Abstract:
A valve timing control system for accelerating activation of a catalytic converter in an internal combustion engine in a cold starting operation mode. The system includes actuators ( 15, 16 ) coupled to cam shafts ( 15 C,  16 C), hydraulic pressure supply units ( 19, 20 ) for driving the actuators ( 15; 16 ), and a controller ( 21 A) for controlling a hydraulic pressure for the actuators ( 15, 16 ) in dependence on engine operation states to change phase of the cam shafts ( 15 C;  16 C) relative to a crank shaft. The actuator ( 15; 16 ) includes a locking mechanism ( 155, 157 ) for setting the relative phase to a lock-up position, and an unlocking mechanism ( 156 ) for releasing the locking mechanism in En response to a predetermined hydraulic pressure. The controller ( 21 A) sets the relative phase to the lock-up position in a starting operation, while controlling the relative phase to be advanced in an engine state succeeding to the starting operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. More particularly, the present invention is concerned with a valve timing control system for an internal combustion engine which system is designed for promoting temperature-rise (or temperature-up) of a catalytic converter disposed in an exhaust pipe of the engine for is realizing a significant reduction of harmful exhaust gas components discharged from the engine by controlling the valve timing toward an advanced phase or state during idling operation of the engine in a cold engine state (i.e., engine state of a low temperature). 
     2. Description of Related Art 
     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. 
     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. 
     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). 
     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 and exhaust valves are so arranged as to be rotationally driven by a crank shaft through the medium of timing belts (or timing chains). 
     Accordingly, the timings for opening and closing the intake and 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. 
     However, in recent years, a valve timing control Ad 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. 
     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). 
     The valve timing control system disclosed in the above-mentioned publication includes a variable valve timing mechanism (also referred to as the VVT 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. 
     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. 
     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. 6 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. 
     Referring to FIG. 6, 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. 
     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 . 
     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 O 2 -sensor  11  and a catalytic converter  12  are disposed in the exhaust pipe  10 . The O 2 -sensor  11  serves for detecting the content of residual oxygen contained in the exhaust gas. 
     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 NO x  (nitrogen oxides) contained in the exhaust gas. 
     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. 
     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. 
     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 and 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. 
     Actuators  15  and  16  for changing adjustably the cam phases are designed to change the timings for driving or actuating the intake and exhaust valves, respectively. 
     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. 
     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). 
     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. 
     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 O 2 -sensor  11 , the crank angle sensor  14  and the cam angle sensors  17  and  18 . 
     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. 
     Next, description will be made of the conventional engine control operation performed by the prior art valve timing control system shown in FIG.  6 . 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. 
     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. 
     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 . 
     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 (NO x ) into harmless carbon dioxide and water (H 2 O). In this way, the engine exhaust gas is purified. 
     In order to make available the maximum purification efficiency of the three-way catalytic converter  12 , the O 2 -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 O 2 -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. 
     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 and exhaust valves are to be driven or actuated. 
     In the following, referring to FIGS. 7 to  14 , description will be made in concrete of the phase angle control operation performed for the cam shafts  15 C and  16 C by the conventional valve timing control system for the internal combustion engine. 
     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. 
     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. 
     FIG. 7 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. 
     In FIG. 7, 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). 
     Referring to FIG. 7, 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. 
     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. 
     FIG. 8 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. 8 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. 
     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. 
     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 and 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. 
     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.  7 . 
     FIGS. 9 to  11  are views showing internal structures of the actuators  15  and  16  which are implemented in a substantially identical structure. More specifically, FIG. 9 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.  7 ), FIG. 10 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.  7 ), and FIG. 11 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.  7 ), respectively. 
     Referring to FIGS. 9,  10  and  11 , 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 . 
     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. 
     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 . 
     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 . 
     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 . 
     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. 
     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 and exhaust valves of the engine cylinders. 
     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. 
     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. 9 to  11 , the amount of oil (i.e., hydraulic pressure) fed to the actuators  15  and  16  is controlled. 
     By way of example, regulation of the cam angle phase to the most retarded position, as illustrated in FIG. 9, 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. 11, can be effectuated by feeding oil into the advancing hydraulic chamber  153 . 
     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. FIGS. 12,  13  and  14  show in side-elevational sectional views the internal structures of the oil control valves  19  and  20  which are implemented substantially identically. 
     Referring to FIGS. 12 to  14 , 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. 
     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. 
     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 . 
     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. 12, the orifice  195  is shown as having been placed in communication with the orifice  196 , while in FIG. 14, the orifice  195  is shown as being communicated to the orifice  197 . 
     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. 12, the orifice  197  is shown as being communicated with the orifice  198 , while in FIG. 14, the orifice  196  is being communicated to the orifice  199 . 
     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.  14 ). 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. 
     FIG. 12 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. 
     Assuming that the oil control valve shown in FIG. 12 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.  9 . 
     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 . 
     On the other hand, assuming that the oil control valve shown in FIG. 12 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.  11 . 
     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 . 
     By virtue of the hydraulic circuit arrangement described above by reference to FIG. 12, 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. 
     In FIG. 14, 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. 
     Assuming, by way of example, that the oil control valve shown in FIG. 14 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 . 
     On the other hand, in the case where the oil control valve shown in FIG. 14 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 . 
     FIG. 13 shows the state corresponding to the  1 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.  10 ). 
     In the state illustrated in FIG. 13, 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.  10 ). 
     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 . 
     At this juncture, it should be mentioned that the predetermined unlocking hydraulic pressure mentioned above may be set at a necessary minimum value. 
     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 . 
     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. 
     Referring to FIG. 8, 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. 7) 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. 7) is indicated by B. 
     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. 
     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 . 
     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. 
     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 . 
     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.  13 . 
     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.  13 . 
     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 . 
     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. 
     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.  10 . 
     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. 
     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. 
     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. 
     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. 
     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.  10 . 
     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.  13 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
     In the light of the state of the art described above, it is an object of the present invention to provide a valve timing control system for an internal combustion engine in which the actuator is locked at an intermediate position between the most retarded position and the most advanced position in the engine starting operation state or mode while being unlocked after the engine operation has been started and in which the valve timing is controlled to be advanced particularly in the idling operation in the cold engine state (i.e., in the cold-state idling operation mode) to thereby realize effectively acceleration or promotion of the temperature-rise of the catalyst and reduction of harmful exhaust gas components. 
     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 a sensor means for detecting operating states of an internal combustion engine, intake and exhaust cam shafts for driving intake and 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 the cam shafts for driving the intake and exhaust valves, respectively, a hydraulic pressure supply unit for feeding a hydraulic pressure to drive the actuator, and a control means for controlling the hydraulic pressure fed from the hydraulic pressure supply unit to the actuator in dependence on the operating states of the internal combustion engine while changing relative phase of the cam shaft relative to the crank shaft. The actuator is comprised of 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 t o a lock-up position within the adjustable range, and a supply unit. The control means is designed for driving the locking mechanism for thereby controlling the relative phase to be set to the lock-up position when the internal combustion engine is detected as being in a starting operation state, while when the internal combustion engine is in a state which succeeds to the starting operation state, the locking mechanism is released by means of the unlocking mechanism with the hydraulic pressure fed from the hydraulic pressure supply unit to the retarding hydraulic chamber and the advancing hydraulic chamber being controlled for thereby executing a retarding control and an advancing control, respectively, for the relative phase. Thus, the relative phase is so controlled as to be advanced when the engine is operating in a cold-state idling operation mode. 
     By virtue of the arrangement described above, there is provided a valve timing control system for an internal combustion engine in which temperature rise behavior of the catalyst and reduction of harmful emission fumes can significantly be enhanced. 
     In a preferred mode for carrying out the invention, the lock-up position may be established as a phase position suited for the control performed in the starting operation mode of the internal combustion engine and in an operation state which immediately succeeds to the starting operation mode. 
     Owing to the feature described above, there can be realized the valve timing control system for the internal combustion engine whose starting performance can remarkably be improved. 
     In another preferred mode for carrying out the invention, the hydraulic pressure supply unit may be so designed as to validate the predetermined hydraulic pressure at least when it is detected that the internal combustion engine is operating in the cold-state idling operation mode (i.e., in the idling operation mode in the cold state). 
     With the arrangement of the valve timing control system described above, the valve timing advancing control can be performed by the actuator in the cold-state idling operation mode of the engine since the locking mechanism is released in that mode. Thus, the temperature rise of the catalyst and reduction of the harmful exhaust gas components can advantageously be accelerated or promoted. 
     In yet another preferred mode for carrying out the invention, the control means may be so designed as to control the relative phase to be set to the lock-up position when it is detected that the internal combustion engine is operating in an warm-up idling operation mode. 
     With the arrangement of the valve timing control system described above, the engine operation performance can significantly be improved in the idling operation of the engine in the warm-up state (i.e., in the warm-up idling operation mode) which approximates to the engine starting operation state. 
     In still another preferred mode for carrying out the invention, the control means may be so designed as to control an ignition timing of the internal combustion engine to be retarded when it is detected that the internal combustion engine is operating in the cold-state idling operation mode. 
     Owing to the arrangement of the valve timing control system described above, the temperature rise of the catalyst and hence the reduction of the harmful exhaust gas components can further be promoted to a great advantage. 
     In a further preferred mode for carrying out the invention, the control means may be so designed as to control quantity of a fuel supplied to the internal combustion engine to be decreased when it is detected that the internal combustion engine is operating in the cold-state idling operation mode. 
     With the arrangement of the valve timing control system described above, the temperature rise of the catalyst and hence the reduction of the harmful exhaust gas components can also be much promoted. 
     In another preferred mode for carrying out the invention, the control means may be so designed as to carry out the relative phase retarding or alternatively advancing control in dependence on the engine operating states at least when it is detected that the internal combustion engine is operating in other operation mode(s) than the warm-up idling operation mode after the engine has been warmed. 
     With the arrangement of the valve timing control system described above, performance of the internal combustion engine in the ordinary operation modes can considerably be improved to another advantage. 
     The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the course of the description which follows, reference is made to the drawings, in which: 
     FIG. 1 is a schematic block diagram showing generally a configuration of a valve timing control system for an internal combustion engine according to a first embodiment of the present invention; 
     FIG. 2 is a flow chart for illustrating control operation of the valve timing control system according to the first embodiment of the present invention; 
     FIG. 3 is a flow chart for illustrating control operation of a valve timing control system according to a second embodiment of the present invention; 
     FIG. 4 is a flow chart for illustrating control operation of the valve timing control system according to a third embodiment of the present invention; 
     FIG. 5 is a flow chart for illustrating control operation of the valve timing control system for the internal combustion engine according to a fourth embodiment of the present invention; 
     FIG. 6 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; 
     FIG. 7 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; 
     FIG. 8 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; 
     FIG. 9 is a perspective view showing an internal structure of a conventional actuator at a most retarded timing position; 
     FIG. 10 is a perspective view showing an internal structure of the conventional actuator at a lock-up position; 
     FIG. 11 is a perspective view showing an internal structure of the conventional actuator at a most advanced timing position; 
     FIG. 12 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; 
     FIG. 13 is a side-elevational sectional view showing an internal structure of the conventional oil control valve unit in a lock-up state; and 
     FIG. 14 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 
     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. 
     Embodiment 1 
     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. 
     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. 6 are denoted by like reference characters as those used in this figure and detailed description thereof is omitted. 
     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. 7, 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.  8 . 
     Further, the structure of the actuators  15  and  16  are essentially identical with that shown in FIGS. 9,  10  and  11 . Besides, the structures of the oil control valves (OCV)  19  and  20  are also essentially identical with those described hereinbefore in conjunction with FIGS. 12,  13  and  14 . 
     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. 
     Further, in the valve timing control system now under consideration, the ECU  21 A includes a cold-state idling operation control means for controlling the phase angles of the cam shafts  15 C and  16 C to advance relative to the crank shafts by means of the actuators  15  and  16  when it is detected that the engine  1  is operating in the idling mode in the cold engine state (i.e., the cold-state idling operation mode). 
     The aforementioned unlocking control means incorporated in the ECU  21 A is so designed as to generate a predetermined hydraulic pressure from the oil pump to release the actuators from the locked state at least when it is decided that the engine  1  is operating in the cold-state idling operation mode. 
     Furthermore, the ECU  21 A of the valve timing control system now concerned includes a warm-up idling operation control means for setting the actuators  15  and  16  to the lock-up position or state when it is detected that the engine  1  is operating in the idling mode in the warm-up state (i.e., the warm-up idling operation mode). 
     In this conjunction, it should be mentioned that the locked position of the actuators  15  and  16  is so set as to be a position desirable for the engine starting operation as well as for the engine state immediately succeeding thereto. 
     In other words, the position of the vane  152  locked by the lock pin  155  (see FIG. 10) is so set as to realize the valve timing suited for the engine starting operation. 
     As described hereinbefore, in the engine starting operation as well as in the state immediately succeeding thereto, both the overretarding and the overadvancing of the valve timing are accompanied with degradation of the engine starting performance. Thus, the relative position between the lock pin  155  and the locking recess  157  is previously so set that the valve timings suited for the engine starting operation as well as for the immediately succeeding engine operation without being necessarily limited to the mid position described hereinbefore by reference to FIG.  10 . 
     In the cold-state idling operation succeeding to the engine start, the lock pins  155  of the actuators  15  and  16  have to be released from the locking recesses  157  for allowing the valve lift timing to be controlled to advance. 
     In this case, the lubricating oil pressure of the engine  1  is also made use of for actuation of the actuators  15  and  16  inclusive of the operation for releasing the lock pin  155 ). In this conjunction, it is to be noted that the lubricating oil pressure will change in dependence of the engine rotation speed (rpm), oil temperature and the like. 
     As described heretofore, at least in the case of performing the advancing control in the cold-state idling operation mode of the engine, it is required to make effective the hydraulic pressure in order to release the lock pin  155 . 
     Additionally, after completion of the advancing control in the cold-state idling operation mode, the actuators  15  and  16  are so controlled as to be set to the locked position. 
     In that case, the feedback control may be carried out for sustaining substantially the lock-up position while holding the hydraulic pressure for clearing the lock-up state. Alternatively, the lock pin  155  may be engaged with the recess at the lock-up position. 
     When the acceleration pedal is depressed for starting to run the motor vehicle from this state, the rotation speed (rpm) of the engine increases, as a result of which the lock-up state is cleared while the engine control can be performed at the retarded or advanced position (not the lock-up position) in dependence on the operating state of the engine  1 . 
     Next, referring to a flow chart shown in FIG. 2 together with FIGS. 7 to  14  mentioned hereinbefore, description will be made of the operation of the valve timing control system according to the first embodiment of the present invention. 
     The processing routine illustrated in FIG. 2 is executed internally of the ECU  21 A at a predetermined timing. 
     Referring to FIG. 2, it is firstly decided by the ECU  21 A whether the engine  1  is operating in the starting state or in the stall state (step Si). 
     When decision is made in the step S 1  that the engine  1  is in the starting state or in the stall state (i.e., when the decision step Si results in the affirmation “Yes”), the current supplied to the solenoids or coils  193  of the oil control valves  19  and  20  is set to a minimum current level or value MIN (step S 2 ), whereupon the procedure exits from the processing routine illustrated in FIG.  2 . 
     The minimum current value MIN may be the nonconducting state current value (i.e., 0 mA). It is however referred to maintain the minimum current value MIN on the order of 100 mA as a stand-by current for the succeeding operation. 
     On the other hand, when it is decided in the step Si that the engine is neither in the starting state nor in the stall state (i.e., when the decision step Si results in negation “No”), decision is succeedingly made as to whether or not the engine  1  is in the idling state (step S 3 ). 
     The decision in the step S 3  can be made by checking whether the idle switch is on or off or alternatively checking whether the throttle valve is fully closed or not, as is well known in the art. 
     When the decision step S 3  results in that the engine  1  is not in the idling state (i.e., when the step S 3  results “No”), the map data position (retarded or advanced position) which conforms with the operating state of the engine  1  is validated (step S 4 ), whereupon the processing routine illustrated in FIG. 2 comes to an end. 
     In a step S 4 , the feedback control is carried out so that the desired position stored as map data in a ROM (Read-Only Memory) incorporated in the ECU  21 A. 
     As mentioned previously, the map data are so prepared as to be referenced on the basis of the engine rotation speed (rpm) and the engine load both serving as parameters. 
     On the other hand, when it is decided in the step S 3  that the engine  1  is in the idling state (i.e., when the decision step S 3  results in “Yes”), it is then decided whether or not the engine  1  is in the cold state (step S 5 ). 
     At this juncture, it should be mentioned that the term “cold state” means the state in which the temperature of cooling water of the engine  1  does not exceed 40° C. 
     When the decision step S 5  shows that the engine  1  is not in the cold state (i.e., when the decision step S 3  results in “No”), the vanes  152  of the actuators  15  and  16  are so controlled as to be set to the lock-up position (step S 6 ), whereupon the processing routine illustrated in FIG. 2 comes to an end. 
     In this conjunction, control of the actuators  15  and  16  to the lock-up position is so performed that the cam phase which determines the valve timing is fixedly set to a position suitable not only for the state for and immediately after the engine starting operation described previously but also suitable for ensuring the stability of the idling operation. 
     By contrast, when decision is made in the step S 5  that the engine  1  is in the cold state (i.e., when the decision step S 5  results in “Yes”), the valve timing is controlled to advance (step S 7 ), whereupon the control procedure exits the processing routine illustrated in FIG.  2 . 
     When the valve timing is controlled to advance in the step S 7 , the valve timing may be so controlled as to be set to the most advanced position or alternatively to a given position between the lock-up position and the most advanced position. 
     The advanced position set in the step S 7  should preferably be, for example, such position where the temperature-up effect of the catalyst  12  is significant (e.g. position at which the temperature of the exhaust gas rises at a highest rate) and where the quantity of hydrocarbon (HC) contained in the exhaust gas can be suppressed to a minimum. 
     As is apparent from the above, when the engine  1  is operating in the cold-state idling mode after the starting operation, the valve timing is so controlled as to advance (step S 7 ). As a result of this, the temperature of the exhaust gas rises up speedily, which is effective for promoting the temperature rise of the catalyst  12 . 
     In particular, by advancing the valve-open start timing of the exhaust valve, the exhaust gas being still undergoing the combustion is discharged to the exhaust pipe  10 , which contributes to the temperature rise of the catalyst  12 . 
     Further, when the valve timings of both the intake and exhaust valves are simultaneously controlled so as to advance, the valve overlap period can remain unchanged. Accordingly, stability of the engine operation in the idling mode can be enhanced. 
     With the locking mechanism incorporated in the actuators  15  and  16 , the vanes  152  are latched at the lock-up position suitable for the engine starting operation as well as the immediately succeeding state within the timing change control range (exclusive of the most retarded position and the most advanced position). As a result of this, starting performance of the engine can be improved. 
     Furthermore, since the locking mechanism is released under the effect of the predetermined hydraulic pressure supplied from the oil pump at least in the cold-state idling operation mode succeeding to the starting operation, the advancing control of the actuators  15  and  16  can be effectuated. Thus, high-temperature exhaust gas is discharged even in the cold-state idling operation mode, whereby activation of the catalyst  12  is promoted, which is effective for reduction of the harmful exhaust gas components. 
     Moreover, in the warm-up idling operation mode succeeding to the engine starting operation (the state approximating to the engine starting operation state), driving performance can be enhanced by controlling the actuators  15  and  16  to the lock-up position. 
     Additionally, in the operating state except for the idling state succeeding to the warm-up operation of the engine (e.g. in the ordinary running state at the least), the retarding/advancing control of the valve timings suited for the ordinary running operation can be carried out in dependence on the operating states. 
     In the valve timing control system according to the instant embodiment of the invention, which is shown in FIG. 1, the actuators  15  and  16  are disposed in association with both the cam shaft  15 C for the intake valves and the cam shaft  16 C for the exhaust valves, respectively. However, the invention is never restricted to such arrangement. Only one of the actuators  15  and  16  may be disposed in association with one of the cam shafts  15 C and  16 C. 
     Furthermore, in the valve timing control system described above, the actuators  15  and  16  are implemented such that the vanes  152  for changing the phase angle are rotationally moved within the respective housings, as described hereinbefore by reference to FIGS. 9 to  11 . However, the invention is not restricted to the actuator of such structure. It goes without saying that other types of actuators such as helical type or the like actuator may equally be employed. A second embodiment of the present invention concerns the valve timing control system in which the retarding control of the ignition timing is adopted. 
     Embodiment 2 
     In the case of the valve timing control system for the internal combustion engine according to the first embodiment of the invention, the temperature rise of the catalyst  12  and the accelerated purification of the exhaust gas are realized through the valve timing advancing control performed with the aid of the actuators  15  and  16  in the cold-state idling operation mode of the engine  1 . It should however be noted that in addition to such valve timing advancing control, the ignition timing of the engine  1  may be so controlled as to be retarded. 
     In the following, description will be directed to the valve timing control system for the internal combustion engine according to a second embodiment of the present invention, which system is designed to control the ignition timing of the engine  1  so that it is retarded in the idling operation carried out in the engine  1  in the cold state thereof. 
     FIG. 3 is a flow chart for illustrating control operation of the valve timing control system according to the second embodiment of the invention, wherein the processing steps similar to those described previously by reference to FIG. 2 are affixed with like step identification numbers and repeated description in detail of these steps is omitted. 
     Now referring to FIG. 3, when it is decided by the ECU  21 A (see FIG. 1) in the steps S 3  and S 5  that the operating state of the engine  1  is in the idling operation mode in the cold state (i.e., when the decision steps S 3  and  55  result in affirmation “Yes”), the ECU  21 A (see FIG. 1) controls the actuators  15  and  16  so that operation timings thereof are advanced (step S 7 ,) while controlling the ignition timing of the engine  1  so that it is retarded (step S 8 ). Then, the processing routine shown in FIG. 3 comes to an end. 
     In this manner, by controlling in the step S 8  the ignition timing so that it is retarded, speed or rate of the combustion within the combustion chamber of the engine cylinder becomes lowered, the temperature rise of the exhaust gas is promoted. 
     Thus, the temperature of the catalyst  12  rises up effectively by adopting the ignition timing retarding control in combination with the advancing control of the valve timing described hereinbefore. 
     It is also noted that by controlling the valve timings of both the intake and exhaust valves to be advanced, the time during which the intake and exhaust valves overlap with each other can remain unchanged, as described previously. Thus, the stability of the idling operation can be enhanced. 
     As can now be appreciated, by controlling the ignition timing of the engine to be retarded while controlling the valve timing so as to be advanced in the idling operation of the engine  1  in the cold state thereof, the exhaust gas of higher temperature than that described previously can flow into the catalyst  12 . As a result of this, the catalyst  12  is heated to high temperature more speedily when compared with the case of the first embodiment to be thereby activated at an earlier time point. 
     Embodiment 3 
     In the case of the valve timing control system for the internal combustion engine according to the second embodiment of the invention, the ignition timing of the engine is so controlled as to be retarded in addition to the advancing control of the actuators  15  and  16  in the idling operation of the engine  1  in the cold state thereof. It is however to be noted that the quantity of fuel supplied to the cylinders of the engine  1  may also be so controlled as to be decreased substantially to the similar effect. A third embodiment of the present invention concerns the valve timing control system in which the injection fuel quantity decreasing control is adopted. 
     In the following, description will be directed to the valve timing control system for the internal combustion engine according to a third embodiment of the present invention, which system is designed to perform the fuel injection quantity decreasing perform in the cold-state idling operation mode of the engine  1 . 
     FIG. 4 is a flow chart for illustrating the control operation of the valve timing control system according to the third embodiment of the invention, wherein the processing steps similar to those described previously by reference to FIGS. 2 and 3 are affixed with like step numbers and repeated description in detail of these steps is omitted. 
     Now referring to FIG. 4, when it is decided by the ECU  21 A (see FIG. 1) in the steps S 3  and S 5  that the operating state of the engine  1  is in the idling operation mode in the cold state (i.e., when the decision steps S 3  and S 5  result in affirmation “Yes”), the ECU  21 A controls the actuators  15  and  16  so that the valve actuation timings are advanced (step S 7 ) while controlling the fuel quantity supplied or injected into the cylinders of the engine  1  to be decreased with the air-fuel ratio A/F assuming a lean value (step S 9 ). Then, the processing routine illustrated in FIG. 4 comes to an end. 
     In this way, when decision is made that the engine  1  is in the idling operation mode in the cold state (i.e., cold-state idling operation mode), the valve timings are so controlled as to be advanced (step S 7 ) while the air-fuel ratio A/F is so controlled as to become lean through the fuel injection decreasing control (step S 9 ). Then, the amount of the harmful components such as hydrocarbon (HC) and others contained in the exhaust gas is further reduced when compared with the reduction solely under the effect of the temperature rise of the catalyst  12  mentioned above, whereby purification of the exhaust gas can be much promoted. 
     As is apparent from the foregoing, in the colds-tate idling operation mode of the engine, the temperature rise of the catalyst  12  is promoted to be activated at an earlier time point and thus hydrocarbon (HC) and others carried by the exhaust gas can further be decreased. 
     Embodiment 4 
     In the case of the valve timing control system for the internal combustion engine according to the second embodiment of the invention, the ignition timing of the engine is controlled to be retarded while in the system according to the fourth embodiment, the fuel injection quantity is controlled to be decreased in addition to the valve timing advancing control performed by the actuators  15  and  16  in the cold-state idling operation mode of the engine  1 . In this conjunction, it is to be noted that both the ignition timing retarding control and the fuel injection quantity decreasing control may be carried out simultaneously. A fourth embodiment of the present invention concerns the valve timing control system in which simultaneous control of the ignition timing and the fuel injection quantity is adopted. 
     In the following, description will be directed to the valve timing control system for the internal combustion engine according to the fourth embodiment of the present invention, in which both the ignition timing retarding control and the fuel injection quantity decreasing control are carried out simultaneously or concurrently in the cold-state idling operation mode of the engine  1 . 
     FIG. 5 is a flow chart for illustrating control operation of the valve timing control system according to the fourth embodiment of the invention, wherein the processing steps similar to those described previously by reference to FIGS. 2,  3  and  4  are affixed with like step numbers and repeated description in detail of these steps is omitted. 
     Referring to FIG. 5, when it is decided by the ECU  21 A (see FIG. 1) in the steps S 3  and S 5  that the operating state of the engine  1  is in the cold-state idling operation mode (i.e., when the decision steps S 3  and S 5  result in “Yes”), the ECU  21 A controls the actuators  15  and  16  so that valve timings are thereby advanced (step S 7 ) while controlling the ignition timing to be retarded with the air-fuel ratio A/F being controlled to assume a lean level (step S 9 ), whereupon the processing routine shown in FIG. 5 comes to an end. 
     In this way, by carrying out concurrently the ignition timing retarding control and the fuel injection decreasing control simultaneously with the valve timing advancing control of the actuators  15  and  16  in the cold-state idling operation mode of the engine  1 , the temperature rise of the catalyst  12  can be accelerated much more, whereby hydrocarbon (HC) and others contained in the exhaust gas can be reduced more efficiently. 
     Many features and advantages of the present invention are apparent from the detailed description and thus it is intended by the appended claims to cover all such features and advantages of the system which fall within the true spirit and scope of the invention. Further, since numerous modifications and combinations will readily occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation illustrated and described. Accordingly, all suitable modifications and equivalents may be resorted to, falling within the spirit and scope of the invention.