JPH04365920A - Exhaust purifying method - Google Patents

Abstract

PURPOSE:To enhance purifying efficiency by converting nitrogen oxides (NOx) exhausted from cylinders operated under an excessive fuel condition into NH3, merging thereto exhaust air exhausted from the cylinders respectively operated under the excessive and lean fuel conditions so as to purify NOx and NH3 into N2 with a second catalyst. CONSTITUTION:An ECU 2 judges that a first cylinder is operated under a lean fuel condition on the basis of a detection signal output from an oxygen sensor 16 disposed in the first cylinder of a multiple cylinder internal combustion engine 1. In this case, the ECU 2 first increases fuel injected from electronic injection valves 17 disposed on a branch pipe of an intake branch pipe 15 communicated with an intake port of the first cylinder. Meanwhile, second and fourth cylinders are operated under the lean fuel condition. Next, NOx exhausted from a cylinder operated under an excessive fuel condition is converted into NH3 with a first catalyst 3. Subsequently, exhaust air exhausted from the cylinders respectively operated under the excessive and lean fuel conditions is merged in an exhaust manifold 6, and then, NOx and NH3 in an exhaust confluence are purified into N2 with a second catalyst 4.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust purification method for purifying exhaust gas consisting of NOx, CO, HC, etc. discharged from a multi-cylinder internal combustion engine such as an automobile. More specifically, exhaust gas purification that can purify exhaust gas consisting of NOx, CO, HC, etc. with a high purification rate, and also allows a multi-cylinder internal combustion engine to operate under fuel lean conditions (that is, improves the fuel efficiency of the multi-cylinder internal combustion engine) Regarding the method.

0002

[Prior art] N emitted from internal combustion engines such as automobiles
BACKGROUND ART There has been a desire for an exhaust purification method that can efficiently purify exhaust gas consisting of harmful components such as Ox, CO, and HC, and can also reduce the fuel consumption of internal combustion engines, and various methods have been developed. For example, JP-A-61-279749
The publication discloses a so-called feedback control method using a three-way catalyst and an oxygen sensor as a method for efficiently purifying NOx, CO, HC, etc.

This feedback control method uses an oxygen sensor to detect the oxygen concentration in the exhaust gas, and precisely adjusts the air-fuel ratio of the fuel to the chemical equivalence point (ie, the air-fuel ratio, A/F=14.
6), the fuel injection amount, air supply amount, etc. are controlled so that the exhaust gas is treated with precious metals (for example, Pt, Rh, P
The purpose is to purify harmful components in the exhaust gas by contacting it with a three-way catalyst carrying d, etc.). In this feedback control method, the air-fuel ratio is controlled within a range of ±0.05 around the chemical equivalence point, and NO
x, CO, and HC are purified.

0004

The feedback control method described above is theoretically the best method in terms of purifying harmful components. However, in practice, there are the following problems. In other words, ■The biggest problem is to accurately adjust the air-fuel ratio of the exhaust gas to the chemical equivalence point (i.e., A/F = 14.6 ± 0.05).
Therefore, it is not possible to reduce the mixing ratio of fuel to air. In other words, it is not possible to reduce the fuel consumption of the internal combustion engine. ■Since the oxygen concentration in the exhaust gas must be detected using an oxygen sensor and the air-fuel ratio of the fuel must be precisely controlled as described above, an oxygen sensor,
High precision is required for equipment that controls the amount of fuel injection and air supply. Therefore, the control device becomes expensive. For example, in a transient state such as from a fuel surplus region to a fuel lean region, the air-fuel ratio deviates significantly from the chemical equivalence point, and there is a possibility that harmful components cannot be purified. Furthermore, the three-way catalyst requires rare and expensive Rh as a resource.

[0005] Of the above two problems, problem (■) is solved,
In addition, in order to reduce fuel consumption, there is a lean burn system that reduces the amount of fuel supplied and increases the amount of air supplied. but,
If the air-fuel ratio is made larger than 14.6, the three-way catalyst containing Rh will no longer be able to purify NOx, as shown in FIG. Further, as a method for purifying NOx discharged from an internal combustion engine operated in a lean burn mode, there are the following methods. Specifically, (a) a method of introducing ammonia into the exhaust gas and reducing NOx to N2 using a V-Ti catalyst, and (b) a method of reducing NOx to N2 using a Cu-zeolite catalyst. Are known. However, in method (a),
Harmful ammonia must be stored and ammonia must be supplied in an amount commensurate with the amount of NOx being emitted. Therefore, method (a) is inappropriate for purifying NOx emitted from moving exhaust sources such as automobiles. Further, depending on the method (a), CO and HC, which are emitted at the same time as NOx, cannot be sufficiently purified. on the other hand,
In method (b), NOx is reduced by HC contained in the exhaust gas.
However, it has the following problems. That is, the NOx purification rate depends on the HC concentration in the exhaust gas. Furthermore, H
C must be HC having 2 or more carbon atoms. Therefore, if the combustion method is improved, HC in the exhaust will inevitably be
The concentration decreases, and the NOx purification rate decreases. Also,
If gasoline substitute fuels such as methanol, hydrogen, natural gas, etc. are used, method (b) becomes ineffective.

The object of the present invention is to solve the problems of the prior art described above. That is, an object of the present invention is to provide an exhaust gas purification method that can purify NOx, CO, and HC in exhaust gas at a high purification rate and that can operate a multi-cylinder internal combustion engine under fuel lean conditions. Another object of the present invention is to provide an exhaust gas purification method that can purify exhaust gas easily and inexpensively without using Rh, which is rare and expensive as a resource.

[0007]

[Means for Solving the Problems] The exhaust gas purification method of the present invention operates some of the cylinders of a multi-cylinder internal combustion engine under a slightly excess fuel condition, and the remaining cylinders are operated under a fuel lean condition. and at least N in the exhaust gas discharged from the cylinder operated under the excess fuel condition.
After a second step of converting Ox to NH3 with a first catalyst, and combining the exhaust gas that has passed through the second step with the exhaust gas discharged from the cylinder operated under the fuel lean condition to form combined exhaust gas,
This exhaust gas purification method is characterized by comprising a third step of purifying NOx and NH3 in the combined exhaust gas into N2 using a second catalyst.

In the first step of the exhaust purification method of the present invention, some of the cylinders of the multi-cylinder internal combustion engine are operated under a slightly excess fuel condition, and the remaining cylinders are operated under a fuel lean condition. Driving. In addition, in order to improve fuel efficiency by operating the multi-cylinder internal combustion engine as a whole under fuel-lean conditions, less than half of the cylinders of the multi-cylinder internal combustion engine are operated under slightly excess fuel conditions. , the remaining cylinders are preferably operated under fuel lean conditions. The reason why a multi-cylinder internal combustion engine is operated in this way is that, as shown in FIG. , A/F=18). Here, the slightly excess fuel condition means that the A/F value is in the range of 14.0 to 14.6. It is practical in converting NOx in the exhaust gas discharged from the cylinders operated below into NH3 with high efficiency.

[0009] As a method for operating a multi-cylinder internal combustion engine as a whole under fuel lean conditions, the following method of electronically controlling the oxygen sensor and the fuel injection valve can be considered, but the present invention is limited to this method. It's not a thing. In other words, an oxygen sensor is installed in at least one of the exhaust pipes communicating with the exhaust port of a cylinder that is to be operated under a slightly excess fuel condition, and its output (i.e., electromotive force) is sent to an engine control device consisting of a microcomputer. Monitor. If the output of the oxygen sensor is high, the cylinder communicating with that exhaust pipe is under fuel excess conditions (
In other words, it is operated in a reducing atmosphere). Also, if the output of the oxygen sensor is low, the cylinder communicating with that exhaust pipe is under lean fuel conditions (i.e., in an oxidizing atmosphere).
This means that it is being driven by By monitoring the oxygen sensor with the engine control device in this manner, the operating conditions of the cylinders to be operated under fuel lean conditions can be calculated from the operating conditions of cylinders operated under slightly excess fuel conditions.

First, fuel injection control in the cylinders operated under the slightly excess fuel condition is performed as follows. In other words, the fuel injection amount from the fuel injection valve of the cylinder operated under fuel excess conditions is increased by a predetermined amount, and then the injection amount is reduced from the increased injection amount at a predetermined rate and at a predetermined rate. The fuel injection valve is controlled so that the fuel injection valve is Then, when it is determined that the A/F has become lean based on the output signal of the oxygen sensor, the injection amount is immediately returned to the amount increased by the predetermined amount. By controlling the repetition of these steps, on average the cylinder will be operated under slightly overfuel conditions. At this time, during such a control process, there is a possibility that the fuel will be under lean conditions for a very short time, but CeO2 is added to the first catalyst described below.
By adding a cocatalyst capable of storing excess oxygen, such as ZrO2, this fear can be virtually eliminated.

Next, fuel injection control in cylinders operated under fuel lean conditions is performed as follows. That is, the engine control device calculates the fuel injection amount that corresponds to the chemical equivalence point based on the output signal of the oxygen sensor, and reduces the injection amount by a predetermined percentage from the calculated injection amount under fuel lean conditions. This is the injection amount of the cylinder being operated. By operating with the injection amount calculated to be reduced in this manner, these cylinders are operated under fuel lean conditions. At this time, it is preferable that this predetermined percentage reduction is 15 to 50% of the fuel injection amount required to reach the chemical equivalence point.

[0012] In the second step of the exhaust gas purification method of the present invention, at least NOx in the exhaust gas discharged from the cylinders operated under fuel excess conditions is converted into NH3 by the first catalyst. Therefore, the first catalyst absorbs most of the NO in the exhaust gas.
It has the ability to convert x to NH3 with high efficiency. Normally, the exhaust gas emitted from the cylinders of multi-cylinder internal combustion engines such as automobiles also contains CO, HC, etc., so the first catalyst has the ability to oxidize and purify CO, HC, etc. in the exhaust with high efficiency. It may also have The first one that acts like this
The catalyst can be configured to include a normal carrier and a noble metal catalyst supported on the carrier. for example,
The first catalyst can be configured to include activated alumina and at least one of Pt and Pd supported on the activated alumina.

[0013] As shown by the shaded area in Fig. 3, the conventional three-way catalyst containing Rh is used under conditions of slight excess fuel, that is, as described above, when the A/F is 14.0 to 14.6. In purifying exhaust gas emitted from cylinders operated within this range, the conversion rate of NOx to N2 is high, and NH3 is
cannot be converted to with high efficiency. Therefore, R
It is not preferable to include h. In addition, as shown in FIG. 4, a noble metal catalyst (i.e., the first
The catalyst (catalyst) most efficiently converts NOx to NH3 in purifying exhaust gas discharged from cylinders operated within the A/F range of 14.0 to 14.6. A/F is 14.0
If the NOx concentration is less than 1, as shown in FIG.
The catalyst cannot convert NOx to NH3 efficiently, and therefore cannot generate enough NH3. Further, when the A/F exceeds 14.6, as can be seen from FIG. 4, the first catalyst hardly generates NH3, so NOx cannot be purified in the third step described below. Due to the characteristics of the first catalyst, some of the cylinders of the multi-cylinder internal combustion engine are heated under slightly excessive fuel conditions (14.0≦A/F≦) in the first step as described above.
14.6). Additionally, depending on how the cylinders are controlled when operated under slightly overfuel conditions,
As mentioned above, there is a possibility that the fuel lean condition will occur only for a very short period of time. When such a control method is employed, a co-catalyst having the ability to store excess oxygen, such as CeO2 or ZrO2, may be added to the first catalyst.

In the third step of the exhaust gas purification method of the present invention, the exhaust gas that has passed through the second step and the exhaust gas discharged from the cylinder operated under fuel lean conditions are combined to form combined exhaust gas, and then,
NOx and NH3 in the combined exhaust gas are purified to N2 by the second catalyst. Therefore, the second catalyst
It has the ability to purify Ox by reducing it to N2 using NH3. The second catalyst that acts in this way is a NOx
If only purification is considered, a normal denitrification catalyst can be used. However, in a multi-cylinder internal combustion engine, exhaust gas is not discharged from each cylinder at the same time, but is discharged from each cylinder in a predetermined order. In other words, there are almost no conditions under which NOx and NH3 are emitted at the same time. Therefore, the second catalyst preferably has the ability to temporarily store NH3. In addition, the combined exhaust gas contains CO, HC, etc. that could not be oxidized and purified by the first catalyst, as well as CO, HC, etc. in the exhaust gas discharged from cylinders operated under fuel lean conditions. There is. Therefore, it is preferable that the second catalyst also has the ability to oxidize and purify CO, HC, etc. in the combined exhaust gas with high efficiency under fuel lean conditions. Further, the second catalyst preferably has the ability to adapt to various operating conditions of a multi-cylinder internal combustion engine, that is, has high temperature stability.

[0015] As the second catalyst as described above, a high silica-containing zeolite (for example, SiO2 and Al2O3
It is preferable to use a material whose content ratio (SiO2 /Al2 O3) is 20 or more in molar ratio. Such high-silica-containing zeolites contain NH3 in their structure.
It has many strong acid sites that can adsorb . especially,
High silica containing zeolites such as type ZSM-5, ferrierite, mordenite are preferred. Also, Co, Cu,
Ni, Fe, etc. may be supported on the zeolite. By supporting these metals on high-silica-containing zeolite, it is possible to cause the HC and NOx in the combined exhaust to react, so when gasoline is used as fuel, HC and NOx can be purified at a higher purification rate. can. Note that this second catalyst is used under the above-mentioned fuel lean conditions (i.e., 1 for the fuel injection amount required to reach the chemical equivalence point).
Under lean fuel conditions with 5-50% weight loss or A/
It is preferable to use one that efficiently acts on exhaust gas discharged from cylinders operated at F=17 or higher.

Furthermore, depending on the fuel used, in other words, when using a gasoline alternative fuel that contains components that require more oxidation reactions, such as methanol, which easily generates harmful formaldehyde, and natural gas, which consists of methane, which is difficult to oxidize. In the exhaust gas purification method of the present invention, a third catalyst having the ability to oxidize and purify aldehydes, methane, etc. at a high purification rate is further disposed at the exhaust port communicating with the cylinder operated under fuel lean conditions. may be set. Such a third catalyst can be configured to include a normal carrier and a noble metal catalyst or metal oxide catalyst supported on the carrier. For example, the third catalyst may include activated alumina and at least one of Pt and Pd supported on the activated alumina, or activated alumina and La-Sr-Co- supported on the activated alumina.
It can be configured to include an O-based or Cu-Sr-Co-O-based metal oxide catalyst. In addition, when using gasoline alternative fuel as mentioned above, CO, HC
In order to oxidize and purify the exhaust gas at a high purification rate, the third catalyst is preferably disposed under as high a temperature as possible, specifically, on the upstream side of the exhaust flow (that is, as close to the multi-cylinder internal combustion engine as possible).

[0017] When hydrogen is used as fuel, the third catalyst is not necessary because there are no harmful components in the exhaust gas that should be purified by the third catalyst. Furthermore, when gasoline is used as fuel, the third catalyst may not be necessary depending on the operating conditions of the multi-cylinder internal combustion engine. In other words, when gasoline is used as fuel, in the third step, the second catalyst
3 is reacted with NOx to purify it into N2, and in parallel with this reaction of NH3 and NOx, the second catalyst reacts with NOx and HC to simultaneously purify NOx and HC. , it is possible to purify NOx with high efficiency. However, in this case, there are conditions in which a portion of HC is converted to CO, so it is necessary to install a third catalyst in consideration of which of NOx, CO, or HC needs to be purified the most. It is better to decide whether there is or not.

[0018]

[Operations and effects of the invention] First of the exhaust gas purification method of the present invention
In the process, some of the cylinders of a multi-cylinder internal combustion engine are operated under a slightly excess fuel condition, and the remaining cylinders are operated under a fuel lean condition. Therefore, if less than half of the cylinders of a multi-cylinder internal combustion engine are operated under slightly fuel-rich conditions and the remaining cylinders are operated under fuel-lean conditions, the multi-cylinder internal combustion engine as a whole can be It is more preferable because it can be operated under lean conditions and fuel consumption can be reduced.

In the second step of the exhaust gas purification method of the present invention, the first catalyst converts most of the NOx in the exhaust gas discharged from the cylinders operated under slightly excess fuel conditions into NH3 with high efficiency. ing. Therefore, the exhaust gas that has passed through this second step contains NH3 and trace amounts of NOx, CO, and HC.
The exhaust gas is converted to an exhaust gas consisting of Furthermore, depending on the first catalyst used, CO, HC, etc. in the exhaust gas can be oxidized and purified with even higher efficiency.

In the third step of the exhaust gas purification method of the present invention, the exhaust gas that has passed through the second step and the exhaust gas discharged from the cylinder operated under fuel lean conditions are combined to form combined exhaust gas, and then,
NOx and NH3 in the combined exhaust gas are purified to N2 by the second catalyst. As mentioned above, if less than half of the cylinders of a multi-cylinder internal combustion engine are operated under a slightly excess fuel condition and the remaining cylinders are operated under a fuel lean condition, the combined exhaust gas will be This results in fuel lean conditions (ie, an oxidizing atmosphere).

[0021] Here, under fuel lean conditions, NO
The reduction reaction of x (for example, NO) with NH3 can be expressed by the following chemical reaction formula. NO+NH3 + (1/4)O2 →N2 +
(3/2) H2O...(1) That is, in the third step of the exhaust gas purification method of the present invention, (1)
It contains a sufficient amount of NH3 for the reduction reaction of Eq. Therefore, the molar ratio of NO and NH3 in the combined exhaust gas is 1:1.
The second catalyst converts the gas into harmless N2 with high efficiency. Note that if HC with a carbon number of 2 or more is present in the combined exhaust gas, a portion of NO is reduced to N2 by the HC. This reduction reaction can be expressed by the following chemical reaction formula.

[0022] NO+HC→N2 +H2 O+CO2...(2)
As is clear from the above description, in the exhaust gas purification method of the present invention, the first catalyst used in the second step is capable of handling a large amount of NOx in the exhaust gas discharged from cylinders operated under slightly excess fuel conditions. This part is converted to NH3 with high efficiency, and a sufficient amount of NH3 source is used for the reaction of formula (1). Furthermore, the second catalyst used in the third step was the NH3 obtained in the second step.
and N emitted from cylinders operated under fuel-lean conditions.
Ox and NO not converted to NH3 in the second step
x is reacted based on formula (1), and highly efficient and harmless N2
is being purified. Furthermore, if HC with a carbon number of 2 or more is present in the combined exhaust gas, the second catalyst will reduce the NOx in the combined exhaust gas.
A part of the water is purified to N2 based on equation (2). Therefore, the exhaust gas purification method of the present invention can purify NOx discharged from a multi-cylinder internal combustion engine with extremely high efficiency. On the other hand, in the conventional exhaust purification method, mainly (2
) based on the formula, the NOx
The purification rate was about 50% at most.

[0023] Furthermore, the second catalyst used in the third step removes CO and H remaining in the combined exhaust gas that has passed through the second step.
It also oxidizes and purifies CO and HC emitted from cylinders operated under fuel lean conditions with high efficiency, reducing the total amount of CO and HC in the exhaust emitted from multi-cylinder internal combustion engines. Can be done. In addition, if the second catalyst used in the third step has a large number of strong acid sites capable of adsorbing NH3 in its structure, it is possible to use a catalyst that has many strong acid sites capable of adsorbing NH3, such as in automobiles, where there are almost no conditions for NOx and NH3 to be emitted at the same time. Even with a moving exhaust gas source, there is no need to prepare a separate NH3 source for NOx purification.

The present invention can be achieved by disposing a third catalyst capable of oxidizing and purifying CO and HC at a high purification rate at the exhaust port of the exhaust pipe communicating with the cylinder operated under fuel lean conditions. The exhaust gas purification method can also purify exhaust gas emitted from multi-cylinder internal combustion engines that use gasoline alternative fuels such as methanol and natural gas at a high purification rate.

The exhaust gas purification method of the present invention operates as described above in detail, so that it can produce the following effects. First, exhaust gas consisting of NOx, CO, HC, etc. can be purified at a high purification rate, and the multi-cylinder internal combustion engine as a whole can be operated under fuel lean conditions. That is, it is possible to reduce the fuel consumption of automobiles and the like without adversely affecting the environment. Second, since there is no need to use rare and expensive Rh as a resource, exhaust gas can be purified at low cost. Third, fewer oxygen sensors are required compared to conventional feedback control. The engine control device then monitors a small number of oxygen sensors and simply controls the fuel injection valves based on the monitoring results, making it possible to operate the multi-cylinder internal combustion engine as a whole under fuel-lean conditions. . Therefore, the engine control device and its related mechanisms can be made simple and inexpensive.

[0026]

【Example】

(First Embodiment) Hereinafter, an embodiment of the present invention will be described in FIGS. 1 and 2.
This will be explained with reference to. First, a four-cylinder automobile engine with a total displacement of 2,000 cc, equipped with an electronic fuel injection valve, was prepared. This engine was modified as shown in Figure 1. First, the four cylinders of the engine 1 are classified into the first cylinder, the second cylinder, the third cylinder, and the fourth cylinder, and the exhaust pipes communicating with their exhaust ports are the first exhaust pipe 11, the second exhaust pipe 12,
It was divided into a third exhaust pipe 13 and a fourth exhaust pipe 14. Note that the branch pipes of the intake manifold 15 communicate with the intake ports of the first to fourth cylinders, respectively.

An oxygen sensor 16 was provided only in the first exhaust pipe 11. This oxygen sensor 16 is connected to an engine control device (hereinafter referred to as ECU) 2 and outputs the oxygen concentration in the exhaust gas in the first exhaust pipe 11 to the ECU 2. EC
U2 connects the intake manifold 15 according to the output of the oxygen sensor 16.
The fuel injection amount of the electronic fuel injection valve 17 arranged at each branch is controlled. Therefore, the operating conditions for the second to fourth cylinders are calculated from the operating conditions for the first cylinder.

The first cylinder is controlled as follows:
Operated under slightly overfuel conditions. That is,
As shown in FIG. 2, based on the output of the oxygen sensor 16, E
When the CU 2 determines that the first cylinder is operating under fuel lean conditions, the ECU 2 determines that the electronic fuel injection valve 17 installed in the branch pipe of the intake manifold 15 communicating with the intake port of the first cylinder is activated. Control is performed to immediately inject a 5% increased amount of fuel. Then, until the ECU 2 again determines that the first cylinder is being operated under fuel lean conditions, the ECU 2
The electronic fuel injection valve 17 is controlled to inject a reduced amount of fuel at a rate of 3%/second. Also, the second to fourth
The cylinders are operated under fuel lean conditions based on the output of the oxygen sensor 16. That is, the ECU 2 calculates the fuel injection amount that will reach the chemical equivalence point from the output of the oxygen sensor 16, and calculates the amount of fuel that is reduced by 35% for each cylinder (i.e., the amount of fuel injected into the first cylinder). almost 65 in quantity
%) is controlled so that the electronic fuel injection valve 17 disposed in the branch pipe of the intake manifold 15 communicating with the intake ports of the second to fourth cylinders injects the reduced amount of fuel. .

The average A/F value calculated from the exhaust gas discharged from the first cylinder controlled in this manner is 14.3, and from the output waveform of the oxygen sensor 16, 92% of the first cylinder is under the excess fuel condition. there were. Note that the operating conditions of the engine 1 at this time were a rotation speed of 2000 rpm and a torque of 30 Nm. Then, NOx in the exhaust discharged from the first cylinder, TH
The average concentration of C and CO is 2100 ppm each
, 2700ppm, 3.4%, and the average concentrations of NOx, THC, and CO in the exhaust gas discharged from the second to fourth cylinders are 540ppm, 3200ppm, and 0.11%, respectively.
%Met. In addition, the first catalyst 3 and the second catalyst described below
NOx, TH in total exhaust when catalyst 4 is not installed
The average concentration of C and CO was 930 ppm, respectively.
It was 3080 ppm, 0.93%.

[0030]The first exhaust pipe 1 communicates with the first cylinder.
The first catalyst 3 prepared as follows was placed at the discharge port of No. 1. First, a cylindrical honeycomb substrate with a capacity of 0.5 liters was prepared as the substrate of the first catalyst 3. The cylindrical honeycomb substrate was made of 400 mesh cordierite. Next, 80 g of gamma alumina was wash coated onto the cylindrical honeycomb substrate. lastly,
A first catalyst 3 was prepared by supporting Pt as a noble metal catalyst on a cylindrical honeycomb substrate wash-coated with γ-alumina. Furthermore, after the first catalyst 3 prepared in this way was equipped as described above, the average concentration of NOx and NH3 in the exhaust gas at the exhaust port of the first catalyst 3 was measured, and the NOx concentration was 280 ppm. Ta. In addition, the concentration of NH3 was 1090 ppm, and it was found that a sufficient amount of NH3 was generated to reduce and convert NOx to N2 based on the above equation (1), and the remaining 730 ppm of NOx was generated. was reduced and converted to N2. In this way, NOx in the exhaust gas discharged from the first cylinder is converted into NH with high efficiency by the first catalyst 3.
It was found that it was converted to 3. Here, although the third catalyst 5 is illustrated in FIG. 1, the third catalyst 5 is omitted in the first embodiment. For this reason, the discharge port of the first catalyst 3 was connected to the middle of an exhaust branch pipe 6 in which the second to fourth exhaust pipes 12 to 14 were put together as a branch pipe. In this way, the exhaust gas that passed through the first catalyst 3 and the exhaust gas discharged from the second to fourth cylinders operated under fuel lean conditions were combined.

Furthermore, a second catalyst 4 prepared as follows was placed at the outlet of the exhaust branch pipe 6. First, as a base for the second catalyst 4, a honeycomb base having a capacity of 1.3 liters and having an oval cross section was prepared. This honeycomb substrate having an elliptical cross section was made of 400 mesh cordierite. Finally, 3% by weight of CuO was supported on 2
A second catalyst 4 was prepared by wash-coating 60 g of ZSM-5 type zeolite onto a honeycomb substrate having an oval cross section.

The engine 1 equipped with the exhaust purification device having such a configuration is operated under the above-mentioned operating conditions, and the second catalyst 4
The average concentrations of NOx, THC, and CO in the exhaust gas at the outlet of the vehicle were measured. As a result, the average concentrations of NOx, THC and CO were 180ppm and 120ppm, respectively.
pm, 0.07%. In this way, NOx, THC, and CO in the exhaust gas discharged from the engine 1 were purified at a high purification rate, and it was possible to significantly reduce these harmful substances from the exhaust gas. Note that the average concentration of NH3 in the exhaust gas at the exhaust port of the second catalyst 4 is 20 pp.
This was not a harmful concentration. The results of purification of harmful components in exhaust gas are summarized in Table 1, including those of Examples and Comparative Examples below. (Second Example) In the second example, an engine 1 equipped with an exhaust purification device having the same configuration as the first example is operated under the same operating conditions as the first example, and the exhaust gas from the engine 1 is This is the purified exhaust gas. However, in the second example, in addition to the same amount of precious metal catalyst metal Pt as the first catalyst 3 of the first example, a first catalyst 3 was prepared in which 0.2 mol of CeO2 was supported as a co-catalyst. Installed at the exhaust pipe outlet. Everything other than this change is the same as the first embodiment. Note that NOx in the exhaust at the exhaust port of the first catalyst 3
The average concentrations of NOx and NH3 were measured, and the NOx concentration was 70 ppm, and the NH3 concentration was 1430 ppm. Thus, it was found that the ability of the first catalyst 3 to reduce and convert NOx to NH3 was optimized by the oxygen storage ability of the cocatalyst CeO2.

As in the first embodiment, the engine 1 is operated, and the NOx and THC in the exhaust gas at the exhaust port of the second catalyst 4 are
and the average concentration of CO was measured. As a result, NOx,
The average concentration of THC and CO was 75 ppm each.
, 100 ppm, and 0.05%. In the second embodiment, NOx in the exhaust discharged from the engine 1 is
In particular, it was purified with a high purification rate. Furthermore, the purification rate of THC and CO could also be improved. Note that in the second embodiment as well, N in the exhaust gas at the exhaust port of the second catalyst 4
The average concentration of H3 was less than 20 ppm, which, as mentioned above, was not a harmful concentration. (Comparative Example 1) In Comparative Example 1, an engine 1 equipped with an exhaust purification device having a configuration in which the first catalyst 3 of the first example was removed was operated under the same operating conditions as the first example. This is the purified exhaust gas emitted from the Similarly to the first example, when the engine 1 was operated and the average concentrations of NOx, THC, and CO in the exhaust gas at the exhaust port of the second catalyst 4 were measured, they were 480 ppm and 310 ppm, respectively.
, 0.33%. In the purification of harmful components in exhaust gas according to Comparative Example 1, NO.
The purification rates of x, THC, and CO were all poor. (Comparative Example 2) In Comparative Example 2, an engine 1 equipped with an exhaust purification device having the same configuration as that of the first example is operated under the same operating conditions as the first example, and the exhaust gas emitted from the engine 1 is It is purified. However, in Comparative Example 2, instead of the noble metal catalyst Pt of the first catalyst 3 of the first example, a first catalyst supporting 0.2 g of Rh as a noble metal catalyst was used.
A catalyst 3 was prepared and placed at the exhaust port of the first exhaust pipe. Everything other than this change is the same as the first embodiment. In this comparative example 2, N in the exhaust gas at the exhaust port of the second catalyst 4
The average concentrations of Ox, THC, and CO were measured and were 350 ppm, 110 ppm, and 0.07%, respectively. The first catalyst 3 supporting Rh as a noble metal catalyst was able to effectively purify NOx in the exhaust gas discharged from the first cylinder operated under excessive fuel conditions.
The ability to convert NH3 to NH3 was inferior. Therefore, the exhaust gas that has been purified by the first catalyst 3 is the N in the exhaust gas discharged from the second to fourth cylinders operated under fuel lean conditions.
It is thought that this could not contribute to the purification of Ox. (Comparative Example 3) In Comparative Example 3, an engine 1 equipped with an exhaust purification device having a configuration in which a second catalyst 4 made of a Pt-titanium oxide catalyst was installed in place of the second catalyst 4 of the first example was used. The engine 1 was operated under the same operating conditions as in the first embodiment, and the exhaust gas emitted from the engine 1 was purified. Note that the amount of Pt supported was 2.1 g. Similarly to the first embodiment, the engine 1 is operated and the N in the exhaust gas at the exhaust port of the second catalyst 4 is
The average concentrations of Ox, THC and CO were measured;
They were 270 ppm, 40 ppm, and 0.02%, respectively. In this way, the second catalyst 4 used in Comparative Example 3 has N
When Ox and NH3 are introduced at the same time, NOx is reduced to 90
Although the conversion was possible with a high efficiency of more than %, in the purification of harmful components in the exhaust gas according to Comparative Example 3,
Compared to that of the first embodiment, the NOx purification rate is deteriorated,
Moreover, the average concentration of NH3 in the exhaust gas at the outlet of the second catalyst 4 was as high as 170 pm. This is because the second catalyst 4 made of Pt-titanium oxide catalyst used in Comparative Example 3 had
This is thought to be due to the lack of ability to store NH3 and poor NOx purification ability under fuel lean conditions. (Third Embodiment) In a third embodiment, an engine 1 equipped with an exhaust purification device having the same configuration as that of the first embodiment and further provided with a third catalyst 5 is used. The engine 1 is operated under similar operating conditions and the exhaust gas emitted from the engine 1 is purified. That is, as shown in FIG. 1, in the third embodiment, the third catalyst 5 is disposed in the middle of an exhaust branch pipe 6 in which the second to fourth exhaust pipes 12 to 14 are grouped together as a branch pipe. Further, the discharge port of the first catalyst 3 was connected to the vicinity of the discharge port of the exhaust branch pipe 6, and the discharge port of the exhaust branch pipe 6 was connected to the intake port of the second catalyst 4. In this way, the exhaust gas that had passed through the first catalyst 3 and the exhaust gas that had passed through the third catalyst 5 were combined. Note that as the third catalyst 5, a catalyst was used in which the noble metal catalyst Pt of the first catalyst 3 of the first embodiment was replaced with Pd. Here, the amount of Pd supported was 1.6 g.
The other configurations were the same as the first catalyst 3 of the first example.

As in the first embodiment, the engine 1 is operated, and the NOx and THC in the exhaust gas at the exhaust port of the second catalyst 4 are
and the average concentration of CO was measured. As a result, NOx,
The average concentration of THC and CO is 130 ppm, respectively.
It was 30 ppm and 0%. NO in the exhaust gas discharged from the engine 1 is reduced by the third catalyst 5 used in the second embodiment.
The purification rate of harmful components such as x, THC, and CO, especially TH
It can be seen that the values of C and CO could be further improved. Similar to the first embodiment, in the third embodiment,
The average concentration of NH3 in the exhaust gas at the outlet of the second catalyst 4 was 20 ppm or less, which caused no actual damage. (Fourth Example) In the fourth example, the engine 1 equipped with the exhaust purification device having the same configuration as the first example uses a mixed fuel consisting of 50% methanol and the balance gasoline instead of gasoline. The engine 1 was operated under the same operating conditions as in the first embodiment, and the exhaust gas discharged from the engine 1 was purified.

As in the first embodiment, the engine 1 is operated, and the NOx and THC in the exhaust gas at the exhaust port of the second catalyst 4 are
and the average concentration of CO was measured. As a result, NOx,
The average concentration of THC and CO was 150 pp each.
m, 80 ppm, and 0.11%. As is clear from this result, even when mixed fuel is used instead of gasoline, the exhaust purification method of the present invention can remove harmful substances such as NOx, THC, and CO in the exhaust gas discharged from the engine 1. It can be seen that it was possible to purify with a high purification rate. Note that the average concentration of formaldehyde in the exhaust gas at the outlet of the second catalyst 4 was 50 ppm, probably because the formaldehyde purification ability of the second catalyst 4 was slightly inferior. (Fifth Example) In the fifth example, an engine 1 equipped with an exhaust purification device having the same configuration as the third example uses a mixed fuel consisting of 50% methanol and the balance gasoline instead of gasoline. The engine 1 was operated under the same operating conditions as in the third embodiment, and the exhaust gas discharged from the engine 1 was purified.

Similarly to the third embodiment, the engine 1 is operated, and the NOx and THC in the exhaust gas at the exhaust port of the second catalyst 4 are
and the average concentration of CO was measured. As a result, NOx,
The average concentration of THC and CO is 120 ppm, respectively.
It was 25 ppm and 0.03%. Compared to the fourth example, the purification rate of THC and CO was improved in the fifth example. Furthermore, the average concentration of formaldehyde in the exhaust gas at the outlet of the second catalyst 4 was reduced to 8 ppm. This is a third exhaust pipe disposed in the middle of an exhaust branch pipe 6 in which the second to fourth exhaust pipes 12 to 14 are grouped together as a branch pipe.
It is clear that the effect was due to catalyst 5.

[0037]

[Table 1]

[Brief explanation of drawings]

FIG. 1 is a block diagram of an exhaust gas purification device used in the exhaust gas purification method of the present invention.

FIG. 2 is a timing chart illustrating a control method for operating the first cylinder of an engine under a slightly excess fuel condition, which is used in the exhaust gas purification method of the present invention.

FIG. 3 is a purification characteristic curve showing the relationship between the concentration of harmful components in exhaust gas and the air-fuel ratio when a conventional three-way catalyst containing Rh is used.

FIG. 4 is a characteristic curve for purifying harmful components in exhaust gas exhibited by a conventional noble metal-based catalyst that does not contain Rh.

FIG. 5 is a characteristic curve showing the relationship between the concentration of harmful components in exhaust gas and the air-fuel ratio.

[Explanation of symbols]

1: Engine, 11: First exhaust pipe, 12: Second exhaust pipe,
13: third exhaust pipe, 14: fourth exhaust pipe, 15: intake manifold, 16: oxygen sensor, 17: electronic fuel injection valve, 2: engine control device, 3: first catalyst, 4: second catalyst, 5: Third catalyst, 6: Exhaust manifold

Claims (1)

[Claims] 1. A first step of operating some of the cylinders of a multi-cylinder internal combustion engine under a slightly excess fuel condition and operating the remaining cylinders under a fuel lean condition; and the above fuel excess condition. a second step of converting at least NOx in the exhaust gas discharged from the cylinders operated under the lower fuel lean condition into NH3 by a first catalyst; 1. A method for purifying exhaust gas, comprising: merging the combined exhaust gas to form combined exhaust gas, and then purifying NOx and NH3 in the combined exhaust gas into N2 using a second catalyst.