Exhaust Gas Purification Apparatus for an Internal Combustion Engine

1. Field of the Invention

The present invention relates to an exhaust gas purification apparatus which includes a plurality of different kinds of catalysts arranged in series with each other and which can purify nitrogen oxides (hereinafter, NOx) included in exhaust gas from an internal combustion engine at lean air-fuel ratios at a higher NOx purification rate than the total of the NOx purification rates of the individual catalysts.

2. Description of the Related Art

To improve fuel economy and to suppress exhaust of carbon dioxide gas to thereby reduce global warming, engines capable of fuel combustion at lean air-fuel ratios (lean burn engines) are being developed and are partially in actual use. Since a conventional catalyst (three-way catalyst) cannot reduce and purify NOx at lean air-fuel ratios, there is a need to develop a catalyst or system that can purify NOx even at lean air-fuel ratios.

Japanese Patent Publication HEI 1-139145 proposes a copper (Cu)/zeolite catalyst in which copper is ion-exchanged on a zeolite carrier. This catalyst is capable of purifying NOx at lean air-fuel ratios. However, the NOx purification ability of the Cu/zeolite catalyst is poor at stoichiometric and rich air-fuel ratios. Further, the Cu/zeolite catalyst has little ability to purify hydrocarbons (hereinafter, HC) included in exhaust gas.

In order to purify HC at high engine power operating conditions and to decrease the amount of NOx exhausted into the atmosphere. Publication HEI 1-139145 further proposes to install a platinum (Pt)/alumina catalyst for oxidizing HC and CO included in the exhaust gas in a portion of the exhaust conduit located downstream of the Cu/zeolite catalyst. However, in this known apparatus, the ability of the Pt/alumina catalyst to purify whatever NOx has passed through the Cu/zeolite catalyst without being completely reduced by the Cu/zeolite catalyst is small.

The reasons why the NOx purification ability of the Pt/alumina catalyst in the Cu/zeolite-Pt/alumina exhaust gas purification system is small are thought to be as follows:

One reason is that the NOx which flows from the Cu/zeolite catalyst to the Pt/alumina catalyst is mainly in the form of NO, and the Pt/alumina catalyst has little capability for decomposing NO into N.sub.2 and O.sub.2. Another reason is that, in a high power condition, a relatively large amount of HC flows to the Pt/alumina catalyst. The HC will gradually degrades the catalyst and thus shortens its life.

An object of the invention is to provide an exhaust gas purification apparatus for an internal combustion engine which includes a plurality of different kinds of catalysts and which can purify NOx included in exhaust gas from the engine at lean air-fuel ratios at a higher NOx purification rate than the total of the NOx purification rates of the individual catalysts.

The above-described object is achieved by an exhaust gas purification apparatus for an internal combustion engine in accordance with the present invention wherein a transition metal/zeolite first catalyst, a three-way second or intermediate catalyst, and a noble metal-type third catalyst are arranged in that order in the direction of exhaust gas flow in an exhaust conduit connected to the engine.

In this apparatus, the transition metal/zeolite first catalyst reduces most of the NOx in the exhaust gas to at least NO. Then the intermediate three-way catalyst installed upstream of the noble metal-type third catalyst oxidizes the NO into NO.sub.2. The NO.sub.2 flows to the noble metal-type catalyst and is almost completely decomposed into N.sub.2 and O.sub.2, because the noble metal-type catalyst can decompose NO.sub.2 much more effectively than it can decompose NO. As a result, the NOx purification rate of the system is increased despite the inability of the the three-way catalyst by itself to purify NOx in an oxidizing condition.

The above-described object and other objects, features, and advantages of the present invention will become more apparent and will be more readily appreciated from the following detailed description of the preferred embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an exhaust gas purification apparatus for an internal combustion engine in accordance with one embodiment of the invention;

FIG. 2 is a schematic side elevational view of the exhaust gas purification system of FIG. 1 mounted in an automobile;

FIG. 3 is a graphical representation of NOx purification rate or NOx conversion versus catalyst temperature characteristics of a Cu/zeolite catalyst and a Pt/alumina catalyst; and

FIG. 4 is a graph illustrating a relationship between temperatures of catalysts and test running patterns

As illustrated in FIG. 1, an exhaust gas purification apparatus for an internal combustion engine 2 of the invention includes three kinds of catalysts arranged in series in an exhaust conduit 4 of the internal combustion engine, which is capable of fuel combustion at lean air-fuel ratios. More particularly, the three kinds of catalysts are a transition metal/zeolite first catalyst 6, which comprises a zeolite carrier on which at least one kind of transition metal is ion-exchanged, a three-way second catalyst 8 including platinum (Pt) and rhodium (Rh), and a noble metal-type third catalyst 10 including at least one kind of noble metal such as platinum. The transition metal of the transition metal/zeolite catalyst is, for example, copper (Cu) or cobalt (Co). A carrier of the noble metal-type catalyst for carrying platinum is alumina or zeolite.

The three kinds of catalysts are arranged in the order of the transition metal/zeolite first catalyst 6, the three-way second catalyst 8, and the noble metal-type third catalyst 10 in the direction of exhaust gas flow. The arrangement should not be changed from that order

As illustrated in FIG. 2, all three of the catalysts 6, 8, 10 are located under a floor of the automobile 12. A muffler 14 is disposed downstream of the noble metal-type third catalyst 10. The exhaust conduit 4 connected to the engine 2 includes a first portion 4a located between the engine 2 and the transition metal/zeolite first catalyst 6 and a second portion 4b located between the first catalyst 6 and the noble metal-type third catalyst 10.

The length of the first portion 4a of the exhaust conduit 4 is determined so that the exhaust gas temperature decreases due to natural heat loss from 850.degree.-950.degree. C. at the engine outlet to a temperature below 600.degree. C., preferably a temperature in the range of 300.degree. C. to 500.degree. C., at an inlet of the transition metal/zeolite catalyst 6 while flowing through the first portion 4a. Since such a length of the first portion 4a for decreasing the exhaust gas temperature is relatively long, the transition metal/zeolite catalyst 6 is located far from the engine 2 and is located under the automobile floor. The length of the second portion 4b of the exhaust gas conduit 4 is determined so that the exhaust gas temperature further decreases by 100.degree. C.-150.degree. C. while the exhaust gas flows through the second portion 4b.

The reasons for the above-described arrangement of the three kinds of catalysts will now be explained.

There are two reasons for locating the transition metal/zeolite first catalyst 6 upstream of the three-way second catalyst 8 and the noble metal-type third catalyst 10.

One reason is that the transition metal/zeolite catalyst 6 requires HC to reduce NOx, because radicals produced through partial oxidation of HC react with NOx to thereby reduce NOx. Thus, the more HC that is included in the exhaust gas, the greater will be the NOx purification rate. In contrast, the noble metal-type catalyst 10, for example Pt/alumina catalyst, is degraded by excessive HC (hereinafter HC degradation).

If the noble metal-type catalyst were located upstream of the transition metal/zeolite catalyst, the large amount of HC included in the exhaust gas flowing to the noble metal-type catalyst would cause HC degradation, and almost all of the HC would be oxidized by the noble metal-type catalyst. Then, the amount of HC flowing to the transition metal/zeolite catalyst would be too small, resulting in a low NOx purification rate for the transition metal/zeolite catalyst. However, in the invention, the HC included in the exhaust gas from the engine flows directly to the transition metal/zeolite first catalyst 6, so that a portion of the HC will be partially oxidized to produce a sufficient amount of radicals, and the NOx purification rate of the transition metal/zeolite catalyst 6 will be higher. Since most of the HC included in the exhaust gas is either perfectly or partially oxidized by the transition metal/zeolite first catalyst, the amount of HC included in the gas flowing into the noble metal-type third catalyst 10 is small, so that HC degradation of the noble metal-type catalyst 10 is effectively suppressed.

Another reason for locating the transition metal/zeolite catalyst upstream of the three-way and noble metal/type catalysts is that, as illustrated in FIG. 3, in the transition metal/zeolite catalyst 6 the NOx conversion rate peaks at a temperature in the range of 400.degree. C.-500.degree. C., while the noble metal-type catalyst 10 peaks at about 300.degree. C. By locating the transition metal/zeolite catalyst 6 upstream of the noble metal/type catalyst, at the portion of the exhaust conduit where the exhaust gas temperature is at 400.degree. C.-500.degree. C. due to natural heat loss, the exhaust gas temperature at the noble metal-type third catalyst 10 located downstream of the transition metal/zeolite first catalyst will be lower than that temperature and will be at about 300.degree. C. As a result, both catalysts 6 and 10 will operate at their peak NOx conversion rates.

There are also two reasons why the three-way second catalyst 8 is located upstream of the noble metal-type third catalyst 10.

One reason is that most of the NOx which has passed through the transition metal/zeolite catalyst 6 without being completely decomposed is in the form of NO. The NO flowing to the three-way catalyst 8 is oxidized and converted to the form of NO.sub.2 by the three-way catalyst 8. The noble metal-type catalyst 10 can more easily decompose NO.sub.2 into N.sub.2 and O.sub.2 than it can decompose NO into N.sub.2 and O.sub.2. Therefore, the undecomposed NOx remaining in the exhaust gas flowing from the first catalyst can be purified by the noble metal type third catalyst with a high conversion rate. As a result, the NOx purification characteristic of the apparatus having a three-way catalyst located upstream of the noble metal-type catalyst is better than that of an apparatus having no three-way catalyst, even though the three-way catalyst itself has little NOx reduction ability at lean air-fuel ratios.

Another reason for placing the three-way catalyst ahead of the noble metal-type catalyst is that HC having passed the transition metal/zeolite first catalyst flows to the three-way second catalyst where the HC is almost perfectly oxidized by the three-way catalyst 8. Since the amount of HC included in the exhaust gas having passed through the three-way catalyst 8 is very small, HC degradation of the noble metal-type catalyst 10 will be effectively suppressed. As a result, the durability of the noble-metal catalyst 10 and the durability of the apparatus as a whole is greatly improved.

The reason for locating the three-way intermediate catalyst 8 downstream of the transition metal/zeolite first catalyst 6 is to maintain a large amount of HC included in the exhaust gas flowing to the transition metal/zeolite catalyst. If the three-way catalyst were located upstream of the transition metal/zeolite catalyst, the HC included in the exhaust gas would be oxidized by the three-way catalyst and would be insufficient to produce radicals at the transition metal/zeolite catalyst.

The reason for locating the transition metal/zeolite first catalyst 6 under an automobile floor is to maintain the temperature of the exhaust gas flowing into the transition metal/zeolite catalyst at about 400.degree. C.-500.degree. C. even at high engine powers, so that the transition metal/zeolite catalyst 6 can be used in the temperature range near its highest NOx conversion rate. Further, since the transition metal/zeolite catalyst is thermally degraded in a relatively short period of time at temperatures above about 600.degree. C., the under-floor arrangement of the transition metal/zeolite catalyst 6 is desirable from the viewpoint of keeping the catalyst below its thermal degradation temperature.

Operation of the exhaust gas purification apparatus will now be explained.

The exhaust gas from the internal combustion engine 2, which includes a relatively great amount of HC, will decrease in temperature to about 500.degree. C. while passing through the first portion 4a of the exhaust conduit 4. FIG. 4 illustrates the relationship between the catalyst temperature of the apparatus and the vehicle speed in ten-mode and fifteen-mode running tests. From FIG. 4, it is seen that the catalyst temperature will be in the range of 350.degree. C.-400.degree. C. at normal vehicle speeds corresponding to the ten-mode test and will be at about 500.degree. C. at high vehicle speeds corresponding to the fifteen-mode test. Since the catalyst temperature is maintained below about 500.degree. C., the transition metal/zeolite first catalyst 6 can show a high NOx purification rate and further it is not HC-degraded. As a result, the NOx purification characteristic and durability of the apparatus are improved.

Then, the exhaust gas flows into the three-way second catalyst 8. Even in the lean burn engine, the exhaust gas lacks oxygen in a high power operating condition, and the NOx reduction ability of the transition metal/zeolite first catalyst 6 decreases. However, in such a rich air-fuel ratio condition, the NOx included in the exhaust gas in the form of NO is oxidized by the three-way second catalyst 8 into NO.sub.2. Further, since the three-way catalyst 8 also oxidizes HC and CO, the HC and CO emissions will be suppressed.

Then the exhaust gas flows from the three-way second catalyst 8 to the noble metal-type third catalyst 10. Since NO has been converted into the form of NO.sub.2 by the three-way second catalyst 8, the noble metal-type third catalyst 10 can almost perfectly purify the remaining NOx. Further, since the HC has been oxidized by the three-way catalyst 8, the noble metal-type catalyst 10 is almost free from HC degradation. Thus, the NOx purification rate and durability of the apparatus are improved to a great extent.

TABLE 1 summarizes the NOx purification characteristics and durability of various apparatuses including the apparatus of the invention, the apparatus of Japanese Patent Publication HEI 1-139145, a comparison apparatus having a Cu/zeolite catalyst only, a comparison apparatus having a three-way catalyst only, and a comparison apparatus having a Pt/alumina catalyst only.

As seen from TABLE 1, despite the fact that the three-way catalyst has no NOx purification ability at lean air-fuel ratios, provision of the three-way second catalyst between the Cu/zeolite first catalyst and the Pt/alumina third catalyst effectively increases the NOx purification characteristic at the lean air-fuel ratios. Accordingly, the apparatus can exhibit a higher NOx purification characteristic than the total of the NOx purification characteristic of the three-way catalyst only and the NOx purification characteristic of an assembly of the Cu/zeolite catalyst and the Pt/alumina catalyst.

In accordance with the invention, since the transition metal/zeolite first catalyst, the three-way second catalyst, and the noble metal-type third catalyst are arranged in that order in the direction of exhaust gas flow in the exhaust conduit of the internal combustion engine, the NOx purification characteristic and durability of the apparatus are greatly increased.

Although only one embodiment of the invention has been described in detail above, those skilled in the art will appreciate that various modifications and alterations can be made to the particular embodiment shown without materially departing from the novel teachings and advantages of the present invention. Accordingly, all such modifications and alterations are included within the spirit and scope of the invention as defined by the following claims.