Abstract:
A two-step roller finger follower having a movable high-lift portion and a low-lift portion. A lock pin mechanism in the low-lift portion includes a lock pin that may be driven hydraulically into latched engagement with the high-lift portion. The lock pin assembly comprises a lock pin and a separable switching pin. A blocking clip mountable on the associated hydraulic lash adjuster includes first and second ends that extend into a switching pin locking groove to block axial motion thereof. A ramp on the follower body mates with a ramp on the blocking clip such that oscillatory motion of the follower engages and disengages the blocking pin from the locking groove. The clip unblocks the switching pin at only those times in the camshaft rotational cycle when complete locking and unlocking is assured, and ending well before the beginning of the next valve lift event.

Description:
TECHNICAL FIELD 
   The present invention relates to roller finger followers for actuating the valves of internal combustion engines; more particularly, to two-step roller finger followers for controllably activating and deactivating engine valves between high-lift and low-lift modes; and most particularly, to a two-step roller finger follower having a timing mechanism governing locking and unlocking action of a lock pin to prevent partial pin engagement and consequent premature pin ejection during a high-lift valve event. 
   BACKGROUND OF THE INVENTION 
   Two-step roller finger followers (RFF) for controllably activating and deactivating compression valves in a variable valve activation train in an internal combustion engines are well known. An RFF extends between a hydraulic lash adjuster (HLA) and the stem of a valve. Engagement of the RFF with a cam lobe of an engine camshaft causes the RFF to be pivoted about the HLA and thereby to depress the valve stem, opening the valve. 
   A two-step RFF mechanism allows an engine valve to be operated by two different cam lobe profiles, one with first and second portions of the mechanism locked together by a slidable lock pin (typically for high lift) and the other with the mechanism portions unlocked (typically low lift). 
   In prior art RFFs, a known problem exists in that the lock pin may be only partially engaged with the high-lift follower portion of the RFF when a high-lift valve event begins. In some instances, there is enough engagement to begin to open the valve but not enough engagement to complete the full valve event. At some point during the valve event, the load on the lock pin becomes insupportable, ejecting the lock pin from engagement with the high-lift follower portion. The effect of this ejection event is that the valve spring compression energy is instantly released and transferred to either the lower-lift cam profile or to the valve seat. 
   Premature lock pin ejection is highly undesirable because a) the intended valve and engine event is frustrated, resulting in improper engine operation; b) the extreme shock produced in associated engine components may cause damage; and c) repeated ejections can damage the lock pin and the high-lift follower portion such that the RFF cannot function properly and must be replaced. 
   What is needed in the art is a two-step roller finger follower having a timing mechanism to ensure that a switching event can occur only immediately at the completion of a valve event, thereby maximizing the time available for the lock pin to completely translate, either into or out of locking relationship, and thus minimizing the opportunity for a lock pin ejection. 
   It is a principal object of the present invention to prevent lock pin ejections during operation of a two-step switchable roller finger follower in an internal combustion engine. 
   SUMMARY OF THE INVENTION 
   Briefly described, a two-step roller finger follower in accordance with the invention includes a high-lift follower portion that moves relative to a low-lift follower portion about a pivot shaft. The low-lift portion is engaged by and follows one or a pair of low-lift cam lobes, and the high-lift follower portion follows one or a pair of high-lift cam lobes. A variable lock pin mechanism is disposed in the low-lift portion and includes an actuable lock pin that may be driven hydraulically slidably into latched engagement with a nose on the high-lift portion. When the low-lift and high-lift portions are latched together, only the high-lift portion engages the camshaft lobe, thus activating the corresponding engine valve in high-lift mode. When the low-lift and high-lift portions are unlatched, both the high-lift portion and the low-lift portion engage their respective camshaft lobes, but the high-lift portion moves in lost motion and thus the corresponding engine valve is activated in low-lift mode only by the low-lift cam lobe. 
   The lock pin assembly is slidably disposed in a bore. The lock pin assembly comprises a lock pin and a separable switching pin for driving the lock pin into engagement. A resilient blocking clip includes first and second ends that extend into a locking groove in the lock pin assembly to block axial motion of the switching pin in either the pin engagement or pin disengagement direction during times when movement of the locking pin could cause pin ejection. A first ramp on the low-lift RFF portion mates with a second ramp on the blocking clip such that oscillatory motion of the RFF alternately engages and disengages the blocking pin from the locking groove. Correct predetermined rotational positioning of the first and second ramp elements serves to restrict unblocking of the switching pin, and consequent actuation of the lock pin, to only those times in the camshaft rotational cycle when complete engagement and disengagement is assured. Preferably, unblocking of the switching pin occurs at the beginning of a valve lift event to permit pre-loading of the switching and lock pins and ending well before the beginning of the next valve lift event. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a graph showing valve lift profiles as a function of cam rotation angle and RFF lock engagement/disengagement sectors for the prior art and also in accordance with the present invention; 
       FIG. 2  is an elevational transparent view of a two-step RFF, in engagement with a section of the camshaft, in accordance with the invention; 
       FIG. 3  is an isometric view, partially in cutaway, of the RFF shown in  FIG. 2 ; 
       FIG. 4  is a plan view of the RFF as shown in  FIG. 3 , showing the lock pin assembly blocked in the unlatched position; and 
       FIG. 5  is a plan view of the RFF as shown in  FIG. 3 , showing the lock pin assembly blocked in the latched position. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a graph  10  shows exemplary valve lift profiles as a function of cam rotation angle in an internal combustion engine for two revolutions of the cam. The peak lift  12  for a low-lift valve event  14  is arbitrarily defined herein as 0° cam rotation angle. In the present example, the peak lift  16  for a high-lift valve event  18  is about 21° after low-lift peak  12 . The duration of low lift event  14 , shown as segment  23  in  FIG. 1 , is from before about −30 degrees to after about +30 degrees. The duration of high-lift event  18 , shown as segment  24  in  FIG. 1 , is from before about −30 degrees to after about +88 degrees. A two-step roller finger follower is capable of selectively providing either low-lift event  14  or high-lift event  18  in response to a hydraulic signal provided via an electronic engine control module, as is well known in the prior art. 
   As described above, a problem in prior art RFFs is that the cam rotation angle at which the RFF is commanded to lock the RFF in high-lift mode is substantially uncontrolled and may be subject to any of several operational variables. That is, the RFF can be commanded to lock during a valve lift event or at anytime when the valve is closed. Obviously, actual engagement and disengagement may occur only when the contact surface of the RFF high-lift follower portion is in contact with the base circle portion of its respective cam lobe (between successive valve events). When, for example, the command is given at about 80 degrees cam rotation, the lock pin will have ample time to fully engage the nose of the high-lift follower portion before the onset  22  of a commanded high-lift event  18 . However, if the command is given during, for example, a rotational segment  20  just before the onset  22  of a commanded high-lift event  18 , wherein the high-lift element is forced by the cam eccentric to move, the lock pin may not have time to fully engage the nose on the RFF high-lift follower portion and may result in the lock pin being violently ejected from such partial engagement during the high-lift event, thus aborting the event and risking damage to the RFF. 
   Referring now to  FIGS. 2 through 5 , a two-step roller finger follower  100  in accordance with the invention generally comprises prior art high- and low-lift components as follows. A follower body  102  includes a domed seat  104  for receiving the domed head  106  of a hydraulic lash adjuster (HLA)  108  mounted on an engine  110 . In operation, follower  100  pivots vertically on head  106  about a horizontal axis  112  in response to the action of high- lift  105  and low-lift  107  cam lobes of camshaft  109 . Follower  100  has an end  111 , opposite domed seat  104  to actuate valve  113  in either high-lift or low-lift mode. The pivoting action of follower  100  is an essential element of the present invention as described further below. (It should be understood that “vertical” and “horizontal” as used herein refer to  FIG. 2  as an elevational view.) Lifter body  102  includes a central aperture  114  for slidably receiving a high-lift follower portion  116  having a contact surface  118 , such as for example, a slider surface or a roller, for engaging high-lift cam lobe  105  and a slider nose  120  for engaging a lock pin  122  of a lock pin assembly  124  that is actuated in accordance with the invention and as will now be described. 
   Lock pin assembly  124  is slidably disposed in a bore  126  in follower body  102  oriented such that lock pin  122  may selectively engage slider nose  120 . It is an important feature of the present invention that such engagement is permitted, as described below, only immediately after completion of high-lift event  18  (segment  24  in  FIG. 1 ) when lock pin  122  will have ample time to fully engage slider nose  120  and partial pin engagement and subsequent premature ejection cannot occur. 
   Lock pin assembly  124  is shown in  FIGS. 3 and 4  in an unlatched position wherein lock pin  122  is fully retracted from engagement with slider nose  120  (note that in the cutaway view shown in  FIG. 3 , the upper portion of lock pin  122  is cutaway and hence cannot be seen). Assembly  124  is switched into the latched position ( FIG. 5 ) by the controlled provision of pressurized engine oil, as for example, from HLA  108  against first face  128  of switching pin  130 , causing switching pin  130  to translate which in turn causes lock pin  122  to translate, thus urging lock pin  122  into engagement with nose  120 . Return spring  132  is compressed by such translation and serves to disengage lock pin  122  from nose  120  after hydraulic pressure is removed from face  128 . 
   As best seen in  FIGS. 3-5 , switching pin  130  is disposed co-axially with lock pin  122 . A necked portion  134  of lock pin  122  engages a second face  136  of switching pin  130  opposite first face  128 , creating thereby a first annular groove  138 . A second annular groove  140  is provided in the outer surface of switching pin  130 . 
   A blocking assembly  141  includes blocking clip  142  formed from spring wire. Clip  142  comprises a centrally-located partial loop  144  that grips HLA  108  firmly when installed thereupon ( FIGS. 2 and 3 ), and further includes first and second spring portions  146   a,    146   b  that extend alongside RFF body  102 . Portions  146   a,    146   b  terminate in first and second blocking end portions  148   a,    148   b  that enter body  102  through respective clip bores  147   a,    147   b  generally transverse of bore  126  and, in relaxed mode, extend into bore  126 . In relaxed mode, as described below, end portions  148   a,    148   b  may extend into either first annular groove  138  ( FIGS. 3 and 4 ) or second annular groove  140  ( FIG. 5 ), depending upon the currently commanded position of the locking assembly. 
   Blocking assembly  141  also includes first and second inner bosses or “pucks”  150   a,    150   b  rigidly attached to opposing walls  151   a,    151   b  of RFF body  102  so that pucks  150   a,    150   b  rotate with the pivoting of follower  100  about axis  112 . Pucks  150   a,    150   b  have openings aligned with clip bores  147   a,    147   b  through which end portions  148   a,    148   b  enter transverse bore  126 . First and second outer pucks  152   a,    152   b  are disposed outboard of respective inner pucks  150   a,    150   b  and are fixedly mounted onto spring portions  146   a,    146   b,  as shown in  FIGS. 4 and 5  so that pucks  152   a,    152   b  do not rotate with pucks  150   a,    150   b  when follower  100  pivots about axis  112 . Slidable, wedged interfaces or ramps  154   a,    154   b  and  155   a,    155   b  are provided on the inner and outer pucks  150 , 152 , respectively, such that the pucks function in relative rotation similarly to tapered washers. 
   Pivoting of body  102  on HLA  108  about axis  112  during either a low-lift valve event  14  (segment  23  in  FIG. 1 ) or a high-lift valve event  18  (segment  24  in  FIG. 1 ) causes relative rotation between the inner and outer pucks. Because of wedged interfaces  154 ,  155 , outer pucks  152   a,    152   b  are translated outwards of inner pucks  150   a,    150   b,  thus withdrawing blocking end portions  148   a,    148   b  from bore  126  during valve events  14 , 18 . During valve events, because the nose of high-lift portion  116  has rotated below lock pin  122  (low-lift mode) or because of the side load exerted on lock pin  122  by nose  120  (high-lift mode), lock pin  122  cannot engage or disengage but can be pre-loaded for such action such that the action occurs immediately at the end of the valve event. Thus, by restricting withdrawal of end portions  148   a,    148   b  to a range  25 , from about −30 degrees to about +75 degrees, slightly within the duration of high-lift event  18  (segment  24 ), engagement and disengagement of lock pin  122  with nose  120  are restricted only to the immediate end of the valve event when the valve closes and the lifter nose  120  is unloaded (engaged to disengaged switch) or the lifter nose  120  is above locking pin  122  (disengaged to engaged switch) . Because end portions  148   a,    148   b  are reinserted into either of grooves  138 , 140  well before the beginning of the next valve event, movement of the lock pin is prevented when lock pin  122  does not have ample time to fully engage nose  120 , such as for example during region  20 , thus preventing partial engagement of the lock pin to the nose and consequent ejection of the lock pin as load is increased during a valve event. 
   It is an important aspect of the present invention that no special external timing apparatus or software is required. The blocking clip ends are withdrawn and reinserted simply by the oscillatory action of the RFF body, which is mechanically timed by the action of the associated cam lobes. Thus the translatory motion of the clip ends is inherently governed by the position of the cam and the RFF body. 
   The mechanism also times the switch of the lock pin from engaged to disengaged position. When oil pressure is removed from switching pin first face  128 , and if the blocking end portions  148   a,    148   b  are retracted from annual groove  140  (such as during range  25 ), compressed second return spring  156  urges switching pin  130  away from lock pin  122  to the switching pin&#39;s unlatched position ( FIGS. 3 and 4 ). However, lock pin  122  remains engaged with nose  120  in high-lift mode because of the load imposed on the lock pin through high-lift follower portion  116  by the compressed valve spring (not shown) associated with engine valve  113 . As soon as the high-lift valve event is complete (end of segment  24 ), the load exerted by engine valve  113  is removed from lock pin  122 , and return spring  132  translates lock pin  122  out of engagement with nose  120  ( FIG. 5 ) and into renewed contact with switching pin  130  in the unlatched position ( FIGS. 3 and 4 ). Because of the angular orientations of pucks  150 , 152  and wedged interfaces  154 , the delatching motion of the lock pin is timed to begin at the immediate start of the cam base circle (end of valve event), thus maximizing the time available to complete the translation and minimizing the possibility of locking pin ejection. 
   It should be understood, of course, that the RFF components and cam lobes referred to hereinabove as “low-lift” and “high-lift” may be exchanged by appropriate configuration of the RFF and cam lobes such that the unlatched mode is a high-lift mode and the latched mode is a low-lift mode; and both configurations are fully embraced within the scope of the present invention. 
   While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.