Abstract:
A hydraulic phase shifting mechanism is described for an engine camshaft subjected to torque fluctuations during operation. The mechanism employs a magnetorheological fluid as a hydraulic pressure medium and is controlled by selective application of a magnetic field to vary the flow properties of the magnetorheological fluid.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a national stage application filed under 35 USC 371 based on International Application No. PCT/GB2002/005464 filed Dec. 4, 2002, and claims priority under 35 USC 119 of United Kingdom Patent Application No. 0129333.1 filed Dec. 7, 2001. 
   FIELD OF THE INVENTION 
   The present invention relates to a phase shifting mechanism for an engine camshaft. 
   BACKGROUND OF THE INVENTION 
   Many camshaft phase shifting mechanisms (herein also termed phasers) have been proposed, and are in use, which are actuated hydraulically, often using engine oil. While these phasers have been demonstrated to be effective, they can have a high oil demand, sometimes requiring an auxiliary pump and an associated cost penalty. The presence of large amounts of oil in the engine cylinder head can cause problems with oil drainage and aeration, together with the problems associated with belt contamination in the case of belt driven camshafts. 
   Furthermore, the use of hydraulic actuation requires a control valve, which can control the oil flow in response to electrical inputs from the engine ECU, and this can have significant cost and packaging implications. 
   U.S. Pat. No. 5,056,477 discloses a phase shifting mechanism for an engine camshaft that is subjected to torque fluctuations during operation. The mechanism comprises a drive member and a driven member coupled for rotation with one another by means of a closed hydraulic circuit formed of a plurality of variable volume working chambers that are connected to one another in such a manner that the volume of one increases as the volume of another decreases. Flow of fluid between the working chambers, which is controlled by a solenoid valve, causes the phase of the drive member to be shifted relative to the driven member. 
   SUMMARY OF THE INVENTION 
   In accordance with its broadest aspect, the present invention provides a hydraulic phase shifting mechanism for an engine camshaft subjected to torque fluctuations during operation, which mechanism includes a closed hydraulic circuit, characterised in that the hydraulic circuit employs a magnetorheological fluid as a hydraulic pressure medium and in that an electromagnet is provided for selectively applying a magnetic field to vary the flow properties of the magnetorheological fluid, in order to control the flow of the pressure medium in the hydraulic circuit. 
   In accordance with a second aspect of the invention, is there is provided a phase shifting mechanism for an engine camshaft that is subjected to torque fluctuations during operation, the mechanism comprising a drive member and a driven member coupled for rotation with one another by means of a closed hydraulic circuit formed of a plurality of variable volume working chambers that are connected to one another in such a manner that the volume of one increases as the volume of another decreases, flow of fluid between the working chambers causing the phase of the drive member to be shifted relative to the driven member, characterised in that the chambers are filled with a magnetorheological fluid and are connected to one another through passages along which fluid flow is controlled by the selective application of a magnetic field. 
   Magnetorheological fluids are materials that respond to an applied magnetic field with a change in their properties. They have been utilised in both rotary and linear damping systems to provide a damping system whose damping coefficient can be controlled via the magnetic field applied to the fluid. 
   Phasers acting on an engine camshaft are always subjected to torque fluctuations on account of the reaction from the springs of the poppet valves. While a valve is being opened by a rising ramp on a cam, the camshaft offers resistance while the force of the valve spring is being overcome whereas when a valve is being allowed to close the valve spring applies a force tending to accelerate the camshaft. The reaction torques can thus be employed to advance and retard the camshaft relative to the crankshaft provided that a phaser is designed to allow relative movement in one sense while inhibiting movement in the other sense. 
   In a phaser where the drive member, i.e. the member connected for rotation with the crankshaft, is coupled hydraulically to the driven member. i.e. the member connected for rotation with the camshaft, it would be possible to allow movement in one sense but not the other by providing two flow paths between each pair of interconnected working chambers, the two paths containing oppositely biased non-return valves. Such a phaser would however require the application of magnetic fields to control fluid flow along two separate fluid flow paths. 
   Magnetorheological fluids have a very fast response time in that their flow properties change within milliseconds of application or removal of a magnetic field. In a preferred embodiment of the invention, this attribute is used to advantage in that a single flow path is provided between each pair of interconnected working chambers and an electromagnet is provided to apply to the fluid flowing along the flow path a magnetic field of modulated amplitude, the modulation having a frequency related to the speed of rotation of the camshaft and having a phase dependent upon the sense in which the relative phase of the drive and driven members is to be varied. 
   Because a single electromagnet can be used to bring about the desired change of phase, it is possible for the electromagnet to be fixed to the engine and to arrange for its magnetic circuit to traverse the fluid path, which must of necessity rotate with the camshaft. The fact that the electromagnet need not rotate with the camshaft makes for a simpler and more robust construction. 
   It is preferred for the phaser to be of the vane type, i.e. having radial vanes movable in arcuate cavities. In particular, the hydraulic circuit preferably comprises two diametrically opposed arcuate cavities that rotate with one of the drive and driven members, each being divided into two working chambers by a respective one of two radial vanes that rotate with the other of the two members. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view of an assembled phaser of the invention, 
       FIG. 2  is a front view of the phaser in  FIG. 1 , 
       FIG. 3  is a section along the plane A—A in  FIG. 2 , 
       FIG. 4  is a section along the place B—B in  FIG. 3 , 
       FIG. 5  is a section along the plane C—C in  FIG. 3 , and 
       FIG. 6  is an exploded perspective view of the phaser shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The illustrated phaser is similar in construction to the vane-type phasing systems that are often utilised in hydraulically controlled applications. The illustrated embodiment has two vane cavities but it will be appreciated by the person skilled in the art that many other layouts would clearly be possible. The phaser moves under the influence of camshaft torque reversals and hence requires no external source of energy. 
   The phaser  10  comprises a sealed unit filled with a magnetorheological fluid that is fixed by means of a bolt  14  on to the front end of an engine camshaft  12 . The casing of the sealed unit is formed of two parts, namely a sprocket  16  that is driven by the engine crankshaft and a dished cover  18  secured to the sprocket  16  by means of bolts  20 . 
   The flat cylindrical chamber contained between the sprocket  16  and the cover  18  contains a hub  22  which, as shown in  FIG. 4 , is keyed to the front end of the camshaft  12 . Two vanes  24  are located in radial slots in the hub  22 . Two crescent shaped blocks  26  are secured by bolts  28  to the sprocket  16 . The inner surfaces of the blocks  26  seal against the cylindrical radially outer surface of the hub  22  and their outer surfaces seal against the outer wall of the chamber defined by the dished cover  18 . Between them, the described components define, as best shown in  FIG. 4 , two arcuate cavities which are divided by the radial vanes  24  into four working chambers designated  50 ,  52 ,  54  and  56 . 
   The hub  22  and vanes  24  together form the driven member that rotates with the camshaft, while the remaining components shown in  FIG. 4  form part of the drive member that rotates with the engine crankshaft. Rotation of the drive member relative to the driven member is accompanied by a change in the volume of the four working chambers which are connected to one another through passages formed by grooves  34  in the end surface of the cover  18 . The working chambers are connected to each other in pairs, the chamber  50  being connected to the chamber  54  and the chamber  52  being connected to the chamber  56 . Thus, fluid displaced for example from the chamber  52  during a clockwise movement of the vanes  24 , as viewed in  FIG. 4 , will flow into the chamber  56 . 
   Each of the passages connecting a pair of working chambers includes a labyrinth that is defined between pole pieces  36  and  38 . The pole pieces  36 ,  38  are formed as ferromagnetic inserts that are mounted within the cover  18 . The cover  18 , the hub  22  and the blocks  26  are all formed of a non-ferromagnetic material, preferably aluminium, in order not to interfere with the magnetic circuit described in more detail below. 
   The interior of the sealed unit contains a magnetorheological fluid which fills the working chambers  50  to  56 . A seal  30  disposed between the hub  22  and the sprocket  16  and a second seal  44  surrounding the bolt  14  prevent the fluid from escaping. 
   A torque spring  32  acts between the hub  22  and the sprocket  16 . The spring  32  acts in the direction to advance the phase of the camshaft  12  relative to the sprocket  16 . However, while any valve is being opened by the camshaft  12 , it will experience a reaction torque that acts to retard it and overcomes the bias of the spring  32 . Thus at different times in the engine cycle the camshaft will experience reversing torques that act alternately in directions to advance and to retard the phase of the camshaft relative to the sprocket  16 . If fluid flow is permitted between the working chambers of the hydraulic circuit while a torque is acting, then the phase of the camshaft will change under the action of the torque. If on the other hand fluid flow along the passages  34  is prevented, then the phase of the camshaft will remain unaltered. The spring  32  is sized to ensure that the forces acting to advance and retard the camshaft are substantially equal so that adjustments should occur at the same rate in both directions. 
   When the magnetorheological fluid is exposed to a magnetic field within the passage  34 , it acts as a viscous liquid and because of the tortuous path of the labyrinth between the poles pieces  36  and  38 , no flow occurs and the coupling is locked. When the magnetic field is removed on the other hand, the fluid flows freely and permits the camshaft  12  to rotate relative to the sprocket  16 . Hence, by the use of a single stationarily mounted electromagnet supplying a magnetic field to the pole pieces  36 ,  38  it is possible to control the phase. 
   In particular, when a constant magnetic field is applied, then the hydraulic circuit is locked at all times and no change in phase can occur. If the magnetic field is is energised intermittently with a frequency proportional to the speed of rotation of the crankshaft then depending on the direction of the torque reaction on the camshaft at the time that the magnetic field is switched off, the phase of the camshaft can be advanced or retarded. 
   The magnetic field is generated by a stationary coil external to the phaser, mounted on the engine chain cover or similar. The magnetic field is generated between a ferrous ring  42  on the front of the phaser that contacts the outer component  38  of both labyrinths, and the central bolt  14 , which is ferromagnetic and contacts the shared centre section of both labyrinths. 
   The person skilled in the art will appreciate that there are a large number of modifications that could be made to the described phaser. The design and number of vanes and cavities could vary significantly from those proposed. Furthermore, it would be possible to design a similar system using hydraulic cylinders to position the camshaft rather than vanes in cavities. 
   There is an alternative control methodology that could be employed if the connecting passages between the vane cavities were of a different nature. Passages could be designed to connect the vane cavities via a one-way valve that would only allow the fluid to pass in one direction. A second set of passages could be provided allowing fluid to flow in the opposite direction. The camshaft timing could then be controlled by removing the magnetic field across the fluid in one or other set of passages. However, this approach requires two independent magnetic circuits, and makes the design of the unit more complex but it removes the need for the coil switching to be synchronised with the crankshaft rotation directly.