Abstract:
A direct injection diesel engine is provided with a fuel injection nozzle which is capable of performing a multistage injection. In a middle-or-high load region, in order to decrease soot, an after-injection is performed immediately after a main injection. In a case of fuel with a low Cetane number, the after-injection can cause a worsening of soot. Hence, an ignition delay interval (period of time) of the main injection is determined. In a case where the ignition time delay interval (period of time) is equal to or above a threshold value, the after-injection is inhibited.

Description:
TECHNICAL FIELD 
     The present invention relates to control apparatus and control method for a direct injection diesel engine in which a fuel injection nozzle which is capable of performing a multi-stage injection is equipped and an after-injection is performed immediately after a main injection. 
     BACKGROUND 
     In order to suppress a generation of soot which provides a problem in the direct injection diesel engine, a technique in which an after-injection in a relatively short period of time is performed immediately after a main injection and soot generated in association with a combustion of the main injection is combusted together with fuel due to the after-injection is described in Japanese Patent Application First Publication No. 2005-233163 and Japanese Patent Application First Publication No. 2000-227061 and so forth. 
     In such a technique of the after-injection as described above, a certain optimum value is present as an interval from a time at which the main injection is ended to a time at which the after-injection is started. As shown in a characteristic a of  FIG. 5 , as the interval shown in a lateral axis of  FIG. 5  becomes gradually larger, a reduction of soot is seen when the after-injection at an appropriate interval is performed. It should be noted that a left end of a lateral axis of  FIG. 5  indicates a case where the interval is zero, namely, a case where no after-injection is performed (a case where the injection is performed without a split of the injection into the main injection and after-injection is not made). 
     However, in a case where a Cetane number of fuel is low, a tendency of a reduction of soot when the interval is appropriately given is observed as shown in a characteristic b of  FIG. 5 . However, as compared to a case where no after-injection is performed, an emission level of soot rather becomes higher. 
     It can be thought that, according to inventor&#39;s research, this is caused by a low Cetane number of fuel so that an ignition delay of a main combustion (a combustion according to the main injection) is increased and a pre-mixture combustion rate is increased. 
     That is, as a first cause, gas flow within a cavity after the ignition in association with an increase in the pre-mixture combustion rate is increased and a rising speed of gas tried to flow out toward an upward direction from the cavity is increased. Therefore, light weight sprays of the after-injection are caused to flow in a rising flow of gas and are brought in a space at a lower side of a cylinder head (a gap space between the cylinder head and a crown surface of a piston). This space is under a low-temperature atmosphere field in an expansion stroke. Hence, an oxidization speed of soot due to fuel of the after-injection and the main combustion is reduced. 
       FIGS. 6A-6D  show situations in which the sprays according to the after-injection are affected according to a magnitude of the pre-mixture combustion rate and in which gas flow within a combustion chamber is indicated in minute arrow marks.  FIG. 6A  shows a state of the combustion chamber at a time immediately after the after-injection in a case where fuel having a relatively high Cetane number is used. A spray F after the after-injection is advanced toward an inside of the cavity and, at a bottom section of the cavity, a reverse squish flow R toward an outer peripheral side from the inside of the cavity is generated in association with the ignition combustion.  FIG. 6B  shows a state of a stage of combustion chamber at which the piston is slightly lowered from the state of  FIG. 6A . Although spray F collides against reverse squish flow R, the reverse squish flow is not so strong that a quantity of an upward brought out spray is small (refer to a region denoted by an enclosure of an ellipse M). 
     Whereas,  FIGS. 6C and 6D  show situations of the combustion chamber at the same crank angle as the cases of  FIGS. 6A and 6B , respectively, in a case where the Cetane number of fuel is low. As shown in  FIG. 6C , since the pre-mixture combustion rate is increased in association with the ignition delay when the Cetane number is low, reverse squish flow R is strongly generated. Consequently, as shown in  FIG. 6D , light weight spray F of the after-injection is brought out toward the upward direction (refer to the region denoted by the enclosure of ellipse M). It should be noted that those in bubble forms in  FIGS. 6B and 6D  are individually dispersed sprays. 
     In addition, as a second cause, it can be considered that, due to the ignition delay of the main combustion, the after-injection is performed under a low oxygen field during the main combustion and soot is increased due to a deficiency of oxygen. 
     SUMMARY 
     It is an object of the present invention to avoid a worsening of soot due to the after-injection when the Cetane number of fuel is low as described above. 
     According to the present invention, there is provided a control apparatus for a direct injection diesel engine, the diesel engine having a fuel injection nozzle capable of performing a multi-stage injection and in which an after-injection is performed immediately after a main injection, the control apparatus is configured to inhibit the after-injection when a pre-mixture combustion rate in a combustion according to the main injection is large. 
     When the Cetane number of fuel is low, the ignition delay is increased and the pre-mixture combustion rate is increased. In such a case as described above, the after-injection is inhibited. Thus, the worsening of soot can be avoided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural explanatory view of a diesel engine in a preferred embodiment; 
         FIG. 2  is an explanatory view of one example of a split injection; 
         FIG. 3  is a flowchart representing one example of a control; 
         FIG. 4  is a flowchart representing a different example of the control; 
         FIG. 5  is a characteristic graph representing a relationship between an interval of an after-injection and soot; and 
         FIGS. 6A-6D  are explanatory views representing an influence of a magnitude of a pre-mixture combustion rate on sprays. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be explained in details on a basis of attached drawings. 
       FIG. 1  is a structural explanatory view representing a direct injection diesel engine  1  together with its intake and exhaust systems. A piston  4  is slidably fitted to a cylinder  3  formed on a cylinder block  2 . A cylinder head  5  fixed on an upper surface of cylinder block  2  covers an upper end opening of cylinder  3 . 
     A re-entrant type cavity  6  is recessed on a top surface of piston  4 . This cavity  6  is concentrically formed with piston  4  and has a relatively large opening diameter. In addition, a multi-injection-hole fuel injection nozzle  7  is disposed at a center position of cylinder  3  corresponding to a center of cavity  6 . In this embodiment, above-described fuel injection nozzle  7  is disposed along a center axial line of cylinder  3 , namely, is vertically disposed. 
     A pair of intake valves  8  and a pair of exhaust valves  9  are disposed on above-described cylinder head  5 . The pair of intake valves  8  open and close tip opening sections of an intake port  10  and the pair of exhaust valves  9  open and close tip opening sections of an exhaust port  11 . 
     These intake valves  8  and exhaust valves  9  are disposed in vertical postures in which the respective valve stems thereof are parallel to center axial lines of cylinder  3 . A glow plug  12  is disposed adjacently to fuel injection nozzle  7 . 
     Fuel injection nozzle  7  of each cylinder is connected to a diagrammatically shown common rail  13 . When a needle (not shown) of fuel injection nozzle  7  is lifted in response to a drive signal from an engine control unit  16 , fuel under a high pressure supplied within common rail  13  by means of a high pressure fuel pump  14  is injected. The fuel pressure within common rail  13  is pressure regulated to a predetermined pressure which accords with a driving condition by means of engine control unit  16  via a pressure regulator valve  15 . It should, herein, be noted that above-described fuel injection nozzle  7  is a highly responsive characteristic nozzle using a piezoelectric element or so forth. Fuel injection nozzle  7  can inject and split in multiple stages a gross fuel injection quantity determined in accordance with a load. 
     Diesel engine  1 , in this embodiment, is provided with a turbocharger  18 . A turbine  19  of turbocharger  18  is disposed in a passage of an exhaust passage  21  and a compressor  20  of turbocharger  18  is disposed in a passage of an intake passage  22 . A pre-catalytic converter  23  and a main-catalytic converter  24  are serially arranged at a downstream side than turbine  19  of exhaust passage  21 . An airflow meter  25  and an air cleaner  26  are disposed at a downstream side than compressor  20  of intake passage  22 . An intercooler  27  is disposed in intake passage  22  between a collector section  28  which is at the downstream side than compressor  20  and compressor  20 . Furthermore, an exhaust recirculation unit includes: an exhaust recirculation passage  29  which communicates between a more upstream side position than turbine  19  of exhaust passage  21  and intake collector section  28 ; and an exhaust recirculation control valve  30  installed to control an exhaust recirculation percentage to a predetermined exhaust recirculation percentage in accordance with an engine driving condition. 
     Above-described engine control unit  16  inputs detection signals from such sensors as: airflow meter  25  described above; a revolution speed sensor  31  for detecting an engine revolution speed Ne; a rail pressure sensor  32  for detecting a rail pressure Prail within common rail  13 ; an intake temperature sensor  33  for detecting an intake temperature Tin; a boost pressure sensor  34  for detecting a supercharge (boost) pressure Boost; an atmospheric temperature sensor  35  for detecting an atmospheric (air) temperature Tatm; an atmospheric pressure sensor  36  for detecting an atmospheric pressure Patm; a water temperature sensor  37  for detecting a water (coolant) temperature Tw; and an internal cylinder pressure sensor  38  for detecting an internal cylinder pressure Pcyl. 
     In such a structure as described above, gross injection quantity Q to be injected from fuel injection nozzle  7  is determined according to the load of diesel engine  1  and a target torque thereof. In addition, a target value of fuel pressure within common rail  13 , namely, a target rail pressure tPrail is determined on a basis of gross fuel injection quantity Q, revolution speed Ne, water temperature Tw, atmospheric pressure Patm, and the atmospheric (air) temperature Tatm. Specifically, maps with gross injection quantity Q and revolution speed Ne as parameters are preset for each water temperature, for each atmospheric pressure, and for each atmospheric temperature Ne. By searching these maps, target rail pressure tPrail which accords with the driving condition at that moment is determined. 
     Then, fuel injected from fuel injection nozzle  7  is split in the multiple stages and injected in accordance with the driving condition. For example, as shown in  FIG. 2 , in addition to a main injection carried out with a top dead center TDC inserted in the main injection, fuel is split into a pilot injection, a pre-injection, and an after-injection and injected. The after-injection is provided to reduce soot mainly generated due to a combustion by the main injection. In a low load region and in a middle-or-high load region except a full open time, the after-injection is basically carried out. 
     A fuel injection quantity of the main injection and an injection timing of the main injection are determined, with a plurality of maps with gross injection quantity Q and revolution speed Ne parameters preset in accordance with water temperature Tw, atmospheric pressure Patm, and atmospheric (air) temperature Tatm, by searching corresponding values, in the same way as above-described target rail pressure tPrail. 
     An interval Int from an end of the main injection to a start of the after-injection (refer to  FIG. 2 ) is, similarly, determined on a basis of gross injection quantity Q, revolution speed Ne, water temperature Tw, atmospheric pressure Patm, and atmospheric (air) temperature Tatm. 
     Soot generated according to the main injection is combusted together with fuel of the after-injection by carrying out the after-injection at an optimum interval in accordance with the driving condition. Hence, a reduction of soot in the middle-or-high load region can be achieved. 
     However, as described before, in a case where fuel having the low Cetane number is used, the ignition delay of the main combustion is increased and the pre-mixture combustion rate is increased. Consequently, there is a possibility of worsening soot, on the contrary, in association with an execution of the after-injection. Therefore, in this embodiment, when the pre-mixture combustion rate is large, the after-injection is inhibited. 
       FIG. 3  shows a flowchart representing one example of a specific process. At a step  1 , engine control unit  16  determines whether the driving condition of diesel engine  1  is in a steady state. If engine control unit  16  determines that the driving condition is not in the steady state, no process after a step  2  is carried out. It should be noted that the after-injection itself is also carried out at a transient time if the driving condition is within a predetermined driving condition range. 
     If the driving condition is in the steady state at step  1 , the routine goes to a step  2 . At step  2 , engine control unit  16  derives the ignition delay interval (period of time) of the main combustion according to the main injection. For example, an internal cylinder rate of heat release is sequentially determined on a basis of the detection signal of internal cylinder pressure sensor  38  and an interval of time until this rate of heat release exceeds a predetermined threshold value is detected as an ignition time delay interval (period of time). The ignition time delay interval (period of time) may be calculated from other parameters not using the internal cylinder pressure sensor  38  detecting an actual combustion state. The ignition delay interval (period of time) may, for example, be calculated from the parameters such as the injection quantity of the pre-injection, atmospheric pressure Patm, boost pressure Boost, the Cetane number of fuel, and so forth. It should be noted that the Cetane number of fuel can be estimated during the driving by an appropriate method. 
     Next, at a step  3 , engine control unit  16  determines whether the above-described ignition delay interval (period of time) is equal to or above a predetermined threshold value or not. This threshold value is determined from a map with gross injection quantity Q and revolution speed Ne as the parameters. 
     In a case where the ignition delay interval (period of time) is equal to or above the predetermined threshold value, the routine goes to a step  4 . At step  4 , the after-injection is inhibited. As described before, when the Cetane number of fuel is low and the pre-mixture combustion rate becomes large in association with the increase of the ignition delay, the worsening of soot, on the contrary, is observed. Hence, in a case where the ignition delay interval (period of time) is equal to or above the threshold value, the worsening of soot is avoided due to the inhibit of the after-injection. 
     If the ignition delay interval (period of time) is not equal to or above (less than) the threshold value, engine control unit  16  allows the after-injection and the routine goes to a step  5 . At the step  5 , engine control unit  16  calculates a required correction quantity for interval Int from the end of the main injection to the start of the after-injection. This can, for example, be determined on a basis of the parameters of revolution speed Ne, gross injection quantity Q, the injection timing of the main injection, boost pressure Boost, rail pressure Prail, atmospheric pressure Patm, and so forth. 
     Then, at a step  6 , engine control unit  16  determines a final interval Int and the injection quantity of the after-injection. 
     As described above, in the above-described embodiment, the after-injection is inhibited in a case where the ignition delay interval (period of time) is increased due to the low Cetane number of fuel. Hence, unnecessary worsening of soot due to the after-injection can be avoided. 
       FIG. 4  shows a flowchart representing another example of the specific process. At a step  11 , in the same way as above-described step  1 , engine control unit  16  determines whether the driving condition of the diesel engine  1  is in the steady state. 
     If the engine driving condition is in the steady state, the routine goes to a step  12 . At step  12 , engine control unit  16  determines a maximum rate of heat release of the main combustion caused by the main injection. For example, the internal cylinder rate of heat release is sequentially determined on a basis of the detection signal of internal cylinder pressure sensor  38  and is detected with a maximum value during one cycle as the maximum rate of heat release. Without use of internal cylinder pressure sensor  38  detecting the actual combustion state, the ignition delay interval may be calculated from the other parameters. For example, the ignition delay interval (period of time) can be calculated from the parameters of the injection quantity of the pre-injection, the atmospheric pressure Patm, the boost pressure Boost, intake air temperature Tin, rail pressure Prail, the injection timing of the main injection, revolution speed Ne, the Cetane number of fuel, the main injection quantity, and so forth. 
     Next, at a step  13 , engine control unit  16  determines whether the above-described maximum rate of heat release is equal to or above a predetermined threshold value. This predetermined threshold value is determined from a map with gross injection quantity Q and revolution speed Ne as the parameters. 
     In a case where the maximum rate of heat release is equal to or above the predetermined threshold value, the routine goes to a step  14 . At step  14 , the after-injection is inhibited. As described before, when the Cetane number of fuel is low and the pre-mixture combustion rate becomes large, the worsening of soot is, on the contrary, observed. In this example, whether an abrupt combustion such that the pre-mixture combustion rate is large or not is determined on a basis of the maximum rate of heat release. In a case where the abrupt combustion in which the pre-mixture combustion rate is large occurs, the worsening of soot is avoided by the inhibit of the after-injection. 
     If the maximum rate of heat release is not equal to or above (less than) the predetermined threshold value, the after-injection is allowed and the routine goes to a step  15 . At step  15 , the required correction quantity for the interval Int from the end of the main injection to the start of the after-injection is calculated. This can, for example, be determined on a basis of the parameters of revolution speed Ne, gross injection quantity Q, the injection timing of the main injection, boost pressure Boost, rail pressure Prail, atmospheric pressure Patm, and so forth. 
     Then, at a step  16 , engine control unit  16  determines final interval Int and the injection quantity of the after-injection. 
     In this way, in the above-described embodiment, the after-injection is inhibited in a case where the maximum rate of heat release becomes excessive due to the low Cetane number of fuel. The unnecessary worsening of soot according to the after-injection can be avoided.