Abstract:
A control device for an internal combustion engine includes: a variable valve timing mechanism that changes a valve timing; and a control unit that controls a change of the valve timing. The control unit sets an engine stop request-time target valve timing that is a target valve timing at the time when a request for an engine stop is issued, starts control for changing the valve timing coincides with the engine stop request-time target valve timing and causes the internal combustion engine to operate at an idle at the time when the request for the engine stop is issued, starts a process of stopping operation of internal combustion engine at the time when the valve timing has reached a predetermined valve timing.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2011-279574 filed on Dec. 21, 2011 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a control device for an internal combustion engine. 
     2. Description of Related Art 
     A control device for an internal combustion engine, which includes a variable valve timing mechanism that changes a valve timing, is described in, for example, Japanese Patent Application Publication No. 2007-327472 (JP 2007-327472 A). In JP 2007-327472 A, a target valve timing at the time when a request to stop the operation of the internal combustion engine (hereinafter, referred to as engine stop) is issued (hereinafter referred to as an engine stop request-time target valve timing) is set, and control for changing the valve timing such that the valve timing coincides with the engine stop request-time target valve timing (hereinafter referred to as engine stop request-time valve timing control) is started at the time when an engine stop request is issued. In this control, a process of stopping the operation of the internal combustion engine (hereinafter referred to as an engine stop process) is started when a predetermined period of time (hereinafter referred to as a predetermined idling extension time) has elapsed from when the engine stop request is issued. 
     The time that is needed from the start of the engine stop process to the stop of the operation of the engine (hereinafter referred to as an engine stop time) differs depending on the operation state of the internal combustion engine at the time when engine stop is requested. The time that is needed until the valve timing is made to coincide with the engine stop request-time target valve timing through engine stop request-time valve timing control (hereinafter referred to as a valve timing control time) differs depending on the valve timing at the time when engine stop is requested, and the engine stop request-time target valve timing. In Japanese Patent Application Publication No. 2007-327472 (JP-2007-327472 A), the aforementioned predetermined idling extension time is set to a certain time. Thus, in the case where the engine stop time is relatively long or the valve timing control time is relatively short, the valve timing may reach the engine stop request-time target valve timing before the operation of the engine is stopped. In this case, the fuel economy of the internal combustion engine may deteriorate correspondingly to idling operation of the internal combustion engine. On the other hand, in Japanese Patent Application Publication No. 2007-327472 (JP-2007-327472 A), in the case where the engine stop time is relatively short or the valve timing control time is relatively long, the operation of the engine may be stopped before the valve timing reaches the engine stop request-time target valve timing. In this case, it may be impossible to make the valve timing reach the engine stop request-time target valve timing. 
     The invention makes the valve timing coincide with the engine stop request-time target valve timing when the operation of the internal combustion engine is stopped, and restrains the fuel economy of the internal combustion engine from deteriorating. 
     SUMMARY OF THE INVENTION 
     The invention relates to a control device for an internal combustion engine that includes a variable valve timing mechanism that changes a valve timing. In a first aspect of the invention, a control device for an internal combustion engine includes a control unit that controls changing of the valve timing. The control unit sets an engine stop request-time target valve timing that is a target valve timing at a time when engine stop that is stop of operation of the internal combustion engine is requested, starts engine stop request-time valve timing control as control of changing the valve timing such that the valve timing coincides with the engine stop request-time target valve timing when the engine stop is requested, operates the internal combustion engine in an idling state for a predetermined time after the engine stop is requested, and starts an engine stop process as a processing of stopping operation of the internal combustion engine upon a lapse of the predetermined time after the engine stop is requested. The control unit sets the predetermined time such that a sum of the predetermined time and an engine stop time as a time that is needed from start of the engine stop process to stop of operation of the internal combustion engine becomes equal to a valve timing control time as a time that is needed until the valve timing is made to coincide with the engine stop request-time target valve timing through the engine stop request-time valve timing control, or such that a sum of the predetermined time and the engine stop time becomes equal to a shortest time among times longer than the valve timing control time. 
     According to the aforementioned configuration, regardless of the length of the engine stop time or the length of the valve timing control time, the operation of the engine is stopped as soon as or almost as soon as the valve timing coincides with the engine stop request-time target valve timing through the engine stop request-time valve timing control. Thus, according to the configuration, the valve timing can be made to coincide with the engine stop request-time target valve timing when the operation of the internal combustion engine is stopped, and the fuel economy of the internal combustion engine can be restrained from deteriorating. 
     In the aforementioned control device, the control unit may estimate the engine stop time, and set the predetermined time from the estimated engine stop time and the valve timing control time. 
     In the control device, the control unit may estimates the estimated engine stop time as a time that increases as a rotational speed of the internal combustion engine increases. 
     In the aforementioned control device, in the case where a power unit is equipped with the internal combustion engine and an electric motor, and the internal combustion engine and the electric motor are coupled to each other, the control unit may estimate the estimated engine stop time as a time that increases as a rotational speed of the electric motor increases. 
     In the aforementioned control device, in the case where a vehicle is equipped with the internal combustion engine and an electric motor, and the internal combustion engine, the electric motor, and the vehicle are coupled to one another, the control unit may estimate the estimated engine stop time as a time that increases as a speed of the vehicle increases. 
     In a second aspect of the invention, a control method for an internal combustion that includes a variable valve timing mechanism that changes a valve timing. The method includes setting an engine stop request-time target valve timing as a target valve timing at a time when engine stop as stop of operation of the internal combustion engine is requested, starting engine stop request-time valve timing control as control of changing the valve timing such that the valve timing coincides with the engine stop request-time target valve timing when the engine stop is requested, operating the internal combustion engine in an idling state for a predetermined time after the engine stop is requested, and starting an engine stop process as a processing of stopping operation of the internal combustion engine upon a lapse of the predetermined time after the engine stop is requested. The control method also includes setting the predetermined time such that a sum of the predetermined time and an engine stop time as a time that is needed from start of the engine stop process to stop of operation of the internal combustion engine becomes equal to a valve timing control time as a time that is needed until the valve timing is made to coincide with the engine stop request-time target valve tuning through the engine stop request-time valve timing control, or such that a sum of the predetermined time and the engine stop time becomes equal to a shortest time among times longer than the valve timing control time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a view showing an internal combustion engine that is equipped with a control device according to the first embodiment of the invention; 
         FIG. 2  is a view showing an variable intake valve timing mechanism of the first embodiment of the invention; 
         FIG. 3  is a view showing a map that is used to acquire a target valve timing in the first embodiment of the invention; 
         FIG. 4  is a view showing an example of a routine for performing engine stop control of the first embodiment of the invention; 
         FIG. 5  is a view showing another example of the routine for performing engine stop control of the first embodiment of the invention; 
         FIG. 6  is a view showing still another example of the routine for performing engine stop control of the first embodiment of the invention; 
         FIG. 7  is a view showing an example of a relationship that is established between engine rotational speed and engine stop time when the internal combustion engine is operated in an idling state after engine stop is requested in the embodiments of the invention; 
         FIG. 8  is a view showing a power unit of the second embodiment of the invention; 
         FIG. 9A  is a view showing an example of a relationship that is established between rotational speed of a first generator motor and engine stop time when the internal combustion engine is operated in an idling state after engine stop is requested in the second embodiment of the invention; and 
         FIG. 9B  is a view showing an example of a relationship that is established between speed of a vehicle and engine stop time when the internal combustion engine is operated in an idling state after engine stop is requested in the second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A first embodiment of the invention will be described below.  FIG. 1  shows an internal combustion engine that includes a control device according to the first embodiment of the invention. In  FIG. 1 , the internal combustion engine  10 , an internal combustion engine body  20 , a valve actuating mechanism  30 , an intake passage  40 , an exhaust passage  50 , an accelerator pedal  60  and an electronic control unit  70  are also shown in  FIG. 1 . A cylinder  21 , a piston  22 , a connecting rod  23 , a crankshaft  24 , a crank angle sensor  25 , a combustion chamber  26 , an ignition plug  27  and a fuel injection valve  28  are also shown in  FIG. 1 . An intake valve  31 , an intake valve actuating mechanism  32 , an exhaust valve  33  and an exhaust valve actuating mechanism  34  are also shown in  FIG. 1 . An intake port  41 , an intake pipe  42 , a throttle valve  43 , a throttle valve actuator  44 , an exhaust port  51 , an exhaust pipe  52  and an accelerator pedal operation amount sensor  61  are also shown in  FIG. 1 . 
     The electronic control unit  70  includes a microprocessor (CPU)  71 , a read only memory (ROM)  72 , a random access memory (RAM)  73 , a backup RAM (B-RAM)  74  and an interface (IF)  75 . These microprocessor  71 , read only memory  72 , random access memory  73 , backup RAM  74  and interface  75  are electrically connected to one another via a bidirectional bus. 
     The piston  22  is arranged in the cylinder  21  so as to be reciprocally movable within the cylinder  21 . The connecting rod  23  connects the piston  22  to the crankshaft  24 . The crank angle sensor  25  is attached to the internal combustion engine body (hereinafter, referred to as engine body)  20  in proximity to the crankshaft  24 , and has the function of outputting an output value corresponding to the rotation phase of the crankshaft  24 . The ignition plug  27  is mounted on the engine body  20  such that the distal end of the ignition plug  27  is exposed to the inside of the combustion chamber  26 . The fuel injection valve  28  is mounted at the intake pipe  42  in proximity to the intake port  41 . 
     The fuel injection valve  28  is electrically connected to the interface  75 , and injects fuel into the intake port  41  on the basis of a command signal from the electronic control unit  70 . Fuel injected from the fuel injection valve  28  is introduced into the combustion chamber  26  together with air via the intake port  41 . The ignition plug  27  is electrically connected to the interface  75 , and ignites fuel in the combustion chamber  26  on the basis of a command signal from the electronic control unit  70 . The piston  22  is reciprocally moved in the cylinder  21  as fuel combusts in the combustion chamber  26 . The crankshaft  24  is rotated via the connecting rod  23  as the piston  22  reciprocally moves in the cylinder  21 . The crank angle sensor  25  is electrically connected to the interface  75 , and the output value of the crank angle sensor  25  is input to the electronic control unit  70 . The electronic control unit  70  calculates the rotation speed of the internal combustion engine on the basis of the output value of the crank angle sensor  25 . 
     The intake valve  31  is arranged on the engine body  20 , and has the function of opening or closing the intake port  41 . The intake valve actuating mechanism  32  is mounted on the engine body  20 . The intake valve actuating mechanism  32  opens or closes the intake valve  31 , and changes the valve timing of the intake valve  31 . As the intake valve  31  is opened, the intake port  41  is opened. As the intake valve  31  is closed, the intake port  41  is closed. The valve timing of the intake valve  31  means both the valve open timing of the intake valve and the valve close timing of the intake valve. 
     The exhaust valve  33  is arranged on the engine body  20 , and has the function of opening or closing the exhaust port  51 . The exhaust valve actuating mechanism  34  is mounted on the engine body  20 , and has the function of opening or closing the exhaust valve  33 . As the exhaust valve  33  is opened, the exhaust port  51  is opened. As the exhaust valve  33  is closed, the exhaust port  51  is closed. 
     The valve actuating mechanism  30  includes the intake valve  31 , the intake valve actuating mechanism  32 , the exhaust valve  33 , and the exhaust valve actuating mechanism  34 . 
     The intake passage  40  is formed of the intake port  41  and the intake pipe  42 , and has the function of supplying air to the combustion chamber  26 . The intake port  41  is formed in the engine body  20 . One end of the intake pipe  42  is connected to the intake port  41 , and the other end of the intake pipe  42  is open to outside air. The throttle valve  43  is pivotably arranged in the intake pipe  42 , and has the function of changing the flow passage area of the intake pipe  42 . The throttle valve actuator  44  is connected to the throttle valve  43 . 
     The throttle valve actuator  44  is electrically connected to the interface  75 , and actuates the throttle valve  43  such that the flow passage area of the intake pipe  42  becomes a desired flow passage area in response to a control signal that is transmitted from the electronic control unit  70 . 
     The exhaust passage  50  is formed of the exhaust port  51  and the exhaust pipe  52 , and has the function of emitting exhaust gas, which is exhausted from the combustion chamber  26 , to outside air. The exhaust port  51  is formed in the engine body  20 . One end of the exhaust pipe  52  is connected to the exhaust port  51 , and the other end of the exhaust pipe  52  is open to outside air. 
     The accelerator pedal  60  is connected to the accelerator pedal operation amount sensor  61 . The accelerator pedal operation amount sensor  61  has the function of outputting an output value corresponding to the depression amount of the accelerator pedal  60 . The accelerator pedal operation amount sensor  61  is electrically connected to the interface  75 , and the output value of the accelerator pedal operation amount sensor  61  is input to the electronic control unit  70 . The electronic control unit  70  calculates a required torque (that is, a torque that is required as a torque to be output from the internal combustion engine) on the basis of the output value of the accelerator pedal operation amount sensor  61 . 
     A mechanism for changing the valve timing of the intake valve in a valve actuating device according to the present embodiment (hereinafter, referred to as variable intake valve timing mechanism) will be described. The variable intake valve timing mechanism according to the present embodiment is shown in  FIG. 2 . In  FIG. 2 , the variable intake valve timing mechanism  80 , an intake camshaft  81 , a housing  82 , a timing pulley  83  and a hydraulic actuator  84  are shown. 
     The housing  82  is accommodated inside the timing pulley  83  such that the outer peripheral wall surface of the housing  82  is in contact with the inner peripheral wall surface of the timing pulley  83 . The timing pulley  83  is connected to the crankshaft  24  via a timing belt (not shown), and is rotated in a direction indicated by an arrow R via the timing pulley  83  through rotation of the crankshaft  24 . The housing  82  is accommodated inside the timing pulley  83  so as to be non-rotatable with respect to the timing pulley  83 . 
     A plurality of vanes  85  are provided on the outer peripheral wall surface of the intake camshaft  81 . The plurality of vanes  85  extend radially outward to the inner peripheral wall surface of the housing  82 . A plurality of partition walls  86  are provided on the inner peripheral wall surface of the housing  82 . The plurality of partition walls  86  extend radially inward to the outer peripheral wall surface of the intake camshaft  81 . A hydraulic chamber (hereinafter, referred to as advance-side hydraulic chamber)  87  is formed between each vane  85  and one of the two adjacent partition walls  86 . On the other hand, a hydraulic chamber (hereinafter, referred to as a retard-side hydraulic chamber)  88  is formed between each vane  85  and the other one of the two adjacent partition walls  86 . 
     The hydraulic actuator  84  supplies hydraulic fluid to the advance-side hydraulic chambers  87 , and simultaneously drains hydraulic fluid from the retard-side hydraulic chambers  88 . Alternatively, the hydraulic actuator  84  drains hydraulic fluid from the advance-side hydraulic chambers  87 , and simultaneously supplies hydraulic fluid to the retard-side hydraulic chambers  88 . 
     A cam (not shown) is provided on the intake camshaft  81 , and the outer peripheral wall surface of the cam is in contact with the distal end of the intake valve  31 . As the intake camshaft  81  rotates, the cant rotates. The intake valve  31  is opened or closed through the rotation of the cam. On the other hand, the exhaust valve actuating mechanism also includes an exhaust camshaft (not shown). A cam (not shown) is also provided on the exhaust camshaft. The outer periphery of the cam is in contact with the distal end of the exhaust valve  33 . As the exhaust camshaft rotates, the cam rotates. The exhaust valve  33  is opened or closed through the rotation of the cam. 
     As the rotation of the crankshaft  24  is transmitted to the timing pulley  83  via the timing belt, the timing pulley  83  rotates. As the timing pulley  83  rotates, the housing  82  rotates together. As the housing  82  rotates, the partition walls  86  rotate together. Thus, the rotation of the housing  82  is transmitted to the vanes  85  via the advance-side hydraulic chambers  87 . Then, the vanes  85  rotate, and the intake camshaft  81  rotates together with the vanes  85 . By so doing, the intake valve  31  is opened or closed. As the timing pulley  83  rotates, the exhaust camshaft is also rotated. By so doing, the exhaust valve  33  is opened or closed. 
     As hydraulic fluid is supplied to the advance-side hydraulic chambers  87  and simultaneously hydraulic fluid is drained from the retard-side hydraulic chambers  88  by the hydraulic actuator  84 , the intake camshaft  81  relatively rotates in the direction of the arrow R shown in  FIG. 2  with respect to the housing  82 . By so doing, the valve open timing and valve close timing of the intake valve  31  are changed to an earlier timing (that is, advanced). On the other hand, as hydraulic fluid is drained from the advance-side hydraulic chambers  87  and simultaneously hydraulic fluid is supplied to the retard-side hydraulic chambers  88  by the hydraulic actuator  84 , the intake camshaft  81  relatively rotates in a direction opposite to the direction of the arrow R in  FIG. 2  with respect to the housing  82 . By so doing, the valve open timing and valve close timing of the intake valve  31  are changed to a later timing (that is, retarded). 
     In the present embodiment, an appropriate valve open timing of the intake valve is obtained in advance through an experiment, or the like, on the basis of an operating state of the internal combustion engine, which is defined by an engine rotation speed and a required torque. As shown in  FIG. 3 , valve open timings obtained in form of a functional map of an engine rotation speed NE and a required torque TQr are stored in the electronic control unit  70  as target valve timings Tivt. During operation of the internal combustion engine, the target valve timing Tivt corresponding to the engine rotation speed NE at that instance and the required torque at that instance are acquired. The valve open timing of the intake valve is changed by the variable intake valve timing mechanism such that the valve open timing of the intake valve coincides with the acquired target valve timing Tivt. More specifically, when the current valve open timing of the intake valve is later than the target valve timing, hydraulic fluid is supplied to the advance-side hydraulic chambers and simultaneously hydraulic fluid is drained from the retard-side hydraulic chambers by the hydraulic actuator. By so doing, the valve open timing of the intake valve is advanced toward the target valve timing. When the valve open timing of the intake valve coincides with the target valve timing, supply of hydraulic fluid to the advance-side hydraulic chambers and drain of hydraulic fluid from the retard-side hydraulic chambers by the hydraulic actuator are stopped. On the other hand, when the current valve open timing of the intake valve is earlier than the target valve timing, hydraulic fluid is drained from the advance-side hydraulic chambers and simultaneously hydraulic fluid is supplied to the retard-side hydraulic chambers by the hydraulic actuator. By so doing, the valve open timing of the intake valve is retarded toward the target valve timing. When the valve open timing of the intake valve coincides with the target valve timing, drain of hydraulic fluid from the advance-side hydraulic chambers and supply of hydraulic fluid to the retard-side hydraulic chambers by the hydraulic actuator are stopped. 
     In the present embodiment, when the valve open timing of the intake valve  31  is determined, the valve close timing of the intake valve  31  is uniquely determined, so a target valve timing related to the valve close timing of the intake valve  31  is not set. 
     Engine stop control according to the present embodiment will be described below. Engine stop control is control that is started when a request to stop engine operation is issued. In the following description, “idling operation” is “engine operation that is able to keep a minimum required engine rotation speed for maintaining engine operation”. 
     In the present embodiment, a target valve timing at the time when an engine stop request is issued (hereinafter, referred to as engine stop request target valve timing) is determined in advance, When an engine stop request is issued, the engine stop request target valve timing is set to the target valve timing. Control for changing the valve open timing of the intake valve such that the valve open timing of the intake valve coincides with the engine stop request target valve timing (hereinafter, referred to as engine stop request valve timing control) is started. The internal combustion engine is operated in an idling state for a predetermined time (hereinafter referred to as a predetermined idling extension time) after engine stop is requested. Upon the lapse of the predetermined idling extension time after engine stop is requested, a processing of stopping engine operation (hereinafter referred to as an engine stop process) is started. Engine stop request-time valve timing control can be performed until the operation of the engine is stopped, and is not performed when the operation of the engine is stopped. In the engine stop process, for example, the injection of fuel from the fuel injection valve is stopped, and the ignition of fuel by the ignition plug is stopped. 
     The predetermined idling extension time of this embodiment of the invention will be described. In this embodiment of the invention, the predetermined idling extension time is set such that the sum of the predetermined idling extension time and a time that is needed from the start of the engine stop process to the stop of the operation of the engine (hereinafter referred to as an engine stop time) becomes equal to a time that is needed to make the valve-opening timing of the intake valve coincide with the engine stop request-time target valve timing through engine stop request-time valve timing control (hereinafter referred to as a valve timing control time), or such that the sum of the predetermined idling extension time and the engine stop time becomes equal to a shortest time among times longer than the valve timing control time (i.e., such that the sum of the predetermined idling extension time and the engine stop time becomes substantially equal to the valve timing control time while remaining longer than the valve timing control time). 
     According to this embodiment of the invention, the following effect is obtained. That is, the engine stop time differs depending on the engine operation state at the time when the engine stop process is started, and the valve timing control time differs depending on the engine stop request-time target valve timing and the valve-opening timing of the intake valve at the time when engine stop is requested. In the case where the engine operation processing is started upon the lapse of a certain time after engine stop is requested, when the engine stop time is relatively long or when the valve timing control time is relatively short, the valve-opening timing of the intake valve may reach the engine stop request-time target valve timing before the operation of the engine is stopped. In this case, the fuel economy of the internal combustion engine may deteriorate in accordance with idling operation of the internal combustion engine. Alternatively, when the engine stop time is relatively short or when the valve timing control time is relatively long, the operation of the engine may be stopped before the valve-opening timing of the intake valve reaches the engine stop request-time target valve timing. In this case, it may be impossible to make the valve-opening timing of the intake valve reach the engine stop request-time target valve timing. In this embodiment of the invention, the predetermined idling extension time is set such that the sum of the predetermined idling extension time and the engine stop time becomes equal to the valve timing control time, or such that the sum of the predetermined idling extension time and the engine stop time becomes equal to a shortest time among times longer than the valve timing control time. Accordingly, the operation of the engine is stopped as soon as or almost as soon as the valve timing coincides with the engine stop request-time target valve timing through engine stop request-time valve timing control, regardless of the length of the engine stop time. Thus, according to this embodiment of the invention, when the operation of the engine is stopped, the valve-opening timing of the intake valve can be made to coincide with the engine stop request-time target valve timing, and the fuel economy of the internal combustion engine can be restrained from deteriorating. 
     An example of a routine that executes engine stop control according to the present embodiment will be described below. An example of the routine is shown in  FIG. 4 . The routine is started at predetermined intervals. 
     When the routine of  FIG. 4  is started, it is determined in step  100  whether or not engine stop has been requested. If it is determined that engine stop has been requested, the routine proceeds to step  101 . On the other hand, if it is determined that engine stop has not been requested, the routine ends. 
     In step  101 , the engine stop request-time target valve timing is set as the target valve timing Tivt. Subsequently in step  102 , engine stop request-time valve timing control is started. Subsequently in step  103 , idling operation of the internal combustion engine is started. Subsequently, it is determined in step  104  whether or not a time Ti that has elapsed after the start of idling operation in step  103  is equal to or longer than the predetermined idling extension time Tith (Ti≧Tith). If it is determined that Ti≧Tith, the routine proceeds to step  105 . The engine stop process is started, and then the routine ends. On the other hand, if it is determined that Ti&lt;Tith, the routine returns to step  104 . 
     In engine stop control of the foregoing embodiment of the invention, the engine stop process may be started even if the predetermined idling extension time has not elapsed after the start of idling operation of the internal combustion engine when the valve-opening timing of the intake valve reaches the engine stop request-time target valve timing. 
     An example of a routine for performing engine stop control in this case is shown in  FIG. 5 . This routine is started on a predetermined cycle. Because steps  200  to  203  of  FIG. 5  are identical to steps  100  to  103  of  FIG. 4  respectively, the description of these steps is omitted. 
     In step  204  of  FIG. 5 , it is determined whether or not the time Ti that has elapsed after the start of idling operation in step  203  is equal to or longer than the predetermined idling extension time Tith (Ti≧Tith). If it is determined herein that Ti≧Tith, the routine proceeds to step  205 . On the other hand, if it is determined that Ti&lt;Tith, the routine proceeds to step  206 . 
     It is determined in step  206  whether or not the current valve-opening timing Tiv of the intake valve coincides with the target valve timing Tivt (Tiv=Tivt). If it is determined herein that Tiv=Tivt, the routine proceeds to step  205 . On the other hand, if it is determined that Tiv≠Tivt, the routine returns to step  204 . 
     In step  205 , the engine stop process is started, and then the routine ends. 
     In engine stop control of the foregoing embodiment of the invention, even if the valve-opening timing of the intake valve has not reached the engine stop request-time target valve timing before the engine stop process is started, the valve-opening timing of the intake valve that can sufficiently reach this engine stop request-time target valve timing before the operation of the engine is stopped may be set as an engine stop process start permission valve timing. In this case, when the valve-opening timing of the intake valve reaches the engine stop process start permission valve timing, the engine stop process may be started, even if the predetermined idling extension time has not elapsed after the start of idling operation of the internal combustion engine. 
     An example of a routine for performing engine stop control in this case is shown in  FIG. 6 . This routine is started on a predetermined cycle. 
     When the routine of  FIG. 6  is started, it is determined in step  300  whether or not engine stop has been requested. If it is determined that engine stop has been requested, the routine proceeds to step  301 . On the other hand, if it is determined that engine stop has not been requested, the routine ends. 
     In step  301 , the engine stop request-time target valve timing is set as the target valve timing Tivt, and an engine stop process start permission valve timing Tiva is set. Subsequently in step  302 , engine stop request-time valve timing control is started. Subsequently in step  303 , idling operation of the internal combustion engine is started. Subsequently, it is determined in step  304  whether or not the time Ti that has elapsed after the start of idling operation in step  303  is equal to or longer than the predetermined idling extension time Tith (Ti≧Tith). If it is determined that Ti≧Tith, the routine proceeds to step  305 . On the other hand, if it is determined that Ti&lt;Tith, the routine proceeds to step  306 . 
     It is determined in step  306  whether or not the current valve-opening timing Tiv of the intake valve coincides with the engine stop process start permission valve timing Tiva (Tiv=Tiva). If it is determined that Tiv=Tiva, the routine proceeds to step  305 . On the other hand, if it is determined that Tiv≠Tiva, the routine returns to step  304 . 
     In step  305 , the engine stop process is started, and then the routine ends. 
     In the foregoing embodiment of the invention, the method of setting the predetermined idling extension time is not limited in particular. For example, the engine stop time may be estimated from various parameters regarding the operation of the internal combustion engine, and the predetermined idling extension time may be set using the engine stop time thus estimated and the valve timing control time. More specifically, a method of setting a time that is calculated by subtracting the aforementioned estimated engine stop time from the valve timing control time as the predetermined idling extension time may be adopted as the method of setting the predetermined idling extension time. In this case, the valve timing control time may be estimated from various parameters regarding the operation of the internal combustion engine. 
     As the engine rotational speed increases, the friction and inertial force regarding the operation of the engine increase, and the time that is needed from the start of the engine stop process to the stop of the operation of the engine increases. Thus, in the case where the engine stop time is estimated from various parameters regarding the operation of the engine as described above, the aforementioned estimated engine stop time Tes may be increased as the engine rotational speed NEi at the time when the internal combustion engine is operated in an idling state after engine stop is requested increases, as shown in, for example,  FIG. 7 . 
     The foregoing first embodiment of the invention is an embodiment in the case where the invention is applied to the internal combustion engine. In the second embodiment of the invention, a case where the invention is applied to a power unit (or a hybrid system) that is equipped with an internal combustion engine and an electric motor will be described hereinafter. 
     A vehicle that is equipped with the power unit of the second embodiment of the invention is shown in  FIG. 8 . In  FIG. 8 , motor generators MG 1  and MG 2  (hereinafter, referred to as first motor generator and second motor generator), the internal combustion engine  10 , the crankshaft (output shaft)  24 , the crank angle sensor  25 , a power distribution mechanism  90 , an inverter  110 , a battery  111 , the accelerator pedal  60 , the accelerator pedal operation amount sensor  61  and the electronic control unit  70  are shown. Note that the internal combustion engine  10  shown in  FIG. 8  includes the same components as those of the internal combustion engine  10  shown in  FIG. 1 . 
     The power distribution mechanism  90  includes a planetary gear unit  91 . The planetary gear unit  91  includes a sun gear  92 , planetary gears  93  and a ring gear  94 . The planetary gears  93  are in mesh with the sun gear  92 , and are in mesh with the ring gear  94 . The sun gear  92  is connected to a shaft (hereinafter, referred to as first shaft)  100  of the first motor generator MG 1 . Thus, the first motor generator MG 1  can be driven for rotation by torque that is input from the sun gear  92  to the first motor generator MG 1 , and is able to output torque to the sun gear  92 . The first motor generator MG 1  is able to generate electric power as it is driven for rotation by torque that is input from the sun gear  92  to the first motor generator MG 1 . The ring gear  94  is connected to a shaft (hereinafter, referred to as second shaft)  101  of the second motor generator MG 2  via a ring gear carrier  96 . Thus, the second motor generator MG 2  is able to output torque to the ring gear  94 , and can be driven for rotation by torque that is input from the ring gear  94  to the second motor generator MG 2 . The second motor generator MG 2  is able to generate electric power as it is driven for rotation by torque that is input from the ring gear  94  to the second motor generator MG 2 . 
     The planetary gears  93  are connected to the crankshaft  24  via a planetary gear carrier  95 . Thus, the planetary gears  93  are driven for rotation by torque that is input from the crankshaft  24  to the planetary gears  93 . The planetary gears  93  are in mesh with the sun gear  92  and the ring gear  94 . Thus, when torque is input from the planetary gears  93  to the sun gear  92 , the sun gear  92  is driven for rotation by the torque. When torque is input from the planetary gears  93  to the ring gear  94 , the ring gear  94  is driven for rotation by the torque. Conversely, when torque is input from the sun gear  92  to the planetary gears  93 , the planetary gears  93  are driven for rotation by the torque. When torque is input from the ring gear  94  to the planetary gears  93 , the planetary gears  93  are driven for rotation by the torque. 
     The ring gear  94  is connected to an output gear  97  via the ring gear carrier  96 . Thus, the output gear  97  is driven for rotation by torque that is input from the ring gear  94  to the output gear  97 , and the ring gear  94  is driven for rotation by torque that is input from the output gear  97  to the ring gear  94 . 
     The first motor generator MG 1  includes a resolver  102 . The resolver  102  is connected to the interface  75  of the electronic control unit  70 . The resolver  102  outputs an output value corresponding to the rotation angle of the first motor generator MG 1 . The output value is input to the electronic control unit  70 . The electronic control unit  70  calculates the rotation speed (hereinafter, referred to as first MG rotation speed) of the first motor generator on the basis of the output value. The second motor generator MG 2  includes a resolver  103 . The resolver  103  is connected to the interface  75  of the electronic control unit  70 . The resolver  103  outputs an output value corresponding to the rotation angle of the second motor generator. The output value is input to the electronic control unit  70 . The electronic control unit  70  calculates the rotation speed (hereinafter, referred to as second MG rotation speed) of the second motor generator on the basis of the output value. 
     The first motor generator MG 1  is electrically connected to the battery  111  via the inverter  110 . Thus, when the first motor generator MG 1  is generating electric power, electric power generated by the first motor generator MG 1  (hereinafter, referred to as first generated electric power) can be supplied to the battery  111  via the inverter  110 . The first motor generator MG 1  can be driven for rotation by electric power that is supplied from the battery  111 , and the rotation speed of the first motor generator MG 1  is controllable by controlling a control torque (hereinafter, referred to as first control torque) that is applied to the first motor generator MG 1  using electric power that is supplied from the battery  111 . 
     The second motor generator MG 2  is electrically connected to the battery  111  via the inverter  110 . The second motor generator MG 2  can be driven for rotation by electric power that is supplied from the battery  111 , and the rotation speed of the second motor generator MG 2  is controllable by controlling a control torque (hereinafter, referred to as second control torque) that is applied to the second motor generator MG 2  using electric power that is supplied from the battery  111 . When the second motor generator MG 2  is generating electric power, electric power generated by the second motor generator MG 2  (hereinafter, referred to as second generated electric power) can be supplied to the battery  111  via the inverter  110 . The first generated electric power can be directly supplied to the second motor generator MG 2 , and the second generated electric power can be directly supplied to the first motor generator MG 1 . 
     The battery  111  is connected to the interface  75  of the electronic control unit  70 . Information about the amount of electric power that is stored in the battery  111  is input to the interface  75  of the electronic control unit  70 . Although not shown in the drawing, the inverter  110  is connected to the interface  75  of the electronic control unit  70 . The amount of electric power that is supplied from the inverter  110  to the second motor generator MG 2  and the amount of electric power that is supplied from the inverter  110  to the first motor generator MG 1  are controlled by a command that is transmitted from the electronic control unit  70  via the interface  75 . 
     The output gear  97  is connected to a differential gear  105  via a gear train  104 , The differential gear  105  is connected to a drive shaft  106 . Drive wheels  107  are respectively connected to both ends of the drive shaft  106 . Thus, torque from the output gear  97  is transmitted to the drive wheels  107  via the gear train  104 , the differential gear  105  and the drive shaft  106 . 
     In this embodiment of the invention, a required power that is required for the power unit is calculated on the basis of the accelerator pedal operation amount and the vehicle speed. The power unit of this embodiment of the invention is formed of the internal combustion engine  10 , the first motor generator MG 1  and the second motor generator MG 2 . 
     In this embodiment of the invention, a power that is output from the internal combustion engine within the required power is calculated as a required engine power. An engine operation point at which fuel economy is maximum when the required engine power is caused to output from the crankshaft is obtained in advance by an experiment, or the like, as an optimal engine operation point for each required engine power. These optimal engine operation points are plotted on a graph that is defined by an engine torque and an engine rotation speed, and these optimal engine operation points are connected. The thus formed line is obtained as an optimal engine operation line. The optimal engine operation line is stored in the electronic control unit. A required engine power is calculated during engine operation, and an engine operation point in the optimal engine operation line, at which it is possible to output the calculated required engine power from the internal combustion engine, is selected. The engine torque and the engine rotation speed that define the selected engine operation point are respectively set for a target engine torque and a target engine rotation speed. The fuel injection amount and the engine rotation speed are controlled such that the set target engine torque and target engine rotation speed are achieved. 
     When the required engine power calculated during engine operation is zero, the engine operation is stopped, and the required power is output from the power unit using only power from the first motor generator or the second motor generator or both of the first motor generator and the second motor generator. 
     When the second MG rotation speed is constant, as the first MG rotation speed changes, the engine rotation speed also changes. In other words, it is possible to control the engine rotation speed by controlling the first MG rotation speed. Where the first MG rotation speed is denoted by NM 1 , the second MG rotation speed is denoted by NM 2 , the engine rotation speed is denoted by NE and the ratio of the number of teeth of the sun gear to the number of teeth of the ring gear (that is, the number of teeth of the sun gear/the number of teeth of the ring gear) is denoted by ρ, the relationship expressed by the following mathematical expression (1) holds between the first MG rotation speed and the engine rotation speed. Where the target first MG rotation speed is denoted by NM 1   t  and the target engine rotation speed is denoted by NEt, the relationship expressed by the following mathematical expression (2) holds between the target first MG rotation speed and the target engine rotation speed.
 
NM1=(NE−NM2)/ρ+NE  (1)
 
NM1 t =(NE t −NM2)/ρ+NE t   (2)
 
     In this embodiment of the invention, the target first MG rotation speed NM 1   t  is calculated from the above mathematical expression (2) using the target engine rotation speed NEt, which is set in accordance with the engine operation point that is selected in accordance with the required output, and the current second MG rotation speed NM 2 . A deviation (=NM 1   t −NM 1 ) of the current first MG rotation speed NM 1  with respect to the calculated target first MG rotation speed NM 1   t  is calculated. The first control torque is controlled such that the calculated deviation becomes zero. 
     Where an engine torque is denoted by TQE, an engine torque that is input to the ring gear (or the drive wheels) (hereinafter, referred to as ring gear input engine torque) is denoted by TQEr and the ratio of the number of teeth of the sun gear to the number of teeth of the ring gear (that is, the number of teeth of the sun gear/the number of teeth of the ring gear) is denoted by ρ, the relationship expressed by the following mathematical expression (3) holds between the ring gear input engine torque and the engine torque.
 
TQEr=1/(1+ρ)×TQE  (3)
 
     That is, the ring gear input engine torque TQEr is part of the engine torque TQE. Thus, the ring gear input engine torque TQEr is smaller than the required driving torque (that is, torque that should be input to the drive wheels  107 ). In the present embodiment, the second control torque is controlled such that a torque corresponding to the difference between the required driving torque and the ring gear input engine torque TQEr is input from the second motor generator to the ring gear, and a torque equal to the required driving torque is input to the ring gear. 
     The engine rotational speed tends to increase as the rotational speed of the first generator motor increases. As a result, the friction and inertial force regarding the operation of the engine increase, and the time that is needed from the start of the engine stop process to the stop of the operation of the engine increases. In the case where the engine stop time is estimated from various parameters regarding the operation of the engine as described above, the estimated engine stop time Tes may be so set as to increase as a rotational speed of the first generator motor at the time when the internal combustion engine is operated in an idling state after engine stop is requested (preferably, a rotational speed of the first generator motor at a time point that is as late as possible while the internal combustion engine is operated in an idling state after engine stop is requested) NM 1   i  increases as shown in, for example,  FIG. 9A . 
     Alternatively, the estimated engine stop time Tes may be made to increase as a speed of the vehicle at the time when the internal combustion engine is operated in an idling state after engine stop is requested (preferably, a speed of the vehicle at a time point that is as late as possible while the internal combustion engine is operated in an idling state after engine stop is requested) Vi increases as shown in, for example,  FIG. 9B . 
     This embodiment of the invention is an embodiment in the case where the invention is applied to the power unit or the vehicle shown in  FIG. 8 . The invention is widely applicable to a power unit equipped with an internal combustion engine and an electric motor that are coupled to each other, or to a vehicle equipped with this power unit. The invention is applicable to a vehicle that is equipped with an internal combustion engine and an electric motor with the internal combustion engine, the electric motor, and the vehicle coupled to one another. 
     Each of the foregoing embodiments of the invention is an embodiment in the case where the invention is applied to the internal combustion engine equipped with the variable intake valve timing mechanism that changes the valve-opening timing of the intake valve and the valve-closing timing of the intake valve. However, the invention is also applicable to an internal combustion engine equipped with an variable intake valve timing mechanism that changes one of the valve-opening timing of the intake valve and the valve-closing timing of the intake valve. 
     Each of the foregoing embodiments of the invention is an embodiment in the case where the invention is applied to the internal combustion engine that includes the variable intake valve timing mechanism that changes the valve timing by hydraulic pressure. The invention is also applicable to an internal combustion engine that includes a variable intake valve timing mechanism that changes the valve timing by means of other than hydraulic pressure, as long as it takes a certain time to reach the target valve timing from the start of changing the valve timing of the intake valve. 
     Each of the foregoing embodiments of the invention is an embodiment in the case where the invention is applied to the internal combustion engine that includes the variable intake valve timing mechanism that changes the valve timing of the intake valve. The invention is also applicable to an internal combustion engine that includes a variable exhaust valve timing mechanism that changes the valve timing of the exhaust valve instead of the variable intake valve timing mechanism. In this case, the exhaust valve actuating mechanism has the function of opening or closing the exhaust valve and the function of changing the valve timing of the exhaust valve. In this case, the same configuration as the configuration of the variable intake valve timing mechanism described with reference to  FIG. 2  may be, for example, employed as the configuration of the variable exhaust valve timing mechanism. 
     Each of the foregoing embodiments of the inventions an embodiment in the case where the invention is applied to a spark ignition internal combustion engine (so-called gasoline engine). The invention is also applicable to a compression ignition internal combustion engine (so-called diesel engine).