CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a continuation of copending International Application Serial No. PCT/US97/10206, filed Jun. 19, 1997 claiming the benefit of U.S. Provisional Application Ser. No. 60/020,033, filed Jun. 21, 1996. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to ignition systems for internal combustion engines, and particularly, to a multiple spark capacitive discharge ignition system for such an engine. 
     In internal combustion engines, it is known that the physical nature of the fuel or fuel/air charge injected into the cylinder varies depending upon engine operating conditions. Specifically, at low engine speeds, the fuel charge is injected into the cylinder in the form of a stratified cloud of fuel particles. The cloud of fuel particles is termed stratified because the density of the fuel particles within the cloud is not constant, i.e., not homogeneous throughout the charge. At higher engine speeds, the fuel charge is injected into the cylinder in what is termed to be a “homogeneous” cloud of fuel particles. The charge is termed homogeneous because the density of fuel particles in the fuel charge is relatively constant throughout the charge. 
     A single ignition spark or a small number of ignition sparks anywhere within a homogeneous fuel charge will cause complete combustion of the fuel charge. This is not so for a stratified fuel charge. With a stratified injection of fuel, it has been found desirable to provide a greater number of ignition sparks (than is provided under homogeneous conditions) in order to ensure that the stratified fuel charge is adequately or completely ignited. U.S. Pat. Nos. 5,170,760 and 4,653,459 generally illustrate ignition systems for providing a plurality of ignition sparks to ignite a stratified or non-homogeneous fuel charge in the cylinder. 
     SUMMARY OF THE INVENTION 
     The invention provides an ignition system for an internal combustion engine having one or more cylinders. The ignition system generates more ignition sparks per ignition event when the engine is operated in the stratified fuel injected mode than when the engine is operated in the homogeneous fuel injection mode. Generally speaking, the system includes an electronic control unit (“ECU”) for generating ignition signals for the respective cylinders, an input/logic multiplexer for multiplexing the ECU control signals, a direct current to direct current (“DC—DC”) converter for charging an ignition capacitor, a silicon controlled rectifier (“SCR”) for discharging the ignition capacitor, an ignition trigger circuit for triggering the SCR and an ignition distribution network for distributing the energy discharged from the ignition capacitor to the appropriate ignition coil. 
     The DC—DC converter includes a pulse width modulator which generates, in response to the inputs from the ECU, a high frequency output of at least 1000 hertz frequency. Preferably, however, the frequency of the pulse width modulator output is 3.0 khz. The pulse width modulator drives a series of parallel connected high power insulated gate bipolar transistors (“IGBTs”) connected through a transformer to a power supply. The power supply voltage is generated by the alternator. Energizing of the transistors by the pulse width modulator at a rate of approximately 3.0 khz causes a flyback voltage to be generated at the primary of the transformer. The flyback voltage is, through mutual inductance, transferred to the secondary of the transformer and “stepped-up” to approximately 200 to 300 volts. This voltage charges an ignition capacitor to approximately 200 to 300 volts. The ignition capacitor is selectively discharged by triggering the SCR to provide electrical energy to the ignition coil which generates a spark to ignite the fuel charge. 
     The current flowing through the IGBTs is monitored using a current sensing resistor connected in series with the IGBTs. The voltage across the current sensing resistor is “fed back” to the pulse width modulator. The pulse width modulator varies the width of the output pulses generated by the pulse width modulator to compensate for variations in the voltage of the power supply. Thus, as the voltage supplied by the alternator increases, the pulse width of the output of the pulse width modulator decreases. This allows the ignition system to operate effectively from a low voltage of approximately eight volts (which occurs upon engine cranking) to a high voltage of approximately 30 volts (which occurs during high speed engine operation). The use of current sensing to indirectly sense the variations of the supply voltage eliminates the need to compensate the ignition system for variations in the temperature of the system. 
     It is an advantage of the invention to provide a capacitive discharge ignition system that, in general, energizes the engine spark plug or spark plugs at a higher energy level or for a longer duration when the engine is operating under stratified fuel injection conditions than when the engine is operating under homogeneous engine operating conditions. 
     It is another advantage of the invention to provide an ignition system that increases the number of strike opportunities per ignition event when the fuel charge is stratified relative to when the fuel charge is homogeneous. 
     It is another advantage of the invention to provide the ignition sparks at a rate which at least exceeds 1000 hertz. 
     It is another advantage of the invention to provide an ignition system for an internal combustion engine that utilizes as its voltage source the voltage from the alternator. 
     It is another advantage of the invention to provide an ignition system for an internal combustion engine that utilizes a transformer which can accommodate larger voltage ranges. 
     It is another advantage of the invention to provide an ignition system for an internal combustion engine that charges the ignition capacitor using a flyback voltage. 
     It is another advantage of the invention to provide an ignition system which senses the current flowing through the transformer to eliminate the need for temperature compensation of the ignition system and improve the efficiency of the ignition system. 
     Other features and advantages of the invention are set forth in the following detailed description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross section of an internal combustion engine embodying the engine. 
     FIG. 2 is a block diagram of the ignition system for the internal combustion engine. 
     FIG. 3 is a detailed schematic of the input/logic multiplexer of the ignition system. 
     FIG. 4 is a detailed schematic of the DC—DC converter of the ignition system. 
     FIG. 5 is a detailed schematic of the ignition trigger circuit of the ignition system. 
     FIG. 6 is a detailed schematic of the ignition distribution circuit of ignition system. 
     FIG. 7 is a chart which plots ignition coil on time as a function of engine speed and throttle position. 
     FIG. 8 is a chart which plots the maximum ignition coil on time for a given engine speed. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Partially shown in FIG. 1 of the drawings is an internal combustion engine  10  embodying the invention. Although any internal combustion engine is appropriate, the internal combustion engine of the preferred embodiment is a two-stroke, direct injected, internal combustion engine having six cylinders (illustrated schematically and labelled  1 - 6  in FIG.  6 ). Cylinder  1  of the engine is illustrated in detail in FIG.  1 . The engine  10  includes a crankcase  14  defining a crankcase chamber  18  and having a crankshaft  22  rotatable therein. An engine block  26  defines the cylinder  1 . The engine block  26  also defines an intake port  30  communicating between the cylinder  1  and the crankcase chamber  18  via a transfer passage  34 . The engine block  26  also defines an exhaust port  38 . A piston  42  is reciprocally movable in the cylinder  1  and is drivingly connected to the crankshaft  22  by a crank pin  46 . The cylinder head  50  closes the upper end of the cylinder  1  so as to define a combustion chamber  54 . A spark plug  58  is mounted on the cylinder head  50  and extends into the combustion chamber  54 . 
     As shown schematically in FIG. 2 of the drawings, the internal combustion engine  10  also includes an ignition system  62  for providing an ignition spark to the spark plug  58  to ignite fuel in the cylinders  1 - 6 . The ignition system  62  illustrated in FIG. 2 may be used in an internal combustion engine having any number of cylinders. In the preferred embodiment of the invention, the ignition system  62  generates a plurality of ignition sparks (per cylinder, per cycle) when the fuel charge injected into the cylinder is stratified, and generates fewer sparks (per cylinder, per cycle) when the fuel charge injected into the cylinder is homogeneous. 
     In general terms, the ignition system  62  includes an electronic control unit (“ECU”)  66 , an input/logic multiplexer  70  (shown in detail in FIG.  3 ), a direct current to direct current (“DC—DC”) converter  74  (shown in detail in FIG.  3 ), an ignition trigger circuit  78  (shown in detail in FIG.  4 ), a silicon controlled rectifier (“SCR”)  82 , and an ignition distribution circuit  86  (shown in detail in FIG.  6 ). 
     Any ECU for an internal combustion engine could be used to operate the ignition system  62 . The ECU  66  generates an ignition control signal for each of the cylinders of the engine. In the embodiment of the engine shown in the drawings, the engine is a six cylinder engine and, accordingly, the ECU  66  generates six ignition control signals, i.e., one ignition control signal per engine cycle for each of the six cylinders. 
     FIG. 3 illustrates the input/logic multiplexer  70  of the ignition system  62 . As shown in FIG. 3, the ignition control signals from the ECU  66  (for cylinders one through six) are input to the input/logic multiplexer  70  on input lines  90 ,  94 ,  98 ,  102 ,  106 , and  110 . The input lines  90 ,  94 ,  98 ,  102 ,  106 , and  110  are connected to inverters  114 ,  118 ,  122 ,  126 ,  130 , and  134 , respectively. The inverters  114 ,  118 ,  122 ,  126 ,  130 , and  134  have outputs  138 ,  142 ,  146 ,  150 ,  154  and  158 , respectively. The outputs  138 ,  142  and  146  are connected to OR gate  162  and the outputs  150 ,  154  and  158  are connected to OR gate  166 . The outputs  170  and  174  of the OR gates  162  and  166 , respectively, are connected to OR gate  178  and to OR gate  182 . The input/logic multiplexer  70  also includes a delay circuit  190  connected to the output  194  of OR gate  178 . The delay circuit  190  includes resistor R 24 , diode D 10 , capacitor C 1  and resistor R 1 . The output of the delay circuit is connected to the input of OR gate  182  to completely combine or multiplex the ignition control signals from the ECU  66 . The output of OR gate  182  is connected to NAND gate  186  through resistor R 9 . A capacitor C 26  is connected to ground and to the inputs of NAND gate  186 . Resistor R 9  and capacitor C 26  form a time delay circuit. The time delay created by R 9  and C 26  allows the capacitor C 10  to completely discharge before receiving a subsequent energy pulse from the pulse width modulator  206 . If the time delay were not provided, the subsequent energy pulse from the pulse width modulator  206  would reach SCR  82  during the discharge of energy from the capacitor C 10 . This would result in SCR  82  being “held open” by the signal from the pulse width modulator  206 . 
     FIG. 4 illustrates the DC—DC converter  74  of the ignition system  62 . The DC—DC converter  74  includes a pulse width modulator  206 . The pulse width modulator  206  is a conventional component that is commercially available from a number of manufacturers. In the preferred embodiment, the pulse width modulator  206  is manufactured by National Semiconductor, Inc. and is marketed under part number LM2578. As shown in FIG. 4, the output  198  of NAND gate  186  is connected via node B to the oscillating input  202  (pin 3 of the LM2578 chip package) of pulse width modulator  206  through an RC circuit comprising resistors R 2 , R 14  and R 15 , capacitors C 6  and C 7 , and a diode D 11 . The pulse width modulator  206  also includes an inverted input  208  (pin 1 of the LM2578 chip package). In the preferred embodiment, pins 5 and 7 of the LM2578 chip package are connected to ground. The pulse width modulator  206  also has an output  210  (pin 6 of the LM2578 chip package) that is connected to a parallel connected bank of insulated gate bipolar transistors (“IGBTs”) Q 1 , Q 2  and Q 3 , through NAND gate  214 , and through a resistive network including resistors R 13 , R 53 , R 17  and diode D 18 . 
     As shown in the drawings, the IGBTs Q 1 , Q 2  and Q 3  include gates  218 ,  222 , and  226 , drains  230 ,  234  and  238 , and sources  242 ,  246  and  250 , respectively. The gates  218 ,  222  and  226  are connected (through the resistive network) to the output of the NAND gate  214 , and the drains  230 ,  234  and  238  are connected through resistors R 20 , R 21  and R 22 , respectively, to one end  254  of the primary winding  258  of a transformer  262 . The sources  242 ,  246 , and  250  are connected to ground via serially connected resistors R 11  and R 10 , and are also connected to the inverted input  208  of pulse width modulator  206 . 
     The opposite end  264  of the primary winding  258  is connected to a voltage source +V. In the preferred embodiment of the invention, the voltage source +v is the output of the internal combustion engine alternator (not shown). The transformer  262  also includes a secondary winding  266  connected at one end  270  to ground and at the opposite end  274  to diode D 9  and ignition capacitor C 10  through diode D 8 . The ignition capacitor C 10  is connected to the anode  278  of the SCR  82 . In the preferred embodiment, the transformer is a 1:2 step up transformer. 
     FIG. 5 illustrates the ignition trigger circuit  78  of the ignition system  62 . The ignition trigger circuit  78  includes an OR gate  282  having inputs  286  and  290  connected to the output of OR gate  178  via node A. The output  294  of the OR gate  282  is connected through an RC circuit including capacitor C 28  and resistor R 16  to a first input  298  of OR gate  302 . The second input  306  of the OR gate  302  is connected to the output  210  of the pulse with modulator  206  through OR gate  310 , an RC circuit including capacitor C 29  and resistor R 48 , NAND gate  314  and an RC circuit consisting of resistor R 49 , capacitor C 30  and resistor R 50 . The output  318  of the OR gate  302  is connected to one input  322  of NAND gate  326 . The other input  330  of NAND gate  326  is connected to the output of OR gate  178  from the input/logic multiplexer  70  via node A. The output  334  of the NAND gate  326  is connected through an RC circuit including resistors R 52  and R 51  and capacitor C 31  to the primary winding  338  (FIG. 5 only) of isolation transformer  342  (shown in FIGS.  4  and  5 ). Secondary winding  346  (FIG. 4 only) of the isolation transformer  342  is connected in parallel to diode D 31  and to the triggering gate  350  of the SCR  82 . The cathode  354  of the SCR  82  is connected via node D to the ignition distribution circuit  86  of the ignition system  62 . 
     Referring to FIG. 6, the ignition distribution circuit  86  includes ignition triggering modules  358 ,  362 ,  366 ,  370 ,  374  and  378 , for each of the internal combustion engine cylinders  1 ,  2 ,  3 ,  4 ,  5  and  6 , respectively. Each of the modules is identical and accordingly only the module  358  will be described in detail. The cathode  354  of SCR  82  is connected to the anode  382  of SCR  386 . The input  390  to the module  358  is connected to the ECU  66  to receive the ECU ignition control signal for cylinder  1 . The input  390  is connected to the base  394  of transistor Q 4  through the RC circuit which includes resistor R 45  and capacitor C 12 . The transistor Q 4  includes an emitter  398  connected to a voltage supply  402  and a collector  406  connected to ground through resistor R 46 . The collector  406  is also connected to the gate  410  of the SCR  386  through the RC circuit including resistor R 47 , diode D 6 , capacitor C 22  and resistor R 12 . The SCR  362  includes a cathode  414  that is connected to capacitor C 22  and resistor R 12  and to ignition coil  58  and diode  418  for the cylinder  1 . 
     Though other components and arrangements of components are possible, the resistors and capacitors employed in the preferred embodiment have the following values. 
     R 1 —510 Kohm, ⅛/watt; 
     R 2 -R 8 , R 14 , R 18 , R 24 —1 Kohm, ⅛ watt; 
     R 10 , R 11 , R 20 -R 22 —0.01 ohm, 2 watt; 
     R 12 , R 28 , R 32 , R 36 , R 40 , R 44 —100 ohm, ⅛ watt; 
     R 13 , R 53 —47 ohm, ¼ watt; 
     R 15 , R 17 —24 ohm, ⅛ watt; 
     R 16 —82 Kohm, ⅛ watt; 
     R 19 , R 26 , R 30 , R 34 , R 38 , R 42 , R 46 —10 Kohm, ⅛ watt; 
     R 25 , R 29 , R 33 , R 37 , R 41 , R 45 —3.3 Kohm, ⅛ watt; 
     R 27 , R 31 , R 35 , R 39 , R 43 , R 47 —56 ohm, ⅛ watt; 
     R 48 —249 Kohm, ⅛ watt; 
     R 49 —5.1 Kohm, ⅛ watt; 
     R 50 —750 Kohm; 
     R 51 , R 52 —150 ohm, ⅛ watt; 
     C 1 , C 28 -C 30 —0.001 microfarad; 
     C 2 , C 4 , C 5 —100 picofarad; 
     C 3 —330 microfarad; 
     C 6 —4700 picofarad; 
     C 7 , C 8 , C 9 , C 11 -C 13 , C 15 —0.022 microfarad; 
     C 10 —0.68 microfarad; 
     C 14 , C 17 -C 24 , C 31 -C 36 —0.1 microfarad; 
     C 16 , C 25 —100 microfarad. 
     The selection of the particular gates, diodes, SCRs, transistors and other components (employed in the ignition system  62 ) is within the realm of one of ordinary skill in the art. 
     In operation, the inputs  90 ,  94 ,  98 ,  102 ,  106  and  110  are normally at a high voltage level (typically five volts and referred to variously as “high” or “logical ‘1’”). In order to generate an ignition control signal at a particular input  90 ,  94 ,  98 ,  102 ,  106  or  110 , the ECU  66  “pulls” the input to a low voltage level (typically zero volts and referred to variously as “low” or “logical ‘0’”). The inputs  90 ,  94 ,  98 ,  102 ,  106  and  110  are inverted by inverters, respectively, and the ouputs of the inverters are “combined” or multiplexed by OR gates  162 ,  166 ,  178  and  182  and are buffered by NAND gate  186  for inputting to the DC—DC converter  74 . The output of the OR gate  178  is also input to the ignition trigger circuit  78  and to OR gate  182  through delay circuit  190 . The delay circuit  190  creates a time delay that allows the pulse width modulator  206  to continue to run even after the ignition control signal attributable to the previous cycle returns to the high condition. This assures that the ignition capacitor C 10  remains charged for the beginning of the current cycle, i.e., when the next ignition control signal from the ECU  66  “goes low”. 
     In response to the output of the input/logic multiplexer  70  (from NAND gate  186 ) the pulse width modulator  206  generates, on output  210 , an oscillating signal having a frequency of approximately between 1000 hertz and 4500 hertz, but which frequency is preferably approximately 3000 hertz (hz). The oscillating signal drives transistors Q 1 , Q 2 , and Q 3  at the 3000 hz frequency causing current from the alternator to flow through the primary winding  258  of the transformer  262 . 
     The rapid switching of the current through the transformer  262  generates a flyback voltage that is multiplied and transmitted, through mutual inductance of the transformer  262 , to the secondary winding  266  of the transformer  262 . The voltage appearing at the secondary winding  266  is approximately 200 to 300 volts. This voltage is stored momentarily by the ignition capacitor C 10  until the ignition capacitor C 10  is discharged by triggering of SCR  82 . 
     The current flow through the primary winding  258  of transformer  262  is monitored by placing current sensing resistors R 10  and R 11  in the current flow path and inputting the voltage across the resistors R 10  and R 11  to the inverted input  208  of pulse width modulator  206 . The pulse width of the pulse width modulator output  210  is changed or modulated in response to this voltage so that the ignition system  62  is effective through a wide range of alternator voltages, i.e., in the preferred embodiment, the alternator voltage range (through which the ignition circuit  62  is effective) is approximately 8 volts to approximately 30 volts. In effect, at low alternator voltages, the pulse width of the output  210  of the pulse width modulator  206  is increased to assure sufficient charge voltage for the ignition capacitor. As the alternator voltage rises, the pulse width of the output  210  of the pulse width modulator  206  decreases. At the beginning of a cycle, the initial trigger for the SCR  82  is generated by the ignition trigger circuit  78  because there is no output  210  from the pulse width modulator  206  to trigger (via trigger circuit  78 ) the SCR  82 . After the initial triggering event, the pulse width modulator output  210 , which is connected to the SCR  82  through the ignition trigger circuit  78 , is used to trigger the discharge of the ignition capacitor C 10 . 
     The ignition control signals from the ECU  66  are input to the appropriate ignition distribution modules of the ignition distribution circuit  86 . When a particular ignition control signal is generated by the ECU  66 , the ignition control signal triggers the SCR of the respective ignition distribution module and that SCR is “held” open until the ignition control signal is turned off by the ECU  66 . As long as the ignition distribution module SCR is held open, the energy discharged from the ignition capacitor C 10  is transmitted directly to the ignition coil and spark plug connected to that ignition distribution module. 
     The ignition system is capable of generating a varying number of ignition sparks at the spark plug to increase or decrease the total spark duration according to various engine operating conditions such as engine speed, engine load, throttle position etc. Though various combinations of desired total spark duration as a function of engine operating conditions are appropriate depending upon the circumstances, the desired total spark duration of the preferred embodiment is determined as a function of both the engine speed and the throttle position as set forth in the chart shown in FIG.  7 . Moreover, while the invention has been described in terms of generating a higher number of sparks under stratified engine operating conditions, the higher energy level could also be provided under stratified engine operating conditions in the form of a longer spark duration or a higher spark voltage or a combination of longer spark duration, higher spark voltage and higher number of sparks. 
     As shown in FIG. 7, on the “Y” axis of the chart, the numbers zero through one thousand represent relative throttle positions, zero representing the idle position of the throttle, and one thousand representing wide open throttle. The numbers along the “X” axis represent the speed of the engine as measured in crankshaft rotations per minute. The numbers in the body of the chart represent ignition spark on time measured in milliseconds. 
     Generally, the chart shows a trend toward decreasing the total spark duration (ignition coil on time) with increasing engine speed and with increasing throttle position. Based on the ignition coil on times shown in the chart, the highest number of sparks attained, with the pulse width modulator  206  operating at approximately 3000 hertz, is approximately fifteen (at 5.0 ms of ignition coil on time, e.g., at idle throttle position and 200 rpm), and the lowest number of sparks attained is one (at 0.1 ms of ignition coil on time, e.g., at 500 throttle position and 1100 rpm). At wide open throttle and 7000 rpm, two ignition sparks are generated (0.5 ms of ignition coil on time). 
     Though, as stated above, there is a general trend toward decreasing the ignition coil on time with increasing speed and increasing throttle position, the ignition coil on time does not decrease continuously with increasing speed and increasing throttle position. Rather, there exist some discontinuities in the general trend toward decreasing ignition coil on time with increasing engine speed and increasing throttle position. These discontinuities exist as a result of empirical evidence that the precise ignition coil on times shown in the chart result in improved engine performance. 
     FIG. 8 is a chart illustrating the maximum ignition coil on time allowed. Exceeding these on times will result in overlap of the ignition event between cylinders. 
     Various features and advantages of the invention are set forth in the following claims.
An ignition system for an internal combustion engine that generates more ignition sparks per ignition event, i.e., per cylinder, per cycle, when the engine is operated in the stratified fuel injected mode than when the engine is operated in the homogeneous fuel injection mode.