Exhaust emission control device for internal combustion engine

An exhaust emission control device for an internal combustion engine is provided with a main catalyst activated with heat of exhaust gas, a self-exothermic type auxiliary catalyst activated with electric current energy and a silencer, which are connected in series and in sequence from an upstream side to a downstream side in an exhaust system leading from an engine body. Thus, noxious components included in the exhaust gas can be converted and the gas is purified by using the main and auxiliary catalysts in combination but with the auxiliary catalyst being subject to less heat from the exhaust gas than the main catalyst.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an exhaust emission control device for an 
internal combustion engine, which oxidizes and reduces noxious components 
such as Nox, CO and the like in exhaust gas by a catalyst connected to an 
exhaust system in an engine body. 
2. Description of the Prior Art 
There is a conventionally known exhaust emission control device, as shown 
in FIG. 3. Such a prior art device uses, in combination, a main catalyst 
activated with heat from exhaust gas, and an auxiliary or sub catalyst 
activated with heat supplied by an electric current. 
In such an exhaust emission control device, a self-exothermic type 
auxiliary catalyst 07 activated with heat supplied by an electric current, 
a main catalyst 06 activated with the heat from exhaust gas and a silencer 
05 are connected in sequence from an upstream side to a downstream side in 
an exhaust system Ex' leading from an exhaust port in an engine body 01. 
In order to activate, at an early stage, the conventional main catalyst 
which is to be activated with heat from exhaust gas, it is necessary to 
dispose the catalyst in the vicinity of the engine body, so that it may be 
exposed to the heat or hot exhaust gas. On the other hand, the 
self-exothermic type catalyst activated by electric current energy 
supplied from the outside is capable of being maintained at an activating 
temperature without relying on the hot exhaust gas. Therefore, if the 
self-exothermic type catalyst is disposed on the upstream side in the 
exhaust system in the vicinity of the engine body, as in the above prior 
art device, it is subjected to the heat of the hot exhaust gas and thus 
early thermal deterioration to lose its intended function as a catalyst 
whereby, as a result, it acts as a mere heater. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a novel 
exhaust emission control device for an internal combustion engine, wherein 
the exhaust emission control effect is further enhanced by a combination 
of a usual or essential catalyst and a self-exothermic type catalyst, 
while inhibiting any thermal deterioration of the self-exothermic type 
catalyst. 
To achieve the above object, according to the present invention, there is 
provided an exhaust emission control device for an internal combustion 
engine, comprising a main catalyst activated with heat from exhaust gas, a 
self-exothermic type auxiliary catalyst activated with heat supplied by an 
electric current and a silencer, in which these main and auxiliary 
catalysts and the silencer are connected in series and in sequence from an 
upstream side to a downstream side in an exhaust system which leads from 
an engine body. 
With the above construction, the main catalyst can be activated early with 
the heat of hot exhaust gas to enhance its conversion function. On the 
other hand, the auxiliary catalyst can be maintained at an activating 
temperature, while any thermal deterioration thereof due to the heat of 
the exhaust gas is inhibited to the best possible extent. In addition, as 
a result of the early activation of the main catalyst, the time of supply 
of electric current to the auxiliary catalyst can be shortened to 
contribute to a reduction in power consumption. Therefore, it is possible 
to efficiently perform an exhaust emission control by using the main and 
auxiliary catalysts in combination. 
In addition, to achieve the above object, according to an embodiment of the 
present invention, the main catalyst comprises first and second main 
catalysts which are connected in series and in sequence from the upstream 
side to the downstream side in the exhaust system. 
With the above construction, a further early activation of the main 
catalyst, particularly, the first main catalyst disposed in the vicinity 
of the engine body is achieved, thereby making it possible to shorten 
further the time of current supply to the auxiliary catalyst, leading to a 
further reduction in power consumption. 
The above and other objects, features and advantages of the invention will 
become apparent from the following description of preferred embodiments, 
taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described by way of preferred embodiments 
in connection with the accompanying drawings. 
Referring to FIG. 1, an exhaust system Ex for releasing exhaust gas 
discharged from an engine body 1 to the atmosphere is connected to an 
exhaust port 2 in the engine body 1. The exhaust system Ex includes an 
exhaust manifold 3 connected to the exhaust port 2, an exhaust pipe 4 
connected to a downstream end of the exhaust manifold 3, and a silencer 5 
connected to a downstream end of the exhaust pipe 4. 
The exhaust pipe 4 includes an upstream exhaust pipe section 4u and a 
downstream exhaust pipe section 4d. A conventional main catalyst 6 is 
connected between the exhaust pipe section 4u and 4d. The main catalyst 6 
is activated with heat from hot exhaust gas. Further, a self-exothermic 
type catalyst 7 (EHC) serving as an auxiliary catalyst is connected 
between a downstream end of the downstream exhaust pipe section 4d and an 
upstream end of the silencer 5, and is activated with heat supplied by 
electric energy. The self-exothermic type auxiliary catalyst 7 is formed 
of a metal carrier 7.sub.1, which is coated and bonded with an active 
material by means of a binder. The metal carrier 7.sub.1 is connected 
through a switch 9 to a battery 8. When the switch 9 is closed, the metal 
carrier 7.sub.1 is directly heated and activated by the electric current. 
Only when the temperature of the exhaust gas is low as at the start of an 
engine and thus the main catalyst 6 is still not activated, is the switch 
9 turned ON and the auxiliary catalyst 7 is heated by electric current. 
The operation of the first embodiment now will be described. When the 
engine body 1 is operated, the exhaust gas flows from the exhaust port 2 
through the exhaust system Ex and is released to the atmosphere. More 
specifically, the exhaust gas flows through the exhaust manifold 3 and the 
upstream exhaust pipe section 4u into the main catalyst 6, where it is 
primarily purified. Then, the exhaust gas flows into the downstream 
exhaust pipe section 4d into the auxiliary catalyst 7, where it is 
secondarily purified, and it then flows through the silencer 5 and is 
released to the atmosphere. 
The auxiliary catalyst 7 is heated and sufficiently activated by the 
electric current from the battery 8, even when the temperature of the 
exhaust gas is still low as at the start of the engine body 1. Therefore, 
even when the main catalyst 6 is in its unactivated state and hence, even 
before it exhibits its converting function, the exhaust gas containing a 
large amount of unburned components can effectively be purified. If the 
main catalyst 6 has been brought to a heated activated state by a 
continued operation of the engine body 1, the supply of the current to the 
auxiliary catalyst 7 is cut-off, so that the main catalyst 6 functions as 
a conventional or essential catalyst. The main catalyst 6 is located 
upstream in the exhaust system Ex and exposed to the hot exhaust gas, so 
that it is activated relatively early. Therefore, it is possible to 
shorten the time of current supply to the auxiliary catalyst 7, as 
compared with the above-described prior art. The auxiliary catalyst 7 is 
located downstream in the exhaust system Ex, so that the exhaust gas 
flowing into the auxiliary catalyst 7 has a lowered temperature, as 
compared with the prior art. Therefore, any thermal deterioration of the 
auxiliary catalyst 7 is inhibited as much as possible and hence, the 
auxiliary catalyst 7 can exhibit its intended converting function over a 
long period of time. 
A second embodiment of the present invention will now be described in 
connection with FIG. 2. In this embodiment a first main catalyst 6.sub.1 
and a second main catalyst 6.sub.2 are used in place of the main catalyst 
6 of the previous first embodiment. The first main catalyst 6.sub.1 is 
connected between the downstream end of the exhaust manifold 3 and the 
upstream end of the upstream exhaust pipe section 4u. The second main 
catalyst 6.sub.2 is connected between the downstream end of the upstream 
exhaust pipe section 4u and the upstream end of the downstream exhaust 
pipe section 4d. 
The structure of mounting the auxiliary catalyst 7 and the silencer 5 to 
the downstream exhaust pipe section 4d is the same as in the first 
embodiment, whereby they will not be described again at this point. 
The operation of the second embodiment now will be described. When the 
engine is operated, the exhaust gas flows through the exhaust manifold 3 
into the first main catalyst 6.sub.1 and further through the upstream 
exhaust pipe section 4u into the second main catalyst 6.sub.2, so that it 
is primarily purified stepwise by both the main catalysts 6.sub.1 and 
6.sub.2. Then, the exhaust gas flows through the downstream exhaust pipe 
section 4d into the auxiliary catalyst 7, where it is secondarily 
purified, and then flows through the silencer 5 and is released to the 
atmosphere. 
Also in the second embodiment, the first and second main catalysts 6.sub.1 
and 6.sub.2 are located upstream in the exhaust system Ex and sequentially 
exposed to the hot exhaust gas, so that an activating energy is applied to 
the first and second main catalysts 6.sub.1 and 6.sub.2 at an early stage. 
Specifically, the first main catalyst 6.sub.2 is located in the vicinity 
of the engine body 1 and therefore is activated very early, thereby making 
it possible to further shorten the time of current supply to the auxiliary 
catalyst 7 and to reduce the electric power consumed in the auxiliary 
catalyst 7. 
Although two embodiments of the present invention have been described, it 
will be understood that the present invention is not limited to these 
embodiments, and various other embodiments and modifications can be made 
within the scope of the invention. For example, the engine body may be 
either of a multi-cylinder type or a single cylinder type, and the main 
and auxiliary catalysts may be those conventionally known.