Apparatus for purifying and controlling exhaust gases

Abstract

A system having an NOx adsorption catalyst in an exhaust gas flow channel of the internal combustion engine, and adsorbing and capturing NOx 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 NOx is carried out based on an estimated NOx purification rate and the NOx 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.

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 NOx, 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 NOx 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 NOx and, especially, NOx in exhaust gases containing a great amount of oxygen (O2) 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 NOx releasing unit that estimates the amount of NOx absorbed in the NOx absorbent disposed in an exhaust pipe of an internal combustion engine and lowers the oxygen concentration in the exhaust gases flowing into the NOx absorbent when the estimated amount of the absorbed NOx exceeds a predetermined allowable limit thereby releasing NOx from the NOx absorbent.

[0006] However, since this method repeats the NOx reducing treatment in accordance with the amount of NOx absorbed in the NOx absorbent (releasing NOx from the NOx absorbent by lowering the oxygen concentration in the exhaust gases and reducing the released NOx), it suffers from a restriction on the accuracy of maintaining the NOx discharge amount after the NOx 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 NOx non-toxic by adsorption instead of absorption of NOx. According to this method, NOx in the exhaust gases is adsorbed as NO2 on the adsorption catalyst, which is partly reduced directly into N2 by HC or CO in the exhaust gases and partly captured in the form of NO2 as it is on the NOx adsorption catalyst in lean burn running, and then NO2 captured by adsorption is reduced into N2 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 NOx at an accurate timing in a system of purifying exhaust gases by capturing NOx on an NOx adsorption catalyst by adsorption or absorption (particularly, in a system of purifying NOx using an NOx adsorption catalyst)

SUMMARY OF THE INVENTION

[0009] The foregoing subject of this invention can be solved by estimating an NOx purification rate based on the amount of NOx discharged from internal combustion engines and the running conditions of the engines and reducing NOx adsorbed on an NOx adsorption catalyst at the time the estimated NOx purification rate lowers to a predetermined value. Since the reduction treatment of NOx 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 NOx 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 NOx 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 NOx 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 NOx 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 NOx adsorbed on the absorption catalyst with the reducing agent and reducing the NOx to non-toxic N2.

[0011] The term “adsorbing catalyst” means a material having an ability of adsorbing NOx 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 NOx, an ability of catalytically reducing NOx and an ability of catalytically oxidizing HC, CO, etc.

[0012] That is, the NOx adsorption catalyst used in this invention adsorbs NOx in the exhaust gas during a lean burn running as NOx on the adsorption catalyst, directly reduces a portion of NOx to N2 by using HC, CO, etc. in the exhaust gas, while captures a portion of NOx as NO2 on the adsorption catalyst and then reduces the adsorbed and/or captured NO2 to N2 during running at a stoichiometric or fuel rich A/F ratio. The NOx 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 NOx,. 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 NOx as NO2 on the surface of the catalyst, thereby inhibiting absorption in the form of NO3− to the inside of the catalyst.

[0013] The oxidizing agent includes O2, NO, and NO2, 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, H2 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, H2 as reducing agents for reducing NOx to nitrogen, they react with O2 as the oxidizing agent in the exhaust gas to cause combustion reaction. NOx (NO and NO2) 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° 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 NOx to N2 effectively by separating NOx from the exhaust gas (at least from O2 in the exhaust gas) by the use of the absorption catalyst and then catalytically reacting NOx with the reducing agent. In this invention, NOx in exhaust gas is separated from O2 by adsorbing NOx from the lean exhaust gas by the use of the NOx 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 (O2, NOx) and the reducing agent (HC, CO, H2), and the NOx adsorbed on the absorption catalyst is catalytically reacted with the reducing agent such as HC to reduce NOx to N2.

[0016] Now, NOx in the exhaust gas substantially comprises NO and NO2. Since NO2 is more reactive than NO, NO2 can be removed by adsorption and reduced more easily than NO Accordingly, oxidation of NO to NO2 facilitates adsorptive removal and reduction of NOx in the exhaust gas. This invention includes a method of oxidizing NOx present in the lean exhaust gas to NO2 by coexisting O2 and thereby removing NOx, and an oxidizing means for this purpose such as provision of an NO-oxidizing function to the absorption catalyst.

[0017] In the NOx adsorption catalyst used in this invention, the reduction reaction for the chemically adsorbed NOx can be approximately expressed by the following reaction scheme:

MO—NO2+HC→MO+N2+CO2+H2O→MCO3+N2+H2O,

[0018] where M is a metal element (the reason of adapting MCO3 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° C.) can be calculated as follows:

2NaNO3(s)+{fraction (5/9)}C3H6→Na2CO3(s)+N2+⅔CO2+{fraction (5/3)}H2O

[0020] [−&Dgr;H=873 kilojoules/mole]

Ba(NO3)2+{fraction (5/9)}C33H6→BaCO3(s)+N2+⅔CO2+{fraction (5/3)}H2O

[0021] [−&Dgr;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 C3H6 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 NOx-capturing agent is an NOx-absorbent, since the NOx 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 NOx rapidly and lowering the concentration of NOx in the exhaust gas discharged out of the apparatus, it is considered that the gas phase reaction between NO2 and HC does not proceeds so rapidly and, therefore, the amount of the released NOx 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 NOx is yet small, but this increases the frequency for the regeneration of the NOx absorbent and lowers the effect of improving the specific fuel consumption.

[0027] Since the absorption catalyst used in this invention captures NOx 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 NOx can be prevented.

[0028] The NOx-absorption catalyst used in this invention has a feature as a material that captures NOx at the surface thereof by chemical adsorption and does not release NOx by the exothermic reaction in the step of reducing NOx. Further, the NOx adsorption catalyst of this invention has a feature as a material that captures NOx by a chemical adsorption at the surface thereof or by chemical bond in the vicinity of the surface thereof and does not release NOx by the exothermic reaction at the step of reducing NOx.

[0029] The present inventors have found that the above-mentioned features can be realized by an NOx 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 NOx 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 NOx on the NOx 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 NOx adsorbed on the catalyst with the reducing agent to reduce the NO to non-toxic N2.

[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 NOx 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 O2 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 NOx by chemical bonds on or near the surface of the NOx 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 NOx captured on the catalyst with the reducing agent to reduce the NOx to harmless N2.

[0032] As the NOx 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 NOx-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 NOx 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 NOx is calculated based on the signal of the NOx sensor placed to the upstream of the absorption catalyst in the exhaust flow channel and the accumulated amount of NOx has exceeded a predetermined value during lean burn running.

[0046] (4) When the NOx concentration during lean burn running is detected based on the signal of the NOx sensor placed to the downstream of the absorption catalyst in the exhaust flow channel and the NOx concentration has exceeded a predetermined value, or when the NOx purification rate is calculated based on the signal of the NOx sensor placed to the upstream or downstream to the absorption catalyst and the NOx purification rate has been lowered below a predetermined value; and

[0047] (5) When the NOx purification rate of the NOx adsorption catalyst is estimated based on at least one of status amounts, namely, the amount of NOx adsorbed on the NOx 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 NOx 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 NOx 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 NOx 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 NOx purification rate;

[0053] FIG. 4 is a graph illustrating a NOx purification rate in a stoichiometric exhaust gas;

[0054] FIG. 5A and FIG. 5B are graphs illustrating a relation between the NOx concentration at the inlet of an adsorption catalyst and the NOx 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 NOx concentration at the inlet of an adsorption catalyst and the NOx 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° 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° 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° C. for one hour. Finally, it was impregnated with Mg nitrate solution and calcined at 45° 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/Al2O3 supporting Ce, Mg, Na, Ti, Rh and Pt on alumina (Al2O3). The expression “/Al2O3” means that the active components are supported on Al2O3, 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 Al2O3 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/Al2O3 was obtained. Further, control catalyst N—R2, namely 2Mg-(0.2Rh, 2.7Pt)-27Ce/Al2O3 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° 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 NOx-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 NOx purification rate decreased to about 40%) were alternately repeated to obtain the characteristics NOx purification rate with lapse of time in FIG. 2. It can be seen from the figure that NOx can be purified by this absorption catalyst during the lean burn running period. The NOx 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 NOx 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 NOx 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 NOx concentration at the inlet of the catalyst, and the amount of the exhaust gas. Accordingly, it is important to estimate the NOx 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 NOx concentration in the exhaust gas and determine the relation between the NOx concentration and the NOx purification rate of lean exhaust gas as shown in FIG. 3. The NOx purification rate decreases with time, in which the decreasing rate is lower as the NOx concentration is lower. The amounts of NOx captured till the NOx 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 NOx captured is substantially constant regardless of the NOx 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° C.) was 4.5×10−4 mole. This value is equal to about {fraction (1/100)} of the above-mentioned NOx adsorption, demonstrating that Pt is not the main adsorbing medium for NOx.

[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 m2/g, which corresponded to a value of 28.050 m2 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 NaCO3 based on that the catalyst was dissolved in mineral acids with evolution of CO2 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 Na2CO3, the amount of Na2CO3 exposed on the surface is 0.275 mole (since Na2CO3 has a specific gravity of 2.533 g/ml, the volume of one Na2CO3 molecule can be determined (Na2CO3 was assumed as a cube and the area of its one face was calculated to take it as the area occupied by surface Na2CO3). According to the reaction scheme shown above, 0.275 mole of Na2CO3 has an ability of adsorbing 0.55 mole of NOx. However, the amount of NOx 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 Al2O3 other than that of Na2CO3. The evaluation given aboveindicates that the amount of adsorbed NOx is much smaller than the NOx-capturing ability of the Na2CO3 bulk, and NOx is captured at least only on the Na2CO3 surface or in a limited region in the vicinity of the surface.

[0075] In FIG. 3, the NOx adsorbing ability decreases along with increase of the running distance of the vehicle and the decreasing rate of the NOx 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 NOx adsorbing substance to deteriorate the adsorbing ability.

[0076] FIG. 4 illustrates the NOx 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 NOx 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 NOx purification characteristics before and after the change-over from lean burn running to stoichiometric or rich running. FIG. 5 shows the NOx 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 NOx concentration in the exhaust gas at A/F=14.2 is high, the inlet NOx concentration in the lean burn running increases greatly. Although the outlet NOx concentration also increases temporarily therewith, the outlet NOx concentration is usually much lower than the inlet NOx concentration. The regeneration proceeds rapidly, and the outlet NOx 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 NOx concentration is usually much lower than the inlet NOx concentration like that in the case of FIG. 5A, and the outlet NOx 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 NOx 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 NOx concentration is usually much lower than the inlet NOx 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 NOx based on the estimation for the NOx purification rate according to this invention. At least one of amounts of state selected from the amount of NOx adsorbed to the NOx 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 NOx 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 NOx purification rate estimation section of the NOx-adsorption catalyst. When the estimated NOx purification rate has been lowered below a predetermined value, reduction treatment of NOx is carried out.

[0082] The reduction treatment of NOx adsorbed on the NOx 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, NOx adsorbed on the NOx 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 NOx 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 NOx purification rate, compares the estimated values, and reduces NOx on the basis of various sensor signals. At least one of amounts of state selected from the amount of NOx adsorbed on the NOx 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 NOx 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 NOx 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 NOx is carried out. The reduction treatment of NOx adsorbed on the NOx 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, NOx adsorbed on the NOx 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 NOx 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 NOx adsorption catalyst which purifies NOx, HC and CO in the exhaust gas by its ternary catalytic function during stoichiometric running and purifies NOx by its NOx 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 NOx purification ability of the NOx adsorption catalyst is always estimated in terms of the estimated NOx purification rate during lean burn running so as to recover the NOx adsorbing ability of the NOx 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 NOx 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 NOx purification rate of the NOx adsorption catalyst is estimated and, a reduction treatment of NOx adsorbed on the NOx adsorption catalyst is conducted when the estimated value has been lowered below a predetermined value, NOx 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 NOx in the exhaust gas with HC or CO in the exhaust gas in addition to capturing (for example, absorption) of a portion of NOx in the exhaust gas to the NOx catalyst during lean burn running and it is applicable also a system of using the NOx catalyst capable of reducing the captured Nox into N2 during running at a stoichiometrical or fuel rich air/fuel ratio.

Claims

1. An exhaust gas purification apparatus for use in an internal combustion engine in which an NOx adsorption catalyst for chemically adsorbing NOx in a state where the amount of an oxidizing agent is larger than the amount of a reducing agent and catalytically reducing NOx 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 NOx 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 NOx adsorbed on the catalyst with the reducing agent into non-toxic N2, wherein the apparatus has a control unit for estimating the NOx purification rate based on the amount of NOx discharged from the internal combustion engine and the running state of the engine, and carrying out a reducing treatment for NOx adsorbed on the NOx adsorption catalyst when the estimated NOx 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 NOx adsorption catalyst for chemically adsorbing NOx in a state where the amount of an oxidizing agent is larger than the amount of a reducing agent and catalytically reducing NOx 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 NOx 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 NOx adsorbed on the catalyst with the reducing agent and reduce the same into non-toxic N2, wherein the NOx purification rate is estimated based on the amount of NOx discharged from the internal combustion engine and the running state of the engine, and a reducing treatment is conducted for NOx adsorbed on the NOx adsorption catalyst when the estimated NOx purification rate is lowered to a predetermined value.

3. An exhaust gas purification apparatus for use in an internal combustion engine in which an NOx catalyst for capturing NOx discharged from an internal combustion engine and reducing a portion of the captured NOx into N2 during lean burn running and reducing NOx captured as it is during lean burn running into N2 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 NOx purification rate based on the amount of NOx discharged from the internal combustion engine and the running state of the engine, and carrying out a reducing treatment for NOx captured on the NOx catalyst when the estimated NOx 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 NOx catalyst for capturing NOx discharged from an internal combustion engine and reducing a portion of the captured NOx into N2 during lean burn running and reducing NOx captured as it is during lean burn running into N2 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 NOx purification rate is estimated based on the amount of NOx discharged from the internal combustion engine and the running state of the engine, and a reducing treatment is conducted for NOx captured on the NOx catalyst when the estimated NOx 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 NOx purification rate of the NOx adsorption catalyst is estimated, as a method of estimating the NOx purification rate based on the amount of NOx 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 NOx adsorbed on the NOx 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 NOx 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 NOx purification rate of the NOx adsorption catalyst is estimated, as a method of estimating the NOx purification rate based on the amount of NOx 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 NOx captured on the NOx 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 NOx 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.