Patent Publication Number: US-2006016180-A1

Title: Apparatus and method for preventing overheating of exhaust purification filter

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to an apparatus and a method for preventing overheating of an exhaust purification filter that is provided in the exhaust system of an internal combustion engine, which exhaust purification filter filters particulate matter in exhaust gas, and burns and purifies particulate matter accumulated during the filtering.  
      Japanese Laid-Open Patent Publication No. 2002-371889 discloses a technique for purifying exhaust gas, in which a NOx storage reduction catalyst is disposed in the exhaust system of an internal combustion engine. In the configuration of the publication, exhaust gas is set to reducing atmosphere when the internal combustion engine is decelerating, so that NOx stored in a catalyst is reduced. When setting exhaust gas to reducing atmosphere, the intake flow rate is reduced or the amount of exhaust gas recirculation is increased.  
      Instead of or in addition to the catalyst, an exhaust purification filter for filtering particulate matter is disposed in the exhaust system in some cases. In such an exhaust purification filter, particulate matter gradually accumulates in the filter as the operation of the internal combustion engine continues. Therefore, a temperature increase process needs to be executed to prevent the filter from clogging. In the temperature increase process, when a certain amount of particulate matter has accumulated, the accumulated particulate matter is burned to regenerate the exhaust purification filter.  
      Some of the heat generated during the regeneration of the exhaust purification filter is lost to the outside. When an internal combustion engine is decelerating, the flow rate of exhaust gas is reduced. Thus, the amount of heat lost to the outside is reduced, accordingly. Thus, if the internal combustion engine is decelerated during the temperature increase process for regenerating the filter, heat is accumulated in the filter, causing the filter to overheat or deteriorate.  
      Especially, if the intake flow rate is reduced by the intake throttle valve and the amount of exhaust gas recirculation is increased by the exhaust gas recirculation valve when the internal combustion engine is decelerating as in the above publication, exhaust flow rate is further reduced. Thus, an exhaust purification filter becomes more likely to overheat.  
      The temperature increase process for the exhaust purification filter may be stopped immediately when the internal combustion engine starts decelerating. However, even if this is the case, since oxygen exists in the filter in one form or another, it is difficult stop the generation of the heat right away.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an objective of the present invention to provide an apparatus that effectively prevent an exhaust purification filter of an internal combustion engine from overheating.  
      To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an apparatus for preventing a filter for purifying exhaust gas emitted by an internal combustion engine from overheating is provided. The filter filters particulate matter in exhaust gas. Particulate matter that is accumulated in the filter through filtering is burned and purified by executing a temperature increase process in which the filter is heated. The apparatus includes overheat prevention means. During the temperature increase process, the overheat prevention means executes an increase process for increasing the flow rate of exhaust gas when the filter is likely to overheat.  
      The present invention also provides an apparatus for preventing a filter for purifying exhaust gas emitted by an internal combustion engine from overheating. The filter filters particulate matter in exhaust gas. Particulate matter that is accumulated in the filter through filtering is burned and purified by executing a temperature increase process in which the filter is heated. The apparatus includes means for reducing the rate of decrease of the flow rate of exhaust gas when the filter is likely to overheat during the temperature increase process.  
      Another objective of the present invention is to provide a method that effectively prevents an exhaust purification filter of an internal combustion engine from overheating.  
      Accordingly, the present invention provides a method for preventing a filter for purifying exhaust gas emitted by an internal combustion engine from overheating. The method includes: filtering particulate matter in exhaust gas with the filter; burning and purifying particulate matter that is accumulated in the filter through filtering by executing the temperature increase process in which the filter is heated; and executing an increase process for increasing the flow rate of exhaust gas when the filter is likely to overheat during the temperature increase process.  
      Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
       FIG. 1  is a diagrammatic view illustrating an overheat prevention apparatus for a diesel engine according to a first embodiment of the present invention;  
       FIG. 2  is a flowchart showing an overheat prevention process for an exhaust purification filter;  
       FIG. 3  is a graph used in the overheat prevention process shown in  FIG. 2 ;  
       FIG. 4  is a time chart showing one example of the control by the overheat prevention process shown in  FIG. 2 ;  
       FIG. 5  is a flowchart showing a fuel injection amount control process;  
       FIG. 6  is a graph used in the fuel injection amount control process shown in  FIG. 5 ;  
       FIG. 7  is a time chart showing one example of control according to a process of a second embodiment of the present invention; and  
       FIG. 8  is a time chart showing one example of control by a process according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  shows a vehicle diesel engine  2  according to the present invention and its control system. The diesel engine  2  has a plurality of cylinders. In this embodiment the engine  2  is a four-cylinder engine having first to fourth cylinders #1, #2, #3, #4. However, the present invention may be applied to engines having three or less cylinders or five or more cylinders. Each of the cylinders #1 to #4 has a combustion chamber  4  that is connected to an intake port  8 . Each intake port  8  is selectively opened and closed with an intake valve  6 . The intake ports  8  are connected to a surge tank  12  with an intake manifold  10 . An intake passage  13  extends from the surge tank  12 . An intercooler  14  and a compressor  16   a  of a turbocharger (supercharger)  16  are located in the intake passage  13 . An air cleaner  18  is connected to the inlet of the intake passage  13 . The intake manifold  10 , the surge tank  12  and the intake passage  13  form an intake system. An outlet  20   a  of an exhaust gas recirculation passage (hereinafter, referred to as EGR passage)  20  is connected to the surge tank  12 . A throttle valve  22  is disposed in a section of the intake passage  13  between surge tank  12  and the intercooler  14 . An intake flow rate sensor  24  and an intake temperature sensor  26  are disposed in a section of the intake passage  13  between the compressor  16   a  and the air cleaner  18 .  
      The combustion chamber  4  of each of the cylinders #1 to #4 is connected to an exhaust port  30  that is selectively opened and closed by an exhaust valve  28 . The exhaust ports  30  are connected to an inlet of the turbine  16   b  of the turbocharger  16  through an exhaust manifold  32 . The outlet of the turbine  16   b  is connected to an exhaust passage  34 . The turbine  16   b  draws exhaust gas from a section of the exhaust manifold  32  that corresponds to the fourth cylinder #4. The exhaust manifold  32  and the exhaust passage  34  form an exhaust system.  
      Three catalytic converters  36 ,  38 ,  40 , each containing an exhaust purification catalyst, are disposed in the exhaust passage  34 . The first catalytic converter  36  located at the most upstream section contains a NOx storage reduction catalyst  36   a . When exhaust gas is regarded as an oxidizing atmosphere (lean) during a normal operation of the diesel engine  2 , the NOx storage reduction catalyst  36   a  stores NOx. On the other hand, when exhaust gas is regarded as a reducing atmosphere (stoichiometric or lower air-fuel ratio), NOx is released from the NOx storage reduction catalyst  36   a , and is reduced by HC and CO. NOx is purified in this manner.  
      The second catalytic converter  38  containing an exhaust purification filter  38   a  is located at the second position from the most upstream side. The exhaust purification filter  38   a  has a monolithic wall. The wall has pores through which exhaust gas passes. Since a layer of NOx storage reduction catalyst is coated on the surface of the exhaust purification filter  38   a , the exhaust purification filter  38   a  functions as an exhaust purification catalyst, and thus purifies NOx as described above. Further, the pores in the wall of the filter  38   a  trap particulate matter (hereinafter, referred to as “PM”) in exhaust gas. Thus, active oxygen, which is generated in a high-temperature oxidizing atmosphere when NOx is stored, starts oxidizing trapped PM. Further, ambient excessive oxygen further oxidizes the PM. In this manner, the second catalytic converter  38  performs purification of NOx, and burning and purification of PM. In this embodiment, the first catalytic converter  36  and the second catalytic converter  38  are formed integrally.  
      The third catalytic converter  40  is located in the most downstream section. The third catalytic converter  40  contains an oxidation catalyst  40   a , which oxidizes and purifies HC and CO.  
      A first exhaust temperature sensor  44  is located between the NOx storage reduction catalyst  36   a  and the exhaust purification filter  38   a , which are close to each other. Between the exhaust purification filter  38   a  and the oxidation catalyst  40   a , a second exhaust temperature sensor  46  is located in the vicinity of the exhaust purification filter  38   a , and an air-fuel ratio sensor  48  is located in the vicinity of the oxidation catalyst  40   a.    
      The air-fuel ratio sensor  48  detects the air-fuel ratio based on components in exhaust gas, and outputs a continuous voltage signal that is proportionate to the air-fuel ratio. The first exhaust temperature sensor  44  and the second exhaust temperature sensor  46  detect exhaust temperatures thci, thco, respectively, at the corresponding position.  
      Pipes of a differential pressure sensor  50  are connected to a section upstream of the filter  38   a  and a section downstream of the filter  38   a . The differential pressure sensor  50  detects the pressure difference ΔP between the sections upstream and downstream of the exhaust purification filter  38   a , thereby detecting the degree of clogging of the filter  38   a . The degree of clogging represents the degree of accumulation of PM in the filter  38   a.    
      An inlet  20   b  of the EGR passage  20  is connected to the exhaust manifold  32 . The inlet  20   b  is located at a section of the exhaust manifold  32  that is close to the first cylinder #1, which section is opposite to a section of the exhaust manifold  32  at which the turbine  16   b  introduces exhaust gas.  
      An iron based EGR catalyst  52  and an EGR cooler  54  are located in the EGR passage  20  in this order from the inlet  20   b . The iron based EGR catalyst  52  reforms exhaust gas that passes through the EGR passage  20  (hereinafter, referred to as EGR gas). The EGR cooler  54  cools EGR gas. The EGR catalyst  52  also has a function to prevent the EGR cooler  54  from clogging. By adjusting the opening degree of the EGR valve  56 , the amount of exhaust gas (hereinafter referred to as EGR amount) that is recirculated to the intake system through the outlet  20   a  from the EGR passage  20  is adjusted.  
      Each of the cylinders #1 to #4 is provided with a fuel injection valve  58  that directly injects fuel into the corresponding combustion chamber  4 . The fuel injection valves  58  are connected to a common rail  60  with fuel supply pipes  58   a . A variable displacement fuel pump  62 , which is electrically controlled, supplies fuel to the common rail  60 . High pressure fuel supplied from the fuel pump  62  to the common rail  60  is distributed to the fuel injection valves  58  through the fuel supply pipes  58   a . A fuel pressure sensor  64  for detecting the pressure of fuel is attached to the common rail  60 .  
      Further, the fuel pump  62  also supplies low pressure fuel to a fuel adding valve  68  through a fuel supply pipe  66 . The fuel adding valve  68  is provided in the vicinity of the exhaust port  30  of the fourth cylinder #4 and injects fuel to the exhaust turbine  16   b , thereby adding fuel to exhaust gas. A catalyst control mode, which is described below, is executed by such addition of fuel.  
      An electronic control unit (hereinafter, referred to as ECU)  70  is mainly composed of a digital computer having a CPU, a ROM, and a RAM, and drive circuits for driving other devices. The ECU  70  reads signals from the intake flow rate sensor  24 , the intake temperature sensor  26 , the first exhaust temperature sensor  44 , the second exhaust temperature sensor  46 , the air-fuel ratio sensor  48 , the differential pressure sensor  50 , an EGR opening degree sensor (not shown) in the EGR valve  56 , the fuel pressure sensor  64 , and a throttle opening degree sensor  22   a . Further, the ECU  70  reads signals from a pedal position sensor  74  that detects the depression degree ACCP of an accelerator pedal  72 , and a coolant temperature sensor  76  that detects the temperature THW of coolant of the diesel engine  2 . The ECU  70  also reads signals from an engine speed sensor  80  that detects the number of revolutions NE of a crankshaft  78 , and a cylinder distinguishing sensor  82  that distinguishes cylinders by detecting the rotation phase of the crankshaft  78  or the rotation phase of the intake cams.  
      Based on the engine operating state obtained from these signals, the ECU  70  controls the amount and the timing of fuel injection by the fuel injection valves  58 . The ECU  70  controls the EGR valve  56 , a motor  22 B that actuates the throttle valve  22 , the fuel pump  62 , and the fuel adding valve  68 , thereby executing catalyst control such as PM elimination control, sulfur release control, or NOx reduction control, which are discussed below, a process for preventing overheat of the exhaust purification filter, and other processes.  
      The ECU  70  selects one of a normal combustion mode and a low temperature combustion mode according to the operating state of the engine. The low temperature combustion mode refers to a combustion mode in which an EGR opening degree map for the low temperature combustion mode is used for increasing the amount of exhaust gas recirculation to slow down the increase of the combustion temperature in the combustion chamber  4 , thereby simultaneously reducing NOx and smoke. The low temperature combustion mode is executed in a low load, low-to-middle rotation speed region, and air-fuel ratio feedback control is performed by adjusting the throttle opening degree TA based on the air-fuel ratio AF detected by the air-fuel ratio sensor  48 . The other combustion mode is the normal combustion mode, in which a normal EGR control (including a case where no EGR is executed) is performed using an EGR opening degree map for the normal combustion mode.  
      The ECU  70  performs four catalyst control modes, which are modes for controlling the exhaust purification catalysts. The catalyst control modes include a PM elimination control mode, a sulfur release control mode, a NOx reduction control mode, and a normal control mode.  
      In the PM elimination control mode, PM deposited on the exhaust purification filter  38   a  in the second catalytic converter  38  is burned. The PM is then converted into CO 2  and H 2 O and discharged. The PM elimination control mode is executed when an estimated accumulation amount of PM reaches a PM elimination reference value. In this mode, fuel is added to exhaust gas by the fuel adding valve  68  in a state where the air-fuel ratio is higher than the stoichiometric air-fuel ratio, so that the catalyst bed temperature is increased (for example, 600 to 700° C.). Further, an after injection is performed by the fuel injection valves  58  in this mode in some cases. The after injection refers to fuel injection to the combustion chambers  4  during the expansion stroke and the exhaust stroke.  
      In the S release control mode, if the NOx storage reduction catalyst  36   a  and the exhaust purification filter  38   a  are poisoned with sulfur compounds and the NOx storage capacity is lowered, S components (sulfur components) are released so that NOx storage capacity is restored. In this mode, addition of fuel from the fuel adding valve  68  is repeated so that the catalyst bed temperature is increased (for example, to 650° C.). Further, fuel is intermittently added to exhaust gas by the fuel adding valve  68 , so that the air-fuel ratio is changed to the stoichiometric air-fuel ratio or a value slightly lower than the stoichiometric air-fuel ratio. In the S release control mode, after injection may be performed by the fuel injection valves  58 .  
      In the NOx reduction control mode, NOx stored in the NOx storage reduction catalyst  36   a  and the exhaust purification filter  38   a  is reduced, and N 2 , CO 2 , and H 2 O are emitted. In this mode, addition of fuel is intermittently performed at a relatively long interval so that the catalyst bed temperature becomes relatively low (for example, 250 to 500° C.). Accordingly, the air-fuel ratio is lowered to or below the stoichiometric air-fuel ratio.  
      Among the four control modes, the normal control is a control mode other than the above three catalyst control modes. During the normal control, addition of fuel from the fuel adding valve  68  and the after injection by the fuel injection valve  58  are not performed.  
      Among the processes executed by the ECU  70 , a filter overheat prevention process will now be described.  FIG. 2  shows a flowchart of the filter overheat prevention process. This process is repeatedly executed at a predetermined cycle. Steps in the flowchart, each of which corresponds to a stage, is denoted as S.  
      When the routine is started, the ECU  70  determines whether the exhaust purification filter  38   a  is likely to overheat at step S 102 . That is, if the following conditions (1) and (2) are both met, it is determined that the exhaust purification filter  38   a  is likely to overheat.  
      (1) A PM elimination control mode, that is, a process in which the exhaust purification filter  38   a  is heated for burning the deposited PM, is being executed.  
      (2) A expected maximum bed temperature CTmax, which is obtained based on the relationship between a decrease amount ΔGA of the intake flow rate per unit time and a PM accumulation amount in the exhaust purification filter  38   a , is higher than an overheat determination temperature OT. The expected maximum bed temperature CTmax is the highest bed temperature that occurs immediately after the intake flow rate GA starts decreasing.  
      The decrease amount ΔGA(g/s 2 ) of the intake flow rate is obtained as an amount of change of the intake flow rate GA(g/s) detected by the intake flow rate sensor  24  per unit time (s). When the intake flow rate GA is decreased, ΔGA&gt;0.  
      The PM accumulation amount of the exhaust purification filter  38   a  is obtained through PM accumulation amount computation executed by the ECU  70  based on the operating state of the diesel engine  2  (the intake temperature, the air-fuel ratio, the exhaust temperatures thci, thco). More specifically, the PM accumulation amount of the exhaust purification filter  38   a  is obtained by accumulating a value that is obtained through computing the balance between a PM emission amount from the diesel engine  2  in a predetermined cycle computed based on the engine operating state, and a PM loss amount due to oxidation in the exhaust purification filter  38   a.    
      Based on the decrease amount ΔGA of the intake flow rate and the PM accumulation amount, the expected maximum bed temperature CTmax is computed by referring to a map MapCT shown in  FIG. 3 . Whether the expected maximum bed temperature CTmax is higher than the overheat determination temperature OT is determined. If CTmax&gt;OT, the condition (2) is deemed to be met.  
      The map MapCT has been obtained in the following manner. That is, experiments were conducted using the decrease amount ΔGA of the intake flow rate and the PM accumulation amount as parameters. The highest bed temperature of the exhaust purification filter  38   a  was actually measured in a period immediately after the intake flow rate started to decrease during the PM elimination control. As shown in  FIG. 3 , for the same value of the intake flow rate decrease amount ΔGA, the greater the PM accumulation amount, the higher the expected maximum bed temperature CTmax becomes. Also the greater the PM accumulation amount, the greater the rate of increase of the expected maximum bed temperature CTmax relative to an increase of the intake flow rate decrease amount ΔGA becomes. For example, when the intake flow rate decrease amount ΔGA=ΔGA1, and the PM accumulation amount is small, the expected maximum bed temperature CTmax=PM 1   a . When the PM accumulation amount is great, the expected maximum bed temperature CTmax=PM 1   b . In either case, expected maximum bed temperature CTmax is lower than the overheat determination temperature OT.  
      However, when the intake flow rate decrease amount ΔGA=ΔGA2, and the rate of decrease of the exhaust flow rate is increased, the expected maximum bed temperature CTmax=PM 2   a  if the PM accumulation amount is small. That is, the expected maximum bed temperature CTmax is lower than the overheat determination temperature OT. If the PM accumulation amount is great, the expected maximum bed temperature CTmax=PM 2   b . That is, the expected maximum bed temperature CTmax is higher than the overheat determination temperature OT.  
      If the conditions (1) and (2) are both met, it is determined that the exhaust purification filter  38   a  is likely to overheat (YES at S 102 ). The ECU  70  then executes an exhaust flow rate increase process at step S 104 , thereby preventing the exhaust purification filter  38   a  from overheating.  
      The exhaust flow rate increase process includes the following processes (a) and (b).  
      (a) Increase the opening degree TA of the throttle valve  22  relative to that during the normal control.  
      The opening degree TA of the throttle valve  22  is controlled in accordance with the engine operating state such that combustion is properly performed in the diesel engine  2 . In the process (a), for example, the throttle opening degree TA is maximized (100%) or increased compared to that of the normal control by a degree that is determined in advance for preventing overheat of the exhaust purification filter  38   a.    
      In this embodiment, the throttle valve  22  is fully opened. This increases the amount of air drawn into the combustion chambers  4 . As a result, the amount of exhaust gas discharged to the exhaust passage  34  is increased.  
      In this specification, “increase process”, that is, increasing the flow rate of air drawn into the engine  2  and the flow rate of exhaust gas discharged from the engine  2 , refer to processes in which the flow rates are increased relative to those in a case where the normal opening degree control is executed for the throttle valve  22 . That is, when the intake flow rate GA and the exhaust flow rate decrease, the “increase process” includes a process in which the rate of decrease of the intake flow rate GA and the exhaust flow rate is reduced, a process in which the intake flow rate GA and the exhaust flow rate are maintained against reduction, and a process in which the intake flow rate GA and the exhaust flow rate are increased. When the intake flow rate GA and the exhaust flow rate increase, the “increase process” includes a process in which the intake flow rate GA and the exhaust flow rate are further increased.  
      (b) Reduce the opening degree EGRa of the EGR of the valve  56  relative to that of the normal control.  
      The opening degree of the EGR valve  56  is controlled in accordance with the engine operating state such that the combustion of the diesel the engine  2  is properly conducted. In the process (b), for example, a target EGR opening degree EGRt is reduced to zero (0%), or is reduced compared to that of the normal control by a degree that is determined in advance for preventing overheat of the exhaust purification filter  38   a.    
      In this embodiment, the EGR valve  56  is fully closed. This increases the amount of air drawn into the combustion chambers  4 . As a result, the amount of exhaust gas discharged to the exhaust passage  34  is increased.  
      For example, a case as shown in the time chart of  FIG. 4  will now be discussed. In the case, after the PM elimination control mode is started at time t 0  as shown in the time chart of  FIG. 4 , the exhaust flow rate is reduced due to deceleration of the diesel the engine  2  ( FIG. 4  shows a decrease of the intake flow rate GA). At time t 1 , the expected maximum bed temperature CTmax of the exhaust purification filter  38   a  is determined to exceed the overheat determination temperature OT. In this case, at time t 1 , the throttle valve  22  is fully opened, and the EGR valve  56  is fully closed.  
      As a result, since the rate of decrease of the intake flow rate GA is reduced, the rate of decrease of the exhaust flow rate is reduced, and the catalyst bed temperature, that is, the temperature of the exhaust purification filter  38   a , is not increased to the overheat determination temperature OT. When the throttle opening degree TA is not increased and the EGR opening degree EGRa is not reduced, the intake flow rate GA is quickly reduced as shown by a broken line. At time t 2 , the temperature of the exhaust purification filter  38   a  exceeds the overheat determination temperature OT.  
      Referring back to  FIG. 2 , if at least one the conditions (1) and (2) is not met, it is determined that the exhaust purification filter  38   a  is not likely to overheat (NO at S 102 ). The ECU  70  determines whether the exhaust flow rate increase process is being executed at step S 106 . If the exhaust flow rate increase process is not being executed (NO at S 106 ), the ECU  70  temporarily suspends the current procedure.  
      If the above described exhaust flow rate increase process is being executed (YES at S 106 ), the ECU  70  determines a condition for stopping the exhaust flow rate increase process is met at step S 108 .  
      The stopping condition is deemed to be established when one of the following conditions (e1) and (e2) is met.  
      (e1) Sufficient time has elapsed since the PM elimination control was completed.  
      If the PM elimination control has been completed, generation of heat in the exhaust purification filter  38   a  due to the burning of PM has been stopped. If time sufficient for cooling the exhaust purification filter  38   a  by exhaust gas has elapsed since the PM elimination control was completed, resumption of the normal throttle opening degree control and EGR opening degree control will not cause the exhaust purification filter  38   a  to overheat. The condition (e1) is therefore selected as one of the stopping conditions for the exhaust flow rate increase process.  
      (e2) The expected maximum bed temperature CTmax of the exhaust purification filter  38   a  is sufficiently lower than the overheat determination temperature OT.  
      For example, the condition (e2) is met when the expected maximum bed temperature CTmax satisfies the following formula 1, which expected maximum bed temperature CTmax has been computed based on the current intake flow rate decrease amount ΔGA and PM accumulation amount, by referring to the map MapCT of  FIG. 3 . H represents a constant determined taking the hysteresis into consideration. 
 
 CTmax&lt;OT−H   [Formula 1]
 
      If the formula 1 is satisfied, resumption of the normal throttle opening degree control and EGR opening degree control will not cause hunting.  
      If at least one of the conditions (e1) and (e2) is met (YES at S 108 ), the ECU  70  stops the exhaust flow rate increase process at step S 110 .  
      If neither of the conditions (e1) and (e2) is met (NO at S 108 ), the ECU  70  suspends the current procedure. The exhaust flow rate increase process is thus continued.  
       FIG. 5  is a flowchart of the procedure of fuel injection amount control executed by the ECU  70 . In the fuel injection amount control, if the exhaust flow rate increase process is being executed during idling of the engine  2 , an idle up process is executed. The idle up process is executed in an interrupting manner at every fuel injection. Specifically, since the present invention is applied to the four-cylinder diesel engine  2 , the idle up process is executed at every crank angle of 180°.  
      When the fuel injection amount control is started, the ECU  70  determines whether the current engine operating state is out of a fuel cutoff region for deceleration of the engine  2  at step S 152 . The fuel cutoff region for deceleration is determined according to the engine operating state (for example, the pedal depression degree ACCP and engine speed NE). If the engine operating state is not out of the fuel cutoff region for deceleration (NO at S 152 ), the ECU  70  suspends the current routine. Fuel is not injected from the fuel injection valves  58 .  
      On the other hand, if the engine operating state is out of the fuel cutoff region for deceleration (YES at S 152 ), the ECU  70  computes an idling governor injection amount QGOV 1  and a driving governor injection amount at step S 154  based on a governor pattern map shown in  FIG. 6  that defines the relationship of a governor injection amount to the engine speed NE and the pedal depression degree ACCP. The idling governor injection amount QGOV 1  is an injection amount for a low speed range of the engine  2 , that is, for a state where the engine  2  is mainly idling. The idling governor injection amount QGOV 1  is shown by a broken line in  FIG. 6 . The driving governor injection amount is an injection amount for a high speed range of the engine  2 , that is, for a state where the vehicle is mainly driving. The driving governor injection amount is shown by a solid line in  FIG. 6 .  
      Next, at step S 156 , the ECU  70  determines whether the exhaust flow rate increase process is being executed. If the exhaust flow rate increase process is not being executed (NO at S 156 ), the ECU  70  computes the governor injection amount QGOV at step S 158 . The governor injection amount QGOV computed according to the following formula 2. 
 
 QGOV← Max( QGOV   1 + QII+QIPB+QIPNT, QGOV   2 + QIPB )  [Formula 2]
 
      That is, a value obtained by adding an integration correction amount QII, an expected load correction factor QIPB for idle speed control (ISC) and an ISC expected speed correction factor QIPNT to the idling governor injection amount QGOV 1  is computed. Also, a value obtained by adding the ISC expected load correction factor QIPB to the driving governor injection amount QGOV 2  is computed. The computed values are compared, and the greater one is set as the governor injection amount QGOV.  
      Therefore, when the exhaust flow rate increase process is not being executed, the governor injection amount QGOV is determined as schematically shown in  FIG. 6 . That is, in the low speed region of the engine  2 , a value obtained by adding the integration correction amount QII, the ISC expected load correction factor QIPB and the ISC expected speed correction factor QIPNT to the idling governor injection amount QGOV 1  is selected as the governor injection amount QGOV. On the other hand, in the high speed range of the engine  2 , a value obtained by adding the ISC expected load correction factor QIPB to the driving governor injection amount is selected as the governor injection amount QGOV.  
      In contrast, if the exhaust flow rate increase process is being executed (YES at S 156 ), the ECU  70  computes the governor injection amount QGOV at step S 160 , using the following formula 3. 
 
 QGOV ←Max( QGOV   1 + QII+QINC, QGOV   2 + QIPB )  [Formula 3]
 
      The formula 3 is different from the formula 2 in that a value obtained by adding the integration correction amount QII and an ISC speed correction factor QINC for an idle up process for preventing overheat of the exhaust purification filter  38   a  to the idling governor injection amount QGOV 1  is used as the governor injection amount QGOV.  
      Therefore, when the exhaust flow rate increase process is being executed, the fuel injection amount is increased according to the ISC speed correction factor QINC, so that the engine speed NE is increased relative to that of the normal idling as indicated by broken line denoted as “during an idle up process” in  FIG. 6 . As a result, even if the engine  2  idles during the exhaust flow rate increase process, a sufficient amount of the exhaust flow rate is ensured, so that the exhaust purification filter  38   a  is prevented from overheating.  
      At step  162 , which is after step S 158  or step S 160 , the ECU  70  sets the smaller one of a maximum injection amount QFULL and the governor injection amount QGOV as a final injection amount QFIN. At step S 164 , the ECU  70  computes an injection amount command value (time conversion value) TSP that corresponds to the final injection amount QFIN, and outputs the injection amount command value TSP. The ECU  70  then temporarily suspends the current procedure. Based on the output of the injection amount command value TSP, the fuel injection valve  58  is actuated to perform fuel injection.  
      As shown in the time chart of  FIG. 4 , the throttle opening degree is maximized (fully open), the EGR opening degree is minimized (fully closed), and the idle up process is continued to prevent the exhaust purification filter  38   a  from overheating after time t 3  as long as the expected maximum bed temperature CTmax is not sufficiently low after the engine  2  starts idling.  
      Among the processes executed by the ECU  70 , the overheat prevention process of  FIG. 2  and steps S 156 , S 160  in the fuel injection amount control process of  FIG. 5  correspond to processes executed when the ECU  70  functions as overheat prevention means.  
      The first embodiment described above has the following advantages.  
      (A1) In the overheat prevention process of  FIG. 2 , when the exhaust purification filter  38   a  is likely to overheat, the throttle valve  22  is fully opened, and the EGR valve  56  is fully closed. Accordingly, the intake flow rate and the exhaust flow rate are increased. Then, when steps S 156 , S 160  of the fuel injection amount control of  FIG. 5  are executed, the idle speed is increased during idling. A sufficient exhaust flow rate is thus ensured. Therefore, the heat generated in the exhaust purification filter  38   a  is positively lost to the outside, and the exhaust purification filter  38   a  is effectively prevented from overheating.  
      (A2) Whether the exhaust purification filter  38   a  is likely to overheat is determined by monitoring whether the expected maximum bed temperature CTmax, which is estimated based on the map MapCT of  FIG. 3 , exceeds the overheat determination temperature OT based on the intake flow rate decrease amount ΔGA and the PM accumulation amount.  
      Therefore, when the engine decelerates in various types of operating state, for example, when the driver releases the accelerator pedal  72  while the vehicle driving downhill, the exhaust flow rate is increased before the actual bed temperature of the exhaust purification filter  38   a  exceeds the overheat determination temperature OT. As a result, the exhaust purification filter  38   a  is effectively prevented from overheating.  
      A second embodiment according to the present invention will now be described. The second embodiment is different form the first embodiment only in that the determination condition of step S 102  and the stopping condition of step S 108  in the overheat prevention process in  FIG. 2 , and the other configurations are the same as the first embodiment. Thus,  FIGS. 1, 2 ,  5  are referred to as necessary in the following description.  
      In this embodiment, at step  102  of  FIG. 2 , whether the exhaust purification filter  38   a  is likely to overheat is determined based on whether at least one of the following conditions (1) and (2) is met.  
      (1) The exhaust temperature thci in a section upstream of the exhaust purification filter  38   a  (that is, the exhaust temperature in a section downstream of the NOx storage reduction catalyst  36   a ) is higher than an overheat determination temperature OTi for the upstream section.  
      (2) The exhaust temperature thco in a section downstream of the exhaust purification filter  38   a  is higher than an overheat determination temperature OTo for the downstream section.  
      When at least one of the conditions (1) and (2) is met, it is determined that the exhaust purification filter  38   a  is likely to overheat.  
      In this embodiment, at step  108  of  FIG. 2 , a condition for stopping the exhaust flow rate increase process is determined to be met when the following condition (e1) is met.  
      (e1) The upstream exhaust temperature thci is sufficiently lower than the overheat determination temperature OTi, and the downstream exhaust temperature thco is sufficiently lower than the overheat determination temperature. OTo.  
      Therefore, as shown in the time chart of  FIG. 7 , the throttle valve  22  is fully opened, and the EGR valve  56  is fully closed at time t 11 , where the exhaust flow rate, that is, the intake flow rate GA is reduced due to, for example, deceleration of the engine  2  during the PM elimination control mode, and the downstream exhaust temperature thco exceeds the overheat determination temperature OTo. As a result, since the intake flow rate GA is increased and decrease of the exhaust flow rate is prevented, the catalyst bed temperature, that is, the temperature of the exhaust purification filter  38   a  is not increased to the overheat determination temperature OT. In contrast, if the throttle opening degree TA is not increased, and the EGR opening degree EGRa is not reduced, the intake flow rate GA is quickly reduced as shown by a broken line. At time t 12 , the temperature of the exhaust purification filter  38   a  exceeds the overheat determination temperature OT. After time t 13 , the idle up process is executed during idling as discussed in the fuel injection amount control of  FIG. 5 .  
      The second embodiment as described above has the following advantages.  
      (A1) The second embodiment has the same advantage as the item (A1) of the first embodiment.  
      (A2) Whether the exhaust purification filter  38 . a  is likely to overheat is determined based on the upstream exhaust temperature thci detected by the first exhaust temperature sensor  44  and the downstream exhaust temperature thco detected by the second exhaust temperature sensor  46 .  
      The temperature of the exhaust purification filter  38   a  is affected by the temperature of exhaust gas that flows into the filter  38   a . Therefore, whether the exhaust purification filter  38   a  is likely to overheat can be determined by determining the upstream exhaust temperature thci of the exhaust purification filter  38   a.    
      Particularly, the NOx storage reduction catalyst  36   a , which is another exhaust purification catalyst, is disposed in a section upstream of the exhaust purification filter  38   a . The NOx storage reduction catalyst  36   a  does not function as a filter for PM, but generates heat with fuel added by the fuel adding valve  68  during temperature increase process of the PM elimination control process. When NOx storage reduction catalyst  36   a  upstream of the exhaust purification filter  38   a  is heated, the exhaust temperature thci that flows into the exhaust purification filter  38   a  increases. This increases the possibility of overheat of the exhaust purification filter  38   a.    
      Further, since the downstream exhaust temperature thco is the temperature in the vicinity of the exhaust purification filter  38   a , the likeliness of overheat of the exhaust purification filter  38   a  is substantially directly detected.  
      When at least one of thci&gt;OTi and thco&gt;OTo is satisfied, it is determined that the exhaust purification filter  38   a  is likely to overheat.  
      Particularly, by determining the temperature thci of exhaust gas that flows into the exhaust purification filter  38   a , overheat of the exhaust purification filter  38   a  can be predicted at an early stage. Accordingly, the exhaust flow rate is increased at an early stage, so that overheat of the exhaust purification filter  38   a  is reliably prevented.  
      A third embodiment according to the present invention will now be described. The third embodiment is different form the first embodiment only in that the determination condition of step S 102  and the stopping condition of step S 108  in the overheat prevention process in  FIG. 2 , and the other configurations are the same as the first embodiment. Thus,  FIGS. 1, 2 ,  5  are referred to as necessary in the following description.  
      In this embodiment, at step  102  of  FIG. 2 , whether the exhaust purification filter  38   a  is likely to overheat is determined based on whether the following condition (1) is met.  
      (1) An estimated bed temperature thcf of the exhaust purification filter  38   a  is higher than the overheat determination temperature OTf.  
      The estimated bed temperature thcf is computed by the ECU  70  using the following formula 4 in a predetermined cycle. 
 
 thcf←thcfold +( Cf−Ce )/ Hcp   [Formula 4]
 
      The estimated bed temperature thcfold is the estimated bed temperature thcf that was computed in the preceding cycle.  
      The generated heat amount Cf of the exhaust purification filter  38   a  refers to the amount of heat generated in the exhaust purification filter  38   a  during one cycle in which the estimated bed temperature thcf is computed. In other words, the generated heat amount Cf is the amount of heat that is generated with a portion of the fuel that has been added to exhaust gas by the fuel adding valve  68 , but has not been consumed at the upstream NOx storage reduction catalyst  36   a . The amount of fuel consumed at the upstream NOx storage reduction catalyst  36   a  is estimated based on the upstream exhaust temperature thci of the exhaust purification filter  38   a  and the intake flow rate GA. Therefore, by subtracting the consumed fuel amount from the total amount of fuel added by the fuel adding valve  68 , the amount of fuel that is burned at the exhaust purification filter  38   a  is obtained. Based on the obtained fuel amount, the generated heat amount Cf of the exhaust purification filter is determined.  
      The emitted heat amount Ce refers to the amount of heat that is removed from the exhaust purification filter  38   a  by exhaust gas during one cycle for computing the estimated bed temperature thcf. The emitted heat amount Ce is computed based on the intake flow rate GA, which reflects the exhaust flow rate, the upstream exhaust temperature thci and the estimated bed temperature thcfold of the preceding cycle.  
      Thermal capacity Hcp of the exhaust purification filter is a thermal capacity of the exhaust purification filter  38   a  that has been measured in advance.  
      In the present embodiment, at step  108  of  FIG. 2 , a condition for stopping the exhaust flow rate increase process is determined to be met when the following condition (e1) is met.  
      (e1) The estimated bed temperature thcf of the exhaust purification filter is sufficiently lower than the overheat determination temperature OTf.  
      Therefore, as shown in the time chart of  FIG. 8 , the throttle valve  22  is fully opened, and the EGR valve  56  is fully closed at time t 21 , where the exhaust flow rate, that is, the intake flow rate GA is reduced due to, for example, deceleration of the engine  2  during the PM elimination control mode, and the estimated bed temperature thcf of the exhaust purification filter exceeds the overheat determination temperature OTf. As a result, since the intake flow rate GA is increased and decrease of the exhaust flow rate is prevented, the temperature of the exhaust purification filter  38   a  is not increased to the overheat determination temperature OT. In contrast, if the throttle opening degree TA is not increased, and the EGR opening degree EGRa is not reduced, the intake flow rate GA is quickly reduced as shown by a broken line. At time t 22 , the temperature of the exhaust purification filter  38   a  exceeds the overheat determination temperature OT. After time t 23 , the idle up process is executed during idling as discussed in the fuel injection amount control of  FIG. 5 .  
      The third embodiment as described above has the following advantages.  
      (A1) The third embodiment has the same advantage as the item (A1) of the first embodiment.  
      (A2) Whether the exhaust purification filter  38   a  is likely to overheat is determined based on the estimated bed temperature thcf estimated based on the intake flow rate GA, the amount of added fuel, and the upstream exhaust temperature thci and the downstream exhaust temperature thco of the exhaust purification filter  38   a.    
      As a result, overheat of the exhaust purification filter  38   a  is accurately predicted, and the exhaust flow rate is properly increased. Thus, the exhaust purification filter  38   a  is effectively prevented from overheating.  
      A fourth embodiment according to the present invention will now be described. The fourth embodiment is different form the first embodiment only in that the determination condition of step S 102  and the stopping condition of step S 108  in the overheat prevention process in  FIG. 2 , and the other configurations are the same as the first embodiment. Thus,  FIGS. 1, 2 ,  5  are referred to as necessary in the following description.  
      In this embodiment, at step  102  of  FIG. 2 , whether the exhaust purification filter  38   a  is likely to overheat is determined based on whether the following condition (1) is met.  
      (1) The PM elimination control is being executed, and the PM accumulation amount is greater than a reference accumulation amount.  
      That is, since the amount of heat generated during the PM elimination control is increased when the PM accumulation amount is great, the exhaust purification filter  38   a  is likely to overheat when the exhaust flow rate decreases. The condition in which the PM accumulation amount&gt;the reference accumulation amount is used to determine whether the exhaust purification filter  38   a  is likely to overheat.  
      In the present embodiment, at step  108  of  FIG. 2 , a condition for stopping the exhaust flow rate increase process is determined to be met when one of the following conditions (e1) and (e2) is met.  
      (e1) The PM accumulation amount is sufficiently less than the reference accumulation amount.  
      That is, since the generated heat amount decreases due to decrease of burning PM, it is determined that the exhaust purification filter  38   a  is not likely to overheat even if the exhaust flow rate decreases.  
      (e2) Sufficient time has elapsed since the PM elimination control was completed.  
      Since sufficient time has elapsed since burning of PM stopped, it is determined that the exhaust purification filter  38   a  is not likely to overheat even if the exhaust flow rate decreases.  
      The fourth embodiment as described above has the following advantages.  
      (A1) The fourth embodiment has the same advantage as the item (A1) of the first embodiment.  
      (A2) Since whether the exhaust purification filter  38   a  is likely to overheat is determined based on the PM accumulation amount during the PM elimination control, the determination is performed easily.  
      Embodiment other than the above describe ones will now be described.  
      (a) In the above embodiments, increase of the intake flow rate is carried out by adjusting the opening degree of both of the throttle valve  22  and the EGR valve  56 . However, the increase of the intake flow rate may be carried out only by increasing the opening degree of the throttle valve  22 . Alternatively, the increase of the intake flow rate may be carried out only by reducing the opening degree of the EGR valve  56 .  
      In the illustrated embodiments, when the intake flow rate is increased, the throttle valve  22  is fully opened. However, the throttle valve  22  may be increased relative to that of the normal control by an amount that is set in advance for preventing overheat. Also, when the intake flow rate is increased, the EGR valve  56  is fully closed in the illustrated embodiments. However, the EGR valve  56  may be decreased relative to that of the normal control by an amount that is set in advance for preventing overheat.  
      (b) In the second embodiment, whether the exhaust purification filter  38   a  is likely to overheat may be determined based only on whether the condition (2) is met. In this case, the condition for stopping the exhaust flow rate increase process is met when the downstream exhaust temperature thco of the exhaust purification filter  38   a  is sufficiently lower than the overheat determination temperature OTo.  
      (c) In the first embodiment, it may be determined that the exhaust purification filter  38   a  is likely to overheat when the following conditions (1) and (2) are both met. The condition (2) is the same as that of the first embodiment.  
      (1) The temperature increase process is being executed according to one of the PM elimination control mode and the S release control mode.  
      (2) The expected maximum bed temperature CTmax is higher than the overheat determination temperature OT (CTmax&gt;OT).  
      In this case, the condition for stopping the exhaust flow rate increase process is as follows. The condition (e2) is the same as that of the first embodiment.  
      (e1) Sufficient time has elapsed since the PM elimination control and the S release control were both completed.  
      (e2) The expected maximum bed temperature CTmax is sufficiently lower than the overheat determination temperature OT.  
      In the fourth embodiment, it may be determined that the exhaust purification filter  38   a  is likely to overheat when the following condition (1) is met.  
      (1) The temperature increase process is being executed according to one of the PM elimination control mode and the S release control mode, and the PM accumulation amount at the exhaust purification filter  38   a  is greater than a reference accumulation amount.  
      In this case, the condition for stopping the exhaust flow rate increase process is as follows. The condition (e1) is the same as that of the fourth embodiment.  
      (e1) The PM accumulation amount is sufficiently less than the reference accumulation amount.  
      (e2) Sufficient time has elapsed since the PM elimination control and the S release control were both completed.  
      In this manner, in either of the first and fourth embodiments, the state where the S release control mode is being executed is used as the condition (1). This effectively prevents the exhaust purification filter  38   a  from overheating during the S release control.  
      (d) The state in which the engine is decelerated is not used as the condition for determining whether the exhaust purification filter  38   a  is likely to overheat in the illustrated embodiments. However, the state in which the engine is decelerating may be used as a condition for determining the likeliness of overheat. This configuration is particularly effective when the engine is decelerating.  
      (e) In the illustrated embodiments, when the exhaust purification filter  38   a  is likely to overheat during idling, the idle up process is executed together with a process for increasing throttle opening degree and a process for reducing the EGR opening degree. However, in the same situation, only the idle up process may be executed. Increase of the exhaust flow rate due to the idle up process effectively prevents the exhaust purification filter  38   a  from overheating.  
      (f) At step S 156  in the fuel injection amount control of  FIG. 5 , whether the idle up process should be performed is determined based on whether the exhaust flow rate increase process for preventing the exhaust purification filter  38   a  from overheating is being executed. However, whether the idle up process should be performed may be determined using, at step  156 , the same conditions as the conditions for executing the exhaust flow rate increase process and the conditions for stopping the process in the illustrated embodiments.  
      For example, at step S 156 , whether the expected maximum bed temperature CTmax has been stable and lower than the overheat determination temperature OT for a predetermined period may be determined. If the condition is not met, the idle up process is continued (S 160 ), and if the condition is met, the idle up process is stopped and step S 158  is executed.  
      The present invention may be applied to other types of engine such as a lean combustion gasoline engine if the engine has a similar catalyst system to the system described herein.