Abstract:
When any one of a plurality of variable valve characteristic mechanisms which vary different operating characteristics of an intake valve is failed, a limit which is capable of satisfying predetermined conditions for a change in opening timing of the intake valve at the time when the other operating characteristic is varied by the normally operating variable valve characteristic mechanism, is set and the operating characteristics of the intake valve are controlled while limiting the intake valve opening timing by the limit, by the normally operating variable valve characteristic mechanism.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a fail-safe control technique for when, in an internal combustion engine equipped with a plurality of variable valve characteristic mechanisms which vary different operating characteristics of an intake valve, one of the variable valve characteristic mechanisms is failed. 
   RELATED ART OF THE INVENTION 
   Japanese Unexamined Patent Publication No. 2001-65321 discloses a technique in which there are provided a first variable valve characteristic mechanism (variable valve timing mechanism) which varies valve timing (valve opening/closing timing) of an intake valve and a second variable valve characteristic mechanism (variable valve lift mechanism) which continuously varies a lift amount of the intake valve. 
   In this technique, in the case where one of the variable valve characteristic mechanisms is failed, operating characteristics of the intake valve are fail-safe controlled by the other normal variable valve characteristic mechanism within a range of noninterference with a piston, thereby enabling a fail-safe running while preventing the breakage of the engine due to the piston interference. 
   However, when the normal variable valve characteristic mechanism is fail-safe controlled within the range of noninterference with the piston, if the intake valve opening timing (IVO) is too advanced, an overlap amount of the intake valve with an exhaust valve becomes larger, and thus a remaining burned gas amount (so-called internal EGR amount) is increased. Therefore, it becomes impossible to ensure the combustion stability, resulting in the further deterioration of drivability. 
   On the other hand, if the intake valve opening timing is too retarded at the deceleration time, an engine oil amount, which is sucked into a cylinder to be consumed due to the rise of cylinder negative pressure, is increased, and there occurs an engine oil loss, causing damage to the engine. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to make it possible that, when one of a plurality of variable valve characteristic mechanisms which vary different operating characteristics of an intake valve, is failed, the other normal variable valve characteristic mechanism is appropriately controlled, to suppress the deterioration of drivability and an oil loss to a minimum while preventing the piston interference. 
   In order to accomplish the above object, the present invention is constructed so that, in an internal combustion engine equipped with a plurality of variable valve characteristic mechanisms which vary different operating characteristics of an intake valve, it is detected whether or not the plurality of variable valve characteristic mechanisms is failed, and when it is detected that the variable valve characteristic mechanism which varies one of the operating characteristics is failed, a limit which is capable of satisfying predetermined conditions for a change in opening timing of the intake valve at the time when the other operating characteristic is varied by the normally operating variable valve characteristic mechanism, is set, and the operating characteristics of the intake valve are controlled while limiting the opening timing of the intake valve by the limit, by the normally operating variable valve characteristic mechanism. 
   The other objects and features of the invention will become understood from the following description with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a system structure of a fail-safe control apparatus for an internal combustion engine equipped with variable valve characteristic mechanisms in an embodiment of the present invention. 
       FIG. 2  is a cross section view showing a variable valve characteristic mechanism in the embodiment (A—A cross section view in  FIG. 3 ). 
       FIG. 3  is a side elevation view of the variable valve characteristic mechanism. 
       FIG. 4  is a top plan view of the variable valve characteristic mechanism. 
       FIG. 5  is a perspective view showing an eccentric cam for use in the variable valve characteristic mechanism. 
       FIG. 6  is a cross section view showing an operation of the variable valve characteristic mechanism at a low lift condition (B—B cross section view of  FIG. 3 ). 
       FIG. 7  is a cross section view showing an operation of the variable valve characteristic mechanism at a high lift condition (B—B cross section view of  FIG. 3 ). 
       FIG. 8  is a valve lift characteristic diagram corresponding to a base end face and a cam surface of a swing cam in the variable valve characteristic mechanism. 
       FIG. 9  is a characteristic diagram showing valve timing and a valve lift of the variable valve characteristic mechanism. 
       FIG. 10  is a perspective view showing a rotational driving mechanism of a control shaft in the variable valve characteristic mechanism. 
       FIG. 11  is a block diagram showing the intake valve control in the embodiment. 
       FIGS. 12(A)  and (B) are diagrams showing the intake valve control by a variable valve lift mechanism at the time when a variable valve timing mechanism is failed, in the embodiment. 
       FIGS. 13(A)  and (B) are diagrams showing the intake valve control by the variable valve timing mechanism at the time when the variable valve lift mechanism is failed, in the embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In an intake pipe  102  of an internal combustion engine  101 , an electronically controlled throttle  104  is disposed for driving a throttle valve  103   b  to open and close by a throttle motor  103   a , and air is sucked into a combustion chamber  106  via electronically controlled throttle  104  and an intake valve  105 . 
   A combusted exhaust gas discharged from combustion chamber  106  via an exhaust valve  107  is purified by a front catalyst  108  and a rear catalyst  109 , and then emitted into the atmosphere. 
   Exhaust valve  107  is driven by a cam  111  axially supported by an exhaust side camshaft  110 , to open and close at a fixed valve lift amount and valve operating angle (crank angle of from opening to closing). A valve lift amount and an operating angle of intake valve  105  are varied continuously by a variable valve lift mechanism  112 . Note, the valve lift amount and the operating angle are varied simultaneously so that, when a characteristic of one of the valve lift amount and the operating angle is determined, a characteristic of the other is also determined. 
   On an intake side, a variable valve timing mechanism  201  constituted by a mechanism which continuously and variably controls a rotation phase difference between a crankshaft and an intake side camshaft, to advance or retard valve timing (valve opening/closing timing) of intake valve  105 , and an intake side cam angle sensor  202  for detecting a rotation position of the intake side camshaft, are disposed on both end portions of the intake side camshaft. As described above, variable valve lift mechanism  112  and variable valve timing mechanism  201  are provided as a plurality of variable valve characteristic mechanisms, which respectively varies the lift amount (operating angle) and the valve timing, being different operating characteristics of intake valve  105 . 
   A control unit  114  incorporating therein a microcomputer controls electronically controlled throttle  104 , variable valve lift mechanism  112  and variable valve timing mechanism  201 , according to an accelerator pedal opening detected by an accelerator opening sensor APS  116  and the like, so that a target intake air amount corresponding to an accelerator opening ACC can be obtained based on an opening of throttle valve  103   b  and an opening characteristic of intake valve  105 . 
   Control unit  114  receives various detection signals from an airflow meter  115  detecting an intake air amount Q of engine  101 , a crank angle sensor  117  taking a rotation signal out of the crankshaft, a throttle sensor  118  detecting an opening TVO of throttle valve  103   b , a water temperature sensor  119  detecting a cooling water temperature Tw of engine  101  and the like, in addition to accelerator opening sensor APS  116 , a rotation angle sensor  127  (to be described later) and intake side cam angle sensor  202 . 
   Further, an electromagnetic fuel injection valve  131  is disposed on an intake port  130  at the upstream side of intake valve  105  of each cylinder. Fuel injection valve  131  injects fuel adjusted at a predetermined pressure toward intake valve  105 , when driven to open by an injection pulse signal from control unit  114 . 
     FIG. 2  to  FIG. 4  show in detail the structure of variable valve lift mechanism  112 . 
   Variable valve lift mechanism  112  shown in  FIG. 2  to  FIG. 4  includes a pair of intake valves  105 ,  105 , a hollow camshaft (drive shaft)  13  rotatably supported by a cam bearing  14  of a cylinder head  11 , two eccentric cams (drive cams)  15 ,  15  axially supported by camshaft  13 , a control shaft  16  rotatably supported by cam bearing  14  and arranged in parallel at an upper position of camshaft  13 , a pair of rocker arms  18 ,  18  swingingly supported by control shaft  16  through a control cam  17 , and a pair of independent swing cams  20 ,  20  disposed to upper end portions of intake valves  105 ,  105  through valve lifters  19 , 19 , respectively. 
   Eccentric cams  15 , 15  are connected with rocker arms  18 , 18  by link arms  25 ,  25 , respectively. Rocker arms  18 , 18  are connected with swing cams  20 ,  20  by link members  26 ,  26 . 
   Rocker arms  18 ,  18 , link arms  25 ,  25 , and link members  26 ,  26  constitute a transmission mechanism. 
   Each eccentric cam  15 , as shown in  FIG. 5 , is formed in a substantially ring shape and includes a cam body  15   a  of small diameter, a flange portion  15   b  integrally formed on an outer surface of cam body  15   a . An insertion hole  15   c  is formed through the interior of eccentric cam  15  in an axial direction, and also a center axis X of cam body  15   a  is biased from a center axis Y of camshaft  13  by a predetermined amount. 
   Eccentric cams  15 ,  15  are pressed and fixed to camshaft  13  via camshaft insertion holes  15   c  at outsides of valve lifters  19 ,  19 , respectively, so as not to interfere with valve lifters  19 ,  19 . Also, outer peripheral surfaces  15   d ,  15   d  of cam body  15   a  are formed in the same cam profile. 
   Each rocker arm  18 , as shown in  FIG. 4 , is bent and formed in a substantially crank shape, and a central base portion  18   a  thereof is rotatably supported by control cam  17 . 
   A pin hole  18   d  is formed through one end portion  18   b  which is formed to protrude from an outer end portion of base portion  18   a . A pin  21  to be connected with a tip portion of link arm  25  is pressed into pin hole  18   d . A pin hole  18   e  is formed through the other end portion  18   c  which is formed to protrude from an inner end portion of base portion  18   a . A pin  28  to be connected with one end portion  26   a  (to be described later) of each link member  26  is pressed into pin hole  18   e.    
   Control cam  17  is formed in a cylindrical shape and fixed to a periphery of control shaft  16 . As shown in  FIG. 2 , a center axis P 1  position of control cam  17  is biased from a center axis P 2  position of control shaft  16  by α. 
   Swing cam  20  is formed in a substantially lateral U-shape as shown in  FIG. 2 ,  FIG. 6  and  FIG. 7 , and a supporting hole  22   a  is formed through a substantially ring-shaped base end portion  22 . Camshaft  13  is inserted into supporting hole  22   a  to be rotatably supported. Also, a pin hole  23   a  is formed through an end portion  23  positioned at the other end portion  18   c  of rocker arm  18 . 
   A base circular surface  24   a  of base end portion  22  side and a cam surface  24   b  extending in an arc shape from base circular surface  24   a  to an edge of end portion  23 , are formed on a bottom surface of swing cam  20 . Base circular surface  24   a  and cam surface  24   b  are in contact with a predetermined position of an upper surface of each valve lifter  19  corresponding to a swing position of swing cam  20 . 
   Namely, according to a valve lift characteristic shown in  FIG. 8 , as shown in  FIG. 2 , a predetermined angle range θ1 of base circular surface  24   a  is a base circle interval and a range of from base circle interval θ1 of cam surface  24   b  to a predetermined angle range θ2 is a so-called ramp interval, and a range of from ramp interval θ2 of cam surface  24   b  to a predetermined angle range θ 3  is a lift interval. 
   Link arm  25  includes a ring-shaped base portion  25   a  and a protrusion end  25   b  protrudingly formed on a predetermined position of an outer surface of base portion  25   a . A fitting hole  25   c  to be rotatably fitted with the outer surface of cam body  15   a  of eccentric cam  15  is formed on a central position of base portion  25   a . Also, a pin hole  25   d  into which pin  21  is rotatably inserted is formed through protrusion end  25   b.    
   Link member  26  is formed in a linear shape of predetermined length and pin insertion holes  26   c ,  26   d  are formed through both circular end portions  26   a ,  26   b . End portions of pins  28 ,  29  pressed into pin hole  18   d  of the other end portion  18   c  of rocker arm  18  and pin hole  23   a  of end portion  23  of swing cam  20 , respectively, are rotatably inserted into pin insertion holes  26   c ,  26   d.    
   Snap rings  30 ,  31 ,  32  restricting axial transfer of link arm  25  and link member  26  are disposed on respective end portions of pins  21 ,  28 ,  29 . 
   In such a constitution, depending on a positional relation between the center axis P 2  of control shaft  16  and the center axis P 1  of control cam  17 , as shown in  FIG. 6  and  FIG. 7 , the valve lift amount is varied, and by driving control shaft  16  to rotate, the position of the center axis P 2  of control shaft  16  relative to the center axis P 1  of control cam  17  is changed. 
   Control shaft  16  is driven to rotate within a predetermined rotation angle range by a DC servo motor (actuator)  121  as shown in  FIG. 10 . By varying a rotation angle of control shaft  16  by actuator  121 , the valve lift amount and valve operating angle of each of intake valves  105 ,  105  are continuously varied (refer to  FIG. 9 ). 
   In  FIG. 10 , DC servo motor  121  is arranged so that the rotation shaft thereof is parallel to control shaft  16 , and a bevel gear  122  is axially supported by the tip portion of the rotation shaft. 
   On the other hand, a pair of stays  123   a ,  123   b  are fixed to the tip end of control shaft  16 . A nut  124  is swingingly supported around an axis parallel to control shaft  16  connecting the tip portions of the pair of stays  123   a ,  123   b.    
   A bevel gear  126  meshed with bevel gear  122  is axially supported at the tip end of a threaded rod  125  engaged with nut  124 . Threaded rod  125  is rotated by the rotation of DC servo motor  121 , and the position of nut  124  engaged with threaded rod  125  is displaced in an axial direction of threaded rod  125 , so that control shaft  16  is rotated. 
   Here, the valve lift amount is decreased as the position of nut  124  approaches bevel gear  126 , while the valve lift amount is increased as the position of nut  124  gets away from bevel gear  126 . 
   Further, potentiometer type rotation angle sensor  127  detecting the rotation angle of control shaft  16  is disposed on the tip end of control shaft  16 , as shown in  FIG. 10 . Control unit  114  feedback controls DC servo motor  121  so that an actual rotation angle detected by rotation angle sensor  127  coincides with a target rotation angle. Here, since the lift amount and the operating angle are varied simultaneously by a rotation angle control of control shaft  16 , rotation angle sensor  127  detects the operating angle and at the same time detects the lift amount. 
   The operating characteristics of intake valve  105  are varied by such variable valve characteristic mechanisms, to control an intake amount. In the present invention, when one of variable valve lift mechanism  112  and variable valve timing mechanism  201  being two types of variable valve characteristic mechanisms, is failed, intake valve  105  is fail-safe controlled by the other normal variable valve characteristic mechanism, thereby preventing the interference between intake valve  105  and a piston, the deterioration of combustion stability due to an increase of remaining burned gas amount, and an engine oil loss due to an increase of cylinder negative pressure, to enable the fail-safe running while preventing an engine damage. 
   A control of intake valve  105  by control unit  114  will be described in accordance with a block diagram of  FIG. 11 . 
   In block B 1 , basic controlled variable (rotation angle of control shaft  16 ) TGVEL 0  of variable valve lift mechanism  112  corresponding to a target operating angle of intake valve  105 , at which a target torque can be obtained, is set based on the accelerator opening ACC detected by accelerator opening sensor  116  and an engine rotation speed Ne detected by crank angle sensor  117 , to be output to block B 2 . 
   In block B 3 , basic controlled variable TGVTC 0  of variable valve timing mechanism  201  corresponding to target valve timing of intake valve  105 , at which the target torque can be obtained, is set based on the accelerator opening ACC and the engine rotation speed Ne, to be output to block B 4 . 
   In block B 5 , it is diagnosed whether or not variable valve timing mechanism  201  is failed (fixed), and a diagnosis result (VTC failure judgment flag) is output to block B 2 . 
   In block B 2 , if it is judged in block B 5  that variable valve timing mechanism  201  is normal, the basic controlled variable TGVEL 0  of variable valve lift mechanism  112  set in block B 1  is output just as it is, as target controlled variable TGVEL. If it is judged that variable valve timing mechanism  201  is failed, fail-safe controlled variable FSVEL of variable valve lift mechanism  112  set in each block (to be described below) is output as the target controlled variable TGVEL. 
   In block B 6 , it is diagnosed whether or not variable valve lift mechanism  112  is failed (fixed), and a diagnosis result (VEL failure judgment flag) is output to block B 4 . 
   In block B 4 , if it is judged in block B 6  that variable valve lift mechanism  112  is normal, the target valve timing TGVTC 0  of variable valve timing mechanism  201  set in block B 3  is output just as it is, as target valve timing TGVTC. If it is judged that variable valve lift mechanism  112  is failed, fail-safe controlled variable FSVTC of variable valve timing mechanism  201  set in each block (to be described below) is output as the target controlled variable TGVTC. 
   The description will be made on blocks for executing a fail-safe control by the normal variable valve characteristic mechanism at the time when one of the variable valve characteristic mechanisms is failed. 
   Blocks B 7  to B 13  are for setting opening timing IVO of intake valve  105 , which is set commonly for the cases where either of variable valve characteristic mechanisms are failed. 
   In block B 7 , an advance limit IVOLadv is set as a limit value in advancing of the valve opening timing IVO, at which the remaining burned gas amount due to overlap of intake valve  105  with exhaust valve  107  in a normal state (non-deceleration time) can be maintained at a reference value or less, to be output to block B 13 . To be specific, the advance limit IVOLadv is set to be in the vicinity of intake top dead center (for example, if a crank angle position of intake top dead center=360 °, 380° slightly retarded side of 360°). The advance limit IVOLadv is set to be on the retarded side of an advance limit at which the interference between intake valve  105  and the piston can be prevented. 
   In block B 8 , a retard limit IVOLrtd is set as a limit value in retarding the valve opening timing IVO, at which the cylinder negative pressure can be maintained at a predetermined value or less at the deceleration time, to be output to block B 13 . Here, the retard limit IVOLrtd at the deceleration time can be set in advance based on an experiment, as valve opening timing, at which the cylinder negative pressure does not reach the predetermined value or above (for example, an absolute pressure, −650 mmHg or less). The retard limit IVOLrtd is set to be on the retarded side of the advance limit at which the interference between intake valve  105  and the piston can be prevented. Note, if the operating angle and the lift amount cannot be increased since the center of actual operating angle at the failed time is positioned on the advance side, the cylinder negative pressure is likely to be increased compared with the case where the lift amount is large, and therefore, the retard limit IVOLrtd may be set to be on the further retarded side. 
   In block B 10 , a difference between a present value of the accelerator opening APS detected by accelerator opening sensor  116  and a previous value output from block B 11 , is calculated. In block B 12 , it is judged whether or not it is the deceleration time, based on whether the difference is positive or negative (if the present value−previous value&lt;0, it is judged that it is the deceleration time). 
   In block B 13 , based on a judgment result in block B 10 , the advance limit IVOLadv set at the intake top dead center is output at the non-deceleration time, while the retard limit IVOLrtd set at the angle where the cylinder negative pressure does not reach the predetermined value or above is output at the deceleration time. Namely, the cylinder negative pressure is unlikely to be increased at the non-deceleration time, compared with the deceleration time, and the retard limit at which the cylinder negative pressure reaches the predetermined value or above, is positioned on the retarded side of the advance limit IVOLadv at which the remaining burned gas is limited. Therefore, the advance limit IVOLadv is selected as fail-safe controlled variable at the valve opening timing, so as to ensure the power by advancing the valve opening timing to the advance limit IVOLadv. On the other hand, at the deceleration time in the low power or no-power (fuel cut-off) state, the remaining burned gas amount is not a problem but the engine oil loss is a problem. Therefore, the retard limit IVOLrtd is selected so that the valve opening timing is advanced to the retard limit IVOLrtd at the deceleration time. 
   Then, at the time when one of the variable valve characteristic mechanisms is failed. The opening timing IVO of intake valve  105  is controlled to the advance limit IVOLadv or the retard limit IVOLrtd set in the above manner by the other normal variable valve characteristic mechanism. 
   Firstly, the description will be made on the blocks for setting the fail-safe controlled variable of variable lift mechanism  112  at the time when variable valve timing mechanism  201  is failed. 
   In block B 14 , advance controlled variable VTCNOW output from block B 15  is subtracted from an operating angle center (crank angle position which is intermediate between the opening timing and closing timing, for example 470° in the case of intake top dead center=360°) VTC 0  of intake valve  105 , which is most retarded in a state where the operation of variable valve timing mechanism  201  stops, output from block B 15 , to calculate an actual operating angle center (crank angle position) NGVTC of intake valve  105  in variable valve timing mechanism  201 , which is fixed due to the failure. 
   In block B 16 , the crank angle position equivalent to the advance limit IVOLadv or the retard limit IVOLrtd from block B 13  is subtracted from the actual operating angle center NGVTC at the time when variable valve timing mechanism  201  is failed. 
   The crank angle calculated in block B 16  is equivalent to a half the operating angle of intake valve  105 . Therefore, in block B 17 , the calculated crank angle is multiplied by a gain of two times to calculate the operating angle, and thereafter, in block B 18 , the operating angle is converted into the controlled variable (rotation angle of control shaft  16 ) of variable valve lift mechanism  112  to be output to block B 2  as the fail-safe controlled variable FSVEL. 
   At the time when variable valve timing mechanism  201  is failed, since the fail-safe controlled variable FSVEL is output to variable valve lift mechanism  112  as the target controlled variable TGVEL from block B 2  as described in the above, the operating angle and the lift amount are controlled so that the opening timing IVO of intake valve  105  is made to be the advance limit IVOLadv in the vicinity of the top dead center in the normal state, while at the deceleration time, to be the retard limit IVOLrtd at which the cylinder negative pressure does not reach the predetermined value or above. 
     FIG. 12  shows the fail-safe control by variable valve lift mechanism  112  at the time when variable valve timing mechanism  201  is failed. 
   For example as shown in (A), in the case where the operating angle center NGVTC at the time when variable valve timing mechanism  201  is failed is positioned on either “a” or “b”, the advance limit of valve opening timing IVO in the range where the intake valve does not interfere with the piston is on the advanced side of the intake top dead center ITDC. However, if the valve opening timing is advanced to the advance limit, the remaining burned gas amount due to the overlap of the intake valve with the exhaust valve is increased to deteriorate the combustion stability, so that a fail-safe operation becomes substantially difficult. Therefore, by controlling the operating angle of intake valve  105  by variable valve lift mechanism  112  so that the valve opening timing IVO is limitedly advanced to the advance limit IVOLadv, the remaining burned gas amount can be maintained at the reference value or less while preventing the interference of the intake valve with the piston, to ensure the combustion stability satisfactorily, thereby enabling the fail-safe operation. Note, by advancing the valve opening timing to the advance limit IVOLadv, the lift amount can be increased as large as possible to ensure the power. 
   On the other hand, as shown in (B), the target lift amount of intake valve  105  is set to be rather small at the deceleration time. However, if the operating angle center NGVTC at the time when variable valve timing mechanism  201  is failed is on the retarded side, the valve opening timing IVO is too retarded to increase the cylinder negative pressure, thereby leading a possibility of the engine oil loss. Therefore, by forcibly advancing the valve opening timing IVO to the retard limit IVOLrtd by variable valve lift mechanism  112 , the cylinder negative pressure can be held at the predetermined value or less while preventing the interference of the intake valve with the piston, to reliably prevent the engine oil loss, thereby enabling the fail-safe operation. 
   Next, the description will be made on the blocks for setting the fail-safe controlled variable of variable valve timing mechanism  201  at the time when variable valve lift mechanism  112  is failed. 
   In block B 19 , the actual rotation angle REVEL of control shaft  16  detected by rotation angle sensor  127  is converted into the actual operating angle REEVENT of intake valve  105 . 
   In block B 20 , the actual operating angle REEVENT is multiplied with a half the gain, to calculate the crank angle of from the actual operating angle center to the actual valve opening timing. 
   In block B 21 , the calculated value is added with the valve opening timing IVO (advance limit IVOLadv or retard limit IVOLrtd) from block B 13 , to calculate a target value (crank angle position) of the operating angle center corresponding to the target valve opening timing. 
   In block B 22 , the operating angle center VTC 0  in the state where the operation of variable valve timing mechanism  201  stops from block B 15  is subtracted from the target value of the operating angle center, to calculate controlled variable of variable valve timing mechanism  201  as a negative value (advance amount), and this controlled variable is output to block B 4  as the fail-safe controlled variable FSVTC. 
   At the time when variable valve lift mechanism  112  is failed, as described in the above, the fail-safe controlled variable FSVTC is output from block B 4  to variable valve timing mechanism  201  as the target controlled variable TGVTC. Therefore, the valve timing is controlled, so that the opening timing IVO of intake valve  105  is made to be the advance limit IVOLadv in the vicinity of the top dead center in the normal state (non-deceleration time), while at the deceleration time, to be the retard limit IVOLrtd at which the cylinder negative pressure does not reach the predetermined value or above. 
     FIG. 13  shows the fail-safe control by variable valve timing mechanism  201  at the time when variable valve lift mechanism  112  is failed. 
   For example, as shown in (A), in the case where the operating angle at the time when variable valve timing mechanism  201  is failed is positioned on either “a” or “b”, the advance limit of valve opening timing IVO in the range where the intake valve does not interfere with the piston is on the advanced side of the intake top dead center ITDC. However, if the valve opening timing is advanced to the advance limit, the remaining burned gas amount due to the overlap of the intake valve with the exhaust valve is increased to deteriorate the combustion stability, so that the fail-safe operation becomes substantially difficult. Therefore, by controlling the valve timing of intake valve  105  by variable valve timing mechanism  201  so that the valve opening timing IVO is limitedly advanced to the advance limit IVOLadv, the remaining burned gas amount can be maintained at the reference value or less while preventing the interference of the intake valve with the piston, to ensure the combustion stability satisfactorily, thereby enabling the fail-safe operation. 
   On the other hand, as shown in (B), at the deceleration time, if the operating angle center NGVTC at the time when variable valve timing mechanism  201  is failed is on the retarded side, the valve opening timing IVO is too retarded to increase the cylinder negative pressure, thereby leading a possibility of the engine oil loss. Therefore, by controlling the valve timing of intake valve  105  by variable valve timing mechanism  201  so as to forcibly advance the valve opening timing IVO to the retard limit IVOLrtd, the cylinder negative pressure can be held at the predetermined value or less while preventing the interference of the intake valve with the piston, to reliably prevent the engine oil loss, thereby enabling the fail-safe operation. 
   The entire contents of Japanese Patent Application No. 2003-181091 filed on Jun. 25, 2003, a priority of which is claimed, are incorporated herein by reference. 
   While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. 
   Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for the purpose of limiting the invention as defined in the appended claims and their equivalents.