Abstract:
An engine ECU executes a program including the steps of: determining presence of abnormality in a low-pressure fuel system; ceasing an intake manifold injector when determination is made of abnormality in the low-pressure fuel system; increasing the target purge rate when the engine operation state attains an injection partaking state between an in-cylinder injector and an intake manifold injector; reducing the VVT overlap; and retarding the ignition timing.

Description:
This nonprovisional application is based on Japanese Patent Application No. 2004-319116 filed with the Japan Patent Office on Nov. 2, 2004, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an internal combustion engine including a first fuel injection mechanism (in-cylinder injector) for injecting fuel into a cylinder and a second fuel injection mechanism (intake manifold injector) for injecting fuel towards an intake manifold or intake port. Particularly, the present invention relates to the technique of suppressing torque variation in a state in which fuel must be injected from the first fuel injection mechanism alone in a region partaking in fuel injection. 
   2. Description of the Background Art 
   An internal combustion engine is well known, including an intake manifold injector for injecting fuel into the intake manifold of the engine and an in-cylinder injector for injecting fuel into the engine combustion chamber, wherein the fuel injection ratio of the intake manifold injector to the in-cylinder injector is determined based on the engine speed and engine load. 
   In the event of operation failure due to a malfunction of the in-cylinder injector or the fuel system that supplies fuel to the in-cylinder injector (hereinafter, referred to as high-pressure fuel supply system), fuel injection by the in-cylinder injector will be ceased. 
   On the basis of the fail-safe faculty in such operation failure, it is possible to ensure travel by inhibiting fuel injection from the in-cylinder injector and fix the combustion mode at the uniform combustion mode to effect fuel injection from the intake manifold injector alone. However, since the intake manifold injector is set to take an auxiliary role of the in-cylinder injector, fuel of a quantity corresponding to the intake air at the time of full opening of the throttle valve cannot be supplied, whereby the air-fuel ratio in the fail-safe mode will become lean. There may be the case where the torque is insufficient due to combustion defect. 
   Japanese Patent Laying-Open No. 2000-145516 discloses an engine controlling device that can maintain the air-fuel ratio properly to obtain suitable driving power even during fuel injection control by the intake manifold injector alone in the fail-safe mode caused by operation failure of the in-cylinder injector. This engine controlling device includes an in-cylinder injector that directly injects fuel to the combustion chamber, an intake manifold injector that injects fuel to the intake system, and an electronic control type throttle valve. When the target fuel injection quantity set based on the engine operation state exceeds a predetermined injection quantity of the in-cylinder injector, the engine controlling device compensates for the insufficient quantity by fuel injection from the intake manifold injector. This engine controlling device also includes an abnormality determination unit determining abnormality of the in-cylinder injector and the high-pressure fuel supply system that supplies fuel to the in-cylinder injector, a target fuel correction unit comparing the maximum injection quantity of the intake manifold injector when abnormality is determined with the target fuel injection quantity to fix the target fuel injection quantity at the maximum injection quantity when the target fuel injection quantity exceeds the maximum injection quantity, a target intake air quantity correction unit calculating the target intake air quantity based on the target fuel injection quantity fixed at the maximum injection quantity and the target air-fuel ratio, and a throttle opening indication value calculation unit calculating the throttle opening indication value with respect to an electronic control type throttle valve based on the target intake air quantity. 
   When abnormality is sensed in the in-cylinder injector and the high-pressure fuel supply system that supplies fuel to the in-cylinder injector in this engine controlling device, the maximum injection quantity of the intake manifold injector is compared with the target fuel injection quantity that is set based on the engine operation state. When the target fuel injection quantity exceeds the maximum injection quantity, the target fuel injection quantity is fixed at the maximum injection quantity. The target intake air quantity is calculated based on this fixed target fuel injection quantity and target air-fuel ratio. The throttle opening indication value is calculated with respect to the electronic control type throttle valve based on the calculated target intake air quantity. Accordingly, when abnormality is sensed in the in-cylinder injector system, fuel injection from the in-cylinder injector is inhibited, and fuel is to be injected from only the intake manifold injector. Based on the maximum injection quantity at this stage and the target air-fuel ratio, the target intake air quantity is calculated. The throttle opening indication value with respect to the electronic control type throttle valve is calculated based on the target intake air quantity. In the fail-safe mode caused by failure in the in-cylinder injector system, the throttle opening will open only to the level corresponding to the target air-fuel ratio no matter how hard the acceleration pedal is pushed down. Thus, the air-fuel ratio is maintained properly to obtain suitable driving power. 
   In the engine controlling device disclosed in Japanese Patent Laying-Open No. 2000-145516, fuel injection from the in-cylinder injector is ceased to inject fuel from the intake manifold injector alone when an error occurs at the high-pressure fuel supply system. However, this publication is silent about failure in the intake manifold injector and in the fuel supply system that supplies fuel to the intake manifold injector. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a control apparatus for an internal combustion engine in which a first fuel injection mechanism that injects fuel into a cylinder and a second fuel injection mechanism that injects fuel to an intake manifold partake in fuel injection, maintaining proper operation of the internal combustion engine even in the event of failure at the second fuel injection mechanism side. 
   According to an aspect of the present invention, a control apparatus for an internal combustion engine controls the internal combustion engine that includes a first fuel injection mechanism injecting fuel into a cylinder, a second fuel injection mechanism injecting fuel into an intake manifold, a first fuel supply mechanism supplying fuel to the first fuel injection mechanism, and a second fuel supply mechanism supplying fuel to the second fuel injection mechanism. The control apparatus includes a control unit controlling the first and second fuel injection mechanisms such that the first and second fuel injection mechanisms partake in fuel injection, including a state of injection from one of the first and second fuel injection mechanisms being ceased, and an abnormality determination unit determining presence of abnormality in the second fuel supply mechanism. The control unit effects control such that, when the abnormality determination unit determines presence of abnormality in the second fuel supply mechanism, fuel injection is conducted from the first fuel injection mechanism, and not from the second fuel injection mechanism. 
   When determination is made of abnormality in the intake manifold injector in an internal combustion engine including a first fuel injection mechanism (in-cylinder injector) injecting fuel into a cylinder and a second fuel injection mechanism (intake manifold injector) injecting fuel into an intake manifold, fuel is injected from the in-cylinder injector, and fuel injection from the intake manifold injector is inhibited. Accordingly, in the case of abnormality such as disconnection of a harness or the like that establishes connection between the intake manifold injector and the control apparatus, a normal operation of the internal combustion engine can be maintained, based on fuel injection from the in-cylinder injector. Thus, there is provided a control apparatus for an internal combustion engine in which a first fuel injection mechanism injecting fuel into a cylinder and a second fuel injection mechanism injecting fuel into an intake manifold partake in fuel injection, maintaining a proper operation of the internal combustion engine even in the event of failure at the second fuel injection mechanism side. 
   Preferably, the control apparatus further includes a purge control unit controlling a purge mechanism provided at the internal combustion engine to increase the purge rate when fuel injection from the second fuel injection mechanism is not conducted as a result of determination of abnormality in the second fuel supply mechanism by the abnormality determination unit as compared to a case where determination is not made of abnormality in the second fuel supply mechanism. 
   In accordance with the present invention, reduction in the fuel quantity from the intake manifold injector in which abnormality is sensed is compensated for from the intake system apparently by increasing the purge rate. Accordingly, fuel can be compensated for from the intake system with a favorable mixing state of the intake air and fuel. Therefore, combustion variation can be suppressed in the case where fuel cannot be injected from the intake manifold injector. 
   Further preferably, the control apparatus further includes an adjustment unit adjusting a variable valve timing mechanism provided at the internal combustion engine such that overlap of intake valves and exhaust valves is reduced when fuel injection is not conducted from the second fuel injection mechanism as a result of determination of abnormality in the second fuel supply system by the abnormality determination unit as compared to a case where determination is not made of abnormality in the second fuel supply system. 
   In accordance with the present invention, the overlap of the intake valves and exhaust valves can be reduced to suppress intake blow-back from the combustion chamber. Accordingly, accumulation of deposits at the intake manifold injector or intake port caused by the PM (Particulate Matters) included in the blow-back can be suppressed. Since the state of no accumulation of deposits can be maintained when fuel injection from the intake manifold injector is not conducted as long as the intake manifold injector itself is absent of failure, that intake manifold injector can be .used even after, for example, repair of the harness. Further, since the combustion state is improved due to reduction in the internal EGR (Exhaust Gas Recirculation) factor when the overlap of the intake valves and exhaust valves is reduced, torque variation can be suppressed. 
   Further preferably, the control apparatus further includes an adjustment unit adjusting an ignition timing variable mechanism provided at the internal combustion engine such that the ignition timing is retarded when fuel injection is not conducted from the second fuel injection mechanism as a result of determination of abnormality in the second fuel supply system by the abnormality determination unit as compared to the case where determination is not made of abnormality in the second fuel supply mechanism. 
   In accordance with the present invention, the ignition timing is retarded and the combustion temperature is reduced to suppress generation of NOx. By retarding the ignition timing as compared to the case where the ignition timing is set in the vicinity of MBT (Minimum spark advance for Best Torque) where the combustion pressure is highest and the combustion temperature is also high, the combustion pressure and the combustion temperature are reduced, allowing suppression of NOx generation. By such reduction in combustion temperature and suppression of NOx, accumulation of deposits at the injection hole of the in-cylinder injector can be suppressed. Therefore, accumulation of deposits towards the injection hole at the leading end of the in-cylinder injector can be suppressed even in the case where fuel injection is to be conducted from only the in-cylinder injector and not from the intake manifold injector in the region where the in-cylinder injector and the intake manifold injector partake in fuel injection. 
   Further preferably, the control apparatus further includes an adjustment unit adjusting the variable valve timing mechanism or ignition timing variable mechanism such that the condition for deposits accumulating at the leading end of the first fuel injection mechanism in the event of not conducting fuel injection from the second fuel injection mechanism as a result of determination of abnormality in the second fuel supply mechanism by the abnormality determination unit will not become worse than the condition for deposits accumulating at the leading end of the first fuel injection mechanism in the event of the first and second fuel injection mechanisms partaking in fuel injection under the same operation condition. 
   In accordance with the present invention, the adjustment unit adjusts the variable valve timing mechanism or ignition timing variable mechanism. The adjustment unit adjusts these mechanisms such that the condition for deposits accumulating at the leading end of the in-cylinder injector is not worse when fuel injection is conducted from the in-cylinder injector alone than when the intake manifold injector and the in-cylinder injector partake in fuel injection. For example, the ignition timing is retarded. The ignition timing is retarded based on a favorable combustion state by reducing the valve overlap to lower the internal EGR rate. Thus, the combustion temperature is reduced and generation of NOx is suppressed. Accordingly, accumulation of deposits at the injection hole of the in-cylinder injector can be suppressed. 
   Further preferably, the condition includes the condition about the temperature at the leading end of the first fuel injection mechanism. 
   In accordance with the present invention, based on the condition for the temperature at the leading end of the in-cylinder injector (for example, the condition that the temperature at the leading end of the in-cylinder injector is substantially equal or below between the case where the intake manifold injector and in-cylinder injector partake in fuel injection and the case where fuel injection is conducted only by the in-cylinder injector), the ignition timing is retarded and the combustion temperature is reduced to suppress accumulation of deposits at the injection hole of the in-cylinder injector. 
   Further preferably, the control apparatus further includes an adjustment unit adjusting the variable valve timing mechanism or ignition timing variable mechanism such that the condition for deposits accumulating at the second fuel injection mechanism or a neighborhood thereof in the event of not conducting fuel injection from the second fuel injection mechanism as a result of determination of abnormality in the second fuel supply system by the abnormality determination unit will not become worse than the condition for deposits accumulating at the leading end of the second fuel injection mechanism or a neighborhood thereof in the event of the first and second fuel injection mechanisms partaking in fuel injection under the same operation condition. 
   In accordance with the present invention, the adjustment unit adjusts the variable valve timing mechanism or ignition timing variable mechanism. The adjustment unit adjusts the mechanism such that the condition for deposits accumulating at the leading end of the intake manifold injector or at the neighborhood thereof when fuel injection is conducted from the in-cylinder injector alone will not become worse than in the case where the intake manifold injector and in-cylinder injector partake in fuel injection. For example, the valve overlap is reduced. Further, the ignition timing is retarded based on a favorable combustion state by reducing the valve overlap and reducing the internal EGR rate. Thus, the overlap of the intake valves and exhaust valves can be reduced to suppress intake blow-back from the combustion engine. Accordingly, accumulation of deposits at the intake manifold injector or intake port caused by the PM included in the blow-back can be suppressed. 
   Further preferably, the first fuel injection mechanism is an in-cylinder injector, and the second fuel injection mechanism is an intake manifold injector. 
   In accordance with the present invention, there is provided a control apparatus for an internal combustion engine in which a in-cylinder injector identified as the first fuel injection mechanism and an intake manifold injector identified as the second fuel injection mechanism, each provided individually, partake in fuel injection, conducting fuel injection from the in-cylinder injector and obviating accumulation of deposits at the intake manifold injector and in-cylinder injector while suppressing torque variation even in the case of failure such as disconnection of the harness establishing connection between the control apparatus and the intake manifold injector. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing a structure of an engine system under control of a control apparatus according to an embodiment of the present invention. 
       FIG. 2  is a flow chart of a control structure of a program executed by an engine ECU that is the control apparatus according to an embodiment of the present invention. 
       FIG. 3  represents a DI ratio map corresponding to a warm state of an engine to which the control apparatus of an embodiment of the present invention is suitably adapted. 
       FIG. 4  represents a DI ratio map corresponding to a cold state of an engine to which the control apparatus of an embodiment of the present invention is suitably adapted. 
       FIG. 5  represents a DI ratio map corresponding to a warm state of an engine to which the control apparatus of an embodiment of the present invention is suitably adapted. 
       FIG. 6  represents a DI ratio map corresponding to a cold state of an engine to which the control apparatus of an embodiment of the present invention is suitably adapted. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described hereinafter with reference to the drawings. The same components have the same reference characters allotted, and their designation and function are also identical. Therefore, detailed description thereof will not be repeated. 
     FIG. 1  is a schematic view of a structure of an engine system under control of an engine ECU (Electronic Control Unit) identified as a control apparatus for an internal combustion engine according to an embodiment of the present invention. Although an in-line 4-cylinder gasoline engine is indicated as the engine, the present invention is not limited to such an engine. 
   As shown in  FIG. 1 , the engine  10  includes four cylinders  112 , each connected to a common surge tank  30  via a corresponding intake manifold  20 . Surge tank  30  is connected via an intake duct  40  to an air cleaner  50 . An airflow meter  42  is arranged in intake duct  40 , and a throttle valve  70  driven by an electric motor  60  is also arranged in intake duct  40 . Throttle valve  70  has its degree of opening controlled based on an output signal of an engine ECU  300 , independently from an accelerator pedal  100 . Each cylinder  112  is connected to a common exhaust manifold  80 , which is connected to a three-way catalytic converter  90 . 
   Each cylinder  112  is provided with an in-cylinder injector  110  for injecting fuel into the cylinder and an intake manifold injector  120  for injecting fuel into an intake port or/and an intake manifold. Injectors  110  and  120  are controlled based on output signals from engine ECU  300 . Further, in-cylinder injector  110  of each cylinder is connected to a common fuel delivery pipe  130 . Fuel delivery pipe  130  is connected to a high-pressure fuel pump  150  of an engine-driven type, via a check valve  140  that allows a flow in the direction toward fuel delivery pipe  130 . Although an internal combustion engine having two injectors separately provided is explained in the present embodiment, the present invention is not restricted to such an internal combustion engine. For example, the internal combustion engine may have one injector that can effect both in-cylinder injection and intake manifold injection. 
   As shown in  FIG. 1 , the discharge side of high-pressure fuel pump  150  is connected via an electromagnetic spill valve  152  to the intake side of high-pressure fuel pump  150 . As the degree of opening of electromagnetic spill valve  152  is smaller, the quantity of the fuel supplied from high-pressure fuel pump  150  into fuel delivery pipe  130  increases. When electromagnetic spill valve  152  is fully open, the fuel supply from high-pressure fuel pump  150  to fuel delivery pipe  130  is ceased. Electromagnetic spill valve  152  is controlled based on an output signal of engine ECU  300 . 
   Specifically, the closing timing during a pressurized stroke of electromagnetic spill valve  152  provided at the pump intake side of high-pressure fuel pump  150  that applies pressure on the fuel by the vertical operation of a pump plunger through a cam attached to a cam shaft is feedback-controlled through engine ECU  300  using a fuel pressure sensor  400  provided at fuel delivery pipe  130 , whereby the fuel pressure in fuel delivery pipe  130  (fuel pressure) is controlled. In other words, by controlling electromagnetic spill valve  152  through engine ECU  300 , the quantity and pressure of fuel supplied from high-pressure fuel pump  150  to fuel delivery pipe  130  are controlled. 
   Each intake manifold injector  120  is connected to a common fuel delivery pipe  160  at the low pressure side. Fuel delivery pipe  160  and high-pressure fuel pump  150  are connected to an electromotor driven type low-pressure fuel pump  180  via a common fuel pressure regulator  170 . Low-pressure fuel pump  180  is connected to fuel tank  200  via fuel filter  190 . When the fuel pressure of fuel ejected from low-pressure fuel pump  180  becomes higher than a predetermined set fuel pressure, fuel pressure regulator  170  returns a portion of the fuel output from low-pressure fuel pump  180  to fuel tank  200 . Accordingly, the fuel pressure supplied to intake manifold injector  120  and the fuel pressure supplied to high-pressure fuel pump  150  are prevented from becoming higher than the set fuel pressure. 
   Engine ECU  300  is based on a digital computer, and includes a ROM (Read Only Memory)  320 , a RAM (Random Access Memory)  330 , a CPU (Central Processing Unit)  340 , an input port  350 , and an output port  360  connected to each other via a bidirectional bus  310 . 
   Air flow meter  42  generates an output voltage in proportion to the intake air. The output voltage from air flow meter  42  is applied to input port  350  via an A/D converter  370 . A coolant temperature sensor  380  producing an output voltage in proportion to the engine coolant temperature is attached to engine  10 . The output voltage from coolant temperature sensor  380  is applied to input port  350  via an A/D converter  390 . 
   A fuel pressure sensor  400  producing an output voltage in proportion to the fuel pressure in high pressure delivery pipe  130  is attached to high pressure delivery pipe  130 . The output voltage from fuel pressure sensor  400  is applied to input port  350  via an A/D converter  410 . An air-fuel ratio sensor  420  producing an output voltage in proportion to the oxygen concentration in the exhaust gas is attached to exhaust manifold  80  upstream of 3-way catalytic converter  90 . The output voltage from air-fuel ratio  420  is applied to input port  350  via an A/D converter  430 . 
   Air-fuel ratio sensor  420  in the engine system of the present embodiment is a full-range air-fuel ratio sensor (linear air-fuel sensor) producing an output voltage in proportion to the air-fuel ratio of air-fuel mixture burned at engine  10 . Air-fuel ratio sensor  420  may be an O 2  sensor that detects whether the air-fuel ratio of air-fuel mixture burned at engine  10  is rich or lean to the stoichiometric ratio in an on/off manner. 
   An accelerator pedal position sensor  440  producing an output voltage in proportion to the pedal position of an accelerator pedal  100  is attached to accelerator pedal  100 . The output voltage from accelerator pedal position sensor  440  is applied to input port  350  via an A/D converter  450 . A revolution speed sensor  460  generating an output pulse representing the engine speed is connected to input port  350 . ROM  320  of engine ECU  300  stores the value of the fuel injection quantity set corresponding to an operation state, a correction value based on the engine coolant temperature, and the like that are mapped in advance based on the engine load factor and engine speed obtained through accelerator pedal position sensor  440  and revolution speed sensor  460  set forth above. 
   A canister  230  that is a vessel for trapping fuel vapor dispelled from fuel tank  200  is connected to fuel tank  200  via a paper channel  260 . Canister  230  is further connected to a purge channel  280  to supply the fuel vapor trapped therein to the intake system of engine  10 . Purge channel  280  communicates with a purge port  290  that opens downstream of throttle valve  70  of intake duct  40 . As well known in the field of art, canister  230  is filled with an adsorbent (activated charcoal) adsorbing the fuel vapor. An air channel  270  to introduce air into canister  230  via a check valve during purging is formed in canister  230 . Further, a purge control valve  250  controlling the amount of purging is provided in purge channel  280 . The opening of purge control valve  250  is under duty control by engine ECU  300 , whereby the amount of fuel vapor that is to be purged in canister  230 , and in turn the quantity of fuel introduced into engine  10  (hereinafter, referred to as purge fuel quantity), is controlled. The purge rate can be calculated based on the purge fuel quantity. Alternatively, the purge fuel quantity is calculated from the target purge rate, and engine ECU  300  effects duty control of the opening of purge control valve  250  such that the calculated purge fuel quantity can be realized. 
   A control structure of a program executed by engine ECU  300  identified as the control apparatus of the present embodiment will be described with reference to  FIG. 2 . The program in this flow chart is executed at a predetermined interval of time, or at a predetermined crank angle of engine  10 . 
   At step (hereinafter, step abbreviated as S)  100 , engine ECU  300  determines whether abnormality in the low-pressure fuel system is sensed or not. For example, abnormality in the low-pressure fuel system is sensed when the harness connecting intake manifold injector  120  and engine ECU  300  is disconnected such that deviation in feedback control on the part of intake manifold injector  120  cannot be eliminated. It is to be noted that abnormality in the low-pressure fuel system does not include the inoperative state of low-pressure fuel pump  180 . When abnormality in the low-pressure fuel system is sensed (YES at S 100 ), control proceeds to S 110 , otherwise (NO at S 100 ), control proceeds to S 200 . 
   At S 110 , engine ECU  300  inhibits fuel injection from intake manifold injector  120 . At S 120 , engine ECU  300  determines whether the current operation state of engine  10  is within the injection partaking region between in-cylinder injector  110  and intake manifold injector  120 . This determination is based on a map that will be described afterwards. When the current operation state of engine  10  is within the injection partaking region between in-cylinder injector  110  and intake manifold injector  120  (YES at S 120 ), control proceeds to S 130 , otherwise (NO at S 120 ), control proceeds to S 160 . 
   At S 130 , engine ECU  300  increases the target purge rate. The opening of purge control valve  250  is under duty control such that the target purge rate can be realized (to realize the purge fuel quantity corresponding to the target purge rate). Accordingly, the purge quantity is increased to compensate for the quantity of fuel injected from intake manifold injector  120 . Since the fuel from the intake system is sufficiently mixed with intake air, torque variation can be suppressed in the case where fuel injection from intake manifold injector  120  is inhibited. 
   At S 140 , engine ECU  300  reduces the overlap of intake valves and exhaust valves by VVT (Variable Valve Timing). Since the overlap of intake valves and exhaust valves is reduced, the intake blow-back from the combustion chamber can be suppressed. Accordingly, accumulation of deposits at the intake manifold injector or intake port caused by the PM included in the blow-back can be suppressed. Further, since the internal EGR rate is reduced when the overlap of intake valves and exhaust valves is reduced, the combustion state becomes favorable, allowing suppression in torque variation. 
   At S 150 , engine ECU  300  retards the ignition timing. Accordingly, reduction in the combustion temperature and NOx can be realized. The temperature at the leading end of in-cylinder injector  110  can be reduced. Thus, accumulation of deposits towards the injection hole at the leading end of in-cylinder injector  110  can be suppressed. 
   At S 160 , engine ECU  300  effects duty control on the opening of purge control valve  250  such that the target purge rate can be realized in a normal manner (realize the purge fuel quantity corresponding to the target purge rate). 
   At S 200 , engine ECU  300  controls engine  10  to execute normal operation. 
   An operation of engine  10  under control of engine ECU  300  identified as a control apparatus for an internal combustion engine according to the present embodiment based on the structure and flow chart set forth above will be described here. 
   When an error occurs at the low-pressure fuel system (YES at S 100 ), intake manifold injector  120  is ceased (S 110 ). When the current operation state of engine  10  is within the injection partaking region between in-cylinder injector  110  and intake manifold injector  120  (YES at S 120 ), the target purge rate is increased (S 130 ). Since fuel from the intake system with a favorable state of intake air can be employed for compensation, torque variation reflecting combustion variation can be suppressed even in the case where fuel injection from the intake manifold injector is ceased. 
   Further, control is effected such that the VVT overlap amount is reduced (S 140 ). In response to reduction in the overlap of intake valves and exhaust valves, the intake blow-back from the combustion chamber is suppressed. Therefore, accumulation of deposits at the intake manifold injector and/or intake port can be suppressed. Further, reduction in the overlap of intake valves and exhaust valves causes the internal EGR rate to be lowered, leading to a favorable combustion state. Thus, torque variation is suppressed. 
   Then, control is effected to retard the ignition timing (S 150 ). By retarding the ignition timing, the combustion temperature is reduced to allow suppression of NOx. By reduction of the combustion temperature and suppression of NOx, accumulation of deposits at the injection hole of the in-cylinder injector can be suppressed. 
   Thus, control is effected such that the VVT overlap is reduced, and the ignition timing is retarded. Both develop synergistically the effect of suppressing accumulation of deposits at the injection hole of in-cylinder injector  110 , the effect of suppressing accumulation of deposits at the injection hole of in-cylinder injector  110  and in the proximity of intake manifold injector  120 , and the effect of suppressing torque variation caused by intake manifold injector  120  being ceased. This will be described specifically taking the temperature at the leading end of in-cylinder injector  110  as an example. The retarded amount of ignition timing or the overlapping amount of intake valves and exhaust valves by VVT is determined to avoid reaching the upper limit value of the temperature at the leading end corresponding to the level at which deposits will not accumulate at the injection hole of the in-cylinder injector in the case of in-cylinder injector  110  and intake manifold injector  120  partaking in fuel injection. In view of deposits at intake manifold injector  120  or in the proximity of intake manifold injector  120 , the retarded amount of ignition timing or the overlapping amount of intake valves and exhaust valves is determined such that deposits are not accumulated at intake manifold injector  120  or the neighborhood thereof taking into consideration deposits that are washed away by the fuel injected from the intake manifold injector  120  in the case of in-cylinder injector  110  and intake manifold injector  120  partaking in fuel injection. 
   The variable valve timing mechanism or ignition timing variable mechanism is adjusted such that the condition for deposits accumulating at the leading end of in-cylinder injector  110  in the event of not conducting fuel injection from intake manifold injector  120  will not become worse than the condition for deposits accumulating at the leading end of in-cylinder injector  110  in the event of in-cylinder injector  110  and intake manifold injector  120  partaking in fuel injection under the same operation condition. Similarly, the variable valve timing mechanism or ignition timing variable mechanism is adjusted such that the condition for deposits accumulating at intake manifold injector  120  or the neighborhood thereof in the event of not conducting fuel injection from intake manifold injector  120  will not become worse than the conduction for deposits accumulating at the leading end of intake manifold injector  120  or the neighborhood thereof in the event of in-cylinder injector  110  and intake manifold injector  120  partaking in fuel injection under the same operation condition. 
   Thus, although the intake manifold injector is ceased when there is an error at the low-pressure fuel system, the purge amount is increased to compensate for the fuel from the intake system that has favorable mixing. The overlap of intake valves and exhaust valves by VVT is reduced to lower the blow-back. Therefore, generation of deposits caused by the PM included in the blow-back at the intake manifold injector and the neighborhood thereof can be suppressed. Further, the ignition timing is retarded to lower the combustion temperature, and generation of NOx is suppressed to obviate generation of deposits at the in-cylinder injector. 
   &lt;Engine ( 1 ) to Which Present Control Apparatus can be Suitably Applied&gt; 
   An engine ( 1 ) to which the control apparatus of the present embodiment is suitably adapted will be described hereinafter. 
   Referring to  FIGS. 3 and 4 , maps indicating a fuel injection ratio (hereinafter, also referred to as DI ratio (r)) between in-cylinder injector  110  and intake manifold injector  120 , identified as information associated with an operation state of engine  10 , will now be described. The maps are stored in an ROM  300  of an engine ECU  300 .  FIG. 3  is the map for a warm state of engine  10 , and  FIG. 4  is the map for a cold state of engine  10 . 
   In the maps of  FIGS. 3 and 4 , the fuel injection ratio of in-cylinder injector  110  is expressed in percentage as the DI ratio r, wherein the engine speed of engine  10  is plotted along the horizontal axis and the load factor is plotted along the vertical axis. 
   As shown in  FIGS. 3 and 4 , the DI ratio r is set for each operation region that is determined by the engine speed and the load factor of engine  10 . “DI RATIO r=100%” represents the region where fuel injection is carried out from in-cylinder injector  110  alone, and “DI RATIO r=0%” represents the region where fuel injection is carried out from intake manifold injector  120  alone. “DI RATIO r≠0%”, “DI RATIO r≠100%” and “0%&lt;DI RATIO r&lt;100%” each represent the region where in-cylinder injector  110  and intake manifold injector  120  partake in fuel injection. Generally, in-cylinder injector  110  contributes to an increase of power performance, whereas intake manifold injector  120  contributes to uniformity of the air-fuel mixture. These two types of injectors having different characteristics are appropriately selected depending on the engine speed and the load factor of engine  10 , so that only homogeneous combustion is conducted in the normal operation state of engine  10  (for example, a catalyst warm-up state during idling is one example of an abnormal operation state). 
   Further, as shown in  FIGS. 3 and 4 , the DI ratio r of in-cylinder injector  110  and intake manifold injector  120  is defined individually in the maps for the warm state and the cold state of the engine. The maps are configured to indicate different control regions of in-cylinder injector  110  and intake manifold injector  120  as the temperature of engine  10  changes. When the temperature of engine  10  detected is equal to or higher than a predetermined temperature threshold value, the map for the warm state shown in  FIG. 3  is selected; otherwise, the map for the cold state shown in  FIG. 4  is selected. In-cylinder injector  110  and/or intake manifold injector  120  are controlled based on the engine speed and the load factor of engine  10  in accordance with the selected map. 
   The engine speed and the load factor of engine  10  set in  FIGS. 3 and 4  will now be described. In  FIG. 3 , NE( 1 ) is set to 2500 rpm to 2700 rpm, KL( 1 ) is set to 30% to 50%, and KL( 2 ) is set to 60% to 90%. In  FIG. 4 , NE( 3 ) is set to 2900 rpm to 3100 rpm. That is, NE( 1 )&lt;NE( 3 ). NE( 2 ) in  FIG. 3  as well as KL( 3 ) and KL( 4 ) in  FIG. 4  are also set appropriately. 
   In comparison between  FIG. 3  and  FIG. 4 , NE( 3 ) of the map for the cold state shown in  FIG. 4  is greater than NE( 1 ) of the map for the warm state shown in  FIG. 3 . This shows that, as the temperature of engine  10  becomes lower, the control region of intake manifold injector  120  is expanded to include the region of higher engine speed. That is, in the case where engine  10  is cold, deposits are unlikely to accumulate in the injection hole of in-cylinder injector  110  (even if fuel is not injected from in-cylinder injector  110 ). Thus, the region where fuel injection is to be carried out using intake manifold injector  120  can be expanded, whereby homogeneity is improved. 
   In comparison between  FIG. 3  and  FIG. 4 , “DI RATIO r=100%” in the region where the engine speed of engine  10  is NE( 1 ) or higher in the map for the warm state, and in the region where the engine speed is NE( 3 ) or higher in the map for the cold state. In terms of load factor, “DI RATIO r=100%” in the region where the load factor is KL( 2 ) or greater in the map for the warm state, and in the region where the load factor is KL( 4 ) or greater in the map for the cold state. This means that in-cylinder injection  110  alone is used in the region of a predetermined high engine speed, and in the region of a predetermined high engine load. That is, in the high speed region or the high load region, even if fuel injection is carried out through in-cylinder injector  110  alone, the engine speed and the load of engine  10  are so high and the intake air quantity so sufficient that it is readily possible to obtain a homogeneous air-fuel mixture using only in-cylinder injector  110 . In this manner, the fuel injected from in-cylinder injector  110  is atomized within the combustion chamber involving latent heat of vaporization (or, absorbing heat from the combustion chamber). Thus, the temperature of the air-fuel mixture is decreased at the compression end, so that the anti-knocking performance is improved. Further, since the temperature within the combustion chamber is decreased, intake efficiency improves, leading to high power. 
   In the map for the warm state in  FIG. 3 , fuel injection is also carried out using in-cylinder injector  110  alone when the load factor is KL( 1 ) or less. This shows that in-cylinder injector  110  alone is used in a predetermined low-load region when the temperature of engine  10  is high. When engine  10  is in the warm state, deposits are likely to accumulate in the injection hole of in-cylinder injector  110 . However, when fuel injection is carried out using in-cylinder injector  110 , the temperature of the injection hole can be lowered, in which case accumulation of deposits is prevented. Further, clogging at in-cylinder injector  110  may be prevented while ensuring the minimum fuel injection quantity thereof Thus, in-cylinder injector  110  solely is used in the relevant region. 
   In comparison between  FIG. 3  and  FIG. 4 , the region of “DI RATIO r=0%” is present only in the map for the cold state of  FIG. 4  . This shows that fuel injection is carried out through intake manifold injector  120  alone in a predetermined low-load region (KL( 3 ) or less) when the temperature of engine  10  is low. When engine  10  is cold and low in load and the intake air quantity is small, the fuel is less susceptible to atomization. In such a region, it is difficult to ensure favorable combustion with the fuel injection from in-cylinder injector  110 . Further, particularly in the low-load and low-speed region, high power using in-cylinder injector  110  is unnecessary. Accordingly, fuel injection is carried out through intake manifold injector  120  alone, without using in-cylinder injector  110 , in the relevant region. 
   Further, in an operation other than the normal operation, or, in the catalyst warm-up state during idling of engine  10  (an abnormal operation state), in-cylinder injector  110  is controlled such that stratified charge combustion is effected. By causing the stratified charge combustion only during the catalyst warm-up operation, warming up of the catalyst is promoted to improve exhaust emission. 
   &lt;Engine ( 2 ) to Which Present Control Apparatus is Suitably Adapted&gt; 
   An engine ( 2 ) to which the control apparatus of the present embodiment is suitably adapted will be described hereinafter. In the following description of the engine ( 2 ), the configurations similar to those of the engine ( 1 ) will not be repeated. 
   Referring to  FIGS. 5 and 6 , maps indicating the fuel injection ratio between in-cylinder injector  110  and intake manifold injector  120  identified as information associated with the operation state of engine  10  will be described. The maps are stored in ROM  320  of an engine ECU  300 .  FIG. 5  is the map for the warm state of engine  10 , and  FIG. 6  is the map for the cold state of engine  10 . 
     FIGS. 5 and 6  differ from  FIGS. 3 and 4  in the following points. “DI RATIO r=100%” holds in the region where the engine speed of engine  10  is equal to or higher than NE( 1 ) in the map for the warm state, and in the region where the engine speed is NE( 3 ) or higher in the map for the cold state. Further, “DI RATIO r=100%” holds in the region, excluding the low-speed region, where the load factor is KL( 2 ) or greater in the map for the warm state, and in the region, excluding the low-speed region, where the load factor is KL( 4 ) or greater in the map for the cold state. This means that fuel injection is carried out through in-cylinder injector  110  alone in the region where the engine speed is at a predetermined high level, and that fuel injection is often carried out through in-cylinder injector  10  alone in the region where the engine load is at a predetermined high level. However, in the low-speed and high-load region, mixing of an air-fuel mixture produced by the fuel injected from in-cylinder injector  110  is poor, and such inhomogeneous air-fuel mixture within the combustion chamber may lead to unstable combustion. Thus, the fuel injection ratio of in-cylinder injector  110  is increased as the engine speed increases where such a problem is unlikely to occur, whereas the fuel injection ratio of in-cylinder injector  110  is decreased as the engine load increases where such a problem is likely to occur. These changes in the DI ratio r are shown by crisscross arrows in  FIGS. 5 and 6 . In this manner, variation in output torque of the engine attributable to the unstable combustion can be suppressed. It is noted that these measures are substantially equivalent to the measures to decrease the fuel injection ratio of in-cylinder injector  110  in connection with the state of the engine moving towards the predetermined low speed region, or to increase the fuel injection ratio of in-cylinder injector  110  in connection with the engine state moving towards the predetermined low load region. Further, in a region other than the region set forth above (indicated by the crisscross arrows in  FIGS. 5 and 6 ) and where fuel injection is carried out using only in-cylinder injector  110  (on the high speed side and on the low load side), the air-fuel mixture can be readily set homogeneous even when the fuel injection is carried out using only in-cylinder injector  110 . In this case, the fuel injected from in-cylinder injector  110  is atomized within the combustion chamber involving latent heat of vaporization (by absorbing heat from the combustion chamber). Accordingly, the temperature of the air-fuel mixture is decreased at the compression end, whereby the antiknock performance is improved. Further, with the decreased temperature of the combustion chamber, intake efficiency improves, leading to high power output. 
   In the engine described in conjunction with  FIGS. 3–6 , the fuel injection timing of in-cylinder injector  110  is preferably achieved in the compression stroke, as will be described hereinafter. When the fuel injection timing of in-cylinder injector  1   10  is set in the compression stroke, the air-fuel mixture is cooled by the fuel injection while the temperature in the cylinder is relatively high. Accordingly, the cooling effect is enhanced to improve the antiknock performance. Further, when the fuel injection timing of in-cylinder injector  110  is set in the compression stroke, the time required starting from fuel injection to ignition is short, which ensures strong penetration of the injected fuel. Therefore, the combustion rate is increased. The improvement in antiknock performance and the increase in combustion rate can prevent variation in combustion, and thus, combustion stability is improved. 
   Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.