Device for purifying exhaust gas of an engine

An exhaust manifold of an engine is connected to a three way (TW) catalyst, and the TW catalyst is connected to an NH.sub.3 adsorbing and oxidizing (NH.sub.3 -AO) catalyst, such as the Cu-zeolite catalyst. The engine performs the lean and the rich operations alternately and repeatedly. When the engine performs the rich operation, the TW catalyst synthesizes NH.sub.3 from NO.sub.x in the inflowing exhaust gas, and the NH.sub.3 is then adsorbed in the NH.sub.3 -AO catalyst. Next, when the engine performs the lean operation, NO.sub.x passes through the TW catalyst, and the adsorbed NH.sub.3 is desorbed and reduces the inflowing NO.sub.x. When the rich operation is in process, or is to be started, the exhaust gas temperature flowing into the NH.sub.3 -AO catalyst is detected. If the temperature is equal to or higher than the upper threshold representing the rich endurance temperature, the lean or the stoichiometric operation is performed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for purifying exhaust gas of an 
engine. 
2. Description of the Related Art 
The air-fuel ratio of an air-fuel mixture in a combustion chamber of an 
internal combustion engine can be referred as an engine air-fuel ratio. 
Japanese unexamined patent publication No. 4-365920 discloses an exhaust 
gas purifying device for an internal combustion engine with 
multi-cylinders, the engine having a first and a second cylinder groups, 
in which the device is provided with: an engine operation control device 
to continuously make each cylinder of the first cylinder group a rich 
engine operation in which the engine air-fuel ratio is rich, and to 
continuously make each cylinder of the second cylinder group a lean engine 
operation in which the engine air-fuel ratio is lean; a first exhaust 
passage connected to each cylinder of the first cylinder group; a second 
exhaust passage connected to each cylinder of the second cylinder group 
and different from the first exhaust passage; an NH.sub.3 synthesizing 
catalyst arranged in the first exhaust passage for synthesizing ammonia 
NH.sub.3 from at least a part of NO.sub.x in the inflowing exhaust-gas; an 
interconnecting passage interconnecting the first exhaust passage 
downstream of the NH.sub.3 synthesizing catalyst and the second exhaust 
passage to each other; and an exhaust gas purifying catalyst arranged in 
the interconnecting passage to react NO.sub.x and NH.sub.3 flowing therein 
to each other to thereby purify NO.sub.x and NH.sub.3 simultaneously. In 
this exhaust gas purifying device, for example, a three way catalyst is 
used as the NH.sub.3 synthesizing catalyst, and a so-called zeolite 
catalyst, which is comprised of a zeolite carrying cobalt Co, copper Cu, 
nickel Ni, iron Fe, or the like, is used as the exhaust gas purifying 
catalyst. If a ratio of the total amount of air fed into the intake 
passage, the combustion chamber, and the exhaust passage upstream of a 
certain position in the exhaust passage to the total amount of fuel fed 
into the intake passage, the combustion chamber, and the exhaust passage 
upstream of the above-mentioned position is referred to as an exhaust gas 
air-fuel ratio of the exhaust gas flowing through the certain position, 
such a zeolite catalyst has a lean endurance temperature, which is an 
endurance temperature when the exhaust gas air-fuel ratio of the inflowing 
exhaust gas is lean, and a rich endurance temperature, which is an 
endurance temperature when the exhaust gas air-fuel ratio of the inflowing 
exhaust gas is rich, and the lean endurance temperature is generally 
higher than the rich endurance temperature. Note that if the temperature 
of the zeolite catalyst is higher than an endurance temperature thereof, 
the catalyst will remarkably deteriorate. 
On the other hand, the larger number of the cylinders of the second 
cylinder group which performs the lean engine operation is preferable for 
decreasing the fuel consumption rate. In this case, the exhaust gas 
air-fuel ratio of the majority of the exhaust gas flowing into the zeolite 
catalyst is lean. Thus, it may be considered that, if the temperature of 
the zeolite catalyst is controlled to be lower than the lean endurance 
temperature by, for example, controlling the temperature of the inflowing 
exhaust gas, the durability of the zeolite catalyst is ensured, to thereby 
ensure good purification of the exhaust gas. However, in this case, even 
though the temperature of the zeolite catalyst is made lower than the lean 
endurance temperature thereof, the temperature of the zeolite catalyst may 
be higher than the rich endurance temperature. However, microscopically, 
the exhaust gas of which the exhaust gas air-fuel ratio is lean and the 
exhaust gas of which the exhaust gas air-fuel ratio is rich flow into the 
zeolite catalyst alternately and repeatedly. Accordingly, if the 
temperature of the zeolite catalyst is higher than the rich endurance 
temperature thereof when the exhaust gas air-fuel ratio of the inflowing 
exhaust gas is rich, the zeolite catalyst may deteriorate remarkably, and 
thus good purification of the exhaust gas cannot be ensured. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a device for purifying an 
exhaust gas of an engine which can ensure the durability of the catalyst. 
According to the present invention, there is provided a device for 
purifying an exhaust gas of an engine having an exhaust passage, 
comprising: an exhaust gas purifying catalyst arranged in the exhaust 
passage, of which an endurance temperature, when an exhaust gas air-fuel 
ratio of the inflowing exhaust gas is lean or stoichiometric, is higher 
than a rich endurance temperature which is an endurance temperature when 
an exhaust gas air-fuel ratio of the inflowing exhaust gas is rich; 
exhaust gas air-fuel ratio control means for controlling the exhaust gas 
air-fuel ratio of the exhaust gas flowing into the exhaust gas purifying 
catalyst; making-rich means adapted for controlling the exhaust gas 
air-fuel ratio control means to make the exhaust gas air-fuel ratio of the 
exhaust gas flowing into the exhaust gas purifying catalyst rich; and 
avoiding-rich means for controlling the exhaust gas air-fuel ratio control 
means to make the exhaust gas air-fuel ratio of the exhaust gas flowing 
into the exhaust gas purifying catalyst lean or stoichiometric when the 
making-rich operation of the making-rich means is to be performed and when 
a temperature representing a temperature of the exhaust gas purifying 
catalyst is equal to or higher than the rich endurance temperature. 
The present invention may be more fully understood from the description of 
preferred embodiments of the invention set forth below, together with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In general, nitrogen oxides (NO.sub.x) include nitrogen monoxide NO, 
nitrogen dioxide NO.sub.2, dinitrogen tetroxide N.sub.2 O.sub.4, 
dinitrogen monoxide N.sub.2 O, etc. The following explanation is made 
referring NO.sub.x mainly as nitrogen monoxide NO and/or nitrogen dioxide 
NO.sub.2, but a device for purifying an exhaust gas of an engine according 
to the present invention can purify the other nitrogen oxides. 
FIG. 1 shows the case where the present invention is applied to an internal 
engine of the spark ignition type. However, the present invention may be 
applied to a diesel engine. Also, the engine shown in FIG. 1 can be used 
for an automobile, for example. 
Referring to FIG. 1, an engine body 1, which is a spark-ignition type 
engine, has four cylinders, i.e., a first cylinder #1, a second cylinder 
#2, a third cylinder #3, a fourth cylinder #4. Each cylinder #1 to #4 is 
connected to a common surge tank 3, via a corresponding branch 2, and the 
surge tank 3 is connected to a air-cleaner (not shown) via an intake duct 
4. In each branch 2, a fuel injector 5 is arranged to feed fuel, such as 
gasoline, to the corresponding cylinder. Further, a throttle valve 6 is 
arranged in the intake duct 4, an opening of which becomes larger as the 
depression of the acceleration pedal (not shown) becomes larger. Note that 
the fuel injectors 5 are controlled in accordance with the output signals 
from an electronic control unit 20. 
On the other hand, each cylinder is connected to a common exhaust manifold 
7, and the exhaust manifold 7 is connected to a catalytic converter 9 
housing an NH.sub.3 synthesizing catalyst 8 therein. The catalytic 
converter 9 is then connected to a muffler 11 housing an exhaust gas 
purifying catalyst 10 therein. The muffler 11 is then connected to a 
catalytic converter 13 housing an NH.sub.3 purifying catalyst 12 therein. 
Further, as shown in FIG. 1, a secondary air supplying device 14 is 
arranged in the exhaust passage between the muffler 11 and the catalytic 
converter 13, for supplying a secondary air to the NH.sub.3 purifying 
catalyst 12, and is controlled in accordance with the output signals from 
the electronic control unit 20. 
The electronic control unit 20 comprises a digital computer and is provided 
with a ROM (read only memory) 22, a RAM (random access memory) 23, a CPU 
(micro processor) 24, an input port 25, and an output port 26, which are 
interconnected by a bidirectional bus 21. Mounted in the surge tank 3 is a 
pressure sensor 27 generating an output voltage proportional to the 
pressure in the surge tank 3. The output voltage of the sensor 27 is input 
via an AD converter 28 to the input port 25. The intake air amount Q is 
calculated in the CPU 24 on the basis of the output signals from the AD 
converter 28. Further, mounted in the collecting portion of the exhaust 
manifold 7 is an air-fuel ratio sensor 29 generating an output voltage 
proportional to the exhaust gas air-fuel ratio of the exhaust gas flowing 
through the collecting portion of the exhaust manifold 7. The output 
voltage of the sensor 29 is input via an AD converter 30 to the input port 
25. Mounted in the exhaust passage around inlets of the catalytic 
converter 8 and the muffler 11 are temperature sensors 33a and 33b, each 
generating an output voltage proportional to the temperature of the 
exhaust gas passing therethrough. The output voltages of the sensors 33a 
and 34b is input via corresponding AD converters 34a and 34b to the input 
port 25. Further, connected to the input port 25 is a crank angle sensor 
31 generating an output pulse whenever the crank shaft of the engine 1 
turns by, for example, 30 degrees. The CPU 24 calculates the engine speed 
N in accordance with the pulse. On the other hand, the output port 26 is 
connected to the fuel injectors 5, and the secondary supplying device 14, 
via corresponding drive circuits 32. 
In the embodiment shown in FIG. 1, the NH.sub.3 synthesizing catalyst 8 is 
comprised of a three-way catalyst 8a, which is simply expressed as a TW 
catalyst, here. The TW catalyst 8a is comprised of precious metals such as 
palladium Pd, platinum Pt, and rhodium Rh, carried on a layer of, for 
example, alumina, formed on a surface of a base. 
FIG. 2 illustrates the purifying efficiency of the exhaust gas of the TW 
catalyst 8a. FIG. 2A shows that the TW catalyst 8a passes the inflowing 
NO.sub.x therethrough when the exhaust gas air-fuel ratio of the inflowing 
exhaust gas is lean with respect to the stoichiometric air-fuel ratio 
(A/F)S, which is about 14.6 and the air-excess ratio .lambda.=1.0, and the 
TW catalyst 8a synthesizes NH.sub.3 from a part of the inflowing NO.sub.x 
when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich. 
The NH.sub.3 synthesizing function of the TW catalyst 8a is partly 
unclear, but it can be considered that some of NO.sub.x in the exhaust gas 
of which the exhaust gas air-fuel ratio is rich is converted to NH.sub.3 
according to the following reactions (1) and (2), that is: 
EQU 5H.sub.2 +2NO.fwdarw.2NH.sub.3 +2H.sub.2 O (1) 
EQU 7H.sub.2 +2NO.sub.2 .fwdarw.2NH.sub.3 +4H.sub.2 O (2) 
On the contrary, it is considered that the other NO.sub.x is reduced to the 
nitrogen N.sub.2 according to the following reactions (3) to (6), that is: 
EQU 2CO+2NO.fwdarw.N.sub.2 +2CO.sub.2 (3) 
EQU 2H.sub.2 +2NO.fwdarw.N.sub.2 +2H.sub.2 O (4) 
EQU 4CO+2NO.sub.2 .fwdarw.N.sub.2 +4CO.sub.2 (5) 
EQU 4H.sub.2 +2NO.sub.2 .fwdarw.N.sub.2 +4H.sub.2 O (6) 
Accordingly, NO.sub.x flowing in the TW catalyst 8a is converted to either 
NH.sub.3 or N.sub.2 when the exhaust gas air-fuel ratio of the inflowing 
exhaust gas is rich, and thus NO.sub.x is prevented from being discharged 
from the TW catalyst 8a. 
As shown in FIG. 2, an efficiency ETA of the NH.sub.3 synthesizing of the 
TW catalyst 8a becomes larger as the exhaust gas air-fuel ratio of the 
inflowing exhaust gas becomes smaller or richer from the stoichiometric 
air-fuel ratio (A/F)S, and is kept constant when the exhaust gas air-fuel 
ratio of the inflowing exhaust gas becomes even smaller. In the example 
shown in FIG. 2, the NH.sub.3 synthesizing efficiency ETA is kept constant 
when the exhaust gas air-fuel ratio of the inflowing exhaust gas equals or 
is smaller than about 13.8, where the air-excess ratio .lambda. is about 
0.95. Note that, in the engine shown in FIG. 1, it is desired to 
synthesize as much NH.sub.3 as possible, when the exhaust gas air-fuel 
ratio of the exhaust gas flowing the TW catalyst 8a is rich, because of 
the reasons described below. Accordingly, a TW catalyst carrying palladium 
Pd or cerium Ce is used as the TW catalyst 8a. In particular, a TW 
catalyst carrying palladium Pd can also enhance an HC purifying 
efficiency, when the exhaust air-fuel ratio of the inflowing exhaust gas 
is rich. Further, note that a TW catalyst carrying rhodium Rh suppresses 
NH.sub.3 synthesizing therein, and a TW catalyst without rhodium Rh must 
be used as the TW catalyst 8a. 
On the other hand, in the embodiment shown in FIG. 1, the exhaust gas 
purifying catalyst 10 consists of an NH.sub.3 adsorbing and oxidizing 
catalyst 10a, which is simply expressed as a NH.sub.3 -AO catalyst. The 
NH.sub.3 -AO catalyst 10a is comprised of a so-called zeolite denitration 
catalyst, such as zeolite carrying copper Cu thereon, which is expressed 
as the Cu-zeolite catalyst, zeolite carrying copper Cu and platinum Pt 
thereon, and zeolite carrying iron Fe thereon, which is carried on a 
surface of a base. Alternatively, the NH.sub.3 -AO catalyst 10a is 
comprised of a solid acid such as zeolite, silica, silica-alumina, and 
titania, carrying the transition metals such as iron Fe and copper Cu or 
precious metals such as palladium Pd, platinum Pt and rhodium Rh. 
The NH.sub.3 -AO catalyst 10a adsorbs NH.sub.3 in the inflowing exhaust 
gas, and desorbs the adsorbed NH.sub.3 when the NH.sub.3 concentration in 
the inflowing exhaust gas becomes lower, or when the inflowing exhaust gas 
includes NO.sub.x. At this time, if the NH.sub.3 -AO catalyst 10a is in an 
oxidizing atmosphere, that is, if the exhaust gas air-fuel ratio of the 
inflowing exhaust gas is lean, the NH.sub.3 -AO catalyst 10a oxidizes all 
of NH.sub.3 desorbed therefrom. Also, if the inflowing exhaust gas 
includes both NH.sub.3 and NO.sub.x, the NH.sub.3 -AO catalyst 10a 
oxidizes NH.sub.3 by NO.sub.x. In these cases, the NH.sub.3 oxidizing 
function is not completely clear, but it can be considered that the 
NH.sub.3 oxidation occurs according to the following reactions (7) to 
(10), that is: 
EQU 4NH.sub.3 +70.sub.2 .fwdarw.4NO.sub.2 +6H.sub.2 O (7) 
EQU 4NH.sub.3 +50.sub.2 .fwdarw.4NO+6H.sub.2 O (8) 
EQU 8NH.sub.3 +6NO.sub.2 .fwdarw.12H.sub.2 O+7N.sub.2 (9) 
EQU 4NH.sub.3 +4NO+O.sub.2 .fwdarw.6H.sub.2 O+4N.sub.2 (10) 
The reactions (9) and (10), which are denitration, reduce both NO.sub.x 
produced in the oxidation reactions (7) and (8), and NO.sub.x in the 
exhaust gas flowing in the NH.sub.3 -AO catalyst 10a. 
It has been found, by experiment, that the NH.sub.3 -AO catalyst 10a of the 
Cu-zeolite catalyst performs good oxidation and denitration when the 
temperature of the inflowing exhaust gas is about 280.degree. to 
500.degree. C. On the other hand, it has been found that if the catalyst 
is arranged in the muffler 11, the temperature of the exhaust gas passing 
through the muffler 11 is about 280.degree. to 500.degree. C. Therefore, 
in this embodiment, the NH.sub.3 -AO catalyst 10a is arranged in the 
muffler 11 to thereby ensure good performance of the NH.sub.3 -AO catalyst 
10a. 
The NH.sub.3 purifying catalyst 12 muffler 11 is comprised of transition 
metals such as iron Fe and copper Cu, or precious metals such as palladium 
Pd, platinum Pt, and rhodium Rh, carried on a layer of, for example, 
alumina, formed on a surface of a base. The NH.sub.3 purifying catalyst 12 
purifies NH.sub.3 in the inflowing exhaust gas, if the catalyst 12 is in 
an oxidizing atmosphere, that is, if the exhaust gas air-fuel ratio of the 
inflowing exhaust gas is lean. In this case, it is considered that the 
oxidation and denitration reactions (7) to (10) mentioned above occur in 
the catalyst 12 and thereby NH.sub.3 is purified. In this embodiment, 
basically, the NH.sub.3 amount exhausted from the NH.sub.3 -AO catalyst 
10a is kept zero, but the NH.sub.3 purifying catalyst 12 prevents NH.sub.3 
from being discharged to the outside air, even if NH.sub.3 is discharged 
from the NH.sub.3 -AO catalyst 10a without being purified. 
In the engine shown in FIG. 1, the fuel injection time TAU is calculated 
using the following equation: 
EQU TAU=TB.multidot.((A/F)S/(A/F)T).multidot.FAF 
TB represents a basic fuel injection time suitable for making the engine 
air-fuel ratio of each cylinder equal to the stoichiometric air-fuel ratio 
(A/F)S, and is calculated using the following equation: 
EQU TB=(Q/N).multidot.K 
where Q represents the intake air amount, N represents the engine speed, 
and K represents a constant. Accordingly, the basic fuel injection time TB 
is the product of an intake air amount per unit engine speed and the 
constant. 
(A/F)T represents a target value for the control of the engine air-fuel 
ratio. When the target value (A/F)T is made larger to make the engine 
air-fuel ratio lean with respect to the stoichiometric air-fuel ratio, the 
fuel injection time TAU is made shorter and thereby the fuel amount to be 
injected is decreased. When the target value (A/F)T is made smaller to 
make the engine air-fuel ratio rich with respect to the stoichiometric 
air-fuel ratio, the fuel injection time TAU is made longer and thereby the 
fuel amount to be injected is increased. 
FAF represents a feedback correction coefficient for making the actual 
engine air-fuel ratio equal to the target value (A/F)T. The feedback 
correction coefficient FAF is determined on the basis of the output 
signals from the air-fuel ratio sensor 29. The exhaust gas air-fuel ratio 
of the exhaust gas flowing through the exhaust manifold 7 and detected by 
the sensor 29 conforms to the engine air-fuel ratio. When the exhaust gas 
air-fuel ratio detected by the sensor 29 is lean with respect to the 
target value (A/F)T, the feedback correction coefficient FAF is made 
larger and thereby the fuel amount to be injected is increased. When the 
exhaust gas air-fuel ratio detected by the sensor 29 is rich with respect 
to the target value (A/F)T, FAF is made smaller and thereby the fuel 
amount to be injected is decreased. In this way, the actual engine 
air-fuel ratio is made equal to the target value (A/F)T. Note that the 
feedback correction coefficient FAF fluctuates around 1.0. 
For detecting the exhaust gas air-fuel ratio more precisely, an additional 
air-fuel ratio sensor may be arranged in the exhaust passage between the 
TW catalyst 8a and the NH.sub.3 -AO catalyst 10a, or in the exhaust 
passage between the NH.sub.3 -AO catalyst 10a and the NH.sub.3 purifying 
catalyst 12, to compensate for the deviation of the engine air-fuel ratio 
from the target value (A/F)T due to the deterioration of the sensor 29. 
For the sensor 29 and the additional sensor, an air-fuel ratio sensor 
generating an output voltage which corresponds to the exhaust gas air-fuel 
ratio over the broader range of the exhaust gas air-fuel ratio may be 
used, while a Z-output type oxygen concentration sensor, of which an 
output voltage varies drastically when the detecting exhaust gas air-fuel 
ratio increases or decreases across the stoichiometric air-fuel ratio, may 
also be used. Additionally, the deterioration of the catalyst(s) located 
between the sensors may be detected on the basis of the output signals 
from the sensors. 
In the engine shown in FIG. 1, there is no device for supplying secondary 
fuel or secondary air in the exhaust passage, other than the secondary air 
supplying device 14. Thus, the engine air-fuel ratio in the exhaust 
passage upstream of the secondary air supplying device 14 conforms to the 
engine air-fuel ratio. In other words, the exhaust gas air-fuel ratio of 
the exhaust gas flowing in the TW catalyst 8a conforms to the engine 
air-fuel ratio, and the exhaust gas air-fuel ratio of the exhaust gas 
flowing in the exhaust gas purifying catalyst 10 also conforms to the 
engine air-fuel ratio. Contrarily, in the exhaust passage downstream of 
the secondary air supplying device 14, the exhaust gas air-fuel ratio 
conforms to the engine air-fuel ratio when the supply of the secondary air 
is stopped, and is made lean with respect to the engine air-fuel ratio 
when the secondary air is supplied. 
Next, the basic method for purifying the exhaust gas in the engine shown in 
FIG. 1 will be explained with reference to FIGS. 3, 4A, and 4B. 
In the engine shown in FIG. 1, an exhaust gas portion of which the exhaust 
gas air-fuel ratio is lean, and an exhaust gas portion of which the 
exhaust gas air-fuel ratio is rich, are formed from the exhaust gas of the 
engine 1, alternately and repeatedly. Then, the exhaust gas portions are 
introduced to, in turn, the TW catalyst 8a, the exhaust gas purifying 
catalyst 10, and the NH.sub.3 purifying catalyst 12. In other words, the 
exhaust gas air-fuel ratio of the exhaust gas flowing in the catalysts 8a 
and 10a is made lean and rich alternately and repeatedly, as shown in FIG. 
3. When the exhaust gas air-fuel ratio of the inflowing exhaust gas is 
made rich, the TW catalyst 8a converts NO.sub.x in the inflowing exhaust 
gas to NH.sub.3 or N.sub.2, as shown in FIG. 4A, according to the 
above-mentioned reactions (1) and (2). The NH.sub.3 synthesized in the TW 
catalyst 8a then flows into the NH.sub.3 -AO catalyst 10a. At this time, 
the concentration of NH.sub.3 in the inflowing exhaust gas is relatively 
high, and thus almost all of the NH.sub.3 in the inflowing exhaust gas is 
adsorbed in the NH.sub.3 -AO catalyst 10a. Even though NH.sub.3 flows out 
the NH.sub.3 -AO catalyst 10a without being adsorbed, the NH.sub.3 then 
flows into the NH.sub.3 purifying catalyst 12 and is purified or oxidized, 
because the catalyst 12 is kept under the oxidizing atmosphere by the 
secondary air supplying device 14. In this way, NH.sub.3 is prevented from 
being discharged to the outside air. 
Contrarily, when the exhaust gas air-fuel ratio of the inflowing exhaust 
gas is made lean, the TW catalyst 8a passes the inflowing NO.sub.x 
therethrough, as shown in FIG. 4B, and the NO.sub.x then flows into the 
NH.sub.3 -AO catalyst 10a. At this time, the NH.sub.3 concentration in the 
inflowing exhaust gas is substantially zero, and thus NH.sub.3 is desorbed 
from the NH.sub.3 -AO catalyst 10a. At this time, the NH.sub.3 -AO 
catalyst 10a is under the oxidizing atmosphere, and thus the desorbed 
NH.sub.3 acts as a reducing agent, and reduces and purifies NO.sub.x in 
the inflowing exhaust gas, according to the above-mentioned reactions (7) 
to (10). Note that, even if the NH.sub.3 amount desorbed from the NH.sub.3 
-AO catalyst 10a exceeds the amount required for reducing the inflowing 
NO.sub.x, the excess NH.sub.3 is purified in the NH.sub.3 -AO catalyst 10a 
or the NH.sub.3 purifying catalyst 12. Accordingly, NH.sub.3 is prevented 
from being discharged to the outside air. Note that, in this case, the 
secondary air is unnecessary. 
When the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, 
hydrocarbon HC, carbon monoxide CO, or hydrogen H.sub.2 may pass through 
the TW catalyst 8a and may flow into the NH.sub.3 -AO catalyst 10a. It is 
considered that the HC, CO, etc. act as the reducing agent, as well as 
NH.sub.3, and reduce a part of NO.sub.x on the NH.sub.3 -AO catalyst 10a. 
However, the reducing ability of NH.sub.3 is higher than those of HC, CO, 
etc., and thus NO.sub.x can be reliably purified by using NH.sub.3 as the 
reducing agent. 
In this way, NO.sub.x exhausted from the engine is reduced to N.sub.2 or 
adsorbed in the NH.sub.3 -AO catalyst 10a in the form of NH.sub.3 when the 
exhaust gas air-fuel ratio of the exhaust gas flowing into the catalysts 
8a and 10a is rich, and is reduced to N.sub.2 by NH.sub.3 desorbed from 
the NH.sub.3 -AO catalyst 10a when the exhaust gas air-fuel ratio of the 
exhaust gas flowing into the catalysts 8a and 10a is lean. Accordingly, 
NO.sub.x is prevented from being discharged to the outside air, regardless 
whether the exhaust gas air-fuel ratio of the exhaust gas flowing into the 
catalysts 8a and 10a is rich or lean. 
Note that, as mentioned above, it is desired that the NH.sub.3 purifying 
catalyst 12 is kept under the oxidizing atmosphere to ensure good NH.sub.3 
purification. In this embodiment, the secondary air supplying device 14 
supplies the secondary air to make the exhaust gas air-fuel ratio of the 
exhaust gas flowing into the NH.sub.3 purifying catalyst 12 equal to about 
15.3 (.lambda.=1.05). 
As long as the exhaust gas air-fuel ratio of the exhaust gas flowing into 
the TW catalyst 8a is kept lean, unburned hydrocarbon HC and/or carbon 
monoxide, etc. in the inflowing exhaust gas are oxidized and purified at 
the TW catalyst 8a. Contrarily, when the exhaust gas air-fuel ratio of the 
inflowing exhaust gas is rich, there may be the case where the HC and/or 
the CO passes through the TW catalyst 8a and the NH.sub.3 -AO catalyst 
10a. However, the HC and/or the CO then flows into the NH.sub.3 purifying 
catalyst 12 and are oxidized and purified sufficiently, because the 
catalyst 12 is kept in an oxidizing atmosphere, as mentioned above. 
To form the exhaust gas portions of which the exhaust gas air-fuel ratios 
are lean and rich respectively, there may be provided a secondary air 
supplying device for supplying the secondary air in, for example, the 
exhaust manifold 7. In this case, while the engine air-fuel ratio is kept 
rich, the supply of the secondary air is stopped to thereby form the 
exhaust gas portion of which exhaust gas air-fuel ratio is rich, and the 
secondary air is supplied to thereby form the exhaust gas portion of which 
exhaust gas air-fuel ratio is lean. Or, there may be provided with a 
secondary fuel supplying device for supplying the secondary fuel in, for 
example, the exhaust manifold 7. In this case, while the engine air-fuel 
ratio is kept lean, the supply of the secondary fuel is stopped to thereby 
form the exhaust gas portion of which exhaust gas air-fuel ratio is lean, 
and the secondary fuel is supplied to thereby form the exhaust gas portion 
of which exhaust gas air-fuel ratio is rich. 
However, as mentioned above, the exhaust gas air-fuel ratio of the exhaust 
gas flowing into the catalysts 8a and 10a conforms to the engine air-fuel 
ratio, in the engine shown in FIG. 1. Therefore, the engine air-fuel ratio 
is controlled to be lean and rich alternately and repeatedly to thereby 
make the exhaust gas air-fuel ratio of the exhaust gas flowing into the 
catalysts 8a and 10a lean and rich alternately and repeatedly. Namely, the 
engine 1 operates a lean engine operation in which the engine air-fuel 
ratio is lean to thereby make the exhaust gas air-fuel ratio of the 
exhaust gas flowing into the catalysts 8a and 10a lean, and the engine 1 
operates a rich engine operation in which the engine air-fuel ratio is 
rich to thereby make the exhaust gas air-fuel ratio of the exhaust gas 
flowing into the catalysts 8a and 10a rich, and the engine 1 operates the 
lean and rich engine operations alternately and repeatedly. 
If a target value of the exhaust gas air-fuel ratio of the exhaust gas 
flowing into the catalysts 8a and 10a is referred as a target air-fuel 
ratio (A/F)T, the actual exhaust gas air-fuel ratio of the exhaust gas 
flowing into the catalysts 8a and 10a is made equal to the target air-fuel 
ratio (A/F)T, by making the target value of the engine air-fuel ratio 
equal to the target air-fuel ratio (A/F)T. Therefore, in the embodiment, 
the target value of the engine air-fuel ratio is conformed to the target 
air-fuel ratio (A/F)T. The target air-fuel ratio (A/F)T is made equal to a 
lean air-fuel ratio (A/F)L which is lean with respect to the 
stoichiometric air-fuel ratio (A/F)S, and equal to a rich air-fuel ratio 
(A/F)R which is rich with respect to the stoichiometric air-fuel ratio 
(A/F)S, alternately and repeatedly, to thereby make the exhaust gas 
air-fuel ratio of the exhaust gas flowing into the catalysts 8a and 10a 
lean and rich alternately and repeatedly. Note that, if an engine 
operation period during which the engine performs the lean engine 
operation is referred as a lean operation period TL, and if an engine 
operation period during which the engine performs the rich engine 
operation is referred as a rich operation period TR, one lean operation 
period TL and one rich operation period TR, next to each other, form a 
cycle. 
In other words, NO.sub.x exhausted from the engine 1 is purified 
sufficiently and is prevented from being discharged to the outside air, by 
the engine operating in the lean and rich engine operating condition 
alternately and repeatedly. 
The lean air-fuel ratio (A/F)L and the rich air-fuel ratio (A/F)R may be 
determined in accordance with the engine operating condition, 
respectively. However, in the present embodiment, the lean air-fuel ratio 
(A/F)L is set constant at about 25.0, and the rich air-fuel ratio (A/F)R 
is set constant at about 14.0, regardless the engine operating condition. 
Therefore, the target air-fuel ratio (A/F)T is made equal to about 25.0 
when the lean engine operation is to be performed, and is made equal to 
about 13.8 when the rich engine operation is to be performed. 
If the air-fuel mixture spreading over the entire combustion chamber is 
uniformly formed when the engine air-fuel ratio is very lean, such as 
25.0, the spark plug 15 cannot ignite the air-fuel mixture, because the 
air-fuel mixture is very thin, and misfiring may occur. To solve this, in 
the engine shown in FIG. 1, an ignitable air-fuel mixture is formed in a 
restricted region in the combustion chamber and the reminder is filled 
with only the air or only the air and the EGR gas, and the air-fuel 
mixture is ignited by the spark plug 15, when the lean engine operation is 
to be performed. This prevents the engine from misfiring, even though the 
engine air-fuel ratio is very lean. Alternatively, the misfiring may be 
prevented by forming a swirl flow in the combustion chamber, while forming 
a uniform air-fuel mixture in the combustion chamber. 
As mentioned at the beginning, a smaller fuel consumption rate is desired, 
and thus it is desired to make the lean operation period TL as long as 
possible, and to make the rich operation period TR as short as possible. 
In particular, it is preferable that TL/TR is equal to or larger than 3, 
for the smaller fuel consumption rate. However, as the lean operation 
period TL becomes longer, the NH.sub.3 amount desorbed from the NH.sub.3 
-AO catalyst 10a becomes smaller. Thus, too, a longer lean operation 
period TL may lead to make the NH.sub.3 amount smaller than that required 
for purifying NO.sub.x in the NH.sub.3 -AO catalyst 10a, and thus NO.sub.x 
is discharged to the outside air without the reduction. To solve this 
problem, in this embodiment, an NH.sub.3 amount adsorbed in the NH.sub.3 
-AO catalyst 10a S(NO.sub.x) is obtained by obtaining an NH.sub.3 amount 
desorbed from the NH.sub.3 -AO catalyst 10a during the lean engine 
operation, and the lean engine operation is stopped and the rich engine 
operation started when the adsorbed NH.sub.3 amount S(NH.sub.3) becomes 
smaller than a predetermined minimum amount MIN(NH.sub.3). This prevents 
NO.sub.x flowing into the NH.sub.3 -AO catalyst 10a from being discharged 
to the outside air without being reduced. 
On the other hand, a shorter rich operation period is preferable. However, 
if the rich operation period TR is made too short, the adsorbed NH.sub.3 
amount S(NH.sub.3) may be smaller than that required for the sufficient 
reduction of NO.sub.x, and thereby NO.sub.x may be discharged without 
being reduced when the NO.sub.x amount flowing into the NH.sub.3 -AO 
catalyst 10a increases drastically. Further, too short a rich operation 
period may lead to frequent changes in the target air-fuel ratio (A/F)T 
between the lean and rich air-fuel ratios, and thus an undesired 
deterioration of the drivability may occur. However, if the rich operation 
period TR becomes longer, the NH.sub.3 -AO catalyst 10a is saturated with 
NH.sub.3, and a large amount of NH.sub.3 is discharged therefrom. To solve 
this problem, in this embodiment, the NH.sub.3 amount adsorbed in the 
NH.sub.3 -AO catalyst 10a during the rich engine operation is obtained to 
thereby obtain the adsorbed NH.sub.3 amount S(NH.sub.3), and the rich 
engine operation is stopped and the lean engine operation started when the 
adsorbed NH.sub.3 amount S(NH.sub.3) becomes larger than a maximum amount 
MAX(NH.sub.3), which is determined in accordance with the adsorbing 
capacity of the NH.sub.3 -AO catalyst 10a. In this way, the lean and the 
rich operation periods TL and TR are determined in accordance with the 
adsorbed NH.sub.3 amount S(NH.sub.3) of the NH.sub.3 -AO catalyst 10a, in 
the present embodiment. 
It is difficult to directly determine the adsorbed NH.sub.3 amount in the 
NH.sub.3 -AO catalyst 10a. Therefore, in this embodiment, the adsorbed 
NH.sub.3 amount is estimated on the basis of the NH.sub.3 amount 
synthesized in the TW catalyst 8a or flowing into the NH.sub.3 -AO 
catalyst 10a. In this case, a sensor for detecting the NH.sub.3 amount 
flowing into the NH.sub.3 -AO catalyst 10a may be arranged in the exhaust 
passage between the TW catalyst 8a and the NH.sub.3 -AO catalyst 10a. 
However, in the embodiment, considering the applicability, the synthesized 
NH.sub.3 amount is estimated on the basis of the NO.sub.x amount flowing 
into the TW catalyst 8a, and then the adsorbed NH.sub.3 amount is 
estimated on the basis of the synthesized NH.sub.3 amount. That is, the 
synthesized NH.sub.3 amount per unit time becomes larger as the NO.sub.x 
amount flowing into the TW catalyst 8a per unit time becomes larger. Also, 
the synthesized NH.sub.3 amount per unit time becomes larger as the 
synthesizing efficiency ETA becomes higher. 
On the other hand, the NO.sub.x amount exhausted from the engine per unit 
time becomes larger as the engine speed N becomes higher, and thus the 
NO.sub.x amount flowing into the TW catalyst 8a per unit time becomes 
larger. Also, the exhaust gas amount exhausted from the engine becomes 
larger and the combustion temperature becomes higher as the engine load 
Q/N (the intake air amount Q/the engine speed N) becomes higher, and thus 
the NO.sub.x amount flowing into the TW catalyst 8a per unit becomes 
larger as the engine load Q/N becomes higher. 
FIG. 6A illustrates the relationships, obtained by experiment, between the 
NO.sub.x amount exhausted from the engine per unit time Q(NO.sub.x), the 
engine load Q/N, and the engine speed N, with the constant lean or rich 
air-fuel ratio (A/F)L, (A/F)R. In FIG. 6A, the curves show the identical 
NO.sub.x amount. As shown in FIG. 6A, the exhausted NO.sub.x amount 
Q(NO.sub.x) becomes larger as the engine load Q/N becomes higher, and as 
the engine speed N becomes higher. Note that the exhausted NO.sub.x amount 
Q(NO.sub.x) is stored in the ROM 22 in advance in the form of a map as 
shown in FIG. 6B. 
The NH.sub.3 synthesizing efficiency ETA varies in accordance with the 
temperature TTC of the exhaust gas flowing into the TW catalyst 8a, which 
represents the temperature of the TW catalyst 8a. That is, as shown in 
FIG. 7, the synthesizing efficiency ETA becomes higher as the exhaust gas 
temperature TTC becomes higher when TTC is low, and becomes lower as TTC 
becomes higher when TTC is high, with the constant rich air-fuel ratio 
(A/F)R. The synthesizing efficiency ETA is stored in the ROM 22 in advance 
in the form of a map as shown in FIG. 7. 
Note that the exhausted NO.sub.x amount from the engine per unit time 
Q(NO.sub.x) varies in accordance with the engine air-fuel ratio. 
Therefore, if the lean or rich air-fuel ratio (A/F)L, (A/F)R is changed in 
accordance with, for example, the engine operating condition, the 
exhausted NO.sub.x amount Q(NO.sub.x) obtained by the map shown in FIG. 6B 
is required to be corrected on the basis of the actual lean or rich 
air-fuel ratio (A/F)L, (A/F)R. Further, the synthesizing efficiency ETA 
also varies in accordance with the exhaust gas air-fuel ratio of the 
exhaust gas flowing into the TW catalyst 8a, that is, the rich air-fuel 
ratio (A/F)R, as shown in FIG. 2. Therefore, if the rich air-fuel ratio 
(A/F)R is changed in accordance with, for example, the engine operating 
condition, the synthesizing efficiency ETA obtained by the map shown in 
FIG. 7 must also be corrected on the basis of the actual rich air-fuel 
ratio (A/F)R. 
The product of Q(NO.sub.x) calculated using the engine load Q/N and the 
engine speed N and the synthesizing efficiency ETA calculated using the 
exhaust gas temperature TTC represents the NH.sub.3 amount flowing into 
the NH.sub.3 -AO catalyst 10a per unit time. Accordingly, during the rich 
engine operation, the NH.sub.3 amount adsorbed in the NH.sub.3 -AO 
catalyst 10a is calculated using the following equation: 
EQU S(NH.sub.3)=S(NH.sub.3)+Q(NO.sub.x).multidot.ETA.multidot.DELTAa 
where DELTAa represents the time interval of calculation of Q(NO.sub.x) and 
ETA. Thus, Q(NO.sub.x).multidot.ETA.multidot.DELTAa represents the 
NH.sub.3 amount adsorbed in the NH.sub.3 -AO catalyst 10a from the last 
calculation of Q(NO.sub.x) and ETA until the present calculation. 
FIG. 8A illustrates the NH.sub.3 amount D(NH.sub.3) desorbed from the 
NH.sub.3 -AO catalyst 10a per unit time, when the exhaust gas air-fuel 
ratio of the exhaust gas flowing into the NH.sub.3 -AO catalyst 10a is 
changed from rich to lean, as obtained by experiment. In FIG. 8A, the 
curves show the identical desorbed NH.sub.3 amount. As shown in FIG. 8A, 
the desorbed NH.sub.3 amount D(NH.sub.3) becomes larger as the adsorbed 
NH.sub.3 amount S(NH.sub.3) becomes larger. Also, D(NH.sub.3) becomes 
larger as the temperature TAC of the exhaust gas flowing into the NH.sub.3 
-AO catalyst 10a, which represents the temperature of the NH.sub.3 -AO 
catalyst 10a, becomes higher. The desorbed NH.sub.3 amount D(NH.sub.3) is 
stored in the ROM 22 in advance in the form of a map as shown in FIG. 8B. 
Accordingly, during the lean engine operation, the adsorbed NH.sub.3 amount 
S(NH.sub.3) is calculated using the following equation: 
EQU S(NH.sub.3)=S(NH.sub.3)-D(NH.sub.3).multidot.DELTAd 
where DELTAd represents the time interval of the calculation of 
D(NH.sub.3), and thus D(NH.sub.3).multidot.DELTAd represents the NH.sub.3 
amount desorbed from the NH.sub.3 -AO catalyst 10a, from the last 
calculation of D(NH.sub.3) until the present calculation. 
To obtain the temperature TTC of the exhaust gas flowing into the TW 
catalyst 8a, and the temperature TAC of the exhaust gas flowing into the 
NH.sub.3 -AO catalyst 10a, temperature sensors 33a and 33b are arranged in 
the exhaust passage directly upstream of the TW catalyst 8a and directly 
upstream of the NH.sub.3 -AO catalyst 10a, respectively, in the engine 
shown in FIG. 1. Alternatively, the exhaust gas temperatures can be 
estimated on the basis of the engine operating condition, such as the 
engine load Q/N, the engine speed N, and the engine air-fuel ratio. 
In this embodiment, one lean operation period TL is performed for several 
minutes, and one rich operation period is performed for several seconds, 
for example. Therefore, in this embodiment, the engine 1 performs the lean 
engine operation basically, and performs the rich engine operation 
temporarily. In this case, a plurality of cylinders perform the lean 
engine operation during the lean engine operation, and a plurality of 
cylinders perform the rich engine operation during the rich engine 
operation. Note that the lean and the rich operation periods may be 
predetermined in the form of a time. 
If an endurance temperature of a catalyst is defined as a temperature that 
the catalyst quickly deteriorates and the durability thereof is remarkably 
lowered if the catalyst temperature is higher than that temperature, it 
has been found by the inventors of the present invention that the 
endurance temperature of the Cu-zeolite catalyst as the NH.sub.3 -AO 
catalyst 10a varies in accordance with the exhaust gas air-fuel ratio of 
the inflowing exhaust gas. Namely, it has been found that the rich 
endurance temperature, which is the endurance temperature when the exhaust 
gas air-fuel ratio of the inflowing exhaust gas is rich, of the Cu-zeolite 
catalyst is about 500.degree. C., and that the lean endurance temperature, 
which is the endurance temperature when the exhaust gas air-fuel ratio of 
the inflowing exhaust gas is lean, of the Cu-zeolite catalyst is higher 
than the rich endurance temperature of the Cu-zeolite catalyst and is 
about 600.degree. C. Further, it has been found that a stoichiometric 
endurance temperature, which is the endurance temperature when the exhaust 
gas air-fuel ratio of the inflowing exhaust gas is stoichiometric, of the 
Cu-zeolite catalyst is also higher than the rich endurance temperature of 
the Cu-zeolite catalyst. Note that, while the following explanation will 
be made by using an example where the NH.sub.3 -AO catalyst is constructed 
as the Cu-zeolite catalyst, the similar explanation will be made even if 
the NH.sub.3 -AO catalyst is constructed as those as described above. 
In the Cu-zeolite catalyst, copper Cu is carried in the form of copper 
oxide CuO. Thus, it is considered that, when the exhaust gas air-fuel 
ratio of the inflowing exhaust gas is rich, that is, when the Cu-zeolite 
catalyst is under the reducing atmosphere, if the catalyst temperature 
becomes higher than the endurance temperature, copper oxide CuO is reduced 
to copper Cu and the copper particles fall out from the carrier, and 
thereby the deterioration of the Cu-zeolite catalyst proceeds faster than 
that in the usual use. 
In this embodiment, the exhaust gas portions of which the exhaust gas 
air-fuel ratios are lean and rich flow in to the Cu-zeolite catalyst 
alternately and repeatedly. In this condition, when the exhaust gas 
portion of which the exhaust gas air-fuel ratio is lean flows into the 
Cu-zeolite catalyst, the catalyst will not remarkably deteriorate as long 
as the catalyst temperature is lower than the lean endurance temperature, 
even though the catalyst temperature is higher than the rich endurance 
temperature. However, the Cu-zeolite catalyst will deteriorate remarkably, 
if the catalyst temperature exceeds the rich endurance temperature when 
the exhaust gas portion of which the exhaust gas air-fuel ratio is rich is 
to be continuously introduced into the catalyst, or if the catalyst 
temperature has been higher than the rich endurance temperature when the 
exhaust gas air-fuel ratio of the exhaust gas portion flowing into the 
catalyst is to be changed from lean to rich. Thus, if the catalyst 
temperature becomes higher than the rich endurance temperature or has been 
higher than the rich endurance temperature when the exhaust gas air-fuel 
ratio of the inflowing exhaust gas is to be made rich, the remarkable 
deterioration of the catalyst is prevented by prohibiting the exhaust gas 
air-fuel ratio of the inflowing exhaust gas from being made rich. In other 
words, if the exhaust gas air-fuel ratio is made lean or stoichiometric, 
the endurance temperature of the Cu-zeolite catalyst is made higher and 
the catalyst temperature at this time becomes lower than the endurance 
temperature, and therefore the remarkable deterioration of the catalyst is 
prevented. 
There may be provided a device for feeding secondary air to the Cu-zeolite 
catalyst and an additional air-fuel ratio sensor for detecting the exhaust 
gas air-fuel ratio of the inflowing exhaust gas, and the exhaust gas 
air-fuel ratio of the inflowing exhaust gas may be controlled not to be 
rich by controlling the amount of the secondary air on the basis of the 
output signals of the additional air-fuel ratio sensor. However, as 
mentioned above, the exhaust gas air-fuel ratio of the exhaust gas flowing 
the Cu-zeolite catalyst conforms to the engine air-fuel ratio in the 
engine shown in FIG. 1. Further, the temperature of the inflowing exhaust 
gas TAC represents the catalyst temperature. Thus, in this embodiment, if 
the exhaust gas is equal to or higher than a predetermined, upper 
threshold temperature UTR when the rich engine operation is to be 
performed, the lean engine operation is performed. Namely, if 
TAC.gtoreq.UTR when the rich operation is performed or when the engine 
operation is to be changed from the lean operation to the rich operation, 
the engine operation is changed to the lean operation or is kept the lean 
operation. Note that the upper threshold UTR is obtained in advance, by 
experiment, so that the actual catalyst temperature is equal to or higher 
than the rich endurance temperature when the exhaust gas temperature TAU 
is equal to or higher than the upper threshold UTR. 
As mentioned above, the adsorbed NH.sub.3 amount S(NH.sub.3) decreases when 
the engine performs the lean operation. Even if the adsorbed amount 
S(NH.sub.3) falls below the minimum amount MIN(NH.sub.3), the rich 
operation is prohibited as long as TAC.gtoreq.UTR, to ensure the catalyst 
durability. However, if the lean operation is continued after 
S(NH.sub.3)&lt;MIN(NH.sub.3), the NH.sub.3 amount becomes insufficient to 
purify the NO.sub.x flowing into the Cu-zeolite catalyst, and to discharge 
the NO.sub.x without the reduction. Therefore, in this embodiment, if 
S(NH.sub.3)&lt;MIN(NH.sub.3) when the rich operation is prohibited, the 
target air-fuel ratio (A/F)T is set to the stoichiometric air-fuel ratio 
(A/F)S, that is, a stoichiometric engine operation is performed. When the 
target air-fuel ratio (A/F)T is set to the stoichiometric air-fuel ratio 
(A/F)S, NO.sub.x, HC, and CO in the exhaust gas are purified in the TW 
catalyst 8a sufficiently and simultaneously, as shown in FIG. 2. 
On the other hand, when the exhaust gas temperature TAC becomes lower than 
the upper threshold UTR, the rich operation is started or resumed. As a 
result, the synthesizing of NH.sub.3 used for purifying NO.sub.x is 
performed. Namely, the basic method of the exhaust gas purification of the 
embodiment is performed, and good purification of the exhaust gas is 
ensured. 
The lean air-fuel ratio for the lean operation which is performed when the 
rich operation is prohibited may be set to any air-fuel ratio, as long as 
the catalyst durability is ensured. However, if the lean air-fuel ratio is 
made slightly lean with respect to the stoichiometric air-fuel ratio, the 
exhaust gas temperature TAC becomes higher than that when the rich 
operation is performed with the rich air-fuel ratio of 14.0. As a result, 
the exhaust gas temperature TAC does not become lower than the upper 
threshold UTR, and the rich operation cannot be resumed. Thus, in this 
embodiment, the lean air-fuel ratio for the lean operation performed when 
the rich operation is prohibited is set to make the exhaust gas 
temperature TAC lower than that in the rich operation. Namely, the lean 
air-fuel ratio is set and kept constant, regardless the engine operation, 
to about 25.0, in this embodiment. When the lean air-fuel ratio is made 
very lean, such as 25.0, the exhaust gas temperature is considerably lower 
than that in the rich operation. As a result, the Cu-zeolite catalyst is 
quickly cooled and the temperature thereof quickly becomes lower than the 
rich endurance temperature. Thus, the rich operation is quickly resumed. 
FIGS. 9A to 9E illustrate a routine for executing the control of the engine 
operation periods. The routine is executed by interruption every 
predetermined crank angle. 
Referring to FIGS. 9A to 9E, first, in step 40 in FIG. 9A, it is judged 
whether FSTOIC is made 1. FSTOIC is made 1 when the rich operation is 
prohibited and the stoichiometric operation is to be performed, and is 
made zero in the other situation. FSTOIC is usually made zero and the 
routine goes to step 41, where it is judged whether FLEAN is made 1. FLEAN 
is made 1 when the rich operation is prohibited and the lean operation is 
to be performed, and is made zero in the other situation. FLEAN is usually 
made zero and the routine goes to step 42, where it is judged whether 
FRICH is made 1. FRICH is made 1 when the rich operation is to be 
performed, and is made zero when the lean operation is to be performed. If 
FRICH is set to 1, the routine goes to step 43, where adsorbed NH.sub.3 
amount S(NH.sub.3) is calculated. When the routine goes to step 43 with 
FRICH=1, the rich operation is performed and the adsorbed NH.sub.3 amount 
is increased. Thus, in the step 43, an increment of the adsorbed NH.sub.3 
amount is performed. That is, the increment process shown in FIG. 9D, and 
described below, is performed in the step 43. In the following step 44, it 
is judged whether the adsorbed NH.sub.3 amount S(NH.sub.3) is larger than 
the maximum amount MAX(NH.sub.3). If S(NH.sub.3)&gt;MAX(NH.sub.3), the 
routine goes to step 45, where FRICH is made zero, and then the processing 
cycle is ended. Namely, if S(NH.sub.3)&gt;MAX(NH.sub.3), the adsorbed 
NH.sub.3 amount is sufficient to purify NO.sub.x, and the rich operation 
is stopped and the lean operation is started. Accordingly, the rich 
operation period TR is a period from when FRICH is made 1 until 
S(NH.sub.3)&gt;MAX(NH.sub.3). 
If S(NH.sub.3).ltoreq.MAX(NH.sub.3), in step 44, the routine jumps to step 
48. 
If FRICH=0, in step 42, the routine goes to step 46, where the adsorbed 
NH.sub.3 amount S(NH.sub.3) is calculated. When the routine goes to step 
46 with FRICH=0, the lean operation is performed and the adsorbed NH.sub.3 
amount is decreased. Thus, in the step 46, a decrement of the adsorbed 
NH.sub.3 amount is performed. That is, the decrement process shown in FIG. 
9E, and described below, is performed in the step 46. In the following 
step 47, it is judged whether the adsorbed NH.sub.3 amount S(NH.sub.3) is 
smaller than the minimum amount MIN(NH.sub.3). If 
S(NH.sub.3).gtoreq.MIN(NH.sub.3), the processing cycle is ended. Namely if 
S(NH.sub.3).gtoreq.MIN(NH.sub.3), the adsorbed NH.sub.3 amount S(NH.sub.3) 
is judged to be still large to purify NO.sub.x, and thus the lean 
operation is continued. If S(NH.sub.3)&lt;MIN(NH.sub.3), the routine goes to 
step 48. Accordingly, the routine goes to step 48 when the rich operation 
is performed and S(NH.sub.3) is smaller than MAX(NH.sub.3), that is, the 
rich operation is to be continued, from step 44, or when the lean 
operation is performed and S(NH.sub.3) becomes smaller than MIN(NH.sub.3), 
that is, the rich operation is to be started, from step 47. 
In step 48, the exhaust gas temperature TAC is obtained. In the following 
step 49, the it is judged whether the exhaust gas temperature TAC is equal 
to or higher than the upper threshold UTR. If TAC&lt;UTR, the routine goes to 
step 50, where FRICH is set or kept to 1, and then the processing cycle is 
ended. Namely, when TAC&lt;UTR, the durability of the Cu-zeolite catalyst is 
judged to be ensured, and thus the rich operation is continued or started. 
That is, when the routine goes from step 44 to step 50, via steps 48 and 
49, S(NH.sub.3) is equal to or smaller than MAX(NH.sub.3). In this 
condition, the adsorbed NH.sub.3 amount is judged to be still insufficient 
to purify NO.sub.x, and thus the rich operation is continued. On the other 
hand, when the routine goes from step 47 to step 50, via steps 48 and 49, 
S(NH.sub.3) is smaller than MIN(NH.sub.3). In this condition, the adsorbed 
NH.sub.3 amount is judged to be insufficient to purify NO.sub.x, and thus 
the rich operation is started. Accordingly, the lean operation period TL 
is from when the FRICH is made zero until S(NH.sub.3)&lt;MIN(NH.sub.3). 
Contrarily, if TAC.gtoreq.UTR, the routine goes to step 51, where FLEAN is 
made 1 and the processing cycle is ended. Namely, if TAC.gtoreq.UTR, the 
durability is judged to be lowered by the rich operation, and the rich 
operation is prohibited. 
When FLEAN=1, the routine goes from step 41 to step 52 in FIG. 9B, where 
the adsorbed NH.sub.3 amount is calculated. When FLEAN=1 and FSTOIC=0, the 
lean operation is performed and the adsorbed NH.sub.3 amount is 
decreasing. Thus, in the step 52, the decrement of the adsorbed NH.sub.3 
amount shown in FIG. 9E is performed. In the following step 53, the 
exhaust gas temperature TAC is calculated. In the following step 54, it is 
judged whether TAC.gtoreq.UTR. If TAC&lt;UTR, the routine goes to step 55, 
where FLEAN is set to zero, and then the processing cycle is ended. 
Namely, when TAC&lt;UTR, it is judged that the catalyst endurance is not 
lowered even if the rich operation is performed, and the lean operation is 
stopped. In this condition, FRICH=1 and the thus rich operation is 
started. 
If TAC.gtoreq.UTR in step 54, the routine goes to step 56, where it is 
judged whether the adsorbed NH.sub.3 amount S(NH.sub.3) is smaller than 
the minimum amount MIN(NH.sub.3). If S(NH.sub.3).gtoreq.MIN(NH.sub.3), the 
processing cycle is ended. Namely, if S(NH.sub.3).gtoreq.MIN(NH.sub.3), 
the adsorbed NH.sub.3 amount S(NH.sub.3) is judged to be still large to 
purify NO.sub.x, and thus the lean operation is continued. If 
S(NH.sub.3)&lt;MIN(NH.sub.3), the routine goes to step 57, where FSTOIC is 
set to 1, and then the processing cycle is ended. Namely if 
S(NH.sub.3)&lt;MIN(NH.sub.3), the adsorbed NH.sub.3 amount S(NH.sub.3) is 
judged to be insufficient to purify NO.sub.x, and the stoichiometric 
operation is started. 
When FSTOIC=1, the routine goes from step 40 to step 58 in FIG. 9C, where 
the adsorbed NH.sub.3 amount is calculated. When FSTOIC=1, the adsorbed 
NH.sub.3 amount is decreasing, and thus, in the step 52, the decrement of 
the adsorbed NH.sub.3 amount shown in FIG. 9E is performed. In the 
following step 59, the exhaust gas temperature TAC is calculated. In the 
following step 60, it is judged whether TAC.gtoreq.UTR. If TAC&lt;UTR, the 
routine goes to step 61, where FSTOIC is set to zero, and then the 
processing cycle is ended. Namely, when TAC&lt;UTR, it is judged that the 
catalyst endurance is not lowered even if the rich operation is performed, 
and the stoichiometric operation is stopped. In this condition, FRICH=1 
and the thus rich operation is started. If TAC.gtoreq.UTR, in step 60, the 
processing cycle is ended. Namely, the stoichiometric operation is 
continued. 
FIG. 9D shows an increment processing of S(NH.sub.3) performed in step 43 
in FIG. 9A. 
Referring to FIG. 9D, in step 71, the exhausted NO.sub.x amount Q(NO.sub.x) 
is calculated using the map shown in FIG. 6B, on the basis of the engine 
load Q/N and the engine speed N. In the following step 72, the exhaust gas 
temperature TTC is obtained. In the following step 73, the NH.sub.3 
synthesizing efficiency ETA is calculated using the map shown in FIG. 7. 
In the following step 74, the adsorbed NH.sub.3 amount S(NH.sub.3) is 
calculated using the following equation: 
EQU S(NH.sub.3)=S(NH.sub.3)+Q(NO.sub.x).multidot.ETA.multidot.DELTAa 
where DELTAa is a time interval from the last processing cycle until the 
present processing cycle, and is obtained by, for example, a timer. Then, 
the processing cycle is ended. 
FIG. 9E shows a decrement processing of S(NH.sub.3) performed in steps 46 
in FIG. 9A, 55 in FIG. 9B, and 58 in FIG. 9C. 
Referring to FIG. 9E, in step 75, the exhaust gas temperature TAC is 
obtained. In the following step 76, the desorbed NH.sub.3 amount 
D(NH.sub.3) is calculated using the map shown in FIG. 8B, on the basis of 
TAC and the present S(NH.sub.3). In the following step 77, the adsorbed 
NH.sub.3 amount S(NH.sub.3) is calculated using the following equation: 
EQU S(NH.sub.3)=S(NH.sub.3)-D(NH.sub.3).multidot.DELTAd 
where DELTAd is a time interval from the last processing cycle until the 
present processing cycle. Then, the processing cycle is ended. 
FIG. 10 illustrates the routine for calculating the fuel injection time 
TAU. 
Referring to FIG. 10, first, in step 80, the basic fuel injection time TB 
is calculated using the following equation, on the basis of the engine 
load Q/N and the engine speed N: 
EQU TB=(Q/N).multidot.K 
In the following step 81, the feedback correction coefficient FAF is 
calculated. In the following step 82, it is judged whether FRICH, which is 
controlled in the routine shown in FIGS. 9A to 9E, is made 1. If FRICH=0, 
that is, if the lean operation is to be performed, the routine goes to 
step 83, where the lean air-fuel ratio (A/F)L is calculated. In this 
embodiment, the lean air-fuel ratio (A/F)L is kept constant at 25.0 
regardless the engine operating condition, and thus the lean air-fuel 
ratio (A/F)L is made 25.0 in step 83. In the following step 84, the lean 
air-fuel ratio (A/F)L is memorized as the target air-fuel ratio (A/F)T. 
Thus, the lean operation is performed. Next, the routine goes to step 90. 
If FRICH=1 in step 82, that is, if the rich operation is to be performed, 
the routine goes to step 85, where it is judged whether FLEAN, which is 
controlled in the routine shown in FIGS. 9A to 9E, is made 1. If FLEAN=1, 
that is, if the rich operation is to be prohibited and the lean operation 
is to be performed, the routine goes to steps 83 and 84. Thus, the lean 
operation is performed. 
If FLEAN=0 in step 85, that is, if the rich operation is to be prohibited 
and the lean operation is not to be performed, the routine goes to step 
86, where it is judged whether FSTOIC, which is controlled in the routine 
shown in FIGS. 9A to 9E, is made 1. If FSTOIC=1, that is, if the rich 
operation is to be prohibited and the stoichiometric operation is to be 
performed, the routine goes to step 87, where the target air-fuel ratio 
(A/F)T is set to the stoichiometric air-fuel ratio (A/F)S. Thus, the 
stoichiometric operation is performed. Then, the routine goes to step 90. 
If FSTOIC=0, that is, if the rich operation is to be performed, more 
precisely, if the rich operation is to be performed and if there is no 
need to prohibit the rich operation, the routine goes to step 88, where 
the rich air-fuel ratio (A/F)R is calculated. In this embodiment, the rich 
air-fuel ratio (A/F)R is kept constant at 14.0 regardless the engine 
operating condition, and thus the rich air-fuel ratio (A/F)R is made 14.0 
in step 88. In the following step 64, the rich air-fuel ratio (A/F)R is 
memorized as the target air-fuel ratio (A/F)T. Next, the routine goes to 
step 90. 
In step 90, the fuel injection time TAU is calculated using the following 
equation: 
EQU TAU=TB.multidot.((A/F)S/(A/F)T).multidot.FAF 
Each fuel injector 5 injects the fuel for the fuel injection time TAU. 
In the embodiment mentioned above, the exhaust gas can be purified 
sufficiently using a single exhaust passage, that is, without providing a 
plurality of exhaust passages. Accordingly, the structure of the exhaust 
gas purifying device is kept small and simple. 
On the other hand, if a ratio of the number of the cylinders which performs 
the lean engine operation to the number of the cylinders which performs 
the rich engine operation in one cycle (see FIG. 5) is referred as a 
cylinder number ratio RATIO, it is desired to make the cylinder number 
ratio RATIO as large as possible, to thereby make the fuel consumption 
rate as small as possible. However, if a part of the cylinders performs 
the rich engine operation and the other performs the lean engine operation 
as in the prior art device mentioned at the beginning, the cylinder number 
ratio RATIO is limited. That is, in the four-cylinders engine, for 
example, the ratio RATIO is limited to 3 and cannot be made larger than 3. 
Thus, the decrease of the fuel consumption rate is limited, with the 
identical lean and rich air-fuel ratio (A/F)L and (A/F)R. Contrarily, in 
the embodiment, the ratio RATIO is allowed to be made larger until the 
NO.sub.x amount flowing into the NH.sub.3 -AO catalyst 10a exceeds the 
NH.sub.3 amount desorbed from the catalyst 10a. In particular, the 
cylinder number ratio RATIO can be made larger than 3 in a four-cylinder 
engine. As a result, the fuel consumption rate can be made lower. 
Further, if the first cylinder #1 continuously performs the rich operation 
and the second, third, and fourth cylinders #2, #3, and #4 continuously 
perform the lean operation, for example, as in the prior art, a large 
temperature difference between the exhaust gases exhausted from the 
cylinders #1 to #4 may occur, and may lead a larger temperature drop in 
the engine body or in the exhaust manifold 7, to thereby lead a large 
thermal distortion therein. Furthermore, in this example, a large amount 
of the deposition may exist in the first cylinder #1 which performs the 
rich operation continuously. Contrarily, in this embodiment, a cylinder in 
which the lean or rich operation is to be performed is not specified, that 
is, every cylinder performs both of the lean and rich operations. 
Accordingly, the large thermal distortion in the engine body or in the 
exhaust manifold 7 is prevented, and the large amount of the deposition on 
the particular cylinder is also prevented. 
Additionally, the exhaust gas purifying method according to the present 
embodiment may be used in a single cylinder engine. 
Further, the NH.sub.3 -AO catalyst 10a is comprised of the Cu-zeolite 
catalyst. Alternatively, the NH.sub.3 -AO catalyst may be comprised of any 
catalyst comprising zeolite carrying a metal, or any catalyst comprising 
palladium Pd. 
According to the present invention, it is possible to provide a device for 
purifying an exhaust gas of an engine which can ensure the durability of 
the catalyst. 
While the invention has been described by reference to specific embodiments 
chosen for purposes of illustration, it should be apparent that numerous 
modifications could be made thereto by those skilled in the art without 
departing from the basic concept and scope of the invention.