Patent Publication Number: US-9413314-B2

Title: Corona ignition with self-tuning power amplifier

Description:
CLAIM FOR PRIORITY 
     This application is a Continuation-in-Part and claims the benefit of U.S. patent application Ser. No. 12/777,105, filed May 10, 2010, which claims the benefit of U.S. provisional application No. 61/298,442, filed Jan. 26, 2010, and U.S. provisional application No. 61/176,614, filed May 8, 2009, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to igniters used for igniting air/fuel mixtures in automotive application and the like, and in particular to a self-tuning power amplifier for use in a corona ignition system. 
     2. Related Art 
     U.S. Pat. No. 6,883,507 discloses an igniter for use in a corona discharge air/fuel ignition system. According to an exemplary method used to initiate combustion, an electrode is charged to a high, radio frequency (“RF”) voltage potential to create a strong RF electric field in the combustion chamber. The strong electric field in turn causes a portion of the fuel-air mixture in the combustion chamber to ionize. The process of ionizing the fuel-air gas can be the commencement of dielectric breakdown. But the electric field can be dynamically controlled so that the dielectric breakdown does not proceed to the level of an electron avalanche which would result in a plasma being formed and an electric arc being struck from the electrode to the grounded cylinder walls or piston. The electric field is maintained at a level where only a portion of the fuel-air gas is ionized—a portion insufficient to create the electron avalanche chain reaction described previously which results in a plasma. However, the electric field is maintained sufficiently strong so that a corona discharge occurs. In a corona discharge, some electric charge on the electrode is dissipated through being carried through the gas to the ground as a small electric current, or through electrons being released from or absorbed into the electrodes from the ionized fuel-air mixture, but the current is very small and the voltage potential at the electrode remains very high in comparison to an arc discharge. The sufficiently strong electric field causes ionization of a portion of the fuel-air mixture to facilitate the combustion reaction(s). The ionized fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining fuel-air mixture. 
       FIG. 1  illustrates a capacitively coupled RF corona discharge ignition system. The system is termed “capacitively coupled” since the electrode  40  does not extend out of the surrounding dielectric material of the feedthru insulator  71   b  to be directly exposed to the fuel-air mixture. Rather, the electrode  40  remains shrouded by the feedthru insulator  71   b  and depends upon the electric field of the electrode passing through part of the feedthru insulator to produce the electric field in the combustion chamber  50 . 
       FIG. 2  is a functional block diagram of the control electronics and primary coil unit  60  according to an exemplary embodiment of the invention. As shown in  FIG. 2 , the control electronics and primary coil unit  60  includes a center tapped primary RF transformer  20  which receives via line  62  a voltage of 150 volts, for example, from the DC source. A high power switch  72  is provided to switch the power applied to the transformer  20  between two phases, phase A and phase B at a desired frequency, e.g., the resonant frequency of the high voltage circuit  30  (see  FIG. 1 ). The 150 volt DC source is also connected to a power supply  74  for the control circuitry in the control electronics and primary coil unit  60 . The control circuitry power supply  74  typically includes a step down transformer to reduce the 150 volt DC source down to a level acceptable for control electronics, e.g., 5-12 volts. The output from the transformer  20 , depicted at “A” in  FIGS. 1 and 2 , is used to power the high voltage circuit  30  which is housed in the secondary coil unit, according to an exemplary embodiment of the invention. 
     The current and voltage output from the transformer  20  are detected at point A and conventional signal conditioning is performed at  73  and  75 , respectively, e.g., to remove noise from the signals. This signal conditioning may include active, passive or digital, low pass and band-pass filters, for example. The current and voltage signals are then full wave rectified and averaged at  77 ,  79 , respectively. The averaging of the voltage and current, which removes signal noise, may be accomplished with conventional analog or digital circuits. The averaged and rectified current and voltage signals are sent to a divider  80  which calculates the actual impedance by dividing the voltage by the current. The current and voltage signals are also sent to a phase detector and phase locked loop (PLL)  78  which outputs a frequency which is the resonant frequency for the high voltage circuit  30 . The PLL determines the resonant frequency by adjusting its output frequency so that the voltage and current are in phase. For series resonant circuits, when excited at resonance, voltage and current are in phase. 
     The calculated impedance and the resonant frequency are sent to a pulse width modulator  82  which outputs two pulse signals, phase A and phase B, each having a calculated duty cycle, to drive the transformer  20 . The frequencies of the pulse signals are based on the resonant frequency received from the PLL  78 . The duty cycles are based on the impedance received from the divider  80  and also on an impedance setpoint received from a system controller  84 . The pulse width modulator  82  adjusts the duty cycles of the two pulse signals to cause the measured impedance from the divider  80  to match the impedance setpoint received from the system controller  84 . 
     The system controller  84 , in addition to outputting the impedance setpoint, also sends a trigger signal pulse to the pulse width modulator  82 . This trigger signal pulse controls the activation timing of the transformer  20  which controls the activation of the high voltage circuit  30  and electrode  40  shown in  FIG. 1 . The trigger signal pulse is based on the timing signal  61  received from the master engine controller  86 , not shown. The timing signal  61  determines when to start the ignition sequence. The system controller  84  receives this timing signal  61  and then sends the appropriate sequence of trigger pulses and impedance setpoint to the pulse width modulator  82 . This information tells the pulse width modulator when to fire, how many times to fire, how long to fire, and the impedance setpoint. The desired corona characteristics (e.g., ignition sequence and impedance setpoint) may be hard coded in the system controller  84  or this information can be sent to the system controller  84  through signal  63  from the master engine controller  86 . The system controller  84  may send diagnostics information to the master engine controller  86 , as is customary in modern engine controls and ignition systems. Examples of diagnostic information may include under/over voltage supply, failure to fire as determined from the current and voltage signals, etc. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     A power amplifier circuit that has an inductor and capacitor connected to one end of the output winding of an RF transformer. The other end of the output winding is connected to a resistor that in turn is connected to ground. The transformer has two primary windings. Both primary windings have one end connected to a variable DC voltage supply. The other end of each primary winding is attached to a MOSFET. All three windings are wound around a ferrite core. The two primary windings are arranged so that current flowing from the DC voltage supply to the MOSFET causes a magnetic flux in the ferrite core in opposing directions. To initiate oscillation of the circuit one of the MOSFETs or switches is turned on briefly causing the inductor and capacitor to ring. As a result, a voltage is generated on the secondary winding current sensor that is fed to a circuit that filters out all noise and leaves a voltage at the natural frequency of the inductor capacitor. The current sensor includes at least one of a resistor, diode, an inductor, and a capacitor. This voltage is fed back to the MOSFETS, controlling on and off timing. In this way the need to measure and record natural frequency is eliminated. 
     In one embodiment of the invention, there is a power amplifier circuit for a corona ignition system, including an RF transformer with an output winding and two primary windings, the output winding and the two primary windings wound around a core; an inductor and capacitor connected to one end of the output winding; and a current sensor connected to another end of the output winding, wherein current induced in the output winding generates a magnetic flux in the core in opposing directions. 
     In one aspect of the invention, the two primary windings each have one end connected to a variable DC voltage supply, and the other end of each of the two primary windings are attached to first and second switches, such that the first and second switches on and off timing are controlled. 
     In another aspect of the invention, the amplifier circuit further includes a sense winding which provides a feedback signal to compensate for varying capacitance, and wherein the output winding provides an output signal to a corona igniter. 
     In another aspect of the invention, ends of the secondary winding are respectively connected to two switches which drive the circuit to operating the corona ignition system, thereby igniting a corona igniter. 
     In yet another embodiment of the invention, there is an internal combustion engine includes a cylinder head with an igniter opening extending from an upper surface to a combustion chamber having and a corona igniter, including a control circuit configured to receive a signal from an engine computer; and a power amplifier circuit to generate an alternating current and voltage signal to drive an igniter assembly at its resonant frequency, the igniter assembly including an inductor, capacitor and current sensor forming an LCR circuit with one end of the inductor connected through a firing end assembly to an electrode crown in the combustion chamber of the combustion engine which ignites the corona igniter. 
     In one aspect of the invention, the power amplifier circuit includes an RF transformer with an output winding and two primary windings, the output winding and the two primary windings wound around a core; the inductor and capacitor connected at one end of the output winding; and the current sensor connected to another end of the output winding, wherein current induced in the output winding generates a magnetic flux in the core in opposing directions. 
     In another aspect of the invention, the control circuit determines a voltage to apply to the power amplifier circuit, the power amplifier circuit drives current through the windings and provides a feedback signal of the resonant frequency of the igniter assembly, and the igniter assembly resonates at a specified frequency when a capacitance at the capacitor, a resistance at the current sensor and an inductance at the inductor are combined. 
     In still another aspect of the invention, the two primary windings each have one end connected to a variable DC voltage supply, and the other end of each of the two primary windings are attached to first and second switches, such that the first and second switches on and off timing are controlled. 
     In yet another aspect of the invention, the amplifier circuit further includes a sense winding which provides a feedback signal to compensate for varying capacitance, and wherein the output winding provides an output signal to the corona igniter. 
     These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment. The drawings that accompany the detailed description are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary corona discharge ignition system in the prior art. 
         FIG. 2  shows a functional block diagram of the control electronics and primary coil unit in accordance with the prior art system. 
         FIG. 3  illustrates a self-tuning circuit in accordance with the invention. 
         FIG. 4  is a block diagram illustrating implementation of the circuit of  FIG. 3  in a corona ignition system according to one exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     A power amplifier circuit that has an inductor and capacitor connected to one end of the output winding of an RF transformer. The other end of the output winding is connected to a current sensor that in turn is connected to ground. The current sensor includes at least one of a resistor diode, an inductor, and a capacitor. The transformer has two primary windings. Both primary windings have one end connected to a variable DC voltage supply. The other end of each primary winding is attached to a MOSFET. All three windings are wound around a ferrite core. The two primary windings are arranged so that current flowing from the DC voltage supply to the MOSFET causes a magnetic flux in the ferrite core in opposing directions. To initiate oscillation of the circuit one of the MOSFETs is turned on briefly causing the inductor and capacitor to ring. As a result, a voltage is generated on the secondary winding current sensor that is fed to a circuit that filters out all noise and leaves a voltage at the natural frequency of the inductor capacitor. This voltage is fed back to the MOSFETs, controlling on and off timing. In this way the need to measure and record natural frequency is eliminated. 
     The circuit illustrated in  FIG. 3  includes a transformer, mosfets to drive the transformer, and a feedback circuit to tune the frequency of operation of the transformer. The transformer has, in one example, a ferrite core with four sets of windings around the core. Inductors L 1  and L 2  are the primary windings, which are joined together at a point that is connected to a DC voltage supply. The circuit can be designed to operate with a range of voltage supply voltages, in this embodiment the voltage will be set to 60 VDC. The other ends of inductors L 1  and L 2  are each connected to a switch, which is shown as a MOSFET. Other types of switches may be used, as readily understood by the skilled artisan. Each switch can also be referred to as oscillator. 
     Inductor L 3  is the secondary or output inductor of the transformer. One end of L 3  is connected through a low value resistance. The other end is connected to the inductor of a corona igniter. The fourth inductor, L 6 , is a sense inductor which provides a feedback signal to compensate for the varying capacitance of different length attachment cables. 
     The ignition system is comprised of three sub-assemblies: a control circuit, a power amplifier and an igniter assembly. 
     Control Circuit: 
     This circuit receives a signal from the engine computer (ECU) that tells the system when to start and end corona in the cylinder. This circuit determines what voltage to apply to the power amplifier transformer. Part of this circuit generates the DC voltage that is applied to the power amplifier transformer. 
     Power Amplifier Circuit: 
     This circuit generates an alternating current and voltage signal to drive the igniter assembly at its resonant frequency. It receives a command from the control circuit to begin and end oscillation. The power amplifier circuit includes circuits to drive current through a transformer and a circuit to feed back the resonant frequency of the igniter assembly. This feedback signal includes a signal related to inductor resonance, a signal related to primary winding voltage, and a feedback signal related to the secondary winding voltage. 
     Igniter Assembly: 
     The igniter assembly attaches to the cylinder head in a manner similar to a spark plug. The assembly includes an inductor and a firing end subassembly which includes an electrode inside the combustion chamber. The igniter assembly has an inductor, capacitor and current sensor wired together as an LCR assembly. When a voltage is applied to one end of the inductor the LCR assembly resonates. The inductor is part of the igniter. The second end of the inductor is connected through a firing end assembly to an electrode crown in the combustion chamber. The firing end assembly and the combustion chamber form a capacitance and resistance that when combined with the inductance resonate at a specific frequency. 
     In operation, a device such as the engine computer (ECU) sends a signal to the control circuit. This signal tells the control circuit when to start and end corona on each igniter. The control circuit sends a normally high signal to the power amplifier that goes low to start the corona event. The signal stays low for as long as corona is desired, and returns high to end the corona event. This signal is applied to node A which is the emitter of Q 13 . This change in the voltage at A causes node N to go from high to low. Node N is then sent to two places. 
     One destination is the collector of Q 12  and the bases of Q 12  and Q 7 . This drop at N causes Q 12  and Q 7  to turn on, allowing current to flow to node Z. The second destination is C 3 , which sends a brief voltage drop through R 13  and diode  1  to node R, the base of Q 9 . This in turn briefly drops the voltage at node T. This dip in the base turns Q 5  on, drawing current from node Z, and raising node B from negative to positive. This turns Q 11  on and Q 17  off, which causes Q 1  to turn on and Q 2  to turn off. This pulls their emitters up, which are connected through R 16  and diode  2  to node C, the gate of M 1 . Node C goes from negative to positive, turning M 1  on. The drain of M 1  is connected to L 2 , and its source is connected to ground. Turning on M 1  causes current to flow through L 2 , which in turn induces a magnetic flux to flow through the ferrite inside the transformer. 
     As M 1  continues to stay on, current is conducted through L 2 , until the voltage at node T returns to a value that shuts Q 5  off. This causes the current flowing through node Z to transfer from R 11  into R 18 , raising node H from negative to positive. This turns Q 8  on and Q 20  off which causes Q 4  to turn on and Q 3  to turn off. This pulls their emitters up, which are connected through R 17  and diode  3  to node F, the gate of M 4 . Node F goes from negative to positive, turning M 4  on. Turning on M 4  causes current to flow through L 1 , which in turn induces a magnetic flux to flow in the opposite direction to the flux caused by L 2 , through the ferrite inside the transformer. 
     The transformer ferrite magnetic flux generates a current through the transformer secondary winding L 3  that in turn creates a voltage across its two ends. One end of L 3  is connected to R 14  which is attached to ground. The other end of L 3  is attached to the inductor in the igniter assembly. The rapidly changing voltage applied to the igniter LCR assembly induces it to resonate. When current flows through R 14  the voltage at node L rises. This voltage is fed through R 15  into node A 2 . The current from node A 2  goes through L 5 , which is connected to C 5  and R 19 . These components form a low pass filter and provide a phase shift in the current of less than 180°, and remove frequencies outside the range of interest. This signal is clipped by D 7  and D 8 , and then passed through C 7  to drive Q 10 . When Q 10  is turned on, current flows through R 18  and stops flowing through R 11 . This switches M 1  off and M 4  on, and vice versa. 
       FIG. 4  illustrates an implementation of the circuit of  FIG. 3  in a corona ignition system according to one exemplary embodiment. The system of  FIG. 4  includes a pulse generator A, a comparator block B, switches C and D, a transformer E, a current sensor F, a low pass filter G, and a clamp H. The corona igniter (not shown) is connected to the transformer E. 
     Operation of the system is initiated by a command signal or “enable signal”  1  asserted by an external source, such as an engine control unit. The “enable signal”  1  corresponds to point A in the circuit of  FIG. 3 . The “start” pulse generator A receives the enable signal  1  and transmits a non-inverting output  2  which initiates oscillation of the current flowing through the system and through the corona igniter in response to the enable signal  1 . The pulse generator A corresponds to components C 3 , R 13 , R 12 , and D 1  of the circuit of  FIG. 3 . 
     The comparator block B receives the enable signal  1 , as well as the non-inverting input  2  from the pulse generator A, and an inverting input  3  from the low pass filter G and clamp H. The signal received by the inverting input  3  of the comparator block B represents the phase of the current of the corona igniter. The non-inverting input  2  corresponds to Q 9  and the inverting input  3  corresponds to Q 10  of  FIG. 3 . The comparator block B then creates control signals for the switches C and D. The control signals provided by the comparator block B are based on information in the enable signal  1 , the non-inverting input  2 , and the inverting input  3 . The inverting input  3  is also referred to as a feedback signal from the low pass filter G and the clamp H. The comparator block B provides the control signals to the switches C and D as a normal output  4  and an inverted output  5 . The normal output  4  corresponds to point H and the inverted output  5  corresponds to point G of  FIG. 3 . The outputs  5  and  4  of the comparator block B also correspond to Q 5  and Q 6  of  FIG. 3 . 
     The switches C and D receive the normal output  4  and the inverted output  5 . The first switch C receives the normal output  4  and the second switch D receives the inverted output  5 . The first switch C corresponds to Q 3 , Q 4 , Q 9 , Q 22 , and Q 101   FIG. 3 , and the second switch D corresponds to Q 1 , Q 2 , Q 11 , Q 17 , and Q 102 . In response to the outputs  4  and  5 , the switches C and D each apply a voltage through signals  6  and  7  to the transformer E, which is connected to the corona igniter through output  9 , and thus causes oscillation of the current of the corona igniter. 
     The transformer E receives the voltage from the switches C and D and, in addition to causing oscillation of the corona igniter, the transformer E also increases the drive voltage of the corona igniter. When the circuit is on, voltage is applied from the transformer E to the corona igniter at all times. A positive voltage should be applied whenever current flows into the corona igniter, and a negative voltage should be applied whenever current flows out of the corona igniter. Switching from positive to negative or back should occur as close to zero current as possible. In one possible scheme, the transformer E has three windings wound around a magnetic core  12 , corresponding to L 1 , L 2 , and L 3  of  FIG. 3 . There are two primary windings L 1  and L 2  which each have one end attached to the power supply and the other end attached to one of the switches C, D; and one secondary winding which has one end attached to the corona igniter and the other attached to the current sensor F. L 1  and L 2  are arranged with switches C and D such that they create opposing magnetic fields in the magnetic core  12  when energized. The voltage output created by the transformer E is a balanced square wave output, symmetrical about zero. 
     The current sensor F of the system receives the current at the output of the transformer E through signal  10  and measures the current at the output of the transformer E, which is also the current of the corona igniter. The current sensor F includes at least one of a resistor, diode, an inductor, and a capacitor. The current sensor F of  FIG. 3  is a resistor located at R 14 . The current measurement obtained by the current sensor F is ultimately used to control the voltage of the secondary winding L 3 , such that the voltage of the secondary winding is “in phase” with the current of the corona igniter. The term “in phase” means that the voltage and the current peak at the same time, which means that the corona igniter is operating at the resonant frequency. More specifically, the block comparator B uses the information obtained by the current sensor F to instruct the switches C, D to apply a voltage to the primary windings of the transformer E at a certain time. The voltage applied to the primary windings L 1 , L 2  is timed such that it causes the secondary winding L 3  to have a voltage in phase with the current of the corona igniter. 
     More specifically, when current is transmitted from the transformer E and into the corona igniter, the current being transmitted to the corona igniter is sensed by the current sensor F. In response, the current sensor F transmits a signal, ultimately to the second switch D, to apply a positive voltage, and thus push more current from the transformer E into the corona igniter. The signal from the current sensor F to the switch D indicates the time at which the current being transmitted to the igniter goes through zero. The switch D turns on, causing the transformer E to provide the positive voltage, and thus provide more current, to the corona igniter, precisely when the current flowing into the corona igniter is at or about zero. Switching from positive to negative voltage or back should occur as close to zero current as possible. 
     Likewise, when current is traveling out of the corona igniter, through the transformer E and to ground, the current traveling out of the corona igniter is also sensed by the current sensor F. In response, the current sensor F transmits a signal, ultimately to the first switch C, causing the switch to close and apply a negative voltage, and thus pull more current, out of the corona igniter. The signal from the current sensor F to the switch C indicates the time at which the current traveling out of the igniter goes through zero. The switch C in turn closes, causing the transformer E to apply a negative voltage, and thus pull more current from the corona igniter, precisely when the current traveling out of the corona igniter is at or about zero. 
     Switching between transmitting current to the corona igniter and pulling current from the corona igniter at a time when the current is nominally at zero allows the system to operate at resonant frequency. In the example of  FIGS. 3 and 4 , when the current is traveling to the corona igniter, the voltage at the current sensor is negative, and when the current is traveling out of the corona igniter, the voltage at the current sensor is positive. In addition, R 1 , L 6 , and C 6  of  FIG. 3  compensate for the length of a cable between the circuit and corona igniter. 
     The low pass filter G of the system receives a voltage signal representing the current from the transformer E and removes or filters unwanted frequencies or frequencies outside of the range of interest. The low pass filter G also creates a phase shift in the current of at least 120° but less than 180°. As alluded to above, the low pass filter G also provides the feedback signal ultimately to the comparator block B, which includes the phase of the current of the corona igniter, indicating whether the current is positive, negative, or at zero. The low pass filter G corresponds to L 5 , C 5 , R 9 , and R 10  of  FIG. 3 . 
     The clamp H receives the feedback signal from the low pass filter G and truncates the signal before transmitting the feedback signal, i.e. inverting input  3 , to the comparator block B. The feedback signal provided to the comparator block B provides for zero crossing current detection only. In  FIG. 3 , the clamp H is located at D 7  and D 8 . 
     The operation of the system and signals sent between the components of the system will now be described in more detail. Initially, before operation of the system begins by the enable signal  1  being transmitted to the comparator block B, the comparator block B is disabled, and the normal output  4  and the inverted output  5  are off. At this point an HV power supply  8  is enabled and ready to provide power to the transformer E. The HV power supply  8  is external to the system. In  FIG. 3 , the HV power supply is connected to COM+. However, no current flows in the transformer E before operation of the system begins. 
     As indicated above, operation of the system begins by the enable signal  1  supplying power to the comparator block B. The enable signal  1  also causes the pulse generator A to generate the non-inverting input  2 , which includes a short pulse that forces the comparator block B out of balance. This causes the normal output  4  to briefly enable the first switch C, which causes current to flow from the HV power supply  8  through the primary winding of the transformer E and to signal  7 . The output  9  of the transformer E is driven negative and current continues to flow through the transformer E and the current sensor F to ground. 
     The current flowing through the transformer E and the current sensor F to ground causes the voltage to rise at signal  10 , reflecting the current flow of the corona igniter. However, the voltage at signal  10  includes high frequency components due to charging and discharging of parasitic capacitance in the system, particularly in the connecting cables. The filter block G removes these unwanted frequencies and provides the phase shift. The phase shift is at least 120°, and it is preferably close to but less than 180°. Therefore, the low pass filter G provides a clean sinusoidal current signal at  11 , reflecting but almost in antiphase with the current of the corona igniter. A further 180° phase shift is provided by using the inverting input  3  of the comparator block B. An unavoidable delay in the comparator block B and switches C and D make for a total phase shift of 360°. This is a condition required for stable oscillation. 
     The clamp H clips the size of the current signal  11  and converts the signal  11  to a square wave. This square wave is fed to the inverting input  3 , i.e. feedback signal, and to the comparator block B. The phase shift causes the inverting input  3  provided to the negative input of comparator block B to be a positive feedback around the entire loop. The positive feedback is a condition required for oscillation of the system and the corona igniter. 
     At this point, due to the resonant LC action of the corona igniter attached to the transformer E through signal  9 , the current flowing through signal  9  into the corona igniter peaks, drops back to zero, and then passes through zero. This causes the voltage at the signal  10  from the transformer E to the current sensor F to reverse its sign. The reverse signal causes the comparator block B to change state of the normal output  4  and the inverted output  5 , swapping the conductance from the first switch C to the second switch D, and reversing the current flow through the system. The current drives the other way, generating a negative half wave at the signal  10 . This process continues until the “enable” signal  1  is removed. 
     After the first cycle, steady state operation is attained, and the short pulse from the pulse generator A provided in the non-inverting input  2  is finished, and the voltage at the non-inverting input  2  is at quiescent level. The voltage at the inverting input  3  describes a small amplitude square wave around the quiescent level, and the small amplitude square wave is in antiphase with the current in the corona igniter. 
     The phasing of the current and applied voltage, through the switches C, and D and transformer E forces the current and voltage of the corona igniter to be in phase. This provides the condition needed for resonance of a series LC circuit, such as the corona igniter, which is a series LC circuit. According, implementation of the circuit of  FIG. 3 , according to the system of  FIG. 4 , enforces operation of the corona igniter at resonant frequency, and forces the corona ignition system to operate at the resonant frequency of the corona igniter. 
     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.