Patent Publication Number: US-2009229256-A1

Title: Control unit for exhaust gas purifying apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a control unit of an exhaust gas purifying apparatus that purifies exhaust gas generated by an engine using a catalyst. 
     BACKGROUND ART 
     An exhaust gas purifying apparatus that purifies exhaust gas caused by an internal combustion engine (hereinafter, referred to also as an engine) such as a diesel engine includes, for example, a NOx storage reduction catalyst and a particulate filter, which collects particulate matter (hereinafter, referred to as PM) from the exhaust gas. 
     The NOx storage reduction catalyst stores NOx if the content of oxygen in the exhaust gas is great and reduces NOx to NO 2  or NO and releases the substance if the content of oxygen in the exhaust gas is small and the amount of reducing agent (for example, unburned elements of fuel (HC)) is great. As the particulate filter (hereinafter, referred to as the filter), which collects PM, a DPF (diesel particulate filter) or a DPNR (diesel particulate-NOx reduction system) catalyst is employed. 
     The exhaust gas purifying apparatus, which includes the NOx storage reduction catalyst and the filter arranged in an exhaust passage, involves various types of control (hereinafter, referred to generally as catalyst control) including NOx reduction control, sulfur release control, and PM elimination control. 
     In the NOx reduction control, which is one type of the catalyst control, for example, fuel is fed to the NOx storage reduction catalyst. NOx stored in the catalyst is thus caused to react with the fuel elements (HC) and reduced through such reaction. 
     In the sulfur release control, the NOx storage reduction catalyst is recovered from sulfur poisoning by desorbing SOx from the NOx storage reduction catalyst. Specifically, to desorb sulfur elements from the NOx storage reduction catalyst, it is effective to expose the NOx storage reduction catalyst in the atmosphere in which the air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio after the catalyst has been heated to a predetermined temperature (for example, 600 to 700° C.). Thus, in this regard, fuel is supplied to the catalyst after the catalyst has been heated through, for example, switching of combustion states of the engine. In this manner, the fuel is exposed to the atmosphere of a rich air-fuel ratio, and the sulfur components are desorbed from the catalyst. Also, the sulfur release control may involve supply of fuel to the catalyst to increase the temperature of the catalyst. 
     In the PM elimination control, the temperature of the catalyst (catalyst bed temperature) is raised by, for example, regulating the combustion state of the engine. This promotes oxidization (burning) of the PM deposited on the catalyst such as a DPNR catalyst. Specifically, to burn and remove the PM from the catalyst according to the PM elimination control, it is necessary to increase the temperature of the catalyst to a predetermined temperature (which is, for example, approximately 600 to 700° C.). Thus, there are cases in which the PM elimination control involves supply of fuel to the catalyst. 
     As a type of diesel engine, there is a V type multicylinder engine that includes left and right banks each having a plurality of cylinders. The sets of the cylinders, each of which forms the corresponding one of the banks, are connected to exhaust passages of different systems. The V type multicylinder engine includes an intake manifold, or a portion of an intake passage, which is provided commonly for the left and right banks to maintain the amounts of the intake air supplied to the banks at equal levels. This type of engine also performs the PM elimination control, the sulfur release control, and the NOx reduction control, which have been described so far, in response to a request for the catalyst control. 
     Patent Document 1, which is listed below, describes a method as a technique related to the catalyst control of the V type multicylinder engine. The technique of Patent Document 1 performs control in such a manner as to suppress variation of flow rates of exhaust gas between multiple cylinders. This prevents delay in recovery of exhaust gas purifying performance and wasteful consumption of energy. 
     In the V type multicylinder engine, the flow rates of the exhaust gas flowing to the catalysts of the systems of the left and right banks may vary between the systems due to a difference (which is, for example, varied sizes of fine pores defined in the catalysts, varied flow characteristics, and varied performances of turbochargers) between the systems. Such variation of the exhaust gas flow rates varies the speeds of deterioration of the catalysts depending on the deposit amount of PM and the degree of sulfur poisoning between the left and right systems. As a result, recovery of the catalyst, or the catalyst control, is requested at different timings between the left and right banks. In this case, if the catalyst control is requested only for one of the systems, it is impossible to switch the combustion state of the bank of the system requesting such control separately from the other system. Specifically, as has been described, the intake manifold, which forms a portion of the intake passage, of the V type diesel engine is provided commonly for the banks in order to maintain the intake air amounts of the banks at equal levels. This makes it impossible to switch the combustion state of one of the banks independently from the other through air-fuel ratio control (rich control) performed by adjusting a throttle valve (an intake air throttle valve). 
     In other words, if the catalyst control is requested for one of the left and right banks, the combustion state of the engine needs to be switched despite the fact that the recovery is not requested for the other bank. Specifically, the PM elimination control and the sulfur release control each involve PM elimination combustion and sulfur release combustion. This increases the fuel consumption. Thus, if the PM elimination/sulfur release combustion is carried out to recover the catalyst of the system that requests the catalyst control every time such request is generated, the recovery is performed for repeated times and the fuel consumption is increased. 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-036663 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a control unit that suppresses increase of fuel consumption caused by PM elimination and sulfur release of an exhaust gas purifying apparatus in which different exhaust systems are connected to a plurality of sets of cylinders of an engine and an exhaust gas purifying catalyst is provided in each of the exhaust systems. 
     To achieve the foregoing objective, the present invention provides a control unit of an exhaust gas purifying apparatus used in an internal combustion engine having a plurality of sets of cylinders. Different exhaust systems are each connected to one of the sets of the cylinders, and the exhaust gas purifying apparatus includes catalysts each provided in one of the exhaust systems to purify exhaust gas. The control unit performs a PM elimination control or a sulfur release control on the catalysts using a common procedure. 
     The present invention also provides a method for controlling an exhaust gas purifying apparatus used in an internal combustion engine having a plurality of sets of cylinders. Different exhaust systems are each connected to one of the sets of the cylinders. The method includes: purifying exhaust gas by means of catalysts each provided in one of the exhaust systems; and performing a PM elimination control or a sulfur release control on the catalysts using a common procedure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a schematic diagram showing one embodiment of the present invention; 
         FIG. 2  is a flowchart representing the content of PM elimination/sulfur release control executed by an ECU; 
         FIG. 3  is a flowchart representing the content of NOx reduction control performed by the ECU; 
         FIG. 4  is a timing chart representing the energization duration, the requested number of multiple addition cycles, and the addition interval for addition of fuel; 
         FIG. 5  is a map representing a bed temperature correction coefficient; and 
         FIG. 6  is a schematic view showing an engine having four systems of exhaust passages. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     One embodiment of the present invention will now be described with reference to the attached drawings. 
     Engine 
     An example of an engine in which the present invention is employed will hereafter be explained with reference to  FIG. 1 . 
     In this example, an engine  1  is a V type eight cylinders diesel engine having a left bank  2 L and a right bank  2 R, each of which is. configured by four cylinders  3 . The banks  2 L,  2 R are arranged in a V shaped manner. The engine  1  includes injectors  4 , each of which injects fuel directly into a combustion chamber of the corresponding one of the cylinders  3  (cylinders #1, #2, #3, #4, #5, #6, #7, #8). Each injector  4  is an electromagnetic on-off valve that opens when energized (supplied with voltage). The timings at which the injectors  4  become open are regulated by an ECU (electronic control unit)  100 . 
     The engine  1  has an intake manifold commonly provided for the left and right banks  2 L,  2 R, and an exhaust manifold  11 L and an exhaust manifold  11 R, which are arranged for the left bank  2 L and the right bank  2 R, respectively. In the following description, the exhaust system of the left bank  2 L may be referred to as a first system and the exhaust system of the right bank  2 R may be referred to as a second system. 
     An intake passage  5  is connected to the engine  1  to introduce intake air to the cylinders  3 . An air cleaner  50  is connected to the intake passage  5 . The intake passage  5  is branched into a branch line  5 L and a branch line  5 R at a position downstream from the air cleaner  50 . A compressor portion  7   a  of a turbocharger  7 L and a compressor portion  7   a  of a turbocharger  7 R are arranged in the branch line  5 L and the branch line  5 R, respectively. An intercooler  6  is connected to the branch lines  5 L,  5 R at a position downstream from the compressor portions  7   a.  The intercooler  6  is connected to the intake manifold  10  through a common intake passage  5 C. An electronically controlled throttle valve (an intake air throttle valve)  9  is provided in the common intake passage  5 C. EGR lines  14 L,  14 R, which will be explained later, are connected to the common intake passage  5 C at a position downstream from the throttle valve  9 . An air flow meter  8 L is arranged in the branch line  5 L at a position upstream from the compressor portion  7   a  of the turbocharger  7 L. An air flow meter  8 R is arranged in the branch line  5 R at a position upstream from the compressor portion  7   a  of the turbocharger  7 R. 
     A fuel adding valve  12 L is arranged in the exhaust manifold  11 L connected to the left bank  2 L. A fuel adding valve  12 R is connected to the exhaust manifold  11 R connected to the right bank  2 R. Each of the fuel adding valves  12 L,  12 R is an electromagnetic on-off valve that opens when energized (supplied with voltage) to add fuel to the exhaust system of the corresponding one of the left and right banks  2 L,  2 R. The timings at which each fuel adding valve  12 L,  12 R becomes open are regulated by the ECU  100 . 
     An exhaust passage  13 L and an exhaust passage  13 R are connected to the exhaust manifold  11 L and the exhaust manifold  11 R, respectively. A turbine portion  7   b  of the turbocharger  7 L is provided in the exhaust passage  13 L. A turbine portion  7   b  of the turbocharger  7 R is provided in the exhaust passage  13 R. Each of the turbochargers  7 L,  7 R is a variable nozzle type turbocharger and has a variable nozzle vane mechanism at the side corresponding to the corresponding one of the turbine portions  7   b.  By adjusting the opening degree of the variable nozzle vane mechanism, charging pressure of the engine  1  is changed. The ECU  100  regulates the opening degree of the variable nozzle vane mechanism. 
     An EGR line  14 L and an EGR line  14 R connect the exhaust manifold  11 L and the exhaust manifold  11 R, respectively, to the common intake passage  5 C. The EGR line  14 L receives an EGR cooler  15 L cooling EGR gas and an EGR valve  16 L, which adjusts the flow rate of EGR. The EGR line  14 R accommodates an EGR cooler  15 R cooling the EGR gas and an EGR valve  16 R adjusting the flow rate of EGR. The opening degree of each of the EGR valves  16 L,  16 R is adjusted by the ECU  100 . 
     An NSR (NOx storage reduction) catalyst  17 L, a DPNR catalyst  18 L, and a sweeper  19 L are arranged in the exhaust passage  13 L at positions downstream from the turbocharger  7 L. An NSR catalyst  17 R, a DPNR catalyst  18 R, and a sweeper  19 R are arranged in the exhaust passage  13 R at positions downstream from the turbocharger  7 R. 
     Each of the NSR catalysts  17 L,  17 R is a NOx storage reduction catalyst formed by, for example, alumina (Al 2 O 3 ) as a carrier supporting alkaline metal such as potassium (K), sodium (Na), lithium (Li), or cesium (Cs), alkaline earth such as barium (Ba) or calcium (Ca), rare earth such as lantern (La) and yttrium (Y), and precious metal such as platinum (Pt). 
     Each NSR catalyst  17 L,  17 R stores NOx if the content of oxygen in the exhaust gas is great and reduces NOx to NO 2  or NO, and releases the substance if the content of oxygen in the exhaust gas is small and the content of reducing agents (such as unburned components of fuel (HC)) is great. The NOx released as No 2  or NO is further reduced to N 2  through rapid reaction with HC or CO contained in the exhaust gas. Through reduction of NO 2  or NO, HC and CO are oxidized to H 2 O or CO 2 . 
     Each of the DPNR catalysts  18 L,  18 R is formed by, for example, a porous ceramic structure supporting a NOx storage reduction catalyst and collects PM from the exhaust gas as PM passes through a porous wall. If the air-fuel ratio of the exhaust gas is lean, NOx contained in the exhaust gas is stored by the NOx storage reduction catalyst. If the air-fuel ratio is rich, the stored NOx is reduced and released. Each DPNR catalyst  18 L,  18 R supports a catalyst (for example, an oxidation catalyst including precious metal such as platinum as a main component) that oxidizes and burns the collected PM. Each of the sweeper  19 L,  19 R is an oxidation catalyst and oxidizes HC and CO to purify the exhaust gas. 
     A first exhaust gas temperature sensor  21 L is arranged between the NSR catalyst  17 L and the DPNR catalyst  18 L. A first exhaust gas temperature sensor  21 R is provided between the NSR catalyst  17 R and the DPNR catalyst  18 R. A second exhaust gas temperature sensor  22 L and an air-fuel ratio sensor  23 L are provided downstream from the DPNR catalyst  18 L. A second exhaust gas temperature sensor  22 R and an air-fuel ratio sensor  23 R are provided downstream from the DPNR catalyst  18 R. A pressure difference sensor  24 L detects the difference in pressure (upstream-downstream pressure difference) between the upstream side and the downstream side of the DPNR catalyst  18 L. A pressure difference sensor  24 R detects the difference in pressure (upstream-downstream pressure difference) between the upstream side and the downstream side of the DPNR catalyst  18 R. Detection signals generated by the first exhaust gas temperature sensors  21 L,  21 R, the second exhaust gas temperature sensors  22 L,  22 R, the air-fuel ratio sensors  23 L,  23 R, and the pressure difference sensors  24 L,  24 R are input to the ECU  100 . 
     ECU 
     The ECU  100  includes a CPU, a ROM, a RAM, and a backup RAM. The ROM stores various control programs and maps, with reference to which the control programs are executed. The CPU performs calculation procedures in accordance with the control programs and the maps, which are stored in the ROM. The RAM is a memory that temporarily stores results of calculations by the CPU and data provided by the sensors. The backup memory is a non-volatile memory that stores data to be maintained after the engine  1  is stopped. 
     As illustrated in  FIG. 1 , the air flow meters  8 L, BR, the first exhaust gas temperature sensors  21 L,  21 R, the second exhaust gas temperature sensors  22 L,  22 R, the air-fuel ratio sensors  23 L,  23 R, the pressure difference sensors  24 L,  24 R, a coolant temperature sensor that detects the temperature of coolant of the engine  1 , a crank position sensor that detects the speed of the engine  1 , and various other sensors including an accelerator pedal position sensor are connected to the ECU  100 . Based on the outputs provided by the above-listed sensors, the ECU  100  carries out various control procedures by controlling operations of the injectors  4 , the throttle valve  9 , the variable nozzle vane mechanisms of the turbochargers  7 L,  7 R, and the EGR valves  16 l,  16 R. Also, the ECU  100  performs the following catalyst control. 
     Catalyst Control 
     The ECU  100  performs PM elimination control, sulfur release control, and NOx reduction control. Specifically, in the PM elimination control, PM deposited on the DPNR catalysts  18 L,  18 R are oxidized. In the sulfur release control, the NOx storage reduction catalysts of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R are recovered from S poisoning. In the NOx reduction control, NOx stored in the NOx storage reduction catalysts of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R are reduced. In the following, the PM elimination control, the sulfur release control, and the NOx reduction control will be explained. 
     [PM Elimination Determination] 
     The ECU  100  estimates the deposit amount of PM deposited on the DPNR catalysts  18 L,  18 R. In one method of such estimation, a map is defined from PM discharge amounts of the engine corresponding to the engine speed and the fuel injection amount, which are determined in advance through tests or the like. The PM deposit amount is estimated by integrating the PM discharge amounts of the engine, which is obtained with reference to the map. 
     In another method for estimating the PM deposit amount, the PM deposit amount is estimated based on an integrated value of the intake air amount. In this method, the PM deposit amount is estimated using one of the DPNR catalyst  18 L,  18 R exhibiting the greater PM deposit amount as a reference. This provides an estimated PM deposit amount corresponding to a greatest possible value, in order to prevent incomplete burning of the PM deposited on the DPNR catalysts  18 L,  18 R. Specifically, between the DPNR catalysts  18 L,  18 R, the catalyst representing a smaller value of the intake air amount (the greater PM deposit amount), which is determined based on detection signals of the air flow meters  8 L,  8 R arranged in the branch lines  5 L,  5 R of the intake passage  5 , is used as the reference. The PM deposit amount is estimated by integrating the intake air amounts of the reference catalyst. 
     The ECU  100  determines that the DPNR catalysts  18 L,  18 R needs to be immediately recovered if the estimated PM amount exceeds a predetermined reference value (a threshold deposit amount). At this stage, the ECU  100  performs the PM elimination control, which will be explained later. 
     The ECU  100  monitors signals output by the pressure difference sensors  24 L,  24 R arranged in the first and second systems. The ECU  100  compares the upstream-downstream pressure differences of the DPNR catalysts  18 L,  18 R obtained from the output signals of the pressure difference sensors  24 L,  24 R with a predetermined threshold value. If the upstream-downstream pressure difference of either one of the DPNR catalysts  18 L,  18 R exceeds the threshold value before the estimated PM deposit amount reaches the reference value, the ECU  100  starts the PM elimination at this stage. 
     [Sulfur Release Determination] 
     The ECU  100  estimates the S poisoning amounts of the NOx storage reduction catalysts of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R. In one method of such estimation, a map is defined from the S poisoning amounts corresponding to the engine speed and the fuel injection amount, which are determined in advance through tests or the like. The S poisoning amounts are estimated by integrating the S poisoning amount, which is obtained with reference to the map. The ECU  100  determines that recovery from S poisoning needs to be immediately performed if an estimated value of the S poisoning amount exceeds a predetermined value (a threshold estimation amount). The ECU  100  then carries out the sulfur release control, which will be described later. 
     [PM Elimination/Sulfur Release Control] 
     The PM elimination/sulfur release control, which is carried out by the ECU  100 , will now be explained with reference to the flowchart of  FIG. 2 . The PM elimination/sulfur release control routine is performed repeatedly at predetermined intervals. 
     In step ST 1 , it is determined whether the PM elimination or the sulfur release needs to be immediately carried out using the above-described determination method. If the determination is negative, the routine is suspended. If positive determination is made in step ST 1 , it is determined whether the control that has been determined to need to be carried out corresponds to the “PM elimination control” in step ST 2 . If it is determined that the “PM elimination control” needs to be immediately performed, step ST 3  is carried out. If the determination of step ST 2  is negative, it is determined that the “sulfur release control” needs to be immediately carried out and step ST 11  is performed. 
     In step ST 3 , the catalyst bed temperatures of the DPNR catalysts  18 L,  18 R are estimated using the output signals of the first exhaust gas temperature sensors  21 L,  21 R. It is then determined whether the lower value of these estimated catalyst bed temperatures corresponds to a temperature required for the PM elimination (which is, for example, approximately 350° C.). If the determination of step ST 3  is positive, it is determined that the PM elimination can be carried out smoothly. Step ST 5  is then performed. 
     If the determination of step ST 3  is negative, the combustion state of the engine  1  is switched (to a PM elimination combustion mode) in step ST 4 , in order to increase the catalyst bed temperatures. Step ST 5  is then carried out. In the PM elimination combustion mode, operations of the first and second systems are controlled in accordance with a common procedure. To switch to the PM elimination combustion mode, the air-fuel ratio (A/F) may be decreased by reducing the intake air amount by means of the throttle valve  9 . Alternatively, in combination with such method, the EGR amount may be increased or the fuel injection timings may be retarded. 
     Next, in step ST 5 , a PM elimination amount is calculated. The PM elimination amount is obtained with reference to a map defined through tests and calculations using the catalyst bed temperatures and a PM oxidization speed as parameters. To prevent incomplete burning of the PM in the DPNR catalysts  18 L,  18 R, the PM elimination amount is determined using the estimated catalyst bed temperature value of one of the catalysts exhibiting the lower catalyst bed temperature, the DPNR catalyst  18 L of the first system or the second DPNR catalyst  18 R of the second system. A requested fuel addition amount is then obtained from the calculated PM elimination amount. Based on the requested fuel addition amount, energization durations (fuel adding durations) of the fuel adding valves  12 L,  12 R, a requested number of multiple addition cycles, and an addition interval (see  FIG. 4 ) are determined (step ST 6 ). 
     In step ST 7 , based on the energization duration, the requested number of multiple addition cycles, and the addition interval, which have been obtained in step ST 6 , operation of the fuel adding valve  12 L of the first system and operation of the fuel adding valve  12 R of the second system are controlled in accordance with a common procedure to carry out the PM elimination. In other words, the PM elimination control is carried out in the first and second systems in accordance with the common procedure. 
     In step ST 8 , it is determined whether a condition for ending the PM elimination control is satisfied in step ST 8 . Specifically, it is determined whether the amount of the fuel added since starting of the PM elimination control has reached the requested fuel addition amount. If the determination is positive, the fuel addition is ended and the routine is also suspended. 
     If the determination of step ST 2  is negative and it is determined that the “sulfur release control” needs to be immediately carried out, the catalyst bed temperatures of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R are estimated based on the output signals of the first exhaust gas temperature sensors  21 L,  21 R in step ST 11 . It is then determined whether the lowest value of the estimated bed temperature values reaches a temperature required for the sulfur release (which is, for example, approximately 350° C.). If such determination is positive, it is determined that the sulfur release should be carried out smoothly and step ST 13  is performed. 
     If the determination of step ST 11  is negative, the combustion state of the engine  1  is switched (to a sulfur release combustion mode) in step ST 12 , so as to raise the catalyst bed temperatures. Step ST 13  is then carried out. In the sulfur release combustion mode, operations of the first and second systems are controlled in accordance with a common procedure. To switch to the sulfur release combustion mode, the air-fuel ratio (A/F) may be decreased by reducing the intake air amount by means of the throttle valve  9 . Alternatively, in combination with such method, the EGR amount may be increased or the fuel injection timings may be retarded. 
     In step ST 13 , it is determined whether the catalyst bed temperatures of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R, which are estimated based on the output signals of the first exhaust gas temperature sensors  21 L,  21 R, and the air-fuel ratio obtained from the output signals of the air-fuel ratio sensors  23 L,  23 R satisfy a rich spike condition (fuel addition condition), based on which the recovery from S poisoning is carried. out. If the determination of step ST 13  is positive, step ST 15  is performed. In step ST 13 , determination is performed using the lowest value of the estimated bed temperatures of the catalysts as the reference catalyst bed temperature. Further, the higher value of the air fuel-ratios of the first and second systems is employed as the air-fuel ratio in such determination. 
     If the determination of step ST 13  is negative, fuel addition (opening of the fuel adding valves  12 L,  12 R) is carried out in step ST 14  in order to adjust the catalyst bed temperatures. In this manner, the catalyst bed temperature is increased and the air-fuel ratio is enriched in such a manner as to satisfy the condition for carrying out the recovery from S poisoning. Step ST 15  is then performed. The rich spike condition for permitting the recovery from S poisoning is, for example, that the catalyst bed temperature is 350° C. or greater and the air-fuel ratio has reached  22 . 
     Subsequently, in step ST 15 , a sulfur release amount is calculated. The sulfur release amount is obtained with reference to a map defined in advance through tests and calculations using the catalyst bed temperatures and an S poisoning reduction speed as parameters. To maximally suppress thermal deterioration of the catalysts and ensure sufficient sulfur release, the sulfur release amount is determined using an average value of the estimated catalyst bed temperature of the DPNR catalyst  18 L of the first system and the estimated catalyst bed temperature of the DPNR catalyst  18 R of the second system. Next, in step ST 16 , the requested fuel addition amount is determined based on the sulfur release amount obtained in step ST 15 . Based on the requested fuel addition amount, the energization duration (the fuel adding duration) of the fuel adding valves  12 L,  12 R, the requested number of multiple addition cycles, and the addition interval (see  FIG. 4 ) are calculated. 
     In step S 17 , based on the energization duration, the requested number of multiple addition cycles, and the addition interval, which are calculated in step ST 16 , operation of the fuel adding valve  12 L of the first system and operation of the fuel adding valve  12 R of the second system are controlled in accordance with a common procedure. That is, the fuel is intermittently added to the exhaust gas by the fuel adding valves  12 L,  12 R at constant time intervals. In this manner, the sulfur release is executed by performing the rich spike, in which the exhaust gas in the vicinity of the NOx storage reduction catalysts is temporarily held in a state in which the oxygen content is small and the content of unburned fuel component is great. In other words, the sulfur release control is performed in the first system and the second system in accordance with the common procedure. 
     It is then determined whether a condition for ending the sulfur release control is satisfied in step ST 18 . Specifically, it is determined whether the amount of the fuel added since starting of the sulfur release control has reached the requested fuel addition amount. If the determination is positive, the rich spike is ended and the routine is also suspended. 
     If the PM elimination and the sulfur release both need to be immediately carried out in the above-described PM elimination/sulfur release control, the PM elimination control, for example, is performed by priority. 
     As has been described, according to the PM elimination/sulfur release control of this example, regardless of the states of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R of the first and second systems, the PM elimination combustion or the sulfur release combustion is carried out for both of the first and second systems in accordance with the common procedure. This reduces the number of recovery cycles compared to a case in which the PM elimination/sulfur release control is performed each time the catalyst control is requested for the catalyst of the first system or the second system. The fuel consumption is thus prevented from increasing due to the PM elimination/sulfur release. 
     Further, the PM elimination is performed using the catalyst exhibiting the lower catalyst bed temperature (the greater PM deposit amount) of the DPNR catalysts  18 L,  18 R of the systems as the reference. The PM is thus completely burned on both DPNR catalysts  18 L,  18 R, preventing incomplete burning of PM. Also, the sulfur release is performed using the average value of the catalyst bed temperatures of the multiple catalysts (the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R) of the first and second systems as the reference, to suppress thermal deterioration of the catalysts and ensure effective release of sulfur. This minimizes the thermal deterioration of each catalyst and allows sufficient release of sulfur. 
     Also, as has been described, the PM elimination is performed using the catalyst exhibiting the lowest catalyst bed temperature as the reference. The sulfur release is carried out using the average of the catalyst bed temperatures of the catalysts. In this manner, the fuel addition for the PM elimination/sulfur release is carried out in both systems in accordance with the common procedure. This further suppresses increase of the fuel consumption caused by the PM elimination/sulfur release. 
     [NOx Reduction Control] 
     In diesel engines, the air-fuel ratio of exhaust gas is lean in most of the operating ranges. Thus, in a normal operating state, the content of oxygen is great in the atmosphere around the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R. This causes the NOx storage reduction catalysts of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R to store NOx of the exhaust gas. However, since the oxygen content in the atmosphere around the catalysts hardly becomes small, the stored NOx cannot be reduced easily. The NOx storage reduction performance of each NOx storage reduction catalyst thus easily reaches a saturated level. 
     To solve this problem, in this example, fuel is supplied to the NOx storage reduction catalysts, including those of the DPNR catalysts, to adjust the air-fuel ratio of the exhaust gas. In this manner, the temperature in the atmosphere around each catalyst is raised or such atmosphere is switched to a reductive atmosphere. This reduces the NOx stored in the NOx storage reduction catalysts to N 2 , CO 2 , and H 2 O and releases the substance. A specific example will hereafter be explained with reference to the flowchart of  FIG. 3 . The NOx reduction control of  FIG. 3  is performed by the ECU  100 . The NOx reduction routine is carried out repeatedly at certain time intervals. 
     In step ST 21 , it is determined whether NOx reduction needs to be immediately carried out for the first system or the second system. If the determination is negative, the routine is suspended. If the determination of step ST 21  is positive, step ST 22  is performed. 
     Such determination is carried out by, for example, estimating the NOx storage amounts of the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R of both systems. Specifically, if an estimated NOx storage amount exceeds a predetermined reference value (a threshold estimated amount), it is determined that the NOx reduction needs to be immediately performed. In one method for estimating the NOx storage amounts, a map is defined from the NOx storage amounts corresponding to the engine speed and the fuel injection amount, which are determined in advance through tests or the like. The NOx storage amounts are then estimated by integrating the NOx storage amount, which is obtained with reference to the map. 
     In step ST 22 , it is determined whether the system for which the NOx reduction needs to be immediately performed corresponds to “the first system”. If the determination is positive, step ST 23  is performed. If the determination of step ST 22  is negative, it is determined that the NOx reduction needs to be immediately carried out in “the second system”. In this case, step ST 31  follows. 
     In step ST 23 , a NOx reduction basic fuel addition amount is calculated based on the difference between the actual fuel-air ratio, which is obtained using the detection signal of the air-fuel ratio sensor  23 L of the first system, and a target air-fuel ratio. The NOx reduction basic fuel addition amount is then multiplied by a bed temperature correction coefficient to obtain a NOx reduction fuel addition amount, with which the NOx reduction control is performed. The bed temperature correction coefficient is determined with reference to the bed temperature correction coefficient map shown in  FIG. 5 , using the lower value of the catalyst bed temperatures of the NSR catalysts  17 L and the DPNR catalyst  18 L, which are estimated from the detection signal of the first exhaust gas temperature sensor  21 L of the first system. The bed temperature correction coefficient map of  FIG. 5  is made while taking into consideration the fact that, if fuel addition is carried out when the catalyst bed temperature is 200° C. or below, the NOx reduction performance of the catalyst remains low and thus HC may pass through the catalyst. Further, in range A in which the catalyst bed temperature is close to 600° C., the NOx reduction performance is low and the fuel consumption is increased by the fuel addition. Thus, in the range A, the bed temperature correction coefficient is set to “zero”. 
     In step ST 24 , the energization duration (the fuel adding duration) of the fuel adding valve  12 L of the first system, the requested number of multiple addition cycles, and the addition interval (see  FIG. 4 ) are calculated based on the NOx reduction fuel addition amount obtained in step ST 23 . In step ST 25 , operation of the fuel adding valve  12 L of the first system is controlled in accordance with the energization duration, the requested number of the multiple addition cycles, and the addition interval, which have been determined in step ST 24 . In this manner, fuel is intermittently added to the exhaust gas by the fuel adding valve  12 L at certain time intervals. The NOx reduction is thus performed by performing the rich spike, in which the atmosphere around each NOx storage reduction catalyst is temporarily held in a state in which the oxygen content is small and the content of unburned fuel component is great. Then, in step ST 26 , it is determined whether a condition for ending the NOx reduction control is satisfied. Specifically, it is determined whether the amount of the fuel added since starting of the NOx reduction control reaches the NOx reduction fuel addition amount. If the determination is positive, the rich spike is ended and the routine is suspended. 
     If the determination of step ST 22  is negative, indicating that the system for which the NOx reduction needs to be immediately performed corresponds to “the second system”, step ST 31  is performed. That is, the NOx reduction basic fuel addition amount is calculated using the difference between the actual air-fuel ratio, which is obtained from the detection signal of the air-fuel ratio sensor  23 R of the second system, and the target air-fuel ratio. The determined NOx reduction basic fuel addition amount is then multiplied by the bed temperature correction coefficient to obtain the NOx reduction fuel addition amount. The bed temperature correction coefficient is determined with reference to the bed temperature correction coefficient map of  FIG. 5 , as in the above-described case. 
     Subsequently, in step ST 32 , the energization duration (=the fuel adding duration) of the fuel adding valve  12 R of the second system, the requested number of multiple addition cycles, and the addition interval (see  FIG. 4 ) are calculated using the NOx reduction fuel addition amount obtained in step ST 31 . In step ST 33 , based on the energization duration, the requested number of multiple addition cycles, and the addition interval, which are determined in step ST 32 , operation of the fuel adding valve  12 R of the second system is controlled. In this manner, the fuel is intermittently added to the exhaust gas by the fuel adding valve  12 R at certain time intervals. The NOx reduction is thus carried out by performing the rich spike, in which the atmosphere around each NOx storage reduction catalyst is temporarily held in a state in which the oxygen content is small and the content of the unburned fuel component is great. Then, in step ST 33 , it is determined whether a condition for ending the NOx reduction control is satisfied. Specifically, it is determined whether the amount of the fuel added since starting of the NOx reduction control reaches the NOx reduction fuel addition amount. If positive determination is made, the rich spike is ended and the routine is suspended. 
     If the NOx reduction needs to be immediately performed for the first system and the second system at the same time in the above-described NOx reduction control, the NOx reduction may be carried out for one of the first system or the second system by priority over the other or for both in parallel. Further, if the PM elimination or the sulfur release needs to be immediately performed at the same time as the NOx reduction control, the PM elimination control or the sulfur release control, for example, is carried out by priority. 
     Other Embodiments 
     In the above-described example, the present invention is employed in the V type eight cylinders diesel engine having two exhaust systems. However, the invention is not restricted to this use but may be employed in a diesel engine having any number of cylinders and three or more exhaust systems, such as a diesel engine having a total of four exhaust systems in which two exhaust passages  201 L,  202 L are provided in the left bank  2 L and two exhaust passages  201 R,  202 R are defined in the right bank  2 R as is illustrated in  FIG. 6 . Also, the invention may be used in an engine other than the V type, for example, in a horizontal opposed type or a straight type. Further, the diesel engine in which the invention is used does not necessarily have to be an in-cylinder direct injection type but may be other types of diesel engines. 
     In the above examples, the NSR catalysts  17 L,  17 R and the DPNR catalysts  18 L,  18 R are arranged in the corresponding exhaust systems. However, an exhaust gas purifying apparatus may be formed by providing an NSR catalyst or an oxidation catalyst and a DPF in each of the exhaust systems.