阅读说明：本技术 内燃机的控制方法以及控制装置 (Method and device for controlling internal combustion engine ) 是由 前田洋史 葛西理晴 于 2017-05-24 设计创作，主要内容包括：一种内燃机的控制方法,在从进气行程至压缩行程前半段为止的期间、和压缩行程后半段分别进行至少1次燃料喷射,由此在燃烧室内形成分层混合气体而进行分层燃烧,其中,在因压缩行程后半段喷射的燃料喷雾的能量而使得火花塞周围的流动能量增大时,相对较大的放电电流在所述火花塞流动而开始火花点火,然后,相对地减小放电电流而在规定期间进行放电。(A control method of an internal combustion engine, wherein fuel injection is performed at least 1 time each during a period from an intake stroke to a first half of a compression stroke and during a second half of the compression stroke, thereby forming a stratified charge in a combustion chamber and performing stratified combustion, wherein when flow energy around a spark plug increases due to energy of fuel spray injected during the second half of the compression stroke, a relatively large discharge current flows through the spark plug to start spark ignition, and thereafter, the discharge current is relatively reduced to perform discharge for a predetermined period.)

1. A control method of an internal combustion engine for performing stratified combustion by forming a stratified mixture in a combustion chamber by performing at least 1 fuel injection during a period from an intake stroke to a first half of a compression stroke and during a second half of the compression stroke, wherein,

when the flow energy around the spark plug increases due to the energy of the fuel spray injected in the latter half of the compression stroke, a relatively large discharge current flows at the spark plug to start spark ignition,

then, the discharge current is relatively reduced and the discharge is performed for a predetermined period.

2. The control method of an internal combustion engine according to claim 1,

a part of a low/medium rotation speed and a low/medium load region of an internal combustion engine is set to a lean combustion region,

the stratified combustion is performed in an operating region where the load of the lean combustion region is relatively high,

in an operating region where the load in the lean combustion region is relatively low, fuel injection is performed at least 1 time during a period from an intake stroke to a first half of a compression stroke, whereby a homogeneous mixture gas is formed in the combustion chamber and homogeneous combustion is performed,

the discharge waveform of the spark plug is made different at the time of the stratified combustion and at the time of the homogeneous combustion.

3. The control method of an internal combustion engine according to claim 2,

the air excess ratio of the entire combustion chamber is controlled to 2 in any case at the time of the stratified combustion and at the time of the homogeneous combustion.

4. The control method of an internal combustion engine according to claim 2 or 3, wherein,

the duration of discharge of the spark plug during the homogeneous combustion is extended as compared with the duration of discharge of the spark plug during the stratified combustion.

5. The control method of an internal combustion engine according to claim 4,

the ignition energy at the time of the homogeneous combustion is increased as compared with the ignition energy at the time of the stratified combustion.

6. A control device for an internal combustion engine, comprising:

a fuel injection valve that directly injects fuel into the combustion chamber;

a spark plug that performs spark ignition of a mixture gas formed in the combustion chamber;

a drive device that drives the spark plug; and

a control unit that controls the fuel injection valve and the drive device, wherein,

the control part is used for controlling the operation of the motor,

at least 1 fuel injection is performed during a period from an intake stroke to a first half of a compression stroke and during a second half of the compression stroke, respectively, whereby a stratified mixture is formed in the combustion chamber,

when the flow energy around the spark plug increases due to the energy of the fuel spray injected in the latter half of the compression stroke, a relatively large discharge current flows at the spark plug to start spark ignition,

then, the discharge current is relatively reduced and the discharge is performed for a predetermined period.

Technical Field

The present invention relates to control of an internal combustion engine in which stratified charge gas mixtures are formed by performing at least 1 fuel injection during a period from an intake stroke to a first half of a compression stroke and during a second half of the compression stroke, and spark ignition is performed during a period in which flow energy around a spark plug is increased by energy of a fuel spray injected in the second half of the compression stroke.

Background

There is known so-called stratified combustion in which a combustible gas mixture is formed around a spark plug and a lean gas mixture is formed in other portions of the spark plug. JP1999-303721a1 discloses control such that, when stratified combustion is performed in low load operation of the internal combustion engine, the discharge period is made longer than that in homogeneous combustion. The control disclosed in the above-mentioned document is for avoiding a situation where no combustible mixture gas is present around the spark plug during the discharge.

Disclosure of Invention

However, in the case of stratified combustion, the equivalence ratio around the spark plug is increased as compared with the case of homogeneous combustion in which combustion is performed by forming a homogeneous combustible gas mixture in the entire combustion chamber. That is, in the stratified combustion, the mixture gas around the spark plug is in a state of being easily ignited compared to the homogeneous combustion. Therefore, the history of the discharge current for achieving stable combustion in the stratified combustion is different from the history of the discharge current in the homogeneous combustion.

However, although the above document describes the ignition timing and the discharge period, the history of the discharge current suitable for the stratified charge combustion is not described. Therefore, the control of the above document has room for improvement.

Therefore, an object of the present invention is to provide a method for controlling the history of discharge current in stratified combustion so as to be suitable for stratified combustion.

According to one aspect of the present invention, there is provided a control method for an internal combustion engine, in which stratified combustion is performed by forming a stratified charge gas in a combustion chamber by performing at least 1 fuel injection during a period from an intake stroke to a first half of a compression stroke and during a second half of the compression stroke. In this control method, when the flow energy around the spark plug increases due to the energy of the fuel spray injected in the second half of the compression stroke, a relatively large discharge current flows through the spark plug to start spark ignition, and then the discharge current is relatively reduced to perform discharge for a predetermined period.

Drawings

Fig. 1 is an explanatory diagram of the overall structure of an internal combustion engine system.

Fig. 2 is an explanatory view of the flow formed in the vicinity of the spark plug.

Fig. 3 is a diagram showing an injection mode of the fuel injection valve.

Fig. 4 is a diagram for explaining the spray beam.

Fig. 5 is a diagram showing the arrangement of the ignition plug and the fuel injection valve.

Fig. 6 is a diagram showing a relationship between the discharge region and the spray beam.

Fig. 7 is a diagram for explaining the contracted flow.

Fig. 8 is an explanatory diagram of the tumble flow generated in the cylinder.

Fig. 9 is an explanatory diagram of tumble flow in the compression stroke.

Fig. 10 is a graph showing changes in turbulence intensity around the spark plug.

Fig. 11 is an explanatory diagram of a spark plug discharge passage in the vicinity of the spark plug.

Fig. 12A is a diagram showing a relationship between the fuel injection timing and the ignition timing.

Fig. 12B is a diagram showing the relationship between the fuel injection timing and the ignition timing.

Fig. 13 is a combustion pattern map.

Fig. 14 is a diagram showing an example of the variable compression ratio mechanism.

Fig. 15 is a graph of the gas flow rate in the discharge gap and the air-fuel ratio in the discharge gap at the time of homogeneous lean combustion.

Fig. 16 is a graph showing a relationship between the elapsed time and the secondary current with respect to the ignition timing at the time of homogeneous lean combustion.

Fig. 17 is a graph of the gas flow rate in the discharge gap and the air-fuel ratio in the discharge gap at the time of stratified lean combustion.

Fig. 18 is a graph showing a relationship between the elapsed time and the secondary current with respect to the ignition timing in the stratified lean combustion.

Fig. 19 is a flowchart showing a control flow stored in the controller.

Fig. 20 is a diagram showing a relationship between the secondary current, the discharge time, the secondary voltage, and the ignition energy in the lean combustion region and the load of the internal combustion engine.

Fig. 21 is a diagram showing the relationship between the air-fuel ratio, the mechanical compression ratio, and the fuel efficiency of the entire combustion chamber in the lean combustion region and the load of the internal combustion engine.

Detailed Description

Embodiments of the present invention will be described below with reference to the drawings.

Fig. 1 is an explanatory diagram of the overall structure of an internal combustion engine system. In the internal combustion engine system 1, the internal combustion engine 10 is connected to an intake passage 51. Further, the internal combustion engine 10 is connected to an exhaust passage 52.

The tumble control valve 16 is provided in the intake passage 51. The tumble control valve 16 closes a part of the flow path cross section of the intake passage 51 to generate a tumble flow in the cylinder.

A header tank 46 is provided in the intake passage 51. An EGR passage 53b is also connected to the header tank 46.

An air flow meter 33 is provided in the intake passage 51. The controller 50 connected to the air flow meter 33 acquires the amount of intake air in the intake passage 51 from the air flow meter 33. Further, an intake air temperature sensor 34 is provided in the intake passage 51. The controller 50 connected to the intake air temperature sensor 34 acquires the temperature of the air passing through the intake passage 51 from the intake air temperature sensor 34.

An electronically controlled throttle valve 41 is provided in the intake passage 51, and the throttle opening is controlled by a controller 50.

Exhaust gas catalysts 44 and 45 for purifying exhaust gas are provided in the exhaust passage 52. For the exhaust gas catalysts 44, 45, a three-way catalyst or the like is used. Further, the exhaust passage 52 branches into an EGR passage 53 connected to the header tank 46 midway therein.

The EGR passage 53 is provided with an EGR cooler 43. Further, the EGR passage 53 is provided with an EGR valve 42. The EGR valve 42 is connected to the controller 50. The opening degree of the EGR valve 42 is controlled by the controller 50 according to the operating conditions of the internal combustion engine 10.

The internal combustion engine 10 has an ignition plug 11, a fuel injection valve 12, an intake-side variable valve mechanism 13, an exhaust-side variable valve mechanism 14, and a fuel injection pump 15. The fuel injection valve 12 is a straight upper type injection valve and is provided in the vicinity of the ignition plug 11.

The ignition plug 11 is driven by a drive device 17 to perform spark ignition in a combustion chamber of the internal combustion engine 10. The ignition plug 11 is connected to a controller 50, and the controller 50 as a control unit controls the ignition timing. The "ignition timing" in the present embodiment refers to the timing at which spark ignition is started. The spark plug 11 also operates as a flow rate sensor 23 that detects the flow rate of gas between the discharge gaps.

The driving device 17 causes the ignition plug 11 to generate a discharge voltage in accordance with an ignition signal from the controller 50. The driving device 17 includes a circuit for applying a voltage (hereinafter, also referred to as a superimposed voltage) in the same direction as the discharge voltage to the electrodes of the spark plug 11 during the discharge period, in addition to a circuit for performing spark discharge at the start of discharge. A structure for applying a superimposed voltage is well known (for example, JP2016-53312a1), and thus detailed description is omitted.

The discharge time can be extended by applying the superimposed voltage during the discharge. In other words, the discharge period can be arbitrarily controlled by controlling the superimposed voltage.

The fuel injection valve 12 injects fuel directly into a combustion chamber of the internal combustion engine 10. The fuel injection valve 12 is connected to a controller 50, and the controller 50 as a control unit controls the fuel injection timing. In the present embodiment, so-called multi-stage injection is performed in which a plurality of fuel injections are performed including the intake stroke. The fuel injection pump 15 supplies pressurized fuel to a fuel supply pipe connected to the fuel injection valve 12.

The intake variable valve mechanism 13 changes the opening/closing timing of the intake valve. The exhaust variable valve mechanism 14 changes the opening/closing timing of the exhaust valve. The intake-side variable valve mechanism 13 and the exhaust-side variable valve mechanism 14 are connected to the controller 50. The opening and closing timing is controlled by the controller 50. Here, the intake side variable valve mechanism 13 and the exhaust side variable valve mechanism 14 are shown, but any one may be provided.

The internal combustion engine 10 is provided with a crank angle sensor, an in-cylinder pressure sensor, and an accelerator opening sensor, which are not shown. The crank angle sensor detects a crank angle of the internal combustion engine 10. The crank angle sensor is connected to the controller 50, and transmits the crank angle of the internal combustion engine 10 to the controller 50.

The in-cylinder pressure sensor detects the pressure of the combustion chamber of the internal combustion engine 10. The in-cylinder pressure sensor is connected to the controller 50. Also, the pressure of the combustion chamber of the internal combustion engine 10 is sent to the controller 50.

The accelerator opening sensor detects a depression amount of an accelerator pedal by a driver.

In addition, the internal combustion engine 10 may have a knock sensor 21, a fuel pressure sensor 24. The controller 50 reads outputs from the various sensors described above and other sensors not shown, and controls ignition timing, valve timing, air-fuel ratio, and the like based on the outputs. The internal combustion engine 10 further includes a variable compression ratio mechanism that changes the mechanical compression ratio, and the controller 50 also controls the variable compression ratio mechanism. The details of the variable compression ratio mechanism will be described later.

Fig. 2 is a diagram for explaining a positional relationship between the ignition plug 11 and the fuel injection valve 12. As described above, the fuel injection valve 12 is a straight upper type injection valve and is provided in the vicinity of the ignition plug 11. Therefore, a part of the injected fuel passes through the vicinity of the discharge gap, and thus a flow can be generated in the vicinity of the spark plug. The generation of the flow will be described later.

Fig. 3 shows the form of the fuel spray injected from the fuel injection valve 12. Fig. 4 is a view of a plane including a circle a in fig. 3 as viewed from the direction of an arrow IV in fig. 3.

The fuel injection valve 12 in the present embodiment injects fuel from 6 nozzle holes. When the fuel spray (hereinafter, also referred to as spray beam) injected from 6 injection holes is B1-B6, each spray beam is formed in a conical shape having a spray cross section that increases as the distance from the injection hole increases. Further, the cross-sections of the spray beams B1-B6 cut by the plane including the circle a are arranged at equal intervals in a ring shape as shown in fig. 4.

FIG. 5 is a diagram showing the positional relationship between spray beams B1-B6 and the ignition plug 11. The fuel injection valve 12 is disposed on the one-dot chain line C, which is a bisector of an angle formed by the central axis B2C of the spray beam B2 and the central axis B3C of the spray beam B3.

Fig. 6 is a view showing a positional relationship between the ignition plug 11 and the spray beam B3 when viewed from the direction of arrow VI in fig. 5. In fig. 6, the discharge region sandwiched between center electrode 11a and outer electrode 11B is arranged in a range sandwiched between the upper outer edge and the lower outer edge of spray beam B3 in the drawing. Although not shown, if fig. 5 is viewed from the direction opposite to arrow VI, the positional relationship between spark plug 11 and spray beam B2 is similar to that of fig. 6, and the discharge region is disposed in the range between the upper outer edge and the lower outer edge of spray beam B2. That is, spark plug 11 is disposed such that the discharge region is disposed in a range sandwiched by a plane including the upper outer edge of spray beam B2 and the upper outer edge of spray beam B3, and a plane including the lower outer edge of spray beam B2 and the lower outer edge of spray beam B3.

Fig. 7 is a diagram for explaining the effect of the spray beams B1-B6 and the spark plug 11 in the positional relationship shown in fig. 5 and 6.

The fuel injected from the fuel injection valve 12 is broken into droplets and changed into a spray, and the surrounding air is entrained and advanced as indicated by thick line arrows in the figure. This causes turbulence in the airflow around the spray.

In addition, in the case where an object (including a fluid) is present around, the fluid is attracted to and flows along the object due to a so-called coanda effect. That is, a so-called converging flow in which spray beam B2 and spray beam B3 are attracted to each other as indicated by thin line arrows in fig. 7 is generated. As a result, a very strong turbulent flow is generated between the spray beam B2 and the spray beam B3, and the intensity of the turbulent flow around the spark plug 11 increases.

Here, a change in the intensity of the tumble flow will be described.

Fig. 8 is an explanatory diagram of the tumble flow generated in the cylinder. Fig. 9 is a diagram for explaining the attenuation of the tumble flow. In these drawings, an intake passage 51, an exhaust passage 52, an ignition plug 11, a fuel injection valve 12, and a tumble control valve 16 are shown. Further, a center electrode 11a and an outer electrode 11b of the spark plug 11 are shown. Also, the tumble flow in the cylinder in the intake stroke is shown by the arrow in fig. 8. The tumble flow in the cylinder of the compression stroke is shown by the arrows in fig. 9.

If the tumble control valve 16 is closed during the intake stroke, the intake air flows into the cylinder while being deflected upward in the drawing of the intake passage 51. As a result, a tumble flow swirling in the longitudinal direction is formed in the cylinder as shown in the figure. Then, in the compression stroke, the piston ascends to contract the combustion chamber in the cylinder. As the combustion chamber shrinks, the tumble flow is crushed and its flow gradually diminishes (fig. 9), and soon breaks down.

Therefore, in the case where stratified charge combustion in which a combustible mixture exists around the ignition plug 11 and a lean mixture exists in the other portion is formed and stratified combustion in which the ignition timing is retarded until the latter half of the compression stroke is performed, the flow around the ignition plug 11 is weakened at the ignition timing. Therefore, an arc (hereinafter, also referred to as a spark plug discharge path CN) generated between the electrodes 11a and 11b of the spark plug 11, that is, in a discharge gap is not sufficiently extended, and there is a possibility of causing a misfire or partial combustion. The "periphery of the spark plug 11" referred to herein also includes a discharge gap of the spark plug 11.

Therefore, in the present embodiment, the spark plug discharge passage CN is elongated by utilizing the characteristic that the turbulence intensity around the spark plug 11 is increased by injecting the fuel.

Fig. 10 is a timing chart showing changes in the turbulence intensity around the spark plug 11 in the case where fuel injection is performed in the latter half of the compression stroke. In fig. 10, the horizontal axis represents the crank angle and the vertical axis represents the turbulence intensity around the spark plug 11. The broken line in the figure indicates the change in the turbulence intensity in the case where the fuel injection in the latter half of the compression stroke is not performed.

As described above, since the intensity of the tumble flow gradually decreases, the intensity of turbulence around the spark plug 11 also decreases accordingly. However, if the fuel injection is performed in the latter half of the compression stroke, the turbulence intensity increases for a predetermined period after the fuel injection. During the period in which the turbulence intensity is increased by the fuel injection, the spark plug discharge passage CN is likely to elongate. In particular, the timing C1 at which the turbulence intensity peaks is suitable as the ignition timing. On the other hand, in the case of performing homogeneous lean combustion described later, fuel injection in the latter half of the compression stroke is not performed, and therefore, combustion becomes slower than stratified combustion. Therefore, the timing C2 that is earlier than the timing C1 is suitable as the ignition timing in the case of homogeneous lean combustion.

Fig. 11 is an explanatory diagram of the spark plug discharge passage CN. The center electrode 11a and the outer electrode 11b of the spark plug 11, and the elongated spark plug discharge passage CN are shown in fig. 11. Note that, here, the case of the spark plug discharge passage CN is focused on, and therefore, the fuel injection valve 12 is omitted. Further, if the flow is generated around the ignition plug in such a manner that the ignition plug discharge passage CN is sufficiently elongated, the tip end of the fuel injection valve 12 may not face the ignition plug 11. For example, an embodiment may be such that the injected fuel is reflected in the combustion chamber to flow around the spark plug.

The more the tumble flow is reduced, the less the flow around the spark plug 11 is. Therefore, if spark ignition is performed, the spark plug discharge passage CN is generally generated so as to extend substantially linearly between the center electrode 11a and the outer electrode 11 b. However, in the present embodiment, spark ignition is performed in a state where the flow around the ignition plug 11 is intensified by fuel injection based on the fuel injection valve 12. Thereby, as shown in fig. 11, the spark plug discharge passage CN between the center electrode 11a and the outer electrode 11b is elongated.

In this way, the flow is generated around the ignition plug 11 after the tumble flow is weakened, and the ignition plug discharge passage CN can be elongated, so that partial combustion and misfire can be suppressed to improve combustion stability.

Fig. 12A and 12B are diagrams showing examples of fuel injection patterns for extending the spark plug discharge passage CN. Fuel of 90% or more of the total injection amount is injected in the intake stroke in any case of fig. 12A, 12B. The remaining fuel may be injected 2 times in the latter half of the compression stroke (fig. 12A), or may be injected 1 time (fig. 12B). Further, the total injection amount referred to herein means the amount of fuel injected per 1 cycle.

Further, as described above, in the stratified charge combustion in the present embodiment, the amount of fuel injected in the latter half of the compression stroke to form a combustible mixture around the ignition plug 11 is 10% or less of the total injection amount. Therefore, the combustible mixture gas formed around the spark plug 11 is only a very small part of the entire combustion chamber. Such stratified combustion may be referred to as "weakly stratified combustion" in order to distinguish it from stratified combustion in which a large amount of fuel is injected in the latter half of the compression stroke.

Here, the control performed by the controller 50 will be explained.

First, switching of the combustion method will be described.

The controller 50 switches the combustion method according to the operating state of the internal combustion engine 10. The operating state referred to herein is the rotational speed and the load of the internal combustion engine 10. The rotation speed may be calculated by a known method based on the detection value of the crank angle sensor. The load may be calculated by a known method based on the detection value of the accelerator opening degree sensor.

Fig. 13 is a map showing a combustion mode executed in each operating state. In fig. 13, the vertical axis represents the load and the horizontal axis represents the rotation speed.

As shown in fig. 13, a part of the low and medium speed and low and medium load regions is a lean combustion region, and the other regions are homogeneous stoichiometric combustion regions. Further, the lean combustion region is divided such that the region with a relatively high load is a stratified lean combustion region and the region with a relatively low load is a homogeneous lean combustion region with the load Q1 as a boundary. The term "stratified lean combustion" as used herein refers to the aforementioned stratified combustion. Homogeneous stoichiometric combustion refers to combustion in which a mixture gas of a stoichiometric air-fuel ratio is formed in the entire combustion chamber. The load Q1 is set according to the specifications of the internal combustion engine 10 to which the present embodiment is applied.

In any case of the stratified lean combustion as well as the homogeneous lean combustion, the controller 50 basically controls the air excess ratio λ of the entire combustion chamber to 2. However, the air excess ratio λ is not strictly limited to 2, and includes a range of approximately 2. In order to ensure the ignitability and the like in accordance with an increase in load, the controller 50 may correct the excess air ratio λ to a leaner side than 2.

In the following description, the air-fuel ratio a/F may be used instead of the excess air ratio λ. In this case, the excess air ratio λ ≈ 2 indicates an air-fuel ratio a/F ≈ 30.

In addition, the controller 50 decreases the mechanical compression ratio in order to suppress the occurrence of knocking in accordance with an increase in the load of the internal combustion engine 10. However, at the time of the stratified lean combustion, the controller 50 controls the mechanical compression ratio to be higher than in the case where the homogeneous lean combustion is assumed to be performed under the same operating conditions. This is because stratified lean combustion is faster in combustion speed and more difficult to produce knocking than homogeneous lean combustion.

Here, the variable compression ratio mechanism will be explained. The variable compression ratio mechanism may be of a known structure. Here, an example of a known variable compression ratio mechanism will be described.

Fig. 14 shows a variable compression ratio mechanism in which a piston 25 and a crankshaft 30 are connected by a plurality of connecting rods, and the top dead center position of the piston 25 can be variably controlled.

The piston 25 is connected to a crankshaft 30 via an upper connecting rod 26 and a lower connecting rod 27. The upper link 26 has one end rotatably connected to the piston 25 and the other end rotatably connected to the lower link 27. The lower link 27 is rotatably coupled to a crank pin 30A of the crankshaft 30 at a position different from the coupling portion with the upper link 26. One end of the control link 28 is rotatably coupled to the lower link 27. The other end of the control link 28 is connected to a position offset from the rotation center of the control shaft 29.

In the variable compression ratio mechanism having the above-described configuration, the mechanical compression ratio can be changed by rotating the control shaft 29 by an actuator or the like, not shown. For example, if the control shaft 29 is rotated by a predetermined angle counterclockwise in the drawing, the lower link 27 is rotated counterclockwise in the drawing about the crank pin 30A via the control link 28. As a result, the top dead center position of the piston 25 is raised, and the mechanical compression ratio is raised. In contrast, if the control shaft 29 is rotated by a predetermined angle in the clockwise direction in the figure, the lower link 27 is rotated in the clockwise direction in the figure about the crank pin 30A via the control link 28. As a result, the top dead center position of the piston 25 is lowered, and the mechanical compression ratio is lowered.

Next, the ignition energy at the time of homogeneous lean combustion and at the time of stratified lean combustion will be described.

Fig. 15 is a graph showing changes in the gas flow rate in the discharge gap and the air-fuel ratio a/F in the discharge gap during homogeneous lean combustion. The horizontal axis in fig. 15 represents the crank angle [ deg ], and indicates the timing C2 and thereafter in fig. 10.

Fig. 16 is a graph showing a relationship between an elapsed time with respect to the ignition timing and the secondary current flowing through the ignition plug 11 at the time of the homogeneous lean combustion.

Fig. 17 is a graph showing changes in the gas flow rate in the discharge gap and the air-fuel ratio a/F in the discharge gap during homogeneous lean combustion. The horizontal axis in fig. 17 represents the crank angle [ deg ], and indicates the timing C1 and thereafter in fig. 10.

Fig. 18 is a graph showing a relationship between an elapsed time with respect to the ignition timing and the secondary current flowing through the ignition plug 11 in the stratified lean combustion. Note that the broken line in the figure is a graph at the time of the homogeneous lean combustion of fig. 16.

The "gas flow rate of the discharge gap" in fig. 15 and 17 is synonymous with the turbulence intensity described in fig. 10.

In the homogeneous lean combustion, the gas flow rate of the discharge gap decreases as the crank angle increases. In addition, since the air excess ratio λ of the entire combustion chamber is controlled to 2, that is, the air-fuel ratio a/F is controlled to substantially 30 at the time of the homogeneous lean combustion, it is needless to say that the air-fuel ratio a/F of the discharge gap is substantially 30.

In contrast, in the stratified lean combustion, spark ignition is performed after fuel injection is performed in the second half of the compression stroke. Therefore, the gas flow rate in the discharge gap is higher at the ignition timing than at the homogeneous lean combustion. However, since the effect of increasing the gas flow rate by fuel injection gradually decreases, the gas flow rate in the discharge gap is almost the same as that in the homogeneous lean combustion.

Further, the air-fuel ratio a/F of the discharge gap at the ignition timing becomes richer than that at the time of homogeneous lean combustion by fuel injection in the latter half of the compression stroke. However, the fuel injected in the latter half of the compression stroke is diffused by its own penetration force and tumble flow, and the air-fuel ratio a/F in the discharge gap gradually returns to 30.

In the homogeneous lean combustion, the a/F of the discharge gap is considerably leaner than the stoichiometric value, and is approximately 30, so that the gas mixture in the discharge gap is less likely to ignite than in the stratified lean combustion. In addition, in the homogeneous lean combustion, the combustion speed is slower than in the stratified lean combustion. Therefore, in the homogeneous lean combustion, in order to achieve stable combustion, it is necessary to cause a relatively large secondary current to continuously flow.

On the other hand, in the stratified lean combustion, since the gas flow rate in the discharge gap at the ignition timing is higher than that in the homogeneous combustion, it is necessary to increase the secondary current than that in the homogeneous lean combustion in order to form the initial flame kernel without losing the gas flow. However, as described above, the mixture gas in the discharge gap is more likely to ignite than in the case of homogeneous lean combustion, and therefore if combustion is once started, stable combustion can be obtained even if the secondary current is reduced. Therefore, at the time of stratified lean combustion, the secondary current can be reduced after the ignition timing. Further, as described above, since the mixture gas in the discharge gap is more likely to ignite than in the homogeneous lean combustion, the discharge time can be shortened in the stratified lean combustion as compared with the homogeneous lean combustion.

In the stratified lean combustion, as described above, by reducing the secondary current after the ignition timing or shortening the discharge time, the ignition energy consumed per 1 cycle can be made smaller than that in the homogeneous lean combustion.

As described above, the discharge waveform suitable for stratified lean combustion is different from the discharge waveform suitable for homogeneous lean combustion. The discharge waveform here refers to the history of the secondary current shown in fig. 16 and 18.

Therefore, the controller 50 controls the drive device 17 in such a manner that a constant secondary current is caused to flow at the time of homogeneous lean combustion, and a relatively large secondary current is caused to flow at the ignition timing and then the secondary current is reduced at the time of stratified lean combustion, respectively.

The waveform of the secondary current shown in fig. 18 is an example, and may be another waveform if the ignition energy is relatively large at the ignition timing and relatively small thereafter and is reduced as compared with that at the time of the homogeneous lean combustion. For example, the following various waveforms and the like can be considered: a waveform in which the secondary current gradually decreases in accordance with the elapsed time with respect to the ignition timing; the secondary current has a waveform that is constant for a predetermined time at which the self-ignition timing is initiated and that decreases stepwise after the predetermined time has elapsed.

Fig. 19 is a diagram showing the control contents described above as a control flow. The control flow is programmed by the controller 50.

In step S10, the controller 50 reads the operation state. Specifically, the rotational speed and the load of the internal combustion engine 10 are read.

In step S20, the controller 50 determines whether or not the current operating region is a lean combustion region, using the operating state read in step S10 and the map of fig. 13. The controller 50 executes the process of step S30 if it is a lean combustion region, and the controller 50 executes the process of step S60 if it is a homogeneous stoichiometric combustion region.

In step S30, the controller 50 determines whether the current operating region is a stratified lean combustion region. The controller 50 executes the process of step S40 if it is a stratified lean combustion region, and the controller 50 executes the process of step S50 if it is a homogeneous lean combustion region.

In step S40, the controller 50 controls the drive device 17 so as to form the above-described discharge waveform for stratified lean combustion.

In step S50, the controller 50 controls the drive device 17 so as to form the discharge waveform for homogeneous lean combustion described above.

In step S60, the controller 50 controls the drive device 17 so as to form a discharge waveform for homogenous stoichiometric combustion. The discharge waveform for homogeneous stoichiometric combustion is basically the same as the discharge waveform for homogeneous lean combustion, but has a smaller secondary current and a shorter discharge time than the discharge waveform for homogeneous lean combustion.

Next, the operation and effect of executing the control flow will be described.

Fig. 20 is a diagram showing a relationship between the secondary current, the discharge time, the secondary voltage, and the ignition energy in the lean combustion region and the load of the internal combustion engine 10. The load Q1 in the figure is the same as the load Q1 in fig. 13. In the figure, values in the case where homogeneous lean combustion is assumed to be performed even in the entire region of the lean combustion region, that is, the region of a relatively high load, are shown by broken lines for comparison. Note that the secondary current in fig. 20 is a current value of the ignition timing. As described above, the controller 50 performs control to reduce the secondary current after the ignition timing.

The secondary current at the time of stratified lean combustion is higher than that in the case where homogeneous lean combustion is performed in this region. However, since the controller 50 controls the secondary current to decrease after the ignition timing, the secondary current in the stratified lean combustion region is smaller than that in the case where the homogeneous lean combustion is performed in the region from the middle stage to the late stage of the discharge period.

The discharge time in the stratified lean combustion is lower than that in the case where the homogeneous lean combustion is performed in this region.

In the entire lean combustion region, the secondary voltage increases in accordance with an increase in the load, and the ignition energy also increases accordingly. However, in the region of a relatively high load, since stratified lean combustion is performed by controlling the secondary current and the discharge time as described above, the ignition energy is reduced as compared with the case of performing homogeneous lean combustion in this region.

Fig. 21 is a diagram showing the relationship between the air-fuel ratio, the mechanical compression ratio, and the fuel consumption of the entire combustion chamber in the lean combustion region and the load of the internal combustion engine 10. The load Q1 in the figure is the same as the load Q1 in fig. 13. In the figure, values in the case where homogeneous lean combustion is assumed to be performed even in the entire region of the lean combustion region, that is, the region of a relatively high load, are shown by broken lines for comparison.

As the load increases, the controller 50 makes the air-fuel ratio of the entire combustion chamber richer than 30 in order to ensure ignitability and the like. However, in the case of stratified lean combustion, the equivalence ratio around the spark plug 11 is increased by fuel injection in the latter half of the compression stroke, and ignition becomes easy. Therefore, in the case of the stratified lean combustion, the air-fuel ratio of the entire combustion chamber can be made leaner than in the case of performing the homogeneous lean combustion in the same region.

In addition, as the load increases, the controller 50 lowers the mechanical compression ratio in order to suppress the occurrence of knocking. However, in the case of stratified lean combustion, the fuel injection in the latter half of the compression stroke increases the equivalence ratio around the spark plug 11, whereby the propagation of flame is accelerated, and thus knocking is less likely to occur. Therefore, in the case of the stratified lean combustion, the mechanical compression ratio can be made higher than in the case of performing the homogeneous lean combustion in the same region.

As described above, if stratified lean combustion is performed in a region of a relatively high load, the air-fuel ratio of the entire combustion chamber can be made leaner and the mechanical compression ratio can be further increased, as compared with the case where homogeneous lean combustion is performed in the same region. As a result, fuel efficiency in a relatively high load region is improved as compared with the case where homogeneous lean combustion is performed in the same region.

As described above, the control method of the internal combustion engine 10 according to the present embodiment is a control method of an internal combustion engine in which stratified charge is formed in a combustion chamber and stratified combustion is performed by performing at least 1 fuel injection during a period from an intake stroke to the first half of a compression stroke and during the second half of the compression stroke. In the present embodiment, when the flow energy around the spark plug 11 increases due to the energy of the fuel spray injected in the second half of the compression stroke, a relatively large secondary current (also referred to as a discharge current) flows through the spark plug 11 to start spark ignition, and then the secondary current is relatively reduced to perform discharge for a predetermined period. The secondary current at the time of starting spark ignition is relatively increased in order to form a discharge channel rather than the flow around the ignition plug 11 enhanced by fuel injection in the latter half of the compression stroke. The reason why the secondary current is reduced is that the fuel injection in the latter half of the compression stroke increases the equivalence ratio of the air-fuel mixture around the spark plug 11, and the air-fuel mixture is easily combusted, whereby stable combustion can be achieved with a small amount of ignition energy. In this way, by controlling the discharge waveform to a waveform suitable for stratified combustion during stratified lean combustion, ignition energy during stratified lean combustion can be reduced, and fuel efficiency can be improved.

In the present embodiment, in the lean combustion region, stratified charge combustion is performed in an operating region where the load on the internal combustion engine 10 is relatively high, and homogeneous lean combustion is performed in an operating region where the load on the internal combustion engine 10 is relatively low. Further, the discharge waveform of the spark plug is made different at the time of stratified combustion and at the time of homogeneous lean combustion. This makes it possible to set a discharge waveform suitable for each combustion mode at the time of stratified combustion and at the time of homogeneous lean combustion.

In the present embodiment, the air excess ratio λ of the entire combustion chamber is controlled to 2 in both cases of stratified combustion and homogeneous lean combustion. Thus, the lean operation region is enlarged as compared with the case where stoichiometric combustion is performed in all regions except the stratified lean combustion region, and therefore fuel efficiency is improved.

In the present embodiment, the discharge time (discharge continuation period) of the ignition plug 11 at the time of the homogeneous lean combustion is made longer than the discharge time at the time of the stratified combustion. In the homogeneous lean combustion, ignitability is reduced and combustion is slowed compared to the stratified lean combustion, but the combustion is stabilized by extending the discharge time. As a result, fuel consumption can be improved and emissions can be reduced.

In the present embodiment, the ignition energy at the time of homogeneous lean combustion is made larger than the ignition energy at the time of stratified combustion. In order to increase the ignition energy, for example, the discharge time may be increased or the accumulated value of the secondary current in the discharge period may be increased by increasing the secondary current. This stabilizes combustion during homogeneous lean combustion, and therefore improves fuel efficiency and reduces emissions.

While the embodiments of the present invention have been described above, the above embodiments are merely illustrative of some application examples of the present invention, and the technical scope of the present invention is not limited to the specific configurations of the above embodiments.