CROSS REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims priority from Japanese Patent Applications 2006-21962, filed Jan. 31, 2006, and 2006-198667, filed Jul. 20, 2006, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ignition system for an internal combustion engine, and particularly to a multiple-spark ignition system for igniting the fuel in the engine by multiple electric ignition sparks each ignition timing. 
   2. Description of the Related Art 
   P2003-521619A (WO01/055588) discloses a multiple-spark ignition system, in which a switching means is turned on or off according to the amount of the primary current flowing from a battery through the primary coil and the amount of the secondary current (discharging current) flowing through the secondary coil of an ignition coil. In more detail, the switching means turns off to generate multiple ignition sparks at a spark plug when the amount of the primary current increases to a threshold value, and turns on to start charging electric energy in the primary coil when the amount of the secondary current decreases to another threshold value. 
   However, if the battery voltage fluctuates when the primary coil is charged, the charging time of the primary coil varies. If the charging time increases, a ignition-less period in which the ignition spark is not generated increases, resulting in that the fuel ignition performance gets worse. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the invention is to provide an improved ignition system that can give a good fuel ignition performance even if the battery voltage fluctuate. 
   Another object of the invention is to provide an ignition system for a lean burn engine that can ignite swirled lean air-fuel mixture gas passing by a spark plug at a high speed. 
   According to a feature of the invention, an ignition system for an internal combustion engine includes an ignition coil, electric power supply means for supplying primary current to the primary coil of the ignition coil at a prescribed voltage, switching means for switching on and off the primary current at controlled timings to discharge the electric energy from the secondary coil to the spark plug, engine condition detecting means for detecting a signal relating to flow speed of air-fuel-mixture gas in the engine, ignition control means for controlling the switching means to provide multiple ignition sparks in a predetermined ignition period. The above ignition control means controls the switching means to maintain each of the ignition sparks according to the signal relating to the flow speed of air-fuel-mixture gas in the engine. 
   In the above featured ignition system: the electric power supply means may include a battery and a boosting DC-DC converter; the ignition control means may include an ignition control circuit for controlling the switching means to maintain each of the ignition sparks until the signal relating to flow speed of air-fuel mixture gas becomes a predetermined value. 
   In this ignition system: the engine condition detecting means may include a resistor connected in series with the secondary coil of the ignition coil and the ignition control circuit controls the switching means to maintain each of the ignition sparks until the amount of the secondary current of the ignition coil detected by the resistor becomes a prescribed amount; or the ignition control circuit may change the prescribed amount according to the signal detected by the engine condition detecting means or the prescribed amount as the predetermined ignition period nears its end. 
   In the above ignition system, the ignition control circuit may turn on the switching means to charge the ignition coil with electric energy for a prescribed charging time before providing each of the multiple ignition sparks that are generated when the switching means is turned off to discharge the electric energy from the ignition coil. In this ignition system, the ignition control circuit increases the prescribed charging time as engine rotation speed becomes lower. 
   In the above featured system: the electric power supply means may include a battery, a power supply circuit and a capacitor discharge circuit that stores electric energy supplied from the battery and discharges the electric energy into the ignition coil and the ignition control circuit may control the capacitor discharge circuit to store the electric energy for a prescribed charging time before each of the multiple ignition sparks that are generated when the switching means is turned on to discharge the electric energy from the capacitor discharge circuit into the ignition coil. 
   In the above ignition system the capacitor discharge circuit may include an energy accumulation coil connected to the battery, a second switching means for switching on and off current supplied to the energy accumulation coil and an energy storing capacitor for storing electric energy of the energy accumulation coil generated by the switching operation of the second switching means. 
   According to another feature of the invention, an ignition system for an internal combustion engine includes an ignition coil, electric power supply means for supplying primary current to the primary coil of the ignition coil at a prescribed voltage, switching means for switching on and off the primary current at controlled timings to discharge the electric energy from the secondary coil of the ignition coil to the spark plug, a secondary current detecting element for detecting secondary current; an ignition control means for controlling the switching means to provide multiple ignition sparks in a predetermined period at the spark plug. The ignition control means is arranged to control the switching means to maintain each of the ignition sparks until the amount of the secondary current becomes a prescribed amount. 
   According to another feature of the invention, an ignition system for an internal combustion engine includes an ignition coil, a battery, an electric power supply circuit for supplying current to the primary coil of the ignition coil at a voltage higher than a voltage of the battery, switching means for switching on and off the primary current at multiple controlled timings to charge and discharge the ignition coil with electric energy, a spark plug connected to the secondary coil, a secondary current detecting element for detecting secondary current supplied from the secondary coil to the spark plug, and ignition control means for controlling the switching means to provide multiple ignition sparks in a predetermined period at the spark plug. The ignition control means controls the switching means to maintain each of the ignition sparks until the amount of the secondary current detected by the secondary current detecting element becomes a prescribed amount. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
       FIG. 1  is a schematic circuit diagram illustrating an ignition system according to the first embodiment of the invention; 
       FIG. 2  is a time diagram showing operation of main portions of the ignition system; 
       FIG. 3A  is a time diagram showing a relation between a drive signal and secondary current of an ignition coil of the ignition system; 
       FIG. 3B  is a time diagram showing a relation between a drive signal and primary current of the ignition coil; 
       FIG. 4  is a graph showing a relation between the flow rate of fuel gas and the amount of spark keeping current; 
       FIG. 5  is a graph showing a relation between voltage applied to the primary coil of an ignition coil and energy-charging time of the ignition coil; 
       FIG. 6A  is a graph showing a relation between engine rotation speed and energy charging time; 
       FIG. 6B  is a graph showing a relation between an ignition period in which multiple sparks are generated and the energy charging time; 
       FIG. 7  is a schematic circuit diagram illustrating an ignition system according to the second embodiment of the invention; 
       FIG. 8  is a time diagram showing operation of main portions of the ignition system according to the second embodiment; and 
       FIGS. 9A ,  9 B and  9 C are graphs showing waveforms of secondary current of other modified ignition systems. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention will be described with reference to the appended drawings. 
   An ignition system according to the first embodiment of the invention will be described with reference to  FIGS. 1-6A  and  6 B. 
   As shown in  FIG. 1 , the ignition system includes an ignition coil  10 , a power supply circuit  11 , a battery  12 , an insulated gate bipolar transistor (hereinafter referred to as the IGBT)  13 , an ignition control circuit  14 , a spark plug  15 , a zener diode  16 , a current measuring resistor  17 , an ECU  20 , etc. 
   The ignition coil  10  has a primary coil  10   a  and a secondary coil  10   b . The primary coil  10   a  has a pair of ends, one of which connected via the power supply circuit  11  to a high side (e.g. 12 V) terminal of the battery  12  and the other of which is connected to a ground via the IGBT  13 . 
   The IGBT  13  has a gate connected to the ignition control circuit  14 , which controls the switching operation of the IGBT  13 . The power supply circuit  11  is constituted of a common boosting DC-DC converter that includes an inductor, a switching element and a capacitor to provide a voltage to be applied to the primary coil  10   a . The secondary coil  10   b  has a pair of ends, one of which is connected to the spark plug  15  and the other of which is connected to the ground via the zener diode  16  and the current measuring resistor  17 . The voltage signal of the current measuring resistor  17  is inputted to the ignition control circuit  14 . 
   The ECU  20  includes a microcomputer constituted of a CPU, a RAM, a ROM, etc. and executes various control programs that are stored in the ROM to control an internal combustion engine. The ECU  20  inputs engine operation data such as an engine rotation speed and an accelerator position and calculate a suitable ignition timing and ignition period in which multiple sparks are generated based on the data to output an ignition timing signal IGt and ignition period signal IGw to the ignition control circuit  14 . 
   The ignition control circuit  14  provides a drive signal IG to control the switching operation of the IGBT  13  according to the ignition timing signal IGt and the ignition period signal IGw. 
   In more detail, the ignition control circuit  14  turns off the IGBT  13  according to the ignition timing signal Igt to generate the first ignition spark at the ignition timing. Thereafter, the ignition control circuit  14  turns on and off the IGBT  13  repeatedly to generate multiple ignition sparks at the spark plug  15 . 
   As shown in  FIG. 2 , the first ignition spark (secondary voltage V 2 ) is generated at timing t 11 , and the multiple ignition sparks are generated in the period between timing t 11  and t 14 , in which the IGBT  13  is cyclically turned on and off. At a timing t 10  before the first spark timing t 11 , the level of the ignition timing signal IGt becomes high (H). Consequently, the level of the drive signal IG becomes H to turn on the IGBT  13 . Accordingly, primary current I 1  flows in the primary coil  10   a  to charge electric energy into the ignition coil  10 . When the level of the ignition timing signal IGt becomes low (L) at the spark timing t 11 , the level of the drive signal IG becomes L to turn off the IGBT  13 . As a result, the first ignition voltage V 2  is generated by the secondary coil  10   b  to cause the first ignition spark at the spark plug  15 , so that the secondary current I 2  flows across the spark plug  15 . 
   The multiple-spark signal IGw is also rises up to the H level at the spark timing t 11 . Therefore, the drive signal IG rises up to the H level at timing t 12  to turn on the IGBT  13  for a period Tc until timing t 13  to make the primary current flow in the primary coil  10   a , thereby charging the ignition coil  10  with a sufficient electric energy. Subsequently, the drive signal falls down to the L-level to turn off the IGBT  13  again, thereby discharging the electric energy to generate ignition spark again. Thereafter, the drive signal IG repeatedly changes its level to turn on and off the IGBT  13  to generate multiple sparks at the spark plug  15  until the level of the multiple-spark signal IGw becomes L at timing t 14 . 
   Incidentally, the electric energy necessary for generating ignition spark at the spark plug  15  changes as the flow speed of air-fuel-mixture-gas in the engine cylinder changes. If the flow speed of the air-fuel-mixture-gas becomes higher, the amount of the secondary current decreases as the on-off operation of the IGBT  13  is repeated, as shown in  FIG. 3A  . On the other hand, the amount of the secondary current increases as the on-off operation of the IGBT  13  is repeated, as shown in  FIG. 3B . 
   The inventor has noticed that the amount of the secondary current (hereinafter referred to as the spark maintenance current) Ik that is necessary to maintain stable ignition sparks increases as the flow speed of the air-fuel mixture gas increases, as shown in  FIG. 4 . Further, the period in which a certain amount of electric energy is charged into the ignition coil becomes shorter as the input voltage applied to the primary coil  10   a  increases, as shown in  FIG. 5 . Therefore, it is effective to provide the primary coil  10   a  with a suitable amount of primary current in the charging time Tc (e.g. 0.4 m sec) that the input voltage V 1  is controlled according to the amount of the secondary current. 
   Thus, the ignition control circuit  14  is arranged to make the level of the drive signal IG high to turn on IGBT  13  to supply the primary current I 1  when the amount of the secondary current (or spark current) becomes as large as a threshold value (hereinafter referred to as the maintenance current value) Ik, which provide secondary voltage for causing multiple ignition sparks, as shown in  FIG. 2 . At the same time, the power supply circuit  11  is arranged to provide supply voltage of a level Vo that is sufficient to provide the primary current  11  for charging the primary coil with the electric energy in the charging time Tc even if the battery voltage lowers to a minimum level (e.g. 12 V). Incidentally, the,.supply voltage Vo can be controlled to provide a suitable electric energy by the ignition control circuit  14  according to the amount of the secondary current that is measured by the current measuring resistor  17 . 
   In the ignition system according to the first embodiment of the invention, the charging time may be changed according to the engine rotation speed. As shown in  FIG. 6A , the suitable charging time Tc decreases as the engine rotation speed increases. As shown in  FIG. 6B , available time for charging the ignition coil  10  increases as the ignition period increases. 
   The ignition control circuit  14  may provide a charging time control means (or program) for increasing the charging time Tc as the engine rotation speed decreases. This embodiment is effective to reduce working loads of the power supply circuit  11 , the battery  12 , the IGBT  13 , the spark plug  15 , etc. 
   The current measuring resistor  17  may be replaced by some other means that measures a value representing the flow speed of air-fuel-mixture-gas in the engine cylinder, such as the engine rotation speed, the cylinder charging efficiency. 
   An ignition system according to the second embodiment of the invention will be described with reference to  FIGS. 7 and 8 . 
   Incidentally, the same reference numeral as the first embodiment represents the same or substantially the same portion, part or component as the first embodiment, hereafter. 
   As shown in  FIG. 7 , the ignition system includes a capacitor-discharge circuit (hereinafter referred to as the CD circuit)  30  in addition to the ignition coil  10 , power supply circuit  11 , battery  12 , the IGBT  13 , the ignition control circuit  14 , the spark plug  15 , the current measuring resistor  17  and the an ECU  20 . The zener diode  16  that is connected in series with the current measuring resistor  17  is replaced by a backflow prevention diode  35  that is connected in series with the primary coil  10  and the IGBT  13 . It may be considered that the CD circuit  30  is included in the power supply circuit  11 . 
   The CD circuit  30  includes a series circuit of an energy accumulation coil  31  and a second IGBT  32 , a diode  33  and, an energy accumulation capacitor  34 . The first series circuit is connected between the battery  12  and the ground. The diode  33  has the anode connected with the series circuit between the coil  33  and the IGBT  32  and the cathode connected with the end of the primary coil  10   a  that is connected to the power supply circuit  11  through the backflow prevention diode  35 . 
   When the ignition control circuit  14  provides the gate of the second IGBT  32  with a second drive signal DS, the IGBT  32  turns on to charge the energy accumulation coil  31  with an amount of electric energy. In more detail, the ignition control circuit  14  turns on the IGBT  32  according to the second drive signal Ds to introduce current into the energy accumulation coil  31 . Thereafter, the ignition control circuit  14  turns off the IGBT  32  to discharge the electric energy accumulated by the coil  31  to the energy accumulation capacitor  34 , which also stores the electric energy. The diode  35  prevents back flow of the current from the capacitor  34  to the power supply circuit  11 . 
   As shown in  FIG. 8 , the first ignition spark is generated at timing t 11 , and the multiple ignition sparks are generated in the period between timing t 11  and t 14 , in which the main IGBT  13  and the second IGBT  32  are,cyclically turned on and off. Incidentally, the second IGBT  32  turns off to discharge the energy accumulation coil  31  when the main IGBT  13  turns on to provide an ignition spark. 
   At a timing t 10  before the first spark timing t 11 , the level of the ignition timing signal IGt to make the second drive signal DS high (H) so that the second IGBT  32  can turn on to charge the energy accumulation coil  31 . Subsequently, at the spark timing t 11 , the level of the main drive signal IG becomes H to turn on the IGBT  13 , while the level of the second drive signal Ds becomes L to turn off the second IGBT  32 . Accordingly, electric energy of the capacitor  34  is discharged into the ignition coil  10  to generate the first ignition voltage V 2  to cause the first ignition spark at the spark plug  15 . That is, the secondary current I 2  flows across the spark plug  15 , while the energy accumulation coil  31  is charged. 
   The ignition control circuit  14  is arranged to make the level of the second drive signal Ds high to turn on the second IGBT  32  and the level of the main drive signal IG low to turn off the main IGBT  13  at timing t 12  where the amount of the secondary current (or spark current) decreases and becomes as large as the maintenance current value Ik. As a result, the energy accumulation coil  31  is charged again with an amount of electric energy that is sufficient to generate the ignition spark. The ignition control circuit  14  repeats the above control operation to generate multiple sparks at the spark plug  15  until the level of the multiple-spark signal IGw becomes L at timing t 14 . 
   The power supply circuit  11  is also arranged to provide supply voltage of a level Vo sufficient to provide the current for charging the energy accumulation coil  31  with the electric energy in the charging time Tc even if the battery voltage lowers to a minimum level. 
   The supply voltage Vo can be controlled to provide a suitable electric energy by the ignition control circuit  14  according to the amount of the secondary current that is measured by the current measuring resistor  17 . 
   As shown in  FIGS. 9A and 9B , the ignition control circuit  14  can linearly or non-linearly increase the level (absolute value) of the maintenance current Ik as the multiple-ignition sparks are continued. This is to increase the electric energy that is discharged into the primary coil  10   a  of the ignition coil  10  as the engine piston gets close to the upper dead center, at which the flow speed of the air-fuel-mixture gas in the engine cylinder is maximum. The ignition control circuit  14  can also increase the charging time Tc in addition to the level of the maintenance current Ik as shown in  FIG. 9C . 
   The level of the maintenance current Ik may be controlled according to the flow speed of the air-fuel-mixture gas by taking the relation between the maintenance current and the flow speed of the air-fuel-mixture gas shown in  FIG. 4  into account. 
   In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.
An ignition system for an internal combustion engine includes an ignition coil, electric power supply circuit, a switching transistor, engine condition detecting element or unit that detects a signal relating to flow speed of air-fuel-mixture gas in the engine, and ignition control unit that controls the switching transistor to provide multiple ignition sparks in a predetermined ignition period. The ignition control unit controls the switching transistor to maintain each of the ignition sparks according to the signal relating to the flow speed of air-fuel-mixture gas in the engine to maintain sufficient spark energy for igniting the air-fuel mixture gas.