Patent Publication Number: US-2007113813-A1

Title: Two-step rocker arm having roller element cam followers

Description:
TECHNICAL FIELD  
      This invention relates to a two-step switchable rocker arm for an internal combustion engine.  
     BACKGROUND OF THE INVENTION  
      Some prior art valvetrains are selectively adjustable to vary the amount of valve travel during opening. Typically, such valvetrains are selectively adjustable between a low-lift mode, in which the valvetrain causes an engine poppet valve to open a first predetermined amount (with lost motion), and a high-lift mode, in which the valvetrain causes the poppet valve to open a second predetermined amount that is greater than the first predetermined amount. This transitioning is accomplished with the use of a two-step rocker arm having an inner and an outer rocker arm assembly operating in contact with a camshaft having two distinct cam profiles, a low lift cam and a high lift cam. Prior art two-step rocker arms employ a roller element cam follower cam interface at the inner rocker arm, which is typically utilized for low lift mode. Additionally, prior art two-step rocker arms employ a sliding cam interface for the outer rocker arm, which is typically selectively lockable to the inner rocker arm to allow the high lift mode of operation.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a valvetrain configuration in an internal combustion engine in which a plurality of two-step rockers enable the engine poppet valves to switch between two lift profiles.  
      Provided is a two-step rocker arm assembly having a movable outer rocker arm characterized by two rail portions spaced a distance apart from one another and defining an open space therebetween. Each of the rail portions has a generally cylindrical roller element cam follower rotatably attached thereto for engagement with a high-lift cam. An inner rocker arm is movably connected to the outer rocker arm such that at least a portion of the inner rocker arm is in the open space. The inner rocker arm has another generally cylindrical roller element cam follower rotatably attached thereto for engagement with a low-lift cam. The outer rocker arm and the inner rocker arm are selectively lockable for unitary movement.  
      Also provided is a valvetrain having a camshaft with a low-lift cam and two high-lift cams, the two high-lift cams being on opposite sides of the low-lift cam. Also included is a movable outer rocker arm characterized by two rail portions spaced a distance apart from one another and defining an open space therebetween. Each of the rail portions has a generally cylindrical roller element cam follower rotatably attached thereto for engagement with a respective one of the high-lift cams. Additionally, an inner rocker arm is movably connected to the outer rocker arm such that at least a portion of the inner rocker arm is in the open space. The inner rocker arm has a generally cylindrical roller element cam follower rotatably attached thereto for engagement with the low-lift cam. The outer rocker arm and the inner rocker arm are selectively lockable for unitary movement.  
      The above features and advantages, and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a vertical cross-sectional fragmentary view of a valvetrain assembly illustrating a two-step rocker arm in accordance with the present invention; and  
       FIG. 2  is a partial isometric view of the valvetrain assembly shown in  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      According to the recent art on switchable rockers, such as commonly assigned U.S. Pat. No. 6,769,387, entitled “Compact Two-Step Rocker Arm”, and hereby incorporated by reference in its entirety, the basic kinematic arrangement for a switchable rocker includes two arms pivoted at one end using a pin joint, therefore, having a freedom of relative rotation with respect to each other. This rotational motion takes place against a biasing spring placed between the arms, preloaded in assembly urging each arm to rotate away from each other. Each arm encompasses a follower surface or cam interface in contact with a respective cam lobe. The two cam lobes, defined as the high-lift cam lobe and the low-lift cam lobe, act simultaneously on their respective cam interface. The arm that follows the motion of the low-lift cam lobe is pivoted at one end and contacts the engine poppet valve at the other end. In the low-lift mode of operation of the engine poppet valve, the low-lift cam lobe is the controlling motion generator. In this mode, the high-lift cam lobe displaces its respective arm to idle in relative rotational motion centered around the pin joint. In the high-lift mode of operation of the engine poppet valve, a mechanical-locking device is hydraulically actuated to prevent the relative rotational motion of the two arms with respect to each other. Because the displacement of the high-lift cam lobe is larger than the low-lift cam lobe, the high-lift cam lobe becomes the controlling motion generator. The locking device may be a circular pin located on one of the arms, hydraulically actuated against a biasing spring, and urged towards a receiving circular opening located on the other arm. Upon decrease of the actuation pressure, the actuation pin is retracted out of the receiving opening, thereby switching the control of the engine poppet valve motion back to the low-lift cam lobe.  
      The present invention specifically relates to a valvetrain configuration employing a two-step rocker arm having roller element cam followers.  
      Referring to  FIGS. 1 and 2 , the valvetrain assembly  10  includes a camshaft  12  engaged with a two-step rocker arm (or two-step rocker arm assembly)  14  for actuating a poppet valve  16  against the force of a poppet valve spring  18 . A hydraulic lash adjuster  20  is positioned within a bore or aperture  22  in a cylinder head  24 , shown in  FIG. 1 , to engage the two-step rocker arm assembly  14  in a manner to compensate for lash and, with certain designs, selectively deactivate the poppet valve  16 . The  FIGS. 1 and 2  show a single two-step rocker arm  14 , poppet valve  16 , and hydraulic lash adjuster  20 , but, of course, in an operating engine, multiples of each would be provided for each cylinder.  
      The camshaft  12  includes low lift cams  26  and high lift cams  28 . The two-step rocker arm assembly  14  includes an inner rocker arm assembly  30  and an outer rocker arm assembly  32  which are pivotably joined by a generally cylindrical pivot shaft  34 . The inner rocker arm assembly  30  includes an inner rocker arm  36 . The outer rocker arm assembly  32  includes an outer rocker arm  38  characterized by two rail portions  40  longitudinally oriented with respect to the rocker arm assembly  14 , spaced a distance apart from one another, and forming an open space  42  therebetween. Tie bar portions  44  of the outer rocker arm assembly  30  interconnects the two rail portions  40 . The inner rocker arm  36  and the outer rocker arm  38  are preferably investment cast. The inner rocker arm assembly  30  is at least partially located within the open space  42 .  
      The pivot shaft  34  is press fit into the inner rocker arm  36 . The pivot shaft  34  has a close, but non-interference fit, through bores or apertures  46  in each of the rail portions  40  of the outer rocker arm  38 . The inner rocker arm  36  includes a valve stem contact pad  48  at a first end  50  adjacent to the pivot shaft  34 . The press fit design for the pivot shaft  34  allows for a traditional valve to rocker arm interface by virtue of the geometry at the valve stem contact pad  48 . Alternatively, the pivot shaft  34  may be press fit into the outer rocker arm apertures  46  and have a close, but non-interference fit, to the inner rocker arm  36 . The poppet valve  16  has a rotational symmetry about an axis A, shown in  FIG. 1 . The pivot shaft  34  is disposed in relation to the contact pad  48  such that the axis A substantially intersects the pivot shaft  34 .  
      The inner rocker arm assembly  30  also includes a roller element cam follower  52 , shown in  FIG. 2 , rotatably disposed in an opening defined by the inner rocker arm  36 . The inner rocker arm assembly  30  also includes locking pins (not shown) used to selectively prevent relative motion between the inner rocker arm  36  and the outer rocker arm  38 , thereby selectively providing lost motion as described above. Accordingly, lost motion may be achieved in the two-step rocker arm assembly  14  thereby providing low-lift, high-lift and zero-lift modes. The operation of a two-step rocker arm is described in more detail in the above referenced U.S. Pat. No. 6,769,387, entitled “Compact Two-Step Rocker Arm”.  
      The inner rocker arm  36  also includes a valve stem guide ear  54  on each side of the valve contact pad  48 . The inner rocker arm  36  also defines a boss portion, or pivot interface,  56  against which the hydraulic lash adjuster  20  abuts, and about which the inner rocker arm  36  is pivotable. Within the scope of the claimed invention, rail portions  40  and tie bar portions  44  may or may not be part of a one-piece outer rocker arm. For example, the rail portions  40  and tie bar portions  44  may be separate members rigidly connected to one another to form the outer rocker arm  38 .  
      The roller element cam follower  52 , shown in  FIG. 2 , is configured for engagement with the low-lift cam  26 , which causes the inner rocker arm assembly  30  to pivot about the hydraulic lash adjustor  20  at the pivot interface  56 . The roller element cam follower  52  is rotatable with respect to the inner rocker arm  36  on an axle  58 , shown in phantom in  FIG. 1 . The axle  58  is preferably laser welded in place for increased structural rigidity of the inner rocker arm assembly  30 . A torsion spring  60 , shown in  FIG. 1 , engages both the inner and outer rocker arms  36 ,  38  for biasing the inner and outer rocker arms  36 ,  38  in an opening direction about the pivot shaft  34 .  
      The outer rocker arm assembly  32  also includes a roller element cam followers  62  located in openings defined by each rail portion  40  of the outer rocker arm  38 . Each of the roller element cam followers  62  is rotatable with respect to the rail portions  40  on axles  64 . Each of the axles  64  are preferably laser welded to the respective rail portion  40  for increased structural rigidity of the outer rocker arm assembly  32 . The roller element cam followers  62  contact the high lift cams  28  for maximum lift of the poppet valve  16  when the two-step rocker arm assembly  14  is locked, that is, when the two-step rocker arm assembly  14  is not operating in “lost motion”, or low lift, mode. Those skilled in the art will recognize other methods of securing the axles  58  and  64  while still remaining within the scope of the present invention. In the preferred embodiment the roller element cam followers  52  and  62  will have an equal diameter.  
      By employing the roller element cam followers  52  and  62  within both the inner and outer rocker arm assemblies  30  and  32 , respectively, the friction characteristics of the cam interface may be reduced over a sliding cam interface. Additionally, by employing the roller element cam followers  52  and  62 , the two-step-rocker arm assembly  14  of the present invention may require less lubrication than a design that employs both a sliding cam interface and roller element cam followers. The two-step rocker arm  14  of the present invention enables the material properties of the camshaft  12  to be optimized for contact with a single type of material, the roller element cam followers  52  and  62 , rather than two different materials, e.g. a roller element cam follower of one material and a sliding cam interface of another material.  
      While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.