Exhaust gas purifying apparatus and method for internal combustion engine

Abstract: To complete a regeneration process of an adsorbent (42) adsorbing unburnt gas components, for a short period of time without making an operational condition of an internal combustion engine (1) unstable and to prevent an ability of the adsorbent (42) from being degraded, an exhaust gas purifying apparatus includes a first exhaust flow path (A) and a second exhaust flow path (B) branched downstream of the catalyst (3a,3b); an adsorbent disposed in the first exhaust flow path for adsorbing unburnt gas components contained in the exhaust gas; and a recirculating device (5,10) for recirculating at least part of the exhaust gas discharged from the adsorbent (42) to an intake side of the internal combustion engine (1). A operational condition of the internal combustion engine is detected. A recirculation amount of the exhaust gas is changed in accordance with the detected operational condition.

Description: BACKGROUND OF THE INVENTION The present invention relates to an exhaust gas purifying apparatus and method for purifying exhaust gas emitted from an internal combustion engine. In an automotive internal combustion engine or the like, it is required to purify the components of exhaust gas, such as carbon monoxide (CO), nitrogen oxide (NOx), and hydrocarbon (HC) before their emission to the atmosphere. To meet such a requirement, an exhaust gas purifying apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open No. Hei 5-171929 is known. In this exhaust gas purifying apparatus, there is provided an exhaust system in which an exhaust pipe downstream of a catalyst is branched into a first branch flow path and a second branch flow path and these branch flow paths are again merged into a single flow on the downstream side. Then, an adsorbent for adsorbing the hydrocarbon is provided in the first branch flow path, and an exhaust gas recirculation path is provided downstream of the adsorbent for introducing a part of the exhaust gas to the intake side of the internal combustion engine. In the exhaust gas purifying apparatus, in the case where the catalyst is kept under non-active conditions as in the case where the internal combustion engine is started in a cold condition, the exhaust gas from the internal combustion engine is caused to flow from the catalyst to the first branch flow path and to a muffler through the adsorbent until the catalyst is activated. Thus, the hydrocarbon (HC) contained in the exhaust gas is adsorbed to the adsorbent. Thereafter, when the catalyst is heated by the heat of the exhaust gas and the temperature of the catalyst reaches the activation temperature, the exhaust gas purifying apparatus renders the exhaust gas, which has been purified by the catalyst, to flow through both first branch flow path and second branch flow path so that the exhaust gas, which has been introduced in the first branch flow path, may flow through the adsorbent to the exhaust gas recirculation flow path and the exhaust gas, which has been introduced into the second branch flow path, may flow to the muffler. The exhaust gas introduced into the first branch flow path causes the adsorbent to be heated and removes therefrom the hydrocarbon (HC) adsorbed to the adsorbent. The hydrocarbon (HC) separated from the adsorbent is introduced into the exhaust gas recirculation flow path together with the exhaust gas and recirculated to the intake side of the internal combustion engine. However, in such an exhaust gas purifying apparatus, there is a fear of problems as follows. When the recirculation amount of the exhaust gas to the intake side is increased, the combustion temperature of the mixture is lowered so that the combustion rate within the combustion chamber is delayed. As a result, the variation of the combustion period for each cycle is remarkable to cause the increase of the torque variation and to induce an uncompleted combustion or a misfire in the combustion chamber. There is a problem that the fuel consumption rate is degraded and the amount of the discharged unburnt hydrocarbon is increased. On the other hand, if the recirculation amount of the exhaust gas is decreased, since the amount of the exhaust gas passing through the adsorbent is decreased, it takes a long time to make the temperature of the adsorbent rise. Then, in some cases, it takes a long time to separate the hydrocarbon away from the adsorbent, and the engine is at a standstill before the completion of the separation. In the case where the engine is at a standstill before the completion of the separation, the hydrocarbon is still adhered to the adsorbent, there is a problem that the adsorbing performance is degraded. Document US-A-5 307 627 discloses a HC trap adsorbing HC which is not oxidized by the catalyst. The adsorbed HC is desorbed to allow a recirculation of the desorbed HC to the upstream side of the catalyst or the intake side of the internal combustion engine. According to this document, the exhaust gas shall only be recirculated in a steady engine state avoiding a stall of the engine. As preferred range for the exhaust gas recirculation a percentage of 5 to 20% is cited. Document US 5 388 405 discloses an exhaust gas purifying apparatus for an internal combustion engine allowing a change of the amount of recirculated exhaust gas. Furthermore, this system comprises an adsorbent provided upstream of the catalyst for adsorbing unburned hydrocarbons, especially in the initial period from starting the engine until the catalyst becomes active. In this document it is disclosed that the cooling water temperature is taken into account for the control of the system. SUMMARY OF THE INVENTION The object underlying the invention has to be seen in achieving an apparatus and a method for purifying exhaust gas of an internal combustion engine comprising an adsorbing means for adsorbing unburned gas components and a recirculating means for recirculating exhaust gas containing unburned gas components which are desorbed from said adsorbing means such that the adsorbing means can be regenerated as early as possible without causing unstable operational conditions of the internal combustion engine. This object is solved by the features of claims 1 and 4. According to the invention, a water temperature detecting means is used for detecting the temperature of the cooling water for the internal combustion engine. The recirculating amount changing means compensates for the amount of the exhaust gas to be recirculated in accordance with the temperature of the cooling water so that the amount of the exhaust gas to be recirculated is decreased in case that the temperature of the cooling water is lower than the predetermined temperature. This allows a regeneration of the adsorbing means as quick as possible but avoids unstable operational conditions of the internal combustion engine which might occur, especially after the engine has been started. In the exhaust gas purifying apparatus for an internal combustion engine according to the invention, the recirculation control is performed in accordance with the operational condition of the internal combustion engine. Namely, the exhaust gas purifying apparatus for the internal combustion engine opens the first exhaust flow path and closes the second exhaust flow path by the path switching means so that the exhaust gas from the catalyst is caused to pass through the first exhaust flow path. In this case, the exhaust gas which is introduced into the first exhaust flow path is caused to pass through the adsorbing means and the unburnt gas components contained in the exhaust gas and not purified in the catalyst is adsorbed into the adsorbing means. Then, when the unburnt gas components adsorbed to the adsorbing means is separated, the path switching means causes major part of the exhaust gas from the internal combustion engine to flow through the second exhaust flow path and the rest part of the exhaust gas to flow through the first exhaust flow path. Then, the recirculating means starts the recirculation of the exhaust gas introduced into the first exhaust flow path. In this case, the exhaust gas introduced into the first exhaust path is recirculated through the adsorbing means to the intake side of the internal combustion engine by the recirculating means. As a result, the adsorbing means is heated by the exhaust gas. Then, the temperature of the adsorbing means rises and reaches the temperature range in which the unburnt gas components adsorbed are separated from the adsorbing means. At this time, the unburnt gas components separated away from the adsorbing means is recirculated to the intake side of the internal combustion engine by the exhaust gas introduced into the first exhaust flow path. Meanwhile the recirculation amount changing means changes the amount of the exhaust gas to be recirculated to the intake side in accordance with the operational condition detected by the operation condition detecting means. Thus, for example, in the case where the operational condition of the internal combustion engine is unstable, the recirculation amount changing means reduces the amount of the exhaust gas to be recirculated in order to stabilize the operational condition of the internal combustion engine. On the other hand, in the case where the operational condition of the internal combustion engine is stable, the recirculation changing means increases the amount of the recirculation in order to accelerate the temperature rise. Accordingly, even before the completion of the warming-up of the internal combustion engine, since the exhaust gas recirculation may be performed without making the operation of the internal combustion engine unstable, it is possible to complete the regeneration process of the adsorbing means in an early stage. It is thus possible to avoid the engine stall and to prevent the ability degradation of the adsorbing means. Also, in the exhaust gas purifying apparatus for the internal combustion engine, it is more preferably that it further includes an activation discriminating means for discriminating whether or not the catalyst is activated, and said recirculating means starts the recirculation of the exhaust gas under the condition that the activation discriminating means judges that the catalyst is activated. In this case, in the exhaust gas purifying apparatus for the internal combustion engine, the recirculation is started under the condition that the catalyst is activated. For example, in the case where it is judged by the activation discriminating means that the catalyst is not activated, the exhaust gas from the catalyst is caused to flow through the first exhaust flow path by the path switching means, and at the same time the recirculation is not carried out by the recirculating means. In this case, the unburnt gas components contained in the exhaust gas which is not purified by the catalyst is adsorbed by the adsorbing means. On the other hand, in the case where it is judged by the activation discriminating means that the catalyst is activated, the path switching means causes major part of the exhaust gas from the internal combustion engine to flow through the second exhaust flow path and the rest part of the exhaust gas to flow through the first exhaust flow path. At the same time, the recirculating means starts the recirculation of the exhaust gas introduced into the first flow path. As a result, the exhaust gas separates the unburnt gas components adsorbed to the adsorbing means and recirculates the separated unburnt gas components to the intake side of the internal combustion engine. Subsequently, the following structure for exhaust gas purifying apparatus for the internal combustion engine may be used according to the present invention. Namely, an exhaust gas purifying apparatus for an internal combustion engine may comprise: an exhaust pipe connected to the internal combustion engine; a catalyst mounted on said exhaust pipe; a first exhaust flow path and a second exhaust flow path branched downstream of said catalyst; a path switching means for introducing the exhaust gas into at least one of said first exhaust flow path and said second exhaust flow path; an adsorbing means disposed in said first exhaust flow path for adsorbing unburnt gas components contained in the exhaust gas; and a recirculating means for recirculating at least part of the exhaust gas discharged from said adsorbing means to an intake side of the internal combustion engine. In this case, in the constructed exhaust gas purifying apparatus for an internal combustion engine, the recirculation control is performed in accordance with the combustion condition of the internal combustion engine. For example, the exhaust gas purifying apparatus for the internal combustion engine opens the first exhaust flow path and closes the second exhaust flow path by the path switching means so that the exhaust gas from the catalyst is caused to pass through the first exhaust flow path. The exhaust gas which is introduced into the first exhaust flow path is caused to pass through the adsorbing means and the unburnt gas components contained in the exhaust gas which is not purified in the catalyst is adsorbed into the adsorbing means. Then, when the unburnt gas components adsorbed to the adsorbing means is separated, the path switching means of the exhaust gas purifying apparatus for the internal combustion engine causes major part of the exhaust gas from the internal combustion engine to flow through the second exhaust flow path and the rest part of the exhaust gas to flow through the first exhaust flow path. Then, the recirculating means starts the recirculation of the exhaust gas introduced into the first exhaust flow path. In this case, the exhaust gas introduced into the first exhaust path is recirculated through the adsorbing means to the intake side of the internal combustion engine by the recirculating means. As a result, the adsorbing means is heated by the exhaust gas. Then, the temperature of the adsorbing means rises and reaches the temperature range in which the unburnt gas components adsorbed are separated from the adsorbing means. At this time, the unburnt gas components separated away from the adsorbing means is recirculated to the intake side of the internal combustion engine by the exhaust gas introduced into the first exhaust flow path. As a result, when the combustion condition of the internal combustion engine is unstable due to exhaust gas recirculation, the exhaust gas purifying apparatus for the internal combustion engine according to the present invention can perform the feedback control for reducing the amount of exhaust gas recirculation in order to stabilize the combustion condition, so that even before the completion of the warming-up of the internal combustion engine, the exhaust gas recirculation may be performed without making the operation of the internal combustion engine unstable. Accordingly, it is compatible to establish the advancement of the regeneration process of the adsorbing means in an early stage and the stabilization of the combustion of the internal combustion engine. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a schematic structure view showing an exhaust gas system of an automotive vehicle to which an exhaust gas purifying apparatus for an internal combustion engine according to the present invention is applied; Fig. 2 is a cross-sectional view showing an internal structure of an adsorbent sleeve; Fig. 3 is a view showing a structure of a flow rate control valve; Fig. 4 is a schematic structure view showing a control system of an automotive vehicle to which an exhaust gas purifying apparatus for an internal combustion engine according to the present invention is applied; Fig. 5 is a view showing a relationship between a temperature THW of engine cooling water and a compensation coefficient kptemp; Fig. 6 is a flowchart showing an example of an exhaust gas recirculation (EGR) control routine; Fig. 7 is a view showing an EGR execution region in the off-idle condition and an exhaust recirculation execution region according to the present invention; Fig. 8 is a view showing temperature characteristics of the adsorbent; DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 An exhaust purifying apparatus for an internal combustion engine according to the present invention will now be described with reference to the accompanying drawings. The exhaust gas purifying apparatus for an internal combustion engine according to the present invention is realized by an automotive exhaust system and a controller system for controlling the exhaust system. The automotive exhaust system for a vehicle according to the present embodiment will now first be described. Fig. 1 is a schematic view showing the automotive exhaust system to which the exhaust gas purifying apparatus for an internal combustion engine according to the present invention is applied. As shown in Fig. 1, in the exhaust system, after two exhaust pipes 2a and 2b connected to an engine 1 are once emerged into a single pipe, the exhaust pipes are separated into two exhaust pipes 8a and 8b. Each of the exhaust pipes 8a and 8b is connected to a muffler 9a, 9b located in the rear side of the chassis. Catalysts 3a and 3b for purifying the exhaust gas are provided in the two exhaust gas pipes 2a and 2b. An adsorbent sleeve 4 is provided at a portion in which the exhaust pipes 2a and 2b are merged into one downstream of the catalysts 3a and 3b. As shown in Fig. 2, the interior of the adsorbent sleeve 4 is divided into two flow paths A and B. These flow paths A and B correspond to the first gas exhaust flow path and the second exhaust gas flow path according to the present invention, respectively. For instance, a zeolite system adsorbent 42 is provided in the flow path A as the adsorbent of the present invention. The adsorbent 42 adsorbs the hydrocarbon (HC) contained in the exhaust gas. A recirculation pipe 5 is connected to the downstream side of the adsorbent 42 in the flow path A. The recirculation pipe 5 is connected to a serge tank 11 provided in an intake pipe of the engine and is used for the exhaust gas recirculation (EGR). A flow rate control valve (EGR valve) 10 is mounted in the midway of the recirculation pipe 5. As shown in Fig. 3, the flow rate control valve 10 is provided with a stator 14 composed of a four-phase magnetically exciting coil 13, a magnetized cylindrical rotor 15 which rotates within the stator 14, and a stepper motor 17 having a rod 16 which threadedly engages with in the cylindrical rotor 15 and projects and retracts in accordance with the rotation of the cylindrical rotor 15. A movable plate 18 which is spring-biased in a direction in which the rod 16 is projected is mounted at the tip end of the rod 16 of the stepper motor 17. Furthermore, a valve 22 which is seated to the seat within the recirculation pipe 5 is provided at the tip end of a valve rod 19 which is in contact with or out of contact with the movable plate 18. In the thus constructed flow rate control valve 10, the movable plate 18 and the rod 16 are normally biased in the advance direction and the valve 22 of the valve rod 19 is seated to the seat to interrupt the flow path within the recirculation pipe 5. Then, when the electric supply is effected, in order, to each phase of the four-phase magnetically exciting coil 13 of the stepper motor 17, in the flow rate control valve 10, the cylindrical rotor 15 is rotated to thereby retract the rod 16. Corresponding to this movement, the valve 22 is opened. In this case, by controlling the order of the electric supply to each phase of the magnetically exciting coil 13, it is possible to change the prcjecting/retracting direction and the amount of the movement (opening degree of the valve 22) of the rod 16, and to control the flow path opening area of the recirculation pipe 5, i.e., the flow rate of the recirculation pipe 5. The recirculation pipe 5 and the flow rate control valve 10 realize the recirculation means according to the present invention. Also, a bypass valve 40 for selectively opening/closing the flow path A and the flow path B is mounted in the outlet portion C of the adsorbent sleeve 4. The bypass valve 40 is connected through a lever 43 and a diaphragm chamber 41 mounted outside the adsorbent sleeve 4. The lever 43 rotates about a furculum 44. When the end of the lever 43 on the side of the diaphragm chamber 41 is lowered, the bypass valve 40 is lifted by the end portion of the lever 43 on the side of the bypass valve 40. As a result, the flow path B is opened and the flow path A is closed. On the other hand, when the end of the lever 43 on the side of the diaphragm chamber 41 is lifted, the bypass valve 40 is lowered by the end portion of the lever 43 on the side of the bypass valve 40. As a result, the flow path A is opened and the flow path B is closed. Here, as shown in Fig. 1, a pipe 7 whose one end is connected to an intake pipe (not shown) of the engine is connected at the other end to the above-described diaphragm chamber 41 so that a negative pressure within the intake pipe may be introduced to the engine. In the midway of the pipe 7, there are connected a check valve 6 for allowing only the negative pressure to be applied from intake pipe side to the diaphragm chamber side, and a vacuum switching valve VSV for closing the pipe 7 when a current is supplied thereto and for closing the pipe 7 when a current is not supplied thereto. If no electric power is, for example, supplied to the vacuum switching valve VSV (during non-electric power supply), the vacuum switching valve VSV closes the pipe 7, so that the negative pressure to the diaphragm chamber 41 is interrupted and the diaphragm chamber 41 is released to the atmosphere. In this case, the end portion of the lever 43 on the side of the diaphragm chamber 41 is lowered by the spring force of the diaphragm per se. As a result, the end portion of the lever 43 on the bypass valve side lifts the bypass valve 40. As a result, the bypass valve 40 closes the flow path A and opens the flow path B. On the other hand, if an electric power is supplied to the vacuum switching valve VSV (during electric power supply), the vacuum switching valve VSV opens the pipe 7, so that the negative pressure is introduced to the diaphragm chamber 41. In this case, the diaphragm chamber 41 lifts the end portion of the lever 43 on the side of the diaphragm chamber 41. When the end portion of the lever 43 on the diaphragm chamber 41 is lifted, the lever 43 is rotated about the furculum 44, and the end portion of the lever 43 on the side of the bypass valve 40 lowers the bypass valve. The bypass valve 40 opens the flow path A and closes the flow path B. The above-described bypass valve 40, diaphragm chamber 41 and vacuum switching valve VSV realize the passage switching means according to the present invention. Subsequently, the controller system for controlling the above-described exhaust system will be described with reference to Fig. 4. The controller system includes a water temperature sensor 108, a throttle opening degree sensor 109, an air flow meter 110, and a crank position sensor 111 each of which is connected to an electronic control unit (ECU) 100. The water temperature sensor 108 realizes the water temperature detecting means according to the present invention and detects a temperature of cooling water for the engine 1. The throttle opening degree sensor 109 outputs a voltage in proportion to the opening degree of the throttle valve provided in the intake pipe of the engine 1. The output signal is used for discriminating whether or not the engine 1 is in the idle condition. The air flow meter 110 realizes the operational condition detecting means together with the crank position sensor 111 to be described later and outputs a voltage in proportion to an air amount Ga sucked to the engine 1. The crank position sensor 111 outputs a pulse signal for every predetermined angular rotation of the crank shaft. The output signal is used for calculating the revolutions per minute NE of the engine 1. Furthermore, the output signal of the crank position sensor 111 is used for calculating the intake air amount GN per one turn of the engine 1 together with the output signal of the air flow meter 110. The calculated RPM NE and the intake air amount GN per one turn are used as data representative of the load condition of the engine 1. Thus, the above-described position sensor 111 and the air flow meter 110 are used to realize the operational condition detecting means according to the present invention. The ECU 100 controls the above-described exhaust system on the basis of the signals from the respective sensors. For this, an ECU for a fuel injection control may be used or an ECU specialized for the exhaust gas purifying apparatus may be used. The ECU 100 is provided with CPU 101, ROM 102, RAM 103 and input/output (I/O) port 105. These are connected to bus 104. A drive circuit 107 and an A/D converter 106 are connected to the input port (I/O) 105. The above-described ROM 102 stores therein various control maps and application programs to be executed by the CPU 101. A control map B is shown in Fig. 5 which represents a relationship between a temperature THW of the cooling water and an coefficient kptemp. The coefficient kptemp is a compensation coefficient multiplied to the target step number specified by the map A. For instance, when the cooling water temperature THW is low as in the case where the engine 1 is started in the cold condition, the combustion of the engine 1 is unstable and if the large recirculation amount is effected, the combustion temperature of the engine 1 is lowered so that the torque is varied and the engine is stalled. It is therefore necessary to compensate for the recirculation amount toward the decreased side. On the other hand, the engine 1 combustion is stable as the cooling water temperature THW rises. Accordingly, it is necessary to compensate for the recirculation amount as the cooling water temperature rises. The above-described maps A and B and the CPU 101 realize the recirculation amount changing means according to the present invention. Furthermore, the ROM 102 stores, as a control map, a map C (not shown) representative of a relationship between the cooling water temperature THW when the engine is started and a period of time (hereinafter referred to as a catalyst activation time) from a time of the engine start until the catalysts 3a and 3b are activated. This map C and the CPU 101 constitute the activation discriminating means according to the present invention. Namely, the operation time period from the start of the engine 1 reaches the catalyst activation time, it is judged that the catalysts 3a and 3b are activated, whereas the operation time period of the engine is shorter than the catalyst activation time, it is judged that the catalysts 3a and 3b are not activated. The CPU 101 operates in accordance with the application program of the ROM 102 and controls the flow rate control valve 10 and the vacuum switching valve VSV through the above-described drive circuit 107. The RAM 103 stores therein, the signals from the temperature sensor 108, the throttle opening degree sensor 109, the air flow meter 110 and the crank position sensor 111 and the calculation results of the CPU 101 (for example, the intake air amount GN per one revolution, the engine RPM NE and the like). The signals from the respective sensors are rewritten as newest signals for every constant time period (for example, at every output of the crank position sensor 111). Also, the RAM 103 has a region for an idle identification flag Xidle for identifying whether the engine 1 is in the idle condition or not (in the idle condition: Xidle=1, in the off idle condition: Xidle=0), warming-up completion flag Xthwon representing whether the temperature of the cooling water exceeds the warming-up completion temperature or not (after warming-up completion: Xthwon=1, before warming-up completion Xthwon=0), and an EGR flag XEGR for identifying whether the execution conditions for the exhaust gas recirculation (EGR) are met or not (when the execution conditions are met: XEGR=1, when the execution conditions are not met: XEGR=0). These flags are set or reset by the CPU 101. For the method for judgement of the idle condition, there is provided a method for discriminating whether the signal from the throttle opening degree sensor 109 shows "0" or not (namely, whether the opening degree of the throttle valve is "0" or not), a method for discriminating whether the idle switch is turned on or not using an idle switch mounted on the throttle opening degree sensor 109, or the like. Then, for the method of judgement of the warming-up completion condition, there is a method for discriminating whether or not the signal from the temperature sensor 108 exceeds a threshold value (for example, 80°C) set in advance. Also, with respect to the execution conditions of the EGR, for instance, there are exemplified conditions whether a predetermined period of time has lapsed from the engine start or not, whether the warming-up of the engine has been completed or not, whether the RPM of the engine 1 is equal to or less than the predetermined RPM or not, whether the engine is in the off-idle condition or not, or the like. Furthermore, the RAM 103 has a region for storing therein the target step number read out from the map A of the ROM 102, or the coefficient kptemp read out from the map B. The A/D converter 106 converts the output signals from the water temperature sensor 108, the throttle opening degree sensor 109, and the air flow meter 110 from the analog signals into the digital signals and inputs the converted signals into the I/O port 105. The drive circuit 107 is connected to the flow rate control valve 10 and the vacuum switching valve VSV. Then, when the drive circuit 107 receives from the CPU 101 an electric supply order comparison signal for driving the flow rate control valve 10 up to the target step number, the drive circuit 107 drives the flow rate control valve 10 in accordance with the signal. Also, when the drive circuit 107 receives from the CPU 101 a switching signal between an electric supply and non-electric supply of the vacuum switching valve, the drive circuit 107 drives the vacuum switching valve VSV in accordance with the signal. The operation of the embodiment will now be described. The prerequisite operation of the exhaust gas purifying apparatus for an internal combustion engine in accordance with the embodiment will now be described. When the engine 1 is started in the cold state (when the catalysts 3a and 3b are not activated), the CPU 101 supplies the vacuum switching valve VSV with an electric power from the drive circuit 107. The vacuum switching valve VSV which is in the electric supply condition receiving the electric power opens the flow path A within the adsorbent sleeve 4 and closes the flow path B. Incidentally, in the start of the engine 1, the flow rate control valve 10 of the recirculation pipe 5 is kept under the fully closed condition. At this time, all the exhaust gas discharged from the catalysts 3a and 3b is discharged to the exhaust pipes 8a and 8b through the flow path A. The hydrocarbon (HC) which has not been oxidized by the catalysts is adsorbed to the adsorbent 42 in the flow path A. Also, in the engine start, the CPU 101 reads out the output signal (cooling water temperature) from the water temperature sensor 108. Then, the CPU 101 accesses the map C of the ROM 102, reads out the catalyst activation time corresponding to the cooling water temperature and compares the catalyst activation time with the operation time period from the engine start until the current point. When the operational time period of the engine 1 reaches the catalyst activation time, the CPU 101 interrupts the electric supply from the drive circuit 107 to the vacuum switching valve. The vacuum switch valve VSV to which the electric supply is interrupted and kept in the non-electric supply condition closes the flow path A and opens the flow path B within the adsorbent sleeve 4. In this case, the exhaust gas purified by the activated catalysts 3a and 3b is introduced into the exhaust pipes 8a and 8b through the flow path B within the adsorbent sleeve 4 and flows into the mufflers 9a and 9b. In this case, it is necessary to separate the hydrocarbon (HC), adsorbed in the adsorbent 42, away from the adsorbent and to perform the exhaust gas recirculation (EGR) to the serge tank 11 of the engine 1. However, if a large amount of exhaust gas recirculation is effected not in view of the operational condition of the engine 1, problems such as a torque variation or the like are induced. Inversely, if the amount of the exhaust recirculation is lowered, it is disadvantageous that it takes a long period of time to separate the hydrocarbon (HC). For this reason, it is preferable to control the amount of the recirculation exhaust gas in correspondence with the operational condition. Accordingly, in the embodiment, the CPU 101 executes the EGR control routine shown in Fig. 6. First of all, the CPU 101 discriminates whether or not the bypass valve 40 closes the flow path A and the opens the flow path B; namely whether or not the catalysts 3a and 3b are activated (step 701). In this case, if the CPU 101 judges that the bypass valve 40 opens the flow path A and closes the flow path B; namely that the catalysts 3a and 3b are not activated, the target opening degree (target step number) of the flow rate control valve 10 is set to "0" (step 708), and the closed condition of the flow rate control valve 10 is maintained (step 707). Namely, the recirculation is not performed. On the other hand, in the above-described step 701, if the CPU 101 judges that the bypass valve 40 closes the flow path A and at the same time opens the flow path B; namely, that the catalysts 3a and 3b are activated, the process is advanced to step 702. In step 702, the CPU 101 accesses the RAM 103 and reads out the intake air amount GN per one revolution of the engine 1 and the RPM NE. Then, the CPU 101 accesses the map A of the ROM 102, and reads out the target step number (opening degree of the flow rate control valve 10) corresponding to the intake air amount GN per one revolution and the RPM NE. Furthermore, the CPU 101 writes the read-out target step number as the target opening degree in the RAM 103. Subsequently, referring to the idle identification flag Xidle of the RAM 103, the CPU 101 discriminates whether the engine 1 is in the idle condition (Xidle=1) or in the off-idle condition (Xidle=0) (step 703). If the idle identification flag of the RAM 103 is Xidle=0, referring to the warming-up completion flag Xthwon of the RAM 103, the CPU 101 discriminates whether the warming-up of the engine 1 is completed (Xthwon=1) or the warming-up of the engine 1 is not completed (Xthwon=0) (step 704). If the warming-up completion flag of the RAM 103 is Xthwon=1, referring to the EGR flag XEGR of the RAM 103, the CPU 101 discriminates whether the EGR execution conditions are met (XEGR=1), or the EGR execution conditions are not met (XEGR=0) (step 705). In this case, if the EGR flag of the RAM 103 is XEGR=1, the CPU 101 inputs into the drive circuit 107 the supply order comparison signal corresponding to the target opening degree (target step number) written in the RAM 103 and drives the flow rate control valve 10 to the target step number (step 707). Namely, the recirculation pipe 5 is opened in correspondence with the number of the target step number. The process of the above-described steps 701, 702, 703 and 707 shows the regular exhaust gas recirculation (EGR) to be performed when the EGR execution conditions are met after the warming-up of the engine 1). Also, in the above-described step 703, in the case where the idle identification flag of the RAM 103 is Xidle=1 (under the condition that the engine is kept in the idle condition), the CPU 101 jumps to the above-described step 707, inputs into the drive circuit 107 the supply order comparison signal corresponding to the target opening degree (target step number) written in the RAM 103 and drives the flow rate control valve 10 to the target step number. Namely, the recirculation pipe 5 is opened in correspondence with the number of the target step number. Furthermore, in the above-described step 704, in the case where the warming-up completion flag of the RAM 103 is Xthwon=0, or in the above-described step 705, the EGR flag of the RAM 103 is XEGR=0 (in the case where the EGR execution conditions are not met), the CPU 101 reads out the cooling water temperature THW stored in the RAM 103, and thereafter accesses the map B of the ROM 102. Then, the CPU 101 reads out the coefficient kptemp corresponding to the cooling water temperature THW from the map B. Furthermore, the CPU 101 multiplies the target opening degree (target step number) of the RAM 103 by the coefficient kptemp to thereby calculate the new target opening degree (new target step number) (step 706). Then, the CPU 101 inputs into the drive circuit 107 the supply order comparison signal corresponding to the newly calculated target opening degree (new target step number) in the same order as in the above-described step 707, and drives the flow rate control valve 10 up to the target step number. As a result, in the case where the engine 1 warming-up has not been completed, or also in the case where the EGR execution conditions are not met, it is possible to perform the exhaust gas recirculation corresponding to the level of the cooling water temperature THW and the load condition of the engine 1 at that time. The relationship between the load condition of the engine 1 and the exhaust gas recirculation in the off-idle condition is shown in Fig. 7. In Fig. 7, the ordinate axis represents the intake air amount GN per one revolution of the engine 1 and the abscissa axis represents the RPM NE of the engine 1. Then, the region A in Fig. 7 shows a region where the normal exhaust gas recirculation (EGR) is to be performed, and the region B shows a region where the exhaust gas recirculation according to the present invention is to be performed. In the normal exhaust gas recirculation (EGR), unless the conditions of the steps 704 and 705 shown in Fig. 7 are met, the exhaust gas recirculation is not performed. However, in the exhaust gas purifying apparatus for an internal combustion engine according to the embodiment, even if the conditions of the steps 704 and 705 are not met, it is possible to perform the exhaust gas recirculation to an extent corresponding to the engine condition at that time the load condition and cooling water temperature THW). In other words, in the exhaust gas purifying apparatus for an internal combustion engine according to the embodiment, since the exhaust gas recirculation is performed even in the load region (region B in Fig. 7) where the exhaust gas recirculation (EGR) is not to be performed in the normal exhaust gas recirculation process, it is possible to perform the exhaust gas recirculation in the broad range. It is possible to early separate and recirculate the unburnt gas components of the adsorbent 42. Also, in the embodiment, in the region B in Fig. 7, since the exhaust gas recirculation is performed to the extent corresponding to the condition of the engine 1, it is possible to stabilize the engine 1. Thus, in accordance with the embodiment, it is possible to perform the exhaust gas recirculation without unstabilizing the operational condition in the broad range of the operational condition of the engine. The temperature variation of the adsorbent 42 in the case where only the regular exhaust gas recirculation (EGR) process is only performed, and the temperature variation of the adsorbent 42 in the case where the exhaust gas recirculation process according to the embodiment is performed are shown in Fig. 8 in which the ordinate axis represents the temperature of the adsorbent 42 and the abscissa axis represent the time. In the case where only the normal exhaust gas recirculation (EGR) process is performed, since all the exhaust gas is caused to flow through the adsorbent 42 from the start of the engine 1 (i in Fig. 8) until the time when the catalysts 3a and 3b are activated (ii in Fig. 8), the temperature rise rate of the adsorbent 42 is high. However, since the exhaust gas is not caused to flow through the adsorbent 42 from the time when the catalysts 3a and 3b are activated (ii in Fig. 8) until the time of completion of the warming-up (iii in Fig. 8), the adsorbent 42 is heated only the heat transfer from the exhaust gas that flows through the flow path B, so that it takes a long time until the temperature reaches the separation temperature of the hydrocarbon (HC). In contrast thereto, in the exhaust gas recirculation according to the embodiment, since the EGR is started from the time when the catalysts 3a and 3b are activated (ii in Fig. 8, before the completion of the warming-up), even before the warming-up, the part of the exhaust gas (the amount of the exhaust gas corresponding to the condition of the engine 1) is caused to flow through the adsorbent 42. In this case, the adsorbent 42 is directly heated by the part of the exhaust gas and the temperature reaches the separation temperature of the hydrocarbon (HC) for a short period of time. In the exhaust gas purifying apparatus for an internal combustion engine according to the embodiment, it is possible to perform the exhaust gas recirculation from the early stage of the start of the engine 1 in view of the condition of the engine 1, and it is therefore possible to complete the regeneration process of the adsorbent in the earlier stage. Thus, a fear of the engine stall before the completion of the separation of the hydrocarbon (HC) is suppressed and it is thus possible to avoid the degradation of the performance of the adsorbent. Embodiment 2 In the foregoing embodiment, the case where the compensation is effected in correspondence with the target opening degree determined in accordance with the load condition of the engine 1 or the cooling water temperature THW for the target opening degree has been explained. In this embodiment, the case where the target opening degree is compensated for by feeding back the combustion condition of the engine 1 will now be described. For the method for judging the combustion condition of the engine 1, it is possible to exemplify the case where a period of time needed for the crank shaft to rotate through a predetermined angle immediately after the ignition of the mixture is monitored so that the combustion condition of the engine 1 is judged. For instance, a MPU type magnetic pickup is used as a crank position sensor 111 for outputting a signal whenever the crank shaft of the engine 1 is rotated through 30° from a top dead center, and a period T30Dn of time from the time when the crank position sensor 111 detects the top dead center until the time when a next signal (outputted when the crank shaft is rotated from the top dead center through 30°) is outputted is written in the RAM 103 of the ECU 10. Then, a difference DTDCP (=T30Dn-T30Dn-1) between the time T30Dn-1 which has been expended for the previous rotation and the time T30Dn which has been expended for the current rotation is calculated. In this case, if the current combustion condition is worse than the previous combustion condition, the difference DTDCP shows a positive value. However, since the value of the difference DTDCP shows a positive value depending upon the engine load change in some cases, in order to distinguish the case where the combustion condition is worse and the case where the engine load is changed, it is necessary to clarify the value that the difference DTDCP takes according to the change of the engine load.

Claims: An exhaust gas purifying apparatus for an internal combustion engine, comprising: an exhaust pipe connected to the internal combustion engine; a catalyst mounted on said exhaust pipe; a first exhaust flow path and a second exhaust flow path branched downstream of said catalyst; a path switching means for introducing the exhaust gas into at least one of said first exhaust flow path and said second exhaust flow path; an adsorbing means disposed in said first exhaust flow path for adsorbing unburned gas components contained in the exhaust gas; a recirculating means for recirculating at least part of the exhaust gas discharged from said absorbing means to an intake side of the internal combustion engine; an operational condition detecting means for detecting an operational condition of the internal combustion engine; a recirculation amount changing means for changing an amount of the exhaust gas to be recirculated by said recirculating means in accordance with the operational condition detected by said operational condition detecting means; and a water temperature detecting means for detecting the temperature of cooling water for the internal combustion engine, wherein said recirculating amount changing means compensates for the amount of the exhaust gas to be recirculated in accordance with the temperature of cooling water detected by said water temperature detecting means, wherein said recirculating amount changing means compensates for the recirculating amount so that the amount of the exhaust gas to be recirculated is decreased in the case where the temperature of the cooling water is lower than a predetermined temperature. The exhaust gas purifying apparatus according to claim 1, further comprising an activation discriminating means for discriminating whether or not the catalyst is activated, wherein said recirculating means starts the recirculation of the exhaust gas under the condition that the activation discriminating means judges that the catalyst is activated. The exhaust gas purifying apparatus according to claim 2, wherein said activation discriminating means judges that the catalyst is activated when an operation time period from the start of the internal combustion engine is equal to or longer than a predetermined time period, and judges that the catalyst is not activated when the operation time period from the start of the internal combustion engine is shorter than the predetermined time period. An exhaust gas purifying method for an internal combustion engine, comprising the following steps of: introducing exhaust gas from the internal combustion engine into at least one of two exhaust flow paths branched downstream of a catalyst; adsorbing unburned gas components contained in the exhaust gas by an adsorbing means provided in said one of exhaust flow paths; recirculating at least part of the exhaust gas from said adsorbing means to an intake side of the internal combustion engine; detecting an operational condition of the internal combustion engine; changing an amount of the exhaust gas to be recirculated to the intake side of the internal combustion engine in accordance with the operational condition of the internal combustion engine; and a water temperature detecting means for detecting the temperature of cooling water for the internal combustion engine, wherein said recirculating amount changing means compensates for the amount of the exhaust gas to be recirculated in accordance with the temperature of cooling water detected by said water temperature detecting means, wherein said recirculating amount changing means compensates for the recirculating amount so that the amount of the exhaust gas to be recirculated is decreased in the case where the temperature of the cooling water is lower than a predetermined temperature.

Inventors: ITOU TAKAAKI, TANAKA HIROSHI, ITOU, TAKAAKI, TANAKA, HIROSHI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1997
Main classification: F01N3
IPC codes: B01D53, F02D21, F01N3, F02M25