An exhaust gas purification device for an engine

Abstract: The exhaust gas purification device according to the present invention utilizes two NO x absorbents (7a,7b) in order to remove NO x from the exhaust gas of an engine (1) operated at a lean air-fuel ratio. The NO x absorbents (7a,7b) are disposed, in series, in the exhaust passage (3) of the engine and a nozzle (12a) for supplying a reducing agent is disposed in the exhaust passage at a position between the two NO x absorbents. Further, the device includes switching valves (5) for changing the direction of the exhaust gas flow in the exhaust gas passage. When the switching valves are set to direct the exhaust gas flow to one direction, the exhaust gas first flows through one of the NO x absorbent (first NO x absorbent) (7a), and after passing through the first NO x absorbent, the reducing agent is supplied to the exhaust gas before it flows into the other NO x absorbent (second NO x absorbent) (7b). Thus, the first NO x absorbent (7a) absorbs NO x in the exhaust gas, and the NO x absorbed by the second NO x absorbent is released from the second NO x absorbent. When the switching valves (5) are switched, the exhaust gas flows in the opposite direction, and flows through the second NO x absorbent (7b) and the first NO x absorbent (7a) in this order. In this case, the second NO x absorbent (7b) absorbs NO x in the exhaust gas, and the NO x absorbed by the first NO x absorbent (7a) is released and reduced to N 2 . Therefore, by reversing the direction of the exhaust gas periodically, the absorbing operation of NO x and releasing/reducing operation of NO x are performed by two NO x absorbents alternately. Thus, it becomes possible to remove NO x in the exhaust gas continuously while the NO x absorbents are prevented from being saturated by the absorbed NO x .

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an engine. More specifically, the present invention relates to a device which is capable of effectively removing an NO x component from exhaust gas. 2. Description of the Related Art An exhaust gas purification device capable of removing an NO x component 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 x components 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 x absorbent is disposed in each of the branch exhaust passages, namely, two NO x absorbents are disposed in the exhaust gas passage in parallel. The NO x absorbent is capable of absorbing NO x component in the exhaust gas under 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 x components contained in the exhaust gas is absorbed by the NO x absorbents. When the NO x absorbent absorbs a certain amount of NO x , the ability of the NO x absorbent to absorb NO x in the exhaust gas falls. To regain the absorption ability, the absorbed NO x in the absorbent must be discharged and reduced periodically, for example, by supplying a reducing agent to the NO x absorbent, i.e., the NO x absorbents must be regenerated to regain the initial ability for absorbing NO x . In the device in JPP '620, two NO x absorbents are disposed in parallel in the exhaust passage in order to enable regeneration of the NO x absorbent without stopping the purification of the exhaust gas. In JPP '620, two NO x absorbents are used alternately, in a normal operation of the engine, i.e., only one of the NO x absorbents is used for absorbing the NO x in 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 x in the exhaust gas is absorbed by the NO x absorbent disposed in that exhaust branch passage. While absorbing NO x in the exhaust gas by one NO x absorbent, the other NO x absorbent disposed on the closed exhaust branch passage is regenerated by supplying a reducing agent, such as hydrogen gas. According to the device disclosed in JPP '620, by using two NO x absorbents disposed in the exhaust passage in parallel, the NO x absorbents can be regenerated alternately while continuing the removal of NO x component from the exhaust gas. However, the device disclosed in JPP '620 has a disadvantage that the reducing agent must be supplied to both of the 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 to switching the supply of the reducing agent between two nozzles. Further, elaborate control of the three-way flow control valve is required to enable the appropriate regeneration of the NO x absorbents. SUMMARY OF THE INVENTION In view of the problems in the related 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 x components from the exhaust gas without requiring complicated devices and controls. The above object is achieved by the exhaust gas purification device for an internal combustion engine having the features of claim 1. When the flow direction of exhaust gas is switched to the first direction, a lean air-fuel ratio exhaust gas from the engine first flows into the first NO x absorbent and NO x components in the exhaust gas are absorbed by the first NO x absorbent. After passing through the first NO x absorbent, a reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the second NO x absorbent. Therefore, the air-fuel ratio of the exhaust gas flowing into the second NO x absorbent becomes rich due to the reducing agent. Thus, in the second NO x absorbent, the absorbed NO x is released and reduced to nitrogen by the reducing agent. Namely, when the exhaust gas flow is switched to the first direction, NO x component in the exhaust gas is absorbed by the first NO x absorbent and removed from the exhaust gas, and the NO x in the second NO x absorbent is released and reduced to nitrogen. 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 x absorbent and NO x components in the exhaust gas are absorbed by the second NO x absorbent. Then, the reducing agent is supplied to the exhaust gas in the exhaust connecting passage before -flowing into the first NO x absorbent. Therefore, the rich exhaust gas flows into the first NO x absorbent, and absorbed NO x is released from the first NO x absorbent 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 when an amount of NO x absorbed in the NO x absorbent reaches a predetermined value, absorption of NO x in the exhaust gas and releasing and reducing of the absorbed NO x is performed in each NO x absorbent alternatively without stopping the absorption of NO x in the exhaust gas as a whole. When 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 concentration of oxygen and a relatively high concentration 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 x absorbent, and the absorbed NO x is released from the first NO x absorbent and reduced to nitrogen by HC and CO components in the exhaust gas. After passing through the first NO x absorbent, the secondary air is supplied to the exhaust gas in the exhaust connecting passage before it flows into the second NO x absorbent, and the air-fuel ratio of the exhaust gas flowing into the second NO x absorbent becomes lean. Therefore, a small amount of NO x which passes through the first NO x absorbent is absorbed by the second NO x absorbent and removed completely from the exhaust gas. Namely, when the direction of the exhaust gas flow is switched to the first direction, the absorbed NO x is released from the first NO x absorbent and reduced to nitrogen, and at the same time, NO x in the exhaust gas is absorbed by the second NO x absorbent. Similarly, when the direction of the exhaust gas flow is switched to the second direction, the absorbed NO x is released from the second NO x absorbent and reduced to nitrogen, and NO x in the exhaust gas is absorbed by the first NO x absorbent at the same time. Therefore, using the device of the claimed invention, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, absorption of NO x in the exhaust gas and releasing and reducing of the absorbed NO x is performed in each NO x absorbent alternatively without stopping the absorption of NO x in the exhaust gas as a whole. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the description as set forth hereinafter, with reference to the accompanying drawings, in which: 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; Fig. 2 is a sectional view of the exhaust passage in Fig. 1 taken along a line II-II in Fig. 1; Figs. 3A and 3B are views for explaining the absorbing and releasing operation of NO x by the NO x absorbent; Fig. 4 is a drawing schematically illustrating another embodiment of the exhaust gas purification device according to the present invention; Figs. 5A and 5B are drawings illustrating another embodiment of the exhaust gas prification device according to the present invention; and Fig. 6 is a drawing illustrating another embodiment of the exhaust gas purification device according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 schematically illustrates an embodiment of an exhaust gas purification device, according to the present invention, when it is applied to an automobile engine. 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 at least in a 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 3a and 3b, and a NO x absorbent (designated by 7a and 7b) is connected to each branch exhaust passage. The NO x absorbents 7a and 7b are connected each other at the ends opposite to the exhaust branch passage 3a and 3b by an exhaust connecting passage 4. A switching valve 5 is disposed on the exhaust gas passage 3 at the point where the branch exhaust passages 3a and 3b diverges. 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 3a and 3b by closing the inlet of one of the branch exhaust passages 3a and 3b. On the respective branch exhaust passages 3a and 3b, a port 6a, 6b and 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 x absorbents 7a and 7b. Fig. 2 schematically illustrates the section of the branch exhaust passages 3a and 3b taken 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 3a and 3b. The exhaust outlet passage 9 communicates the branch exhaust passages 3a and 3b through the ports 6a and 6b, respectively. Namely, when the shut-off valve 8a is opened, the branch exhaust passage 3a is connected to the exhaust outlet passage 9 via the port 6a, and when the shut-off valve 8b is opened, the branch exhaust passage 3b is 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. In this embodiment, the switching valve 5 and the shut-off valves 8a and 8b are 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 actuator which can drive the switching valve 5 and shut-off valves 8a and 8b synchronously in accordance with the control signal from the ECU 20. In this embodiment, the shut-off valves 8a and 8b are operated through the mechanical link 10 in such a manner that the shut-off valve 8b opens the port 6b and the shut-off valve 8a closes the port 6a when 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 8a opens the port 6a and the shut-off valve 8b closes the port 6b when the switching valve 5 is at a position closing the inlet of the branch exhaust passage 3a. 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 3a and flows through the NO x absorbent 7a, the exhaust connecting passage 4 and the NO x absorbent 7b in this order, and, after leaving the NO x absorbent 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. On the contrary, 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 3b and flows through the NO x absorbent 7b, the exhaust connecting passage 4 and the NO x absorbent 7a in this order, and, after leaving the NO x absorbent 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 x absorbents 7a, 7b and the exhaust connecting passage 4 is reversed by switching the position of the switching valve 5. 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 of a pressurized reducing agent, such as a pressurized container of the reducing agent or a reducing agent supply pump, and a nozzle 12a for injecting the reducing agent into the exhaust connecting passage 4. The reducing agent supply unit is provided with a flow control valve 12b which controls the injection rate of the reducing agent in accordance with a control signal from the ECU 20. In this embodiment, substances which generate components such as hydrogen H 2 , 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. 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) I/O (input and output) interface, all connected each other by a bi-directional bus. The ECU 20 performs a basic control of the engine 1 such as fuel injection control. In addition, the ECU 20 controls the absorption and releasing operation of NO x absorbents 7a and 7b as 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 12b of the reducing agent supply unit 12 in order to supply the control signals. The NO x absorbents 7a and 7b use 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 x absorbents 7a and 7b absorb NO x in the exhaust gas when the air-fuel ratio of the exhaust gas flowing through the NO x absorbents is lean, and release the absorbed NO x when 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 x absorbents. Therefore, when no air and fuel (or reducing agent) is supplied in the exhaust passage upstream of the NO x absorbents 7a and 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). Though the mechanism of this absorption and releasing operation of the NO x absorbent 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. Namely, when the air-fuel ratio of the exhaust gas flowing into the NO x absorbent is lean, the concentration of oxygen in the exhaust gas is relatively high. In this case, as shown in Fig. 3A, the oxygen O 2 is deposited on the surface of platinum Pt in the form of O 2 - or O 2 - . The NO in the exhaust gas reacts with O 2 - or O 2 - on the surface of the platinum Pt and becomes NO 2 (2NO + O 2 → 2NO 2 ). Then, a part of the produced NO 2 is oxidized on the platinum Pt and absorbed into the NO x absorbent. While bonding with the barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO 3 - as shown in Fig. 3A. In this way, NO x is absorbed in the NO x absorbent. As long as the oxygen concentration in the exhaust gas is high, the NO x is produced on the surface of the platinum Pt, and as long as the NO x is produced on the surface of the platinum Pt, and as long as the NO x absorption capacity of the absorbent is not saturated by the absorbed NO x , the NO x is absorbed into the NO x absorbent and nitric acid ions NO 3 - are produced. On the other hand, when the oxygen concentration in the exhaust gas flowing into the NO x absorbent is lowered, the production of NO 2 is lowered and the reaction proceeds in an inverse direction (NO 3 - → NO 2 ), and thus nitric acid ions NO 3 - in the absorbent are released in the form of NO 2 from the NO x absorbent. In this case, if the components such as HC, CO exist in the exhaust gas, these components react with the oxygen O 2 in the exhaust gas and oxygen O 2 or O 2 - on the platinum Pt and are oxidized. By this oxidation of the HC, CO components, the oxygen component existing near the NO x absorbent is consumed, and the concentration of oxygen in the atmosphere around the NO x absorbent is lowered. Also, the NO 2 released from the NO x absorbent reacts with the HC and CO components in the exhaust gas as shown in Fig. 3B and is reduced to N 2 . In this way, when the NO 2 on the surface of the platinum Pt reacts with HC and CO in the exhaust gas, and when the NO 2 no longer exist on the surface of the platinum Pt, the NO 2 is successively released from the absorbent. Accordingly, if components such as HC, CO exist in the exhaust gas, the NO x is released from NO x absorbent and reduced to N 2 in a short time. Namely, the HC and CO components in the reducing agent supplied to NO x absorbent immediately react with the O 2 - or O 2 - on the platinum Pt and are oxidized, and subsequently if the HC and CO still remain after the O 2 - or O 2 - on the platinum Pt are consumed, the NO x released from the absorbent is reduced to N 2 . In the embodiment in Fig. 1, the absorbing and releasing operations of the respective NO x absorbents 7a and 7b are 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. 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 x absorbent 7a first, and then flows into NO x absorbent 7b after passing through the NO x absorbent 7a. In this case, the reducing agent injected into the exhaust connecting passage 4 from the nozzle 12a of the reducing agent supply unit 12 flows into the NO x absorbent 7a. 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 x absorbent 7a contains a relatively high concentration of oxygen, and the NO x in the exhaust gas is absorbed by the NO x absorbent 7a as explained in Fig. 3A and thus removed from the exhaust gas. Thus, an exhaust gas having very low concentration of NO x component is discharged from the NO x absorbent 7a and 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 12a of the reducing agent supply unit 12, and a mixture of the exhaust gas and the reducing agent flows into the NO x absorbent 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 x absorbent 7b becomes 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 x absorbent 7b, and the oxygen concentration in the exhaust gas in the NO x absorbent 7b becomes lower. Thus, the NO x absorbed in the NO x absorbent 7b is released from the absorbent and reduced by the HC and CO components in the exhaust gas as explained in Fig. 3B. Further, the NO x components, in the exhaust gas which has passed the NO x absorbent, though very little, are also reduced to N 2 in the NO x absorbent 7b. Therefore, the exhaust gas which has passed through the NO x absorbent 7b and which is discharged to atmosphere through the port 6a and the exhaust outlet passage 9, is almost completely free from NO x components. When the switching valve 5 is switched to the position in which the inlet of the branch exhaust passage 3a is closed, the exhaust gas flows in the second direction, and flows into the NO x absorbent 7b first. In this case, contrary to the above, absorption of the NO x in the exhaust gas is performed in the NO x absorbent 7b, and the releasing and reducing of the absorbed NO x is performed on the NO x absorbent 7a. 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 x absorbents 7a and 7b can absorb and release NO x alternately without stopping the exhaust gas flow. Thus, it becomes possible to remove NO x components in the exhaust gas continuously while preventing the NO x absorbents 7a and 7b from being saturated by the absorbed NO x . 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 x absorbent, an amount of NO x absorbed in the NO x absorbent and the discharge rate of NO x (i.e., the temperature of the NO x absorbent). Therefore, it is preferable to determine the amount of the injection of the reducing agent by, for example, experiment using the actual NO x absorbent used for the engine. 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 x in the NO x absorbent reaches a predetermined value (for example, about 60% of the maximum absorbing capacity of the NO x absorbent) so that the releasing and reducing operation of NO x absorbent is performed before the NO x absorbent is saturated with absorbed NO x . In this case, for example, the amount of NO x generated 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 x generated by the engine at regular intervals based on the engine load conditions. The amount of NO x absorbed by the NO x absorbent during the intervals is estimated as a value obtained by multiplying the amount of the generated NO x by a predetermined constant. The total amount of NO x absorbed by the NO x absorbent is obtained by accumulating the value obtained by the above calculation. The ECU 20 performs the estimation of the amount of NO x absorbed by the NO x absorbent 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. Usually, the time required for the NO x absorbent to release and reduce the absorbed NO x is substantially shorter than the time required for the amount of absorbed NO x to reach the predetermined value. For example, usually it takes a few minutes to more than one hour for the absorbed NO x to reach the predetermined value while the releasing and reducing operation of NO x requires 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 x absorbent 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 x absorbent to return to lean air-fuel ratio and the downstream NO x absorbent becomes available for absorbing NO x in the exhaust gas. Therefore, after the releasing and reducing operation is completed, even very small amount of NO x which is not absorbed by the upstream NO x absorbent can be absorbed by the downstream NO x absorbent, and the NO x components in the exhaust gas is almost completely removed even after the supply of the reducing agent is stopped. 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 3a and 3b, i.e., it is not required to dispose the reducing agent injection nozzle and flow control valve on the respective branch exhaust passages 3a and 3b. Therefore, the construction of the reducing agent supply unit and the control thereof can be simplified. Next, another embodiment of the present invention is explained with reference to Fig. 4. In Fig. 4, the same reference numerals as those in Fig. 1 represent same elements as in Fig. 1, and a detailed explanation of these elements is omitted in order to avoid repetition. 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 3a and 3b. The three-way reducing and oxidizing catalyst 41 is capable of removing HC, CO and NO x components in the exhaust gas simultaneously when the air-fuel ratio of the exhaust gas is close to a stoichiometric air-fuel ratio. 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 42a for supplying secondary air to the exhaust gas in the exhaust connecting passage 4 and a flow control valve 42b for controlling the injection flow rate of the secondary air. 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 x generated by the engine increases. Therefore, the exhaust gas in this embodiment includes a larger amount of NO x than 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 x components 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 x components increases while the capability for removing HC and CO components decreases. Therefore, when the exhaust gas from the engine passes through the three-way catalyst 41, a large part of NO x and 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 3a and 3b contains a certain amount of HC and CO components and a small amount of NO x and oxygen components. 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 3a and flows into the NO x absorbent 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 x as explained by Fig. 3B occurs in the NO x absorbent 7a. Thus, the NO x absorbed by the NO x absorbent 7a and the NO x in the exhaust gas which is not removed by the three-way catalyst 41 are reduced by the NO x absorbent 7a. Through the three-way catalyst 41, a portion of NO x is converted to NH 3 when the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio, and this NH 3 is also consumed to reduce the NO x released from the NO x absorbent 7a. After the exhaust gas passes through the NO x absorbent 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 x absorbent 7b becomes 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 x absorbent 7b, and controls the flow control valve 42b so that the calculated amount of secondary air is injected from the nozzle 42a. Thus, the exhaust gas having lean air-fuel ratio flows into the NO x absorbent 7b, and the absorbing of NO x is performed in the NO x absorbent 7b. Therefore, the NO x component which was not removed by the three-way catalyst 41 and the NO x absorbent 7a is absorbed by the NO x absorbent 7b and removed from the exhaust gas. Further, HC and CO components in the exhaust gas which is not consumed by the NO x absorbent 7a during the releasing and reducing operation are also oxidized by the NO x absorbent 7b. Therefore, the exhaust gas after passing through the NO x absorbent 7b, which is discharged to atmosphere through the port 6a and the exhaust outlet passage 9, is almost completely free from HC, CO and NO x components. When the switching valve 5 is switched to the position which 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 x absorbent 7b first, and the releasing and reducing of NO x is performed in the NO x absorbent 7b and the absorbing of is performed in the NO x absorbent 7b and the absorbing of NO x is performed in the NO x absorbent 7a. Therefore, by switching the switching valve 5 at an appropriate intervals, the NO x absorbents 7a and 7b also perform the releasing and reducing operations and the absorbing operations alternately in this embodiment, and it becomes possible to remove NO x components in the exhaust gas continuously while preventing the NO x absorbents 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 x reaches 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 x absorbent must be maintained at a lean air-fuel ratio. Further, though the three-way catalyst 41 is provided for removing a large portion of the NO x generated by the engine 40, the three-way catalyst 41 may be omitted if the NO x absorbents 7a and 7b have sufficient capacities for absorbing the NO x components generated by the engine 40. 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. Next, another embodiment of the present invention is described with reference to Figs. 5A and 5B. In the previous embodiments, branch exhaust passages 3a and 3b are required to accommodate the NO x absorbents 7a and 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 x absorbents in a single housing. Fig. 5A shows a sectional view of a main part of the exhaust gas purification device according to the present embodiment. 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, 57b are NO x absorbents disposed in the space between the outer surface of the exhaust gas pipe 59 and the inner surface of the housing 50. The NO x absorbents 57a and 57b are disposed near the center portion of the housing, and spaces 51 and 52 are provided at both ends of the housing 50. 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 x absorbents 57a and 57b. Further, the partitioning plate 53 divides the space 51 at the end of the housing 50 into two separate spaces 51a and 51b. On the other hand, since the partitioning plate terminates at the end face of the NO x absorbents 57a and 57b at the other end of the housing, the space 52 is not divided by the partitioning plate 53. In the portion of the exhaust gas pipe 59 located in the space 51, ports 55a, 56a and 55b, 56b are provided. The ports 55a and 56a open to the space 51a of the housing 50, while the ports 55b and 56b open to the space 51b of the housing 50, respectively. Further, a switching valve 58 is provided in the exhaust gas pipe 59 at the portion between the ports 56a, 56b and the ports 55a, 55b. The switching valve 58 is driven by an appropriate actuator via a driving shaft 58a so 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. When the switching valve 58 takes the first position, the port 55a is opened and the port 56a is closed on the space 5la side of the exhaust gas pipe 59, and the port 55b is closed and the port 56b is opened on the space 51b side of the exhaust gas pipe 59. On the other hand, when the switching valve 58 takes the second position, the port 55a is closed and the port 56a is opened on the space 51a side of the exhaust gas pipe 59, and the port 55b is opened and the port 56b is closed on the space 51b side. Numeral 59b in 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. 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 5la through the port 55a. Since the port 56a is closed by the switching valve 58, the exhaust gas flowing into the space 51a passes through the NO x absorbent 57a and flowing into the space 52 at the other end of the housing 50. This exhaust gas, then, flows into the NO x absorbent 57b from the space 52 and flows into the space 51b of 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 56b and flows down the exhaust gas pipe 59. 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 x absorbent 57b, the space 52, the NO x absorbent 57a, the port 56a and 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 x and the absorbing operation of NO x can be performed alternately in the NO x absorbents 57a and 57b without stopping the absorption of NO x in the exhaust gas. According to the embodiment in Figs. 5A and 5B, since two NO x absorbents 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. 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 51a and 51b through the ports 56a and 56b, respectively, and flow into the exhaust gas pipe from the spaces 51a and 51b through the ports 55a and 55b, respectively. 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 scope of the present invention as defined in the appended claim 1. Especially, other arrangements of the NO x absorbents 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 x absorbents. For example, Fig. 6 shows another arrangement of the NO x absorbents and the exhaust gas passages. In Fig. 6, the NO x absorbents 7a and 7b are disposed, in series, in an exhaust connecting passage 60 connecting the branch exhaust passages 3a and 3b. In this embodiment, two switching valves 65a and 65b are disposed at the points where the exhaust connecting passage 60 merges the branch exhaust passages 3a and 3b. The switching valves 65a and 65b are operated synchronously to switch the direction of the exhaust gas flow in the exhaust connecting passage 60. Namely, when the switching valves 65a and 65b are 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 3a and passes the NO x absorbents 7a and 7b in this order, and flows out to the branch exhaust passage 3b downstream of the switching valve 65b. On the other hand, when the switching valves 65a and 65b are set at the position indicated by dotted lines in Fig. 6, the exhaust gas flows through the branch exhaust passage 3a, the NO x absorbents 7b and 7a on the exhaust connecting passage 60 in this order and flows into the branch exhaust passage 3a downstream 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 x absorbents 7a and 7b, a similar operation to that explained in Figs. 1 and 4 are obtained. The embodiment in Fig. 6 is advantageous since it requires only two valves 65a and 65b while three valves 5, 8a and 8b are required in the embodiments in Figs. 1 and 4. The exhaust gas purification device according to the present invention utilizes two NO x absorbents in order to remove NO x from the exhaust gas of an engine operated at a lean air-fuel ratio. The NO x absorbents are disposed, in series, in the exhaust passage of the engine and a nozzle for supplying a reducing agent is disposed in the exhaust passage at a position between the two NO x absorbents. Further, the device includes switching valves for changing the direction of the exhaust gas flow in the exhaust gas passage. When the switching valves are set to direct the exhaust gas flow to one direction, the exhaust gas first flows through one of the NO x absorbent (first NO x absorbent), and after passing through the first NO x absorbent, the reducing agent is supplied to the exhaust gas before it flows into the other NO x absorbent (second NO x absorbent). Thus, the first NO x absorbent absorbs NO x in the exhaust gas, and the NO x absorbed by the second NO x absorbent is released from the second NO x absorbent. When the switching valves are switched, the exhaust gas flows in the opposite direction, and flows through the second NO x absorbent and the first NO x absorbent in this order. In this case, the second NO x absorbent absorbs NO x in the exhaust gas, and the NO x absorbed by the first NO x absorbent is released and reduced to N 2 . Therefore, by reversing the direction of the exhaust gas periodically, the absorbing operation of NO x and releasing/reducing operation of NO x are performed by two NO x absorbents alternately. Thus, it becomes possible to remove NO x in the exhaust gas continuously while the NO x absorbents are prevented from being saturated by the absorbed NO x .

Claims: An exhaust gas purification device for an internal combustion engine having an exhaust passage, the device comprising: a first (7a; 57a) and a second (7b; 57b) NO x absorbent disposed in the exhaust passage (3; 59) of the engine and connected to each other in series by an exhaust connecting passage (4, 52; 60), each of the NO x absorbents (7a; 57a, 7b; 57b) absorbing NO x components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NO x absorbents (7a; 57a, 7b; 57b) is lean, and releasing the absorbed NO x when the oxygen concentration in the exhaust gas becomes lower; a reducing agent supply means (12; 54; 62) for supplying a reducing agent into the exhaust gas in the exhaust connecting passage (4; 52; 60) between the first (7a; 57a) and the second (7b; 57b) NO x absorbents for an engine being operated at an air-fuel ratio higher than a stochiometric air-fuel ratio or a secondary air supply means (42; 54; 62) for supplying a secondary air into the exhaust gas in the exhaust connecting passage (4; 52; 60) between the first (7a; 57a) and the second (7b: 57b) NO x absorbents for an engine being operated at an air-fuel ratio equal to or lower than a stochiometric air-fuel ratio; a switching means (5; 58; 65a, 65b) for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NO x absorbent (7a; 57a), the exhaust connecting passage (4; 52; 60) and the second NO x absorbent (7b; 57b) in this order, and a second direction in which the exhaust gas flows through the second NO x absorbent (7b; 57b), the exhaust connecting passage (4; 52; 60) and the first NO x absorbent (7a; 57a) in this order; and a control means (20) for controlling the switching means (5; 58; 65a, 65b), 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.

Inventors: IGARASHI KOUHEI, ITOU TAKAAKI, KINUGASA YUKIO, TAKAOKA TOSHIFUMI, IGARASHI, KOUHEI, ITOU, TAKAAKI, KINUGASA, YUKIO, TAKAOKA, TOSHIFUMI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1995
Main classification: F01N3
IPC codes: F01N7, F01N3, F02B1, B01D53