Ignition control system

An ignition control system performs discharge generation control, in which a discharge spark is generated, once or a plurality of times during a single combustion cycle. The ignition control system successively calculates an approximate energy density based on a secondary current and a discharge path length. During a predetermined period after blocking of a primary current is performed during a single combustion cycle, the ignition control system calculates an integrated value by integrating the discharge path length at this time, based on the approximate energy density being greater than a predetermined value. The ignition control system performs the discharge generation control again based on the calculated integrated value being less than a first threshold.

BACKGROUND

Technical Field

The present disclosure relates to an ignition control system that is used in an internal combustion engine.

Background Art

In recent years, to improve fuel efficiency of internal combustion engines for automobiles, technologies related to combustion control (lean burn engines) of lean fuel and exhaust gas recirculation (EGR) in which a combustible air-fuel mixture is circulated to a cylinder of the internal combustion engine are being studied. In these technologies, as an ignition system for effectively burning fossil fuel that is contained in the air-fuel mixture, a multiple ignition system in which a spark plug continuously performs discharge a plurality of times at an ignition timing of the internal combustion engine is sometimes used.

SUMMARY

The present disclosure is an ignition control system that is applied to an internal combustion engine that includes a spark plug, an ignition coil including a primary coil and a secondary coil, a voltage value detecting unit, and a secondary current detecting unit. The ignition control system performs discharge generation control, in which a discharge spark is generated, once or a plurality of times during a single combustion cycle. The ignition control system successively calculates an approximate energy density based on a secondary current and a discharge path length. During a predetermined period after blocking of a primary current is performed during a single combustion cycle, the ignition control system calculates an integrated value by integrating the discharge path length at this time, based on the approximate energy density being greater than a predetermined value. The ignition control system performs the discharge generation control again based on the calculated integrated value being less than a first threshold.

DESCRIPTION OF THE EMBODIMENTS

In this multiple ignition system, a problem arises in that electrode wear in the spark plug and power consumption in an ignition coil that supplies a high voltage to the spark plug increase by an extent amounting to the execution of a plurality of discharge operations during a single ignition cycle. In addition, wastefulness in terms of energy, that is, discharge operations being unnecessarily repeated even in cases in which the air-fuel mixture can be favorably ignited by an initial discharge also occurs.

As a countermeasure against the foregoing issues, the following related technology is known. That is, when a voltage peak of a secondary voltage that is applied to an ignition coil exceeds a determination threshold during a capacitive discharge period, an accumulated time of an exceedance interval during which the voltage peak exceeds the determination threshold or an accumulated value of the secondary voltage during the exceedance intervals is measured. Then, based on the measured accumulated time of the exceedance interval or the accumulated value of the secondary voltage during the exceedance intervals, whether an air-fuel mixture is in a combustion state or a misfire state is determined.

The related technology describes that, during execution of capacitive discharge, between cases in which the air-fuel mixture is burned and cases in which misfire occurs, the secondary voltage that is detected when the air-fuel mixture is burned is lower. A reason for this is thought to be that combustion ions are produced as a result of the air-fuel mixture being ignited by the discharge that is generated in the spark plug. As a result of the combustion ions being present between the electrodes in the spark plug, a secondary current easily flows between the electrodes of the spark plug. Consequently, discharge resistance decreases. In accompaniment, the secondary voltage that is applied to the spark plug decreases.

Here, in a high flow field in which a flow rate of airflow inside a combustion chamber is high, an assumption can be made that the combustion ions that are produced by the air-fuel mixture being ignited are carried away by the airflow and the combustion ions that are present between the electrodes of the spark plug decrease. In this state, the discharge resistance hardly decreases. In accompaniment, the secondary voltage that is applied to the spark plug hardly decreases. In this case, in the related technology, even when the air-fuel mixture is in the combustion state, because the secondary voltage applied to the spark plug is in a high state, an erroneous determination that the air-fuel mixture is in a misfire state may be made. In this regard, there is room for improvement in determination control for determining a combustion state of an air-fuel mixture.

It is thus desired to provide an ignition control system that is capable of more accurately estimating a combustion state of a combustible air-fuel mixture, and improving the combustion state of the combustible air-fuel mixture by causing a spark plug to perform re-discharge, as required.

An exemplary embodiment of the present disclosure provides an ignition control system that is applied to an internal combustion engine that includes: a spark plug that generates, between a pair of discharge electrode, a discharge spark for igniting a combustible air-fuel mixture inside a cylinder of the internal combustion engine; an ignition coil that includes a primary coil and a secondary coil, and applies a secondary voltage to the spark plug through the secondary coil; a voltage value detecting unit that detects a voltage value of at least either of a primary voltage that is applied to the primary coil and the secondary voltage that is applied to the spark plug; and a secondary current detecting unit that detects a secondary current that flows to the spark plug.

The ignition control system includes a primary current control unit, a discharge path length calculating unit, an approximate energy density calculating unit, and an integrated value calculating unit.

The primary current control unit performs discharge generation control, in which the discharge spark is generated in the spark plug, once or a plurality of times during a single combustion cycle by causing blocking of a primary current to the primary coil to be performed after conduction of the primary current is performed.

The discharge path length calculating unit successively calculates a discharge path length as a length of the discharge spark that is formed between the discharge electrodes based on the voltage value detected by the voltage value detecting unit.

The approximate energy density calculating unit that successively calculates an approximate energy density that serves as an approximate value of energy density that is energy per unit length of the discharge spark by dividing the secondary current detected by the secondary current detecting unit by the discharge path length calculated by the discharge path length calculating unit.

During a predetermined period after blocking of the primary current is performed during the single combustion cycle, the integrated value calculating unit calculates an integrated value by integrating the discharge path length calculated at this time by the discharge path length calculating unit, based on the approximate energy density calculated by the approximate energy density calculating unit being greater than a predetermined value.

The primary current control unit performs the discharge generation control again based on the integrated value calculated by the integrated value calculating unit being less than a first threshold.

In the present disclosure, it has been found that a discharge spark of which the energy density of the discharge spark that is calculated by discharge energy determined by a product of the secondary current and the secondary voltage being divided by the discharge path length is greater than a predetermined value contributes to the combustion of the combustible air-fuel mixture, whereas a discharge spark of which the energy density is less than the predetermined value hardly contributes to the combustion of the combustible air-fuel mixture. In addition, a variation width of the secondary current during a discharge period in which discharge is generated in the spark plug is large at about 200 to 0 [mA].

Meanwhile, a variation width of a secondary induction discharge voltage (maintained voltage) is small at about 0.5 to 10 [kV]. From the foregoing, it has been found that variations in the secondary voltage in a tip portion of a spark of which the current is large is moderate (in other words, the variation width of the second voltage is small), and the secondary current is a more dominant parameter in terms of determining the magnitude of the value of discharge energy.

In addition, in accompaniment with the findings, it has been found that the energy density of the discharge spark can be approximated by dividing the secondary current by the discharge path length. In addition, when the energy density of the discharge spark is the same, a relationship is such that the discharge energy of the discharge spark increases and a surface area of the discharge spark increases as the discharge path length increases. From this relationship, it is clear that the discharge path length is a parameter that accurately reflects the magnitude of the discharge energy of the discharge spark.

Based on the foregoing, it has been found that whether the discharge spark that is generated in the spark plug contributes to the combustion of the combustible air-fuel mixture can be estimated from the approximate energy density. Furthermore, whether the combustion state of the combustible air-fuel mixture is favorable can be more accurately estimated based on the integrated value of the discharge path length of the discharge spark of which the approximate energy density is greater than the predetermined value.

Therefore, in the present ignition control system, the approximate energy density calculating unit is provided. The approximate energy density calculating unit successively calculates the approximate energy density that is an approximate value of the energy density of the discharge spark by dividing the secondary current detected by the secondary current detecting unit by the discharge path length calculated by the discharge path length calculating unit.

In addition, under a condition that the approximate energy density calculated by the approximate energy density calculating unit is greater than the predetermined value during the predetermined period after blocking of the primary current is performed during a single combustion cycle, the integrated value calculating unit calculates the integrated value by integrating the discharge path length that is calculated by the discharge path length calculating unit. That is, the calculated integrated value is an integrated value of the discharge path length of the discharge spark that contributes to the combustion of the combustible air-fuel mixture during the predetermined period.

Therefore, when the integrated value that is integrated during the predetermined period is less than the first threshold, the combustion state of the combustible air-fuel mixture can be estimated as not being favorable. Therefore, under a condition that the integrated value calculated by the integrated value calculating unit is less than the first threshold, the discharge generation control is performed again by the primary current control unit. As a result, the combustion state of the combustible air-fuel mixture can be made favorable. Meanwhile, when the integrated value calculated by the integration value calculating unit is greater than the first threshold, the combustion state of the combustible air-fuel mixture can be estimated as being favorable.

Therefore, as a result of the discharge generation control not being performed by the primary current control unit again, energy being unnecessarily consumed in the spark plug can be suppressed. In addition, as a result of the present control being performed through use of the approximate energy density instead of the energy density, a calculation step for discharge energy can be omitted (in other words, a calculation step for calculating a product of the secondary current and the secondary voltage can be omitted). Furthermore, a calculation circuit that is required to perform the present control can be simplified.

With reference toFIG. 1, an engine system10includes an engine11that is a spark-ignition-type multiple-cylinder internal combustion engine. Here,FIG. 1shows an example of only a single cylinder among the plurality of cylinders provided in the engine11.

The engine system10performs control to change an air-fuel ratio of an air-fuel mixture to a rich side or a lean side in relation to a theoretical air-fuel ratio, based on an operation state of the engine11. For example, the engine system10performs control to change the air-fuel ratio of the air-fuel mixture to the lean side when the operation state of the engine11is within an operation range of low rotation and low load.

A combustion chamber11band a water jacket11care formed inside an engine block11athat configures a main body portion of the engine11. The engine block11ais provided so as to house a piston12capable of back-and-forth motion. The water jacket11cis a space through which a cooling liquid (also referred to as cooling water) can flow. The water jacket11cis provided so as to surround the periphery of the combustion chamber11b.

An intake port13and an exhaust port14are formed so as to be communicable with the combustion chamber11b, in a cylinder head that is an upper portion of the engine block11a. In addition, an intake valve15, a discharge valve16, and a valve driving mechanism17are provided in the cylinder head. The intake valve15is provided to control a communication state between the intake port13and the combustion chamber11b. The exhaust valve15is provided to control a communication state between the exhaust port14and the combustion chamber11b. The valve driving mechanism17is provided to enable the intake valve15and the exhaust valve16to perform opening/closing operations at predetermined timings.

An intake manifold21ais connected to the intake port13. The intake manifold21aincludes an electromagnetic-drive-type injector18to which high-pressure fuel is supplied from a fuel supply system. The injector18is a port-injection-type fuel injection valve that sprays fuel towards the intake port13in accompaniment with energization.

A surge tank21bis arranged on an upstream side of the intake manifold21ain an intake flow direction. An exhaust pipe22is connected to the exhaust port14.

An EGR passage23is provided so as to be capable of introducing a portion of exhaust gas that is discharged from the exhaust pipe22into intake air (hereafter, the exhaust gas that is introduced into the intake air is referred to as EGR gas) by connecting the exhaust pipe22and the surge tank21b. An EGR control valve24is disposed on the EGR passage23. The EGR control valve24is provided so as to be capable of controlling an EGR rate (a mixing ratio of the EGR gas in gas before combustion that is drawn into the combustion chamber11b) based on a degree of opening thereof. Therefore, the EGR passage23and the EGR control valve24correspond to an exhaust recirculation mechanism.

A throttle valve25is disposed in an intake pipe21on an upstream side of the surge tank21bin the intake flow direction. A degree of opening of the throttle valve25is controlled by an operation of a throttle actuator26, such as a direct-current (DC) motor. In addition, an airflow control valve (corresponding to an airflow generating portion)27for generating a swirl flow and a tumble flow is provided near the intake port13.

A catalyst41, such as a three-way catalyst, for purifying CO, HC, NOx, and the like in the exhaust gas is provided in the exhaust pipe22. An air-fuel ratio sensor40(such as a linear air-fuel [A/F] sensor) for detecting the air-fuel ratio of the air-fuel mixture, with the exhaust gas as a detection target, is provided on an upstream side of the catalyst41.

The engine system10includes an ignition circuit unit31, an electronic control unit32, and the like.

The ignition circuit unit31is configured to generate a discharge spark in a spark plug19. The discharge spark is generated to ignite fuel air-fuel mixture inside the combustion chamber11b. The electronic control unit32is a so-called engine electronic control unit (ECU). The electronic control unit32is configured to control operations of each unit including the injector18and the ignition circuit unit31, based on the operation state of the engine11that is acquired based on output from various sensors such as a crank angle sensor33(referred to, hereafter, in an abbreviated manner as “engine parameters”).

Regarding ignition control, the electronic control unit32is configured to generate an ignition signal IGt based on the acquired engine parameters and output the ignition signal IGt. The ignition signal IGt prescribes an optimal ignition timing and a discharge current (spark discharge current) based on a state of the gas inside the combustion chamber11band the output of the engine11that is required (that change based on the engine parameters).

The crank angle sensor33is a sensor for outputting a rectangular crank angle signal at every predetermined crank angle (such as at a 30° CA cycle) of the engine11. The crank angle sensor33is mounted in the engine block11a. A cooling water temperature sensor34is a sensor for detecting (acquiring) a cooling water temperature that is a temperature of the cooling liquid that flows inside the water jacket11. The cooling water temperature sensor34is mounted in the engine block11a.

An airflow meter35is a sensor for detecting (acquiring) an intake air amount (a mass flow rate of intake air that flows through the intake pipe21and is introduced into the combustion chamber11b). This airflow meter35is mounted in the intake pipe21on the upstream side of the throttle valve25in the intake flow direction. An intake pressure sensor36is a sensor for detecting (acquiring) intake pressure that is pressure inside the intake pipe21. The intake pressure sensor36is mounted in the surge tank21b.

A throttle opening sensor37is a sensor that generates output that corresponds to the degree of opening of the throttle valve25(throttle opening). The throttle opening sensor37is provided within the throttle actuator26. An accelerator position sensor38is provided so as to generate output that corresponds to an accelerator operation amount.

<Configuration of the Periphery of the Ignition Circuit Unit>

With reference toFIG. 2, the ignition circuit unit31includes an ignition coil311, an insulated-gate bipolar transistor (IGBT)312(corresponding to a switching element), a power supply unit313, and an ignition control circuit314.

The ignition coil311includes a primary coil311A, a secondary coil311B, and a core311C. A first end of the primary coil311A is connected to the power supply unit313. A second end of the primary coil311A is connected to a collector terminal of the IGBT312. In addition, an emitter terminal of the IGBT312is connected to a grounding side. A diode312dis connected in parallel to both ends (the collector terminal and the emitter terminal) of the IGBT312.

A first end of the secondary coil311B is connected to a current detection path L1with a diode316therebetween. A resistor317for secondary current detection is provided on the current detection path L1. A first end of the resistor317is connected to the first end of the secondary coil311B with the diode316therebetween. A second end of the resistor317is connected to the grounding side.

The ignition control circuit314, described hereafter, is connected to the resistor317. An anode of the diode316is connected to the first end side of the secondary coil311bso as to prohibit a flow of current in a direction towards a second end side of the secondary coil311B via the resistor317B from the grounding side and control a secondary current (discharge current) I2to a direction towards the secondary coil311B from the spark plug19.

The second end of the secondary coil311B is connected to the spark plug19. A voltage detection path (corresponding to a voltage value detecting unit) L3is connected to a path L2that connects the second of the secondary coil311B and the spark plug19. Resistors318A and318B for voltage detection are provided on the voltage detection path L3. One end of the resistor318A is connected to the path L2, and the other end is connected to the resistor318B. One end of the resistor318B is connected to the resistor318A and the other end is connected to the grounding side. In addition, a node (reference number omitted) between the resistor318A and the resistor318B is connected to the ignition control circuit314, described hereafter. A secondary voltage V2that is applied to the spark plug19is detected by the voltage detection path L3such as this.

The electronic control unit32generates the ignition signal IGt based on the acquired engine parameters, as described above. The electronic control unit32then transmits the generated ignition signal IGt to the ignition control circuit314. The ignition control circuit314performs ignition control based on the ignition signal IGt received from the electronic control unit32.

In the ignition control, the ignition control circuit314outputs a drive signal IG for performing opening/closing control of the IGBT312to a gate terminal of the IGBT312, and controls the IGBT312to perform conduction of a primary current I1that flows to the primary coil311A. The ignition control is control performed for the spark plug19that is provided inside the cylinder that includes the ignition control circuit314. In other words, ignition control of the spark plug19that is provided in each cylinder is performed by the ignition control circuit314that is provided in the same cylinder.

The ignition control circuit314stops output of the drive signal IG to the gate terminal of the IGBT312by the electronic control unit32stopping the output of the ignition signal IGt after the elapse of a first predetermined amount of time. As a result, conduction of the primary current I1that flows to the primary coil311A is blocked in the IGBT312and a high voltage is induced in the secondary coil311B. Dielectric breakdown occurs in the gas in a spark gap portion of the spark plug19, and a discharge spark is thereby generated in the spark plug19.

The ignition control circuit314successively detects the secondary voltage V2that is applied to the voltage detection path L3and calculates a discharge path length L of the discharge spark that is generated in the spark plug19based on the detected secondary voltage V2. In addition, the ignition control circuit314successively detects a secondary current I2that flows to the current detection path L1and calculates an approximate energy density D based on the detected secondary current I2and the calculated discharge path length L of the discharge spark.

Therefore, the current detection path L1and the ignition control circuit314correspond to a secondary current detecting unit. The voltage detection path L3and the ignition control circuit314correspond to the voltage value detecting unit. In addition, the ignition control circuit314corresponds to a primary current control unit, a discharge path length calculating unit, an approximate energy density calculating unit, and an integrated value calculating unit.

Conventionally, when a combustible air-fuel mixture that is present inside the combustion chamber11bis burned by a discharge spark being generated in the spark plug19, a combustion state of the combustible air-fuel mixture is estimated based on changes in the secondary voltage V2that is applied to the spark plug19.

Specifically, when a voltage peak of the secondary voltage V2of the discharge spark generated in the spark plug19exceeds a determination threshold and falls below the determination threshold, an accumulated time of an exceedance interval during which the voltage peak exceeds the determination threshold or an accumulated value of the secondary voltage V2during the exceedance intervals is measured. Then, whether the combustible air-fuel mixture is in a combustion state or a misfire state is determined based on the measured accumulated time of the exceedance interval or accumulated value of the secondary voltage V2during the exceedance intervals.

Here, in the engine system10according to the present embodiment, the airflow control valve27is provided near the intake port13. When homogeneous lean burn is performed, airflow, such as a swirl flow or a tumble flow, is generated inside the combustion chamber11bby the airflow control valve27. Turbulence (disturbance) is induced and a combustion rate is improved.

At this time, because a speed of the airflow inside the combustion chamber11bincreases, it is assumed that combustion ions that are generated as a result of the combustible air-fuel mixture being ignited are swept away by the airflow, and the combustion ions present between the electrodes of the spark plug19decrease. In this state, discharge resistance hardly decreases. In accompaniment, the secondary voltage V2applied to the spark plug19hardly decreases.

Therefore, should the combustion state of the combustible air-fuel mixture be estimated based on the secondary voltage V2, even when the combustible air-fuel mixture is in the combustion state, because the secondary voltage V2that is applied to the spark plug19is in a high state, an erroneous estimation that the combustible air-fuel mixture is in a misfire state may be made.

As a countermeasure against the foregoing, according to the present embodiment, the combustion state of the combustible air-fuel mixture is estimated based on the approximate energy density D of the discharge spark and the discharge path length L of the discharge spark.

In the present disclosure, it has been found that a discharge spark of which the energy density of the discharge spark is greater than a predetermined value Th contributes to the combustion of the combustible air-fuel mixture, and a discharge spark of which the energy density of the discharge spark is less than the predetermined value Th hardly contributes to the combustion of the combustible air-fuel mixture. The energy density of the discharge spark is calculated by discharge energy that is determined by a product of the secondary current I2and the secondary voltage V2being divided by the discharge path length L.

In addition, as shown inFIG. 3, in comparison to a variation width of the secondary current I2being large (about 200 to 0 [mA]) during a discharge period in which the spark plug19is generating discharge, a variation width of the secondary voltage V2is small (about 0.5 to 10 [kV]). In light of the foregoing, it has been found that variations in the secondary voltage in a distal end portion of the discharge spark of which the current value is large is moderate (in other words, the variation width of the secondary voltage is small), and the secondary current I2is a more dominant parameter in terms of determining the magnitude of the value of discharge energy.

In addition, in accompaniment with this finding, it has been found that the energy density of the discharge spark can be approximated by dividing the secondary current I2by the discharge path length L. Furthermore, when the energy density of the discharge spark is the same, a relationship is such that the discharge energy of the discharge spark increases and a surface area of the discharge spark increases as the discharge path length L increases. From this relationship, it has been found that the discharge path length L is a parameter that accurately reflects the magnitude of the discharge energy of the discharge spark.

As a result of the foregoing, whether the discharge spark that is generated in the spark plug19contributes to the combustion of the combustible air-fuel mixture can be estimated from the approximate energy density D. In addition, the discharge path length L of the discharge spark of which the approximate energy density D is greater than the predetermined value Th can be considered to be the discharge path length L of the discharge spark that contributes to the combustion of the combustible air-fuel mixture (provides the combustible air-fuel mixture with energy for combustion).

Therefore, it has been found that a sum of energy for combustion that is provided to the combustible air-fuel mixture can be estimated from an integrated value of the discharge path length L of the discharge spark. Furthermore, it has been found that the combustion state of the combustible air-fuel mixture can be accurately determined from the integrated value of the discharge path length L of the discharge spark.

Based on the foregoing findings, in the ignition control circuit314according to the present embodiment, combustion state determination control described below is performed. In the combustion state determination control, under a condition that, during a predetermined period from when conduction of the primary current I1flowing to the primary coil311A is blocked in the IGBT312, the approximate energy density D calculated by a calculation method described hereafter is greater than the predetermined value Th, an integration process for integrating the discharge path length L of the discharge spark at this time is performed.

In addition, a combustion state determination control regarding the combustible air-fuel mixture described hereafter is performed based on the integrated value of the discharge path length L of the discharge spark calculated by the integration process upon elapse of the predetermined period.

According to the present embodiment, as shown in expression (1), the approximate energy density D is calculated by the secondary current I2being divided by the discharge path length L that serves as a length of the discharge spark.
D=I2÷L(1)

Regarding the discharge path length L, as shown inFIG. 4, it has been found that a relationship between the secondary voltage V2and the discharge path length L can be more accurately approximated by a natural logarithm. Therefore, as shown in expression (2), the discharge path length L is calculated based on a natural logarithm of an absolute value of the secondary voltage V2. Here, a and b are constants that appropriately prescribe the relationship between the secondary voltage V2and the discharge path length L.
L=a×ln(V2)+b(2)

The discharge path length L is successively calculated based on the detected secondary voltage V2. The approximate energy density D is also successively calculated based on the detected secondary current I2and the calculated discharge path length L.

The combustion state determination control will be described with reference toFIG. 5.FIG. 5shows changes in time series in the approximate energy density D of the discharge spark and the discharge path length L subsequent to the discharge spark being generated in the spark plug19by conduction of the primary current I2flowing to the primary coil311A being blocked in the IGBT312.

During the predetermined period (see time t1to t3) from when conduction of the primary current I1flowing to the primary coil311A is blocked in the IGBT312, the calculated discharge path length L of the discharge spark at this time is integrated until the approximate energy density D becomes less than the predetermined value Th (see time t2). An integration expression for the discharge path length L of the discharge spark of which the approximate energy density D is greater than the predetermined value Th is determined by a product of a step function u of a value obtained by the approximate energy density D being subtracted by the predetermined value Th and the discharge path length L being integrated, as shown in expression (3).
V=∫L×u(D−Th)dt(3)
The combustion state determination control is performed upon the elapse of the predetermined period. Specifically, under a condition that the approximate energy density D that is calculated in the integration process is greater than the predetermined value Th, whether the integrated value (referred to, hereafter, as the integrated value of the discharge path length L of which the approximate energy density D is large) of the discharge path length L obtained by the discharge path length L of the discharge spark at this time being integrated is less than a first threshold is determined.

When the integrated value of the integrated discharge path lengths L of which the approximate energy density D is large is determined to not be less than the first threshold, the discharge spark is determined to be sufficiently contributing to the combustion of the combustible air-fuel mixture. Therefore, the combustion state of the combustible air-fuel mixture is determined to be favorable and discharge control is then ended.

Meanwhile, when the integrated value of the integrated discharge path lengths L of which the approximate energy density D is large is determined to be less than the first threshold, the discharge spark is determined to not be sufficiently contributing to the combustion of the combustible air-fuel mixture. The combustion state of the combustible air-fuel mixture is determined to be poor and re-discharge control is performed.

In the re-discharge control, first, the discharge spark that is being generated in the spark plug19is ended by the drive signal IG being outputted to the gate terminal of the IGBT312again. As a result, energy is supplied to the primary coil311A from the power supply unit313. Then, after the elapse of a second predetermined amount of time, the ignition control circuit314stops output of the drive signal IG to the gate terminal of the IGBT312and controls the spark plug19to perform re-discharge.

Here, the second predetermined amount of time is set to be shorter than the first predetermined amount of time. A reason for this is that, when the discharge spark that is being generated in the spark plug19is ended, electric power is assumed to still be stored in the primary coil311A. The amount of time required for the electric power required to generate a re-discharge in the spark plug19to be stored is assumed to be short.

According to the present embodiment, the determination of the combustion state of the combustible air-fuel mixture is performed even when the re-discharge control is performed. As a result of the re-discharge control being performed, the discharge spark that is generated again in the spark plug19continuously heats the combustible air-fuel mixture that has been heated by the discharge spark that has been generated in the spark plug19up to this point.

Therefore, when the re-discharge control is performed, the integrated value of the discharge path length L of which the approximate energy density D is large that has been calculated during the predetermined period is added to the integrated value of the discharge path length L that has been calculated up to this point during a single combustion cycle. When the sum that is calculated as a result is less than the first threshold, the combustion state of the combustible air-fuel mixture is assumed to still not be favorable. Therefore, the re-discharge control is performed.

Meanwhile, when the calculated sum is not less than the first threshold, the combustion state of the combustible air-fuel mixture is assumed to have become favorable. Therefore, discharge generation control is not performed again. As a result of control such as this being performed, the integrated value of the discharge path length L can be controlled so as to be greater than the first threshold. In addition, the number of times that the discharge generation control is performed can be kept to a required minimum, so as to cause the combustion state of the combustible air-fuel mixture to be favorable.

Here, the combustible air-fuel mixture becomes more difficult to burn as the air-fuel ratio inside the combustion chamber leans toward the lean side. Therefore, to enable the combustible air-fuel mixture to favorably burn, the discharge spark of which the approximate energy density D is greater than the predetermined value Th is required to be generated for a longer amount of time. Therefore, the ignition control circuit314sets the first threshold to be greater as the air-fuel ratio increases (leans toward the lean side).

In addition, in the engine11in which the EGR passage23is provided as according to the present embodiment, the percentage of EGR gas in the combustion chamber increases as the EGR rate increases. Therefore, combustion of the combustible air-fuel mixture becomes more difficult. When the EGR gas content is high, the discharge spark of which the approximate energy density D is greater than the predetermined value Th is required to be generated for a longer amount of time to enable the combustible air-fuel mixture to favorably burn. Therefore, the ignition control circuit314sets the first threshold to be greater as the EGR rate increases.

When the discharge spark is generated in the spark plug19by the primary current I1being blocked, noise is assumed to be generated in the secondary voltage V2that is applied to the voltage detection path L3and the secondary current I2that flows to the current detection path L1. During a period in which the noise is generated, error being included in the calculated approximate energy density D and discharge path length L can be considered. Therefore, the above-described combustion state determination control is preferably not performed during this period. Taking this into consideration, according to the present embodiment, a predetermined mask period is set with a point immediately after the conduction of the primary current I1flowing to the primary coil311A being blocked in the IGBT312as a starting point. The above-described predetermined period is set so as to exclude the mask period.

In addition, when the period over which the discharge spark is generated in the spark plug19increases, the discharge spark stretches into a “U” shape as a result of airflow inside the combustion chamber11b. At this time, when a location at which a distance between spark discharges that face each other is a short distance is present, the spark discharges may become joined at this location, and a discharge short in which a stretched portion of the discharge spark following the location is extinguished may occur. Noise is generated in the secondary voltage V2and the secondary current I2when the discharge short occurs as well. Therefore, the above-described predetermined period is set so as not to overlap with a period in which the probability of a short occurring in the discharge spark generated in the spark plug19increases.

According to the present embodiment, the combustion state determination control shown inFIG. 6, described hereafter, is performed by the ignition control circuit314. The combustion state determination control shown inFIG. 6is repeatedly performed at a predetermined cycle by the ignition control circuit314, during a discharge period that is a period during which the spark plug19is made to perform discharge that is started by the conduction of the primary current I1flowing to the primary coil311A being blocked in the IGBT312.

First, at step S100, the ignition control circuit314determines whether a current time is included within the mask period. When determined that the current time is not included within the mask period (NO at step S100), the ignition control circuit314proceeds to step S110.

At step S110, the ignition control circuit314detects the secondary voltage V2that is applied to the voltage detection path L3. At step S120, the ignition control circuit314detects the secondary current I2that flows to the current detection path L1.

At step S130, the ignition control circuit314calculates the discharge path length L based on the natural logarithm of the absolute value of the secondary voltage V2. At step S140, the ignition control circuit314calculates the approximate energy density D by dividing the secondary current I2by the discharge path length L.

At step S150, the ignition control circuit314determines whether the approximate energy density D calculated at step S140is greater than the predetermined value Th. When determined that the approximate energy density D is not greater than the predetermined value Th (NO at S150), the ignition control circuit314proceeds to step S170, described hereafter. When determined that the approximate energy density D is greater than the predetermined value Th (YES at S150), the ignition control circuit314proceeds to step S160. At step S160, the ignition control circuit314integrates the discharge path length L calculated at step S130.

At step S170, the ignition control circuit314determines whether the predetermined period for integrating the discharge path length L has elapsed. When determined that the predetermined period has elapsed (YES at S170), the ignition control circuit314proceeds to step S180. At step S180, the ignition control circuit314sets the first threshold based on the air-fuel ratio detected by the air-fuel ratio sensor40and the EGR rate calculated based on the degree of opening of the EGR control valve24. At step S190, the ignition control circuit314determines whether the integrated value of the discharge path length L integrated at step S160is less than the first threshold.

When determined that the integrated value of the discharge path length L is not less than the first threshold (NO at S190), the ignition control circuit314proceeds to step S200. The ignition control circuit314determines that the combustion state of the combustible air-fuel mixture is favorable and ends the present control. When determined that the integrated value of the discharge path length L is less than the first threshold (YES at S190), the ignition control circuit314proceeds to step S210. The ignition control circuit314determines that the combustion state of the combustible air-fuel mixture is poor and proceeds to step S220. At step S220, the ignition control circuit314performs the re-discharge control and returns to step S100.

When determined that the current time is included within the mask period (YES at S100), or when determined that the predetermined period for integrating the discharge path length L has not elapsed (NO at S170), the ignition control circuit314returns to step S100.

Here, in the combustion state determination control performed during the re-discharge control, a portion of the control content thereof is changed. Specifically, in the determination process at step S190, the determination process is changed to that in which whether a sum of the integrated value of the discharge path length L integrated at step S160and the integrated value of the discharge path length L calculated up to this point during a single combustion cycle is less than the first threshold is determined. Other steps are identical to the steps in the combustion state determination control during the initial discharge.

Here, the process at step S130corresponds to a process as the discharge path length calculating unit. The process at step S140corresponds to a process as the approximate energy density calculating unit. The processes at step S150and step S160correspond to a process as the integrated value calculating unit.

Next, an aspect of the combustion state determination control according to the present embodiment will be described with reference toFIG. 7.

InFIG. 7, “IG” indicates whether the drive signal IG is outputted to the gate terminal of the IGBT312by high/low. “I1” indicates the value of the primary current I1that flows to the primary coil311A. “V1” indicates the value of the primary voltage V1that is applied to the primary coil311A. In addition, “V2” indicates the value of the secondary voltage V2that is applied to the spark plug19. “I2” indicates the value of the secondary current I2that flows to the spark plug19.

The drive signal IG is transmitted to the gate terminal of the IGBT312(see time t10) by the ignition control circuit314that receives the ignition signal IGt from the electronic control unit32. As a result, the IGBT312enters a closed state and the primary current I1flows to the primary coil311A. Then, after the elapse of the first predetermined amount of time, the output of the ignition signal IGt to the ignition control circuit314from the electronic control unit32is stopped. Therefore, in accompaniment, the output of the drive signal IG to the gate terminal of the IGBT312by the ignition control circuit314is stopped (see time t11). As a result, the IGBT312enters an open state and the conduction of the primary current I1flowing to the primary coil311A is blocked. The secondary voltage V2is induced in the secondary coil311B and dielectric breakdown occurs in the gas in the spark gap portion of the spark plug19. As a result, the discharge spark is generated in the spark plug19.

The approximate energy density D is not calculated until the predetermined mask period elapses (see time t11to t12) from when the discharge spark is generated in the spark plug19(from when the conduction of the primary current I1flowing to the primary coil311A is blocked). During the predetermined period (see time t12to t13) provided after the predetermined mask period, the approximate energy density D is calculated by the detected secondary current I2being divided by the discharge path length L of the discharge spark that is calculated based on the detected secondary voltage V2. Then, under a condition that the calculated approximate energy density D is greater than the predetermined value Th, the discharge path length L of the discharge spark at this time is integrated.

After the elapse of the predetermined period (see time t13), whether the integrated value of the discharge path length L of which the approximate energy density D is large that has been integrated during the predetermined period is less than the first threshold is determined. Then, as a result of the integrated value of the discharge path length L of which the approximate energy density D is large that has been integrated during the predetermined period being determined to be less than the first threshold, the ignition control circuit314transmits the drive signal IG to the gate terminal of the IGBT312again (see time t14). Subsequently, the output of the drive signal IG to the gate terminal of the IGBT312is stopped as a result of the elapse of the second predetermined amount of time (see time t14to t15). As a result, the discharge spark is generated again in the spark plug19.

In a manner similar to that during the initial discharge, during the re-discharge as well, the predetermined mask period is provided. The approximate energy density D is not calculated until the elapse of the predetermined mask period (see time t15to t16) from when the discharge spark is generated in the spark plug19. Then, under a condition that the calculated approximate energy density D is greater than the predetermined value Th during the predetermined period provided after the predetermined mask period, the discharge path length L of the discharge spark at this time is integrated (see time t16to t17).

After the elapse of the predetermined period (see time t17), whether the sum of the integrated value of the discharge path length L of which the approximate energy density D is large that has been integrated during the predetermined period and the integrated value of the discharge path length L of which the approximate energy density D is large that has been integrated up to this point during a single combustion cycle is less than the first threshold is determined. As a result of the sum being determined to not be less than the first threshold, the re-discharge control is not performed and the discharge control is immediately ended.

Here, during an interval from time t13to t14, large variations occur in the primary voltage V1, the secondary voltage V2, and the secondary current I2. The variations are thought to occur as a result of a short in the discharge spark that is generated in the spark plug19. When a discharge short occurs in this manner, large variations occur in the primary voltage V1, the secondary voltage V2, and the secondary current I2. Therefore, an end point of the predetermined period is preferably set before a period during which the likelihood of the discharge short occurring increases.

As a result of the above-described configuration, according to the present embodiment, the following effects are achieved.

The re-discharge control is performed under a condition that the integrated value of the discharge path length L calculated during the predetermined period is less than the first threshold. As a result, the combustion state of the combustible air-fuel mixture can be made favorable.

FIG. 8andFIG. 9show that the combustion state of the combustible air-fuel mixture is actually improved by the re-discharge control being performed.

FIG. 8is a comparison between data when the discharge spark is generated only once in the spark plug19and data when the discharge spark is generated twice in the spark plug19according to the present embodiment, the data being that regarding the extent of variation in a torque variation rate of the engine11as the air-fuel ratio inside the combustion chamber11bleans towards the lean side. Based onFIG. 8, it is clear that the torque variation rate increases as the air-fuel ratio increases (the air-fuel ratio leans toward lean), when the discharge spark is generated only once in the spark plug19.

That is, it is suggested that the frequency of misfire occurring in the engine11increases as the air-fuel ratio increases. Meanwhile, when the discharge spark is generated twice in the spark plug19according to the present embodiment, compared to the data when the discharge spark is generated only once in the spark plug19, the torque variation rate when the air-fuel ratio increases can be reduced. Based on the foregoing, it is suggested that the frequency of misfire occurring in the engine11can be reduced when the discharge spark is generated twice in the spark plug19.

FIG. 9shows, by (a), a comparison of data between when the discharge spark is generated only once in the spark plug19and when the discharge spark is generated twice in the spark plug19according to the present embodiment, in an environment in which the air-fuel ratio inside the combustion chamber11bleans toward the rich side.

FIG. 9shows, by (b), a comparison of data between when the discharge spark is generated only once in the spark plug19and when the discharge spark is generated twice in the spark plug19according to the present embodiment, in an environment in which the air-fuel ratio inside the combustion chamber11bleans further toward the lean side than in a case shown inFIG. 9by (a).

In each graph shown inFIG. 9by (a) and (b), a vertical axis indicates crank angles that are passed until 2% of the mass of the combustible air-fuel mixture is burned from the ignition timing. Therefore, as the value of the vertical axis increases, the amount of time until the combustion air-fuel mixture is burned increases. The combustible air-fuel mixture is unable to be burned within the discharge period and the likelihood of a misfire occurring is high.

As shown inFIG. 9by (a), in an environment in which the air-fuel ratio inside the combustion chamber11bleans toward the rich side, even when the discharge spark is generated only once in the spark plug19, the combustible air-fuel mixture can be burned in an amount of time that is equal to that when the discharge spark is generated twice in the spark plug19according to the present embodiment.

However, as shown inFIG. 9by (b), in an environment in which the air-fuel ratio inside the combustion chamber11bleans towards the lean side, among cases in which the discharge spark is generated only once in the spark plug19, particularly regarding discharge sparks of which the integrated value of the discharge path length L of which the approximate energy density D is large, a significant amount of time tends to be required until the combustible air-fuel mixture is burned.

That is, it is suggested that, even when the discharge spark is generated only once in the spark plug19, when the integrated value of the discharge path length L of which the approximate energy density D is large is large, the combustible air-fuel mixture can be favorably burned. Meanwhile, when the integrated value of the discharge path length L of which the approximate energy density D is large is small, the combustion state of the combustible air-fuel mixture tends to be poor.

In contrast, when the discharge spark is generated twice in the spark plug19according to the present embodiment in an environment in which the air-fuel ratio inside the combustion chamber11leans toward the lean side, the integrated value of the discharge path length L of which the approximate energy density D is large can be increased compared to that when the discharge spark is generated only once. Therefore, the combustion state of the combustible air-fuel mixture can be made favorable within the discharge period.

Consequently, as a result of the present combustion state determination control being performed, as a result of the re-discharge control being performed under a condition that the integrated value of the discharge path length L of which the approximate energy density D is large is less than the first threshold, the combustion state of the combustible air-fuel mixture can be improved.

In addition, when the integrated value of the discharge path length L of which the approximate energy density D is large that is calculated during the predetermined period is less than the first threshold, the combustion state of the combustible air-fuel mixture can be estimated as being favorable. Therefore, as a result of the re-discharge control not being performed, energy being unnecessarily consumed in the spark plug19can be suppressed.

FIG. 10shows, by (a), data indicating the value of the discharge path length L of the discharge spark that is integrated under a condition that the energy density that is calculated from the ignition timing until 2% of the mass of the combustible air-fuel mixture is burned is greater than the predetermined value Th.

FIG. 10shows, by (b), data indicating the value of the discharge path length L of the discharge spark that is integrated under a condition that the approximate energy density D that is calculated from the ignition timing until 2% of the mass of the combustible air-fuel mixture is burned is greater than the predetermined value Th. The results shown inFIG. 10by (a) and the results shown inFIG. 10by (b) substantially coincide. Therefore, the approximate energy density D is able to favorably approximate the energy density of the discharge spark. Here, experiments shown inFIG. 10by (a) and (b) are both performed in equivalent environments.

As a result of the present combustion state determination control being performed through use of the approximate energy density D instead of the energy density, a calculation step for discharge energy can be omitted (in other words, a calculation step for calculating a product of the secondary current I2and the secondary voltage V2can be omitted). Furthermore, a calculation circuit that is required to perform the present control can be simplified.

The discharge spark of which the approximate energy density D is greater than the predetermined value is thought to contribute to the combustion of the combustible air-fuel mixture. However, the combustion state of the combustible air-fuel mixture differs (for example, combustion is promoted as heat that is provided increases) based on a total area of the combustible air-fuel mixture facing the discharge spark (a total amount of combustible air-fuel mixture that is provided with heat by the discharge spark). Therefore, as a result of the integrated value of the discharge path length L of which the approximate energy density D is large being calculated, the total area over which the combustible air-fuel mixture faces the discharge spark can be ascertained. Furthermore, the combustion state of the combustible air-fuel mixture can be estimated.

As a result of the discharge path length L being calculated based on the natural logarithm of the absolute value of the secondary voltage V2as shown in expression (2), a map or the like that prescribes the relationship therebetween in advance is not required to be prepared. The discharge path length L can be calculated by a calculation formula.

The first threshold is set to be greater as the air-fuel ratio of the combustible air-fuel mixture increases. As a result, the combustion state of the air-fuel mixture can be more accurately estimated.

The first threshold is set to be greater as the EGR gas increases. As a result, the combustion state of the air-fuel mixture can be more accurately estimated.

The predetermined period is set so as to exclude the predetermined mask period immediately after the conduction of the primary current I1flowing to the primary coil311A is blocked in the IGBT312. As a result, error included in the integrated value of the discharge path length L of which the approximate energy density D is large can be reduced.

In the present combustion state determination control, the combustion state of the combustible air-fuel mixture is estimated based on the integrated value of the discharge path length L of the discharge spark in a state in which the approximate energy density D is greater than the predetermined value Th. Therefore, erroneous estimation of the combustion state of the combustible air-fuel mixture can be suppressed even in an environment in which the flow rate of gas inside the combustion chamber11is high.

The above-described embodiment can also be carried out with modifications such as those below.

According to the above-described embodiment, the combustion state determination control is performed by the ignition control circuit314. Regarding this point, the combustion state determination control may be performed by the electronic control unit32. Alternatively, the combustion state determination control may be performed by the electronic control unit32and the ignition control circuit314in cooperation. In addition, a separate circuit that is not limited to the electronic control unit32or the ignition control circuit314may perform the combustion state determination control.

According to the above-described embodiment, the secondary voltage V2that is applied to the voltage detection path L3is calculated. The discharge path length L and the approximate energy density D are calculated through use of the detected secondary voltage V2. Here, symbols of the secondary voltage V2and the primary voltage V1are inverted, the magnitudes of the values differ. However, as shown inFIG. 11, an aspect of change in the primary voltage V1tends to take on an aspect of change that is similar to that of the secondary voltage V2.

Therefore, the primary voltage V1may serve as a substitute for the secondary voltage V2. Specifically, the ignition control unit31may be configured to include a voltage detection path that detects the primary voltage V1that is applied to the primary coil311A, instead of the voltage detection path L3. The discharge path length L may be calculated through use of the detected primary voltage V1.

According to the above-described embodiment, the approximate energy density D is calculated by the secondary current I2being divided by the discharge path length L. Regarding this point, for example, the approximate energy density D may be calculated by a current value amounting to noise being subtracted from the secondary current I2, and the value thereof being divided by the discharge path length L. Alternatively, a map that indicates the relationship among the secondary current I2, the discharge path length L, and the approximate energy density D may be generated in advance. The approximate energy density D may be acquired from the secondary current I2and the discharge path length L with reference to the map.

According to the above-described embodiment, the discharge path length L is calculated based on the natural logarithm of the absolute value of the secondary voltage V2as shown in expression (2). Regarding this point, a map that prescribes the relationship between the secondary voltage V2and the discharge path length L in advance may be prepared. The discharge path length L may be estimated from the detected secondary voltage V2with reference to the map.

According to the above-described embodiment, the ignition control circuit314sets the first threshold. Regarding this point, the ignition control circuit314is not required to set the first threshold. For example, the electronic control unit32may set the first threshold.

According to the above-described embodiment, the first threshold that serves as a threshold for determining whether the combustion state of the combustible air-fuel mixture is favorable is set to be greater as the air-fuel ratio increases (leans toward the lean side) or the EGR rate increases. Regarding this point, the first threshold may be a fixed value.

According to the above-described embodiment, the present combustion state determination control is performed even when the re-discharge control is performed. Regarding this point, when the re-discharge control is performed, the combustion state of the combustible air-fuel mixture may be considered improved and the present combustion state determination control may not be performed. In this case, the frequency of execution of the combustion state determination control can be reduced. Reduction of load placed on the ignition control circuit314becomes possible.

According to the above-described embodiment, the predetermined mask period is set with the point immediately after the conduction of the primary current I1flowing to the primary coil311A being blocked in the IGBT312as the starting point. Regarding this point, the mask period may not be set. The predetermined period may be set immediately after the conduction of the primary current I1flowing to the primary coil311A is blocked in the IGBT312.

The ignition circuit unit31according to the above-described embodiment is mounted in the engine11in which airflow, such as a swirl flow or a tumble flow, is generated inside the combustion chamber11aby the airflow control valve27that is provided near the intake port13, when homogeneous lean burn is performed. Regarding this point, the ignition circuit unit31according to the above-described embodiment is not necessarily required to be mounted in the engine11in which the airflow control valve27is provided.

According to the above-described embodiment, the discharge path length L is calculated based on expression (3). Regarding this point, the discharge path length L is not necessarily required to be calculated based on expression (3). For example, as shown inFIG. 12, the discharge path length L of the discharge spark that is generated in the spark plug19may be calculated each time a third predetermined amount of time (such as 0.02 ms) elapses during the predetermined period. All of the discharge path lengths L calculated each time the third predetermined amount of time elapses may be added upon the elapse of the predetermined period, and the integrated value of the discharge path length L may be calculated. Here, regarding a graph shown inFIG. 12, at least the discharge sparks during the predetermined period are assumed to be in a state in which the approximate energy density D is higher than the first threshold.

The discharge spark that is generated in the spark plug19may be extinguished (discharge ended) before the elapse of the predetermined period as a result of the discharge spark generated in the spark plug19being blown out due to the flow rate inside the cylinder being high, carbon that is produced by incomplete combustion of fuel attaching to an electrode outer circumferential portion of the spark plug19and flashover discharge being generated between the carbon and an attachment fixture of the spark plug19, or the like.

In this case, discharge is assumed to end before the combustible air-fuel mixture is sufficiently heated. The likelihood of the combustion state of the combustible air-fuel mixture not being favorable is high. As a countermeasure, the re-discharge control is immediately performed when the absolute value of the secondary current I2that flows to the current detection path L1becomes less than a second threshold during the predetermined period.

FIG. 13is a modification of the flowchart shown inFIG. 6. That is, step S430is newly added as a step to which the ignition control circuit314proceeds when the ignition control circuit314determines NO in a determination process at step S370that corresponds to step S170inFIG. 6.

At step S430, the ignition control circuit314determines whether the absolute value of the secondary current I2detected at step S320that corresponds to step S120is less than the second threshold. When determined that the absolute value of the secondary current I2is not less than the second threshold (NO at S430), the ignition control circuit314returns to step S300. When determined that the absolute value of the secondary current I2is less than the second threshold (YES at S430), the ignition control circuit314proceeds to step S420that corresponds to step S220.

Regarding other steps, processes at each of steps S300,310,330,340,350,360,380,390,400, and410inFIG. 13are respectively identical to the processes at each of steps S100,110,130,140,150,160,180,190,200, and210inFIG. 6.

As a result, even should the discharge spark that is generated in the spark plug19be extinguished during the predetermined period, as a result of the re-discharge control being immediately performed, the discharge spark can be generated again in the spark plug19. Furthermore, an interval from when discharge is ended until the discharge spark is generated again can be shortened.

As shown inFIG. 14, the torque variation rate can be decreased even in an environment in which the EGR rate is high, as the discharge interval when discharge is performed twice is shortened. A reason for this is thought to be that, because the combustible air-fuel mixture that has been heated by the discharge spark that has been generated first can be heated again by the second discharge spark that is generated by the re-discharge control, deterioration of the ignitability of the combustible air-fuel mixture and the combustion state can be suppressed.

In another example, the re-discharge control is immediately performed when the absolute value of the second current I2that flows to the current detection path L1becomes less than the second threshold during the predetermined period. Regarding this point, the determination may be performed based on the absolute value of the primary voltage V1, the absolute value of the secondary voltage V2, or the approximate energy density D, instead of the absolute value of the secondary current I2.

Specifically, the configuration may be such that the re-discharge control is immediately performed when the absolute value of the primary voltage V1or the absolute value of the secondary voltage V2becomes less than a third threshold that is provided to identify 0 during the predetermined period. Alternatively, the configuration may be such that the re-discharge control is immediately performed when the approximate energy density D becomes less than a fourth threshold during the predetermined period.

Here, a relationship among the predetermined value Th and the first threshold to third threshold is as follows. The predetermined threshold Th is a threshold for determining whether the discharge spark that is generated in the spark plug19contributes to the combustion of the combustible air-fuel mixture.

The first threshold is a threshold for determining that the discharge spark sufficiently contributes to the combustion of the combustible air-fuel mixture and, therefore, the combustion state of the combustible air-fuel mixture is favorable, based on the discharge path length L.

The second threshold is a threshold for determining whether the discharge spark that is generated in the spark plug19has been extinguished during the predetermined period, based on the absolute value of the secondary current I2.

The third threshold is a threshold for determining whether the discharge spark that is generated in the spark plug19has been extinguished during the predetermined period, based on the absolute value of the primary voltage V1or the absolute value of the secondary voltage V2.

The fourth threshold is a threshold for determining whether the discharge spark that is generated in the spark plug19has been extinguished during the predetermined period, based on the absolute value of the approximate energy density D.

At this time, because the re-discharge control is immediately performed when the discharge spark that is generated in the spark plug19is determined to be extinguished during the predetermined period, it can be said, in other words, that the second threshold to fourth threshold are all thresholds that determine whether the re-discharge control is to be immediately performed. Therefore, the third threshold corresponds to the second threshold in the scope of claims.

While the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure is intended to cover various modification examples and modifications within the range of equivalency. In addition, various combinations and configurations, and further, other combinations and configurations including more, less, or only a single element thereof are also within the spirit and scope of the present disclosure.