Patent Publication Number: US-2007095314-A1

Title: Control apparatus and control method for internal combustion engine

Description:
This nonprovisional application is based on Japanese Patent Application No. 2005-318509 filed with the Japan Patent Office on Nov. 1, 2005, the entire contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to control of an internal combustion engine. In particular, the invention relates to the technique of advancing or retarding a phase in which a valve is closed, according to a torque exerted on a camshaft that drives the valve.  
      2. Description of the Background Art  
      An internal combustion engine has been known that has such valves as intake valves and exhaust valves opened and closed by rotation of camshafts. In such an internal combustion engine, as the camshaft rotates, a reaction force from the valve causes a torque acting on the camshaft (the torque is also referred to as cam torque hereinafter). As the valve is opened, the cam torque is exerted in the direction opposite to the rotational direction of the camshaft (the cam torque is exerted in the direction that hinders rotation of the camshaft). In contrast, as the valve is closed, the cam torque is exerted in the rotational direction of the camshaft (the cam torque is exerted in the direction that helps rotation of the camshaft). Therefore, the torque necessary for rotating the camshaft may vary to a significant degree while the camshaft makes one rotation. Thus, for VVT (Variable Valve Timing) that changes the phase in which the valve is opened/closed (valve opening/closing timing) by rotation of the camshaft that drives the valve (relative rotation with respect to the sprocket for example), the cam torque could influence the VVT. In other words, depending on the direction in which the cam torque is exerted, the phase of the valve may be easy to change or difficult to change by rotation of the camshaft. Therefore, it is necessary to take the cam torque into account for controlling the phase.  
      Japanese Patent Laying-Open No. 2005-076518 discloses a control apparatus for a variable valve timing mechanism that changes the phase according to the cam torque. The control apparatus disclosed in Japanese Patent Laying-Open No. 2005-076518 controls the variable valve timing mechanism that changes the rotational phase of the camshaft relative to the crankshaft of the internal combustion engine to vary the valve timing of an intake valve or an exhaust valve. The control apparatus detects the state of the cam torque generated on the camshaft due to operation of the cam driving to open/close the intake valve or the exhaust valve and, when the cam torque is generated in the direction opposite to the direction of changing the rotational phase, decreases the degree of change in rotational phase or maintains the rotational phase as it is.  
      Regarding the control apparatus for the variable valve timing mechanism disclosed in the above-referenced publication, when the rotational phase is to be changed and the cam torque is generated in the direction that hinders the change of the rotational phase, the change (degree of change) of the rotational phase is decreased or the change in rotational phase is stopped. Thus, an increase in engine load due to the cam torque can be prevented.  
      As for a V-type 8-cylinder internal combustion engine employing a double-plane (also referred to as dual-plane) crankshaft with crankpins arranged at 90° therebetween, it is known that the right and left banks cannot be fired alternately and firing is successively caused in one of the banks. In such a V8 internal combustion engine, the cylinders are not fired at regular intervals. Thus, valves provided to respective cylinders have respective phases (opening/closing timings) that are not at regular intervals as well. Therefore, there may be the case where a valve of one of the cylinders is closed in a certain phase (at a certain timing) while opening operation of another cylinder in the same bank may be started in that phase (at that timing) and there may be the case where the above-described valve state does not occur. Accordingly, in a phase (at a timing) in (at) which valves of some of the cylinders are closed, the cam torque exerted in the direction opposite to the camshaft rotational direction increases. If the cam torque exerted in the direction opposite to the camshaft rotational direction is large, the phase in which the valve is actually closed is later than or delayed relative to a phase determined under control due to influences of deformation or the like of such components as a chain coupling the crankshaft and camshafts and other parts. In contrast, there may be the case where the intake valve of any cylinder is closed in a certain phase while the intake valve of another cylinder in the same bank is in the transition from the opening operation to the closing operation in that phase and there may be the case where such a valve state does not occur. Accordingly, in a phase in which some of the cylinders are closed, the cam torque exerted in the camshaft rotational direction increases. If the cam torque exerted in the camshaft rotational direction is large, the phase in which the valve is actually closed is earlier than or advanced relative to a phase determined under control, due to influences of deformation or the like of such components as the chain coupling the crankshaft and camshafts and other parts. In the cylinder having its intake valve closed in the delayed phase, the quantity of air pushed back from the cylinder into the intake manifold as the piston is lifted increases, resulting in a decrease in final quantity of air sucked into the cylinders. On the contrary, in the cylinder having its intake valve closed in the advanced phase, the quantity of air pushed back from the cylinder into the intake manifold as the piston is lifted decreases, resulting in an increase in final quantity of air sucked into the cylinder. Therefore, some cylinders are smaller in sucked or intake air quantity than other cylinders. Further, in a cylinder having its exhaust valve closed in a delayed phase, the quantity of exhaust gas sucked back from the exhaust manifold into the cylinder as the piston is moved downward increases (namely internal EGR (Exhaust Gas Recirculation) quantity increases). On the contrary, in a cylinder having its exhaust valve closed in an advanced phase, the quantity of exhaust gas sucked back from the exhaust manifold into the cylinder as the piston is moved downward decreases. Therefore, the cylinders are nonuniform in internal EGR quantity. For such an internal combustion engine as described above, if the control apparatus for the variable valve timing mechanism disclosed in Japanese Patent Laying-Open No. 2005-076518 is used, the difference in quantity of air taken into cylinders as well as the difference in internal EGR quantity could be increased. In other words, for such a cylinder having its intake valve or exhaust valve closed in a delayed phase, in the phase in which the intake valve or exhaust valve is closed, the cam torque is exerted in the direction opposite to the direction in which the phase is advanced (camshaft rotational direction), so that advance of the phase is restrained or stopped. On the contrary, in such a cylinder having its intake valve or exhaust valve closed in an advanced phase, in the phase in which the intake valve or exhaust valve is closed, the cam torque is exerted in the direction of advancing the phase, so that the advance of the phase is continued. Consequently, the phase in which the intake valve or exhaust valve is actually closed is further displaced from a phase determined under control, which could increase differences in air quantity and internal EGR for example between cylinders.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a control apparatus or the like for an internal combustion engine that can restrain occurrence of nonuniformity of cylinders with respect to each other in terms of intake air quantity and internal EGR quantity.  
      A control apparatus for an internal combustion engine according to the present invention controls the internal combustion engine including a camshaft driving a valve and a change mechanism changing a phase in which the valve is closed. The control apparatus includes an operation unit. The operation unit controls the change mechanism in a manner that the phase is advanced in a case where a torque exerted on the camshaft by rotation of the camshaft acts in a direction opposite to a rotational direction of the camshaft, and controls the change mechanism in a manner that the phase is retarded in a case where the torque acts in the rotational direction of the camshaft.  
      In accordance with the present invention, in the case where the torque acts in the opposite direction to the camshaft rotational direction, the phase in which the valve is closed is advanced and, in the case where the torque acts in the camshaft rotational direction, the phase is retarded. Thus, for a cylinder, in the case where the torque acting in the opposite direction to the camshaft rotational direction could cause delay of the phase (timing) in (at) which the intake valve or exhaust valve is closed, the phase in which the intake valve or exhaust valve is closed can be advanced. Accordingly, occurrence of delay of the phase in which the intake valve or exhaust valve is closed can be restrained. Further, for a cylinder, in the case where the torque acting in the camshaft rotational direction could cause advance of the phase in which the intake valve or exhaust valve is closed, the phase in which the intake or exhaust valve is closed can be retarded. Accordingly, occurrence of advance of the phase in which the intake valve or exhaust valve is closed can be restrained. Thus, the control apparatus for the internal combustion engine can be provided that can restrain occurrence of displacement of the phase in which the intake valve or exhaust valve of each cylinder is actually closed with respect to a phase determined under control, and can restrain occurrence of nonuniformity of cylinders with respect to each other in terms of quantity of air taken into the cylinder and internal EGR quantity.  
      Preferably, the operation unit controls the change mechanism in a manner that the phase is advanced to a greater extent as the torque acting in the direction opposite to the rotational direction of the camshaft is larger, and controls the change mechanism in a manner that the phase is retarded to a greater extent as the torque acting in the rotational direction of the camshaft is larger.  
      In accordance with the present invention, as the torque acting in the opposite direction to the camshaft rotational direction is larger, the phase of the intake valve or exhaust valve is advanced to a greater extent. Further, as the torque acting in the camshaft rotational direction is larger, the phase of the intake valve or the exhaust valve is retarded to a grater extent. Accordingly, as delay in phase in which the intake valve or exhaust valve is closed is larger, the phase in which the intake valve or exhaust valve is closed can be advanced to a greater extent. Further, as advance in phase in which the intake valve or exhaust valve is closed is larger, the phase in which the intake valve or exhaust valve is closed can be retarded to a greater extent. Thus, for each cylinder, occurrence of displacement of the phase in which the intake valve or exhaust valve is actually closed with respect to a phase determined under control can be restrained, and occurrence of nonuniformity of cylinders with respect to each other in terms of intake air quantity and internal EGR quantity can be restrained.  
      Still preferably, the operation unit controls the change mechanism in a manner that the phase is advanced to a greater extent as the camshaft has a higher rotational speed, and controls the change mechanism in a manner that the phase is retarded to a greater extent as the camshaft has a higher rotational speed.  
      In accordance with the present invention, as a higher camshaft rotational speed causes a larger torque acting in the opposite direction to the camshaft rotational direction, the phase of the intake valve or exhaust valve is advanced to a greater extent. Further, as a higher camshaft rotational speed causes a larger torque acting in the camshaft rotational direction, the phase of the intake valve or exhaust valve is retarded to a greater extent. Accordingly, as delay in phase in which the intake valve or exhaust valve is closed is larger, the phase in which the intake valve or exhaust valve is closed can be advanced to a greater extent. Further, as advance in phase in which the intake valve or exhaust valve is closed is larger, the phase in which the intake valve or exhaust valve is closed can be retarded to a greater extent. Thus, for each cylinder, occurrence of displacement of the phase in which the intake valve or exhaust valve is actually closed with respect to a phase determined under control can be restrained, and occurrence of nonuniformity of cylinders with respect to each other in terms of intake air quantity and internal EGR quantity can be restrained.  
      Still preferably, the operation unit controls the change mechanism in a manner that the phase is advanced to a greater extent as the internal combustion engine has a higher load, and controls the change mechanism in a manner that the phase is retarded to a greater extent as the internal combustion engine has a higher load.  
      In accordance with the present invention, as a higher load of the internal combustion engine causes a larger torque acting in the opposite direction to the camshaft rotational direction, the phase of the intake valve or exhaust valve is advanced to a greater extent. Further, as a higher load of the internal combustion engine causes a larger torque acting in the camshaft rotational direction, the phase in which the intake valve or exhaust valve is closed can be retarded to a greater extent. Accordingly, as delay in phase in which the intake valve or exhaust valve is closed is larger, the phase in which the intake valve or exhaust valve is closed can be advanced to a greater extent. Further, as advance in phase in which the intake valve or exhaust valve is closed is larger, the phase in which the intake valve or exhaust valve is closed can be retarded to a greater extent. Thus, for each cylinder, occurrence of displacement of the phase in which the intake valve or exhaust valve is actually closed with respect to a phase determined under control can be restrained, and nonuniformity of cylinders with respect to each other in terms of intake air quantity and internal EGR quantity can be restrained.  
      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  schematically shows a configuration of an engine of a vehicle on which an ECU is mounted that is a control apparatus according to an embodiment of the present invention.  
       FIG. 2  is a map defining target values of the phase of an intake camshaft.  
       FIG. 3  is a perspective view showing a cylinder block.  
       FIG. 4  is a table showing the firing order of the engine.  
       FIG. 5  shows changes of a cam torque exerted on the intake camshaft.  
       FIG. 6  shows a map for correcting the phase of an intake valve.  
       FIG. 7  is a flowchart showing a control structure of a program executed by the ECU in  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      With reference to the drawings, embodiments of the present invention are hereinafter described. In the following description, like components are denoted by like reference characters. They are also named identically and function identically. Therefore, a detailed description thereof is not repeated.  
      Referring to  FIG. 1 , a description is given of an engine of a vehicle on which a control apparatus is mounted, according to an embodiment of the present invention. The control apparatus in the present embodiment is implemented for example by means of a program executed by an ECU (Electronic Control Unit)  4000  shown in  FIG. 1 .  
      Engine  1000  is a V-type 8-cylinder engine having an “A” bank  1010  and a “B” bank  1012  each including a group of four cylinders. Here, any engine other than the V8 engine may be used.  
      Into engine  1000 , air is sucked from an air cleaner  1020 . The quantity of sucked air is adjusted by a throttle valve  1030 . Throttle valve  1030  is an electronic throttle valve driven by a motor.  
      The air is mixed with fuel in a cylinder  1040  (combustion chamber). Into cylinder  1040 , the fuel is directly injected from an injector  1050 . In other words, injection holes of injector  1050  are provided within cylinder  1040 .  
      The fuel is injected in the intake stroke. The fuel injection timing is not limited to the intake stroke. Further, in the present embodiment, engine  1000  is described as a direct-injection engine having injection holes of injector  1050  that are provided within cylinder  1040 . However, in addition to direct-injection (in-cylinder) injector  1050 , a port injector may be provided. Moreover, only the port injector may be provided.  
      The air-fuel mixture in cylinder  1040  is ignited by a spark plug  1060  and accordingly burned. The air-fuel mixture after burned, namely exhaust gas, is cleaned by a three-way catalyst  1070  and thereafter discharged to the outside of the vehicle. The air-fuel mixture is burned to press down a piston  1080  and thereby rotate a crankshaft  1090 .  
      At the top of cylinder  1040 , an intake valve  1100  and an exhaust valve  1110  are provided. Intake valve  1100  is driven by an intake camshaft  1120 . Exhaust valve  1110  is driven by an exhaust camshaft  1130 . Intake camshaft  1120  and exhaust camshaft  1130  are coupled by such parts as a chain and gears to be rotated at the same rotational speed.  
      Intake valve  1100  has its phase (opening/closing timing) controlled by an intake VVT mechanism  2000  provided to intake camshaft  1120 . Exhaust valve  1110  has its phase (opening/closing timing) controlled by an exhaust VVT mechanism  3000  provided to exhaust camshaft  1130 .  
      In the present embodiment, intake camshaft  1120  and exhaust camshaft  1130  are rotated by the VVT mechanisms to control respective phases of intake valve  1100  and exhaust valve  1110 . It is noted that the method of controlling the phase is not limited to the aforementioned one.  
      Intake VVT mechanism  2000  is operated by an electric motor. Exhaust VVT mechanism  3000  is hydraulically operated. It is noted that intake VVT mechanism  2000  may be hydraulically operated while exhaust VVT mechanism  3000  may be driven by an electric motor. Further, since any well-known art may be applied to implement the VVT mechanism, a detailed description thereof is not given here.  
      To ECU  4000 , signals indicating the rotational speed and the crank angle of crankshaft  1090  are input from a crank angle sensor  5000 . Further, to ECU  4000 , signals indicating respective phases of intake camshaft  1120  and exhaust camshaft  1130  (phase: the camshaft position in the rotational direction) are input from a cam position sensor  5010 .  
      Furthermore, to ECU  4000 , a signal indicating the water temperature (coolant temperature) of engine  1000  from a coolant temperature sensor  5020  as well as a signal indicating the quantity of intake air (quantity of air taken or sucked into engine  1000 ) of engine  1000  from an airflow meter  5030  are input.  
      Based on these signals input from the sensors as well as a map and a program stored in a memory (not shown), ECU  4000  controls the throttle opening position, the ignition timing, the fuel injection timing, the quantity of injected fuel, the phase of intake valve  1100  and the phase of exhaust valve  1110  for example, so that engine  1000  is operated in a desired operating state.  
      In the present embodiment, ECU  4000  determines the phase of intake valve  1100  based on the map as shown in  FIG. 2  that uses the engine speed NE and the intake air quantity KL as parameters. A plurality of maps for respective coolant temperatures are stored for determining the phase of intake valve  1100 .  
      Referring to  FIG. 3 , a further description of engine  1000  is given. In “B” bank  1012  of a cylinder block  1002  of engine  1000 , cylinders  1040  to which respective numbers # 1 , # 3 , # 5  and # 7  are allocated are formed and arranged successively from the front side to the rear side of the vehicle.  
      Further, in “A” bank  1010  of cylinder block  1002 , cylinders  1040  to which respective numbers # 2 , # 4 , # 6  and # 8  are allocated are formed and arranged successively from the front side to the rear side of the vehicle.  
      As shown in  FIG. 4 , firing is caused in the cylinders in the order of # 1  cylinder, # 8  cylinder, # 7  cylinder, # 3  cylinder, # 6  cylinder, # 5  cylinder, # 4  cylinder, and # 2  cylinder. The firing interval is 90° in crank angle (CA).  
      While crankshaft  1090  makes two rotations (720° in crank angle), one cycle of engine  1000  is completed that is comprised of the four steps: intake stroke→compression stroke→power stroke→exhaust stroke. Therefore, between two cylinders one of which precedes the other with another cylinder therebetween in firing order, namely between two cylinders at a firing interval of 180° in crank angle, there is a difference in cycle corresponding to one stroke in the cycle.  
      Thus, when # 1  cylinder is in the transition from the intake stroke to the compression stroke, # 7  cylinder is in the transition from the exhaust stroke to the intake stroke. Here, in the phase (timing) in which intake valve  1100  of # 1  cylinder is closed, intake valve  1100  of # 7  cylinder starts opening operation. Therefore, the cam torque increases that is exerted in the direction opposite to the direction in which intake camshaft  1120  rotates.  
      The cam torque exerted in the direction opposite to the rotational direction of intake camshaft  1120  increases in the phase in which intake valve  1100  of # 3  cylinder is closed, in addition to the phase in which intake valve  1100  of # 1  cylinder is closed.  
      Further, regarding two cylinders that are successive in firing order, namely two cylinders at a firing interval of 90° in crank angle, in the phase in which intake valve  1100  of one cylinder preceding in firing order is closed, intake valve  1100  of the other cylinder following in firing order is in the transition from the opening operation to the closing operation.  
      In the present embodiment, in the phase in which intake valve  1100  of # 7  cylinder is closed, intake valve  1100  of # 3  cylinder is in the transition from the opening operation to the closing operation. Therefore, the cam torque exerted in the direction in which intake camshaft  1120  is rotated increases.  
      A similar state to the above-described one may also occur in “A” bank  1010 . As shown in  FIG. 5 , in the phase in which respective intake valves  1100  of # 2  cylinder and # 6  cylinder are closed, the cam torque exerted in the direction opposite to the rotational direction of intake camshaft  1120  is larger. Further, in the phase in which intake valve  1100  of # 4  cylinder is closed, the cam torque exerted in the rotational direction of intake camshaft  1120  is larger.  
      It is noted that, in  FIG. 5 , the solid line represents the cam torque exerted on intake camshaft  1120  provided to “B ” bank  1012 . The broken line represents the cam torque exerted on intake camshaft  1120  provided to “A” bank  1010 . The dots each represent the cam torque at a crank angle at which intake valve  1100  of each cylinder  1040  is closed.  
      Moreover, in  FIG. 5 , the cam torque exerted in the direction opposite to the rotational direction of intake camshaft  1120  is represented by a positive value and the cam torque exerted in the rotational direction of intake camshaft  1120  is represented by a negative value.  
      If the cam torque exerted in the direction opposite to the rotational direction of intake camshaft  1120  is larger, the phase in which intake valve  1100  is actually closed is later than a phase determined under control, due to influences of deformation or the like of such components as the chain coupling the crankshaft and camshafts and other parts. Accordingly, the phase in which respective intake valves  1100  of # 1  cylinder, # 3  cylinder, # 2  cylinder and # 6  cylinder are actually closed is later than the phase in which respective intake valves  1100  of other cylinders are closed.  
      On the contrary, if the cam torque exerted in the rotational direction of intake camshaft  1120  is larger, the phase in which intake valve  1100  is actually closed is earlier than a phase determined under control, due to influences of deformation or the like of such components as the chain coupling the crankshaft and camshafts and other parts. Accordingly, the phase in which respective intake valves  1100  of # 7  cylinder and # 4  cylinder are actually closed is earlier than the phase in which respective intake valves  1100  of other cylinders are closed.  
      In cylinder  1040  having intake valve  1100  closed in a phase as delayed, the quantity of air pushed back from cylinder  1040  into the intake manifold as piston  1080  is lifted increases, resulting in a decrease in final quantity of air sucked into cylinder  1040 .  
      On the contrary, in cylinder  1040  having intake valve  1100  closed in a phase as advanced, the quantity of air pushed back from cylinder  1040  into the intake manifold as piston  1080  is lifted decreases, resulting in an increase in final quantity of air sucked into cylinder  1040 .  
      Thus, the cylinders differ from each other in intake air quantity. In this case, fluctuation of the rotation of crankshaft  1090  (fluctuation of the rotational speed while one rotation is made) increases, and accordingly vibrations and noise of engine  1000  could increase.  
      Then, in the present embodiment, the phase in which intake valve  1100  is closed is corrected so that the difference in quantity of air sucked into cylinder  1040  is decreased as much as possible. The phase in which intake valve  1100  is closed is corrected, as shown in  FIG. 6 , based on the map defining, according to the cam torque, the extent to which the phase should be corrected.  
      The phase in which intake valve  1100  is closed is corrected, in the case where the crank angle is a crank angle at which the cam torque is exerted in the direction opposite to the rotational direction of intake camshaft  1120 , so that the phase is advanced relative to a reference phase defined by the map shown in  FIG. 2  (the phase determined from engine speed NE and intake air quantity KL).  
      In contrast, the phase in which intake valve  1100  is closed is corrected, in the case where the crank angle is a crank angle at which the cam torque is exerted in the rotational direction of intake camshaft  1120 , so that the phase is retarded relative to the reference phase defined by the map shown in  FIG. 2 .  
      Referring to  FIG. 7 , a description is given of a control structure of a program executed by ECU  4000  that is a control apparatus according to the present embodiment.  
      In step (hereinafter step is abbreviated as S)  100 , ECU  4000  detects the crank angle based on a signal (pulse signal) transmitted from crank angle sensor  5000 .  
      In S 200 , ECU  4000  corrects, based on the detected crank angle and the aforementioned map (see  FIG. 6 ), the phase in which intake valve  1100  of each cylinder  1040  is closed to a phase that is advanced or retarded relative to the reference phase. Intake VVT mechanism  2000  is controlled so that the corrected phase is implemented. After this, this process is ended.  
      A description is given of an operation of ECU  4000  that is the control apparatus in the present embodiment based on the above-described structure and flowchart.  
      While the engine is operating, the crank angle is detected (S  100 ) and, at a crank angle at which the cam torque is exerted in the direction opposite to the rotational direction of intake camshaft  1120 , the phase in which intake valve  1100  is closed is advanced (S 200 ).  
      Thus, in the case where the cam torque exerted in the direction opposite to the rotational direction of intake camshaft  1120  could retard the phase in which intake valve  1100  is actually closed, the phase of intake valve  1100  can be advanced. Accordingly, occurrence of delay in phase in which intake valve  1100  is actually closed can be restrained.  
      Further, at a crank angle at which the cam torque is exerted in the rotational direction of intake camshaft  1120 , the phase in which intake valve  1100  is closed is retarded (S 200 ). Thus, in the case where the cam torque exerted in the rotational direction of intake camshaft  1120  could advance the phase in which intake valve  1100  is actually closed, the phase of intake valve  1100  can be retarded. Accordingly, occurrence of advance in phase in which intake valve  1100  is actually closed can be restrained.  
      As discussed above, with the ECU identified as the control apparatus in the present embodiment, at a crank angle at which the cam torque is exerted in the direction opposite to the rotational direction of the intake camshaft, the intake VVT mechanism is controlled so that the phase in which the intake valve is closed is advanced. Further, at a crank angle at which the cam torque is exerted in the rotational direction of the intake camshaft, the intake VVT mechanism is controlled so that the phase in which the intake valve is closed is retarded. Thus, for a cylinder, in the case where the cam torque exerted in the direction opposite to the rotational direction of the intake camshaft could retard the phase in which the intake valve of the cylinder is actually closed, the phase in which the intake valve is closed can be advanced. Accordingly, occurrence of delay in phase in which the intake valve is actually closed can be restrained. In contrast, for a cylinder, in the case where the cam torque exerted in the rotational direction of the camshaft could advance the phase in which the intake valve of the cylinder is actually closed, the phase in which the intake valve is closed can be retarded. Accordingly, occurrence of advance in phase in which the intake valve is actually closed can be restrained. In this way, occurrence of displacement can be restrained of the phase in which the intake valve of each cylinder is actually closed, with respect to a phase determined under control, and occurrence of nonuniformity of cylinders with respect to each other in terms of quantity of air sucked into the cylinder can be restrained.  
     Other Embodiments  
      In such cases where the rotational speed of intake camshaft  1120  is high and where the load of engine  1000  is high, namely the case where the absolute value of the cam torque is relatively large, the extent to which the phase is corrected may be larger than the one used in the case where the absolute value of the cam torque is relatively small. In other words, in the case where the cam torque exerted in the direction opposite to the direction in which intake camshaft  1120  is rotated is relatively large, the phase in which intake valve  1100  is closed may be advanced to a greater extent than the one used in the case where the aforementioned cam torque is relatively small. Further, in the case where the absolute value of the cam torque exerted in the rotational direction of intake camshaft  1120  is relatively large, the phase in which intake valve  1100  is closed may be retarded to a greater extent than the one used in the case where the aforementioned absolute value is relatively small.  
      In this way, as the phase in which intake valve  1100  is actually closed is delayed to a greater extent, the phase of intake valve  1100  may be advanced to a greater extent. Further, as the phase in which intake valve  1100  is actually closed is advanced to a greater extent, the phase of intake valve  1100  may be retarded to a greater extent. Thus, occurrence of displacement of the phase in which the intake valve of each cylinder is actually closed with respect to a phase determined under control can be restrained, and occurrence of nonuniformity of cylinders with respect to each other in terms of intake air quantity can be restrained.  
      Furthermore, in addition to or instead of intake valve  1100 , exhaust valve  1110  may be advanced or retarded in phase according to the cam torque. This is because of the fact that a cylinder having exhaust valve  1100  closed in a delayed phase has an increased internal EGR quantity while a cylinder having exhaust valve  1100  closed in an advanced phase has a decreased internal EGR quantity, resulting in nonuniformity of cylinders with respect to each other in internal EGR quantity and increased rotational fluctuations of engine  1000 .  
      Thus, for cylinder  1040 , in the case where the cam torque exerted in the direction opposite to the rotational direction of exhaust camshaft  1130  could delay the phase in which exhaust valve  1110  is actually closed, the phase of exhaust valve  1110  can be advanced. Accordingly, occurrence of delay of the phase in which exhaust valve  1110  is actually closed can be restrained.  
      Further, for cylinder  1040 , in the case where the cam torque exerted in the rotational direction of exhaust camshaft  1130  could advance the phase in which exhaust valve  1110  is actually closed, the phase of exhaust valve  1110  can be retarded. Accordingly, occurrence of advance of the phase in which exhaust valve  1110  is actually closed can be restrained. Consequently, for each cylinder, occurrence of displacement of the phase in which exhaust valve  1110  is actually closed with respect to a phase determined under control can be restrained, and occurrence of nonuniformity of cylinders with respect to each other in terms of internal EGR quantity can be restrained.  
      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.