Abstract:
A plasma ignitor, or plasma source, for igniting a combustible mixture in an internal combustion engine. The ignitor includes at least two spaced apart electrodes dimensioned and arranged such that an outwardly moving plasma is formed when a voltage is applied across the electrodes. The present invention is characterized by its efficient use of input electrical energy for driving the plasma ignitor and by an ignition plasma kernel which is several orders of magnitude larger than that produced by conventional spark plugs. Outward motion and expansion of the plasma kernel is produced by a combination of Lorentz and thermal forces. Use of very lean combustible mixtures, in which the dilution of the mixture is achieved by use of exhaust gas recirculation, is made possible by the present ignition system. Improvement in engine efficiency, and a major reduction in exhaust gas pollutants are obtained.

Description:
This Application is a 371 of PCT/US97/09240 filed May 29, 1997 and also claim the benefit of Provisional No. 60/018,534 filed May 29, 1996. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to internal combustion engine ignition systems, including the associated firing circuitry and ignitors such as spark plugs. 
     BACKGROUND OF THE INVENTION 
     Automobiles have undergone many changes since their initial development at the end of the last century. Many of these evolutionary changes can be seen as a maturing of technology, with the fundamental principles remaining the same. Such is the case with the ignition system. Some of its developments include the replacement of mechanical distributors by electronic ones, increasing reliability and allowing for easy adjustment of the spark timing under different engine operating conditions. The electronics responsible for creating the high voltage required for the discharge have changed, with transistorized coil ignition (TCI) and capacitive discharge ignition (CDI) systems common today. However, the basic spark plug structure has not changed. Spark plugs today differ from earlier ones mostly in the use of improved materials, but the basic point-to-point discharge remains the same. 
     A spark driven by the force from the interaction of the magnetic field created by the spark current and the current itself is very attractive concept, for enlarging the ignition kernel for a given ignition system input energy. 
     The need for an enhanced ignition source has long been recognized. Many inventions have been made which provide enlarged ignition kernels. The use of plasma jets and Lorentz force plasma accelerators have been the subject of much study and patents. None of these prior inventions have resulted in practical commercially acceptable solutions, though. The primary weakness of the prior inventions has been the requirement for excessive ignition energy, which eliminates any possible efficiency enhancement in the engine in which they are employed. These higher ignition energy requirements have resulted in high rates of ignition electrode erosion, which reduces ignition operating life to unacceptable levels. 
     The concept of enlarging the volume and surface area of the spark initiated plasma ignition kernel is an attractive idea for extending the practical lean limit for combustible mixtures in a combustion engine. The objective is to reduce the variance in combustion delay which is typical when engines are operated with lean mixtures. More specifically, there has been a long felt need to eliminate ignition delay, by increasing the spark volume. While it will be explained in more detail below, note that if a plasma is confined to the small volume between the discharge electrodes (as is the case with a conventional spark plug), its initial volume is quite small, typically about 1 mm 3  of plasma having a temperature of 60,000° K. is formed. This kernel expands and cools to a volume of about 25 mm 3  and a temperature of 2,500° K., which can ignite the combustible mixture. This volume represents about 0.04% of the mixture that is to be burned to complete combustion in a 0.5 liter cylinder at a compression ratio of 8:1. From the discussion below it will be seen that, if the ignition kernel could be increased 100 times, 4% of the combustible mixture would be ignited and the ignition delay would be significantly reduced. This attractive ignition goal has not heretofore been achieved in practical systems, though. 
     The electrical energy required in these earlier systems, e.g., Fitzgerald et al., U.S. Pat. No. 4,122,816, is claimed to be more than 2 Joules per firing (col. 2, lines 55-63). This energy is about 40 times higher than that used in conventional spark plugs. 
     Matthews et al., infra, reports the use of 5.5 Joules of electrical energy per ignition, or more than 100 times the energy used in conventional ignition systems. 
     Consider a six cylinder engine operating at 3600 RPM, which requires firing three cylinders every engine revolution or 180 firings per second. At 2 Joules per firing this is 360 Joules/second. This energy must be provided by the combustion engine at a typical efficiency of about 18% and converted to a suitable higher voltage by power conversion devices with a typical efficiency of about 40%, for a net use of the engine fuel at an efficiency of about 7.2%. Fitzgerald requires a fuel consumption of 360/0.072 Joules/second, or about 5000 Joules/second to run the ignition system. 
     To move a 1250 kg vehicle on a level road at about 80 km/hr (about 50 mph) requires about 9000 Joules/second of fuel energy. At an engine fuel to motive force conversion efficiency of 18%, about 50,000 Joules/second of energy will be consumed. Thus, the system employed by Fitzgerald et al, infra, will consume about 10% of the fuel energy consumed to run the vehicle to run the ignition system. This is greater than the efficiency gain to be expected by use of the Fitzgerald et al. ignition systems. 
     By comparison, conventional ignition systems use about 0.25 percent of the fuel energy to run the ignition system. Further, the high energy employed in these systems causes high levels of erosion to occur in the electrodes of the spark plugs, thus reducing the useful operating life considerably. This shortened life is demonstrated in the work by Matthews et al., infra, where the need to reduce ignition energy is acknowledged although no solution is provided. 
     As an additional attempt at solving this problem, consider the work by Tsao and Durbin (Tsao, L. and Durbin, E. J., “Evaluation of Cyclic Variation and Lean Operation in a Combustion Engine with a Multi-Electrode Spark Ignition System”,  Princeton Univ., MAE Report,  (January, 1984)), where a larger than regular ignition kernel was generated by a multiple electrode spark plug, demonstrating a reduction in cyclic variability of combustion, a reduction in spark advance, and an increase in output power. The increase in kernel size was only six times that of an ordinary spark plug. 
     Bradley and Critchley (Bradley, D., Critchley, I. L., “Electromagnetically Induced Motion of Spark Ignition Kernels”,  Combust. Flame  22, pgs. 143-152 (1974)) were the first to consider the use of electromagnetic forces to induce a motion of the spark, with an ignition energy of 12 Joules. Fitzgerald (Fitzgerald, D. J., “Pulsed Plasma Ignitor for Internal Combustion Engines”,  SAE paper  760764 (1976); and Fitzgerald, D. J., Breshears, R. R., “Plasma Ignitor for Internal Combustion Engine”, U.S. Pat. No. 4,122,816 (1978)) proposed to use pulsed plasma thrusters for the ignition of automotive engines with much less but still substantial ignition energy (approximately 1.6 J). Although he was able to extend the lean limit, the overall performance of such plasma thrusters used for ignition systems was not significantly better than that of regular spark plugs and the sparks they produce. In this system, much more ignition energy was used without a significant increase in plasma kernel size. (Clements, R. M., Smy, P. R., Dale. J. D., “An Experimental Study of the Ejection Mechanism for Typical Plasma Jet Ignitors”,  Combust. Flame  42, pages 287-295 (1981)). More recently Hall et al. (Hall, M. J., Tajima, H., Matthews, R. D., Koeroghlian, M. M., Weldon, W. F., Nichols, S. P., “Initial Studies of a New Type of Ignitor: The Railplug”,  SAE paper  912319 (1991)), and Matthews et al. (Matthews, R. D., Hall, as M. J., Faidley, R. W., Chiu, J. P., Zhao, X. W., Annezer, I., Koening, M. H., Harber, J. F., Darden, M. H., Weldon, W. F., Nichols, S. P., “Further Analysis of Railplugs as a New Type of Ignitor”,  SAE paper  922167 (1992)), have shown that a “rail plug” operated at an energy of over 6 J (2.4 cm long) showed a very substantial improvement in combustion bomb experiments. They also observed improvements in the lean operation of an engine when they ran it with their spark plug at an ignition energy of 5.5 J. They attributed the need of this excessive amount of energy to poor matching between the electrical circuit and the spark plug. This level of energy expended in the spark plug is about 25% of the energy consumed in propelling a 1250 kg vehicle at 80 km/hr on a level road. Any efficiency benefits in engine performance would be more than consumed by the increased energy in the ignition system. 
     SUMMARY OF THE INVENTION 
     A first significant aspect of the invention is a plasma injector, or ignitor, for an internal combustion engine, including at least first and second electrodes; means for maintaining the electrodes in a predetermined, spaced-apart relationship; and means for mounting in an internal combustion engine with active portions of the electrodes installed in a combustion cylinder of the engine. The electrodes are dimensioned and configured, and their spacing is arranged, such that when a sufficiently high voltage is applied across the electrodes while the ignitor is installed in an internal combustion engine, in the midst of a gaseous mixture of air and fuel, a plasma is In formed in the mixture between the electrodes and the plasma moves outwardly from between the electrodes into an expanding volume in the cylinder, under a Lorentz force. The spaced relationship between the electrodes may be maintained by surrounding a substantial portion of the electrodes with a dielectric material such that as the voltage is applied to the electrodes, the plasma forms on or in the vicinity of the surface of the dielectric. The voltage may be reduced, and increased current supplied, to maintain the plasma after its initial formation. 
     As more particularly explained herein, another aspect of the invention is a plasma injector, or ignitor, for an internal combustion engine, one embodiment of which includes two electrodes which are spaced apart and have substantially parallel and circular facing surfaces between which a radially outwardly moving plasma is formed in the fuel-air mixture via a voltage applied across the electrodes. 
     According to another aspect of the invention, a plasma injector, or ignitor, for an internal combustion engine includes at least two spaced apart and substantially parallel longitudinal electrodes, between which a longitudinally outwardly-moving plasma is formed via a high voltage applied across the electrodes. 
     Another aspect of the invention, usable with the two preceding aspects of the invention, is an ignition source which provides an ignition plasma kernel by providing a sufficiently high first voltage for creating a channel formed of plasma between the electrodes and a second voltage of lower potential than the first voltage for sustaining current through the plasma in the channel between the electrodes, such that said current and a magnetic field resulting from a current in at least one of the electrodes arising from the current in the plasma interact to create a Lorentz force upon the plasma that, in combination with thermal expansion forces, causes the plasma to move away from its region of origin and to expand in volume. 
     According to yet another aspect, the invention comprises an ignitor which includes substantially parallel and spaced apart electrodes, including at least first and second electrodes forming a discharge gap between them, wherein the ratio of the sum of the radii of the electrodes to the length of the electrodes is larger than or equal to about four, while the ratio of the difference of these two radii to the length of the electrodes is larger than about one-third; a dielectric material surrounds a substantial portion of the electrodes and the space between them; an uninsulated end of portion of each of the electrodes is free of said dielectric material and in oppositional relationship to one another; and wherein there are means for mounting the ignitor with the free ends of the first and second electrodes installed in a combustion cylinder of a combustion engine. 
     According to still another aspect of the invention, an ignitor is provided which includes at least two parallel and spaced apart electrodes adapted for forming discharge gaps between them, wherein the radius of the largest cylinder which can fit between the electrodes is greater than the length of an electrode divided by six; a dielectric material surrounds a substantial portion of the electrodes and the space between them; an uninsulated end portion of each of the electrodes is free of the dielectric material and in oppositional relationship to one another, the uninsulated end portions being designated the lengths of the electrodes, and further including means for mounting the ignitor with free ends of the electrodes in a combustion cylinder of an engine. 
     A still further aspect of the invention is a traveling spark ignition system for a combustion engine which includes an ignitor and together therewith or separately therefrom electrical means for providing a potential difference between electrodes of the ignitor. The ignitor includes substantially parallel and spaced apart coaxial electrodes which include a least first and second electrodes forming a discharge gap between them, wherein the ratio of the sum of the radii of the electrodes to their lengths is larger than or equal to about four, while the ratio of the difference of these two radii to the lengths of the electrodes is larger than about one-third. A dielectric material, such as a polarizable ceramic, surrounds a substantial portion of the electrodes and the space between them, with an uninsulated end portion of each of the electrodes being free of the dielectric material and in oppositional relationship to one another. Means are included for mounting the ignitor with the free ends of the electrodes installed in a combustion cylinder of an engine. Such means may include threads on one of the electrodes. The electrical means for providing a potential difference between the electrodes initially provides a sufficiently high first voltage for creating a channel formed of plasma in the fuel-air mixture between the electrodes, and thereafter provides a second voltage of lower potential than the first voltage for sustaining a current through the plasma in the channel between the electrodes. As a result, said current in at least one of the electrodes interacts with a magnetic field in a manner which creates a Lorentz force upon the plasma, causing it to move away from its region of origin. 
     According to a further aspect of the invention, there is provided a traveling spark ignition system for a combustion engine which includes an ignitor and electrical means for sequentially providing two potential differences between electrodes of the ignitor. The ignitor includes at least two parallel spaced apart electrodes adapted to form discharge gaps between them, wherein the radius of the largest cylinder which can fit between said electrodes is greater than the length of the electrodes; a dielectric material surrounds a substantial portion of the electrodes and a space between them, which dielectric material may, for example, be a polarizable ceramic material; an uninsulated end portion of each of the electrodes is free of the dielectric material and in oppositional relationship to one another, the uninsulated end portions being the aforesaid lengths of the electrodes; and means being provided for mounting the ignitor with the free ends of the electrodes in a combustion cylinder of an engine, such means being, for example, threads provided on one of the electrodes. The electrical means for sequentially providing potential differences between the electrodes provides a first potential difference which is sufficiently high to create a channel formed of plasma between the electrodes, after which the potential difference is reduced to a second voltage of lower potential than the first voltage for sustaining a current through the plasma in the channel between the electrodes. Said current interacts with a magnetic field arising from a current in a manner which creates a force upon the plasma to cause it to move away from its region of origin, to increase the swept volume of the plasma. 
    
    
     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. 3 is a similar 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. 4 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. 5 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. 6 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. 7 shows a circuit schematic diagram of another ignition circuit embodiment according to the invention. 
     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. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is a traveling spark initiator or ignitor (TSI) in the form of a miniature Marshall gun (coaxial gun), with high efficiency of transfer of electric energy into plasma volume creation. In the embodiment of FIG. 2, a ratio of a sum of the radii (r 2 ) and (r 1 ), of an external electrode and internal electrode, respectively, to the length (l) of the electrodes should be larger than or equal to 4, whereas the ratio of the difference of these two radii (r 2 −r 1 )=g 1  to the length (l) of the electrodes should be larger than ⅓ (preferably larger than ½), as follows:              r   2     +     r   1       l     ≥     4                 and                       r   2     -     r   1       l       &gt;     1   3                            
     and g 1  is the gap spacing between the electrodes. 
     Similar relations are required for the embodiment of FIG. 3, where r 2  and r 1  from FIG. 2 are replaced by R 2  and R 1  as shown, the gap between the electrodes is g 2 , and the length of the electrodes is L. Hence              R   2     +     R   1       L     ≥     4                 and                     g   2     L       &gt;     1   3                            
     The heat transfer to the combustible mixture occurs in the form of the diffusion of ions and radicals from the plasma. The very large increase in plasma volume dramatically increases the rate of heat transfer to the combustible mixture. 
     The principle of the Marshall gun is discussed first. There follows a discussion of the environmental benefits provided by larger spark volumes. The construction details of such a system will then be discussed relative to various embodiments of the invention. 
     The principle of the Marshall gun 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 coaxial plasma gun, where B θ  is the poloidal magnetic field directed along field line  4 . The plasma  16  is moved in a 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 O 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 burns 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 release is less than that lost, the flame will not propagate. An increase in the ignition volume is required to assure that the flame propagation does not slow down as the energy density of the combustible mixture is reduced. 
     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 results in an increase of the spark advance required for achieving the maximum torque, and reduces the amount of output work available. 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 conditions will reduce emissions and increase efficiency, by reducing the number of poor bum cycles, and by extending the operating air fuel ratio range of the engine. 
     Quader determined the mass fraction of the combustible mixture which was burned as a function of the crank angle, for two different start timings (Quader. A., “What Limits Lean Operation in Spark Ignition Engines—Flame Initiation or Propagation?”,  SAE Paper  760760 (1976)). His engine was running very lean (i.e., an equivalence ratio of about 0.7), at 1200 rpm and at 60% throttle. The mass fraction burned did not change in any noticeable way immediately after the spark occurred (there is an interval where hardly any burning can be detected, commonly known as the ignition delay). This is due to the very small volume of the spark, and the slow combustion duration due to the small surface area and relatively low temperature. Once a small percentage of the combustible mixture has burned, the combustion rate increases, slowly at first, and then more rapidly as the flame front grows. The performance of the engine at both of these spark timings is poor. In the case of 60° B.T.D.C. (before top dead center ignition timing), too much of the mixture has burned while the piston is compressing the mixture therefore, negative work is being done. The rise in pressure opposes the compression strokes of the engine. In the case of 40° B.T.D.C. timing, a considerable fraction of the mixture is burned after the expansion strokes have started, thus reducing the output work available. 
     The intersection of a 4% burned line with the curves determined by Quader, Id., shows the potential advantage that a large spark volume, if it were available, would have in eliminating the ignition delay. For the 60° B.T.D.C. spark curve, if the spark timing is changed from 60° to 22° B.T.D.C., a change of nearly 40 degrees, the rate of change of mass fraction burned will be higher because the combustible mixture density will be higher at the moment of ignition. For the 40° B.T.D.C. spark time curve, if the timing is changed from 40° to 14° B.T.D.C., a change of about 25 degrees, the combustible mixture will be completely burned at a point closer to TDC, thus increasing efficiency. 
     The above arguments clearly illustrate the importance of an increase in spark volume for reduced emission and improved fuel economy. With the TSI system of the present invention, the required spark advance for maximum efficiency can be reduced by 20° to 30°, or more. 
     While increasing spark volume, the TSI system also provides for moving the spark deeper into the combustible mixture, with the effect of reducing the combustion duration. 
     The construction of a practical TSI system will now be discussed for various exemplary embodiments of the invention. 
     There are provided, in accordance with the present invention, (a) a small plasma gun or traveling spark ignitor (also known as a TSI) that substitutes for a conventional spark plug and (b) specially matched electronic trigger (i.e., ignition) circuitry. Matching the electronic circuit to the parameters of the plasma gun (e.g., length of electrodes, diameters of coaxial cylinders, duration of the discharge) maximizes the volume of the plasma when it leaves the gun for a given store of electrical energy. By properly choosing the parameters of the electronic circuit it is possible to obtain current and voltage time profiles so that substantially maximum electrical energy is transferred to the plasma. 
     Preferably, the TSI ignition system of the present invention uses no more than about 300 mJ per firing. By contrast, earlier 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. 
     Heretofore, it had been thought that the proper design principle was to generate moving plasma with a very high speed, which would penetrate the combustible mixture to create a high level of turbulence and ignite a large volume of that mixture. This was accomplished by using a relatively long length of electrodes with a relatively small gap between them. For example, an aspect ratio of electrode length to discharge gap more than 3 and preferably 6-10 was proposed by Matthews et al., supra. By contrast, the present invention uses a relatively short length of electrodes with a relatively large gap between them. 
     Consider that the kinetic energy of the plasma is proportional to the product of plasma mass, M p , and its velocity, v p , squared, as follows: 
     
       
         
           K.E.≈M 
           p 
           v 
           p 
           2 
         
       
     
     Doubling the velocity of the plasma multiplies the kinetic energy four-fold. The mass of plasma is ρ p ×Vol p  where ρ p  and Vol p  are the plasma density and plasma volume, respectively. Thus, if the volume of the plasma is doubled at the same velocity, the required energy is only doubled. 
     The present invention increases the ratio of plasma volume to energy required to form the plasma. This is done by quickly achieving a modest plasma velocity. 
     If one assumes a spherical shape for the ignition plasma volume, the surface area of the volume increases as the square of the radius of the volume. Ignition of the combustible mixture occurs at the surface of the plasma volume after the plasma has expanded and cooled to the combustible mixture ignition temperature. Thus, the rate at which the combustible mixture burns initially depends primarily on the plasma temperature and not on its initial velocity. Consequently, maximizing the ratio of plasma volume and temperature to plasma input energy, maximizes the effectiveness of the electrical input energy in speeding up the combustion of the combustible mixture. 
     The drag, D, on the expanding volume of plasma is proportional to the density of the combustible mixture, PC, and the square of the speed of the expanding plasma, v p , as follows: 
     
       
         
           D˜ρ 
           c 
           v 
           p 
           2 
         
       
     
     The magnitude of the electrical force, F, to expand the plasma is proportional to the discharge current, I, squared. Equating these two forces yields the following: 
     
       
         
           F˜I 
           2 
           =D˜ρ 
           c 
           v 
           p 
           2 
         
       
     
     The radius, r, of the plasma volume, Vol p , is proportional to  0 ∫ t     D    v p (t)dt where t D  is the duration of the discharge. The volume of the plasma is proportional to the cube of the radius r, while the radius of the plasma volume is proportional to  0 ∫ t     D    I(t)dt=Q, the electric charge inserted into the plasma. Thus, the volume of the plasma is proportional to Q 3 . 
     If the source of electrical energy is that stored in a capacitor, then Q=VC, where V is the voltage at which the charge Q is stored and C is the capacitance; and the energy stored in the capacitor is E=½ CV 2 . 
     To maximize the plasma volume for given energy, the ratio of plasma volume, Vol p , to electrical energy, E, has to be maximized. Vol p /E is proportional to C 3 V 3 /CV 2 , which is C 2 V. For a given constant energy E=½ CV 2 , C will be proportional to V −2 . Hence, Vol p /E is proportional to V −3 . 
     Therefore, the optimum circuit design is one which stores the desired electric energy in a large capacitor at a low voltage. 
     To enhance efficiency, therefore, the discharge should take place at the lowest possible voltage. To that end, according to the invention the initial discharge of electrical energy takes place on the surface of an insulator, and a power supply is used to raise the gap conductivity near the surface of that insulator, and the main source of discharge energy is stored and provided at the lowest possible voltage that will be effective to create the plasma reliably. 
     A further objective, preferably, is to avoid recombination of the large amount of ions and electrons of the traveling spark (plasma) on the electrode walls. The energy losses due to the recombination of ions and electrons reduce the efficiency of the system. Since recombination processes increase with time, the ion formation should take place quickly to minimize the probability of interaction of ions with the walls. To reduce recombination, therefore, the discharge time should be short. This can be accomplished by achieving the desired velocity on a short travel distance. 
     There is a second loss mechanism: the drag force on the plasma as it impacts the combustible mixture ahead of its path. These losses vary as the square of the velocity. Thus the exit velocity should be as low as possible to reduce or minimize such losses. 
     The high volume that is desired, combined with the need to discharge quickly, leads to a structure characterized by a short length l for plasma travel with a relatively wide gap between electrodes. This requirement is specified geometrically by the two ratio pairs described with reference to FIGS. 2 and 3, above. 
     What does this mean with respect to physical dimensions? If the volume of the plasma in a point-to-point discharge of a conventional spark plug is about 1 mm 3 , it would be desirable, preferably, to create a plasma volume at least 100 times greater, i.e., Vol p ≈100 mm 3 . Thus, using the configuration of FIG. 2, an example satisfying such conditions could be: length l=2.5 mm, the radius (inside) of the larger diameter cylindrical electrode being r 2 =5.8 mm (this would be a typical radius of the cylindrical electrode using the conventional spark gap with a thread diameter of 14 mm) and the radius of the smaller diameter cylindrical electrode being r 1 =4.6 mm. 
     As shown in the embodiments of FIGS. 2 and 3, TSI  17 ,  27 , respectively, share many of the same physical attributes as a standard spark plug, such as standard mounting means or threads  19 , a standard male spark plug connector  21 , and an insulator  23 . The tips or plasma forming portions of the TSI&#39;s  17  and  27 , respectively, differ significantly from conventional is spark plugs, though. In a Traveling Spark Ignitor (TSI) for one embodiment of the present invention as shown in FIG. 2, an internal electrode  18  is placed with a lower portion extending coaxially into the interior open volume of external electrode  20  distal boot connector  21 . The space between the electrodes is filled with an insulating material  22  (e.g., ceramic) except for the last 2 to 3 mm, in this example, at the end of the ignitor  17 , this distance being shown as l. The space or discharge gap g, between the electrodes may have a radial distance of about 1.2 to about 1.5 mm, in this example. These distances for l and g 1  are important in that the TSI preferably works as a system with the matching electronics (discussed below) in order to obtain maximum efficiency. A discharge between the electrodes  18 - 20  starts along the exposed interior surface of the insulator  23 , since a lower voltage is required to initiate a discharge along the surface of an insulator than in the gas some distance away from the insulator surface. When a voltage is applied, the gas (air/fuel mixture) is ionized by the resulting electrical field, creating a plasma  24  which becomes a good conductor and supports a current between the electrodes at a lower voltage. This current ionizes more gas (air/fuel mixture) and gives rise to a Lorenz force which increases the volume of the plasma  24 . In the TSI of FIG. 2, the plasma accelerates out of the “ignitor plug”  17  in the axial direction. 
     FIG. 3 shows a TSI  27  with an internal electrode  25  that is placed coaxially in the external electrode  28 . The space between the electrodes  26  and  28  is filled with an insulating material  30  (e.g., ceramic). The main distinguishing feature for the embodiment of FIG. 3 relative to 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 electrode  26  also 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. Accordingly, the preferred direction of travel for the plasma to obtain maximum interaction with the mixture is from the gap  29  to the cylinder wall 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 l 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 embodiment of FIG. 3, the plasma  32  initiates in discharge gap  29  at the exposed surface of insulator  30 , and grows and expands outwardly in the radial direction of arrows  29 A. This provides several additional 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  30  in the TSI  27  of FIG. 3 provides an additional heat conducting path for electrode  26 . The added insulator material  30  will keep the inner electrode metal  25 ,  26  cooler than electrode  18  in FIG. 2, thereby enhancing the reliability of TSI  27  relative to TSI  17 . Finally, in using TSI  27 , the plasma will not be impinging on and perhaps eroding the associated piston head. 
     FIGS. 5 and 6 illustrate pictorially the differences in plasma trajectories between TSI  17  of FIG. 2, and TSI  27  of FIG. 3 when installed in an engine. In FIG. 5, 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. 6, 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. 
     The electrode materials 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 metal may be of 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 between the electrodes, possibly reducing the required ignition voltage. Also, the electrodes may be made out 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 the end, are separated by an isolator or insulator material which is a high temperature, polarizable electrical 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 with a solder glass frit, for example. As above, the ceramic could also comprise a permanent magnet material such as barium ferrite. 
     In terms of operation of the embodiments of FIGS. 2 and 3, when the electrodes  18 ,  20  and  26 ,  28 , respectively, are connected to the rest of the TSI system, they become part of an electrical system which also comprises an electrical circuit for providing potential differences which are sufficiently high to create a spark in the gap between respective electrode pairs. The resulting current in the plasma channel and a magnetic field arising from a current flowing in at least one of the electrodes due to said current through the plasma interact, creating a Lorentz force on the plasma in the spark channel; this effect causes the point of origin of the spark channel to move, and not to remain fixed in position, thus increasing the cross-sectional area of the spark channels, as previously described. This is in contrast to traditional spark ignition systems, wherein the point of origin of the spark remains fixed. Electronic circuits matched to the TSIs  17  and  27  complete the TSI system for each embodiment, and are discussed in the following examples. 
     EXAMPLE 1 
     FIG. 4 shows TSI plug or ignitor  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 TSI  27 .) A discharge between the two electrodes  18  and  20  starts along the surface  56  of the insulator material  22 . 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 . The electrical circuit shown in FIG. 4 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 gap along the surface  56  of the TSI  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  are necessary to isolate electrically 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 resistor  54  is one way to prevent a low resistance current path between the voltage source  44  and the spark gap of TSI  17 . 
     Note that the circuit of FIG. 4 is simplified, for purposes of illustration. In a commercial application, the circuit of FIG. 7 described below under the heading “Example 2” is preferred for recharging capacitor  46  in a more energy-efficient manner, using a resonant circuit. Furthermore, the conventional ignition system  42 , 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 . 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. 
     The current through the central electrode  18  and the plasma  24  to the external electrode  20  creates around the central 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 electrode  18 . Hence, the current I flowing through the plasma  24  perpendicular to the poloidal magnetic field B generates a Lorentz force F on the charged particles in the plasma  24  along the axial direction z of the cylinders  18 ,  20 . The force is computed as follows: 
     
       
         
           F˜I×B→F 
           Z 
           ˜I 
           r 
           ·B 
           θ 
         
       
     
     This force accelerates the charged particles, which due to collisions with non-charged particles accelerate 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 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 in a distance of a dozen centimeters to a final velocity of about 10 7  cm/sec. The plasma gun used as an engine ignitor herein operates at relatively high gas (air/fuel mixture) pressure. The drag force F v  of such a gas is approximately proportional to the square of the plasma velocity, as shown below: 
     
       
         
           F 
           v 
           ˜V 
           p 
           2 
         
       
     
     The distance over which the plasma accelerates is short (2-3 mm). Indeed, experimentation has shown that increasing the length of the plasma acceleration distance beyond 2 to 3 mm does not increase significantly the plasma exit velocity, although electrical energy stored in the capacitor  46  has to be 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. 
     EXAMPLE 2 
     TSI ignitors  17  and  27  of FIGS. 2 and 3, respectively, can be combined with the ignition electronics shown in FIG.  7 . The ignition electronics can be divided into four parts, as shown: the primary and secondary circuits  77 ,  79 , respectively, and their associated charging circuits  75 ,  81 , respectively. The secondary circuit  79 , in turn, is divided into a high voltage section  83 , and a low voltage section  85 . 
     The primary and secondary circuits  77 ,  79 , respectively, correspond to primary  58  and secondary  60  windings of an ignition coil  62 . When the SCR  64  is turned on via application of a trigger signal to its gate  65 , the capacitor  66  discharges through the SCR  64 , which causes a current in the coil primary winding  58 . This in turn imparts a high voltage across the associated secondary winding  60 , which causes the gas in the spark gap  68  to break down and form a conductive path, i.e. a plasma. Once the plasma has been created, diodes  86  turn on and the secondary capacitor  70  discharges. The spark gap symbol  68  is representative of an ignitor, according to the invention, such as exemplary TSI devices  17  and  27  of FIGS. 2 and 3, respectively. 
     After the primary and secondary capacitors  66  and  70  have discharged, they are recharged by their respective charging circuits  75  and  81 . Both charging circuits  75 ,  81  incorporate an inductor  72 ,  74  (respectively) and a diode  76 ,  78  (respectively), together with a power supply  80 ,  82  (respectively). The function of the inductor  72 ,  74  is to prevent the power supplies from being short-circuited through the ignitor. The function of the diodes  76  and  78  is to avoid oscillations. The capacitor  84  prevents the power supply  82  voltage V 2  from the going through large fluctuations. 
     The power supplies  80  and  82  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. (In experiments conducted by the inventors these power supplies were kept separate to make it easier to vary the two voltages independently.) Power supplies  80  and  82  may be DC-to-DC converters from a CDI (capacitive discharge ignition) system, which can be powered by a 12 volt car battery, for example. 
     An essential part of the ignition circuit of FIG. 7 are one or more high current diodes  86 , which have a high reverse breakdown voltage, larger than the maximum spark gap breakdown voltage of either TSI  17  or TSI  27 , for all engine operating conditions. The function of the diodes  86  is to isolate the secondary capacitor  70  from the ignition coil  62 , by blocking current from secondary winding  60  to capacitor  70 . If this isolation were not present, the secondary voltage of ignition coil  62  would charge the secondary capacitor  70 , and, given a large capacitance, the ignition coil  62  would never be able to develop a sufficiently high voltage to break down the air/fuel mixture in spark gap  68 . 
     Diode  88  prevents capacitor  70  from discharging through the secondary winding  60  when there is no spark or plasma. Finally, the optional resistor  90  may be used to reduce current through secondary winding  60 , thereby reducing electromagnetic radiation (radio noise) emitted by the circuit. 
     In the present TSI system, a trigger electrode can be added between the inner and outer electrodes of FIGS. 2 through 4 to lower the voltage on capacitor  70  in FIG.  7 . Such a three electrode ignitor is shown in FIG. 8, and is described in the following paragraph. 
     In FIG. 8, a three electrode plasma ignitor  100  is shown schematically. An internal electrode  104  is placed coaxially within the external electrode  106 , both having diameters on the order of several millimeters. Radially between the internal electrode  104  and the external  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  100 . 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 very low current discharge, which is sufficient to create enough charged particles on the surface  114  of the insulator  112  for the main capacitor to discharge between electrodes  104  and  106  along surface  114  of dielectric or insulator  112 . 
     As shown in FIGS. 9A,  9 B and  9 C, another embodiment of the invention includes a traveling spark ignitor  120  having parallel rod-shaped electrodes  122  and  124 , as shown. 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 I outwardly from the surface of the bottom end of dielectric  126 . 
     With reference to FIGS. 9B and 9C, the electrodes  122  and  124  are spaced apart a distance 2 r, where r is the radius of the largest cylinder that can fit between the electrodes  122 ,  124  (see FIG.  9 C). 
     Although various embodiments of the invention are shown and described herein, they are not meant to be limiting as they are shown by way of example only. For example, the electrodes  18  and  20  of TSI  17 , and  25  of TSI  27  can be other than cylindrical. Also, the disk shaped electrode  26  can be other than circular—a straight rod, for example. For TSI  17 , the electrodes  18  and  20  may also be other than coaxial, such as parallel rods or parallel elongated rectangular configurations. Although the electrodes are shown as presenting equal lengths, this too may be varied, in which event the term “length” as used in the claims shall refer to the dimension of electrode overlap along the direction of plasma ejection from the ignitor. Those of skill in the art will recognize still further modifications to the embodiments, which modifications are meant to be covered by the spirit and scope of the appended claims.