Abstract:
In an engine valve train equipped with a DHLA, the radius of the base circle of a cam lobe is decreased, relative to the radius of the base circle of a cam lobe in an engine equipped with an HLA, in proportion to the internal lash in the DHLA. The actual decrease in the base-circle radius is calculated by trigonometric relationships among the base circle portion, the length of the rocker pivot arm, and the internal lash of the DHLA. The decrease in the base circle radius is compensated by extension of the valve train lash-adjustment mechanism of the DHLA. After the internal DHLA lash is taken up during valve actuation, the coordinate position of the pivot point of the DHLA rocker arm is then identical with the coordinate position of the pivot point of an HLA rocker arm of a DHLA-equipped engine.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to internal combustion engines having overhead camshafts, hydraulic lash adjusters (HLAs), and rocker arms end-pivoting on the HLAs and following the cams to activate engine combustion valves; more particularly, to such engines having HLAs both with and without means for selectively engaging and disengaging such valve activation; and most particularly, to means for compensating for mechanical lash in a deactivating hydraulic lash adjuster (DHLA) to permit equal valve lift performance in engines having both HLAs and DHLAs. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is well known that overall fuel efficiency in a multiple-cylinder internal combustion engine can be increased by selective deactivation of the engine valves, on one or more cylinders, under certain engine load conditions. For cam-in-head engines, a known approach to providing selective deactivation is to equip the HLAs for those valves with means whereby the rocker arms may be rendered incapable of transferring the cyclic motion of engine overhead cams into reciprocal motion of the associated valves. See, for example, U.S. Pat. No. 6,321,704 B1. Typically, a DHLA includes, in addition to the conventional means for hydraulic lash elimination in the valve train, concentric inner and outer portions which are mechanically responsive to the cam lobe and which may be selectively latched and unlatched hydromechanically to each other, typically by the selective engagement of pressurized engine oil to drive spring-loaded latch pins in the inner portion. 
         [0003]    In manufacturing generally, it is beneficial to maximize the use of standard components. Thus, in the automotive industry it is standard practice to use identical roller finger followers and camshafts having identical lobe base circle diameters on engines having HLAs and DHLAs. However, DHLAs typically are provided with an amount of intended axial mechanical lash in the components to increase the reliability of the locking mechanism extending from the pin housing to engage the locking feature, such as a groove, in the HLA body. The DHLA&#39;s hydraulic lash adjustment mechanism cannot compensate for this built-in mechanical lash. Consequently, since the mechanical lash must be taken up first by the DHLA valve train before the associated valve begins to open, the actual valve lift of a prior art valve train employing locked DHLAs and using a cam lobe designed for a conventional HLA is reduced compared to that of an identical valve train employing HLAs. It is reduced because part of the cam eccentric designed to lift the valve is instead used to compress the length of the DHLA by an amount equal to the mechanical lash. Further, because the pivot center is lowered relative to the HLA counterpart, the timing of the valve opening, maximum lift, and closing are affected as well. Also, the change in effective operating geometry causes changes to the resulting force vectors associated with the DHLA valve trains and leads to further differences in the operating kinematics between the DHLA valve trains and the HLA valve trains. These differences are counter to the objective to reducing variation between the valve trains. 
         [0004]    The current remedy of simply adding lash ramps to both the opening slope and the closing slope of the associated cam lobe (along with an increase in maximum lobe height) to accommodate the additional travel of the pin housing within the body due to the mechanical lash, does nothing to keep the pivot centers from shifting. Thus, while differences in valve lift profiles can be eliminated, the variation of force vectors and operating kinematics caused by the shifting pivot centers remain. In addition, with the current remedy, the cam polar coordinates of the HLA cam lobes and the DHLA cam lobes must be made different in order to produce identical valve motion, and, in order to achieve identical valve lift timing, the reference angle locating the maximum lift point on the DHLA cam lobe must be angularly shifted relative to the reference angle locating the maximum lift point on the HLA cam lobe. These adjustments needlessly complicate the camshaft grind specifications and increase the chance of errors when fabricating the camshaft. 
         [0005]    What is needed in the art is a simple compensation that can be provided inexpensively in engine manufacture such that the valve lift, valve timing, and effective operating geometry of systems with both HLAs and DHLA components are identical. 
         [0006]    It is a principal object of the present invention to provide equal valve lift performance in like engines having HLAs and DHLAs that does not cause the pivot center points of the DHLA rocker arms to shift away from the pivot center points of the HLA rocker arms. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly described, in an engine valve train equipped with a DHLA, in accordance with the present invention the radius of the base circle portion of an associated cam lobe is decreased, relative to the radius of the base circle portion of a cam lobe on the same camshaft associated with an HLA, by an amount proportional to the internal mechanical lash in the DHLA. The surface coordinates of the cam eccentric are unchanged except for the addition of entry and exit ramps which provide the necessary transition to the reduced-diameter base circle portion. The actual decrease in the base-circle radius is calculated by simple trigonometric relationships among the base circle portion, the length of the rocker arm pivot axis, and the internal lash of the DHLA. The decrease in the base circle radius is exactly compensated by extension of the valve train lash-adjustment mechanism of the DHLA with respect to the locked position of the device. The net effect of this improvement is that after the internal DHLA lash is taken up by the rotating cam lobe, the coordinate position of the pivot point of the rocker arm on the associated DHLA is identical to the coordinate position of the pivot point associated with an HLA on the same camshaft at a point when the valve lift event begins to occur. The resulting valve train geometry and operating kinematics is therefore identical for the deactivating and non deactivation valve trains. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0009]      FIG. 1  is an elevational cross-sectional view of a prior art non-deactivating HLA; 
           [0010]      FIG. 2  is an elevational cross-sectional view of a prior art deactivating DHLA, lo substantially as disclosed in U.S. Pat. No. 6,321,704 B1; 
           [0011]      FIG. 3  are schematic drawings of a prior art HLA valve train including the HLA shown in  FIG. 1 ; 
           [0012]      FIGS. 4   a  and  4   b  are schematic drawings of a prior art DHLA valve train including the DHLA shown in  FIG. 2 , showing the effect of mechanical lash within the DHLA; and 
           [0013]      FIGS. 5   a  and  5   b  are schematic drawings of a HLA valve train ( 5   a ) and of a DHLA valve train ( 5   b ), in accordance with the present invention, showing correct compensation for mechanical lash within the DHLA. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring to  FIG. 1 , a prior art HLA  10  having a longitudinal axis  11  is shown. A cylindrical, hollow HLA body  12  receives a slidable piston  14  urged outwards by a bias spring  16  disposed in a high-pressure chamber  18 . Piston  14  includes a low-pressure reservoir  20  that receives pressurized oil from an engine oil gallery (not shown) via connecting ports  22 , 24 . A check valve  26  passes oil from reservoir  20  to chamber  18  as needed to support piston  14  in any lash-adjusting extension within body  12  as required by a valve train in which HLA  10  is installed. Piston  14  includes a hemispherical head  28  for pivotably receiving a hemispherical socket  29  of a rocker arm  31 , such as for example, a roller finger follower, as described below. Rocker arm  31  includes a roller  33 , mounted on axis  34 , for following the base circle portion  35 , having a constant radius, and eccentric portion  37  (entrance ramp  39 , nose  41 , and exit ramp  43 ) of a camshaft lobe  45 . An opposite end of rocker arm  31  includes an arcuate pad  47  for engaging a stem  49  of a combustion valve in an internal combustion engine. Arcuate pad  47  is defined by a radius  51  and a center  53  which, together with the pivot center  55  of socket  29  and head  28 , define a pivot arm  57  for rocker arm  31 . 
         [0015]    Referring specifically to  FIG. 3 , prior art HLA  10  in conjunction with its associated RFF, combustion valve and cam lobe (HLA valve train), as the HLA valve train relates to internal combustion engine  152 , is shown. The left view shows the HLA valve train on base circle of the cam lobe (closed valve) where roller  33  just reaches the beginning of entrance ramp  39  of camshaft lobe  45 ; the right view shows the HLA valve train at maximum valve lift. In both views, distance A is the vertical distance between the pivot center  55  and the axis of rotation  58  of the cam lobe. In both views, dimension A is the same. Circle  59 , shown in the right view, represents an arbitrary fixed reference point on the cam shaft from which the angular position of the maximum cam radius of each cam lobe is positioned. The angle R represents the angle from the reference point to the maximum cam radius of the particular cam lobe shown. 
         [0016]    Referring to  FIG. 2 , prior art DHLA  110  having a longitudinal axis  111  is substantially as disclosed in U.S. Pat. No. 6,321,704 B1, the relevant disclosure of which is incorporated herein by reference. DHLA  110  has a generally cylindrical adjuster body  112 . A pin housing  114  is slidably disposed within a first axial bore  115  in adjuster body  112 . Pin housing  114  itself has a second axial bore  130  for slidably receiving a piston  132  having a hemispherical head  128  for receiving a socket end (not shown) of a roller finger follower in an overhead-cam engine valve train. Pin housing  114  has a transverse bore  134  slidably receivable of two opposed locking pins  136  separated by a pin-locking spring  138  disposed in compression therebetween. First axial bore  115  in adjuster body  112  is provided with a circumferential groove  140  for receiving the outer ends of locking pins  136 , thrust outwards by spring  138  when pins  136  are axially aligned with groove  140 . In such configuration, DHLA  110  is in valve-activation mode. (As shown in  FIG. 2 , DHLA  110  is in valve-deactivation mode.) Upper end  142  of pin housing  114  defines a first seat for a loss-of-motion (LM) return spring  144  disposed within an annular chamber  146  formed between bore  115  and pin housing  114 . LM spring  144  finds a second seat at an annular stop  148  in bore  115 . Groove  140  further defines a reservoir for providing high pressure oil against the outer ends of locking pins  136  to overcome spring  138  and retract the locking pins into bore  134 , thereby unlocking the pin housing from the adjuster body to deactivate the adjuster. Groove  140  is in communication via at least one port  150  with an oil gallery (not shown) in engine  152 , which in turn is supplied with high pressure oil by an engine control module (not shown) under predetermined engine parameters in which deactivation of valves is desired. Piston  132  includes a hydraulic element assembly (HEA)  126  lodged at an inner end thereof. HEA  126  comprises a spring loaded check ball  154  lodged against a seat  156  formed in piston  132  separating a low-pressure oil reservoir  120  from a high-pressure chamber  118  formed between HEA  126  and pin housing  114 . 
         [0017]    The purpose of describing in detail thus far the arrangement of prior art DHLA  110  is to permit description of the source of mechanical lash within DHLA  110 , which is the source of the problem solved by the present invention. 
         [0018]    The outward travel of pin housing  114  within bore  115  is limited by engagement of housing stop  158  with stop  148 . The axial thickness of housing stop  158  is selected such that the lower edges  160  of pins  136  can readily clear the lower edge of groove  140  during locking of DHLA  110 . A desired clearance of typically about 0.250 mm is provided, defining an internal axial mechanical lash within DHLA  110 . 
         [0019]    Referring to  FIGS. 4   a  and  4   b , a prior art DHLA  110  in conjunction with its associated RFF, combustion valve and cam lobe (DHLA valve train) is shown.  FIG. 4   a  shows the DHLA valve train before the internal axial mechanical lash within DHLA  110  is taken up;  FIG. 4   b  shows the DHLA valve train just after the internal axial lash within DHLA is taken up (and immediately before valve  49  begins to open), wherein cam lobe  45  has rotated slightly from its position shown in  FIG. 4   a . For clarity of comparison,  FIG. 4   b  is shown as a mirror image of  FIG. 4   a.    
         [0020]    Referring now to  FIG. 4   a , when the locking pins are engaged in valve-activation mode of DHLA  110 , as roller  33  reaches the beginning of entrance ramp  39  of the cam lobe, the internal axial mechanical lash has not yet been taken up. At that point, dimension A, being the vertical dimension between pivot center  55  and the axis of rotation  58  of the cam lobe, is shown. In the prior art, dimension A at the point of rotation of cam lobe  45 , as shown in  FIG. 4   a , is identical to a dimension A in an HLA (non-deactivating) valve train. Therefore, the coordinate position of the pivot point of the rocker arm on the associated DHLA is identical to the coordinate position of the pivot point associated with an HLA on the same camshaft, assuming that both  FIGS. 4   a  and  4   b  are on the same coordinate systems, that is, the origin of the coordinates being the axis of rotation  58  of the cam lobe and the y axes of the coordinate systems being the same. 
         [0021]    Referring now to  FIGS. 2 and 4   b , the camshaft has continued its rotation and roller  33  has started up entrance ramp  39 . Pin housing  114  and piston  132  will travel against the force of LM return spring  144 , along axis  111  into body  112  until extended pins  136  engage the lower edge of groove  140 , after which head  128  becomes a firm pivot for the rocker arm socket. Pivot center  55 ′ is displaced from a first position A to a second and lower position A+X by the rotational force of the camshaft acting against the LM spring  144 , all before any valve-opening motion is induced in pad  47 . Thus, it will be seen that part of the intended valve-lifting effect of a cam eccentric is lost to the taking up of this mechanical lash at the start of each valve-opening cycle. Therefore, the coordinate position of the pivot point of the rocker arm on the associated DHLA ( 55 ′ in  FIG. 4   b ) is not identical to the coordinate position of the pivot point associated with an HLA ( 55  in  FIG. 4   a ) on the same camshaft (assuming again that their coordinate systems are the same). 
         [0022]    The shift in the coordinate position of pivot center  55 ′ to A+X fundamentally changes the relationship between the pivoting follower and the cam lobe and therefore must be accounted for in order to achieve identical valve lift and valve lift timing between the HLA and DHLA valve trains. It is possible to eliminate any lift and/or timing variation by using the actual A+X pivot point in the calculations of the cam lobe contour and by shifting angle β (that is, using an angle β for the DHLA valve trains that differs from angle β for the HLA valve trains). In this case, the valve lifting portion of the lobe contour and angle β of the cam lobes associated with the DHLA valve trains would be unique thereby complicating the grind parameters of the cam shaft lobes. 
         [0023]    Thus, while it is possible to exactly match the valve lift of the DHLA valve train with the valve lift of the HLA valve train, there is no way to achieve identical geometry and operating kinematics between the DHLA and HLA valve trains because of the shifting pivot point  55 . In other words, the shift in the pivot center from A to A+X causes changes to the resulting force vectors and operating kinematics acting on the DHLA valve train which introduces undesired variation between the DHLA and HLA valve trains. 
         [0024]    By comparing  FIG. 5   a  to  FIG. 5   b , the solution to the lash problem provided by the present invention is shown. (For clarity of comparison,  FIG. 5   b  is shown in mirror image position of  FIG. 5   a ).  FIG. 5   a  shows the HLA (non-deactivating) valve train as shown in  FIG. 3 . Roller  33  is in contact with base circle  35  of the associated cam lobe. Distance A is the vertical distance between the pivot center  55  and the axis of rotation  58  of the cam lobe. 
         [0025]      FIG. 5   b  shows the DHVA valve train in accordance with the invention. Roller  33  is in contact with base circle  35 ′ and before the roller reaches the beginning of entrance ramp  39 ′ of cam lobe  45 ′ (the internal axial mechanical lash has not yet been taken up). The invention provides for the coordinate position of the pivot point of the rocker arm on the associated DHLA relative to the axis of its cam lobe, after lash removal, to be identical to the coordinate position of the pivot point associated with an HLA relative to the axis of its cam lobe. This is done by raising the position of the lash-included pivot point of the DHLA rocker arm to a new pivot point  55 ″  FIG. 5   b , so that the coordinate position of the pivot point of the rocker arm on the associated DHLA, after the mechanical lash is taken up by the rotating cam lobe, is identical to the coordinate position of the pivot point associated with the rocker arm of the HLA on the same camshaft (again assuming that the coordinate systems being used as reference are the same as defined above). This is readily accomplished by reducing proportionally the radius R′ of base circle  35 ′ as compared to R of base circle  35  in  FIG. 5   a , allowing rocker arm roller  33  to assume a new position A−X closer to the axis of rotation  58  of base circle  35 ′ of the cam lobe (through extension of the HLA lash adjusting mechanism  126  ( FIG. 2 )). It will be seen that the amount of radius reduction, proportional to lash allowance mechanical lash and to be applied during manufacture of the camshaft, may be calculated readily by simple trigonometric relationships, depending upon the radii of roller  33  and base circle  35 ′, the distance of roller axis  34  from pivot points  53  and  55  (see  FIGS. 3 ), and the known axial mechanical lash within DHLA  110 . 
         [0026]    Since, presently, the particular grind criteria must be calculated and set for each cam lobe of a given cam shaft, a decrease of the base circle radius (along with the addition of suitable entrance and exit ramps) of each lobe dedicated to a DHLA, coupled to the identical polar coordinates for the high lift portion of the cam lobe may be achieved with no or little cost added to the cost of manufacturing the cam shaft. On the other hand, the improvement allows identical valve lift, valve lift timing, and identical operating geometries and operating kinematics to be achieved regardless of whether the valve is in a DHLA position or in a conventional HLA position. 
         [0027]    While the invention described herein is shown in a DHLA valve train having the HLA pivot point at one end of the rocker arm, the valve stem contact pad at the other end of the rocker arm and the rocker arm roller between the pivot point and pad, it is understood that components of the rocker arm may be in any order or relationship, including the rocker arm roller on one end, the valve stem contact pad on the other end of the rocker arm and the HLA pivot point between the roller and pad. 
         [0028]    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.