Abstract:
A valve timing control system for an internal combustion engine for preventing dispersion of a control amount and unexpected unlocking of a lock pin is provided. The valve timing control system is provided with actuators  15  and  16  connected to cam shafts  15 C and  16 C, hydraulic pressure supply units  19  and  20  for driving the actuators, and a controller  21 A for controlling a hydraulic pressure for the actuators depending on operating states while changing a relative phase of the cam shafts relative to crank shafts. The actuator includes a locking mechanism for setting the relative phase to a lock-up position and unlocking mechanisms for unlocking the locking mechanism in response to a predetermined hydraulic pressure, and the controller restricts a control of valve timing within a predetermined range of said lockup position in the locking mechanism.

Description:
[0001]    This application is based on Application No. 2001-137666, filed in Japan on May 8, 2001, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates in general terms to a valve timing control system for an internal combustion engine for controlling operation timings of intake valves and exhaust valves of the engine in dependence on engine operating states.  
           [0004]    2. Description of Related Art  
           [0005]    In recent years, the statutory regulations imposed in connection with emission of harmful materials or substances contained in the exhaust gas discharged to the atmosphere from the internal combustion engine mounted on a motor vehicle or automobile become more and more severe from the standpoint of environmental protection. Under the circumstances, there exists a great demand for reducing the emission of harmful materials or substances contained in the exhaust gas of the internal combustion engine.  
           [0006]    In general, there have heretofore been known two sorts of methods of reducing the harmful exhaust gas components. One method is directed to reduction of the harmful gas directly discharged from the internal combustion engine (hereinafter also referred to simply as the engine) while the other method is directed to the reduction of the harmful components through posttreatment of the engine exhaust gas with the aid of a catalytic converter (hereinafter also referred to simply as the catalyst) installed within the exhaust pipe of the engine at an intermediate portion.  
           [0007]    As is well known in the art, in the catalyst such as mentioned above, reaction of rendering the harmful gas components to be harmless is difficult or unable to take place unless the temperature of the catalyst has reached a predetermined value. Consequently, it is an important requirement to increase or rise speedily the temperature of the catalyst even when the internal combustion engine is, for example, in the course of starting operation in the cold state (i.e., in the state of low temperature).  
           [0008]    In this conjunction, it is also known that in most of the internal combustion engines known heretofore, cam shafts which plays an essential role in determining the timings for opening and closing the intake or exhaust valves are so arranged as to be rotationally driven by a crank shaft through the medium of timing belts (or timing chains).  
           [0009]    Accordingly, the timings for opening and closing the intake or exhaust valves (which timing may also be referred to as the cam angles) are so controlled as to remain constant relative to the crank angle notwithstanding of the fact that the valve timings as required may change in dependence on the operating states of the engine.  
           [0010]    However, in recent years, a valve timing control system designed to be capable of changing or modifying the valve timings has been adopted for practical applications with a view to enhancing the fuel-cost performance of the engine while ensuring improvement of the exhaust gas quality.  
           [0011]    The valve timing control system of this type is disclosed in, for example, in Japanese Patent Application Laid-Open Publication No. 324613/1997 (JP-A-9-324613).  
           [0012]    The valve timing control system disclosed in the above-mentioned publication includes a variable valve timing mechanism (also referred to as the WT mechanism in short) which is comprised of vanes each disposed rotatably within a housing for changing the phase (or angular position) of the cam shafts which is adapted to drive the intake valves and the exhaust valves. Incidentally, concerning arrangement of the vanes, description will be made later on.  
           [0013]    At this juncture, however, it should be mentioned that in the engine starting operation, the vane of the variable valve timing mechanism is held substantially at a mid position (start corresponding position) for controlling or regulating the relative angular displacement of the cam angle relative to the crank angle and releasing the regulation or control after lapse of a predetermined time.  
           [0014]    For having better understanding of the concept underlying the present invention, description will first be made in some detail of a hitherto known or conventional valve timing control system of an internal combustion engine. FIG. 12 is a functional block diagram showing generally and schematically a configuration of a conventional valve timing control system of an internal combustion engine together with several peripheral parts of the engine.  
           [0015]    Referring to FIG. 12, provided in association with an intake pipe  4  for feeding the air into a combustion chamber(s) defined within the cylinder(s) of the engine  1  are an air cleaner  2  for purifying the intake air, an air flow sensor  3  for measuring the quantity or flow rate of the intake air. Further, installed in the intake pipe  4  are a throttle valve  5  for adjusting or regulating the intake air quantity (i.e., the amount or flow rate of the intake air) to thereby control the output of the engine  1 , an idle speed control valve (also referred to simply as the ISCV in short)  6  for adjusting or regulating the intake air flow which bypasses the throttle valve  5  to thereby effectuate the engine rotation speed (rpm) control in the idling operation mode, and a fuel injector  7  for charging or injecting an amount of fuel which conforms with the intake air quantity.  
           [0016]    Additionally, provided internally of the combustion chamber of the engine cylinder  1  is a spark plug  8  for producing a spark discharge for triggering combustion of the air-fuel mixture charged in the combustion chamber defined within the cylinder. To this end, the spark plug  8  is electrically connected to an ignition coil  9  which supplies electric energy of high voltage to the spark plug  8 .  
           [0017]    An exhaust pipe  10  is provided for discharging an exhaust gas resulting from the combustion of the air-fuel mixture within the engine cylinder. An O2-sensor  11  and a catalytic converter  12  are disposed in the exhaust pipe  10 . The O2-sensor  11  serves for detecting the content of residual oxygen contained in the exhaust gas.  
           [0018]    The catalytic converter or catalyst  12  is constituted by a three-way catalytic converter known by itself is capable of eliminating simultaneously harmful gas components such as HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxides) contained in the exhaust gas.  
           [0019]    A sensor plate  13  designed for detecting the crank angle is mounted on a crank shaft (not shown) so as to corotate therewith. The sensor plate  13  is provided with a projection (not shown) at a predetermined crank angle in the outer periphery thereof.  
           [0020]    A crank angle sensor  14  is installed at a position diametrically opposite to the outer periphery of the sensor plate  13  for the purpose of detecting the angular position of the crank shaft in cooperation with the sensor plate  13 . Thus, the crank angle sensor  14  can generate an electric signal indicative of the crank angle, i.e., the crank angle signal, every time the projection of the sensor plate  13  passes by the crank angle sensor  14 . In this way, the rotating position or angular position (crank angle) of the crank shaft can be detected.  
           [0021]    The engine  1  is equipped with valves for putting into communication the intake pipe  4  and the exhaust pipe  10  to each other, wherein the timings for driving the intake or exhaust valves are determined by the cam shafts which are rotated at a speed equal to a half of that of the crank shaft, as will be described later on.  
           [0022]    Actuators  15  and  16  for changing adjustably the cam phases are designed to change the timings for driving or actuating the intake or exhaust valves, respectively.  
           [0023]    More specifically, each of the actuators  15  and  16  is comprised of a retarding hydraulic chamber and an advancing hydraulic chamber partitioned from each other (described later on) for changing or varying the rotational or angular positions (phases) of the cam shafts  15 C and  16 C, respectively, relative to the crank shaft.  
           [0024]    Cam angle sensors  17  and  18  are disposed at positions diametrically opposite to the outer periphery of cam angle detecting sensor plates (not shown) for the purpose of detecting the angular positions of the cams (i.e., cam angles or phases) through cooperation with the sensor plate. More specifically, each of the cam angle sensors  17  and  18  is designed to generate a pulse signal indicative of the cam angle (i.e., the cam angle signal) in response to a projection formed in the outer periphery of the associated cam angle detecting sensor plate in a similar manner as the crank angle sensor  14  described previously. In this way, it is possible to detect the cam angles (or angular position of the cam shafts).  
           [0025]    Oil control valves (also referred to as the OCV in short)  19  and  20  constitute hydraulic pressure supply units in cooperation with oil pumps (not shown) and serve for controlling or regulating the hydraulic pressure supplied to the individual actuators  15  and  16  for thereby controlling the cam phases. Parenthetically, the oil pump is designed to feed oil at a predetermined hydraulic pressure.  
           [0026]    An electronic control unit (also referred to simply as the ECU)  21  which may be constituted by a microcomputer or microprocessor serves as a control means for the internal combustion engine system. Among others, the ECU  21  is in charge of controlling the fuel injectors  7  and the spark plugs  8  as well as the cam phases (angular positions of the cams) of the actuators  15  and  16  in dependence on the engine operating states detected by the various sensors such as the air-flow sensor  3 , the O2-sensor  11 , the crank angle sensor  14  and the cam angle sensors  17  and  18 .  
           [0027]    Further, provided in association with the throttle valve  5  is a throttle position sensor (not shown in the figure) for detecting the throttle opening degree while a water temperature sensor is provided for the engine  1  for detecting the temperature of cooling water therefor. The throttle opening degree and the cooling water temperature as detected are also inputted to the ECU  21  as the information indicative of the operating state of the engine  1  similarly to the various sensor information mentioned above.  
           [0028]    Next, description will be made of the conventional engine control operation performed by the prior art valve timing control system shown in FIG. 12. Firstly, the air flow sensor  3  measures the air quantity (flow rate of the intake air) fed to the engine  1 , the output of the air-flow sensor  3  being supplied to the ECU  21  as the detection information indicative of the operating state of the engine.  
           [0029]    The electronic control unit or ECU  21  arithmetically determines the fuel quantity or amount which conforms to the air quantity as measured to thereby drive or actuate correspondingly the fuel injector  7 . At the same time, the ECU  21  controls the time duration for electrical energization of the ignition coil  18  as well as the timing for interruption thereof to thereby produce a spark discharge at the spark plug  8  for igniting or firing the air-fuel mixture charged within the combustion chamber defined within the engine cylinder at a proper timing.  
           [0030]    On the other hand, the throttle valve  5  serves for adjusting or regulating the amount of intake air fed to the engine to thereby control correspondingly the output torque or power generated by the engine  1 . The exhaust gas resulting from the combustion of the air-fuel mixture within the cylinder of the engine  1  is discharged through the exhaust pipe  10 .  
           [0031]    In that case, the catalytic converter  12  disposed within the exhaust pipe  10  at an intermediate location thereof converts the harmful components contained in the exhaust gas such as hydrocarbon (HC) (unburned gas), carbon monoxide (CO) and nitrogen oxides (NOx) into harmless carbon dioxide and water (H2O). In this way, the engine exhaust gas is purified.  
           [0032]    In order to make available the maximum purification efficiency of the three-way catalytic converter  12 , the O2-sensor  11  is installed in association with the exhaust pipe  10  for detecting the amount of residual oxygen contained in the exhaust gas. The output signal of the O2-sensor  11  is inputted to the electronic control unit or ECU  21  which responds thereto by regulating in a feedback loop the amount of fuel injected through the fuel injector  7  so that the air-fuel mixture which is to undergo the combustion can assume the stoichiometric ratio.  
           [0033]    In addition, the ECU  21  controls the actuators  15  and  16  (which constitute parts of the variable valve timing mechanism) in dependence on the engine operating state for regulating the timings at which the intake or exhaust valves are to be driven or actuated.  
           [0034]    In the following, referring to FIGS.  13  to  14 , description will be made in concrete of the phase angle control operation preformed for the cam shafts  15 C and  16 C by the conventional valve timing control system for the internal combustion engine.  
           [0035]    By the way, in the case of the conventional internal combustion engine of the fixed valve timing scheme (not shown), torque of the crank shaft is transmitted to the cam shafts through the medium of the timing belts (timing chains) and transmission mechanisms including pulleys and sprockets and coupled operatively to the cam shafts for corotation with the pulleys.  
           [0036]    By contrast, in the case of the internal combustion engine equipped with the variable valve timing mechanism, there are provided the actuators which are designed to change the relative phase position between the crank shaft and the cam shafts in place of the pulleys and the sprockets mentioned above.  
           [0037]    [0037]FIG. 13 is a view for illustrating relation between the crank angle [° CA] and the valve lift stroke (indicating the degree of valve opening [mm], (hereinafter also referred to as the valve opening quantity). In the figure, the top dead center in the compression stroke of the cylinder is designated by reference symbol TDC.  
           [0038]    In FIG. 13, a single-dotted broken line curve represents change of the valve lift stroke delimited mechanically in the most retarded state, a broken line curve represents change of the valve lift stroke delimited mechanically in the most advanced state, and a solid line curve represents change of the valve lift stroke in a locked state set by a locking mechanism (described hereinafter).  
           [0039]    Referring to FIG. 13, it is to be noted that the peak position of the valve lift stroke on the retarded side (right-hand side as viewed in the figure) with reference to the top dead center (TDC) corresponds to the fully opened position of the intake valve while the peak position of the valve lift stroke on the advanced side (left-hand side as viewed in the figure) corresponds to the fully opened position of the exhaust valve.  
           [0040]    Accordingly, difference in the crank angle between the peaks on the retarded side and the advanced side (i.e., difference between the single-dotted line curve and the broken line curve) represents the range within which the valve timing can be changed (i.e., valve timing adjustable range). To say in another way, the valve timing can be changed or adjusted within the crank angle range defined between the broken line curve and the single-dotted line curve in either of the suction and exhaust operation.  
           [0041]    [0041]FIG. 14 is a timing chart for illustrating phase or timing relations between the output pulse signal of the crank angle sensor  14  on one hand and that of the cam angle sensor  17  or  18  on the other hand. More specifically, shown in FIG. 14 are the output pulse signals of the cam angle sensor  17  or  18  in both the most retarded state and the most advanced state, respectively, relative to the output of the crank angle sensor.  
           [0042]    In this conjunction, it should be added that the phase position of the output signal of the cam angle sensor  17  or  18  relative to the output signal of the crank angle sensor  14  (i.e., crank angle signal) becomes different in dependence on the positions at which the cam angle sensors  17  and  18  are mounted.  
           [0043]    At this juncture, it should further be mentioned that retarding of the valve timing means that the valve opening start timings of both the intake or exhaust valves is retarded or delayed relative to (or with reference to) the crank angle, while advancing of the valve timing means that the valve opening start timings of both the valves is advanced relative to the crank angle.  
           [0044]    The opening start timings for the intake valve and the exhaust valves can be changed or modified by means of the actuators  15  and  16  which constitute parts of the variable valve timing mechanism to be thereby so controlled as to assume a given retarded position or advanced position within the aforementioned valve timing adjustable or variable range mentioned hereinbefore by reference to FIG. 13.  
           [0045]    FIGS.  15  to  17  are views showing internal structures of the actuators  15  and  16  which are implemented in a substantially identical structure. More specifically, FIG. 15 shows the same in a state where the cam phase is adjusted to the most retarded position (corresponding to the state indicated by the single-dotted line curve in FIG.  13 ), FIG. 16 shows the same in a state where the cam phase is adjusted to the locked or lock-up position (corresponding to the state indicated by the solid line curve in FIG. 13), and FIG. 17 shows the same in a state where the cam phase is adjusted to the most advanced position (corresponding to the state indicated by the broken line curve in FIG. 13), respectively.  
           [0046]    Referring to FIGS. 15, 16 and  17 , each of the actuators  15  and  16  is comprised of a housing  151  which is rotatable in the direction indicated by an arrow, a vane  152  rotatable together with the housing  151 , retarding hydraulic chambers  153  and advancing hydraulic chambers  154  both defined internally of the housing  151 , a lock pin  155  and a spring  156  which are also provided within the housing  151 , and locking recesses  157  formed in the vane  152 .  
           [0047]    Power or torque is transmitted to the housing  151  from the crank shaft through the medium of a belt/pulley assembly (not shown) with the speed of rotation being reduced by a factor of ½.  
           [0048]    The position (phase position) of the vane  152  is caused to shift within the housing  151  in response to the hydraulic pressure supplied selectively to the retarding hydraulic chamber  153  or the advancing hydraulic chamber  154 .  
           [0049]    The range of operation (hereinafter also referred to as the operation range) of the vane  152  is determined or defined by the retarding hydraulic chamber  153  and the advancing hydraulic chamber  154 .  
           [0050]    The spring  156  resiliently urges the lock pin  155  in the protruding direction while the locking recess  157  is formed at a predetermined vane lock-up position so that the recess  157  faces in opposition to the tip end of the lock pin  155 .  
           [0051]    Parenthetically, an oil feed port (not shown) is formed in the locking recess  157  through which the hydraulic medium (i.e., oil in this case) is supplied interchangeably from either one of the retarding hydraulic chamber  153  and the advancing hydraulic chamber  154  within which a higher hydraulic pressure prevails.  
           [0052]    The vanes  152  designed to operate within the retarding hydraulic chamber  153  and the advancing hydraulic chamber  154  (i.e., operation range of the vane) and shifted in the angular position or phase are operatively coupled to the cam shafts  15 C and  16 C for driving the intake or exhaust valves of the engine cylinders.  
           [0053]    Although not shown in the drawings, the actuator  16  on the exhaust side is provided with a spring for resiliently urging the vane  152  so that it can assume the advanced position against the reaction force of the cam shaft  16 C.  
           [0054]    The actuators  15  and  16  are driven under the hydraulic pressure of a lubricant oil of the engine  1  supplied through the oil control valves  19  and  20 . For controlling the cam angle phases of the actuators  15  and  16  in such manner as illustrated in FIGS.  15  to  17 , the amount of oil (i.e., hydraulic pressure) fed to the actuators  15  and  16  is controlled.  
           [0055]    By way of example, regulation of the cam angle phase to the most retarded position, as illustrated in FIG. 15, can be realized by feeding oil into the retarding hydraulic chamber  153 . On the contrary, regulation of the cam angle phase to the most advanced position, as illustrated in FIG. 17, can be effectuated by feeding oil into the advancing hydraulic chamber  153 .  
           [0056]    The oil control valves  19  and  20  are in charge of selecting either the retarding hydraulic chamber  153  or the advancing hydraulic chamber  154  for the oil supply. FIG. 18, 19 and  21  show in side-elevational sectional views the internal structures of the oil control valves  19  and  20  which are implemented substantially identically.  
           [0057]    Referring to FIGS.  18  to  20 , each of the oil control valves  19  and  20  is comprised of a cylindrical housing  191 , a spool  192  slideably disposed within the housing  191 , a solenoid coil  193  for driving continuously the spool  192  and a spring  194  for resiliently urging the spool  192  in the restoring direction.  
           [0058]    The housing  191  is provided with an orifice  195  which is hydraulically communicated to a pump (not shown), orifices  196  and  197  hydraulically connected to the actuator  15  or  16 , and drain orifices  198  and  199  fluidly communicated to an oil pan.  
           [0059]    The orifice  196  can be communicated to the retarding hydraulic chamber  153  of the actuator  15  or the advancing hydraulic chamber  154  of the actuator  16 . On the other hand, the orifice  197  can be communicated to the advancing hydraulic chamber  154  of the actuator  15  or the retarding hydraulic chamber  153  of the actuator  16 .  
           [0060]    The orifices  196  and  197  are selectively put into communication with the oil feeding orifice  195  in dependence on the axial position of the spool  192  (i.e., the position of the spool in the longitudinal direction thereof). In the state shown in FIG. 18, the orifice  195  is shown as having been placed in communication with the orifice  196 , while in FIG. 20, the orifice  195  is shown as being communicated to the orifice  197 .  
           [0061]    Similarly, the drain orifices  198  and  199  are selectively put into communication with the orifice  197  or  196  in dependence on the axial position of the spool  192 . In the state shown in FIG. 18, the orifice  197  is shown as being communicated with the orifice  198 , while in FIG. 20, the orifice  196  is being communicated to the orifice  199 .  
           [0062]    The oil feed port formed in the locking recess  157  is so arranged as to be supplied with oil when the oil control valves  19  and  20  are in the electrically driven state (see FIG. 20). More specifically, when the hydraulic pressure applied to the locking recess  157  exceeds the spring force of the spring  156 , the lock pin  155  is pushed out from the locking recess  157 , whereby the locked state is cleared.  
           [0063]    [0063]FIG. 18 shows the state in which the electric current flowing through the solenoid or coil  193  is at a minimum value and thus the spring  194  is stretched or relaxed to a maximum extent.  
           [0064]    Assuming that the oil control valve shown in FIG. 18 serves as the oil control valve  19  of the intake side, the hydraulic medium or oil supplied from the pump via the orifice  195  flows into the retarding hydraulic chamber  153  of the actuator  15 , as a result of which the actuators  15  is shifted to the state illustrated in FIG. 15.  
           [0065]    Consequently, the oil resident in the advancing hydraulic chamber  154  of the actuator  15  is forced to flow out through the orifice  197  to be finally discharged to the oil pan by way of the orifice  198 .  
           [0066]    On the other hand, assuming that the oil control valve shown in FIG. 18 serves as the oil control valve  20  on the exhaust side, the situation is reversed. Namely, the hydraulic medium or oil supplied from the pump via the orifice  196  flows into the advancing hydraulic chamber  154  of the actuator  16 , as a result of which the actuators  16  is ultimately set to the state illustrated in FIG. 17.  
           [0067]    In that case, the oil contained in the retarding hydraulic chamber  153  of the actuator  16  is forcibly discharged to the oil pan by way of the orifices  197  and  198 .  
           [0068]    By virtue of the hydraulic circuit arrangement described above by reference to FIG. 18, valve overlap can be suppressed to a minimum even upon occurrence of failure such as shutdown of electric current supply to the oil control valves  19  and  20  disposed at the intake side and the exhaust side, respectively, due to wire breakage or the like. This feature is advantageous from the viewpoint of ensuring high withstandability against the engine stall.  
           [0069]    In FIG. 20, the state is illustrated in which where the current flowing through the coil  193  is of a maximum value and thus the spring  194  is compressed to the minimum length.  
           [0070]    Assuming, by way of example, that the oil control valve shown in FIG. 20 serves as the oil control valve  19  installed on the intake side, the oil fed from the pump is caused to flow into the advancing hydraulic chamber  154  of the actuator  15  via the orifice  197 , whereas the oil in the retarding hydraulic chamber  153  of the actuator  15  is discharged via the orifices  196  and  199 .  
           [0071]    On the other hand, in the case where the oil control valve shown in FIG. 20 serves as the oil control valve  20  on the exhaust side, the oil fed from the pump is forced to flow into the retarding hydraulic chamber  153  of the actuator  16  via the orifice  197 , while the oil in the advancing hydraulic chamber  154  of the actuator  16  is discharged via the orifices  196  and  199 .  
           [0072]    [0072]FIG. 19 shows the state corresponding to the valve timing control end position or lock-up position (mid position). In this state, the vanes  152  of the actuators  15  and  16  are at desired positions, respectively, (see the state illustrated in FIG. 16).  
           [0073]    In the state illustrated in FIG. 19, the orifice  195  provided at the oil supply side is not directly communicated to the orifice  196  or  197  disposed on the actuator side. However, due to oil leakage, oil is supplied to the oil feed port of the locking recess  157  (see FIG. 16).  
           [0074]    Accordingly, even when the vane  152  is at the lock-up position, there may arise such situation in which the hydraulic pressure applied to the oil feed port under the oil leakage overcomes the spring force of the spring  156  (i.e., exceeds the predetermined unlocking hydraulic pressure value). In that case, the lock pin  155  is caused to disengage from the locking recess  157 , allowing the vane  152  to move or operate within the housing  151 .  
           [0075]    At this juncture, it should be mentioned that the predetermined unlocking hydraulic pressure mentioned above may be set at a necessary minimum value.  
           [0076]    Furthermore, the positions (phases) of the vanes  152  of the actuators  15  and  16  which play the role for determining the valve timing can appropriately be controlled by detecting the vane positions by mans of the cam angle sensors  17  and  18 .  
           [0077]    The cam angle sensors  17  and  18  are mounted at the positions which enable these sensors to detect the relative position between the crank shaft on one hand and the cam shafts  15 C and  16 C on the other hand.  
           [0078]    Referring to FIG. 20, the phase difference relative to the output signal of the crank angle sensor at the position where the valve timing is most advanced (see the broken line curve shown in FIG. 14) is indicated by A, whereas the phase difference relative to the output signal of the crank angle sensor at the position where the valve timing is most retarded (see the single-dotted line curve shown in FIG. 14) is indicated by B.  
           [0079]    The ECU  21  is designed or programmed to perform the feedback control so that the phase difference A or B as detected coincides with the desired value, whereby the valve timing control is carried out at given positions.  
           [0080]    More specifically, it is assumed, by way of example only, that on the intake side, the detected position of the cam angle sensor  17  relative to the detection timing of the crank angle sensor  14  is retarded with reference to the desired position arithmetically determined by the ECU  21 . In that case, the detected position (detection timing) of the cam angle sensor  17  has to be to advanced the desired position. To this end, the amount of the electric current flowing through the coil  193  of the oil control valve  19  is regulated in dependence on the difference between the detected position and the desired position, to thereby control correspondingly the spool  192 .  
           [0081]    Further, in the case where the difference between the desired position and the detected position is large, the amount of electric current supplied to the coil  193  of oil control valve  19  is increased in order to allow the desired position to be attained speedily.  
           [0082]    As a result of this, the aperture of the orifice  197  opened into the advancing hydraulic chamber  154  of the actuator  15  is increased, which results in increasing of the amount of oil fed to the advancing hydraulic chamber  154 .  
           [0083]    Subsequently, as the detected position approaches to the desired position, the current supply to the coil  193  of the oil control valve  19  is decreased so that the position of the spool  192  of the oil control valve  19  becomes closer to the state illustrated in FIG. 19.  
           [0084]    At the time point when coincidence is found between the detected position and the desired position, the electric current supply to the coil  193  is so controlled that the oil flow path leading to the retarding hydraulic chamber  153  and the advancing hydraulic chamber  154  of the actuator  15  is intercepted, as can be seen in FIG. 19.  
           [0085]    Incidentally, the desired position in the ordinary engine operation mode (e.g. running state succeeding to the warm-up operation) can be so set or established that optimal valve timing can be realized in accordance with the engine operation state by previously storing, for example, two-dimensional map data values obtained experimentarily in correspondence to the operating states (e.g. engine rotation speeds (rpm) and engine loads), respectively, in a read-only memory or ROM incorporated in the ECU  21 .  
           [0086]    On the other hand, in the engine starting operation mode, the rotation speed of the oil pump which is driven by the engine  1  is not sufficiently high. Consequently, the volume of the oil fed to the actuator  15  is also insufficient. Thus, the control of the valve timing to the advanced position by controlling the hydraulic pressure as described previously is rendered practically impossible.  
           [0087]    Such being the circumstances, jolting or fluttering of the vane  152  due to shortage of the hydraulic pressure has to be prevented by engaging the lock pin  155  with the locking recess  157 , as illustrated in FIG. 16.  
           [0088]    In that case, if the intake valve is actuated excessively retardingly (i.e., if the valve timing is overretarded), the actual compression ratio becomes lowered while excessive advancing of actuation of the intake valve (i.e., overadvancing of the valve timing) will result in increasing of the time period during which the intake valve and the exhaust valve overlap with each other. In other words, overretarded or overadvanced actuation of the intake valve results in increasing of the pumping loss.  
           [0089]    Certainly, the overretarding or overadvancing actuation control of the intake valve can profitably be adopted for increasing the rotation speed in the engine starting operation (e.g. upon cranking) and triggering the initial explosion. However, because the combustion is essentially inadequate, complete combustion or explosion is difficult to realize.  
           [0090]    On the other hand, overretarding of actuation of the exhaust valve will result in increasing of the overlap period during which the intake valve and the exhaust valve overlap with each other, similarly to the case where operation of the intake valve is advanced excessively. By contrast, overadvancing of the exhaust valve actuation will incur lowering of the actual expansion ratio, rendering it impossible to transmit the combustion energy sufficiently to the crank shaft.  
           [0091]    As is apparent from the above, overretarding or overadvancing control of the valve timing in the engine starting operation or immediately thereafter may unwantedly incur degradation of the engine starting performance or the state incapable of starting the engine operation in the worst case.  
           [0092]    Thus, for coping with the problems such as mentioned above in the engine starting operation, the vane  152  is fixedly set at the lock-up position (i.e., nearly mid position between the most retarded position and the most advanced position) by engaging the lock pin  155  into the locking recess  157 , as shown in FIG. 16.  
           [0093]    In that case, since the hydraulic pressure of the lubricating oil increases as the engine rotation speed (rpm) increases in succession to starting operation of the engine, the hydraulic pressure is fed to the actuators  15  and  16  because of the oil leakage described previously even in the state where the spool  192  is at the position shown in FIG. 19.  
           [0094]    Such being the circumstances, when the hydraulic pressure applied to the locking recess  157  overcomes the spring force of the spring  156 , the lock pin  155  is caused to disengage from the locking recess  157 , allowing the vane  152  to move.  
           [0095]    Thus, by controlling the oil control valves  19  and  20  after unlocking of the vanes, the hydraulic pressure fed to the retarding hydraulic chamber  153  and the advancing hydraulic chamber  154  can be regulated, whereby the valve timing retarding or advancing control can be carried out.  
           [0096]    In that case, particularly in the high-speed rotation range of the engine  1 , the valve timing is so controlled as to be retarded more when compared with the engine starting operation for the purpose of realizing the suction inertia effect as well as enhancement of the volumetric efficiency and hence the output performance of the engine.  
           [0097]    As can be appreciated from the foregoing, in the engine starting operation, the lock pins  155  of the actuators  15  and  16  are locked at a nearly mid position between the most retarded position and the most advanced position with a view to enhancing the engine starting performance. On the other hand, once the engine operation has been started after releasing of the locking mechanism, the valve timing is so controlled as to be retarded especially in the high-speed rotation range of the engine.  
           [0098]    However, in the conventional valve timing control system for the internal combustion engine, no consideration has been given to such technical matters as improvement of the exhaust gas quality and promotion of temperature rise of the catalyst.  
           [0099]    The conventional valve timing control system for an internal combustion engine is configured as described above. In an engine starting operation, the valve timing control system engages with a substantially intermediate position between a most advanced position and a most retarded position by the locking mechanism of the actuator, thereby improving a starting performance of the internal combustion engine. After the engine operation has been started, when the locking mechanism is released, the valve timing control system improves an output performance of the internal combustion engine by controlling the valve timing toward a more retarded state than in the starting operation, in particular, in a high rotational range.  
           [0100]    In addition, Japanese Patent Application Laid-open No. Hei 11-210424 describes that, after the lock pin is released, a control of valve timing executes a feedback control for making a detected advance angle amount coincide with a target advance angle amount.  
           [0101]    On the intake side, if the detected advance angle amount is in a more retarded state than the target advance angle amount, the valve timing control system controls the OCVs  19  and  20  to supply oil to the advancing hydraulic chamber of the actuator in order to advance the valve timing. As a result, the OCVs are capable of successively controlling the spool  192  to be set to an arbitrary position by an amount of energizing current to the coil  193  as shown in FIG. 20, thereby successively controlling an amount of oil to be supplied from an oil pump to the actuators  15  and  16 .  
           [0102]    If the detected advance angle amount is in a more advanced state than the target advance angle amount, the valve timing control system controls the OCVs to supply oil to the retarding hydraulic chamber of the actuator as shown in FIG. 18 so that the valve timing is retarded. In addition, if the detected advance angle amount substantially coincides with the target advance angle amount, the valve timing control system controls both the advancing hydraulic chamber  154  and the retarding hydraulic chamber  153  to be set to positions for blocking a passage as shown in FIG. 19.  
           [0103]    If the target advance angle amount is in a pin-lock-up position, the lock pin  155  is in the position of the locking recess  157 , and most of the passages of the OCVs  19  and  20  are blocked. Thus, since a hydraulic pressure decreases by a large degree and a hydraulic pressure applied to the lock pin  155  also decreases, the lock pin  155  is locked in the locking recess  157  if a force caused by the hydraulic pressure applied on the lock pin  155  becomes smaller than a spring force.  
           [0104]    Here, in the case in which an integral control is executed in order to make the detected advance angle amount coincide with the target advance angle amount, the detected advance angle amount is locked by the lock pin  155  if there is only a slight difference between the pin lock-up position and the target advance angle amount when the lock pin  155  is locked. Thus, the detected advance angle amount does not move despite the fact that an integrated value is increased or decreased, and the integrated value is increased or decreased to a limit of a control range. When the target advance angle amount changes and it is intended to make the detected advance angle amount follow the change, the detected advance angle amount may not be able to follow the target advance angle amount promptly because a control value diverges.  
           [0105]    In addition, when passages to the actuators of the OCVs are secured and a hydraulic pressure to the lock pin  155  reaches a hydraulic pressure sufficient to release a lock before the integrated value reaches the limit of the control range, the pin lock is released and a control amount deviates largely due to a movement of the integrated value at this point. Thus, the detected advance angle amount may deviate largely from the target advance angle amount simultaneously with the release of the lock pin.  
         SUMMARY OF THE INVENTION  
         [0106]    In the conventional valve timing control system for the internal combustion engine, no consideration is given to the improvement of the exhaust gas quality and the acceleration or promotion of temperature-rise of the catalyst, as described above. To say in another way, the conventional valve timing control system suffers problems that the catalyst temperature can not sufficiently be increased and that the quality of the exhaust gas yet remains to be improved.  
           [0107]    In view of the above and other objects which will become apparent as the description proceeds, there is provided according to an aspect of the present invention a valve timing control system for an internal combustion engine, which system includes sensor means for detecting engine operation states of an internal combustion engine, intake or exhaust cam shafts for driving intake or exhaust valves, respectively, of the internal combustion engine in synchronism with a rotation of a crank shaft of the internal combustion engine, at least one actuator operatively connected to at least one of cam shafts for driving the intake or exhaust valves, respectively, a hydraulic pressure supply unit for feeding a hydraulic pressure to drive the actuator; and control means for controlling the hydraulic pressure Ted from the hydraulic pressure supply unit to the actuator in dependence on the operating states of the internal combustion engine while changing a relative phase of the cam shaft relative to the crank shaft, wherein the actuator includes a retarding hydraulic chamber and an advancing hydraulic chamber for setting an adjustable range of the relative phase, a locking mechanism for setting the relative phase to a lock-up position within the adjustable range, and an unlocking mechanism for releasing the locking mechanism in response to a predetermined level of hydraulic pressure fed from the hydraulic pressure supply unit, and the control means restricts control of a valve timing within a predetermined range of a lock-up position in the locking mechanism.  
           [0108]    Further, restraining the control means not executing steady control.  
           [0109]    Furthermore, the control means detects a detected advance angle amount that is a phase difference between phases of the crank shaft and the cam shaft and calculates a target advance angle amount that is valve timing suitable for an operating state of the internal combustion engine not to set the target advance angle amount in a predetermined range of the lock-up position of the locking mechanism in the case in which the control means controls such that the detected advance angle amount substantially coincides with the target advance angle amount.  
           [0110]    Still further, the predetermined range is at least a variation range of steady control or the variation range of the steady control and an amount of a clearance by the locking mechanism.  
           [0111]    Yet still further, the steady control is not executed only when the operating state of the internal combustion engine is in a predetermined sate.  
           [0112]    Furthermore, the control means corrects a control amount with respect to a normal time if the detected advance angle amount is within a range allowing for a steady variation and an amount of a clearance from the lock-up position.  
           [0113]    In addition, the correction of the control amount is performed such that an acting speed increases.  
           [0114]    Finally, if there is a delay from the time when the control amount is changed until the time when a detected advance angle amount changes, the control means corrects the control amount earlier by a time corresponding to the delay. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0115]    In the accompanying drawings:  
         [0116]    [0116]FIG. 1 is a block diagram showing a configuration of a valve timing control system for an internal combustion engine in the present invention;  
         [0117]    [0117]FIG. 2 is a flow chart showing control operations of an ECU  21 A in accordance with a first embodiment of the present invention;  
         [0118]    [0118]FIG. 3 is a flow chart showing processing after determining a mode in FIG. 2;  
         [0119]    [0119]FIG. 4 is a diagram illustrating movement of a detected advance angle amount Vd with respect to a target advance angle amount Vt;  
         [0120]    [0120]FIG. 5 is a diagram illustrating a positional relation between a lock pin  155  and a locking recess  157  in the state in which a target advance angle amount Vt approaches a pin lock-up position Vr most (a point in time A of FIG. 4) with the target advance angle amount Vt assumed to be in a position apart from the pin lock-up position Vr by amounts of a steady variation Vc and a clearance Vg;  
         [0121]    [0121]FIG. 6 is a diagram illustrating the case in which the target advance angle amount Vt is on a retard side with respect to the pin lock-up position Vr contrary to the case of FIG. 4;  
         [0122]    [0122]FIG. 7 is a diagram illustrating the case in which the target advance angle amount Vt is on the retard side with respect to the pin lock-up position Vr contrary to the case of FIG. 5;  
         [0123]    [0123]FIG. 8 is a diagram showing that the target advance angle amount Vt changes in a step-like manner in the vicinity of the pin lock-up position Vr;  
         [0124]    [0124]FIG. 9 is a flow chart showing control operations of the ECU  21 A in accordance with a second embodiment of the present invention;  
         [0125]    [0125]FIG. 10 is a flow chart showing control operations of the ECU  21 A in accordance with a third embodiment of the present invention;  
         [0126]    [0126]FIG. 11 is a flow chart showing control operations of the ECU  21 A in accordance with a fourth embodiment of the present invention;  
         [0127]    [0127]FIG. 12 is a functional block diagram showing generally and schematically a configuration of a conventional valve timing control system of an internal combustion engine known heretofore;  
         [0128]    [0128]FIG. 13 is a view for illustrating a phase adjustable range of the conventional valve timing control system in terms of relation between crank angles and valve lift strokes;  
         [0129]    [0129]FIG. 14 is a timing chart for illustrating conventional phase or timing relations between individual output pulse signals of a crank angle sensor and cam angle sensors;  
         [0130]    [0130]FIG. 15 is a perspective view showing an internal structure of a conventional actuator at a most retarded timing position;  
         [0131]    [0131]FIG. 16 is a perspective view showing an internal structure of the conventional actuator at a lock-up position;  
         [0132]    [0132]FIG. 17 is a perspective view showing an internal structure of the conventional actuator at a most advanced timing position;  
         [0133]    [0133]FIG. 18 is a side-elevational sectional view showing an internal structure of a conventional oil control valve unit (hydraulic pressure supply unit) in an electrically deenergized state;  
         [0134]    [0134]FIG. 19 is a side-elevational sectional view showing an internal structure of the conventional oil control valve unit in a lock-up state; and  
         [0135]    [0135]FIG. 20 is a side-elevational sectional view showing an internal structure of the conventional oil control valve unit in an electrically energized state.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0136]    The present invention will be described in detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings. In the following description, like reference characters designate like or corresponding parts throughout the several views.  
         [0137]    Embodiment 1  
         [0138]    In the following, a valve timing control system for an internal combustion engine according to a first embodiment of the present invention will be described in detail by reference to the drawings.  
         [0139]    [0139]FIG. 1 is a schematic block diagram showing generally a configuration of the valve timing control system for the internal combustion engine according to the first embodiment of the invention. In the figure, components same as or equivalent to those mentioned hereinbefore by reference to FIG. 12 are denoted by like reference characters as those used in this figure and detailed description thereof is omitted.  
         [0140]    Accordingly, in the valve timing control system for the internal combustion engine according to the instant embodiment of the invention, the change control range of the valve timings for the intake valve and the exhaust valve is essentially same as shown in FIG. 13, and the relation between the output of the crank angle sensor and that of the cam angle sensor is also same as shown in FIG. 14.  
         [0141]    Further, the structure of the actuators  15  and  16  are essentially identical with that shown in FIGS. 15, 16 and  17 . Besides, the structures of the oil control valves (OCV)  19  and  20  are also essentially identical with those described hereinbefore in conjunction with FIGS. 18, 19 and  20 .  
         [0142]    Now, referring to FIG. 1, an electronic control unit (also referred to as the ECU in short)  21 A shown in FIG. 1 includes a lock control means for setting the actuators  15  and  16  to the lock-up position or state by means of the locking mechanism and an unlock control means for performing retarding or advancing control of the actuators  15  and  16  after the actuators  15  and  16  are released from the lock-up state by means of an unlocking mechanism in succession to the engine starting operation, as described hereinbefore.  
         [0143]    Moreover, the ECU  21 A includes restricting means for not executing but restricting steady control of a valve timing within a predetermined range of a position where the lock pin  155  engages with the locking recess  157 . This prevents a control amount from dispersing due to hitching of the lock pin by not executing the control in a pin lock-up position of an actuator, thereby making full use of engine performance to prevent deterioration of its driveability, and decrease in its mileage and gas exhausting performance.  
         [0144]    In a running mode after warm-up or the like that is a normal driving mode, a target advance angle amount can be an optimum valve timing in each driving mode if, for example, a map of a target advance angle amount that is two-dimensionally mapped by a rotation and a load of an engine is stored in an ROM of the ECU  21  in advance and target advance angle amounts according to driving states are set in the map.  
         [0145]    Since an oil pump is driven by an engine, the number of rotations of the oil pump is not enough in the engine starting operation and an oil amount supplied to the actuator is insufficient. Thus, a control of an advanced position is impossible. Therefore, flopping of the vane  152  due to insufficient hydraulic pressure is prevented by engaging the lock pin  155  with the locking recess  157  as shown in FIG. 16.  
         [0146]    There is a valve timing suitable for starting in the starting operation, and it is intended that an engagement position by the lock pin  155  becomes a valve timing in the starting operation. A valve overlap becomes large if an intake valve is excessively advanced and an actual compression ratio decreases if the intake valve is excessively retarded. In each case, the number of rotation in the cranking operation increases due to the decrease of a pumping loss, which is advantageous to an initial explosion but may not lead to a complete explosion because subsequent explosions are insufficient.  
         [0147]    When an exhaust valve is excessively advanced, the actual compression ratio decreased and combustion energy cannot be transmitted to a crank sufficiently. When the exhaust valve is excessively retarded, a valve overlap becomes large and the same situation arises as in the case in which the intake valve is excessively advanced. In the starting operation or in the operation state which immediately succeeds to the starting operation, the starting performance is deteriorated or starting becomes impossible if the valve timing is either excessively advanced or excessively retarded. Therefore, the valve timing is locked by the lock pin  155  such that it becomes favorable in the starting operation or in the operation state which immediately succeeds to the starting operation.  
         [0148]    After the starting operation, a hydraulic pressure increases in response to the increase of an engine speed and the hydraulic pressure is also supplied to the actuator. When the hydraulic pressure is supplied to the actuator, the hydraulic pressure is also supplied to the locking recess  157 . Then, when the hydraulic pressure overcomes the force of the spring  156 , the lock pin  155  is released from the locking recess  157  and the vane  152  is made to be operable. Thus, the OCVs  19  and  20  are for regulating control the supply of the hydraulic pressure to the retarding hydraulic pressure chamber  153  and the advancing hydraulic pressure chamber  154 , whereby an advance angle and an retard angle can be controlled.  
         [0149]    If a feedback control is executed according to a deviation of a target advance angle and a detected advance angle, a control value at the time of a holding control that is substantially indicated by a situation of FIG. 19 is learnt, and the control is executed on the basis of the learnt value. The leaning is executed in order to stabilize the control even if there are dispersions in which a control value at the time of the holding control varies for each engine. The learning is executed based on an integrated value at the time of the holding control, and if the learning is not executed, the integrated value fluctuates largely due to the dispersions. Thus, a certain degree of range is required for a width of an integral control.  
         [0150]    Depending on engine operating states, the target advance angle amount gets close to the pin lock-up position. When the detected advance angle amount follows the target advance angle amount, the OVC is controlled in the position shown in FIG. 19. In this case, since passages to both an advance angle and a retard angle are blocked and a hydraulic pressure by a leaked amount from the OCV is supplied to the actuator, the hydraulic pressure drops significantly and the force of the sprint  156  overcomes the hydraulic pressure to bring the lock pin  155  in the locking recess  157 . When the integral control is executed in this state, since the detected advance angle amount does not change in spite of changing a control current, the control current disperses. Thus, a control for preventing the dispersion of the control current is required.  
         [0151]    A valve timing control on an intake side according to a first embodiment of the present invention will now be described with reference to a flow chart of FIG. 2 together with the above-mentioned FIGS.  13  to  20 . This processing is executed for each predetermined timing (e.g., 25 [ms]) in the ECU  21 A.  
         [0152]    First, in step S 201 , the ECU  21 A detects a detected advance angle amount Vd that is a phase difference between a phase of a crank shaft and a phase of a cam shaft. This corresponds to A and B in FIG. 14. Then, in step S 202 , the ECU  21 A calculates a target advance angle amount Vt that is a valve timing suitable for an engine operating state from a charging efficiency, which is a loading state in an engine, and an engine speed.  
         [0153]    Then, if it is determined by a determination in step S 203  that the target advance angle amount Vt is smaller than a lock-up position Vr and is larger than a position apart from the lock-up position by an amount that allows for a steady variation Vc and an amount of a clearance Vg between a lock pin  155  and a locking recess  157  (Vr−(Vc+Vg/2)), the ECU  21 A sets the target advance angle amount Vt to a position that allows for the steady variation Vc, the amount of a clearance Vg between the lock pin  155  and the locking recess  157  and at least an amount of 1 LSB α (Vr−(Vc+Vg/2)−α) in step S 204 .  
         [0154]    On the other hand, if it is determined in step S 205  that the target advance angle amount Vt is larger than the lock-up position Vr and smaller than a position apart from the lock-up position by an amount that allows for a steady variation Vc and an amount of a clearance Vg between a lock pin  155  and a locking recess  157  (Vr+(Vc+Vg/2)), the ECU  21 A sets the target advance angle amount Vt to a position that allows for the steady variation Vc, the amount of a clearance Vg between the lock pin  155  and the locking recess  157  and at least the amount of 1 LSB α (Vr+(Vc+Vg/2)+α) in step S 206 .  
         [0155]    That is, if the target advance angle amount Vt enters a range of Vr to Vr−(Vc+Vg/2), the ECU  21 A sets the target advance angle amount Vt to Vr−(Vc+Vg/2)−α, and if the target advance angle amount Vt enters a range of Vr to Vr+(Vc+Vg/2), the ECU  21 A sets the target advance angle amount Vt to Vr+(Vc+Vg/2)+α, thereby not setting the target advance angle amount Vt within a range of Vr−(Vc+Vg/2) to Vr+(Vc+Vg/2).  
         [0156]    The ECU  21 A subtracts the detected advance angle amount Vd from the target advance angle amount Vt to find a control deviation Ver in the next step S 207 . Then, the ECU  21 A determines in step S 208  if the control deviation Ver is within a range of a steady variation (−Vc to Vc). If the control deviation is within the range of the steady variation, the ECU  21 A determines that the valve timing control is in a holding mode in step S 210 . On the other hand, if the control deviation is not within the range of the steady variation, the ECU  21 A determines that the valve timing control is in a PD (proportional differential) mode in step S 209 .  
         [0157]    [0157]FIG. 3 is a flow chart showing processing after determining a mode in FIG. 2. If it is determined in step S 301  that the valve timing control is in the holding mode, the ECU  21 A adds a product of the control deviation Ver and an integral gain Igain to an integrated value Ii to calculate a new integrated value Ii in step S 302 . The integral gain Igain is a value set in advance and stored in an ROM. Then, the ECU  21 A adds the integrated value Ii and a holding current learnt value Ih to calculate a control output value Iout in step S 303 . The holding current learnt value Ih is a value found by learning the control output value lout in the state in which the target advance angle amount Vt and the detected advance angle amount Vd substantially coincide with each other at the time of the holding mode.  
         [0158]    On the other hand, if it is determined in step S 301  that the valve timing control is in the PD mode, the ECU  21 A multiplies the control deviation Ver and a proportional gain Pgain to calculate a proportional value Ip. Then, the ECU  21 A multiplies a difference found by subtracting the last control deviation Ver[i−1] from the control deviation Ver by a differential gain Dgain to calculate a differentiated value Id in step S 305 . The proportional gain Pgain and the differential gain Dgain are values set in advance and stored in an ROM. Then, the ECU  21 A adds the proportional value Ip, the differentiated value Id and the holding current learnt value Ih to find a the control output value Iout in step S 306 . The holding current learnt value Ih is the same as the holding current learnt value Ih in step S 303 .  
         [0159]    The control output value lout calculated in the PD mode or the control output value lout calculated in the holding mode is converted to a duty ratio to be outputted to the OCV and controlled.  
         [0160]    In FIGS.  4  to  7 , the target advance angle amount Vt will be described concerning the necessity of allowing for the steady variation Vc and the clearance Vg between the lock pin  155  and the locking recess  157 . FIG. 4 is a diagram showing a movement of the detected advance angle amount Vd relative to the target advance angle amount Vt. The detected advance angle amount Vd is controlled relative to the target advance angle amount Vt according to a deviation within the range of the variation (−Vc to Vc) by an integral control. A clearance is provided between the lock pin  155  and the locking recess  157 , and a difference of the internal diameter of the locking recess  157  and the external diameter of the lock pin  155  is the clearance Vg. Even if the lock pin  155  is engaged with the locking recess  157 , the amount of the clearance (−Vg/2 to Vg/2) varies around the pin lock position Vr.  
         [0161]    [0161]FIG. 5 shows a positional relation between a lock pin  155  and a locking recess  157  in the state in which a target advance angle amount Vt approaches a pin lock-up position Vr most (a point in time A of FIG. 4) with the target advance angle amount Vt assumed to be in a position apart from the pin lock-up position Vr by amounts of a steady variation Vc and a clearance Vg. In the positional relation of FIG. 5, since the clearance on the advance side becomes zero, the lock pin  155  is engaged with the locking recess  157 .  
         [0162]    [0162]FIGS. 6 and 7 show the case in which the target advance angle amount Vt is on the retard side with respect to the pin lock-up position Vr contrary to FIGS. 4 and 5. A point in time A of FIG. 6 is in the state in which the detected advance angle amount Vd approaches the pin lock-up position Vr most, and the positional relation between the lock pin  155  and the locking recess  157  is as shown in FIG. 7. The clearance on the retard side also becomes zero in this case, so that the lock pin  155  is engaged with the locking recess  157 .  
         [0163]    Thus, since the lock pin  155  is engaged with the locking recess  157  if the target advance angle amount Vt is set within a range of ±(Vc+Vg/2) from the pin lock-up position allowing for the steady variation and the amount of the clearance, the ECU  21 A avoids setting the target advance angle amount Vt within this range. Therefore, the target advance angle amount is set further allowing for the amount of 1 LSB α in addition to ±(Vc+Vg/2) in steps S 204  and S 1006  of FIG. 2.  
         [0164]    In the state in which the target advance angle amount Vt changes in a ramp-like manner, the target advance angle amount Vt changes in a step-like manner in the vicinity of the pin lock-up position Vr as shown in FIG. 8. In this case, since the detected advance angle amount Vd is controlled by calculating a control amount from a deviation of the detected advance angle amount Vd and the target advance angle amount Vt, the control amount becomes large in accordance with the step-like change of the target advance amount. Therefore, the movement of the detected advance angle amount also becomes fast and the speed of the detected advance angle amount passing the pin lock-up position is fast. Thus, the lock pin  155  never hitches the locking recess  157 , and the detected advance angle amount can follow the target advance angle amount.  
         [0165]    Depending on an operating state of an engine, engine performance may be the best when a target advance angle amount is set to a pin lock-up position and controlled. In this case, change of the target advance angle amount by amounts of a steady variation and a pin clearance from the lock-up position results in decrease in the engine performance. However, the decrease in the engine performance can be smaller than the case in which the detected advance angle amount does not follow the target advance angle amount when the lock pin  155  hitches on the locking recess and the target advance angle amount changes or the pin lock is unlocked due to dispersion of an integrated value and the detected advance angle amount deviates largely from the target advance angle amount.  
         [0166]    In this way, the ECU  21 A does not control the target advance angle amount Vt in the range allowing for the steady variation Vc and the amount of the clearance Vg between the lock pin  155  and the locking recess  157  from the lock-up position, thereby preventing the situation in which the lock pin  155  hitches on the locking recess  157 , the deviation between the target advance angle amount Vt and the detected advance angle amount Vd is not eliminated and the control output value lout is dispersed by an integrated value although the control output value lout is changed. In addition, when the lock pin  155  hitches on the locking recess  157  and the integrated value disperses largely, a passage to an actuator of an OCV is secured, the lock pin  155  is unlocked from the locking recess  157  and the detected advance angle amount deviates largely from the target advance angle amount, whereby deterioration of its driveability, mileage and gas exhausting performance is prevented.  
         [0167]    Embodiment 2  
         [0168]    A second embodiment of the present invention will now be described. FIG. 9 is a flow chart showing control operations of the ECU  21 A in accordance with the second embodiment of the present invention. In FIG. 9, steps identical with those in the first embodiment shown in FIG. 2 are given the identical reference numerals and their descriptions are omitted.  
         [0169]    In this second embodiment, as shown in FIG. 9, it is determined in step S 901  if a number of revolutions Ne of an engine is smaller than a predetermined number of revolutions (3000[r/m]) after the detection of the detected advance angle amount Vd and the calculation of the target advance angle amount Vt (steps S 201  and S 202 ) and only when the number of revolutions Ne is smaller, the ECU  21 A moves to step S 203  and performs calculation processing of the target advance angle amount Vt allowing for the steady variation Vc and the clearance Vg between the lock pin  155  and the locking recess  157 . If the number of revolutions Ne is not smaller than the predetermined number of revolutions (3000[r/m]), the ECU  21 A moves to step S 207 . Other procedures are the same as those in the first embodiment.  
         [0170]    In this way, since a hydraulic pressure is sufficiently secured and the lock pin  155  never hitches on the locking recess  157  when an engine speed is equal to or more than a predetermined number of revolutions, it does not cause any problem even if a target advance angle amount is set in the vicinity of a pin lock-up position to perform a positional control. Moreover, if engine performance is the best in a pin lock position, since the control in the pin lock-up position is possible, decrease in engine performance is also eliminated.  
         [0171]    On the other hand, when an engine speed is equal to or less than a predetermined number of revolutions, the ECU  21 A does not execute a control in the vicinity of a pin lock-up position to eliminate hitching of a pin as in the first embodiment. And then, the ECU  21 A prevents a dispersion of a control value and a defect of a detected advance angle amount in following a target advance angle amount, whereby deterioration of its driveability, mileage and gas exhausting performance can be prevented.  
         [0172]    Although it is described in the second embodiment that a target advance angle amount setting that allows for a steady variation and an amount of a pin clearance is not executed when an engine speed is equal to or more than a predetermined number of revolutions, unlocking of a lock pin is determined by a hydraulic pressure and the hydraulic pressure is substantially determined by a number of revolutions and a temperature factor. Thus, if it is intended to execute the target advance angle amount setting more precisely, the target advance angle amount may be corrected according to a water temperature that is a parameter of a warm-up state of an engine, or may be corrected by measuring an oil temperature. Alternatively, the target advance angle amount setting may be executed by directly measuring a hydraulic pressure.  
         [0173]    Embodiment 3  
         [0174]    A third embodiment of the present invention will now be described. FIG. 10 is a flow chart showing control operations of the ECU  21 A in accordance with the third embodiment of the present invention and corresponds to control procedures according to the mode in the first embodiment shown in FIG. 3. And control procedures in FIG. 10 at the time of the PD mode are different from those in FIG. 3. Further, in FIG. 10, steps identical with those in the first embodiment shown in FIG. 3 are given the identical reference numerals and their descriptions are omitted.  
         [0175]    In this third embodiment, as shown in FIG. 10, if it is determined that the detected advance angle amount Vd is within a range allowing for a steady variation and an amount of a clearance from a pin lock-up position (Vr−(Vc+Vg/2)&lt;Vd&lt;Vr+(Vc+Vg/2)) in step S 1001  after it is determined that the valve timing control is in the PD mode in step S 301 , the ECU  21 A sets a correction coefficient Kr to a predetermined value larger than 1.0 (1.2) in step S 1002 . If it is determined that the detected advance angle amount Vd is not within the range in step S 1001 , the ECU  21 A sets the correction coefficient Kr to 1.0 in step S 1003 . Then, the ECU  21 A multiplies a sum of the proportional value Ip and the differentiated value Id by the correction coefficient Kr and adds the holding current learnt value Ih to the product to find the control output value Iout.  
         [0176]    In this way, if a detected angle amount is within a range allowing for a steady variation and a clearance, since a target advance angle amount is corrected such that the control output value lout increases, an acting speed increases and the lock pin  155  passes the locking recess  157  fast. Thus, the lock pin  155  never engages with the locking recess  157  and a defect of a detected advance angle amount in following a target advance angle amount due to hitching of a pin can be prevented, whereby deterioration of its driveability, mileage and gas exhausting performance can be prevented.  
         [0177]    Embodiment 4  
         [0178]    A fourth embodiment of the present invention will now be described. FIG. 11 is a flow chart showing control operations of the ECU  21 A in accordance with the fourth embodiment of the present invention and corresponds to control procedures according to the mode in the first embodiment shown in FIG. 3. And control procedures in FIG. 11 at the time of the PD mode are different from those in FIG. 3. Further, in FIG. 11, steps identical with those in the first embodiment shown in FIG. 3 and those in the third embodiment shown in FIG. 10 are given the identical reference numerals and their descriptions are omitted.  
         [0179]    In this fourth embodiment, as shown in FIG. 11, if it is determined that the target advance angle amount Vt is equal to or more than a position allowing for a steady variation and an amount of a clearance from a lock-up position (Vr+(Vc+Vg/2) and the detected advance angle amount Vd is within a range smaller than the lock-up position by a predetermined value (Vr−10&lt;Vd&lt;Vr−5) in step S 1101  after it is determined that the valve timing control is in the PD mode in step S 301 , the ECU  21 A sets the correction coefficient Kr to predetermined value larger than 1.0 (1.2) in step S 1002  as in the third embodiment shown in FIG. 10.  
         [0180]    On the other hand, if it is determined to the contrary in step S 1101 , the ECU  21  determines in step S 1102  if the target advance angle amount Vt is equal to or less than a position allowing for a steady variation and an amount of a clearance from a lock-up position (Vr−(Vc+Vg/2) and the detected advance angle amount Vd is within a range larger than the lock-up position by a predetermined value (Vr+5&lt;Vd&lt;Vr+10). If it is determined affirmative in step S 1102 , the ECU  21 A sets the correction coefficient Kr to a predetermined value larger than 1.0 (1.2) in step S 1002 . If it is determined negative is step S 1102 , the ECU  21 A sets the correction coefficient Kr to 1.0 in step S 1303  as in the third embodiment shown in FIG. 10. Then, the ECU  21 A multiplies a sum of the proportional value Ip and the differentiated value Id by the correction coefficient Kr in step S 1004  and adds the holding current learnt value Ih to the product to find the control output value Iout.  
         [0181]    In this way, if there is a delay from the time when the control output value lout is changed until the time when a detected advance angle amount changes, since the ECU  21 A corrects the control output value lout such that an acting speed increases before the detected advance angle amount comes to the vicinity of a pin lock-up position, action of the detected advance angle amount in the pin lock-up position gets fast. Thus, since the lock pin  155  passes the locking recess  157  quickly, the pin never hitches on the locking recess, whereby deterioration of its driveability, mileage and gas exhausting performance can be prevented.  
         [0182]    As described above, according to the present invention, since steady control of a valve timing in a position is not executed where a lock pin engages with a locking recess, the lock pin is prevented from being engaged with the locking recess and a defect of a detected advance angle amount in following a target advance angle amount, which is set optimally for engine performance, due to hitching of the lock pin can be prevented, whereby its driveability, mileage and gas exhausting performance can be prevented.  
         [0183]    In addition, since a control value is corrected to a side on which an acting speed increases if a detected advance angle amount passes a pin lock-up position, a lock pin is prevented from engaging with a locking recess and a defect of a detected advance angle amount in following a target advance angle amount, which is set optimally for engine performance, due to hitching of the lock pin can be prevented, whereby its driveability, mileage and gas exhausting performance can be prevented.