Abstract:
An exhaust gas purification device for an internal combustion engine, wherein an amount of fuel supply during a forced regeneration is optimized, and a fuel efficiency is effectively improved. The device includes a filter in an exhaust passage of the internal combustion engine for collecting particulate matter in an exhaust gas, an electrostatic capacity detecting unit that detects an electrostatic capacity of the filter, and a filter regenerating unit that supplies fuel to the filter and executes a forced regeneration to burn and remove the particulate matter in the filter. The filter regenerating unit stops supplying fuel when a rate of decrease in the detected electrostatic capacity per predetermined period of time reaches or falls below a predetermined lower threshold, which indicates a drop in burning efficiency of the particulate matter, while the forced regeneration is being executed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application, which claims the benefit under 35U.S.C. §371 of PCT International Patent Application No. PCT/JP2014/050565, filed Jan. 15, 2014, which claims the foreign priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2013-013389, filed Jan. 28, 2013, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to exhaust purification devices for internal combustion engines, and in particular relates to an exhaust purification device that has a filter for collecting particulate matter in an exhaust gas emitted from an internal combustion engine. 
     BACKGROUND ART 
     A diesel particulate filter (hereinafter, referred to as DPF), for example, is known as a filter for collecting particulate matter (hereinafter, referred to as PM) in an exhaust gas emitted from a diesel engine. 
     The DPF can only collect a limited amount of PM, and therefore a forced regeneration needs to be performed to periodically burn and remove accumulated PM. The forced regeneration is performed by supplying unburned fuel, primarily hydrocarbon (“HC”), to an oxidation catalyst on an upstream side of an exhaust passage by means of in-pipe injection or post-injection, and raising the exhaust gas temperature to a PM-burning temperature with heat produced upon oxidation. 
     For example, Patent Literature 1 discloses an exhaust gas purification system that performs a forced regeneration by raising the temperature of a DPF by means of post-injection as the pressure difference across the DPF between the upstream side and the downstream side of the exhaust passage reaches or exceeds a predetermined value. 
     LISTING OF REFERENCES 
     PATENT LITERATURE 1: Japanese Patent Application Laid-Open Publication(Kokai) No. 2011-247145 
     As burning of accumulated PM progresses through the forced regeneration, the pressure difference across the DPF between the upstream side and the downstream side of the exhaust passage gradually decreases. Thus, the sensitivity of the remaining PM to the pressure difference is lost in particular during the last stage (phase) of the forced regeneration, and a problem arises, i.e., an accurate amount of accumulated PM in the DPF cannot be grasped. 
     In order to prevent PM from remaining in the DPF, it is conceivable to set the safety factor with a relatively large margin and increase an amount of fuel supply (extend the duration of fuel supply). This, however, keeps wasting the fuel in the last stage of the forced regeneration, and the fuel efficiency may be reduced. 
     It is also conceivable to reduce an interval between the end of a forced regeneration and the start of a next forced regeneration (hereinafter, referred to as a regeneration interval) so as to reduce PM accumulation in the DPF. This, however, increases the frequency at which the temperature is raised to a regeneration-possible temperature, and fuel is wasted in raising the temperature. Thus, this may also deteriorate the fuel efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of such problems, and an object of the present invention is to optimize an amount of fuel supply during a forced regeneration and to effectively improve the fuel efficiency. 
     To achieve the above-mentioned object, an exhaust gas purification device for an internal combustion engine according to the present invention includes a filter that is provided in an exhaust passage of the internal combustion engine and is configured to collect particulate matter in an exhaust gas, an electrostatic capacity (capacitance) detecting unit configured to detect an electrostatic capacity (capacitance) of the filter, and a filter regenerating unit configured to supply fuel to the filter to perform a forced regeneration, which burns and removes particulate matter accumulated in the filter. The filter regenerating unit stops supplying the fuel when a rate of decrease in the detected electrostatic capacity per predetermined period of time reaches or falls below a predetermined lower threshold, which indicates a drop in burning efficiency of the particulate matter, while the forced regeneration is being performed. 
     The filter regenerating unit may stop supplying the fuel after reducing an amount of fuel supply stepwise when the rate of decrease in the detected electrostatic capacity per predetermined period of time reaches or falls below the predetermined lower threshold while the force regeneration is being performed. 
     The electrostatic capacity detecting unit may include a pair of electrodes that are disposed in the filter so as to oppose each other with at least one partition wall interposed therebetween. The paired electrodes form a capacitor. 
     The exhaust gas purification device may further include a bypass passage that branches from the exhaust passage at a position upstream of the filter and connects to the exhaust passage at position downstream of the filter so as to bypass the filter, and a second filter that is provided in the bypass passage and collects particulate matter in the exhaust gas flowing through the bypass passage. The paired electrodes may be disposed in the second filter such that the two electrodes oppose each other with at least one partition wall interposed therebetween. 
     When a forced regeneration of the second filter is performed, the paired electrodes may be used as a heater. 
     According to the exhaust gas purification device for an internal combustion engine according to the present invention, an amount of fuel supply during a forced regeneration is optimized, and the fuel efficiency is effectively improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall configuration diagram schematically illustrating an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention. 
         FIG. 2  illustrates changes in the electrostatic capacity and in the pressure difference across a DPF during a forced regeneration by the exhaust gas purification device for the internal combustion engine according to the embodiment of the present invention. 
         FIG. 3 a    illustrates a change in the amount of accumulated PM during a forced regeneration by the exhaust gas purification device for the internal combustion engine according to the embodiment of the present invention,  FIG. 3 b    shows an instruction signal for stopping fuel injection when the regeneration efficiency drops during the forced regeneration, and  FIG. 3 c    is a timing chart of an instruction signal for reducing the amount of fuel injection stepwise when the regeneration efficiency drops during the forced regeneration. 
         FIG. 4  is a flowchart illustrating the content of control by the exhaust gas purification device for the internal combustion engine according to the embodiment of the present invention. 
         FIG. 5  is an overall configuration diagram schematically illustrating an exhaust gas purification device for an internal combustion engine according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1 to 4 , an exhaust gas purification device for an internal combustion engine according to embodiments of the present invention will be described. Identical parts are given identical reference numerals and signs, and their names and functions are identical as well. Therefore, detailed description of such parts will not be repeated. 
     As illustrated in  FIG. 1 , a diesel engine (hereinafter, simply referred to as the engine)  10  includes an intake manifold  10   a  and an exhaust manifold  10   b  . An intake passage  11  for introducing fresh air is connected to the intake manifold  10   a , and an exhaust passage  12  for discharging exhaust gas to the atmosphere is connected to the exhaust manifold  10   b . Furthermore, an in-pipe injection device  13  and an exhaust gas post-treatment device  14  are provided in the exhaust passage  12 . The in-pipe injection device  13  is located upstream of the exhaust gas post-treatment device  14  in the exhaust passage. 
     The in-pipe injection device  13  injects unburned fuel (primarily HC) into the exhaust passage  12  in response to an instruction signal from an engine control unit (“ECU”)  20 , which will be described later. It should be noted that the in-pipe injection device  13  may be dispensed with if post-injection for carrying out multiple-injection to the engine  10  is used. 
     The exhaust gas post-treatment device  14  includes an oxidation catalyst  15  and a DPF  16 . The oxidation catalyst  15  is located upstream of the DPF  16  in the exhaust passage. The oxidation catalyst  15  and the DPF  16  are disposed in a casing  14   a.    
     The oxidation catalyst  15  is formed, for example, of a catalyst component supported on a surface of a ceramic carrier. The ceramic carrier may have a cordierite honeycomb structure. As unburned fuel (primarily HC) is supplied by the in-pipe injection device  13  or through post-injection, the oxidation catalyst  15  oxidizes the unburned fuel, and raises the exhaust gas temperature. 
     The DPF  16  is an example of a filter according to the present invention, and has, for example, a number of cells defined by porous partition walls arranged in a flowing direction of the exhaust gas. The upstream side and the downstream side of these cells are alternately plugged. The DPF  16  collects PM of the exhaust gas in the small cavities of the partition walls and on the surfaces of the partition walls. Upon the amount of accumulated PM reaching a predetermined amount, a so-called forced regeneration for burning and removing the PM is executed. The forced regeneration is performed by supplying unburned fuel (primarily HC) to the oxidation catalyst  15  from the in-pipe injection device  13  or through post-injection and by raising the temperature of the DPF  16  to the PM-burning temperature (e.g., approximately 600 degrees C.) through a rise in the exhaust gas temperature. 
     The DPF  16  of this embodiment also has a pair of electrodes  17   a  and  17   b  that are disposed so as to oppose each other with at least one partition wall interposed therebetween. The electrodes  17   a  and  17   b  form a capacitor. The electrodes  17   a  and  17   b  are electrically connected to the ECU  20 , respectively. 
     The ECU  20  controls fuel injection and other operations of the engine  10  and the in-pipe injection device  13 , and includes known CPU, ROM, RAM, input port, output port, and other components. The ECU  20  also includes an electrostatic capacity calculating unit  21  and a regeneration controlling unit  22  as part of its functional elements. The description continues with a premise that these functional elements are included in the ECU  20 , which is an integrated hardware piece, but some of these functional elements can be provided in a separate hardware piece. 
     In this embodiment, the electrostatic capacity calculating unit  21  and the electrodes  17   a  and  17   b  constitute the electrostatic capacity detecting unit of the present invention. The regeneration controlling unit  22  and the in-pipe injection device  13  (or a fuel injection device (not shown) of the engine  10 ) constitute the filter regenerating unit of the present invention. 
     The electrostatic capacity calculating unit  21  calculates the amount of accumulated PM from an electrostatic capacity C between the pair of electrodes  17   a  and  17   b  in order to estimate the amount of accumulated PM in the DPF  16 . The electrostatic capacity C is given by the following expression  1 , where c represents the dielectric constant of a medium between the electrodes  17   a  and  17   b , and d represents the distance between the electrodes  17   a  and  17   b . The electrostatic capacity C varies as the dielectric constant c or the distance d changes with the accumulated PM. 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     ɛ 
                     × 
                     
                       S 
                       d 
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     For example, when conductive carbon accumulates between the electrodes  17   a  and  17   b , the distance d between the electrodes  17   a  and  17   b  substantially decreases, and the electrostatic capacity C increases. When the accumulation of PM in the medium between the electrodes  17   a  and  17   b  progresses, the dielectric constant ε increases, and the electrostatic capacity C increases accordingly. In other words, a proportional relation is present between the electrostatic capacity C and the amount of accumulated PM, and thus the amount of accumulated PM can easily be estimated by calculating the electrostatic capacity C. In this manner, the electrostatic capacity C is used to estimate the amount of accumulated PM in this embodiment. Therefore, the estimation accuracy can be improved in this embodiment, as compared to using a differential pressure sensor whose sensitivity decreases in the last stage of a forced regeneration. 
     The regeneration controlling unit  22  controls the forced regeneration of the DPF  16  on the basis of the electrostatic capacity C calculated by the electrostatic capacity calculating unit  21 . Specific contents of the control will be described with reference to  FIGS. 2 and 3 . 
     Firstly, the control of starting a forced regeneration will be described. The ECU  20  stores the electrostatic capacity C corresponding to a maximum accumulation amount of PM that can be collected by the DPF  16  as an upper threshold C MAX . The maximum accumulation amount of PM that can be collected by the DPF  16  is obtained in advance through an experiment or the like. As the electrostatic capacity C calculated by the electrostatic capacity calculating unit  21  exceeds the upper threshold C MAX  (see time T 1  in  FIG. 2 ), the regeneration controlling unit  22  generates (outputs) an instruction signal that causes the in-pipe injection device  13  (the fuel injection device of the engine  10  in the case of post-injection) to inject a prescribed amount of fuel (see time T 1  in  FIG. 3 b   ). This instruction signal is generated continually at predetermined intervals until control of ending the forced regeneration starts, which will be described later. In  FIGS. 2 and 3 , the time T 2  indicates the time at which the temperature of the DPF  16  is raised to the PM-burning temperature (e.g., 600 degrees C.) through a rise in the exhaust gas temperature, and the accumulated PM starts burning. 
     The control of ending the forced regeneration will be described. In the forced regeneration, when the amount of accumulated PM decreases to a predetermined amount, PM does not burn any further even if fuel injection is continued, and the burning efficiency of PM (hereinafter, referred to as regeneration efficiency) significantly drops. If fuel injection is continued in this state, the regeneration efficiency corresponding to the amount of fuel supply is not attained. 
     The ECU  20  stores a slope or gradient ΔC/ΔT of the electrostatic capacity C (the rate of decrease in the electrostatic capacity C per predetermined period of time), at which the regeneration efficiency starts dropping significantly, as a lower threshold ΔC/ΔT MIN . The slope ΔC/ΔT of the electrostatic capacity C is obtained in advance through an experiment or the like. The regeneration controlling unit  22  calculates an average (running average) of the slope ΔC/ΔT of the electrostatic capacity C on the basis of the electrostatic capacity C calculated by the electrostatic capacity calculating unit  21 . As the slope ΔC/ΔT reaches or falls below the lower threshold ΔC/ΔT MIN  (see time T 3  in  FIG. 2 ), the regeneration controlling unit  22  stops the instruction signal for fuel injection (see time T 3  in  FIG. 3 b   ). Thus, after the regeneration efficiency of the DPF  16  drops, unnecessary fuel supply is suppressed, and the fuel efficiency is effectively improved. 
     It should be noted that as illustrated in  FIG. 3 c   , after the slope ΔC/ΔT of the electrostatic capacity C has reached or fallen below the lower threshold ΔC/ΔT MIN , the amount of fuel injection may be reduced stepwise (see times T 3  to T 4  in  FIG. 3 c   ). In this configuration, the reduced amount of fuel injection or the timing of ceasing the fuel injection may be set to an optimum value in accordance with the capacity of the DPF  16  or other factors. 
     Referring now to  FIG. 4 , a control process of the exhaust gas purification device according to the embodiment of the present embodiment will be described. This control starts upon turning on of an ignition key. 
     At step  100  (hereinafter, “step” is simply indicated as S), it is determined whether the electrostatic capacity C, which is proportional to the amount of accumulated PM, has exceeded the upper threshold C MAX , which corresponds to the maximum accumulation amount of PM. If the electrostatic capacity C has exceeded the upper threshold C MAX (C&gt;C MAX ), the control proceeds to S 110 . On the other hand, if the electrostatic capacity C is equal to or less than the upper threshold C MAX (C≦C MAX ), the control proceeds to “Return.” 
     At S 110 , to start a forced regeneration, an instruction signal is generated that causes the in-pipe injection device  13  (the fuel injection device of the engine  10  in the case of post-injection) to inject a predetermined amount of fuel. Thereafter, the temperature of the DPF  16  is raised to the PM-burning temperature (e.g., approximately 600 degrees C.), and PM that has accumulated in the DPF  16  starts burning. 
     At S 120 , it is determined whether the slope ΔC/ΔT of the electrostatic capacity C (the decreasing rate of the electrostatic capacity C per predetermined period of time) has reached the lower threshold ΔC/ΔT MIN , which indicates a drop in the regeneration efficiency. If the slope ΔC/ΔT has reached or fallen below the lower threshold ΔC/ΔT MIN  (ΔC/ΔT≦C MIN ), the control proceeds to S 130 . On the other hand, if the slope ΔC/ΔT is greater than the lower threshold ΔC/ΔT MIN  (ΔC/ΔT&gt;C MIN ), the control returns to S 110 . In other words, fuel injection from the in-pipe injection device  13  (or post-injection) continues. 
     At S 130 , to suppress unnecessary fuel consumption, the instruction signal that causes the in-pipe injection device  13  (the fuel injection device of the engine  10  in the case of post-injection) to inject fuel is stopped, or the amount of fuel injection is reduced stepwise. Then, the control proceeds to “Return.” Thereafter, S 100  to S 130  are reiterated until the ignition key is turned off. 
     Advantages of the exhaust gas purification device for an internal combustion engine according to this embodiment will be described. 
     In a conventional forced regeneration, the starting time and the ending time are controlled on the basis of the pressure difference, which is detected by a differential pressure sensor, across the DPF between the upstream side and the downstream side of the exhaust passage. As burning of PM progresses through the forced regeneration, the pressure difference across the DPF between the upstream side and the downstream side of the exhaust passage gradually decreases, and the sensitivity of the remaining PM to the pressure difference tends to decrease in particular in the last stage of the forced regeneration (e.g., see region A in  FIG. 2 ). Thus, an accurate amount of PM remaining in the DPF cannot be grasped. Therefore, it has been necessary to set the safety factor with a large margin and increase the amount of fuel supply or to reduce the regeneration interval. 
     On the other hand, the exhaust gas purification device of this embodiment estimates the amount of accumulated PM on the basis of the electrostatic capacity C with good sensitivity even in the last stage of a forced regeneration, and stops the fuel injection as the slope ΔC/ΔT of the electrostatic capacity C becomes equal to or smaller than the lower threshold ΔC/ΔT MIN . The lower threshold ΔC/ΔT MIN  indicates a drop in the regeneration efficiency. In other words, the timing at which the regeneration efficiency relative to the amount of fuel injection notably drops is grasped with accuracy on the basis of the electrostatic capacity C with good sensitivity, and the forced regeneration is finished reliably when the regeneration efficiency drops. 
     Accordingly, the exhaust gas purification device of this embodiment can suppress unnecessary fuel consumption due to a forced regeneration, and can effectively improve the fuel efficiency. In addition, because an accurate amount of remaining PM is grasped toward the end of the forced regeneration (in the last stage of the forced regeneration), the amount of fuel injection need not be set with a margin, and the regeneration intervals can be set long. 
     It is to be noted that the present invention is not limited to the above-described embodiment, and appropriate changes and modifications may be made to the above-described embodiment without departing from the scope and spirit of the present invention. 
     For example, as illustrated in  FIG. 5 , a bypass passage  18  may be connected to the exhaust passage  12  to bypass the DPF  16 , and a DPF  16   a  for measurement (second filter) with a small capacity may be provided in the bypass passage  18 . In this configuration, a pair of electrodes  17   a  and  17   b  may be disposed in the measurement DPF  16   a  such that the electrodes  17   a  and  17   b  oppose each other with at least one partition wall interposed between the electrodes  17   a  and  17   b , and an orifice (throttle)  18   a  may be provided in the bypass passage  18  for regulating the flow rate of the exhaust gas. In addition, when a forced regeneration of the measurement DPF  16   a  is performed, an electric current may be applied to the electrodes  17   a  and  17   b  to cause the electrodes  17   a  and  17   b  to function as a heater.