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 吉村太 岩渊良彦 于 2017-05-24 设计创作，主要内容包括：一种内燃机的控制方法,该内燃机具有：燃料喷射阀,其将燃料直接喷射至缸内；以及火花塞,其对从燃料喷射阀喷射的燃料直接进行火花点火,其中,将启动内燃机时的内燃机旋转速度的实际的变化动作即实际动作,与预先设定的基准动作进行比较,在实际动作与基准动作不同的情况下,从对从燃料喷射阀喷射而滞留于火花塞周围的燃料喷雾直接进行火花点火的分层燃烧,向在燃烧室内形成均质的混合气体而使燃料燃烧的均质燃烧切换,并且与实际动作和基准动作一致的情况相比,提高内燃机的机械压缩比。(A control method of an internal combustion engine having: a fuel injection valve that directly injects fuel into the cylinder; and an ignition plug for directly performing spark ignition on the fuel injected from the fuel injection valve, wherein an actual operation, which is an actual change operation of an engine rotational speed at the time of starting the internal combustion engine, is compared with a preset reference operation, and when the actual operation is different from the reference operation, stratified combustion in which spark ignition is directly performed on fuel spray injected from the fuel injection valve and accumulated around the ignition plug is switched to homogeneous combustion in which fuel is combusted by forming a homogeneous mixed gas in a combustion chamber, and a mechanical compression ratio of the internal combustion engine is increased as compared with a case where the actual operation and the reference operation coincide with each other.)

1. A control method of an internal combustion engine having:

a fuel injection valve that directly injects fuel into the cylinder; and

a spark plug that directly performs spark ignition on the fuel injected from the fuel injection valve, wherein,

the actual change operation of the engine rotational speed at the time of starting the engine, that is, the actual operation is compared with a preset reference operation,

when the actual operation and the reference operation are different from each other, the stratified combustion in which spark ignition is directly performed on the fuel spray injected from the fuel injection valve and accumulated around the spark plug is switched to homogeneous combustion in which a homogeneous gas mixture is formed in a combustion chamber and the fuel is combusted, and the mechanical compression ratio of the internal combustion engine is increased as compared with a case where the actual operation and the reference operation are matched.

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

the action is a slope of an increase in the engine rotational speed after the internal combustion engine starts combustion.

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

combustion is switched, and deposit removal control for removing deposits adhering to the tip of the fuel injection valve is executed.

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

the deposit removal control is a control for increasing the fuel injection pressure as compared with a case where the actual operation and the reference operation match.

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

if the deposit removal control is executed for a prescribed period, the homogeneous combustion is switched to the stratified combustion.

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

a fuel injection valve that directly injects fuel into the cylinder;

a spark plug that directly performs spark ignition on the fuel injected from the fuel injection valve;

a sensor that acquires an internal combustion engine rotation speed;

a variable compression ratio mechanism that changes a mechanical compression ratio; and

a control portion that controls the fuel injection valve, the ignition plug, and the variable compression ratio mechanism, wherein,

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

the actual variation operation of the engine rotational speed at the time of starting the engine is compared with a preset reference operation,

when the actual changing operation and the reference operation are different from each other, the stratified combustion in which spark ignition is directly performed on the fuel spray injected from the fuel injection valve and accumulated around the spark plug is switched to homogeneous combustion in which a homogeneous gas mixture is formed in a combustion chamber and the fuel is combusted, and the mechanical compression ratio of the internal combustion engine is increased as compared to before the switching of the fuel.

Technical Field

The present invention relates to a control method and a control device for an internal combustion engine, the internal combustion engine including: a fuel injection valve that directly injects fuel into the cylinder; and an ignition plug that directly performs spark ignition on the fuel injected from the fuel injection valve.

Background

There is known a rapid idle speed control in which a stratified charge is formed around a spark plug after a cold start of an internal combustion engine, and stratified combustion is performed by retarding an ignition timing until after compression top dead center. Stratified combustion can raise the exhaust temperature by largely retarding the ignition timing, and is therefore effective for activating the exhaust catalyst as soon as possible.

As a method of forming a stratified mixed gas around a spark plug, a wall surface (wall) guide type is currently mainly used, in which a fuel spray is reflected toward a chamber provided in a piston to form a stratified mixed gas around a spark plug. However, in the wall surface guide type, part of the fuel that collides with the piston tends to remain on the top surface of the piston, and the remaining fuel may be burned to produce coal. Therefore, in recent years, demands for exhaust performance have been increasing, and an injection-guided type in which fuel is injected around a spark plug to form a stratified mixture has attracted attention.

However, in a so-called direct in-cylinder injection internal combustion engine in which fuel is directly injected into a cylinder, the tip end of a fuel injection valve is exposed in a combustion chamber and is easily affected by combustion in the cylinder, and therefore an actual spray pattern is deviated from a design pattern (hereinafter, also referred to as a reference pattern) due to a change with time or the like. In the wall surface guide type, even if the spray pattern is somewhat deviated, the fuel spray advances around the spark plug if it collides with the chamber, whereas in the injection guide type, there is no function of correcting the deviation of the spray pattern such as the wall surface guide type. Therefore, in the injection-guided type, if the spray pattern is deviated from the reference pattern, it is difficult to secure combustion stability.

As a control for solving this problem, JP2001-152931a1 discloses a control for switching to homogeneous combustion by prohibiting stratified combustion when a specific condition is satisfied.

Disclosure of Invention

However, in the case of homogeneous combustion, if the ignition timing is greatly retarded as in the case of stratified combustion, the combustion stability cannot be ensured, and therefore the amount of retardation of the ignition timing is limited compared to the case of stratified combustion. That is, in the case of homogeneous combustion, the exhaust gas temperature cannot be increased as compared with the case of stratified combustion. Therefore, if the stratified combustion is switched to the homogeneous combustion as in the above-described document, activation of the exhaust catalyst is delayed, and exhaust performance is deteriorated. However, in the above-mentioned document, a method of switching to homogeneous combustion to suppress deterioration of exhaust performance is adopted.

Therefore, the present invention aims to suppress deterioration of exhaust performance even when homogeneous combustion is performed by inhibiting stratified combustion in fast idle control.

According to an aspect of the present invention, there is provided a method of controlling an internal combustion engine including: a fuel injection valve that directly injects fuel into the cylinder; and an ignition plug that directly performs spark ignition on the fuel injected from the fuel injection valve. In this control method, an actual operation, which is an actual change operation of the engine rotational speed at the time of starting the engine, is compared with a preset reference operation. When the actual operation and the reference operation are different from each other, the stratified combustion in which spark ignition is directly performed on fuel spray injected from the fuel injection valve and accumulated around the spark plug is switched to the homogeneous combustion in which fuel is combusted by forming a homogeneous mixed gas in the combustion chamber, and when the actual operation and the reference operation are the same, the mechanical compression ratio of the internal combustion engine is increased.

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 diagram for explaining the position of the spark plug and the combustion stability.

Fig. 14 is a diagram showing a relationship between a position of the spark plug and a combustion stability.

Fig. 15 is a flowchart showing a control flow executed by the controller.

Fig. 16 is a diagram showing an example of the variable compression ratio mechanism.

Fig. 17 is a timing chart in the case where the control flow of fig. 16 is executed.

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 performs spark ignition in a combustion chamber of the internal combustion engine 10. The spark plug 11 is connected to a controller 50, and the controller 50 as a control unit controls the spark ignition timing. As described later, the ignition plug 11 also operates as a flow rate sensor 23. The method of detecting the flow rate will be described later.

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 and an in-cylinder pressure 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.

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. The cross-sections of the spray beams B1-B6 cut by the plane including the circle a are arranged at equal intervals in a circular 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 a 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 outer edge on the upper side in the drawing and the outer edge on the lower side in the drawing of spray beam B3. 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 an explanatory diagram of tumble flow collapse. 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, the piston rises in the compression stroke, and the combustion chamber in the cylinder is contracted. If the combustion chamber narrows, the tumble flow is crushed and gradually fails to maintain its flow (fig. 9), and is eventually destroyed.

Therefore, in the case where a stratified charge air-fuel mixture is formed around the ignition plug 11 and stratified rapid idle speed control (hereinafter also referred to as stratified FIR control) is executed until the ignition timing is retarded after compression top dead center, the flow around the ignition plug 11 is weakened at the time of ignition of the ignition plug. 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 is not sufficiently elongated, and thus, a misfire or partial combustion is likely to occur.

Therefore, in the present embodiment, the characteristic that the turbulence intensity around the spark plug 11 is increased by injecting the fuel is utilized, and the state in which the spark plug discharge passage CN is elongated occurs after the tumble flow collapses.

Fig. 10 is a timing chart showing changes in the turbulence intensity around the ignition plug 11 in the case where fuel injection is performed after compression top dead center. In fig. 10, the horizontal axis represents the crank angle and the vertical axis represents the turbulence intensity around the spark plug 11. As described above, the intensity of the tumble flow gradually decreases, and therefore the intensity of turbulence around the spark plug 11 also decreases along with this. However, if the fuel injection is performed after compression top dead center, the turbulence intensity for a prescribed period after the fuel injection is increased. During the period in which the turbulence intensity is increased by this fuel injection, the spark plug discharge passage CN is likely to be elongated. In particular, the timing at which the turbulence intensity peaks is suitable as the ignition timing.

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, and an embodiment may be possible in which the flow is reflected in the combustion chamber and generated in the vicinity of the ignition plug even if it is directed in a different direction.

The flow near the spark plug 11 after the tumble flow collapse is small. 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, the flow is generated in the vicinity of the ignition plug 11 by the fuel injection valve 12 until the spark plug discharge passage CN is generated after the tumble flow collapses. Then, as shown in fig. 11, the spark plug discharge passage CN between the center electrode 11a and the outer electrode 11b is elongated by the generated flow.

Thereby, it is possible to generate a flow in the combustion chamber after the tumble flow collapse and to elongate the spark plug discharge passage CN, so it is possible to suppress local combustion and misfire and improve combustion stability. In particular, even in a situation where flame propagation combustion is difficult compared to normal conditions, such as a case of using EGR or a case of using lean combustion, which will be described later, spark ignition can be stably performed.

Fig. 12A and 12B are diagrams showing examples of fuel injection patterns for extending the spark plug discharge passage CN. In addition to the intake stroke and the expansion stroke of the multi-stage injection described above, the fuel injection may be further performed during the period after the tumble flow is collapsed until the spark plug discharge passage is generated (fig. 12A), or the expansion stroke injection of the multi-stage injection may be performed during the period after the tumble flow is collapsed until the spark plug discharge passage is generated (fig. 12B).

However, since the fuel injection valve 12 that directly injects fuel into the cylinder is exposed to combustion flame or combustion gas, so-called deposits tend to accumulate around the injection hole. Further, if the deposit blocks the traveling path of the fuel spray, for example, as shown in fig. 13, the spray pattern such as the shape and traveling direction of the spray beam is deviated from the reference pattern set for improving the flow intensity around the ignition plug 11 by the fuel injection. As a result, even if fuel is injected, the flow strength around the spark plug 11 is not increased, and the combustion stability in the stratified FIR control may be lowered.

In addition, it is also possible for the exhaust temperature in the stratified FIR control to not reach the target exhaust temperature due to the decrease in combustion stability. Here, the relationship between the combustion stability and the exhaust gas temperature will be described.

Fig. 14 is a diagram for explaining the relationship between the combustion stability and the exhaust gas temperature. The horizontal axis in fig. 14 represents the position [ deg.ca ] of the combustion center of gravity. The "limit of combustion stability" in the figure is the degree of combustion stability when the noise or vibration reaches the upper limit value tolerable by the occupant. The target exhaust gas temperature in the figure is a target value of the exhaust gas temperature in the stratified FIR control, and is a value set from the viewpoint of early activation of the exhaust catalysts 44, 45, and the like. In the figure, a solid line a indicates the case of the reference pattern, and a solid line B indicates the case of being deviated from the reference pattern.

As shown in fig. 14, it is known that the closer the combustion center of gravity is to the retard angle side, the higher the exhaust gas temperature. On the other hand, the closer the combustion center of gravity is to the retard angle side, the lower the combustion stability. In the reference mode (solid line a), the extension of the spark plug discharge passage CN ensures combustion stability to a position closer to the retard angle side. Also, the exhaust temperature is greater than or equal to the target exhaust temperature when the combustion stability limit is reached.

In contrast, if the combustion stability limit deviates from the reference pattern, the combustion stability limit is closer to the advance angle side than the reference pattern. Therefore, when the combustion stability exhibits a characteristic such as the solid line B, for example, the exhaust temperature at the combustion stability limit is lower than the target exhaust temperature.

Therefore, when the combustion stability cannot be ensured in the hierarchical FIR control, some measure needs to be taken. For example, as the FIR control, switching to control for forming a homogeneous mixture gas in the combustion chamber and burning the fuel (hereinafter, also referred to as homogeneous FIR control) may be considered. However, in the case of homogeneous combustion, if the ignition timing is greatly retarded as in the case of stratified combustion, the combustion stability decreases. Therefore, the exhaust temperature cannot be sufficiently raised only by switching to the homogeneous FIR control, which may cause deterioration of the exhaust performance.

Therefore, in the present embodiment, the controller 50 performs the control described below in order to ensure combustion stability and suppress deterioration of exhaust performance.

The inventors also considered that the main cause of the failure to ensure the combustion stability in the stratified FIR control was the change in the fuel spray pattern due to the deposit described above, but the possibility of other causes was not denied.

Fig. 15 is a flowchart showing a control flow executed by the controller 50. The controller 50 is programmed in a manner to execute the control flow. This routine is executed at the time of cold start of the internal combustion engine 10. The following steps are described.

In step S100, the controller 50 calculates dR/dt, which is the gradient of the increase in the engine rotational speed after the start of combustion in the internal combustion engine 10, using the detection value of the crank angle sensor.

In step S110, the controller 50 determines whether or not dR/dt acquired in step S100 is greater than a threshold value X serving as a reference operation. If the spray pattern changes due to the deposit, the combustion stability decreases as compared with the case where no deposit is deposited, and therefore, a delay in the first explosion and a decrease in the output occur, and as a result, the increase in the engine rotational speed becomes slow. Therefore, in step S110, it is determined whether or not deposits have accumulated at the tip of the fuel injection valve 12 by dR/dt. The threshold value X is a value smaller by a predetermined amount than the gradient of the increase in the engine rotational speed after the start of combustion in a state where deposits are not accumulated in the fuel injection valve 12. The value smaller by the prescribed amount is utilized because if the exhaust gas temperature at which the combustion stability reaches the combustion stability limit is greater than or equal to the target exhaust gas temperature, a drop in the combustion stability can be tolerated. Therefore, the predetermined amount is determined based on the characteristics of the change in the combustion stability caused by the accumulation of the deposits. In addition, the parameter for determining whether or not the precipitate is accumulated is not limited to dR/dt. Parameters other than dR/dt are described later.

If it is determined in step S110 that dR/dt is greater than the threshold value X, the controller 50 performs the process of step S120, and if it is determined that dR/dt is less than or equal to the threshold value X, the process of step S130 is performed.

In step S120, the controller 50 executes normal FIR control. The normal FIR control referred to herein is a hierarchical FIR control in which the deposit removal control described later is not executed. Further, the stratified combustion control for starting is executed from the start of cranking until the process of this step is executed. The stratified combustion control for starting is different from the stratified FIR control in that the ignition timing is not retarded.

The controller 50 determines the start of the deposit removal control in step S130, and increases the target value of the fuel injection pressure (hereinafter also referred to as the target fuel pressure) in step S140 as compared to the case where the deposit removal control is not executed. The target fuel pressure is increased in order to increase the fuel flow rate in the vicinity of the nozzle hole of the fuel injection valve 12 and thereby blow away the deposits. The increased target fuel pressure may be, for example, the maximum fuel pressure that can be achieved by the fuel injection pump 15, or may be a fuel pressure that can blow off deposits, which is determined through experiments or the like.

In step S150, the controller 50 increases the mechanical compression ratio of the internal combustion engine 10 and switches from the stratified combustion control for starting to the homogeneous FIR control.

Here, the rise of the mechanical compression ratio is explained.

The mechanical compression ratio is changed by the variable compression ratio mechanism. A known structure may be used for the variable compression ratio mechanism. Here, an example of a known variable compression ratio mechanism will be described.

Fig. 16 shows a variable compression ratio mechanism in which a piston 25 and a crankshaft 30 are connected by a plurality of connecting rods to variably control the top dead center position of the piston 25.

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.

Returning to the description of the flowchart.

The reason why the transition to the homogeneous FIR control is made in step S150 is that even if the pattern of the fuel spray changes due to the deposit, the combustion stability can be ensured in the case of homogeneous combustion. However, in the case of homogeneous combustion, the amount of delay in the ignition timing that can ensure combustion stability is reduced as compared with the case of stratified combustion, and therefore if switching is made to homogeneous FIR control, the exhaust gas temperature is reduced as compared with stratified FIR control. Therefore, the time until the activation of the exhaust catalyst is achieved is prolonged, resulting in a decrease in exhaust performance. On the other hand, if the mechanical compression ratio is made high, it is easy to ensure the combustion stability, so the ignition timing can be further retarded. Therefore, by switching to the homogeneous FIR control and increasing the mechanical compression ratio, the combustion stability and a sufficient ignition timing retardation amount can be ensured and the degradation of the exhaust performance can be suppressed.

In step S160, the controller 50 determines whether or not a predetermined time has elapsed, and if the predetermined time has elapsed, executes the process of step S170. The predetermined time here is a time required for blowing off the deposit while the fuel pressure is increased, which is obtained in advance through an experiment or the like, and is set based on the time, and is, for example, about several seconds.

In step S170, the controller 50 decreases the fuel pressure and the mechanical compression ratio and switches to the stratified FIR control. Then, in step S180, the controller 50 determines whether the combustion stability is less than a target value, that is, whether the combustion stability is ensured. If the combustion stability is ensured, the flow is ended as it is, and if the combustion stability is not ensured, the flow returns to the processing of step S130.

As described above, when the combustion stability cannot be ensured in the stratified FIR control due to the deposit, the controller 50 executes the switching to the homogeneous FIR control and the increase of the mechanical compression ratio, thereby executing the deposit removal control while ensuring the combustion stability and the exhaust performance. Then, if a predetermined time has elapsed, the deposit removal control is terminated, the mechanical compression ratio is decreased, and after returning to the stratified FIR control, it is checked whether or not the combustion stability is ensured. Here, if the combustion stability is not ensured, the control is switched to the homogeneous FIR control again, the mechanical compression ratio is raised, and the deposit removal control is executed.

Fig. 17 is a timing chart in the case where the above-described control flow is executed.

The dotted line in the figure indicates a state where no precipitate is attached (hereinafter also referred to as a normal state), and the solid line indicates a state where a precipitate is attached (hereinafter also referred to as a deteriorated state).

If it is determined that the internal combustion engine is started to start cranking at the timing T0, the engine rotational speed R is raised to a prescribed rotational speed and maintained. Then, combustion is started at timing T1, and the engine rotational speed R starts to rise again. In addition, as the engine rotation speed increases, the rotation speed of the fuel injection pump 15 driven by the internal combustion engine 10 also increases, and therefore the fuel pressure increases.

The engine rotational speed R increases with the start of combustion in both the normal state and the degraded state, and converges to the idle rotational speed after one-end overshoot. The dR/dt is a slope of the rise of the rotation speed from the timing T1 to the timing T2. As described above, in the degraded state, the rising slope is smaller than that in the normal state. The time from the timing T1 to the timing T2 may be set arbitrarily. For example, the time until the engine rotational speed R reaches 1000[ rpm ] in the normal state is measured in advance and set.

Controller 50 calculates dR/dt at timing T2 and determines whether the state is normal or degraded. If the engine is in the normal state, the controller 50 switches the combustion state from the stratified combustion control for starting to the stratified FIR control, and lowers the mechanical compression ratio from the value for starting to the value for stratified FIR control. In contrast, if the engine is in the degraded state, the controller 50 switches the combustion state from the stratified combustion control for starting to the homogeneous FIR control at timing T3, increases the mechanical compression ratio from the value for starting to the value for the homogeneous FIR control, and increases the fuel pressure for the deposit removal control. Thereby, the same combustion stability as that in the normal state is ensured even in the deteriorated state.

Further, the timing T3 is the timing at which the engine rotation speed R in the degraded state reaches the engine rotation speed R in the normal state at the timing T2. This is because if the stratified charge combustion control for starting is ended in a state where the engine rotational speed is not sufficiently increased, the combustion stability cannot be ensured.

At a timing T4 when a predetermined time has elapsed after the switching to the homogeneous FIR control, the combustion state is switched to the stratified FIR control, and the mechanical compression ratio and the fuel pressure are reduced to the values for the stratified FIR control. Also, at the timing T5, the controller 50 determines whether the combustion stability is ensured. If the deposits are removed by the deposit removal control, the determination result that the combustion stability is ensured is obtained by the determination, and the controller 50 continues the hierarchical FIR control as it is. On the other hand, if the precipitates are not removed, the combustion stability decreases as shown in fig. 17. In this case, the controller 50 switches to the homogeneous FIR control again to raise the mechanical compression ratio and the fuel pressure.

As described above, in the present embodiment, the actual operation, which is the actual variation operation of the engine rotational speed at the time of starting the internal combustion engine, is compared with the preset reference operation. When the actual operation and the reference operation are different from each other, the stratified combustion in which spark ignition is directly performed from the fuel spray injected from the fuel injection valve 12 and accumulated around the ignition plug 11 is switched to the homogeneous combustion in which a homogeneous mixed gas is formed in the combustion chamber and the fuel is combusted, and the mechanical compression ratio of the internal combustion engine 10 is increased as compared with the case where the actual operation and the reference operation are matched. By switching to the homogeneous combustion, the combustion stability can be ensured in a situation where the combustion stability cannot be ensured in the stratified combustion, and a sufficient ignition timing retardation can be ensured by increasing the mechanical compression ratio, so that the degradation of the exhaust performance can be suppressed.

In the present embodiment, the operation of comparing the engine rotational speeds is defined as the slope of the increase in the engine rotational speed after the start of combustion in the engine. When deposits accumulate at the tip end of the fuel injection valve 12, a desired stratified mixture cannot be formed at the time of start-up, and the combustion becomes slow, so that the increase in the engine rotational speed is delayed from the normal state. In the present embodiment, the presence or absence of deposit can be reliably determined using this characteristic.

In the present embodiment, the combustion is switched and the deposit removal control for removing the deposits adhering to the tip of the fuel injection valve 12 is executed. This improves the combustion stability.

The deposit removal control in the present embodiment is a control for increasing the fuel injection pressure as compared with the case where the actual operation and the reference operation match. Since fuel is injected at a high fuel pressure to blow off the deposits, the operation of the internal combustion engine 10 does not need to be stopped, and a new device does not need to be added, so that the combustion stability can be improved.

In the present embodiment, if the deposit removal control is executed for a predetermined period, the homogeneous combustion is switched to the stratified combustion. That is, after the removal of the deposit, the compression ratio is reduced to return to the same hierarchical FIR control as the normal state. Thereby, the same exhaust performance as that in the normal state can be obtained.

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.