NOx purification apparatus for an internal combustion engine

A plurality of lean NOx catalysts are installed in passages of a dual passage portion of an exhaust conduit of an internal combustion engine. A space velocity changing means including a valve is provided at a connecting portion of the passages so that the amount of exhaust gas flowing through the lean NOx catalysts is altered periodically. When a space velocity of exhaust gas at the lean NOx catalyst changes from a low velocity to a high velocity, an NOx purification rate of the lean NOx catalyst increases momentarily. By repeatedly generating the NOx purification rate increased conditions, the NOx purification rate of the NOx purification apparatus including the plurality of lean NOx catalysts is greatly increased.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a nitrogen oxides (NOx) purification 
apparatus provided with a "lean" NOx catalyst which is defined as an NOx 
purification catalyst capable of purifying NOx under excess-oxygen 
conditions such as in exhaust gas from an internal combustion engine 
operated at lean air-fuel ratios. 
2. Description of the Prior Art 
To improve fuel economy and to suppress exhaust of carbon dioxide, thereby 
reducing global warming, engines capable of fuel combustion at lean 
air-fuel ratios (lean burn engines) are being developed and are in actual 
use today. Since a conventional catalyst (three-way catalyst) cannot 
reduce and purify NOx at lean air-fuel ratios, there is a need to develop 
a catalyst or system that can purify NOx even at lean air-fuel ratios. 
Japanese Patent Publication HEI 1-139145 proposes a copper (Cu)/zeolite 
catalyst in which copper is exchanged on a zeolite carrier and which is 
capable of purifying NOx at lean air-fuel ratios in the presence of 
hydrocarbons (HC). To use such a lean NOx catalyst as an NOx purification 
apparatus for internal combustion engines, a system wherein the lean NOx 
catalyst can operate at a high NOx purification rate should be developed. 
In this meaning, Japanese Patent Application HEI 2-317664 filed Nov. 26, 
1990 proposes an exhaust gas purification system wherein two lean NOx 
catalysts are arranged in parallel with each other in an exhaust system of 
an internal combustion engine and exhaust gas flow is switched so that, 
when exhaust gas is flowing through one lean NOx catalyst, exhaust gas 
flow through the other lean NOx catalyst is stopped. When the exhaust gas 
flow is switched to flow through one of the two catalysts, the temperature 
of the one catalyst increases accompanied by a momentary increase in the 
NOx purification rate. By repeating switching of exhaust gas flow, the 
increase in the NOx purification rates is repeatedly produced so that the 
NOx purification rate of the system increases. 
However, it has been found in further tests that almost no increase in the 
NOx purification rate of the system is seen just after the space velocity 
of exhaust gas at the lean NOx catalyst changes from a low velocity to a 
high velocity such as occurs just after an idling condition changes to an 
acceleration condition. This is true even though the exhaust gas is merely 
switched between the two lean NOx catalysts. 
This suggests that an NOx purification rate (NOx conversion) of a lean NOx 
catalyst is affected not only by a change in the catalyst temperature but 
also by a space velocity of exhaust gas at the catalyst. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an NOx purification 
apparatus for an internal combustion engine wherein an NOx purification 
rate of a lean NOx catalyst is increased making good use of the effect 
that a change in the space velocity has on the NOx purification rate of 
the lean NOx catalyst. 
The above-described object is achieved by an NOx purification apparatus for 
an internal combustion engine in accordance with the present invention. 
The apparatus in accordance with the present invention includes an 
internal combustion engine having an exhaust system, a plurality of lean 
NOx catalysts including a first lean NOx catalyst and a second lean NOx 
catalyst arranged in parallel with each other in the exhaust system of the 
internal combustion engine, and space velocity changing means for changing 
alternately and periodically a first velocity of exhaust gas at the first 
lean NOx catalyst and a second velocity of exhaust gas at the second lean 
NOx catalyst so that when the first velocity is high, the second velocity 
is low, and when the first velocity is low, the second velocity is high. 
In tests executed by the inventors it has been found that when the space 
velocity of exhaust gas at a lean NOx catalyst (a ratio of the volume of 
exhaust gas to the volume of the catalyst) changes, the NOx purification 
rate of the lean NOx catalyst changes for a few minutes and then returns 
to the original NOx purification rate. More particularly, when the space 
velocity at the lean NOx catalyst changes from a high velocity to a low 
velocity, the NOx purification rate of the lean NOx catalyst is 
momentarily increased to a great extent, and when the space velocity at 
the lean NOx catalyst changes from a low velocity to a high velocity, the 
NOx purification of the lean NOx catalyst shows almost no change and in 
some cases is slightly decreased. 
When the exhaust gas flow ratio is changed between a plurality of lean NOx 
catalysts arranged in parallel with each other, the NOx purification rate 
of the system increases. More particularly, the NOx purification rate of 
the system is increased when the space velocity at one of the lean NOx 
catalysts changes from a high velocity to a low velocity so that the NOx 
purification rate of the catalyst greatly increases, while the space 
velocity at another lean NOx catalyst changes from a low velocity to a 
high velocity so that the NOx purification rate of the catalyst slightly 
decreases. In this instance, since the magnitude of the increase in the 
NO.sub.x purification rate is greater than the magnitude of the decrease 
in the NOx purification rate, the total NOx purification rate of the 
system is improved. 
The increase in the NOx purification rate of the system is seen for only a 
few minutes after the changing of the space velocity, and after these few 
minutes the NOx purification rate of the system returns to the original, 
normal NOx purification rate. By repeating the changing of the space 
velocity, the NOx purification rate increased conditions are repeatedly 
generated so that a high NOx purification rate of the system is obtained 
for a long period of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Four embodiments of the present invention will be described. The first 
embodiment is illustrated in FIGS. 1-4, the second embodiment is 
illustrated in FIG. 5, the third embodiment is illustrated in FIGS. 6-13, 
and the fourth embodiment is illustrated in FIGS. 14-18. Throughout all 
the embodiments, the same structural portions are denoted with the same 
reference numerals. 
FIRST EMBODIMENT 
As illustrated in FIG. 1, a dual passage portion 6 is provided in an 
exhaust conduit 4 of an internal combustion engine 2 capable of fuel 
combustion at lean air-fuel ratios (which may be a lean burn gasoline 
engine or a diesel engine). The dual passage portion includes a first 
passage 6a and a second passage 6b which are connected in parallel to each 
other. The dual passage portion 6 may be replaced by a plural passage 
portion which includes three or more passages. 
Two lean NOx catalysts 8, that is, a first lean NOx catalyst 8a and a 
second lean NOx catalyst 8b are installed in the dual passage portion 6. 
More particularly, the first lean NOx catalyst 8a is installed in the 
first passage 6a and the second lean NOx catalyst 8b is installed in the 
second passage 6b. When the plural passage portion includes three or more 
passages, the lean NOx catalysts 8 are installed in respective passages, 
and the lean NOx catalysts are grouped into two groups, a first group of 
lean NOx catalysts 8a and a second group of lean NOx catalysts 8b. 
Each lean NOx catalyst 8 preferably comprises a transition metal/zeolite 
catalyst which comprises a zeolite carrier on which at least one kind of 
transition metal is ion-exchanged. The transition metal is, for example, 
copper or cobalt. 
A space velocity changing means 10 is installed in an upstream side 
connecting portion of the the passages 6a and 6b. The space velocity 
changing means 10 comprises a valve for changing alternately and 
periodically a first space velocity of exhaust gas at the first lean NOx 
catalyst 8a and a second space velocity of exhaust gas at the second lean 
NOx catalyst 8b so that when one velocity of the first and second space 
velocities is high, the other velocity is low, and when the one velocity 
changes to be low, the other velocity changes to be high. 
More particularly, the space velocity changing means 10 includes a valve 
body 10a and an actuator 10b for moving the valve body 10a periodically. 
The valve body 10a does not close the first and second passages 6a and 6b 
perfectly so that some amount of exhaust gas is always flowing in the 
first and second passages 6a and 6b during operation of the engine. A 
first amount of exhaust gas flowing through the first passage 6a and a 
second amount of exhaust gas flowing through the second passage 6b alter 
periodically, but the total amount of the first amount and the second 
amount remains substantially constant. 
The alteration of the space velocity at each of the first lean NOx catalyst 
8a and the second lean NOx catalyst 8b is shown in FIG. 2 in the case 
where the amount of exhaust gas exhausted from the internal combustion 
engine is constant. The space velocity alters periodically between a high 
space velocity and a low space velocity. When a large portion (for 
example, eighty percent) of exhaust gas flows through the first lean NOx 
catalyst 8a and the remaining twenty percent of exhaust gas flows through 
the second lean NOx catalyst 8b, the space velocity at the first lean NOx 
catalyst 8a is high and the space velocity at the second lean NOx catalyst 
8b is low. In contrast, when a large portion (for example, eighty percent) 
of exhaust gas flows through the second lean NOx catalyst 8b and the 
remaining twenty percent of the exhaust gas flows through the first lean 
NOx catalyst 8a, the space velocity at the second lean NOx catalyst 8b is 
high and the space velocity at the first lean NOx catalyst 8a is low. 
The cycle of the alteration of the space velocity is preferably set at 
about thirty seconds to two minutes. The space velocity is preferably 
altered only when the exhaust gas temperature at the catalyst is in the 
range of 330.degree. C.-470.degree. C. This is because significantly less 
increase in the NOx purification rate is seen if the alteration of the 
space velocity is conducted at temperatures outside this range. 
The space velocity changing means 10 may be replaced by fluidics means. 
More particularly, control flow supply ports for supplying small amounts 
of flow may be provided in opposite side surfaces of the upstream side 
connecting portion of the passages 6a and 6b so that the main flow of 
exhaust gas passing through the connecting portion is controlled by the 
control flow. In the control, the characteristic that the exhaust gas 
tends to flow along the side surface of the passage when the control flow 
to the side surface is cut (Coanda effect), is utilized. 
The space velocity changing means 10 may alternatively be provided at a 
downstream side connecting portion of the first passage 6a and the second 
passage 6b. 
Operation of the first embodiment will now be explained. 
It has been found by the inventor that when the space velocity at the lean 
NOx catalyst 8 changes, the NOx purification rate of the lean NOx catalyst 
8 also causes a change which continues for a few minutes. 
More particularly, when the space velocity of exhaust gas at a lean NOx 
catalyst 8 changes from a high velocity to a low velocity, the NOx 
purification rate of the lean NOx catalyst 8 temporarily increases to a 
great extent. Contrarily, when the space velocity of exhaust at the lean 
NOx catalyst 8 changes from a low velocity to a high velocity, the NOx 
purification rate of the lean NOx catalyst 8 shows almost no increase and 
in some cases decreases slightly. 
The reason why the NOx purification rate of the lean NOx catalyst 8 
increases when the space velocity changes to a low velocity is thought to 
be as follows: Since, at high velocity, exhaust gas flows through the 
catalyst without sufficiently contacting the surface of porosities of the 
catalyst, activated points on the surface of the catalyst tend not to be 
excessively consumed so that the number of the activated points increases 
temporarily when the velocity is high. However, when the space velocity 
changes from the high velocity to a low velocity, exhaust gas sufficiently 
contacts the surface of porosities of the catalyst and consumes the 
activated points which have increased during the high space velocity 
operation. As a result, the NOx purification rate of the lean NOx catalyst 
temporarily increases. When the exhaust gas has consumed almost all of the 
activated points, the NOx purification rate of the lean NOx catalyst 
returns to the original NOx purification rate of the normal condition. The 
reason why the NOx purification rate of the lean NOx catalyst 8 does not 
increase when the space velocity changes to a high velocity is thought to 
be as follows: Since the activated points of the lean NOx catalyst are not 
generated during a low space velocity operation, the NOx purification rate 
of the lean NOx catalyst does not improve even if the space velocity 
changes from a low velocity to a high velocity. In some cases, the NOx 
purification rate may decrease. 
The space velocity of the first lean NOx catalyst 8a changes to a low space 
velocity while the space velocity at the second lean NOx catalyst 8b 
changes to a high velocity, and vice versa. Since the NOx purification 
rate increasing effect is larger than the NOx purification rate decreasing 
effect, the NOx purification rate of the total system improves. 
For example, when a first condition where eighty percent of exhaust gas 
flows through the first lean NOx catalyst 8a and the remaining twenty 
percent of exhaust gas flows through the second lean NOx catalyst 8b 
changes to a second condition where twenty percent of exhaust gas flows 
through the first lean NOx catalyst 8a and the remaining eighty percent of 
exhaust gas flows through the second lean NOx catalyst 8b, suppose that 
the NOx purification rate of the first lean NOx catalyst 8a momentarily 
changes from 45% to 85% and that the NOx purification rate of the second 
lean NOx catalyst 8b changes from 45% to 42%. The NOx purification rate of 
the total system increases due to the increase in the NOx purification 
rate of the first lean NOx catalyst 8a by a first amount: 
(85%-45%).times.0.2=8%, and decreases due to the decrease in the NOx 
purification rate of the second lean NOx catalyst 8b by a second amount: 
(42%-45%).times. 0.8=-2.4%. As a result, the NOx purification rate of the 
total system increases by the amount: 8%-2.4%=5.6%. 
SECOND EMBODIMENT 
As illustrated in FIG. 5, the dual passage portion 6 of the exhaust conduit 
4 of the internal combustion engine includes a first passage 6a' and a 
second passage 6b' which are connected in parallel to each other. A space 
velocity changing means 10' is installed in one of the passages 6a' and 
6b', for example in the passage 6b'. The space velocity changing means 10' 
comprises a valve which varies the amount of exhaust gas flowing through 
the passage 6b'. However, the valve 10' does not completely close the 
passage 6b'. Therefore, some amount of exhaust gas always flows through 
the passage 6b'. The amount of exhaust gas flowing through the second 
passage 6b' can vary from fifty percent to about twenty percent of the 
total amount of exhaust gas, while the amount of exhaust gas flowing 
through the first passage 6a' can vary from fifty percent to about eighty 
percent of the total amount of exhaust gas. Other structure and operation 
of the second embodiment are the same as that of the first embodiment. 
THIRD EMBODIMENT 
As illustrated in FIG. 6, an NOx purification apparatus for an internal 
combustion engine of the third embodiment of the invention includes all 
structures of the apparatus of the first embodiment of the inventions. The 
apparatus of the third embodiment of the invention further includes a 
space velocity changing means control means for controlling the space 
velocity changing means so that when an exhaust gas temperature is high, a 
space velocity changing interval is short, and when the exhaust gas 
temperature is low, the space velocity changing interval is longer. The 
space velocity changing means control means may control not only the space 
velocity changing interval but also a space velocity alteration amplitude. 
The space velocity changing means control means preferably comprises the 
control routine of FIG. 9 and maps of FIGS. 10 to 12, which are stored in 
a memory of a computer. 
More specifically, as illustrated in FIG. 6, the NOx purification apparatus 
of the third embodiment includes an electronic control unit (ECU) 12. The 
apparatus includes an engine load sensor 14, an engine speed sensor 16, 
and an exhaust gas temperature sensor 18, outputs of which are fed to the 
ECU 12. The actuator 10b of the valve 10 is electrically connected to the 
ECU 12 so that the valve 10 is operated in accordance with instructions 
from the ECU 12. 
The ECU 12 comprises an electronic computer. The computer includes an input 
interface which receives the outputs of the sensors 14, 16 and 18, an 
output sending the instructions from the ECU to the actuator 10b, a read 
only memory (ROM) storing the control routine of FIG. 9 and the maps of 
FIGS. 10 to 12, a random access memory (RAM), and a central processor unit 
(CPU) for executing calculations using the control routine and the maps. 
As illustrated in FIGS. 7 and 8, the period of the space velocity 
alteration cycle is set to be short, for example thirty seconds to one 
minute, at a high temperature portion within the predetermined temperature 
range (330.degree. C.-470.degree. C.), and the period is set to be long, 
for example one minute to two minutes, at a low temperature portion within 
the predetermined temperature range. 
Further, as illustrated in FIGS. 7 and 8, the amplitude of the space 
velocity alteration is set to be large, for example so as to be altered 
between 95% and 5%, at a high temperature portion within the predetermined 
temperature range (330.degree. C.-470.degree. C.), and the amplitude is 
set to be small, for example so as to be altered between 60% and 40% at a 
low temperature portion within the predetermined temperature range. 
The space velocity changing means 10 is controlled in accordance with 
instructions from the ECU 12 so that the above-described space velocity 
changing patterns of FIGS. 7 and 8 are obtained. More particularly, the 
control routine of FIG. 9 is entered at predetermined intervals. At step 
102, a current engine load PM, which is the output of the engine load 
sensor 14, and a current engine speed NE, which is the output of the 
engine speed sensor 16, are entered. Then, at step 104, a decision is made 
using the map of FIG. 9 as to whether the current engine operating 
condition is in a space velocity control range. If the current engine 
operating condition is in the space velocity control range, the routine 
proceeds to step 106, and if the current engine operating conditions is 
not in the space velocity control range, the routine proceeds to an end 
step where the cycle ends. The exhaust gas temperature is high when PM and 
NE are high, and the exhaust gas temperature is low when PM and NE are 
low. When the current exhaust gas temperature is out of the predetermined 
temperature range, it is difficult to increase the NOx purification rate 
of the lean NOx catalyst even if the space velocity is altered. Therefore, 
only when the current exhaust gas temperature is within the predetermined 
temperature range of FIG. 10, the routine proceeds to step 106 so that the 
space velocity is altered. 
At step 106, a decision is made as to whether or not the previous space 
velocity alteration cycle has ended, that is, whether or not the time (T) 
which has been counted since the beginning of the previous space velocity 
alteration cycle has exceeded the previous space velocity alteration 
period (S). If T has not exceeded S, the routine proceeds to step 114 
where the time (T) is increased by a predetermined time increment (delta 
T) per cycle. If T has exceeded S, the routine proceeds from step 106 to 
step 108 where the time (T) is cleared to zero. 
The routine proceeds from step 108 to step 110 where the current space 
velocity alteration period (S) corresponding to the current engine load 
and engine speed is determined using the map of FIG. 11. As shown in the 
map of FIG. 11, in the predetermined engine operation range, the higher 
the engine load and the engine speed are, (that is, the higher the exhaust 
gas temperature is) the shorter the predetermined the space velocity 
alteration period (S), and the lower the engine load and the engine speed 
are, (that is, the lower the exhaust gas temperature is,) the longer the 
predetermined space velocity alteration period (S). 
Further, at step 110, the current space velocity alteration amplitude (D) 
is also determined using the map of FIG. 12. As shown in the map of FIG. 
12, in the predetermined engine operation range, the higher the engine 
load and the engine speed are, (that is, the higher the exhaust gas 
temperature is) the larger the predetermined space velocity alteration 
amplitude (D), and the lower the engine load and the engine speed are, 
(that is, the lower the exhaust gas temperature is) the lower the 
predetermined space velocity alteration amplitude (D). 
Then, the routine proceeds to step 112 where the space velocity changing 
means 10 including the valve body 10a and the actuator 10b is driven so 
that the current one cycle of space velocity alteration is executed on the 
basis of the determined period (S) and amplitude (D). Then, the routine 
proceeds from step 112 to the end step where the cycle ends. 
Operation of the third embodiment will now be explained. In the case where 
the space velocity alteration period and amplitude are constant 
independently of a change in the exhaust gas temperature (as in the first 
embodiment), the NOx purification rate improvement due to the alteration 
of the space velocity is seen only in the predetermined medium exhaust gas 
temperature range, and almost no NOx purification rate improvement is seen 
in the exhaust gas temperature ranges higher than and lower than the 
predetermined medium exhaust gas temperature range. 
The reason is presumed to be as follows: FIG. 9 shows the relationship 
between the amount of radicals generated through partial oxidation of 
hydrocarbons at the lean NOx catalyst and the elapsed time since the space 
velocity of exhaust gas at the lean NOx catalyst changes from a high 
velocity to a low velocity. As seen from FIG. 9, in a high temperature 
range, radicals are relatively swiftly generated and consumed as shown by 
curve A, and in a low temperature range, radicals are relatively slowly 
generated and consumed as shown by curve B. Curve C shows the generation 
and consumption characteristic of radicals of a medium temperature range. 
If the period (t.sub.0) of the space velocity alteration is set based on 
the characteristic of curve C, in a high exhaust gas temperature condition 
it is difficult to obtain a high NOx purification rate, because generated 
radicals are swiftly burned in the high temperature range and so the space 
velocity is altered in the condition of a small amount of radicals. In a 
low exhaust gas temperature range also, it is difficult to obtain a high 
NOx purification, because generation of radicals is slow and the space 
velocity is altered in a condition where radicals have not yet been 
generated sufficiently. 
In the third embodiment of the invention, the period (t.sub.0) of the space 
velocity alteration cycle is not constant. More particularly, when the 
exhaust gas temperature is high, the period is shifted to the right side 
in FIG. 13 to be shorter so that the period approaches the peak point of 
the amount of radicals at high exhaust gas temperatures. In contrast, when 
the exhaust gas temperature is low, the period is shifted to the left side 
in FIG. 13 to be longer so that the period approaches the peak point of 
the amount of radicals at low exhaust gas temperatures. As a result, a 
sufficient amount of radicals can be utilized both at high exhaust gas 
temperatures and at low exhaust gas temperatures so that a high NOx 
purification rate due to the space velocity alteration is maintained. 
A similar effect can be obtained by controlling the amplitude of the space 
velocity alteration. In this instance, increasing the amplitude of the 
space velocity alteration corresponds to shortening the period of space 
velocity alteration cycle, because a large amplitude and a short period 
operate so as to promote generation of radicals. Also, decreasing the 
amplitude corresponds to lengthening the period, because a small amplitude 
and a long period operate so as to delay generation of radicals. Thus, 
when the exhaust gas temperature is high, the amplitude is controlled to 
be larger, and when the exhaust gas temperature is low, the amplitude is 
controlled to be small. As a result, a more sufficient amount of radicals 
is generated both at high exhaust gas temperatures and at low exhaust gas 
temperatures. 
Controlling the period and amplitude of the space velocity alteration 
widens the exhaust gas temperature range wherein the NOx purification 
improvement effect due to the space velocity alteration is obtained as 
compared with the effective temperature range of the first embodiment. 
FOURTH EMBODIMENT 
As illustrated in FIG. 14, an NOx purification apparatus for an internal 
combustion engine of the fourth embodiment of the invention includes all 
structures of the apparatus of the third embodiment of the invention. 
In addition to the same structures as those of the third embodiment, the 
apparatus of the fourth embodiment of the invention further includes: a 
plurality of HC injection means, provided upstream of lean NOx catalysts 
8a and 8b, respectively, for injecting hydrocarbons into the passages 6a 
and 6b, respectively, of the exhaust conduit 4; and an HC injection means 
control means for controlling the HC injection means so that just before a 
space velocity of exhaust gas at one of the plurality of lean NOx 
catalysts 8a and 8b changes from a low velocity to a high velocity, 
hydrocarbons are injected momentarily into a portion of the exhaust 
conduit 4 upstream of the one lean NOx catalyst. 
More particularly, as illustrated in FIG. 14, an HC injection means is 
provided to each passage 6a, 6b of the dual passage portion 6 of the 
exhaust conduit 4 for supplying HC into a portion of each passage 6a, 6b 
upstream of each lean NOx catalyst 8a, 8b. Each HC injection means 
includes an HC supply port 20a, 20b provided in a portion of each passage 
6a, 6b upstream of each lean NOx catalyst 8a, 8b, an HC control valve 22a, 
22b installed in a pipe connecting each HC supply port 20a, 20b to an HC 
source 24 (for example, an assembly of a fuel tank and a fuel pump) so 
that the HC control valve 22a, 22b turns the supply of HC on and off. 
The HC injection means control means controls each HC control valve 22a, 
22b so that each HC control valve 22a, 22b opens to supply hydrocarbons to 
each passage 6a, 6b for a predetermined period of time just before the 
space velocity of exhaust gas at each lean NOx catalyst 8a, 8b changes 
from a low velocity to a high velocity. During other time periods, each HC 
control valve 22a, 22b closes to stop the supply of hydrocarbons. The 
period of supply of hydrocarbons varies in accordance with the length of 
the space velocity alteration cycle as shown in FIGS. 15 and 16. More 
particularly, the longer the space velocity alteration cycle (S), the 
longer the HC supply period (delta S). 
The HC injection means control means is stored in the ECU 12. The HC 
injection means control means comprises a control routine of FIG. 17 which 
is stored in the ECU 12 and is the same as the routine of FIG. 9 (the 
third embodiment) except steps 110' and 112'. At step 110', in addition to 
calculation of the period S and the amplitude of the space velocity 
alteration, an HC supply period (data S) is determined. For example, the 
HC supply period is determined by the following equation: 
EQU delta S=d.times.S 
where, k is a factor selected from the range of 0.01 to 0.25. Then, the 
routine proceeds from step 110' to step 112'. At step 112', the space 
velocity is altered by controlling the space velocity changing means 10 
(including the valve body 10a and the actuator 10b) based on the S and D 
determined at step 110'. Further, at step 112', hydrocarbons are injected 
for a period delta S into the upstream portion of the lean NOx catalyst 
just before the space velocity of the lean NOx catalyst is changed from a 
low velocity to a high velocity. Then, the routine proceeds from step 112' 
to the end step where the cycle ends. 
Operation of the fourth embodiment of the invention includes all of the 
operation of the third embodiment of the invention. The fourth embodiment 
of the invention further includes the following operation: Since 
hydrocarbons are injected for a period of delta S into the portion of the 
passage upstream of the lean NOx catalyst just before the space velocity 
at the lean NOx catalyst changes from a low velocity to a high velocity, 
lack of hydrocarbons just after the space velocity changes from a low 
velocity to a high velocity is solved and the number of activated points 
of the lean NOx catalyst increases temporarily until the activated points 
generated due to the supply of HC are finally consumed. As a result, as 
shown by curve F in FIG. 18, the NOx concentration of the exhaust gas 
decreases and the NOx purification rate of the lean NOx catalyst increases 
temporarily to a great extent just after the space velocity at the lean 
NOx catalyst changes from a low velocity to a high velocity. In this 
instance, curve E shows the characteristic of the third embodiment of the 
invention where no hydrocarbons are injected into the exhaust conduit 4. 
The NOx purification rate of the lean NOx catalyst is improved in the 
fourth embodiment more than in the third embodiment. 
Since the supply of hydrocarbons into the exhaust conduit in the fourth 
embodiment of the invention is momentary and not constant, the HC amount 
consumed in the HC injection is not too large. 
In accordance with any embodiment of the invention, since the space 
velocity of exhaust gas at each lean NOx catalyst is altered periodically, 
the NOx purification rate of each lean NOx catalyst is increased 
repeatedly so that the NOx purification rate of the system including the 
lean NOx catalyst is greatly improved. 
Although several embodiments of the invention have been described in detail 
above, those skilled in the art will appreciate that various modifications 
and alterations can be made to the particular embodiments shown without 
materially departing from the novel teachings and advantages of the 
present invention. Accordingly, all such modifications and alterations are 
included within the spirit and scope of the invention as defined by the 
following claims.