Abstract:
Proposed is an igniting device having a radio frequency resonator. Moreover, the resonator can be designed as a strip waveguide on a printed-circuit board. Several resonators can be connected in a pattern to the RF source via p-i-n diodes. At the cold end, the resonator is electrically isolated but connected to ground, in terms of the radio frequency, via a capacitor. In this manner, ion currents can be simply coupled in subsequent to the application of an auxiliary voltage.

Description:
BACKGROUND INFORMATION 
     In internal combustion engines with externally supplied combustion ignition, usually spark plugs are installed in the combustion chamber of the internal combustion engine, the spark plugs essentially being composed of terminal stud, insulator, shell, and electrodes. The insulator is inserted in the tubular metallic shell, in the central bore of the insulator, in turn, an inner conductor arrangement being inserted which is composed of a central electrode on the combustion chamber side and of the terminal stud, which is distant from the combustion chamber. In this context, the rotationally symmetric axes of the shell, of the insulator, and of the inner conductor arrangement coincide. Mounted to the shell are the at least one ground electrode on the combustion chamber side so that an ignition spark forms between the central electrode and the ground electrode in response to the application of a high voltage, the spark assuring the ignition of the combustible mixture in the combustion chamber of an internal combustion engine. Usually, the ignition voltage is made available inductively by an ignition coil which assures that the voltage at the electrodes of the spark plug increases very heavily in response to disconnecting the ignition coil charging space. The function of the spark plug is to introduce the ignition energy into the combustion chamber, and to initiate the combustion of the air/fuel mixture by the electric spark between the electrodes. During the operation of the spark plug, voltages of up to over thirty Kilovolts can occur. The residues separating from the combustion process such as soot, oil, carbon, and ash from fuel and oil, are electrically conductive given certain thermal conditions. Nevertheless, no sparkovers or breakdowns may occur across the insulator in these conditions. For this reason, the electrical resistance of the insulator must be sufficiently high up to 1000° C. and may not change during the service life of the spark plugs. 
     Besides providing the ignition voltage inductively, it is known to generate an ignition spark by radio frequency ignition as described in SAE paper 970071 “Investigation of a Radio Frequency Plasma Ignitor for Possible Internal Combustion Engine Use”. Here, the possibility of generating ignition sparks using radio frequency ignition is described. In such a radio frequency ignition, which is also called microwave ignition, a high voltage is generated by low-resistance infeed at the hot end of a quarter-wave line of an RF resonator. 
     SUMMARY OF THE INVENTION 
     In contrast, the igniting device according to the present invention has the advantage of a simple coupling out for an ion current and of a particularly simple design, respectively. Both the oscillator and the high-voltage section are advantageously seated on a shared substrate. The capacitor can likewise be arranged between the waveguide patterns on the substrate. Thus, a simple manufacture is possible, and the requirements for high-voltage strength can be taken into consideration by a corresponding form design and/or insulation level. 
     It is particularly advantageous to use a flexfilm as the substrate for jointly mounting the high-voltage section and the oscillator portion. Such a flexfilm offers the possibility of very simple and cost-effective manufacture. 
     DRAWING 
     FIG. 1 shows the principle of the radio frequency ignition. 
     FIG. 2 illustrates one embodiment of the present invention. 
     FIG. 3 illustrates another embodiment of the present invention. 
     FIG. 4 illustrates another embodiment of the present invention. 
     FIG. 5 illustrates another embodiment of the present invention. 
     FIG. 6 illustrates another embodiment of the present invention. 
     FIG. 7 illustrates another embodiment of the present invention. 
     FIG. 8 illustrates another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts the functional principle of the igniting device according to the present invention. The igniting device has a metallic shell  10  including a thread used for screwing in the wall of a cylinder of an internal combustion engine. In this context, the metal shell  10  is designed as a conventional spark plug on the combustion chamber side, i.e. an insulator  11  is provided in metal shell  10 , the insulator being used for electrically insulating a high-voltage feed-through lead for a central electrode  14 . Arranged opposite central electrode  14  is a ground electrode  15  which is connected to metallic shell  10  in an electrically conductive manner. In response to the application of a sufficiently high voltage, an ignition spark used for igniting the gasoline/air mixture in the combustion chamber of a cylinder of the internal combustion engine sparks over in the small gap between central electrode  14  and ground electrode  15 . 
     On the side of metal shell  10  facing away from the combustion chamber, provision is made for a radio frequency resonator used for generating the ignition voltage. The radio frequency resonator or microwave resonator has a first waveguide pattern  12  which is separated from a second waveguide pattern  16  by a dielectric  17 . First waveguide pattern  12  is electrically connected to central electrode  14 . Waveguide pattern  12  is contacted by a supply lead  18  through which radio frequency signals can be injected. In this context, supply lead  18  is arranged in the immediate vicinity of combustion chamber-distant end  13  of waveguide pattern  12 . This end is frequently referred to as the cold end of the resonator since no high voltage is present here. At the opposed hot end, however, a high-voltage signal develops which can discharge by an ignition spark via the electrodes. 
     During the injection of high-voltage signals on supply lead  18 , radio frequency waves form in the resonator because of the geometric conditions. If the frequency is selected correctly in proportion to the geometric dimensions, a high voltage forms at central electrode  14  which is electrically connected to waveguide  12 . The geometric dimensions are to be selected such that the effective length of waveguide  12  and central electrode  14  electrically connected thereto correspond exactly to one quarter of the wavelength of the injected radio frequency. Here, effective length is to be understood as a numerical value which also allows for the dielectric properties of insulator  11  and of dielectric  17 , respectively, in addition to the linear dimensions of waveguide patterns  12 ,  16  and of central electrode  14 . In many cases, this effective length quarter-wave cannot be ascertained by calculation but by experiments only. 
     At end  13 , which is distant from the combustion chamber, waveguide  12  is electrically connected to second waveguide  16  via a capacitor  30 . With regard to the radio frequency, capacitor  30  acts as a short-circuit but is used for coupling out a current signal (ion current) via lead  31 . To check whether a combustion has taken place and whether this combustion was normal or knocking, it is usual to apply a voltage of several 100 Volts to the spark plug after the ignition spark ends. The current flowing then is 
     a) a measure for the occurred ignition with corresponding ionization, and 
     b) the A.C. components in a specific frequency range indicate if the combustion was knocking. 
     The description following now is dedicated primarily to the expedient and simple design of first waveguide  12 , of second waveguide  16 , and of supply lead  18 . 
     FIG. 2 shows a top view of a first example, and FIG. 3 depicts a cross-section along marked line III—III of FIG.  2 . As is well-discernible in the cross-section of FIG. 3, here, the construction is composed of a supporting plate or printed-circuit board  100  on whose upper side patterned metal layers are applied. The top view of FIG. 2 shows that waveguide  12  is formed as a strip waveguide on the upper side of supporting plate  100 . Supply lead  18  is likewise designed as a strip waveguide which meets with strip waveguide  12  perpendicularly. Also on the upper side of supporting plate  100 , waveguide pattern  16  is formed in such a manner that it surrounds strip waveguide  12  and strip waveguide  18  on both sides. Waveguide pattern  16  is also formed of a superficial conducting layer, preferably of metal, the conducting layer being applied to supporting plate  100 . 
     Supporting plate  100  is an insulating dielectric material. The whole arrangement is preferably formed of one printed-circuit board having a superficial metal layer over the entire surface. The patterns such as strip waveguide  12 , supply lead  18 , and strip waveguide  16  are then formed by incorporating trenches. Since such printed-circuit boards are regularly suitable for mounting electrical components, as well, the elements needed for controlling the individual igniting devices can be mounted directly onto dielectric plates  100 . In this context, it is also possible for a capacitor between the first and the second waveguide pattern, as is described with reference to FIG. 1, to be mounted directly onto the surface of the printed-circuit board. Furthermore, there are dielectric plates  100  which are flexible. This makes it possible to provide a one-part plate  100  for several cylinders of an internal combustion engine, several igniting device then being formed thereon. 
     FIG.  4  and FIG. 5 depict a further example for the construction of the igniting device according to the present invention. In this context, FIG. 5 shows a cross-section along line V—V of FIG.  4 . FIGS. 4 and 5 illustrate an embodiment using a dielectric plate or printed-circuit board  100  coated on both sides. In this context, as is discernible in FIG. 5, supply lead  18  is formed on the upper side which is isolated by a trench pattern from the rest of the superficial metal layer which forms a waveguide pattern  16 . A via  101  (see FIG. 5) is used to connect supply lead  18  arranged on the upper side to waveguide  12  arranged on the lower side. Waveguide  12  extends on the lower side of printed-circuit board  100  along line V—V. In the edge area of printed-circuit board  100 , provision is made for an edge contacting  102  which provides an electrical contacting of the waveguide pattern  16  on the upper side to waveguide pattern  12  on the lower side of printed-circuit board  100 . In the top view of FIG. 4, strip waveguide  12  arranged on the lower side of the printed-circuit board is not discernible. 
     In FIGS. 6 and 7, a further embodiment of the igniting device is shown. As is discernible in the cross-section of FIG. 7, this embodiment is a multilayer printed-circuit board  100  having an upper insulating layer  110  and a lower insulating layer  111  including an intermediate metallic conductor strip layer. Furthermore, another metallic conductor strip layer is provided on the upper side and on the lower side of printed-circuit board  100 . In the top view of FIG. 6, supply lead  18  is discernible again. For supply lead  18  which is also designed as a waveguide and strip waveguide  12 , waveguide pattern layer  16  acts as the second line of the waveguide. As shown in the cross-section of FIG. 7, provision is made again for a via  101  connecting the upper and lower side of printed-circuit board  100 . Thus, an electrical contact is made between supply lead  18  designed as strip waveguide and strip waveguide  12  arranged on the lower side. In the metallic conducting layer between the two insulating layers  110 ,  111 , waveguide pattern  16  is formed which is short-circuited to waveguide  12  by an edge contacting  102 . 
     All examples, as are described in FIGS. 2 through 7, are preferably formed by flexible printed-circuit boards which make it possible for several igniting devices for several different cylinders to be formed in one piece from a single printed-circuit board trimmed correspondingly. Thus, the manufacturing effort for igniting devices for several cylinders is strongly simplified. 
     FIG. 8 is a schematic representation of such a printed-circuit board  100  which includes igniting devices for four cylinders. For reasons of simplification, waveguide patterns  12  and supply leads  18  as well as other lines are shown here only as simple lines. Waveguide patterns  16  are all interconnected electrically. The individual cylinders are designated by letters A, B, C, and D. A radio-frequent signal is applied to an oscillator terminal  53 . Via a distribution line  54 , this radio-frequency signal is fed to the individual igniting devices for cylinders A, B, C, D. Each of these igniting devices has a capacitor  51  which is connected to distribution line  54 . Capacitor  51  is then connected to supply lead  18  via a p-i-n diode  52 . Provided between capacitors  51  and p-i-n diodes  52  is in each case one control current terminal. Capacitor  51  represents a short-circuit for the radio-frequency signal, whereas p-i-n diodes  52  keep the radio-frequency signal away from supply leads  18  or waveguides  12 . By applying a direct voltage to terminals A, B, C, and D, p-i-n diodes  52  are switched into conduction so that then, the radio-frequency signal is also applied to supply leads  18  or waveguides  12 . In this manner, the radio-frequency signal can be fed to the leads or waveguide  12  selectively for each individual cylinder. 
     Capacitors  51  and p-i-n diodes  52  can be affixed to the printed-circuit boards as customary, surface-mounted components. Moreover, the circuit for generating the radio-frequency signal can be applied directly to the printed-circuit board. Furthermore, the regions of the printed-circuit boards which form supply lead  18  can be designed to have different lengths to guarantee the supply to the individual cylinders which may be located at different distances. This is exemplarily depicted in FIG. 8 in that, for cylinder A and D, longer regions for supply leads  18  are shown.