Exhaust gas purification device for an engine

[0002] 1. Field of the Invention

[0003] The present invention relates to an exhaust gas purification device for an engine. More specifically, the present invention relates to a device that is capable of effectively removing an NO_xcomponent from exhaust gas.

[0004] 2. Description of the Related Art

[0005] An exhaust gas purification device capable of removing an NO_xcomponent from engine exhaust gas is, for example, disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 62-117620. The device disclosed in JPP '620 is used for removing NO_xcomponents from the exhaust gas of a lean burn engine, i.e., an engine operated at an air-fuel ratio higher than a stoichiometric air-fuel ratio. In the device disclosed in JPP '620, an exhaust passage of the engine is diverged into two branch exhaust passages, and a NO_xabsorbent is disposed in each of the branch exhaust passages, namely, two NO_xabsorbents are disposed in the exhaust gas passage in parallel. The NO_xabsorbent is capable of absorbing an NO_xcomponent in the exhaust gas in the presence of oxygen. Since the engine is operated on a lean air-fuel ratio, the exhaust gas from the engine contains relatively high concentration of oxygen, therefore, NO_xcomponents contained in the exhaust gas are absorbed by the NO_xabsorbents.

[0006] When the NO_xabsorbent absorbs a certain amount of NO_x, the ability of the NO_xabsorbent to absorb NO_xin the exhaust gas is reduced. To regain the absorption ability, the absorbed NO_xin the absorbent must be discharged and reduced periodically. For example, by supplying a reducing agent to the NO_xabsorbent, the NO_xabsorbents are regenerated to regain the initial NO_xabsorption ability. In the device in JPP '620, two NO_xabsorbents are disposed in parallel in the exhaust passage in order to enable regeneration of the NO_xabsorbent without stopping the purification of the exhaust gas. In JPP '620, two NO_xabsorbents are used alternately during normal engine operation, i.e., only one of the NO_xabsorbents is used for absorbing the NO_xin the exhaust gas. Namely, in the normal operation, one of the branch exhaust passage is closed, and the exhaust gas from the engine is directed to the other branch exhaust passage where the NO_xin the exhaust gas is absorbed by the NO_xabsorbent disposed in that exhaust branch passage. While absorbing NO_xin the exhaust gas by one NO_xabsorbent, the other NO_xabsorbent disposed on the closed exhaust branch passage is regenerated by supplying a reducing agent, such as hydrogen gas.

[0007] According to the device disclosed in JPP '620, by using two NO_xabsorbents disposed in the exhaust passage in parallel, the NO_xabsorbents can be regenerated alternately while continuing to remove the NO_xcomponent from the exhaust gas.

[0008] However, a disadvantage of the device disclosed in JPP '620 has is that the reducing agent must be supplied to both exhaust branch passages. This requires two injection nozzles, one in each of the exhaust branch passages, and associated piping connecting the nozzles to the supply source of the reducing agent, and a three-way flow control valve for switching the supply of the reducing agent between the two nozzles. Further, elaborate control of the three-way flow control valve is required to enable the appropriate regeneration of the NO_xabsorbents.

[0009] In view of the problems in the prior art set forth above, the object of the present invention is to provide an exhaust gas purification device for an internal combustion engine which is capable of removing NO_xcomponents from the exhaust gas without requiring complicated devices and controls.

[0010] The above object is achieved by the present invention, an exhaust gas purification device, for an internal combustion engine. The engine is operated with an air-fuel ratio higher than a stoichiometric air-fuel ratio. The device comprises (a) first and second NO_xabsorbents disposed in the exhaust passage of the engine and connected in series by an exhaust connecting passage, each of the NO_xabsorbents absorbs NO_xcomponents in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NO_xabsorbents is lean, and releases the absorbed NO_xwhen the oxygen concentration in the exhaust gas becomes lower; (b) a reducing agent supply means for supplying a reducing agent to the exhaust gas in the exhaust connecting passage between the first and the second NO_xabsorbents; (c) a switching means for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NO_xabsorbent, the exhaust connecting passage and the second NO_xabsorbent in this order, and a second direction in which the exhaust gas flows through the second NO_xabsorbent, the exhaust connecting passage and the first NO_xabsorbent in this order; (d) and a control means for controlling the switching means so that the direction of the exhaust gas flow is switched between the first and the second direction each time when a predetermined condition is satisfied.

[0011] When the flow direction of exhaust gas is switched to the first direction, the lean air-fuel ratio exhaust gas from the engine first flows into the first NO_xabsorbent and NO_xcomponents in the exhaust gas are absorbed by the first NO_xabsorbent. After passing through the first NO_xabsorbent, a reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the second NO_xabsorbent. Therefore, the air-fuel ratio of the exhaust gas flowing into the second NO_xabsorbent becomes rich due to the reducing agent. Thus, in the second NO_xabsorbent, the absorbed NO_xis released and reduced to nitrogen by the reducing agent. Namely, when the exhaust gas flow is switched to the first direction, NO_xcomponents in the exhaust gas are absorbed by the first NO_xabsorbent and removed from the exhaust gas, and the NO_xin the second NO_xabsorbent is released and reduced to nitrogen.

[0012] On the other hand, when the exhaust gas flow is switched to the second direction, the lean exhaust gas from the engine first flows into the second NO_xabsorbent and NO_xcomponents in the exhaust gas are absorbed by the second NO_xabsorbent. Then, the reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the first NO_xabsorbent. Therefore, the rich exhaust gas flows into the first NO_xabsorbent, and absorbed NO_xis released from the first NO_xabsorbent and reduced to nitrogen by the reducing agent. Therefore, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, for example, each time an amount of NO_xabsorbed in the NO_xabsorbent reaches a predetermined value, absorption of NO_xin the exhaust gas and releasing and reducing of the absorbed NO_xis performed in each NO_xabsorbent alternatively without stopping the absorption of NO_xin the exhaust gas as a whole.

[0013] According to another aspect of the present invention, there is provided an exhaust gas purification device for an internal combustion engine having an exhaust passage. The engine is operated at an air-fuel ratio equal to, or lower than, a stoichiometric air-fuel ratio. The device comprises (a), first and second NO_xabsorbents disposed in the exhaust passage of the engine and connected each other in series by an exhaust connecting passage, each of the NO_xabsorbents absorbs NO_xcomponents in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NO_xabsorbents is lean, and releases the absorbed NO_xwhen the oxygen concentration in the exhaust gas becomes; (b) a secondary air supply means for supplying secondary air to the exhaust gas in the exhaust connecting passage between the first and the second NO_xabsorbents; (c) a switching means for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NO_xabsorbents, the exhaust connecting passage and the second NO_xabsorbent in this order, and a second direction in which the exhaust gas flows through the second NO_xabsorbent, the exhaust connecting passage and the first NO_xabsorbent in this order; and (d) a control means for controlling the switching means so that the direction of the exhaust gas flow is switched between the first and the second direction each time when a predetermined condition is satisfied.

[0014] In this aspect of the invention, the air-fuel ratio of the exhaust gas from the engine is rich, i.e., lower than, or equal to, the stoichiometric air-fuel ratio, and relatively low concentrations of oxygen and a relatively high concentrations of HC and CO components are contained in the exhaust gas. Therefore, when the direction of exhaust gas flow is switched to the first direction, the exhaust gas with a rich air-fuel ratio flows into the first NO_xabsorbent, and the absorbed NO_xis released from the first NO_xabsorbent and reduced to nitrogen by HC and CO components in the exhaust gas. After passing through the first NO_xabsorbent, the secondary air is supplied to the exhaust gas in the exhaust connecting passage before it flows into the second NO_xabsorbent, and the air-fuel ratio of the exhaust gas flowing into the second NO_xabsorbent becomes lean. Therefore, a small amount of NO_xwhich passes through the first NO_xabsorbent is absorbed by the second NO_xabsorbent and removed completely from the exhaust gas.

[0015] Namely, when the direction of the exhaust gas flow is switched to the first direction, the absorbed NO_xis released from the first NO_xabsorbent and reduced to nitrogen, and at the same time, NO_xin the exhaust gas is absorbed by the second NO_xabsorbent.

[0016] Similarly, when the direction of the exhaust gas flow is switched to the second direction, the absorbed NO_xis released from the second NO_xabsorbent and reduced to nitrogen, and NO_xin the exhaust gas is absorbed by the first NO_xabsorbent at the same time.

[0017] Therefore, in this aspect of the present invention, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, absorption of NO_xin the exhaust gas and releasing and reducing of the absorbed NO_xis performed in each NO_xabsorbent alternatively, without stopping absorption of NO_xin the exhaust gas as a whole.

[0018] The present invention will be better understood from the description as set forth hereinafter, with reference to the accompanying drawings, in which:

[0019] FIG. 1 is a drawing schematically illustrating an embodiment of the exhaust gas purification device according the present invention when applied to an automobile engine;

[0020] FIG. 2 is a sectional view of the exhaust passage in FIG. 1 taken along a line II—II in FIG. 1;

[0021] FIGS. 3A and 3B are views for explaining the absorbing and releasing operation of NO_xby the NO_xabsorbent;

[0022] FIG. 4 is a drawing schematically illustrating another embodiment of the exhaust gas purification device according to the present invention;

[0023] FIGS. 5A and 5B are drawings illustrating another embodiment of the exhaust gas purification device according to the present invention; and

[0024] FIG. 6 is a drawing illustrating another embodiment of the exhaust gas purification device according to the present invention.

[0025] FIG. 1 schematically illustrates an embodiment of an exhaust gas purification device, according to the present invention, as applied to an automobile engine.

[0026] In FIG. 1, reference numeral 1 designates an internal combustion engine such as a diesel engine or a lean-burn gasoline engine. The engine 1 is operated at an air-fuel ratio higher than the stoichiometric air-fuel ratio some paint in time during the normal operating range. Numeral 3 designates an exhaust gas passage of the engine 1. The exhaust gas passage 3 diverges into two branch exhaust passages 3aand 3b, and a NO_xabsorbent (designated by 7aand 7b) is connected to each branch exhaust passage. The NO_xabsorbents 7aand 7bare connected to each other at opposite ends of to the exhaust branch passage 3aand 3bby an exhaust connecting passage 4.

[0027] A switching valve 5 is disposed on the exhaust gas passage 3 at the point where the branch exhaust passages 3aand 3bdiverge. The switching valve 5 is actuated by a control signal from an engine control circuit (ECU) 20 which is explained later. The switching valve 5 directs the exhaust gas flow to either of the branch exhaust passages 3aand 3bby closing the inlet of one of the branch exhaust passages 3aand 3b. On the respective branch exhaust passages 3aand 3b, a port 6a, 6band a shut-off valve 8a, 8b, which closes the port 6a, 6b, respectively, are disposed at the portion between the switching valve 5 and the NO_xabsorbents 7aand 7b.

[0028] FIG. 2 schematically illustrates the section of the branch exhaust passages 3aand 3btaken along the line II—II in FIG. 1. As shown in FIG. 2, an exhaust outlet passage 9 is attached to the branch exhaust passages 3aand 3b. The exhaust outlet passage 9 communicates the branch exhaust passages 3aand 3bthrough the ports 6aand 6b, respectively. Namely, when the shut-off valve 8ais opened, the branch exhaust passage 3ais connected to the exhaust outlet passage 9 via the port 6a, and when the shut-off valve 8bis opened, the branch exhaust passage 3bis connected to the exhaust outlet passage 9 via the port 6b. The exhaust outlet passage 9 opens to the atmosphere downstream of a muffler (not shown) disposed therein.

[0029] In this embodiment, the switching valve 5 and the shut-off valves 8aand 8bare functionally connected to each other by a mechanical link 10 and driven by a common actuator 11. The actuator 11 may be a solenoid actuator, a vacuum actuator, or any other appropriate type of actuator that can drive the switching valve 5 and shut-off valves 8aand 8bsynchronously in accordance with the control signal from the ECU 20. In this embodiment, the shut-off valves 8aand 8bare operated through the mechanical link 10 in such a manner that the shut-off valve 8bopens the port 6band the shut-off valve 8acloses the port 6awhen the switching valve 5 is at a position closing the inlet of the branch exhaust passage 3b(i.e., the position shown in FIG. 1), and in such a manner that the shut-off valve 8aopens the port 6aand the shut-off valve 8bcloses the port 6bwhen the switching valve 5 is at a position closing the inlet of the branch exhaust passage 3a.

[0030] Therefore, when the switching valve 5 is set at the position closing the inlet of the branch exhaust passage 3b, the exhaust gas from the engine flows into the branch exhaust passage 3aand flows through the NO_xabsorbent 7a, the exhaust connecting passage 4 and the NO_xabsorbent 7bin this order, and, after leaving the NO_xabsorbent 7b, flows into the exhaust outlet passage 9 through the port 6b. This direction of the exhaust gas flow is called a first direction hereinafter.

[0031] On the other hand, when the switching valve 5 is set at the position closing the inlet of the branch exhaust passage 3a, the exhaust gas from the engine flows into the branch exhaust passage 3band flows through the NO_xabsorbent 7b, the exhaust connecting passage 4 and the NO_xabsorbent 7ain this order, and, after leaving the NO_xabsorbent 7a, flows into the exhaust outlet passage 9 through the port 6a. This direction of the exhaust gas flow is called a second direction hereinafter. It will be understood from the above description, that the direction of the exhaust gas flow in the NO_xabsorbents 7a, 7band the exhaust connecting passage 4 is reversed by switching the position of the switching valve 5.

[0032] In FIG. 1, numeral 12 designates a reducing agent supply unit which supplies a reducing agent to the exhaust connecting passage 4. The reducing agent supply unit 12 includes a supply source 12cof a pressurized reducing agent, such as a pressurized container of the reducing agent or a reducing agent supply pump, and a nozzle 12afor injecting the reducing agent into the exhaust connecting passage 4. The reducing agent supply unit is provided with a flow control valve 12bwhich controls the injection rate of the reducing agent in accordance with a control signal from the ECU 20.

[0033] In this embodiment, substances which generate components such as hydrogen H₂, hydrocarbon HC, and carbon monoxide CO can be used as the reducing agent. Therefore, reducing gases such as hydrogen gas or carbon monoxide gas, gaseous or liquid hydrocarbons such as propane, propylene or butane, and liquid fuels such as gasoline, gas oil or kerosene, etc., can be used as the reducing agent in the present invention.

[0034] The engine control unit 20 in FIG. 1 comprises, for example, a digital computer including a ROM (read only memory), a RAM (random access memory), a CPU (central processing unit), and I/O (input and output) interface, all connected each other by a bi-directional bus. The ECU 20 performs the basic control of the engine 1 such as fuel injection control. In addition, the ECU 20 controls the absorption and releasing operation of NO_xabsorbents 7aand 7bas explained later in detail. For this purpose, signals representing engine operating conditions, such as an engine speed signal, an accelerator signal which represents the depression of the accelerator pedal, and an inlet air flow amount signal, are fed to the I/O interface from the respective sensors (not shown). Further, the I/O interface of the ECU 20 is connected to the actuator 11 of the link 10 and the flow control valve 12bof the reducing agent supply unit 12 in order to supply the control signals.

[0035] The NO_xabsorbents 7aand 7buse alumina as a carrier, and on this carrier, precious metals such as platinum, and at least one substance selected from alkali metals such as potassium K, sodium Na, lithium Li and cesium Cs; alkali-earth metals such as barium Ba and calcium Ca; and rare-earth metals such as lanthanum La and yttrium Y are carried. The NO_xabsorbents 7aand 7babsorb NO_xin the exhaust gas when the air-fuel ratio of the exhaust gas flowing through the NO_xabsorbents is lean, and release the absorbed NO_xwhen the oxygen concentration in the exhaust gas becomes low. In this specification, the term “air-fuel ratio of the exhaust gas” means a ratio of the air and the fuel (or reducing agent and other hydrocarbons) which are supplied to the engine or exhaust passages upstream of the NO_xabsorbents. Therefore, when no air and fuel (or reducing agent) is supplied in the exhaust passage upstream of the NO_xabsorbents 7aand 7b, the air-fuel ratio of the exhaust gas becomes the same as the operating air-fuel ratio of the engine (i.e., the air-fuel ratio of the air-fuel mixture supplied to combustion chambers of the engine).

[0036] Though the mechanism of this absorption and releasing operation of the NO_xabsorbent is not clear, it is thought that the absorption and releasing operation is conducted by the mechanism shown in FIGS. 3A and 3B. FIGS. 3A and 3B explain the mechanism of the absorption and the releasing operation in the case where platinum Pt and barium Ba are carried on the carrier, as an example, but it is thought that a similar mechanism also applies even if other precious metals, alkali metals, alkali earth metals, or rare earth metals are used.

[0037] Namely, when the air-fuel ratio of the exhaust gas flowing into the NO_xabsorbent is lean, the concentration of oxygen in the exhaust gas is relatively high. In this case, as shown in FIG. 3A, the oxygen O₂is deposited on the surface of platinum Pt in the form of O₂⁻ or O²⁻. The NO in the exhaust gas reacts with O₂⁻ or O²⁻ on the surface of the platinum Pt and becomes NO₂(2NO+O₂→2NO₂). Then, a part of the produced NO₂is oxidized on the platinum Pt and absorbed into the NO_xabsorbent. While bonding with the barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO₃⁻ as shown in FIG. 3A. In this way, NO_xis absorbed in the NO_xabsorbent. As long as the oxygen concentration in the exhaust gas is high, the NO_xis produced on the surface of the platinum Pt, and as long as the NO_xis produced on the surface of the platinum Pt, and as long as the NO_xabsorption capacity of the absorbent is not saturated by the absorbed NO_x, the NO_xis absorbed into the NO_xabsorbent and nitric acid ions NO₃⁻ are produced.

[0038] On the other hand, when the oxygen concentration in the exhaust gas flowing into the NO_xabsorbent is lowered, the production of NO₂is lowered and the reaction proceeds in an inverse direction (NO₃⁻→NO₂), and thus nitric acid ions NO₃⁻ in the absorbent are released in the form of NO₂from the NO_xabsorbent.

[0039] In this case, if components such as HC, CO exist in the exhaust gas, these components react with the oxygen O₂in the exhaust gas and oxygen O₂⁻ or O² on the platinum Pt and are oxidized. By this oxidation of the HC, CO components, the oxygen component existing near the NO_xabsorbent is consumed, and the concentration of oxygen in the atmosphere around the NO_xabsorbent is lowered. Also, the NO₂released from the NO_xabsorbent reacts with the HC and CO components in the exhaust gas as shown in FIG. 3B and is reduced to N₂. In this way, when the NO₂on the surface of the platinum Pt reacts with HC and CO in the exhaust gas, and when the NO₂no longer exist on the surface of the platinum Pt, the NO₂is successively released from the absorbent.

[0040] Accordingly, if components such as HC, CO exist in the exhaust gas, the NO_xis released from NO_xabsorbent and reduced to N₂in a short time.

[0041] Namely, the HC and CO components in the reducing agent supplied to NO_xabsorbent immediately react with the O₂⁻ or O² on the platinum Pt and are oxidized, and subsequently if the HC and CO still remain after the O₂⁻ or O² on the platinum Pt are consumed, the NO_xreleased from the absorbent is reduced to N₂.

[0042] In the embodiment in FIG. 1, the absorbing and releasing operations of the respective NO_xabsorbents 7aand 7bare controlled by switching the direction of the exhaust gas flow between the first and the second direction using the switching valve 5 as explained above.

[0043] Next, the operation of the exhaust gas purification device in FIG. 1 is explained. First, the case in which the switching valve 5 is set at the position closing the branch exhaust passage 3b(i.e., the position shown in FIG. 1) is explained. In this case, the exhaust gas flows in the first direction as explained before and flows into the NO_xabsorbent 7afirst, and then flows into NO_xabsorbent 7bafter passing through the NO_xabsorbent 7a. In this case, the reducing agent injected into the exhaust connecting passage 4 from the nozzle 12aof the reducing agent supply unit 12 flows into the NO_xabsorbent 7a.

[0044] Since the engine 1 in the present embodiment is operated at a lean air-fuel ratio, the air-fuel ratio of the exhaust gas from the engine 1 is substantially lean. Therefore, the exhaust gas flowing into the NO_xabsorbent 7acontains a relatively high concentration of oxygen, and the NO_xin the exhaust gas is absorbed by the NO_xabsorbent, 7aas explained in FIG. 3A, and thus removed from the exhaust gas. Thus, an exhaust gas having very low concentration of NO_xcomponent is discharged from the NO_xabsorbent 7aand flows into the exhaust connecting passage 4. In the exhaust connecting passage 4, the reducing agent is supplied to the exhaust gas by the nozzle 12aof the reducing agent supply unit 12, and a mixture of the exhaust gas and the reducing agent flows into the NO_xabsorbent 7b. As explained later, the injection rate of the reducing agent is determined in such a manner that the air-fuel ratio of the mixture of the exhaust gas and the reducing agent flowing into the NO_xabsorbent 7bbecomes lower than, or at least equal to, the stoichiometric air-fuel ratio. Therefore, the oxygen in the exhaust gas reacts with HC and CO components generated by the reducing agent in the NO_xabsorbent 7b, and the oxygen concentration in the exhaust gas in the NO_xabsorbent 7bbecomes lower. Thus, the NO_xabsorbed in the NO_xabsorbent 7bis released from the absorbent and reduced by the HC and CO components in the exhaust gas as explained in FIG. 3B. Further, the NO_xcomponents, in the exhaust gas, which has passed the NO_xabsorbent, though very little, are also reduced to N₂in the NO_xabsorbent 7b. Therefore, the exhaust gas, which has passed through the NO_xabsorbent 7band which is discharged to atmosphere through the port 6aand the exhaust outlet passage 9, is almost completely free from NO_xcomponents.

[0045] When the switching valve 5 is switched to the position in which the inlet of the branch exhaust passage 3ais closed, the exhaust gas flows in the second direction, and flows into the NO_xabsorbent 7bfirst. In this case, contrary to the above, absorption of the NO_xin the exhaust gas is performed in the NO_xabsorbent 7b, and the releasing and reducing of the absorbed NO_xis performed on the NO_xabsorbent 7a.

[0046] From the above explanation, it will be understood that, if the switching valve 5 is operated so that it switches the direction of the exhaust gas flow at the appropriate time, the NO_xabsorbents 7aand 7bcan absorb and release NO_xalternately without stopping the exhaust gas flow. Thus, it becomes possible to remove NO_xcomponents in the exhaust gas continuously while preventing the NO_xabsorbents 7aand 7bfrom being saturated by the absorbed NO_x.

[0047] In this embodiment, the ECU 20 estimates the air-fuel ratio of the exhaust gas at the engine outlet based on the amount of the inlet air flow and the amount of the fuel injection. Then, the ECU 20 calculates the amount of the reducing agent required to adjust the air-fuel ratio of the exhaust gas to a value lower than the stoichiometric air-fuel ratio, and controls the flow control valve so that the calculated amount of the reducing agent is injected into the exhaust connecting passage 4. The required amount of the reducing agent varies in accordance with types of NO_xabsorbent, an amount of NO_xabsorbed in the NO_xabsorbent and the discharge rate of NO_x(i.e., the temperature of the NO_xabsorbent). Therefore, it is preferable to determine the amount of the injection of the reducing agent by, for example, experimenting with the actual NO_xabsorbent used for the engine.

[0048] Though the switching valve may be switched at regular intervals, it is preferable to switch the valve 5 each time when the amount of absorbed NO_xin the NO_xabsorbent reaches a predetermined value (for example, about 60% of the maximum absorbing capacity of the NO_xabsorbent) so that the releasing and reducing operation of NO_xabsorbent is performed before the NO_xabsorbent is saturated with absorbed NO_x.

[0049] In this case, for example, the amount of NO_xgenerated by the engine 1 is measured beforehand in the respective engine load conditions, and stored in the ROM in the ECU 20 as a function of the parameter representing the engine load conditions (such as, the amount of inlet air flow and engine speed, or the accelerator signal and the engine speed). The ECU 20 calculates the amount of the NO_xgenerated by the engine at regular intervals based on the engine load conditions. The amount of NO_xabsorbed by the NO_xabsorbent during the intervals is estimated as a value obtained by multiplying the amount of the generated NO_xby a predetermined constant. The total amount of NO_xabsorbed by the NO_xabsorbent is obtained by accumulating the value obtained by the above calculation. The ECU 20 performs the estimation of the amount of NO_xabsorbed by the NO_xabsorbent during each interval and calculates the accumulated value thereof at regular intervals, and when the accumulated value reaches a predetermined value, the ECU 20 switches the switching valve 5.

[0050] Usually, the time required for the NO_xabsorbent to release and reduce the absorbed NO_xis substantially shorter than the time required for the amount of absorbed NO_xto reach the predetermined value. For example, usually it takes a few minutes to more than one hour for the absorbed NO_xto reach the predetermined value while the releasing and reducing operation of NO_xrequires only less than one second to ten seconds. In actual operation of the engine 1, the reducing agent is injected only for a short time after the switching valve 5 is switched. Therefore, the amount of consumption of the reducing agent is relatively small in actual operation. Thus, even if the fuel of the engine 1 is used as the reducing agent, the fuel consumption of the engine does not increase significantly. When the releasing and reducing operation of NO_xabsorbent is completed, the supply of the reducing agent is stopped. This causes the air-fuel ratio of the exhaust gas flowing into the downstream side NO_xabsorbent to return to lean air-fuel ratio and the downstream NO_xabsorbent becomes available for absorbing NO_xin the exhaust gas. Therefore, after the releasing and reducing operation is completed, even very small amount of NO_xwhich is not absorbed by the upstream NO_xabsorbent can be absorbed by the downstream NO_xabsorbent, and the NO_xcomponents in the exhaust gas are almost completely removed even after the supply of the reducing agent is stopped.

[0051] Further, the reducing agent is supplied only to the exhaust connecting passage 4 in this embodiment and it is not required to supply the reducing agent to the respective branch exhaust passages 3aand 3b, i.e., it is not required to dispose the reducing agent injection nozzle and flow control valve on the respective branch exhaust passages 3aand 3b. Therefore, the construction of the reducing agent supply unit and the control thereof can be simplified.

[0052] Next, another embodiment of the present invention is explained with reference to Pig. 4. In FIG. 4, the same reference numerals as those in FIG. 1 represent the same elements as in FIG. 1, and a detailed explanation of these elements is omitted in order to avoid repetition.

[0053] In FIG. 4, reference numeral 40 designates an internal combustion engine. However, contrary to the previous embodiment, the engine 40 is operated at a rich air-fuel ratio, i.e., the air-fuel ratio is lower than, or equal to, the stoichiometric air-fuel ratio at least in a normal operating range in this embodiment. For example, the engine 40 in FIG. 4 is operated at a slightly rich air-fuel ratio, corresponding stoichiometry, of 0.98 to 0.99 in order to prevent an increase in fuel consumption. Further, a three-way reducing and oxidizing catalyst 41 of a known type is disposed on the exhaust gas passage 3 upstream of the branch exhaust passages 3aand 3b. The three-way reducing and oxidizing catalyst 41 is capable of removing HC, CO and NO_xcomponents in the exhaust gas simultaneously when the air-fuel ratio of the exhaust gas is close to a stoichiometric air-fuel ratio.

[0054] In FIG. 4, instead of the reducing agent supply unit 12, a secondary air supply unit 42 is provided. The secondary air supply unit 42 comprises a pressurized air source (such as an air pump) 42c, an air injection nozzle 42afor supplying secondary air to the exhaust gas in the exhaust connecting passage 4 and a flow control valve 42bfor controlling the injection flow rate of the secondary air.

[0055] When an internal combustion engine is operated at an air-fuel ratio lower than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls rapidly and the concentrations of HC and CO rises rapidly. Since the engine 40 is operated at a slightly rich air-fuel ratio in the normal operating range, the exhaust gas from the engine 40 contains HC and CO at relatively high concentrations and oxygen at a low concentration. Further, when an internal combustion engine is operated at an air-fuel ratio near the stoichiometric air-fuel ratio, the amount of NO_xgenerated by the engine increases. Therefore, the exhaust gas in this embodiment includes a larger amount of NO_xthan the exhaust gas in the embodiment in FIG. 1. As stated before, the three-way reducing and oxidizing catalyst is capable of removing HC, CO and NO_xcomponents at high efficiency when the air-fuel ratio of exhaust gas is close to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio, the capability of the three-way catalyst for removing NO_xcomponents increases while the capability for removing HC and CO components decreases.

[0056] Therefore, when the exhaust gas from the engine passes through the three-way catalyst 41, a large part of NO_xand portions of HC and CO are removed from the exhaust gas by the three-way catalyst 41. Thus, the exhaust gas flowing into the branch exhaust passages 3aand 3bcontains a certain amount of HC and CO components and a small amount of NO_xand oxygen components.

[0057] For example, when the switching valve 5 is set at the position which closes the inlet of branch exhaust passage 3b, the exhaust gas flows in the first direction as explained before, i.e., the exhaust gas passing through the three-way catalyst 41 is directed to the branch exhaust passage 3aand flows into the NO_xabsorbent 7a. Since the concentration of oxygen of this exhaust gas is low, and HC and CO are contained in this exhaust gas, the releasing and reducing of the absorbed NO_xas explained by FIG. 3B occurs in the NO_xabsorbent 7a. Thus, the NO_xabsorbed by the NO_xabsorbent 7aand the NO_xin the exhaust gas, which is not removed by the three-way catalyst 41 are reduced by the NO_xabsorbent 7a. Through the three-way catalyst 41, a portion of NO_xis converted to NH₃when the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio, and this NH₃is also consumed to reduce the NO_xreleased from the NO_xabsorbent 7a.

[0058] After the exhaust gas passes through the NO_xabsorbent 7a, secondary air is supplied at the exhaust connecting passage 4 from the secondary air supply unit 42. The amount of the secondary air supplied to the exhaust gas is determined so that the air-fuel ratio of the exhaust gas flowing into the NO_xabsorbent 7bbecomes higher than the stoichiometric air-fuel ratio. The ECU 20 calculates the air-fuel ratio of the engine 10 from the inlet air flow amount and the fuel injection amount. Further, the ECU 20 calculates the amount of the secondary air injection rate required to maintain the air-fuel ratio higher than the stoichiometric air-fuel ratio at the inlet of the NO_xabsorbent 7b, and controls the flow control valve 42bso that the calculated amount of secondary air is injected from the nozzle 42a.

[0059] Thus, the exhaust gas having lean air-fuel ratio flows into the NO_xabsorbent 7b, and the absorbing of NO_xis performed in the NO_xabsorbent 7b. Therefore, the NO_xcomponent which was not removed by the three-way catalyst 41 and the NO_xabsorbent 7ais absorbed by the NO_xabsorbent 7band removed from the exhaust gas. Further, HC and CO components in the exhaust gas which is not consumed by the NO_xabsorbent 7aduring the releasing and reducing operation are also oxidized by the NO_xabsorbent 7b. Therefore, the exhaust gas after passing through the NO_xabsorbent 7b, which is discharged to atmosphere through the port 6aand the exhaust outlet passage 9, is almost completely free from HC, CO and NO_xcomponents.

[0060] When the switching valve 5 is switched to the position that closes the inlet of the branch exhaust passage 3a, the exhaust gas flows in the second direction. In this case, the exhaust gas after it passes through the three-way catalyst 41 flows into the NO_xabsorbent 7bfirst, and the releasing and reducing of NO_xis performed in the NO_xabsorbent 7band the absorbing of NO_xis performed in the NO_xabsorbent 7a. Therefore, by switching the switching valve 5 at an appropriate intervals, the NO_x, absorbents 7aand 7balso perform the releasing and reducing operations and the absorbing operations alternately in this embodiment, and it becomes possible to remove NO_xcomponents in the exhaust gas continuously while preventing the NO_xabsorbents from being saturated by the absorbed NO_x. Also, in this embodiment, the interval of the switching operation of the switching valve 5 may be constant, or the switching valve may be switched each time when the amount of the absorbed NO_xreaches a predetermined value, as explained in the previous embodiment. In this embodiment, however, it is different from the embodiment in FIG. 1 in that the secondary air is supplied to the exhaust connecting passage 4 continuously during the engine operation since the air-fuel ratio of the exhaust gas flowing into the downstream side NO_xabsorbent must be maintained at a lean air-fuel ratio.

[0061] Further, though the three-way catalyst 41 is provided for removing a large portion of the NO_xgenerated by the engine 40, the three-way catalyst 41 may be omitted if the NO_xabsorbents 7aand 7bhave sufficient capacities for absorbing the NO_xcomponents generated by the engine 40.

[0062] According to the present embodiment, the secondary air is supplied only to the exhaust connecting passage 4, therefore, the construction and control of the secondary air supply unit 42 is largely simplified.

[0063] Next, another embodiment of the present invention is described with reference to FIGS. 5A and 5B. In the previous embodiments, branch exhaust passages 3aand 3bare required to accommodate the NO_xabsorbents 7aand 7b. This arrangement is disadvantageous since it requires larger space for installation. This is true especially when the device is applied to automobile engines which have limited space for installation. In this embodiment, this problem is solved by containing the two branch exhaust passages and the two NO_xabsorbents in a single housing.

[0064] FIG. 5A shows a sectional view of a main part of the exhaust gas purification device according to the present embodiment.

[0065] In FIG. 5A, numeral 50 is a housing having a substantially cylindrical shape, 59 is an exhaust gas pipe penetrating the housing 50 along its central axis, and 57a, 57bare NO_xabsorbents disposed in the space between the outer surface of the exhaust gas pipe 59 and the inner surface of the housing 50. The NO_xabsorbents 57aand 57bare disposed near the center portion of the housing, and spaces 51 and 52 are provided at both ends of the housing 50.

[0066] FIG. 5B shows a cross sectional view taken along the line B—B in FIG. 5A. AS Seen from FIGS. 5A and 5B, a partitioning plate 53 is provided between the inner surface of the housing 50 and the outer surface of the exhaust gas pipe 59 in order to separate the NO_xabsorbents 57aand 57b. Further, the partitioning plate 53 divides the space 51 at the end of the housing 50 into two separate spaces 51aand 51b. On the other hand, since the partitioning plate 53 terminates at the end face of the NO_xabsorbents 57aand 57bat the other end of the housing 50, the space 52 is not divided by the partitioning plate 53.

[0067] In the portion of the exhaust gas pipe 59 located in the space 51, ports 55a, 56aand 55b, 56bare provided. The ports 55aand 56aopen to the space 51aof the housing 50, while the ports 55band 56bopen to the space 51bof the housing 50, respectively. Further, a switching valve 58 is provided in the exhaust gas pipe 59 at the portion between the ports 56a, 56band the ports 55a, 55b. The switching valve 58 is driven by an appropriate actuator via a driving shaft 58aso that it takes either a first position as indicated by solid line in FIG. 5A or a second position as indicated by dotted line in FIG. 5A.

[0068] When the switching valve 58 takes the first position, the port 55ais opened and the port 56ais closed on the space 51aside of the exhaust gas pipe 59, and the port 55bis closed and the port 56bis opened on the space 51bside of the exhaust gas pipe 59. On the other hand, when the switching valve 58 takes the second position, the port 55ais closed and the port 56ais opened on the space 51aside of the exhaust gas pipe 59, and the port 55bis opened and the port 56bis closed on the space 51bside. Numeral 59bin FIG. 5A shows another partitioning plate which blocks the exhaust gas pipe 59 at the position of the switching valve 58. Numeral 54 in FIG. 5A is a nozzle which injects the reducing agent or the secondary air into the space 52 of the housing 50.

[0069] In this embodiment, the exhaust gas from the engine flows into the housing 50 from the direction indicated by an arrow A in FIG. 5A. When the switching valve 58 is set at the first position (shown by the solid line in FIG. 5A), the exhaust gas in the exhaust gas pipe 59 flowing into the space 51athrough the port 55a. Since the port 56ais closed by the switching valve 58, the exhaust gas flowing into the space 51apasses through the NO_xabsorbent 57aand flowing into the space 52 at the other end of the housing 50. This exhaust gas, then, flows into the NO_xabsorbent 57bfrom the space 52 and flows into the space 51bof the housing 50. From the space 51b, the exhaust gas flows into the portion of the exhaust gas pipe downstream of the switching valve 58 through the port 56band flows down the exhaust gas pipe 59.

[0070] On the contrary, when the switching valve 58 is switched to the second position (indicated by the dotted line in FIG. 5A), the exhaust gas from the engine flows through the port 55b, the NO_xabsorbent 57b, the space 52, the NO_xabsorbent 57a, the port 56aand exhaust gas pipe 59 in this order. Thus, the direction of the exhaust gas flow can be reversed by switching the switching valve 58 between the first and the second position. Therefore, also in this embodiment, the releasing and reducing operation of NO_xand the absorbing operation of NO_xcan be performed alternately in the NO_xabsorbents 57aand 57bwithout stopping the absorption of NO_xin the exhaust gas. According to the embodiment in FIGS. 5A and 5B, since two NO_xabsorbents and branch exhaust passages can be contained in a single housing, the construction of the device can be more compact and simple than the devices in FIGS. 1 and 4.

[0071] Though the exhaust gas in the exhaust gas pipe 59 flows in the direction shown by the arrow A in FIG. 5A in the above embodiment, the direction of the exhaust gas flow in the exhaust gas pipe 59 may be revered, i.e., the exhaust gas may flow into the spaces 51aand 51bthrough the ports 56aand 56b, respectively, and flow into the exhaust gas pipe from the spaces 51aand 51bthrough the ports 55aand 55b, respectively.

[0072] Although in the above explanation, the invention has been described with reference to specific embodiments chosen for the purpose of illustration, it should be understood that numerous modifications could be applied by those skilled in the art without departing from the basic concept and scope of the present invention. Especially, other arrangements of the NO_xabsorbents and the exhaust gas passages can be applied for the present invention as long as it enables to switch the direction of the exhaust gas flow between two NO_xabsorbents.

[0073] For example, FIG. 6 shows another arrangement of the NO_xabsorbents and the exhaust gas passages. In FIG. 6, the NO_xabsorbents 7aand 7bare disposed, in series, in an exhaust connecting passage 60 connecting the branch exhaust passages 3aand 3b. In this embodiment, two switching valves 65aand 65bare disposed at the points where the exhaust connecting passage 60 merges the branch exhaust passages 3aand 3b. The switching valves 65aand 65bare operated synchronously to switch the direction of the exhaust gas flow in the exhaust connecting passage 60.

[0074] Namely, when the switching valves 65aand 65bare set at the position shown by solid lines in FIG. 6, the exhaust gas flows into the exhaust connecting passage 60 from the branch exhaust passage 3aand passes the NO_xabsorbents 7aand 7bin this order, and flows out to the branch exhaust passage 3bdownstream of the switching valve 65b. On the other hand, when the switching valves 65aand 65bare set at the position indicated by dotted lines in FIG. 6, the exhaust gas flows through the branch exhaust passage 3a, the NO_xabsorbents 7band 7aon the exhaust connecting passage 60 in this order and flows into the branch exhaust passage 3adownstream of the switching valve 65a. Therefore, by injecting the reducing agent or the secondary air into the exhaust connecting passage 60 using a nozzle 62 disposed on the passage 60 at the portion between the NO_xabsorbents 7aand 7b, a similar operation to that explained in FIGS. 1 and 4 is obtained.

[0075] The embodiment in FIG. 6 is advantageous since it requires only two valves 65aand 65bwhile three valves 5, 8aand 8bare required in the embodiments in FIGS. 1 and 4.