Exhaust gas purifying system for internal combustion engine

It is an object of the present invention to provide an exhaust gas purifying system for an internal combustion engine in which the rise of the temperature of an HC adsorbent is adequately retarded to improve the purification of HC at the start of the engine. In an exhaust system of the internal combustion engine, there are provided a three-way catalyst provided in the downstream side of an exhaust manifold, an adsorbent-catalyst provided under the floor of an automotive vehicle and containing at least zeolite in its composition, and a catalyst provided in the downstream side of the adsorbent-catalyst. In particular, the heat capacity of an exhaust gas passage which connects between the three-way catalyst and the adsorbent-catalyst is increased greater than the heat capacity of the exhaust manifold. More particularly, the thickness of a wall of the exhaust gas passage is greater than that of the exhaust manifold. A substrate, a partition, or a muffler is provided across the exhaust gas passage. The exhaust tube in the exhaust gas passage has at least an inner side or an outer side thereof shaped to a convex and concave configuration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an exhaust gas purifying system for an 
internal combustion engine and particularly to an exhaust gas purifying 
system for an internal combustion engine, which has an improved capability 
of purifying hydrocarbons (referred to as HC hereinafter) in the exhaust 
gas at the start of the internal combustion engine. 
2. Description of the Related Art 
A variety of studies for purifying exhaust gases from an internal 
combustion engine e.g. of an automotive vehicle have been developed to 
prevent pollution of the atmosphere. Known harmful substances in the 
exhaust gas from the vehicle are HC, CO, and NOx. For example, one of 
schemes for purifying HC is disclosed in Japanese Patent Laid-open 
Publication Hei 5-59942. 
Disclosed in the Publication is a conventional exhaust gas purifying system 
which includes, in this order from the upstream of an exhaust gas passage 
of the internal combustion engine, a first three-way catalyst, an HC 
adsorbent, and a second three-way catalyst for purifying HC while the 
temperature of the gas is relatively low just after the start of the 
engine. As well known, three-way catalyst has a lower capability of 
reactively purifying HC before its temperature rises up to an activating 
level (300.degree. C.). Its capability will then increase as the 
temperature reaches the activation level. HC adsorbent has a higher 
capability of adsorbing HC when the temperature is low. When its 
temperature exceeds a certain level, the HC adsorbent starts desorbing HC. 
In the conventional exhaust gas purifying system, the first three-way 
catalyst is provided for extending a period before the temperature of the 
HC adsorbent reaches the certain level. During the extended period, the 
three-way catalyst is heated up to improve the purification of HC at the 
start of the engine. 
Another scheme is disclosed in Japanese Patent Laid-open Publication Hei 
5-31359. The scheme has a zeolite adsorbent, a honeycomb heater, and a 
main monolithic catalyst provided in an exhaust gas passage of an 
automotive vehicle so that HC in the exhaust gas of a lower temperature at 
the start of an engine is adsorbed by the adsorbing action of the zeolite 
adsorbent and further purified through energizing the honeycomb heater to 
instantaneously activate the catalyst on the heater. 
However, in the former conventional system, the speed of rising the 
temperature of the exhaust gas is slowed down by the first three-way 
catalyst located at the upstream side. The speed of rising the temperature 
of the exhaust gas received by the HC adsorbent is not successfully 
retarded and the period before the temperature rises up to the level at 
which HC is desorbed from the HC adsorbent can hardly be extended to 
desired length. 
In the latter conventional system, the catalyst is activated by heating 
with the honeycomb heater before the desorption of HC from the zeolite 
adsorbent starts. Since the catalyst is heated up within a short time, the 
supply of electric power to the honeycomb heater has to be increased. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an exhaust gas 
purifying system for an internal combustion engine in which the rise of 
the temperature of the HC adsorbent is properly retarded to improve the 
purification of HC at the start of the engine. 
For achievement of the object of the present invention, an exhaust gas 
purifying system for an internal combustion engine having in an exhaust 
system of the internal combustion engine a three-way catalyst provided in 
the downstream side of an exhaust manifold, an adsorbent-catalyst provided 
under the floor of an automotive vehicle and containing at least zeolite 
in its composition, and a catalyst provided in the downstream side of the 
adsorbent-catalyst is characterized in that the heat capacity of an 
exhaust passage which connects between the three-way catalyst and the 
adsorbent-catalyst is increased greater than the heat capacity of the 
exhaust manifold. As featured, the temperature of the exhaust gas is 
absorbed by the action of the heat capacity of the exhaust passage as well 
as of the three-way catalyst provided in the downstream side of the 
exhaust manifold and hence, the rise of the temperature of the 
adsorbent-catalyst can further be retarded. As the rise of the temperature 
of the adsorbent-catalyst is retarded, HC can be adsorbed in the 
adsorbent-catalyst for a longer period of time, thus improving the 
purification of HC at the start of the engine. 
Also, the present invention is featured in that the radiation of heat from 
the exhaust gas passage which connects between the three-way catalyst and 
the adsorbent-catalyst is increased higher than that of the manifold. As 
featured, the temperature of the exhaust gas is dropped down by the 
radiation of heat from the exhaust gas passage as well as the absorbing 
action of the three-way catalyst at the downstream side of the exhaust 
manifold and hence, the rise of the temperature of the adsorbent-catalyst 
can further be retarded. As the rise of the temperature of the 
adsorbent-catalyst is retarded, HC can be adsorbed in the 
adsorbent-catalyst for a longer period of time, thus improving the 
purification of HC at the start of the engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will be described in more detail referring to the 
accompanying drawings. FIG. 1 is a schematic view of an exhaust gas 
purifying system for an internal combustion engine showing one embodiment 
of the present invention. 
As shown, a series of an exhaust manifold 2, a first converter case 3, a 
first exhaust tube 4, a second converter case 5, and a second exhaust tube 
6 are connected in the form of an exhaust gas passage on the exhaust side 
of an engine 1. Also, a plurality of air/fuel ratio sensors 7a to 7d, e.g. 
oxygen sensors, are disposed at given location of the exhaust gas passage. 
The air/fuel ratio sensors 7a may preferably be of a linear air fuel ratio 
sensor. 
A three-way catalyst (TWC) 11 is mounted in the center of the first 
converter case 3 located at the downstream of the exhaust manifold 2. In 
this embodiment, the three-way catalyst 11 has about 1200 cells per square 
inch in cross section. The second converter case 5 contains, from 
upstream, a substrate 13, an adsorbent-catalyst 14, electrically heated 
catalysts 15a AND 15b, and a three-way catalyst 16. 
The exhaust manifold 2 which was once made of cast iron is fabricated in 
the embodiment by pressing a 0.5 mm thick stainless steel to a shape which 
is thus lower in the heat capacity than conventional one. The exhaust 
manifold 2 is coated with an exhaust manifold case 2a which is made by 
pressing a 1.5 mm thick stainless steel so that an air layer 2b is 
provided between the exhaust manifold 2 and the exhaust manifold case 2a 
forming a thermal insulating structure and is thus minimized in the 
radiation of heat. Accordingly, since the heat capacity and the heat 
radiation of the exhaust manifold 2 are minimized, the conduction of heat 
to the three-way catalyst 11 will be improved thus accelerating the 
activation of the three-way catalyst 11. 
The substrate 13 is made of a honeycomb structure of a ceramic material or 
preferably of a metallic material which is higher in the absorption of 
heat. The metallic structure of the substrate 13 may be implemented by 
rolling a plain sheet and a corrugate sheet of highly heat-resistant 
ferrite stainless steel a number of times. In the embodiment, the metal 
substrate 13 incorporates a honeycomb structure having about 300 cells per 
square inch in cross section. To prevent increase of the exhaust 
resistance, the honeycomb structure is profiled relatively coarse as a 
role in the exhaust system. 
The adsorbent-catalyst 14 has a structure having an HC adsorbent such as 
zeolite, e.g. ZSM-5, or layers of HC adsorbent and three-way catalyst 
provided on a honeycomb substrate. The honeycomb substrate of the 
adsorbent-catalyst 14 has about 1200 cells per square inch in cross 
section. As understood, the adsorbent-catalyst 14 can adsorb HC before the 
temperature reaches a particular degree and when heated up over the 
degree, it will desorb the adsorbed HC. 
The electrically heated catalysts 15a and 15b are identical in the shape. 
It may be a honeycomb heater coated with the catalyst such as disclosed in 
Japanese Patent Laid-open Publication Hei 9-192453. 
In this embodiment, the cross section includes about 500 cells per square 
inch. A pair of electrodes 17 and 18 are connected on the outer side of 
the honeycomb heater for supply of electric power. The electrodes 17 and 
18 are fed with currents from a power source not shown. The electrically 
heated catalyst 15a contains platinum in its composition while the 
electrically heated catalyst 15b contains palladium in its composition, 
hence effectively purifying HC produced from the engine just after the 
start up. The power source for the electrically heated catalysts 15a and 
15b may be a battery or alternator commonly mounted in an automotive 
vehicle or a specific battery or alternator provided for use with the 
electrically heated catalysts 15a and 15b. 
When the engine 1 is started, an exhaust gas including HC runs through the 
exhaust manifold 2, the first converter case 3, the first exhaust tube 4, 
the second converter case 5, and the second exhaust tube 6 in the exhaust 
gas purifying system for internal combustion engine. The exhaust gas is 
first low in the temperature and the catalyst located in the exhaust gas 
passage remains below a level of activation and performs absorption of 
heat. Since the exhaust gas entering the adsorbent-catalyst 14 has a low 
temperature, it adsorbs HC. When the temperature of the exhaust gas rises 
with time, the catalysts in the exhaust gas passage are heated up. The 
higher the temperature of the adsorbent in the adsorbent-catalyst 14, the 
faster the desorption of HC is promoted. When the desorbing speed exceeds 
the adsorbing speed, HC is desorbed from the adsorbent. 
The three-way catalyst 11 is heated up by heat from the exhaust gas and the 
electrically heated catalysts 15a and 15b are heated and activated by the 
currents supplied via the electrodes 17 and 18 from the unshown power 
source. If the period before the temperature rises up to a level at which 
the HC adsorbent starts desorbing HC is not long, the temperature of the 
three-way catalyst 11 may not reach its activating level when the 
temperature of the HC adsorbent rises up to the level for desorbing HC. 
This will require the electrically heated catalysts 15a and 15b to be 
supplied with a greater supply of electric power for heating up within a 
short time thus stressing the power source. 
For extending the period before the temperature of the HC adsorbent rises 
up to its HC desorbing level in the embodiment, an exhaust gas passage 
between the three-way catalyst 11 and the adsorbent-catalyst 14 (which is 
more particularly a passage extending from the downstream side of the 
three-way catalyst 11 to the upstream side of the adsorbent-catalyst 14 
and including the first exhaust tube 4) is arranged greater in the heat 
capacity than the exhaust manifold 2. 
In a first example for increasing the heat capacity in the exhaust gas 
passage, the thickness d of a wall of the exhaust tube 4 in the exhaust 
gas passage is increased greater than the thickness of a wall of the 
exhaust manifold 2. More specifically, the thickness d is 1.5 mm in the 
embodiment. FIGS. 2A and 2B are cross sectional views taken along the line 
A--A of FIG. 1. As a second example, a substrate 13 which is high in the 
absorption of heat is provided in the exhaust gas passage. The substrate 
13 may be accompanied with a catalyst. For instance, a three-way catalyst 
is used as the second catalyst for improving the capability of purifying 
the exhaust gas in normal conditions. 
According to the two, first and second examples, the member provided in the 
exhaust gas passage is high in the heat capacity and can thus absorb heat 
from the exhaust gas at a higher efficiency even if the temperature of the 
exhaust gas entering the exhaust manifold 2 is increased with time after 
the start of the engine. The speed of increasing the temperature of the 
exhaust gas which runs into the adsorbent-catalyst 14 will hence be slowed 
down as compared with the conventional system with only the three-way 
catalyst 11. 
This increases the period before the temperature of the HC adsorbent rises 
up to its HC desorbing level. The heat capacity of the exhaust manifold 2 
is smaller and the speed of increasing the temperature of the exhaust gas 
entering the three-way catalyst 11 increases hence reducing the period 
before the temperature of the three-way catalyst 11 rises up to its 
activating level. Also, the period before the electrically heated 
catalysts 15a and 15b are activated is increased and the supply of 
electric power to the electrically heated catalysts 15a and 15b needs not 
to be increased, relieving the power source. 
A third example of the embodiment is explained. The third example has a 
partition provided in the exhaust gas passage for allowing heat of the 
exhaust gas to stay, as shown in FIG. 3. The partition may be made of a 
small-diameter inner tube 4a provided in the first exhaust tube 4 and 
extending lengthwisely of the same as shown in FIG. 3A. The inner tube 4a 
has a multiplicity of holes therein for reducing the resistance of the 
exhaust gas. Also, another partition may be a group of partition sheets 20 
made of a metal mesh material as shown in FIG. 3B. The partition sheets 20 
are arranged at intervals of a proper distance in the exhaust tube 4 as 
shown in FIG. 3C. The partition sheets may be metal plates having a number 
of perforations. 
A second embodiment of the present invention will now be described. The 
first embodiment previously described is featured in that the heat 
capacity of the exhaust gas passage is increased greater than that of the 
exhaust manifold 2. In the second embodiment, the radiation of heat from 
the exhaust tube is increased while the heat capacity of the exhaust gas 
passage is kept high, or the radiation of heat from the exhaust tube is 
only increased. 
In a first example of this embodiment, the first exhaust tube 4 has an 
inner side or an outer side thereof shaped to a convex and concave 
configuration as shown in FIG. 2B. The both sides of the exhaust tube 4 
may be shaped to a convex and concave configuration. The convex and 
concave configuration may be implemented by cutting parallel linear slots 
or a spiral slot lengthwisely of the exhaust tube 4 or simply making pits 
and lands. Preferably, the convex and concave configuration on the inner 
side of the first exhaust tube 4 is made in the respect of exhaust 
resistance by cutting parallel linear slots lengthwisely. The process of 
making the convex and concave configuration may be carried out by bulge 
forming, pressing, cutting, drawing, injection, or any other appropriate 
technique. 
This permits the inner side of the exhaust tube to be increased in the area 
of contact with the exhaust gas thus improving the transfer of heat to the 
exhaust tube or the outer side of the exhaust tube to be increased in the 
area of contact with the outside air thus improving the radiation of heat. 
Accordingly, the rise of the temperature of the adsorbent-catalyst 14 can 
be retarded. 
A second example of the embodiment employs a muffler 22 provided in the 
exhaust tube as shown in FIG. 4. In this example, the heat capacity and 
the radiation of heat on the exhaust tube can be improved. The muffler 22 
may be of known resonance, expansion, or their combination type. 
A third example uses fins 4d provided on the outer side of the exhaust 
tubes 4b and 4c as shown in FIG. 5. This increases the radiation of heat 
from the exhaust tube 4 and thus retards the rise of the temperature of 
the adsorbent-catalyst 14. 
It would be understood that the foregoing examples are provided in any 
combination to have more favorable effects than that of each example. 
According to the present invention, the heat capacity of the exhaust gas 
passage connecting between the three-way catalyst on the downstream side 
of the exhaust manifold and the adsorbent-catalyst under the vehicle floor 
is increased greater than that of the exhaust manifold. Hence, the heat of 
the exhaust gas is absorbed by means of the heat capacity of the exhaust 
gas passage as well as the action of the three-way catalyst and the rise 
of the temperature of the adsorbent-catalyst can further be retarded. 
Since the rise of the temperature of the adsorbent-catalyst is retarded 
successfully, HC is adsorbed in the adsorbent-catalyst for a longer period 
and the purification of HC at the start of the engine will thus be 
improved. 
Also, the radiation of heat from the exhaust gas passage between the 
three-way catalyst and the adsorbent-catalyst is increased higher than 
that of the exhaust manifold. Accordingly, the temperature of the exhaust 
gas is dropped down through the absorbing action of the three-way catalyst 
on the downstream side of the exhaust manifold and the radiating action of 
the exhaust gas passage, and the rise of the temperature of the 
adsorbent-catalyst can further be retarded. Since the rise of the 
temperature of the adsorbent-catalyst is retarded successfully, HC is 
adsorbed in the adsorbent-catalyst for a longer period and the 
purification of HC at the start of the engine will thus be improved. 
The embodiments of the present invention offer the following advantages. 
(1) The electrically heated catalysts located at the downstream side of the 
adsorbent-catalyst are activated by a lower power in the period before the 
desorption of HC from the adsorbent-catalyst is started, and the HC 
desorbed from the adsorbent-catalyst can be purified. 
(2) Because the substrate is provided across the exhaust gas passage, the 
rise of the temperature of the adsorbent-catalyst can be retarded with the 
use of a simple construction. 
(3) Because the second catalyst is provided in the exhaust gas passage to 
improve the purification of the exhaust gas, the rise of the temperature 
of the adsorbent-catalyst can further be retarded. 
(4) Because the partition which allows heat of the exhaust gas to stay is 
provided in the exhaust gas passage, the rise of the temperature of the 
adsorbent-catalyst can further be retarded. 
(5) Because the muffler is provided across the exhaust gas passage to 
increase the heat capacity and the radiation of heat, the rise of the 
temperature of the adsorbent-catalyst can further be retarded. 
(6) Because the fins for radiation of heat is provided in the exhaust gas 
passage to increase the radiation of heat, the rise of the temperature of 
the adsorbent-catalyst can further be retarded. 
(7) Because the thickness of a wall of the exhaust tube in the exhaust gas 
passage is increased greater than that of the exhaust manifold, the heat 
capacity of the exhaust gas passage is higher than that of the exhaust 
manifold and the rise of the temperature of the adsorbent-catalyst can be 
retarded while the conduction of heat to the catalyst at the downstream 
side of the exhaust manifold is improved. 
(8) Because at least the inner or outer side of the exhaust gas passage is 
shaped to a convex and concave configuration so that the area of contact 
with the exhaust gas on the inner side or with the outside air on the 
outer side is increased to improve the conduction of heat to the exhaust 
tube or the radiation of heat, the rise of the temperature of the 
adsorbent-catalyst can be retarded successfully.