JPH07194941A - Exhaust gas purification apparatus for vehicle - Google Patents

Abstract

PURPOSE:To obtain an exhaust gas cleaning apparatus keeping conbustion efficiency of an internal combustion engine and with excellent durability. CONSTITUTION:Durability of an exhaust gas cleaning apparatus is improved by providing a Pd catalyst or a mixed catalyst of Pd and Rh with excellent durability to high temp. on the upstream side of a gas exhaust system and an exhaust gas cleaning apparatus with high cleaning rate to the exhaust gas of the internal combustion engine wherein a fuel is burnt by a theoretical air- fuel ratio is obtd. by providing a trimetal catalyst with excellent activity from low temp. to high temp. on the downstream side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle exhaust gas purifying apparatus for an internal combustion engine in a normal combustion state.

[0002]

2. Description of the Related Art When an internal combustion engine is burned (lean combustion) at a place thinner than the stoichiometric air-fuel ratio, not at the stoichiometric air-fuel ratio which is a normal combustion state, NO _X contained in the exhaust gas discharged from the internal combustion engine. It is known that NO _X increases and HC and CO decrease in the amount of HC, CO. Therefore, in the exhaust gas purifying apparatus that uses this lean combustion property, by controlling the combustion, the amounts of HC and CO can be made to values sufficiently satisfying the pollution regulations, and only NO _X is purified. The advantage is that you only have to do it.

As an exhaust gas purifying apparatus utilizing this lean combustion property, there is one disclosed in, for example, Japanese Patent Laid-Open No. 5-7737. That is, on the upstream side of the exhaust system of the internal combustion engine
Rh-based catalyst or Pd-based catalyst or a mixed catalyst thereof is provided, and a Pt-based catalyst is provided on the downstream side of the
In order to take advantage of the large NOx reduction activity of the system catalyst, on the other hand, in order to solve the drawback that N ₂ O increases when NOx is reduced by the Pt system catalyst, this N ₂
The CO in the exhaust gas that causes the generation of O is removed by the Rh-based catalyst or Pd-based catalyst or its mixed catalyst provided on the upstream side, which is accompanied by the production of N ₂ O, which is a drawback of the Pt-based catalyst. Instead, it reduces NOx by the HC contained in the exhaust gas.

Next, when the internal combustion engine is burned at the stoichiometric air-fuel ratio, NO contained in the exhaust gas discharged from the internal combustion engine.
It is known that the amounts of _X , HC and CO are all high values, and therefore purification of NO _X , HC and CO is required. Therefore, as an exhaust gas purifying apparatus for an internal combustion engine that burns at the stoichiometric air-fuel ratio, there is one disclosed in, for example, Japanese Patent Publication No. 5-47262. That is, this exhaust gas purification apparatus is provided with a mixed catalyst of Pt and Rh on the upstream side of the exhaust system of the internal combustion engine and a Pd catalyst on the downstream side thereof.

[0005]

In the above conventional exhaust gas purifying apparatus, the exhaust gas purifying apparatus for lean combustion has a problem that the combustion efficiency of the internal combustion engine is low. In addition, an Rh-based catalyst or Pd-based catalyst or a mixed catalyst thereof is provided on the upstream side (high temperature side) of the exhaust system of the internal combustion engine, and a Pt-based catalyst is provided on the downstream side (low temperature side) thereof is effective for lean combustion. However, when burning at the stoichiometric air-fuel ratio, NO
_{In order} to purify all of _X , HC and CO, a satisfactory purification rate cannot be obtained yet, and there is a problem to be improved. Next, in the above conventional exhaust gas purifying apparatus for an internal combustion engine that burns at a stoichiometric air-fuel ratio, the combustion efficiency of the internal combustion engine is good,
Generally, a mixed catalyst of Pt and Rh has a property of being weak against heat, and providing the mixed catalyst of Pt and Rh on the high temperature side has a problem in terms of durability of the catalyst.

The present invention provides an exhaust gas purifying apparatus which burns an internal combustion engine at a stoichiometric air-fuel ratio to maintain the combustion efficiency of the internal combustion engine and is excellent in purification and durability.

[0007]

Means for Solving the Problems According to the present invention for solving the above problems, in an exhaust gas purifying apparatus provided in an exhaust system of a vehicle, a Pd catalyst or a mixture of Pd and Rh is provided upstream of the exhaust system. The catalyst is provided, and the trimetal catalyst is provided on the downstream side.

[0008]

Since the present invention is constructed as described above, it has the following effects. That is, the durability of the exhaust gas purification device is improved by providing a Pd catalyst or a mixed catalyst of Pd and Rh that has excellent durability against high temperatures on the upstream side of the exhaust system, and it has excellent activity from low temperature to high temperature on the downstream side. By providing the trimetal catalyst, it becomes possible to obtain an exhaust gas purifying apparatus having a high purification rate for exhaust gas of an internal combustion engine that burns at a stoichiometric air-fuel ratio.

[0009]

EXAMPLE An example of the present invention will be described below. 1 and 2 schematically show an exhaust system of an internal combustion engine, in which 1 is an engine and 2 is an exhaust manifold. In the example shown in FIG. 1, a Pd catalyst or a mixed catalyst 3 of Pd and Rh is provided immediately after the exhaust manifold 2, and a trimetal catalyst (Pd / Pt / Rh) 4 is provided under the floor of the vehicle, as shown in FIG. Examples are Pd catalysts or
A mixed catalyst 3 of Pd and Rh and a trimetal catalyst (Pd / Pt /
Rh) 4 are provided close to each other.

FIG. 3 shows durability tests of various exhaust gas purifying apparatuses. The durability conditions in this experiment are as follows. That is, the high temperature side (first stage) close to the engine 1 and the engine 1
The endurance conditions were divided into the low temperature side (second step) away from. The first stage durability condition was a temperature of 1200 K for 50 hours, and the second stage durability condition was a temperature of 1100 K for 50 hours.

The operating mode as a test method is the American LA-4 mode which employs a transient mode including an acceleration / deceleration operating state in which the vehicle actually travels in a city, and is performed from the engine start time (cold). It was As the analysis method, the direct sampling method is used, the exhaust gas exhausted from the engine alone is branched into a plurality of pipelines, and the exhaust gas is evenly distributed. The second stage was set according to the endurance condition, and the endurance test of each exhaust gas purifying device was simultaneously performed under the same condition. A chassis dynamo was used as the test device. The catalyst used in the test was a monolith type, and the same catalyst was used in each test. The numerical values shown in FIG. 3 are average purification rates from cold to hot.

[Comparison 1] As can be seen from FIG. 3, in comparison of sample numbers 1 and 4 using the same catalyst Pd / Rh in the first stage, the one using the Pd / Pt / Rh catalyst in the second stage was compared. Is Pt /
Compared with the case of using Rh catalyst, the purification rate is superior in all of HC, CO, and NO _X , and the Pd / Rh + Pd / Pt / Rh exhaust gas purification device is better than the Pd / Rh + Pt / Rh exhaust gas purification device. It can be said that the durability is excellent from cold to hot. What can be said in this comparison is that, as an exhaust gas purification device for an internal combustion engine with stoichiometric air-fuel ratio combustion, it is better to place a Pd / Pt / Rh catalyst in the second stage in order to obtain a high purification rate from cold to hot. I understand.

[Comparison 2] Next, the same catalyst Pd / Pt is used in the second stage.
In the comparison of sample numbers 1 and 3 using / Rh, the same purification rate was obtained for NO _X , but for HC and CO, the one using the Pd / Rh catalyst in the first step was Pt / It is superior to the one using Rh, and the Pd / Rh + Pd / Pt / Rh exhaust gas purifier is colder than the Pt / Rh + Pd / Pt / Rh exhaust gas purifier from the cold in the internal combustion engine of stoichiometric air-fuel ratio combustion. It can be said that it has excellent durability against hot weather. What can be said in this comparison is that it is better to arrange the Pd / Rh catalyst, which has excellent durability against high temperatures, in the first stage.

[Comparison 3] Similarly, in the comparison between Sample Nos. 2 and 3 using the same catalyst Pd / Pt / Rh in the second stage, the one using the Pd catalyst in the first stage has a higher Pt / Rh. Although it shows a lower purification rate for NO _X than that used,
For C and CO, Pt using the Pd catalyst in the first stage is more
/ Rh is better than the one using Pd + Pd / Pt / Rh, and the durability from cold to hot is better than the exhaust gas purifier of Pt / Rh + Pd / Pt / Rh. be able to. Also in this comparison, it is similarly understood that it is better to arrange the Pd catalyst, which has excellent durability against high temperature, in the first stage.

[Comparison 4] Further, in comparison with Sample Nos. 3 and 5 using Pt / Rh having low heat resistance in the first stage, a Pd / Pt / Rh catalyst was arranged in the second stage. One is Pt /
The purification rate is higher than that with Rh installed. As an exhaust gas purification device for internal combustion engines with stoichiometric air-fuel ratio combustion, in order to obtain a high purification rate from cold to hot, the Pd / Pt / Rh catalyst is used in the second stage. Shows how good it is to place.

Comprehensively comparing the above comparisons, sample number 1
It can be seen that the exhaust gas purifying devices (2) and (2) are suitable as an exhaust gas purifying device for an internal combustion engine of stoichiometric air-fuel ratio combustion, which is rich in durability from cold to hot and has a high purification rate. The reason why such an experiment was obtained is that the Pd-based catalyst used in the first stage has a low activation temperature and is excellent in the exhaust gas performance at the low temperature start of the engine. Is excellent in the oxidation characteristics of HC and CO, and the Pd-based catalyst is resistant to heat. Therefore, by using this in the first stage, the characteristics can be utilized more effectively. The trimetal catalyst (Pd / Pt / Rh) used in the second stage is superior to the Pt-based catalyst in purifying aromatic hydrocarbons and paraffins, and is considered to exert a remarkable effect on HC purifying activity. is there.

It is generally known that Pd-based catalysts are more susceptible to poisoning with P, S, Pb, etc. than Pt-based catalysts.
At present, engine oil is being made low in phosphorus, gasoline is being made lead-free, and sulfur is being made low, and the environment against poisoning and deterioration of Pd-based catalysts is improving.In the future, Pd-based catalysts will be installed in the first stage. It also becomes possible.

[0018]

As described in detail above, according to the present invention, the Pd catalyst or the Pd catalyst which has excellent durability against high temperature is provided on the upstream side of the exhaust system.
By providing a mixed catalyst of Pd and Rh to improve the durability of the exhaust gas purification device, and by providing a trimetal catalyst with excellent activity from low temperature to high temperature on the downstream side, the exhaust gas of an internal combustion engine that burns at a theoretical air-fuel ratio As a result, an exhaust gas purification device with a high purification rate can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an arrangement of an exhaust gas purifying apparatus according to the present invention.

FIG. 2 is a schematic view showing another arrangement of the exhaust gas purifying apparatus according to the present invention.

FIG. 3 is a table showing test results.

[Explanation of sign]

 1 engine 2 exhaust manifold 3 Pd catalyst or mixed catalyst of Pd and Rh (first stage catalyst) 4 trimetal catalyst (second stage catalyst)

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Claims (1)

[Claims] 1. An exhaust gas purifying apparatus provided in an exhaust system of a vehicle, characterized in that a Pd catalyst or a mixed catalyst of Pd and Rh is provided upstream of the exhaust system and a trimetal catalyst is provided downstream thereof. Exhaust gas purification device.