WO2012108043A1 - Control device for internal combustion engine - Google Patents

Abstract

The objective of the present invention is to prevent NOx from slipping through a NOx catalyst when switching between lean running and rich running with a control device for an internal combustion engine for which NOx absorbed by the NOx catalyst is removed by means of a rich spike. This control device for an internal combustion engine is equipped with: an internal combustion engine capable of executing lean running, wherein the running air-fuel ratio, which is the ratio between the amount of new air and the amount of fuel, is greater than the theoretical air-fuel ratio; a NOx catalyst capable of absorbing NOx in the exhaust gas during lean running; an EGR device capable of executing EGR, wherein a portion of the exhaust gas is recirculated to the intake air as EGR gas; a rich spike means that temporarily executes rich running wherein the running fuel-air ratio is made equal to or less than the theoretical air-fuel ratio when the NOx absorbed in the NOx catalyst is reduced; and a running control means that, when transitioning between lean running and rich running, shifts the running air-fuel ratio and the gas-fuel ratio, which is the ratio between the sum of the amount of new air and the amount of EGR gas and the amount of the fuel, along a predetermined path.

Description

Control device for internal combustion engine

The present invention relates to a control device for an internal combustion engine.

In an internal combustion engine that performs a lean operation in which the air-fuel ratio is larger than the stoichiometric air-fuel ratio, oxygen in the exhaust gas becomes excessive, so that the three-way catalyst cannot sufficiently purify NOx. Therefore, in an internal combustion engine that performs lean operation, a NOx catalyst (occlusion reduction type NOx catalyst) having a function of storing NOx is used. The NOx catalyst can absorb and store NOx in the exhaust gas during lean operation. In order to purify NOx stored in the NOx catalyst, it is necessary to perform a rich spike in which the air-fuel ratio is temporarily set to a rich air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio. By performing this rich spike, exhaust gas containing reducing agent components such as HC and CO is supplied from the internal combustion engine to the NOx catalyst, and the stored NOx is reduced and purified by this reducing agent component and released as N _2. Is done.

Japanese Unexamined Patent Publication No. 2000-54824 Japanese Unexamined Patent Publication No. 2002-97976

As described above, an internal combustion engine that performs lean operation switches from lean operation to rich operation each time a rich spike is performed. At this time, a large amount of NOx tends to be discharged from the internal combustion engine due to a change in the combustion state accompanying the change in the air-fuel ratio between the lean air-fuel ratio and the rich air-fuel ratio. Therefore, there is a problem that the NOx passes through the NOx catalyst without being completely treated with the NOx catalyst. Further, in order to suppress the slipping of this NOx, it is required to increase the capacity of the NOx catalyst, and there is a problem that the amount of expensive noble metal used increases and the cost increases.

The present invention has been made to solve the above-described problems, and provides an internal combustion engine control device capable of suppressing NOx from passing through a NOx catalyst when switching between lean operation and rich operation. With the goal.

In order to achieve the above object, a first invention is a control device for an internal combustion engine,
An internal combustion engine capable of performing a lean operation in which the operating air-fuel ratio that is the ratio of the amount of fresh air and the amount of fuel in the air-fuel mixture in the cylinder is greater than the theoretical air-fuel ratio;
A NOx catalyst disposed in an exhaust passage of the internal combustion engine and capable of absorbing NOx in exhaust gas during the lean operation;
An EGR device capable of executing EGR to recirculate a part of the exhaust gas as EGR gas to the intake air;
Rich spike means for temporarily performing rich operation in which the operating air-fuel ratio is equal to or lower than the theoretical air-fuel ratio when reducing NOx stored in the NOx catalyst;
A ratio of the amount of fuel and the sum of the operating air-fuel ratio, the amount of fresh air in the air-fuel mixture in the cylinder, and the amount of EGR gas when transitioning between the lean operation and the rich operation Operation control means for changing the gas fuel ratio along a predetermined route;
It is characterized by providing.

The second invention is the first invention, wherein
The path is defined such that an EGR rate increases with an increase in the operating air-fuel ratio when the lean operation is shifted to the rich operation.

The third invention is the first or second invention, wherein
The path is set according to a combustion limit for EGR.

According to the first aspect of the present invention, the operating air-fuel ratio and the gas fuel ratio can be changed along a predetermined route when shifting between the lean operation and the rich operation. As a result, the lean operation and the rich operation can be switched while the combustion state of the internal combustion engine is maintained in an ideal state. For this reason, since the NOx emission amount at the time of switching between the lean operation and the rich operation accompanying the execution of the rich spike can be reliably suppressed, it is ensured that NOx passes through the NOx catalyst and is discharged into the atmosphere. It can be avoided. Further, since the NOx slip-through can be avoided without increasing the capacity of the NOx catalyst so much, the amount of expensive noble metal used can be suppressed, which contributes to cost reduction.

According to the second aspect of the present invention, the above-mentioned route is determined so that the EGR rate increases with a change in the operating air-fuel ratio when shifting from the lean operation to the rich operation, so that at the time of switching between the lean operation and the rich operation The amount of NOx emission can be further reduced.

According to the third invention, by setting the above path according to the combustion limit for EGR, the EGR amount is set as much as possible while avoiding instability of combustion when switching between lean operation and rich operation. Can do a lot. For this reason, the amount of NOx emissions when switching between lean operation and rich operation is further reduced while reliably preventing adverse effects such as an increase in torque fluctuation due to instability of combustion and deterioration of emissions other than NOx. Can do.

It is a figure for demonstrating the system configuration | structure of Embodiment 1 of this invention. 6 is a map that defines a path followed by an operating air-fuel ratio A / F and a gas fuel ratio G / F when transitioning between a lean operation and a rich operation. It is a figure which shows the relationship between an operating air fuel ratio A / F, an EGR rate, and the NOx discharge | emission amount from an internal combustion engine.

FIG. 1 is a diagram for explaining a system configuration according to the first embodiment of the present invention. As shown in FIG. 1, the system according to the first embodiment of the present invention includes a spark ignition type internal combustion engine 10. The internal combustion engine 10 can be preferably used as a power source of a vehicle, for example. The number of cylinders and the cylinder arrangement of the internal combustion engine 10 are not particularly limited. Each cylinder of the internal combustion engine 10 is provided with a piston 12, an intake valve 14, an exhaust valve 16, a spark plug 18, and a fuel injector 20 that directly injects fuel into the cylinder. In this internal combustion engine 10, the operating air-fuel ratio (combustion air-fuel ratio), which is the ratio of the amount (weight) of fresh air in the air-fuel mixture in the cylinder and the amount (weight) of fuel, is set to a lean air-fuel ratio greater than the stoichiometric air-fuel ratio. This is a lean burn engine that can perform lean operation. Each cylinder is connected to an intake passage 22 and an exhaust passage 24.

The internal combustion engine in the present invention is not limited to the in-cylinder injection engine such as the illustrated internal combustion engine 10, but uses a port injection engine that injects fuel into the intake port, or a combination of in-cylinder injection and port injection. It may be an engine.

The internal combustion engine 10 of the present embodiment has a supercharger 28. The supercharger 28 includes an intake compressor 281 and an exhaust turbine 282. The intake compressor 281 is connected to the intake passage 22, and the exhaust turbine 282 is connected to the exhaust passage 24. An air flow meter 30 that can detect the amount [g / s] of fresh air sucked into the internal combustion engine 10 (hereinafter referred to as “Ga”) is installed in the intake passage 26 upstream of the intake compressor 281. Has been. In the intake passage 22 on the downstream side of the intake compressor 281, an intercooler 32 and an electronically controlled throttle valve 34 for controlling the amount of fresh air Ga to be sucked are installed. An intake pressure sensor (not shown) for detecting the intake pressure downstream of the throttle valve 34 may be provided at a position downstream of the throttle valve 34 (for example, a surge tank). In this case, the fresh air amount Ga can be obtained based on the detected value of the intake pressure sensor, or can be calculated by combining the detected value of the air flow meter 30 and the detected value of the intake pressure sensor.

An occlusion reduction type NOx catalyst 38 is installed in the exhaust passage 36 on the downstream side of the exhaust turbine 282. An air-fuel ratio sensor 40 that detects the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 10 is installed in the exhaust passage 24 upstream of the exhaust turbine 282.

The internal combustion engine 10 is further provided with an EGR device for performing EGR (Exhaust Gas Recirculation) for recirculating a part of the exhaust gas to the intake air. The EGR device of the present embodiment has an EGR passage 42 that connects the exhaust passage 24 and the intake passage 22. The EGR passage 42 has an EGR valve 44 for controlling the amount of exhaust gas recirculated to the intake air (hereinafter referred to as “EGR gas”), an EGR cooler 46 for cooling the EGR gas, and a flow rate of the EGR gas. And a flow rate sensor 48 is provided.

The system of this embodiment includes an ECU (Electronic Control Unit) 50, an accelerator position sensor 52 for detecting a load command from the driver for the internal combustion engine 10, a rotation angle of the crankshaft of the internal combustion engine 10 and the engine. A crank angle sensor 54 for detecting the rotational speed is further provided. The various sensors and actuators described above are electrically connected to the ECU 50. The ECU 50 controls the operation of the internal combustion engine 10 by driving each actuator based on information detected by each sensor.

The ECU 50 controls the fuel injection amount of the fuel injector 20 based on the injection time, whereby the amount of fuel [g / s] (hereinafter represented by the symbol “Gf”) supplied from the fuel injector 20 into the cylinder. Can be controlled. The operating air-fuel ratio (hereinafter represented by the symbol “A / F”) can be calculated as a ratio between the fresh air amount Ga and the fuel amount Gf. That is, the following equation holds.
A / F = Ga / Gf (1)

Further, the ECU 50 can calculate the amount [g / s] of EGR gas introduced into the intake air (hereinafter represented by the symbol “Gegr”) based on the signal from the flow sensor 48. The EGR gas amount Gegr can also be obtained based on the detection value of the intake pressure sensor. Further, the EGR gas amount Gegr can be calculated by combining the detection value of the air flow meter 30 and the detection value of the intake pressure sensor. Then, the ECU 50 can calculate an EGR rate (hereinafter referred to as “Regr”) defined by the following equation based on the EGR gas amount Gegr and the fresh air amount Ga.
Regr = Gegr / (Ga + Gegr) (2)

Note that the EGR rate Regr can also be calculated by a known method based on the opening degree of the EGR valve 44, the engine speed, the engine load, and the like. Thus, when calculating the EGR rate Regr, the flow sensor 48 may not be provided.

When the internal combustion engine 10 is performing a lean operation, NOx in the exhaust gas is absorbed by the NOx catalyst 38 and stored. The ECU 50 can calculate the amount of NOx stored in the NOx catalyst 38 (NOx occlusion amount) by a known method. The ECU 50 temporarily performs a rich operation in which the operating air-fuel ratio A / F is set to a rich air-fuel ratio equal to or lower than the theoretical air-fuel ratio before the calculated NOx occlusion amount reaches the limit value. This is called a rich spike. By executing this rich spike, exhaust gas having a rich air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio is supplied from the internal combustion engine 10 to the NOx catalyst 38. NOx stored in the NOx catalyst 38 is reduced and purified to N ₂ and released by reducing agent components such as HC and CO contained in the rich air-fuel ratio exhaust gas. After the reduction of NOx stored in the NOx catalyst 38 is completed, the ECU 50 returns the operation state of the internal combustion engine 10 from the rich operation to the lean operation.

Thus, the internal combustion engine 10 switches between the lean operation and the rich operation every time the rich spike is performed. That is, every time the rich spike is performed, the operating air-fuel ratio A / F is switched between the lean air-fuel ratio and the rich air-fuel ratio. Conventionally, a large amount of NOx tends to be discharged from the internal combustion engine 10 due to a change in the combustion state at the time of switching of the operating air-fuel ratio A / F. If this large amount of NOx is discharged, there is a problem that the NOx catalyst 38 cannot process NOx, and NOx passes through the NOx catalyst 38 and is discharged into the atmosphere. In addition, in order to suppress this NOx slip-through, it is required to increase the capacity of the NOx catalyst 38, and there is a problem that the amount of expensive noble metal used increases and the cost increases.

In this embodiment, in order to solve the above-described problem, this transition is performed while introducing a large amount of EGR gas into the intake air when transitioning between lean operation and rich operation during the execution of rich spike. By introducing a large amount of EGR gas into the intake air, the combustion temperature is lowered, so that the amount of NOx discharged from the internal combustion engine 10 can be sufficiently reduced. However, there is a flammability limit for high volume EGR. That is, there is a limit to the amount of EGR that can establish stable combustion of the internal combustion engine 10. If the EGR is introduced into the intake air beyond the combustion limit, the combustion of the internal combustion engine 10 becomes unstable, causing adverse effects such as an increase in torque fluctuation and deterioration of emissions other than NOx. On the other hand, if the EGR amount is small, the combustion temperature is not sufficiently lowered, and the NOx emission amount from the internal combustion engine 10 cannot be sufficiently reduced. Therefore, in order to sufficiently reduce the NOx emission amount from the internal combustion engine 10 by the large amount EGR, it is important to maintain the combustion state close to the combustion limit.

The amount of EGR that becomes the combustion limit changes according to the operating air-fuel ratio A / F. If the operating air-fuel ratio A / F can be changed while transitioning between lean operation and rich operation while maintaining the combustion state close to the combustion limit, the internal combustion engine does not cause adverse effects due to excessive EGR. NOx emissions from 10 can be sufficiently reduced. In order to realize this ideal, in this embodiment, the gas fuel ratio is controlled together with the operating air-fuel ratio A / F when shifting between the lean operation and the rich operation. The gas fuel ratio (hereinafter referred to as the symbol “G / F”) is the total gas amount in the air-fuel mixture in the cylinder (that is, the sum of the amount of fresh air (weight) and the amount of EGR gas (weight)) and the fuel. It is a ratio to the amount (weight). The gas fuel ratio G / F can be calculated by the following equation.
G / F = (Ga + Gegr) / Gf (3)

Further, the gas fuel ratio G / F can be calculated by the following equation by modifying the above equation (3) using the relationship between the above equations (1) and (2).
G / F = A / F × {1 / (1-Regr)} (4)

FIG. 2 is a map that defines the path followed by the operating air-fuel ratio A / F and the gas fuel ratio G / F when transitioning between lean operation and rich operation. This map is stored in the ECU 50 in advance. When the ECU 50 shifts the operation state of the internal combustion engine 10 between the lean operation and the rich operation in performing the rich spike, the operation air-fuel ratio A / F and the gas fuel ratio G / F along the path shown in FIG. The operating air-fuel ratio A / F and the gas / fuel ratio G / F are controlled so that. At this time, the ECU 50 can control the operating air-fuel ratio A / F by adjusting the fuel injection amount from the fuel injector 20. The fuel injection amount may be corrected based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor 40. Further, the ECU 50 can control the gas fuel ratio G / F calculated by the above equation (3) or (4) by adjusting the EGR gas amount Gegr according to the opening of the EGR valve 44.

The paths of the operating air-fuel ratio A / F and the gas fuel ratio G / F shown in FIG. 2 are set to comply with the following conditions.
(1) The EGR rate increases with the operating air-fuel ratio A / F when shifting from lean operation to rich operation. That is, as the operating air-fuel ratio A / F increases, the EGR rate also increases.
(2) In each value of the operating air-fuel ratio A / F, the value of the gas fuel ratio G / F is determined according to the combustion limit of EGR.

FIG. 3 is a diagram showing the relationship among the operating air-fuel ratio A / F, the EGR rate, and the NOx emission amount from the internal combustion engine 10. As shown in FIG. 3, when the EGR rate is small, the NOx emission amount increases rapidly as the operating air-fuel ratio A / F changes from the lean side toward the rich side. On the other hand, the larger the EGR rate, the more the NOx emission increase when the operating air-fuel ratio A / F changes from the lean side toward the rich side can be suppressed. As described above, the route shown in FIG. 2 is determined such that the EGR rate increases with the operating air-fuel ratio A / F when shifting from lean operation to rich operation. For this reason, in the combustion state along the path shown in FIG. 2, the NOx emission amount from the internal combustion engine 10 can be sufficiently suppressed even when the operating air-fuel ratio A / F shifts to the rich side. it can. In the route shown in FIG. 2, as described above, the value of the gas fuel ratio G / F is determined according to the combustion limit of EGR. Therefore, in the combustion state along the path shown in FIG. 2, the combustion does not become unstable, and adverse effects such as an increase in torque fluctuation and deterioration of emissions other than NOx can be avoided.

In the present embodiment, when the ECU 50 shifts between the lean operation and the rich operation when the rich spike is performed, the operation air-fuel ratio A / F and the gas fuel ratio G along the path shown in FIG. Control so that / F changes. Control is performed so that the operating air-fuel ratio A / F and the gas fuel ratio G / F change in the lean air-fuel ratio direction along the path shown in FIG. 2 even when the reduction of NOx is completed and the rich operation returns to the lean operation. It is desirable to do.

While shifting between the lean operation and the rich operation by changing the operating air-fuel ratio A / F and the gas fuel ratio G / F along the path shown in FIG. 2, the combustion state of the internal combustion engine 10 has been described above. In such an ideal state, that is, the NOx emission amount is sufficiently suppressed by a large amount of EGR, and a state in which there is no harmful effect such as an increase in torque fluctuation due to instability of combustion and deterioration of emissions other than NOx is maintained. The For this reason, the NOx emission amount at the time of switching between the lean operation and the rich operation accompanying the execution of the rich spike can be surely suppressed, and it is ensured that NOx passes through the NOx catalyst 38 and is discharged into the atmosphere. It can be avoided. In addition, the NOx slip-through can be avoided without increasing the capacity of the NOx catalyst 38 so much, so that the amount of expensive noble metal used can be suppressed, which contributes to cost reduction.

In the internal combustion engine 10, it is desirable that the operating air-fuel ratio A / F and the gas fuel ratio G / F in the normal lean operation (lean operation between two rich spikes) are always on the path shown in FIG. . In the present invention, however, the normal lean operation region may include the operating air-fuel ratio A / F and the gas fuel ratio G / F that are not on the path shown in FIG. In this case, when the operation state before starting the rich spike is not on the route shown in FIG. 2, the ECU 50 first adjusts the EGR amount to change the gas fuel ratio G / F, thereby changing the internal combustion engine. 10 operating states are shifted on the route shown in FIG. Thereafter, the rich spike may be executed by shifting the operating air-fuel ratio A / F in the rich air-fuel ratio direction along the path shown in FIG.

In the first embodiment described above, the ECU 50 executes the rich spike control as described above, thereby realizing the “rich spike means” and the “operation control means” in the first invention.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 14 Intake valve 16 Exhaust valve 18 Spark plug 20 Fuel injector 22, 26 Intake passage 24, 36 Exhaust passage 28 Supercharger 281 Intake compressor 282 Exhaust turbine 30 Air flow meter 34 Throttle valve 38 NOx catalyst 40 Air-fuel ratio sensor 42 EGR Passage 44 EGR valve 46 EGR cooler 48 Flow rate sensor 50 ECU
52 Accelerator position sensor 54 Crank angle sensor

Claims (3)

An internal combustion engine capable of performing a lean operation in which the operating air-fuel ratio that is the ratio of the amount of fresh air and the amount of fuel in the air-fuel mixture in the cylinder is greater than the theoretical air-fuel ratio;
A NOx catalyst disposed in an exhaust passage of the internal combustion engine and capable of absorbing NOx in exhaust gas during the lean operation;
An EGR device capable of executing EGR to recirculate a part of the exhaust gas as EGR gas to the intake air;
Rich spike means for temporarily performing rich operation in which the operating air-fuel ratio is equal to or lower than the theoretical air-fuel ratio when reducing NOx stored in the NOx catalyst;
A ratio of the amount of fuel and the sum of the operating air-fuel ratio, the amount of fresh air in the air-fuel mixture in the cylinder, and the amount of EGR gas when transitioning between the lean operation and the rich operation Operation control means for changing the gas fuel ratio along a predetermined route;
A control device for an internal combustion engine, comprising: 2. The control of an internal combustion engine according to claim 1, wherein the path is determined such that an EGR rate increases with an increase in the operating air-fuel ratio when the lean operation is shifted to the rich operation. apparatus. 3. The control device for an internal combustion engine according to claim 1 or 2, wherein the path is set according to a combustion limit for EGR.

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