Abstract:
An internal combustion engine radio frequency spark plug including two plasma-generating electrodes, separated by an insulator, that constitute respectively an outer shell enclosing the insulator and a central electrode housed in a central bore of the insulator. The spark plug includes a deep opening over the entire circumference of the shell, forming a heat-exchanger chamber inside the spark plug shell, opening outwards.

Description:
BACKGROUND 
     The present invention relates to a plasma-generating spark plug used in particular for the ignition of internal combustion engines using electric sparks between the electrodes of a spark plug. 
     More specifically, the invention relates to an internal combustion engine radio-frequency spark plug comprising two plasma-generating electrodes separated by an insulator, it being possible for one of the two electrodes to consist of the entirety of the cylinder head and of the shell of the spark plug. 
     Plasma-generating spark plugs are high-frequency multi-spark ignition systems capable of providing ignition in spark-ignition engines under the best conditions while at the same time reducing polluting emissions, particularly under lean burn conditions. They are liable to coking, particularly when cold. 
     Like all spark plugs, they are characterized by a thermal index. This index takes account of their thermal behavior at particular engine operating points. In particular, it provides an indication as to their ability to withstand temperatures that are high enough to avoid coking by pyrolysis, without suffering from “pre-ignition”. 
     Publications FR 2859830, FR 2859869 and FR 2859831 disclose a multi-spark spark plug known as a cold spark plug because it does not come up to temperature quickly enough to avoid coking. Indeed, the buildup of a deposit of carbon or coke on the electrodes is observed with such spark plugs and this significantly reduces the insulation needed between the tip of the central electrode and the shell. With poor insulation, there is a risk that the high-voltage power applied to the spark plug might not be high enough to be able to cause the necessary “breakdowns” that trigger the sparks. 
     To avoid coking, particularly when cold, of the spark plug electrode exposed to the atmosphere of the combustion chamber, one solution might be to increase the temperature of the insulator, to encourage the destruction of deposits through a pyrolysis phenomenon. This temperature is dependent on the thermal resistance of the spark plug as a whole, including its insulator. 
     The steps usually taken to increase the temperature of the insulator are limited by the onset of “pre-ignition” at the spark plugs, when these reach excessively high temperatures during operation. 
     BRIEF SUMMARY 
     It is an object of the present invention to regulate the thermal index of a multi-spark spark plug so it can come up to temperature quickly, without the risk of suffering from pre-ignition later. 
     To this end, the invention anticipates forming a deep opening around the entire circumference of the shell, this opening forming a heat exchange chamber inside the shell of the spark plug open to the outside. 
     According to a preferred embodiment of the invention, the chamber is positioned between the shell and the insulator. 
     According to the invention, the chamber may contain an expansion piece capable of opening or closing its inlet to hot gases. 
     The proposed steps make it possible to limit the cooling of the ceramic during the start-up phase without increasing its operating temperature. This then yields a non-linear thermal index which corresponds to rapid heating of the spark plug but without the risk of pre-ignition when hot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood from reading the following description of some non-limiting embodiments thereof, with reference to the attached drawings, in which: 
         FIG. 1  illustrates the known state of the art, 
         FIGS. 2 ,  3 ,  4 A- 4 B and  5 A- 5 B illustrate four embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a multi-spark spark plug  1  of known type comprising two plasma-generating electrodes  2 ,  3  separated by an insulator  4  made of a dielectric material such as a ceramic. The two electrodes  2 ,  3  respectively constitute an outer shell  3  surrounding the insulator, and a central electrode  2  housed in a central bore of the insulator  4 . The shell  3  in the conventional way has an external screw thread  3   a  so that the spark plug can be screwed into the engine cylinder head. As mentioned earlier, when the insulator  4  has not reached a high enough temperature, coke deposits disrupt the operation of the spark plug by creating current leakage paths. Above and beyond a certain temperature, of the order of 400° C., the deposited coke is destroyed by pyrolysis. 
     The spark plug in  FIG. 2  additionally has a dead volume  6  constituting a chamber open to the outside. The chamber  6  runs between the shell  3  and the insulator  4 . According to the diagram, the chamber may advantageously have a tubular first sector  6   a  connected to a circular second sector  6   b  open to the outside. 
     According to another feature of the invention, demonstrated in  FIG. 2 , the walls of the chamber  6  may be metalized. The metal layer or sleeve  7  applied to the insulator is then in direct contact with the hot gases, which are also particularly oxidizing under lean burn conditions, of the combustion chamber. Metalizing the walls of the chamber  6  in particular makes it possible to prevent plasma from being created between the ceramic of the insulator and the shell. This metallic layer  7  may for example, consist of a sleeve brazed onto the ceramic, which will give the latter the ability to withstand the oxidizing gases. In practice, the thickness of the sleeve may be a compromise between its ability to withstand thermochemical erosion, its thermal resistance and the cost of producing it. Indeed, if the sleeve is too thick, its thermal resistance will be too low, and the ceramic will not heat up enough to destroy the deposits by pyrolysis. The material of which the sleeve is made must also be chosen according to its conductivity and its expansion coefficient, which needs to be compatible with that of the ceramic and with the mechanical properties thereof. Finally, without department from the scope of the invention, the metallic layer may itself be protected by an inert coating, a thin layer of ceramic or some other metallic coating particularly well able to withstand oxidation, such as nickel. 
       FIG. 3  illustrates a second embodiment of the invention whereby the chamber  6  is a simple tubular opening formed in the mass of the shell  3 . In this case, the chamber no longer extends between the shell and the insulator as before but constitutes a cutout in the mass of the shell. The application of a metallic layer  7  is essential to prevent a plasma from forming. Here, the metalization can be applied simply to the interface between the ceramic  4  and the shell  3 , independently of the chamber  6 . 
       FIGS. 4A to 5B  illustrate additional arrangements that allow the behavior of the chamber to adapt automatically to the temperature conditions of the spark plug so as further to improve the “non-linear” regulation of the thermal index of the spark plug, particularly to make it behave like a very hot spark plug when the engine is still cold and like a warm spark plug when the engine is hot, particularly under heavy load. 
     As indicated in these figures, the chamber  6  may contain an expansion piece  8 ,  9  capable of opening or closing its inlet to hot gases. When the temperature is low, the expansion piece is contracted and opens the passage to the hot gases which supply a stream of heat accelerating the operation of the spark plug. Once the spark plug has reached its operating temperature, the piece is expanded and closes the passage to hot gases. Thus, the spark plug reaches its thermal equilibrium at a lower temperature than if the chamber had remained open. 
     In  FIGS. 4A and 4B  the expansion piece  8  is a corrugated sleeve, one end of which is fixed and the other end of which carries a cylindrical shutter  8   a  that closes off the inlet to the chamber  6  when the sleeve has expanded. 
     In  FIGS. 5A and 5B , the expansion piece  9  is a double-walled sleeve containing a metal  9   a  that melts at a relatively low temperature: expansion of the liquid metal  9   a  causes the sleeve  9  to swell, thus blocking off the passage to the hot gases. 
     These two arrangements are nonlimiting and of course, other types of shutter based, for example, on flanges acting as shutters, or on the use of shape memory alloys or a bi-material strip, may also be envisioned. 
     In conclusion, it must be emphasized that all the measures proposed by the invention rely on the creation of an empty space, or open chamber, between the insulator and the shell, making it possible to regulate the thermal index of the spark plug and, in particular to obtain a non-linear thermal index. Furthermore, metalizing the walls of the chamber is a solution particularly well-suited to lean-burn running, because it protects the ceramic from the oxidizing agents in the combustion gases.