EP1949514B1 - Sparkplug for an internal combustion engine - Google Patents

Abstract

A spark plug for an internal combustion engine of a motor vehicle, having a substantially cylindrical general form, includes an essentially capacitive lower part and an essentially inductive upper part. The lower part includes two coaxial electrodes, including a central electrode with an axis and a shell electrode that surrounds the central electrode, and an insulator inserted between the central electrode and the shell electrode. The upper part includes a central mandrel surrounded by a coil, a bottom end part of which overlaps a top end part of the central electrode in a radial direction, an external casing, and an insulator inserted radially between the casing and the coil. The top end part of the central electrode has a coating of material which is more electrically conductive than the material of the central electrode and has no ferromagnetic properties.

Description

The invention relates to a plasma generator spark plug, used in particular for igniting internal combustion engines by electric spark.

• une partie inférieure essentiellement capacitive comportant :
  • deux électrodes coaxiales : une électrode interne d'axe appelée électrode centrale (3) et une électrode externe appelée culot entourant l'électrode centrale
  • un bloc isolant électriquement, appelé isolant interposé entre l'électrode centrale et le culot,
• une partie supérieure essentiellement inductive comportant :
  • un mandrin central entouré d'un bobinage, dont
  • une partie d'extrémité basse entoure une partie d'extrémité haute de l'électrode centrale
  • une enveloppe externe,
  • un isolant interposé radialement entre l'enveloppe et le bobinage.

The spark plug for the internal combustion engine of a motor vehicle, of generally cylindrical general shape, comprising:

• a substantially capacitive lower part comprising:
  • two coaxial electrodes: an internal axis electrode called a central electrode (3) and an external electrode called a cap surrounding the central electrode
  • an electrically insulating block, called an insulator interposed between the central electrode and the base,
• an essentially inductive upper part comprising:
  • a central mandrel surrounded by a winding, of which
  • a low end portion surrounds a high end portion of the central electrode
  • an outer envelope,
  • an insulator interposed radially between the casing and the coil.

The publications, FR2859830 , FR2859569 , FR2859831 relate to such a spark multi-spark incorporating a series resonator. The single coil winding of the winding ensures a high quality coefficient which is the ratio between the energy stored in the structure and the ohmic and dielectric losses. All the energy is stored in magnetic form and transferred to the essentially capacitive part. Furthermore, the low end portion of the coil surrounds the high end portion of the central electrode. This Electromagnetic overlap zone induces eddy currents in the central electrode which have the effect of creating additional resistance and thus reducing the overvoltage coefficient of the winding-plug assembly.

In order to overcome these drawbacks, the invention aims at reducing the electromagnetic energy dissipation induced by the eddy currents in the central electrode so as to optimize the properties of the spark plug, in particular a high overvoltage coefficient.

For this purpose, the invention proposes a candle of the type mentioned above, characterized in that the high end portion of the central electrode comprises a coating of material which has an electrical conductivity greater than that of the material of the central electrode and which has no ferromagnetic properties.

According to other characteristics of the invention, the thickness of the coating is at least equal to the initial skin thickness of the central electrode.

According to other features of the invention, the axial height of the coating is at least equal to the height of the high end portion of the central electrode.

Other features and advantages of the invention will appear on reading the description of exemplary embodiments with reference to the appended figures.

The figure 1 is a schematic sectional view along the Z axis of a radiofrequency plasma candle according to the state of the art.

The figure 2 is a schematic sectional view along the Z axis of a portion of a radiofrequency plasma candle having a low end portion of the coil and a high end portion of the central electrode according to the state of the technical.

The figure 3 represents a sectional view of a central electrode according to the invention.

Identical or similar elements are designated by the same reference numerals.

As represented in figure 1 , a radiofrequency plasma candle 1 of substantially cylindrical general shape Z axis mainly comprises a substantially capacitive bottom portion C and a substantially inductive upper part I, the parts C and I being substantially elongated in shape, connected in series.

The 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 electrically insulating block, called "insulator" 4 is placed between the base 2 and the central electrode 3. In a manner well known in the state of the art, the base 2 has, on the outer face of its lower part. the closest to the cylinder head of the internal combustion engine equipped with the spark plug 1, a form suitable for the introduction, maintenance and tightening of the spark plug 1 on the cylinder head (for example and without limitation, as shown on the figure 1 : a thread).

The essentially inductive part I of the spark plug 1 has from inside to outside: a central mandrel 8, a winding 5, an insulator 7, an outer envelope 6.

The central mandrel 8 is generally cylindrical in shape with a circular section whose axis is substantially coincident with the Z axis of the candle 1. It is made of insulating and non-magnetic material.

The winding 5 is of cylindrical general shape with circular section. It is composed of a wire of diameter D wound and forming contiguous turns 51 surrounding the central mandrel 8 from a first turn 51a to a last turn 51b, which constitute the two end turns 51a, 51b of the winding 5. The first turn 51a is connected to the connector 12 and the last turn 51b is connected by appropriate means 14 to an inner end of the central electrode 3.

As represented in figure 2 a low end portion 57 of the coil 5 surrounds a high end portion 31 of the central electrode 3 which is inserted into the mandrel 8.

As shown in figure 1 , the insulator 7 surrounding the coil 5 is of generally cylindrical shape. It can be chosen in different materials such as silicone.

The outer casing 6 is of generally cylindrical shape. It is connected to a mass and surrounds the winding 5. The envelope 6 has an electromagnetic shielding function. The envelope 6 may be chosen from a high-conductivity non-ferrous material such as copper.

As shown in figure 3 , according to one embodiment of the invention, the central electrode 3 comprises a coating 9. The coating 9 is located axially on the upper end portion 31 of the central electrode 3. This coating 9 may be deposited for example by electrolysis.

The coating is characterized by an electrical conductivity greater than that of the material of the central electrode 3 and does not exhibit ferromagnetic properties. For example, the central electrode 3 may be chosen from nickel whose conductivity is 14.3 * 10 ⁶ S / m (Siemens / meter) and the coating 9 may be chosen from silver whose electrical conductivity is 63 * 10 ⁶ S / m.

• une épaisseur E et
• une hauteur H.

Regarding the dimensions of the coating 9, it is characterized by:

• a thickness E and
• a height H.

où :

• µ : perméabilité
• µ = µ₀.µr µr : perméabilité relative du matériau, µ₀ perméabilité dans le vide
• σ (S/m) : conductivité électrique
• f (Hz) : fréquence en Hz

The radial thickness E is at least equal to the skin thickness of a central electrode without a coating 9. It should be recalled that the skin thickness is defined as the thickness in the material under consideration, at the end of which the induced currents are reduced by a factor "e" (with ln (e) = 1). The skin thickness can be calculated with the equation below: $$\delta =\sqrt{\frac{1}{\pi \cdot \mu \cdot \sigma \cdot f}}$$

or :

• μ: permeability
• μ = μ ₀ .μr μr: relative permeability of material, μ ₀ permeability in vacuum
• σ (S / m): electrical conductivity
• f (Hz): frequency in Hz

The height H is at least equal to the height of the high end portion 31 of the central electrode 3.

The low end portion 57 of the coil 5 surrounding the high end portion 31 of the central electrode 3 is called the electromagnetic overlap zone A. The latter A induces eddy currents in the central electrode 3. The losses by Foucault current is proportional to the square of the frequency. The skin thickness decreases in proportion to the square root of the permeability, frequency, electrical conductivity and size of the room. The thicker the piece in the plane perpendicular to the magnetic field than the skin thickness, the higher the induced currents will be.

The eddy currents being located in the skin thickness of the material of the central electrode 3, an embodiment of the invention provides a coating 9 which replaces this skin thickness. This coating 9 makes it possible to reduce the eddy currents and the skin thickness. In this way, the resistance of the central electrode 3 is decreased. The overvoltage coefficient is therefore increased and consequently the performance of the candle is improved.

This invention is not limited to the described and illustrated embodiment which has been given by way of example.

Claims (3)

1. Spark plug (1) for the internal combustion engine of a motor vehicle, having a substantially cylindrical general form, comprising:

   - an essentially capacitive lower part (C) comprising:

   two coaxial electrodes: one internal electrode with an axis (Z) known as the central electrode (3) and one external electrode known as the shell (2) surrounding the central electrode (3), and

   an electrically insulating block known as the insulator (4) inserted between the central electrode (3) and the shell (2); and

   - an essentially inductive upper part (I) comprising:

   a central mandrel (8) surrounded by a coil (5), a bottom end part (57) of which surrounds a top end part (31) of the central electrode (3),

   an external casing (6), and

   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:

   - is more electrically conductive than the material of the central electrode (3); and

   - has no ferromagnetic properties.
2. Spark plug (1) according to Claim 1, characterized in that the radial thickness (E) of the coating (9) is at least equal to the initial skin thickness of the central electrode (3).
3. Spark plug (1) according to either of the preceding claims, characterized in that the axial height (H) of the coating (9) is at least equal to the height of the top end part (31) of the central electrode (3).

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