JP5594022B2 - Gasoline engine combustion control device - Google Patents

Description

 The present invention relates to a combustion control device for a gasoline engine that performs HCCI (Homogeneous-Charge Compression-Ignition) combustion, that is, so-called 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 is adopted in the present invention. That is, as described in claim 1 in the claims,
A combustion control device for a gasoline engine to which a fuel containing at least a gasoline is supplied,
In the engine high load region, the front stage fuel injection for HCCI (premixed compression ignition) combustion is performed before the compression top dead center, while the rear stage fuel injection is performed after the compression top dead center.
The pre-stage fuel injection is divided into an initial injection for HCCI combustion and a second injection for an ignition source of the HCCI combustion, and the injection amount of the second injection is greater than the injection amount of the initial injection. Less,
The injection timing of the latter fuel injection is set so that the fuel injected by the latter fuel injection starts to burn at an interval from the end of the HCCI combustion ,
In the load region where the fuel injection amount is small, the crank angle interval between the front fuel injection timing and the rear fuel injection timing is shortened compared to the high load region.
It is like that. According to the above solution, it is possible to improve fuel efficiency by executing premixed compression ignition by pre-stage fuel injection. In addition, the combustion is a combustion mode that is divided into combustion by premixed compression ignition and combustion by subsequent fuel injection performed thereafter. The generation of NOx can be suppressed as well as the suppression. In particular, since the injection amount of the second injection in the front stage fuel injection is made smaller than the injection amount of the first injection, the problem of NOx generation caused by the second injection can also be avoided. Further, by advancing the ignition timing of HCCI combustion by the second injection in the front stage fuel injection, it is possible to secure a time for the fuel injected by the rear stage fuel to vaporize without delaying the rear stage fuel injection timing. It is possible to suppress the occurrence of smoke in combustion by the subsequent fuel injection while ensuring thermal efficiency, that is, torque. In addition to the above, when the load is relatively low where the occurrence of smoke is unlikely to be a problem, the combustion is preferably performed as close to the compression top dead center as possible as a whole in order to improve fuel efficiency.

A preferred mode based on the above solution is as described in claim 2 and the following claims. That is,
The second fuel injection is performed before the compression top dead center (corresponding to claim 2). In this case, ignition is performed before the compression top dead center, which is preferable in terms of improving fuel efficiency in combustion by the pre-stage fuel injection.

  After the second fuel injection and before compression top dead center, ignition assist is performed by a spark plug (corresponding to claim 3). In this case, by using the spark plug, the fuel injected before the compression top dead center can be ignited more reliably, and the effect corresponding to claim 2 can be more fully exhibited.

  The G / F (the total amount of gas in the cylinder / the amount of fuel supplied into the cylinder) in the high load range is set to 20 or more (corresponding to claim 4). In this case, both fuel efficiency improvement and NOx suppression can be satisfied at a high level.

The total injection amount of the initial injection amount and the second injection amount is set to be equal to or less than the injection amount of the subsequent fuel injection (corresponding to claim 5 ). In this case, the amount of fuel burned by HCCI is reduced as much as possible, which is more preferable in suppressing NOx generation.

The total injection amount of the initial injection amount and the second injection amount is limited to a range in which the maximum temperature of the HCCI combustion is not more than a predetermined temperature at which NOx generation does not become a problem ( Corresponding to claim 6 ). In this case, NOx generation due to HCCI combustion can be reliably suppressed.

 According to the present invention, it is possible to suppress NOx generation while improving fuel efficiency by performing HCCI combustion in a high load region. Moreover, high thermal efficiency can be ensured while reducing smoke.

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 the fuel-injection timing in the area | region B, a heat generation rate, and cylinder temperature. The figure which shows the relationship between the fuel injection timing in the area | region C, and a heat generation rate.

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.

FIG. 2 shows an example of a control system of the engine 1. In FIG. 2, U is a controller 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.

FIG. 3 shows a setting example of the combustion mode of the engine 1 in accordance with the operating state except when cold. Note that, when the engine is cold, the fuel is injected before the compression top dead center, and the combustion mode of a normal gasoline engine in which ignition by the spark plug 11 is performed before the compression top dead center. In FIG. 3, the operation region is set with the engine rotation and the engine load as parameters. The operation region is divided into four regions by α rays, β rays and γ rays in the figure.

 The α ray, β ray, and γ ray are set linearly, the β ray is set to a higher load region than the α ray, and the γ ray is set to a higher load region than the β ray. Yes. The α ray, β ray, and γ ray are set so that the load becomes lower as the engine speed increases.

A region A below the α ray, a region B between the α ray and the β ray, and a region C between the β ray and the γ ray are regions where HCCI combustion is performed. Further, in the region D where the load is higher than the γ-ray, HCCI combustion is not performed as in the case of a normal gasoline engine, but the combustion mode is performed by executing the ignition of the spark plug 11 (the fuel is injected before the compression top dead center). , Ignition by the spark plug 11 before compression top dead center).

 In regions A, B, and C where HCCI combustion is performed, fuel injection for premixed compression ignition is performed before compression top dead center. In region A, fuel for premixed compression ignition is performed. Only injection is performed. Since the region A is a considerably low load region, the in-cylinder temperature does not rise to a predetermined temperature (for example, 1800 K) as a threshold temperature at which NOx generation becomes a problem.

 In the region B, the load is higher than that in the region A, and when the required fuel amount is all premixed compression ignition, the in-cylinder temperature rises and the problem of NOx occurs. For this reason, part of the required fuel amount is used for premix compression ignition before compression top dead center (pre-stage fuel injection), and the remaining fuel amount is burned after compression top dead center and by premix compression ignition The fuel is injected after the start of fuel (second-stage fuel injection). That is, after the HCCI combustion by the front stage fuel injection is completed, the rear stage fuel injection is performed (combustion peaks occur twice in total). The latter-stage fuel injection is performed in a range that is not largely retarded from the compression top dead center, which is preferable for improving fuel efficiency. Note that the injection amount of the front stage fuel injection is limited so that the in-cylinder temperature determined based on the in-cylinder pressure detected by the pressure sensor S3 does not exceed a predetermined temperature (for example, 1800 K) at which NOx generation is a problem. . Further, the fuel injection amount is preferably set so that the post-stage fuel injection amount is equal to or higher than the pre-stage fuel injection amount in order to surely prevent the in-cylinder temperature from exceeding a predetermined temperature at which NOx generation becomes a problem. .

FIG. 4 shows the relationship between the fuel injection timing, the heat generation amount per unit crank angle (heat generation ratio), and the in-cylinder temperature in the region B. 4A shows the fuel injection timing, the symbol F shows the fuel injection timing for premixed compression ignition, and the symbol R shows the timing of the subsequent fuel injection. FIG. 4 (b) shows the amount of heat generated per unit crank angle, that is, the heat generation rate. Symbol Fa corresponds to combustion in premixed compression ignition, and symbol Ra corresponds to combustion in post-stage fuel injection. . FIG. 4C shows the in-cylinder temperature, where symbol Fb corresponds to combustion in premixed compression ignition and symbol Rb corresponds to combustion in post-stage fuel injection. In FIG. 4C, the temperature indicated by KX is a predetermined temperature (for example, 1800 K) as a threshold value at which NOx generation becomes a problem. As can be understood from FIG. 4, the in-cylinder temperature is suppressed to a range equal to or lower than a predetermined temperature at which NOx generation becomes a problem. In the region B, G / F (total in-cylinder gas amount / fuel amount supplied into the cylinder) is 20 or more. The in-cylinder total gas amount is the total amount of the new amount and the EGR gas amount, but when the EGR gas amount is substantially zero, the air-fuel ratio is 20 or more.

Here, when each fuel injection is performed, the fuel injection timing and the fuel are set so that the in-cylinder temperature determined according to the pressure detected by the pressure sensor S3 is always equal to or lower than a predetermined temperature at which NOx generation is a problem. The injection amount can be set. In this case, as described above, during the pre-stage fuel injection for premixed compression ignition, the temperature range in which the self-ignition is stably performed while limiting the fuel injection amount so that the in-cylinder temperature is equal to or lower than the predetermined temperature. It is preferable to obtain combustion (for example, 1200K to 1800K). In addition, it is preferable to select the fuel injection timing and the fuel injection amount so that the best torque is obtained in the range where the in-cylinder temperature is equal to or lower than the predetermined temperature.

 In the region C, as in the region B, the pre-stage fuel injection for premixed compression ignition in which fuel is injected before the compression top dead center and the post-stage fuel injection in which fuel is injected after the compression top dead center are performed. In particular, in region C, the combustion by the subsequent fuel injection is started after a sufficient interval from the end of the combustion by the previous fuel injection. In the region C, the combustion by the front stage fuel injection is advanced as compared with the case of the region B, and the time required for vaporizing the fuel injected by the rear stage fuel is sufficiently secured to suppress the generation of smoke. It is.

 In the region C, two divided injections are performed in order to advance the start of combustion by the preceding stage fuel injection. That is, the initial injection is for HCCI combustion, and is performed before the compression top dead center. The second injection is for stratified combustion, and is performed after the first injection and before the compression top dead center. The second injection is performed to promote ignition of HCCI combustion by the first injection, and the injection amount of the second injection is sufficiently smaller than the injection amount of the first injection. By this second injection, the fuel injected for the first time (more specifically, the fuel for the first injection + the fuel for the second injection) is ignited early. In order to more reliably advance the ignition of the fuel injected by the front stage fuel, it is possible to perform an ignition assist by the spark plug 11 (activation of the air-fuel mixture by the operation of the spark plug 11 is not ignition by the ignition itself). . In order to avoid the problem of NOx generation due to combustion by the front stage fuel injection, that is, HCCI combustion, the in-cylinder temperature detected by the pressure sensor S3 and determined according to the in-cylinder pressure is equal to or lower than the predetermined temperature. It is limited to be in the range.

 The post-stage fuel injection in the region C is injected at substantially the same time or slightly later than the post-stage fuel injection in the region B. As a result, the combustion of the fuel injected by the rear stage fuel is started after an interval from the end of the combustion of the fuel injected by the front stage fuel. The fuel injected by the latter stage fuel is sufficiently vaporized to suppress the generation of smoke. The injection timing of the post-stage fuel injection is preferably set as close to the compression top dead center as much as possible within a range in which sufficient fuel vaporization is ensured from the viewpoint of improving thermal efficiency, that is, securing torque. Also in the region C, the problem of NOx generation can be avoided while performing HCCI combustion, and the generation of smoke can also be suppressed.

FIG. 5 shows the fuel injection timing and the heat generation rate in the region C (the in-cylinder temperature depends on the heat generation rate). That is, in FIG. 5A showing the fuel injection timing, FX1 indicates the initial injection in the front stage fuel injection, FX2 indicates the second injection, and RX indicates the rear stage fuel injection. Further, in FIG. 5B showing the heat generation rate, FXa indicates combustion by the front stage fuel injection (corresponding to fuel injection at FX1 + FX2), and RXa indicates combustion by the rear stage fuel injection (corresponding to fuel injection by RX) . Further, in FIG. 5B, for comparison, the heat generation rate in the preceding stage fuel injection in the region B is indicated by a broken line (corresponding to the sign Fa in FIG. 4B), and in the region C, the region B It is understood that the combustion by the front stage fuel injection is advanced as compared with the case of.

 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). Of course, from the standpoint of securing torque, etc., the object of the present invention is not limited to what is explicitly stated, but implicitly includes providing what is substantially preferred or expressed as an advantage.

 The present invention can be applied to, for example, an automobile engine, and can improve fuel consumption, suppress NOx generation, improve thermal efficiency, and suppress smoke in a high load range.

1: Engine 2: Cylinder 3: Piston 4: Combustion chamber 5: Recess 6: Intake passage 7: Exhaust passage 8: Intake valve 9: Exhaust valve 10: Fuel injection valve 11: Spark plug A: Region (premixed compression ignition only )
B: Area (one front fuel injection + rear fuel injection)
C: Region (front-stage fuel injection divided into two times + rear-stage fuel injection)

Claims (6)

A combustion control device for a gasoline engine to which a fuel containing at least a gasoline is supplied,
In the engine high load region, the front stage fuel injection for HCCI (premixed compression ignition) combustion is performed before the compression top dead center, while the rear stage fuel injection is performed after the compression top dead center.
The pre-stage fuel injection is divided into an initial injection for HCCI combustion and a second injection for an ignition source of the HCCI combustion, and the injection amount of the second injection is greater than the injection amount of the initial injection. Less,
The injection timing of the latter fuel injection is set so that the fuel injected by the latter fuel injection starts to burn at an interval from the end of the HCCI combustion ,
In the load region where the fuel injection amount is small, the crank angle interval between the front fuel injection timing and the rear fuel injection timing is shortened compared to the high load region.
A combustion control device for a gasoline engine. In claim 1,
2. A gasoline engine combustion control apparatus, wherein the second fuel injection is performed before compression top dead center. In claim 2,
A combustion control apparatus for a gasoline engine, wherein ignition assistance is performed by a spark plug after the second fuel injection and before compression top dead center. In claim 1,
G / F (total amount of gas in cylinder / amount of fuel supplied to the cylinder) in the high load range is 20 or more, a combustion control apparatus for a gasoline engine, In claim 1,
The gasoline engine combustion control apparatus, wherein a total injection amount of the initial injection amount and the second injection amount is equal to or less than an injection amount of the rear fuel injection. In any one of Claims 1 thru | or 5 ,
The total injection amount of the injection amount of the first injection and the injection amount of the second injection is limited to a range where the maximum temperature of the HCCI combustion is not more than a predetermined temperature at which NOx generation does not become a problem. Gasoline engine combustion control device.

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