Abstract:
This invention relates to an actuator assembly for a turbocharger pressure control valve assembly and a method of assembling a pressure control assembly of a turbocharger. The assembly comprises an actuator coupled to an actuator rod which in use is coupled to the valve assembly to control the position thereof. The actuator rod comprises an elongate member at least a portion of which is flexible. The actuator rod may be a resilient member which may be substantially straight when unstressed. In use the actuator rod may extend in a substantially straight line between the actuator and the valve assembly.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a turbocharger incorporating a wastegate and wastegate actuator, and in particular relates to the manner in which the actuator is connected to the wastegate.  
         BACKGROUND OF THE INVENTION  
         [0002]    Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. For instance, in a centripetal turbine the turbine housing defines an annular inlet passageway around the turbine wheel and a generally cylindrical axial outlet passageway extending from the turbine wheel. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.  
           [0003]    Turbochargers incorporating wastegates are also well known. A wastegated turbocharger has a bypass passageway between the exhaust inlet and exhaust outlet portions of the turbine housing to enable control of the turbocharger boost pressure. A wastegate valve is located in the passageway and is controlled to open the passageway when the pressure level of the boost air increases to a predetermined level, thus allowing some of the exhaust gas to bypass the turbine wheel preventing the boost pressure from rising further. The wastegate valve is generally actuated by a pneumatic actuator operated by boost air pressure delivered by the compressor wheel. The position of the wastegate valve, and thus the amount of exhaust gas permitted to bypass the turbine wheel, is thus controlled in direct response to variations in the boost pressure.  
           [0004]    A conventional pneumatic actuator comprises a spring loaded diaphragm or sliding seal housed within a canister (referred to as the wastegate actuator can) which is mounted to the compressor housing. The diaphragm seal acts on a connecting rod which actuates the wastegate valve assembly which is mounted in the turbine housing.  
           [0005]    The actuator can is connected to the compressor outlet via a hose to deliver boost air to the can which acts on the diaphragm (or sliding seal) to oppose the spring bias. The spring is selected, and the actuator and wastegate valve initially set, so that under low boost conditions the wastegate valve remains closed. However, when the boost pressure reaches a predetermined maximum the diaphragm seal is moved against the action of the spring and operates to open the wastegate valve (via the connecting actuator rod) thereby allowing some exhaust gas to bypass the turbine wheel.  
           [0006]    In conventional arrangements the wastegate valve is mounted on a valve stem which extends through the turbine housing and which is rotated to open and close the valve. Rotation of the valve stem is achieved by the reciprocal motion of the actuator rod via a lever arm which links the end of the actuator rod to the valve stem. To accommodate the motion of the actuator rod there is a pivotable joint between the lever arm and the actuator rod, the opposite end of the actuator lever being secured (typically by welding) to the end of the valve stem. For accurate operation of the actuator it is also important that the diaphragm seal maintains alignment within the canister, and thus that the rod maintains its alignment along the axis of the actuator can. It is therefore known to design the pivotal joint between the actuator rod and the lever arm to allow a slight amount of movement along the axis of the actuator lever to limit the tendency of the actuator rod to pulled off-line as it reciprocates.  
           [0007]    The “lift off point”, i.e. that pressure at which the wastegate valve begins to open, is critical to operation of the wastegate and therefore must be very carefully set when the actuator and wastegate are assembled to the turbocharger. The precise actuator can pressure at which the diaphragm begins to move is dependent upon the preload of the spring used. Unfortunately, because tolerances to which springs can practically be manufactured mean that there can be variations in spring rate from one spring to the next, it is necessary to set the lift off point of each turbocharger individually.  
           [0008]    One method of carrying out the initial set up of the conventional actuator assembly described above, is a process known as “weld to set”. The actuator can, actuating rod and actuator lever are pre-assembled, and mounted to the turbocharger. The wastegate valve is then clamped shut from within the turbine housing and the actuator can is pressurised to the desired lift off pressure. With the diaphragm, actuator rod and valve thus held in their respective relative positions immediately prior to lift off, the end of the actuator lever welded to the valve stem. Accordingly, any increase in the pressure supplied to the actuator above the predetermined lift off pressure will cause the valve to open.  
           [0009]    A known alternative to the above is to use an adjustable length actuator rod, typically comprising a threaded rod and rod end. The set point is achieved by adjusting the length of the rod, either by turning the rod end or a nut captured in the rod end assembly.  
           [0010]    European patent publication number 0 976 919 discloses a two-part actuator rod which overcomes many of the disadvantages of the conventional actuator rod described above. The two parts of the rod are connected via a spherical joint located towards the wastegate end of the rod which allows rotational freedom between the end of the rod connected to the actuator and the end of the rod connected to the actuator lever. This arrangement greatly simplifies initial set up by obviating the need to pre-assemble the lever to the actuator rod (the lever instead being pre-assembled to the wastegate valve stem) or the need to provide an adjustable length rod. However, both of the above actuator assemblies suffer the disadvantage that the respective pivotable joints add to the component cost and provide a site for wear to occur.  
         OBJECT OF THE INVENTION  
         [0011]    Accordingly, it is an object of the present invention to provide an actuating assembly for a turbocharger wastegate which overcomes problems associated with conventional arrangements discussed above. In particular it is an object of the present invention to provide an improved actuator rod.  
         SUMMARY OF THE INVENTION  
         [0012]    According to a first aspect of the present invention there is provided an actuator assembly for a turbocharger pressure control valve assembly, the actuator assembly comprising an actuator coupled to an actuator rod which in use is coupled to the valve assembly to control the position thereof, wherein the actuator rod comprises an elongate member at least a portion of which is flexible.  
           [0013]    According to a second aspect of the present invention there is provided a turbocharger including a pressure control assembly comprising an actuator, a valve assembly, and an actuating rod connected between the actuator and the valve assembly, the actuator controlling operation of the valve assembly via the actuator rod, wherein the actuator rod comprises an elongate member at least a portion of which is flexible.  
           [0014]    Preferably the actuator rod comprises a multi-strand cable e.g. a steel cable.  
           [0015]    The provision of a flexible actuator rod offers a number of advantages over conventional rigid actuator rods as is discussed in more detail in the following description. It is to be understood that the degree of flexibility could vary from one embodiment of the invention to another. Preferably the rod is sufficiently flexible to flex under normal operating conditions as it moves to actuate the valve assembly, but not, for instance, to flex significantly under force of gravity if held at one end only. The rod need not be flexible along its entire length although preferred embodiments are flexible at least along a substantial portion of the length of the rod.  
           [0016]    According to a third aspect of the present invention there is provided a method of assembling a pressure control assembly of a turbocharger, the turbocharger comprising a turbine housing and a compressor, the pressure control assembly comprising a valve assembly mounted within the turbine housing, a pneumatic actuator mounted to the turbocharger to receive pressurised air from the compressor or an external air supply, a flexible actuator rod extending from the pneumatic actuator, and a lever arm extending from the valve assembly and the turbine housing and linking the actuator rod to the valve assembly, the method comprising:  
           [0017]    assembling the valve assembly and lever arm on the turbine housing;  
           [0018]    assembling the pneumatic actuator and actuator rod as a sub-assembly;  
           [0019]    mounting the pneumatic actuator/actuating rod sub-assembly to the turbocharger; and  
           [0020]    securing the end of the actuator rod remote from the pneumatic actuator to the lever arm.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is an axial cross-section through a conventional turbocharger illustrating the major components of a turbocharger;  
         [0023]    [0023]FIG. 2 illustrates details of a conventional turbocharger wastegate valve and actuator assembly;  
         [0024]    [0024]FIG. 3 is a perspective view of a wastegate valve actuator assembly in accordance with an embodiment of the present invention;  
         [0025]    [0025]FIG. 4 shows a detail of the actuator assembly of FIG. 3;  
         [0026]    [0026]FIG. 5 is a further perspective view of the actuator assembly of FIG. 3, looking in the direction of arrow A on FIG. 3 and with part of the actuator can cut away to reveal internal detail; and  
         [0027]    [0027]FIG. 6 shows internal details of the actuator can of FIGS. 4 and 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    Referring first to FIG. 1, this illustrates the basic components of a conventional centripetal type turbocharger. The turbocharger comprises a turbine  1  joined to a compressor  2  via a central bearing housing  3 . The turbine  1  comprises a turbine housing  4  which houses a turbine wheel  5 . Similarly, the compressor  2  comprises a compressor housing  6  which houses a compressor wheel  7 . The turbine wheel  5  and compressor wheel  7  are mounted on opposite ends of a common shaft  8  which is supported on bearing assemblies  9  within the bearing housing  3 .  
         [0029]    The turbine housing  4  is provided with an exhaust gas inlet  10  and an exhaust gas outlet  11 . The inlet  10  directs incoming exhaust gas to an annular inlet chamber  12  which forms a volute surrounding the turbine wheel  5 . The exhaust gas flows through the turbine and into the outlet  11  via a circular outlet opening which is coaxial with the turbine wheel  5 .  
         [0030]    Referring now to FIG. 2, this illustrates components of a conventional prior art wastegate valve and actuator assembly which is not visible from FIG. 1. The turbine housing  4  is provided with a bypass passageway (not shown) which communicates between the exhaust inlet  10  and the exhaust outlet  11  bypassing the turbine wheel  5 . The bypass passageway communicates with the exhaust inlet  10  via a circular opening (not shown) which is opened and closed by a valve member  13   a  of a wastegate valve  13  provided for controlling the flow therethrough. In FIG. 2 the part of the turbine housing  4  defining the inlet  10  is shown partially cut away to reveal details of the wastegate valve  13  which is operated by rotation of a valve stem  14  which extends through a bush  14   a  in the turbine housing  5 .  
         [0031]    Rotation of the valve stem  14 , and thus actuation of the wastegate valve  13 , is achieved by an actuator assembly mounted on the outside of the turbocharger and comprising a spring loaded pneumatic actuator  15  which is linked to the valve stem  14  via a connecting rod  16  and actuating lever  17 . The actuator can  15  receives compressed air from the outlet of the compressor  2  and thus is conventionally mounted to the outside of the compressor housing  6  by an appropriate bracket (not shown). Details of an appropriate mounting arrangement are omitted from FIG. 2 for simplicity but may be entirely conventional.  
         [0032]    The actuator  15  comprises a diaphragm or sliding valve (e.g. piston), not shown, mounted within a cylindrical canister (the actuator can)  18  on one end of the actuator rod  16 . The actuator rod  16  extends from the front of the actuator can  15  towards the turbine housing  4  and wastegate valve  13 . In FIG. 2 the actuator can is shown cut away to reveal detail of a coil spring  19  mounted coaxially around the actuator rod  16  and acting between the diaphragm/sliding seal (not shown) and the front end of the actuator can  18 . The coil spring  19  thus biases the actuator rod towards the rear of the actuator can  18 .  
         [0033]    The actuator rod connects the diaphragm/seal of the actuator  15  to the wastegate valve  13  via the lever arm  17 . The end of the actuator rod  16  is connected to the actuator lever  17  via a pivotal connection  20  to allow relative movement between the two whereas the lever  17  is fixed to the valve stem  14 , typically by welding as mentioned above.  
         [0034]    In use, the wastegate valve  13  will in low boost conditions be held closed by the action of the spring  19 . However, once the pressure in the compressor outlet reaches a predetermined limit, the pressurised air transmitted to the actuator  15  forces the diaphragm/seal to move against the action of the spring  19  thereby opening the wastegate valve  13  to allow inlet exhaust gas to bypass the turbine. In this way, the maximum boost pressure produced by the turbocharger can be controlled and limited.  
         [0035]    In the illustrated example, the actuator rod  16  is of adjustable length. That is, the actuator rod  16  has two portions which can be moved relative to each other by rotation of an adjusting nut  16   a  to lengthen or shorten the rod. This allows the length of the rod to be adjusted at initial set up to ensure that at the “lift off” pressure, i.e. that pressure at which the valve begins to open, the actuator rod  16  is properly aligned with the axis of the actuator  15  (provided the actuator can is itself correctly mounted to the compressor).  
         [0036]    FIGS.  3  to  6  illustrate an embodiment of an actuator assembly in accordance with the present invention which overcomes a number of disadvantages of the conventional arrangement described above. Essentially, the actuator assembly comprises a pneumatic actuator  21  an actuator rod  22  and an actuating lever  23 . The actuator  21  may be entirely conventional and may be mounted to the compressor housing in a conventional way so as to receive air from the compressor outlet via an inlet pipe  21   a . The fundamental differences between the actuator assembly of the invention and the conventional system described above lie in the nature of the actuating rod  22  and its manner of connection to the suitably modified lever  23 .  
         [0037]    In accordance with the invention, the actuating rod  22  is flexible, comprising in this embodiment a multi-strand steel cable. The actuator end of the rod  22  is secured within one end of a steel sheath  24  (for instance by crimping) the other end of which is secured to a piston rod  25  (again by crimping or welding etc). The piston rod  25  supports a piston  26  which is slidable along the axis of the actuating can  27 . A coil spring  28  is mounted between the piston  26  and the front end of the actuating can  27  to bias the piston  26 , and thus actuator rod  24 , towards the rear of the actuator can  27  thereby holding the valve assembly in a closed position. The actuator is provided with an actuator rod guide  29  in the form of a cylindrical boss which receives the end of the sleeve  24  which extends into the actuator can  27 . This ensures that the actuator rod is effectively orientated along the axis of the actuator can  27  at its point of entry into the actuator. This helps ensure that the force exerted on the piston rod  25 , and thus piston  26 , by the tension in the actuator rod  22  acts along the axis of the actuator can  27 , i.e. in the direction of movement of the piston  26 , even if the actuator is misaligned with the actuating lever  23 . An additional guide  30  surrounds the piston  26  to further prevent the piston being pulled off-axis as it moves within the actuator can  27 .  
         [0038]    The opposite end of the actuator rod  22  is secured to the lever  23  which is formed as a clamp. Pressure is applied to the end of the actuator rod  22  to hold it firmly in position by tightening a clamping nut  24 . The lever  23  further defines an aperture  25  enabling connection of the lever arm to the wastegate valve stem, for instance by the conventional method of welding the lever to the valve stem  14 .  
         [0039]    Provision of a flexible actuator rod in accordance with the invention is based on the realisation that the actuator rod is in fact always in tension since the wastegate valve is always biased towards an open position against the action of the actuator spring by the gas pressure within the exhaust gas inlet. Thus, when moving the valve from a closed to an open position it is not actually necessary to push the valve open (as with the action of a conventional rigid actuator rod) but rather simply to allow the valve to open under the pressure of the exhaust inlet gas. Adoption of a flexible actuator rod therefore has no detrimental effect on the basic operation of the actuator assembly and instead provides a number of benefits over conventional rigid actuating rods.  
         [0040]    The ability of the actuator rod to flex obviates the need to provide a pivotal connection between the actuator rod and the actuating lever as with the conventional arrangement described above, or to provide a spherical joint as with the actuator rod described in European patent publication number 0 976 919 (as mentioned above). This reduces the number of components needed in manufacture of the actuating assembly and removes the pivotal joints which can wear during use.  
         [0041]    The need for an adjustable length actuator rod is also obviated since the effective length of the rod (i.e. the length of the rod extending between actuating the lever and the actuator) is determined at set up.  
         [0042]    The flexible nature of the actuator rod allows for a certain degree of missalignment between the actuator  15  and the wastegate valve assembly without exerting significant off-axis force on the actuator diaphragm/sliding seal etc. This further reduces the cost and complexity of manufacture by increasing manufacturing tolerances of the various components and simplifying assembly.  
         [0043]    A further significant advantage of a flexible actuating rod is that it can provide a degree of damping between the wastegate valve assembly and the actuator. In the particular embodiment described in which the actuator rod is a multi-strand cable, energy is dissipated through friction as strands of the cable rub together as the cable flexes. This damps the vibration caused by fluttering of the wastegate valve as the pressure within the exhaust gas inlet which fluctuates is a recognised problem of conventional wastegate actuator assemblies.  
         [0044]    It will be appreciated that many modifications may be made to the embodiment of the invention described above. For instance, the actuator rod end may be connected to the actuating lever by any appropriate means and not just by clamping as illustrated. Preferred means allow the effective length of the actuator rod to be determined at initial set up and include alternatives such as welding (or otherwise bonding) or crimping. The actuator lever itself can be of any desired configuration suitable for the chosen connection method and the location of the valve assembly.  
         [0045]    The actuator rod itself need not be a multi-strand steel cable. Multi-strand cables are preferred because of the improved damping offered by interaction of the cable strands. However, other cable materials may be employed including other metals and synthetic materials including plastics fibres. The cable may for instance be a composite of fibres of different materials.  
         [0046]    As mentioned above, the actuator rod need not be a cable but could comprise an essentially single piece rod but having the required flexibility. Such a rod may have the dimensions of a conventional rod but be fabricated from material with the required inherent flexibility. Alternatively, a suitable rod can be manufactured from a conventional material, such as steel, but made thinner than a conventional rod to again provide the required flexibility. The appropriately skilled person will appreciate the many possible variations.  
         [0047]    Moreover, the rod need not be flexible along its entire length. For instance a rod comprising one or more flexible portions joined to one or more rigid sections could be used. An essentially single piece flexible rod, possibly with connecting end pieces (such as the sleeve  24  of the illustrated example), is however preferred for mechanical simplicity.  
         [0048]    As mentioned above, the actuator may be entirely conventional. However, in the illustrated embodiment the actuator is modified by the provision of the actuator rod guide  29  and piston guide  30 . It will be appreciated that the exact form of these guides may vary considerably. Furthermore one or both of these guides could be omitted, particularly if steps are taken to ensure the actuator is correctly aligned with the actuator lever as with conventional assemblies. In addition, whereas in the illustrated example the actuator rod  22  is connected to the piston  26  via a piston rod  25  it will be appreciated that the actuator rod  22  could extend through the sleeve  24  and be connected directly to the piston. Indeed, the sleeve  24  could be omitted entirely.  
         [0049]    Again, it will be appreciated that many modifications could be made to the detailed design of the actuator can. For instance, the sliding piston actuator could be replaced by a diaphragm actuator. Other possible modifications will be readily apparent to the appropriately skilled person.