Abstract:
An internal combustion engine includes a cylinder with a spark plug, an electric DC voltage supply, and an ignition circuit. The ignition circuit includes a switching device and a transformer having a primary winding coupled to the supply via the switching device, and a secondary winding coupled to the spark plug. After having generated an initial breakdown of a breakdown path through a gas mixture between electrodes of the spark plug, switching occurs repeatedly per combustion to produce pulses at the primary winding, with a repeat frequency which is at least sufficiently high that the breakdown path remains conductive between consecutive switches per combustion. The switching on and switching off provides heating of the breakdown path to ignite the gas mixture. The transformer has an air core around which the primary and secondary windings are arranged concentrically.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of International Patent Application Number PCT/NL02/00774, filed Nov. 29, 2002, entitled “Combustion Engine and Ignition Circuit for a Combustion Engine” and designating, inter alia, the United States, which claims priority to Netherlands Patent Application Serial No. 1019448, filed Nov. 29, 2001, both of which applications are herein incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   The invention relates to an internal combustion engine comprising a cylinder with a spark plug, an electric DC voltage supply and an ignition circuit. 
   Internal combustion engines provided with spark plugs for igniting a gas mixture in a cylinder are generally known. Such engines are provided with an ignition coil, which is essentially a transformer with a switch between a primary winding of the transformer and an electric supply such as an accumulator and a coupling of the secondary winding to the spark plug. The ignition coil provides for a high voltage across the electrodes of the spark plug, which leads to a spark-over so that the gas mixture is ignited. 
   U.S. Pat. No. 5,456,241 discloses an ignition circuit for an internal combustion engine. This ignition circuit includes a capacitor which forms a resonant circuit with a series connection of the primary winding and a supplementary coil. To generate high voltages, first the capacitor is charged, while a switch keeps the current path through the primary winding and the supplementary coil interrupted. Next, the switch is rendered conductive. This leads to a dampened oscillation with voltage peaks which, via the transformer, lead to the desired high voltages across the electrodes of the spark plug. Upon the first voltage peak, an electric breakdown through the gas mixture occurs, so that a breakdown path through the gas mixture between the electrodes becomes conductive. Next, the rest of the oscillations lead to an oscillating current through the breakdown path. Thus, the gas mixture is heated to a temperature at which combustion occurs. An important parameter of such an ignition is the energy thereby transferred to the spark plug. Further, erosion of the spark plugs resulting from the discharge impact is an important parameter. U.S. Pat. No. 5,456,241 describes how the switch is made conductive and non-conductive several times for the same combustion. Thus, the capacitor can be charged several times and more energy is transferred to the spark plug than through switching the switch a single time. 
   U.S. Pat. No. 5,456,241 further describes how the amount of energy that can be transferred from the supply to the gas mixture decreases according as the speed of the motor increases. While this effect is counteracted by increasing the frequency with which the switch is switched on and off at higher speeds, the amount of energy nonetheless decreases at higher speeds. This is because at high speeds the time between consecutive combustions of the gas mixture more and more approximates the charging time needed to charge the resonant circuit. This charging time is fairly great, because a fairly large self-induction of the primary winding is needed for the storage of sufficient energy for igniting the gas mixture. 
   U.S. Pat. No. 4,677,960 describes a capacitive discharge ignition circuit using a high efficiency voltage doubling ignition coil. The ignition is used with a high pulse rate, multiple pulse ignition box providing rapid pulsed plasma ignition sites. The circuit may include an energy storage capacitor but the coil will also operate with standard or electronic ignition excepting that full advantage cannot be taken of the high current/voltage capabilities of the coil since these ignitions cannot store high energy rapidly. Because the coil comprises a core made of ferrite, the possible frequencies applied to a spark plug are limited to approximately 30 kHz. 
   U.S. Pat. No. 5,842,456 shows a multi-firing ignition circuit with which several pulses per combustion cycle can be supplied to a spark plug. Concrete circuits are not shown, but the patent does mention that here too use is made of energy from a capacitance in a “capacitive discharge coil-on-plug” circuit. The patent does not speak of the use of an existing breakdown path to generate an oscillating current through the gas mixture with several pulses. 
   European patent application No. 893600 shows the use of several pulses per combustion. The patent application does not speak of the use of an existing breakdown path to generate an oscillating current through the gas mixture with several pulses. 
   European patent application 0913897, German patent application DE10037536and PCT patent application 0133073 describes the use of plasma ignitions, in which very high-frequency electromagnetic signal sources, up to inter alia laser and microwave sources, are used to heat gas mixtures. European patent application 0913897 describes a plasma ignition circuit in which the gas mixture is ignited by thermal energy from a high-frequency field. This publication provides for the ignition of different plasma filaments one after the other, by means of a special kind of spark plug. Details about the high-frequency circuit are not given. U.S. Pat. No. 4,366,801 discloses an ignition circuit with a resonant circuit for generating pulses. DE10037536 utilizes signals of about 1 Ghz, but further does not describe any circuits. PCT patent applications 0133073 describes the use of laser light or microwaves of a plasma, after this has been rendered conductive with an electric pulse. 
   U.S. Pat. No. 4,366,801 discloses the use of a number of transformers, each for its own spark plug, and each in series with a capacitor, with which the transformer forms a resonant circuit, so that a resonant signal is generated across the spark plug. PCT patent application WO0050747 describes an ignition which, after a breakdown generating pulse, generates a sequence of further voltage oscillations across the electrodes of the spark plug, whereby a reactance circuit causes the voltage of the voltage oscillations to decrease once the breakdown path is conductive. If the breakdown path is interrupted, the voltage increases again. This publication shows an oscillator to generate the pulses with a frequency in the band of 1-100 kHz. The pulse forming circuit comprises the transformer and a capacitor which jointly work as a resonant circuit. 
   As far as these ignition devices work with spark plugs that have electrodes between which electric fields are generated, all these devices utilize a transformer of which the secondary winding is coupled to the electrodes and of which the primary winding is included in a resonant circuit. This enables efficient transfer of the needed energy to the electrodes. However, it also has as an effect that the maximum rise speed of the field across the electrodes is related to the pulse repeat frequency. When effecting the initial breakdown between the electrodes, the circuit must deliver a voltage so high as to be sufficient to create a breakdown path. 
   U.S. Pat. No. 4,846,129 describes an ignition system wherein a detector is employed to sense the first or “breakdown” phase of spark discharge across a spark plug which causes a short duration high current flow across the plug gap. The detection of the breakdown current enables control over a number of ignition system functions. A pulse transformer is used which enables extremely short duration energization of the spark plug at controllable voltages. The spark plug may be caused to multiply discharge within a short duration which has been found to increase the lean burn limit of the engine. Rapid multiple firing of the plug is achieved by sensing the existence of breakdown current which signifies the discharge event. This signal is used to immediately curtail that discharge cycle and begin another firing cycle, enabling multiple discharges to occur in a very short time duration. However, at each discharge the electrodes of the spark plug are eroded by the repeatedly particle impacts resulting in a relatively short spark plug lifetime. 
   It is one object of the invention to provide an improved internal combustion engine and an ignition circuit for such an internal combustion engine. 
   Therefore, the invention relates to an internal combustion engine comprising a cylinder with a spark plug, an electric DC voltage supply and an ignition circuit, which ignition circuit is provided with a switching device and a transformer having a primary winding, which is coupled to the supply via the switching device, and a secondary winding which is coupled to the spark plug, the switching device being arranged for (i.e., configured such that), after having generated an initial breakdown of a breakdown path through a gas mixture between electrodes of the spark plug, switching repeatedly per combustion to produce pulses at the primary winding, with a repeat frequency which is at least so high that the breakdown path remains conductive between consecutive switches per combustion, so that the switching on and switching off of a high current leads to a heating of the breakdown path in order to ignite the gas mixture, wherein the transformer comprises an air core around which the primary and secondary winding are arranged concentrically. 
   This arrangement is preferable when transforming high frequency pulses. No energy is stored in the core, resulting in a transformer wherein the rise time of the pulses in the secondary winding can be as high as 2 kV/ns or even higher. This very high rise speed leads to a greater energy content in the gas mixture by means of dielectric heating. As a result, a less high voltage can suffice for generating the initial breakdown. 
   The primary winding is provided with energy from the DC voltage supply repeatedly during the combustion, without making use of energy which is stored in a resonant circuit. By doing this with a sufficiently high frequency, sufficient energy for the combustion can be supplied without energy from a resonant circuit being necessary. Thus, it can suffice to use a primary winding with a low self-induction, which can be charged again faster, so that fewer problems arise at high speeds. A resonant circuit is now not necessary anymore to generate sufficient energy for the combustion. 
   Further, also, the erosion of the electrode of the spark plug is less because the kinetic energy of the impacting particles is not high enough to disrupt the structural integrity of the electrode material. As the rest of the pulses of the signal lead to an alternating current through the existing breakdown path, without a new breakdown being necessary in the respective combustion cycle, there does not further arise any erosion either. The minimum repeat frequency needed for this purpose depends on the engine design, typical values being, for instance, between 100 kHz and 10 MHz. In the combustion engine according to the invention, the gas mixture is heated by the high frequency current pulses to a temperature at which combustion occurs. 
   Preferably, in substantially every period of the repeated switching, the same fraction of the energy for combustion of the gas mixture is supplied. 
   Thus, the energy is supplied, for instance, equally distributed over fifty or more pulses per combustion, or even over eighty or more pulses and preferably a hundred or more pulses per combustion. As a result, no great current per pulse is needed. 
   The duty cycle and/or the frequency with which the switching device connects the supply with the primary winding may be modulatable. Thus, the amount of energy that is supplied to the spark plug can be set, for instance depending on the condition of the engine, a measured degree of combustion of previous gas mixtures, the available supply voltage, and so forth. Also, the amount of energy of consecutive periods in which the connection is made conductive can be made variable in a single combustion, for instance by supplying more energy per time unit with later pulses to thereby reduce spark erosion. 
   The frequency with which current pulses from the supply are led through the primary winding is preferably high above the speed of the engine, preferably as high as is practically possible, for instance in a range of 100 kHz-10 MHz. This is simple to realize with a high frequency generator. Thus, a small transformer can suffice, and high speeds of the engine do not entail any decrease of the energy that is supplied to the spark plug. 
   In an embodiment, the ignition circuit is arranged to produce a high frequency pulse train at the secondary winding wherein a first pulse of the pulse train has an amplitude of at least 2 Kilovolt and subsequent pulses of the pulse train have amplitudes of less than 300 Volt. After having generated an initial breakdown path in the gas mixture, the voltage of subsequent pulses can be much lower than the initial pulse. The number of subsequent lower pulses per combustion may vary, so that the amount of energy transferred to the gas mixture can be regulated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objectives and advantages of the internal combustion engine and ignition according to the invention will be further discussed with reference to the following figures. 
       FIG. 1  shows an ignition circuit; 
       FIG. 2  shows a cross section of an embodiment of a transformer; 
       FIG. 3  shows a switching pattern; 
       FIG. 4  shows a high-frequency generator. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an ignition circuit. The ignition circuit includes an engine management system  10 , a supply  12 , a number of high-frequency generators  14   a-d , a number of transformers  16   a-d,  a number of spark plugs  18   a-d  in cylinders  19   a-d . Each high-frequency generator  14   a-d  has an output which is coupled to a primary winding of a respective transformer  16   a-d.  The secondary windings of the transformers  16   a-d  are coupled to electrodes of the spark plugs  18   a-d  via resistors  15   a-d  connected in parallel to capacitors  17   a-d . Resistors  15   a-d  may have values of for example 700 Ω and the capacitors may have values of for example 10 nF. 
   In operation, periodically, gas mixtures in the cylinders  19   a-d  are caused to combust through sufficient heating of the breakdown paths in the spark plugs  18   a-d . The engine management system  10  determines when which spark plug  18   a-d  must cause a gas mixture to combust. When this is the case, the engine management system  10  sends a control signal to the high-frequency generator  14   a-d  that is coupled to the respective spark plug  18   a-d.  The respective high-frequency generator  14   a-d  thereupon generates a sequence of pulses in which the primary winding is connected with the supply  12 . According to the invention, the transformers  16   a-d  are arranged as to produce very steep pulses at the secondary windings. The slope of a pulse at the secondary winding is about 2 kV/ns or more. These steep pulses can be produced using a transformer comprising two windings which transfer energy without storing magnetic energy in a coil.  FIG. 2  shows a cross section of an embodiment of the transformer  40  comprises a main yoke  50  having an air core  51 , around which a primary winding  52  is wound and, isolated from the primary winding  51 , a secondary winding  52 . Because of the low inductance of the core  41 , no energy is stored at high frequencies, i.e. higher than 100 kHz. By providing a steep pulse to the gas mixture, a breakdown path will act as a resistance that can be heated by providing multiple subsequent pulses after the initial pulse within a certain time frame. Only a relatively low voltage (e.g. 2 kV) is needed to cause this effect. The resistors  15   a-d  in  FIG. 1  ensure a limited current during breakdown resulting in less erosion of the electrodes of the plug. The capacitors  17   a-d  are provided to conduct the high frequency current. 
     FIG. 3  shows a switching pattern of the ignition of  FIG. 1. A  first number of signals  20   a-d  show control pulses  21  of the engine management system  10  for respective high-frequency generators  14   a-d  in different signals. The engine management system  10  sends a pulse  21  each time when a gas mixture in the cylinder  19  concerned is to be caused to combust. A second number of signals  22   a-d  show pulses  23  each indicating a time interval in which high-frequency generators  14   a-d  connect the primary windings of the different transformers  16   a-d  with the supply  12 . For each pulse  21  of the engine management system  10 , a high-frequency generator  14   a-d  generates a large number of such pulses  23 , i.e. a pulse train. As a result, each time energy from the supply is led into the primary winding of the transformer  16   a-d  and via the secondary winding to the spark plug  18   a-d . Preferably, the amplitude of the first pulse in the pulse train  23  is higher than the amplitude of subsequent current pulses in the pulse train  23 , see FIG.  3 . The first pulse may for example result in a pulse in the secondary winding having an amplitude above 2 kV, while the other pulses result in secondary winding pulses with amplitudes as low as 300 V or even lower. 
   As shown, the circuit does not include a capacitor parallel with the primary winding of the transformers  16   a-d.  This is not necessary because the circuit is not based on oscillating exchange of energy between the primary windings and a capacitor. The current through the primary windings is completely determined by the high-frequency generators  14   a-d.  If there is a resonant circuit at all at the output of the high-frequency generators  14   a-d  of which a primary winding forms a part (for instance as a result of parasitic capacitances), the resonant frequency is far beyond the frequency with which the high-frequency generators  14   a-d  deliver the pulses  23 . 
   As in this way the full energy does not need to be stored at a single time in the primary winding, a relatively small self-induction of, for instance, around 10 milliHenry (or in the range of 3-30 milliHenry) will suffice for these windings. For such relatively small self-inductions, it is easy to make transformers that have few losses up to fairly high frequencies up to above 100 kHz. This permits working with frequencies for the pulses  23  coming from the high-frequency generators  14   a-d  in a range of 10 to at least 100 Kilohertz. 
     FIG. 4  shows an example of a high-frequency generator for use in an ignition such as shown in FIG.  1 .  FIG. 4  shows the transformer  30 , with the primary winding  31  and the secondary winding  32 . The primary winding  31  has connections  33   a, b  at the terminal ends and a connection  34  in the middle. 
   The circuit further comprises a voltage source  36 , transistors  35   a, b  and a control circuit  37 . The voltage source  36 , for instance a storage battery, is coupled between the central connection  34  and to a common connection  38 . The control circuit  37  has control outputs coupled to the control electrodes of the transistors  35   a, b.  Each transistor  35   a, b  has a main current channel which is coupled between one of the end connections  33   a, b  and the common connection  38 . 
   In operation, the control circuit  37  receives a synchronization signal that indicates when the gas mixture in a cylinder is to be caused to combust. The control circuit  37  thereupon generates a sequence of pulses on the control electrodes of the output transistors  35   a, b,  in each case first a pulse on the control electrode of a first output transistor  35   a,  virtually directly followed by a pulse on the control electrode of a second output transistor  35   b.  The pulses render the main current channel of the output transistors  35   a, b  in question conductive during the pulse. Thus, alternately, pulse-shaped currents flow in mutually opposite directions through a part of the primary winding  31 . These currents generate magnetic fields which generate voltage in the secondary winding  32  which is applied across the electrodes of a spark plug (not shown). 
   By making use of two output transistors  35   a, b  which alternately generate opposite fields, the energy dissipation in the output transistors  35   a, b  is limited. Preferably, the transformer  30  includes a magnetic core, for instance of ferrite, to couple the primary winding  31  and the secondary winding  32 . Through the alternate use of the output transistors  35   a, b,  it is moreover ensured that the field in the magnetic core is averagely zero, so that the magnetic core can be optimally driven to full output. Without deviating from the invention, however, it may suffice to use a single output transistor, this output transistor and the voltage source  36  being coupled to opposite terminal ends  31  of the primary winding. 
   It will be clear that the output transistor or transistors of the high-frequency generator preferably switches between a condition in which this transistor is conductive as much as possible and a condition in which it is conductive as little as possible. Thus, a minimum of energy is dissipated in the high-frequency generator. Without deviating from the invention, however, a more gradual switching is possible, whereby the voltage drop across the output transistor is controlled more gradually by the control signal on the control electrode. 
   The control circuit  37  controls the amount of power which is supplied to the spark plug by setting of the width of the pulses on the control electrodes of the transistors  35   a, b.  In principle, the control circuit  37  can then keep the repeat frequency with which it delivers the pulses constant, or vary it independently of the pulse width. Also, the distance between the pulses on the control electrodes of different transistors  35   a, b  can be set freely, though preferably with avoidance of overlap between the pulses, since that does not lead to energy transfer. 
   More generally, the duty cycle, that is, the fraction of the time when the high-frequency generator  14   a-d  connects the primary winding with the supply  12 , is modulatable. Thus, the amount of energy which is supplied to the spark plug  18   a-d  can be regulated. Also, the amount of energy can be regulated by regulation of the frequency of the pulses  23  given a fixed pulse width. 
   Such regulation or regulations are preferably under the control of a signal of the engine management system  10 , which controls the duty cycle, for instance depending on the condition of the engine (temperature, speed, etc. ), to provide for an optimum combustion of the gas mixture with a minimal energy consumption from the supply.