JP2002038929A - Exhaust gas purification device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately sense a deterioration of NOX catalyst installed in the exhaust passage of a lean combustion type internal combustion engine. SOLUTION: An exhaust emission control device of an internal combustion engine is equipped with a NOX catalyst of storage reduction type installed in the exhaust passage of the engine and an absorption and emission control means to drop the exhaust air-fuel ratio for regenerating the NOX absorbing ability of the catalyst, wherein the air-fuel ratio in the exhaust gas flowing into the catalyst is lowered from the case in which the NOX storage ability is regenerated temporarily, and deterioration of the catalyst is judged on the basis of the time in which the air-fuel ratio in the exhaust gas flowing out from the catalyst at this time of the ratio being lowered indicates a rich air-fuel ratio.

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 nitrogen oxide (N) in exhaust gas.
The present invention relates to an exhaust gas purification device that effectively purifies Ox).

[0002]

2. Description of the Related Art In an internal combustion engine mounted on an automobile or the like, particularly in a lean-burn internal combustion engine capable of burning an air-fuel mixture in an excess oxygen state, a technique for purifying nitrogen oxides (NOx) in exhaust gas has been developed. 2. Description of the Related Art There is known an exhaust gas purification device in which a storage reduction type NOx catalyst is disposed in an exhaust passage. NOx storage reduction type
The catalyst occludes nitrogen oxides (NOx) in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is high, and absorbs the nitrogen oxides (NOx) when the air-fuel ratio of the exhaust gas is low and the reducing agent is present. while releasing NOx) is a catalyst which allowed to reduced to nitrogen (N _2).

[0003] Since the NOx absorption capacity of the NOx storage reduction catalyst is limited, in order to effectively use the capacity of the NOx storage reduction catalyst, the NOx absorption capacity of the NOx storage reduction catalyst must be appropriately adjusted before the NOx absorption capacity of the NOx storage catalyst is saturated. At the right time
It is necessary to release and reduce nitrogen oxides (NOx) absorbed by the x catalyst.

[0004] On the other hand, in a conventional exhaust gas purifying apparatus, fuel as a reducing agent is added to the exhaust gas upstream of the NOx storage reduction catalyst at a suitable timing in a short period of time.
A reducing agent is supplied to the NOx storage-reduction catalyst while temporarily reducing the air-fuel ratio of exhaust gas flowing into the NOx storage-reduction catalyst, whereby nitrogen oxides (NOx) absorbed by the NOx storage-reduction catalyst are reduced. Is released and reduced, that is, a so-called rich spike control is executed.

On the other hand, in the exhaust gas purifying apparatus as described above,
It is also important to accurately detect the deterioration of the storage reduction type NOx catalyst. In response to such a demand, conventionally, during rich spike control for releasing and purifying nitrogen oxides (NOx) stored in the NOx storage reduction catalyst, the NOx storage reduction NOx
A method has been proposed in which the air-fuel ratio of the exhaust gas flowing out of the x catalyst is measured, and the deterioration of the NOx storage reduction catalyst is determined based on the time during which the measured air-fuel ratio is maintained near the stoichiometric air-fuel ratio.

In order to realize such a method, an O ₂ sensor is disposed in an exhaust passage downstream of the NOx storage reduction catalyst, and fuel as a reducing agent is added to an exhaust passage upstream of the NOx storage reduction catalyst. A reducing agent addition device will be arranged. Then, based on the time during which the output signal value of the O ₂ sensor is maintained near the stoichiometric air-fuel ratio when the reducing agent is added to the exhaust gas from the reducing agent device, the storage reduction type N
The state of deterioration of the Ox catalyst will be determined.

[0007]

In the case where the reducing agent is supplied directly to the exhaust passage upstream of the NOx storage reduction catalyst, the output of the O ₂ sensor provided downstream of the catalyst does not indicate a rich state. Even if there is, NOx can be reduced and purified. This is because a rich space exists locally even if the exhaust gas is lean as a whole. Therefore, it is desirable to add a minimum amount of reducing agent capable of purifying NOx to the exhaust passage. However, in such a rich spike control with a reducing agent amount in which the output of the downstream O ₂ sensor does not indicate rich, the storage reduction type is based on the time during which the output signal of the O ₂ sensor is maintained at the stoichiometric air-fuel ratio. The deterioration state of the NOx catalyst cannot be detected. Further, even if rich spike control indicating that the output of the downstream O ₂ sensor is rich is performed, the time during which the air-fuel ratio of the exhaust gas is maintained near the stoichiometric air-fuel ratio is extremely short, making accurate determination of deterioration difficult.

Accordingly, the present invention has been made in consideration of the above points, and has as its object to provide an exhaust gas purifying apparatus for an internal combustion engine that can more reliably and accurately determine the state of deterioration of a NOx catalyst.

[0009]

Means for Solving the Problems The invention according to the present application employs the following means in order to solve the above technical problems. That is, the exhaust gas purification device for an internal combustion engine according to the present invention is provided in the exhaust passage of the internal combustion engine, and when the air-fuel ratio of the inflowing exhaust gas is high, absorbs the nitrogen oxides in the exhaust gas and reduces the air-fuel ratio of the inflowing exhaust gas. When the NOx catalyst releases and reduces the nitrogen oxides absorbed, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is reduced to a predetermined air-fuel ratio in order to purify the nitrogen oxides absorbed by the NOx catalyst. An air-fuel ratio control means for temporarily lowering the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to an air-fuel ratio lower than the predetermined air-fuel ratio; and an air-fuel ratio of the exhaust gas flowing into the NOx catalyst. Air-fuel ratio detecting means for detecting the air-fuel ratio of the exhaust gas flowing out of the NOx catalyst when the air-fuel ratio is lowered below the predetermined air-fuel ratio; and a detection value of the air-fuel ratio detecting means indicating a value lower than a reference air-fuel ratio. time The NO Based
x deterioration determining means for determining deterioration of the catalyst.

In the exhaust gas purifying apparatus for an internal combustion engine configured as described above, when it is determined that the NOx catalyst has deteriorated, N
The air-fuel ratio of the exhaust gas flowing into the Ox catalyst is made lower than when the nitrogen oxides (NOx) absorbed by the NOx catalyst are released. In this case, the air-fuel ratio of the exhaust gas flowing out of the NOx catalyst differs from the case where the NOx catalyst has not deteriorated by N
Since a clear difference is likely to occur between the case where the Ox catalyst is deteriorated, it is easy to determine the deterioration of the NOx catalyst.

The reference air-fuel ratio used as a reference when determining deterioration can be arbitrarily set, and may be, for example, a variable value whose setting is changed as appropriate based on the operating state of the internal combustion engine.
Further, the variable value may be a variable value whose setting is changed as appropriate based on the ambient temperature of the NOx catalyst. Further, it may be a fixed value determined in a preliminary experiment or the like.

In the deterioration judging means according to the invention of the present application, the reference air-fuel ratio is set as a stoichiometric air-fuel ratio, and a time period in which the air-fuel ratio detected by the air-fuel ratio detecting means shows a value lower than the stoichiometric air-fuel ratio is set in advance. The predetermined time may be compared, and if the time during which the value detected by the air-fuel ratio detecting means indicates a value lower than the stoichiometric air-fuel ratio is shorter than the predetermined time, it may be determined that the NOx catalyst has deteriorated. That is, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is reduced by the above-described air-fuel ratio control means, the air-fuel ratio is controlled so that the air-fuel ratio of the exhaust gas flowing out of the NOx catalyst surely reaches the stoichiometric air-fuel ratio or less. In addition, a clear criterion at the time of deterioration determination is set to facilitate deterioration.

The predetermined period is a value which is set based on preliminary experiments conducted under various conditions using a new NOx catalyst which has not deteriorated. Is a period in which the air-fuel ratio of the exhaust gas flowing out of the new NOx catalyst shows a value equal to or lower than the stoichiometric air-fuel ratio when the air-fuel ratio of the new NOx catalyst is greatly reduced.

Further, the internal combustion engine according to the present invention includes a reducing agent adding means for adding a reducing agent to the exhaust gas upstream of the NOx catalyst, and the absorption / release control means controls a predetermined amount of the reducing agent from the reducing agent adding means to the exhaust gas. Is added, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is configured to be reduced to a predetermined air-fuel ratio. The air-fuel ratio of the exhaust gas flowing into the NOx catalyst may be made lower than a predetermined air-fuel ratio by adding a reducing agent.

The means for adding a reducing agent according to the present invention includes N
A device for directly adding a reducing agent to an exhaust passage provided with an Ox catalyst can be exemplified. In the present invention, examples of the internal combustion engine capable of lean combustion include a lean-burn gasoline engine and a diesel engine. In the present invention, examples of the reducing agent include those containing hydrocarbon HC such as light oil and gasoline.

[0016]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. The embodiment described below shows an example in which the exhaust emission control device according to the present application is applied to a compression ignition type internal combustion engine, that is, a diesel engine.

<Outline of Diesel Engine> As shown in FIG. 1, the engine body 1 forms a combustion chamber 5 between a cylinder block 2 containing a piston 4 and a piston 4 provided above the cylinder block 2. A cylinder head 3, an electrically controlled fuel injection valve 6 provided in the cylinder head 3 for injecting fuel into the combustion chamber 5, an intake port 8 and an intake valve 7 for supplying air into the combustion chamber 5, An exhaust port 10 and an exhaust valve 9 for discharging the later exhaust gas from the combustion chamber 5 are provided.

The intake port 8 is connected to a surge tank 12 via an intake branch pipe 11.
2 is connected to the compressor 15 side of the turbocharger 14 via the intake duct 13. A throttle valve 17 driven by a step motor 16 is provided in the intake duct 13. Further, a cooling device 18 is provided around the intake duct 13, and the intake air flowing through the intake duct 13 is cooled by the cooling device 18. The engine cooling water is guided into the cooling device 18, and the intake air is cooled by the engine cooling water.

On the other hand, the exhaust port 10 is connected to an exhaust turbine 21 side of the turbocharger 14 via an exhaust branch pipe 19 and an exhaust pipe 20. An outlet of the exhaust turbine 21 has a casing 51 containing a storage-reduction NOx catalyst 50 (hereinafter, simply referred to as a NOx catalyst), and an oxidation catalyst 52.
Is connected to an exhaust gas purifying device constituted by a casing 53 or the like in which is incorporated. The NOx catalyst 50 and the oxidation catalyst 5
The exhaust gas purification device provided with 2 will be described later in detail.

The exhaust branch pipe 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation passage 24 (hereinafter, referred to as an EGR passage). The EGR passage 24 is
A cooling device 26 for cooling the EGR gas flowing through the passage 24 and an electric control type EGR control valve 25 for interrupting the flow of the EGR gas flowing through the EGR passage 24 are provided.
In addition, the engine cooling water is guided to the cooling device 26 and the EGR
The gas is cooled by the engine cooling water.

The electric fuel injection valve 6 provided on the cylinder head 3 is connected to a common rail 27 via a fuel supply pipe 6a.
(Accumulation chamber). In addition, common rail 27
Is connected to a fuel pump 28 capable of arbitrarily adjusting a fuel discharge amount.
The fuel a is supplied to the common rail 27. Also,
The fuel supplied to the common rail 27 is supplied to the electric fuel injection valve 6 via the fuel supply pipe 6a. The discharge amount of the fuel pump 28 is adjusted based on an output signal of a fuel pressure detection sensor 29 provided on the common rail 27 and detecting the fuel pressure in the common rail 27.

The electronic control unit 30 is composed of a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, and a CPU (central control unit) 3 connected to each other by a bidirectional bus 31.
4, a plurality of input ports 35 and output ports 36 are provided.

An input port 35 provided in the electronic control unit 30 has a fuel pressure detection sensor 29, a load sensor 41 for generating an output voltage proportional to the depression amount L of the access pedal 40, and a crankshaft (not shown). Crank angle sensor 4 that outputs a pulse signal corresponding to the rotation of the crankshaft
2, etc., are connected via the corresponding A / D converter 37. The electronic control unit 30 grasps the current engine operating state based on output signals from various sensors input to the input port 35.

On the other hand, a plurality of drive circuits 38 are provided on the output port 36 side, and the electric fuel injection valve 6, the throttle valve driving step motor 16 and the EGR control valve 2 are provided.
5. Various devices such as the fuel pump 28 are connected to the electronic control unit 30 via the corresponding drive circuits 38. Each device is appropriately controlled (driven) by an instruction from the electronic control unit 30.

The control performed by the electronic control unit 30 will be described below.
It performs fuel supply control to supply fuel. Hereinafter, the fuel supply control will be described with reference to FIG. FIG. 3A shows the required torque TQ and the accelerator pedal 4.
The relative relationship between the stepping amount L of 0 and the engine speed N is shown. Each curve represents an iso-torque curve, and T
The curve indicated by Q = 0 indicates that the torque is zero. The remaining curves are TQ = a, TQ = b, TQ =
The required torque gradually increases in the order of c and TQ = d.

The required torque TQ is stored in the ROM 32 in advance in the form of a map as a function consisting of the depression amount L of the accelerator pedal 40 and the engine speed N as shown in FIG. In the present embodiment, a required torque TQ corresponding to the depression amount L of the accelerator pedal 40 and the engine speed N is first calculated based on the map shown in FIG. 3B, and the fuel injection amount is calculated based on the required torque TQ. Is calculated. Then, the control of the electric fuel injection valve 6 and the fuel pressure in the common rail 27 are controlled so that the fuel corresponding to the calculated fuel injection amount is supplied into the combustion chamber 5.

<Structure of Exhaust Purification Device> An exhaust purification device provided in the diesel engine 1 will be described below. The exhaust purification device is connected to an upstream exhaust pipe 54 connected to the outlet of the exhaust turbine 21 and has a NOx catalyst 50 therein.
, A casing 53 connected downstream of the casing 51 and containing an oxidation catalyst 52 therein, and a downstream exhaust pipe 54 a connected downstream of the casing 53 containing the oxidation catalyst 52. Have.

The upstream exhaust pipe 54 and the downstream exhaust pipe 54a are provided with air-fuel ratio sensors (A / F sensors) 55 and 56 for detecting the air-fuel ratio of the exhaust gas flowing through each exhaust pipe. Further, a catalyst temperature sensor 57 for detecting the ambient temperature of the NOx catalyst 50 built in the casing 51 is provided in the casing 51 containing the NOx catalyst 50. Further, there is provided a reducing agent adding device 60 for adding a fuel as a reducing agent to exhaust gas flowing into a casing 51 containing the NOx catalyst 50. That is, the reducing agent addition device 60, the air-fuel ratio sensor 55, the NOx
Each component is provided in series in the order of the catalyst 50, the oxidation catalyst 53, and the air-fuel ratio sensor 56. Hereinafter, each component will be described in detail.

The NOx catalyst 50 accommodated in the casing 55 connected to the upstream exhaust pipe 54
It has a function of mainly purifying NOx (nitrogen oxide) in the exhaust gas that flows in more. In the present embodiment, NOx
The storage reduction type NOx catalyst 50 is employed as the catalyst 50.

The storage-reduction type NOx catalyst 50 uses, for example, alumina Al ₂ O ₃ as a carrier and, on this carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, barium Ba, and calcium Ca. At least one selected from rare earths such as alkaline earth, lanthanum La and yttrium Y, and platinum Pt
NOx catalyst 5
When the air-fuel ratio of the exhaust gas flowing into the exhaust gas is higher than a predetermined air-fuel ratio, NOx in the exhaust gas is occluded, and when the air-fuel ratio of the inflow exhaust gas is lower than the predetermined air-fuel ratio, the stored NOx is released to release nitrogen. It has the function of reducing and purifying gas N ₂ . In addition,
The NOx purification mechanism of the NOx storage reduction catalyst 50 will be described later in detail.

The air-fuel ratio of the inflowing exhaust gas is defined as the amount of air filled in the exhaust passage, the combustion chamber, and the intake passage located upstream of the NOx catalyst 50, and the amount of fuel component present in the exhaust passage, the combustion chamber, and the intake passage. (Reducing agent). Therefore, as long as no fuel component, reducing agent or air is supplied to the exhaust passage upstream of the NOx catalyst 50, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture in the combustion chamber 5.

A casing 51 containing the NOx catalyst 50
The catalyst temperature detecting sensor 57 provided at the center of the casing 51 is attached at substantially the center of the casing 51, and outputs an output signal corresponding to the ambient temperature of the NOx catalyst 50. The output signal is input to the electronic control unit 30 via the corresponding A / D converter 37.

An air-fuel ratio sensor 55 attached to the upstream exhaust pipe 54 outputs an output signal corresponding to the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 50. The output signal is sent to the electronic control unit 3 via the corresponding A / D converter 37.
0 has been entered.

The reducing agent adding device 60 is for adding a fuel (light oil) as a reducing agent to the exhaust gas flowing into the NOx catalyst 50. The reducing agent adding device 60 is provided in the cylinder head 3 so that the injection hole faces the inside of the exhaust port 10. A reducing agent injection nozzle 61 for opening a valve when the fuel pressure acts to inject fuel into the exhaust branch pipe 19;
The reducing agent supply path 62 for guiding the fuel discharged from the fuel pump 28 to the reducing agent injection nozzle 61 and the flow rate of the fuel provided in the reducing agent supply path 62 and flowing through the reducing agent supply path 62 are adjusted. And a flow control valve 63 to be operated.

Then, at the same time that the flow control valve 61 is opened, the high-pressure fuel discharged from the fuel pump 6 flows into the reducing agent injection nozzle 61 via the reducing agent supply path 62 and acts on the reducing agent injection nozzle 61. When the generated fuel pressure reaches or exceeds the valve opening pressure, the reducing agent injection nozzle 61 opens and fuel as the reducing agent is injected into the exhaust branch pipe 19.

On the other hand, the reducing agent injected into the exhaust branch pipe 19 is discharged together with the exhaust gas discharged from the combustion chamber 5 into the exhaust turbine 2.
Flow into 1. The exhaust gas and the reducing agent that have flowed into the exhaust turbine 21 are uniformly stirred by the rotation of the exhaust turbine 21 and flow into the casing 51 in which the NOx catalyst 50 is housed as exhaust gas having a low air-fuel ratio. Then, the NOx catalyst 50
The NOx catalyst 50 that has been occluded is released for reduction and purification. The correlation between the addition of the reducing agent and the NOx purification action, and the timing of the addition of the reducing agent will be described later in detail.

When the flow control valve 63 is closed and the fuel supply to the reducing agent injection nozzle 61 is stopped, the fuel pressure acting on the reducing agent injection nozzle 61 becomes lower than the valve opening pressure of the reducing agent injection nozzle 61. As a result, the reducing agent injection nozzle 61 closes, and fuel addition into the exhaust branch pipe 19 is stopped.

The mounting position of the reducing agent injection nozzle 61 and the injection direction of the reducing agent at the reducing agent injection nozzle 61 depend on the EGR passage 2 provided in the exhaust branch pipe 19.
4 so that the fuel injected from the reducing agent injection nozzle 61 does not flow in and the reducing agent reaches the exhaust turbine 21 without stagnation in the exhaust branch pipe 19.
Further, the reducing agent injection nozzle 61 penetrates a water jacket (not shown) formed in the cylinder head 3 or is mounted adjacent to the water jacket, and is cooled by cooling water flowing through the water jacket. .

The oxidation catalyst 53 incorporated in the casing 54 connected downstream of the NOx catalyst 50 has a function of mainly purifying hydrocarbons HC and carbon monoxide CO in the exhaust gas discharged from the engine body 1, For example, a noble metal composed of only palladium Pb and platinum Pt, or palladium Pb, for example, alumina Al ₂ O ₃ or cordierite is supported on a carrier. Then, the hydrocarbon HC and carbon monoxide CO in the exhaust gas flowing into the oxidation catalyst are oxidized and purified into harmless carbon dioxide gas CO ₂ and water vapor H ₂ O (HC + CO + O ₂ → CO ₂ + H ₂ O).

The air-fuel ratio sensor 56 attached to the downstream exhaust pipe 54a outputs an output signal corresponding to the air-fuel ratio of exhaust flowing into the downstream exhaust pipe 54a via the NOx catalyst 50 and the oxidation catalyst 53. The output signal is input to the electronic control unit 30 via the corresponding A / D converter 37.

In the following description, the upstream exhaust pipe 54 will be described.
May be referred to as an inflow exhaust air-fuel ratio sensor 55. Further, the air-fuel ratio sensor 56 provided in the downstream side exhaust pipe 54a is connected to the outflow exhaust air-fuel ratio sensor 55.
It may also be called. Further, the exhaust gas flowing into the NOx catalyst 50 is referred to as inflow exhaust gas, while the NOx catalyst 50
The exhaust gas flowing out to the downstream exhaust pipe 54a via the oxidation catalyst 53 may be referred to as outflow exhaust gas.

<NOx Purification Mechanism> Subsequently, the NOx storage reduction catalyst provided in the exhaust gas purification apparatus will be described.
The NOx purification mechanism will be described. The NOx catalyst 50 stores NOx in the exhaust gas when the air-fuel ratio of the inflow exhaust gas is higher than a predetermined air-fuel ratio, and conversely when the air-fuel ratio of the inflow exhaust gas is lower than the predetermined air-fuel ratio. NOx that releases NOx and reduces and purifies it to nitrogen N ₂
It has a purifying action.

Here, the predetermined air-fuel ratio is a value generally set near the stoichiometric air-fuel ratio.
The composition can be slightly changed by changing the composition. In the present embodiment, the stoichiometric air-fuel ratio is a predetermined air-fuel ratio. Therefore, when the inflow exhaust gas has a lean air-fuel ratio higher than the stoichiometric air-fuel ratio, NOx is stored. Conversely, when the inflow exhaust gas has a rich air-fuel ratio lower than the stoichiometric air-fuel ratio, NOx is released to perform reduction purification.

It is considered that the NOx reduction and purification action of the NOx catalyst 50 is performed by the purification mechanism shown in FIG. The purification mechanism shown in FIG.
Although an example in which platinum Pt and barium Ba are supported on the carrier of the NOx catalyst 50 is shown, the same mechanism is obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.

[0045] First, at high lean air-fuel ratio of the exhaust gas flowing into the NOx catalyst 50 is far from the stoichiometric air-fuel ratio, FIG. 4 oxygen O ₂ in the inflowing exhaust gas as shown in (A) is O ₂ ^-
Alternatively, it adheres to the surface of platinum Pt supported on a carrier in the form of O ²⁻ . The nitrogen oxide NO contained in the inflow exhaust gas is
It reacts with O ₂ ⁻ or O ²⁻ on the platinum Pt supported on the carrier to form nitrogen dioxide NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, the N formed on the surface of platinum Pt
O ₂ is further oxidized on platinum Pt and combines with barium Ba supported on the same carrier. More specifically, barium oxide B oxidized by oxygen O ₂ in the inflow exhaust gas
diffuses in the NOx catalyst 50 in the form of ^- bound nitrate ions NO ₃ while the aO-.

The exhaust gas flowing into the NOx catalyst 50
At a rich air-fuel ratio where the fuel ratio becomes less than the stoichiometric air-fuel ratio,
Oxygen O contained in inflow exhaust_TwoWhite because the number of
Nitrogen dioxide NO generated on the surface of gold Pt_TwoThe amount of
Decrease. In the NOx catalyst 50, the reaction in the reverse direction
Nitrate ion NO diffused into the NOx catalyst 50_Three
^- Is nitrogen dioxide NO_Two(NO_Three ^-→ N
O_Two). And finally, nitrogen dioxide NO_TwoOr monoacid
Released in the form of NO from the NOx catalyst 50 into exhaust gas
You.

On the other hand, at a rich air-fuel ratio, hydrocarbon HC (fuel component) and carbon monoxide CO, which are contained in exhaust gas in a large amount, have an extremely high binding force with oxygen O _2, and oxygen O ₂ ^- or O ₂ on platinum Pt. Immediately combined with ^2- and oxidized, it becomes water vapor H ₂ O and carbon dioxide gas CO ₂ and diffuses into the exhaust gas.

At this time, even if the oxygen O ₂ ^- or O ^2- on the platinum Pt is exhausted by the oxidation of the hydrocarbon HC and the carbon monoxide CO, the unburned hydrocarbon remains in the inflowing exhaust gas. HC
And if carbon monoxide CO remains, the nitrogen dioxide NO ₂ or NO released from the NOx catalyst 50 is:
The excess unburned hydrocarbon HC and unburned carbon monoxide CO
, And is reduced to harmless nitrogen N ₂ and diffused into the exhaust gas (see FIG. 4B).

As described above, the NOx storage reduction catalyst 50 stores NOx in the inflowing exhaust gas when the air-fuel ratio of the inflowing exhaust gas becomes a lean air-fuel ratio, and stores the NOx in the catalyst 50 when the air-fuel ratio of the inflowing exhaust gas becomes a rich air-fuel ratio. to release the NOx that has been occluded in a short period of time, it is reduced to nitrogen N _2. Therefore, emission of NOx into the atmosphere can be prevented.

Incidentally, in the diesel engine 1 mounted on a vehicle or the like, the engine is operated at a lean air-fuel ratio higher than the stoichiometric air-fuel ratio (A / F = 13 to 14). Therefore, in a normal engine operating state, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 50 is extremely high, and the NOx in the exhaust gas discharged from the engine body 1 is stored in the NOx catalyst 50, and the NOx catalyst 50 There is little release.

On the other hand, in a spark ignition type engine such as a gasoline engine, by making the air-fuel mixture supplied to the combustion chamber 5 a rich air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio, the oxygen concentration in the inflowing exhaust gas is deliberately reduced, and the NOx catalyst 50 NOx stored in the fuel cell can be released. However, in the diesel engine 1 as shown in the present embodiment, if the air-fuel mixture supplied to the combustion chamber is set to a rich air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio, fine particles such as soot are generated during combustion of the air-fuel mixture. I will.

Therefore, in a diesel engine, NOx
In order not to saturate the NOx storage capacity of the catalyst 50, the air-fuel ratio of the inflow exhaust gas flowing into the NOx catalyst 50, instead of the air-fuel mixture, needs to be a rich air-fuel ratio, and the NOx stored in the NOx catalyst 50 needs to be released in a timely manner. . Therefore, in the present embodiment, as described above, the reducing agent addition device 60 (absorption / release control means) is provided in the exhaust passage 54 on the upstream side of the NOx catalyst 50 to cope with the problem. That is, by adding fuel as a reducing agent to the inflowing exhaust gas, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 50 is reduced, and the NOx stored in the NOx catalyst 50 is released and reduced and purified. In the following description, control for intentionally releasing NOx from the NOx catalyst 50 is referred to as rich spike control or regeneration of the NOx catalyst.

<Rich Spike Control> In this rich spike control, first, the CPU 34 of the electronic control unit 30 determines whether or not the conditions for adding the fuel as the reducing agent are satisfied. Here, the conditions for adding the reducing agent include, for example, whether the output signal of the catalyst temperature sensor 57 has reached the activation temperature of the NOx catalyst 50, and whether the NOx catalyst 50 Conditions such as whether the temperature increase control has not been performed, whether the number of traveling distances of the vehicle has reached a predetermined number of traveling distances, or whether the driving time of the vehicle has reached a predetermined time length that is preset. .

When the above-mentioned reducing agent addition condition is satisfied, the CPU 34 performs addition of reducing agent (addition of fuel) to the inflowing exhaust gas by the reducing agent adding device 60 at a predetermined timing, so that the inflowing exhaust gas becomes empty. The fuel ratio is set to a spike rich air-fuel ratio in a relatively short cycle. And
The NOx stored in the NOx catalyst 50 is released in a short cycle to reduce and purify the NOx.

At this time, the CPU 34 reads the engine speed, the output signal (accelerator opening) of the load sensor 41, the combustion consumption, and the like stored in the RAM 33, and reads these engine speed, engine load, fuel injection amount, and the like. As a parameter,
The control map of the reducing agent adding device 60 prepared in advance is accessed in the ROM 32, and the adding time and amount of the reducing agent in the reducing agent adding device 60 are calculated. And CP
In U <b> 34, the opening time of the flow rate control valve 63 is adjusted based on the addition condition and the addition amount of the reducing agent, and the reducing agent is injected from the reducing agent injection nozzle 61.

The reducing agent injected from the reducing agent injection nozzle 61 into the exhaust branch pipe 19 mixes with the exhaust gas flowing from the upstream side of the exhaust branch pipe 19 to form an exhaust having a rich air-fuel ratio. Exhaust gas with a rich air-fuel ratio flows into the NOx catalyst 50. Thus, the rich air-fuel ratio exhaust is
When the flow into the Ox catalyst 50, NOx that has been occluded in the NOx catalyst 50 as described above is released and further reduced combines with the reducing agent in the exhaust into nitrogen N _2.

As described above, when the NOx catalyst is used for a long period of time, the NOx storage capacity decreases. Eventually, NOx may not be occluded.
Therefore, when the NOx catalyst is used effectively, N
It is necessary to periodically determine how much the storage capacity of Ox has decreased. Therefore, in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the deterioration of the NOx catalyst 50 is determined by the method described below.

<Method of Determining Deterioration of NOx Catalyst> First, the principle of the determination of deterioration of the NOx catalyst 50 according to the gist of the present application will be described. NOx catalyst 5
0 is hydrocarbon H as a reducing agent in the inflow exhaust gas as described above.
It has a function of consuming C and carbon monoxide CO and oxidizing it into water vapor H ₂ O and carbon dioxide gas CO ₂ . At the same time, when the hydrocarbon HC and carbon monoxide CO remain in the inflow exhaust gas without being oxidized, the unburned hydrocarbon HC and carbon monoxide CO are consumed and the NOx catalyst 50 is consumed.
The NOx stored has a function allowed to reduced to nitrogen N ₂ in.

Therefore, when an amount of reducing agent sufficient to regenerate the NOx catalyst 50 is added to the inflow exhaust gas, the N
Until the NOx stored in the Ox catalyst 50 is completely released, nitrogen gas N ₂ , water vapor H ₂ O, and carbon dioxide gas CO ₂ continue to be generated in and around the NOx catalyst 50, and FIG.
As shown in the figure, the oxygen concentration (air-fuel ratio) of the exhaust gas flowing out of the NOx catalyst 50 by the nitrogen gas N ₂ and the water vapor H ₂ O-carbon dioxide CO ₂ shows a value sufficiently lower than the stoichiometric air-fuel ratio for a certain period. That is, the O ₂ storage effect can be recognized.

Incidentally, these nitrogen gas N ₂ and water vapor H ₂
In generating O and carbon dioxide gas CO ₂ , NOx and oxygen O ₂ stored in the NOx catalyst 50 are required.
Therefore, these nitrogen gas N ₂ , water vapor H ₂ O, carbon dioxide gas C
A period T (hereinafter, referred to as a rich time) during which the oxygen concentration (air-fuel ratio) of the outflow exhaust gas is reduced by the generation of O _2.
Is measured to determine the amount of N that can be stored in the NOx catalyst 50.
The amount of Ox, that is, the NOx storage capacity can be evaluated. More specifically, the deterioration state of the NOx catalyst 50 can be determined by comparing the rich time T1 of the new NOx catalyst with the rich time T2 calculated by actual measurement.

Therefore, in the exhaust gas purifying apparatus according to the present application,
In the rich spike control described above, the rich time T2
And the NOx based on the length of the rich time T2
The deterioration of the catalyst 50 is determined. In addition, at the time of this deterioration determination, the amount of the reducing agent added in the normal rich spike control is smaller than that in the normal rich spike control so that the air-fuel ratio of the outflow exhaust gas detected downstream of the NOx catalyst has a remarkable influence on the O ₂ storage effect. A large amount of reducing agent is added to the inflow and exhaust.

The length of time during which the air-fuel ratio of the outflow exhaust gas is temporarily reduced by the O ₂ storage effect is such that the outflow exhaust gas is close to the stoichiometric air-fuel ratio while NOx is being released from the NOx catalyst 50. It is much longer than the time maintained. Than also using the O ₂ storage effect described above deteriorated state of the NOx catalyst 50 Thus by measuring the time that the air-fuel ratio of the outflowing exhaust gas in the vicinity of the stoichiometric air-fuel ratio is maintained to determine the deteriorated state of the NOx catalyst 50 Is more accurately and more reliably determined.

In the following description, the addition amount of the reducing agent added during normal rich spike control is used as the basic addition amount, and the addition amount of the reducing agent added during rich spike control, which also serves as a deterioration judgment, is used as the deterioration judgment value. Distinguish as the amount added. Further, a large amount of reducing agent addition operation accompanying the deterioration determination may be referred to as deterioration determination injection. Hereinafter, the deterioration determination control performed based on the above-described method for determining the deterioration of the NOx catalyst 50 will be described with reference to a flowchart shown in FIG.

<NOx Catalyst Deterioration Judgment Control> FIG.
5 is a flowchart showing a series of flows relating to the deterioration determination of the Ox catalyst 50. Note that the deterioration determination processing routine shown in this flowchart is performed in the ROM 3 of the electronic control unit 30.
2 is a routine that is stored in advance in the storage unit 2 and is repeatedly executed at predetermined time intervals.

<Step 101> First, before executing the deterioration judgment of the NOx catalyst 50, it is judged whether or not the deterioration judgment execution condition is satisfied (step 10).
1). Here, a condition that can be exemplified as the deterioration determination execution condition is described. For example, it is determined whether the number of traveling distances of the vehicle has reached a predetermined number of traveling distances, or whether the driving time of the vehicle has reached a predetermined time period set in advance. In this embodiment, for example, in the present embodiment, whether the operating state of the engine body 1 is in the steady state, whether the steady state has continued for a preset specified time, and whether the ambient temperature of the NOx catalyst 50 is the activation temperature N is reached when various conditions such as
The deterioration of the Ox catalyst 50 is determined.

The steady state of the engine body 1 refers to an operating state in which the engine load and the engine speed are substantially constant, and the idling operating state is also included in the steady operating state. In addition, when the engine body 1 is not in the steady operation state, the air-fuel ratio is unstable in both the inflow exhaust gas and the outflow exhaust gas, and a correct deterioration determination cannot be performed. Also, N
When the ambient temperature of the Ox catalyst 50 is lower than the activation temperature, the purifying action of the NOx catalyst 50 is slow or not functioning, and a correct deterioration determination cannot be performed. As described above, when the negative determination is made in step 101, the deterioration determination processing routine is temporarily ended.

<Step 102> On the other hand, step 101
If an affirmative determination is made in step, the routine proceeds to step 102, where a reducing agent (fuel) corresponding to the deterioration determination addition amount is injected into the exhaust branch pipe 19 as shown in the above-described deterioration determination method (air-fuel ratio control means). Here, the addition amount of the reducing agent at the time of the deterioration determination injection, that is, the deterioration determination addition amount, is about several times, several tens times, or several hundred times as much as the basic addition amount during the normal rich spike control.

In this embodiment, the deterioration determination addition amount is determined so that the air-fuel ratio of the outflow exhaust gas surely indicates a value equal to or lower than the stoichiometric air-fuel ratio. The deterioration determination addition amount was determined based on preliminary experiments performed under various conditions, and the deterioration determination addition amount determined by the preliminary experiment was created using the catalyst atmosphere temperature, the current engine load, and the like as parameters. The information is recorded in a ROM 32 in the electronic control unit 30 in the form of a map.

On the other hand, the amount of the reducing agent added at the time of the deterioration determination injection is large. Therefore, in the NOx catalyst 50, not all of the hydrocarbon HC and carbon monoxide CO in the inflowing exhaust gas may be completely purified. Therefore, in the present embodiment, the oxidation catalyst 52 is provided downstream of the NOx catalyst 50,
The hydrocarbon HC and carbon monoxide CO that have flowed out through the catalyst 50 are converted into harmless steam H by the oxidation catalyst 52.
_It is oxidized to ₂ O and carbon dioxide gas CO ₂ for purification. Therefore, the HC emission does not deteriorate even when the deterioration is determined.

<Step 103> Subsequently, the electronic control unit 30 proceeds to step 103, in which the elapsed time after executing the deterioration determination injection is measured, and whether or not the elapsed time has reached a predetermined time set in advance. Is determined. Here, the predetermined time period is defined as the time when the reducing agent injected from the reducing agent injection nozzle 60 into the exhaust branch pipe 19 flows down the exhaust turbine 21 and the upstream exhaust pipe 54 and flows into the NOx catalyst 50. And the time required for the NOx catalyst 50 to release NOx and perform reduction purification. The specified time is determined based on preliminary experiments performed under various conditions, and the deterioration determination addition amount determined by the preliminary experiments is based on a map created using the current engine speed, engine load, and the like as parameters. ROM in electronic control unit 30 in form
32.

If the result in step 103 is negative, the routine for determining deterioration is temporarily ended. In addition,
Even if the operating state of the engine body 1 has deviated from the steady state during execution of step 103, this step 1
03 is denied, and the deterioration determination processing routine is temporarily ended. Therefore, in this case, the deterioration determination injection is temporarily terminated, and the elapsed time after executing the deterioration determination injection serving as the basis of the determination in step 103 is also reset to “0”.

<Step 104> Step 10
3 is affirmative, the NOx catalyst 5
Air-fuel ratio of the outflowing exhaust gas flowing out from 0 to calculate the time that is maintained in a rich air-fuel ratio by the O ₂ storage effect. More specifically, first, the inflow exhaust air-fuel ratio sensor 55
, The air-fuel ratio of the exhaust gas flowing into the NOx catalyst 50 is measured.
The air-fuel ratio of the exhaust gas flowing out is measured. Subsequently, a rich time during which the detected value is maintained at the rich air-fuel ratio is calculated based on the detected value detected by each of the air-fuel ratio sensors 55 and 56, and a rich time corresponding to the inflow exhaust is set to correspond to the outflow exhaust. NOx catalyst 5 by the quotient divided by the rich time
A comparison value used for determining the deterioration of 0 is obtained.

Here, the comparison value will be described in detail. As described above, the higher the NOx storage capacity of the NOx catalyst 50, the longer the time during which the air-fuel ratio of the exhaust flowing out of the NOx catalyst 50 is maintained at the rich air-fuel ratio. . Therefore,
The NOx catalyst 5 is obtained by calculating a quotient obtained by dividing the rich time corresponding to the inflow exhaust gas by the rich time corresponding to the outflow exhaust gas.
The NOx storage capacity of 0 can be specifically quantified.

<Step 105> Step 10
When the comparison value is obtained in step 4, the electronic control unit 30 proceeds to step 105 and determines the deterioration state of the NOx catalyst 50. In this step 105, the comparison value determined based on the actual measurement value in the previous step 104 is compared with a deterioration determination reference value that is an index when performing the deterioration determination of the NOx catalyst 50, and the comparison value is compared with the reference value. It is determined whether it is negative (deterioration determination means).

The deterioration determination reference value serving as an index when performing the deterioration determination of the NOx catalyst 50 is a value determined based on preliminary experiments performed under various conditions using the new NOx catalyst 50. The deterioration determination reference value is recorded in the ROM 32 of the electronic control unit 30 in the form of a map created using the engine load, the catalyst atmosphere temperature, and the like as parameters. Therefore, the deterioration state of the NOx catalyst 50 can be accurately determined by comparing the deterioration determination reference value developed on this map with a comparison value obtained by actual measurement.

If the determination in step 105 is affirmative, the CPU 34 of the electronic control unit 30 sets N
It recognizes that the Ox catalyst 50 is in a deteriorated state (Step 1)
06). In the case of a negative determination, it is determined that the NOx catalyst is functioning normally, and the present deterioration determination processing routine is terminated.

As described above, in the exhaust gas purifying apparatus for an internal combustion engine according to the present application, the deterioration determination of the NOx catalyst is performed based on the time in which the air-fuel ratio of the outflow exhaust gas is temporarily reduced due to the effect of the O ₂ storage effect. I have.

[Other Embodiments] In the above embodiment, the comparison value is calculated based on the relationship between the rich time corresponding to the outflow exhaust gas and the rich time corresponding to the inflow exhaust gas, and the NOx catalyst is calculated based on the comparison value. 50 degradation judgments have been implemented,
The determination of the deterioration of the NOx catalyst according to the present application is necessary only if the rich time corresponding to the outflow exhaust gas can be calculated.

That is, the comparison value applied in the above-described deterioration determination processing routine is simply defined by the rich time corresponding to the outflow exhaust, and the deterioration determination reference value serving as an index at the time of the deterioration determination is defined in units of time. Then, the rich time corresponding to these outflow exhausts is compared with a deterioration determination reference value, and when the rich time becomes shorter than the deterioration determination reference value, the NOx catalyst can be considered to be in a deteriorated state.

In the above-described embodiment, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 50 is actually measured using the inflow exhaust air-fuel ratio sensor 55. From the engine body 1
May be stored in advance in the ROM 32 of the electronic control unit 30, and the rich time of the inflowing exhaust gas may be estimated with reference to this map. Then, the above-described deterioration determination processing routine may be performed based on the estimated rich time.

In the above-described embodiment, the outflow exhaust air-fuel ratio sensor 56 is mounted downstream of the oxidation catalyst 52. However, the outflow exhaust air-fuel ratio sensor 56 may be provided immediately after the NOx catalyst 50. . Further, the air-fuel ratio sensor 55,
The air-fuel ratio may be detected by an oxygen O ₂ sensor or the like instead of 56.

In the above-described embodiment, the description has been given by taking the compression ignition type internal combustion engine as an example. However, the exhaust gas purifying apparatus for an internal combustion engine according to the present application is of course applicable to a spark ignition type internal combustion engine such as a gasoline engine. , Is applicable. Also,
When the internal combustion engine is a gasoline engine, the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber of the internal combustion engine is set to a rich air-fuel ratio and burned, so that the air-fuel ratio of the inflow exhaust gas flowing into the NOx catalyst is set to a rich air-fuel ratio. 10 can be supplied with hydrocarbon HC as a reducing agent. That is, the reducing agent supply unit can be realized by the fuel injection control unit.

When the storage-reduction NOx catalyst described in the above embodiment is used for a long period of time, it is poisoned by sulfur oxides SOx in the exhaust gas, and the NOx purification rate decreases. Therefore, when SOx poisoning has progressed to some extent, it is necessary to perform a regeneration process to recover the NOx catalyst from SOx poisoning. During this regeneration process, NOx is released and
It is necessary to supply a large amount of reducing agent to the NOx catalyst, which is larger than the basic addition amount added during the rich spike control for reduction purification. Therefore, the determination of the deterioration of the NOx catalyst according to the present application can be performed even during the regeneration processing of SOx poisoning.

In the above embodiment, the storage reduction type NO
Although the x-catalyst is employed, the same operation and effect as the occlusion-reduction-type NOx catalyst can be obtained even when the selective reduction-type NOx catalyst is employed. In the selective reduction type NOx catalyst, when the inflow exhaust gas has a lean air-fuel ratio and N
It has the function of reducing or decomposing Ox.

[0086]

As described above, according to the present invention, when the deterioration of the NOx catalyst is determined, the air-fuel ratio of the exhaust gas flowing out of the NOx catalyst is greatly reduced by greatly reducing the air-fuel ratio of the exhaust gas flowing into the NOx catalyst. In this case, a clear difference tends to occur between the case where the NOx catalyst is deteriorated and the case where the NOx catalyst is not deteriorated, so that it is possible to make a highly accurate deterioration determination.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram schematically showing an exhaust gas purification device for an internal combustion engine according to the present invention.

FIG. 2 is an enlarged view of a main part of the internal combustion engine shown in FIG.

FIG. 3 is a diagram showing a required torque of an engine;

FIG. 4 is a diagram for explaining the NOx absorbing / releasing action of the NOx storage reduction catalyst;

FIG. 5 is a diagram for explaining a change in an air-fuel ratio when a reducing agent is added.

FIG. 6 is a flowchart illustrating a deterioration determination processing routine for determining deterioration of the NOx catalyst.

[Explanation of symbols]

 Reference Signs List 1 diesel engine (engine body) 5 combustion chamber 54 upstream exhaust pipe 54a downstream exhaust pipe 50 storage reduction type NOx catalyst (NOx catalyst) 52 oxidation catalyst 57 catalyst temperature sensor 55 inflow exhaust air-fuel ratio sensor 56 outflow exhaust air-fuel ratio sensor 60 Reducing agent supply device 61 Reducing agent injection nozzle

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) F02D 45/00 368 F02D 45/00 368G F-term (Reference) 3G084 AA01 AA04 BA09 BA24 DA10 DA22 DA27 EA04 EB08 EB12 EB16 FA10 FA26 FA27 FA30 FA33 FA38 3G091 AA10 AA11 AA17 AA18 AB02 AB06 BA14 BA33 CA18 DA02 DA04 DA08 DB10 EA18 EA30 EA34 EA38 FA12 FA18 FB12 GB02Y GB03Y GB04Y GB05W GB06W GB07W GB10X GB17X HA09 HA36 HA38 HA38 HA38 HA38 HA38 HA38 HA38 HA38 HA

Claims (3)

[Claims] 1. A nitrogen oxidation device provided in an exhaust passage of an internal combustion engine to absorb nitrogen oxides in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is high and absorb nitrogen oxides in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas decreases. A NOx catalyst for releasing and reducing substances, and purifying nitrogen oxides absorbed by the NOx catalyst.
Suction / release control means for lowering the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to a predetermined air-fuel ratio; and air for temporarily lowering the air-fuel ratio of the exhaust gas flowing into the NOx catalyst to an air-fuel ratio lower than the predetermined air-fuel ratio. Fuel ratio control means; air-fuel ratio detection means for detecting the air-fuel ratio of exhaust flowing out of the NOx catalyst when the air-fuel ratio of exhaust flowing into the NOx catalyst is reduced below the predetermined air-fuel ratio; An exhaust purification device for an internal combustion engine, comprising: a deterioration determination unit that determines deterioration of the NOx catalyst based on a time when a detection value of the detection unit is lower than a reference air-fuel ratio. 2. The method according to claim 1, wherein the reference air-fuel ratio is a stoichiometric air-fuel ratio, and the deterioration determination unit determines that the NOx catalyst has a value lower than a predetermined time when a value detected by the air-fuel ratio detection unit is lower than the stoichiometric air-fuel ratio. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein it is determined that the exhaust gas is deteriorated. 3. A reducing agent adding means for adding a reducing agent to exhaust gas upstream of the NOx catalyst, wherein the absorption / release control means adds a predetermined amount of reducing agent to the exhaust gas from the reducing agent adding device. By doing so, the NOx
The air-fuel ratio of the exhaust gas flowing into the catalyst is reduced to a predetermined air-fuel ratio, and the air-fuel ratio control unit controls the NOx catalyst by adding the predetermined amount or more of the reducing agent into the exhaust gas from the reducing agent adding unit. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the inflowing exhaust gas is made lower than the predetermined air-fuel ratio.