Patent Publication Number: US-10787938-B2

Title: Engine with variable valve timing mechanism

Description:
TECHNICAL FIELD 
     The present invention relates to an engine with a variable valve timing mechanism. 
     BACKGROUND ART 
     A known variable valve timing mechanism (hereinafter referred to as a “VVT”) of an engine is a hydraulic VVT described in Patent Document 1. This VVT includes advance chambers and retard chambers defined by a housing that rotates in cooperation with a crank shaft of the engine and a vane body that rotates together with the cam shaft. When an oil pressure is supplied to the advance chambers, a phase angle of the cam shaft with respect to the crank shaft, that is, a valve timing, changes in an advancing direction, whereas when an oil pressure is supplied to the retard chambers, the valve timing changes in a retarding direction. In the engine described in Patent Document 1, hydraulic VVTs are disposed in both an intake side and an exhaust side. 
     To change the phase angle of the cam shaft in the advancing direction, it is necessary to rotate the cam shaft against a biasing force of a valve spring. Thus, in the hydraulic VVTs, the number of advance chambers is generally larger than the number of retard chambers. 
     CITATION LIST 
     Patent Document 
     PATENT DOCUMENT 1: Japanese Patent Application Publication No. 2015-194132 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     It is known that in a low-load to intermediate-load operating range of an engine, when a valve overlap amount in which an open period of an intake valve and an open period of an exhaust valve overlap each other is increased, a pumping loss decreases, and fuel efficiency of the engine is enhanced. 
     On the other hand, in view of enhancement of fuel efficiency of the engine, a discharge oil pressure of an oil pump driven by the engine is set as low as possible. In this case, an oil pressure usable by the VVT is restricted to a low range, and thus, the operating speed of the VVT is also restricted depending on the level of the usable oil pressure. Alternatively, in the case of including a hydraulic valve stop mechanism that performs a reduced-cylinder operation of an engine by stopping intake valves and/or exhaust valves of some cylinders of the engine under an oil pressure by the oil pump, the operating speed of the VVT is restricted in the reduced-cylinder operation in such a manner that an oil pressure supplied from the oil pump to the valve stop mechanism does not decrease below an oil pressure necessary for maintaining the valve stop state. 
     In view of this, in a case where the operating state with a small valve overlap amount transitions to an operating state with a large valve overlap amount by retarding the valve timings of the intake valve and the exhaust valve with an increase in an engine load, for example, it is difficult to increase the valve overlap amount in this transition period. That is, since restriction of the operating speed makes it difficult to increase the retarding speed in the exhaust side relative to the retarding speed in the intake side, the valve timings of the intake side and the exhaust side are retarded with a small valve overlap amount. Thus, in the transition period, a pumping loss does not decrease, and a fuel efficiency deteriorates. In addition, restriction of the operating speed requires a time for changing the valve timings, and thus, a pumping loss further deteriorates fuel efficiency. 
     It is therefore an object of the present invention to reduce a pumping loss in a transition period in which a valve overlap amount is changed by advancement or retardation of a valve timing under restriction of an oil pressure usable by a VVT. 
     Solution to the Problem 
     According to the present invention, advance chambers and retard chambers of an intake-side VVT and an exhaust-side VVT are configured such that a pumping loss in the transition period decreases. 
     A VVT-equipped engine disclosed here includes: an intake VVT serving as a variable valve timing mechanism that changes a phase angle of an intake cam shaft with respect to a crank shaft and; an exhaust VVT serving as a variable valve timing mechanism that changes a phase angle of an exhaust cam shaft with respect to the crank shaft, wherein each of the intake VVT and the exhaust VVT is a hydraulic VVT including advance chambers for changing the phase angle in an advancing direction by supply of an oil pressure and retard chambers for changing the phase angle in a retarding direction by supply of an oil pressure, each of the advance chambers and the retard chambers is defined by a housing configured to rotate in cooperation with the crank shaft and a vane body configured to rotate together with the cam shaft, and the number of the advance chambers is larger than or equal to the number of the retard chambers in the intake VVT and the number of the retard chambers is larger than or equal to the number of the advance chambers in the exhaust VVT or the number of the advance chambers is larger than the number of the retard chambers in the intake VVT and the number of the retard chambers is larger than or equal to the number of the advance chambers in the exhaust VVT. 
     The intake cam shaft and the exhaust cam shaft lift the intake valve and the exhaust valve by cams against biasing forces of valve springs by rotating in the advancing direction. Thus, the biasing forces of the valve springs are exerted on the cam shafts in the retarding direction. Thus, a driving force necessary for rotating the cam shafts in the retarding direction is smaller than that in the advancing direction. That is, as long as oil pressures applied to the vane bodies of the VVTs are the same, the retarding speed is higher than the advancing speed. 
     The configuration of the VVT-equipped engine “the number of the advance chambers is larger than or equal to the number of the retard chambers in the intake VVT and the number of the retard chambers is larger than the number of the advance chambers in the exhaust VVT” means that the advancing speed is not retarded in the intake side and the retarding speed is further increased in the exhaust side. 
     In this case, regarding the advancing speed, since the number of the advance chambers is larger than or equal to the number of the retard chambers in the intake VVT and the number of the advance chambers is smaller than the number of the retard chambers in the exhaust VVT, the advancing speed in the intake side can be made higher than the advancing speed in the exhaust side. Accordingly, in a transition period in which the opening and closing timings (valve timings) of the intake valve and the exhaust valve are advanced to shift the valve overlap amount from a large state to a small state, the advancing speed in the intake side is made higher than the advancing speed in the exhaust side so that the state with a large valve overlap amount can be continued for a while. Consequently, an increase in a pumping loss is suppressed so that fuel efficiency can be enhanced. 
     On the other hand, regarding the retarding speed, in the exhaust VVT, the exhaust cam shaft is biased to rotate in the retarding direction by the valve spring, and in addition, the number of the retard chambers is larger than the number of the advance chambers. Thus, the retarding speed can be further increased. Accordingly, in a transition period in which the valve timings of the intake valve and the exhaust valve are retarded to shift the valve overlap amount from a small state to a large state, the retarding speed in the exhaust side is made higher than the retarding speed in the intake side so that the valve overlap amount can be quickly increased. As a result, a pumping loss can be reduced so that fuel efficiency can be enhanced. 
     Next, a case where “the number of the advance chambers is larger than the number of the retard chambers in the intake VVT and the number of the retard chambers is larger than or equal to the number of the advance chambers in the exhaust VVT” in the VVT-equipped engine will be described. 
     Regarding the advancing speed, in this case, since the number of the advance chambers is larger than the number of the retard chambers in intake VVT and the number of the advance chambers is larger than or equal to the number of the retard chambers in the exhaust VVT, the advancing speed in the intake side can be made higher than the advancing speed in the exhaust side, in a manner similar to the former case. Accordingly, in a transition period in which the opening and closing timings of the intake valve and the exhaust valve are advanced to shift the valve overlap amount from a large state to a small state, the state with a large valve overlap amount can be continued for a while, and thereby, an increase in a pumping loss can be suppressed so that fuel efficiency can be enhanced. 
     Regarding the retarding speed, this case includes a case where the number of the retard chambers is equal to the number of the advance chambers in the exhaust VVT. In this case, however, since a biasing force of the valve spring is exerted on the exhaust cam shaft in the retarding direction as described above, in a transition period in which the opening and closing timings of the intake valve and the exhaust valve are retarded to shift the valve overlap amount from a small state to a large state, the retarding speed in the exhaust side can be made higher than the retarding speed in the intake side so that the valve overlap amount can be increased quickly. As a result, a pumping loss can be reduced so that fuel efficiency can be enhanced. 
     In one aspect, the engine may include a transfer unit that drives the housing of the intake VVT and the housing of the exhaust VVT such that the housing of the intake VVT and the housing of the exhaust VVT rotate in opposite direction by the crank shaft, wherein the number of the advance chambers in the intake VVT may be equal to the number of the retard chambers in the exhaust VVT, and the number of the retard chambers in the intake VVT may be equal to the number of the advance chambers in the exhaust VVT. 
     The expression in which the housing of the intake VVT and the housing of the exhaust VVT rotate in opposite directions means the following configuration. In a configuration employing, as an intake VVT, a hydraulic VVT including a first operating chamber for pivoting a vane body in one direction and a second operating chamber for pivoting the vane body in the other direction, the first operating chamber serves as an advance chamber and the second operating chamber serves as a retard chamber, whereas in a configuration employing the hydraulic VVT as an exhaust VVT, the first operating chamber serves as a retard chamber and the second operating chamber serves as an advance chamber, in a manner opposite to the case of the intake VVT. 
     In view of this, in this embodiment, the housing of the intake VVT and the housing of the exhaust VVT are rotated in opposite directions under conditions where the number of the advance chambers in the intake VVT is equal to the number of the retard chambers in the exhaust VVT, and the number of the retard chambers in the intake VVT is equal to the number of the advance chambers in the exhaust VVT. Thus, the hydraulic VVT with the same configuration can be employed for both of the intake VVT and the exhaust VVT. Accordingly, it is unnecessary to provide a hydraulic VVT for each of the intake VVT and the exhaust VVT, which is advantageous in reducing manufacturing costs. 
     In one aspect, the engine may include a high-pressure fuel pump that serves as an auxiliary machine of the engine and supplies fuel to a combustion chamber of the engine, the number of the advance chambers may be larger than the number of the retard chambers in the intake VVT, and the intake cam shaft may include a cam portion that drives the fuel pump. 
     In a case where cam driving of the fuel pump is performed by using the cam shaft, the cam shaft is under a heavy rotation load in the advancing direction. On the other hand, in the intake VVT, since the number of advance chambers is larger than the number of retard chambers, the rotation load on the intake cam shaft in the advancing direction has a margin, as compared to the exhaust VVT. In view of this, in this embodiment, cam driving of the fuel pump is performed by using the intake cam shaft. Accordingly, the fuel pump can be easily operated with stability without hindering a change of opening and closing of the intake valve and the timings of opening and closing the intake valve. In addition, the fuel pump can be easily disposed in the intake side of the engine, which is advantageous in safety. 
     Advantages of Invention 
     According to the present invention, in a case where the number of advance chambers is larger than or equal to the number of retard chambers in the intake VVT, the number of retard chambers is larger than the number of advance chambers in the exhaust VVT, and in a case where the number of advance chambers is larger than the number of retard chambers in the intake VVT, the number of retard chambers is larger than or equal to the number of advance chambers in the exhaust VVT. Thus, in a transition period in which the opening and closing timings of the intake valve and the exhaust valve are advanced to shift the valve overlap amount from a large state to a small state, a state with a large valve overlap amount can be continued for a while, and in a transition period in which the opening and closing timings of the intake valve and the exhaust valve are retarded to shift the valve overlap amount from a small state to a large state, the valve overlap amount can be increased quickly. As a result, in a situation where oil pressures that can be used by the intake VVT and the exhaust VVT are restricted, a pumping loss in a transition period in which the valve overlap amount is changed by advancing or retarding the opening/closing timing can be reduced, which is advantageous for enhancing fuel efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A cross-sectional view illustrating a schematic configuration of a VVT-equipped engine. 
         FIGS. 2A-2C  Cross-sectional views illustrating a configuration and operating states of a valve stop mechanism. 
         FIG. 3  A plan view schematically illustrating an arrangement of an engine device concerning a VVT. 
         FIG. 4  A side view schematically illustrating a driving system of an intake and exhaust VVTs and intake and exhaust cams. 
         FIG. 5  A lateral cross-sectional view of the exhaust VVT in a most retarded state. 
         FIG. 6  A lateral cross-sectional view of the exhaust VVT in a most advanced state. 
         FIG. 7  A cross-sectional view illustrating a relationship between the exhaust VVT and an oil pressure control valve. 
         FIG. 8  A lateral cross-sectional view of the intake VVT in a most advanced state. 
         FIG. 9  A lateral cross-sectional view of the intake VVT in a most retarded state. 
         FIG. 10  A view illustrating an engine oil supply system. 
         FIG. 11  A control block diagram of the exhaust VVT. 
         FIG. 12  A graph showing an example of changes of opening and closing timings at which a valve overlap amount changes from a large state to a small state. 
         FIG. 13  A graph showing an example of changes of opening and closing timings at which the valve overlap amount changes from a small state to a large state. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments for carrying out the present invention will be described with reference to the drawings. The following embodiments are merely preferred examples in nature, and are not intended to limit the invention, applications, and use of the applications. 
     Engine Configuration 
     The engine  2  illustrated in  FIG. 1  is, for example, an inline four-cylinder gasoline engine in which first through fourth cylinders are arranged in series in a direction perpendicular to the drawing sheet of  FIG. 1 , and is mounted on a vehicle such as an automobile. 
     In the engine  2 , a head cover  3 , a cylinder head  4 , a cylinder block  5 , a crank case (not shown), and an oil pan  6  (see  FIG. 10 ) are coupled vertically. A piston  8  slidable in each of four cylinder bores  7  formed in the cylinder block  5  is coupled to a crank shaft  9  rotatably supported on the crank case by a connecting rod  10 . The cylinder bore  7  in the cylinder block  5 , the piston  8 , and the cylinder head  4  form a combustion chamber  11  for each cylinder. 
     The cylinder head  4  has an intake port  12  and an exhaust port  13  each communicating with the combustion chamber  11 . The intake port  12  and the exhaust port  13  are provided with an intake vale  14  and an exhaust valve  15  that open and close the intake port  12  and the exhaust port  13 , respectively. The intake valve  14  and the exhaust valve  15  are biased in closing directions (upward in  FIG. 1 ) by valve springs  16  and  17 , respectively. Cam portions  18   a  and  19   a  disposed on outer peripheries of an intake cam shaft  18  and an exhaust cam shaft  19  push cam followers  20   a  and  21   a  rotatably disposed substantially on center portions of swing arms  20  and  21  downward. Accordingly, the swing arms  20  and  21  swing about vertexes of pivot mechanisms  25   a  each disposed at one end of each of the swing arms  20  and  21 . In this manner, at the other end of each of the swing arms  20  and  21 , the intake valve  14  and the exhaust valve  15  are opened while being pushed downward against biasing forces of the valve springs  16  and  17 . 
     A known hydraulic lash adjuster  24  (hereinafter referred to as an HLA  24 ) that automatically adjusts a valve clearance to zero by an oil pressure is provided as pivot mechanisms (having a configuration similar to that of a pivot mechanism  25   a  of an HLA  25  described later) in the swing arms  20  and  21  of the second cylinder and the third cylinder located at a center portion in the cylinder line of the engine  2 . The HLA  24  is shown only in  FIG. 10 . 
     The HLA  25  equipped with a valve stop mechanism (hereinafter referred to as a valve stop mechanism-equipped HLA  25 ) including the pivot mechanism  25   a  is provided on each of the swing arms  20  and  21  of the first cylinder and the fourth cylinder at the ends of the cylinder line of the engine  2 . The pivot mechanism  25   a  of the valve stop mechanism-equipped HLA  25  is configured to automatically adjust a valve clearance to zero by an oil pressure in a manner similar to the HLA  24 . In addition, the valve stop mechanism of the HLA  25  stops operations (i.e., stops open and close operations) of the intake and exhaust valves  14  and  15  of the first cylinder and the fourth cylinder in a reduced-cylinder operation in which operations of the first cylinder and the fourth cylinder as a part of all the cylinders of the engine  2  are suspended, and operates (i.e., performs open and close operations of) the intake and exhaust valves  14  and  15  of the first cylinder and the fourth cylinder in an all-cylinder operation in which all the cylinders (four cylinders) are operated. The intake and exhaust valves  14  and  15  of the second cylinder and the third cylinder operate in both the reduced-cylinder operation and the all-cylinder operation. The reduced-cylinder operation and the all-cylinder operation are switched to each other when necessary in accordance with the operating state of the engine  2 . 
     Intake- and exhaust-side portions of the cylinder head  4  corresponding to the first and fourth cylinders respectively have attachment holes  26  and  27  for inserting and attaching lower end portions of the valve stop mechanism-equipped HLAs  25 . Intake- and exhaust-side portions of the cylinder head  4  corresponding to the second cylinder and the third cylinder have attachment holes for inserting and attaching lower end portions of the HLAs  24 . In addition, two oil passages  61  and  63  and two oil passages  62  and  64  are formed to pierce the cylinder head  4  and respectively communicate with the attachment holes  26  and  27  for the valve stop mechanism-equipped HLAs  25 . An oil pressure (operating pressure) is supplied from the oil passages  61  and  62  to valve stop mechanisms  25   b  (see  FIGS. 2A through 2C ) in the valve stop mechanism-equipped HLAs  25  in a state where the valve stop mechanism-equipped HLAs  25  are fitted in the attachment holes  26  and  27 . On the other hand, an oil pressure for enabling the pivot mechanisms  25   a  of the valve stop mechanism-equipped HLAs  25  to automatically adjust valve clearances to zero is supplied from the oil passages  63  and  64 . Only the oil passages  63  and  64  communicate with the attachment holes for the HLAs  24 . The oil passages  61  through  64  will be described in detail later with reference to  FIG. 10 . 
     The cylinder block  5  includes a main gallery  54  extending along the cylinder line in a side wall at an exhaust side of the cylinder bores  7 . Near a lower side of the main gallery  54 , oil jets  28  that are used for cooling the pistons and communicate with the main gallery  54  are disposed. Each oil jet  28  has a nozzle portion  28   a  disposed under the piston  8 , and injects oil (engine oil) toward the back side of a vertex portion of the piston  8  from the nozzle portion  28   a.    
     Oil showers  29  and  30  constituted by pipes are disposed above the cam shafts  18  and  19 , respectively. Lubricating oil is dropped from the oil showers  29  and  30  onto the underlying cam portions  18   a  and  19   a  of the cam shafts  18  and  19  and further underlying contact portions between the swing arms  20  and  21  and the cam followers  20   a  and  21   a.    
     Here, the valve stop mechanism  25   b  will be described with reference to  FIGS. 2A to 2C . The valve stop mechanism  25   b  stops an operation of at least one of the intake and exhaust valves  14  and  15  (both of the valves in this embodiment) of the first cylinder and the fourth cylinder that are a part of all the cylinders of the engine  2 . In a reduced-cylinder the of the engine  2 , the valve stop mechanisms  25   b  stop opening and closing operations of the intake and exhaust valves  14  and  15  of the first cylinder and the fourth cylinder. In an all-cylinder operation of the engine  2 , stopping of operations of the valves by the valve stop mechanisms  25   b  is canceled, and opening and closing operations of the intake and exhaust valve  14  and  15  of the first cylinder and the fourth cylinder are performed. 
     As illustrated in  FIG. 2A , each of the valve stop mechanisms  25   b  includes a lock mechanism  250  that locks an operation of the pivot mechanism  25   a . The lock mechanism  250  includes a pair of lock pins  252  (lock members). The lock pins  252  are disposed to be inserted and extracted into/from two through holes  251   a  that are radially opposed to each other in a side surface of a bottomed outer cylinder  251  that houses the pivot mechanism  25   a  such that the pivot mechanism  25   a  is axially slidable. The pair of lock pins  252  is biased radially outward by a spring  253 . A lost motion spring  254  that presses and biases the pivot mechanism  25   a  upward from the outer cylinder  251  is disposed between the inner bottom of the outer cylinder  251  and the bottom of the pivot mechanism  25   a.    
     In a case where the lock pins  252  are fitted in the through holes  251   a  of the outer cylinder  251 , the pivot mechanism  25   a  located above the lock pins  252  is fixed while projecting upward. In this case, the vertex portion of the pivot mechanism  25   a  serves as a fulcrum of swing of each of the swing arms  20  and  21 , and thus, when the cam portions  18   a  and  19   a  push the cam followers  20   a  and  21   a  downward with rotation of the cam shafts  18  and  19 , the intake and exhaust valves  14  and  15  are pushed downward against biasing forces of the valve springs  16  and  17  to be opened. In this manner, the lock pins  252  cause the valve stop mechanism  25   b  to be fitted in the through holes  251   a  in the first cylinder and the fourth cylinder so that the engine  2  can thereby perform an all-cylinder operation. 
     On the other hand, as illustrated in  FIGS. 2B and 2C , when the outer end surfaces of the lock pins  252  are pushed by an operating oil pressure, the lock pins  252  move rearward toward the radially inside of the outer cylinder  251  against a biasing force of the spring  253  such that the lock pins  252  approach each other. Consequently, the lock pins  252  are released from the through holes  251   a  of the outer cylinder  251 , and thus, the pivot mechanism  25   a  located above the lock pins  252  moves downward to be axially under the outer cylinder  251  together with the lock pins  252  so that the valves come to be in a valve stop state. 
     That is, the valve springs  16  and  17  that bias the intake and exhaust valves  14  and  15  upward are configured to generate biasing forces greater than that of the lost motion spring  254  that biases the pivot mechanism  25   a  upward. Accordingly, when the cam portions  18   a  and  19   a  respectively push the cam followers  20   a  and  21   a  downward with rotation of the cam shafts  18  and  19 , the vertex portions of the intake and exhaust valves  14  and  15  serve as fulcrums of swing of the swing arms  20  and  21 . Consequently, with the intake and exhaust valves  14  and  15  closed, the pivot mechanisms  25   a  are pushed downward against biasing forces of the lost motion springs  254 . As a result, the lock pins  252  are released from the through holes  251   a  by an operating oil pressure so that a reduced-cylinder operation can be performed. 
     Intake VVT and Exhaust VVT 
     As illustrated in  FIG. 3 , the intake cam shaft  18  and the exhaust cam shaft  19  extend along a line of cylinders  115 . An intake VVT  32  is disposed at one end of the intake cam shaft  18 , and an exhaust VVT  33  is disposed at one end of the exhaust cam shaft  19 . Gears  101  and  102  that mesh with each other are fixed to housings  201  (see  FIGS. 5, 6, 8, and 9 ) described later of the intake VVT  32  and the exhaust VVT  33 . The meshing of the gears  101  and  102  causes the intake VVT  32  and the exhaust VVT  33  to rotate in opposite directions together with the cam shafts  18  and  19 . 
     A cam angle sensor  74  that detects rotation phases of the cam shafts  18  and  19  and, based on the cam angles thereof, detects phase angles of the VVTs  32  and  33  are disposed near the other end of each of the intake cam shaft  18  and the exhaust cam shaft  19 . In addition, a pump cam  106  for driving a high-pressure fuel pump  81  that supplies fuel to the combustion chamber  11  of the engine  2  is disposed at the other end of the intake cam shaft  18 . The pump cam  106  drives a plunger  81   a  of the fuel pump  81 , and the fuel pump  81  supplies high-pressure fuel to a fuel injection valve that supplies fuel to the combustion chamber  11  of the engine  2 . 
     Then, as illustrated in  FIG. 4 , a timing chain  108  is wound around a cam pulley (sprocket)  203  and a crank shaft pulley (sprocket)  9 A fixed to the housing  201  of the exhaust VVT  33 . An intermediate sprocket  111 , a hydraulic chain tensioner  112 , and a chain guide  113  are disposed between the crank shaft pulley  9 A and the cam pulley  203 . 
     The gears  101  and  102  and the timing chain  108  constitute a transfer unit that drives the housing  201  of the intake VVT  32  and the housing  201  of the exhaust VVT  33  to rotate in opposite directions by the crank shaft  9 . 
     Configuration of Exhaust VVT 
     First, the exhaust VVT will be described.  FIGS. 5 through 7  illustrate the exhaust VVT  33 .  FIG. 7  also illustrates an exhaust-side oil pressure control valve  35  that controls an operation of the exhaust VVT  33  by an oil pressure. 
     The exhaust VVT  33  is operated by an oil pressure, and includes the substantially annular housing  201  and a vane body  202  housed in the housing  201 . The housing  201  is coupled to be rotatable together with a cam pulley  203  that rotates in synchronization with the crank shaft  9 , and rotates in cooperation with the crank shaft  9 . The vane body  202  includes a plurality of vanes  202   a , and as illustrated in  FIG. 7 , is coupled to the exhaust cam shaft  19  by a fastening bolt  205  such that the vane body  202  is rotatable together with the exhaust cam shaft  19 . 
     In the housing  201 , a plurality of advance chambers  207  and a plurality of retard chambers  208  are defined by the housing  201  and the vane body  202 . As illustrated in  FIG. 7 , the advance chambers  207  and the retard chambers  208  are connected to an exhaust-side oil pressure control valve (first direction switching valve)  35  through an advance-side oil passage  211  and a retard-side oil passage  212 , respectively. The exhaust-side oil pressure control valve  35  is connected to a variable displacement oil pump  36 . In the cam shaft  19  and the vane body  202 , advance-side oil passages  215  and retard-side oil passages  216  constituting parts of the advance-side oil passage  211  and the retard-side oil passage  212  are formed. 
       FIG. 5  illustrates a state where each of the vanes  202   a  is held in a most retarded position with respect to the cam pulley  203 , that is, the crank shaft  9 , by oil supplied through the retard-side oil passages  216 . In contrast,  FIG. 6  illustrates a state where each of the vanes  202   a  is held in a most advanced position with respect to the cam pulley  203  by oil supplied through the advance-side oil passages  215 . 
     The advance-side oil passages  215  extend radially from a vicinity of the center of the vane body  202  and are connected to the advance chambers  207 . The retard-side oil passages  216  extend radially from a vicinity of the center of the vane body  202  and are connected to the retard chamber  208 . 
     A chamber  207   a  illustrated in  FIG. 6  does not communicate with the advance-side oil passages  215 , and no oil is supplied. Thus, no rotation torque is generated on the vanes  202   a . That is, the chamber  207   a  does not constitute an advance chamber. Thus, the number of advance chambers  207  is smaller than that of retard chambers  208  by one. The exhaust VVT  33  according to this embodiment includes three advance chambers  207  and four retard chambers  208 . 
     As illustrated in  FIG. 7 , the exhaust VVT  33  includes a lock mechanism  230  for locking an operation of the exhaust VVT  33 .  FIGS. 5 and 6  do not show the lock mechanism  230 . The lock mechanism  230  includes a lock pin  231  for locking a phase angle of the exhaust cam shaft  19  with respect to the crank shaft  9  at an intermediate phase angle between a most advanced angle and a most retarded angle. 
     The lock pin  231  is slidable in the radial direction of the housing  201 . A spring holder  232  is fixed to a portion of the housing  201  radially outside the lock pin  231 . A lock pin biasing spring  233  that biases the lock pin  231  radially inward of the housing  201  is disposed between the spring holder  232  and the lock pin  231 . While the fitting recess  202   b  formed in a portion of the outer peripheral surface of the vane body  202  where no vanes  202   a  are formed is at a position facing the lock pin  231 , the lock pin biasing spring  233  causes the lock pin  231  to be fitted in the fitting recess  202   b , that is, to be in a locked state. Accordingly, the vane body  202  is fixed to the housing  201 , and the phase angle of the exhaust cam shaft  19  with respect to the crank shaft  9  is locked. 
     As illustrated in  FIG. 7 , the exhaust-side oil pressure control valve  35  is a solenoid valve having three ports and three positions, a supply port  351  is connected to the oil pump  36 , and output ports  352  and  353  are connected to the advance-side oil passages  215  and the retard-side oil passages  216 , respectively. In  FIG. 7 , reference numeral  354  denotes a solenoid that exerts an electromagnetic force on a spool  356 . 
       FIG. 7  illustrates a state where the supply port  351  communicates with the output port  352 . Oil in an amount in accordance with the communication degree of the supply port  351  is supplied to the advance chambers  207  of the VVT  33 . Accordingly, the vane body  202  rotates in the advancing direction so that the volume of the retard chambers  208  is reduced. With this volume reduction, oil discharged from the retard chambers  208  is drained from the output port  353  to the oil pan  6  through a drain port  357 . 
     When the spool  356  moves forward (moves downward in  FIG. 7 ) against a biasing force of the return spring  359  to reach a neutral position in which both the output ports  352  and  353  are closed, oil supply to the advance chambers  207  and the retard chambers  208  are blocked. 
     When the spool  356  further moves forward against a biasing force of the return spring  359 , the supply port  351  communicates with the output port  353 . Accordingly, oil is supplied to the retard chambers  208  of the exhaust VVT  33  and the vane body  202  pivots in the retarding direction, and oil discharged from the advance chambers  207  with volume reduction of the advance chambers  207  is drained from the output port  352  to the oil pan  6  through a drain port  358 . 
     As described above, the exhaust-side oil pressure control valve  35  controls oil supply to the advance chambers  207  and the retard chambers  208  of the exhaust VVT  33  so that opening and closing timings of the exhaust side can be changed. Specifically, when oil is supplied in a larger amount (under a higher oil pressure) to the advance chambers  207  than to the retard chambers  208 , the exhaust cam shaft  19  pivots in the rotation direction of the cam shaft  19  (direction indicated by arrows in  FIGS. 5 and 6 ) with respect to the housing  201 , and the opening timing in the exhaust valve  15  is advanced. On the other hand, when oil is supplied in a larger amount (under a higher oil pressure) to the retard chambers  208  than to the advance chambers  207 , the cam shaft  19  pivots in a direction opposite to the rotation direction of the cam shaft  19 , and the opening timing of the exhaust valve  15  is retarded (see  FIG. 5 ). 
     Configuration of Intake VVT  32   
       FIGS. 8 and 9  illustrate the intake VVT  32 . The intake VVT  32  employs a hydraulic VVT having the same configuration as that of the exhaust VVT  33 . In this case, since the intake VVT  32  and the exhaust VVT  33  rotate in opposite directions as described above, elements constitutes the advance chambers  207  of the exhaust VVT  33  serve as the retard chambers  208  in the intake VVT  32 , and elements constituting the retard chambers  208  of the exhaust VVT  33  serve as the advance chambers  207  in the intake VVT  32 . Similarly, elements constituting the advance-side oil passages  215  of the exhaust VVT  33  serve as the retard-side oil passages  216  in the intake VVT  32 , and elements constituting the retard-side oil passages  216  of the exhaust VVT  33  serve as the advance-side oil passages  215  in the intake VVT  32 . 
     Thus, in the intake VVT  32 , the number of advance chambers  207  is four, and the number of the retard chambers  208  is three. The intake VVT  32  is connected to an intake-side oil pressure control valve (first direction switching valve)  34  illustrated in  FIG. 10 . The intake-side oil pressure control valve  34  is a solenoid valve having three ports and three positions similar to the exhaust-side oil pressure control valve  35 . Although not shown specifically, in the intake-side oil pressure control valve  34 , a port corresponding to the output port  352  of the exhaust-side oil pressure control valve  35  illustrated in  FIG. 7  serves as a retarding output port, and a port corresponding to the output port  353  serves as an advancing output port. 
     Oil Supply Device 
     As illustrated in  FIG. 10 , an oil supply device  1  that supplies oil to the engine  2  includes a variable displacement oil pump  36  that is driven by rotation of the crank shaft  9 , and an oil supply passage  50  (oil supply path) that is connected to the oil pump  36  and guides oil whose pressure has been increased by the oil pump  36  to a lubricating part of the engine  2  and the hydraulic operating devices such as the exhaust VVT  33 . 
     The oil supply passage  50  is constituted by a first communication path  51 , a main gallery  54 , a second communication path  52 , a third communication path  53 , and a plurality of oil passages  61  through  69 . 
     The first communication path  51  extends from an outlet  361   b  of the oil pump  36  to a branch point  54   a  in the cylinder block  5 . The main gallery  54  extends along the cylinder line in the cylinder block  5 . The second communication path  52  extends from a branch point  54   b  on the main gallery  54  to the cylinder head  4 . The third communication path  53  extends substantially horizontally between an intake side and an exhaust side in the cylinder head  4 . The plurality of oil passages  61  through  69  are branched from the third communication path  53  in the cylinder head  4 . 
     The oil pump  36  includes a housing  361 , a driving shaft  362 , a pump element, a cam ring  366 , a spring  367 , and ring members  368 . 
     The housing  361  is constituted by a pump body having an opening at one end and including a pump accommodating chamber including a hollow space that is circular in cross section, and a cover member covering the opening at the end of the pump body. The driving shaft  362  is rotatably supported by the housing  361 , penetrates substantially a center portion of the pump accommodating chamber, and is driven to rotate by the crank shaft  9 . The pump element is constituted by a rotor  363  rotatably housed in the pump accommodating chamber and coupled to the driving shaft  362  at a center portion thereof, and vanes  364  individually retreatably housed in a plurality of slits formed by radially cutting out an outer peripheral portion of the rotor  363 . The cam ring  366  is eccentrically disposed with respect to a rotation center of the rotor  363  at the outer periphery of the pump element, and defines pump chambers  365  as a plurality of hydraulic oil chambers together with the rotor  363  and its adjacent vanes  364 . The spring  367  is a biasing member that is housed in the pump body and constantly biases the cam ring  366  in a direction in which an eccentricity of the cam ring  366  with respect to the rotation center of the rotor  363  increases. The ring members  368  are a pair of ring-shaped members slidably disposed at each inner peripheral side of the rotor  363  and each having a smaller diameter than the rotor  363 . 
     The housing  361  includes an inlet  361   a  through which oil is supplied to inner pump chambers  365  and the outlet  361   b  through which oil is discharged from the pump chambers  365 . In the housing  361 , a pressure chamber  369  is defined by the inner peripheral surface of the housing  361  and the outer peripheral surface of the cam ring  366 , and the pressure chamber  369  has an introduction hole  369   a.    
     As described above, the oil pump  36  is configured such that introduction of oil into the pressure chamber  369  through the introduction hole  369   a  causes the cam ring  366  to swing with respect to a fulcrum  361   c  and causes the rotor  363  to be eccentric with respect to the cam ring  366  so that the discharge capacity of the oil pump  36  changes. 
     An oil strainer  39  facing the oil pan  6  is connected to the inlet  361   a  of the oil pump  36 . In the first communication path  51  communicating with the outlet  361   b  of the oil pump  36 , an oil filter  37  and an oil cooler  38  are disposed in this order from an upstream side to a downstream side. Oil stored in the oil pan  6  is pumped by the oil pump  36  through the oil strainer  39 , then filtered by the oil filter  37  and cooled by the oil cooler  38 , and then introduced to the main gallery  54  in the cylinder block  5 . 
     The main gallery  54  is connected to the oil jet  28  for injecting cooling oil to the back surfaces of the four pistons  8  described above, oil supply portions  41  of metal bearings disposed in five main journals rotatably supporting the crank shaft  9 , and oil supply portions  42  of metal bearings disposed on crank pins of the crank shaft  9  rotatably coupling four connecting rods. Oil is constantly supplied to the main gallery  54 . 
     An oil supply portion  43  for supplying oil to a hydraulic chain tensioner and an oil passage  40  for supplying oil from the introduction hole  369   a  into the pressure chamber  369  of the oil pump  36  through a linear solenoid valve  49  are connected to a downstream side of a branch point  54   c  on the main gallery  54 . 
     An oil supply system at the exhaust valve side will be described. An oil passage  68  branching from a branch point  53   a  of the third communication path  53  is connected to the oil pressure control valve  35  of the exhaust VVT  33 . An oil passage  64  branching from the branch point  53   a  is connected to oil supply portions  45  (see white triangles in  FIG. 10 ), the HLAs  24  (see black triangles in  FIG. 10 ), and valve stop mechanism-equipped HLAs  25  (white oval in  FIG. 10 ). The oil supply portions  45  supply oil to a cam journal of the exhaust-side cam shaft  19 . The oil passage  64  is constantly supplied with oil. In addition, an oil passage  66  branching from a branch point  64   a  of the oil passage  64  is connected to the oil shower  30  that supplies lubricating oil to an exhaust-side swing arm  21 . The oil passage  66  is also constantly supplied with oil. 
     Next, an oil supply system at the intake valve side will be described. An oil passage  67  branching from a branch point  53   c  of the third communication path  53  is connected to the intake-side oil pressure control valve  34 . The intake-side oil pressure control valve  34  is controlled such that oil is supplied to the advance chambers  207  and the retard chambers  208  of the intake VVT  32  through an advance-side oil passage  211  and a retard-side oil passage  212 , respectively. The oil passage  67  is provided with an oil pressure sensor  70  that detects an oil pressure of the oil passage  67 . The oil passage  63  branching from a branch point  53   d  is connected to oil supply portions  44  of a cam journal of the intake cam shaft  18  (see white triangles in  FIG. 10 ), the HLAs  24  (see black triangles in  FIG. 10 ), the valve stop mechanism-equipped HLAs  25  (see white ovals in  FIG. 10 ), the fuel pump  81 , and a vacuum pump  82 . The vacuum pump  82  is driven by the cam shaft  18 , and obtains a pressure of a brake master cylinder. In addition, an oil passage  65  branching from a branch point  63   a  of the oil passage  63  is connected to the oil shower  29  that supplies lubricating oil to the intake-side swing arm  20 . 
     The oil passage  69  branching from a branch point  53   c  of the third communication path  53  is provided with a check valve  48  that restricts an oil flow direction to one direction from an upstream side to a downstream side. At a branch point  69   a  downstream of the check valve  48 , the oil passage  69  branches to the two oil passages  61  and  62  communicating with the attachment holes  26  and  27  for the valve stop mechanism-equipped HLAs  25 . The oil passages  61  and  62  are connected to the valve stop mechanisms  25   b  of the intake-side and exhaust-side valve stop mechanism-equipped HLAs  25  at the intake side and the exhaust side through an intake-side second direction switching valve  46  and an exhaust-side second direction switching valve  47 , respectively. In this configuration, the intake- and exhaust-side second direction switching valves  46  and  47  are controlled to supply oil to the valve stop mechanisms  25   b.    
     After lubrication and cooling, lubricating oil and cooling oil supplied to the metal bearing rotatably supporting the crank shaft  9 , the pistons  8 , and the cam shafts  18  and  19 , for example, are dropped in the oil pan  6  through an unillustrated drain oil passage and is circulated by the oil pump  36  again. 
     Control System 
     An operation of the engine  2  is controlled by a controller  100 . The controller  100  receives detection information from sensors that detect an operating state of the engine  2 . The controller  100  detects a rotation angle of the crank shaft  9  by a crank angle sensor  71  and determines an engine speed based on the detection signal, for example. An action position sensor  72  detects a pressing amount (accelerator opening angle) of an accelerator pedal by a passenger of the vehicle on which the engine  2  is mounted. Based on the pressing amount, a required torque is calculated. In addition, the oil pressure sensor  70  detects a pressure of the oil passage  67 . An oil temperature sensor  73  disposed substantially at the same position as the oil pressure sensor  70  detects an oil temperature in the oil passage  67 . The oil pressure sensor  70  and the oil temperature sensor  73  may be disposed on any location of the oil supply passage  50 . The cam angle sensor  74  causes the oil pressure control valves  31  and  35  of the VVTs  32  and  33  to operate such that the detected phase angles of the VVTs  32  and  33  reach target phase angles set in accordance with the operating state of the engine, based on detected current phase angles of the VVTs  32  and  33 . A water temperature sensor  75  detects a temperature of cooling water for cooling the engine  2  (hereinafter referred to as a water temperature). 
     The controller  100  is a control device based on a known microcomputer, and includes a signal receiving section that receives detection signals from sensors (e.g., the oil pressure sensor  70 , the crank angle sensor  71 , a throttle position sensor  72 , the oil temperature sensor  73 , the cam angle sensor  74 , and the water temperature sensor  75 ), a computation section that performs a computation process for control, a signal output section that outputs control signals to devices to be controlled (e.g., the oil pressure control valves  35 ,  46 , and  47  and the linear solenoid valve  49 ), and a storage section that stores programs and data necessary for control (e.g., an oil pressure control map and a duty ratio map). 
     The linear solenoid valve  49  is a flow rate (discharge rate) control valve for controlling the discharge rate of the oil pump  36  in accordance with the operating state of the engine  2 . In this configuration, oil is supplied to the pressure chamber  369  of the oil pump  36  while the linear solenoid valve  49  is open. The configuration of the linear solenoid valve  49  itself is already known, and thus, will not be described here. 
     The controller  100  transmits, to the linear solenoid valve  49 , a control signal of a duty ratio in accordance with the operating state of the engine  2 , and controls a pressure of oil to be supplied to the pressure chamber  369  of the oil pump  36  through the linear solenoid valve  49 . Based on the oil pressure of the pressure chamber  369 , an eccentricity of the cam ring  366  is controlled so that the amount of change of the internal volume of the pump chambers  365  is controlled to thereby control the flow rate (discharge rate) of the oil pump  36 . That is, the volume of the oil pump  36  is controlled by using the duty ratio. 
     Control of VVTs  32  and  33   
       FIG. 11  is a block diagram illustrating a method for controlling the exhaust VVT  33 . From an exhaust VVT request advance map C 01  set in an engine operating state (an engine speed and an air charging efficiency), a request advance amount of the exhaust VVT  33  is acquired in accordance with the engine operating state. The acquired map request advance amount is input to an exhaust VVT speed limit request block C 04 . 
     In a block C 02 , a limit value of an operating speed of the exhaust VVT  33  is acquired based on an engine oil temperature. Oil temperature-speed limit tables are previously created for a reduced-cylinder operation and an all-cylinder operation individually, and the limit value of the operating speed of the exhaust VVT  33  is acquired from these tables. 
     The speed limit value acquired from each table is input to a switch block C 03 . The switch block C 03  receives “reduced-cylinder operation determination” in a reduced-cylinder operation and “no speed limit” for maintaining valve stop in an all-cylinder operation, in addition to the speed limit value from each table. In the reduced-cylinder operation, the speed limit value acquired from the oil temperature-speed limit table for the reduced-cylinder operation is input to the exhaust VVT speed limitation request block C 04 . In the all-cylinder operation, the speed limit value acquired from the oil temperature-speed limit table for the all-cylinder operation is input to the exhaust VVT speed limitation request block C 04 . 
     The exhaust VVT speed limitation request block C 04  outputs an exhaust VVT request advance amount. A difference between this exhaust VVT request advance amount and a current exhaust VVT actual advance amount is calculated. From this difference, a deviation between a request value (target value) of an advance amount and an actual advance amount is calculated, and is input to an advance F/B control block C 05 . 
     In the advance F/B control block C 05 , based on the input advance amount target/actual value deviation, an OCV drive duty ratio in accordance with the limit value of the operating speed of the exhaust VVT  33  is obtained by, for example, a proportional-integral-differential (PID) method. 
     Although not shown, the method for controlling the intake VVT  32  is similar to that for the exhaust VVT  33 , and an operation of the intake VVT  32  is controlled by using an intake VVT request advance map set in accordance with the engine operating state (the engine speed and the air charging efficiency), and oil temperature-speed limit tables set for a reduced-cylinder operation and an all-cylinder operation individually in accordance with the engine oil temperature. 
     Example of Valve Timing Change 
       FIG. 12  shows changes of opening and closing timings of the intake and exhaust valves  14  and  15  set based on the VVT request advance map when the engine  2  shifts from an intermediate-rotation and intermediate-load operating state to a low-rotation and low-load operating state. In  FIG. 12 , thin solid lines indicate opening and closing timings before shift, and bold solid lines indicate opening and closing timings after the shift. This is a case where the opening and closing timings of the intake and exhaust valves  14  and  15  are advanced, and an operating state with a large valve overlap amount shifts to an operating state with a small valve overlap amount. 
     As described above, the number of advance chambers  207  is “four” and the number of retard chambers  208  is “three” in the intake VVT  32 , whereas the number of advance chambers  207  is “three” and the number of retard chambers  208  is “four” in the exhaust VVT  33  That is, the number of advance chambers  207  in the intake VVT  32  is larger than that in the exhaust VVT  33 . Thus, when oil pressures applied to the VVTs  32  and  33  are the same, the advancing speed of the opening/closing timing of the intake valve  14  is higher than the advancing speed of the opening/closing timing of the exhaust valve  15 . 
     Thus, in operating the VVTs  32  and  33  at the same time, as shown in  FIG. 12 , when the opening/closing timing of the exhaust valve  15  is only slightly advanced from a position indicated by the thin solid line to a position indicated by a broken line, for example, the opening/closing timing of the intake valve  14  is greatly advanced from a position indicated by the thin solid line to a position indicated by a bold solid line. Accordingly, in a transition period in which the valve overlap amount shifts from a large state to a small state, the state with a relatively large valve overlap amount continues for a while (where the valve overlap amount can be temporarily increased). As a result, an increase in a pumping loss can be suppressed in this transition period so that fuel efficiency can be enhanced. 
       FIG. 13  shows changes of the opening and closing timings of the intake and exhaust valves  14  and  15  set based on the VVT request advance map when the engine  2  shifts from a low-rotation and low-load operating state to an intermediate-rotation and intermediate-load operating state. In  FIG. 12 , thin solid lines indicate opening and closing timings before shift, and bold solid lines indicate opening and closing timings after the shift. This is a case where the opening and closing timings of the intake and exhaust valves  14  and  15  are retarded, and an operating state with a small valve overlap amount shifts to an operating state with a large valve overlap amount. 
     Since the number of retard chambers  208  in the exhaust VVT  33  is larger than that in the intake VVT  32  as described above, oil pressures applied to the VVTs  32  and  33  are the same, the retarding speed of the opening/closing timing of the exhaust valve  15  is higher than the retarding speed of the opening/closing timing of the intake valve  14 . 
     Thus, in operating the VVTs  32  and  33  at the same time, as shown in  FIG. 13 , when the opening/closing timing of the intake valve  14  is only slightly retarded from a position indicated by the thin solid line to a position indicated by a chain line, for example, the opening/closing timing of the exhaust valve  15  is greatly retarded from a position indicated by a thin solid line to a position indicated by a broken line. Accordingly, in a transition period in which the valve overlap amount shifts from a small state to a large state, the valve overlap amount increases quickly. As a result, a pumping loss can be reduced so that fuel efficiency can be enhanced. 
     As described above, in a situation where oil pressures that can be used by the intake VVT  32  and the exhaust VVT  33  are restricted and the operating speeds of the VVTs  32  and  33  are limited, a pumping loss in a transition period in which the valve overlap amount is changed by advancing or retarding the opening/closing timing can be reduced, which is advantageous for enhancing fuel efficiency. 
     In this embodiment, although the number of advance chambers is smaller than the number of retard chambers in the exhaust VVT  33 , since the number of advance chambers is larger than the number of retard chambers in the intake VVT  32 , the intake cam shaft  18  has a margin for a rotation load in the advancing direction as compared to the exhaust cam shaft  19 . The embodiment uses this configuration to perform cam driving of the fuel pump  105  by using the intake cam shaft  18 . Thus, the fuel pump  105  can be easily operated with stability without hindering a change of opening and closing of the intake valve  14  and the timings of opening and closing the intake valve  14 . In addition, the fuel pump  105  can be easily disposed at the intake side of the engine  2 , which is advantageous in safety. 
     In this embodiment, the number of advance chambers is larger than that of retard chambers in the intake VVT  32  and the number of retard chambers is larger than that of advance chambers in the exhaust VVT  33 . Alternatively, if the number of advance chambers is larger than that of retard chambers in the intake VVT  32 , the number of retard chambers may be equal to that of advance chambers in the exhaust VVT  33 . If the number of retard chambers is larger than that of advance chambers in the exhaust VVT  33 , the number of advance chambers may be equal to that of retard chambers in the intake VVT  32 . 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
         
           
               1  oil supply device 
               2  engine 
               8  piston 
               14  intake valve 
               15  exhaust valve 
               18  intake cam shaft 
               19  exhaust cam shaft 
               25  valve stop mechanism-equipped HLA 
               25   a  pivot mechanism 
               25   b  valve stop mechanism 
               28  oil jet 
               32  intake VVT 
               33  exhaust VVT 
               34  oil pressure control valve 
               35  oil pressure control valve 
               36  oil pump 
               207  advance chamber 
               208  retard chamber