JP2002038933A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress fuel consumption by performing S-poisoning recovering process when an internal combustion engine is in decelerative operation while the function of NOX catalyst is well secured. SOLUTION: An exhaust emission control device of an internal combustion engine is equipped with a NOX catalyst of storage reduction type to absorb NOX in the exhaust gas when the exhaust air-fuel ratio is lean, emit the absorbed NOX when the oxygen concentration in the exhaust gas is reduced and perform reductive purification of the NOX by addition of a reducing agent, and an ECU 35 as a temperature raising means to raise the temperature of the NOX catalyst within the range to suit NOX purification in the case the S-poisoning of NOX catalyst is to be recovered when the engine 1 is in the accelerative operation or in the steady running condition and to turn the temperature of the NOX catalyst into a temperature suitable for S-poisoning recovery by turning lean the exhaust air-fuel ratio when the engine 1 is in the decelerative operation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus having a NOx catalyst for removing nitrogen oxides (hereinafter referred to as "NOx" unless otherwise specified) in exhaust gas. About.

[0002]

2. Description of the Related Art As an exhaust gas purifying device for purifying exhaust gas of an internal combustion engine, alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earths such as barium Ba, calcium Ca, lanthanum La, 2. Description of the Related Art An exhaust emission control device using a NOx catalyst comprising at least one selected from rare earth elements such as yttrium Y and a noble metal such as platinum Pt is known.

[0003] The NOx storage reduction catalyst is a catalyst capable of purifying exhaust gas discharged from a compression ignition type internal combustion engine such as a diesel engine or a lean burn type internal combustion engine such as a lean burn gasoline engine.

[0004] The NOx storage reduction catalyst uses an air-fuel ratio of exhaust gas flowing into the catalyst (hereinafter referred to as "exhaust air-fuel ratio (exhaust A / A
F) ". ) Is lean, absorbs NOx, and when the oxygen concentration in the inflow exhaust decreases, the N
NOx is released and released by releasing Ox.

In the compression ignition type internal combustion engine and the lean burn type internal combustion engine, the exhaust air-fuel ratio in their steady operation state is lean. NOx is NOx
Absorbed by the catalyst. However, its absorption capacity is limited, so it eventually saturates and cannot be absorbed any more, and NO
x may leak.

[0006] In order to avoid leaks, the NOx absorption capacity must be restored. For this purpose, it is necessary to release the NOx that has been absorbed by the NOx storage reduction catalyst to restore the NOx absorption capacity of the NOx storage reduction catalyst.

Therefore, before the storage reduction type NOx catalyst becomes saturated, the air-fuel ratio of the inflowing exhaust gas is made rich at a predetermined timing to reduce the oxygen concentration extremely, thereby reducing the storage reduction type NOx.
By releasing NOx from the catalyst, the NOx absorption capacity is restored (hereinafter referred to as "regeneration of the NOx storage reduction catalyst").

[0008] However, even if the NOx catalyst can be regenerated only by the simple release of NOx, NOx purification, which is the original function of the NOx catalyst, cannot be performed, leading to deterioration of emission. Therefore, for example, a fuel injection valve is provided at the exhaust port,
The released NOx by adding from the fuel injection valve in the NOx storage reduction catalyst the engine fuel is injected continuously or intermittently engine fuel in accordance with the operation state of the internal combustion engine as a reducing agent to N ₂ It is reduced, and NOx purification is performed.

[0009] Since engine fuel contains a large amount of sulfur components, it burns to produce sulfur oxides (hereinafter referred to as "SOx" unless otherwise specified). And SOx is N
It is absorbed by the NOx storage reduction catalyst by the same mechanism as Ox.

SOx generally forms a stable sulfate, so it is generally difficult to decompose and release it, and SOx is easily accumulated in the NOx storage reduction catalyst.

Then, the amount of absorbed and reduced N
A phenomenon in which the NOx purification rate of the Ox catalyst is reduced, which hinders NOx purification, which is a function of the NOx storage reduction catalyst, is called S
Poisoning may occur. Therefore, it is necessary to recover the NOx storage reduction catalyst in the S poisoning state, and the process for this is called S poisoning recovery processing of the NOx storage reduction catalyst.

However, since the sulfate accumulated in the NOx storage reduction catalyst is relatively stable as described above, the temperature at which the NOx storage reduction catalyst performs regeneration (for example,
(About 500 ° C.), it is difficult to release SOx absorbed by the NOx storage reduction catalyst.

[0013] To perform the S poisoning recovery process, the storage reduction type N
For example, 600 higher than the temperature at which the Ox catalyst is regenerated.
This problem is dealt with by increasing the temperature of the NOx storage reduction catalyst to a high temperature of about 700 ° C. and periodically making the exhaust air-fuel ratio stoichiometric or rich.

On the other hand, the exhaust temperature of a diesel engine is lower than that of a conventional gasoline engine, and it is difficult to achieve the high temperature required for performing the S poisoning recovery process.

If the exhaust air-fuel ratio is set to stoichiometric or rich when the diesel engine is in a high-load operation state, that is, when the diesel engine is operating at a high temperature, the unburned fuel becomes Fine particles such as so-called particulate matter (hereinafter referred to as "PM" unless otherwise specified), which is particulate matter such as components, are easily discharged. Therefore, the exhaust air-fuel ratio cannot be made stoichiometric or rich at the high temperature.

Therefore, the temperature of the NOx storage reduction catalyst (hereinafter referred to as "catalyst bed temperature") is raised by some method at the high temperature, and the exhaust air-fuel ratio is increased during a light load operation in which particulates such as soot are hardly discharged. To recover S poisoning.

[0017]

However, when the diesel engine is accelerated, for example, to secure the temperature suitable for performing the S poisoning recovery process, the fuel injection amount is maintained in order to maintain the engine output in the operating state. Increases, which is not desirable in terms of fuel efficiency.

The present invention has been made in view of the above circumstances, and a problem to be solved is to perform the S poisoning recovery process when the internal combustion engine is running at a reduced speed while ensuring the function of the NOx catalyst. The purpose is to suppress fuel consumption.

[0019]

In order to solve the above-mentioned problems, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention employs the following means.

(1) The exhaust gas purifying apparatus for an internal combustion engine according to the present invention comprises:
When the exhaust air-fuel ratio is lean, NOx in the exhaust gas is absorbed, and when the oxygen concentration in the exhaust gas drops, the NO
When the internal combustion engine is in an accelerating operation state or a steady operation state, the NOx catalyst is used to release NO and reduce and purify NOx by adding a reducing agent. The temperature of the catalyst is raised within a temperature range suitable for NOx purification, and when the internal combustion engine is in a deceleration operation state, the exhaust air-fuel ratio is made lean to make the N
Temperature raising means for setting the temperature of the Ox catalyst to a temperature suitable for recovery from S poisoning.

Here, an ECU for controlling the entire internal combustion engine is provided.
And a brief description of the components of this section.

The ECU comprises a digital computer, as is well known, interconnected by a bidirectional bus.
It comprises a CPU as a central processing controller, a ROM as a read-only memory, a RAM as a random access memory, an input port, an output port and the like.

The input ports are electrically connected to various sensors mounted on the internal combustion engine or the vehicle. When output signals of these various sensors enter the ECU through the input ports, the parameters related to these sensors are temporarily changed. Is stored in the RAM.

When the CPU executes the various application programs stored in the dedicated memory ROM, the CPU stores the parameters stored in the random access memory RAM via the bidirectional bus and the map stored in the ROM. It is called up as necessary and performs the necessary arithmetic processing by the CPU based on the parameters. As a result of this arithmetic processing, various components of the internal combustion engine are operated via the output port.

As the "NOx catalyst", a selective reduction type NO
x catalysts and NOx storage reduction catalysts can be exemplified. The reducing agent for purifying NOx used in these catalysts preferably contains a HC component such as light oil or gasoline, that is, an engine fuel used for combustion of an internal combustion engine.

When the internal combustion engine is in an accelerating operation state or a steady operation state, the NOx catalyst is raised within a temperature range suitable for NOx purification, and the internal combustion engine is brought into a deceleration operation state. At one time, an application program set to make the exhaust air-fuel ratio lean so that the temperature of the NOx catalyst becomes a temperature suitable for S poisoning recovery. The application program is executed by the CPU, and the attribute of the CPU is in the ECU. Therefore, the ECU can also be referred to as a temperature raising unit.

"The internal combustion engine is in a steady operation state"
This means an engine operating state in which the accelerator depression amount is constant and the engine speed is constant.

The "temperature range suitable for NOx purification" includes, for example, a temperature of about 200 to 500 ° C.

The "temperature suitable for recovery from S poisoning" is, for example, 6
"Internal combustion engine" capable of raising a temperature of about 00 to 700 ° C
Examples thereof include a lean burn gasoline engine which is a lean burn type internal combustion engine and a diesel engine which is a compression ignition type internal combustion engine.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention as described above, when the S poisoning of the NOx catalyst is to be recovered, when the internal combustion engine is in an acceleration operation state or a steady operation state, the NOx catalyst is deactivated. Since the temperature is raised within a temperature range suitable for NOx purification, NOx can be reduced and purified by adding a reducing agent at that time.

Further, when the internal combustion engine is in a deceleration operation state, the amount of oxygen in the exhaust gas can be increased by making the exhaust air-fuel ratio lean, so that if the increased oxygen is supplied to the NOx catalyst, the oxygen is oxidized. The reaction can be accelerated. As a result,
As a result, the reaction heat increases and the temperature of the NOx catalyst can be increased. Therefore, if the application program is set so as to increase the amount of oxygen so that the reaction heat can be increased to a temperature suitable for recovery from S poisoning, it is intended to increase the catalyst temperature by burning fuel. No need. As a result, even with a small amount of fuel, the temperature of the NOx catalyst can be raised to a temperature sufficient for performing the S poisoning recovery, so that the fuel consumption can be reduced.

(2) After the temperature of the NOx catalyst reaches a temperature suitable for recovery from S poisoning, the air-fuel ratio of the exhaust gas flowing through the NOx catalyst is made rich for a predetermined time, and It is preferable to maintain the temperature of the NOx catalyst at a temperature suitable for the S poisoning recovery process.

Here, the "predetermined time" refers to a time sufficient for recovering the NO reduction catalyst in the S poisoned state.

For a predetermined period of time, the air-fuel ratio of the exhaust gas flowing through the NOx catalyst is made rich, and
By maintaining the temperature of the NOx catalyst at a temperature suitable for the S poisoning recovery process, the S poisoning recovery process can be reliably executed.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to the accompanying drawings.

First, an overall configuration of an internal combustion engine employing the exhaust gas purifying apparatus for an internal combustion engine of the present invention will be described with reference to FIG.

The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-stroke cycle diesel engine having four cylinders 2.

The internal combustion engine 1 has an injector 3 for directly injecting fuel into a combustion chamber of each cylinder 2, and each injector 3 is connected to an accumulator (common rail) 4 for accumulating fuel up to a predetermined pressure. . The common rail 4 includes a common rail pressure sensor 4 a that outputs an electric signal corresponding to the pressure of the fuel in the common rail 4, and communicates with a fuel pump 6 via a fuel supply pipe 5.

The fuel pump 6 operates using the rotational torque of the output shaft (crankshaft) of the internal combustion engine 1 as a drive source.
Pump pulley 6 attached to the input shaft of this fuel pump 6
a and a crank pulley 1 a attached to an output shaft (crankshaft) of the internal combustion engine 1 are connected by a belt 7.

When the crankshaft rotates and its rotational torque is transmitted to the input shaft of the fuel pump 6 via the crank pulley 1a-belt 7-pump pulley 6a, the fuel pump 6
Generates a pump pressure corresponding to the magnitude of the rotational torque transmitted from the crankshaft to the input shaft of the fuel pump 6, and sends the amount of fuel corresponding to the pump pressure to the common rail 4 via the fuel supply pipe 5.

The fuel sent to the common rail 4 is accumulated to a predetermined pressure and distributed to the injectors 3 of each cylinder 2.
When a drive current is applied to the injector 3, the injector 3 opens and fuel is injected from the injector 3 into the cylinder 2.

Next, an intake branch pipe 8 is connected to the internal combustion engine 1, and each branch pipe of the intake branch pipe 8 is connected to a combustion chamber of each cylinder 2 via an intake port (not shown).

The intake branch pipe 8 is connected to an intake pipe 9, and the intake pipe 9 has an air cleaner box 10 at the start end thereof. An air flow meter 11 that outputs an electric signal corresponding to the mass of the intake air flowing through the intake pipe 9 and an electricity corresponding to the temperature of the intake air flowing through the intake pipe 9 are provided downstream of the air cleaner box 10 in the intake pipe 9. An intake air temperature sensor 12 for outputting a signal is attached.

An intake throttle valve 13 for adjusting the flow rate of intake air flowing through the intake pipe 9 is provided near the upstream of the intake branch pipe 8 in the intake pipe 9. The intake throttle valve 13 is connected to an intake throttle actuator 14 composed of a stepper motor or the like, and is opened and closed by the operation of the intake throttle actuator 14.

A compressor housing 15a of a centrifugal supercharger (turbocharger) 15, which operates using heat energy of exhaust gas as a drive source, is provided between the air flow meter 11 and the intake throttle valve 13 in the intake pipe 9. The intake air that has flowed into the compressor housing 15a is compressed by rotation of a compressor wheel provided inside the compressor housing 15a, and becomes high-temperature intake air. The intake air is cooled by an intercooler 16 provided in the intake pipe 9 downstream of the compressor housing 15a. As a result, a decrease in the air density is prevented, and the power of the internal combustion engine is reduced. The compressor wheel is another component of the centrifugal supercharger 15 and is coaxially connected to the turbine wheel of the turbine housing 15b disposed in the exhaust system.

The intake air cooled by passing through the intercooler 16 is flow-regulated by the intake throttle valve 13 and flows into the intake branch pipe 8. The intake air flowing into the intake branch pipe 8 is:
The fuel is distributed toward the combustion chamber of each cylinder 2 via each branch pipe, and is used for combustion of fuel injected from the injector 3 of each cylinder 2.

On the other hand, the internal combustion engine 1 has an exhaust branch pipe 18 connected to the combustion chamber of each cylinder via an exhaust port (not shown) of each cylinder 2.

The exhaust branch pipe 18 is connected to the turbine housing 15 b of the centrifugal turbocharger 15, and the turbine housing 15 b is connected to the exhaust pipe 19. And the exhaust pipe 1
A muffler (not shown) is attached to the downstream of 9.

In the middle of the exhaust pipe 19, there is arranged a catalytic converter 20 as an exhaust gas purifying device which contains an exhaust gas purifying catalyst for purifying harmful gas components in the exhaust gas. An air-fuel ratio sensor 23 that outputs an electric signal corresponding to the exhaust air-fuel ratio and an exhaust temperature sensor 24 that outputs an electric signal corresponding to the exhaust temperature are mounted downstream of the catalytic converter 20 in the exhaust pipe 19. It is.

The air-fuel ratio sensor 23 and the exhaust temperature sensor 2
An exhaust throttle valve 21 for adjusting the exhaust flow rate in the exhaust pipe 19 is mounted downstream of the exhaust pipe 4. Exhaust throttle valve 2
Reference numeral 1 is connected to an exhaust throttle actuator 22 constituted by a stepper motor or the like, and is opened and closed when the exhaust throttle actuator 14 operates.

The air-fuel mixture burned in each cylinder 2 becomes exhaust gas, is discharged to the exhaust branch pipe 18 via the exhaust port, and then enters the turbine housing 15b of the centrifugal supercharger 15 from the exhaust branch pipe 18. The exhaust gas flowing into the turbine housing 15b rotates a turbine wheel rotatably supported in the turbine housing 15b, and rotates the compressor wheel of the compressor housing 15a which is coaxially connected to the turbine wheel. To a high pressure.

The exhaust gas discharged from the turbine housing 15b flows into the catalytic converter 20 via the exhaust pipe 19, and removes or purifies harmful gas components contained in the exhaust gas. Exhaust gas from which harmful gas components have been removed or purified by the catalytic converter 20 is adjusted in flow rate as required by an exhaust throttle valve 21, and is discharged into the atmosphere after passing through a muffler.

The exhaust branch pipe 18 and the intake branch pipe 8 recirculate a part of the exhaust flowing through the exhaust branch pipe 18 to the intake branch pipe 8 (hereinafter referred to as an “EGR passage”). ) 25 are connected. The EGR passage 25 has a flow rate adjusting valve (hereinafter, “EGR gas”) that changes the flow rate of exhaust gas recirculated gas (hereinafter, referred to as “EGR gas”) flowing therethrough.
"Valve". ) 26. The EGR valve 26 is configured by an electromagnetic valve or the like, and its opening is determined according to the magnitude of the applied power, and the flow rate of the EGR gas changes according to the opening.

The EGR valve 26 in the EGR passage 25
An EGR cooler 27 for cooling the EGR gas flowing in the EGR passage 25 is provided further upstream.

When the EGR valve 26 is opened, the EGR passage 25 becomes conductive, and a part of the exhaust gas flowing in the exhaust branch pipe 18 flows into the EGR passage 25 and passes through the EGR cooler 27 to the intake branch pipe 8. Flows.

At this time, in the EGR cooler 27, heat exchange is performed between the EGR gas flowing in the EGR passage 25 and a predetermined refrigerant to cool the EGR gas.

The EGR gas recirculated from the exhaust branch pipe 18 to the intake branch pipe 8 via the EGR passage 25 is guided to the combustion chamber of each cylinder 2 while mixing with fresh air flowing from the upstream of the intake branch pipe 8. , The fuel injected by the injector 3 is used for combustion. However, EGR gas contains water (H ₂ O)
Since EGR gas contains an endothermic inert gas component such as carbon dioxide and carbon dioxide (CO ₂ ) that does not burn itself and is endothermic, the combustion of the air-fuel mixture The temperature decreases, thereby suppressing the generation amount of nitrogen oxides (NOx).

Further, when the EGR gas is cooled by the EGR cooler 27, the temperature of the EGR gas decreases, and the volume of the EGR gas decreases. For this reason, when the EGR gas is supplied into the combustion chamber, the temperature of the atmosphere in the combustion chamber is prevented from rising, and the amount of fresh air supplied into the combustion chamber does not extremely decrease. There is no hindrance to the operation of the engine 1.

The internal combustion engine 1 has a catalytic converter 20
There is provided a reducing agent supply mechanism for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage located further upstream.

As shown in FIG. 1, this reducing agent supply mechanism is mounted on the cylinder head of the internal combustion engine 1 so that its injection hole faces the exhaust branch pipe 18, and when fuel of a predetermined pressure or more is applied. A reducing agent injection valve 28 that opens; a reducing agent supply passage 29 that guides the fuel discharged from the fuel pump 6 to the reducing agent injection valve 28; and a reducing agent supply passage that is provided in the middle of the reducing agent supply passage 29. A flow control valve 30 for adjusting the flow rate of the fuel flowing through the inside of the fuel cell 29;
And a shutoff valve 31 for shutting off the flow of fuel in the fuel cell 9.

The reducing agent injection valve 28 is configured such that the injection hole of the reducing agent injection valve 28 is located downstream of the connection portion of the EGR passage 25 in the exhaust branch pipe 18, and a set of four branch pipes in the exhaust branch pipe 18. It is preferable that the projection is protruded from the exhaust port of the cylinder 2 closest to the cylinder head and is attached to the cylinder head so as to face the collecting part of the exhaust branch pipe 18.

This prevents the reducing agent (unburned fuel component) injected from the reducing agent injection valve 28 from flowing into the EGR passage 25, and also prevents the reducing agent from being centrifuged in the exhaust branch pipe 18 without centrifugation. This is for the purpose of reaching the turbine housing 15b of the supercharger 15.

In the example shown in FIG. 1, the first (# 1) cylinder 2 of the four cylinders 2 of the internal combustion engine 1 is located at the position closest to the gathering portion of the exhaust branch pipe 18, so that the first (# 1) cylinder 2 # 1) Cylinder 2
The reducing agent injection valve 28 is attached to the exhaust port of
When a cylinder 2 other than the first (# 1) cylinder 2 is located at the position closest to the collecting portion of the exhaust branch pipe 18, the reducing agent injection valve 28 is attached to the exhaust port of the cylinder 2.

In such a reducing agent supply mechanism, when the flow control valve 30 is opened, the high-pressure fuel sent from the fuel pump 6 is applied to the reducing agent injection valve 28 via the reducing agent supply passage 29. When the pressure of the fuel applied to the reducing agent injection valve 28 reaches a predetermined pressure or more, the reducing agent injection valve 28 opens to inject fuel as a reducing agent into the exhaust branch pipe 18.

The reducing agent injected from the reducing agent injection valve 28 into the exhaust branch pipe 18 flows into the turbine housing 15b together with the exhaust gas flowing from the upstream of the exhaust branch pipe 18.
The exhaust gas and the reducing agent that have flowed into the turbine housing 15b are agitated by the rotation of the turbine wheel and are homogeneously mixed to become exhaust gas having a rich air-fuel ratio.

This exhaust gas flows from the turbine housing 15b into the catalytic converter 20 via the exhaust pipe 19, and releases nitrogen oxides (NOx) stored by the exhaust purification catalyst of the catalytic converter 20 while releasing nitrogen (N _2). ).

Thereafter, when the flow control valve 30 is closed and the supply of the reducing agent from the fuel pump 6 to the reducing agent injection valve 28 is stopped, the pressure of the fuel applied to the reducing agent injection valve 28 becomes lower than the predetermined pressure. As a result, the reducing agent injection valve 28 closes and the addition of the reducing agent into the exhaust branch pipe 18 is stopped.

The internal combustion engine 1 has an electronic control unit (ECU: Electronic Control Unit) for controlling the internal combustion engine 1.
nit) 35. The ECU 35 is a unit that controls the operating state of the internal combustion engine 1 in accordance with the operating conditions of the internal combustion engine 1 and a driver's request, and is connected to a central processing control unit CPU (not shown) via a bidirectional bus. ,
Read only memory ROM, random access memory R
It comprises an AM, an input interface circuit, an output interface circuit and the like.

The input interface circuit includes a common rail pressure sensor 4a, an air flow meter 11, an intake air temperature sensor 12, an intake pipe pressure sensor 17, an air-fuel ratio sensor 23,
It is electrically connected to various sensors such as an exhaust gas temperature sensor 24, a reducing agent pressure sensor 32, a crank position sensor 33, a water temperature sensor 34, and an accelerator opening sensor 36.
When output signals of these various sensors enter the ECU 35 from the input interface circuit, these parameters are temporarily stored in the random access memory RAM. When the CPU executes the necessary arithmetic processing based on these parameters according to various application programs stored in the read-only memory ROM, the random access memory RAM stores the random access memory RAM via the bidirectional bus. The above parameters and ROM
Call the map stored in as needed.

The output interface circuit includes an injector 3 that operates based on output signals of the various sensors.
Intake throttle actuator 14, exhaust throttle actuator 22, EGR valve 26, flow control valve 30, shut-off valve 31
Are electrically connected to various devices such as
It operates appropriately based on the calculation result of the CPU.

Next, the catalytic converter 20, which is an exhaust gas purifying apparatus, will be specifically described.

The catalytic converter 20 purifies nitrogen oxides (NOx) in exhaust gas in the presence of a reducing agent in the case.
The NOx catalyst includes an Ox catalyst. Examples of such a NOx catalyst include a selective reduction type NOx catalyst and an occlusion reduction type NOx catalyst. Here, the occlusion reduction type NOx catalyst will be described as an example.

The storage-reduction type NOx catalyst uses, for example, alumina (Al ₂ O ₃ ) as a carrier and, on the carrier, for example, potassium (K), sodium (Na), lithium (Li), cesium (Cs) or the like. Alkali metal and barium (B
a) alkaline earths such as calcium (Ca),
It is configured to support at least one selected from rare earths such as lanthanum (La) and yttrium (Y) and a noble metal such as platinum (Pt).

When the air-fuel ratio of the exhaust gas is the lean air-fuel ratio, the NOx storage-reduction catalyst having such a configuration stores nitrogen oxides (NOx) in the exhaust gas and reduces the amount of exhaust gas flowing into the NOx storage-reduction catalyst. When the oxygen concentration is reduced and a reducing agent is present, reduction and purification are performed while releasing nitrogen oxides (NOx) that have been stored up to that time.

Here, the exhaust air-fuel ratio is defined as the sum of the amount of air supplied to the exhaust pipe 19, the combustion chamber, the intake pipe 9 and the like upstream of the catalytic converter 20, and the total amount of fuel (hydrocarbon). Shall mean the ratio. Therefore, as long as no fuel, reducing agent, or air is supplied to the exhaust system upstream of the catalytic converter 20, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber.

Here, the NOx absorption / desorption mechanism of the NOx storage reduction catalyst will be described with reference to an example of the NOx storage reduction catalyst in which platinum (Pt) and barium (Ba) are supported on a carrier made of alumina.

It is considered that the NOx absorbing / releasing action of the NOx storage-reduction catalyst is performed by a mechanism shown in FIG.

First, in the NOx storage reduction catalyst, when the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes a lean air-fuel ratio and the oxygen concentration in the exhaust gas increases, as shown in FIG. In addition, oxygen (O ₂ ) in the exhaust gas changes to O ₂ ^- or O ^2-
And adheres to the surface of platinum (Pt) in the form of, and the nitrogen monoxide (NO) in the exhaust gas becomes O ₂ ^- or O ^2- on the surface of platinum (Pt).
To form nitrogen dioxide (NO ₂ ) (2NO +
O ₂ → 2NO ₂ ).

The nitrogen dioxide (NO ₂ ) is oxidized on the surface of platinum (Pt) and combines with barium oxide (BaO) to form nitrate ions (NO ₃ ⁻ ). Thus, the nitrogen oxides (NOx) in the exhaust gas are stored in the storage-reduction NOx catalyst as nitrate ions ( _NO3- ).

Such an action is called a NOx occlusion action.
The NOx storage action is such that the exhaust air-fuel ratio flowing into the storage reduction type NOx catalyst is lean and the NOx of the storage reduction type NOx catalyst
It is continued as long as the Ox storage capacity is not saturated.

On the other hand, the oxygen concentration of the inflow exhaust gas decreases.
Then, nitrogen dioxide (NO_Two)
Nitrate ions bound to barium oxide (BaO)
(NO _3-) Is conversely nitrogen dioxide (NO_Two) And nitric oxide
(NO) and desorbs from the NOx storage reduction catalyst.

That is, when the oxygen concentration of the exhaust gas flowing into the NOx storage reduction catalyst decreases, nitrate ions (NO ₃₋ )
Nitrogen oxide (NOx) stored in the NOx storage reduction catalyst in the form of nitrogen dioxide (NO ₂ ) or nitrogen monoxide (N
O) and is released from the NOx storage reduction catalyst.

As shown in FIG. 2B, nitrogen oxides (NOx) released from the NOx storage reduction catalyst are reduced on the reducing components (for example, platinum (Pt) of the NOx storage reduction catalyst) contained in the exhaust gas. Reacts with oxygen O ₂₋ or O ²⁻ (an active species such as partially oxidized hydrocarbon (HC) or carbon monoxide (CO)) and is reduced to nitrogen (N ₂ ).

That is, hydrocarbon (HC) and monoacid in exhaust gas
Carbonized (CO) is converted to O 2 on platinum (Pt)._Two ^-Or O^2-When
Reacts and oxidizes, thereby causing O 2 on platinum (Pt)_Two ^-Also
Is O ^2-Hydrocarbons (HC) and carbon monoxide
If hydrogen (CO) remains, these hydrocarbons (H
C) and carbon monoxide (CO) from the NOx storage reduction catalyst
The released nitrogen oxides (NOx) and the emissions from the internal combustion engine 1
The released nitrogen oxides (NOx) are converted to nitrogen (N_Two)
You.

Accordingly, by setting the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst to the stoichiometric air-fuel ratio or the rich air-fuel ratio, the nitrogen oxides (NOx) stored in the NOx storage reduction catalyst can be released. It is possible to reduce.

Since the NOx storage capacity of the NOx storage reduction catalyst is limited, when exhaust gas having a lean air-fuel ratio flows into the NOx catalyst over a long period of time, the NOx storage capacity of the NOx storage reduction catalyst is saturated. In addition, nitrogen oxides (NOx) in the exhaust gas are released to the atmosphere without being removed or purified by the NOx storage reduction catalyst.

However, in a diesel engine, for example, a mixture having a lean air-fuel ratio is burned in most of the operating range, and the air-fuel ratio of the exhaust gas becomes a lean air-fuel ratio in most of the operating range. NOx
There is a possibility that the NOx storage capacity of the catalyst is saturated, the catalyst cannot be absorbed any more, and NOx leaks.

In order to avoid leakage, the NOx absorption capacity must be restored, and for that purpose, the NOx absorption capacity of the storage reduction type NOx catalyst is released by releasing the NOx absorbed by the storage reduction type NOx catalyst. You need to recover.

Therefore, before the occlusion-reduction type NOx catalyst becomes saturated, the air-fuel ratio of the inflowing exhaust gas is made rich at a predetermined timing to reduce the oxygen concentration extremely, thereby reducing the occlusion-reduction NOx.
It is necessary to perform a regeneration process of the NOx storage reduction catalyst for recovering the NOx absorption ability by releasing NOx from the catalyst.

However, the mere release of NOx can regenerate the NOx storage reduction catalyst, but cannot purify NOx, which is the original function of the NOx catalyst, which leads to deterioration of emission. Therefore, for example, a fuel injection valve is provided at the exhaust port, and the engine fuel is continuously or intermittently injected from the fuel injection valve according to the operation state of the internal combustion engine, and the engine fuel is added as a reducing agent to the NOx storage reduction catalyst. doing, performs NOx purification the released NOx with reduced to N _2.

Incidentally, since the engine fuel contains a large amount of sulfur components, it burns to produce sulfur oxides (hereinafter referred to as "SOx" unless otherwise specified). And SOx is N
It is absorbed by the NOx storage reduction catalyst by the same mechanism as Ox.

Since SOx forms a stable sulfate,
It is not easily decomposed and released, and easily accumulates in the NOx storage reduction catalyst. Then, the NOx purification rate of the NOx storage reduction catalyst is reduced by an amount corresponding to the absorption of the SOx by the NOx storage reduction catalyst, and a phenomenon that interferes with the NOx purification function of the NOx storage reduction catalyst, that is, a so-called S poisoning occurs. There is a fear. Therefore, it is necessary to recover the NOx storage reduction catalyst that is in the S poisoning state.
This is called poisoning recovery processing.

However, since the sulfate accumulated in the NOx storage reduction catalyst is relatively stable as described above, the temperature at which the storage reduction NOx catalyst performs regeneration (for example,
(250-500 ° C).
It is difficult to release SOx absorbed by the Ox catalyst.

To perform the S poisoning recovery process, the storage reduction type N
For example, 600 higher than the temperature at which the Ox catalyst is regenerated.
This problem is dealt with by increasing the temperature of the NOx storage reduction catalyst to a high temperature of about 700 ° C. and periodically making the exhaust air-fuel ratio stoichiometric or rich.

On the other hand, the exhaust temperature of a diesel engine is lower than that of a conventional gasoline engine, and it is difficult to achieve the high temperature required for performing the S poisoning recovery process.
Also, when the diesel engine is in a high load operation state, for example, when the exhaust air-fuel ratio is made stoichiometric or rich when the diesel engine is operating at a high temperature for the diesel engine, soot, unburned fuel components, etc. Fine particles such as particulate matter, such as PM, are easily discharged. Therefore, the exhaust air-fuel ratio cannot be made stoichiometric or rich at the high temperature.

Therefore, by raising the catalyst bed temperature which is the temperature of the NOx storage reduction catalyst at the high temperature and making the exhaust air-fuel ratio stoichiometric or rich at the time of light load operation in which particulates such as soot are hardly discharged, S poisoning is reduced. It is possible to recover.

However, when the diesel engine is accelerated, for example, in order to secure the temperature suitable for performing the S poisoning recovery process, the fuel injection amount increases in order to maintain the engine output in the operating state, and therefore the fuel consumption is reduced. Is not preferred. The above problems are as already described in the description of the prior art.

On the other hand, in the internal combustion engine 1 according to the present embodiment, while maintaining the function of the NOx storage reduction catalyst, the S poisoning recovery process is performed when the internal combustion engine is running at a reduced speed, thereby reducing fuel consumption. It is a technology that makes it better.

That is, when the internal combustion engine 1 is in an acceleration operation state or a steady operation state, the NOx catalyst is raised within a temperature range suitable for NOx purification, and when the internal combustion engine 1 is in a deceleration operation state, the exhaust gas is exhausted. By making the air-fuel ratio lean, N
The internal combustion engine 1 has a temperature raising means for setting the temperature of the Ox catalyst to a temperature suitable for recovery from S poisoning.

It should be noted that "in the steady operation state" of "the internal combustion engine 1 is in the acceleration operation state or the steady operation state" means that the accelerator pedal depression amount by the driver is constant and the engine speed is constant. Means the operating state of the engine.

Next, referring to the flowchart of FIG.
A program for realizing the S poisoning recovery process execution routine of the engine 1 by U35 will be described.

This program comprises steps 101 to 108 described below. A program including these steps is stored in the ROM of the ECU 35 and is called up as needed. All the processes in the above steps are performed by the CPU of the ECU 35. Note that the symbol S is used.
It is abbreviated as 01.

From the fuel consumption, the S value of the NOx storage reduction catalyst was determined.
The poisoning amount is estimated, and if the poisoning amount has reached a predetermined value or more, it is determined that the occlusion reduction type NOx catalyst should recover S poisoning, and the routine shifts to this routine.

That is, it is determined in S101 whether or not the S poisoning recovery process is necessary. If the determination is affirmative, the process proceeds to S102. If the determination is negative, the control is terminated. In order to recover S poisoning, the exhaust gas air-fuel ratio should be 14.
5 or less, and if the condition is not satisfied, S
Poisoning cannot be recovered.

In S102, first, the catalyst bed temperature is detected,
When the temperature is 0 ° C or lower, the temperature rise control is performed. That is, 50
It is determined whether the temperature is higher than 0 ° C, and if a negative determination is made, S
Proceeding to 103, if a positive determination is made, this process is repeated until a negative determination is made.

If the catalyst bed temperature is higher than 500 ° C.,
The reason for not proceeding to the processing after 103 is that when the internal combustion engine 1 is in the deceleration operation state, the exhaust air-fuel ratio is made lean so that the temperature of the NOx catalyst becomes S poisoning without expecting fuel combustion. This is because the technical idea of the present invention that the temperature is set to 600 ° C. or more suitable for recovery is also taken.

Note that the condition for recovery from S poisoning is 60.
By setting the determination value at 500 ° C, which is not so far from 0 ° C, useless deterioration of fuel efficiency is prevented. Also 500
° C is a limit value of deterioration of the reduction performance of the NOx catalyst 20, and when the temperature becomes higher than that, the reduction performance deteriorates.
x Temperature suitable for purification. In addition, 250-500 degreeC is mentioned as a temperature range suitable for NOx purification.

In S103, when the catalyst bed temperature is 500 ° C. or lower, the temperature rise control is executed.
Temperature increase control by fuel injected from the reducing agent injection valve 28 according to the operation state of the internal combustion engine, temperature increase control by so-called post-injected fuel which is a secondary injection other than the main injection injected by the injector 3, air-fuel ratio The temperature rise control according to the engine operating state is performed by any one of the so-called engine rich control or the combination thereof, which makes the engine rich.
By this temperature increase control, the NOx storage reduction catalyst is raised to a temperature range suitable for NOx purification.

In S104, it is determined whether the internal combustion engine 1 is in an accelerating operation state or a steady operation state. If the judgment is affirmative, the process returns to S102. If the negative judgment is made, that is, if the internal combustion engine 1 is in a deceleration operation state. Goes to S105.

In S105, which proceeds when the internal combustion engine 1 is in the deceleration operation state, the exhaust air-fuel ratio is made instantaneously lean. In this manner, the NOx storage reduction catalyst is instantaneously placed in an oxygen-excess atmosphere. That is, since the amount of oxygen in the exhaust gas increases, the increased oxygen is stored in the NOx storage-reduction type.
It is supplied to the catalyst. As a result, the oxidation reaction is accelerated, and the reaction heat is increased accordingly, and the occlusion reduction type N
The temperature of the Ox catalyst can be raised.

In S106, it is determined whether or not the catalyst bed temperature is higher than 600 ° C. necessary for recovery from S poisoning. If the determination is affirmative, the process proceeds to S107; if the determination is negative, S105 is performed.
And the process is repeated until an affirmative determination is made in S106. By doing so, the catalyst bed temperature is raised to the above 600 ° C.
Control the temperature up to S106 is a process performed when the exhaust air-fuel ratio is instantaneously lean when the internal combustion engine 1 is in the deceleration operation state (see S104 and S105). Therefore, S10
4 to S106 can be said to be temperature raising means for setting the temperature of the NOx storage reduction catalyst to a temperature suitable for recovery from S poisoning by making the exhaust air-fuel ratio lean when the internal combustion engine 1 is in the deceleration operation state. The program including S104 to S106 is stored in the ROM, and the attribute of the ROM is in the ECU 9. Therefore, when the internal combustion engine 1 is in the deceleration operation state, the ECU 35 sets the exhaust air-fuel ratio to lean so that the storage reduction NOx This can be referred to as a temperature raising means for setting the temperature of the catalyst to a temperature suitable for recovery from S poisoning. The temperature raising means is also a temperature raising means for maintaining the NOx catalyst at a temperature suitable for NOx purification when the internal combustion engine 1 is in an acceleration operation state or a steady operation state (see S102, S103, S104).

In S107, by performing the fuel injection from the reducing agent injection valve 28 or executing the engine rich, the exhaust air-fuel ratio is made rich for a predetermined time by performing the same temperature control as that performed in S103.

In S108, the predetermined time in S107 is counted by an appropriate timer (not shown). This predetermined time is obtained by an experiment and by using an appropriate arithmetic expression. Then, after the elapse of the predetermined time, S
The poisoning recovery control ends.

According to the internal combustion engine 1 having such a configuration, the following operational effects can be obtained.

In the case where the S poisoning of the NOx catalyst is to be recovered, when the internal combustion engine 1 is in the accelerating operation state or the steady operation state, the NOx storage-reduction catalyst is set to, for example, 500 ° C. which belongs to a temperature range suitable for NOx purification. To C
At that time, reduction and purification of NOx can be performed by adding a reducing agent.

Further, when the internal combustion engine 1 is in the deceleration operation state, the amount of oxygen in the exhaust gas can be increased by making the exhaust air-fuel ratio lean, so that the increased oxygen is stored and reduced.
If supplied to the catalyst, the oxidation reaction can be promoted. Therefore, since the reaction heat increases and the temperature of the NOx catalyst can be increased, the amount of oxygen is increased so that the reaction heat can be increased to a temperature suitable for S poisoning recovery (see S105). It is not necessary to increase the catalyst temperature by burning the fuel as in the technique described in the above. As a result, even with a small amount of fuel, the temperature of the NOx catalyst can be raised to a temperature sufficient for performing the S poisoning recovery process, so that the fuel consumption can be reduced.

The timer counts the predetermined time. During this time, the air-fuel ratio of the exhaust gas flowing through the NOx storage reduction catalyst is made rich, and the ECU 35 serving as the temperature increasing means is configured to store the NOx catalyst in the NOx storage reduction catalyst. The temperature of
By maintaining the temperature suitable for the poisoning recovery process, the S poisoning recovery process can be reliably performed.

[0118]

According to the exhaust gas purifying apparatus for an internal combustion engine of the present application, the fuel consumption is suppressed by performing the S poisoning recovery processing when the internal combustion engine is running at a reduced speed while securing the function of the NOx catalyst. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of an internal combustion engine to which an exhaust gas purification device for an internal combustion engine of the present invention is applied.

FIG. 2 is a diagram showing the NOx storage / release operation of a storage reduction type NOx catalyst.

FIG. 3 is a flowchart for explaining a program for realizing an S-poisoning recovery process execution routine of the internal combustion engine.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 internal combustion engine 2 cylinder 3 injector 4 common rail 4 a common rail pressure sensor 5 fuel supply pipe 6 fuel pump 6 a pump pulley 1 a crank pulley 7 belt 8 intake branch pipe 9 intake pipe 10 air cleaner box 11 air flow meter 12 intake temperature sensor 13 intake throttle valve 14 intake Throttle actuator 15 Centrifugal supercharger 15a Compressor housing 16 Intercooler 15b Turbine housing 18 Exhaust branch pipe 19 Exhaust pipe 20 Catalytic converter 23 Air-fuel ratio sensor 24 Exhaust temperature sensor 22 Exhaust throttle actuator 21 Exhaust throttle valve 25 EGR passage 26 EGR valve 27 EGR cooler 28 Reducing agent injection valve 29 Reducing agent supply path 30 Flow control valve 31 Shutoff valve 35 ECU 17 Intake pipe pressure sensor 32 Reducing agent pressure sensor 33 Clamp Position sensor 34 water temperature sensor 35 ECU (heating device) 36 accelerator opening sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 41/12 355 F02D 41/12 355 43/00 301 43/00 301E 301T F-term (Reference) 3G084 AA01 BA07 BA09 BA20 CA04 CA06 DA02 DA10 FA02 FA07 FA10 FA20 FA27 FA29 3G091 AA10 AA11 AA18 AA28 AB06 BA11 CA01 CA18 CA27 DA02 DA04 DB10 EA08 EA17 EA30 EA34 FA17 FA18 FA19 FB10 FB12 GB02Y GB03Y GB04Y GB05W GB06W GB13 GB10 GB3 LA03 MA01 NE13 NE15 PA01Z PA10Z PB08Z PD03Z PD11Z PE08Z PF03Z

Claims (2)

[Claims] When the exhaust air-fuel ratio is lean, N
When Ox is absorbed and the oxygen concentration in the exhaust decreases, NOx that has been absorbed is released and NOx is added by adding a reducing agent.
A NOx catalyst that performs reduction purification of the NOx catalyst, and when the S poisoning of the NOx catalyst is to be recovered, when the internal combustion engine is in an acceleration operation state or a steady operation state,
The temperature of the NOx catalyst is raised within a temperature range suitable for NOx purification, and when the internal combustion engine is in a deceleration operation state, the exhaust air-fuel ratio is made lean so that the temperature of the NOx catalyst is suitable for S poisoning recovery. An exhaust purification device for an internal combustion engine, comprising: a temperature raising means for raising a temperature. 2. After the temperature of the NOx catalyst has reached a temperature suitable for recovery from S poisoning, the air-fuel ratio of the exhaust gas flowing through the NOx catalyst is made rich for a predetermined time, and the temperature raising means is provided with the NOx catalyst. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the temperature of the catalyst is maintained at a temperature suitable for the S poisoning recovery process.