Valve timing control device for use in an internal combustion engine

A valve timing control device for use in an internal combustion engine includes a valve timing controller for variably controlling the open/close timing of intake and exhaust valves of the internal combustion engine, a drive force estimator provided in the valve timing controller for estimating a drive force of a VVT actuator on the basis of a hydraulic pressure supplied thereto, and a variable control limiter for limiting control made by the valve timing controller when the drive force of the VVT actuator, which is estimated by the drive force estimator, is equal to or less than a variable valve timing control limit boundary value.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an internal combustion engine having a
 variable valve timing mechanism (hereinafter simply referred to as VVT)
 that variably controls the open/close timing (hereinafter simply referred
 to as valve timing) of an intake valve and an exhaust valve of the
 internal combustion engine in response to the operating state of an
 engine, and more particularly to a valve timing control device for
 controlling the operation of the VVT in its abnormal state.
 2. Description of the Related Art
 A conventional valve timing control device for use in an internal
 combustion engine includes, as shown in FIG. 9, an oil pump 1 for forcedly
 feeding a lubricating oil to a valve timing control system, a cam shaft 2
 that is coupled to a crank shaft (not shown) of an engine by means of a
 power transmission mechanism such as a timing belt and is driven to rotate
 in synchronism with the rotation of the crank shaft to cause the intake
 valve and the exhaust valve to open and close through a valve operating
 mechanism, and a variable valve timing control hydraulic actuator 3
 (hereinafter simply referred to as "VVT actuator") for varying the
 rotational phase of the cam shaft with respect to the crankshaft. The
 conventional valve timing control device also includes an oil control
 valve 4 for adjusting the amount of lubricating oil in the form of an
 operating oil supplied to the VVT actuator 3, a crank angle detector 5 for
 detecting the rotational phase of the crank shaft, a cam angle detector 6
 for detecting the rotational phase of the cam shaft, and an engine
 operating state detector 7 for detecting the operating state of the
 internal combustion engine. The conventional valve timing control device
 further includes an oil control valve control circuit 8 that electrically
 drives the oil control valve 4, an electronic control unit (ECU) 10 that
 gives an instruction or command to the oil control valve control circuit
 8, an oil temperature detector 14 for detecting the temperature of the
 lubricating oil supplied to the oil control valve 4, and a hydraulic
 pressure detector 15 for detecting the hydraulic pressure of the
 lubricating oil.
 Subsequently,. the operation of the above-described conventional valve
 timing control device will be described. The ECU 10 calculates the current
 open/close timing (hereinafter simply referred to as "valve timing") of
 the intake and exhaust valves on the basis of an output of the engine
 operating state detector 7 that detects the operating state of the
 internal combustion engine. The ECU 10 calculates the operating amount of
 the oil control valve 4 in accordance with the hydraulic pressure of the
 lubricating oil so as to reduce a deviation between an optimum valve
 timing and the current valve timing, and outputs an instruction or command
 to the oil control valve control circuit 8. The oil control valve control
 circuit 8 adjusts the supply voltage or current to the oil control valve 4
 so that the operating amount instructed by the ECU 10 and the electric
 behavior of the oil control valve 4 coincide with each other.
 The lubricating oil forcedly fed from the oil pump 1 is controlled by the
 oil control valve 4 and then supplied to any one of a spark retarding
 chamber 3a and a valve timing advancing chamber 3b of the VVT actuator 3
 by an appropriate amount in a desired direction. On the other hand, the
 lubricating oil, filled in the other chamber to which the lubricating oil
 is now not being supplied, is returned to an oil pan 12 through a drain 11
 of the oil control valve 4.
 When the oil is supplied to the spark retarding chamber 3a or to the valve
 timing advancing chamber 3b of the VVT actuator 3, a rotor 3c within the
 VVT actuator 3 is driven to rotate in a direction toward a valve timing
 advancing side or a spark retarding side due to hydraulic pressure. The
 cam shaft 2 is coaxially connected to the rotor 3c, and the rotation of
 the rotor 3c allows the rotational phase of the cam shaft 2 to vary with
 respect to the rotational phase of the crank shaft to thereby change the
 valve timing.
 Subsequently, a control method or operation of the control device or ECU 10
 that estimates the drive force of the VVT actuator 3 on the basis of the
 operating state of the engine and controls the valve timing in response to
 the drive force will be described with reference to a flowchart shown in
 FIG. 10.
 First, the ECU 10 reads various sensor signals from the operating state
 detector 7, the crank angle detector 5 that detects the rotational phase
 of the crank angle and the cam angle detector 6 that detects the
 rotational phase of the cam shaft 2 (step S1), and calculates the current
 valve timing .theta. from the rotational phase of the crank angle and the
 rotational phase of the cam shaft 2 thus read (step S2).
 Also, in step S3, an optimal valve timing .theta.T (hereinafter referred to
 as "target valve timing") in the engine operating state is calculated from
 the sensor signals (for example, the number of revolutions per minute
 (rpm) or rotational speed of the engine, the throttle opening degree, the
 charging efficiency, the temperature of an engine coolant or cooling
 water, etc.) indicative of the engine operating state as read in step S1.
 In addition, the ECU 10 reads an oil temperature from the oil temperature
 detector for detecting the temperature of the lubricating oil (hereinafter
 referred to as "oil temperature") and estimates the viscosity of the
 lubricating oil from the oil temperature thus read in (step S4).
 Subsequently, in step S5, the ECU 10 calculates a deviation .theta.e of the
 current valve timing with respect to the target valve timing and estimates
 a force (hereinafter referred to as "an operating force Fc of the cam
 shaft 2") necessary for operating the cam shaft 2 in order to set the
 deviation .theta.e to zero, from the viscosity of the lubricating oil as
 estimated in step S4 and the rotational speed (rpm) of the engine.
 In step S6, the hydraulic pressure is read from the hydraulic pressure
 detector 15 that detects the supply pressure of the lubricating oil
 (hereinafter referred to as "hydraulic pressure") and then a cam shaft
 drive force Fp for driving the cam shaft 2 to rotate is determined from
 the hydraulic pressure supplied to the VVT actuator 3, to thereby estimate
 a force (hereinafter referred to as "ACT drive force Fa") by which the VVT
 actuator 3 changes the rotational phase of the cam shaft 2 with respect to
 the rotational phase of the crank shaft, where Fa=Fp-Fc.
 In step S7, the operation of opening (when
 .vertline..theta.e.vertline.&gt;.theta.D) or closing (when
 .vertline..theta.e.vertline..gtoreq..theta.D) of the oil control valve 4
 is determined on the basis of the relation in magnitude between the
 deviation .theta.e and a dead zone .theta.D. The dead zone .theta.D may b
 set to "0".
 Then, In step S8, the operating amount for opening the oil control valve 4
 is determined. The operating amount is determined on the basis of the
 deviation .theta.e calculated in step S5 and the ACT drive force Fa
 estimated in step S6. For example, if PID control is applied in the
 determination of the operating amount, respective gains are set in
 accordance with the ACT drive force Fa.
 In step S9, the operating amount for closing the oil control valve 4 is
 determined through the same operation as in step S8.
 In step S10, the operating amount determined in step S8 or S9 is converted
 into an electric signal by means of the oil control valve control circuit
 8 to thereby drives the oil control valve 4.
 In the control device that estimates the ACT drive force Fa and controls
 the valve timing in response to the ACT drive force Fa as described above,
 the operating amount of the oil control valve 4 is determined by
 multiplying the deviation between the target valve timing advancing amount
 and the actual valve timing advancing amount by a control gain which is
 set in response to the estimated ACT drive force. In general, a response
 time of the VVT actuator 3 tends to increase as the ACT drive force
 decreases, so the control gain is set to be large with the decreasing ACT
 drive force in order to provide stabilized controllability regardless of
 fluctuations of the ACT drive force (see FIG. 3).
 However, because the hydraulic pressure depends on the temperature of the
 lubricating oil and the rotational number or speed of the engine, there is
 a case in which the hydraulic pressure temporarily remarkably becomes low
 depending on the operating state of the engine, and the ACT drive force
 becomes in the vicinity of or not greater than zero (0). In other words,
 this is a case of Fp&lt;Fc. Also, even if a sufficient ACT drive force exists
 immediately before the VVT operation, there is a case where the ACT drive
 force cannot sufficiently be ensured immediately after the VVT operation
 due to a reduction of the hydraulic pressure accompanied by the VVT
 operation. If the VVT is operated in this state, there arises such a
 problem in that the response of the actual valve timing advancing amount
 with respect to a change in the target valve timing advancing amount is
 remarkably lowered, or that the valve timing advancing amount cannot be
 held to a desired valve timing advancing amount.
 Also, in the case where the valve timing cannot be set to a desired valve
 timing advancing amount, combustion in the internal combustion engine may
 become unstable or drivability and exhaust gas emission may be
 deteriorated.
 Further, there may be a case in which abnormal abrasion occurs in the VVT
 actuator because the VVT actuator heavily vibrates in the vicinity of the
 most spark retarding position.
 In addition, the VVT system having the hydraulic pressure detector 15
 suffers from such a problem that it cannot detect an abnormality (for
 example, leakage of the lubricating oil from the VVT actuator due to
 wearing or the like) at the downstream of the hydraulic oil detector 15
 although it can detect a hydraulic pressure abnormality at the upstream of
 the hydraulic pressure detector 15.
 SUMMARY OF THE INVENTION
 Accordingly, an object of the present invention is to provide a valve
 timing control device for use in an internal combustion engine which is
 capable of overcoming the above-described various problems with the prior
 art.
 In order to achieve the above object, according to the present invention,
 there is provided a valve timing control device for use in an internal
 combustion engine, comprising: a valve timing controller for variably
 controlling the open/close timing of intake and exhaust valves of the
 internal combustion engine; a drive force estimator provided in the valve
 timing controller for estimating a drive force of a VVT actuator on the
 basis of a hydraulic pressure supplied thereto; and a variable control
 limiter for limiting control made by the valve timing controller when the
 drive force of the VVT actuator, which is estimated by the drive force
 estimator, is equal to or less than a variable valve timing control limit
 boundary value.
 In a preferred form of the invention, the variable control limiter has, as
 the variable valve timing limit boundary value, the drive force of the VVT
 actuator that satisfies a given response time under the variable valve
 timing control.
 In another preferred form of the invention, the variable control limiter
 learns the variable valve timing control limit boundary value as an
 initial value.
 In a further preferred form of the invention, the variable control limiter
 updates the valve timing variable control boundary value on the basis of a
 behavior of the actual valve timing advancing amount responsive to a valve
 timing advancing command for variable valve timing control.
 In a still further preferred form of the invention, the variable control
 limiter determines an update amount of the variable valve timing control
 limit boundary value in accordance with a change in the actual valve
 timing advancing amount with respect to time.
 In a yet further preferred form of the invention, the valve timing control
 device further comprises an abnormality determiner for judging an
 abnormality of the VVT system when the variable valve timing control limit
 boundary value exceeds a given value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Now, a description will be given in more detail of preferred embodiments of
 the present invention with reference to the accompanying drawings.
 FIRST EMBODIMENT
 FIG. 1 is a block diagram showing the functional construction of an
 electronic control unit (ECU) 100 in accordance with a first embodiment of
 the present invention. FIG. 2 is a flowchart showing the control operation
 of the ECU 100 in accordance with the first embodiment. The hardware
 configuration of a valve timing control device for use in an internal
 combustion engine in accordance with the present invention is identical
 with the hardware configuration of the conventional example as already
 described and shown in FIG. 9, but the functional construction and
 operation of the electronic control unit (ECU) 100 are different from
 those of the conventional example.
 Referring to FIG. 1, the ECU 100 includes a valve timing controller 100A
 which detects the operating state of the internal combustion engine and
 calculates an optimum valve timing in that operating state to variably
 control the open/close timing of intake and exhaust valves, a drive force
 estimator 100B which is disposed in the valve timing controller 100A and
 estimates the drive force of a VVT actuator on the basis of a hydraulic
 pressure of an operating oil supplied thereto, and a variable control
 limiter 100C which limits control made by the valve timing controller 100A
 when the drive force of the VVT actuator estimated by the drive force
 estimator 100B is equal to or less than a variable valve timing control
 limit boundary value.
 As shown in FIG. 4 which will be described later, the variable control
 limiter 100C has the drive force of the VVT actuator (hereinafter referred
 to as "ACT drive force") which satisfies a given response time as the
 variable valve timing limit boundary value (hereinafter referred to as
 "VVT control limit boundary value") FL under the variable valve timing
 control, and learns the VVT control limit boundary value FL during the
 operation of the engine. The VVT control limit boundary value FL is
 updated on the basis of the behavior of the actual valve timing advancing
 amount (actual valve timing advancing amount) .theta.a responsive to the
 valve timing advancing instruction for the variable valve timing control,
 and its update amount is determined in accordance with a change in the
 actual valve timing advancing amount .theta.a with respect to time.
 It is assumed that the VVT control limit boundary value FL is predetermined
 as an initial value as shown in FIG. 4. In other words, F1 is set to the
 minimum value of the ACT drive force which satisfies a response time
 required when the VVT system is in the operating state, and FL is set to
 the ACT drive force produced when the VVT system is inoperable in this
 operating state. Also, the VVT actuator can be fixed at a given position
 by a stopper pin or the like when no hydraulic pressure is applied
 thereto, and the stopper pin is constructed such that it is released when
 the hydraulic pressure is applied thereto.
 Subsequently, the control operation of the ECU 100 will be described with
 reference to the flowchart shown in FIG. 2. First, if the ACT drive force
 estimated in the same manner as that in step S6 of FIG. 10 is within a VVT
 control limiting range in step S101, the VVT cannot be controlled to a
 desired valve timing advancing amount when the VVT is operated within that
 range. Therefore, the VVT is fixed to the most stabilized position without
 any hydraulic pressure, for example, the most spark retarding position in
 case of the intake valve (step S103). If the estimated ACT drive force is
 out of the control limiting range, processing is then advanced to step
 S102.
 If the ACT drive force is not in the vicinity of the VVT control limit
 boundary value FL {FL&lt;ACT drive force.ltoreq.FL +FLN, where FLN is an
 arbitrary given amount (refer to FIG. 4)} in step S102, processing is
 advanced to step S104 and the VVT is operated without updating the VVT
 control limit boundary value. If the ACT drive force is in the vicinity of
 the VVT control limit boundary value FL in step S102, processing is
 advanced to step S105 where the response time Tres is reset (Tres=0).
 Then, processing is advanced to step S106.
 In step S106, the VVT is operated (during this operation, Tres is added or
 accumulated).
 Subsequently, it is judged in step S107 whether or not the actual valve
 timing advancing amount .theta.a is converged to the target valve timing
 advancing amount .theta.T. Specifically, it is judged whether or not
 .vertline..theta.a-.theta.T.vertline.&lt;.epsilon. (.epsilon. is an arbitrary
 value indicative of a convergent judgement error) is satisfied. If this
 condition is satisfied, it is found that the actual valve timing advancing
 amount .theta.a is converged to the target valve timing advancing amount
 .theta.T at a speed higher than a given time Tpre judged in step S111
 which will be described later. Therefore, it is judged that the response
 is satisfactory, and processing is branched to step S109. On the other
 hand, if the above condition is not satisfied in step S107 (that is,
 .vertline..theta.a-.theta.T.vertline.&gt;.epsilon.), processing is advanced
 to step S108.
 It is judged in step S108 whether the actual valve timing advancing amount
 .theta.a always changes toward the target valve timing advancing amount.
 Specifically, it is judged whether differential of the target valve timing
 advancing amount by time always satisfies d.theta.a/dt.gtoreq.0. If
 d.theta.a/dt&lt;0 is judged, the target valve timing advancing amount is
 changed in a direction to enlarge the deviation in consideration of the
 condition in step S107, although the actual valve timing advancing amount
 .theta.a has a sufficient deviation with respect to the target valve
 timing advancing amount .theta.T. Therefore, it is judged that the actual
 valve timing advancing amount .theta.a is advanced in a direction opposite
 to a desired direction. Processing is then branched to step S110.
 If d.theta.a/dt.gtoreq.0 is satisfied in step S108, processing is advanced
 to step S111.
 In step S109, the update amount .DELTA.FL of the VVT control limit boundary
 value FL is obtained by DEC_MAP in FIG. 5 showing their relation
 (.DELTA.FL=DEC_MAP(Tres)). Specifically, because it can be judged that the
 response is more satisfactory as the response time is shorter, DEC_MAP is
 set such that the update amount .DELTA.FL (a negative value in this
 example) becomes larger (the absolute value becomes smaller) with an
 increase in the value of the response time Tres. The update amount
 .DELTA.FL corresponding to the response time Tres of DEC_MAP is determined
 through an experiment or the like in advance.
 In step S110, the given value FR obtained in the case where the VVT is
 advanced in an opposite direction within the given period of time Tpre is
 set to the update amount .DELTA.FL of the VVT control limit boundary value
 FL. That is, .DELTA.FL=FR is set.
 In step S111, it is judged whether the response time Tres after the VVT
 starts to operate elapses the given period of time Tpre. In this example,
 it is desirable that the given period of time Tpre is set to not greater
 than a response time required by the VVT system. If the response time Tres
 does not exceed the given period of time Tpre, processing is returned to
 step S106. If the response time Tres exceeds the given period of time
 Tpre, it is judged that the response time Tres at the ACT drive force
 cannot satisfy a required value, and processing is advanced to step S112.
 In step S112, it is judged whether the VVT does not at all respond to the
 ACT drive force for which the response time Tres exceeds the given period
 of time Tpre or slowly responds thereto during the given period of time
 Tpre. Specifically, it is judged whether the actual valve timing advancing
 amount .theta.a is equal to or less than a given threshold value .theta.NR
 (.theta.NR is an arbitrary value for judging no response and desirably set
 to a value close to 0), that is, whether .theta.a .ltoreq..theta.NR is
 satisfied. If this condition is satisfied, it is judged that the VVT did
 not respond within the given period of time Tpre at all, and processing is
 branched to step S113. If this condition is not satisfied, it is judged
 that the response is very slow, and processing is branched to step S114.
 In step S113, a given value FNR obtained in the case where the VVT does not
 respond within the given period of time Tpre is set as the update amount
 .DELTA.FL of the VVT control limit boundary value FL. That is,
 .DELTA.FL=FNR is set.
 In step S114, the update amount .DELTA.FL of the VVT control limit boundary
 value FL is obtained from INC_MA shown in FIG. 6. Specifically, because it
 can be judged that the response is lowered more as the amount of movement
 (actual valve timing advancing amount) .theta.a within the given period of
 time Tpre is smaller, INC_MAP is set such that the update amount .DELTA.FL
 (a positive value in this example) becomes smaller with an increase in the
 value of the amount of movement (actual valve timing advancing amount)
 .theta.a. The update amount .DELTA.FL with respect to the amount of
 movement (actual valve timing advancing amount) of INC_MAP is determined
 through an experiment or the like in advance.
 In step S115, the VVT is fixed to a position stabilized even without any
 hydraulic pressure. That is, the processing results immediately before
 step S115 are any one of the following cases.
 1) The VVT does not respond at all within the given period of time Tpre
 (step S113);
 2) The VVT may be advanced in a direction opposite to the desired direction
 (step S110); and
 3) The VVT cannot satisfy the required response time (it is advanced to
 some degree in the given period of time Tpre) (step S114).
 Therefore, it is judged that VVT cannot be stabilized in the ACT drive
 force in that operating state, and the VVT is fixed to a position
 stabilized even without any hydraulic pressure.
 In step S116, the VVT control limit boundary value is updated by use of the
 update amount .DELTA.FL. Specifically, it is updated by the following
 expression.
EQU FL=FL+.DELTA.FL
 The level of deterioration of the response is relieved in the stated order
 of the above items 1) (The VVT does not respond at all within the given
 period of time Tpre), 2) (The VVT may be advanced in a direction opposite
 to the desired direction), and 3) (The VVT cannot satisfy the required
 response time but is advanced to some degree in the given period of time
 Tpre). The values of .DELTA.FL set in step S110, step S113 and step S114,
 respectively, are preferably set as follows:
EQU INC_MAP (.theta.a).ltoreq.FR.ltoreq.FNR
 where INC_MAP (.theta.a) is a map value corresponding to the valve timing
 advancing amount .theta.a obtained by INC_MAP shown in FIG. 6.
 As described above, according to the first embodiment, even when the ACT
 drive force is remarkably lowered, that is, even when the ACT drive force
 becomes equal to or less than the VVT control limit boundary value, the
 VVT actuator is fixed to the most stabilized position by limiting control
 made by the valve timing controller 100A, thereby making it possible to
 prevent the deterioration of drivability and the exhaust gas emission, and
 also making it possible to prevent abnormal wearing of the VVT actuator
 due to vibration.
 Also, since the ACT drive force that satisfies the given response time is
 set as the VVT control limit boundary value under the variable valve
 timing control, the VVT is not operated so that the stabilized
 controllability is obtained even if the ACT drive force is decreased due
 to a lowered hydraulic pressure after the operation of the VVT, in a state
 where the ACT drive force does not reach the VVT control limit boundary
 value after the operation of the VVT.
 Further, since the VVT control limit boundary value is learned as an
 initial value by the variable control limiter 100C, the stabilized
 controllability is obtained without any influence of a variation in
 manufacture of system components and a change over time such as wearing.
 Still further, since the VVT control limit boundary value is updated on the
 basis of the behavior of the actual valve timing advancing amount
 responsive to the valve timing advancing command for the variable valve
 timing control, the stability of the operation of the actual VVT system
 can be reflected to the stabilizing control of the VVT system.
 Consequently, the operation of the VVT system can be stabilized.
 Moreover, since the update amount of the VVT control limit boundary value
 is determined by a change of the actual valve timing advancing amount with
 respect to time, the period of time until the current boundary value is
 converged to a real boundary value can be shortened.
 SECOND EMBODIMENT
 FIG. 7 is a block diagram showing the functional construction of an
 electronic control unit (ECU) 200 in accordance with a second embodiment
 of the present invention. FIG. 8 is a flowchart showing the control
 operation of the ECU 200 in accordance with the second embodiment. The
 hardware configuration in accordance with the second embodiment is
 identical with the hardware configuration of the above-described
 conventional example shown in FIG. 9, and only the functional structure
 and operation of the electronic control unit (ECU) 200 are different from
 those of the conventional example.
 As shown in FIG. 7, the ECU 200 according to the second embodiment includes
 abnormality determiner 100D for judging an abnormality of the valve timing
 control device 100A when the VVT control limit boundary value becomes
 larger than a given value, in addition to the construction of the ECU 100
 of the above-mentioned first embodiment.
 Subsequently, the control operation of the ECU 200 in accordance with the
 second embodiment will be described with reference to the flowchart shown
 in FIG. 8.
 First, in step S100, it is judged whether the abnormality judgement of the
 VVT system has already been conducted. If the abnormality judgement has
 been conducted, the VVT system is fixed to a stabilized position to
 complete processing in step S103. If the abnormality judgement has not
 been conducted, processing is advanced to step S101.
 Processing of from step SlOl to step S116 is identical with that in the
 above-mentioned first embodiment, and therefore its description will be
 omitted.
 In step S117, it is judged whether the VVT control limit boundary value FL
 exceeds a given value FLpre. If FL&lt;FLpre, the abnormality of the VVT
 system is judged, and processing is advanced to step S118. If not, it is
 judged that the VVT system is not abnormal, and processing is completed.
 That is, when the actual ACT drive force is lowered, the VVT control limit
 boundary value FL goes up. Therefore, when the VVT control limit boundary
 value FL exceeds the given value FLpre (FL&lt;FLpre), it can be judged that
 the actual ACT drive force is abnormally lowered.
 In step S118, it is judged that the stabilized control cannot be executed
 because of the abnormality of the VVT system, and the VVT is fixed to a
 position stabilized even without any hydraulic pressure, thereby
 completing processing.
 The second embodiment provides the following advantages in addition to the
 advantages obtained by the above first embodiment. That is, since the VVT
 system is judged as abnormal when the VVT control limit boundary value
 becomes larger than the given value, the abnormality at the VVT system
 downstream of the hydraulic pressure detector which cannot be detected by
 the hydraulic detector 15 can be detected.
 As described above, a valve timing control device for use in an internal
 combustion engine according to the present invention includes: a valve
 timing controller for variably controlling the open/close timing of intake
 and exhaust valves of the internal combustion engine; a drive force
 estimator provided in the valve timing controller for estimating a drive
 force of a VVT actuator on the basis of a hydraulic pressure supplied
 thereto; and a variable control limiter for limiting control made by the
 valve timing controller when the drive force of the VVT actuator, which is
 estimated by the drive force estimator, is equal to or less than a
 variable valve timing control limit boundary value. With the above
 construction, even when the ACT drive force is remarkably lowered (i.e.,
 the ACT drive force becomes equal to or less than the variable valve
 timing control limit boundary value), the VVT actuator can be fixed to the
 most stabilized position by limiting control made by the valve timing
 controller through the variable control limiter, thereby making it
 possible to stabilize the operating state of the internal combustion
 engine. Therefore, the deterioration in drivability and exhaust gas
 emission can be prevented, and also abnormal wearing of the VVT actuator
 due to vibration can be prevented.
 Also, the variable control limiter has, as the variable valve timing limit
 boundary value, a drive force of the VVT actuator that satisfies a given
 response time under the variable valve timing control. With the this
 construction, the VVT is not operated so that the stabilized
 controllability is obtained even if the drive force of the VVT actuator is
 decreased due to a lowered hydraulic pressure after the operation of the
 VVT, in a state where the drive force of the VVT actuator does not reach
 the VVT control limit boundary value after the operation of the VVT.
 Further, since the variable control limiter learns the variable valve
 timing control limit boundary value as an initial value, the stabilized
 controllability is obtained without any influence of a variation in
 manufacture of system components and a change over time such as wearing.
 Still further, since the variable control limiter updates the variable
 valve timing control boundary value on the basis of a behavior of the
 actual valve timing advancing amount responsive to a valve timing
 advancing command for variable valve timing control, the stability of the
 operation of the actual VVT system can be reflected on the stabilizing
 control of the VVT system. Consequently, the operation of the VVT system
 can be stabilized.
 Moreover, since the variable control limiter determines the update amount
 of the variable valve timing control limit boundary value in accordance
 with a change in the actual valve timing advancing amount with respect to
 time, the period of time until the current boundary value is converged to
 a real boundary value can be shortened.
 In addition, since the valve timing control device further includes an
 abnormality determiner for judging an abnormality of the VVT system when
 the variable valve timing control limit boundary value exceeds a given
 value, an abnormality in the VVT system downstream of the hydraulic
 pressure detector, which cannot be detected by the hydraulic detector, can
 be detected.
 The foregoing description of the preferred embodiments of the invention has
 been presented for purposes of illustration and description. It is not
 intended to be exhaustive or to limit the invention to the precise form
 disclosed, and modifications and variations are possible in light of the
 above teachings or may be acquired from practice of the invention. The
 embodiments were chosen and described in order to explain the principles
 of the invention and its practical application to enable one skilled in
 the art to utilize the invention in various embodiments and with various
 modifications as are suited to the particular use contemplated. It is
 intended that the scope of the invention be defined by the claims appended
 hereto, and their equivalents.