Patent Publication Number: US-6662793-B1

Title: Electronic circuits for plasma-generating devices

Description:
This application claims the benefit of provisional application Ser. No. 60/154,908, filed Sep. 21, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to ignition circuitry for use with internal combustion engines and, more particularly, the circuits that may be used to drive a plasma-generating device. 
     2. Related Art 
     There exist several types of ignition systems for creating a spark to ignite a fuel/air mixture in combustion chamber of an internal combustion engine. A conventional ignition system typically provides a single high voltage capable of causing a discharge between the two electrodes of a conventional spark plug. Common systems for providing such a high voltage include transistorized coil ignition (TCI) and capacitive discharge ignition (CDI) systems. These systems are affective in providing the required high voltage for the initial discharge. 
     However, recent study has shown that spark plugs which are capable of producing a volume of plasma between the electrodes and expelling the plasma into a combustion chamber may produce better ignition efficiency as well as reducing the amount of hydrocarbon emissions of an internal combustion engine. Such spark plugs are driven by dual-stage electronics with provide an initial high voltage pulse that causes a breakdown between the electrodes to create an initial plasma kernel. A follow-on low voltage high current pulse is then provided which creates a current through the plasma. The location where the current travels through the plasma is swept outward, along with the plasma, under Lorentz and thermal expansion forces. Examples of such a spark plug as well as the associated dual stage electronics which operate in this manner are disclosed in U.S. Pat. No. 5,704,321 and U.S. patent application Ser. No. 09/204,440, both of which are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the present invention is directed to firing electronics which are used to provide electrical energy to a spark plug. It should be understood that while aspects of the present invention detailed below are directed to traveling spark ignitors, plasma-generating devices, and plasma-producing devices, the electronics disclosed herein may be used in conjunction with conventional spark plugs as well. 
     In one embodiment, an electrical circuit for providing use with a traveling spark ignitor having at least two space apart electrodes and dielectric material that fills a substantial portion of the space between the electrodes, the unfilled portion of the space between the electrodes defining a discharge gap of the ignitor is disclosed. In this embodiment, the circuit includes a conventional high-voltage ignition circuit coupled to the ignitor that provides an initial high voltage between the at least two electrodes to ionize an air/fuel mixture and form a plasma kernel in the discharge gap of the ignitor. The circuit of this embodiment also includes a secondary circuit coupled to the ignitor that a provides a follow-on current through the plasma kernel, after the initial high voltage, that expands the plasma kernel. 
     In some aspects, the circuit may also include a third circuit coupled to ignitor that provides an initial pulse of current to the ignitor during the follow-on current which causes the plasma kernel to begin moving away from an upper surface of the dielectric material. 
     In another embodiment, a circuit for providing a follow-on current between the electrodes of a traveling spark ignitor after an initial break down between the electrodes has occurred is disclosed. In this embodiment, the circuit may include a first capacitor coupled in parallel to a secondary side of an ignition coil and the ignitor and a blocking element serially coupled between the first capacitor and the ignitor. The circuit of this embodiment may also include a second capacitor coupled in parallel with the ignitor and an inductive element serially coupled between the second capacitor and the ignitor. 
     In another embodiment a method of operating a traveling spark ignitor is is disclosed. The method of this embodiment includes steps of: (a) providing a first high voltage between electrodes of the ignitor to form a plasma kernel, (b) providing a follow-on current between the electrodes and through the plasma kernel causing the plasma kernel to expand and be swept out of the ignitor due to a Lorentz force and (c) during step (b) providing a first high current during a first time period of the follow-on current at causes the plasma kernel to begin moving outwardly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments of the invention are illustrated and described below with reference to the accompanying drawings, in which like items are identified by the same reference designation, wherein: 
     FIG. 1 is a cross-sectional view of a cylindrical Marshall gun with a pictorial Illustration of its operation, which is useful in understanding the invention. 
     FIG. 2 is a cross-sectional view of a cylindrical traveling spark ignitor for one embodiment of this invention, taken through the axes of the cylinder, including two electrodes and wherein the plasma produced travels by expanding in the axial direction. 
     FIG. 3A is a detailed view of the tip of a cylindrical traveling spark ignitor for the embodiment shown in FIG.  2 . 
     FIG. 3B is a detailed view of one embodiment if a tip of a cylindrical traveling spark ignitor. 
     FIG. 4 is a three dimensional cross-sectional view further defining one embodiment of the present invention. 
     FIG. 5 is a cross-sectional view of a traveling spark ignitor for another embodiment of the invention wherein the plasma produced travels by expanding in the radial direction. 
     FIG. 6 is a cutaway pictorial view of a traveling spark ignitor for one embodiment of the invention, as installed into a cylinder of an engine. 
     FIG. 7 is a cutaway pictorial view of a traveling spark ignitor for a second embodiment of the invention, as installed into a cylinder of an engine. 
     FIG. 8 shows a cross-sectional view of yet another traveling spark ignitor for an embodiment of the invention. 
     FIG. 9A shows a longitudinal cross-sectional view of another traveling spark ignitor for another embodiment of the invention. 
     FIG. 9B is an end view of the traveling spark ignitor of FIG. 9A showing the free ends of opposing electrodes. 
     FIG. 9C is an enlarged view of a portion of FIG.  9 B. 
     FIG. 10 is an illustration of the ignitor embodiment of FIG. 2 coupled to a schematic diagram of an exemplary electrical ignition circuit to operate the ignitor, according to an embodiment of the invention. 
     FIG. 11 is a high-level block diagram of an ignition circuit according to one embodiment of the present invention. 
     FIG. 12 shows a circuit schematic diagram of another ignition circuit embodiment according to the invention. 
     FIG. 13 shows one embodiment of the secondary electronics of FIG.  11 . 
     FIGS. 14A-14C show alternative embodiments of a primary electronics of FIG.  11 . 
     FIGS. 15A-15C show alternative embodiments of the secondary electronics of FIG.  11 . 
     FIG. 16 shows a high-level block diagram of an electrical ignition circuit of the present invention. 
     FIG. 17 is a more detailed version of the circuit disclosed in FIG.  16 . 
     FIG. 18 is a more detailed version of the secondary circuit disclosed in FIG.  17 . 
     FIG. 19 is a graph representing an example of the voltage between the electrodes of a spark plug with respect to time that may be created by the circuit of FIG.  18 . 
     FIG. 20 is an alternative to the secondary circuit shown in FIG.  18 . 
     FIG. 21 is another alternative to the secondary circuit shown in FIG.  18 . 
     FIG. 22 is a variation of the circuit shown in FIG.  21 . 
     FIG. 23 is series connected version of the circuit disclosed in FIG.  17 . 
     FIG. 24 is a variation of the circuit shown in FIG.  23 . 
     FIG. 25 is another variation of the firing circuitry of the present invention. 
     FIG. 26 is yet another embodiment of the firing circuitry of the present invention. 
     FIG. 27 shows the secondary electronics as included in an add-on unit to be used in combination with a conventional ignition system. 
     FIG. 28 shows how a conventional spark plug may be placed in a combustion chamber. 
     FIG. 29 shows how embodiments of the present invention may be placed in a combustion chamber. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description will describe several embodiments and components of aspects of the present invention. It should be understood that various aspects of the invention may be combined or omitted depending upon the context and that the required elements for each embodiment are included only in the appended claims. 
     I. General Theory of Operation 
     The following discussion will relate to the general operation of a plasma-generating device in order to more clearly explain aspects of the present invention. 
     FIG. 1 shows a simplified embodiment of a prior art Marshall gun (plasma gun) that, with limitation, presents an effective way of creating a large volume of plasma. The schematic presentation in FIG. 1 shows the electric field  2  and magnetic field  4  in an illustrative Marshall gun, where B θ  is the poloidal magnetic field directed along field line  4 . The plasma  16  is moved in an outward direction  6  by the action of the Lorentz force vector F and thermal expansion, with new plasma being continually created by the breakdown of fresh gas as the discharge continues. V Z  is the plasma kernel speed vector, also directed in the z-direction represented by arrow  6 . Thus, the plasma  16  grows as it moves along and through the spaces between electrodes  10 ,  12  (which are maintained in a spaced relationship by isolator or dielectric  14 ). Once the plasma  16  leaves the electrodes  10 ,  12 , it expands in volume, cooling in the process. It ignites the combustibles mixture after it has cooled to the ignition temperature. Fortunately, increasing plasma volume is consistent with acknowledged strategies for reducing emissions and improving fuel economy. Two such strategies are to increase the dilution of the gas mixture inside the cylinder and to reduce the cycle-to-cycle variations. 
     Dilution of the gas mixture, which is most commonly achieved by the use of either excess air (running the engine lean) or exhaust gas recirculation (EGR), reduces the formation of oxides of nitrogen by lowering the combustion temperature. Oxides of nitrogen play a critical role in the formation of smog, and their reduction is one of the continuing challenges for the automotive industry. Dilution of the gas mixture also increases the fuel efficiency by lowering temperature and thus reducing the heat loss through the combustion chamber walls, improving the ratio of specific heats, and by lowering the pumping losses at a partial load. 
     Zeilinger determined the nitrogen oxide formation per horsepower-hour of work done, as a function of the air to fuel ratio, for three different spark timings (Zeilinger, K., Ph.D. thesis, Technical University of Munich (1974)). He found that both the air-to-fuel ratio and the spark timing affect the combustion temperature, and thus the nitrogen oxide formation. As the combustible mixture or air/fuel ratio (A/F) is diluted with excess air (i.e., A/F larger than stoichiometric), the temperature drops. At first, this effect is diminished by the increase in the amount of oxygen. The NO x  formation increases. When the mixture is further diluted, the NO x  formation decreases to values much below those at a stoichiometric mixture because the combustion temperature decline overwhelms the increase in 0 2 . 
     A more advanced spark timing (i.e., initiating ignition more degrees before top dead center) raises the peak temperature and decreases engine efficiency because a larger fraction of the combustible mixture bums before the piston reaches top dead center (TDC) and the mixture is compressed to a higher temperature, hence leading to much higher NO x  levels and heat losses. As the mixture is made lean, the spark timing which gives the maximum brake torque (MBT timing) increases. 
     Dilution of the mixture results in a reduction of the energy density and the flame propagation speed, which affect ignition and combustion. The lower energy density reduces the heat released from the chemical reaction within a given volume, and thus shifts the balance between the chemical heat release and the heat lost to the surrounding gas. If the heat released is less than that lost, the flame will not propagate. Thus, a larger initial flame is needed. 
     Reducing the flame propagation speed increases the combustion duration. Ignition delay results from the fact that the flame front is very small in the beginning, which causes it to grow very slowly, as the quantity of fuel-air mixture ignited is proportional to the surface area. The increase in the ignition delay and the combustion duration leads to an increase of the spark advance and larger cycle-to-cycle variations which reduces the work output and increases engine roughness. A larger ignition kernel will reduce the advance in spark timing required, and thus lessen the adverse effects associated with such an advance. (These adverse effects are an increased difficulty to ignite the combustible mixture, due to the lower density and temperature at the time of the spark, and an increase in the variation of the ignition delay, which causes driveability to deteriorate). 
     Cyclic variations are caused by unavoidable variations in the local air-to-fuel ratio, temperature, amount of residual gas, and turbulence. The effect of these variations on the cylinder pressure is due largely to their impact on the initial expansion velocity of the flame. This impact can be significantly reduced by providing a spark volume which is appreciably larger than the mean sizes of the inhomogeneities. 
     A decrease in the cyclic variations of the engine combustion process will reduce emissions and increase efficiency, by reducing the number of poor burn cycles, and by extending the operating air fuel ratio range of the engine. 
     While increasing spark volume, some embodiments of the present invention may also provide for expelling the spark deeper into the combustible mixture, with the effect of reducing the combustion duration. 
     To achieve these goals, some embodiments of the present invention utilize ignitors having electrodes of relatively short length with a relatively large distance between them; that is, the distance between the electrodes is large relative to electrode length. 
     II. Configuration of the Plasma-Generating Devices (ignitors) 
     The following description will explain various aspects of embodiments of plasma-generating devices according to the present invention. 
     FIG. 2 shows one illustrative embodiment of a TSI  17  according to the present invention. This embodiment has standard mounting means  19  such as threads for mounting the TSI  17  in a combustion chamber such as a piston chamber of an internal combustion engine. These threads may mount the TSI in the combustion chamber such that the electrodes extend specific distances into the combustion chamber. The mounting of the TSI  17  may affect the operation of an internal combustion engine and is discussed in greater detail below. 
     The TSI  17  also contains a standard male spark plug connector  21 , and insulating material  23 . The tip  22  of the TSI  17  varies greatly from a standard spark plug. In one embodiment, the tip  22  includes two electrodes, a first electrode  18  and a second electrode  20 . The particular embodiment shown in FIG. 2 has the first electrode  18  coaxially disposed within the second electrode  20 ; that is, the second electrode  20  surrounds the first electrode  18 . The first electrode  18  is attached to a distal boot connector  21 . The space between the electrodes is substantially filled with insulating material (or dielectric)  23 . 
     Application of a voltage to the TSI  17  between the first and second electrodes,  18  and  20 , causes a discharge originating on the surface of the insulating material  23 . The voltage required for a discharge across the insulating material  23  is lower than for a discharge between the electrodes  18  and  20  some distance away from the insulating material  23 . Therefore, the initial discharge occurs across the insulating material  23 . The location of the initial discharge shall be referred to herein as the “initiation region.” This initial discharge constitutes an ionization of the gas (an air/fuel mixture), thereby creating a plasma  24 . This plasma  24  is a good conductor and supports a current between the first electrode  18  and the second electrode  20  at a lower voltage than was required to form the plasma. The current through the plasma serves to ionize even more gas into a plasma. The current-induced magnetic fields surrounding the electrode and the current passing through plasma the interact to produce a Lorentz force on the plasma. This force causes the point of origin of current though the plasma to move and, thus, creates a larger volume of plasma. This is in contrast to traditional ignition systems wherein the spark initiation region remains fixed. The Lorentz force created also serves to expel the plasma from the TSI  17 . Inherent thermal expansion of the plasma aids in this expulsion. That is, as the plasma heats and expands it is forced to travel outwardly, away from the surface of the dielectric material  23 . 
     The first and seconds electrodes,  18  and  20 , respectively, may be made from materials which may include any suitable conductor such as steel, clad metals, platinum-plated steel (for erosion resistance or “performance engines”), copper, and high-temperature electrode metals such as molybdenum or tungsten, for example. The electrodes (one or both) may be of a metal having a controlled thermal expansion like Kovar (a trademark and product of Carpenter Technology Corp.) and coated with a material such as cuprous oxide so as to give good subsequent seals to glass or ceramics. Electrode materials may also be selected to reduce power consumption. For instance, thoriated tungsten could be used, as its slight radioactivity may help to pre-ionize the air or air-fuel mixture between the electrodes, possibly reducing the required ignition voltage. Also, the electrodes may be made of high-Curie temperature permanent magnet materials, polarized to assist the Lorentz force in expelling the plasma. 
     The electrodes, except for a few millimeters at their ends, are separated by insulating material  23  which may be an isolator or insulating material which is a high temperature dielectric. This material can be porcelain, or a fired ceramic with a glaze, as is used in conventional spark plugs, for example. Alternatively, it can be formed of refractory cement, a machinable glass-ceramic such as Macor (a trademark and product of Corning Glass Company), or molded alumina, stabilized zirconia or the like fired and sealed to the metal electrodes such as with a solder glass frit, for example. As above, the ceramic could also comprise a permanent magnet material such as barium ferrite. 
     It should be appreciated that the second electrode  20  need not necessarily be a complete cylinder that completely surrounds the first electrode  18 . That is, the second electrode  20  may have portions removed from it so that there are spaces separating pieces of the second electrode  20  from other pieces. These pieces, if connected, would create a complete circle that surrounds the first electrode  18 . 
     FIG. 3A is a more detailed cross-sectional view of one possible embodiment for the tip  22  shown in FIG.  2 . The particular embodiment shown here relates to TSI  17 . However, it should be noted that the specific properties of this configuration could be applied to any of the below-discussed embodiments, for example TSI&#39;s  27 ,  101  and  120 , or to any embodiment later discovered. 
     The tip  22 , as shown, includes a first electrode  18  and a second electrode  20 . Between the first and second electrodes is an insulating material  23 . The insulating material  23  fills a substantial portion of the space between the electrodes  18  and  20 . The portion of the space between the electrodes  18  and  20  not filled by the insulating material  23  is referred to herein as the discharge gap. This discharge gap has a width W dg  which is the distance between the electrodes  18  and  20  and is measured at their nearest point. The length by which the first electrode  18  extends beyond the insulating material  23  is denoted herein as l and the length by which the second electrode  20  extends beyond the insulating material is denoted as l 2 . The shorter of l 1  or l 2  shall be referred to herein as the length of the discharge gap. The first electrode  18  has a radius r 1  and the second electrode  20  has a radius r 2 . The difference between the radii of the second and first electrodes, r 2 −r 1 , represents the width of the discharge gap W g . It should be noted however that W g  may also be represented by the distance between two spaced apart non-concentric electrodes. 
     The current through the first electrode  18  and the plasma  24  to the second electrode  20  creates around the first electrode  18  a poloidal (angular) magnetic field B θ  (I, r), which depends on the current and distance (radius r 0 , see FIG. 1) from the axis of the first electrode  18 . Hence, a current I flowing through the plasma  24  perpendicular to the poloidal magnetic field B θ  generates a Lorentz force F on the charged particles in the plasma  24  along the axial direction z of the electrodes  18 ,  20 . The force is approximately computed as follows in equation (1): 
     
       
           F˜I×B→F   z   ˜I   r   ·B   θ   (1) 
       
     
     This force accelerates the charged particles which, due to collisions with non-charged particles, accelerates all the plasma. Note that the plasma consists of charged particles (electrons and ions), and neutral atoms. The temperature is not sufficiently high in the discharge gap to fully ionize all atoms. 
     The original Marshall guns as a source of plasma for fusion devices were operated in a vacuum with a short pulse of gas injection between the electrodes. The plasma created between the electrodes by the discharge of a capacitor was accelerated a distance of a dozen centimeters to a final velocity of about 10 7  cm/sec. The drag force F v  on the plasma is approximately proportional to the square of the plasma velocity, as shown below in equation (2): 
     
       
           F   v   ˜V   p   2   (2) 
       
     
     The distance over which the plasma accelerates is short (1-3 mm). Indeed, experimentation has shown that increasing the length of the plasma acceleration distance beyond 1 to 3 mm does not significantly increase the plasma exit velocity, although electrical energy used to drive such a TSI is increased significantly. At atmospheric pressures and for electrical input energy of about 300 mJ, the average velocity is close to 5×10 4  cm/sec and will be lower at high pressure in the engine. At a compression ratio of 8:1, this average velocity will be approximately 3×10 4  cm/sec. 
     By contrast, if more energy is put into a single discharge of a conventional spark, its intensity is increased somewhat, but the volume of the plasma created does not increase significantly. In a conventional spark, a much larger fraction of the energy input goes into heating the electrodes when the conductivity of the discharge path is increased. 
     Given the above dimensioning constraints, the present invention optimizes the combination of the electromagnetic (Lorentz) and thermal expansion forces when the TSI is configured according to the following approximate condition: 
     
       
         ( r   2   −r   1 )/l x ≧⅓  (3) 
       
     
     where l x  is the length of the shorter one of l 1  or l 2 . It should be noted that the dimensional boundaries just expressed are approximate; small deviations above or below them still yield a functional TSI according to the present invention though probably with less than optimal performance. Also, as these dimensions define only the outer bounds, one skilled in the art would realize that there are many configurations which will satisfy these dimensional characteristics. 
     The quantity (r 2 −r 1 )/ l x  represents the gap-to-length ratio in this representation. A smaller gap-to-length ratio may increase the Lorentz force that drives the plasma out of the TSI for the same input energy (when there is a larger current due to lower plasma resistance). If this gap-to-length ratio is too small, the additional energy provided by the Lorentz force goes primarily into erosion of the electrodes due to an increase of the sputtering process on the electrodes. Further, as described above, an optimally performing TSI should form a large volume plasma. Increasing the gap-to-length ratio for the same electrode length increases the volume in which the plasma may be formed and thereby contributes to the increase of the plasma volume produced. Thus, the TSI of the present invention preferably has a sufficiently large gap-to-length ratio such that there is enough volume within which to form a plasma. This volume constraint also serves to set a lower limit for the gap-to-length ratio. A gap-to-length ratio of approximately ⅓ or higher has been found to create an optimal balance between these two constraints. 
     Contrary to early attempts where acceleration of plasma led to the input energy loss due to drag forces which grow with the square of velocity, the large gap-to-length ratio provides for the generation of a large volume of plasma which expelled at a lower velocity. The lower velocity reduces the drag force, thereby reducing the required input energy. Reduced input energy results in a lesser degree of electrode erosion, leading, in turn, to a TSI having a previously unattainable lifetime. 
     Preferably, the TSI ignition system of the present invention uses no more than about 400 mJ per firing. By contrast, early plasma and Marshall gun ignitors have not achieved practical utility because they employed much larger ignition energies (e.g., 2-10 Joules per firing), which caused rapid erosion of the ignitor and short life. Further efficiency gains in engine performance were surrendered by increased ignition system energy consumption. 
     FIG. 3B shows an alternative embodiment of a tip  22  portion of a TSI. In this embodiment there exists an air gap  200  in the direct path over the surface of insulating material  23  between the first electrode  18  and the second electrode  20 . This air gap  200  has a width W ag  and a depth D ag . The width W ag  and the depth D ag  may vary between individual TSI&#39;s but are fixed for each individual TSI. The insulating material in this configuration includes a upper surface  204  and a lower surface  205  located at the base bottom of the air gap  200 . An ignitor having an upper surface  204  and lower surface  205  such as that shown in FIG. 3B shall be referred to herein as a “semi-surface discharge” ignitor. It should be appreciated that a semi-surface discharge ignitor need not have the dimensional ratios shown in FIG.  3 B. 
     The air gap  200  serves several distinct purposes but its dominant effect is to increase the lifetime of the TSI. First, the air gap  200  helps to prevent the electrodes  18  and  20  from being short circuited due to a build up of a complete conduction path over the insulating material  23 . Such a conduction path may be created by a number of mechanisms. For example, every time a TSI is fired, a portion of the metal of the electrodes is blasted away. This removal of electrode metal is known as ablation. Ablation of the electrodes produces a film of metal deposits over the surface of the insulating material  23 . This film, over time, may become solid and thick enough to carry a current and thereby become a conduction path. Another way in which a conduction path between the electrodes could be created is from an excessive build up of carbon deposits or the like on the conduction material  204 . If the build up of carbon deposits becomes large enough to carry a current, a short circuit of the electrodes may result. This direct interconnection leads to a greater amount of energy being imparted to and consumed by the TSI  17  without an appreciable increase in plasma volume. The air gap  200  provides a physical barrier which the conduction path must bridge before such a short circuit condition may occur. That is, in order for a short circuit to occur, the air gap would have to be completely bridged with metal or carbon or a combination thereof. 
     The air gap  200  also serves to help reduce electrode wear. In the absence of the air gap  200 , the initial discharge has been found to occur between the same points on the electrodes every time the TSI  17  is used to ignite a plasma kernel. Namely, the initial discharge would occur at the point where the insulating material contacted the second electrode  20  (assuming a discharge from the first electrode  18  to the second electrode  20 ). Because the discharge occurs at the same point, the second electrode  20  wears out quicker at the point of discharge and eventually is destroyed. Introduction of the air gap  200  causes the initial discharge points to vary. By spreading the discharge points across electrode  20 , the wear is spread over a greater surface; this significantly increases electrode life. The second electrode  20  is preferably a substantially smooth surface. This allows for the spark to jump to more places on the second electrode  20  and thereby increases the area over which wear occurs. This is shown schematically and discussed in more detail in relation to FIG.  4 . 
     FIG. 4 is an example of a cut-away side view of one side of a section of a discharge gap of a TSI. This example includes the first electrode  18 , the second electrode  20 , the insulating material  23  and the air gap  200 . As previously discussed, if the air gap  200  did not exist, the initial breakdown point would occur at substantially the same location, i.e., the closest point of contact between the second electrode  20  and the insulating material  23 . This leads to a rapid erosion of the second electrode  20  at that point and limits ignitor life. The air gap  200  helps to overcome this problem by varying the location of the initial discharge such that the second electrode  20  is not worn away (ablated) at the same point every discharge. This is shown graphically in FIG. 4 where an area of ablation  400  is of width W a  and a height H a . The first time the ignitor is fired, the initial breakdown will occur at the point when the two electrodes are closest to one another. At this time, some ablation of the electrode will occur causing that point to no longer be the closest point so, the next breakdown occurs at the “new” closest point (assuming a uniform gas mixture). Thus, the air gap  200  considerably expands the region over which the discharge occurs. When a thing ring of ablation is formed over the entire perimeter of the second electrode  20 , the closest point will be slightly above or below this ring where a new discharge initiation region will be formed. This occurs during the entire life of the ignitor. 
     Eventually, the area of ablation,  400 , is formed; the size of this area is large enough that the ignitor lasts for a commercially practicable time before the second electrode  20  is ablated away. The width of the air gap W ag  is limited to being about one-half the width of the discharge gap W dg  when, if this width is any larger, the effects of breakdown across the insulating material  23  may be lost due to an increase in resistance occasioned by the increase in space between the electrodes. 
     The area of ablation,  400 , leads to another physical constraint for an ignitor according to one embodiment of the invention. In the case of concentric cylindrical electrodes, the inside of the second electrode  20  should be substantially smooth to ensure that the distance between the electrodes is substantially the same throughout the entire length of the discharge gap. Particularly, in the vicinity of the top of the air gap  200 , no portion of the second electrode  20  should be any closer to the first electrode  18  than in any other area of the gap. A substantially smooth surface of the second electrode  20  allows for the ablation of the second electrode  20  to occur around the entire ablation area  400 . 
     Currently, those conventional spark plugs which are concentric in nature and have a center electrode extending beyond a dielectric material have outer electrodes that are not suited to take advantage of the Lorentz force. In these conventional plugs, the bulk of the outer electrode is directed (at least to a certain degree) radially away from the center electrode. In order to generate Lorentz force on the plasma, the outer electrode must provide a return path for the electric current which is substantially parallel to the center electrode. Thus, in some embodiments, it may be desired to have the first and second electrodes arranged such that the facing sides of the electrodes remain substantially parallel at least in the initiation region. In other embodiments, the electrodes should be substantially parallel to one another throughout the length of the discharge gap. That is, the first and second electrodes should be parallel to one another from at least a region near the upper surface  204  to the ends of the electrodes. In other embodiments, the first and second electrodes may remain parallel to one another some distance below the upper surface  204 . For instance, the first and second electrodes may remain parallel to one another a distance below the upper surface  204  which is approximately equal to the width of the discharge gap W dg  or remain parallel to one another for a distance which represents any fraction between zero and one of the width of the discharge gap W dg . It should be appreciated that the electrodes of any of the TSI embodiments disclosed herein may also be so arranged. 
     Referring again to the embodiment of FIG. 3B, there may exist another gap, the expand gap  202 , between the insulating material  23  and the first electrode  18 . The expand gap  202  has an initial width, We, when the TSI  17  is cold. In some embodiments, the expand gap  202  exists between the insulating material  23  and the first electrode  18  for substantially the entire length of the TSI  17 . In other embodiments, the expand gap  202  may only exist in between the first electrode  18  and the dielectric material  23  for a few (e.g. .5-5) cm below the upper surface  204 . 
     One purpose of the expand gap  202  is to provide a space into which the first electrode  18  may expand as it heats up during operation. Without the expand gap  202  any expansion of the first electrode  18  could cause the insulating material  23  to crack. If the insulating material is cracked, its dielectric properties could be altered and thereby reduce the efficiency of the TSI. Further, the expand gap  202  helps to reduce the possibility of short circuits in a manner similar to that for the air gap  200 . It should be understood however, that the embodiment shown in FIG. 3B could be implemented without the expand gap  202 , if a more flexible/less brittle insulating material is discovered. 
     A TSI shown to work well has been made with an air gap width W ag  of about 0.53 mm, an air gap depth D ag  of about 5.00 mm and an expand gap width W e  of about 0.08 mm. These dimensions are implemented in a concentric electrode TSI similar to TSI  17  of FIG. 2 wherein the length of the first electrode  18  is about 2.7 mm, the length of the second electrode  20  is about 1.2 mm and the gap between them (r 2 −r 1 ) is about 2.4 mm. 
     It should be understood that either or both the air gap and the expand gap discussed above may be utilized in any of the embodiments of a TSI discussed below. 
     FIG. 5 is an example of another embodiment of a TSI according to the present invention. TSI  27  includes an internal electrode  25  that is placed coaxially within an external electrode  28 . The space between the electrodes  25  and  28  is substantially filled with an insulating material  23  (e.g., ceramic). A difference between the embodiment in FIG.  5  and that in FIG. 2 is that there is a flat, disk-shaped (circular) electrode surface  26  formed integrally with, or attached to, the free end of the center electrode  25 , extending transversely to the longitudinal axis of electrode  25  and facing electrode  28 . Note further that the horizontal plane of disk  26  is parallel to the associated piston head (not shown) when the plasma ignitor  27  is installed in a piston cylinder. The end surface of electrode  28  which faces disk electrode  26  is a substantially flat circular shape extending parallel to the facing surface of electrode  26 . As a result, an annular cavity  29  is formed between opposing surfaces of electrodes  26  and  28 . More precisely, there are two substantially parallel surfaces of electrodes  26  and  28  spaced apart and oriented to be parallel to the top of an associated piston head, as opposed to the embodiment of FIG. 2 wherein the electrodes run perpendicularly to an associated piston head when in use. Consider that when the air/fuel mixture is ignited, the associated piston “rises” and is close to the spark plug or ignitor  27 , so that it is preferably further from gap  29  of the ignitor  27  to the wall of the associated cylinder than to the piston head. The essentially parallel electrodes  26  and  28  are substantially parallel to the longest dimension of the volume of the combustible mixture at the moment of ignition, instead of being oriented perpendicularly to this dimension and toward the piston head as in the embodiment of FIG. 2, and the prior art. It was discovered that when the same electrical conditions are used for energizing ignitors  17  and  27 , the plasma acceleration lengths I and L, respectively, are substantially equal for obtaining optimal plasma production. Also, for TSI  27 , under these conditions the following dimensions work well: the radius of the disk electrode  26  is R 2 =6.8 mm, the radius of the isolating ceramic is R 1 =4.3 mm, the gap between the electrodes g 2 =1.2 mm and the length L=2.5 mm. 
     In the illustrative embodiment of FIG. 5, the plasma  32  initiates in discharge gap  29  at the exposed surface of insulator  25 , and grows and expands outwardly in the radial direction of arrows  29 A. This may provide advantages over the TSI embodiment of FIG.  2 . First, the surface area of the disk electrode  26  exposed to the plasma  32  is substantially equal to that of the end portion of the outer electrode  28  exposed to the plasma  32 . This means that the erosion of the inner portion of disk electrode  26  can be expected to be significantly less than that of the exposed portion of inner electrode  18  of TSI  17  of FIG. 2, the latter having a much smaller surface area exposed to the plasma. Secondly, the insulator material  23  in TSI  27  provides an additional heat conducting path for electrode  26 . The added insulator material  23  will keep the inner metal of electrodes  25 ,  26  cooler than electrode  18 . In addition, in using TSI  27 , the plasma will not be impinging on and perhaps eroding the associated piston head. 
     FIGS. 6 and 7 illustrate pictorially the differences in plasma trajectories between TSI  17  of FIG. 2, and TSI  27  of FIG. 5 when installed in an engine. In FIG. 6, a TSI  17  is mounted in a cylinder head  90 , associated with a cylinder  92  and a piston  94  which is reciprocating—i.e., moving up and down—in the cylinder  92 . As in any conventional internal combustion engine, as the piston head  96  nears top dead center, the TSI  17  will be energized. This will produce the plasma  24 , which will travel in the direction of arrow  98  only a short distance toward or to the piston head  96 . During this travel, the plasma  24  will ignite the air/fuel mixture (not shown) in the cylinder  92 . The ignition begins in the vicinity of the plasma  24 . In contrast to such travel of plasma  24 , the TSI  27 , as shown in FIG. 7, provides for the plasma  32  to travel in the direction of arrows  100 , resulting in the ignition of a greater amount of air/fuel mixture than provided by TSI  17 , as previously explained. 
     A trigger electrode can be added between the inner and outer electrodes of FIGS. 2 through 5 to lower the voltage required to cause an initial breakdown between the first and second electrodes. FIG. 8 shows such a three electrode plasma ignitor  101  schematically. Also shown in FIG. 8 is a simplified version of the electronics which may drive a TSI. An internal electrode  104  is placed coaxially within the external electrode  106 , both having diameters on the order of several millimeters. Radially placed between the internal electrode  104  and the external electrode  106  is a third electrode  108 . This third electrode  108  is connected to a high voltage (HV) coil  110 . The third electrode  108  initiates a discharge between the two main electrodes  104  and  106  by charging the exposed surface  114  of the insulator  112 . The space between all three electrodes  104 ,  106 ,  108  is filled with insulating material  112  (e.g., ceramic) except for the last 2-3 mm space between electrodes  104  and  106  at the combustion end of the ignitor  101 . A discharge between the two main electrodes  104  and  106 , after initiation by the third electrode  108 , starts along the surface  114  of the insulator  112 . The gas (air-fuel mixture) is ionized by the discharge. This discharge creates a plasma, which becomes a good electrical conductor and permits an increase in the magnitude of the current. The increased current ionizes more gas (air-fuel mixture) and increases the volume of the plasma, as previously explained. 
     The high voltage between the tip of the third electrode  108  and the external electrode  106  provides a low current discharge, which is sufficient to create enough charged particles on the surface  114  of the insulator  112  for an initial discharge to occur between electrodes  104  and  106 . 
     As shown in FIGS. 9A,  9 B and  9 C, another embodiment of the invention includes a TSI  120  having parallel rod-shaped electrodes  122  and  124 . The parallel electrodes  122 ,  124  have a substantial portion of their respective lengths encapsulated by dielectric insulator material  126 , as shown. A top end of the dielectric  126  retains a spark plug boot connector  21  that is both mechanically and electrically secured to the top end of electrode  122 . The dielectric material  126  rigidly retains electrodes  122  and  124  in parallel, and a portion rigidly retains the outer metallic body  128  having mounting threads  19  about a lower portion, as shown. Electrode  124  is both mechanically and electrically secured to an inside wall of metallic body  128  via a rigid mount  130 , as shown, in this example. As shown in FIG. 9A, each of the electrodes  122  and  124  extends a distance l 1  and l 2 , respectively, outwardly from the surface of the bottom end of dielectric  126 . 
     With reference to FIGS. 9B and 9C, the electrodes  122  and  124  may be parallel rods that are spaced apart a distance G, where G is understood to represent the width of the discharge gap between the electrodes  122 ,  124  (see FIG.  9 C). 
     It has been discovered that, while operating a TSI as described above, a great deal of RF noise may be generated. During the initial high voltage breakdown, current flows in one direction through a first electrode and in another through a second electrode. These opposite flowing currents generate the RF noise. In conventional spark plugs this is not an issue because a resistive element may be placed within the plug in the incoming current path. However, due to the large currents experienced during the high current stage of operation of the present invention, such a solution is not feasible because such a resistor would not allow enough current to flow to generate a large plasma kernel. 
     Such RF noise may interfere with various electronic devices and may violate regulations if not properly shielded. As such, and referring again to FIG. 9A, the TSI  120  may also include a co-axial connector  140  for attaching a co-axial cable (not shown) to the TSI  120 . The co-axial connector  140  may be threads, a snap connection, or any other suitable connectors for attaching a co-axial cable to an ignitor. It should be understood that while not illustrated in the above embodiment, such a co-axial connector  140  could be included in any of the above embodiments. Furthermore, the co-axial connector  140  may be included in any semi-surface ignitor currently available or later produced. Cables of this sort will typically provide electricity to the boot connector  21 , surround the dielectric  126  and mate with the body  128  to provide a ground. The cable should be able to withstand high voltages (during the primary discharge), carry a high current (during the secondary discharge) and survive the hostile operating environment in an engine compartment. One suitable co-axial cable is a RG-225 Teflon co-axial cable with a double braided shield. Other suitable cables include those disclosed in PCT Published Application WO 98/10431, entitled High Power Spark Plug Wire, filed Sep. 7, 1997, which is hereby incorporated by reference. 
     III. The Firing Circuitry 
     The following description will focus on various embodiments of the firing circuitry which may lead to effective utilization of the plasma-generating devices disclosed above. It should be appreciated that the application of the firing circuitry electronics disclosed below are applicable to other types of spark plugs as well. 
     FIG. 10 shows a TSI  17  with a schematic of the basic elements of an electrical or electronic ignition circuit connected thereto, which supplies the voltage and current for the discharge (plasma). (The same circuitry and circuit elements may be used for driving any embodiment of a TSI disclosed herein or later discovered.) A discharge between the two electrodes  18  and  20  starts along the surface  56  of the dielectric material  23 . The gas air/fuel mixture is ionized by the discharge, creating a plasma  24  which becomes a good conductor of current and permits current between the electrodes at a lower voltage than that which initiated the plasma. This current ionizes more gas (air/fuel mixture) and increases the volume of the plasma  24 . 
     As shown, the discharge travels from first electrode  18  to the second electrode  20 . One of ordinary skill would realize that the polarity of the electrodes could be reversed. However, there are advantages to having the discharge travel from the first electrode  18  to the second electrode  20 . Physical constraints, namely the fact that the second electrode  20  surrounds the first electrode  18  in this embodiment, allow for the second electrode  20  to have a greater total surface area. The greater the surface area of an electrode the more resistant to ablation the electrode is. Having the second electrode  20  be the target of the positive ion bombardment, because of its greater resistance to ablation, allows for the production of a TSI  17  having a longer useful life. 
     The electrical circuit shown in FIG. 10 includes a conventional ignition system  42  (e.g., capacitive discharge ignition (CDI) or transistorized coil ignition (TCI)), a low voltage (V s ) supply  44 , capacitors  46  and  48  diodes  50  and  52 , and a resistor  54 . The conventional ignition system  42  provides the high voltage necessary to break down, or ionize, the air/fuel mixture in the discharge gap along the surface  56  of the dielectric material  23   17 . Once the conducting path has been established, the capacitor  46  quickly discharges through diode  50 , providing a high power input, or current, into the plasma  24 . The diodes  50  and  52  electrically isolate the ignition coil (not shown) of the conventional ignition system  42  from the relatively large capacitor  46  (between 1 and 4 μF). If the diodes  50 ,  52  were not present, the coil would not be able to produce a high voltage, due to the low impedance provided by capacitor  46 . The coil would instead charge the capacitor  46 . The function of the resistor  54 , the capacitor  48 , and the voltage source  44  is to recharge the capacitor  46  after a discharge cycle. The use of resistor  54  is one way to prevent a low resistance current path between the voltage source  44  and the spark gap of TSI  17 . 
     FIG. 11 is a high level block diagram of one illustrative embodiment of a firing circuit  200  according to the present invention. The circuit of this embodiment includes a primary circuit  202 , an ignition coil  300 , and a secondary circuit  208 . 
     In one embodiment, the primary circuit  202  includes a power supply  210 . The power supply  210  may be, for example, a DC to DC converter with an input of  12  volts and an output of 400-500 volts. In other embodiments, the power supply  210  could be an oscillating voltage source. The primary circuit  202  may also include a charging circuit  212  and a coil driver circuit  214 . The charging circuit charges a device, such as a capacitor (not shown), in order to supply the coil driver circuit  214  with a charge to drive the ignition coil  300 . In one embodiment, the power supply  210 , the charging circuit  212 , and the coil driver  214  may be a CDI circuit. However, it should be understood that these three elements could be combined to form any type of conventional ignition circuit capable of causing a discharge between two electrodes of a spark plug, for example, a TCI system. The coil driver circuit  214  is connected to a low voltage winding of the ignition coil  300 . The high voltage winding of the ignition coil  300  is electrically coupled to the secondary circuit  208 . 
     In the embodiment of FIG. 11, the secondary circuit  208  includes a spark plug and associated circuitry  220 , a secondary charging circuit  222 , and a power supply  224 . The spark plug and associated circuitry  220  may include a capacitor (not shown) which is used to store energy in the secondary circuit  208 . The two power supplies,  210  and  224 , for the primary and secondary circuits,  202  and  208 , respectively, may be derived from a single power source. It should be appreciated that the term “spark plug” as used in relation to the following firing circuitry may refer to any plug capable of producing a plasma, such as the plasma-generating and plasma expelling devices described above. 
     FIG. 12 is a more detailed version of the circuit described above in relation to FIG.  10 . In a commercial application, the circuit of FIG. 12 is preferred for recharging capacitor  46  (FIG. 10) in a more energy-efficient manner, using a resonant circuit. Furthermore, the conventional ignition system  42  (FIG.  10 ), whose sole purpose is to create the initial breakdown, is modified so as to use less energy and to discharge more quickly than has been conventional. Almost all of the ignition energy is supplied by capacitor  46  (FIG.  10 ). The modification is primarily to reduce high voltage coil inductance by the use of fewer secondary turns. This is possible because the initiating discharge can be of a much lower voltage when the discharge occurs over an insulator surface. The voltage required can be about one-third that required to cause a gaseous breakdown in air for the same distance. 
     Matching the electronic circuit to the parameters of the TSI (length of electrodes, diameters of coaxial cylinders, duration of the discharge) maximizes the volume of the plasma when it leaves the TSI for a given store of electrical energy. By choosing the parameters of the electronic circuit properly, it is possible to obtain current and voltage time profiles that transfer substantially maximum electrical energy to the plasma. 
     The ignition electronics can be divided into four parts, as shown: the primary and secondary circuits,  202  and  208 , respectively, and their associated charging circuits,  212  and  222 , respectively. The primary circuit  202  also includes a coil driver circuit  214 . The secondary circuit  208  may include spark plug and associated electronics circuitry  220  which may be broken down into a high voltage section  283 , and a low voltage section  285 . 
     The primary and secondary circuits,  202  and  208 , respectively, correspond to primary  258  and secondary  260  windings of an ignition coil  300 . When the SCR  264  is turned on via application of a trigger signal to its gate  265 , the capacitor  266  discharges through the SCR  264 , which causes a current in the coil primary winding  258 . This in turn imparts a high voltage across the associated secondary winding  260 , which causes the gas in a region near the spark plug  206  to break down and form a conductive path, i.e. a plasma. Once the plasma has been created, diodes  286  turn on and the secondary capacitor  270  discharges. 
     After the primary and secondary capacitors  266  and  270 , respectively, have discharged, they are recharged by their respective charging circuits  212  and  222 . Both charging circuits  212  and  222  incorporate an inductor  272 ,  274  (respectively) and a diode  276 ,  278  (respectively), together with a power supply  210 ,  224  (respectively). The function of the inductors  272  and  274  is to prevent the power supplies from being short-circuited through the spark plug  206 . The function of the diodes  276  and  278  is to avoid oscillations. The capacitor  284  prevents the power supply  224  voltage V 2  from the going through large fluctuations. 
     The power supplies  210  and  224  both supply on the order of 500 volts or less for voltages V 1  and V 2 , respectively. They could be combined into one power supply. Power supplies  210  and  224  may be DC-to-DC converters from a CDI (capacitive discharge ignition) system, which can be powered by a 12-volt automobile electrical system, for example. 
     The high current diodes  286  connected in series have a high total reverse breakdown voltage, larger than the maximum spark plug breakdown voltage of any of the above disclosed plasma-generating devices, for all engine operating conditions. The function of the diode  286  is to isolate the secondary capacitor  270  from the ignition coil  300 , by blocking current from secondary winding  260  to capacitor  270 . If this isolation were not present, the secondary voltage of ignition coil  300  would charge the secondary capacitor  270 ; and, given a large capacitance, the ignition coil  300  would never be able to develop a sufficiently high voltage to break down the air/fuel mixture in a region near the spark plug  206 . 
     Diode  288  prevents capacitor  270  from discharging through the secondary winding  260 . Finally, the optional resistor  290  may be used to reduce current through secondary winding  260 , thereby reducing electromagnetic radiation (radio noise) emitted by the circuit. 
     FIGS. 13-15 detail general various alternative secondary circuits  208  which may be used according to the present invention. 
     FIG. 13 shows an example of one embodiment of a secondary circuit  208  according to the present invention. This circuit provides for a fast initial breakdown across the spark plug  206  followed by a slow follow-on current between the electrode of the spark plug  206  due to the inductor L 1 . As such, this circuit may be thought of as a “fast-slow” circuit. 
     The secondary (high voltage) winding  260  of the ignition coil  300  receives electrical energy from the primary circuit (not shown), which is attached to the low side winding (not shown) of the ignition coil  300 , in order to charge capacitor C 1  which is connected in parallel with the ignition coil  300 . When the voltage across the capacitor C 1  becomes large enough to cause a breakdown over both the spark gap  302  and between the electrodes of the spark plug  206 , the capacitor C 1  is discharged through the spark gap  302  and the spark plug  206 . The capacitor C 1  is prevented from discharging into capacitor C 2  by inductor L 1  which acts as a large resistance to a rapidly changing current. 
     This initial breakdown caused by the discharge of capacitor C 1  is the initial phase which begins the formation of a plasma kernel between the electrodes of the spark plug. 
     It should be understood that the spark gap  302  could be replaced by a diode or other device capable of handling the high voltage across the secondary winding  260  and blocking a large current from discharging into the secondary winding  260 . From time to time in the following description and in the attached figures, the spark gap  302  will be described and shown as a diode to illustrate their theoretical interchangeability for certain analytical purposes. 
     Before the initial breakdown occurs, the capacitor C 2  is charged by the power supply  124 . The power supply  224  is sized such that it does not create a large enough voltage across capacitor C 2  in order to cause a breakdown across the spark plug  206 . After the capacitor C 1  has started to discharge through the spark plug  206 , capacitor C 2  then discharges through the spark plug  206 . This discharge is a lower voltage, higher current discharge than that provided by the discharge of capacitor C 1 . The capacitor C 2  is prevented from discharging through the secondary coil  260  by the spark gap  302 . As discussed above, the spark gap  302  could be replaced by a diode capable of enduring the high voltage across capacitor C 1  and blocking the high current discharge of capacitor C 2  from traveling to the secondary winding  260  and while still allowing for a fast discharge (e.g., a break-over diode or self-triggered SCR). The discharge of capacitor C 2  through the spark plug  206  is the follow-on low-voltage, high-current pulse which causes the plasma kernel to expand and be swept out from between the electrodes of the spark plug  206  as described above. 
     The discharge of capacitor C 2  through the spark plug  206  is slower than the discharge of capacitor C 1 . The reason that the discharge is slower is due to the inductor L 1 , which serves to slow down the rate which capacitor C 2  may discharge through the spark plug  206 . In one embodiment, capacitor C 2  is larger than capacitor C 1  and, as is known in the art, its discharge is thus slower. 
     Resistor R 1  serves as a current limiting resistor so that the power supply does not provide a continuous current through the spark plug  206  after capacitor C 2  has discharged and limits the charging current to capacitor C 2 . It should be appreciated that the connection between resistor R 1  and the power supply  224  is the Thevenin equivalent of a current limited power supply. It should also be appreciated that resistor R 1  could be replaced with a suitably sized inductor to prevent a continuous current from the power supply  224  from persisting through the spark plug  206  and limits the charging current of capacitor C 2 . The combination of resistor R 1  and power supply  224  may from time to time be referred herein to generally as a secondary charging circuit. 
     Suitable values for the components described in relation to FIG. 13 include C 1 =200 pF, L 1 =200 μH, C 2 =2 μf, and R 1 =2 K ohms, when power supply  224  provides 500V. 
     FIGS. 14A-14C show various circuit schematics for different variations of the primary circuit. All of them use a capacitor  620  which is charged by the primary charging circuit  212  through the coil primary winding  258 . All of the embodiments shown in FIGS. 14A-14C also include an SCR  264  which is used to rapidly discharge the capacitor  620  through winding  258 , which creates the high voltage on the secondary winding  260 . The three circuits have diode  622  in different places. 
     FIG. 14A has the SCR  264  in parallel with the primary winding  258 . Once the capacitor  620  is completely discharged and begins to recharge in the opposite polarity, the diode  264  becomes conductive, and a current through the primary winding  258  continues through the diode  622  until it is dissipated by the resistances of the primary winding and the diode,  258  and  622  respectively, and the energy transfer to the secondary winding. Thus the coil current and secondary voltage (high voltage) do not change polarity. 
     FIG. 14B has the diode connected in parallel to the SCR  264 . When the SCR  264  fires, the capacitor  620  discharges, and then recharges in the opposite polarity due to the inductance of the primary coil  258 . Once the capacitor  620  is charged to the maximum voltage, the current reverses, passing through the diode  622 . This cycle is then repeated until all of the energy is dissipated. The coil current and high voltage thus oscillate. 
     The circuit of FIG. 14C is designed to give a single pass of current through the primary winding  258 , recharging the capacitor  620  in the opposite direction. The second pass of current in the opposite direction then occurs through the diode  622  and the inductor  624  (which are connected in series between the cathode of the SCR  264  and ground), at a slower rate, so that the capacitor is recharged after the spark in the spark plug (not shown) has been extinguished. The diode  622  and inductor  624  function as an energy recovery circuit. 
     FIGS. 15A-15C show further embodiments of the secondary circuit  208 . The embodiments shown in FIGS. 15A-15C include the spark plug and associated circuitry  220  (FIG.  11 ). 
     The embodiment of FIG. 15A includes a single diode  626 . It should be appreciated that diode  626  could be replaced by a plurality of series connected diodes. The diode  626  provides a low impedance path for the capacitor  626  to discharge. In this embodiment it is preferably that the two windings,  258  and  260 , be completely separated. 
     FIG. 15B is an example of a thru-circuit. This embodiment includes the capacitor C 2  which discharges through the secondary winding  260 . Ordinarily this would result in a very slow discharge due to the large inductance of the secondary winding  260 . However, if the coil core  628  saturates, dramatically reducing the coil inductance, then the discharge can occur more rapidly. 
     FIG. 15C shows another embodiment of a secondary circuit. In this embodiment, the inductor  632  is in a parallel arrangement with the second winding  260 . The spark gap  630  is in series between the secondary winding  260  and the spark plug  206 . 
     In the above described embodiments, the nature of the discharge may be described as being of a dual-stage nature. However, in some situations it may be desirable to add a third stage to the discharge. It has been discovered that an initial high-current burst may be required to allow the current channel to begin moving away from the upper surface of the dielectric material between the electrodes of a plasma-generating device. However, if this initial high-current burst delivers the energy too quickly, the plasma may not move for a long enough time to create a large kernel. That is, if the current is large enough to create a Lorentz force sufficient to cause the spark to travel, such a current may discharge all of the stored energy to quickly to allow the spark to travel far enough to generate an enlarged plasma kernel. Furthermore, large currents lead to increased electrode ablation. These drawbacks may be alleviated by lengthening the discharge or lowering the amount of current for a given discharge. However, if the current is reduced to achieve a longer discharge, the resultant Lorentz force may not be strong enough to cause the spark to move away from the location when the spark originated (e.g., the upper surface of the dielectric). The following examples discuss various circuits which overcome these problems, and others, by combining the initial breakdown with a fast high-current discharge to get the spark moving and longer lower-current discharge to grow the plasma kernel while minimizing electrode ablation. 
     FIG. 16 shows an example what shall be referred to herein as a parallel three circuit ignition system  700 . This system includes a conventional high-voltage circuit  702 , a secondary circuit  704  and a third circuit  706 . The high-voltage circuit  702  and the secondary  704  circuit are connected in parallel with the spark plug  206 . The parallel connection is similar to those described above. The high-voltage circuit  702  may be any conventional ignition circuit such as a CDI circuit, a TCI circuit or a magneto ignition system. The high-voltage circuit  702  provides the initial high voltage to ionize the air/fuel mixture in the discharge gap of a plasma-generating device. In the following examples, it should be understood that the high voltage circuit includes both the primary and secondary windings of the ignition coil. The secondary circuit  704  provides the follow-on current that serves to expand the plasma kernel. The embodiment of FIG. 16 also includes a third circuit  706  connected to the secondary circuit  704 . In some embodiment, the third circuit  706  may be a sub-circuit of the secondary circuit  704 . The third circuit  706  provides an initial pulse of current during the follow-on current which enables the initial current channel (and the surrounding plasma) to move away from the upper surface of the dielectric. 
     FIG. 17 shows a more detailed example of the circuit shown in FIG.  16 . This circuit includes a high-voltage circuit  702 , secondary circuit  704  and the third circuit  706 . 
     Connected in parallel with the high-voltage circuit  702  is the first capacitor C 1 . The function of the first capacitor C 1  is to enhance the initial spark between the electrodes of the spark plug  206  by providing a rapid, high-voltage discharge. In some embodiments, the first capacitor C 1  may be omitted. For purposes of this discussion, the combination of capacitor C 1  and high-voltage circuit should be called the primary circuit  708 . 
     The primary circuit  708  may also include a first sub-circuit SC 1  connected between the capacitor C 1  and the spark plug  206 . The first sub-circuit SC 1  may be any device capable of preventing the capacitors of the second circuit  704  and the third circuit  706  from discharging into the first capacitor C 1  after capacitor C 1  has discharged. An additional feature of the first sub-circuit SC 1  may be to reduce the rise time of the high voltage. Suitable elements that may be used for the first sub-circuit SC 1  include, but are not limited to, diodes, bread-over diodes and spark gaps. 
     The secondary circuit  704  includes a second capacitor C 2 , and inductor L 1 , and the second sub-circuit SC 2 . Attached to the second circuit  704  is the secondary charger  710  which include resistor R 1  and voltage supply  224 . 
     The inductor L 1  serves to slow down the discharge of the second capacitor C 2 . As discussed below, this allows for the desired three stage voltage to produce increased plasma growth. The second sub-circuit SC 2  serves to isolate the secondary circuit  704  from the high voltage created in the primary circuit  708  to both protect the secondary circuit  704  as well as to provide a high impedance to force the primary circuit  708  to generate a high enough voltage to cause an initial breakdown between the electrodes of the spark plug  206 . To this end, the second sub-circuit SC 2  may be a high voltage diode or an inductor. 
     The third circuit  706  includes a third capacitor C 3  connected in parallel with the spark plug  206 . The third circuit  706  may optionally also include a third sub-circuit SC 3 . The third capacitor C 3  provides an initial pulse of current, which allows the plasma to move away from the region of the initial breakdown. The optional third sub-circuit SC 3  may be used to prevent the rapid recharging of the third capacitor C 3 . If the third sub-circuit SC 3  is omitted, the third capacitor C 3  may form an oscillatory circuit with the second capacitor C 2  and the inductor L 1 . Possible implementation of the third sub-circuit SC 3  include, but are not limited to, a diode connected in parallel with either an inductor or a resistor or just a single diode. Of course, the diode would be connected such that its anode is connected to the third capacitor C 3  and its cathode is connected to the inductor L 1 . 
     FIG. 18 shows another embodiment of a secondary circuit  208 . This circuit provides an initial “snap” high voltage across the spark plug  206  followed by a first high current discharge and a slower discharge. FIG. 18 will be used to further explain the operation of a three stage circuit. As discussed above, the high-voltage circuit (not shown) delivers power to the secondary coil  260  of the ignition coil  300 . When the voltage across the secondary coil  260  exceeds the breakdown voltage between the electrodes of the spark plug  206 , an initial discharge of a high voltage occurs between the electrodes. In this embodiment, the first and second sub-circuits have been replaced by diodes D 1  and D 2 . 
     The initial voltage discharged across the spark plug  206  may be in the range of 500V. Thus, the diode D 1  should be able to sustain a voltage drop across it of close to 500V. However, 500V is given by way of example only and as one of ordinary skill in the art will readily realize, this voltage could be higher or lower depending upon the application. 
     The initial high voltage serves several functions. First, this high voltage may help knock loose any carbon and/or metal deposits present between the electrodes of the spark plug  206 . In addition, this high voltage may also begin forming the plasma kernel. 
     During the time that the primary circuit is charging the coil  300 , the power supply  224  is charging capacitors C 3  and C 2 . The diode D 2  keeps the secondary coil  260  from discharging through either capacitor C 3  or capacitor C 2 . 
     After the initial discharge of the secondary coil  260  through the spark plug  206 , both capacitors C 2  and C 3  begin to discharge through the spark plug  206 . The discharge of capacitor C 3  is a fast discharge as compared to the discharge of capacitor C 2  due to the inductor L 1  placed between the two. Thus, capacitor C 3  provides a fast, high current discharge through spark plug  206  which serves to cause the plasma kernel between the electrodes of the spark plug  206  to expand and travel outwardly between the electrodes. Due to the inductor L 1 , the discharge of capacitor C 2  is slower than that of capacitor C 3  and sustains a current between the electrode even after capacitor C 3  has discharged. Capacitor C 2  is prevented from discharging through, and thereby charging, capacitor C 3  by blocking diode D 3 . 
     FIG. 19 is a graph of voltage across the electrodes of the spark plug  206  as a function of time. From time t 0  to time t 1  the voltage across the electrodes of the spark plug  206  rises as the voltage across the secondary coil  260  increases until time t 1 . At time t 1 , the voltage has increased to a level where a breakdown can occur between the electrodes of the spark plug  206 . In addition, because there is no inductor between capacitor C 3  and the spark plug, capacitor C 3  also begins to discharge which adds to the current through the spark plug and lead to “the snap” across the electrodes. Both the secondary coil  260  and capacitor C 3  are allowed to discharge freely. Thus, the voltage drops quickly between time t 1  and t 2  At time t 2 , capacitor C 2  (whose discharge was delayed by inductor L 1 ) begins to discharge through the spark plug  206 . The combined discharges of the secondary winding  260  and of capacitors C 2  and C 3  accounts for the flatness of the voltage curve between times t 2  and t 3 . By time t 3 , capacitor C 3  and the secondary winding  260  have fully discharged and capacitor C 2  is allowed to discharge on its own and provide a current through the plasma between the electrode for an extended time period (i.e., until it fully discharges or a new cycle begins). 
     Suitable values for the components of the circuit in FIG. 18 have been found to be C 2 =2 μF, C 3 =0.2 μF, L 1 =200 μH, and R 1 =2 K ohms with the power supply  224  providing 500V. 
     It should be understood that the preceding functional explanation may apply to any of the three stage circuits described herein. 
     FIG. 20 shows another embodiment of a secondary circuit  208 . This embodiment is substantially the same as the one discussed in relation to FIG. 18 with the addition of the third sub-circuit SC 3 . In this example, the third sub-circuit SC 3  includes a diode D 3  connected in parallel with an inductor L 3 . The cathode of the diode D 3  is connected between D 2  and L 1  and its anode is connected to the capacitor C 3 . C 1  has been omitted for clarity but may be included as one of ordinary skill will readily realize. 
     FIG. 21 shows a circuit similar to that of FIG. 18 except that diodes D 1  and D 2  have been replaced, respectively, by a spark gap  712  and inductor L 2 . This embodiment functions in much the same manner as FIG.  18 . The spark gap  712  and inductor L 2  provide the same functionality as the diodes D 1  and D 2  which they replace albeit in a different manner. The spark gap  712  provides an impedance so that C 3  and C 2  do not discharge in to the secondary coil  260  or charge C 1  instead of the spark plug  206  and inductor L 2  provides a similar impedance to keep the voltage from the secondary coil  260  from charging capacitors C 2  and C 3  instead of discharging across the electrodes of the spark plug  206 . The inductor L 2  provides this functionality due to inherent characteristics of inductors as well as the characteristic frequency of the break down across the spark gap  712 . The inductor L 2  should be sized such that it provides a high enough impedance at the characteristic frequency of the air gap breakdown (about 10 MHz) while still allowing both C 3  and C 2  to discharge through L 2 . In some embodiments, the spark gap  712  may be replace by solid-state elements that operate in manners similar to a spark gap such as a break-over diode or a self-triggered SCR. In other respects the multi-stage discharge is the same as described above. 
     Of course, and as shown in FIG. 22, the secondary circuit could include the third sub-circuit SC 3  described above. In the embodiment of FIG. 22, the third sub-circuit SC 3  includes a diode D 3  connected in parallel with an inductor L 3  where the cathode of diode D 3  is connected between D 2  and L 1  and its anode is connected to the capacitor C 3 . Of course, SC 3  could just include diode D 3 . 
     FIG. 23 is an alternative embodiment of a circuit which provides a three stage discharge through the spark plug  206 . In this embodiment, a conventional high-voltage circuit  702  may be connected directly to the spark plug  206 . The blocking diode  720  is connected between the output terminals  722  and  724  of the high voltage circuit  702  and serves to keep the high voltage circuit from charging capacitors C 2  and C 3 . Capacitor C 3  is connected between the anode of the blocking diode  720  and ground. Connected in parallel with capacitor C 3  is the series connection of inductor L 1  and capacitor C 3 . After the initial break down between the electrodes of the spark plug  206  caused by the high voltage of the conventional high-voltage circuit  702 , as described above, C 3  quickly discharges through the spark plug  206  while the discharge of C 2  is slowed by inductor L 1 . The discharge in this embodiment is similar to that disclosed in FIG.  19 . Of course, and as discussed above, the circuit of FIG. 23 also includes a charging circuit  726  to charge capacitors C 2  and C 3  before each discharge. 
     FIG. 24 shows an embodiment similar to that shown in FIG. 23 with the addition of the third sub-circuit SC 3 . In this embodiment, includes a diode D 3  connected in parallel with an inductor L 3  where the cathode of diode D 3  connected between D 2  and L 1  and its anode is connected to the capacitor C 3 . 
     FIG. 25 is an example of another embodiment of a secondary circuit  208  according to the present invention. This embodiment differs from the prior embodiments in at least two respects. First, this embodiment does not utilize a spark gap or diode in order to prevent the capacitor C 2  of the secondary circuit  208  from being charged by the voltage across the secondary winding  260  of the ignition coil  300 . Second, the power supply  210  of the primary circuit  202  supplies an oscillating voltage. In one embodiment, power supply  210  may oscillate at an RF frequency. 
     The ignition coil  300  in this case has a primary winding  402  which has fewer turns than the secondary winding  260 . In a preferred embodiment, the secondary winding  260  of the ignition coil  300  has a self-resonance approximately equal to the oscillation frequency f 0  of the oscillating power supply  210 . Because the primary winding  402  of the ignition coil  300  has fewer turns than the secondary winding, its resonant frequency does not match that of the oscillating power supply  210 . As such, an appropriately sized capacitor C 5  is used to tune the primary winding  402  to the resonant frequency of the oscillating power supply  210 . Thus, at node  404  there exists an oscillating high voltage. The diode D 1 , as discussed above, prevents the discharge of capacitor C 2  into the secondary winding  260 . The diode D 1  also serves as a half-wave rectifier. As one of ordinary skill in the art would readily realize, however, the diode D 1  could be replaced with a capacitor which will pass the full oscillating signal while still blocking the DC discharge from capacitor C 2 . 
     In contrast to the prior embodiments discussed above, the voltage across winding  260  is prevented from discharging into capacitor C 2  by the parallel connection of inductor L 1  and capacitor C 4  instead of by a diode. The inductor L 1  preferably has a high Q factor which allows it to provide, theoretically, infinite impedance at its resonant frequency. Capacitor C 4  is used to tune inductor L 1  so that its resonant frequency matches that of the oscillating power supply  210 . In this manner, the oscillating voltage is prevented from passing through to the capacitor C 2 . 
     As discussed above, when the voltage at node  404  exceeds the breakdown voltage across the electrodes of the spark plug  206 , the secondary winding  260  is discharged through the electrodes of the spark plug  206 . Then capacitor C 2  provides the follow-on current which causes the plasma kernel to expand and be expelled from between the electrodes of the spark plug  206 . The parallel combination of capacitor C 4  and inductor L 1  does not affect the discharge of capacitor C 2  because this discharge is at a lower frequency. 
     FIG. 26 shows another alternative embodiment circuitry that may be used to provide a multi-stage discharge to a plasma-expelling device. This embodiment includes a first transformer  730  which is typically part of a high-voltage ignition system. Connected to and in parallel with the secondary side  732  of the first transformer  730  is a peaking capacitor  734 . The peaking capacitor  734  is connected in parallel with the series connection of a spark gap  736  and the primary side  738  of a second transformer  740 . In one embodiment, the second transformer  740  is a torodial transformer (e.g., metal core) having a greater number of turns on its secondary side  742  than on the primary side  738  (e.g., a turns ratio of 4 to 1 may be appropriate). 
     When a sufficient voltage is stored in the peaking capacitor  734 , a rapid breakdown across the spark gap  736  may occur. The rapid breakdown induces a high voltage in the secondary side  742  of the second transformer  740 . The high voltage induced in the secondary side  742  causes the initial breakdown between electrodes of the spark plug  206  which is connected between the a first terminal  744  of the secondary side  742  and ground. Connected between the second terminal  746  of the secondary side  748  and ground is a the third capacitor C 3 . The third capacitor C 3  is connected in parallel to the series combination of inductor L 1  and capacitor C 2 . A charging circuit  748  may be connected to a point between inductor L 1  and capacitor C 2  to charge capacitors C 2  and C 3  (such a charging circuit, as discussed above, may include a power source and a resistor, the resistor being connected to the point between inductor L 1  and capacitor C 2 ). 
     After the initial breakdown between the electrode of the spark plug  206 , capacitors C 3  and C 2  begin to discharge (e.g., current begins to flow from) through secondary side  742  of the second transformer  742  to the spark plug  206 . The current through the secondary side  742  causes the core of the second transformer  740  to saturate and thereby reduces the effective impedance of the secondary side  742 . As before, the inductor L 1  slows the discharge of capacitor C 2  to create an discharge through the spark plug  206  similar to that shown in FIG.  19 . In one embodiment, the first and second sides,  732  and  742 , respectively, should be phased such the at the induced current in the secondary side  742  due to the initial breakdown flows in the same direction as the discharge from capacitors C 2  and C 3 . This avoids having to reverse the magnetic field in the core and thereby avoids losses associated with such a reversal. 
     Examples of values of components described in relation to FIG. 26 are C 1 =200 pF, C 2 =2.2 μF, C 3 =0.67 μF and L 1 =200 μF. 
     IV. Add-On Units 
     Any of the above described secondary circuit embodiments may be implemented as an add-on unit to be used in conjunction with a conventional ignition system installed on an internal combustion engine in order to allow such engines to operate a plasma-generating device in an effective manner. For example, and referring now to FIG. 27, the secondary circuit  208  could be totally encapsulated in a small package which is connected to the output of the primary electronics (circuit)  202  (which could be any conventional ignition system and, as shown, includes the ignition coil  300 ). In one embodiment, the add-on unit includes the two diodes D 1  and D 2  or alternatively, spark gaps discussed above could be provided in their place. Between the cathodes of diodes D 1  and D 2  is the spark plug  206 . The follow-on current producer  602  may contain any of the above described secondary circuits as viewed from the right of the blocking element D 2 . It should be appreciated that D 2  may be replaced by the parallel LC combination disclosed above if the primary electronics utilize an alternating voltage source. Furthermore, the power supply  224  could be co-located or receive power from the power source of the primary electronics. 
     In one embodiment, the secondary electronics  208  may be turned off to allow the primary electronics only to control the spark plug. This may be advantageous for some engine operating conditions. For example, when the engine is running at high RPM&#39;s due to the fuel/air mixing provided by a carburetor at these speeds. Thus, the switch  604  may open when it is determined that the engine is operating at high enough RPM&#39;s to have a good mixture and a follow-on voltage is not needed to create a larger plasma kernel. 
     V. Placement of a Plasma-Generating Device in a Combustion Chamber 
     Optimal placement of an ignitor will be discussed in relation to FIGS. 26-27 below. Generally, when operating on systems containing stratified mixtures, the ignitor should be mounted in the combustion chamber so that it does not contact the fuel plume introduced into the combustion chamber, but rather, expels the plasma into the fuel plume from a distance. 
     FIG. 28 is an example of a conventional ignition setup for an internal combustion engine. A fuel injector  802  periodically injects a fuel plume  804  into a combustion chamber  806 . After the fuel plume  804  has been injected, the combustion chamber  806  contains a stratified mixture having a fuel rich region (the fuel plume  804 ) and a region without a  808  substantial amount of fuel. A spark plug such as conventional spark plug  810  ignites the fuel plume  804  by creating an electrical discharge (spark) between the first electrode  812  and a second electrode  814 . The spark causes the fuel plume  804  to ignite and drive the piston  816  in the downward direction. 
     As discussed above, there are several problems associated with such a system. Namely, the location of the fuel plume  804  must be directed such that there is a minimum amount of fuel near the walls of the combustion chamber  806  in order to avoid quenching of the flame by the walls of the combustion chamber  806 . In addition, the discharge between the first and second electrodes  812  and  814  must be positioned so that it contacts the fuel plume  804  or the fuel plume  804  may fail to ignite. Placing the electrodes  812  and  814  directly in the path of the fuel plume  804  may lead to the spark being blown out by passing fuel or create a significant amount of fouling of the plug  810 . 
     FIG. 29 illustrates by example a way to avoid these problems utilizing the teachings contained herein. As before, the fuel injector  802  injects a stratified mixture (i.e., a fuel plume  804 ) into the combustion chamber  806 . Thus, the combustion chamber  806  includes a stratified mixture of the fuel plume  804  and a region  808  that does not contain a significant amount of fuel. It should be appreciated that the fuel injector may introduce the fuel plume  804  into the combustion chamber  806  by a variety of methods, such as direct fuel injection. 
     A plasma-generating device  820  is displaced in the combustion chamber so that the ends of its electrodes  822  and  824  are flush or nearly flush with the wall of the combustion chamber  106 . In one embodiment, the end of the longer electrode  822  or  824  extends less than about 2.54 cm (1 inch) into the combustion chamber  806 . In other embodiments, the electrodes may extend from any distance between about 0 and 2.54 cm into the combustion chamber  806 . The plasma-generating device  820  generates a volume of plasma  832 , as described above, which is expelled from between the electrodes  822  and  824  into the fuel plume  804  and ignites the fuel plume  804 . Such a system allows the ignition system designer to integrate a plasma-generating device that is flush or nearly flush with an optimized combustion chamber. Instead of extending the spark plug reach (and incurring many of the aforementioned problems) into the fuel plume  804 , one embodiment of the present invention uses a combination of special dual-energy electronics  830  (as described above) and an appropriately designed plasma-generating device to form a plasma  832  and inject it into the fuel plume  804 . 
     At high speeds, engines are generally run in a homogenous mixture mode of operation where the fuel injector injects the fuel plume  804  into the combustion chamber  806  early in the cycle to provide a uniform mixture throughout the combustion chamber  806 , when combustion initiates near top dead center of the engine cycle. The ignition system of the present invention proves advantageous in this mode as well. First, the plasma-generating device  820  may be flush or nearly flush with the cylinder wall, which reduces hydrocarbon emissions and partial burn that result from flame quenching around protruding sparkplugs. Secondly, the plasma-generating device  820  is by design a “cold” spark plug, eliminating potential pre-ignition problems resulting from protruding plug designs used in stratified mixture engines today. Third, the present invention allows the combustion chamber to be designed more optimally for performance at higher speed. 
     Finally, the present invention, in some embodiments, may be operated in a conventional mode (as opposed to the dual-stage mode discussed above). In these embodiment, the system may include a disabling element (either external or built-in; possibly inherent to the electronics) for controlling the application of TSI operation vs. conventional operation, according to which areas of operation require a higher-energy ignition kernel. The disabling element serves to disable the follow-on current provider (e.g., secondary electronics) or, alternatively, to prevent the current generated in the provider from discharging through the ignitor. In either case, the net effect is to prevent the follow-on current from being transmitted to the ignitor. 
     The system may switch modes based upon engine RPM, throttle position, the rate at which the RPM&#39;s are changing, or any other available engine condition that may give insight to how well the fuel is mixed. One simple way to implement such a system includes, as referring back to FIG. 27 by way of example only, including an additional element (such as a thyristor) between the portion of the circuit which generates the follow on current (e.g., to the left of D 2 ) which only allows the follow on portion to be provide when the element is active. Such an element, in effect, blocks the current from the follow-on current provider. Alternatively, and as discussed above, the switch  604  could serve to disconnect the follow on current producer when such a follow on current is not needed. Either the switch  604  or the additional element, as one will readily realize, may be controlled by a circuit which determines the best mode of operation depending upon the operating conditions discussed above, as well as others. 
     Having now described a few embodiments, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention.