Abstract:
The present invention provides a fuel control for an internal-combustion engine having an exhaust gas purifying device that is disposed in an exhaust system of the engine. A reducing agent is supplied to the exhaust gas purifying device by injecting fuel in an expansion stroke or an exhaust stroke of the engine. An intake air amount supplied to the engine is controllable. The fuel control includes decreasing the intake air amount supplied to the engine during the fuel injection for the supply of the reducing agent. The fuel control also includes advancing an injection timing of the fuel injection for the supply of the reducing agent when an actual intake air amount is less than a desired intake air amount or when an actual boost pressure is higher than a desired boost pressure. The advancement of the injection timing improves the combustibility of engine while decreasing the HC exhaust amount.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a control of fuel to be supplied to an internal-combustion engine. 
         [0002]    Conventionally, in order to regenerate an exhaust gas purifying device provided in an exhaust system of an engine, an air/fuel ratio is switched from lean to rich at a predetermined timing. Japanese Patent Application Publication No. 2003-322015 discloses a technique for performing a post-injection of a small amount of fuel after a main injection so as to reduce NOx trapped in a NOx trapping catalyst disposed in an exhaust system of an engine. According to this technique, a fuel amount to be injected in the post-injection and its injection timing are determined based on a delay of an actual value with respect to a desired value for the intake air amount. 
         [0003]    The post-injection is capable to enrich the air/fuel ratio to reduce NOx without any torque variation. However, if such a post-injection is performed under a state of air shortage, the amount of unburned fuel to be exhausted, that is, the exhaust amount of HC (hydrocarbon) may increase because the post-injection is performed in an expansion stroke or an exhaust stroke. 
         [0004]    Considering these problems, the present invention aims at suppressing an increase of the HC exhaust amount even when an injection for reducing an exhaust gas purifying device is performed under a state of air shortage. 
       SUMMARY OF THE INVENTION 
       [0005]    According to one aspect of the invention, a fuel control for an engine is provided. The engine has an exhaust gas purifying device that is disposed in an exhaust system of the engine. A reducing agent is supplied to the exhaust gas purifying device by injecting fuel in an expansion stroke or an exhaust stroke of the engine. An intake air amount supplied to the engine is controllable. The fuel control includes decreasing the intake air amount supplied to the engine during the fuel injection for the supply of the reducing agent. The fuel control also includes advancing an injection timing of the fuel injection for the supply of the reducing agent when an actual intake air amount is less than a desired intake air amount or when an actual boost pressure is higher than a desired boost pressure. 
         [0006]    When an actual intake air amount is less than a desired intake air amount or when an actual boost pressure is higher than a desired boost pressure by decreasing the intake air amount, there may occur a shortage of the amount of air with respect to the amount of fuel, which may cause an increase in the unburned fuel and eventually an increase in the HC exhaust amount. According to this invention, in such a situation, the injection timing of the fuel injection performed for the reduction is advanced. Therefore, an atmosphere where the temperature and the pressure inside a cylinder are high is created to improve the ignitability, which can more surely burn the fuel injected for the reduction. Thus, an increase in the HC exhaust amount due to the unburned fuel can be suppressed. Further, according to this invention, because the combustibility of the fuel injected for the reduction is improved, a supply of CO (carbon monoxide) required as a reducing agent of a catalyst that traps NOx (nitrogen oxides) can be secured while suppressing an increase in the HC exhaust amount. Because the amount of fuel injection required for supplying the equivalent amount of CO can be decreased, the fuel efficiency can be improved. 
         [0007]    According to one embodiment of the present invention, the fuel control further includes setting a limit on an amount of the advancement of the injection timing of the fuel injection for the supply of the reducing agent. Thus, because the advance angle amount of the injection timing is limited, occurrence of smoke can be restricted to an allowable level. 
         [0008]    According to one embodiment of the present invention, an amount of the advancement of the injection timing is determined by adding an advance angle amount determined based on a difference between the actual intake air amount and the desired intake air amount and an advance angle amount determined based on a difference between the actual boost pressure and the desired boost pressure. Thus, the injection timing can be determined such that an increase in the HC exhaust amount is suppressed in terms of both of the intake air amount and the boost pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram showing an engine and its control unit in accordance with one embodiment of the present invention. 
           [0010]      FIG. 2  is a graph showing an increase in an HC exhaust amount caused by an enrichment operation. 
           [0011]      FIG. 3  is a graph for explaining that an HC exhaust amount can be decreased by advancing a post-injection timing. 
           [0012]      FIG. 4  is a block diagram of a control apparatus in accordance with one embodiment of the present invention. 
           [0013]      FIG. 5  is a flowchart of a control process in accordance with one embodiment of the present invention. 
           [0014]      FIG. 6  shows maps for defining an advance angle amount based on an intake air amount difference and a boost pressure difference in accordance with one embodiment of the present invention. 
           [0015]      FIG. 7  shows an effect of a control in accordance with one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Preferred embodiments of the present invention will be described referring to the attached drawings.  FIG. 1  is an overall system structure of an internal-combustion engine (which will be hereinafter referred to as an “engine”) and its control unit in accordance with one embodiment of the present invention. 
         [0017]    An electronic control unit (hereinafter referred to as an “ECU”)  1  is a computer having a central processing unit (CPU) and a memory. The memory can store one or more computer programs for implementing various controls for a vehicle and data required for executing the programs. The ECU  1  receives data sent from each section of the vehicle and performs operations using the received data to generate control signals for controlling each section of the vehicle. 
         [0018]    In this embodiment, an engine  2  is a diesel engine. The engine  2  comprises, for example, four cylinders. One of the cylinders is illustrated in  FIG. 1 . 
         [0019]    An intake manifold  3  and an exhaust manifold  4  are connected to the engine  2 . A combustion chamber  5  is formed between a piston  6  and a cylinder head  7 . A fuel injection valve  8  is attached in such a manner as to protrude into the combustion chamber  5 . The fuel injection valve  8  is connected to a high pressure pump  9  and a fuel tank (not shown in the figure) via a common rail (not shown in the figure). The high pressure pump  9  raises a pressure of fuel in the fuel tank and then sends the fuel to the fuel injection valve  8  via the common rail. The fuel injection valve  8  injects the received fuel into the combustion chamber  5 . An injection pressure of the fuel (which is referred to as a “fuel pressure”) can be changed by controlling the high pressure pump  9  through a control signal from the ECU  1 . The fuel pressure is detected by a fuel pressure sensor that is disposed in the common rail. Its detection signal is sent to the ECU  1 . Further, an injection time period (or injection amount) and an injection timing of the fuel injection valve  8  are controlled in accordance with a control signal from the ECU  1 . 
         [0020]    A crank angle sensor  10  is provided in the engine  2 . The crank angle sensor  10  outputs a CRK signal and a TDC signal in accordance with the rotation of a crank shaft  11  to the ECU  1 . The CRK signal is a pulse signal to be output at every predetermined crank angle. In response to the CRK signal, the ECU  1  calculates a rotational speed NE of the engine  2 . The TDC signal is a pulse signal to be output at a crank angle associated with a top dead center (TDC) position of the piston  6  at a start time of the intake stroke. In case of the 4-cylinder engine, the TDC signal is output at every 180 degrees of the crank angle. 
         [0021]    A supercharger  12  is provided. The supercharger  12  has a rotatable compressor  13  disposed in the intake manifold, a rotatable turbine  14  disposed in the exhaust manifold  4  and a shaft  15  connecting the compressor  13  and the turbine  14 . The turbine  14  is driven to rotate by the kinetic energy of the exhaust gas. The rotational movement of the turbine  14  drives the compressor  13  to rotate to compress the intake air. 
         [0022]    The turbine  14  has a plurality of rotatable variable vanes  16  (only two vanes are shown in the figure). An actuator  17  is connected to each of the variable vanes  16 . The actuator  17  changes the opening degree of the variable vane  16  (which is referred to as a “vane opening degree”) in accordance with a control signal from the ECU  1 . By changing the vane opening degree, the rotational speed of the turbine  14  can be changed. As the vane opening degree is smaller, the rotational speed of the turbine  14  is greater and hence the boost pressure increases. 
         [0023]    In the intake manifold  3 , an air flow sensor  20  is disposed upstream of the compressor  13 , and a water-cooled intercooler  21  and a boost pressure sensor  22  are disposed downstream of the compressor  13 . The air flow sensor  20  detects an amount of intake air introduced into the intake manifold  3 . The boost pressure sensor  22  detects a pressure (boost pressure) inside of the intake manifold  3 . These detection signals are sent to the ECU  10 . The intercooler  21  operates to cool down the intake air when, for example, a temperature of the intake air rises due to the boost operation by the supercharger  12 . 
         [0024]    A throttle valve  23  is disposed downstream of the boost pressure sensor  22 . An actuator  24  is connected to the throttle valve  23  to control the opening degree of the throttle valve  23  in accordance with a control signal from the ECU  1 . 
         [0025]    In the downstream of the throttle valve  23 , the intake manifold branches corresponding to respective cylinders. Each intake manifold branch extends to a combustion chamber of the corresponding cylinder through an intake port. The intake manifold is partitioned into two passages  25 ,  26 . In one passage  25 , a swirl valve  27  is provided and connected with an actuator  28 , which can change the opening degree of the swirl valve  27  in accordance with a control signal from the ECU  1 . The strength of the swirl that occurs in the combustion chamber  5  can be controlled by the opening degree of the swirl valve  27 . 
         [0026]    An EGR pipe  31  is provided between the intake manifold  3  and the exhaust manifold  4 , more specifically, between the passage  26  in a collecting portion for the intake manifold branches and the upstream of the turbine  14  in the exhaust manifold  4 . Through the EGR pipe  31 , a part of the exhaust gas of the engine  2  is recirculated as an EGR gas. Such recirculation decreases the combustion temperature in the combustion chamber  5  and hence can decrease NOx in the exhaust gas. 
         [0027]    An EGR control valve  32  is disposed in the EGR pipe  31 . In one example, the EGR control valve  32  is formed by a linear electromagnetic valve so that a lift amount of the EGR control valve  32  can be linearly changed in accordance with a control signal from the EGU  1 . According to the lift amount of the EGR control valve  32 , an amount of the EGR gas to be recirculated can be controlled (such an amount is referred to as an “EGR amount”). 
         [0028]    A switching valve  35  and an EGR cooler  36  are disposed in the EGR pipe  31 . A passage  37  is a bypass passage for bypassing the EGR cooler  36 . The switching valve  35  selectively switches the downstream of the switching valve  35  between the EGR pipe  31  and the bypass passage  37  in accordance with a control signal from the EGU  1 . When the bypass passage  37  is selected, the EGR gas is introduced into the bypass passage  37  and then recirculated into the intake manifold  3 . When the EGR pipe  31  is selected, the EGR gas is cooled by the EGR cooler  36  and then recirculated into the intake manifold  3 . 
         [0029]    A three-way catalyst  41  and a NOx catalyst  42  are disposed downstream of the turbine  14  of the exhaust manifold  4 . When the air/fuel ratio is a theoretical (stoichiometric) air/fuel ratio, the three-way catalyst  41  oxidizes HC and CO while reducing NOx to purify the exhaust gas. When the air/fuel ratio is lean and the oxygen concentration in the exhaust gas is relatively high, the NOx catalyst  42  captures NOx contained in the exhaust gas. When the air/fuel ratio is rich and the concentration of the reducing agent (HC, CO) in the exhaust gas is relatively high, the NOx catalyst  42  reduces the captured NOx to purify the exhaust gas. 
         [0030]    Further, a LAF sensor  43  is disposed upstream of the three-way catalysts  41 . The LAF sensor  43  linearly detects an oxygen concentration in the exhaust gas over a wide range of the air-fuel ratio from rich to lean. Based on the oxygen concentration detected by the LAF sensor  43 , the ECU  1  calculates an actual air/fuel ratio of an actual air-fuel mixture that has burned in the combustion chamber  5 . 
         [0031]    An accelerator pedal opening degree sensor  46  is further connected to the ECU  1  to output a detection signal representing an operation amount (which is referred to as an accelerator opening degree) of an accelerator pedal (not shown in the figure). 
         [0032]    In response to the above-described input signals and in accordance with one or more programs and data (including one or more maps) stored in the memory, the ECU  1  detects an operating condition of the engine  2  and controls the fuel injection amount, the fuel injection timing, the EGR amount, the intake air amount, the boost pressure and so on. 
         [0033]    In a normal operation of the engine  2 , the air/fuel ratio is set to a value on the lean side with respect to the theoretical air/fuel ratio. As described above, the NOx catalyst  42  operates to trap NOx contained in the exhaust gas when the air/fuel ratio is lean. Therefore, in order to regenerate this catalyst, it is required to switch the air/fuel ratio to rich at a predetermined timing so as to supply the reducing agent (HC, CO) into the exhaust gas and desorb the trapped NOx from the catalyst. 
         [0034]    On the other hand, one combustion cycle consists of an intake stroke for taking air into the combustion chamber  5  from the vicinity of the top dead center (TDC) to the vicinity of the bottom dead center of the piston, a compression stroke for compressing the intake air by the rising piston  6  from the vicinity of the bottom dead center to the vicinity of the top dead center of the piston  6 , an expansion stroke for pushing down the piston  6  by the combustion of the air-fuel mixture, and an exhaust stroke for exhausting the gas in the combustion chamber from the vicinity of the bottom dead center to the vicinity of the top dead center of the piston  6 . An injection for causing the engine to output a desired torque (such an injection is referred to as a “main injection”) is typically performed in the vicinity of the top dead center in the compression stroke. In this embodiment of the present invention, the air/fuel ratio is enriched by an injection performed in the expansion stroke or the exhaust stroke after the main injection (such an injection is referred to as a “post-injection”). The fuel injected through the post-injection supplies the reducing agent (CO, HC) into the exhaust gas, which can reduce the NOx catalyst  42 . By using the post-injection, the NOx catalyst  42  can be regenerated while variations in the output torque are suppressed. 
         [0035]      FIG. 2(   a ) shows an example of a behavior of an actual intake air amount and an HC exhaust amount in a certain engine operating condition without applying the present invention. 
         [0036]    In this example, a fuel injection amount that is injected by the fuel injection valve is feedback-controlled such that the actual air/fuel ratio converges to a desired air/fuel ratio. The EGR amount is also feedback-controlled such that the actual intake air amount converges to a desired intake air amount. The opening degree of the throttle valve and the boost pressure are set to corresponding desired values that are determined based on the engine operating condition. 
         [0037]    At time t 1 , in response to satisfaction of a predetermined enrichment condition, the air/fuel ratio is switched from lean to rich. The enrichment operation is implemented by increasing the fuel by performing the post-injection in addition to the main injection while decreasing the opening degree of the throttle valve to a desired throttle opening. 
         [0038]    When the opening degree of the throttle valve is decreased, the amount of air taken into the engine is temporarily and abruptly decreased. Although the feedback control for the EGR amount tries to cause the actual intake air amount to converge to the desired intake air amount, the actual intake air amount may temporarily fall below (undershoot) the desired intake air amount. If such undershooting occurs, a difference temporarily occurs between the desired intake air amount and the actual intake air amount (time t 1  through t 2 ). Because the actual intake air amount falls short of the desired intake air amount, the unburned fuel increases and hence the amount of HC that is to be exhausted to the atmosphere temporarily increases as shown in the figure. 
         [0039]    Even if such undershooting does not occur, the HC exhaust amount may increase when the opening degree of the throttle valve is decreased at the enrichment operation. In this regard, referring to  FIG. 2  ( b ), an example of a behavior of an actual boost pressure and an HC exhaust amount in a certain engine operating condition without applying the present invention is shown. Similarly to  FIG. 2(   a ), the opening degree of the throttle valve is decreased so as to perform the enrichment operation at time t 1 . 
         [0040]    By decreasing the throttle opening degree, the pressure of air upstream of the throttle valve, which can be detected by the boost pressure sensor  22  of  FIG. 1 , temporarily rises. As a result, as shown in the figure, the actual boost pressure may exceed (overshoot) a desired boost pressure. Due to such overshooting, a difference occurs between the desired boost pressure and the actual boost pressure (time t 1  through t 2 ). The magnitude of the difference reflects the amount of new air that has decreased by narrowing the opening degree of the throttle valve. In order to cause the actual intake air amount to converge to the desired intake air amount, the EGR amount corresponding to the decreased amount of the new air is introduced into the engine. Because a ratio of the EGR amount with respect to the desired intake air amount relatively increases, the unburned fuel increases and hence the amount of HC that is to be exhausted to the atmosphere temporarily increases as shown in  FIG. 2(   b ). 
         [0041]    Thus, a problem may occur that the HC exhaust amount increases when the opening degree of the throttle valve is made smaller in switching the air/fuel ratio from lean to rich. The present invention suppresses such an increase in the HC exhaust amount by advancing the injection timing of the post-injection. By advancing the post-injection timing, the fuel injected by the post-injection can be burned under a condition where the temperature and the pressure inside of the combustion chamber are high. Therefore, the injected fuel can be more surely burned. As a result, an occurrence of the unburned fuel can be suppressed and hence an increase in the HC exhaust amount can be suppressed. 
         [0042]    In this regard,  FIG. 3  shows an example of a simulation result of a rate of heat release (ROHR) for different post-injection timings when the actual intake air amount is less than the desired intake air amount. As the value of the ROHR is higher, the fuel is burned more actively. Reference numeral  51  indicates an ROHR in the case where the post-injection timing is 40 degrees after the compression TDC (that is, ATDC 40 degrees). Reference numeral  52  indicates an ROHR in the case where the post-injection timing is 55 degrees after the compression TDC (that is, ATDC 55 degrees). As clearly seen from a comparison of both ROHRs around 60 degrees after the compression TDC, the ROHC when the post-injection timing is 40 degrees after the compression TDC is higher than the ROHC when the post-injection timing is 55 degrees after the compression TDC. That is, the fuel is burned more actively if the post-injection timing is advanced. As the fuel is burned more actively, an occurrence of the unburned fuel is suppressed and hence the HC exhaust amount can be decreased. 
         [0043]      FIG. 4  shows functional blocks of a control apparatus in accordance with one embodiment of the present invention. These functional blocks are implemented in the ECU  1 . 
         [0044]    An enrichment control part  61  starts an enrichment control that involves a post-injection in response to satisfaction of a predetermined condition so as to perform a process for reducing the NOx catalyst  42 . The condition for performing this control is, for example, that an absorption amount of the NOx catalyst, which is estimated in accordance with any appropriate method (for example, refer to Japanese Patent Application Publication No. 2006-242170), is greater than a predetermined amount. Alternatively, the enrichment control may be started in a predetermined time interval or in a predetermined traveling distance of the vehicle. 
         [0045]    If the enrichment control that involves the post-injection is permitted, the enrichment control part  61  refers to a predetermined map (not shown in the figure) based on the engine rotational speed NE and the accelerator opening degree AP (which represents a requested torque) to determine a desired throttle valve opening degree THCMD. This map is defined such that a desired throttle valve opening degree is smaller than an opening degree (for example, a full opening) at a normal operation of the engine when the air/fuel ratio is lean. The actuator  24  controls the opening degree of the throttle valve  23  in accordance with the desired throttle opening THCMD thus determined. Thus, the intake air amount that is taken into the engine  2  through the throttle valve  23  is decreased. 
         [0046]    Further, the enrichment control part  61  refers to a predetermined map (not shown in the figure) based on the engine rotational speed NE and the accelerator opening degree AP (which represents a requested torque) to determine a desired boost pressure BPCMD. The actuator  17  controls the variable vanes  16  in accordance with the desired boost pressure BPCMD thus determined. 
         [0047]    The enrichment control part  61  determines, through a predetermined feedback control for the EGR amount, a desired EGR gas amount EGRCMD for causing an actual intake air amount GA to converge to a desired intake air amount GACMD. The desired intake air amount GACMD is determined by referring to a predetermined map (not shown in the figure) based on the engine rotational speed NE and the accelerator opening degree AP. The EGR control valve  32  is driven in accordance with the desired EGR gas amount EGRCMD thus determined. 
         [0048]    The enrichment control part  61  further refers to a predetermined map (not shown in the figure) based on the engine rotational speed NE and the accelerator opening degree AP to determine a desired fuel injection amount POSTCMD for the post-injection. In this map, a desired fuel injection amount POSTCMD for the post-injection is defined such that a combination of the desired fuel injection amount for the post-injection and a desired fuel injection amount for the main injection achieves a desired air/fuel ratio. Here, the desired air/fuel ratio is set to a value on the rich side with respect to the theoretical air/fuel ratio. Further, the enrichment control part  61  determines a basic post-injection timing POSTTIM for the post-injection by referring to a predetermined map (not shown in the figure) based on the engine rotational speed NE and the accelerator opening degree AP. This map is defined such that the basic post-injection timing POSTTIM is a timing value within the expansion stroke or the exhaust stroke. 
         [0049]    A first advance angle amount determining part  62  determines a first advance angle amount ΔPOST 1  corresponding to a difference between the actual and desired intake air amounts when the actual intake air amount GA is less than the desired intake air amount GACMD. The first advance angle amount ΔPOST 1  is determined such that it is greater with an increase in the difference. Thus, the post-injection timing is more advanced as the shortage of the air amount is greater. As a result, the combustibility is improved and the HC exhaust amount is decreased. 
         [0050]    A second advance angle amount determining part  63  determines a second advance angle amount ΔPOST 2  corresponding to a difference between the actual and desired boost pressures when the actual boost pressure BPA is higher than the desired boost pressure BPCMD. The second advance angle amount ΔPOST 2  is determined such that it is greater with an increase in the difference. Thus, the post-injection timing is more advanced as the shortage of the air amount is greater. As a result, the combustibility is improved and the HC exhaust amount is decreased. 
         [0051]    An addition part  64  adds the first advance angle amount ΔPOST 1  and the second advance angle amount ΔPOST 2  to calculate a total advance angle amount ΔPOST as in the following equation: 
         [0000]      ΔPOST=ΔPOST1+ΔPOST2 
         [0052]    A limiting part  65  applies a limiting process to the total advance angle amount ΔPOST by using a predetermined threshold value. As the advance angle amount for the post-injection timing increases, the HC exhaust amount decreases. However, it may cause more smoke because the combustion is more active. Therefore, it is preferable to limit the advance angle amount by the predetermined threshold value such that the amount of smoke does not exceed a predetermined allowable level. The threshold value can be pre-established through a simulation or the like. 
         [0053]    In accordance with the total advance angle amount ΔPOST, a correction part  66  corrects the basic post-injection timing POSTTIM determined for the post-injection to determine a final post-injection timing FPOSTTIM. Thus, the post-injection by the fuel injection valve  8  is performed to inject the above-determined desired fuel injection amount POSTCMD in accordance with the final post-injection timing FPOSTTIM thus determined through the correction based on the shortage of the air amount. 
         [0054]    In this embodiment, the final post-injection timing is determined based on the total advance angle amount calculated by adding the first advance angle amount and the second advance angle amount. As described above referring to  FIG. 2 , by using both the first and second advance angle amounts, an increase in the HC exhaust amount can be suppressed not only in the case where the actual intake amount undershoots the desired intake air amount but also in the case where the actual boost pressure overshoots the desired boost pressure by narrowing the opening of the throttle valve. Alternatively, one of the first and second advance angle amounts may be used to control the post-injection timing. In this alternative case, one of the first advance angle amount determining part  62  and the second advance angle amount determining part  63  is provided and the addition part  64  is not provided. The limiting part  65  applies the above-described limiting process to the first or second advance angle amount. The correction part  66  corrects the basic post-injection timing in accordance with the limited first or second advance angle amount to determine the final post-injection timing. One of the first advance angle amount and the second advance angle amount that is used to correct the basic post-injection timing may be selected in accordance with the operating condition of the engine  2 . 
         [0055]    Referring to  FIG. 5 , a process for implementing a fuel control according to one embodiment of the invention will be described. In this example, the process is performed by the ECU 1 , more specifically, by the first advance angle amount determining part  62 , the second advance angle amount determining part  63 , the addition part  64 , the limiting part  65  and the correction part  66  in  FIG. 4 . In this example, the process is performed in synchronization with an input of the TDC signal. 
         [0056]    In step S 1 , an enrichment flag, which is to be set to a value of one when a predetermined condition for starting an enrichment control involving a post-injection is satisfied, is examined. If the decision in step S 1  is “No”, the process terminates here. If the decision in step S 1  is “Yes”, it indicates that the enrichment control involving the post-injection is being carried out. The process proceeds to step S 3 . 
         [0057]    Step S 3  is a process performed by the first advance angle amount determining part  62 . According to the following equation, a difference ΔQair between the actual intake air amount GA that is detected by the air flow meter  20  and a desired intake air amount GACMD is calculated. 
         [0000]      Δ Q air= GACMD−GA    
         [0058]    A map as shown in  FIG. 6(   a ) is referred to based on the intake air amount difference ΔQair to determine the first advance angle amount ΔPOST 1  for the post-injection timing corresponding to the difference ΔQair. This map may be stored in the memory of the ECU  1 . This map is defined such that the advance angle amount is greater as the intake air amount difference ΔQair is greater. In other words, this map is defined such that the advance angle amount is greater, as the shortage of the actual intake air amount with respect to the desired intake air amount, which is caused by decreasing the opening degree of the throttle valve, is greater. Thus, as the shortage of the intake air amount is greater, the advance angle amount is more increased so that the combustibility of the fuel is improved. 
         [0059]    Step S 4  is a process performed by the second advance angle amount determining part  62 . According to the following equation, a difference ΔBPA between the actual boost pressure BPA that is detected by the boost pressure sensor  22  and a desired boost pressure BPCMD is calculated. 
         [0000]      Δ BPA=BPA·BPCMD    
         [0060]    A map as shown in  FIG. 6(   b ) is referred to based on the boost pressure difference ΔBPA to determine the second advance angle amount ΔPOST 2  for the post-injection timing corresponding to the difference ΔBPA. This map may be stored in the memory of the ECU  1 . This map is defined such that the advance angle amount is greater, as the boost pressure difference ΔBPA is greater. In other words, this map is defined such that the advance angle amount is greater, as the amount of new air that has been decreased by decreasing the opening degree of the throttle valve is greater. Thus, as the amount of a decrease in the new air is greater, the advance angle amount is more increased so that the combustibility of the fuel is improved. 
         [0061]    Steps S 3  and S 4  may be performed in parallel or in inverse order. In step S 5 , the first advance angle amount ΔPOST 1  and the second advance angle amount ΔPOST 2  are added to determine the total advance angle amount ΔPOST as described above. In step S 6 , a limiting process using a predetermined threshold value is applied to the total advance angle amount ΔPOST. For example, assuming that the threshold value is set to a crank angle of “30” degrees, the total advance angle amount ΔPOST is set to this threshold value of “30” when the total advance angle amount is greater than “30”. 
         [0062]    In step S 7 , a final post-injection timing FPOSTTIM is calculated by subtracting the limited total advance angle amount ΔPOST from the basic post-injection timing POSTTIM as expressed in the following equation: 
         [0000]        F POST TIM =POST TIM −ΔPOST 
         [0063]    Thus, the post-injection by the fuel injection valve  8  is carried out in accordance with the final post-injection timing FPOSTTIM. 
         [0064]    Alternatively; a threshold value may be applied to the intake air amount difference ΔQair and the boost pressure difference ΔBPA. For example, the first advance angle amount may be determined when the intake air amount difference ΔQair is equal to or greater than a predetermined value. The second advance angle amount may be determined when the boost pressure difference ΔBPA is equal to or greater than a predetermined value. 
         [0065]      FIG. 7(   a ) shows an example of a simulation result regarding the actual intake air amount and the HC exhaust amount when the above-described technique in accordance with the present invention is applied. As clearly seen from a comparison with  FIG. 2(   a ), even when the actual intake amount undershoots the desired intake air amount by decreasing the opening degree of the throttle valve in the enrichment control, an increase in the HC exhaust amount is suppressed because an occurrence of the unburned fuel is suppressed by advancing the post-injection timing. Further,  FIG. 7(   b ) shows an example of a simulation result regarding the actual boost pressure and the HC exhaust amount when the above-described technique in accordance with the present invention is applied. As clearly seen from a comparison with  FIG. 2(   b ), even when the actual boost pressure overshoots the desired boost pressure by decreasing the opening degree of the throttling valve in the enrichment control, an increase in the HC exhaust amount is suppressed because an occurrence of the unburned fuel is suppressed by advancing the post-injection timing. 
         [0066]    Although the embodiments of the present invention have been described above with reference to a diesel engine as an example, the present invention can be applied to a gasoline engine or the like. Further, the present invention can be applied to a general-purpose internal-combustion engine (such as an outboard motor or the like).