Abstract:
An ignition circuit comprising storage means to store electrical energy, first and second switching devices, means for charging the storage means and means for switching each of the switching devices such that charge on the storage means is transferred to cause firing of an igniter in such a way that the voltage across each switching device is only a fraction of the total applied to the igniter. In this way, standard solid state switches may be employed rather than the more costly devices required to handle the full voltage to be applied to the igniter.  
     More than one such ignition circuit may be combined to provide a scanning multiple spark system with a plurality of igniters, each in conjunction with a pair of switching devices and means for switching each of the switching devices as described above.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to ignition circuits.  
           [0002]    High energy ignition systems employ a capacitor to store electrical energy which is then rapidly discharged to an igniter or spark plug to produce an intense spark sufficient to light a fuel-air mixture. A typical solid-state igniter may require up to 2000 volts to cause break-over. Once the spark has commenced, the igniter voltage collapses to near zero while a current of approximately 2000 amperes flows for the duration of the spark until the energy in the capacitor has dissipated. Normally this cycle of charging and discharging of the stored energy is repeated many times until satisfactory ignition of the fuel occurs.  
           [0003]    Some high energy ignition systems employ a gas discharge tube, which breaks over at a point when the voltage on the charging storage capacitor reaches the desired level to ‘dump’ the accumulated charge into the igniter. For various technical reasons including, life expectancy, synchronisation, mechanical robustness and reliability, it is desirable to use solid state electronic switching of the discharge; the most suitable component for this is a thyristor. At present, suitable devices are not cheaply available to handle 2000 volts directly and the high currents in this application. However, devices to switch comfortably at 1000 volts are easy to obtain and are relatively low cost.  
           [0004]    It is an object of the present invention to provide an alternative ignition circuit.  
         SUMMARY OF THE INVENTION  
         [0005]    According to one aspect of the present invention there is provided an ignition circuit including storage means to store electrical energy, first and second switching devices, means for charging the storage means, and means for turning on each of the switching devices so that charge on the storage means is transferred to cause firing of an igniter in such a way that the voltage across each switching device is limited to a fraction of the total applied to the igniter. This makes it possible to use relatively inexpensive switching devices, capable of handling a moderate voltage, whilst providing the igniter with a sufficiently high voltage to cause sparking.  
           [0006]    Preferably, the storage means comprises a double storage means and provides a reduced voltage point, compared to the total voltage applied to the igniter, which limits the voltage applied across each switching device.  
           [0007]    The storage means preferably includes two storage capacitors. The voltage across each switching device is preferably substantially half the total voltage applied to the igniter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    Several embodiments of ignition circuit will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 is a diagram of a first embodiment of the circuit;  
         [0010]    [0010]FIG. 2 is a diagram of a modified form of the circuit shown in FIG. 1;  
         [0011]    [0011]FIGS. 3 and 4 are diagrams of further embodiments of the circuit; and  
         [0012]    [0012]FIG. 5 is a diagram of a scanning multiple spark system employing the circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    With reference first to FIG. 1, the circuit includes a transformer  1 , with a centre tapped secondary winding  2 . The transformer  1  may be either the output transformer of a switched-mode power converter, or the secondary of a mains step-up transformer. An input supply  3  is connected to the primary winding of the transformer  1 , this being current limited to prevent damage to the supply when momentary overloads are applied during the operating cycle of the system. The supply  3  preferably also has an overvoltage limit to prevent overcharge of the storage capacitors should discharge of ignition sparks not be requested or fail to occur. Two rectifier diodes  4  and  5  are connected in opposite senses to opposite ends of the secondary winding  2 . The cathode and anode of the diodes  4  and  5  are connected respectively to a series connection of two storage capacitors  6  and  7 , the junction  8  between the two capacitors being connected to a centre tapping  9  of the secondary winding  2 . The junction  10  between diode  4  and capacitor  6  connects to the anode of a first thyristor  11 . The junction  12  between the other diode  5  and capacitor  7  connects to a 0 volts reference point  13 . A second thyristor  14  is connected in series between the first thyristor  11  and a first output terminal  15  of the circuit. The circuit&#39;s other output terminal  16  is connected with the 0 volts reference point  13 . A resistor  17  is connected between the two output terminals  15  and  16 . A high current diode  18  is connected at its anode to the junction  8  between the two capacitors  6  and  7 . The cathode of the diode  18  connects both to a junction  19  between the two thyristors  11  and  12  and to one end of a resistor  20 . The other end of the resistor  20  connects to a junction between the resistor  17  and the output terminal  15 .  
         [0014]    The circuit also includes a first trigger circuit  21  connected to the trigger of thyristor  14  and a second trigger circuit  22  connected to the trigger of thyristor  11 . The second trigger circuit  22  is operated in response to the output from a detector  23  connected across the thyristor  14 , which responds when this thyristor turns on.  
         [0015]    The output terminals  15  and  16  of the circuit are connected across the electrodes of an igniter or spark plug  30 .  
         [0016]    The trigger circuit  21  initiates the request for a spark and may be either a free-running clock oscillator producing regular thyristor gate-pulses, or a pulse shaping circuit that produces a single output pulse in response to an external timing signal to initiate a spark.  
         [0017]    In operation, assuming both thyristors  11  and  14  are initially off (open-circuit or high impedance), the alternating output current from the transformer winding  2  charges each storage capacitor  6  and  7  through diodes  4  and  5 . Typically, this voltage rises to 1000 volts on each capacitor  6  and  7  so that the voltage across their series extremities  10  and  12  is 2000 volts. The voltage across resistor  17  will be virtually zero at this time since its value is low enough to bleed away any thyristor leakage current and also the flow through the resistor  20 . The small current which does flow through the resistor  20  ensures that the diode  18  is forward biased and the voltage on its cathode is therefore virtually identical to that at the junction  8  of the capacitors  6  and  7 , that is, 1000 volts relative to 0 volts. By this means, the thyristors  11  and  14  have 2000 volts across them in total, but only 1000 volts across each device.  
         [0018]    When the trigger circuit  21  requests a spark, it turns on the thyristor  14  so this becomes effectively a short circuit between its anode and its cathode. This directly connects the 1000 volts from the capacitor  7 , via the diode  18  and the thyristor  14  to the igniter  30 . Most igniters are unlikely to break down at this voltage so the resistor  17  provides a current path to maintain sufficient hold-on current for the thyristor  14 . It can be seen that the voltage across the thyristor  11  at this time remains safely at 1000 volts. The detector circuit  23  detects when the thyristor  14  turns on from the collapse of voltage across it; this causes the trigger circuit  22  to generate a trigger pulse for the other thyristor  11 . When the thyristor  11  turns on, the full 2000 volts, from the series arrangement of the two capacitors  6  and  7 , is applied to the igniter  30  resulting in the initiation of a spark. The diode  18  becomes reverse biased, which prevents current flowing back into the transformer centre tap  9 . The high current discharge through both thyristors  11  and  14  continues until virtually all the stored energy in the capacitors  6  and  7  is depleted and the thyristors both switch off because of the lack of hold-on current.  
         [0019]    Since the capacitors  6  and  7  are of the same value, they tend to discharge together with only minor imbalance. However, there is always likely to be some tendency for one or other of the capacitors  6  or  7  to drain first and so exceed the zero limit and develop a reverse charge. Whilst this may not be disastrous, it puts severe strain on the associated rectifier diode  4  or  5  and in some converter circuits may cause saturation of the transformer core producing circuit failure. This effect can be reduced by adding some series resistance into the transformer secondary winding  2 , or by connecting a reverse protection diode across either half of the winding to clamp any reverse swing.  
         [0020]    It is possible that the igniter  30  will break-over and commence sparking after the turn-on of the first thyristor  14  but before the firing of the second  11 . In this circuit, the resulting collapse in voltage across the capacitor  7  will simultaneously cause a step shift in the voltage at both ends of the capacitor  6 , so maintaining the 1000 volts charge from the capacitor  6  across the thyristor  11 , which it is able to withstand safely. Although the spark commences at an earlier point in the circuit&#39;s operation, the subsequent turn-on of the thyristor  11  still ensures that virtually all the stored energy from both capacitors  6  and  7  is available for the igniter  30 .  
         [0021]    With reference now to FIG. 2, this shows a modification of the circuit of FIG. 1, equivalent components being given the same reference number with the addition of  100 .  
         [0022]    The circuit of FIG. 2 differs from that of FIG. 1 in that capacitor  106  is connected across the 2000 volts developed in the full winding  102  of transformer  101  and that the values and voltage ratings of capacitors  106  and  107  are adjusted appropriately. Capacitor  106  now becomes the single main energy store for the circuit. Since it is operating at the 2000 volts, for a given energy capacity its size and cost may be considerably reduced compared with the dual versions previously described. The other capacitor  107  provides only the initialising voltage for the igniter  130  and reservoir for the mid-rail voltage and will typically be only between 0.5% and 1% of the capacitance value of the main capacitor  106 .  
         [0023]    In operation, assuming both thyristors  111  and  114  are off, the alternating output current from the centre tapped winding  102  of transformer  101  charges capacitor  106  to typically 2000 volts and capacitor  107  to 1000 volts through diodes  104  and  105  respectively. The series arrangement of the two thyristors  111  and  114  is subjected to the full 2000 volts from the main capacitor  106  but the voltage across each device is limited to 1000 volts as held by the voltage on the other capacitor  107 .  
         [0024]    When the trigger circuit  121  turns on the thyristor  114  this directly connects the 1000 volts at the junction  119  of the two thyristors to the igniter  130 . The other trigger circuit  122  responds to breakdown of voltage across the thyristor  114  rapidly to turn on the other thyristor  111 . When the thyristor  111  turns on, the full 2000 volts from the main capacitor  106  is applied to the igniter  130  resulting in the initiation of a spark. The high current discharge through both thyristors  111  and  114  continues until virtually all the stored energy in the capacitor  114  is depleted and the thyristors both switch off because of the lack of hold-on current.  
         [0025]    With this alternative unbalanced capacitor arrangement, the voltage stress on the thyristor  111  is significantly increased if the igniter  130  discharges after the thyristor  114  turns on but before the thyristor  111  turns on. However, since the turn-on of thyristor  114  immediately ‘requests’ the turn on of thyristor  111 , and because typical circuit resistances in leads and the like naturally cause a finite time for the voltage to increase across the main capacitor  106 , it can be arranged for the ‘crowbar’ effect of thyristor  111  to self limit the possibility of overvoltage.  
         [0026]    Since there is only one main energy storage capacitor in this implementation, reversal of capacitor voltage is not likely and so an extra protection diode is usually unnecessary.  
         [0027]    With reference now to FIG. 3 there is shown another modification of the circuit in FIG. 1. In this circuit equivalent components have been given the same reference number with the addition of  200 .  
         [0028]    In this circuit, the transformer  210  has two separate windings  202  and  202 ′ in place of the single centre-tapped arrangement. Also, the thyristors  211  and  214  are not connected together directly. Instead, one thyristor  211  is connected in the series connection of the two capacitors  206  and  207  and the other thyristor  214  is connected between the terminal  210  at the end of the capacitor series and one output terminal  215 . The circuit has an additional diode  227  connected across the series connection of the capacitor  207  and the thyristor  211  directly to the output terminal  215  of the circuit. These two diodes  218  and  227  enable the circuit to operate whichever thyristor  211  or  214  is fired first, thus permitting an alternative thyristor drive arrangement. By way of example, FIG. 3 shows a modified trigger circuit  221  that provides two virtually simultaneous outputs which are applied to the thyristors  211  and  214  together thus eliminating the need for a turn-on detector.  
         [0029]    In operation, assuming that both thyristors  211  and  214  are off, the output of each transformer winding  202  and  202 ′ will charge the associated energy storage capacitors  206  and  207  via diodes  204  and  205  respectively to 1000 volts. If, for example, thyristor  211  happens to turn on first in response to its signal from the trigger circuit  221  the 1000 volt charge on the capacitor  207  will be applied to the igniter  230  through thyristor  211  and diode  218 . Normally this is not sufficient to cause break over of the igniter  230 . The resistor  217  provides a path to ensure sufficient hold-on current for the thyristor  211 . When the other thyristor  214  turns on, in response to its own trigger pulse from the circuit  221 , the further 1000 volts charge on capacitor  206  is now added to that of capacitor  207  by nature of its series connection, thereby increasing the voltage applied to the igniter  230  to 2000 volts and initiating a spark. The high current discharge through both thyristors  211  and  214  continues until virtually all the stored energy in the capacitors  206  and  207  is depleted and the thyristors both switch off due to lack of hold-on current.  
         [0030]    If the igniter  230  breaks over to commence sparking following the turn-on of the first thyristor  211  but before the firing of the second thyristor  214 , the circuit inherently avoids subjecting the second thyristor to any increase in voltage beyond the 1000 volt level, since the voltage on the capacitor  206  is unaffected by the turn-on of the thyristor  211 . If the thyristors are triggered so that thyristor  214  turns on before thyristor  211 , the diode  218  will carry the initial 1000 volt application from the capacitor  206  to the igniter  230  in place of the diode  227 .  
         [0031]    With reference now to FIG. 4, there is shown another modified circuit. In this circuit equivalent components are given the same reference number as in FIG. 1 but with the addition of  300 .  
         [0032]    This circuit has two secondary windings  302  and  302 ′ and the two thyristors  314  and  311  are connected across respective windings via the diodes  304  and  305 . One capacitor  306  is connected between the junction  310  of the diode  304  with the thyristor  313  and an output terminal  315  of the circuit. The other capacitor  307  is connected between the two thyristors  311  and  314 . This circuit uses a ‘shunt’ method of high current switching. As in the circuits of FIGS. 1 and 3, individual storage capacitors  306  and  307  are employed at 1000 volts to govern the voltage imposed on each thyristor  311  and  314 . The capacitor  306  charges through the diodes  304  and  327  whilst the capacitor  307  charges through the diodes  305  and  327 . If identical value capacitors  306  and  307  are used, the diode  327  could in practice be replaced with a resistor. When the trigger circuit  322  turns on the thyristor  311 , the charge on the capacitor  307  is applied to the terminal  315  through the diode  318 . It should be noted that the voltage applied to the igniter  330  is negative in polarity relative to the 0 volts shown in the diagram. As before, when the thyristor  314  turns on, the additional 1000 volts on the capacitor  306  is added and applied to the igniter  330  to initiate a spark. This circuit provides the advantage that instantaneous protection may be provided against overvoltage of the transformer in the event of a disconnected or faulty igniter since triggering the thyristors will immediately clamp the winding voltages to zero. As in previous embodiments, the charging circuit associated with the transformer should be protected against over current in this condition.  
         [0033]    [0033]FIG. 5 shows how circuits of the kind in the arrangement of FIG. 1 can be used in a scanning multiple spark system. The components in FIG. 5 equivalent to those in FIG. 1 are given the same reference number with the addition of  400 . The system has a single pair of storage capacitors  406  and  407  charged via diodes  404  and  405  from a transformer  401  and power supply  403  in the same manner as in the arrangement of FIG. 1. Instead of having a single switching circuit, the system of FIG. 5 has multiple switching circuits, in this example three circuits indicated A, B and C, which switch charge from the capacitors  406  and  407  to respective ones of three different igniters  430 A,  430 B and  430 C. Each circuit A to C has a pair of thyristors as in the arrangement of FIG. 1 but these are triggered by signals from a single trigger circuit  421  common to the three circuits A to C. The trigger circuit  421  is a scanning trigger source with an individual output for each switching circuit A, B and C. The trigger circuit  421  triggers each switching circuit A, B and C in turn, one after the other. The interval between each trigger output is chosen to be long enough to allow replenishment of the stored capacitor energy. It will be appreciated that different numbers of switching circuits could be used to fire different numbers of igniters.  
         [0034]    The present invention enables low cost electronic switching devices to be used without risk of damage.