Abstract:
A method for controlling the power supply of a radiofrequency spark plug in an internal combustion engine up to an electric voltage sufficient for generating a highly branched spark. To this end, the electric voltage for powering the spark plug is increased step by step up to an adequate voltage adapted for ignition.

Description:
BACKGROUND 
     This involves a method for electrically powering an ignition spark plug up to an electric voltage ensuring the generation of a branched ignition spark in particular of an internal combustion engine. 
     Also involved is a device for powering such a spark plug, this device comprising means for powering the spark plug with electrical energy up to a voltage ensuring the generation of a branched ignition spark. 
     In order to have better control over igniting the flammable mixture in an internal combustion engine, it is known to be preferable to use an electric spark of considerable size. Specifically, the larger the spark, the greater the probability of there being a meeting between the hot electric arc and the cloud of fuel and the more efficient the ignition. For a conventional ignition spark plug, the size of the spark (of the order of one mm cubed) is limited by the distance between two electrodes of the spark plug. 
     In order to increase the size of the spark of an ignition spark plug, it has already been proposed:
         in U.S. Pat. No. 5,623,179, to increase the distance between the electrodes of the spark plug; however such a solution requires a particularly high powering voltage,   which is directly proportional to the distance between the electrodes,   in EP-A-1202411 or EP-A-1526618, to use the electric arc which slides over the insulation of the spark plug, which makes it possible to lengthen the spark without increasing the electric voltage by too much; however, in such a solution, the lengthening of the spark remains relatively short and the insulating surface touched by the hot arc quickly degrades;   in FR-A-2886776 or FR-A-2878086, to form a multifilament radio frequency spark developing from a single pointed electrode; this makes it possible to increase notably the length of the spark, but in the known method of this solution, the number of filaments formed simultaneously is limited (2-3 at most).       

     BRIEF SUMMARY 
     The object of the present invention is to prevent the performance limitations of the solutions of the prior art. 
     Another object is to increase notably the degree of branching of the radio frequency spark (that is to say the total number of filaments generated simultaneously) and thus increase this spark and therefore its efficiency in igniting the mixture entering its environment. 
     One solution proposed for at least approaching this (these) object(s) is that the electric power supply of the spark plug (in particular a radio frequency spark plug) comprises a step of increasing by stages (therefore with at least one such stage) the power-supply voltage of this spark plug up to the adapted ignition voltage. 
     In terms of device, it is also proposed that the means for supplying the spark plug with electrical energy be adapted to generate a first voltage for igniting the spark and subsequently to increase this first electric voltage by stage(s) up to said adapted ignition voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed description of the invention follows, with reference to the accompanying drawings supplied in a nonlimiting manner and in which: 
         FIG. 1  schematizes a radio frequency spark plug mounted on an internal combustion engine, 
         FIG. 2  schematizes a typical time/voltage evolution on RF spark plugs controlled in the conventional manner, 
         FIGS. 3 ,  4  schematize an example of time/voltage evolution according to the invention on an RF spark plug controlled in a different manner, 
       and  FIG. 5  schematizes a branched spark that can be obtained with the control according to  FIGS. 3 ,  4 ; as compared with the spark of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a radio frequency (RF) resonant spark plug  1  mounted on the cylinder head  3  of an internal combustion engine  5 . The tip  1   a  of the spark plug leads into the combustion chamber  7  of the engine into which the mixture to be ignited is injected. 
     This RF plasma spark plug  1  is excited by a low-voltage RF power supply  9  controlled by a computer  11  onboard the vehicle provided with said engine. Each multifilament spark  13  is therefore formed from the single tip  1   a  of the spark plug. 
     The general known operating mode of such a spark plug is described for example in FR-A-2878086, FR-A-2886776 or FR-A-2888421. 
     As schematized in  FIG. 2 , which therefore illustrates the prior art, there are typically two main phases for electrically powering the RF spark plug  1 : 
     During the initial phase  15   a , which begins at the moment t_ 0  on applying voltage, the electric voltage U applied to the spark plug increases continuously so that the thin electric channels  13  form from the tip  1   a  of the spark plug. 
     Once formed, such a multifilament structure is, during the next phase  15   b  (between t_ 1  and t_ 2 ,  FIG. 1 ), heated up to several thousands of ° C. by the electric current supplied by the controlled RF power supply  9 . The electric voltage (substantially Um) applied to the spark plug remains (about) constant throughout this second phase. 
     At the end of this heating phase (portion  15   b   1  up to t_ 2 ), the hot filaments cause the mixture to ignite in the cylinder of the internal combustion engine with which the combustion chamber  7  is associated. 
     Then, during the final phase  15   c  of this cycle for igniting the mixture via the spark plug (between t_ 2  and t_ 3 ,  FIG. 1 ), the electric voltage applied to this spark plug again reduces continuously until it disappears. 
     The length L (of the order of one cm;  FIG. 1 ) of the filaments  13  formed at the end of the phase  15   b   1  depends only on the maximum amplitude of the voltage U applied to the tip  1   a.    
     So long as, during the heating phase  15   b / 15   b   1 , the amplitude of the RF voltage Um, corresponding to the maximum electric voltage (or adapted ignition voltage) applied to the tip of the spark plug, is kept stable (constant), the length of the filaments  13  and their number no longer change or virtually no longer change. 
     The inventors have noted that, in this known operating mode, the degree of branching (that is to say the number of bifurcation points, as marked  13   a ,  13   b ,  FIG. 1 ) of the RF spark  13  remains relatively low: the filaments formed during the formation phase are rather straight with few bifurcation points (typically 2-3 at most) which limits the size of the spark. 
     In order to increase the degree of branching of the multifilament spark, the inventors propose to modify the method of electrically powering the RF spark plug  1 , as illustrated in particular in  FIG. 3 . 
     Therefore, instead (as in  FIG. 2 ) of applying to the tip of the electrode  1   a  of the spark plug a voltage such that at a moment t_ 1  (end of the initial phase  15   a ) immediately following t_ 0 , the maximum voltage Um (the adapted ignition voltage for combustion) is present there after a continuous increase in this voltage from the beginning of supplying power (moment t_ 0 ), a step of increasing by stage(s), up to said maximum voltage Um, the electric voltage for powering the spark plug will be applied. 
       FIG. 3  shows such a voltage increase in several stages, in this instance two: 17.1 and 17.2. 
     Consequently it is found that, with the solution of the invention, and in the exemplary embodiment shown in  FIG. 3 , the electric voltage will initially, between t_ 0  and t_ 10 , increase only up to a value U 1  that is just necessary for the formation of the 1 st -generation filaments  130 , namely those marked “a” notably in  FIG. 5 , which all originate from the tip  1   a  of the electrode of the spark plug. 
     At the moment t_ 10 , that is to say typically a few μs after the beginning of excitation at t_ 0  (from 5 to 10 μs in the proposed embodiment), the RF power supply stabilizes the amplitude of the applied voltage and holds it substantially at U 1  for a few μs (from 2 to 5 μs in the proposed embodiment) until the moment t_ 20 . 
     It is the 1 st  heating phase corresponding to the stage  17 . 1 . 
     Advantageously, the value U 1  of the electric voltage at this first voltage stage  17 . 1  will be just necessary for the formation, at the free end  1   a  of the electrode, of electric filaments originating from this end. 
     During this period of time, the temperature of the primary filaments  130  “a” reaches 1000-5000° C., the gas inside the channels becomes heavily ionized, its electrical resistivity falls from infinity to a few kOhms only. As a result, the voltage of the spark plug is applied to the ends of the filaments “a” that have become conducting (the solid points in  FIG. 5 ). 
     Between the moments t_ 20  and t_ 30 , the RF power supply again (continuously) increases the amplitude of the voltage of the spark plug up to the intermediate voltage U 2  (where naturally U 2  is greater than U 1 ). 
     Preferably, the voltage difference between the zero voltage and the U 1  voltage of the first voltage stage will be greater than the electric voltage difference between the electric voltage U 1  of the first voltage stage and said adapted ignition voltage Um, as schematized in  FIGS. 3 ,  4 . 
     Because the diameter of the ionized filaments  130  (typically of the order of 50-100 μm) is substantially smaller than that of the tip (typically of the order of 500 μm), all that is needed is a small increase in the electric voltage U applied for the local electric field at the ends of the filaments  130  “a” (inversely proportional to the square of their diameter) to be great enough to cause the formation of the 2 nd -generation filaments. This time, the new filaments, marked  130  “b”, still in  FIG. 3 , originate from the ends of the filaments “a” and no longer from the tip  1   a  of the spark plug. 
     During the period of time between t_ 30  and t_ 40  the filaments “b” are heated. The voltage is again stabilized, in this instance at U 2 , which corresponds to the second stage  17 . 2 . The potential of the tip is then at the ends of the latter (the open points in  FIG. 5 ). 
     Again between the moments t_ 40  and t_ 50 , the RF power supply again increases the voltage of the spark plug  1   a , causing the birth of the 3 rd  generation of filaments  130  “c” from the ends of the filaments of the previous generation. 
     The process could continue further. In  FIGS. 3 ,  4 ,  5  it has been considered that it stops there, since it was supposed that the adapted ignition voltage Um was reached at the moment t_ 50 . 
     Therefore, according to a worthwhile feature of the invention for achieving the intended objects, between the initial moment t_ 0  of beginning to electrically power the spark plug and the stabilized application of the maximum voltage at t_ 50 , at least one stage of stabilized electric voltage has been produced for a period of between 1 and 10 μs. 
     Once formed with its branches of successive generations of filaments  130   a, b, c  (initial phase  150   a  of increasing voltage by stages), such a multifilament structure is, during the next phase  150   b , heated (as before) up to several thousands of ° C. by the electric current supplied by the controlled RF power supply  9 . The electric voltage (Um) applied to the spark plug remains (substantially) constant throughout this second phase, as shown in  FIG. 3 . 
     Again as in the conventional operating mode, at the end of this heating phase (portion  150   b   1  up to the moment t_ 60 ), the hot filaments cause the ignition of the mixture in the cylinder of the internal combustion engine with which the combustion chamber  7  is associated. 
     And, during the final phase  150   c  of this cycle for igniting the mixture via the spark plug, the electric voltage applied to this spark plug again reduces continuously until it disappears (moment t_ 70 ). 
     Preferably, a period of voltage stages will be applied between two voltage increases (such as t_ 10 −t_ 20  and t_ 30 −t_ 40 )—that is greater than the elapsed time between two successive stages of increase of said voltage (such as t_ 20 −t_ 30 ). 
     The “formation of filaments→their heating→increase in voltage→formation . . . →heating . . . →increase . . . ” cycle can be repeated as many times as necessary. On each further increase in the voltage, the new bifurcation points appear. 
     Therefore, the means for powering with electrical energy  9 ,  11  will have been adapted relative to the prior situation of  FIG. 2  in order, progressively with the stages  17 . 1  . . . beyond the first voltage U 1  for igniting the spark, to generate the creation of new branches  130   b  . . . at the (round, solid) end(s) of the electric spark created at the first stage. 
     Finally, the spark  130  generally formed in this way is characterized by a degree of branching that is much greater than in the case of the conventional excitation schematized in  FIG. 2 . It is possible to estimate the total number of filaments at 
                 N   total     ≈       ∑     k   =   1     n     ⁢     N   0   k         ,         
where N 0  is the number of filaments of one generation and n the number of cycles. Therefore, in the situation illustrated in  FIG. 5  where N 0 ≈3 and n=3 Ntotal≈39 or approximately 10 times more than in the case of conventional RF excitation. Even though the average length of the filaments of each new generation is increasingly short, the total overall length of the spark at the end of its powering is much greater than in the case of the conventional powering (see  FIGS. 1 and 5 ). This increases the probability of an encounter between the hot arc and the fuel/air mixture and therefore makes the ignition more efficient.
 
     Naturally, it will have been noted in  FIGS. 2 to 4  that the electric voltages in question (Um, U 1  . . . ) are alternatives, the sinusoidal curve of evolution of the voltage U schematized on the left, with its first alternations, being clear in this respect.