U.S. patent application number 09/989204 was filed with the patent office on 2002-05-02 for apparatus for purifying and controlling exhaust gases.
This patent application is currently assigned to HITACHI, LTD.. Invention is credited to Kuroda, Osamu, Manaka, Toshio, Miura, Kiyoshi.
Application Number | 20020050135 09/989204 |
Document ID | / |
Family ID | 16797841 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020050135 |
Kind Code |
A1 |
Manaka, Toshio ; et
al. |
May 2, 2002 |
Apparatus for purifying and controlling exhaust gases
Abstract
A system having an NO.sub.x adsorption catalyst in an exhaust
gas flow channel of the internal combustion engine, and adsorbing
and capturing NO.sub.x in an oxidative atmosphere of an exhaust gas
during lean burn running and then producing a reductive atmosphere
thereby regenerating the adsorption catalyst, wherein a reduction
treatment of NO.sub.x is carried out based on an estimated NO.sub.x
purification rate and the NO.sub.x purification rate in the lean
burn exhaust gas of the internal combustion engine is always
maintained at or about a predetermined level thereby decreasing the
amount of exhaust gas discharged.
Inventors: |
Manaka, Toshio;
(Hitachinaka, JP) ; Kuroda, Osamu; (Hitachi,
JP) ; Miura, Kiyoshi; (Naka-gun, JP) |
Correspondence
Address: |
McDermott, Will & Emery
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
HITACHI, LTD.
Tokyo
JP
|
Family ID: |
16797841 |
Appl. No.: |
09/989204 |
Filed: |
November 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09989204 |
Nov 21, 2001 |
|
|
|
09631627 |
Aug 4, 2000 |
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Current U.S.
Class: |
60/277 ; 60/285;
60/295; 60/301 |
Current CPC
Class: |
Y02T 10/40 20130101;
Y02T 10/44 20130101; F02D 2200/0811 20130101; F02D 41/405 20130101;
Y02T 10/12 20130101; F01N 3/0842 20130101; F02D 41/0275 20130101;
Y02T 10/24 20130101 |
Class at
Publication: |
60/277 ; 60/285;
60/295; 60/301 |
International
Class: |
F01N 003/00; F01N
007/00; F01N 003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 1999 |
JP |
11-223418 |
Claims
What is claimed is:
1. An exhaust gas purification apparatus for use in an internal
combustion engine in which an NO.sub.x adsorption catalyst for
chemically adsorbing NO.sub.x in a state where the amount of an
oxidizing agent is larger than the amount of a reducing agent and
catalytically reducing NO.sub.x adsorbed on the catalyst in a state
where the amount of the reducing agent, is equal to or larger than
the amount of the oxidizing agent in a redox stoichiometric
relation between each of components in an exhaust gas is disposed
in an exhaust gas flow channel, and which produces a state where
the amount of the oxidizing agent is larger than the amount of the
reducing agent in a redox stoichiometric relation between each of
components to chemically adsorb NO.sub.x on the adsorption catalyst
and then produces a state where the amount of the reducing agent is
equal to or larger than the amount of the oxidizing agent to
catalytically react and reduce NO.sub.x adsorbed on the catalyst
with the reducing agent into non-toxic N.sub.2, wherein the
apparatus has a control unit for estimating the NO.sub.x
purification rate based on the amount of NO.sub.x discharged from
the internal combustion engine and the running state of the engine,
and carrying out a reducing treatment for NO.sub.x adsorbed on the
NO.sub.x adsorption catalyst when the estimated NO.sub.x
purification rate is lowered to a predetermined value.
2. An exhaust gas purification and control apparatus for use in an
internal combustion engine in which an NO.sub.x adsorption catalyst
for chemically adsorbing NO.sub.x in a state where the amount of an
oxidizing agent is larger than the amount of a reducing agent and
catalytically reducing NO.sub.x adsorbed on the catalyst in a state
where the amount of the reducing agent is equal to or larger than
the amount of the oxidizing agent, in a redox stoichiometric
relation between each of components in an exhaust gas is disposed
in an exhaust gas flow channel, and which produces a state where
the amount of the oxidizing agent is larger than the amount of the
reducing agent in a redox stoichiometric relation between each of
components to chemically adsorb NO.sub.x on the adsorption catalyst
and then produces a state where the amount of the reducing agent is
equal to or larger than the amount of the oxidizing agent and
catalytically reacting NO.sub.x adsorbed on the catalyst with the
reducing agent and reduce the same into non-toxic N.sub.2, wherein
the NO.sub.x purification rate is estimated based on the amount of
NO.sub.x discharged from the internal combustion engine and the
running state of the engine, and a reducing treatment is conducted
for NO.sub.x adsorbed on the NO.sub.x adsorption catalyst when the
estimated NO.sub.x purification rate is lowered to a predetermined
value.
3. An exhaust gas purification apparatus for use in an internal
combustion engine in which an NO.sub.x catalyst for capturing
NO.sub.x discharged from an internal combustion engine and reducing
a portion of the captured NO.sub.x into N.sub.2 during lean burn
running and reducing NO.sub.x captured as it is during lean burn
running into N.sub.2 when an air/fuel ratio of the internal
combustion engine is set to a theoretical air/fuel ratio or a
stoichiometrical ratio is disposed in an exhaust gas flow channel,
wherein the apparatus has a control unit for estimating the
NO.sub.x purification rate based on the amount of NO.sub.x
discharged from the internal combustion engine and the running
state of the engine, and carrying out a reducing treatment for
NO.sub.x captured on the NO.sub.x catalyst when the estimated
NO.sub.x purification rate is lowered to a predetermined value.
4. A control apparatus for exhaust purification for use in an
internal combustion engine in which an NO.sub.x catalyst for
capturing NO.sub.x discharged from an internal combustion engine
and reducing a portion of the captured NO.sub.x into N.sub.2 during
lean burn running and reducing NO.sub.x captured as it is during
lean burn running into N.sub.2 when an air/fuel ratio of the
internal combustion engine is set to a theoretical air/fuel ratio
or a stoichiometrical ratio is disposed in an exhaust gas flow
channel, wherein the NO.sub.x purification rate is estimated based
on the amount of NO.sub.x discharged from the internal combustion
engine and the running state of the engine, and a reducing
treatment is conducted for NO.sub.x captured on the NO.sub.x
catalyst when the estimated NO.sub.x purification rate is lowered
to a predetermined value.
5. An exhaust gas purification apparatus for use in an internal
combustion engine as defined in claim 2, wherein the NO.sub.x
purification rate of the NO.sub.x adsorption catalyst is estimated,
as a method of estimating the NO.sub.x purification rate based on
the amount of NO.sub.x discharged from the internal combustion
engine and the running state of the engine, based on one or more of
amounts of states selected from the amount of NO.sub.x adsorbed on
the NO.sub.x adsorption catalyst, the temperature of the exhaust
gas, the temperature of the adsorption catalyst, the poisoning
amount of sulfur, the running distance of the vehicle, the degree
of degradation of the catalyst, the air/fuel ratio, the
concentration of the unburnt hydrocarbons, the NO.sub.x
concentration before the catalyst, the period of time for the lean
burn running having lapsed from the change over from the
stoichiometrical (theoretical air/fuel ratio) or fuel rich running
to lean burn running, the number of rotation of the internal
combustion engine, the load on the engine, the amount of intake
air, and the amount of the exhaust gas.
6. An exhaust gas purification apparatus for use in an internal
combustion engine as defined in claim 4, wherein the NO.sub.x
purification rate of the NO.sub.x adsorption catalyst is estimated,
as a method of estimating the NO.sub.x purification rate based on
the amount of NO.sub.x discharged from the internal combustion
engine and the running state of the engine, based on one or more of
amounts of states selected from the amount of NO.sub.x captured on
the NO.sub.x adsorption catalyst, the temperature of the exhaust
gas, the temperature of the adsorption catalyst, the poisoning
amount of sulfur, the running distance of the vehicle, the degree
of degradation of the catalyst, the air/fuel ratio, the
concentration of the unburnt hydrocarbons, the NO.sub.x
concentration before the catalyst, the period of time for the lean
burn running having lapsed from the change over from the
stoichiometrical (theoretical air/fuel ratio) or fuel rich running
to lean burn running, the number of rotation of the internal
combustion engine, the load on the engine, the amount of intake
air, and the amount of the exhaust gas.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention concerns an apparatus for purifying
and controlling exhaust gases discharged from internal combustion
engines, for example, of automobiles and, more in particular, it
relates to an apparatus for purifying and controlling exhaust gases
discharged from automobiles equipped with internal combustion
engines which can be driven at a lean air/furl ratio (lean burn
running).
[0002] Carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides
(NO), etc contained in exhaust gases discharged from internal
combustion engines of automobiles cause various problems as
atmospheric pollutants, so that a great endeavor has been made so
far for decreasing the discharge of them. It has been developed a
method of decreasing the generation of exhaust gases by improving
the combustion method in internal combustion engines, as well as a
method of purifying the discharged exhaust gases by the use of
catalysts, which have achieved favorable results. In the field of
automobiles using gasoline engines, use of ternary catalysts
comprising Pt and Rh as main active components has become
predominant for simultaneous oxidation of HC and CO and reduction
of NO.sub.x, thereby making them non-toxic.
[0003] However, ternary catalysts are effective, by their nature,
only to exhaust gases formed from combustion near the theoretical
air/fuel ratio referred to as window. Although the air/fuel ratio
varies with the running conditions of automobiles, the range of the
variation has hitherto been regulated, as a rule, so as to fall in
the vicinity of the theoretical air/fuel ratio. The theoretical
air/fuel ratio A/F is about at 14.7 (by weight) in a case of
gasoline. In this specification, the theoretical air/fuel ratio A/F
is typically represented as A/F=14.7, even though it may vary
depending on the kind of fuel. However, if an engine can be
operated at a more lean air/fuel ratio than the theoretical
air/fuel ratio, specific fuel consumption will be improved.
Accordingly, the technique of lean burn combustion has been
developed, and the internal combustion engines in many automobiles
are operated at present in the lean burn zone at an air/fuel ratio
of 18 or above.
[0004] However, the existent ternary catalysts used for
purification of lean-burnt exhaust gases can not effectively purify
NO.sub.x by reduction although can purify HC and CO by oxidation.
Accordingly, a technique for purifying exhaust gases capable of
coping with lean burn running is necessary in order to apply the
lean burn technique to large-sized automobiles and prolong the lean
burn combustion period (extension for the range of the lean burn
running zone). In view of the above, a technique for purifying
exhaust gases capable of coping with the lean burn, namely a
technique for purifying HC, NO and NO.sub.x and, especially,
NO.sub.x in exhaust gases containing a great amount of oxygen
(O.sub.2) has now been under vigorous development.
[0005] Japanese Patent No. 2586739 (U.S. Pat. No. 5,437,153),
proposes an apparatus for purifying exhaust gases provided with an
NO.sub.x releasing unit that estimates the amount of NO.sub.x
absorbed in the NO.sub.x absorbent disposed in an exhaust pipe of
an internal combustion engine and lowers the oxygen concentration
in the exhaust gases flowing into the NO.sub.x absorbent when the
estimated amount of the absorbed NO.sub.x exceeds a predetermined
allowable limit thereby releasing NO.sub.x from the NO.sub.x
absorbent.
[0006] However, since this method repeats the NO.sub.x reducing
treatment in accordance with the amount of NO.sub.x absorbed in the
NO.sub.x absorbent (releasing NO.sub.x from the NO.sub.x absorbent
by lowering the oxygen concentration in the exhaust gases and
reducing the released NO.sub.x), it suffers from a restriction on
the accuracy of maintaining the NO.sub.x discharge amount after the
NO.sub.x absorbent to lower than the regulated discharge gas
level.
[0007] Further, Japanese Published Unexamined Patent Application
No. Hei 10-212933 (WO97/47864) proposes a method of making NO.sub.x
non-toxic by adsorption instead of absorption of NO.sub.x.
According to this method, NO.sub.x in the exhaust gases is adsorbed
as NO.sub.2 on the adsorption catalyst, which is partly reduced
directly into N.sub.2 by HC or CO in the exhaust gases and partly
captured in the form of NO.sub.2 as it is on the NO.sub.x
adsorption catalyst in lean burn running, and then NO.sub.2
captured by adsorption is reduced into N.sub.2 during running at a
stoichiometrical or fuel rich air/fuel ratio.
[0008] The present invention intends to provide an apparatus for
purifying and controlling exhaust gases capable of reducing
NO.sub.x at an accurate timing in a system of purifying exhaust
gases by capturing NO.sub.x on an NO.sub.x adsorption catalyst by
adsorption or absorption (particularly, in a system of purifying
NO.sub.x using an NO.sub.x adsorption catalyst)
SUMMARY OF THE INVENTION
[0009] The foregoing subject of this invention can be solved by
estimating an NO.sub.x purification rate based on the amount of
NO.sub.x discharged from internal combustion engines and the
running conditions of the engines and reducing NO.sub.x adsorbed on
an NO.sub.x adsorption catalyst at the time the estimated NO.sub.x
purification rate lowers to a predetermined value. Since the
reduction treatment of NO.sub.x adsorbed on the adsorption catalyst
can be started at a timing not worsening the exhaust gases, the
level of the exhaust gases can be maintained always below the
regulation level.
[0010] The NOX adsorption catalyst used in this invention
chemically adsorbs NO.sub.x from exhaust gases in a state where the
amount of an oxidizing agent is larger than the amount of a
reducing agent, and catalytically reduces the adsorbed NO.sub.x in
a state where the amount of the reducing agent is equal with or
larger than the amount of the oxidizing agent in the redox
stoichiometric relation between each of components of the exhaust
gas. The NO.sub.x adsorption catalyst is disposed in an exhaust gas
flow channel. The exhaust gas-purification apparatus of this
invention produces a state where the amount of the oxidizing agent
is larger than the amount of the reducing agent in a redox
stoichiometric relation between each of the components of exhaust
gas, thereby chemically adsorbing NO.sub.x on the absorption
catalyst and then produces a state where the amount of the reducing
agent is equal with or larger than the amount of the oxidizing
agent, thereby catalytically reacting NO.sub.x adsorbed on the
absorption catalyst with the reducing agent and reducing the
NO.sub.x to non-toxic N.sub.2.
[0011] The term "adsorbing catalyst" means a material having an
ability of adsorbing NO.sub.x and, at the same time, having a
catalytic function. In the present specification, the term means a
material having an ability of adsorbing and capturing NO.sub.x, an
ability of catalytically reducing NO.sub.x and an ability of
catalytically oxidizing HC, CO, etc.
[0012] That is, the NO.sub.x adsorption catalyst used in this
invention adsorbs NO.sub.x in the exhaust gas during a lean burn
running as NO.sub.x on the adsorption catalyst, directly reduces a
portion of NO.sub.x to N.sub.2 by using HC, CO, etc. in the exhaust
gas, while captures a portion of NO.sub.x as NO.sub.2 on the
adsorption catalyst and then reduces the adsorbed and/or captured
NO.sub.2 to N.sub.2 during running at a stoichiometric or fuel rich
A/F ratio. The NO.sub.x adsorption catalyst used in this invention
is described specifically in WO 97/47864 (U.S. Ser. No. 09/202,243,
entitled as "Exhaust Gas Purification Apparatus of Internal
Combustion Engine and Catalyst for Purifying Exhaust Gas of
Internal Combustion Engine"), filed by the present applicant
(assignee). The adsorption catalyst contains K, Na, Mg, Sr, etc. as
a base material for adsorbing NO.sub.x,. which is combined with Ti,
Si, to form a composite oxide. The adsorbing ability is controlled
by adjusting the solid basicity so as to adsorb and/or capture
NO.sub.x as NO.sub.2 on the surface of the catalyst, thereby
inhibiting absorption in the form of NO.sub.3.sup.- to the inside
of the catalyst.
[0013] The oxidizing agent includes O.sub.2, NO, and NO.sub.2,
being mainly oxygen. The reducing agent includes HC supplied to an
internal combustion engine, derivatives thereof formed in the
course of combustion such as HC (including oxygen-containing
hydrocarbon) CO, H.sub.2 and, further, reducing substances such as
HC to be added to the exhaust gas as a reducing component which
will be explained later.
[0014] When a lean exhaust gas is brought into contact with HC, CO,
H.sub.2 as reducing agents for reducing NO.sub.x to nitrogen, they
react with O.sub.2 as the oxidizing agent in the exhaust gas to
cause combustion reaction. NO.sub.x (NO and NO.sub.2) are also
reacted therewith and reduced to nitrogen. Since both the reactions
usually proceed in parallel, the utilization rate of the reducing
agent is low in the presence of oxygen. Particularly, when the
reaction temperature is as high as 500.degree. C. or above
(dependent on the kind of catalyst material), a proportion of the
latter reaction is considerably high. Thus, it becomes possible to
carry out the reduction of NO.sub.x to N.sub.2 effectively by
separating NO.sub.x from the exhaust gas (at least from O.sub.2 in
the exhaust gas) by the use of the absorption catalyst and then
catalytically reacting NO.sub.x with the reducing agent. In this
invention, NO.sub.x in exhaust gas is separated from O.sub.2 by
adsorbing NO.sub.x from the lean exhaust gas by the use of the
NO.sub.x adsorption catalyst.
[0015] Then, in this invention, it produces a state where the
amount of the reducing agent is equal to or larger than the amount
of the oxidizing agent in a redox system constituted with the
oxidizing agent (O.sub.2, NO.sub.x) and the reducing agent (HC, CO,
H.sub.2), and the NO.sub.x adsorbed on the absorption catalyst is
catalytically reacted with the reducing agent such as HC to reduce
NO.sub.x to N.sub.2.
[0016] Now, NO.sub.x in the exhaust gas substantially comprises NO
and NO.sub.2. Since NO.sub.2 is more reactive than NO, NO.sub.2 can
be removed by adsorption and reduced more easily than NO
Accordingly, oxidation of NO to NO.sub.2 facilitates adsorptive
removal and reduction of NO.sub.x in the exhaust gas. This
invention includes a method of oxidizing NO.sub.x present in the
lean exhaust gas to NO.sub.2 by coexisting O.sub.2 and thereby
removing NO.sub.x, and an oxidizing means for this purpose such as
provision of an NO-oxidizing function to the absorption
catalyst.
[0017] In the NO.sub.x adsorption catalyst used in this invention,
the reduction reaction for the chemically adsorbed NO.sub.x can be
approximately expressed by the following reaction scheme:
MO--NO.sub.2+HC.fwdarw.MO+N.sub.2+CO.sub.2+H.sub.2O.fwdarw.MCO.sub.3+N.sub-
.2+H.sub.2O,
[0018] where M is a metal element (the reason of adapting MCO.sub.3
as the reduction product is to be described later).
[0019] The reaction described above is an exothermic reaction. If
an alkali metal and an alkaline earth metal are used for the metal
M and (typically represented by Na and Ba, respectively), the heat
of reaction in the normal state (1 atmosphere, 25.degree. C.) can
be calculated as follows:
2NaNO.sub.3(s)+{fraction
(5/9)}C.sub.3H.sub.6.fwdarw.Na.sub.2CO.sub.3(s)+N-
.sub.2+2/3CO.sub.2+{fraction (5/3)}H.sub.2O
[0020] [-.DELTA.H=873 kilojoules/mole]
Ba(NO.sub.3).sub.2+{fraction
(5/9)}C.sub.33H.sub.6.fwdarw.BaCO.sub.3(s)+N.-
sub.2+2/3CO.sub.2+{fraction (5/3)}H.sub.2O
[0021] [-.DELTA.H=751 kilojoules/mole]
[0022] where s is solid and g is gas.
[0023] As the thermodynamic quantities of the adsorbed species, the
values of corresponding solids are used.
[0024] Additionally, the heat of combustion of 5/9 mole of
C.sub.3H.sub.6 is 1,070 kilojoules, so that the heat of combustion
of each of the reactions described above is comparable to the heat
of combustion of HC. Naturally, this generated heat is transferred
to the exhaust gas in contact therewith, and local rise of
temperature on the absorption catalyst surface can be
suppressed.
[0025] In a case where the NO.sub.x-capturing agent is an
NO.sub.x-absorbent, since the NO.sub.x captured in the bulk mass of
the absorbent is also reduced, the generation of heat increases.
Since the transfer of heat to the exhaust gas is limited, this
brings about a rise in the temperature of the absorbent. This heat
generation shifts the equilibrium of the following absorbing
reaction to the releasing side: 1
[0026] Even if the concentration of the reducing agent is increased
with an aim of reducing the released NO.sub.x rapidly and lowering
the concentration of NO.sub.x in the exhaust gas discharged out of
the apparatus, it is considered that the gas phase reaction between
NO.sub.2 and HC does not proceeds so rapidly and, therefore, the
amount of the released NO.sub.x cannot sufficiently be decreased by
the increase in the amount of the reducing agent. Further, it may
be considered to carry out the reduction reaction in a stage where
the amount of NO.sub.x is yet small, but this increases the
frequency for the regeneration of the NO.sub.x absorbent and lowers
the effect of improving the specific fuel consumption.
[0027] Since the absorption catalyst used in this invention
captures NO.sub.x only in the vicinity of the surface, the heat of
generation is small as an absolute value. Further, since the heat
is rapidly transferred to the exhaust gas, the absorption catalyst
shows less temperature rise. Accordingly, release of the once
captured NO.sub.x can be prevented.
[0028] The NO.sub.x-absorption catalyst used in this invention has
a feature as a material that captures NO.sub.x at the surface
thereof by chemical adsorption and does not release NO.sub.x by the
exothermic reaction in the step of reducing NO.sub.x. Further, the
NO.sub.x adsorption catalyst of this invention has a feature as a
material that captures NO.sub.x by a chemical adsorption at the
surface thereof or by chemical bond in the vicinity of the surface
thereof and does not release NO.sub.x by the exothermic reaction at
the step of reducing NO.sub.x.
[0029] The present inventors have found that the above-mentioned
features can be realized by an NO.sub.x adsorption catalyst
containing, as a portion of its components, at least one element
selected from the group consisting of potassium (K), sodium (Na),
magnesium (Mg), strontium (Sr) and calcium (Ca).
[0030] The exhaust gas purification apparatus for purifying an
exhaust gas of an internal combustion engine to which this
invention is applied has a feature in that it has an NO.sub.x
adsorption catalyst containing, as a portion of the components
thereof, at least one element selected from the group consisting of
potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and
calcium (Ca) disposed in an exhaust gas flow channel, and in that
it produces a state where the amount of an oxidizing agent is
larger than the amount of a reducing agent in a redox
stoichiometric relation between each of the components of the
exhaust gas, thereby chemically adsorbing NO.sub.x on the NO.sub.x
adsorption catalyst, and then produces a state where the amount of
the reducing agent is equal with or larger than the amount of the
oxidizing agent, thereby catalytically reacting the NO.sub.x
adsorbed on the catalyst with the reducing agent to reduce the NO
to non-toxic N.sub.2.
[0031] The exhaust gas purification apparatus for purifying an
exhaust gas of an internal combustion engine to which this
invention is applied has a feature in that it has an NO.sub.x
adsorption catalyst containing, as a portion of the components
thereof, at least one element selected from the group consisting of
potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and
calcium (Ca) disposed in an exhaust gas flow channel, and in that
it produces a state where the amount of an oxidizing agent such as
O.sub.2 is larger the amount of reducing agent such as HC in a
redox stoichiometric relation between each of the components of the
exhaust gas thereby NO.sub.x by chemical bonds on or near the
surface of the NO.sub.x adsorption catalyst, and then produces a
state where the amount of the reducing agent is equal with or
larger than the amount of the oxidizing agent, thereby
catalytically reacting the NO.sub.x captured on the catalyst with
the reducing agent to reduce the NO.sub.x to harmless N.sub.2.
[0032] As the NO.sub.x adsorption catalyst used in this invention,
the following compositions can be used preferably:
[0033] A composition comprising metals and metal oxides (or
composite oxides) containing at least one element selected from the
group consisting of potassium (K), sodium (Na), magnesium (Mg),
strontium (Sr) and calcium (Ca), at least one element selected from
rare earth elements such as cerium and at least one element
selected from noble metals such as platinum, rhodium and palladium
and a composition prepared by supporting the above-mentioned
composition on a porous, heat-resistant metal oxide. These
compositions have an excellent NO.sub.x-adsorbing performance and,
in addition, excellent SOx resistance.
[0034] In this invention, the state where the amount of the
reducing agent is equal with or larger than the amount of the
oxidizing agent can be produced by the following method.
[0035] In an internal combustion engine, the condition of
combustion is adjusted to a theoretical air/fuel ratio or a
fuel-rich ratio state. Alternatively, a reducing agent is added to
a lean burnt exhaust gas.
[0036] The former can be achieved by the following method:
[0037] A method of controlling the amount of fuel injected, for
example, in accordance with the output of an oxygen concentration
sensor and the output of an intake gas flow rate sensor disposed in
an exhaust gas duct. This method also includes a method of bringing
a portion of a plurality of cylinders into a fuel-rich state while
bringing the remaining cylinder into a fuel-lean state, and
producing a state where the amount of the reducing agent is equal
with or larger than the amount of the oxidizing agent in the redox
stoichiometric relation for the components in a mixed exhaust gas
discharged from whole cylinders.
[0038] The latter can be achieved by the following method:
[0039] A method of adding a reducing agent to the upstream of the
absorption catalyst in the exhaust gas stream. The reducing agent
can include, for example, gasoline, light oil, kerosene, natural
gas or modified products thereof which are used as the fuel of
internal combustion engines, as well as hydrogen, alcohol and
ammonia.
[0040] A method of guiding a blow-by gas or canister purge gas to
the upstream of the absorption catalyst and adding the reducing
agent contained in the gas such as hydrocarbon or the like is also
effective. In a direct fuel injection type internal combustion
engine, it is effective to inject a fuel in the exhausting stroke
and charge the fuel as the reducing agent.
[0041] The adsorption catalyst used in this invention can be used
in a variety of forms. The catalyst is applicable in a honeycomb
shape prepared by coating a honeycomb structure made of a metallic
material such as cordierite or stainless steel with absorption
catalyst components, as well as in the shape of pellet, plate,
granule, and powder.
[0042] The timing for producing a state where the amount of the
reducing agent is equal with or larger than the amount of the
oxidizing agent can be established according to each of the
following methods, the methods (4) and (5) being preferred for
deciding the time at a high accuracy in order to satisfy the
regulation values for the exhaust gas.
[0043] (1) When the NO.sub.x discharge amount during lean burn
running is estimated based, for example, on the air/fuel ratio
setting signal, engine rpm signal. Intake air amount signal, air
intake pipe pressure signal, speed signal, opening degree of
throttle and exhaust gas temperature determined by ECU (Engine
Control Unit) and the accumulated values have exceeded
predetermined values;
[0044] (2) When the accumulated oxygen amount is detected based on
the signal of the oxygen sensor (or A/F sensor) placed in the
upstream or down-stream to the absorption catalyst in the exhaust
gas flow channel and the accumulated oxygen amount has exceeded a
predetermined value or, as a modified embodiment thereof, when the
accumulated oxygen amount during lean burn running has exceeded a
predetermined value.
[0045] (3) When the accumulated amount of NO.sub.x is calculated
based on the signal of the NO.sub.x sensor placed to the upstream
of the absorption catalyst in the exhaust flow channel and the
accumulated amount of NO.sub.x has exceeded a predetermined value
during lean burn running.
[0046] (4) When the NO.sub.x concentration during lean burn running
is detected based on the signal of the NO.sub.x sensor placed to
the downstream of the absorption catalyst in the exhaust flow
channel and the NO.sub.x concentration has exceeded a predetermined
value, or when the NO.sub.x purification rate is calculated based
on the signal of the NO.sub.x sensor placed to the upstream or
downstream to the absorption catalyst and the NO.sub.x purification
rate has been lowered below a predetermined value; and
[0047] (5) When the NO.sub.x purification rate of the NO.sub.x
adsorption catalyst is estimated based on at least one of status
amounts, namely, the amount of NO.sub.x adsorbed on the NO.sub.x
adsorption catalyst, the temperature of the exhaust gas, the
temperature of the absorption catalyst, the amount of sulfur
poisoning, the running distance of automobile, the degree of
deterioration of catalyst, the air/fuel ratio, the concentration of
unburnt hydrocarbon, the NO.sub.x concentration before catalyst,
the time of lean burn running lapsed from the point of change from
running at a theoretical air/fuel ratio or a fuel rich running to
the lean burn running, the number of rotation of internal
combustion engine, the load on the engine, the amount of intake air
and the amount of exhaust gas and the NO.sub.x purification rate
has been lowered below a predetermined value.
[0048] As described above, the period of time for keeping the state
where the amount of the reducing agent is equal with or larger than
the amount of the oxidizing agent or the amount of the reducing
agent to be charged for keeping the state can be determined
previously by taking the characteristics of the absorption catalyst
and the factors and characteristics of the internal combustion
engine into account. They can be decided by increasing the amount
of the fuel injected from a fuel injection valve into the
cylinders, by injecting the fuel into the cylinders during the
expanding stroke of the internal combustion engine or by supplying
the fuel into the exhaust pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] Other objects and advantages of the invention will become
apparent during the following discussion of the accompanying
drawings, wherein:
[0050] FIG. 1 is a constitutional view of an apparatus for
purifying and controlling exhaust gases by the method of this
invention, which shows a typical embodiment of this invention;
[0051] FIG. 2 is a graph illustrating characteristics for a
NO.sub.x purification rate with lapse of time upon alternately
repeating fuel rich running and lean burn running by the apparatus
of this invention;
[0052] FIG. 3 is a graph illustrating a relation between a running
distance of an automobile and an NO.sub.x purification rate;
[0053] FIG. 4 is a graph illustrating a NO.sub.x purification rate
in a stoichiometric exhaust gas;
[0054] FIG. 5A and FIG. 5B are graphs illustrating a relation
between the NO.sub.x concentration at the inlet of an adsorption
catalyst and the NO.sub.x concentration at the outlet of the
adsorption catalyst at the time of change-over from a fuel rich
(stoichiometric) running to a lean burn running;
[0055] FIG. 6A and FIG. 6B are graphs illustrating a relation
between the NO.sub.x concentration at the inlet of an adsorption
catalyst and the NO.sub.x concentration at the outlet of the
adsorption catalyst at the time of change-over from a fuel rich
(stoichiometric) running to a lean turn running; and
[0056] FIG. 7 is an outlined view illustrating an engine
controlling system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0057] This invention will be explained in more details by
referring to concrete embodiments of the invention. Needless to
say, this invention is not limited to the embodiments and examples
to be described below, and a variety of embodiments can be included
within the scope of the technical idea of the invention.
[0058] [Adsorption Catalyst]
[0059] The characteristics of the adsorption catalyst used in the
method of this invention will be explained below. Characteristics
of N--N9 containing Na as the alkali metal and N--K9 containing K
as the alkali metal are as shown below.
[0060] <<Method for Preparing Adsorption Catalyst>>
[0061] Adsorption catalyst N--N9 was prepared by the following
method.
[0062] A nitric acid-acidified alumina slurry was prepared by
mixing an alumina sol as a binder which was obtained by nitric
acid-peptization of alumina powder and boehmite.
[0063] A honeycomb structure was dipped in the coating fluid thus
obtained and then the structure was immediately taken out. After
removing the fluid kept in the cells by air blow, the structure was
dried and calcined at 450.degree. C. This procedure was repeated,
to coat 150 g of alumina per one liter of the apparent volume of
the honeycomb. Then, catalytic active components were supported on
the alumina-coated honeycomb to obtain a honeycomb-form absorption
catalyst. For example, a honeycomb structure was impregnated with a
solution of cerium nitrate (Ce nitrate), dried and then calcined at
600.degree. C. for one hour. Successively, it was impregnated with
a mixed solution containing a solution of sodium nitrate (Na
nitrate), a titania sol solution and a solution of magnesium
nitrate (Mg nitrate), and dried and calcined in the same manner.
Further, it was impregnated with a mixed solution containing a
dinitrodiamine Pt nitrate solution and a solution of rhodium
nitrate (Rh nitrate) and calcined at 450.degree. C. for one hour.
Finally, it was impregnated with Mg nitrate solution and calcined
at 45.degree. C. for one hour. With the procedures described above,
there was obtained a honeycomb-form absorption catalyst of 2
Mg-(0.2Rh, 2.7Pt)-(18Na, 4Ti, 2Mg)-27Ce/Al.sub.2O.sub.3 supporting
Ce, Mg, Na, Ti, Rh and Pt on alumina (Al.sub.2O.sub.3). The
expression "/Al.sub.2O.sub.3" means that the active components are
supported on Al.sub.2O.sub.3, and the numerical figures preceding
the symbols of elements express the weight (g) of each indicated
metallic component supported per one liter of apparent volume of
the honeycomb. The order in the arrangement indicates the order of
supporting. That is, components were supported in the order of
components indicated closer to Al.sub.2O.sub.3 and components
remote therefrom. Components indicated together in one parenthesis
are of the supported simultaneously. Additionally, the amount of
each active components to be supported can be varied by changing
the concentration of each active component in the impregnating
solution.
[0064] The adsorption catalyst N--K9 was prepared by the following
method.
[0065] In accordance with the same procedures as those for the
adsorption catalyst N--N9, excepting that the solution of Na
nitrate used in the preparation of N--N9 was replaced by a solution
of potassium nitrate (K nitrate), N--K9, namely, 2Mg-(0.2Rh,
2.7Pt)-(18K, 4Ti, 2Mg)-27Ce/Al.sub.2O.sub.3 was obtained. Further,
control catalyst N--R2, namely 2Mg-(0.2Rh,
2.7Pt)-27Ce/Al.sub.2O.sub.3 was also prepared by the same procedure
as above.
[0066] <<Method for Evaluation of Performance>>
[0067] The adsorption catalysts obtained by the methods described
above were heat-treated at 700.degree. C. for 5 hours in an
oxidative atmosphere, and then their characteristics were evaluated
in the following method.
[0068] A honeycomb-form absorption catalyst having a volume of 1.7
L prepared according to the method of this invention was mounted on
a passenger car equipped with a gasoline engine of lean
burn-specification having a 1.8L displacement, and the
NO.sub.x-cleaning characteristics were evaluated.
[0069] <<Characteristics of Adsorption Catalyst>>
[0070] The adsorption catalyst N--N9 was mounted and a fuel rich
running at A/F=13.3 for 30 seconds and a lean burn running at
A/F=22 for about 20 minutes (period of time till the NO.sub.x
purification rate decreased to about 40%) were alternately repeated
to obtain the characteristics NO.sub.x purification rate with lapse
of time in FIG. 2. It can be seen from the figure that NO.sub.x can
be purified by this absorption catalyst during the lean burn
running period. The NO.sub.x purification rate gradually decreased
during the lean burn running period, and the purification rate
which was 100% at the initial stage decreased to about 40% with
lapse of time is each case. However, the lowered purification rate
was recovered to 100% by fuel rich running for 30 seconds or by
injection of fuel into the cylinder in the expanding stroke or
exhausting stroke of the engine. When the lean burn running was
carried out again, the NO.sub.x purification ability was recovered
and the NO purification rate lowered in the same manner as above.
When the lean burn running and the fuel rich running were repeated
by a plurality of times, the lowering rate of the NO.sub.x
purification rate during lean burn running varied depending on the
temperature of the catalyst, the amount of sulfur poisoning, the
running distance of automobile, the NO.sub.x concentration at the
inlet of the catalyst, and the amount of the exhaust gas.
Accordingly, it is important to estimate the NO.sub.x purification
rate at high accuracy in accordance with such running
conditions.
[0071] At a constant running speed of about 40 km/h (the space
velocity (SV) of the exhaust gas was constant at about 20,000/h)
and the ignition timing was changed to vary the NO.sub.x
concentration in the exhaust gas and determine the relation between
the NO.sub.x concentration and the NO.sub.x purification rate of
lean exhaust gas as shown in FIG. 3. The NO.sub.x purification rate
decreases with time, in which the decreasing rate is lower as the
NO.sub.x concentration is lower. The amounts of NO.sub.x captured
till the NO.sub.x purification rate lowered to 50% and 30%,
respectively, were determined from the figure as shown in Table
1.
1 TABLE 1 NOx conc. in inlet NOx purified till NOx purified till
exhaust 50% purification 30% purification (ppm) rate (mol) rate
(mol) About 50 ppm 0.030 0.041 About 120 ppm 0.031 0.047 About 230
ppm 0.030 0.045 About 450 ppm 0.030 0.042 About 550 ppm 0.026
0.038
[0072] The amount of NO.sub.x captured is substantially constant
regardless of the NO.sub.x concentration. It is characteristic
feature of the chemical adsorption that the amount of adsorption is
independent of the concentration (pressure) of the substance
adsorbed.
[0073] In the tested absorption catalyst, the substance which can
be considered at first as the adsorption medium is Pt particles.
When the amount of CO adsorption was evaluated as is frequently
employed as a means for evaluating the amount of exposed platinum,
the amount of adsorbed CO (at 100.degree. C.) was
4.5.times.10.sup.-4 mole. This value is equal to about {fraction
(1/100)} of the above-mentioned NO.sub.x adsorption, demonstrating
that Pt is not the main adsorbing medium for NO.sub.x.
[0074] On the other hand, the BET specific surface area of this
adsorption catalyst (measured by nitrogen adsorption) measured
together with cordierite was about 25 m.sup.2/g, which corresponded
to a value of 28.050 m.sup.2 per 1.7 L of the honeycomb. When the
chemical structure of Na in the absorption catalyst of the
invention was examined, it could be judged that Na existed
predominantly as NaCO.sub.3 based on that the catalyst was
dissolved in mineral acids with evolution of CO.sub.2 gas, and
based on the value of inflection point on its neutralizing
titration curve with the mineral acid. If it is assumed that the
whole surface is occupied by Na.sub.2CO.sub.3, the amount of
Na.sub.2CO.sub.3 exposed on the surface is 0.275 mole (since
Na.sub.2CO.sub.3 has a specific gravity of 2.533 g/ml, the volume
of one Na.sub.2CO.sub.3 molecule can be determined
(Na.sub.2CO.sub.3 was assumed as a cube and the area of its one
face was calculated to take it as the area occupied by surface
Na.sub.2CO.sub.3). According to the reaction scheme shown above,
0.275 mole of Na.sub.2CO.sub.3 has an ability of adsorbing 0.55
mole of NO.sub.x. However, the amount of NO.sub.x actually removed
by the absorption catalyst of this invention was approximately 0.04
mole, which is less than {fraction (1/10)} of the above-mentioned
value. This difference is attributable to that the BET method
evaluates the physical surface area and it evaluates also the
surface area such as of Al.sub.2O.sub.3 other than that of
Na.sub.2CO.sub.3. The evaluation given aboveindicates that the
amount of adsorbed NO.sub.x is much smaller than the
NO.sub.x-capturing ability of the Na.sub.2CO.sub.3 bulk, and
NO.sub.x is captured at least only on the Na.sub.2CO.sub.3 surface
or in a limited region in the vicinity of the surface.
[0075] In FIG. 3, the NO.sub.x adsorbing ability decreases along
with increase of the running distance of the vehicle and the
decreasing rate of the NO.sub.x purification rate is increased
after change-over from the stoichiometrical running to the lean
burn running. This is because the poisoning substance (such as SOx)
contained in the exhaust gas reacts with the NO.sub.x adsorbing
substance to deteriorate the adsorbing ability.
[0076] FIG. 4 illustrates the NO.sub.x purification rate just after
the change-over from running at lean ratio to running at
stoichiometric ratio. It can be seen that the absorption catalyst
of this invention gives an NO.sub.x purification rate of 90% or
higher from just after the change-over to the running at
stoichiometric ratio.
[0077] FIG. 5 and FIG. 6 illustrate the NO.sub.x purification
characteristics before and after the change-over from lean burn
running to stoichiometric or rich running. FIG. 5 shows the
NO.sub.x concentrations at the inlet and outlet of the adsorption
catalyst N--N9, in which FIG. 5A illustrates a case of
changing-over the air/fuel ratio from a lean burn running at A/F=22
to a rich running at A/F=14.2. At the time of starting the
regeneration just after the change-over to rich running, since the
NO.sub.x concentration in the exhaust gas at A/F=14.2 is high, the
inlet NO.sub.x concentration in the lean burn running increases
greatly. Although the outlet NO.sub.x concentration also increases
temporarily therewith, the outlet NO.sub.x concentration is usually
much lower than the inlet NO.sub.x concentration. The regeneration
proceeds rapidly, and the outlet NO.sub.x concentration reaches
approximately zero in a short period of time. FIG. 5B illustrates a
case of changing over the air/fuel ratio from a lean burn running
at A/F=22 to a rich running at A/F=13.2. Also in this case, the
outlet NO.sub.x concentration is usually much lower than the inlet
NO.sub.x concentration like that in the case of FIG. 5A, and the
outlet NO.sub.x concentration reaches approximately zero in a
shorter period of time.
[0078] As is apparent from the foregoings, the A/F value as a
condition of regeneration gives an influence on the time required
for regeneration. The A/F value, time and the amount of the
reducing agent suitable to regeneration undergo the effect of the
composition, shape and temperature of the absorption catalyst, the
SV value, the kind of the reducing agent, and the shape and length
of exhaust gas flow channel. Accordingly, the conditions of
regeneration should be decided collectively considering these
factors.
[0079] FIGS. 6A and 6B show the NO.sub.x concentration at the inlet
and the outlet of the absorption catalyst N--K9, in which FIG. 6A
is a case of changing over the air/fuel ratio from a lean burn
running at A/F=22 to a rich running at A/F=14.2, and FIG. 6B is a
case of changing over the air/fuel ratio from a lean burn running
at A/F=22 to a rich running at A/F=13.2. Like that in the case of
the absorption catalyst N--N9, the outlet NO.sub.x concentration is
usually much lower than the inlet NO.sub.x concentration and
regeneration of the absorption catalyst progresses in a short
period of time.
[0080] [Apparatus for Purifying and Controlling Exhaust Gases]
[0081] FIG. 1 is an example of an apparatus for reducing NO.sub.x
based on the estimation for the NO.sub.x purification rate
according to this invention. At least one of amounts of state
selected from the amount of NO.sub.x adsorbed to the NO.sub.x
adsorption catalyst, the temperature of the exhaust gas, the
temperature of the adsorption catalyst, the amount of sulfur
poisoning, the running distance of the vehicle, the degradation
degree of the catalyst, the air/fuel ratio, the concentration of
unburnt hydrocarbon, the NO.sub.x concentration before the
catalyst, the period of time for the lean burn running from the
change-over from the stoichiometric (theoretical air/fuel ratio) or
rich running to lean burn running, the number of rotation of the
internal combustion engine, the load on the engine, the amount of
intake air and the amount of the exhaust gas is input to the
NO.sub.x purification rate estimation section of the
NO.sub.x-adsorption catalyst. When the estimated NO.sub.x
purification rate has been lowered below a predetermined value,
reduction treatment of NO.sub.x is carried out.
[0082] The reduction treatment of NO.sub.x adsorbed on the NO.sub.x
adsorption catalyst is carried out by increasing the concentration
of unburnt hydrocarbons in the exhaust gas flowing into the
catalyst. Concretely the concentration of unburnt hydrocarbons is
increased by making the air/fuel ratio lower than the theoretical
air/fuel ratio (namely, increasing the amount of injected fuel) or,
additionally injecting the fuel in the expanding stroke or
exhausting stroke of the engine in the case of injection into
cylinders. Due to this increase, NO.sub.x adsorbed on the NO.sub.x
adsorption catalyst is reduced by the unburnt hydrocarbons and made
non-toxic. When the concentration of unburnt hydrocarbons
increases, since the output torque of the engine or the final
running torque of the driving wheels may possibly vary, this
variation is regulated by using any one of means of the ignition
timing, the amount of intake air, the amount of the exhaust gas to
be mixed into inlet air of the internal combustion engine (EGR
rate), the amount of the injected fuel, the timing of fuel
injection, the electric motor assisting the output of the internal
combustion engine, the load of a generator placed in the engine and
braking on the output side of the engine.
[0083] FIG. 7 is a diagram illustrating the system for controlling
the internal combustion engine to realize the condition described
above.
[0084] The apparatus of this invention comprises an engine 99 which
can be worked at lean burn ratio, an air suction system having an
air flow sensor 2 and an electronically controlled throttle valve
3, an exhaust gas system having an oxygen concentration sensor
(or-A/F sensor) 19, an exhaust gas temperature sensor 17 and an
NO.sub.x adsorption catalyst 18, and a controlling unit (ECU) 25.
The ECU comprises I/O LSI as an input/output interface, a computing
unit MPU, memory devices RAM and ROM storing a number of
controlling programs, and a timer counter. The ECU houses
controlling programs executing the following processing of this
invention, and conducts estimation for the NO.sub.x purification
rate, compares the estimated values, and reduces NO.sub.x on the
basis of various sensor signals. At least one of amounts of state
selected from the amount of NO.sub.x adsorbed on the NO.sub.x
adsorption catalyst, the temperature of the exhaust gas, the
temperature of the absorption catalyst, the poisoning amount of
sulfur, the running distance of the automobile, the degree of
deterioration of the catalyst, the air/fuel ratio, the
concentration of unburnt hydrocarbons, the NO.sub.x concentration
before the catalyst, the period of time of lean burn running having
passed from the time of change-over from stoichiometric running (at
theoretical air/fuel ratio) or rich running to lean burn running,
the number of rotation of the internal combustion engine, the load
on the engine, the amount of intake air, and the amount of the
exhaust gas is input into the NO.sub.x purification rate estimating
portion of the NO adsorption catalyst. When the estimated NO
purification rate has lowered below a predetermined value, the
reduction treatment of NO.sub.x is carried out. The reduction
treatment of NO.sub.x adsorbed on the NO.sub.x adsorption catalyst
is carried out by increasing the concentration of unburnt
hydrocarbons in the exhaust gas flowing into the catalyst.
Concretely, the concentration of the unburnt hydrocarbons is
increased by making the air/fuel ratio lower than the theoretical
air/fuel ratio (namely, increasing the amount of injected fuel) or
by additionally injecting fuel in the expanding stroke or
exhausting stroke of the engine thereby increasing the
concentration of unburnt hydrocarbons in the case of injection into
cylinders. Due to this increase, NO.sub.x adsorbed on the NO.sub.x
adsorption catalyst is reduced by the unburnt hydrocarbons and made
non-toxic. When the concentration of unburnt hydrocarbons is
increased, since the output torque of the engine or the final
running torque of the driving wheels may possibly vary, this
variation is suppressed by regulating any one of the ignition time,
the amount of intake air, the amount of the exhaust gas to be mixed
into intake air of the internal combustion engine (EGR rate, EGR
valve 27), the amount of injected fuel (injector 5), the timing of
fuel injection, the electric motor assisting the output of the
internal combustion engine, the load on the generator placed in the
engine and the braking on the output side of the engine.
[0085] The apparatus for purifying and controlling the exhaust gas
described above functions as follows. That is, air taken into the
engine is filtered by an air cleaner 1, metered by an air flow
sensor 2, passed through an electronically controlled throttle
valve 3, supplied with the fuel injected from an injector 5, and
fed to the engine 99 as a gas mixture. Signals from the air flow
sensor and other sensors are input to the ECU (engine control
unit).
[0086] The ECU evaluates the running state of the internal
combustion engine and the state of the NO.sub.x adsorption catalyst
by the method mentioned later, determines the air/fuel ratio and
controls the injection time of the injector 5 to set the fuel
concentration in the gas mixture to a predetermined value. The
injector 5 may be attached so as to enable cylinder injection like
that in diesel engine, instead of setting at the position of the
air intake port of the engine in FIG. 7. Alternatively, fuel
concentration in the gas mixture may be set to a prescribed value
by decreasing the amount of intake air by controlling the opening
degree (throttle actuator 31) of the electronically controlled
throttle valve 3 while keeping the amount of injected fuel
constant. The gas mixture taken into cylinders is ignited with an
ignition plug 6 controlled by the signals from the ECU 25 and
burnt. The exhaust gas of combustion is led to the exhaust gas
purification system. The exhaust gas purification system is
provided with an NO.sub.x adsorption catalyst which purifies
NO.sub.x, HC and CO in the exhaust gas by its ternary catalytic
function during stoichiometric running and purifies NO.sub.x by its
NO.sub.x adsorbing function and, at the same time, purifies HC and
CO by its burning function during lean burn running. Further, based
on the judgement of ECU and control signals, the NO.sub.x
purification ability of the NO.sub.x adsorption catalyst is always
estimated in terms of the estimated NO.sub.x purification rate
during lean burn running so as to recover the NO.sub.x adsorbing
ability of the NO.sub.x adsorption catalyst by shifting the
air/fuel ratio of combustion to the fuel rich side or injecting the
fuel into cylinders in the expanding stroke or exhausting stroke
when the NO.sub.x purification ability has been lowered. By the
operations described above, the apparatus of the invention
effectively purifies the exhaust gas under all the engine
combustion conditions including lean burn running and
stoichiometric running (including fuel rich running).
[0087] In FIG. 7, are illustrated an accelerating pedal 7, a load
sensor 8, a suction air temperature sensor 9, a fuel pump 12, a
fuel tank 13, an adsorption catalyst temperature sensor 20, an
exhaust gas concentration sensor 21, a knocking detecting sensor
26, an EGR valve 27, a water temperature sensor 28 and a crank
angle sensor 29.
[0088] According to the apparatus of this invention since the
NO.sub.x purification rate of the NO.sub.x adsorption catalyst is
estimated and, a reduction treatment of NO.sub.x adsorbed on the
NO.sub.x adsorption catalyst is conducted when the estimated value
has been lowered below a predetermined value, NO.sub.x can be
purified at a high efficiency over a long period of time without
increasing the amount of toxic exhaust gas.
[0089] While the use of the NOX adsorption catalyst has been
explained, this invention can directly reduce a portion of NO.sub.x
in the exhaust gas with HC or CO in the exhaust gas in addition to
capturing (for example, absorption) of a portion of NO.sub.x in the
exhaust gas to the NO.sub.x catalyst during lean burn running and
it is applicable also a system of using the NO.sub.x catalyst
capable of reducing the captured Nox into N.sub.2 during running at
a stoichiometrical or fuel rich air/fuel ratio.
* * * * *