Patent Publication Number: US-9425586-B2

Title: Method for producing a corona ignition device

Description:
RELATED APPLICATIONS 
     This application claims priority to DE 10 2013 104 061.6, filed Apr. 22, 2013, and DE 10 2014 102 230.0, filed Feb. 2, 2014, both of which are hereby incorporated herein by reference in their entireties. 
     BACKGROUND 
     The invention relates to a method for producing a corona ignition device. 
     Corona ignition devices comprise a housing tube in which a coil is arranged, said coil being connected to a center electrode. The center electrode is stuck in (plugs into) an insulator that is fastened by means of a mount to a front end of the housing tube. At its rear end, the housing tube carries a plug connector, via which the corona ignition device can be connected by means of a suitable mating plug connector to the on-board power supply system of a vehicle. 
     The center electrode, together with the insulator and the mount, provides a capacitance, which together with the coil forms an oscillating circuit. If the oscillating circuit is resonantly excited, this leads to a voltage step-up between the center electrode and the walls of the combustion chamber or the housing tube of the corona ignition device. This leads to the formation of a corona discharge in the combustion chamber. Fuel in the combustion chamber of an engine can thus be ignited by means of a corona discharge starting from the center electrode. 
     Compared to conventional spark plugs, which ignite fuel/air mixtures by means of arc discharges, corona ignition devices have the advantage of a much lower burn-up of the electrodes or ignition tips. Corona ignition devices therefore have the potential of a much longer service life compared to conventional spark plugs. 
     A common cause for premature failure of corona ignition devices are shunts and dielectric breakdowns in the interior of the housing tube. In order to prevent this, housing tubes of corona ignition devices are filled up with electrically insulating casting compound. Another possibility, which is described in EP 1 662 626 B1, consists of filling the housing tube with insulating gas. 
     SUMMARY 
     The present invention provides a way in which a corona ignition device having a long service life can be manufactured economically. 
     According to this disclosure, insulating gas is introduced into the housing tube through a bore. The bore can be drilled into the housing tube or into the plug connector which closes the rear end of the housing tube. 
     If the bore is drilled into the housing tube, it is typically in a rear end section of the housing tube, specifically behind the rear end of the coil with respect to the longitudinal direction of the housing tube. The rear end is particularly suitable for filling insulating gas into the housing tube since a bore at the rear end can be closed in a gastight manner without impairing the electrical properties of the corona ignition device. When closing a bore, unevenness on an inner surface can only be avoided with difficulty. Any unevenness, edge or the like may lead to a local increase of the electric field and may thus increase the risk of dielectric breakdowns in the interior of the corona ignition device. Field peaks in a portion of the housing tube surrounding the coil therefore lead easily to shunts between the coil and housing tube, that is to say to a premature failure of the corona ignition device. By contrast, field peaks behind the rear end of the coil are largely uncritical due to the greater distance from the coil. Any unevenness behind the rear end of the coil that might be produced when closing an opening is therefore largely unproblematic. 
     A bore in the plug connector is advantageous since the electric properties of the housing tube are not influenced thereby. In addition, the housing of the plug connector can be manufactured without significant costs with a wall thickness that is greater than the wall thickness of the housing tube. A bore can be welded shut all the more easily, the thicker the wall in which it is located. 
     In an embodiment of this disclosure, the bore is closed with a peg that is welded to the rim of the bore. The peg can be inserted into the bore before insulating gas is filled into the housing tube. If the peg has a non-circular cross-section it can be plugged into the bore without closing it in a gas-tight manner. Insulating gas can then flow along the peg through the bore. The bore is only closed gas tight when the peg is welded to the rim of the bore. A peg with a non-circular cross-section can be made by pressing a cylindrical pin flat or by cutting a strip from sheet metal, for example. 
     The bore may also be closed without a peg by welding. However, the bore can usually be closed more reliably in a gas tight manner if a peg is used. This is because insulating gas can react with molten metal and thus cause the weld to become brittle. The material of a peg can be chosen in consideration of the insulating gas so that it does not form a brittle weld. The peg may for example be made of an alloy based on nickel. The tube housing and the plug connector, more precisely the plug connector housing wherein the bore may be drilled, may for example be made of steel. Preferably the peg is made of a different material than the tube housing and/or the plug connector although the peg may also be made of the same material. 
     The peg may be pressed into the bore. In this case the peg is held in the bore by friction until welding. It is also possible to insert the peg loosely into the bore and to fix it only later by welding. 
     According to an advantageous refinement of this disclosure, the peg has a head. When the peg is inserted into the bore the head abuts the rim of the bore. The head keeps the peg from being pushed too far into the bore. 
     For example, nitrogen, carbon dioxide, noble gases and/or sulphur hexafluoride can be used as insulating gas. Helium can be mixed with the insulating gas in order to facilitate a leakage test. Instead of helium, hydrogen can also be used as a tracer gas, for example. 
     The insulating gas is preferably filled into the housing tube at a pressure of at least 2 bar, preferably at least 4 bar. A gas pressure of preferably at least two bar or more thus prevails in the housing tube of the finished corona ignition device. The higher the gas pressure, the better the electric insulation produced by the insulating gas. 
     In accordance with an advantageous refinement of this disclosure, air is suctioned from the housing tube before said housing tube is filled with insulating gas. Air is preferably suctioned off through the same bore through which insulating gas is subsequently filled into the housing tube. The gastight closing of the housing tube is thus advantageously easier once the housing tube has been filled with insulating gas. For example, the corona ignition device can be introduced into a pressure chamber which is then evacuated and subsequently flooded with insulating gas. The corona ignition device can be introduced completely into a pressure chamber or may protrude via its front end from the pressure chamber, which then surrounds the housing tube in a gastight manner. 
     It is also possible however to fill insulating gas into the housing tube through a first bore and to remove air through a second bore. Then two openings must be closed in a gastight manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a sectional view of an illustrative embodiment of a corona ignition device; 
         FIG. 2  shows a detail of  FIG. 1  with a peg in a not yet sealed bore  10 ; 
         FIG. 3  shows a cross-section along line III-III of  FIG. 2 ; 
         FIG. 4  shows a schematic illustration of the corona ignition device in a pressure chamber for filling with insulating gas; and 
         FIG. 5  shows a schematic illustration of the corona ignition device in a further pressure chamber for filling with insulating gas. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of this disclosure. 
     The corona ignition device shown in  FIG. 1  has a center electrode  1 , which is surrounded by an insulator  2  and leads to one or more ignition tips  3 . The center electrode  1  is connected to a coil  4 , which is arranged in a housing tube  5 . At its front end, arranged on the side of the combustion chamber, the housing tube  5  carries a mount  6  for the insulator  2 , and at its rear end carries a plug connector  7  for connection of the corona ignition device to a voltage source. 
     The mount  6  surrounds the insulator  2  in a gastight manner and is welded to the housing tube  5 . The mount  6  may have an outer thread for screwing into an engine block. Corona ignition devices can also be mounted differently however to an engine block, and therefore an outer thread is not absolutely necessary. The mount  6  is formed in the shown illustrative embodiment as a sleeve which has a smaller outer diameter than the housing tube  5  and has a flange for fastening to the housing tube  5 . 
     A portion of the center electrode  1  can be formed as a glass body which is electrically conductive due to the addition of metal particles or graphite particles and seals off a channel leading through the insulator  2 . 
     The plug connector  7  has a metal housing which forms the outer conductor of a coaxial plug connector, a metal inner conductor  8 , and an electrically insulating glass body  9 , which seals off an annular gap between the inner conductor  2  and the outer conductor  1 . The glass body  9  can form a compression glass seal for the inner conductor  2 . In the embodiment, the glass body  9  is used simultaneously as an insulating support for the inner conductor  2 , such that it is possible to dispense with further components. 
     The housing tube  5  is connected in a gastight manner to the outer conductor of the plug connector  7 , for example by welding. The inner conductor  8  of the plug connector is connected to the coil  4 , for example in that the coil  4  is wound onto a coil former, which, at its end, carries a socket into which the inner conductor  8  is plugged. 
     The mount  6 , together with the center electrode  7  and the insulator  6 , forms a capacitor. This capacitor is connected in series with the coil  4  and forms an electric oscillating circuit therewith. By exciting this oscillating circuit, a corona discharge can be generated starting from the ignition tips  3 . 
     In order to reduce the risk of dielectric breakdowns in the interior of the housing tube  1 , the housing tube  5  is filled with insulating gas. The gas pressure is increased with respect to the atmospheric pressure, for example to a value of more than two bar. Values from 5 bar to 30 bar are generally well suited. 
     For example, nitrogen, sulphur hexafluoride, dry air, in particular air with less than 0.001 vol. % of water vapor, noble gas and/or carbon dioxide can be used as insulating gas. Insulating gases such as nitrogen, sulphur hexafluoride and carbon dioxide are particularly well suited. In particular, gas mixtures that contain sulphur hexafluoride, for example 5% (based on the total number of gas molecules or gas atoms) or more, enable excellent gas insulation. In order to facilitate a leakage test, the insulating gas may contain helium. Low helium proportions are sufficient for this, for example 5 less. 
     The following steps are carried out when producing the corona ignition device: The center electrode  1  is plugged into the insulator  2  and connected to the coil  4 , the coil  4  is arranged in the housing tube  5 , a mount  6  for the insulator  2  is fastened to a front end of the housing tube  5 , and a plug connector  7  is fastened to a rear end of the housing tube  5 . These steps are preferably carried out in the above-mentioned order, but can also be carried out in a different order. 
     After these steps, insulating gas is introduced into the housing tube  5  through a bore  10  that is was drilled either into the plug connector  7  or into the housing tube  5 , e.g., in a section behind the rear end of the coil  4  with respect to the longitudinal direction of the housing tube  5 . In this context, it should be noted that the term “coil” merely denotes the wire windings themselves. The coil former onto which the coil  4  is wound is not part of the coil. 
     Two examples of possible positions of such bores  10  are indicated in  FIG. 1 . The bores  10  in each case can run transverse to the longitudinal direction of the housing tube  5  and may have a diameter from 0.1 mm to 1.5 mm, for example 0.2 mm to 0.5 mm. Air is initially suctioned off from the housing tube  5  through such a bore  10  and the housing tube  5  is then filled with insulating gas. The bore  10  is then sealed by welding. 
     The bore  10  can be sealed by welding simply by melting material of the housing tube  5  or the plug connector  7  around the bore  10  without adding additional material or parts. A more reliable sealing can be achieved with a peg  18  that is plugged into the bore  10  as shown in  FIG. 2 . The bore  10  is then sealed by welding the peg  18  to the rim of the bore  10 . In this case material of the peg  18  and of the housing tube  5  or the plug connector  7  is molten to seal the bore  10 . 
     The peg  18  can be plugged in to the bore  10  before the housing tube  5  is evacuated and then filled with insulating gas if the peg  18  has a non-circular cross-section. Gas can then flow through a gap  19  between the peg  18  and the surrounding wall of the bore  10 . The gap  19  is shown schematically in  FIG. 3 . The peg&#39;s insertion section that is in the bore  10  can be produced from an initially cylindrical section by flattening. For example, a cylindrical peg or section of a peg can be flattened by pressing or hammering. The flattened shape may be an elliptical or angled shape, for example. 
     The peg  18  has an insertion section that is arranged inside the bore  10  and may additionally have a head that covers a rim of the bore  10 . The head of the peg can be shaped like the head of a nail. The length of the insertion section can be shorter than the length of the bore  10 . Then the peg  18  does not protrude out of the bore  10  into the interior of the housing tube  5 . Thus the peg  18  ends on the inside either flush with the bore  10  or the peg  18  ends inside the bore  10 . 
     The peg  18  can consist of a nickel-based alloy, e.g., Inconel. The tube housing and the plug connector  7 , more precisely the plug connector housing wherein the bore  10  may be drilled, can be made of steel, for example. 
       FIG. 4  schematically shows a corona ignition device which is arranged in a pressure chamber  11  for filling the corona ignition device with insulating gas. In this illustrative embodiment the opening for filling the housing tube  5  with insulating gas is a bore  10  drilled into the plug connector  7 . As explained before, the bore may also be drilled into the housing tube  5 . A peg  18  is plugged into the bore  10  and the ignition device is then placed in the pressure chamber  11 . 
     The pressure chamber  11  is evacuated, and in so doing air is suctioned off from the housing tube  5 . The pressure chamber  11  is then flooded with insulating gas. The bore  10  is then closed by laser welding. To this end, a laser beam  12  is guided through a window  13  into the pressure chamber  11 . The pressure chamber  11  may contain a mount for the corona ignition device so that said corona ignition device is positioned precisely for the welding process. 
       FIG. 5  shows a further illustrative embodiment of a corona ignition device with a pressure chamber  11  for filling the corona ignition device with insulating gas. In this illustrative embodiment the corona ignition device protrudes from the schematically illustrated pressure chamber  11 . The pressure chamber  11  surrounds the housing tube  5  or an annular area of the plug connector  7  in a gastight manner. The volume of the pressure chamber  11  to be evacuated or to be filled is thus reduced. In the illustrative embodiment of  FIG. 4  also, a smaller pressure chamber  11  can be used, from which the corona ignition device protrudes. The part of the corona ignition device protruding from the pressure chamber  11  can be arranged in a second pressure chamber which adjoins the pressure chamber  11 . A desired pressure in the pressure chamber  11  can thus be maintained more easily, in spite of any leakage points between the housing tube  5  and a seal of the pressure chamber  11 . 
     In the illustrative embodiments in  FIG. 5 , air is suctioned out from the housing tube  5  through a bore in the plug connector  7 , and insulating gas is then introduced through this bore. 
     In all embodiments the pressure chamber  11  can be connected to a vacuum chamber  15  via a valve  14 , said vacuum chamber having a larger volume than the pressure chamber  11 . Since the valve  14  is opened with respect to the vacuum chamber  15 , air can be suctioned off very quickly from the corona ignition device. The valve  14  is then closed with respect to the vacuum chamber  15  and a valve  16  is opened with respect to a compressed gas container  17 , such that the pressure chamber  11  and the interior of the housing tube  5  are flooded with insulating gas. 
     The advantage of a vacuum chamber  15  is that a vacuum chamber can be pumped empty continuously by a pump. With a vacuum chamber  15  that is larger than the pressure chamber  11 , preferably at least twice as large, air can therefore be suctioned off very quickly from the pressure chamber  11  and the corona ignition device, even with a small pumping capacity, by opening the valve  14  of the vacuum chamber  15 . A comparatively quick suctioning-off of air without a vacuum chamber  15  would thus require a much higher pumping capacity. 
     While exemplary embodiments have been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of this disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.