MX2008006072A - Sparkplug for an internal combustion engine - Google Patents

Abstract

Sparkplug (1) for the internal combustion engine of a motor vehicle, of substantially cylindrical overall shape, comprising:- an essentially capacitive lower part (C) comprising:âÇ¢two coaxial electrodes:an internal electrode of axis (D) termed the central electrode (3) and an external electrode termed the shell (2) surrounding the central electrode (3)âÇ¢an electrically insulating block termed the insulator (4) inserted between the central electrode (3) and the shell (2), - an essentially inductive upper part (I) comprising:âÇ¢a central sleeve (8) surrounded by a coil (5), a bottom end part (57) of which surrounds a top end part (31) of the central electrode (3)âÇ¢an external casing (6),âÇ¢an insulator (7) inserted radially between the casing (6) and the coil (5) characterized in that the top end part (31) of the central electrode (3) has a coating (9) of material which:- has greater electrical conductivity than the material of the central electrode (3), - has no ferromagnetic properties.

Description

SPARK PLUG FOR AN INTERNAL COMBUSTION ENGINE FIELD OF THE INVENTION The invention relates to a spark plug generating plasma, used especially for the ignition of internal combustion engines by means of electric sparks.

BACKGROUND OF THE INVENTION The spark plug for the internal combustion engine of a motor vehicle, having a substantially cylindrical general shape, comprises: an essentially capacitive lower part comprising: two coaxial electrodes: an internal electrode with an axis known as an electrode central (3) and an external electrode known as the lining surrounding the central electrode, and an electrically insulating block known as the insulator inserted between the central electrode and the liner; and an essentially inductive upper portion comprising: a central mandrel surrounded by a coil, a lower end portion which surrounds an upper end portion of the central electrode, an external armature, and an insulator radially inserted between the armature and the coil. The publications FR 2 859 830, FR 2 859 569 and FR 2 859 831 are related to a multibuy of this type that integrates a series of resonators. The simple winding back with the coil allows a high quality coefficient to be achieved, which is the ratio between the energy stored in the structure and the ohmic and dielectric losses. All energy in turn is stored in magnetic form and transferred to the essentially capacitive part.

In addition, the lower end portion of the coil surrounds the upper end portion of the central electrode. This zone of electromagnetic superposition induces parasitic currents in the central electrode, which have the effect of creating extra resistance and thus reducing the overvoltage coefficient of the coil / spark plug assembly.

SUMMARY OF THE INVENTION In order to alleviate these disadvantages, the invention has the purpose of decreasing the dissipation of the electromagnetic energy induced by the eddy currents in the central electrode in order to optimize the properties of the spark plug nozzle, in particular a high overvoltage coefficient.

For this purpose, the invention provides a spark plug of the type mentioned hereinabove, characterized in that the upper end part of the central electrode has a coating of material which is more electrically conductive than the material of the central electrode and which does not have ferromagnetic properties. According to another feature of the invention, the coating thickness is at least equal to the thickness of the initial coating of the central electrode. According to another feature of the invention, the axial height of the coating is at least equal to the height of the upper end portion of the central electrode.

BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will become apparent upon reading the description of exemplary embodiments with reference to the appended figures. Figure 1 shows a schematic sectional view along the Z axis of the radio frequency plasma spark plug according to the prior art. Figure 2 shows a schematic sectional view along the Z axis of the part of a radio frequency plasma spark plug comprising a lower end portion of the coil and an upper end portion of the central electrode according to the prior art .

Figure 3 shows a sectional view of a central electrode according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES OF THE INVENTION Identical or similar elements were designated by the same reference numerals. As shown in Figure 1, a radio frequency plasma plug 1 having a substantially cylindrical general shape with a Z axis mainly includes a substantially capacitive lower part C and an essentially inductive upper part I connected in series, the parts C being I of substantially elongated shape. The essentially capacitive part C includes in particular a liner 2 for connecting to ground and surrounding a substantially cylindrical central electrode 3 with a Z axis, which is the high voltage electrode. An electrically insulating block, known as the "insulator" 4, is placed between the liner 2 and the central electrode 3. As is well known in the prior art, the liner 2 has, on the outer face of its lower part, the which is closer to the cylindrical head of the internal combustion engine provided with the spark plug 1, a shape suitable for installing, holding and holding the spark plug 1 in the cylindrical head (for example, and in a non-limiting manner, as shown in FIG. Figure 1: a thread). The essentially inductive part I of the spark plug 1 has, from the inside out: a central mandrel 8, a coil 5, an insulator 7 and an external reinforcement 6. The central mandrel 8 has a generally cylindrical shape with a circular cross section, the axis of which coincides approximately with the Z axis of the spark plug 1.

This is made of an insulating and non-magnetic material. The coil 5 has a generally cylindrical shape with a circular cross section. This consists of a coiled wire of diameter D forming contiguous turns 51 surrounding the central mandrel 8 starting from a first turn 51a to a last turn 51b, those two end turns 51a, 51b of coil 5 constituting the first turn 51a is connected to the connector 12 and the last turn 51b is connected by means of suitable means 14 to an inner end of the central electrode 3. As shown in Figure 2, a lower end portion 57 of the coil 5 surrounds an upper end part 31 of the central electrode 3 which is placed on the mandrel 8. As shown in Figure 1, the insulator 7, which surrounds the coil 5, has a general cylindrical shape.

It can be chosen from various materials such as silicon. The external armature 6 has a generally cylindrical shape. This is connected to earth and surrounds coil 5. Armature 6 has an electromagnetic protection function. Armature 6 can be chosen from a non-ferrous, highly conductive material such as copper. As shown in Figure 3, according to one embodiment of the invention, the central electrode 3 has a coating 9. The coating 9 is axially on the upper end part 31 of the central electrode 3. This coating 9 can be deposited by example by electrolysis. The coating is characterized by being more electrically conductive than the material of the central electrode 3 and by having non-ferromagnetic properties. For example, the central electrode 3 can be chosen from nickel, whose conductivity is 14.3 * 106 S / m (Siemens / meter), and the coating 9 can be chosen from silver, whose electrical conductivity is 63 * 105 S / m. With respect to the dimensions of the cover 9, it is characterized by: a thickness E and a height H. The radial thickness E is at least equal to the film thickness of a central electrode without coating 9. It will be recalled that the film thickness it will be defined as a thickness of the material in question, after which the induced currents are reduced by an "e" factor (where ln (e) = l). The film thickness can be calculated using the following equation: d = p - μ - s - f where: μ = permeability; μ = μ0. μrr where μr = relative material permeability and μ0 = permeability in a vacuum; s (S / m) = electrical conductivity; and f (Hz) = frequency in Hz. The height in H is at least equal to the height of the upper end portion 31 of the central electrode 3. The lower end portion 57 of the coil 5 surrounding the upper end portion 31 of the central electrode 3 is known as the electromagnetic superposition zone A. This zone A induces eddy currents in the central electrode 3.

The losses caused by eddy currents are proportional to the square of the frequency. The thickness of the film decreases in proportion to the square root of the permeability, frequency, and electrical conductivity and the dimensions of the piece in question. The thicker the piece in the perpendicular plane the magnetic field in relation to the film thickness, the greater the induced currents will be. Since the eddy currents are in the film thickness of the core electrode material 3, one embodiment of the invention provides a coating 9 which replaces this film thickness. This coating 9 allows a reduction in eddy currents and film thickness. In this way, the resistance of the central electrode 3 decreases. The overvoltage coefficient simply in this way and as a result improves the performance of the spark plug. This invention is not limited to the described and illustrated embodiment that has been given by way of example.

Claims (3)

CLAIMS 1. A spark plug for the internal combustion engine of a motor vehicle, having a substantially cylindrical general shape, comprising: - an essentially capacitive lower part
1. (C) comprising: two coaxial electrodes: an internal electrode with an axis (Z) known as the central electrode and an external electrode known as the coating surrounding the central electrode, and an electrically insulating block known as the insulator inserted between the electrode central and the coating; and an essentially inductive upper part (I) comprising: a central mandrel surrounded by a coil, a lower end portion which surrounds an upper end portion of the central electrode, an external reinforcement, and an insulator radially inserted between the coating and the coil; characterized in that the upper end part of the central electrode has a coating of material which: is more electrically conductive than the material of the central electrode; and it does not have ferromagnetic properties.
2. 2. The spark plug according to claim 1, characterized in that the radial thickness (E) of the coating is at least equal to the initial film thickness of the central electrode. The spark plug according to any of the preceding claims, characterized in that the axial height (H) of the coating is at least equal to the height of the upper end portion of the central electrode.