and for reasons of simplicity of manufacture and cost reduction, said thin conductive layer 18 may be deposited on a polymer film itself vacuum-plated on said insulator 7. Advantageously in the two cases just mentioned, said thin conductive layer 18 will be protected from the outside, and, in particular, oxidation or corrosion phenomena that can degrade its electrical performance, by a conductive thin layer of a suitable barrier material (for example and without limitation: a layer of varnish or a polymer film).

In all the embodiments previously described, the electrical continuity of the assembly of said spark plug 1 according to the invention is finalized by said connecting element 13 intended for the connection of said connector 12 to the power supply of said spark plug 1.

Advantageously, said element 13, bound in a rigid form, in particular, to said envelope 6 (for example and in a nonlimiting manner, thanks to a means of the type of those just described for the connection of said envelope 6 with said base 2), has a shape adapted to the use of a simple clamping tool for the introduction and tightening of said spark plug 1 in the cylinder head of the engine.

It thus appears that a spark plug according to the invention can be put in place and removed from its access well without the need to slide a specific tool around said candle: it is therefore not necessary to provide, in said access shaft to said candle, a space necessary for the passage of said specific tool and used only for the assembly and disassembly operations of said candle.

It should be noted that the principle of rigid connection both mechanical and electrical between the two parts of a spark plug (candle itself and its coil), here described in its application to a radiofrequency plasma candle, and preferentially intended for this type of candle, can be transposed to any type of conventional candle.

Claims (14)

Spark plug (1) of an internal combustion engine of a motor vehicle, of substantially cylindrical general shape, of the type comprising mainly a substantially capacitive part (C), the closest to the cylinder head of said engine, and a substantially inductive part (I), furthest from said cylinder head of said engine, said essentially capacitive (C) and essentially inductive (I) portions being connected in series to form an LC resonator and having a common longitudinal axis substantially coincident with the longitudinal axis (Z ) of said spark plug (1); wherein said substantially inductive portion (I) is surrounded by an envelope (6) whose dimensions and materials are, in particular, suitable for producing an electromagnetic shielding of said essentially inductive portion (I), characterized in that said essentially capacitive part (C) and said essentially inductive part (I) of said spark plug (1) are mechanically and electrically interconnected by a single rigid device. Spark plug (1) according to claim 1, wherein the substantially capacitive portion (C) comprises a base (2) surrounding a substantially cylindrical central electrode (3) and acting as a high voltage electrode, characterized in that said rigid single device makes it possible to transmit a tightening torque of said essentially inductive part (I) to said essentially capacitive part (C). Spark plug (1) according to any one of claims 1 to 2, characterized in that said transmission of said tightening torque is performed by the perimeter of said essentially inductive portion (I) of said spark plug (1) . Spark plug (1) according to claim 3, wherein said casing (6) is metallic, characterized in that said casing (6) and said cap (2) constitute a single piece. Spark plug (1) according to claim 3, wherein said casing (6) is a metal foil, characterized in that said casing (6) is welded to said base (2). Spark plug (1) according to claim 4 or 5, characterized in that the inner face of said casing (6) is coated with a conductive thin layer of a material with high electrical conductivity. Spark plug (1) according to claim 3, characterized in that said casing (6) is a sleeve made of polymer material, and in that the outer face of said casing (6) is coated with a conductive thin layer of a material with high electrical conductivity. Spark plug (1) according to claim 7, characterized in that the portion of said base (2) intended to be in contact with said envelope (6) comprises a cavity (16) complementary to the cavity (15) produced on the section of the envelope (6), and in that the conductive thin layer extends to the impression made on the section of the envelope (6). Spark plug (1) according to any one of claims 1 to 2, wherein said inductive portion (I) comprises, in particular, a coil (5) formed around a central element (8) and surrounded by a insulating sleeve (7), said insulating sleeve (7) being itself surrounded by said envelope (6), characterized in that said transmission of said tightening torque is performed by said central element (8). Spark plug (1) according to claim 9, characterized in that said central element (8) comprises, at its end closest to said base (2), an enlarged part (17) of shape complementary to the shape of the inner face of the base to provide a rigid mechanical connection with said base (2). Spark plug (1) according to claim 9, characterized in that said enlarged portion (17) of said central element (8) and said base (2) comprise a common housing for receiving an element ensuring the connection between said enlarged part (17) and said base (2). Spark plug (1) according to any one of claims 9 to 11, wherein said envelope (6) is a sheet of a few tens of microns thick of a conductive material, characterized in that said envelope (6 ) is wrapped in a polygonal shape around said insulating sleeve (7). Spark plug (1) according to any one of claims 9 to 12, characterized in that said casing (6) is coated on its inner face with a conductive thin layer of a material having a high electrical conductivity . Spark plug (1) according to any one of claims 9 to 13, characterized in that the outer face of said insulating sleeve (7) is coated with a conductive thin layer (18) of a material having a high conductivity electric.

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