US20070283686A1

US20070283686A1 - Exhaust gas purification apparatus

Info

Publication number: US20070283686A1
Application number: US11/723,753
Authority: US
Inventors: Hirosuke Ohkubo; Takashi Iwamoto; Yoshiaki Matsuzono; Takahiro Shinkai
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2006-04-03
Filing date: 2007-03-21
Publication date: 2007-12-13
Also published as: EP1843017B1; JP2007278100A; EP1843017A1; CN101050717B; DE602007011140D1; CN101050717A

Abstract

The present invention relates to an exhaust gas purification apparatus which has an enhanced property of purifying exhaust gas from an internal combustion engine for vehicles, and which is configured to decrease the exhaust gas pressure of the engine. The exhaust gas purification apparatus includes an engine head block, where exhaust pipes combine, has a side on which an exhaust outlet is formed, a first catalytic converter that holds a first catalyst and that is to be coupled to a side of the engine head block, a second catalytic converter that holds a second catalyst and that is coupled to a rear of the first catalytic converter, an exhaust pipe that couples the engine head block to the first catalytic converter. Moreover, the first and second catalysts withstand a temperature equal to/more than 800° C.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application 2006-102545 filed on Apr. 3, 2006, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
Generally, the present invention relates to exhaust gas purification apparatuses. More specifically, the present invention relates to an exhaust gas purification apparatus which has an enhanced property of purifying exhaust gas from an internal combustion engine for vehicles, and which is configured to decrease the exhaust gas pressure of the engine.
2. Description of the Related Art
First, with reference to FIG. 6, a description will be given below, of a process for purifying exhaust gas from an engine 100 b for a commercial vehicle X.
The engine 100 b discharges exhaust gas through cylinders within a cylinder block 101. Subsequently, the exhaust gas flows through an engine head block 102 and an exhaust manifold 103 b where multiple exhaust pipes are joined together. Finally, the exhaust gas reaches a catalytic converter 109, and it is purified there.
The catalytic converter 109 includes catalysts 106 b and 107 b. FIGS. 7A and 7B show the detail of a catalyst of this type. Referring to these figures, an exhaust gas catalyst 110 has a honeycomb structure 111 made of ceramic or metal which includes a catalyst support 113 and a wash coat layer 114. In addition, the wash coat layer 114 supports noble metal, such as platinum (Pt), palladium (Pd) or rhodium (Rh), that serves as a catalyst. Specifically, the exhaust gas catalyst 110 is formed by coating and baking the wash coat layer 114 on the honeycomb structure 111. The catalyst support 113 and the wash coat layer 114 define multiple cells 112. When exhaust gas passes through the cells 112, non methane organic gas (NMOG) and NOx, etc. contained in the exhaust gas are removed.
On the other hand, for the purpose of reducing environmental pollution, regulations against harmful substances such as NMOG and NOx are becoming stringent year by year. To give one example, Partial Zero Emission Vehicle (PZEV), which has been introduced by California, the U.S.A., is currently one of the strictest regulations for commercial vehicles. Only a handful of vehicles observe this regulation at present. Accordingly, exhaust gas purification apparatuses which exhibit an enhanced property of purifying exhaust gas have been highly in demand.
In order to comply with such strict regulations against harmful substances, various attempts to improve exhaust gas purification apparatuses have been made. Some examples of those attempts are as follows:
1) to enlarge an area where exhaust gas comes into contact with the wash coat layer 114 (of FIG. 7B);
2) to grow the number of cells 112 in the honeycomb structure 111, that is, increase the density of the cells in each of the catalysts 106 b and 107 b; and
3) to raise the amount of the noble metal in the wash coat layer 114.
Thanks to the above attempts, the property of purifying exhaust gas by exhaust gas purification apparatuses has been enhanced.
As a result, for example, in a test vehicle Y (see FIG. 2) equipped with an engine of 2.4 litter displacement, one catalyst has 900 cells/in²and the other has 600 cells/in². In addition, the vehicle contains noble metal of 12 g.
Moreover, Japanese Unexamined Patent Application Publication 2004-330025 discloses a catalyst provided with a wash coat layer having multiple voids. With these voids presented in the wash coat layer, exhaust gas is diffused widely, and an area where the exhaust gas comes into contact with the catalyst is made large. This results in the improvement of the purification property.
However, when the density of the cells in a catalyst increases for the above reason, the exhaust gas resistance of the catalyst is prone to be high. This may cause the rise in the pressure of the exhaust gas, which lowers the output power of the engine. In addition, an amount of noble metal in a catalyst may increase, thus leading to high manufacturing costs for vehicles.
Moreover, when the catalyst disclosed by JP2004-330025 is applied to exhaust gas purification apparatuses, the property of eliminating NMOG is enhanced to some degree. Meanwhile, the property of removing NOx is not improved significantly (as will be described in detail in this specification).
Taking the above description into account, the present invention has been conceived. An object of the present invention is to provide an exhaust gas purification apparatus for engines, which achieves an improved purification property with a small amount of noble metal, and which is configured to decrease the resistance of exhaust gas.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided, an exhaust gas purification apparatus for purifying exhaust gas from an internal combustion engine for vehicles, including:
a1a) an engine head block where multiple exhaust pipes combine, and which has a side on which an exhaust outlet is formed;
a2) a first catalytic converter which holds a first catalyst, and which is to be coupled to a side of the engine head block;
a3) a second catalytic converter which holds a second catalyst, and which is coupled to a rear of the first catalytic converter; and
a4) an exhaust pipe which couples the engine head block and the first catalytic converter.
In addition, the first and second catalysts withstand a temperature equal to/more than 800° C.
In the above exhaust gas purification apparatus, the exhaust gas fed to the first catalytic converter is relatively hot. Therefore, the catalytic converter operates at relatively high temperatures. As a result, it is possible to enhance the property of purifying the exhaust gas, as well as to reduce the number of the cells, and decrease the amount of the expensive noble metal. This enables the exhaust resistance of the engine to be lowered.
According to a second aspect of the present invention, there is provided, the exhaust gas purification apparatus in which each of the first and second catalysts includes a catalyst support, a wash coat layer being formed on the catalyst support, and noble metal being supported by the wash coat layer, and the wash coat layer has a thickness equal to/less than 0.2 mm.
In the above exhaust gas purification apparatus, the density of the noble metal increases. This enables the purification property to be further enhanced.
According to a third aspect of the present invention, there is provided, the exhaust gas purification apparatus in which the wash coat layer includes a first wash coat layer and a second wash coat layer, and each of the first and second wash coat layers has multiple voids therein.
In the above exhaust gas purification apparatus, the exhaust gas diffuses widely over the cells of the catalyst in each catalytic converter, thereby enlarging the area where the exhaust gas comes into contact with the wash coat layer. This enables the purification property to be still further enhanced.
Other aspects, features and advantages of the present invention will become apparent upon reading the following specification and claims when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For more complete understanding of the present invention and the advantages hereof, reference is now made to the following description taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a perspective view depicting an exhaust gas purification apparatus according to first and second embodiments of the present invention, which is to be applied to an internal combustion engine for vehicles;
FIG. 2 is a perspective view depicting an exhaust gas purification apparatus in an internal combustion engine for a test vehicle Y;
FIG. 3 is a cross-section view of a catalyst in an exhaust gas purification apparatus of the first embodiment;
FIG. 4 is a graph and a table which both show a test result (an amount of NMOG) of Examples and Comparative examples of the exhaust gas purification apparatus according to the first and second embodiments;
FIG. 5 is a graph and a table which both show a test result (an amount of NOx) of Examples and Comparative examples of the exhaust gas purification apparatus according to the first and second embodiments;
FIG. 6 is a perspective view depicting an exhaust gas purification apparatus in an internal combustion engine for a test vehicle X;
FIG. 7A is a perspective view depicting a catalyst supported by a catalytic converter apparatus; and
FIG. 7B is an enlarged cross-section view of an internal structure of the catalyst.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION

First, a description will be given, of structures of exhaust gas purification apparatuses according to first and second embodiments of the present invention. Second, their behaviors and effects will be explained.

Structure of Exhaust Gas Purification Apparatus of First Embodiment

A description will be given below, of a structure of an exhaust gas purification apparatus according to a first embodiment of the present invention, with reference to FIGS. 1 to 3.
Referring to FIG. 1, an exhaust gas purification apparatus 20 of this embodiment, which is applied to an engine 1, includes an engine head block 3, a first catalytic converter 4 holding a catalyst 9 a and being coupled to a side of the engine, a second catalytic converter 5 holding catalyst 9 b and being attached to a rear of the first catalytic converter 4, and exhaust pipes 6 and 7.
The engine head block 3 is secured through a gasket to an upper portion of the cylinder block 2 that contains cylinders of the engine, thus defining an upper structure of the cylinders. In addition, the engine head block 3 accommodates intake and exhaust breather valves, camshaft (not shown), etc. On the engine head block 3, an upper wall over the cylinders is formed, and exhaust pipes connected to the corresponding cylinders are arranged beneath the upper wall. The exhaust pipes combine to form a single space within the engine head block 3, and a single exhaust outlet 8 is formed on an extruded portion 3 a on the side of the engine head block 3.
On the other hand, a conventional engine head block differs from the engine head block 3, in that the exhaust pipes do not combine therein, and multiple exhaust outlets are formed on the side of the engine head block.
The engine head block 3 is formed, for example, by a casting. In this case, the portion where the exhaust pipes combine within the engine head block 3 is formed by an inner mold of a casting.
The exhaust pipe 6 has an L shape, and couples the engine head block 3 to the first catalytic converter 4. On the exhaust pipe 6, the distance between each cylinder and the first catalytic converter 4 is shorter than that of an exhaust manifold 103 b (see FIG. 6). In the exhaust manifold 103 b, in order to join four exhaust pipes together, the exhaust pipe needs to be longer than a predetermined length, for example, equal to/more than 200 mm. In contrast, the exhaust pipe 6 does not have such limitation. Thus, the exhaust pipe 6 can be shorter than that of the exhaust manifold 103 b. In this embodiment, the length of the exhaust pipe 6 is equal to/less than 200 mm. Hence, the first catalytic converter 4 can be positioned close to a side of the engine 1. The first catalytic converter 4 and a second catalytic converter 5 to which the first catalytic converter 4 is coupled through an exhaust pipe 7 are adapted to be arranged under a floor of a vehicle.
Since the exhaust system of a test vehicle Z is disposed inside the engine head block 3, the distance between each cylinder in the cylinder block 2 and the first catalytic converter 4 can be short. This makes it possible to reduce the heat dissipation of the exhaust gas, thus raising the temperature of the first catalytic converter 4. Consequently, a catalyst 9 a held by the first catalytic converter 4 operates at high temperatures, so that the property of purifying the exhaust gas by the catalyst 9 a is enhanced.
In an exhaust system of a commercial vehicle X of FIG. 6, exhaust gas from an engine head block 102 passes through an exhaust manifold 103 b and, then reaches a catalytic converter 109 to be located under a floor. The heat dissipation of the exhaust gas between the engine head block 102 and the catalytic converter 109 is deemed to be great. In this case, the temperature of the catalyst 106 b could not be high. In other words, the purification property of the catalyst 106 b cannot be sufficiently exhibited.
Referring to FIG. 2, the exhaust gas purification apparatus 108 a for the test vehicle Y is used as a comparative example of the exhaust gas purification apparatus 20 for the test vehicle Z. In the engine 100 for the test vehicle Y, the exhaust gas from the cylinders in a cylinder block 101 passes through an exhaust manifold 103 a where exhaust pipes combine. Then, the exhaust gas is fed to a first catalytic converter apparatus 104 holding a catalyst 106 a, and then, to the second catalytic converter apparatus 105 holding the catalyst 107 a. In this case, since the first catalytic converter apparatus 104 is positioned close to a jointed end of the exhaust manifold 103 a, the distance between the engine head block 102 and the first catalytic converter apparatus 104 is relatively short. However, it is still longer than that of the exhaust gas purification apparatus 20 shown in FIG. 1. The length of the exhaust manifold 103 a is, for example, more than 200 mm. The heat dissipation of the exhaust gas between the engine head block 102 and the first catalytic converter apparatus 104 is smaller than that between the engine head block 102 and the catalytic converter 109 as shown in FIG. 6. However, it is still higher than that in the exhaust gas purification apparatus 20 of FIG. 1. Accordingly, the catalyst 106 a held by the first catalytic converter apparatus 104 cannot have the same temperature as or higher temperature than that of the catalyst 9 a supported by the first catalytic converter 4 of this embodiment as shown in FIG. 1. Thus, it can be considered that the exhaust gas purification apparatus 20 is superior to the exhaust gas purification apparatus 108 a.
Referring to FIG. 1, in the exhaust gas purification apparatus 20 for the test vehicle Z, the first catalytic converter 4 and the second catalytic converter 5 hold the catalysts 9 a and 9 b, respectively.
In the catalysts 106 a and 107 a of FIG. 2, the number of the cells per square inch is, for example, 900 and 600, respectively. In contrast, each of the catalysts 9 a and 9 b in the exhaust gas purification apparatus 20 can have the fewer cells. This makes it possible to lower the exhaust gas pressure of the apparatus 20. The reason why the number of the cells in the catalysts 9 a and 9 b can decrease is due to the arrangement where the first catalytic converter 4 is located close to the engine 1 and the improvement of the catalysts as will be described later.
Referring to FIG. 3, a catalyst corresponds to any of the catalysts 9 a and 9 b in the exhaust gas purification apparatus 20 as shown in FIG. 1. The wash coat layer 11 in a cell 16 has a function of a catalyst, and it is formed on a catalyst support 10 having a honeycomb structure. The wash coat layer 11 b on a corner is thicker than the wash coat layer 11 a on a line. The wash coat layer 11 a is 0.06 mm in average thickness. The wash coat layer 11 b is 0.15 mm in average thickness and 0.2 mm in maximum thickness. The wash coat layer 11 is about 30% thinner than the wash coat layer of the catalysts 106 a and 107 a of catalytic converter 109 (see FIG. 6) or of the first and second catalytic converter apparatuses 104 and 105 (see FIG. 2).
The wash coat layer 11 of each of the catalysts 4 and 5 is composed of two wash coat layers; one is a first wash coat layer 12 formed on the catalyst support 10 and the other is a second wash coat layer 13 formed on the first wash coat layer 12. Each of the first and second wash coat layers 12 and 13 is formed of a porous structure 14 of which main component includes aluminum oxide (alumina), zirconium oxide (zirconia), and ceric oxide (ceria). The porous structure 14 supports noble metal 17 made of at least one of Pt, Pd and Rh. This noble metal 17 serves as a catalyst.
A porous structure 14 forming the first and second wash coat layers 12 and 13 has multiple voids, that is, spaces therein. Each void 15 is 0.004 mm±0.002 mm in diameter. The voids occupy about 20% of each whole volume of the first and second wash coat layers 12 and 13. However, this occupancy ratio is not limited to 20%. In this embodiment, the occupancy ratio of the voids may range from 10% to 60%. These voids 15 contribute to the diffusing of the exhaust gas, and to the improvement of the purification property. Thanks to the voids 15, the noble metal 17 are exposed to the cell 16 sufficiently. Hence, the ratio of the noble metal 17 which serves as a catalyst is made high, thus leading to the advancement of the purification property. On the other hand, the wash coat layer of the catalysts 106 a and 107 a of FIG. 2 has no voids.
Sintered alumina in the porous structure 14 (see FIG. 3) is stable at high temperatures and has a large surface area. Therefore, the noble metal 17 is distributed widely in the porous structure 14. Consequently, a ratio at which the noble metal 17 serves as a catalyst in the wash coat layer 11 is relatively high. This enables the purification property of the catalysts 9 a and 9 b to be made relatively high.
Ceria in the porous structure 14 has the oxidation-reduction property, depending on the valence of ceria ions contained therein. If oxygen excessively resides in exhaust gas, then ceria decreases the oxygen partial pressure by being coupled to the oxygen atoms in the excess oxygen. Meanwhile, if a small amount of oxygen is contained in exhaust gas, then the oxygen atoms coupled to ceria is liberated, thereby increasing the oxygen partial pressure in the exhaust gas. Such a property of ceria reduces the rapid change in the oxygen partial pressure of the exhaust gas, in the case where the catalytic converter apparatus 1 is applied to an engine that controls an air-fuel ratio based on stoichiometry (e.g. stoichiometry type engine). As a result, the oxygen partial pressure in the exhaust gas is kept at a certain level at which reducing hydrocarbon, nitric oxide and oxidizing nitrogen oxides are removed at the same time.
Zirconia in the porous structure 14 has a property of enhancing the thermal stability of ceria. Due to this property, the oxidation-reduction property of ceria is maintained, even if the porous structure 14 of the wash coat layer 11 is exposed to high temperatures for a long time.
The noble metal 17 on the first wash coat layer 12 in the catalyst 4 is the same as that of the first wash coat layer 12 in the catalyst 5. Specifically, barium (Ba) and palladium (Pd) are held by the above porous structure 14. In this case, Ba acts to enhance the thermal stability of Pd, while preventing the rough enlargement of Pd particles.
On the other hand, the noble metal 17 of the second wash coat layer 12 in the catalyst 4 is different from that of the second wash coat layer 12 in the catalyst 5. The noble metal 17 on the second wash coat layer 13 of the catalyst 4 is Rh, while the noble metal 17 on the second wash coat layer 13 of the catalyst 5 is Pt and Rh.
As described above, the wash coat layer 11 of the catalysts 9 a and 9 b held by the exhaust gas purification apparatus 20 of this embodiment is about 30% thinner than the wash coat layer of the catalysts 106 a and 107 a of FIG. 2. Hence, the density of the noble metal 17 in the catalysts 9 a and 9 b is greater than that in the catalysts 106 a and 107 a. Thus, the purification property of the catalysts 9 a and 9 b is more excellent than that of the catalysts 106 a and 107 a. Since the wash coat layer 11 is thin, the ratio at which the aperture occupies the cell 16 in the catalysts 9 a and 9 b is great. Accordingly, the exhaust resistance of the catalysts 9 a and 9 b is lower than that of the catalysts 106 a and 107 a.
The wash coat layer 11 is fabricated by the following process. Material powder for the porous structure 14, component of the noble metal 17 and pyrolytic or flammable spherical molding body are dispersed into a solvent (e.g. water), thereby forming a slurry. Note that a material of the molding body includes foaming resin and activated charcoal. Next, a honeycomb base material made of cordierite, which is to make up the catalyst support 10, is dipped into the slurry. Following this, it is taken away therefrom and dried. Finally, the base material is sintered, thereby forming the first wash coat layer 12 on the catalyst support 10. The density of the slurry is adjusted such that the wash coat layer has a predetermined thickness. While being sintered, the molding body is decomposed, whereby the voids 15 are formed in the first wash coat layer 12. By repeating the above process, the second wash coat layer 13 having the voids 15 is also formed on the first wash coat layer 12. The catalyst fabricated by the above processes withstands a temperature equal to/more than 800° C., because all materials thereof have high thermal resistance.
In the exhaust gas purification apparatus 20 of this embodiment, the exhaust gas fed to the first catalytic converter 4 has a higher temperature than that of the apparatus 108 a (FIG. 2) or 108 b (FIG. 6). Accordingly, the first catalytic converter 4 needs to possess a higher heat resistance. Preferably, it is equal to/more than 800° C.

Structure of Exhaust Gas Purification Apparatus of Second Embodiment

Next, a description will be described below, of a structure of an exhaust gas purification apparatus according to a second embodiment of the present invention.
An exhaust system of this apparatus is similar in structure to that of the apparatus of the first embodiment. Specifically, a structure and an arrangement of the engine head block 3, the exhaust pipes 6 and 7 and the first and second catalytic converter apparatuses 4 and 5 are identical to each other. However, catalysts in the apparatus of the second embodiment are identical to the catalysts 106 a and 107 a in the apparatus 100 of FIG. 2. Thus, the wash coat layer in the apparatus of the second embodiment is not thinner. In addition, the wash coat layer has no voids. Consequently, the exhaust gas purification apparatus of this embodiment includes the catalysts 106 a and 107 a of FIG. 2 instead of the catalysts 9 a and 9 b of FIG. 1.
Next, a detailed description will be given below, of behavior and effect of the catalytic converter apparatus 1 according to the first and second embodiments.
The purification property of the catalytic converter apparatus 1 was evaluated by amounts of NMOG and NOx. In addition, the purification property was tested based on the following three parameters.
1) CT (mg/km):
Started up an engine of a vehicle;
Ran the vehicle over a predetermined distance while the engine was cool;
Measured an amount of residual harmful substance contained in exhaust gas from the engine; and
Corrected the measured amount by multiplying it by a predetermined constant, thereby determining CT. Note that the predetermined constant was determined according to a certain driving condition of “cool start” in the modeled driving for evaluating the exhaust system.
2) CS (mg/km):
Started up and warmed up an engine of a vehicle;
Ran the vehicle over a predetermined distance after the engine was completely warmed;
Measured an amount of residual harmful substance contained in exhaust gas from the engine; and
Corrected the measured amount by multiplying it by a predetermined constant, thereby determining CS. Note that the predetermined constant was determined according to a certain driving condition of “warm start” in the modeled driving for evaluating the exhaust system.
3) HT (mg/km):
Started up and warmed up an engine of a vehicle;
Stopped the engine after the engine was completely warmed;
Started up the engine again;
Ran the vehicle over a predetermined distance while the engine was warm;
Measured an amount of residual harmful substance contained in exhaust gas from the engine; and
Corrected the measured amount by multiplying it by a predetermined constant. Note that the predetermined constant was determined according to a certain driving condition of “warm re-start” in the modeled driving for evaluating the exhaust system.
The sum of these parameters “CT+CS+HT” was defined as a reference for determining an amount of harmful substance. By using this parameter, the purification properties of Examples and Comparative examples were determined. As the value “CT+CS+HT” was lower, the purification property was considered to be superior.
FIGS. 4 and 5 show results of testing amounts of residual NMOG and NOx in exhaust gas from the engine equipped with apparatuses of examples and comparative examples, respectively.
In FIGS. 4 and 5, an apparatus of an Example according to the first embodiment (hereinafter, called an “Example 1”) corresponded to an exhaust gas purification apparatus G. An apparatus of an Example according to the second embodiment (hereinafter, called an “Example 2”) corresponded to an exhaust gas purification apparatus H. In addition, the Example 1 was equivalent to the exhaust gas purification apparatus 20 for the test vehicle Z of FIG. 1. Comparative examples 1 to 6 corresponded to the exhaust gas purification apparatuses A to F, respectively. The Comparative example 1 was equivalent to the exhaust gas purification apparatus 108 a for the test vehicle Y of FIG. 2. The wash coat layer of the Comparative example 1 was thicker than that of the Example 1. Furthermore, the wash coat layer of the Comparative example 1 had no voids therein. The Comparative example 1 had more cells than those of the Example 1, and held much noble metal than that of the Example 1.
It is clear from the results of FIGS. 4 and 5 that the Example 1 (exhaust gas purification apparatus G) and the Example 2 (exhaust gas purification apparatus H) exhibit a more excellent property of removing NMOG and NOx than that of the Comparative example 1 (exhaust gas purification apparatus A). Moreover, the Example 1 enables an exhaust gas pressure to be made lower than the Comparative example 1 does, as well as holds less noble metal than the Comparative example 1 does.
The Comparative example 2 was different from the Comparative example 1 in an amount of the noble metal. Specifically, the Comparative example 1 held the noble metal of 12 g, while the Comparative example 2 held the noble metal of 4 g. Considering the result of the Comparative examples 1 and 2 in FIGS. 4 and 5, the amount of the noble metal in the Comparative example 1 cannot decrease without degrading its purification property.
The Comparative examples 3 to 6 (exhaust gas purification apparatuses C to F) had an identical exhaust system to that of the Comparative example 1 or 2. Specifically, it had a system equipped with the exhaust manifold 103 a. However, the Comparative examples 3 to 6 had the improved wash coat layer 11 shown in FIG. 3, and their cells were as many as those in the Example 1. Among the Comparative examples 3 to 6, the amounts of the noble metal were different from one another. They varied a range from 4 g to 12 g.
The test results of these Comparative examples reveal the following facts.
As for an apparatus having the exhaust system equipped with the exhaust manifold 103 a, the amount of NMOG exhausted from the system can be reduced by providing the catalyst with the improved wash coat layer (of FIG. 3) and by increasing the amount of the noble metal in the catalyst. However, even though the catalyst with the improved wash coat layer is used and the amount of the noble metal in the catalyst increases, the amount of NOx from the system with the exhaust manifold cannot be reduced.
In conclusion, in order to decrease the amounts of both NMOG and NOx in the exhaust gas from an apparatus, the exhaust system without the exhaust manifold must be applied to the apparatus.
In addition, by applying the improved catalysts to the apparatus, the number of the cells in the catalysts can decrease. This makes it possible to lower the exhaust gas pressure of the apparatus and to decrease the amount of the noble metal in the catalysts.

As described above, the effect achieved by the exhaust system without the exhaust manifold is due to the high temperature of the exhaust gas fed to the first catalytic converter. Actually, temperatures of the different catalysts were measured. The measuring result was shown in a table 1.

TABLE 1


	20 sec after	20 sec after
	starting system	re-starting system
SAMPLE	with being cool	with being warm

COMPARATIVE EXAMPLE 1	410° C.	359° C.
EXAMPLE 1	442° C.	402° C.

According to the table 1, the temperature of the Example 1 was greater than that of the Comparative example 1 by 30° C. or more.
Therefore, it proves that the purification property of the exhaust gas purification apparatus of the first or second embodiment is attributed to the high temperature of the first catalytic converter 4.
In view of the above result, with the exhaust gas purification apparatus of the first embodiment or the second embodiment, the purification property can be improved. In addition, the exhaust gas purification apparatus of the first embodiment makes it possible to lower the exhaust gas pressure, and to decrease the amount of the noble metal supported by the catalyst.
Note that the decrease in the exhaust gas pressure results in the improvement of the output performance of engines, that is, the enhancement of the commercial value of vehicles.
Be sure to note that the present invention is not limited to the aspects of the Examples 1 and 2 in the embodiments.
The Example 1 (exhaust gas purification apparatus A) included the first catalytic converter of the 400 cells/in²and the second catalytic converter of 350 cells/in². In addition, the Example 1 had the noble metal of 4 g. However, the present invention is not limited to this configuration. It is obvious that the number of the cells or the amount of the noble metal in exhaust gas purification apparatuses can be varied without departing from the spirit and scope of the claims.

Claims

1. An exhaust gas purification apparatus for purifying exhaust gas from an internal combustion engine for vehicles, said exhaust gas purification apparatus comprising:

an engine head block where a plurality of exhaust pipes combine, the engine head block having a side on which an exhaust outlet is formed;

a first catalytic converter for holding a first catalyst, the first catalytic converter being to be coupled to a side of the engine head block;

a second catalytic converter for holding a second catalyst, the second catalytic converter being coupled to a rear of the first catalytic converter;

an exhaust pipe for coupling the engine head block and the first catalytic converter; and

the first and second catalysts withstanding a temperature equal to/more than 800° C.

2. The exhaust gas purification apparatus according to claim 1,

wherein each of the first and second catalysts comprises a catalyst support, a wash coat layer being formed on the catalyst support, and noble metal being supported by the wash coat layer, and the wash coat layer has a thickness equal to/less than 0.2 mm.

3. The exhaust gas purification apparatus according to claim 2,

wherein the wash coat layer comprises a first wash coat layer and a second wash coat layer, and each of the first and second wash coat layers has a plurality of voids therein.

4. The exhaust gas purification apparatus according to claim 3,

wherein each of voids is 0.004 mm±0.002 mm in diameter.

5. The exhaust gas purification apparatus according to claim 3,

wherein the voids occupy 10% to 60% of a whole volume of each of the first and second wash coat layers.