Nitrogen oxides decreasing apparatus for internal combustion engine

Abstract: In an exhaust purification apparatus of an internal combustion engine having an NOx adsorption and release material (2) in an exhaust conduit (6), a heater (24) is provided close to the NOx adsorption and release material (2). When an NOx adsorption operation continues for a predetermined period, the heater (24) is switched on to raise the temperature of the NOx adsorption and release material (2) above a predetermined temperature, for example, 500°C. When the temperature of the NOx adsorption and release material (2) is high, the NOx adsorption and release material (2) releases absorbed NOx and is regenerated. Released NOx is contacted with a reduction catalyst (4).

Description: The present invention relates to a nitrogen oxides (NOx) decreasing apparatus for an internal combustion engine. More particularly, the present invention relates to an apparatus for effectively purifying NOx included in exhaust gas from a lean burn engine, using a combination of an NOx absorption and release material and an NOx purification catalyst. To improve fuel economy and to suppress exhaust of carbon dioxide (CO₂) to thereby reduce global warming, engines capable of fuel combustion at lean air-fuel ratios (lean burn engines which include lean burn gasoline engines and diesel engines) are being developed and are partially in actual use. Since a conventional catalyst (three-way catalyst) cannot reduce and purify NOx at lean air-fuel ratios, there is a need to develop a catalyst or system that can purify NOx even at lean air-fuel ratios, that is, under oxidizing gas conditions (excess-oxygen conditions). As an apparatus capable of purifying NOx under oxidizing gas conditions, the present applicant filed Japanese Patent Application HEI 3-289046 an October 4, 1991, in which an NOx absorption and release material capable of absorbing NOx at lean air-fuel ratios and releasing the absorbed NOx at rich air-fuel ratios is installed in an exhaust conduit of a lean burn engine. The air-fuel ratio of the engine is momentarily changed to a rich air-fuel ratio before the NOx absorption and release material is saturated so that the the NOx absorption and release material is regenerated. However, NOx release operation of the NOx absorption and release material is affected not only by the amount of excess oxygen in the exhaust gas but also by exhaust gas temperature. Therefore, the NOx absorption and release material may release the absorbed NOx at some temperatures even under oxidizing gas conditions. On the other hand, the NOx absorption and release material may release little NOx at other temperatures even at rich air-fuel ratios, and therefore, the NOx release time period may have to be prolonged or the frequency of NOx release may have to be increased, which will be accompanied by degradation of driveability. Further, if it is intended to maintain the NOx absorption and release material at appropriate temperatures from the viewpoint of NOx absorption and release, location of the NOx absorption and release material will be limited and freedom of design will be restricted. JP-A-03 135 417 illustrates an NOx decreasing apparatus having the features of the preamble of Claim 1. An object of the invention is to provide an NOx decreasing device for an internal combustion engine where a temperature of an NOx absorption and release material can be controlled intentionally without being restricted by location of the NOx absorption and release material. The invention provides an NOx decreasing apparatus in accordance with Claim 1. In the above-described NOx decreasing apparatus, since the NOx absorption and release material can be heated intentionally by the heater to a temperature above the predetermined temperature, release of NOx from the NOx absorption and release material is executed even though the exhaust gas temperature is low, without being affected by the location of the NOx absorption and release material and the operating condition of the internal combustion engine. The above-described object and other objects, features, and advantages of the present invention will become more apparent and will be more readily appreciated from the following detailed description of the preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which FIG. 1 is a schematic system diagram of an NOx decreasing apparatus for an internal combustion engine in accordance with a first embodiment of the invention; FIG. 2 is a more detailed system diagram of the apparatus of FIG. 1; FIG. 3 is a flow chart of regeneration control of an NOx absorption and release material used in the apparatus of FIG. 1; FIG. 4 is a flow chart for controlling various flags so that the routine does not proceed to NOx regeneration steps 104'-110 before a predetermined time period SNe0 has elapsed since regeneration of the NOx absorption and release material; FIG. 5 is a schematic side elevational view of a vehicle mounting the NOx decreasing apparatus of FIG. 1; Fig. 6 is a graphical representation of an NOx concentration versus exhaust gas temperature characteristic of the NOx absorption and release material used in the apparatus of Fig. 1; Fig. 7 is a schematic system diagram of an NOx decreasing apparatus for an internal combustion engine in accordance with a second embodiment of the invention; Fig. 8 is a flow chart of control for regeneration start of an NOx absorption and release material of Fig. 7; FIG. 9 is a flow chart of control for regeneration finish and reset of an NOx absorption and release material provided in one conduit of the apparatus of FIG. 7; FIG. 10 is a flow chart of control for regeneration finish and reset of an NOx absorption and release material provided in another conduit of the apparatus of FIG. 7; FIG. 11 is a schematic system diagram of an NOx decreasing apparatus for an internal combustion engine in accordance with a third embodiment of the invention; FIG. 12 is a flow chart of control for regeneration start of the NOx decreasing apparatus of FIG. 11; FIG. 13 is a flow chart of control for regeneration finish and reset of an NOx absorption and release material provided in one conduit of the apparatus of FIG. 11; and FIG. 14 is a flow chart of control for regeneration finish and reset of an NOx absorption and release material provided in another conduit of the apparatus of FIG. 11. Three preferred embodiments of the invention will be explained with reference to the drawings. FIGS. 1-6 illustrate a first embodiment of the invention: FIGS. 7-10 illustrate a second embodiment of the invention: and FIGS. 11-14 illustrate a third embodiment of the invention. FIRST EMBODIMENT As illustrated in FIG. 1, an internal combustion engine capable of fuel combustion at lean air-fuel ratios has an exhaust conduit 6. An NOx absorption and release material 2 and an NOx decomposition catalyst 4 are disposed in the exhaust conduit 6 in that order in the direction of exhaust gas flow. An electric heater 24 operable to heat the NOx absorption and release material 2 is located in the exhaust conduit 6. Preferably, the heater 24 is located upstream of and close to the NOx absorption and release material 2. A bypass conduit 8 bypassing the heater 24 and the NOx decomposition catalyst 4 is provided to the exhaust conduit 6. A bypass valve 10 is disposed at a diverging portion of the bypass conduit 8 and the exhaust conduit 6 to permit control of the ratio of the amount of exhaust gas flowing through the NOx absorption and release catalyst to the amount of exhaust gas flowing through the bypass conduit 8. Switching of the heater 24 between "ON" and "OFF" is controlled by an NOx absorption and release material regeneration means 26. Operation of the bypass valve 10 is controlled by a bypass valve control means 12. When the NOx absorption and release material 2 has continuously absorbed NOx for a predetermined time period, the heater 24 is switched to "ON" to heat the NOx absorption and release material 2. When the temperature T of the NOx absorption and release catalyst has exceeded a predetermined temperature T0, the bypass valve 10 throttles the exhaust gas flowing through the NOx absorption and release material 2 and the NOx decomposition catalyst 4 for a predetermined period, for example, ten seconds. FIG. 2 illustrates the system in more detail. In FIG. 2, an electronic control unit (ECU) 14 is constituted by a micro computer. The NOx absorption and release material regeneration means 26 and the bypass valve control means 12 comprise means defined in programs (see FIGS. 3 and 4) stored in the ECU 14. The ECU 14 includes a central processor unit (CPU) 14a for executing calculations, a read-only memory (ROM) 14b, a random access memory (RAM) 14c, an analog/digital converter 14d for converting analog signals to digital signals, an input interface 14e, and an output interface 14f. A timer (described later) 14g is connected to the ECU 14. In the exhaust conduit 6, an exhaust gas temperature sensor (or an NOx absorption and release material temperature sensor) 16 is installed in a portion of the exhaust conduit 6 downstream of the heater 24. The output of the sensor 16 is fed to the analog/digital converter 14d and is used as an exhaust gas temperature T in calculation. The results of the calculation executed at the CPU 14a are sent via the output interface 14f to the heater 24 and an actuator of the bypass valve 10 so that the heater 24 and the bypass valve 10 are controlled. The NOx absorption and release material 2 is disposed in a portion of the exhaust conduit where the exhaust gas temperature is lower than 500°C. Such an exhaust conduit portion is usually spaced apart from the engine 18 and is located under a floor of a vehicle 20, as shown in FIG. 5. A member denoted with reference numeral 22 is a muffler. The NOx absorption and release material 2 comprises a material capable of absorbing NOx included in exhaust gas at temperatures below a predetermined temperature (for example, 500°C) and releasing absorbed NOx at temperature above the predetermined temperature. Such an NOx absorption and release material comprises a composite oxide of alkali earth and copper (Ba-Cu-O base), or a combination of rare-earth element and noble metal. The composite oxide of alkali earth and copper is, for example, MnO₂.BaCuO₂, and the combination of rare-earth element and noble metal is, for example, a combination of lanthanum (La) and platinum (Pt). The Ba-Cu-O base NOx absorption and release material converts NO into NO₂ at temperatures below about 500°C and can absorb stably the converted NO₂ in the crystal structure of the NOx absorption and release material. The Ba-Cu-O base NOx absorption and release material 2 loses its NOx absorption ability at temperatures above about 500°C and releases absorbed NO₂, which the NOx absorption and release material 2 has absorbed for several hours, within a short period of time, for example, ten seconds. The Ba-Cu-O base NOx absorption and release material 2 is produced by the following methods: Production method 1 (Pt carried): Equal moles of copper nitrate and barium nitrate were mixed and the mixture was calcined in air at 650°C for four hours. Then, the resulting sinstered mixture was placed into 8-platinum nitrate solution so that platinum was deposited onto the sintered mixture. Then, the platinum-coated mixture was roasted at 500°C in a flow of nitrogen. Production method 2 (cerium carried): Equal moles of copper nitrate and barium nitrate were mixed and the mixture was calcined in air at 650°C for four hours. This sintered mixture was placed into cerium nitrate solution so that cerium was deposited onto the mixture. Then, the cerium-coated mixture was roasted at 500°C in a flow of air. Formation into a monolith: Powder of one of the above-described roasted mixtures, silica-sol, and water were mixed at a ratio of 100 : 100 : 10 to produce a slurry. A monolith constructed of sintered cordierite was immersed in the slurry and then was taken out of the slurry and was dried in a stream of hot air at 250°C. This coating procedure was repeated several times, and then the coated monolith was roasted at 500°C to obtain an NOx absorption and release material. FIG. 6 illustrates an NOx concentration versus temperature characteristic of the exhaust gas at an outlet of the Ba-Cu-O (Ba : Cu = 1 : 13) base NOx absorption and release material in a condition of no oxygen (solid line) and at a space velocity or specific volume rate (volumes of exhaust gas per volume of NOx absorption and release material) of 6,000/hour. In the presence of oxygen, the NOx release characteristic shifts by 50 - 100°C toward the right (dashed line) in FIG. 6. From FIG. 6, it is seen that the Ba-Cu-O base NOx absorption and release material releases NOx under oxidizing gas conditions at lean air-fuel ratios above about 500°C, for example, when the heater is switched to "ON", and absorbs NOx at temperatures below about 500°C, for example, when the heater is switched to "OFF". Further, if the air-fuel ratio is changed to a stoichiometric air-fuel ratio or rich ratios, the exhaust gas approaches the non-oxygen condition and the NOx release characteristic shifts to a lower temperature so that NOx release is promoted. The NOx decomposition catalyst 4 comprises a transition metal/zeolite catalyst, a noble metal/alumina catalyst, or a cobalt/alumina catalyst. The transition metal/zeolite catalyst includes zeolite and a transition metal, for example copper, ion-exchanged and deposited onto the zeolite. The noble metal/alumina catalyst includes a carrier of alumina (Al₂O₃) and a noble metal, for example platinum, deposited onto the alumina carrier. The cobalt/alumina catalyst includes an alumina carrier and cobalt deposited onto the alumina carrier. These catalysts can decompose NOx into N₂ and O₂ at low space velocities in the presence of an appropriate amount of HC, or without HC. The amount of HC should be appropriate, because too much HC would decrease the NOx decomposition rate of the catalyst. Also, the space velocity should be low, because the NOx decomposition rate of the NOx absorption and release material is low at high space velocities. Therefore, when the heater 24 is switched to "ON" and the temperature of the exhaust gas at the NOx absorption and release material is higher than the predetermined temperature T0 (for example, 500°C), the bypass valve 10 should throttle the amount of exhaust gas flowing through the NOx absorption and release material 2 and the NOx decomposition catalyst 4 so that the space velocity at the NOx decomposition catalyst 4 is low. FIGS. 3 and 4 illustrate control routines for controlling the heater 24 and the bypass valve 10 so that the heater 24 is switched to "ON" to heat the NOx absorption and release material 2 at a predetermined condition and at the same time the bypass valve 10 throttles the amount of exhaust gas flowing through the NOx decomposition catalyst 4 to cause the NOx decomposition catalyst 4 to decompose NOx at a high rate. These routines are stored in the ROM 14b and are entered into the CPU 14a where calculations are executed. The control routines of FIGS. 3 and 4 are entered at intervals of predetermined periods or at every predetermined crank angle. In the routine of FIG. 3, at step 102 a decision is made as to whether a heater and bypass valve operation flag Fdb is "1" or not. The state that flag Fdb is "1" means that the heater 24 is in an "ON" state and the bypass valve 10 throttles the exhaust gas flowing through the NOx absorption and release material 2 and the NOx decomposition catalyst 4 and causes a large portion of the exhaust gas to flow through the bypass conduit 8. In contrast, the state that flag Fdb is "0" means that the heater 24 is in an "OFF" state and all of the exhaust gas flows through the NOx absorption and release material 2 and the NOx decomposition catalyst 4. When it is determined at step 102 that flag Fdb is "1", since the heater 24 is in an "ON" state and the bypass valve 10 is in a throttling operation, the heater 24 need not be switched to "ON" and the bypass valve 10 need not be changed in operation to throttle the amount of the exhaust gas flowing through the NOx absorption and release material 2 by causing the routine to proceed through steps 104 - 110. Thus, the routine proceeds directly to a return step. In contrast, when it is determined at step 102 that flag Fdb is "0", the routine proceeds to step 102 where a decision is made as to whether rotation accumulation flag Fdg is "1" or not. The state that flag Fdg is "1" means that the accumulated engine rotations SNe counted since the last regeneration of the NOx absorption and release material 2 do not exceed a predetermined number of rotations SNe0 and the NOx absorption and release material 2 is not yet saturated. In contrast, the state that flag Fdg is "0" means that SNe is larger than SNe0 and that the NOx absorption and release material 2 is already saturated or is going to be saturated. When it is determined at step 104 that flag Fdg is "1", since a long period has not elapsed since last release of NOx from the NOx absorption and release material 2, the NOx absorption and release material 2 need not be regenerated, and so the routine proceeds to the return step. In contrast, when it is determined at step 104 that flag Fdg is "0", the routine proceeds to step 104', where the heater 24 is switched to "ON". Then, the routine proceeds to step 106, where a decision is made as to whether or not the exhaust gas temperature (or NOx absorption and release material temperature) T which is raised through heating by the heater 24 exceeds a predetermined temperature T0. T0 is defined as a temperature forming a boundary between an NOx absorption temperature range and an NOx release temperature range, and T0 is about 500°C. When it is determined at step 106 that T is equal to or smaller than T0, since the NOx absorption and release material 2 has not yet been raised in temperature to an NOx release beginning temperature after the switching-on of the heater 24, the routine proceeds to the return step. In contrast, when it is determined at step 106 that T exceeds T0, the routine proceeds to steps 108 and 110 to control the bypass valve 10 so that the NOx decomposition catalyst 4 decomposes at high decomposition rates the NOx which the NOx absorption and release material 2 begins to release. Since the heater 24 is in an "ON" state and the bypass valve 10 is in a throttling condition when the routine proceeds through the route including steps 108 and step 110, flag Fdb is set to "1" at step 108. Then, the routine proceeds to step 110, where the bypass valve 10 is switched to throttle the exhaust gas flowing though the NOx absorption and release material 2 and the NOx decomposition catalyst 4, and then the timer 14g is switched on. The timer 14g is preset to switch off the heater 24, to clear the accumulated rotation SNe to zero, and to reset flag Fdb to "0", after about ten seconds has elapsed since the switching on of the timer 14g. Accordingly, the timer 14g maintains the heater 24 at an "ON" state for about ten seconds after the exhaust gas temperature T exceeds the predetermined temperature T0 subsequent to switching the heater on, so that NOx release from the NOx absorption and release material 2 is completed during the period of about ten seconds. After the period of ten seconds, flag Fdb is cleared and the timer 14g is reset. In the above-described structure, steps 104 and 104' and the timer 14g constitute the NOx absorption and release material regeneration means 26, and steps 106 and 110 and the timer 14g constitute the bypass valve control means 12. Following the routine of FIG. 3, a routine of FIG. 4 is entered. The routine of FIG. 4 is a flag control routine for controlling flags Fdb and Fdg so that the heater 24 and the bypass valve 10 are prevented from operation before the NOx absorption and release material 2 is saturated, even though exhaust gas temperature T exceeds the predetermined temperature T0. The routine of FIG. 4 assures that exhaust gas flow through the bypass conduit 8 to atmosphere without being purified will not continue for a long period of time. More particularly, at step 202, a decision is made as to whether flag Fdb is "1". When it is determined at step 202 that flag Fdb is "0", the routine proceeds to step 204, where the number of engine rotations counted since last regeneration of the NOx absorption and release material 2 is increased by every entry to the routine of FIG. 4. When it is determined at step 202 that Fdb is "1", the routine proceeds directly to a return step. If Fdb is "0", the routine proceeds from step 202 to 204, and the engine rotation number SNe is accumulated. Then, the routine proceeds to step 206, where a decision is made as to whether the accumulated rotation SNe exceeds the predetermined rotation SNe0. If SNe is equal to or less than SNe0 at step 202, the NOx absorption and release material 2 may be deemed to have not yet been saturated, and the routine proceeds to step 212, where flag Fdg is set to "1". If SNe exceeds SNe0 at step 202, the routine proceeds to steps 208 and 210 where flags Fdb and Fdg are set to "1" in preparation for execution of control according to the routine of FIG. 3. Operation of the first embodiment will now be explained. In a standard operation range, all of the exhaust gas flows through the NOx absorption and release material 2 and the NOx decomposition catalyst 4. Since the heater 24 is in an "OFF" state and the exhaust gas temperature at the inlet of the NOx absorption and release material 2 is below about 500°C, NOx included in the exhaust gas is absorbed by the NOx absorption and release material 2. The volume of the NOx absorbing and release material 2 is selected so that the material has enough capacity to continuously absorb NOx included in the exhaust gas for several hours. After continuous operation for several hours and the NOx absorption and release material 2 approaches the saturation, the heater 24 is switched on and the exhaust gas temperature (NOx absorption and release material temperature) increases to above 500°C at the inlet of the NOx absorption and release material 2. In this high temperature condition, the NOx absorption and release material 2 releases the NOx which had been absorbed, so as to be finally regenerated. During the NOx release period of the NOx absorption and release material 2, the bypass valve 10 throttles the exhaust gas flowing to the NOx decomposition catalyst 4 so that the space velocity at the NOx decomposition catalyst 4 is low and NOx is decomposed into N₂ and O₂ by the NOx decomposition catalyst 4 at high rates. Release of absorbed NOx from the NOx absorption and release material 2 is achieved in about ten seconds. During the regeneration of the NOx absorption and release material 2, most of the exhaust gas flows through the bypass conduit 8 and the NOx included in the exhaust gas is exhausted to atmosphere without being decomposed. However, this state continues for only about ten seconds and NOx exhausted during this short period is small, which will cause no problem from the viewpoint of environmental pollution. The above-described absorption of NOx by the NOx absorption and release material 2 and regeneration of the NOx absorption and release material 2 by operating the heater 24 do not require high exhaust gas temperature, so the location of the NOx absorption and release material can be spaced from the engine. Accordingly, there is increased design freedom for locating the exhaust gas purification system. SECOND EMBODIMENT As illustrated in FIG. 7, an exhaust conduit 30 of a lean burn engine (diesel engine in the third embodiment) includes a dual passage portion 32 having a first passage 32A and a parallel second passage 32B. Diesel particulate filter and NOx absorption and release materials (hereinafter, NOx absorption and release materials) 34A and 34B are disposed in first and second passages 32A and 32B, respectively. Each NOx absorption and release material 34A, 34B functions not only as a diesel particulate filter for capturing diesel particulates but also as an NOx absorption and release material. More particularly, each NOx absorption and release material 34A, 34B absorbs NOx included in exhaust gas from the engine at temperatures below a predetermined temperature and releases absorbed NOx at temperatures above the predetermined temperature. Each NOx absorption and release material 34A, 34B includes a carrier constituted of alumina which carries at least one kind of material selected from the group consisting of alkali metal including potassium (K), sodium (Na), and lithium (Li); alkali-earth including barium (Ba), cesium (Ce), and calcium (Ca); and rare-earth element including lanthanum (La) and yttrium (Y), and further carries at least one kind of metal selected from the group consisting of noble metals including platinum (Pt) and transition metals including copper (Cu) and iron (Fe). This NOx absorption and release material absorbs NOx when the exhaust gas is in oxidizing gas conditions, and releases NOx when the oxygen concentration of the exhaust gas decreases and/or when the the exhaust gas temperature rises to a temperature above the predetermined temperature. NOx decomposition catalysts 36A and 36B with a heater are disposed in portions of the first and second passages 32A and 32B, respectively, located downstream of the NOx absorption and release materials 34A and 34B. For the NOx decomposition catalysts 36A and 36B, any of the following three kinds of catalysts can be used: a catalyst which is constituted by zeolite ZSM-5 and copper ion-exchanged and deposited onto the zeolite; a catalyst which is constituted by alumina Al₂O₃ and Cu deposited onto the alumina; and a catalyst which is constituted by alumina Al₂O₃ and platinum or platinum and rhodium deposited onto the alumina. Electric heaters 38A and 38B are disposed in portions of the first and second passages 32A and 32B, respectively, located upstream of the NOx absorption and release materials 34A and 34B. When the heaters 38A and 38B are switched on, the heaters 38A and 38B burn diesel particulates captured by the NOx absorption and release materials 34A and 34B, and heat the NOx absorption and release material 34A and 34B to a temperature above the predetermined temperature to cause the NOx absorption and release material 34A and 34B to release absorbed NOx. As a result, the NOx absorption and release materials 34A and 34B are regenerated both as a diesel particulate filter and as an NOx absorption material. Flow switching valves 40A and 40B for switching exhaust gas flow between the first passage 32A and the second passage 32B are disposed in the first and second passages 32A and 32B, respectively, upstream of the heaters 38A and 38B, respectively. The flow switching valves 40A and 40B are driven by actuators 42A and 42B, respectively. When one of the switching valves 40A and 40B is open, the other of the switching valves 40A and 40B is closed. Secondary air supply ports 46A and 46B for supplying secondary air into the first and second passages 32A and 32B are provided at portions of the first and second passages 32A and 32B located between the switching valves 40A and 40B and the NOx absorption and release materials 34A and 34B. The secondary air supply ports 46A and 46B are connected to an air pump 50 via secondary air conduits in which solenoid valves 48A and 48B for switching on and off supply of secondary air are disposed. The heaters 38A and 38B provided so as to heat the NOx absorption and release materials 34A and 34B and the heaters provided so as to heat the NOx decomposition catalysts 36A and 36B are connected via an electric circuit to a battery 86 so that electricity is supplied to the heaters, and an electric relay device 88 is provided on the electric circuit so as to selectively switch on and off the heaters. The switching on and off of the flow switching valves 40A and 40B and the switching on and off of the electric heaters for the NOx decomposition catalysts 38A and 38B are controlled by an electronic control unit (ECU) 90. Like the cases of the first and second embodiments, the ECU 90 includes a CPU, a ROM, a RAM, and input and output interfaces, and analog signals are converted into digital signals by an A/D converter and are fed to the input interface. A pressure sensor 92 is installed in a portion of the exhaust conduit 30 located upstream of the dual passage portion 32 and another pressure sensor 94 is installed in a portion of the exhaust conduit 30 located downstream of the dual passage portion 32. A differential between outputs of these two pressure sensors 92 and 94 corresponds to the amount of diesel particulates captured by the NOx absorption and release materials 34A and 34B, so that the need for regeneration as a diesel particulate filter can be detected. The outputs of the pressure sensors 92 and 94 are fed to the ECU 90. Output signals of the engine speed sensor 96 and the engine load sensor 98 are also fed to the ECU 90. Control routines of FIGS. 8 - 10 for controlling the flow switching valves 40A and 40B and heaters 38A and 38B are stored in the ROM, and calculation according to the control routines is executed in the CPU. The routine of FIG. 8 controls beginning regeneration of the NOx absorption and release material, and the routines of FIGS. 9 and 10 control the regeneration operation periods using a timer to perform regeneration and then resetting at optimum timings. The routine of FIG. 8 is entered at step 500 at intervals of predetermined periods, for example, at every eight milliseconds. Then, at step 502, the current engine speed Ne, the current engine load T (for example, throttle valve opening degree), the current pressure Pa of the pressure sensor 92, and the current pressure Pb of the pressure sensor 94 are entered. Then, at step 504, a predetermined allowable pressure differential Pd between the upstream side and the downstream side of the NOx absorption and release material for the current engine operating condition is determined using a map. Then, at step 506, a decision is made as to whether or not the current pressure differential (Pa - Pb) exceeds the allowable pressure differential Pd. If the actual pressure differential is equal to or smaller than Pd, the NOx absorption and release material need not be regenerated, and so the routine proceeds to a return step. In contrast, if the actual pressure differential exceeds Pd, the routine proceeds to step 508 to regenerate the NOx absorption and release material by burning diesel particulates. To execute the regeneration, at step 508 a decision is made as to which passage of the first and second passages is open to exhaust gas flow. The condition that passage A flag PAF is "1" means valve 40a is currently open so that the exhaust gas has been flowing through the first passage 32A, and therefore the NOx absorption and release material to be regenerated is the NOx absorption and release material 34A. If it is determined at step 508 that flag PAF is "1", the routine proceeds to step 510A, where a decision is made as to whether or not the current cycle is a first cycle for passing through the route including step 510A. The condition that flag C is "1" corresponds to the first cycle. When it is determined at step 510A that flag C is "1", the routine proceeds to steps 512A - 522A, where regeneration of the NOx absorption and release material 34A begins. More particularly, at step 512A the flow switching valve 40A is closed, at step 514A the timer is switched on, at step 516A the air pump 50 is turned on, and at step 518A the heater of the NOx decomposition catalyst 36A is switched on by operating the relay 88. As a result, the heater 38A burns the particulates to regenerate the NOx absorption and release material. During the regeneration, secondary air supplied from the air pump 50 flows through the NOx absorption and release material 34A at low speeds. Since the temperature of the exhaust gas flowing through the NOx absorption and release material is high due to combustion energy of the particulates, the NOx absorption and release material 34A releases absorbed NOx. Then, at step 524A, flag C is set to "0", and then the routine proceeds to step 526 where the cycle ends. In the successive cycle, when the routine proceeds to step 510A, the routine will proceed from step 51A to step 526. As a result, the regeneration by steps 512A - 524A of the previous cycle holds until the regeneration has been completed in the routines of FIGS. 9 and 10. If it is determined at step 508 that passage A flag PAF is "0", this means valve 40a is currently shut so that the exhaust gas has been flowing through the passage 32B, and the NOx absorption and release material to be regenerated is the NOx absorption and release material 34B. Thus, the routine proceeds from step 508B to step 510B so that regeneration of the NOx absorption and release material 34B is executed. Structures and functions of steps 510B - 524B are the same as those of steps 510A - 524A except that suffix A of each reference numeral is changed to B. The regeneration of the NOx absorption and release material 34A continues for a predetermined period according to the routine of FIG. 9. The routine of FIG. 9 is entered at step 600A at intervals of predetermined periods, for example, at every eight milliseconds. Then, at step 602A, a decision is made as to whether the timer count time exceeds a predetermined period T1 (for example, three minutes). If the timer count time exceeds the predetermined period T1 at step 602A, the routine proceeds to step 604A, where the relay 88 is switched off to switch off the heater 38A. Then, the routine proceeds to step 606A, where a decision is made as to whether the timer count time exceeds a predetermined period T2 (for example, fifteen minutes). If the timer count time exceeds the predetermined period T2, the routine proceeds to step 608A, where the air pump 50 and the air switching valve 48A are switched off, so that supply of secondary air stops. Then, the routine proceeds to step 612A, where a decision is made as to whether or not a timer count time exceeds a predetermined period T3 (for example, fifteen minutes). If the timer count time exceeds the period T3, the routine proceeds to step 614A, where the heater provided to the NOx decomposition catalyst 36A is switched off. In this way, when the period of the maximum of T1, T2, and T3 has elapsed, the regeneration is completed. Then, at step 616A, a decision is made as to whether or not all of the relay 88, the air pump 50, the air switching valve 48A, and the heater 36A are switched off, that is, whether or not the regeneration has been completed. If the regeneration has not yet been completed, the routine proceeds to a return step 624A, and if the regeneration has been completed, the routine proceeds to steps 618A - 622A, where passage A flag PAF is reset to "0", flag C is reset to "1", and the timer count time is cleared to zero. Regeneration and resetting of the NOx absorption and release material 34B are executed according to a routine of FIG. 10 which is similar to that of FIG. 9. Steps 600B - 624B of the routine of FIG. 10 correspond to steps 600A - 624A of the routine of FIG. 9 except that suffix A is changed to suffice B. In the above-described structures, the routines of FIGS. 8, 9, and 10 constitute the NOx absorption and release material regeneration means of the second embodiment of the invention. Operation of the second embodiment of the invention will now be explained. When the pressure differential (Pa - Pb) between the pressures detected by the pressure sensors 92 and 94 exceeds the predetermined value Pd, it is assumed that the amount of diesel particulates captured by the NOx absorption and release material is large and it is a time for a regeneration cycle. Then, the flow switching valve 40A or 40B which has been open is switched to be closed and secondary air begins to be supplied into the passage where the now shut flow switching valve 40A, 40B is installed. Further, a corresponding heater 38A or 38B is switched to "ON" so that the diesel particulates which have been captured by the NOx absorption and release material are burned. The heater of the corresponding NOx decomposition catalyst 36A or 36B located downstream of the NOx absorption and release material is also switched on. When the temperature of the NOx absorption and release material 34A, 34B is high due to the thermal energy which the captured diesel particulates produces when burned, the NOx absorption and release material 34A, 34B releases absorbed NOx. The released NOx flows to the NOx decomposition catalyst 36A, 36B where the NOx is decomposed. Thus, the NOx absorption and release material 34A, 34B is regenerated not only as a captured diesel particulate filter and but also as an NOx absorption material. In the second embodiment, since flow through the NOx absorption and release material 34A, 34B during regeneration is at a low space velocity and at a high temperature, the regeneration is smooth. Consequently, one of the NOx absorption and release materials 34A, 34B is always in a non-saturated condition capable of absorbing NOx, so NOx exhaust to atmosphere is always prevented. THIRD EMBODIMENT As illustrated in FIGS. 11 - 14, an apparatus according to the third embodiment of the invention comprises an apparatus where a reduction agent such as hydrocarbons (HC) and hydrogen is supplied into the passage upstream of the NOx decomposition catalyst 36A, 36B in an apparatus otherwise similar to that of the second embodiment, so that NOx conversion rate of the NOx decomposition catalyst 36A, 36B is increased. Since other structures and operation of the third embodiment of the invention are the same as those of the second embodiment of the invention, the same portions are denoted with the same reference numerals as those of the second embodiment, and portions different from the second embodiment only will be explained below. In FIG. 11, between the NOx absorption and release material 34A, 34B and the NOx decomposition catalyst 36A, 36B, an HC injection apparatus for injecting fuel (light oil) of the diesel engine into the exhaust conduit is installed. The HC injection apparatus includes an HC supply valve 28A, 28B which supplies HC when it is open. Opening and closing of the HC supply valve 28A, 28B is controlled by the ECU 90. FIGS. 12 - 14 illustrate routines for controlling regeneration of the NOx absorption and release materials 34A and 34B, which constitute the NOx absorption and release material regeneration means of the third embodiment of the invention. The routines of FIGS. 12 - 14 are the same as those of FIGS. 8 - 10, respectively, except that steps 700A, 700B, 702A, 704A, 702B, 704B are added in the routines of FIGS. 12 - 14. More particularly, in FIG. 12, step 700A is added between step 522A and step 524A. At step 700A, HC supply valve 28A is opened so that HC is supplied. The NOx decomposition catalyst 36A, 36B needs HC as a reduction agent to decompose NOx. The supplied HC helps the released NOx to be decomposed at the NOx decomposition catalyst 36A. Similarly, step 700B is added between step 522B and 524B. At step 700B, HC supply valve 28B is opened so that HC is supplied. The supplied HC helps the released NOx to be decomposed at the NOx decomposition catalyst 36B. When regeneration of the NOx absorption and release material 34A or 34B is completed, supply of HC is stopped. More particularly, in FIG. 13, steps 702A and 703A are added between step 614A and 616A. When the time counted by the timer exceeds a predetermined period T4 (for example, fifteen minutes), the HC supply valve 28A is switched off and supply of HC is stopped. Similarly, in FIG. 14, steps 702B and 704B are added and supply of HC is stopped by switching off the HC supply valve 28B. With respect to operation of the third embodiment of the invention, the NOx conversion rate of the NOx decomposition catalysts 36A and 36B is increased to a great extent. More particularly, injected HC is thermally decomposed into HC of desirable sizes which is partially oxidized to generate radicals to promote decomposition of NOx at the NOx decomposition catalysts 36A and 36B. The NOx absorption and release material 34A, 34B begins to release absorbed NOx not only when the exhaust gas temperature exceeds a predetermined temperature but also when the oxygen concentration of the exhaust gas lowers. When the air-fuel ratio is stoichiometric or rich, the exhaust gas includes almost no oxygen while unburned HC increases. The unburned HC is consumed in decomposing NOx, because the HC rapidly takes oxygen atoms from the NOx that is released from the NOx absorption and release material to be oxidized. Since the NOx absorption and release materials 34A and 34B have an NOx absorption characteristic under a lean burn condition and an NOx decomposition characteristic at low oxygen concentrations, the NOx decomposition catalysts 36A and 36B are not absolutely necessary in this embodiment. In accordance with the present invention, since the heater operable to heat the NOx absorption and release material is provided, the temperature of the NOx absorption and release material can be intentionally controlled independently of the engine operating condition and the location of the NOx absorption and release material, so that controllability and freedom of design are improved.

Claims: An NOx decreasing apparatus comprising: an exhaust conduit (6) for connection to an internal combustion engine; an NOx absorption and release material (2), installed in a first portion of the exhaust conduit (6), for absorbing NOx included in exhaust gas from the engine under oxidizing gas conditions at temperatures below a predetermined temperature and for releasing absorbed NOx at temperatures above the predetermined temperature; a first heater (24) installed in the first portion of the exhaust conduit (6) and operable to heat the NOx absorption and release material (2) to a temperature above the predetermined temperature; and an NOx decomposition catalyst (4) capable of decomposing NOx under oxidizing gas conditions and installed in the first portion of the exhaust conduit (6) downstream of the NOx absorption and release material (2); characterized by means (10,40A,40B,46A,46B) provided in the exhaust conduit (6) for throttling an amount of exhaust gas flowing through the NOx absorption and release material (2) when the NOx absorption and release material (2) is releasing absorbed NOx. An NOx decreasing apparatus according to Claim 1, further comprising air-fuel ratio control means for decreasing an air-fuel ratio of the internal combustion engine from a lean ratio to at least a stoichiometric ratio when the NOx absorption and release material (2) is releasing NOx. An NOx decreasing apparatus according to claim 1, further comprising: NOx absorption and release material regeneration means (26) for temporarily operating the heater (24) to heat the NOx absorption and release material (2) to a temperature above the predetermined temperature to cause the NOx absorption and release material (2) to release absorbed NOx. An NOx decreasing apparatus according to claim 3, wherein the NOx absorption and release material (2) comprises a composite oxide of alkaline earth and copper. An NOx decreasing apparatus according to claim 4, wherein the composite oxide of alkali earth and copper is MnO₂.BaCuO₂. An NOx decreasing apparatus according to claim 3, wherein the NOx absorption and release material (2) comprises a combination of a rare-earth element and a noble metal. An NOx decreasing apparatus according to Claim 6, wherein the rare-earth element is lanthanum and the noble metal is platinum. An NOx decreasing apparatus according to any one of Claims 1 - 7, wherein the NOx decomposition catalyst (4) is a zeolite-type catalyst including zeolite and copper ion-exchanged and deposited onto the zeolite. An NOx decreasing apparatus according to any one of Claims 1 - 7, wherein the NOx decomposition catalyst (4) is a three-way catalyst. An NOx decreasing apparatus according to any one of Claims 1 - 9, wherein the predetermined temperature above which the NOx absorption and release material (2) releases NOx is about 500°C. An NOx decreasing apparatus according to any one of claims 1 - 10 wherein said means provided in the exhaust conduit (6) comprises: a bypass conduit (8) bypassing the portion of the exhaust conduit (6) where the NOx absorption and release material (2) and the NOx decomposition catalyst (4) are installed; a bypass valve (10) installed at a connection point between the bypass conduit (8) and the first portion of the exhaust conduit (6) and operable to throttle exhaust gas flowing through the first portion of the exhaust conduit (6) containing the NOx absorption and release material (2) and the NOx decomposition catalyst (4), while allowing the remainder of the exhaust gas to flow through the bypass conduit (8); and bypass valve control means for controlling the bypass valve so that the bypass valve throttles the exhaust gas flowing through the NOx absorption and release material (2) and the NOx decomposition catalyst (4) while the NOx absorption and release material regeneration means is causing the heater (24) to heat the NOx absorption and release material (2). An NOx decreasing apparatus according to claim 11 wherein the NOx absorption and release material regeneration means comprises a timer (14g) which is switched on to begin to count time when an accumulated number of engine rotations exceeds a predetermined number and the exhaust gas temperature measured at an inlet of the NOx absorption and release material (2) exceeds a predetermined temperature and which switches off the heater (24) when the counted period of time exceeds a predetermined time period. An NOx decreasing apparatus according to claim 12, wherein the predetermined time period set in the timer (14g) is about ten seconds. An NOx decreasing apparatus according to any one of claims 1 - 13 wherein the exhaust conduit comprises a single conduit (54), the first portion of the exhaust conduit containing the NOx absorption and release material (56) being located under a vehicle floor and the NOx decomposition catalyst comprising a three-way catalyst (58). An NOx decreasing apparatus according to claim 14, further comprising: another three-way catalyst (64) installed in the exhaust conduit between the internal combustion engine (52) and the NOx absorption and release material (56). An NOx decreasing apparatus according to claim 14 wherein the NOx absorption and release material regeneration means comprises: first decision means for determining whether or not a lean burn condition has continued for a first predetermined time period; second decision means for determining whether or not an exhaust gas temperature at the NOx absorption and release material (56) is higher than the predetermined temperature; air-fuel ratio control means for temporarily reducing the air-fuel ratio of the engine to at least a stoichiometric ratio when the first decision means determines that a lean burn condition has continued for the first predetermined time period and the second decision means determines that the exhaust gas temperature is higher than the predetermined temperature; and a timer for counting time since the air-fuel ratio control means reduces the air-fuel ratio to at least a stoichiometric ratio, the timer switching off the heater and clearing the air-fuel ratio control means when the time counted by the timer exceeds a second predetermined time period. An NOx decreasing apparatus according to claim 16, wherein the second predetermined time period is about twenty seconds. An NOx decreasing apparatus according to claim 1, wherein the engine is a diesel engine, the exhaust conduit (30) including a second portion (32B) parallel to the first portion (32A), a second one of said NOx absorption and release material (34B) being disposed in the second portion (32B) of the exhaust conduit, the first and second NOx absorption and release materials (34A, 34B) operating also as diesel particulate filters, said first heater (38A) being disposed in the first portion (32A) of the exhaust conduit upstream of the first NOx absorption and release material (34A) and a second heater (38B) being disposed in the second portion (32B) of the exhaust conduit upstream of the second NOx absorption and release material (34B). An NOx decreasing apparatus according to claim 18, further comprising: a first flow switching valve (40A) disposed in the first portion (32A) of the exhaust conduit and a second flow switching valve (40B) disposed in the second portion (32B) of the exhaust conduit; a first secondary air supply port (46A) located in the first portion (32A) of the exhaust conduit between the first flow switching valve (40A) and the first NOx absorption and release material (34A), and a second secondary air supply port (46B) located in the second portion (32B) of the exhaust conduit between the second flow switching valve (40B) and the second NOx absorption and release material (34B); a first pressure sensor (92) disposed in the exhaust conduit upstream of the first and second NOx absorption and release materials (34A, 34B), and a second pressure sensor (94) disposed in the second conduit downstream of the first and second NOx absorption and release materials (34A, 34B); and NOx absorption and release material regeneration means for temporarily and alternately operating the first and second heaters (38A, 38B) to burn diesel particulates captured by the first and second NOx absorption and release materials (34A, 34B) and causing the first and second NOx absorption and release materials (34A, 34B) to release absorbed NOx. An NOx decreasing apparatus according to claim 19, further comprising: a first NOx decomposition catalyst (36A) disposed in the first portion (32A) of the exhaust conduit downstream of the first NOx absorption and release material (34A), and a second NOx decomposition catalyst (36B) disposed in the second portion (32B) of the exhaust conduit downstream of the second NOx absorption and release material (34B). An NOx decreasing apparatus according to claim 19 or claim 20, wherein said NOx absorption and release material regeneration means includes: a timer; means for determining whether or not a difference between the outputs of the first and second pressure sensors (92, 94) exceeds a predetermined value; means for determining which one of the first and second portions (32A, 32B) of the exhaust conduit is currently open to exhaust gas flow; and means for closing the flow switching valve (40A, 40B) in the determined portion of the exhaust conduit, injecting secondary air into the determined portion, switching on the heater (38A, 38B) in the determined portion, and switching on the timer to cause the timer to count time. An NOx decreasing apparatus according to claim 21, wherein said NOx absorption and release material regeneration means further includes: means for determining whether or not a time counted by the timer exceeds a first predetermined time period and switching off the heater (38A, 38B) when the counted time exceeds the first predetermined time; and means for determining whether or not a time counted by the timer exceeds a second predetermined time period and stopping the supply of secondary air when the counted time exceeds the second predetermined time. An NOx decreasing apparatus according to claim 22, wherein the first time period is three minutes and the second time period is fifteen minutes. An NOx decreasing apparatus according to claim 20, further comprising: a first HC supply port disposed in the first portion (32A) of the exhaust conduit between the first NOx absorption and release material (34A) and the first NOx decomposition catalyst (36A), and a second HC supply port disposed in the second portion (32B) of the exhaust conduit between the second NOx absorption and release material (34B) and the second NOx decomposition catalyst (36B). An NOx decreasing apparatus according to claim 24, wherein said NOx absorption and release material regeneration means includes: means for supplying HC into one of the first and second portions (32A, 32B) of the exhaust conduit through one of the HC supply ports provided in the one of the first and second portions while one of the first and second NOx absorption and release materials (34A, 34B) installed in the one of the first and second portions (32A, 32B) is being regenerated so that supplied HC promotes reduction of NOx by one of the first and second NOx decomposition catalysts (36A, 36B) installed in the one of the first and second portions (32A, 32B).

Inventors: HIROTA SHINYA C O TOYOTA JIDOS, SETO SATOMI C O TOYOTA JIDOSHA, TAKESHIMA SHINICHI C O TOYOTA, HIROTA, SHINYA, C/O TOYOTA JIDOSHA K.K., SETO, SATOMI, C/O TOYOTA JIDOSHA K.K., TAKESHIMA, SHINICHI, C/O TOYOTA JIDOSHA K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1992
Main classification: F01N3
IPC codes: F01N3, B01D53, F01N7, F02B1