Abstract:
A variable valve timing control apparatus is provided which comprises an operation angle control mechanism capable of varying an intake operation angle, a phase control mechanism capable of varying an intake maximum lift phase, and a control unit that controls the operation angle control mechanism and the phase control mechanism in accordance with an engine operating condition, wherein the control unit has an engine temperature estimating section that estimates an engine temperature and controls the intake operation angle so that when the engine is operating in an extremely low-load range including idling, the intake operation angle when an engine temperature is extremely low is set larger than that when the engine is cold, the extremely low-engine temperature being lower than the engine temperature when the engine is cold. A method of controlling a variable valve timing of an internal combustion engine is also provided.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a variable valve timing control apparatus for an internal combustion engine which includes both of an operation angle control mechanism capable of varying an operation angle of an intake valve (hereinafter referred to as intake operation angle) and a phase control mechanism capable of varying a maximum lift phase of an intake valve (hereinafter referred to as intake maximum lift phase). The present invention further relates to a method of controlling a variable valve timing of an internal combustion engine. 
   For the purpose of improving the output and the fuel consumption of an internal combustion engine and reducing the exhaust emission, various variable valve timing control apparatuses for varying the opening and closing characteristics (valve lift characteristics) of intake and exhaust valves have heretofore been proposed. For example, a variable valve timing control apparatus that includes a valve lift control mechanism capable of varying a valve lift and an operation angle of an intake valve in two stages and a phase control mechanism capable of varying a maximum lift phase of an intake valve continuously are used jointly is disclosed in Japanese Patent Provisional Publication No. 2000-18056. 
   SUMMARY OF THE INVENTION 
   In such a variable valve timing control apparatus, it is desired to make smaller the intake operation angle when the engine is operating in an extremely low-load range including idling with a view to improving the fuel consumption and reducing the exhaust emission. However, at extremely low temperature, i.e., in case the engine temperature falls beyond −20° C. in a cold district, etc., the friction of the engine becomes extremely high mainly due to an increase of the viscosity of engine oil. Accordingly, if the intake operation angle is made smaller as described when the engine is to operate in the extremely low-load range and at extremely low-engine temperature, it cannot be obtained an engine torque that is sufficiently large enough for the engine to start against the above-described engine friction, thus possibly lowering the engine startability. The present invention has been made with a view to solving such a problem. 
   To achieve the above object, there is provided according to an aspect of the present invention a variable valve timing control apparatus for an internal combustion engine comprising an operation angle control mechanism capable of varying an intake operation angle of an intake valve, a phase control mechanism capable of varying an intake maximum lift phase of the intake valve, and a control unit that controls the operation angle control mechanism and the phase control mechanism in accordance with an engine operating condition, wherein the control unit has an engine temperature estimating section that estimates an engine temperature and controls the operation angle control mechanism so that when the engine is operating in an extremely low-load range including idling, the intake operation angle when an engine temperature is extremely low is set larger than that when the engine is cold, the extremely low-engine temperature being lower than the engine temperature when the engine is cold. 
   According to another aspect of the present invention, there is provided a method of controlling a variable valve timing of an internal combustion engine, the engine having a variable valve timing control apparatus that includes an operation angle control mechanism capable of varying an intake operation angle of an intake valve, a phase control mechanism capable of varying an intake maximum lift phase of the intake valve, and a control unit that controls the operation angle control mechanism and the phase control mechanism in accordance with an engine operating condition, the control unit having an engine temperature estimating section that estimates an engine temperature, the method comprising controlling the operation angle control mechanism so that when the engine is operating in an extremely low-load range including idling, the intake operation angle when an engine temperature is extremely low is set larger than that when the engine is cold, the extremely low-engine temperature being lower than the engine temperature when the engine is cold. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic perspective view of a variable valve timing control apparatus according to an embodiment of the present invention; 
       FIG. 2  is a sectional view of an operation angle control mechanism of the variable valve timing control apparatus of  FIG. 1 ; 
       FIG. 3  is an enlarged sectional view of a phase control mechanism of the variable valve timing control apparatus of  FIG. 1 ; 
       FIG. 4  is a flowchart of a control routine of determining and controlling the intake operation angle and intake maximum lift phase; 
       FIG. 5  is a diagram showing an intake operation angle in relation to engine temperature when the engine is operating in an extremely low-load range; 
       FIG. 6  is a diagram showing an intake maximum lift phase in relation to engine temperature when the engine is operating in an extremely low-load range; 
       FIG. 7  is a diagram showing various valve lift characteristics when the engine is operating in an extremely low-load range; 
       FIG. 8  illustrates intake valve lift characteristics when the engine is under various operating conditions; 
       FIG. 9  is a diagram showing the valve lift characteristics upon acceleration from idling of cold engine; 
       FIG. 10  is a diagram showing the valve lift characteristics upon acceleration from idling after warm-up; 
       FIG. 11  is a diagram showing the valve lift characteristics upon acceleration from idling after warm-up; and 
       FIG. 12  is a flowchart of a control routine of determining and controlling the priority of the operation angle control mechanism and the phase control mechanism. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , each cylinder (not shown) is provided with a pair of intake valves  2 . Above intake valves  2  is disposed hollow intake drive shaft  3  that extends in the direction along which cylinders (not shown) are arranged. Rotatably installed on intake drive shaft  3  are oscillation cams  4  that are drivingly connected to each other by a sleeve (no numeral) so as to oscillate or pivot together and abuttingly engaged with valve lifters  2   a  so as to drive intake valves  2 , respectively. 
   Between intake drive shaft  3  and one oscillation cam  4  is disposed operation angle control mechanism  10  for continuously varying the operation angle and valve lift of intake valves  2 . To an end portion of intake drive shaft  3  is provided phase control mechanism  20  that is capable of continuously varying an intake maximum lift phase that is a phase of intake valve  2  when the lift of intake valve  2  is maximum, by varying the phase of intake drive shaft  3  relative to a crankshaft (not shown). 
   As shown in  FIGS. 1 and 2 , operation angle control mechanism  10  includes circular drive cam  11  eccentrically and fixedly mounted on intake drive shaft  3 , pivotal link  12  pivotally mounted on drive cam  11 , control shaft  13  extending in parallel with intake drive shaft  3  and in the direction in which the cylinders (not shown) are arranged, circular control cam  14  eccentrically and fixedly provided to control shaft  13 , rocker arm  15  pivotally mounted on control cam  14  and having an end portion pivotally connected to a protruded arm portion of pivotal link  12 , and connecting link  16  having an upper end portion pivotally connected to another end portion of rocker arm  15  and a lower end portion pivotally connected to one oscillation cam  4 . Control shaft  13  is driven to rotate within a predetermined control range by means of electric actuator  17  and by way of gear unit  18 . 
   With the above-described structure, when intake drive shaft  3  is rotated in timed relation to the crankshaft, drive cam  11  causes pivotal link  12  to move up and down. Movement of pivotal link  12  causes rocker arm  15  to pivot about the axis of control cam  14 . Oscillation cams  4  are caused to oscillate by way of connecting link  16 , thus driving intake valves  2  to open and close. Further, by varying the rotational or angular position of control shaft  13 , the axis of control cam  14  that is the pivotal axis of rocker arm  15  is varied, thus causing the position of oscillation cams  4  to be varied. This enables the intake operation angle and valve lift to be varied continuously with the intake maximum lift phase being maintained nearly constant. 
   Such operation angle control mechanism  10  has a good durability and reliability in operation since the connecting portions of the constituent parts such as the bearing portions of drive cam  11  and control cam  14  are structured so as to be in surface-to-surface contact with each other and therefore lubrication thereof can be attained with ease. Further, since oscillation cam  4  that drives intake valve  2  is disposed concentrically with intake drive shaft  3 , operation angle control mechanism  10  can be more accurate in control and more compact in size so as to be installed on the engine more easily as compared with the mechanism where the oscillation cam is installed on a shaft different from intake drive shaft  3 . Particularly, operation angle control mechanism  10  can be applied to the direct drive type valve operating system without requiring a large change of layout. Further, since a biasing means such as return springs is not necessitated, the friction of the valve operating mechanism can be held low. 
   ECU (Engine control Unit)  30  executes the following variable control of the operation angle and maximum lift phase of intake valves  2  in addition to a general engine control such as a fuel injection control and an ignition timing control based on the angular positions of intake drive shaft  3  and control shaft  13  that are detected by angle detecting sensor  31  and further on the engine operating conditions such as crank angle, engine speed, load and engine temperature that are detected or estimated by various sensors or the like. Further, ECU  30  includes engine temperature estimating means  36  that estimates the engine temperature (oil/water temperature) based upon at least one of the cooling water temperature detected by water temperature sensor  34  and the oil temperature detected by oil temperature sensor  35 . Based on the engine temperature, ECU  30  can determine whether the engine is in a warm-up condition, i.e., whether the engine temperature is extremely low or whether the engine is cold or after warm-up, accurately. 
     FIG. 3  shows electric phase control mechanism  20 . Phase control mechanism  20  includes first rotor  21  fixedly attached to cam sprocket  25  that rotates in timed relation to the crankshaft, second rotor  22  fixedly attached to an end of intake drive shaft  3  to rotate together with intake drive shaft  3 , and intermediate gear  23  meshed with the inner circumferential surface of first rotor  21  and the outer circumferential surface of second rotor  22  by means of helical splines  26 . To intermediate gear  23  is connected drum  27  by way of three-start threads  28 . Between drum  27  and intermediate gear  23  is disposed coil spring  29 . Intermediate gear  23  is urged by coil spring  29  in the direction to retard (in the left-hand direction in  FIG. 3 ) and is caused to move in the direction to advance (in the right-hand direction in  FIG. 3 ) by way of drum  27  and three-start threads  28  when a voltage is applied to electromagnetic retarder  24  to produce a magnetic force. Depending upon the axial position of intermediate gear  23 , the relative phase of rotors  21 ,  22  is varied thereby varying the phase of intake drive shaft  3  relative to the crankshaft. Above-described electromagnetic retarder  24  is controlled depending upon a control signal from ECU  30 . 
     FIG. 4  shows a flowchart of a control routine for determination and control of the intake operation angle and intake maximum lift phase at start of the engine and when the engine is operating in an extremely low-load range, which constitutes an important part of this embodiment. The control routine is executed by ECU  30 . When it is determined in step S 1  that the engine is in a starting condition or it is determined in step S 2  that the engine is operating in an extremely low-load range including idling, the program proceeds to step S 3  where the engine temperature that is estimated by engine temperature estimating means  36  is read. In the memory of ECU  30  are stored tables or maps corresponding to (a) of FIG.  5  and (b) or (c) of FIG.  6 . In step S 4 , the target values of the intake operation angle and the intake maximum lift phase are calculated by reference to those tables or maps. In step S 5 , control signals corresponding to those target values are outputted to electric actuator  17  of operation angle control mechanism  10  and electromagnetic retarder  24  of phase control mechanism  20 . According to those control signals, the intake operation angle and intake maximum lift phase are controlled independently. When the determination in step S 2  is negative, the program proceeds to step S 6  where determination and control of the intake operation angle and intake maximum lift phase according to the engine speed and engine load is executed by another control routine that is not shown. 
     FIGS. 5  to  7  show intake valve lift characteristics with relation to the engine temperature when the engine is in the above-described starting condition or the engine is operating in the extremely low-load range including idling. Indicated by (a) of  FIG. 5  are the intake operation angle characteristics, and indicated by (b) of  FIG. 6  are the intake maximum lift phase characteristics. In the meantime, the phrase “when the engine is cold” is herein used to indicate that the engine is of the normal temperature before warm-up and typically the engine temperature is 20° C. The phrase “when the engine temperature is extremely low” is herein used to indicate that the temperature of the engine in cold districts or the like is lower than that normally resulting when the engine is cold and typically −20° C. or lower. 
   When the engine temperature is extremely low, the viscosity of engine oil that serves as lubricant becomes higher as compared with that when the engine is cold and therefore the friction of the engine becomes larger. This results in the necessity of making the engine produce a larger torque for maintaining at least idle engine speed against the friction. Thus, in this embodiment, when the engine temperature is extremely low, the intake operation angle  40   a  is set at about 180° CA (crank angle) and the intake maximum lift phase  40   b  is set at about 90° ATDC (after top dead center). Namely, the intake valve opening timing (IVO) of intake valve characteristics  40  (refer to  FIG. 7 ) is set at a point adjacent TDC and the intake valve closing timing (IVC) is set at a point adjacent BDC. This enables intake valve  2  to open in proper quantities on intake stroke, i.e., no overlap and no minus overlap are caused, thus enabling the engine to produce a sufficient torque for maintaining idle engine speed against the above-described friction. Accordingly, it becomes possible to obtain a good engine startability and a rapid warm-up operation even when the engine temperature is extremely low. 
   When the engine is cold, intake operation angle  42   a  is set at a minimum operation angle, i.e., at a value ranging from about 80° to 100° CA and preferably 90° CA, and intake maximum lift phase  42   b  is set at a maximumly retarded phase, i.e., at 180° ATDC, namely, set at a point adjacent BDC, and the intake valve opening timing (IVO) of intake valve lift characteristics  42  is largely retarded than TDC to enlarge the retard limits of the ignition timing thereby accelerating a rise of exhaust temperature and shortening the time for catalyst temperature rise for thereby improving the exhaust emission. Further, by minimizing the intake operation angle, the friction of the valve operating system can be kept minimum while enhancing the gas flow thereby accelerating atomization of fuel. 
   After warm-up, intake operating angle  44   a  is kept minimum similarly to the time when the engine is cold, and intake maximum lift phase  44   b  is advanced than that when the engine is cold thereby decreasing the pumping loss and improving the fuel consumption. 
   At the transition from the time when the engine temperature is extremely low to the time when the engine is cold, intake operation angle  41   a  is made smaller gradually and intake maximum lift phase  41   b  is retarded gradually as the engine temperature rises. Further, at the transition from the time when the engine is cold to the time after warm-up, intake maximum lift phase  43   b  is advanced gradually as the engine temperature rises. Accordingly, in case, for example, the engine is started when the engine temperature is extremely low and idling of the engine is continued until warm-up is completed, it becomes possible to vary the intake operation angle and intake maximum lift phase smoothly to those that can effect such characteristics that are advantageous from the exhaust emission and the fuel consumption as the engine temperature rises, while attaining a good engine startability. 
   The above-described setting example of the intake maximum lift phase represented by (b) of  FIG. 6  is suited for use with electric phase control mechanism  20  that has a good responsiveness and can set the variable amount sufficiently large. In contrast to this, the setting example of the intake maximum lift phase represented by (c) of  FIG. 6  is suited for use with a hydraulic drive type phase control mechanism that is inferior in the responsiveness and the variable amount to the above-described electric type but is superior in the cost. 
   The setting (c) of  FIG. 6  differs from the setting (b) of  FIG. 6  in that the set value  42   c  when the engine is cold is made equal to the set value  44   c  after warm-up. The set values  40   c ,  44   c  when the engine temperature is extremely low and after warm-up are equal to the set values  40   b ,  44   b  of (b) of FIG.  6 . According to the setting (c) of  FIG. 6 , it is unnecessary to vary the intake maximum lift phase at the transition from the time when the engine is cold to the time after warm-up and a variation ΔD of the intake maximum lift phase at the transition from the time when the engine temperature is extremely low to the time when the engine is cold can be small. For this reason, a variation of the intake maximum lift phase can be smaller as compared with the setting (b) of  FIG. 6  though improvement of the exhaust emission by considerable retardation of the intake maximum lift phase when the engine is cold cannot be attained, thus making it possible to attain, for example, a good startability when the engine temperature is extremely low and vary the intake maximum lift phase suitably according to increase of the engine temperature. 
     FIG. 8  shows an example of setting of the intake operation angle and intake maximum lift phase at various engine operating conditions. In the meantime, intake maximum lift phases P 1  to P 5  that will be described later have a relation of P 1 &lt;P 2 &lt;P 3 &lt;P 4 &lt;P 5  when the advance side is regarded as positive. 
   Firstly, the valve lift characteristics after warm-up will be described. In the extremely low-load range (a 1 ) including idling, the intake maximum lift phase is set at a predetermined retarded phase P 2  and the intake operation angle is set at a minimum operation angle thereby setting the intake valve opening timing (IVO) at a point after TDC and the intake valve closing timing (IVC) at a point adjacent BDC. By this, the remaining gas is reduced and the upper surface of the piston is not exposed to the intake vacuum from TDC so that the intake valve opens after the piston moves a certain amount from TDC and vacuum is produced inside the cylinder, thus making it possible to reduce the pumping loss. Further, since the intake operation angle is minimized, the friction is reduced and the gas flow is enhanced to accelerate atomization of fuel. As a result, it becomes possible to improve the fuel consumption and exhaust efficiency. The above-described minimum operation angle ranges, for example, from 80° to 90° CA and the above-described retarded phase P 2  is a value on the retard side of at least 90° ATDC (after top dead center). 
   In the medium-load range (c), the intake vale opening timing is set at a point before TDC for the purpose of reducing the pumping loss mainly by the effect of increase of the remaining gas and improving the combustion by the effect of the high-temperature remaining gas, and at the same time the intake valve closing timing (IVC) is set at a point before BDC for the purpose of reducing the pumping loss by mainly reducing the intake air amount (charging efficiency). Thus, the operation angle is set at a predetermined small operation angle larger than the above-described minimum operation angle and the intake maximum lift phase is set at a most advanced phase P 5 . 
   In the low-load range (b) where the intake air amount is smaller than that in the above-described medium-load range, the intake operation angle is set at a value between the above-described minimum operation angle and the small operation angle mainly for the purpose of improving the combustion and reducing the remaining gas, and at the same time the intake maximum lift phase is set at a predetermined advanced phase P 4 . By this, it becomes possible to reduce the pumping loss by the effect of increase of the effective compression ratio thereby improving the fuel consumption. The above-described advanced phase P 4  is the value on the retard side of the above-described most advanced phase P 5  and on the advance side of 90° ATDC (after top dead center). 
   In the full-throttle range (d) to (f), mainly for the purpose of improving the charging efficiency, the intake maximum lift phase is set at or adjacent a predetermined intermediate phase P 3  and the intake operation angle is increased with increase of the engine speed. For example, in the full-throttle and low-speed range (d), the intake valve opening timing (IVO) is set nearly at TDC and the intake valve closing timing (IVC) is set at a point after BDC. The intermediate phase P 3  is, for example, 90° ATDC (after top dead center). 
   On the other hand, at an engine operating condition in the extremely low-load range (a 1 ) such as starting or idling when the engine is cold, i.e., when the engine temperature is lower than a predetermined value, it is hard to obtain sufficient warm-up of catalyst so that for the purpose of improving the combustion and thereby purifying the exhaust emission and raising the exhaust temperature the intake operation angle is set minimum and the intake maximum lift phase is set at a most retarded phase P 1  thereby retarding the intake valve opening timing (IVO) than TDC considerably. By such setting, the gas flow is enhanced to accelerate atomization of the fuel and retardation of the intake valve opening timing (IVO) causes the intake valve to open after the vacuum within the cylinder has been developed sufficiently, thus allowing the gas flow to be enhanced further at the time of opening of the intake valve. 
   In the meantime, though pot shown, in the low to medium load range when the engine is cold, there is a possibility of the combustion being deteriorated if the lift characteristics are set equal to those (b), (d) after warm-up. Thus, it is necessitated, for example, to set the lift characteristics nearly equal to those (d) in the low-speed and full-throttle range. 
   In the meantime, differing from the setting (a 2 ) of  FIG. 8 , the operation angle of the intake valve in the extremely low-load range, e.g., at start can be set smaller when the engine is cold than after warm-up. In this instance, since the operation angle becomes smaller at cold engine start (i.e., start when the engine is cold) than after warm-up, the gas flow is enhanced and the combustion is improved. On the other hand, since the operation angle becomes relatively larger at hot engine start (i.e., start when the engine is hot) as compared with that at cold engine start, it becomes possible to suppress the intake resistance and improve the fuel consumption. 
   Then, referring to  FIGS. 9  to  11 , description will be made as to the examination on the case where acceleration is made from various operating conditions. In the meantime, L 1  indicates the standard characteristics corresponding to the standard setting of the intake operation angle and the intake maximum lift phase under the operating condition before acceleration. Further, L 2  indicates the target characteristics corresponding to the target operation angle and the target phase, L 3  indicates the characteristics under the condition where only the intake operation angle is varied by a predetermined amount toward the target operation angle with respect to the standard characteristics L 1 , and L 4  indicates the characteristics under the condition where on the intake maximum lift phase is varied by a predetermined amount toward the target phase with respect to the standard characteristics L 1 . 
   Firstly, referring to  FIG. 9 , consideration will be made with respect to acceleration from an extremely low-load range when the engine is cold (cold engine idling condition). Under the cold engine idling condition, if only the intake operation angle is increased, there is a possibility of the torque being decreased temporarily for some reason such as one that the intake valve closing timing (IVC) is extremely delayed. For example, since in an engine speed range lower than the first engine speed N 1  the torque of the characteristics L 3  after the operation angle is increased is lower than that of the standard torque L 1 , the torque will be decreased temporarily if only the intake operation angle is varied. 
   On the other hand, at the time of acceleration from such an extremely low-load range, advance of only the intake maximum lift phase causes the torque to increase assuredly. Accordingly, at acceleration from such an extremely low-load and extremely low-engine speed range, advance of the intake maximum lift phase by means of phase control mechanism  20  is carried out by priority. Namely, only phase control mechanism  20  is driven or phase control mechanism  20  is controlled so that the amount of variation of the intake maximum lift phase by means of phase control mechanism  20  is sufficiently larger than the amount of variation of operation angle control mechanism  10 . By this, the torque in transition of acceleration trends assuredly toward increase, thus making it possible to avoid reduction of the torque in transition assuredly. 
   In the meantime, the standard setting (minimum operation angle and most advanced phase) L 1  at cold engine idling (i.e., idling when the engine is cold) is used in the engine speed range higher in engine speed than the extremely low-engine speed range mainly for the purpose of improving the combustion. However, as the engine speed becomes higher, the suction time decreases if the intake operation angle is the same. Thus, only advance of the intake maximum lift phase cannot increase the full-throttle torque effectively. Accordingly, in the extremely low-engine speed range (e.g., the engine speed range lower than the engine speed N 2  at which the characteristics L 3  under the condition where the intake operation angle is increased and the characteristics L 4  under the condition where the intake maximum lift phase is advanced are reversed in torque), the intake maximum lift phase is advanced by priority as described above, and in the low-engine speed range (e.g., the engine speed range exceeding the second engine speed N 2 ) the intake operation angle is increased by priority, thereby making it possible to increase the engine torque most efficiently. 
   Then, by reference to  FIG. 10 , consideration will be made with respect to acceleration from the extremely low-load range after warm-up. In the extremely low-load range after warm-up, mainly for the purpose of suppressing the intake resistance thereby improving the fuel consumption, the intake maximum lift phase is set at the retarded phase P 2  (refer to  FIG. 8 ) that is for use after warm-up and that is advanced than the most retarded phase P 1 . Namely, in order to make higher the effective compression ratio thereby improving the combustion, the intake valve closing timing (IVC) is advanced than that when the engine is cold. Accordingly, if only the intake maximum lift phase is advanced, the effective compression ratio and the charging efficiency are lowered, possibly causing such a case where the torque cannot be increased effectively. Thus, at the time of acceleration from the extremely low-load range after warm-up, the intake operation angle is increased by priority thereby making it possible to increase the torque efficiently. 
   Thus, even in the case of acceleration from the same load range, one of operation angle control mechanism  10  and phase control mechanism  20  is driven by priority based on the engine speed or engine temperature (cold engine or hot engine) thereby making it possible to increase the torque efficiently and improve the drivability of the vehicle. 
   Then, by reference to  FIG. 11 , consideration will be made with respect to acceleration from the low-load range after warm-up. At acceleration from the low-load range, as is apparent that in  FIG. 11  both of the characteristics L 3  and L 4  are higher in torque than the standard characteristics L 1 , the torque can be increased by either increasing the operation angle or retarding the phase. However, as is apparent that in  FIG. 11  the characteristics L 3  under the condition where the operation angle is increased is always higher in torque than the characteristics L 4  under the condition where the phase is retarded, the torque can be increased efficiently by increasing the intake operation angle by means of operation angle control mechanism  10  prior to retardation of the intake maximum lift phase  20  by means of phase control mechanism  20 . 
   In the meantime, though not shown, at the time of acceleration from the medium-load range such as (c) of  FIG. 8 , it is preferable to carry out retardation of the intake maximum lift phase by means of phase control mechanism  20  by priority irrespective of the engine speed since if increase of the operation angle is carried out by priority the intake valve opening timing (IVO) becomes too earlier to cause a possibility that the intake valve and the piston come very close to each other. 
   As described above, since phase control mechanism  20  is structured so as to be of the electrically-driven type, it becomes possible to vary the intake maximum lift phase rapidly irrespective of the engine temperature (whether the engine is cold or hot). Namely, as compared with the hydraulic drive that is liable to cause a delay in variation of the phase when the engine is cold, it becomes possible to vary the phase rapidly even when the engine is cold. By this, the intake valve opening timing (IVO) can be retarded largely when the engine that is cold is to be started to enhance the gas flow thereby improving the combustion and the exhaust gas purification. Further, after warm-up, the intake maximum lift phase is advanced a little for the purpose of reducing the intake resistance thereby improving the fuel consumption. By this, both of the purification of the exhaust emission when the engine is cold and the improvement of the fuel consumption after warm-up can consist and be maintained at the high level. 
   Further, since operation angle control mechanism  10  is of the electrically-driven type, the variable width can be set sufficiently large, e.g., 80° to 280° CA and the intake operation angle can be varied assuredly and rapidly even at cold engine start or at extremely low-engine speed. Namely, by employing such electrically-driven type operation angle control mechanism  10 , operation angle control mechanism  10  can be driven by priority even when the engine is operating in a low-engine speed range. Further, the operation angle can be increased rapidly irrespective of the engine temperature (whether the engine is cold or after warm-up). For this reason, as compared with the hydraulically-driven type that is liable to cause a delay of increase of the operation angle when the engine is cold, the minimum operation angle can be set sufficiently small thereby making it possible to effectively enhance the gas flow at cold engine start and improve the combustion, thus making it possible to further purify the exhaust emission. 
   Since both of control mechanisms  10 ,  20  are of the electrically-driven type, a control of switching the priority of the both control mechanisms  10 ,  20  can be done even in transition of acceleration. 
   In the meantime, in case of the structure where, as in the embodiment, the actual measurement value (actual maximum lift phase) of intake drive shaft  3  relative to the crank angle based on the detection signal from sensor  31  for detecting an angle of intake drive shaft  3 , the intake maximum lift phase is detected every one rotation of intake drive shaft  3 . On the other hand, in case of the structure where the actual measurement value of the intake operation angle (actual operation angle) is detected based on the detection signal from sensor  32  for detecting an angle of control shaft  13 , the interval between detections can be set freely so that the actual operation angle can be detected at an arbitrary timing. Accordingly, by detecting the actual operation angle at the timing at which the actual intake phase is detected, it becomes possible to execute a control of setting the target value of the intake operation angle and the intake maximum lift phase, or the like based on the actual intake phase and the actual operation angle that are detected at the same time, thus making it possible to improve the accuracy in control. 
   The flow of such control will be described with reference to the flowchart of FIG.  12 . In step S 11 , the actual intake phase is detected based on the detection signal from sensor  31  for detecting an angle of intake drive shaft  3 . Then, the program proceeds to step S 12  where the actual operation angle is detected based on the detection signal of sensor  32  for detecting an angle of control shaft  13 . In step S 13 , it is determined whether the engine is in an accelerating condition. If it is determined in step S 13  that the engine is in an accelerating condition, i.e., the answer is Yes, the program proceeds to step S 14  where it is determined based on the engine temperature or the like whether the engine is cold or in a condition after warm-up. If the engine is cold, the program proceeds to step S 16  where it is determined based on the engine speed whether the engine is operating in an extremely low-engine speed or low-engine speed range. If the engine is operating in an extremely low-engine speed range, the program proceeds to steep S 17  where a control of driving phase control mechanism  20  by priority is executed. On the other hand, if the engine is operating in a low-engine speed range, the program proceeds to step S 18  where a control of driving operation angle control mechanism  10  by priority is executed. 
   Further, if it is determined in step S 14  that the engine is in a condition after warm-up, the program proceeds to step S 15  where it is determined whether the intake valve opening timing (IVO) is excessively early. If the intake valve opening timing (IVO) is excessively early, the program proceeds to step S 17  where a control of driving phase control mechanism  20  by priority is executed. If it is determined in step S 15  that the intake valve opening timing (IVO) is relatively late, the program proceeds to step S 16  where it is determined based on the engine speed which one of control mechanisms  10 ,  20  is to be driven by priority. 
   The entire contents of Japanese Patent Applications P2002-71226 (filed Mar. 15, 2002) are incorporated herein by reference. 
   Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.