JPH05187229A - Exhaust emission purifier for internal combustion engine - Google Patents

Abstract

PURPOSE:To suppress the increase of an exhaust amount of transient NOx when an air-fuel ratio is changed from lean to stoic. CONSTITUTION:An NOx catalyst 6 containing quantities of La (0.03mol/1cat or more) and carrying platinum is disposed in the exhaust gas passage 4 of a lean combustible internal combustion engine 2. A three-component catalyst 8 is disposed on the downstream of the NOx catalyst. When an air-fuel ratio is changed from lean to stoic, NOx separated from the NOx catalyst 6 is purified by means of a three-component catalyst 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, which can purify NOx in exhaust gas of an internal combustion engine capable of lean combustion at a high NOx purification rate.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of protecting the global environment, CO ₂ emitted from automobiles has become a problem, and lean burn is promising as a solution. However, since the three-way catalyst has almost no NOx purification capacity under oxygen-rich exhaust gas conditions in the lean air-fuel ratio region, it is desired to develop a NOx catalyst and system for purifying NOx even under oxygen-rich conditions.

One of the NOx catalysts that are currently being researched as a NOx catalyst exhibiting a relatively high NOx purification rate even under an oxygen excess condition is a zeolite-based catalyst in which a transition metal such as Cu is ion-exchanged and supported on a zeolite (Japanese Patent Laid-Open No. HEI-1). No. 139145), Pt-based catalysts in which a noble metal such as Pt is supported on alumina or zeolite, and the like. Pt-based catalysts are disclosed in
As disclosed in Japanese Laid-Open Patent Publication No. 112715, it is usually L
It contains rare earth elements such as a. However, the content of La in the conventional catalyst is 0.01 mol / lcat (mo
1 / lcat indicates the number of moles of La per 1 liter of the catalyst volume) and is small.

In the catalyst development test research conducted by the inventors, when the amount of rare earth element such as La in the catalyst is increased, for example, 0.03 mol / lcat or more, the NOx purification rate of the NOx catalyst is increased and the NOx purification is increased. The rate was found to be particularly high during the transition of the air-fuel ratio from stoichiometric (theoretical air-fuel ratio) to lean (lean air-fuel ratio), and a patent application for such NOx catalyst was filed (Japanese Patent Application No. 3- issue).

[0005]

However, the NOx catalyst with an increased loading of rare earth elements such as La shows that when the air-fuel ratio changes from stoichiometric to lean, according to further test studies by the inventors. The NOx purification rate is significantly higher than that of the conventional lean NOx catalyst, but it was found that the NOx purification rate transiently decreases when the air-fuel ratio changes from lean to stoichiometric.

An object of the present invention is to purify NOx in the exhaust gas of an internal combustion engine that can burn in a lean air-fuel ratio range by using a NOx catalyst carrying a larger amount of rare earth elements such as La than before. An object of the present invention is to provide an exhaust gas purification device (system) for an internal combustion engine that can suppress NOx emission to the outside air even if the air-fuel ratio changes from lean to stoichiometric and the NOx purification rate of the NOx catalyst transiently decreases. ..

[0007]

The above object is achieved by the following exhaust gas purification apparatus for an internal combustion engine according to the present invention.
That is, a lean air-fuel ratio region in which an internal combustion engine capable of burning in a lean air-fuel ratio region and its exhaust passage and a rare earth element installed in the exhaust passage at a ratio of 0.03 mol / lcat or more and platinum is carried NOx catalyst capable of purifying NOx in the exhaust of the exhaust gas, and the NOx in the exhaust passage.
An exhaust emission control device for an internal combustion engine, comprising: a three-way catalyst installed on a downstream side of the catalyst.

[0008]

The air-fuel ratio of the lean burn internal combustion engine is lean during steady running and acceleration, and is considered to be stoichiometric during idling (misfire if lean during idling). During steady running and acceleration, NOx is purified by the NOx catalyst. What is important is that the air-fuel ratio is controlled to lean during steady running and acceleration, whereas the NOx catalyst has NOx purification capacity, while the three-way catalyst has NOx reduction capacity in the exhaust gas with lean air-fuel ratio. Because there is hardly any.

During the transition of the air-fuel ratio, when the air-fuel ratio changes from stoichiometric to lean, rare earth elements such as La adsorb and decompose NOx, so NOx is a NOx catalyst.
It is transiently adsorbed at a high NOx purification rate, and is decomposed by promoting decomposition of NOx in the presence of Pt (platinum). When the air-fuel ratio changes from lean to stoichiometric, some rare earth elements such as La release the adsorbed NOx, but the downstream three-way catalyst has stoichiometric NOx purification capability, so
The NOx released by the x catalyst flows to the three-way catalyst where it is purified and suppressed from being released to the outside air. Therefore, the release of NOx to the outside air is suppressed in all operating conditions.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. 1 to 4 relate to a first embodiment of the present invention, and FIGS. 5 to 9 relate to a second embodiment of the present invention. The first embodiment is a case where the release of NOx to the outside air is suppressed under all operating conditions, and the second embodiment forcedly changes the air-fuel ratio in order to further suppress the release of NOx to the outside air. In this case, the NOx purification rate of the system is improved. First, the first embodiment will be described. In FIG. 1, an internal combustion engine 2 is an engine capable of burning in a lean air-fuel ratio range, and may be a gasoline engine or a diesel engine. In the case of a lean-burn gasoline engine, the air-fuel ratio is operated lean during steady running and acceleration / deceleration, and is operated stoichiometric (theoretical air-fuel ratio) during idling.

In the exhaust passage 4 of the internal combustion engine 2, preferably, a rare earth element such as La (lanthanum) is contained in the underfloor portion of the vehicle in an amount of 0.03 mol / lcat (mol / lcat is the amount supported per 1 liter of catalyst volume) or more. A NOx catalyst 6 capable of purifying NOx in the exhaust gas in a lean air-fuel ratio region (a region where the air-fuel ratio is leaner than stoichiometric) that carries Pt and Pt is arranged. The carrier of the NOx catalyst 6 is of alumina or silica type, but other types of carrier may be used. The amount of platinum supported is not particularly limited, but, for example, about 0.5 g / lcat may be supported.

A three-way catalyst 8 is disposed in a portion of the exhaust passage 4 of the internal combustion engine 2 downstream of the NOx catalyst 6. The three-way catalyst is capable of purifying NOx in stoichiometric and air-fuel ratio rich exhaust gas. This three-way catalyst is composed of Pt (platinum), Rh (rhodium),
It carries one or more active metals such as Pd (palladium) and Ce (cerium).

FIG. 2 shows the catalyst portion of the first embodiment of the present invention. The NOx catalyst 6 is La (lanthanum) as a rare earth element, contains La in a proportion of 0.03 mol / lcat or more, and is composed of a platinum-containing catalyst (hereinafter, also referred to as La-based catalyst). The carrier of the NOx catalyst 6 is alumina, on which La is carried, for example in the form of La2 O3, and Pt (platinum) is carried on La. The carrier of the three-way catalyst 8 is alumina, and Ce (cerium), Pt
And Rh (rhodium).

FIG. 4 shows the NOx purification rate-air-fuel ratio characteristics of the above-mentioned NOx catalyst and various conventionally known catalysts. As shown in A of FIG. 4, the three-way catalyst CC _RO has almost no NOx purification capacity in the lean region of the air-fuel ratio. Figure 4
As shown in B of Pt, Pt in which only Pt is supported on alumina
The NOx catalyst composed of / alumina catalyst (which does not have Rh unlike a three-way catalyst) has a good NOx purification rate in the lean region of the air-fuel ratio and stoichiometry. However, since the Pt / alumina catalyst does not have Rh, NO in the air-fuel ratio rich region
x Purification ability drops sharply compared to three-way catalysts. As shown in C of FIG. 4, the Cu / zeolite catalyst has an excellent NOx purification ability in the air-fuel ratio lean region, but the NOx purification ability does not improve even if the air-fuel ratio is changed.

The La-based catalyst has a La content of 0.03 mo.
When it is as large as 1 / lcat or more (D in FIG. 4), when the air-fuel ratio changes from stoichiometric to lean, as shown by the thick line in FIG. 3, the NOx purification rate is significantly improved over several minutes. For example, when the NOx catalyst in the steady state is about 35%, when the air-fuel ratio changes from stoichiometric to lean, it transiently remarkably improves to about 95%. Therefore, the NOx catalyst 6 transiently exhibits a high NOx purification rate as shown in FIG. 4 when the air-fuel ratio changes from stoichiometric to lean. However, the amount of La is 0.01m as before.
When it is as small as ol / lcat or less, the improvement in the NOx purification rate when the air-fuel ratio changes from stoichiometric to lean is small, as shown by the thin line in FIG. 3, and is the purification capacity at the level B in FIG.

0.03 mol / l of rare earth element such as La
As shown in FIG. 3, the NOx catalyst carried at a ratio of cat or more, that is, the NOx catalyst 6, has a transiently high NOx purification when the air-fuel ratio changes from stoichiometric to lean (when changing from idle to acceleration). The NOx purification rate transiently drops when the air-fuel ratio changes from lean to stoichiometric. The reason is that La adsorbs and decomposes NOx when the air-fuel ratio changes from lean to stoichiometric, but when the adsorbing capacity is saturated within a few minutes, the NOx purification rate returns to that at the steady state, and the air-fuel ratio changes from lean to stoichiometric. It is presumed that this is because when it changes to, the NOx that has been adsorbed up to that point and not yet decomposed is transiently desorbed.

Further, as shown in FIG. 3, the NOx catalyst containing a rare earth element such as La has a NOx purifying ability even during a stoichiometric operation such as idling, and the NOx concentration of the gas discharged from the catalyst is equal to the NOx concentration of the input gas. Compared to. The reason is that in idle, the space velocity SV is small, so NOx is easily purified by the NOx catalyst. Also, when the NOx concentration is high to some extent, the NOx purification rate is improved, but the exhausted gas is exhausted by NOx separated during idle. It is presumed that the NOx concentration of NOx increases temporarily and the NOx purification rate temporarily improves.

Utilizing the above characteristics of the NOx catalyst containing a rare earth element such as La, the NOx catalyst 6 containing a large amount of La is arranged on the upstream side, and the three-way catalyst 8 is arranged on the downstream side. NOx catalyst 6 when changing from lean to stoichiometric
This is because the three-way catalyst 8 purifies the NOx that is temporarily released at.

Next, the operation of the first embodiment will be described. In actual vehicle operation, idling, acceleration, steady running, deceleration, and idling are repeated, so the air-fuel ratio is repeatedly changed between lean and stoichiometric. During a steady state where the lean state or idle state continues continuously for a long time, NOx is mainly purified by the NOx catalyst 6, and even if it passes through without being decomposed by the NOx catalyst 6, it is purified by the three-way catalyst 8 at the time of stoichiometry. ..

The air-fuel ratio transient state in which the air-fuel ratio changes has different properties when the air-fuel ratio changes from lean to stoichiometric and when it changes from stoichiometric to lean. When the air-fuel ratio changes from stoichiometric to lean, the NOx purification rate of the La-based NOx catalyst transiently rises for several minutes as shown in FIG. 3, so that the NOx purification rate of the NOx catalyst 6 transiently increases significantly, The NOx purification rate of the entire system transiently improves.

On the other hand, when the air-fuel ratio changes from lean to stoichiometric, NOx is released from the NOx catalyst 6 and the NOx concentration of the input gas of the three-way catalyst 8 increases temporarily, but when the three-way catalyst 8 is stoichiometric, it increases. Further, since the NOx is sufficiently purified, the amount of NOx discharged to the outside can be suppressed sufficiently low.

Next, a second embodiment will be described. When the air-fuel ratio is changed from the rich side (air-fuel ratio higher than the theoretical air-fuel ratio) side to the lean side, the NOx purification rate improving effect appears as described in FIG. Since the effect of improving the NOx purification rate is greater than the increase in the NOx concentration when the air-fuel ratio changes from the lean side to the rich side, when the air-fuel ratio is changed, the NOx purification efficiency becomes larger than the steady state. .. Focusing on this point, in the second embodiment, the air-fuel ratio is forcibly changed alternately between the rich side and the lean side during lean operation. However, if the air-fuel ratio is shaken too much, a torque shock will occur, so it is not necessary to shake it greatly from lean to stoichiometric.

In FIG. 5, the internal combustion engine 2, the exhaust passage 4,
Since the NOx catalyst 6 and the three-way catalyst 8 conform to the description in the first embodiment, the same reference numerals as those in the first embodiment are used to
The description is omitted. Intake system of internal combustion engine 2 or internal combustion engine 2
A fuel injection valve 10 is provided to supply fuel to the cylinders. In order to perform feedback control of the air-fuel ratio, the exhaust system 4 is provided with an air-fuel ratio sensor 14 (for example, an oxygen sensor), the output of which is input to an electronic control unit (ECU) 12 including a microcomputer. .. Further, an intake pressure sensor 18 and a throttle opening sensor 20 are provided in the intake system in order to know the engine operating conditions used for the calculation in the ECU 12, and the engine driven by the distributor is driven by the engine crankshaft. Rotation speed sensor 16 (crank angle sensor)
Built-in, each sensor 18, 20, 16
Is output to the ECU 12.

The ECU 12 includes a CPU, ROM, RAM,
It has an A / D converter, an input interface, and an output interface. The input values that change from moment to moment from the various sensors are converted from analog signals into digital signals by the A / D converter, and the digital signals are input to the input interface as they are and temporarily stored in the RAM to update the data. , And is read by the CPU to execute the operation. The ROM stores programs and maps as shown in FIGS. 6 to 8, and these programs are CPs.
It is read by U and the operation is executed. The signal of the fuel injection amount obtained by the calculation is sent to the fuel injection valve 10 via the output interface, and the fuel injection valve 10 is opened for a time corresponding to the signal to execute the fuel injection.

FIG. 6 shows an air-fuel ratio control means, which is stored in the ROM, is read out by the CPU, and is used for the arithmetic operation, and which is composed of a program for controlling the air-fuel ratio. The program of FIG. 6 is the same as the conventional air-fuel ratio control program means except for step 58 which calls a lean combustion (F / B: feedback) target air-fuel ratio variation subroutine.

The program of FIG. 6 will be described. Step 5
At 0, the operating state of the engine, for example, engine speed NE (output of engine speed sensor 16) and intake pipe negative pressure PM (output of intake pressure sensor 18, signal corresponding to load) are read. Subsequently, the routine proceeds to step 52, where the basic fuel injection amount TP is obtained based on the engine operating state using the map of FIG. TP is a fuel injection amount corresponding to the stoichiometric operation, and at the time of idling or the like, fuel is injected by the amount of this TP.

Subsequently, the routine proceeds to step 54, where it is judged whether or not the current operating condition is the lean burn condition. During stoichiometric operation such as idling, proceed to step 74
When AU = TP, the routine proceeds to step 76, where fuel injection for a fuel injection time of TAU (fuel injection time corresponding to the fuel injection amount) is executed, but at the time of lean burn conditions, the lean correction coefficient KLEAN is calculated at step 55. After that, the process proceeds to step 56. In step 56, a lean burn (F / B) target air-fuel ratio A / FT (where T is a target).
Is calculated using a lean combustion map (not shown). Conventionally, the process proceeds to step 60-72 as it is, and the feedback control is performed so that the actual air-fuel ratio A / F becomes the target air-fuel ratio A / F-T, but in the present invention, from step 56 to step Proceed to 58, for lean burn (F /
B) Proceed to the F / B target air-fuel ratio variation subroutine for forcibly changing the target air-fuel ratio alternately between the rich side and the lean side, and the calculation of this subroutine is executed. This F / B target air-fuel ratio subroutine constitutes lean combustion target air-fuel ratio changing means. This lean combustion target air-fuel ratio varying means is
One example thereof is shown in FIG. 8, which will be described later.

After step 58, the target air-fuel ratio A
/ FT is in a state of alternately changing to the rich side and the lean side. Then, in step 60, the air-fuel ratio sensor (A /
The output of the F sensor) 14 is read, and the output is calculated in step 62 to obtain the current and actual air-fuel ratio A / F. Then, the routine proceeds to step 64, where the target air-fuel ratio A / F-
The difference D between T and the actual air-fuel ratio A / F is obtained. If D is positive in step 66, the routine proceeds to step 68, where the correction coefficient FAF is reduced by a predetermined value α, and D is calculated in step 66.
If is negative, the routine proceeds to step 70, where the correction coefficient FAF is increased by α, and the routine proceeds to step 72. In step 72, the fuel injection time TAU is calculated by TP * KLEAN * FAF. Subsequently, the routine proceeds to step 66, where the injection execution processing of the fuel injection corrected to the lean target air-fuel ratio is executed.

The air-fuel ratio control means shown in FIG. 6 controls the air-fuel ratio to stoichiometric (theoretical air-fuel ratio) during output operation, and to the lean burn (F / B) target air-fuel ratio A / FT under lean burn conditions. Controlled. However, under the lean burn condition, the target air-fuel ratio A / F-T is alternately changed to the rich side and the lean side by the lean combustion target air-fuel ratio changing means by passing through step 58.

FIG. 8 shows a lean burn (F / B) target air-fuel ratio varying means. In FIG. 8, in step 102,
By determining whether the lean correction flag FLEAN is 1 or 0, it is determined whether the target air-fuel ratio is in the lean side or the rich side.

At step 102, the lean correction flag FL is set.
If it is determined that the EAN is 1, that is, the lean side should be corrected, the process proceeds to step 104. Step 10
In No. 4, the target air-fuel ratio A / FT for lean burn (F / B)
Is corrected to the lean side by a predetermined amount per interrupt. That is, the A / F-T is increased by a predetermined amount. While passing through the step 104 of this subroutine several times, the A / FT becomes gradually larger.

Subsequently, the routine proceeds to step 106, where the lean correction flag FLEAN is set to 1 and at the same time, the count time of the timer which started counting has passed a predetermined time, that is, the target air-fuel ratio is corrected to the lean side. It is determined whether or not the continued time has exceeded a predetermined time.
Here, since the predetermined time varies depending on the catalyst capacity, the exhaust gas flow rate, etc., it is experimentally obtained in advance so that the transient effect as shown in FIG. 3 can be utilized. Step 106
Then, if the predetermined time is not exceeded, the process directly returns and the lean correction is continued. If it is determined in step 106 that the predetermined time has been exceeded, the process proceeds to step 108, the lean correction flag FLEAN is reset to 0, the rich correction timer is turned on to start counting, and then the process returns.

In step 102, the lean correction flag FL
If it is determined that the EAN is 0, that is, the state where the EAN should be corrected to the rich side, the process proceeds to step 110. Step 11
At 0, the lean burn (F / B) target air-fuel ratio AF / T is set to 1
A predetermined amount per interrupt is corrected to the rich side. Next, the routine proceeds to step 112, where it is determined whether or not the rich side correction timer count time has passed a predetermined time. If not, the routine returns to continue the rich side correction,
When the time has elapsed, the routine proceeds to step 114, where the lean correction flag FLEAN is set to 1 and the lean side correction timer starts counting.

As shown in FIG. 9, the target air-fuel ratio is divided into the rich side and the lean side with the target air-fuel ratio calculated in step 56 of FIG. 7 as the center by the lean burn target air-fuel ratio changing means of FIG. , And are forcibly changed alternately. Then, when the lean correction flag FLEAN is 1, it is changed to the lean side, and when FLEAN is 0, it is changed to the rich side.

As described above, the target air-fuel ratio is alternately changed to the rich side and the lean side. By swinging the target air-fuel ratio, a transient state of the air-fuel ratio is created, and the temporary NOx purification rate improvement of FIG. 3 is obtained. By repeating this, the NOx is leaned over the entire operating time.
Purification rate is increased.

[0036]

According to the present invention, N containing rare earth elements such as La in a large amount of 0.03 mol / lcat as compared with the prior art.
An Ox catalyst is placed in the exhaust passage of the internal combustion engine, and downstream of that,
Since a three-way catalyst is installed, even if the air-fuel ratio changes from lean to stoichiometric and NOx is released from the NOx catalyst, it can be sufficiently purified by the three-way catalyst, so NOx is reduced in all operating conditions.
Emissions can be suppressed.

[Brief description of drawings]

FIG. 1 is a system diagram of an exhaust gas purification device for an internal combustion engine according to a first embodiment of the present invention.

FIG. 2 is a layout view of a NOx catalyst and a three-way catalyst according to the first embodiment of the present invention.

FIG. 3 is a NOx concentration vs. time characteristic diagram of the La-based catalyst when the air-fuel ratio changes.

FIG. 4 is a NOx purification rate versus air-fuel ratio characteristic diagram of various catalysts.

FIG. 5 is a system diagram of an exhaust gas purifying apparatus for an internal combustion engine according to a second embodiment of the present invention.

FIG. 6 is a control flowchart of air-fuel ratio control in the second embodiment of the present invention.

FIG. 7 is a map used in the flowchart of FIG. 6 when determining a basic fuel injection amount from engine operating conditions.

FIG. 8 is a flowchart of a lean combustion target air-fuel ratio variation subroutine in a second embodiment of the present invention.

FIG. 9 is a time chart of changes in the target air-fuel ratio in the second embodiment of the present invention.

[Explanation of symbols]

 2 Internal combustion engine 4 Exhaust passage 6 NOx catalyst 8 Three-way catalyst

Front Page Continuation (72) Inventor Etuta Inuta Toyota City, Toyota, Aichi Prefecture, 1st Toyota Motor Co., Ltd. (72) Inventor Hidetaka Nohira, Toyota City, Aichi Prefecture, Toyota City, 1st Toyota Motor Co., Ltd. (72) Inventor Kiyo Nakanishi, Toyota City, Toyota City, Aichi Prefecture, Toyota Motor Co., Ltd. (72) Inventor, Satoshi Iguchi Toyota City, Aichi Prefecture Toyota City, Toyota City Co., Ltd. (72) Inventor, Tetsuro Kihara Toyota City, Aichi Prefecture 1st Toyota Town Toyota Motor Co., Ltd. (72) Inventor Hideaki Muraki 1st 41st Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture 1 Toyota Central Research Institute Co., Ltd.

Claims (1)

[Claims] 1. An internal combustion engine capable of combusting in a lean air-fuel ratio region and its exhaust passage, and a rare earth element installed in said exhaust passage, carrying a rare earth element at a ratio of 0.03 mol / lcat or more and a platinum. NOx in the exhaust gas in the air-fuel ratio range
An exhaust purification device for an internal combustion engine, comprising: a NOx catalyst capable of purifying the exhaust gas; and a three-way catalyst installed in a portion of the exhaust passage on the downstream side of the NOx catalyst.