Patent Publication Number: US-2022213857-A1

Title: Control Device for Internal Combustion Engine

Description:
TECHNICAL FIELD 
     The present invention relates to a control device for an internal combustion engine. 
     BACKGROUND ART 
     In recent years, to improve the fuel efficiency of a vehicle, a control device for an internal combustion engine has been developed, the control device utilizing newly introduced techniques, such as a technique of operating the internal combustion engine by burning an air-fuel mixture thinner than an air-fuel mixture with a theoretical air-fuel ratio and a technique of recovering part of an exhaust gas resulting from combustion and introducing the exhaust gas into the air-fuel mixture again. 
     According to this type of control device for an internal combustion engine, the amount of fuel or air in a combustion chamber deviates from a theoretical value, and, consequently, a failure by an ignition plug in igniting the fuel is apt to occur. One method of solving this problem is to extend a discharge path generated between the electrodes of the ignition plug by increasing a discharge current of the ignition plug and suppress ignition failures by the extended discharge path. However, to increase the discharge current of the ignition plug, an ignition device increases the amount of charge/discharge. This leads to an increase in the calorific value or the volume of the ignition device. 
     Patent Literature 1 discloses a control device for an internal combustion engine according to which, by using two ignition coils, the number of ignition coils to be actuated is changed according to the likelihood of occurrence of an ignition failure under each operation condition. 
     CITATION LIST 
     Patent Literature 
     
         
         PLT 1: WO 2017/010310 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In general, a gas flow velocity in a cylinder increases as the number of revolutions of an engine and a filling factor increase. When the gas flow velocity is high, it is necessary that a longer discharge path be formed by outputting a large amount of power in a short time to increase opportunities for the gas and the discharge path to come in contact with each other. When the gas flow velocity is low, the discharge path cannot be made longer. It is necessary in this case that a small amount of power be outputted for a long time to form a shorter discharge path that lasts for a longer time, which increases opportunities for the gas and the discharge path to come in contact with each other. However, according to the technique disclosed in Patent Literature 1, because the likelihood of occurrence of an ignition failure needs to be reduced regardless of the flow velocity, a large amount of power is outputted for a long time. The calorific value and the volume of the ignition device, therefore, cannot be suppressed. 
     The present invention has been conceived in view of the above problems, and it is therefore an object of the invention to suppress the power consumption, the calorific value, and the volume of an ignition device in an internal combustion engine while suppressing failures in igniting fuel by an ignition plug. 
     Solution to Problem 
     A control device for an internal combustion engine according to the present invention includes an ignition control unit that controls energization of an ignition coil that gives electric energy to an ignition plug that discharges in a cylinder of the internal combustion engine to ignite fuel. The ignition control unit controls energization of the ignition coil so that first electric energy is released from the ignition coil while second electric energy is released as energy superposed on the first electric energy, the second electric energy changing based on a gas state around the ignition plug. 
     Advantageous Effects of Invention 
     According to the present invention, the power consumption, the calorific value, and the volume of an ignition device in an internal combustion engine can be suppressed as failures in igniting fuel by an ignition plug is suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram for explaining a configuration of a principle part of an internal combustion engine and a control device for the internal combustion engine according to an embodiment. 
         FIG. 2  is a partially enlarged view for explaining an ignition plug. 
         FIG. 3  is a functional block diagram for explaining a functional configuration of the control device according to the embodiment. 
         FIG. 4  is a diagram for explaining an electric circuit including the ignition coil according to the embodiment. 
         FIG. 5  is a diagram for explaining a relationship between an operating state of the internal combustion engine and a gas flow velocity around the ignition plug. 
         FIG. 6  is a diagram for explaining a relationship between a discharge path and a flow velocity, the relationship being observed between electrodes of the ignition plug. 
         FIG. 7  is a diagram for explaining a change in output power from the ignition coil, the change being caused by execution or non-execution of superposing discharge. 
         FIG. 8  is a diagram for explaining first superposing discharge control. 
         FIG. 9  is a diagram for explaining second superposing discharge control. 
         FIG. 10  is a diagram for explaining a relationship between a gas flow velocity between the electrodes and set values for ignition signals in the second superposing discharge control. 
         FIG. 11  is an example of a flowchart for explaining a method of controlling the ignition coil. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A control device for an internal combustion engine according to embodiments of the present invention will hereinafter be described. 
     Hereinafter, a control device  1 , which is one mode of the control device for the internal combustion engine according to one embodiment of the present invention, will be described. In the present embodiment, a case where the control device  1  controls discharge (ignition) of an ignition plug  200  provided in each of cylinders  150  making up a four-cylinder internal combustion engine  100  will be described exemplarily. 
     Hereinafter, in the embodiment, a combination of some or all of constituent elements of the internal combustion engine  100  and some or all of constituent elements of the control device  1  will be generally referred to as the control device  1  of the internal combustion engine  100 . 
     [Internal Combustion Engine] 
       FIG. 1  is a diagram for explaining a configuration of a principle part of the internal combustion engine  100  and an ignition device for the internal combustion engine. 
       FIG. 2  is a partially enlarged view for explaining electrodes  210  and  220  of an ignition plug  200 . 
     In the internal combustion engine  100 , external air taken into the internal combustion engine  100  flows through an air cleaner  110 , an air intake pipe  111 , and an air intake manifold  112 , and flows into each cylinder  150  when an air intake valve  151  opens. The amount of air flowing into each cylinder  150  is adjusted by a throttle valve  113 , and the amount of air adjusted by the throttle valve  113  is measured by a flow sensor  114 . 
     The throttle valve  113  is provided with a throttle opening sensor  113   a  that detects a degree of opening of a throttle. Information on the degree of opening of the throttle valve  113 , the degree of opening being detected by the throttle opening sensor  113   a , is outputted to the control device (electronic control unit or ECU)  1 . 
     As the throttle valve  113 , an electronic throttle valve driven by an electric motor is used. However, a different type of valve may also be used as the throttle valve  113 , providing that it can adjust an air flow rate properly. 
     The temperature of a gas flowing into each cylinder  150  is detected by an intake air temperature sensor  115 . 
     A crank angle sensor  121  is provided outside a ring gear  120  attached to a crankshaft  123  in such a way as to be on an extension of the radius of the ring gear  120 . The crank angle sensor  121  detects a rotation angle of the crankshaft  123 . In the embodiment, the crank angle sensor  121  detects the rotation angle of the crankshaft  123 , for example, in every 10-degree shift and in every combustion cycle. 
     A water jacket (not illustrated) of a cylinder head is provided with a water temperature sensor  122 . This water temperature sensor  122  detects the temperature of cooling water for the internal combustion engine  100 . 
     The vehicle is equipped with an accelerator position sensor (APS)  126  that detects an amount of displacement of an accelerator pedal  125  (amount of stepping on the accelerator pedal  125 ). The accelerator position sensor  126  detects the driver&#39;s required torque. The driver&#39;s required torque detected by the accelerator position sensor  126  is outputted to the control device  1 , which will be described later. The control device  1  controls the throttle valve  113 , based on the required torque. 
     Fuel stored in a fuel tank  130  is sucked and pressurized by a fuel pump  131 , flows through a fuel pipe  133  provided with a pressure regulator  132 , and is led to a fuel injection valve (injector)  134 . The fuel flowing out of the fuel pump  131  is adjusted in pressure by the pressure regulator  132  into the fuel with a given pressure, which is injected from the fuel injection valve (injector)  134  into each cylinder  150 . As a result of the pressure adjustment by the pressure regulator  132 , excess fuel is returned to the fuel tank  130  via a return pipe (not illustrated). 
     The cylinder head (not illustrated) of the internal combustion engine  100  is provided with a combustion pressure sensor (cylinder pressure sensor (CPS), which is also referred to as a cylinder internal pressure sensor)  140 . The combustion pressure sensor  140  is disposed in each cylinder  150 , and detects the internal pressure (combustion pressure) of the cylinder  150 . 
     As the combustion pressure sensor  140 , a piezoelectric-type or gauge-type pressure sensor is used. The combustion pressure sensor  140  is capable of detecting a combustion pressure (cylinder internal pressure) in the cylinder  150  over a wide temperature range. 
     Each cylinder  150  is fitted with an exhaust valve  152  and with an exhaust manifold  160  for discharging a gas (exhaust gas) resulting from combustion out of the cylinder  150 . A three-way catalyst  161  is provided on the exhaust side of the exhaust manifold  160 . When the exhaust valve  152  is opened, the exhaust gas is discharged from the cylinder  150  into the exhaust manifold  160 . The exhaust gas flows through the exhaust manifold  160 , is purified by the three-way catalyst  161 , and then is discharged into the air. 
     An upstream side air-fuel ratio sensor  162  is provided on the upstream side of the three-way catalyst  161 . The upstream side air-fuel ratio sensor  162  continuously detects the air-fuel ratio of the exhaust gas discharged from each cylinder  150 . 
     A downstream side air-fuel ratio sensor  163  is provided on the downstream side of the three-way catalyst  161 . The downstream side air-fuel ratio sensor  163  outputs a switch-like detection signal upon detecting an air-fuel ratio close to a theoretical air-fuel ratio. In the embodiment, the downstream side air-fuel ratio sensor  163  is provided as, for example, an  02  sensor. 
     The ignition plug  200  is provided above each cylinder  150 . Discharge by the ignition plug  200  causes a spark (ignition), which ignites an air-fuel mixture in the cylinder  150 , thus causing an explosion in the cylinder  150 , and the explosion pushes a piston  170  down. The piston  170  being pushed down causes the crankshaft  123  to rotate. 
     The ignition plug  200  is connected to an ignition coil  300  that generates electric energy (voltage) supplied to the ignition plug  200 . A voltage generated by the ignition coil  300  causes discharge between a center electrode  210  and an outer electrode  220  of the ignition plug  200  (see  FIG. 2 ). 
     As shown in  FIG. 2 , in the ignition plug  200 , the center electrode  210  is supported in an insulated state by an insulator  230 . A prescribed voltage (e.g., 20,000V to 40,000V according to the embodiment) is applied to the center electrode  210 . 
     The outer electrode  220  is grounded. When the prescribed voltage is applied to the center electrode  210 , discharge (ignition) occurs between the center electrode  210  and the outer electrode  220 . 
     It should be noted that in the ignition plug  200 , a voltage that causes dielectric breakdown of a gas component to induce discharge (ignition) fluctuates, depending on a state of a gas present between the center electrode  210  and the outer electrode  220  or a cylinder internal pressure. This voltage that induces discharge is referred to as a dielectric breakdown voltage. 
     Discharge control (ignition control) over the ignition plug  200  is carried out by an ignition control unit  83  of the control device  1 , which ignition control unit  83  will be described later. 
       FIG. 1  is referred to again. Output signals from various sensors mentioned above, such as the throttle opening sensor  113   a , the flow sensor  114 , the crank angle sensor  121 , the accelerator position sensor  126 , the water temperature sensor  122 , and the combustion pressure sensor  140 , are outputted to the control device  1 . Based on the output signals from these various sensors, the control device  1  detects an operation state of the internal combustion engine  100 , and controls an amount of air sent into the cylinder  150 , an amount of fuel injection, and ignition timing of the ignition plug  200 , and the like. 
     [Hardware Configuration of Control Device] 
     An overall hardware configuration of the control device  1  will then be described. 
     As shown in  FIG. 1 , the control device  1  includes an analog input unit  10 , a digital input unit  20 , an analog/digital (A/D) converter  30 , a random access memory (RAM)  40 , a micro-processing unit (MPU)  50 , a read only memory (ROM)  60 , an input/output (I/O) port  70 , and an output circuit  80 . 
     The analog input unit  10  receives incoming analog output signals from various sensors, such as the throttle opening sensor  113   a , the flow sensor  114 , the accelerator position sensor  126 , the upstream side air-fuel ratio sensor  162 , the downstream side air-fuel ratio sensor  163 , the combustion pressure sensor  140 , and the water temperature sensor  122 . 
     To the analog input unit  10 , the A/D converter  30  is connected. Analog output signals from various sensors inputted to the analog input unit  10  are subjected to signal processing, such as noise removal, are converted into digital signals by the A/D converter  30 , and are stored in the RAM  40 . 
     To the digital input unit  20 , a digital output signal from the crank angle sensor  121  is inputted. 
     The digital input unit  20  is connected to the I/O port  70 , and the digital output signal inputted to the digital input unit  20  is stored in the RAM  40  via the I/O port  70 . 
     Each output signal stored in the RAM  40  is arithmetically processed by the MPU  50 . 
     The MPU  50  executes a control program (not illustrated) stored in the ROM  60 , thereby arithmetically processing the output signal stored in the RAM  40  according to the control program. According to the control program, the MPU  50  calculates a control value that defines an operation amount of each actuator (e.g., the throttle valve  113 , the pressure regulator  132 , the ignition plug  200 , and the like) that drives the internal combustion engine  100 , and temporarily stores the control value in the RAM  40 . 
     The control value that defines the operation amount of the actuator, the control value being stored in the RAM  40 , is outputted to the output circuit  80  via the I/O port  70 . 
     The output circuit  80  includes a function of the ignition control unit  83  (see  FIG. 3 ) that controls a voltage applied to the ignition plug  200 . 
     [Functional Block of Control Device] 
     A functional configuration of the control device  1  according to the embodiment of the present invention will then be described. 
       FIG. 3  is a functional block diagram for explaining a functional configuration of the control device  1  according to one embodiment of the present invention. Functions of the control device  1  are each implemented by the output circuit  80 , for example, when the MPU  50  executes a control program stored in the ROM  60 . 
     As shown in  FIG. 3 , the output circuit  80  of the control device  1  according to a first embodiment includes an overall control unit  81 , a fuel injection control unit  82 , and an ignition control unit  83 . 
     The overall control unit  81  is connected to the accelerator position sensor  126  and to the combustion pressure sensor (CPS)  140 , and receives a required torque (acceleration signal S 1 ) from the accelerator position sensor  126  and an output signal S 2  from the combustion pressure sensor  140 . 
     Based on the required torque (acceleration signal S 1 ) from the accelerator position sensor  126  and on the output signal S 2  from the combustion pressure sensor  140 , the overall control unit  81  carries out overall control of the fuel injection control unit  82  and the ignition control unit  83 . 
     The fuel injection control unit  82  is connected to a cylinder identifying unit  84  that identifies each cylinder  150  of the internal combustion engine  100 , to an angle information creating unit  85  that measures a crank angle of the crankshaft  123 , and to a number-of-revolutions information creating unit  86  that measures the number of revolutions of the engine, and receives cylinder identifying information S 3  from the cylinder identifying unit  84 , crank angle information S 4  from the angle information creating unit  85 , and engine number-of-revolutions information S 5  from the number-of-revolutions information creating unit  86 . 
     The fuel injection control unit  82  is connected also to an air intake amount measuring unit  87  that measures an amount of air taken into the cylinder  150 , to a load information creating unit  88  that measures an engine load, and to a water temperature measuring unit  89  that measures the temperature of engine cooling water, and receives air intake amount information S 6  from the air intake amount measuring unit  87 , engine load information S 7  from the load information creating unit  88 , and cooling water temperature information S 8  from the water temperature measuring unit  89 . 
     The fuel injection control unit  82  calculates an injection amount and an injection time (fuel injection valve control information S 9 ) of the fuel injected from the fuel injection valve  134 , based on each piece of information received, and controls the fuel injection valve  134 , based on the calculated injection amount and injection time of the fuel. 
     The ignition control unit  83  is connected to the overall control unit  81  and to the cylinder identifying unit  84 , the angle information creating unit  85 , the number-of-revolutions information creating unit  86 , the load information creating unit  88 , and the water temperature measuring unit  89  as well, and receives pieces of information from these units. 
     Based on pieces of information received, the ignition control unit  83  calculates an amount of current supplied to a primary coil (not illustrated) of the ignition coil  300  (energization angle), an energization start time, and an energization end time at which current supply to the primary coil is cut off. The ignition coil  300  of the present embodiment has two types of primary coils, which will be described later. The ignition control unit  83 , therefore, calculates the energization angle, the energization start time, and the energization end time of each of these two types of primary coils. 
     Based on the calculated energization angle, energization start time, and energization end time, the ignition control unit outputs an ignition signal SA and an ignition signal SB respectively to the two primary coils of the ignition coil  300 , thereby carrying out discharge control (ignition control) of controlling discharge by the ignition plug  200 . 
     The function of the ignition control unit  83  of carrying out ignition control over the ignition plug  200  using the ignition signals SA and SB at least corresponds to the control device for the internal combustion engine according to the present invention. 
     [Electric Circuit for Ignition Coil] 
     An electric circuit  400  including the ignition coil  300  according to the embodiment of the present invention will then be described. 
       FIG. 4  is a diagram for explaining the electric circuit  400  including the ignition coil  300  according to one embodiment of the present invention. In the electric circuit  400 , the ignition coil  300  includes two types of primary coils, i.e., primary coils  310  and  360  each having a winding tuned a given number of times, and a secondary coil  320  having a winding turned a number of times greater than the number of times the winding of each of the primary coils  310  and  360  is turned. Now, at the time of ignition by the ignition plug  200 , power from the primary coil  310  is first supplied to the secondary coil  320 , and then power from the primary coil  360  is supplied to the secondary coil  320 , as power superposed on the power from the primary coil  310 . 
     Hereinafter, in view of this process, the primary coil  310  will be referred to as a “primary main coil”, and the primary coil  360  will be referred to as a “primary sub-coil”. In addition, a current flowing through the primary main coil  310  will be referred to as “primary main current”, and a current flowing through the primary sub-coil  360  will be referred to as “primary sub-current”. 
     One end of the primary main coil  310  is connected to a DC power supply  330 . Hence a given voltage (according to the embodiment, for example, 12V) is applied to the primary main coil  310 . 
     The other end of the primary main coil  310  is connected to an igniter  340 , and is grounded via the igniter  340 . The igniter  340  is provided as a transistor, a field effect transistor (FET), or the like. 
     A base (B) terminal of the igniter  340  is connected to the ignition control unit  83 . The ignition signal SA outputted from the ignition control unit  83  is inputted to the base (B) terminal of the igniter  340 . The ignition signal SA inputted to the base (B) terminal of the igniter  340  puts a collector (C) terminal and an emitter (E) terminal of the igniter  340  in a conductive state to each other, thus causing a current flow between the collector (C) terminal and the emitter (E) terminal. As a result, the ignition signal SA outputted from the ignition control unit  83  flows to the primary main coil  310  of the ignition coil  300  via the igniter  340 , which causes a primary main current to flow through the primary main coil  310 , where power (electric energy) is accumulated. 
     When outputting the ignition signal SA from the ignition control unit  83  ceases to cut off the primary main current flow in the primary main coil  310 , a high voltage is generated at the secondary coil  320 , the high voltage corresponding to a ratio of the number of turns of the secondary coil  320  to the number of turns of the primary main coil  310 . 
     One end of the primary sub-coil  360  is connected to the DC power supply  330  through a common node to which one end of the primary main coil  310  is connected. Thus, the given voltage (according to the embodiment, for example, 12V) is applied also to the primary sub-coil  360 . 
     The other end of the primary sub-coil  360  is connected to an igniter  350  and is grounded via the igniter  350 . The igniter  350  is provided as a transistor, a field effect transistor (FET), or the like. 
     A base (B) terminal of the igniter  350  is connected to the ignition control unit  83 . The ignition signal SB outputted from the ignition control unit  83  is inputted to the base (B) terminal of the igniter  350 . The ignition signal SB inputted to the base (B) terminal of the igniter  350  puts a collector (C) terminal and an emitter (E) terminal of the igniter  350  in a conductive state to each other, the conductive state corresponding to a change in voltage of the ignition signal SB, thus causing a current to flow between the collector (C) terminal and the emitter (E) terminal, the current corresponding to the change in voltage of the ignition signal SB. As a result, the ignition signal SB outputted from the ignition control unit  83  flows to the primary sub-coil  360  of the ignition coil  300  via the igniter  350 , which causes a primary sub-current to flow through the primary sub-coil  360 , where power (electric energy) is generated. 
     When output of the ignition signal SB from the ignition control unit  83  changes to cause a change in the primary sub-current flowing through the primary sub-coil  360 , a high voltage is generated at the secondary coil  320 , the high voltage corresponding to a ratio of the number of turns of the secondary coil  320  to the number of turns of the primary sub-coil  360 . 
     The high voltage generated at the secondary coil  320  by the ignition signal SB is added to the high voltage generated at the secondary coil  320  by the ignition signal SA, and the sum of both high voltages is applied to the ignition plug  200  (the center electrode  210 ). This creates a potential difference between the center electrode  210  and the outer electrode  220  of the ignition plug  200 . When the potential difference created between the center electrode  210  and the outer electrode  220  becomes equal to or larger than a dielectric breakdown voltage Vm for the gas (air-fuel mixture in the cylinder  150 ), the dielectric strength of the gas component breaks down, causing discharge between the center electrode  210  and the outer electrode  220 , which ignites the fuel (air-fuel mixture). 
     Through the operation of the electric circuit  400  as described above, the ignition control unit  83  controls energization of the ignition coil  300 , using the ignition signals SA and SB. Through this energization control, the ignition control unit  83  carries out ignition control of controlling the ignition plug  200 . 
     [Controlling Energization of Ignition Coil] 
     Controlling energization of the ignition coil  300  according to one embodiment of the present invention will then be described. The ignition control unit  83  outputs the ignition signal SA and the ignition signal SB respectively to the igniter  340  and the igniter  350 , thereby controlling energization of the primary main coil  310  and the primary sub-coil  360 . In this energization control, a gas state around the ignition plug  200  in the cylinder  150  is estimated, and based on the estimated gas state, energization of the primary main coil  310  and the primary sub-coil  360  is controlled so that electric energy is released from the primary main coil  310  to the secondary coil  320  while electric energy is released from the primary sub-coil  360  to the secondary coil  320 , as electric energy superposed on the electric energy released from the primary main coil  310 . This energization control (which will hereinafter be referred to as superposing discharge control) by the ignition control unit  83  will hereinafter be described. 
       FIG. 5  is a diagram for explaining a relationship between an operating state of the internal combustion engine  100  and a gas flow velocity around the ignition plug  200 . As shown in  FIG. 5 , in general, as the number of revolutions of the engine and load current get higher, a gas flow velocity in the cylinder  150  gets higher and therefore a gas flow velocity around the ignition plug  200  too gets higher. Thus, the gas flows at high speed between the center electrode  210  and the outer electrode  220  of the ignition plug  200 . In the internal combustion engine  100  in which exhaust gas recirculation (EGR) is performed, an EGR rate is set according to a relationship between the number of revolutions of the engine and the load, for example, in a manner as shown in  FIG. 5 . Expanding a high EGR area, in which the EGR rate is set higher, achieves low fuel consumption and less exhaust gas but leads to more frequent ignition failures by the ignition plug  200 . 
       FIG. 6  is a diagram for explaining a relationship between a discharge path and a flow velocity, the relationship being observed between the electrodes of the ignition plug  200 . 
     When a high voltage is generated in the secondary coil  320  of the ignition coil  300  and dielectric breakdown occurs between the center electrode  210  and the outer electrode  220  of the ignition plug  200 , a discharge path is formed and remains between the electrodes of the ignition plug  200  until a current flowing between these electrodes becomes equal to or less than a certain value. When the combustible gas comes in contact with the discharge path, a flame core grows and develops into combustion. Because the discharge path moves under the influence of a gas flow between the electrodes, a higher gas flow velocity results in formation of a longer discharge path in a shorter time while a lower gas flow velocity results in formation of a shorter discharge path.  FIG. 6( a )  shows an example of a discharge path  211  that is formed when the gas flow velocity is high, and  FIG. 6( b )  shows an example of a discharge path  212  that is formed when the gas flow velocity is low. 
     When the internal combustion engine  100  is operated at a high EGR rate, the probability that the flame core grows as a result of the combustible gases&#39; coming into contact with the discharge path becomes lower. In this case, therefore, an opportunity for the combustible gas to come in contact with the discharge path needs to be increased. Because the discharge path is generated by breaking the dielectric strength of the gas, as described above, if the current necessary for maintaining the discharge path is constant, power output corresponding to the length of the discharge path is required. When the gas flow velocity is high, therefore, it is preferable that energization of the ignition coil  300  be controlled in such a way as to allow the ignition coil  300  to output large power to the ignition plug  200  in a short time, and that as a result of such energization control, the long discharge path  211  shown in  FIG. 6( a )  be formed to give the discharge path an opportunity to come in contact with the gas in a wider space. When the gas flow velocity is low, on the other hand, it is preferable that energization of the ignition coil  300  be controlled in such a way as to cause the ignition coil  300  to keep outputting small power to the ignition plug  200  for a long time, and that as a result of such energization control, the short discharge path  212  shown in  FIG. 6( b )  be maintained to give the discharge path an opportunity to come in contact with the gas flowing near the electrodes of the ignition plug  200  for a long time. 
     According to the present embodiment, the ignition coil  300  including the primary main coil  310  and the primary sub-coil  360 , which have been described with reference to  FIG. 4 , is adopted, and superposing discharge control using the ignition signals SA and SB is carried out on the ignition coil  300  to allow the ignition plug  200  to perform the above-described discharge process. 
       FIG. 7  is a diagram for explaining a change in output power from the ignition coil  300 , the change being caused by execution or non-execution of superposing discharge.  FIG. 7( a )  shows a relationship between the output waveform of the ignition signal SA, output power from the ignition coil  300 , and power required for gas combustion in a case of executing no superposing discharge, and  FIG. 7 ( b )  shows a relationship between the output waveforms of the ignition signals SA and SB, output power from the ignition coil  300 , and power required for gas combustion in a case of executing superposing discharge. 
     As described above, while the ignition control unit  83  keeps outputting the ignition signal SA, the primary main coil  310  accumulates electric energy. As a result, as shown in  FIGS. 7( a ) and 7( b ) , output power  71  from the primary main coil  310  of the ignition coil  300  gradually increases. At this time, the primary main current flows in the primary main coil  310  because of a constant voltage supplied from the power supply, and heat is generated in an amount corresponding to a time the current keeps flowing. When output of the ignition signal SA ends, the electric energy having been accumulated in the primary main coil  310  is released, which starts power supply to the ignition plug  200  via the secondary coil  320 . As a result, as shown in  FIGS. 7 ( a ) and 7( b ) , the output power  71  from the primary main coil  310  decreases as the amount of electric energy in the primary main coil  310  decreases. 
     In the case of executing superposing discharge, on the other hand, the following process results. While the ignition control unit  83  keeps outputting the ignition signal SB, the primary sub-coil  360  releases electric energy corresponding in size to the primary sub-current flowing through the primary sub-coil  360 , thus supplying power to the ignition plug  200  via the secondary coil  320 . As a result, as shown in  FIG. 7( b ) , the output power  71  from the primary main coil  310  and output power  72  from the primary sub-coil  360  of the ignition coil  300  are superposed together, and the sum of these output power is supplied to the ignition plug  200 . 
     To allow discharge by the ignition plug  200  to cause gas combustion, two kinds of power, i.e., power for dielectric breakdown and power for maintaining the discharge path are basically required. The power required for maintaining the discharge path, as described above, varies depending on the gas flow velocity between the electrodes, and large power supplied for a short time is needed when the gas flow velocity is high, while small power supplied for a long time is needed when the gas flow velocity is low. In  FIGS. 7( a ) and 7( b ) , a  FIG. 73  represents power for dielectric breakdown, a  FIG. 74  represents power required for maintaining the discharge path when the gas flow velocity is high, and a  FIG. 75  represents power required for maintaining the discharge path when the gas flow velocity is low. 
     In the example shown in  FIG. 7( a ) , the  FIGS. 74 and 75  both stick out from a figure representing the output power  71 . This demonstrates a fact that required power is not supplied in both cases of high flow velocity and low flow velocity. Consequently, the ignition plug  200  cannot maintain the discharge path during its discharge process, in which case the discharge path short-circuits. As a result, the distance and the maintaining time of the discharge path becomes insufficient, which leads to a shortage of opportunities for the discharge path and the gas to come in contact with each other, thus causing a failure in gas combustion. To solve this problem only by the output power  71  from the primary main coil  310 , the primary main coil  310  of a large size is needed because a sufficient amount of electric energy must be ensured. This, however, poses a problem that a charge time increases and, consequently, heat generation by the ignition coil  300  increases. 
     In the example shown in  FIG. 7( b ) , on the other hand, the  FIGS. 74 and 75  are both within an area representing the sum of the output power  71  and the output power  72 . This demonstrates a fact that required power is supplied in both cases of high flow velocity and low flow velocity. In other words, by executing superposing discharge using two types of primary coils (the primary main coil  310  and the primary sub-coil  360 ), the occurrence of a combustion failure in the internal combustion engine  100  can be suppressed in both cases of high flow velocity and low flow velocity. Besides, because such superposing discharge can be made executable by merely adding a control board to the ignition coil  300 , executing superposing discharge is more efficient in suppressing an increase in the volume of the ignition coil  300  than the case of increasing the amount of electric energy the primary main coil  310  is charged with. 
     However, in the example of  FIG. 7( b ) , the output time of the ignition signal SA and of the ignition signal SB is t=6, and the sum of these signal output times is Σt=12. This is two times as large as the output time of the ignition signal SA shown in  FIG. 7( a ) . In this manner, in superposing discharge depicted in  FIG. 7( b ) , a difference between discharge power from the ignition coil  300  and power required for forming and maintaining the discharge path between the electrodes of the ignition plug  200  is large. As a result, power efficiency turns out to be low. 
     In the present embodiment, to improve power efficiency in superposing discharge, the ignition control unit  83  estimates a gas state around the ignition plug  200  in the cylinder  150 , and changes the output time of the ignition signal SA and the output time and output timing of the ignition signal SB, based on the estimated gas state. As a result, energization of the ignition coil  300  is controlled so that electric energy accumulated by the primary main coil  310  is released from the ignition coil  300  while electric energy accumulated by the primary sub-coil  360 , the electric energy changing based on a gas state around the ignition plug  200 , is released as electric energy superposed on the electric energy accumulated by the primary main coil  310 . 
     [First Superposing Discharge Control] 
     First superposing discharge control according to one embodiment of the present invention will then be described. In the first superposing discharge control, the output time and output timing of the ignition signal SB are changed in a manner described below, based on a gas flow velocity around the ignition plug  200 . 
       FIG. 8  is a diagram for explaining the first superposing discharge control.  FIG. 8 ( a )  shows a relationship between the output waveforms of the ignition signals SA and SB, output power from the ignition coil  300 , and power required for gas combustion in the low flow velocity case where the gas flow velocity is low, and  FIG. 8 ( b )  shows a relationship between the output waveforms of the ignition signals SA and SB, output power from the ignition coil  300 , and power required for gas combustion in the high flow velocity case where the gas flow velocity is high. 
     Generally, when the internal combustion engine  100  is operated at a low EGR rate, time of ignition is delayed because the phase of a combustion centroid needs to be corrected as a combustion speed increases. As a result of delaying the time of ignition, the volume of a combustion chamber at the time of ignition reduces, which makes the gas in the cylinder  150  the gas with a low flow velocity. In this case, therefore, the power for dielectric breakdown, which is represented by the  FIG. 73 , and the power required for maintaining the discharge path in the case of low flow velocity, which is represented by the  FIG. 75 , need to be supplied from the ignition coil  300  to the ignition plug  200 , as shown in  FIG. 8( a ) . 
     In the first superposing discharge control, the ignition signal SB is output following output of the ignition signal SA in the case of low flow velocity, as shown in  FIG. 8( a ) . In this case, in comparison with the case of  FIG. 7( b ) , the output time of the ignition signal SB is reduced to t=2, which gives a signal output time Σt=8, the signal output time being the sum of the output times of the ignition signals SA and SB. This improves power efficiency. In  FIG. 8( a ) , however, a part of the  FIG. 75  sticks out of the area representing the sum of the output power  71  and the output power  72 . This means that the discharge path cannot be maintained for a necessary period in the case of low flow velocity, which raises a concern that a failure in gas combustion may occur. 
     When the internal combustion engine  100  is operated at a high EGR rate, in general, the time of ignition is advanced because the phase of the combustion centroid needs to be corrected as the combustion speed decreases. As a result of advancing the time of ignition, the volume of the combustion chamber at the time of ignition increases, which makes the gas in the cylinder  150  the gas with a high flow velocity. In this case, therefore, the power for dielectric breakdown, which is represented by the  FIG. 73 , and the power required for maintaining the discharge path in the case of high flow velocity, which is represented by the  FIG. 74 , need to be supplied from the ignition coil  300  to the ignition plug  200 , as shown in  FIG. 8( b ) . 
     According to the first superposing discharge control, in the case of high flow velocity, a phase difference is set between the ignition signal SA and the ignition signal SB, and the ignition signal SB is output at a point of time delayed from the end of output of the ignition signal SA by a time span equivalent to the phase difference, as shown in  FIG. 8( b ) . At this time, the output time of the ignition signal SB in the case of  FIG. 7( b )  is reduced by the time span equivalent to the phase difference, to t=4. This improves power efficiency. In  FIG. 8( b ) , however, a part of the  FIG. 74  sticks out of the area representing the sum of the output power  71  and the output power  72 . This means that a long discharge path cannot be formed in the case of high flow velocity, which raises a concern that a failure in gas combustion may occur. In addition, the signal output time that is the sum of the output times of the ignition signal SA and the ignition signal SB is Σt=10, which is larger than the signal output time Σt=8 shown in  FIG. 8( a ) . 
     As described above, in the first superposing discharge control, power efficiency can be improved when the gas flow velocity is low, but the sufficient discharge path cannot be formed in both cases of low flow velocity and high flow velocity. In addition, a difference in the gas flow velocity results in a difference in the signal output time that is the sum of the output times of the ignition signal SA and the ignition signal SB. As a result, due to design-related considerations, a measure against heat generation by the ignition coil  300  needs to be taken in accordance with a condition under which the longer signal output time results. This leads to lower hardware efficiency. 
     [Second Superposing Discharge Control] 
     Second superposing discharge control according to one embodiment of the present invention will then be described. In the second superposing discharge control, the output time of the ignition signal SA and the output time and output timing of the ignition signal SB are changed in a manner described below, based on a gas flow velocity around the ignition plug  200 . 
       FIG. 9  is a diagram for explaining the second superposing discharge control.  FIG. 9( a )  shows a relationship between the output waveforms of the ignition signals SA and SB, output power from the ignition coil  300 , and power required for gas combustion in the low flow velocity case where the gas flow velocity is low, and  FIG. 9( b )  shows a relationship between the output waveforms of the ignition signals SA and SB, output power from the ignition coil  300 , and power required for gas combustion in the high flow velocity case where the gas flow velocity is high. 
     According to the second superposing discharge control, in the case of low flow velocity, the output time of the ignition signal SA is reduced to t=4 as a phase difference is set between the ignition signal SA and the ignition signal SB, and the ignition signal SB is output at a point of time delayed from the end of output of the ignition signal SA by a time span equivalent to the phase difference, as shown in  FIG. 9( a ) . At this time, the signal output time that is the sum of the output times of the ignition signal SA and the ignition signal SB is Σt=8. In  FIG. 9( a ) , the  FIGS. 73 and 75  are both within the area representing the sum of the output power  71  and output power  72 . This indicates that the discharge path can be maintained for a necessary period in the case of low flow velocity. 
     In the case of high flow velocity, on the other hand, the output time of the ignition signal SA is set t=6 while the output time of the ignition signal SB is reduced to t=2, as shown in  FIG. 9( b ) . Zero phase difference is set between the ignition signal SA and the ignition signal SB, so that the ignition signal SB is output right after output of the ignition signal SA. At this time, the signal output time that is the sum of the output times of the ignition signal SA and the ignition signal SB is Σt=8, which is the same as the signal output time in the case of low flow velocity. In  FIG. 9( b ) , the  FIGS. 73 and 74  are both within the area representing the sum of the output power  71  and output power  72 . This indicates that a long discharge path can be formed in the case of high flow velocity. 
     As described above, according to the second superposing discharge control, the primary sub-current is controlled, by adjusting the output time and output timing of the ignition signal SB, so that as the gas flow velocity around the ignition plug  200  becomes higher, timing of the primary sub-current&#39;s flowing through the primary sub-coil  360  is made earlier while a period of the primary sub-current&#39;s flowing through the primary sub-coil  360  is made shorter. In addition, the primary main current is controlled, by adjusting the output time of the ignition signal SA, so that as the gas flow velocity around the ignition plug  200  becomes higher, a period of the primary main current&#39;s flowing through the primary main coil  310  is made longer. It is preferable in this case that the primary main current and the primary sub-current be controlled so that the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  becomes equal to or shorter than the discharge period of the primary main coil  310 . Hence, in both cases of low flow velocity and high flow velocity, a difference between discharge power from the ignition coil  300  and power required for forming and maintaining the discharge path between the electrodes of the ignition plug  200  is reduced, which allows formation of a sufficient discharge path while improving power efficiency. 
     Further, according to the second superposing discharge control, the primary main current and the primary sub-current are controlled so that even when the gas flow velocity changes, the signal output time given by summing up the output times of the ignition signal SA and the ignition signal SB, that is, the sum of the period of the primary main current&#39;s flowing through the primary main coil  310  and the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  becomes constant. As a result, a measure against heat generation by the ignition coil  300  set under the same condition can be taken, regardless of the gas flow velocity. This improves hardware efficiency. 
       FIG. 10  is a diagram for explaining a relationship between a gas flow velocity between the electrodes and set values for the ignition signals SA and SB in the second superposing discharge control. 
       FIG. 10( a )  shows a relationship between the gas flow velocity and a charge time of the primary main coil  310 . As indicated in  FIG. 10( a ) , the ignition control unit  83  sets the output time of the ignition signal SA so that the charge time of the primary main coil  310  becomes longer as the gas flow velocity between the electrodes becomes higher. In a case where the gas flow velocity remains the same, the output time of the ignition signal SA is set so that the charge time of the primary main coil  310  becomes longer as the EGR rate becomes higher. 
       FIG. 10( b )  shows a relationship between the gas flow velocity and a superposing discharge time of the primary sub-coil  360 . As indicated in  FIG. 10( b ) , the ignition control unit  83  sets the output time of the ignition signal SB so that the superposing discharge time of the primary sub-coil  360  during discharge by the primary main coil  310  becomes shorter as the gas flow velocity between the electrodes becomes higher. In a case where the gas flow velocity remains the same, the output time of the ignition signal SB is set so that the superposing discharge time of the primary sub-coil  360  becomes longer as the EGR rate becomes higher. 
       FIG. 10( c )  shows a relationship between the gas flow velocity and a phase difference between a discharge start time of the primary main coil  310  and a discharge start time of the primary sub-coil  360 . As indicated in  FIG. 10( c ) , the ignition control unit  83  sets output timing of the ignition signal SA and of the ignition signal SB so that as the gas flow velocity between the electrodes becomes higher, the phase difference between the discharge start time of the primary main coil  310  and the discharge start time of the primary sub-coil  360  becomes shorter and, consequently, timing of starting discharge by the primary sub-coil  360  becomes earlier. 
     As described above, by determining respective output times and output timing of the ignition signals SA and SB according to the gas flow velocity between the electrodes, power required for ignition, the power changing depending on the gas flow velocity between the electrodes, can be supplied as power with less excess or shortage, from the ignition coil  300  to the ignition plug  200 . 
     It should be noted that in setting the ignition signals SA and SB according to the gas flow velocity between the electrodes under the second superposing discharge control, as described above, any given pattern of setting may be selectively carried out. For example, the ignition signal SB may be set so that the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  is set constant and timing of the primary sub-current&#39;s flowing is made earlier as the gas flow velocity around the ignition plug  200  becomes higher. Alternatively, the ignition signal SB may be set so that timing of the primary sub-current&#39;s flowing through the primary sub-coil  360  is set constant and the period of the primary sub-current&#39;s flowing is made shorter as the gas flow velocity around the ignition plug  200  becomes higher. Through these approaches, power required for ignition, the power changing depending on the gas flow velocity between the electrodes, can be supplied as power adjusted within a certain range, from the ignition coil  300  to the ignition plug  200 . 
     [Method for Controlling Ignition Coil] 
     A method of controlling the ignition coil  300  by the ignition control unit  83  when the first and second superposing discharge controls are carried out will then be described. FIG.  11  is an example of a flowchart for explaining a method of controlling the ignition coil  300  by the ignition control unit  83  according to one embodiment of the present invention. In the present embodiment, when the ignition switch of the vehicle is turned on to supply power to the internal combustion engine  100 , the ignition control unit  83  starts controlling the ignition coil  300  according to the flowchart of  FIG. 11 . It should be noted that steps shown in the flowchart of  FIG. 11  represent steps for one cycle of the internal combustion engine  100 , and that the ignition control unit  83  executes the steps shown in the flowchart of  FIG. 11  for each cycle. 
     At step S 201 , the ignition control unit  83  detects an operation condition for the internal combustion engine  100 , and estimates a flow velocity and an EGR rate of the gas. Specifically, for example, the ignition control unit  83  stores values for a gas flow velocity and an EGR rate that are determined in advance for each operation condition, as map information, and substitutes the detected number of revolutions of the engine and an estimated load in the map information, thereby obtaining values for a gas flow velocity and an EGR rate that correspond to a current operation state of the internal combustion engine  100 . 
     At step S 202 , the ignition control unit  83  calculates a coil charging period. Specifically, for example, the ignition control unit  83  stores the relationship between the gas flow velocity and the charge time of the primary main coil  310 , the relationship being shown in  FIG. 10( a ) , as map information, and substitutes the flow velocity and the EGR rate obtained at step S 201  in the map information, thereby obtaining a value for the charge time of the primary main coil  310 . 
     At step S 203 , the ignition control unit  83  calculates a superposing discharge period. Specifically, for example, the ignition control unit  83  stores the relationship between the gas flow velocity and the superposing discharge time of the primary sub-coil  360 , the relationship being shown in  FIG. 10( b ) , as map information, and substitutes the flow velocity and the EGR rate obtained at step S 201  in the map information, thereby obtaining a value for the superposing discharge time of the primary sub-coil  360 . 
     At step S 204 , the ignition control unit  83  calculates a phase difference. Specifically, for example, the ignition control unit  83  stores the relationship between the gas flow velocity and the phase difference between the discharge start time of the primary main coil  310  and the discharge start time of the primary sub-coil  360 , the relationship being shown in  FIG. 10( c ) , as map information, and substitutes the flow velocity and the EGR rate obtained at step S 201  in the map information, thereby obtaining a value for a phase difference between discharge by the primary main coil  310  and discharge by the primary sub-coil  360 . 
     At step S 205 , the ignition control unit  83  sets calculated values. Specifically, the ignition control unit  83  stores respective values for the coil charge period, the superposing discharge period, and the phase difference, the values being calculated at steps S 202  to S 204 , in a storage area of the ignition control unit  83 , so that the ignition signals SA and SB reflecting these calculated values are output in the next and subsequent cycles of ignition control. After setting the calculated values at step S 205 , the ignition control unit  83  ends control of the ignition coil  300  that is executed according to the flowchart of  FIG. 11 . 
     The embodiment of the present invention described above offers the following effects. 
     (1) The control device  1  for the internal combustion engine includes the ignition control unit  83  that controls energization of an ignition coil  300  that gives electric energy to the ignition plug  200  that discharges in the cylinder  150  of the internal combustion engine  100  to ignite the fuel. The ignition control unit  83  controls energization of the ignition coil  300  so that first electric energy is released from the ignition coil  300  while second electric energy is released as electric energy superposed on the first electric energy, the second electric energy changing based on a gas state around the ignition plug  200 . Hence the power consumption, the calorific value, and the volume of the ignition coil  300  in the internal combustion engine  100  can be suppressed as failures in igniting the fuel by the ignition plug  200  are suppressed. 
     (2) The ignition coil  300  has the primary main coil  310  and the primary sub-coil  360  that are disposed respectively on the primary side, and the secondary coil  320  disposed on the secondary side. The ignition control unit  83  controls the primary main current flowing through the primary main coil  310 , and controls also the primary sub-current flowing through the primary sub-coil  360 , based on the gas state around the ignition plug  200 . Specifically, the ignition control unit  83  controls the primary sub-current so that as the gas flow velocity around the ignition plug  200  becomes higher, timing of the primary sub-current&#39;s flowing through the primary sub-coil  360  is made earlier and the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  is made shorter. In addition, the ignition control unit  83  controls the primary main current so that as the gas flow velocity around the ignition plug  200  becomes higher, the period of the primary main current&#39;s flowing through the primary main coil  310  is made longer. Hence, in both cases of low flow velocity and high flow velocity, the difference between discharge power from the ignition coil  300  and power required for forming and maintaining the discharge path between the electrodes of the ignition plug  200  is reduced, which allows formation of a sufficient discharge path while improving power efficiency. 
     (3) The ignition control unit  83  controls the primary main current and the primary sub-current so that even if the gas flow velocity around the ignition plug  200  changes, the sum of the period of the primary main current&#39;s flowing through the primary main coil  310  and the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  remains constant. As a result, a measure against heat generation by the ignition coil  300  set under the same condition can be taken, regardless of the gas flow velocity. This improves hardware efficiency. 
     (4) It is preferable that the ignition control unit  83  control the primary main current and the primary sub-current so that the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  becomes equal to or shorter than the discharge period of the primary main coil  310 . This limits the period in which the primary sub-coil  360  carries out superposing discharge to a period of a necessary length, thus allowing power saving. 
     (5) The ignition control unit  83  controls the primary main current and the primary sub-current so that as the EGR rate of the internal combustion engine  100  becomes higher, the period of the primary main current&#39;s flowing through the primary main coil  310  and the period of the primary sub-current&#39;s flowing through the primary sub-coil  360  are made longer. At this time, the primary sub-current is controlled so that even if the EGR rate of the internal combustion engine  100  changes, timing of the primary sub-current&#39;s flowing through the primary sub-coil  360  remains constant. Hence, in the internal combustion engine  100  in which exhaust gas recirculation is performed, optimum power according to the EGR rate can be supplied from the ignition coil  300  to the ignition plug  200 . 
     In the embodiment described above, each of the functional components of the control device  1  described with reference to  FIG. 3  may be provided, as stated above, in the form of software executed by the MPU  50  or in the form of hardware, such as a field-programmable gate array (FPGA). These software-based components and hardware-based components may be used in combination. 
     Embodiments and various modifications are described above as examples. Other modifications may be made providing that such modifications do not impair the features of the invention. These embodiments and modifications have been described herein, but the present invention is not limited to these embodiments and modifications. Other modes that can be conceived in the range of technical concept of the present invention are also included in the scope of the present invention. 
     REFERENCE SIGNS LIST 
     
         
           1  control device 
           10  analog input unit 
           20  digital input unit 
           30  A/D converter 
           40  RAM 
           50  MPU 
           60  ROM 
           70  I/O Port 
           80  output circuit 
           81  overall control unit 
           82  fuel injection control unit 
           83  ignition control unit 
           84  cylinder identifying unit 
           85  angle information creating unit 
           86  number-of-revolutions information creating unit 
           87  air intake amount measuring unit 
           88  load information creating unit 
           89  water temperature measuring unit 
           100  internal combustion engine 
           110  air cleaner 
           111  air intake pipe 
           112  air intake manifold 
           113  throttle valve 
           113   a  throttle opening sensor 
           114  flow sensor 
           115  intake air temperature sensor 
           120  ring gear 
           121  crank angle sensor 
           122  water temperature sensor 
           123  crankshaft 
           125  accelerator pedal 
           126  accelerator position sensor 
           130  fuel tank 
           131  fuel pump 
           132  pressure regulator 
           133  fuel pipe 
           134  fuel injection valve 
           140  combustion pressure sensor 
           150  cylinder 
           151  air intake valve 
           152  exhaust valve 
           160  exhaust manifold 
           161  three-way catalyst 
           162  upstream side air-fuel ratio sensor 
           163  downstream side air-fuel ratio sensor 
           170  piston 
           200  ignition plug 
           210  center electrode 
           220  outer electrode 
           230  insulator 
           300  ignition coil 
           310  primary main coil 
           320  secondary coil 
           330  DC power supply 
           340 ,  350  igniter 
           360  primary sub-coil 
           400  electric circuit