JP5477246B2 - Combustion control method and combustion control apparatus for gasoline engine for performing combustion - Google Patents

Description

 The present invention relates to a combustion control method and a combustion control device for a gasoline engine that performs HCCI (premixed compression ignition) combustion.

 Some gasoline engines to which a fuel containing at least gasoline is supplied perform HCCI combustion as described in Patent Document 1. In this HCCI combustion, ignition and combustion can be performed even in an atmosphere in which the air-fuel ratio is considerably leaner than the stoichiometric air-fuel ratio, and since rapid combustion is performed, this is a very preferable combustion mode for improving fuel consumption. In the technique described in Patent Document 1, a stratified region is formed by second fuel injection after performing first fuel injection for premixed compression ignition, and any fuel injection is before ignition (ignition) starts. It is to be done. In the high load region where NOx generation is a problem, the injection amount of the first fuel injection is set to zero.

JP 2009-74488 A

By the way, in HCCI combustion, in a high load region where the fuel injection amount is large, combustion pressure becomes extremely high due to rapid combustion, so that not only noise but also combustion temperature becomes extremely high and NOx is generated. Is a big problem. For this reason, it is necessary to substantially prevent HCCI combustion in a high load region or to separately provide a NOx purification catalyst in the exhaust passage.

 The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a combustion control apparatus for a gasoline engine capable of suppressing NOx generation while performing HCCI combustion in a high load range. is there.

In order to achieve the above object, the following solution technique is adopted in the combustion control method of the present invention. That is, as described in claim 1 in the claims,
A method for controlling combustion in a gasoline engine to which fuel containing at least a gasoline is supplied,
The engine has a piston crown formed along the ceiling of the combustion chamber, a recess formed in the center of the upper surface of the piston, and a geometric compression ratio of 15 or more.
In the compression top dead center position, the volume ratio of the recess to the total combustion chamber volume is set to 10 to 20%,
In the engine high load range, split injection of front-stage fuel injection that injects fuel in the intake stroke and rear-stage fuel injection that injects fuel into the recesses in the compression stroke, and immediately before compression top dead center or compression top dead center The fuel in the recess is compressed and self-ignited, and the fuel other than the recess is ignited with a delay from the compression self-ignition, so that the first peak of the heat generation rate of combustion that generates torque is the expansion stroke. It is a combustion mode in which the heat generation rate increases again after a period when the heat generation rate does not increase after the piston descending timing has elapsed.
It is like that.

According to the above solution, the combustion is performed in such a way that the heat generation rate has two peaks. That is, first, the fuel in the recess is compressed and ignited to start combustion. At this time, the fuel in the gap between the piston crown surface other than the recess and the combustion chamber ceiling surface takes heat to the combustion chamber wall surface. Will be ignited. As the piston descends, the gap between the combustion chamber ceiling surface and the piston crown surface widens, so that the fuel in this gap is subsequently burned. As a result, an increase in the in-cylinder temperature can be suppressed and NOx generation can be suppressed as compared with a combustion mode in which all fuels are burned simultaneously and the peak of the heat generation rate occurs only once. it can. Further, since the fuel is injected at least in the intake stroke, the fuel and air are mixed well, which is preferable in terms of CO reduction and smoke reduction. In addition to the above, it is preferable for reliably performing the compression self-ignition of the fuel in the recess, and also preferable for avoiding the ignition delay by appropriately setting the compression self-ignition timing.

A preferred mode based on the above solution is as set forth in claims 2 to 3 in the claims. That is,
The compression self-ignition timing of the fuel in the recess is adjusted so that the combustion temperature at the peak of the heat generation rate falls below a predetermined temperature (corresponding to claim 2). In this case, it is preferable to control the combustion temperature and more reliably suppress NOx.

By operating the spark plug, compression self-ignition of the fuel in the recess is promoted (corresponding to claim 3 ). In this case, it is preferable to perform compression self-ignition more reliably.

In order to achieve the above object, the following solution is adopted in the combustion control device of the present invention. That is, as described in claim 4 in the claims,
A combustion control device for a gasoline engine to which a fuel containing at least a gasoline is supplied,
A crown surface of the piston is formed along the ceiling surface of the combustion chamber, and a recess is formed at the center of the upper surface of the piston.
The geometric compression ratio of the engine is set to 15 or higher,
In the compression top dead center position, the volume ratio of the recess to the total combustion chamber volume is set to 10 to 20%,
A fuel injection valve for injecting fuel into the cylinder and a controller for controlling the fuel injection valve;
The controller controls the fuel injection valve so as to perform split injection of pre-stage fuel injection in which fuel is injected in an intake stroke and post-stage fuel injection in which fuel is injected into the recess in a compression stroke in an engine high load range. As a result, the fuel in the concave portion is compressed and self-ignited immediately before or at the compression top dead center, and the fuel other than the concave portion is ignited after the compression self-ignition to generate torque. The first peak of the heat generation rate of combustion is the piston lowering timing of the expansion stroke, and after that, after a period in which the heat generation rate does not increase, the heat generation rate increases again.
It is like that. According to the said solution technique, the combustion control apparatus for performing the combustion control method described in Claim 1 is provided.

 According to the present invention, generation of NOx can be suppressed while improving fuel efficiency by performing HCCI combustion in a high load range.

The schematic block diagram which shows the state of the combustion chamber vicinity of the engine to which this invention was applied. The figure which shows the example of a control system of this invention in a block diagram. The figure which shows the example of a setting of an operation area | region. The figure which shows the relationship between a heat generation rate and in-cylinder temperature.

FIG. 1 schematically shows the vicinity of the combustion chamber of the engine 1. In the embodiment, the engine 1 is an in-line four-cylinder automobile gasoline engine that is a reciprocating type. The engine 1 is not limited to gasoline 100% as long as it contains fuel, and may contain other fuel such as ethanol, for example. In FIG. 1, 2 is a cylinder, and 3 is a piston slidably fitted in the cylinder 2. A combustion chamber 4 is defined above the piston 3, and a recess 5 is formed at the center of the upper surface of the piston 3 in order to ensure the combustion chamber volume at the compression top dead center position.

An intake port 6 and an exhaust port 7 are opened in the cylinder 2, that is, in the combustion chamber 4. The intake port 6 is opened and closed by an intake valve 8, and the exhaust port 7 is opened and closed by an exhaust valve 9. Further, a fuel injection valve 10 and a spark plug 11 are disposed so as to face the combustion chamber 4 (recessed portion 5) near the compression top dead center. The fuel injection valve 10 is of a porous type and is dispersed in the radial direction of the combustion chamber 4 (recess 5) to inject fuel.

 In the exhaust passage (not shown) connected to the exhaust port 7, a three-way catalyst as an exhaust gas purification catalyst is disposed, but in the exhaust passage, a NOx catalyst is not disposed. . That is, in this embodiment, as described later, even if HCCI (premixed compression ignition) combustion is performed with a lean air-fuel ratio, the generation of NOx itself is suppressed, so a NOx catalyst is not required. It is set.

The engine 1 has a high compression ratio as a gasoline engine. In the embodiment, the volume per cylinder is approximately 500 cc, and the geometric compression ratio is 18. In addition, when the volume (displacement amount) per cylinder, which is common as an automobile engine, is 400 cc to 600 cc, the geometric compression ratio is appropriately set within a range of, for example, 15 to 21 (preferably 17 to 20). be able to.

The crown surface (upper surface) of the piston 3 is set to a shape along the ceiling surface of the combustion chamber 4. In the embodiment, the combustion chamber 4 is a pent roof type in which the ceiling surface has two inclined surfaces 4a and 4b. Correspondingly, the crown surface of the piston 1 has an inclined surface 3a parallel to the inclined surface 4a and an inclined surface 3b parallel to the inclined surface 4b. Thereby, in the compression top dead center position, the gap between the inclined surfaces 4a and 3a and the inclined surfaces 4b and 3b facing each other are extremely small. And the combustion chamber volume in a compression top dead center position is ensured by the recessed part 5 formed in the center part of the piston 1. FIG. In addition, in the compression top dead center position, the volume ratio of the recessed part 5 which occupies in the whole combustion chamber volume can be set to the range of 10%-20%, for example.

FIG. 2 shows an example of a control system of the engine 1. In FIG. 2, U is a controller (control means) configured using a microcomputer. The controller U receives signals from various sensors S1 to S4. The sensor S1 is a load sensor that detects an engine load. The sensor S2 is a rotational speed sensor that detects the engine rotational speed. The sensor S3 is a pressure sensor that detects in-cylinder pressure, and is incorporated in the spark plug 11 in the embodiment. Since the in-cylinder pressure has a substantially proportional relationship with the in-cylinder temperature, the detection of the in-cylinder pressure is also the detection of the in-cylinder temperature. The sensor S4 is a water temperature sensor that detects the engine coolant temperature. The controller U controls the fuel injection amount and fuel injection timing from the fuel injection valve 10 described above, and controls the ignition timing of the spark plug 11 and the like.

FIG. 3 shows a setting example of the combustion mode of the engine 1 in accordance with the operating state except when cold. When the engine is cold, the combustion mode of a normal gasoline engine in which fuel is injected during the intake stroke and ignition by the spark plug 11 is performed. In FIG. 3, the operation region is set with the engine rotation and the engine load as parameters. The operation region is divided into two regions A and B by α line in the figure. The α ray is set linearly, and is set so that the load decreases as the engine speed increases.

A region A below the α line is a region where HCCI combustion is performed. Further, in the region D where the load is higher than the α-ray, HCCI combustion is not performed as in the case of a normal gasoline engine, but the combustion is performed by executing ignition of the spark plug 11 (for example, fuel is injected into the intake stroke and compressed) Ignition execution by spark plug 11 before top dead center).

 The region A where HCCI combustion is performed is expanded to a considerably high load region as compared with the conventional case. In other words, normally, the HCCI combustion is performed up to a region where the load is considerably lower than the α-ray, but conventionally, the HCCI combustion is performed up to the region where ignition is performed by the spark plug 11. Yes.

 In the region A where HCCI combustion is performed (particularly in the high load region), the combustion mode is as follows. That is, fuel injection from the fuel injection valve 10 is executed in the intake stroke. The fuel injected in the intake stroke is widely dispersed throughout the combustion chamber 4 when approaching compression top dead center. That is, the fuel is present in the recess 5, in the gap between the inclined surfaces 4a and 3a, and in the gap between the inclined surfaces 4b and 3b. The fuel in the recess 5 is compressed and self-ignited at or immediately before the compression top dead center, but the fuel in the gap between the inclined surfaces 4a and 3a and the gap between 4b and 3b passes through the wall surface (ceiling surface) of the combustion chamber 4. Since the heat is taken away by the piston crown surface), it is not ignited at the compression top dead center position or at a point before that.

 The fuel in the concave portion in which the fuel in the concave portion 5 has been subjected to compression self-ignition, the rate of heat generation (a calorific value per unit crank angle) at the timing when the piston descends in the expansion stroke after the compression top dead center is reached. ) Peak (first peak). As the piston 3 descends from the compression top dead center position, the gap between the inclined surfaces 4a and 3a and the gap between the inclined surfaces 4b and 3b widen, and the fuel in this gap is compressed by self-ignition. It is ignited and burned (diffusion combustion) in response to the high heat of combustion resulting from this, and the next peak (second peak) of the heat generation rate is generated. The time when the heat generation rate becomes the second peak is later than the time when the first peak is reached. That is, when the combustion period in which the heat generation rate does not increase beyond the first peak due to the combustion of the fuel in the compression self-ignited recess 5, the heat generation rate due to the combustion of the fuel in the gap increases. The second peak of the heat generation rate is generated.

FIG. 4 shows a relationship between the heat generation rate in the region A and the in-cylinder temperature. 4A shows the heat generation rate, and FIG. 4B shows the in-cylinder temperature. In FIG. 4 (a), symbol F indicates the peak heat generation rate when the fuel in the recess 5 is burned (at the first peak), and symbol R indicates the inclined surfaces 4a and 3a and 4b and 3b. The peak time (second peak time) of the heat generation rate when the fuel in the gap is burned. Further, in FIG. 4A, the symbol X indicates the heat generation rate when all the fuel is combusted by compression self-ignition all at once. It is understood that the size of the peak of the heat generation rate, F and R, are sufficiently smaller than X.

The in-cylinder temperature shown in FIG. 4B corresponds to the reference F in FIG. 4A, the reference Fb corresponds to the reference R in FIG. 4A, and the reference Xa corresponds to the reference F in FIG. ). In FIG. 4B, the temperature indicated by KX is a predetermined temperature (for example, 1800 K as a threshold temperature) at which NOx generation becomes a problem. It is understood that the sign Xa greatly exceeds the predetermined temperature and NOx generation becomes a problem, whereas the signs Fa and Ra are suppressed to a predetermined temperature or less and the NOx generation is suppressed.

Here, the in-cylinder temperature determined in accordance with the pressure detected by the pressure sensor S3 (in-cylinder temperature indicated by reference signs Fa and Ra in FIG. 4) is always equal to or lower than a predetermined temperature at which NOx generation does not cause a problem. The fuel injection mode, EGR amount adjustment, and the like can be performed. Further, the fuel injection can be divided injection of a front stage fuel injection in which fuel is injected in the intake stroke and a rear stage fuel injection in which fuel is injected into the recess 5 in the compression process. By performing the post fuel injection, the compression self-ignition property can be enhanced. In order to lower the sign Fa in FIG. 4, the ratio of the subsequent fuel injection amount may be reduced, and in order to lower the sign Ra in FIG. Further, by delaying the post-stage fuel injection timing, the compression self-ignition timing can be retarded, and the signs Fa and Ra in FIG. 4 can be lowered. Further, the post-stage fuel injection amount can be determined so that combustion in a temperature range (for example, 1200K to 1800K) in which self-ignition is stably performed can be obtained.

 In order to reliably perform compression self-ignition and improve combustion robustness, the spark plug 11 can be activated to activate the air-fuel mixture (instead of ignition by ignition of the spark plug 11, the air-fuel mixture is activated). Giving enough energy). The in-cylinder temperature can be suppressed to a predetermined temperature or less by adjusting the EGR amount.

 Although the embodiments have been described above, the present invention is not limited to the embodiments, and appropriate modifications can be made within the scope of the claims. When premixed compression ignition is performed, the air-fuel mixture may be activated by operating the spark plug 11 particularly at low temperatures or at an intermediate temperature when the engine 1 is not sufficiently warmed up (by ignition of the spark plug 11). Not ignition). From the viewpoint of securing torque, it is preferable from the viewpoint of securing torque to increase the combustion ratio in compression self-ignition as much as possible up to a temperature range close to a predetermined temperature at which NOx generation does not become a problem. Of course, the object of the present invention is not limited to what is explicitly stated, but also implicitly includes providing what is substantially preferred or expressed as an advantage.

 The present invention is preferably applied to, for example, an automobile engine to suppress NOx generation while improving fuel consumption.

1: Engine 2: Cylinder 3: Piston 3a, 3b: Inclined surface 4: Combustion chamber 4a, 4b: Inclined surface 5: Recessed portion 6: Intake passage 7: Exhaust passage 8: Intake valve 9: Exhaust valve 10: Fuel injection valve 11 : Spark plug A: Area (premixed compression ignition)
B: Area (ignition by spark plug)
F: first peak of heat generation rate R: second peak of heat generation rate Fa: first peak of in-cylinder temperature Ra: second peak of in-cylinder temperature

Claims (4)

A method for controlling combustion in a gasoline engine to which fuel containing at least a gasoline is supplied,
The engine has a piston crown formed along the ceiling of the combustion chamber, a recess formed in the center of the upper surface of the piston, and a geometric compression ratio of 15 or more.
In the compression top dead center position, the volume ratio of the recess to the total combustion chamber volume is set to 10 to 20%,
In the engine high load range, split injection of front-stage fuel injection that injects fuel in the intake stroke and rear-stage fuel injection that injects fuel into the recesses in the compression stroke, and immediately before compression top dead center or compression top dead center The fuel in the recess is compressed and self-ignited, and the fuel other than the recess is ignited with a delay from the compression self-ignition, so that the first peak of the heat generation rate of combustion that generates torque is the expansion stroke. It is a combustion mode in which the heat generation rate increases again after a period when the heat generation rate does not increase after the piston descending timing has elapsed.
A combustion control method for a gasoline engine characterized by the above. In claim 1,
A combustion control method for a gasoline engine, characterized in that the compression self-ignition timing of the fuel in the recess is adjusted so that the combustion temperature at the peak of the heat generation rate falls below a predetermined temperature. In claim 1,
A method for controlling combustion in a gasoline engine, characterized by accelerating compression self-ignition of fuel in the recess by operating a spark plug. A combustion control device for a gasoline engine to which a fuel containing at least a gasoline is supplied,
A crown surface of the piston is formed along the ceiling surface of the combustion chamber, and a recess is formed at the center of the upper surface of the piston.
The geometric compression ratio of the engine is set to 15 or higher,
In the compression top dead center position, the volume ratio of the recess to the total combustion chamber volume is set to 10 to 20%,
A fuel injection valve for injecting fuel into the cylinder and a controller for controlling the fuel injection valve;
The controller controls the fuel injection valve so as to perform split injection of pre-stage fuel injection in which fuel is injected in an intake stroke and post-stage fuel injection in which fuel is injected into the recess in a compression stroke in an engine high load range. As a result, the fuel in the concave portion is compressed and self-ignited immediately before or at the compression top dead center, and the fuel other than the concave portion is ignited after the compression self-ignition to generate torque. The first peak of the heat generation rate of combustion is the piston lowering timing of the expansion stroke, and after that, after a period in which the heat generation rate does not increase, the heat generation rate increases again.
A combustion control device for a gasoline engine.

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