JP2008280013A - Exhaust gas purification system for internal combustion engine - Google Patents

Abstract

In an exhaust gas purification system for an internal combustion engine that is applied to a hybrid vehicle that travels with two types of power sources, that is, an internal combustion engine and an electric motor, when performing reduction control on the NOx catalyst, the combustion state in the internal combustion engine Provides a technique capable of improving the purification efficiency of NOx while suppressing instability.
When a reduction request for NOx is issued, the EGR opening degree Degr is increased from Degr0 to Degr1 at t1, and the combustion mode is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode, and the combustion state of the air-fuel mixture is stabilized. The fuel injection amount Qf is decreased from Qf0 to Qf1 to maintain the combustible state. As a result, the assist torque TQa assists the torque that the engine torque TQe is insufficient with respect to the required torque TQr.
[Selection] Figure 3

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine applied to a hybrid vehicle that runs with two types of power sources, that is, an internal combustion engine and an electric motor.

  A so-called hybrid vehicle is known that combines the output of an internal combustion engine and the output of an electric motor to output a required output. In such a hybrid vehicle, it is also known to dispose a NOx catalyst in the exhaust passage in order to purify nitrogen oxide (hereinafter referred to as NOx) in exhaust gas discharged from an internal combustion engine capable of lean combustion.

The NOx catalyst stores (absorbs and absorbs) NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, reduces the oxygen concentration of the inflowing exhaust gas, and reduces the reducing agent (for example, hydrocarbon (HC), etc.). When present, the stored NOx catalyst releases NOx that has been stored and is reduced to nitrogen (N ₂ ), or when a reducing agent (eg, HC, urea water, etc.) is present in an oxygen-excess atmosphere. A selective reduction type NOx catalyst for reduction is known.

  In an exhaust gas purification system for an internal combustion engine that is applied to the hybrid vehicle exemplified above and includes a NOx catalyst, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst by supplying a reducing agent to the exhaust gas (hereinafter simply referred to as “inflowing exhaust air-fuel ratio”) Also, reduction control for temporarily setting the air / fuel ratio to a rich air / fuel ratio is executed.

In Patent Document 1, in an exhaust gas purification system for an internal combustion engine that is applied to a hybrid vehicle and includes an NOx catalyst, the output of the internal combustion engine is reduced as much as possible and the inflow exhaust air-fuel ratio is made rich. Thus, a technique for reducing the supply amount of the reducing agent necessary for reducing the inflow exhaust air-fuel ratio to the rich air-fuel ratio is disclosed.
JP 2002-195064 A JP 2000-8835 A Japanese Patent Laid-Open No. 11-62653

  However, according to the technique disclosed in Document 1, the fuel efficiency associated with the reduction control is increased as the base air-fuel ratio, which is the ratio of the intake air amount sucked into the internal combustion engine to the amount of fuel supplied to the internal combustion engine, becomes richer. However, the combustion of the air-fuel mixture in the combustion chamber becomes unstable, which may increase smoke discharged from the internal combustion engine and deteriorate drivability.

  The present invention has been made in view of the above-described prior art, and an object of the present invention is an internal combustion engine that is applied to a hybrid vehicle that travels with two types of power sources, that is, an internal combustion engine and an electric motor, and that includes a NOx catalyst. To provide a technology capable of improving the NOx purification efficiency while suppressing the combustion state of the air-fuel mixture in the internal combustion engine from becoming unstable when executing the reduction control on the NOx catalyst in the exhaust purification system. It is.

In order to achieve the above object, an exhaust gas purification system for an internal combustion engine according to the present invention employs the following means.
That is,
An exhaust gas purification system for an internal combustion engine applied to a hybrid vehicle that travels with two types of power sources, an internal combustion engine and an electric motor,
A NOx catalyst provided in an exhaust passage of the internal combustion engine;
Reduction executing means for executing a reduction control for temporarily reducing the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a rich air-fuel ratio by reducing the NOx in the NOx catalyst by supplying a reducing agent to the exhaust gas passing through the exhaust passage. When,
Prior to execution of the reduction control, the air-fuel ratio of the air-fuel mixture is changed from the normal air-fuel ratio mode from the normal air-fuel ratio mode in which the air-fuel ratio of the air-fuel mixture is set to a predetermined normal air-fuel ratio. Combustion mode switching means for switching to a low air-fuel ratio mode for reducing
Engine torque adjusting means for adjusting engine torque within a range in which the combustion state of the internal combustion engine is maintained in a predetermined stable state when the combustion mode switching means switches the combustion mode;
Assist torque adjusting means for adjusting the assist torque output by the electric motor to output the shortage of the engine torque relative to the required torque to the electric motor when the engine torque is insufficient with respect to the required torque;
It is characterized by providing.

  In the above configuration, the distribution of the torque (engine torque and assist torque) output from the internal combustion engine, which is the power source (drive source) of the hybrid vehicle, and the electric motor is determined according to the required torque requested by the driver. The driving state of the vehicle is controlled. The “required torque” in the present invention is a torque required for the entire hybrid vehicle.

  During normal operation of the hybrid vehicle according to the present invention (for example, an operating state in which the reduction control by the reduction execution unit is not executed), the intake air amount and the engine torque required for the internal combustion engine are output to the internal combustion engine. A fuel injection amount is determined. That is, the air-fuel ratio of the air-fuel mixture during normal operation is determined. The “predetermined normal air-fuel ratio” in the present invention is the air-fuel ratio of the air-fuel mixture during normal operation, and the “normal air-fuel ratio mode” is the combustion mode when the air-fuel ratio of the air-fuel mixture is controlled to the normal air-fuel ratio. Means.

  In the present invention, the combustion mode switching means switches the combustion mode of the internal combustion engine from the normal air-fuel ratio mode to the low air-fuel ratio mode in which the air-fuel ratio of the air-fuel mixture is lower than that in the normal air-fuel ratio mode prior to execution of the reduction control. Thereby, the amount of oxygen remaining in the exhaust at the time of being discharged from the internal combustion engine can be reduced. As a result, the NOx reduction reaction in the NOx catalyst is more suitably performed, and the NOx reduction efficiency can be improved.

  Further, by setting the combustion mode of the air-fuel mixture to the low air-fuel ratio mode, the amount of reducing agent supplied to reduce the inflow exhaust air-fuel ratio to the target air-fuel ratio (rich air-fuel ratio) is reduced, and the fuel consumption related to the supply of reducing agent is reduced. Can be improved.

  Various conventionally known methods can be adopted as a method of switching the combustion mode of the air-fuel mixture from the normal air-fuel ratio mode to the low air-fuel ratio mode. For example, the combustion mode switching means may reduce the air-fuel ratio of the air-fuel mixture by reducing the amount of intake air taken into the internal combustion engine. Further, when the exhaust gas purification system for an internal combustion engine according to the present invention includes an EGR device that recirculates part of the exhaust gas in the exhaust system to the intake system, the air-fuel ratio of the air-fuel mixture is decreased by increasing the EGR gas amount. May be.

Here, after the combustion mode switching means switches the combustion mode from the normal air-fuel ratio mode to the low air-fuel ratio mode, if the required torque is obtained only by the engine torque, the combustion state of the air-fuel mixture may become excessively unstable. is there. If the fuel injection amount is increased while maintaining the combustion mode in the low air-fuel ratio mode, a location where oxygen is locally insufficient (for example, a region near the fuel injection valve) is generated in the air-fuel mixture. As a result, the combustion state becomes unstable, and there is a possibility that the internal combustion engine may be misfired, the torque is rapidly reduced, and the smoke emission amount is increased.

  Therefore, in the present invention, the engine torque is adjusted within a range in which the combustion state of the internal combustion engine is maintained in a predetermined stable state when the combustion mode is the low air-fuel ratio mode. The “predetermined stable state” is a state where the combustion of the air-fuel mixture is stable, and is a state where the deterioration of the combustion state does not become excessive in each combustion cycle of the internal combustion engine. The deterioration of the combustion state is a concept including torque reduction, misfire, increase in the amount of smoke discharged from the internal combustion engine, increase in combustion noise, and the like.

  This suppresses unstable combustion even when the combustion mode is switched to the low air-fuel ratio mode. In the “predetermined stable state”, for example, an allowable limit for deterioration of the combustion state may be set in advance, and the engine torque may be adjusted so that the deterioration of the combustion state is within the allowable limit range.

  As described above, when the combustion mode is the low air-fuel ratio mode, the engine torque is adjusted within a range in which the combustion state of the air-fuel mixture is maintained in a predetermined stable state. Therefore, the required torque cannot be satisfied only by the engine torque. There is a case (that is, the engine torque is insufficient with respect to the required torque).

  In such a case, in the present invention, the assist torque adjusting means adjusts the torque (assist torque) that is output to the electric motor, and causes the electric motor to assist the shortage of the engine torque. That is, the assist torque is adjusted so that the sum of the engine torque and the assist torque (hereinafter also referred to as “total output torque”) substantially matches the required torque. As a result, it is possible to output the torque of the entire hybrid vehicle as required by the driver while ensuring the stability of the combustion state of the air-fuel mixture.

  As described above, according to the present invention, the NOx purification efficiency can be improved while suppressing the instability of the combustion state in the internal combustion engine when performing the reduction control on the NOx catalyst. That is, it is possible to improve the NOx reduction efficiency in the NOx reduction control while suppressing the occurrence of problems such as torque shock, misfire, increase in smoke emission, and combustion noise.

  Further, when the required torque changes after the combustion mode of the air-fuel mixture is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode, either engine torque or assist torque, or engine torque and assist torque, Therefore, it is necessary to make the total output torque (the sum of the engine torque and the assist torque) coincide with the required torque.

  However, when the engine torque is adjusted according to the required torque, the combustion state of the internal combustion engine may not be maintained in the stable state. In particular, when the required torque is increased, the combustion state is likely to deteriorate. Therefore, in the present invention, when the required torque changes after the combustion mode is switched to the low air-fuel ratio mode, the engine torque adjusting means maintains the combustion state of the internal combustion engine in the stable state. The engine torque may be adjusted so that the sum of the engine torque and the assist torque is approximately equal to the required torque after the change. Thereby, it can suppress reliably that the combustion state of air-fuel mixture deteriorates and becomes unstable.

Further, in the above, when the engine torque adjusting means adjusts the engine torque, the adjustment of the assist torque by the assist torque adjusting means is not prohibited. Adjusting both the engine torque and the assist torque according to the change in the required torque is particularly effective when the required torque is increased and the amount of increase is large (for example, when sudden acceleration is requested). is there.

  For example, even if the engine torque is increased to the maximum within the range in which the combustion state is maintained in the stable state, the total output torque may be insufficient with respect to the required torque. It is also possible to compensate for the insufficient torque by increasing.

  In addition, as described above, when the required torque changes after the combustion mode is switched to the low air-fuel ratio mode, maintaining the engine torque of the internal combustion engine substantially constant may deteriorate the combustion state of the air-fuel mixture. It is considered preferable from the viewpoint of suppressing the above.

  Therefore, in the present invention, when the required torque changes in a state after the combustion mode is switched to the low air-fuel ratio mode, the engine torque adjusting means maintains the engine torque substantially constant and the assist torque. The torque adjusting means may adjust the assist torque so that the sum of the engine torque and the assist torque substantially matches the required torque after the change.

  This makes it possible to respond to changes in the required torque while reliably suppressing deterioration of the combustion state in the internal combustion engine (misfire of the internal combustion engine, increase in smoke emission, increase in combustion noise, torque shock, etc.). Further, since the fuel injection amount is not changed by maintaining the engine torque substantially constant, it is possible to suppress fluctuations in the air-fuel ratio of the air-fuel mixture in the combustion chamber of the internal combustion engine. That is, even when the required torque changes, the NOx reduction efficiency can be maintained at a high level.

  By the way, the magnitude of the assist torque that can be output by the motor depending on the state of charge of the battery serving as the power source of the motor or the value of the assist torque that the motor can output to the maximum even when the battery is sufficiently charged. Has its limits. Therefore, when the required torque increases rapidly, the motor cannot output the required assist torque, and it may be difficult for the motor to assist the shortage of the engine torque with respect to the required torque.

  Therefore, in the present invention, there is further provided an assist determination means for determining whether or not the electric motor can output a shortage of the engine torque with respect to the required torque, and the shortage of the engine torque is supplied to the electric motor. When it is determined that the output cannot be performed, switching to the low air-fuel ratio mode or maintenance of the low air-fuel ratio mode by the combustion mode switching means may be prohibited.

  In other words, if it is determined that the assist torque sufficient to compensate for the engine torque shortage cannot be output to the motor before the combustion mode is switched from the normal air fuel ratio mode to the low air fuel ratio mode, the combustion mode is switched. Is prohibited. Further, when the above determination is made after the combustion mode has already been switched from the normal air-fuel ratio mode to the low air-fuel ratio mode, the combustion mode is returned from the low air-fuel ratio mode to the normal air-fuel ratio mode.

After it is determined that the shortage of the engine torque relative to the required torque cannot be output to the electric motor, the combustion mode of the internal combustion engine is controlled as the normal air-fuel ratio mode in any of the above cases. In that case, there is no possibility that the combustion state becomes unstable even if the engine torque is adjusted so as to match the required torque. Therefore, the engine torque can be adjusted so that the total output torque becomes equal to the required torque. For example, the assist torque adjusting means may set the assist torque to substantially zero, and the engine torque adjusting means may adjust the engine torque so that the engine torque and the required torque substantially match.

  As a result, the required torque can be reliably output while suppressing the deterioration of the combustion state of the air-fuel mixture. Further, even when the combustion mode of the air-fuel mixture is returned to the normal air-fuel ratio mode as described above, NOx reduction control is executed by the reduction execution means supplying the reducing agent, so that the reduction of NOx is ensured. Control can be performed.

  The present invention is applied to a hybrid vehicle that travels with two types of power sources, that is, an internal combustion engine and an electric motor. In an exhaust gas purification system for an internal combustion engine that includes a NOx catalyst, the internal combustion engine performs reduction control on the NOx catalyst. The NOx reduction efficiency can be improved while suppressing the unstable combustion state of the air-fuel mixture in the engine. That is, when the NOx reduction control is executed, the NOx purification efficiency can be improved while suppressing the increase of smoke emission in the internal combustion engine, the misfire of the internal combustion engine, and the deterioration of drivability. .

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings. It should be noted that the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are intended to limit the technical scope of the invention only to those unless otherwise specified. is not.

<System configuration of hybrid vehicle>
FIG. 1 is a block diagram showing a system configuration of a hybrid vehicle to which an exhaust gas purification system for an internal combustion engine according to this embodiment is applied. This system includes an engine 1 that functions as the main power of the hybrid vehicle, a motor generator (hereinafter simply referred to as “MG”) 2 that functions as auxiliary power, a main ECU 3, T / M4, and T / MECU5 that control the entire system. , A battery 6, an inverter 7, and a battery ECU 8.

  The MG 2 is configured to function as an electric motor that assists the driving force of the engine 1 or as a generator for charging the battery 6. In this embodiment, MG2 corresponds to the electric motor in the present invention.

  For example, when the hybrid vehicle travels using only the power of either engine 1 or MG2, the power of either engine 1 or MG2 is transmitted to wheels 9 via T / M4 in response to a command from T / MECU. When the vehicle travels using both as power sources, the power of the engine 1 and MG2 is transmitted to the wheels 9 via the T / M4.

  The battery 6 is a rechargeable storage battery configured to be able to function as a power source for driving the MG 2. The battery 6 is provided with a battery ECU 8 that detects the state of charge (that is, the amount of charge) of the battery 6, and is electrically connected to the main ECU 3. The inverter 7 is configured to convert DC power extracted from the battery 6 into AC power and supply it to the MG 2, and to convert AC power generated by the MG 2 into DC power and supply it to the battery 6. Has been.

<Schematic configuration of exhaust purification system>
FIG. 2 is a diagram showing a schematic configuration of an exhaust gas purification system for an internal combustion engine in the present embodiment. The engine 1 shown in FIG. 2 is a diesel engine having four cylinders 12. The engine 1 includes a fuel injection valve 13 that directly injects fuel into the combustion chamber of the cylinder 12 in each cylinder. In this embodiment, the engine 1 corresponds to the internal combustion engine in the present invention.

<Intake system>
An intake manifold 18 is connected to the engine 1, and each branch pipe of the intake manifold 18 communicates with a combustion chamber of each cylinder 12 through an intake port. The intake manifold 18 is connected to an intake passage 19, and an intercooler 14 that cools the gas flowing through the intake passage 19 is provided in the intake passage 19.

  Furthermore, a compressor housing 17a of a turbocharger 17 that operates using exhaust energy as a drive source is provided upstream of the intercooler 14 in the intake passage 19. An air flow meter 15 that outputs an electrical signal corresponding to the amount of intake air flowing through the intake passage 19 is disposed upstream of the compressor housing 17a, and an air cleaner 16 is provided further upstream of the air flow meter 15. It has been. Further, an intake throttle valve 11 capable of adjusting the flow rate of intake air flowing through the intake passage 19 is provided in the vicinity of the connection portion between the intake passage 19 and the intake manifold 18.

  In the intake system of the engine 1 configured as described above, dust or dust in the intake air is removed by the air cleaner 16 and then flows into the compressor housing 17a through the intake passage 19. The intake air flowing into the compressor housing 17a is compressed by the rotation of a compressor (not shown) built in the compressor housing 17a. The intake air that has been compressed to a high temperature is cooled by the intercooler 14 and then flows into the intake manifold 18. The intake air flowing into the intake manifold 18 is distributed to each cylinder 12 via each intake port (not shown). The intake air distributed to each cylinder 12 is combusted using the fuel injected from the fuel injection valve 13 as an ignition source.

<Exhaust system>
An exhaust manifold 28 is connected to the engine 1, and each branch pipe of the exhaust manifold 28 is connected to a combustion chamber of each cylinder 12 via an exhaust port (not shown). A turbine housing 17 b of the turbocharger 17 is connected to the exhaust manifold 28. An exhaust passage 29 is connected to the turbine housing 17b, and the exhaust passage 29 is connected to a muffler (not shown) downstream.

  An occlusion reduction type NOx catalyst (hereinafter referred to as “NOx catalyst”) 20 is provided in the middle of the exhaust passage 29. Further, a fuel addition valve 21 is provided upstream of the turbine housing 17b in the exhaust passage 29 to open a valve as a command signal from the main ECU 3 and add fuel as a reducing agent into the exhaust. An exhaust throttle valve 22 capable of adjusting the flow rate of the exhaust gas flowing through the exhaust passage 29 is provided downstream of the NOx catalyst 20 in the exhaust passage 29, and an air-fuel ratio sensor is provided downstream of the exhaust throttle valve 22. 25 is provided.

  In the exhaust system of the engine 1 configured as described above, the burned gas burned in each cylinder 12 of the engine 1 is discharged to the exhaust manifold 28 and then flows from the exhaust manifold 28 into the turbine housing 17b of the turbocharger 17. The exhaust gas flowing into the turbine housing 17b rotates a turbine (not shown) rotatably supported in the turbine housing 17b using the thermal energy of the exhaust gas. At that time, the rotational torque of the turbine (not shown) is transmitted to the compressor (not shown) of the compressor housing 17a.

  Then, after the NOx is occluded in the NOx catalyst 20, the flow rate of the exhaust gas flowing out of the turbine housing 17b is adjusted by the exhaust throttle valve 22 as necessary, and released into the atmosphere via the muffler.

<EGR device>
The engine 1 is provided with an EGR device 30 that recirculates a part of the exhaust gas passing through the exhaust manifold 28 to the intake manifold 18. The EGR device 30 includes an EGR passage 31 that connects the exhaust manifold 28 and the intake manifold 18, an EGR valve 32 that can adjust the flow rate of exhaust (hereinafter referred to as “EGR gas”) flowing in the EGR passage 31, and An EGR cooler 33 that cools the EGR gas that flows upstream of the EGR valve 32 in the EGR passage 31 is provided.

  In the EGR device 30 configured as described above, when the EGR valve 32 is opened, the EGR passage 31 is in a conductive state, and a part of the exhaust gas in the exhaust manifold 28 flows into the intake manifold 18 and is re-applied to the engine 1. Circulated.

  In the main ECU 3 described above, sensors related to control of the operating state of the engine 1 such as the air flow meter 15, the crank position sensor 23 for detecting the engine speed Ne, and the accelerator position sensor 24 for detecting the accelerator opening degree are electrically connected. They are connected via wiring, and their output signals are input to the main ECU 3. On the other hand, an intake throttle valve 11, a fuel injection valve 13, a fuel addition valve 21, an exhaust throttle valve 22, an EGR valve 32, and the like are connected to the main ECU 3 through electrical wiring, and are controlled by the main ECU 3. It has become.

  The main ECU 3 includes a CPU, a ROM, a RAM, and the like. In the ROM, a program and data for performing overall control of the hybrid system related to traveling of the hybrid vehicle and reduction control of the NOx catalyst 20 are stored. Stored map is stored. A control routine described later is one of programs stored in the ROM of the main ECU 3.

  Here, the operation control of the hybrid vehicle in the above-described embodiment will be described. In the hybrid system in the present embodiment, the distribution of driving power between the MG 2 functioning as an electric motor and a generator and the engine 1 is controlled by the main ECU 3, and the traveling state of the hybrid vehicle is controlled. Torque to be output to the engine 1 (hereinafter referred to as “engine torque”) TQe and torque to be output to the MG 2 (hereinafter referred to as “assist torque”) TQa so that the sum of the engine torque and assist is equal to the required torque TQr. The torque TQa is controlled by the ECU 3. Hereinafter, driving force distribution for each traveling state of the hybrid vehicle according to the present embodiment will be exemplarily described.

  For example, at the time of starting the hybrid vehicle, MG2 driven using the electric energy of battery 6 functions as an electric motor. With this power, the engine 1 is cranked and the engine 1 is started.

  At the start of the hybrid vehicle, two types of modes can be adopted depending on the state of charge of the battery 6. The main ECU 3 grasps the state of charge of the battery 6 based on the output signal of the battery ECU 8. For example, when the state of charge is good, there is no need to charge the battery 6 with the MG 2, so the engine 1 gives priority to promoting warm-up. On the other hand, when the state of charge is not good, MG2 functions as a generator by the power of engine 1, and battery 6 is charged.

  In a driving region where the efficiency of the engine 1 is relatively good as in normal running, the hybrid vehicle runs mainly by the power of the engine 1. At this time, the MG 2 may be driven by a part of the power of the engine 1 to generate power in the MG 2 functioning as a generator, and the battery 6 may be charged.

Further, at the time of braking of the hybrid vehicle, the MG is driven by the power transmitted from the wheels 9.
Rotate 2 to function as a generator. Thereby, the kinetic energy of the wheel 9 is converted into electric energy and the battery 6 is charged (so-called “regeneration”).

<NOx reduction control>
Next, NOx reduction control for reducing NOx stored in the NOx catalyst 20 of this embodiment will be described. In the NOx reduction control in the present embodiment, when a reduction request for NOx is issued, the main ECU 3 causes the fuel addition valve 21 to add fuel as a reducing agent (hereinafter referred to as “added fuel”) into the exhaust, and the exhaust. Reduce the oxygen concentration.

In this way, NOx is reduced to nitrogen (N ₂ ) by temporarily setting the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 (hereinafter referred to as “inflow exhaust air-fuel ratio”) to a rich air-fuel ratio. In this embodiment, the NOx reduction control corresponds to the reduction execution means in the present invention. Further, the main ECU 3 that executes the NOx reduction control corresponds to the reduction execution means in the present invention.

<Combustion mode switching control>
By the way, since the engine 1 according to the present embodiment is a diesel engine, the air-fuel ratio of the air-fuel mixture in the combustion chamber of the engine 1 (hereinafter, this air-fuel ratio is referred to as “base air-fuel ratio”) is theoretically empty in most operating regions. The engine is operated on the lean side of the fuel ratio (stoichiometric). Therefore, even if the inflow exhaust air-fuel ratio is made rich by NOx reduction control, a large amount of oxygen may remain in the exhaust flowing into the NOx catalyst 20. In such a case, the NOx reduction reaction in the NOx catalyst 20 hardly occurs, and the NOx purification efficiency may be deteriorated.

  Therefore, in this embodiment, prior to the execution of the reduction control, the combustion mode of the engine 1 is switched to the low air-fuel ratio mode in which the base air-fuel ratio is lowered compared to the base air-fuel ratio during normal operation. Hereinafter, the combustion mode switching control in the present embodiment will be described.

  “During normal operation” in the present embodiment refers to an operation state in which the reduction control is not executed. Hereinafter, when the combustion mode during normal operation is referred to as “normal air-fuel ratio mode” for convenience, when the combustion mode is the normal air-fuel ratio mode, the engine 1 outputs only the engine torque TQe required for the engine 1. The intake air amount Ga, the opening degree of the EGR valve 32 (hereinafter simply referred to as “EGR opening degree”) Degr, the fuel injection amount Qf, and the like are controlled so as to be adapted. In this embodiment, the base air-fuel ratio during normal operation corresponds to the predetermined normal air-fuel ratio in the present invention.

  On the other hand, when NOx reduction control is executed, the base air-fuel ratio of the air-fuel mixture is reduced in advance compared to the normal air-fuel ratio mode. Specifically, the EGR rate Regr is increased as compared with the normal operation. That is, the main ECU 3 issues a command to the EGR valve 32 and changes the EGR opening degree Degr to the open side to lower the base air-fuel ratio of the air-fuel mixture.

  In the low air-fuel ratio mode, the degree of lowering the base air-fuel ratio compared to the normal air-fuel ratio mode may be determined experimentally in advance, and the EGR rate Regr is controlled accordingly. Specifically, for example, the base air-fuel ratio in the low air-fuel ratio mode may be determined so that the oxygen concentration in the exhaust discharged from the engine 1 is sufficiently reduced. Further, the EGR rate Regr during normal operation is required to achieve the target NOx reduction rate based on the target NOx reduction rate determined in consideration of, for example, the exhaust gas regulation value and the conformity margin for the regulation value. The EGR rate may be determined.

According to the combustion mode switching control in the present embodiment, the amount of oxygen contained in the exhaust gas is reduced at the time when it is exhausted from the engine 1, so that the NOx reduction efficiency in the NOx catalyst 20 can be improved. Further, since the air-fuel ratio of the exhaust discharged from the engine 1 is lower than that during normal operation, the main ECU 3 adds the added fuel to the fuel addition valve 21, and the inflow exhaust air-fuel ratio is set to the target air-fuel ratio (rich air-fuel ratio). It is also possible to save the amount of fuel added when the pressure is reduced to a minimum. In this embodiment, the main ECU 3 that executes the combustion mode switching control corresponds to the combustion mode switching means in the present invention.

<Engine torque adjustment control>
After the combustion mode is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode, if the required torque TQr is obtained only by the engine torque TQe, the combustion state of the air-fuel mixture may become excessively unstable and unstable. For example, when the required torque TQr is relatively high, the base air-fuel ratio is low and the fuel injection amount increases, so that an oxygen shortage occurs in the region near the fuel injection valve 13. As a result, misfire of the engine 1, sudden torque reduction, and increase in smoke emission may occur.

  Therefore, when the combustion mode is the low air-fuel ratio mode, the engine torque TQe (that is, the fuel injection amount Qf) is adjusted within a range in which the combustion state of the engine 1 is maintained in the “stable combustion possible state”. Hereinafter, engine torque adjustment control in this embodiment will be described. The “stable combustion possible state” is a state in which the combustion of the air-fuel mixture in the combustion chamber of the engine 1 is stable, for example, within a range in which torque fluctuations in each combustion cycle of the engine 1 are within an allowable value. You may define with the combustion state of time.

  The above-described allowable value may be obtained experimentally in advance, so that the torque fluctuation falls within the allowable value according to the operating state of the engine 1, that is, the combustion state of the air-fuel mixture can be maintained in a stable combustible state. Engine torque TQe is determined. The fuel injection amount Qf is controlled according to the engine torque TQe. This suppresses the combustion state from becoming unstable due to the main ECU 3 switching the combustion mode of the air-fuel mixture to the low air-fuel ratio mode. In the present embodiment, the main ECU 3 that executes engine torque adjustment control corresponds to the engine torque adjustment means in the present invention.

<Assist control>
As described above, in this embodiment, when the combustion mode of the air-fuel mixture is switched to the low air-fuel ratio mode, the engine torque TQe is adjusted within a range in which the combustion state can maintain the stable combustion possible state. May be insufficient with respect to the required torque TQr. Therefore, in this embodiment, assist control is performed to compensate for the shortage of the engine torque TQe with the assist torque TQa that causes the MG2 to output. Thus, the sum of the engine torque TQe and the assist torque TQa can be matched with the required torque TQr, and the driver's request can be satisfied. In the present embodiment, the main ECU 3 that executes assist control corresponds to the assist torque adjusting means in the present invention.

  Next, the command values of the respective parameters when the above-described NOx reduction control and each control (combustion mode switching control, engine torque adjustment control, assist control) executed in accordance with the NOx reduction control are executed are shown in FIG. Will be described with reference to FIG. FIG. 3 is a time chart showing command values of each parameter when the NOx reduction control is executed in the present embodiment. 3 (a) shows the EGR opening degree Degr, FIG. 3 (b) shows the fuel injection amount Qf, FIG. 3 (c) shows the engine torque TQe, FIG. 3 (d) shows the assist torque TQa, and FIG. It is the time chart which showed the open / close (ON-OFF) state of the fuel addition valve which adds. The EGR opening degree Degr, the fuel injection amount Qf, the engine torque TQe, and the assist torque TQa in the figure represent command values (target values) issued from the main ECU 3, and the command signal issued from the main ECU 3 for the open / closed state of the fuel addition valve. Represents. Moreover, the broken line shown in FIG.3 (c) represents the request torque TQr.

In each figure, on the vertical axis, the symbol with “0” at the end represents each command value when the combustion mode is the normal air-fuel ratio mode, and the symbol with “1” represents the combustion mode. Indicates each command value in the low air-fuel ratio mode. For example, “Degr0” in FIG. 3A indicates the EGR opening degree Degr in the normal air-fuel ratio mode, and “Degr1” indicates the EGR opening degree Degr in the low air-fuel ratio mode.

  On the other hand, the horizontal axis in each figure represents time. In the figure, “t0” represents the time when a NOx reduction request is issued, and “t1” and “t2” represent the start and end of fuel addition by the fuel addition valve 21. At t0, the combustion mode is switched from the normal air fuel ratio mode to the low air fuel ratio mode, and at t2, the combustion mode is switched again from the low air fuel ratio mode to the normal air fuel ratio mode.

  When a NOx reduction request is issued at time t0, the combustion mode is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode prior to the addition of the added fuel. That is, the EGR opening degree Degr is increased from Degr0 to Degr1 (changed to the opening degree on the more open side), and the fuel injection amount Qf is reduced from Qf0 to Qf1.

  As a result, the engine torque TQe decreases from TQe0 to TQe1, so that the assist torque TQa1 is output to the MG2 by the amount that the engine torque TQe is insufficient with respect to the required torque TQr. Since the purpose and operation effect of these controls have already been described, a detailed description thereof will be omitted.

  Then, the fuel addition valve 21 is opened from time t1 to time t2, and a predetermined amount of added fuel is added to the exhaust gas. The reason for delaying the start of fuel addition from t0 is to consider the time lag from when the combustion mode is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode until the actual base air-fuel ratio decreases.

  That is, the response delay time from when the EGR opening degree Degr is increased until the amount of EGR gas actually recirculated to the engine 1 increases is taken into consideration. It should be noted that the interval between the times t0 and t1 may be determined according to experimental confirmation in advance of the response when changing the EGR rate Regr.

  The combustion mode is switched from the low air-fuel ratio mode to the normal air-fuel ratio mode at time t2 when the addition of the added fuel into the exhaust is completed and the execution of the NOx reduction control is completed. After the time t2, even if the required torque TQr is output only by the engine torque TQe from the engine 1, there is no possibility that the combustion state of the air-fuel mixture becomes unstable. Accordingly, the fuel injection amount Qf is changed from Qf1 to Qf0 so that the engine torque TQe matches the required torque TQr. Further, since the required torque TQr is output only by the engine 1, the assist torque TQa to be output to the MG2 is changed to zero.

  When changing the fuel injection amount Qf from Qf1 to Qf0, the fuel injection amount Qf is gradually increased in consideration of response delay of the EGR gas amount, supercharging delay, etc., and the assist torque TQa by MG2 is gradually decreased. May be. Thereby, the effect which suppresses deterioration of exhaust emission is acquired. Further, after the time t2, the requested torque TQr is output only by the engine torque TQe and the assist torque TQa is set to 0, which is merely an example of the present embodiment and is not intended to be limited to this. . The distribution of the driving force between the engine 1 and the MG 2 may be determined in accordance with the driving state of the vehicle or the charging state of the battery 6.

  Hereinafter, the NOx reduction control executed by the main ECU 3 will be described with reference to the flowchart of FIG. FIG. 4 is a flowchart showing a NOx reduction control routine in the present embodiment. This routine is a program stored in the ROM in the main ECU 3, and is executed every predetermined period.

  When this routine is executed, first, in step S101, it is determined whether or not an execution condition for NOx reduction control for the NOx catalyst 20 is satisfied. In this case, the main ECU 3 calculates the NOx occlusion amount in the NOx catalyst 20 based on the operation state after the engine 1 starts operation. Then, it is determined whether or not the execution condition is satisfied by determining whether or not the current NOx occlusion amount exceeds a determination value set based on the NOx saturation amount of the NOx catalyst 20.

  In this step, if the current NOx occlusion amount is equal to or smaller than the determination value, it is determined that the NOx reduction control should not be started yet, and this routine is temporarily ended. If the current NOx occlusion amount exceeds the determination value in step S101, it is determined that the NOx reduction control should be started, and the process proceeds to step S102.

  The determination as to whether or not the execution condition for the NOx reduction control is satisfied is based on, for example, the integrated value of the NOx emission amount based on the operation history of the engine 1 since the previous NOx reduction control for the NOx catalyst 20 is completed. You may make it come out. Further, a NOx sensor (not shown) may be provided in the exhaust passage 19 so as to be output based on the output of the NOx sensor and the intake air amount of the engine 1.

  In step S102, the current operating state of the engine 1 is detected. Specifically, the engine speed Ne is obtained from the output value of the crank position sensor 23. Further, from the output values of the accelerator position sensor 24 and the air flow meter 15, the current fuel injection amount Qf0 is obtained based on the engine map stored in the main ECU 3.

  In step S103, a target EGR opening degree Degr1 that is a target value for the EGR opening degree when the combustion mode of the air-fuel mixture is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode is calculated. Specifically, the calculation is made based on the engine map for the low air-fuel ratio mode stored in advance in the main ECU 3 from the engine speed Ne and the output value of the accelerator position sensor 24. Similarly, in the normal air-fuel ratio mode before switching the combustion mode, the target value of the EGR opening degree Degr is obtained based on the engine map for the normal air-fuel ratio mode separately stored in the main ECU 3.

  In step S104, the target fuel injection amount Qf1 is calculated so that the combustion state of the engine 1 is maintained in the “stable combustion possible state” even when the combustion mode of the air-fuel mixture is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode.

  In step S105, a shortage of torque that is insufficient with respect to the required torque TQr of the engine torque TQe that occurs when the current fuel injection amount Qf0 is reduced to the target fuel injection amount Qf1 (TQr-TQe1 in FIG. 3C). Is calculated, and a target assist torque TQa1 for assisting the engine torque TQe by MG2 is calculated. The required torque TQr is calculated based on the output signal of the accelerator position sensor 24 and the engine speed Ne.

  In step S106, the EGR opening degree Degr is increased from the current EGR opening degree Degr0 to the target EGR opening degree Degr1, and the combustion mode is switched to the low air-fuel ratio mode. At the same time, the fuel injection amount Qf is reduced from the current fuel injection amount Qf0 to the target fuel injection amount Qf1. Further, MG2 is supplied with electric power from the battery 6, and MG2 is made to output the assist torque of the target assist torque TQa1.

In step S107, the low air-fuel ratio mode elapsed time Δtaf, which is the elapsed time from the execution of the process of step S106, is counted. In step S108, it is determined whether or not the low air-fuel ratio mode elapsed time Δtaf has passed (exceeded) a first reference elapsed time Δtafb (corresponding to t0 to t1 in FIG. 3). The first reference elapsed time is an elapsed time that allows for a predetermined margin in the time required for the transition period when the combustion mode of the air-fuel mixture completely switches from the normal air-fuel ratio mode to the low air-fuel ratio mode.

  If a positive determination is made in this step, the process proceeds to step S109. On the other hand, if a negative determination is made, the process returns to step S107, and the processes of steps S107 and S108 are continued until the low air-fuel ratio mode elapsed time Δtaf has passed the first reference elapsed time Δtafb. Thereby, since the air-fuel ratio of the exhaust discharged from the engine 1 when the fuel addition by the fuel addition valve 21 is performed in step S109 described later is reliably reduced, the NOx reduction efficiency can be increased.

  In step S109, the fuel addition valve 21 is opened, and a predetermined amount Qad of added fuel is added to the exhaust gas. The predetermined amount Qad is determined as an amount sufficient to suitably reduce the air-fuel ratio of the exhaust discharged from the engine 1 to the target air-fuel ratio (rich air-fuel ratio) and reduce NOx stored in the NOx catalyst 20. It is done. Thereby, NOx stored in the NOx catalyst 20 is reduced.

  In step S110, the fuel addition elapsed time Δthc, which is the elapsed time since the process of step S109 was executed, is counted. In step S111, it is determined whether or not the fuel addition elapsed time Δthc has passed (exceeded) a preset second reference elapsed time Δthcb (corresponding to t1 to t2 in FIG. 3). The second reference elapsed time Δthcb is a time required to add the predetermined amount of Qad added fuel into the exhaust gas, and is experimentally obtained in advance.

  If a positive determination is made in this step, the process proceeds to step S112. On the other hand, if a negative determination is made, the process returns to step S110, and the processes of steps S110 and S111 are continued until the fuel addition elapsed time Δthc has passed the second reference elapsed time Δthcb.

  In step S112, the fuel addition valve 21 is closed to stop the addition of added fuel. Then, the EGR opening degree Degr is changed from the target EGR opening degree Degr1 to the original EGR opening degree Degr0 again. That is, the combustion mode is returned from the low air-fuel ratio mode to the normal air-fuel ratio mode. At the same time, the fuel injection amount Qf is returned from the target fuel injection amount Qf1 to the fuel injection amount Qf0. Further, the supply of power from the battery 6 to the MG2 is stopped, and the assist torque TQa by the MG2 is set to zero.

  In this step, the required torque TQr is acquired based on the output value of the accelerator position sensor 24 and the engine speed Ne, and the engine torque TQe and the assist torque are output so that the required torque TQr is output by the engine 1 and MG2. You may distribute to TQa. When this step is finished, this routine is once ended.

  As described above, according to this routine, in step S106, the oxygen concentration of the exhaust gas discharged from the engine 1 is reduced in advance by switching the combustion mode of the air-fuel mixture from the normal air fuel ratio mode to the low air fuel ratio mode. be able to. As a result, the NOx reduction efficiency of the NOx catalyst 20 can be improved, and the fuel efficiency for reducing the inflow exhaust air-fuel ratio to the rich air-fuel ratio can be improved.

  Further, by adjusting the engine torque TQe according to the switching of the combustion mode, it is possible to suitably suppress the combustion state of the air-fuel mixture becoming unstable and getting worse. Further, since the engine torque TQe that is insufficient at that time is reliably assisted by the assist torque TQa by the MG2, the required torque TQr can be reliably output.

  In step S105 of this routine, when calculating the target assist torque TQa1, it is determined whether or not it is possible to detect the charge amount of the battery 6 from the output value of the battery ECU 8 and cause the MG2 to output the target assist torque TQa1. This is more preferable.

  If it is determined that it is difficult to output the target assist torque TQa1 due to insufficient charge, the engine torque TQe is increased within the range where the combustion state of the engine 1 is maintained in the “stable combustion possible state” (target fuel injection The target assist torque TQa1 to be output to MG2 may be reduced by setting the amount Qf1 to the increase side).

  If it is determined that the amount of charge of the battery 6 is excessively small and the required torque TQr is to be satisfied, the combustion state of the engine 1 cannot be maintained in a stable combustible state, the combustion mode is set to a low air-fuel ratio. The process may proceed to step S109 without switching to the mode, and fuel addition by the fuel addition valve 21 may be executed. That is, the fuel addition control may be executed in the normal air-fuel ratio mode. Thereby, NOx reduction control can be reliably executed while suppressing deterioration of the combustion state.

<Modification>
Next, an embodiment as a modification of the present embodiment will be described. Hereinafter, the control in the case where the required torque TQr from the driver is changed after the combustion mode switching control is executed, that is, after the combustion mode is switched from the normal air-fuel ratio mode to the low air-fuel ratio mode will be described with reference to FIG. To do. When the required torque TQr changes, it is necessary to make the sum of the engine torque TQe and the assist torque TQa coincide with the changed required torque TQr.

FIG. 5 is a time chart showing command values of parameters in which the required torque TQr is increased after the combustion mode is switched to the low air-fuel ratio mode in the present embodiment.
5 (a) shows the EGR opening degree Degr, FIG. 5 (b) shows the fuel injection amount Qf, FIG. 5 (c) shows the engine torque TQe, FIG. 5 (d) shows the assist torque TQa, and FIG. It is the time chart which showed the open / close (ON-OFF) state of the fuel addition valve which adds.

  Of the symbols shown on the vertical and horizontal axes in the figure, the same symbols as those in FIG. 3 represent the same meaning. That is, a reduction request for NOx is issued at t0, and the addition of added fuel is executed from t1 to t2. Here, as shown in FIG. 5C, the required torque TQr (broken line) increases at t3. Here, the value of the required torque TQr before increasing at t3 is indicated by TQr1, the value after increase is indicated by TQr2, and the amount of increase is indicated by ΔTQr (TQr2-TQr1 = ΔTQr).

  In this embodiment, when the required torque TQr1 increases, as shown in FIGS. 5C and 5D, the assist torque TQa is increased by increasing the required torque while keeping the fuel injection amount Qf constant. Increase by ΔTQr. That is, the assist torque TQa is increased from TQa1 to TQa2 at t3 (TQa2−TQa1 = ΔTQr).

  Here, responding to the increase amount ΔTQr of the required torque by changing the assist torque TQa by MG2 is particularly effective for ensuring the stability of the air-fuel mixture in the combustion state. This is because if the response is made by increasing the fuel injection amount Qf with respect to the increase in the required torque TQr, the combustion state may be excessively deteriorated depending on the magnitude of the increase amount ΔTQr in the required torque. That is, since the combustion mode of the air-fuel mixture is the low air-fuel ratio mode, if the fuel injection amount Qf is suddenly increased to respond to the increase in the required torque TQr, the smoke discharged from the engine 1 increases excessively or the torque There is a risk that fluctuations will increase and the combustion state will deteriorate.

  Thereafter, when the fuel addition by the fuel addition valve 21 is completed at t2, the combustion mode is returned to the normal air-fuel ratio mode as described above. Therefore, the assist by MG2 is terminated, and the required torque TQr is output by the engine torque TQe. That is, the assist torque TQa is changed from TQa2 to zero, and the fuel injection amount Qf is increased from Qf1 to Qf2 to increase the engine torque TQe to TQe2 (= TQr2).

  According to the above control, the engine torque TQe does not vary even when the required torque TQr increases. Therefore, the NOx reduction control can be executed while reliably suppressing the deterioration of the combustion state. Further, if this is done, NOx reduction control can be executed continuously, so that NOx stored in the NOx catalyst 20 can be reduced at an early stage. If the increase amount ΔTQr of the required torque is small and the combustion state of the air-fuel mixture can be maintained in a stable combustible state even if the fluctuation of the required torque TQr is absorbed by adjusting the engine torque TQe, the combustion injection amount Qf is increased. Thus, the engine torque TQe may be matched with the required torque TQr.

  In FIG. 5, the case where the required torque TQr is increased is described as an example. However, when the required torque TQr is decreased, the assist torque TQa is set so that the required torque TQr is equal to the sum of the assist torque TQa and the engine torque TQe. And / or the engine torque TQe may be reduced.

  Further, variation control when the required torque TQr increases after switching the combustion mode from the normal air-fuel ratio mode to the low air-fuel ratio mode will be described. For example, at t3 in FIG. 5, whether or not the assist torque TQa can be increased from TQa1 to TQa2 may be determined from the charge amount of the battery 6 or the maximum output of the battery 6. If it is determined that it is difficult to increase the assist torque TQa to TQa2, the maintenance of the combustion mode in the low air-fuel ratio mode is temporarily terminated, the normal air-fuel ratio mode is switched to continue the NOx reduction control. Also good.

  In this case, although the merit by the improvement of NOx reduction efficiency by executing the combustion mode switching control cannot be enjoyed, the combustion state of the air-fuel mixture is deteriorated and the drivability is excessively deteriorated or the amount of smoke discharged NOx can be reliably reduced while suppressing an increase in the amount of NOx.

  Alternatively, even when it is determined that it is difficult to increase the assist torque TQa to TQa2, it is more preferable to perform the following control. When the torque that is still insufficient with respect to TQa2 even if the assist torque TQa is increased to the maximum is referred to as assist insufficient torque, when the engine torque TQe is increased by the insufficient assist torque, the combustion state of the mixture of the engine 1 is “ It is determined whether or not the “stable combustion possible state” is maintained.

  When an affirmative determination is made (when it is determined that the combustion state of the air-fuel mixture is maintained in a state where stable combustion is possible), the assist torque TQa is increased to the maximum, and the engine torque TQe is increased by the assist insufficient torque. increase. As a result, it is possible to respond appropriately to an increase in the required torque TQr while suppressing the combustion state from becoming unstable.

  When a negative determination is made (when it is determined that the combustion state of the air-fuel mixture cannot be maintained in a state where stable combustion is possible), the maintenance of the combustion mode in the low air-fuel ratio mode is temporarily terminated as described above. Alternatively, the NOx reduction control may be continued by switching to the normal air-fuel ratio mode.

As described above, even when the required torque TQr is increased, the combustion mode is maintained in the low air-fuel ratio mode while maintaining the combustion state of the air-fuel mixture in a stable combustible state according to the increase amount ΔTQr of the required torque. , NOx reduction efficiency can be improved. Further, when it is difficult to maintain the combustion state in a stable combustion enabled state, the NOx reduction control can be continued by switching to the normal air-fuel ratio mode, so that NOx can be reliably reduced.

  In the present embodiment, the method of switching the combustion mode from the normal air-fuel ratio mode to the low air-fuel ratio mode by increasing the RGR rate Regr has been described as an example, but various conventionally known methods can be employed. For example, a method of reducing the amount of intake air taken into the engine 1 by controlling the opening of the intake throttle valve 11 to the closed side may be employed. When the turbocharger 17 is a so-called variable capacity turbocharger having a variable nozzle, the amount of EGR gas may be increased by adjusting the opening of the variable nozzle to the throttle side.

  In this embodiment, fuel is added to the NOx catalyst 20 by adding fuel from the fuel addition valve 21 provided in the exhaust passage 29. For example, the fuel from the fuel addition valve 21 is supplied. Instead of the addition, the fuel may be sub-injected from the fuel injection valve 13 during the expansion stroke or exhaust stroke of the engine 1.

  In the present embodiment, the exhaust purification system including the NOx storage reduction catalyst as the NOx catalyst has been exemplarily described. However, the present invention is not limited to this. For example, the present invention can be applied to an exhaust purification system including a selective reduction type NOx catalyst. In that case, urea water or ammonia water as a reducing agent may be added from the fuel addition valve.

1 is a block diagram illustrating a system configuration of a hybrid vehicle to which an exhaust gas purification system for an internal combustion engine according to a first embodiment is applied. 1 is a diagram illustrating a schematic configuration of an exhaust gas purification system for an internal combustion engine in Embodiment 1. FIG. 6 is a time chart showing command values of parameters when NOx reduction control is executed in the first embodiment. (A) is the EGR opening degree Degr, (b) is the fuel injection amount Qf, (c) is the engine torque TQe, (d) is the assist torque TQa, (e) is the open / closed state of the fuel addition valve for adding the added fuel. It is the time chart shown. 3 is a flowchart showing a NOx reduction control routine in Embodiment 1. 3 is a time chart showing command values of parameters in which required torque TQr is increased after switching the combustion mode to the low air-fuel ratio mode in the first embodiment. (A) is the EGR opening degree Degr, (b) is the fuel injection amount Qf, (c) is the engine torque TQe, (d) is the assist torque TQa, (e) is the open / close state of the fuel addition valve for adding the added fuel. It is the time chart shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Engine 2 ... Motor generator (MG)
3 ... Main ECU
4 ... T / M
5 ... T / MECU
6 ... Battery 7 ... Inverter 8 ... Battery ECU
12, · Cylinder 13 ·· Fuel injection valve 17 · Turbocharger 18 · Intake manifold 19 · Intake passage 20 · Occlusion reduction NOx catalyst 21 · Fuel addition valve 23 · Crank position sensor 24 · Accelerator position Sensor 28 ・ Exhaust manifold 29 ・ Exhaust passage 30 ・ EGR device 31 ・ EGR passage 32 ・ EGR valve

Claims (4)

An exhaust gas purification system for an internal combustion engine applied to a hybrid vehicle that travels with two types of power sources, an internal combustion engine and an electric motor,
A NOx catalyst provided in an exhaust passage of the internal combustion engine;
Reduction executing means for executing a reduction control for temporarily reducing the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a rich air-fuel ratio by reducing the NOx in the NOx catalyst by supplying a reducing agent to the exhaust gas passing through the exhaust passage. When,
Prior to execution of the reduction control, the air-fuel ratio of the air-fuel mixture is changed from the normal air-fuel ratio mode from the normal air-fuel ratio mode in which the air-fuel ratio of the air-fuel mixture is set to a predetermined normal air-fuel ratio. Combustion mode switching means for switching to a low air-fuel ratio mode for reducing
Engine torque adjusting means for adjusting engine torque within a range in which the combustion state of the internal combustion engine is maintained in a predetermined stable state when the combustion mode switching means switches the combustion mode;
Assist torque adjusting means for adjusting the assist torque output by the electric motor to output the shortage of the engine torque relative to the required torque to the electric motor when the engine torque is insufficient with respect to the required torque;
An exhaust gas purification system for an internal combustion engine, comprising:   When the required torque changes in a state after the combustion mode is switched to the low air-fuel ratio mode, the engine torque adjusting means is in a range where the combustion state of the internal combustion engine is maintained in the stable state and The exhaust purification system for an internal combustion engine according to claim 1, wherein the engine torque is adjusted so that a sum of the engine torque and the assist torque substantially matches a required torque after the change.   When the required torque changes in a state after the combustion mode is switched to the low air-fuel ratio mode, the engine torque adjusting means maintains the engine torque substantially constant and the assist torque adjusting means 2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the assist torque is adjusted so that a sum of the torque and the assist torque substantially matches the required torque after the change. An assist determination means for determining whether or not the electric motor can output a shortage of the engine torque with respect to the required torque;
When it is determined that the shortage of the engine torque cannot be output to the electric motor, switching to the low air-fuel ratio mode or maintenance of the low air-fuel ratio mode by the combustion mode switching means is prohibited. An exhaust purification system for an internal combustion engine according to any one of claims 1 to 3.

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