PLUG SPARK EQUIPEE OF MEANS OF PREVENTION OF the SHORT-CIRCUITS

A spark plug equipped with means for prevention of courtscircuits

The present invention relates to a spark plug, plasma type radio frequency (RF), for internal combustion engine.

More specifically, the candle is equipped with means to avoid short-circuit.

To increase the dimensions of the spark generated upon ignition, it is known to use a plasma plug having a central electrode at pointed end and brought to a high voltage spark-generating multifilament very large.

Such plug which is described, for example, in the FRs 2,878 086, FR 2,886 776, FR 2,888 421, FR2 895 169 and FR 2,895 170, comprises:

-a cylindrical cup main axis formed in a first electrically conductive material,

a center electrode formed in a second conductive material and extending along the major axis and,

-a body formed of an electrically insulating material, disposed between said base and said electrode, with an inner side face of contact with the electrode and beyond which an end portion of the electrode projects.

However, it is found that as the use of the candles, the size of the spark gradually decreases.

This phenomenon is due to the fact that the external face of the insulating body which is in contact with the gas mixture of the fuel and flame, becomes fouled with the combustion residue.

Furthermore, parasitic a spark appears on the outer face fouled between the central electrode and the shell peripheral mass.

Based on the level of fouling, the spark can, or only consume a portion of the energy available in the radio frequency power supply (RF) thereby reducing, the main volume of the spark, or cause a short circuit which prevents the transmission of the main spark.

The origin of the problem in the profile of the junction between the outer face of the insulating body and the center electrode, where the media dielectric-metal-air are in contact.

More specifically, the body has a central duct in which is housed the center electrode.

For reasons of industrial production, it is impossible to perform a conduit to sharp edge so that the outer face of the dielectric body can be oriented perpendicular to the wall of the electrode and then has a slight inclination or a curvature.

There are consequently in a small free space where the electrostatic field lines pass through a volume of air having the dielectric constant is close to a single unit prior to entering the body having a higher dielectric constant and of the order of 7 to 9.

In these conditions, the intensity of the electric field is reinforced in the local area of body-electrode junction where the threshold for ionization of air is be widely exceeded.

Therefore, the inclined surface or curvilinear of the insulating body in the junction region generates a tangential field component which promotes the propagation of a ionization avalanche phenomenon.

Or, this phenomenon contributes to ignite a spark ground roll driven by the presence of a semiconductor layer bonded carbon to fouling of the external face of the body.

Candles radio frequency of the prior art have only sought to reduce fouling of the body.

To solve the technical problem set forth above by operating on the origin of the problem, i.e., in order to avoid the reinforcement of the electrical field at the junction between the outer face of the dielectric body and the center electrode.

This is achieved, according to the invention, by means of a candle with the dielectric body includes at least one annular cavity central axis, provided on the side face of contact with the electrode so as to locally modify the lines of the electrostatic field at the junction between the outer face of the said body and the electrode.

In accordance with an advantageous feature, said cavity is provided with at least one metal element packing extending, at least partially, along the inner side face of the body in contact with the electrode and whose coefficient of thermal expansion is close to that of the dielectric material of said body.

According to a first alternative embodiment said packing element is formed of a metal layer covering the inner wall of said cavity.

Preferably, said metal element packing is constructed of a single piece.

In a second embodiment, said body is made by sintering a ceramic composition around the electrode carrying said packing element.

Alternatively, said electrode is formed with the metal element of packing metal casting in situ in said cavity.

In yet another embodiment, said metal member lining is attached by brazing.

In another advantageous feature, said packing element is an alloy of one or more metals selected from molybdenum, Tungsten, the Invar and Kovar.

According to the invention, it is also provided that the junction between the outer face of the body and the electrode has a convex curvature which the radius is less than or equal to half the distance between said face of said cavity.

Preferably, the wall of said cavity has a semi-elliptical profile which the semi-minor axis is greater than or equal to three times the radius of curvature of the junction between the outer face of the body and the electrode and the semi-major axis is greater than or equal to six times said radius.

The spark plug of the present invention protect the radio frequency fouling plugs acting on the cause of the phenomenon.

The invention is based on a field weakening at the junction between the external face of the body and the electrode obtained by modifying their internal interface in contact.

Therefore, the operation and the reliability of the candles (RF) are improved and their service life is significantly prolonged.

The invention be included at the reading the description which will follow, with reference to the accompanying drawings on which:

Figure 1 represents a sectional side view of an embodiment of the candle in an ignition phase.

Figures 2A and 2B are enlarged views of detail and cross-section respectively of a RF plug according to the prior art and according to the invention by the field lines.

Figures 3Α , 3B and 3C are half-view detail in cross-section of three embodiments of the candle.

Radio frequency The plug 1 shown in Figure 1 has a coaxial structure comprising:

-a cylindrical cup 2 X main axis formed in a first electrically conductive material,

a center electrode 4 formed in a second conductive material and extending along the central axis X and,

3-a body formed in an electrically insulating material (dielectric), disposed between the base 2 and the electrode 4.

The body 3 has an inner side face 3b 4b of contact with a portion of the electrode and beyond which a tip end portion 4a of the electrode 4 projects.

The electrode 4 is heated to a high RF voltage and is therefore isolated by the body 3 of the cap 2 which is, in turn, connected to ground.

The body 3 is generally made of a dielectric material such as a ceramic.

Under normal conditions RF electrical excitation, the diameter of the spark A multi filaments 5 is in the order of centimetres.

The outer face 3a of the body 3 which is planar over the major part of its surface is exposed to the gas mixture and fuel to the flame and is therefore expected to be fouled by carbon deposit.

The semiconducting properties from this deposition of surface leads to a high risk of short-circuit between the base 2 and the electrode 4.

The figure 2A represents a view to cutting a conventional RF spark plug.

The junction of the outer face 3a of the dielectric body 3 with the electrode 4 has an inclined profile or chamfered or more generally, as in the embodiment represented, a curvilinear profile with a radius of curvature R of one hundred microns (0.1 mm).

Such profile defines an empty space restricted and confined of very intense electrical field (of the order of 3.10⁷ V/m) of occurrence of ionization mechanism of the air that can be propagated along the curve and extend along the face 3a by creating a spark ground roll.

This parasite is all the more likely that the face 3a is heavily exposed to the flame and is gradually covered with a layer of carbon.

The candle shown in Figure 2B is changed, according to the invention, by performing, on the body 3, of at least one annular recess 30 X-axis.

The cavity is provided on the inner side face 3b of the body 3 for contact with the electrode 4.

The cavity 30 is operable to locally change the electrostatic field lines and, in particular, at the junction between the outer face 3a of the body 3 and the electrode 4 so as to avoid the increase of the electric field in this area and to prevent the initiation of ionization.

The connection between the outer face 3a of the body 3 and the electrode 4 has a convex curvature which the radius R is less than or equal to half the distance (c) separating the face 3a of the cavity 30.

Preferably, the wall of the cavity 30 has a semi-elliptical profile which the semi-minor axis (b) is greater than or equal to three times the radius of curvature R of the junction between the outer face 3a of the body 3 and the electrode 4 and the semi-major axis (a) is greater than or equal to six times said radius.

The cavity 30 is provided with at least one metal element packing 5 whose thermal expansion coefficient is close to that of the dielectric material of the body 3.

The metal member packing 5 also extends and, at least partially, along the inner side face 3b of the body 3 in contact with the electrode 4 in the form of a coating 53 for filling the small gap spacer between these parts.

Indeed, if the space between the inner side face 3b of the body 3 and the electrode 4 was not sealed, the gas mixture would then carburized capable of entering said space and come into contact with the walls to high temperature prior to ignite and cause breakage of the body 3.

Furthermore, the coating 53 is in electrical contact with the electrode 4 within the candle 1 so that the whole inner surface of the body 3 is equipotential with the electrode which is in high voltage.

Figures 3A to 3C represent alternative forms of the candle.

In variant of Figure 3A, the packing element 5 is formed of a metal layer 50 covering the inner wall of the cavity 30 and extending across the cavity and along the electrode 4 by the coating 53.

The layer 50 and the coating 53 are made from Molybdenum alloys and/or Tungsten.

In variant of Figure 3B, the coating 53 is added a second packing element 51 made by filling the cavity 30 with a metal alloy based and Kovar/or invar.

Joining is conducted by soldering in order to ensure good contact between the respective facing faces of the body 3 and of the element 51.

In variant of Figure 3C which corresponds to that of Figure 2B, the second packing element 52 is formed integrally with the electrode 4 and is itself provided with a first packing element in the form of the metal coating 53.

In this case, the body 3 can be produced by sintering of a ceramic composition around the electrode 4 carrying the metal element 5.

An alternative may comprise conducting directly, the electrode 4 with a metal element 52 by metal casting in situ in the central conduit and the cavity 30 of the body 3.