Abstract:
A variable camshaft timing (VCT) system for an internal combustion engine is described. In one example, the system includes a spool valve that displaces oil from one phaser chamber to another phaser chamber when the spool valve is moved. The system may respond faster than systems that employ a pump to move oil between phaser chambers.

Description:
FIELD 
     The present description relates to a variable camshaft timing system for an internal combustion engine. 
     BACKGROUND 
     It is known in the art to employ variable camshaft timing (VCT) systems in internal combustion engines for improved fuel economy, emissions, and performance. VCT systems operate to vary the relative phasing between a camshaft and a crankshaft to optimize the cam timing over the range of engine operation. 
     An example of a VCT is a dual oil feed vane-type VCT. A dual oil feed vane-type variable cam timing unit provides an inner member or hub that is fixably connected to an end face of a camshaft. The hub has a series of vanes which are captured in cavities or pressure chambers provided in an outer member which is concentrically mounted on the hub. The outer member incorporates a camshaft timing pulley which is powered by the crankshaft via a belt which is looped over the camshaft pulley and a crankshaft timing gear. The vanes circumferentially divide the pressure chambers into an advance side and a retard side. A spool valve, fluidly communicative with the pressure chambers via the inner member and the camshaft, controls the fluid pressure in the advance side and retard side of the pressure chambers. Hence, by controlling the fluid in the advance and retard pressure chambers, the angular position of the timing pulley versus the crankshaft can be varied. 
     A disadvantage of such a VCT utilizing oil pressure and flow to control the phase of the camshaft is that the VCT response rate is dependent on the oil temperature and engine speed in order to achieve desired fuel economy and emission benefits. 
     The inventor herein has developed a system that improves variable camshaft timing systems and ameliorates the above problem. 
     SUMMARY 
     According to a first aspect of the description there is provided a variable camshaft timing (VCT) system for an internal combustion engine, the VCT system comprising: a housing for accepting drive from a crankshaft of the engine; a rotor coaxially located within the housing for connection to a camshaft, the housing and the rotor defining at least one vane separating a chamber in the housing into a phaser advance chamber and a phaser retard chamber, the vane being capable of rotation to shift the relative angular positions of the housing and the rotor; and a control valve having a spool slidably located within a bore in a valve sleeve, wherein the spool comprises one land dividing the bore into a valve advance chamber and a valve retard chamber, with the valve retard chamber and the valve advance chamber both being connected to a hydraulic source, the valve advance chamber and the valve retard chamber being in hydraulic communication with the phaser advance chamber and the phaser retard chamber respectively through an advance line and a retard line, such that displacements of the spool push oil from a valve chamber to a phaser chamber and rotate the vane. 
     Preferably, the valve retard chamber and the valve advance chamber are connected to the hydraulic source through a first and a second feed line, each of them being provided with a check valve. 
     Preferably, the spool is connected to a control actuator for controlling movement of the spool relative to the valve sleeve based upon various engine parameters. Conveniently, the control actuator is a stepper motor or a solenoid. 
     The VCT system may further include a locking mechanism for locking the spool in position. The locking mechanism may include two solenoid valves disposed within the advance line and the retard line respectively and two additional feed lines, each solenoid valve being connected at one of its ends to its corresponding feed line and at its other end to its corresponding advance or retard line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages described herein will be more fully understood by reading an example of an embodiment, referred to herein as the Detailed Description, when taken alone or with reference to the drawings, wherein: 
         FIG. 1  is a schematic diagram of a VCT system according to the present invention showing a VCT phaser and a control valve each in a respective null position; 
         FIG. 2  is a view similar to  FIG. 1  but with the VCT phaser and the control valve each in a respective fully advanced position; 
         FIG. 3  is a view similar to  FIG. 1  but with the VCT phaser and the control valve each in a respective fully retarded position; and 
         FIG. 4  is a view similar to  FIG. 1  showing a modification to the VCT system showing in  FIGS. 1 to 3 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1 to 3 , there is shown in part a VCT system  10  for an internal combustion engine including in known manner a crankshaft, a camshaft (not shown) and a hydraulic oil supply, typically engine lubricating oil supplied by an engine driven pump. The VCT system  10  includes a vane phaser  11  mounted on the engine camshaft and a control valve  12 . In a conventional way, the vane phaser  11  has a rotor with vanes, in this example one vane  13 , mounted to the end of the camshaft, surrounded by a housing  14  provided with a vane chamber into which the vane  13  fits. Conventionally the housing has a pulley for accepting drive from the crankshaft. The vane chamber is divided into two separate chambers by the vane  13 , respectively a phaser advance chamber  15  and a phaser retard chamber  16 . 
     The control valve  12 , located remotely from the phaser, includes a valve sleeve  17  having a bore  18  in which a stepped cylindrical spool  19  is slidable. The cylindrical spool  19  has one land  20  which cooperates with the bore  18  to divide the bore  18  into two chambers, respectively a valve advance chamber  21  and a valve retard chamber  22 . The phaser advance chamber  15  and the valve advance chamber  21  are in hydraulic communication via an advance line  11 . Similarly the phaser retard chamber  16  and the valve retard chamber  22  are in hydraulic communication via a retard line  23 . 
     Engine oil is pumped to the advance line  11  through a first feed line  24 , which incorporates a check valve  25 , feeding both advance chambers  15 ,  21  with oil. Engine oil is also pumped directly to the retard line  23  through a second feed line  26  feeding thus both retard chambers  16 ,  22  with oil. The second feed line  26  incorporates a check valve  27 . 
     As shown in  FIG. 1 , the volume of the phaser advance chamber  15 , the volume of the advance line  11  and the volume of the valve advance chamber  21  together form an advance volume which is equal to a retard volume formed by the volume of the phaser retard chamber  16 , the volume of the retard line  23  and the volume of the valve retard chamber  22 . It will be appreciated that there is no hydraulic communication between the advance volume and the retard volume and that both these volumes are constant during operation of the spool  19  as it will further explained below. 
     A control actuator  30 , for example a step motor, acts on one end of the spool  19  and controls movement of the spool  19  relative to the valve sleeve  17  under the control of an engine control unit (ECU)  31 . This control actuator  30  is able to lock the spool  19  in position in to thereby lock the phaser vane  13  in position. 
     In operation, the spool  19  can be moved to various positions between the advanced position shown in  FIG. 2  and the retarded position shown in  FIG. 3  based upon various engine parameters monitored by the ECU  31  which utilizes this information to operate the control actuator  30 . Hence, as shown  FIG. 2 , when the ECU  31  controls the control actuator  30 , i.e. the step motor, to pull the spool  19  to the left from its null position ( FIG. 2 ), the oil in the valve advance chamber  21  is pushed to the phaser advance chamber  15  while the oil in the phaser retard chamber  16  is pulled to the valve retard chamber  22 , causing the vane  13  to advance. The control actuator  30  is then locked in position in order to avoid any further motion of the vane  13  toward the advance or retard direction. 
     Similarly, referring now to  FIG. 3 , when the ECU  31  controls the control actuator  30 , i.e. the step motor, to pushes the spool  19  to the right from its null position ( FIG. 1 ), the oil in the valve retard chamber  22  is pushed to the phaser retard chamber  16  while the oil in the phaser advance chamber  15  is pulled to the valve advance chamber  21 , causing the vane  13  to retard. Then the control actuator  30  is locked in position in order to avoid any further motion of the vane  13  toward the retard or advance direction. 
     The present description allows the position of the angle of the phaser vane  13  to be determined directly by the position of the spool  19 . Further, the vane&#39;s moving speed can be increased as compared to a conventional VCT system in which oil is pumped from the phaser advance chamber to the phaser retard chamber because moving the spool is not dependant on the volumetric capacity of the pump. 
     Referring now to  FIG. 4 , there is shown a modification of the VCT system  10  shown in  FIGS. 1 to 3 . The VCT system  10  in  FIG. 4  is further provided with a locking mechanism preventing the vane  13  from retarding or advancing further depending on the engine operations. The locking mechanism includes two solenoid valves  40  respectively disposed within the advance line  11  and the retard line  23  and two additional feed lines  41 ,  42 , each incorporating a check valve  43 ,  44 . The first additional feed line  41  is connected at one end to the first feed line  24  and at the other end to the advance line  11  in such a way that one of the solenoid valve  40  is disposed between the two check valves  44 ,  25 . Similarly the second additional feed line  42  is connected at one end to the second feed line  26  and at the other end to the retard line  23  in such a way that the other solenoid valve  40  is disposed between two check valves  43 ,  27 . 
     It will be noted that in another modification of the VCT shown in  FIG. 4 , the VCT system  10  can be provided with only two check valves, each check valve being located within the first and second feed lines. 
     In operation, when the ECU  31  monitors that the angle of the vane  13  needs to be changed, the ECU  31  commands the two solenoid valves  40  toward the open position and then the spool  19  is moved by the control actuator  30  to thereby move the vane  13  to a new position. At this new position, the ECU  31  closes the two solenoid controlled valves  40  ensuring thus that the VCT phaser is locked. 
     Although the above examples describe a step motor as a control actuator, it will be appreciated that the control actuator may be a solenoid or another type of motor driving through a self-locking system such as a worm gear.