Abstract:
Hydraulically operated camshaft phasers are described which comprise a drive member and a driven member having a fixed range of angular adjustment and a means for mechanically locking the position of the driven member relative to the drive member when there is insufficient hydraulic pressure to operate the phaser. In the invention, the lock operates in an intermediate position within the range of adjustment, and a hydraulic circuit is provided for biasing the phaser towards the intermediate position for the lock to engage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 of United Kingdom Patent Application No. 0607642.6 filed Apr. 19, 2006. 
   FIELD OF THE INVENTION 
   The present invention relates to a hydraulically operated camshaft phaser and is particularly concerned with locking the phaser in a preset position. 
   BACKGROUND OF THE INVENTION 
   It is known for hydraulically operated camshaft phasers to be fitted with a locking system to control the position of the phaser when there is insufficient oil supply pressure to do so. An example of such a system is disclosed in GB 0428063.2. Conventionally, the locking system holds the phaser at one extreme of its operating range such that it will be returned to the locking position either by the camshaft drive torque, or by a simple return spring arrangement. 
   It has also been proposed in the prior art (see for example GB 2372797) to lock the phaser in an intermediate position, as this allows better optimisation of the engine start-up position for the phaser. However, the prior art only discloses the use of a spring to bias the phaser towards the intermediate position in which it can be locked by the locking system. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided a camshaft phaser comprising a drive member and a driven member having a fixed range of angular adjustment, a means for locking the position of the driven member relative to the drive member at an intermediate position within the range of adjustment, and a means for biasing the phaser towards the intermediate position where the locking means will engage, wherein the means for biasing the phaser comprises a hydraulic system allowing unidirectional oil flow within the phaser such that the phaser moves to the lock position under the action of camshaft torque reversals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
       FIGS. 1 and 2  are graphs showing the range of adjustment of different camshaft phasers; 
       FIGS. 3 to 7  show an embodiment of the invention in a locked position;  FIG. 3  being a plan view from above;  FIG. 4  a section along the line IV-IV in  FIG. 3 ;  FIG. 5  a section along the line V-V in  FIG. 3 ;  FIG. 6  a section along the line VI-VI in  FIG. 4 ; and  FIG. 7  a section along the line VII-VII in  FIG. 4 ; 
       FIGS. 8 to 12  are views corresponding to  FIGS. 3 to 7  of the first embodiment of the invention, showing the phaser in an unlocked position; and 
       FIG. 13 to 18  show a second embodiment of the invention;  FIG. 13  being a plan view from above showing the phaser mounted on a camshaft;  FIGS. 14 to 16  sections along the lines XIV-XIV, XV-XV and XVI-XVI in  FIG. 13 , respectively;  FIG. 17  an axial section; and  FIG. 18  a partially exploded view. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention may be used in a phaser for controlling the timing of intake/exhaust valve opening relative to the crankshaft, as shown in  FIG. 1 . The graph  10  shows a valve event when the phaser is at one end of its range and graph  12  the event of the same valve when the phaser is at the opposite end of its range. Normally, the locked position when the oil pressure is too low to operate the phaser is selected to be one or other of these two end positions but in the present invention, the phaser is designed to lock in a central position, such as the position represented by the graph  14 . 
   The invention can also be used in a phaser which controls the valve lift using a cam-summation system as shown in  FIG. 2 . The valve events in the end positions of the phaser are represented in  FIG. 2  by the graphs  20  and  22  and the valve event in the locked position by an intermediate graph  24 . 
   In the illustrated embodiments of the invention, the phaser is a vane-type phaser which is well known in the art. A full description of a similar phaser and locking pin are to be found in GB 2413168 and need not be repeated in the present context. Essentially, the phaser comprises a drive member or stator which is connected for rotation with the engine crankshaft and a driven member or rotor which comprises two end plates connected to vanes which move in and, are sealed relative to, arcuate recesses in the stator, each vane dividing its recess into two opposed working chambers. As oil is pumped into one working chamber and allowed to escape from the other, the rotor is rotated relative to the stator to vary the phase of the camshaft relative to the crankshaft. A locking pin, which is mounted in a bore in the stator, is hydraulically retracted when there is sufficient oil pressure to rotate the rotor relative to the stator. In the absence of sufficient oil pressure, an internal spring expands the locking pin and its end engages in the hole in the end plate to lock the rotor and stator relative to one another. 
   The embodiments of the invention rely on the torque reaction of the valve train rather than a spring to return the phaser to a central position. 
   As a cam of the camshaft attempts to open a valve, the camshaft drive train encounters a retarding torque but when a valve attempts to close and its movement is resisted by a cam, the camshaft encounters an accelerating torque. Consequently the torque reaction of the valve train undergoes periodic reversals. The embodiments of  FIGS. 3 to 18  use one-way valves to allow oil to escape from the double acting arcuate working chambers in response to reaction torques acting in one direction but not the other. 
   In the embodiment shown in a locked position in  FIGS. 3 to 7  and in an unlocked position in  FIGS. 8 to 12 , the construction of the phaser is the same as that described above in terms of the design of the rotor, the stator, the vanes and the end plates. However, instead of a single pin engaging in a hole in one of the end plates, the locking mechanism comprises two separate locking pins  200 ,  202  that each engage in respective slots  204 ,  206  in the opposite end plates  208 ,  210  of the phaser. 
   When the phaser is not in its locked position and the oil pressure in the supply to the phaser drops, only one locking pin  200 ,  202  can engage in its slot whilst the other will run against the inner surface of the end plate  208 ,  210 . As shown in  FIGS. 4 and 5 , the bore of each locking pin  200 ,  202  is connected to a one-way valve  212 ,  214  by an oil drilling  216 ,  218  that also enters the adjacent vane cavity  220 . When a locking pin  200 ,  202  is engaged in its slot  204 ,  206 , oil is able to flow through the drilling  216 ,  218  by passing through a groove  221  in the outer surface of the locking pin. On the other hand, when the locking pin  200 ,  202  is unable to enter its slot, this oil drilling is obscured by the diameter of the pin, preventing any oil flow through the associated one-way valve  212 ,  214 . Therefore, in the situation where the phaser is unlocked, oil may only flow through one of the one-way valves  212 ,  214  and the direction of oil exchange between the working chambers of the adjacent cavities acts to move the phaser to the position where the second locking pin will engage in its slot. 
   The one-way valves thus allow oil to pass from one vane cavity to another under the action of camshaft torque reversals. Disabling one of the valves will therefore allow the phaser to move only in one direction when it is subjected to torque reversals, the hydraulic circuit being arranged to allow the phaser to move only in the direction of the locking position. 
   An embodiment of the invention having a hydraulic circuit with a single locking pin  300  is shown in  FIGS. 13 to 18 . In this case, the locking pin  300  moves radially. 
   As with the previous hydraulic circuit, there are two opposing one-way valves, one of the valves  312  being shown in the section of  FIG. 15 . Both of these circuits are connected to one of the vane cavities  320  such that oil may only flow into the cavity, whilst the other side of the one-way valve is connected to the bore of the locking pin  300 . When the locking pin is engaged, it obscures the oil feeds to both one-way valves, but when it is disengaged, as shown in  FIG. 28  the oil feeds are connected by the reduced diameter portion of the locking pin  302 . 
     FIGS. 16 to 18  illustrate how the oil flow through the one-way valves may be controlled to ensure that the phaser will always return to its locked position. A third drilling  342  also leads into the locking pin bore as shown most clearly in  FIG. 17 , and this hole leads through a thin manifold plate  340  (the centre component of the exploded view in  FIG. 18 ) into a slot  344  in the front plate  346  of the phaser. The slot  344  acts to connect the first hole to two other holes in the manifold plate that are selectively covered and uncovered by one of the phaser vanes, as shown in dotted lines in  FIG. 16 . 
   In the locked position, the vane predominantly obscures both holes—as shown in  FIG. 16 , but any movement of the phaser away from the locked position will uncover one of the holes, allowing oil to flow out of the associated cavity under the action of the camshaft torque reversals, and into the opposing set of cavities via the one-way valve  312 . Thus the phaser will always try to return to the position where both of the holes are obscured under the action of the camshaft torque reversals, allowing the locking pin to engage. 
   In this embodiment, the locking pin  300  is disengaged by a separate oil pressure signal from the front bearing of the camshaft, rather than one of the control oil feeds to the phaser. 
     FIG. 13  also shows how the phaser may be mounted to a single cam phaser camshaft, and SCP camshaft being one in which cams mounted for rotation about the same axis can be phase shifted relative to one another. The camshaft in  FIG. 13  comprises a tubular first shaft which concentrically surrounds and is rotatable relative to a second shaft, relative rotation of the two shafts causing selected cams of the camshaft to rotate relative to other cams of the camshaft. Each of the shafts of the camshaft assembly is connected for a rotation with a different respective one of the two driven of the phaser and the connections between the shafts of the camshaft assembly and the driven members of the phaser are interchangeable. 
   It will of course be clear that the same principle can be applied to phase a solid camshaft relative to the crankshaft.