Abstract:
A locking mechanism for a plural rocker arm valve train assembly is provided. The locking mechanism is adapted for use with a camshaft having a plurality of different cam lobes having a plurality of different profiles, which result in variable valve displacement and duration. A plurality of rocker arms are located on a pivot shaft which runs parallel to the camshaft, each rocker arm having structures configured to be acted upon by respective lobes of the camshaft. An active rocker arm has structure configured to act upon an engine valve or valves. A movable locking element is fully enclosed by the rocker arms and is capable of selectively moving along the pivot shaft to allow the active rocker arm to selectively engage one or more of the other rocker arms for common pivoting, resulting in varied displacement of the engine valve or valves.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a variable valve train for an internal combustion engine having two or more cam lobes per cylinder. 
       BACKGROUND OF THE INVENTION 
       [0002]    The valve train is the mechanical system responsible for operation of gas exchange valves in internal combustion engines. These valves are driven, either directly or indirectly, by cam lobes on a camshaft. The timing of the valve opening and closing is important to vehicle performance, as it affects torque and power output of the engine as well as emissions. Different engine speeds require different valve timing and lift for optimum performance. Generally, low engine speeds require valves to open a relatively small amount over a shorter duration, while high engine speeds and loads require valves to open a relatively larger amount over a longer duration for optimum performance. Engines without some method of variable valve timing must compromise between optimization at either low or high speed and sacrifice some performance in the non-elected range. By adding the ability to choose between different cam profiles, and thus driving the valves differently at different speeds and loads, engines are able to better optimize performance throughout a wider range of engine operating conditions. 
       SUMMARY OF THE INVENTION 
       [0003]    A locking mechanism for a plural rocker arm valve train assembly is adapted for use with a camshaft having a plurality of different cam lobe profiles. The plurality of rocker arms and the locking mechanism are supported by a pivot shaft that is parallel to the camshaft and that defines a common axis about which the rocker arms are rotatable. Each of the rocker arms is directly or indirectly acted upon by a corresponding cam lobe; each cam lobe has a different profile configured for varying valve lift and timing according to specific engine needs. One of the rocker arms is an active rocker arm which directly or indirectly operates at least one engine valve. A mechanism for selectively locking one or more secondary rocker arms to the active rocker arm is contained within the rocker housing, and operable to slide axially along the common axis. 
         [0004]    The locking mechanism operates via a male element housed within a female cavity within the rocker arms. When hydraulic pressure is changed in response to changing engine conditions, the male elements slide between predetermined positions within the female cavities. This axial change of position causes the male elements to selectively lock or unlock the active rocker arm to an adjoining secondary arm so that the two move together as a unit or move independently of one another. Selectively locking the active rocker arm to a secondary rocker arm results in changing the cam profile which is controlling valve operation. Hydraulic fluid to actuate the system is supplied via parallel, axial galleries within the pivot shaft. 
         [0005]    Placement of axially-sliding locking elements inside the rocker arms and around the pivot shaft enables a compact and lighter-weight rocker design. It also avoids the need for carrying pins, springs, machined holes, and oil-feed galleries located on the outer structures of the rocker arms, which can add mass and complexity to the rocker arms and actuation mechanism. Additional benefits include a system that is compact and imparts low torque on the locking mechanism. 
         [0006]    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 
         [0007]      FIG. 1  is a perspective view of a plural rocker arm valve train assembly. 
           [0008]      FIG. 2  is an exploded view of a portion of the locking mechanism for a plural rocker arm valve train assembly shown in  FIG. 1 . 
           [0009]      FIG. 3  is cross section view of a portion of a locking mechanism for the plural rocker arm valve train assembly shown in  FIG. 1 . 
           [0010]      FIG. 4  is a perspective view of an alternate embodiment of a plural rocker arm valve train assembly. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in  FIG. 1  a sliding-pivot locking mechanism employed in a valve train  10 ; which is a center-pivoted configuration driving two engine valves  11 , with valve stems  12 , valve springs  13 , and valve seats  15 . An active rocker  14 , with a T-shaped valve-end  16 , pushes on the two valves  11  of the same cylinder (not shown). The valve train could be alternately designed where the active arm actuates one engine valve, as will be recognized by those skilled in the art. In either one or two-valve embodiments, lash compensation is performed by hydraulic lash adjusters (not shown) placed at the valve-end  16  of the active rocker  14 . Oil feed to the lash adjusters is communicated through a transfer passage (not shown in  FIG. 1 , shown as reference  56  of  FIG. 3 ) inside the active rocker  14 . 
         [0012]    Straddling the active rocker  14  are two secondary rockers  18  and  20 , which follow higher-lift cam lobes. In the embodiment of  FIG. 1 , in the order of increasing lift, the three lobes are indicated by: cam  22 , cam  24 , and cam  26 , located on camshaft  27 . Rollers  28  located at the cam-end of each rocker  14 ,  18 , and  20  provide low-friction contact between the rockers  14 ,  18 , and  20 , and their respective cams  22 ,  24 , and  26 . All three rockers  14 ,  18 , and  20  are pivotable around a stationary pivot shaft  30 , which acts as a journal bearing support for the rockers. Those skilled in the art will recognize that this embodiment is a three step valve train having three different cam lobe profiles, and therefore three different valve displacements, from which to choose. Those skilled in the art will further recognize other valve train configurations within the scope of the claimed invention. 
         [0013]    In operation, if neither of the secondary rocker arms  18  and  20  is locked to the active arm  14 , then the engine valves  11  follow the input motion from cam  22 , which is the lowest lift among the three lobes  22 ,  24 , and  26 . Inactive secondary arms  18  and  20  idle against their respective biasing springs  19  and  21  (not shown in  FIG. 1 , shown in  FIG. 3 ) while riding along the lobes of cams  24  and  26 , respectively. 
         [0014]    If one of the secondary arms is locked to the active arm, the active and locked secondary arm pivot commonly and the valves  11  follow input from the higher of the two respective cam lobes, while the remaining (unlocked) secondary rocker arm idles against its biasing spring. For example, if the secondary arm  18  is locked to the active arm  14 , the active arm  14  and secondary arm  18  pivot commonly and the valve follows input from cam  24 —because that is the higher of cams  22  and  24 —while the secondary rocker arm  20  idles against its biasing spring  21  (not shown in  FIG. 1 , shown in  FIG. 3 ) as it follows cam  26 . If desired, both secondary arms  18  and  20  can simultaneously be locked to the active arm  14 , in which case the highest lift cam lobe—cam  26  in the embodiment of FIG.  1 —will control and the valve follows its input motion. Operation of the locking mechanism is described in more detail below in relation to  FIGS. 2 and 3 . 
         [0015]    A valve train having this rocker configuration is advantageous in terms of the reduced overall height of the valve train mechanism. This architecture also enables shortening the distance between engine valves&#39; line of action and the pivot shaft centerline, thereby reducing the torque on the locking mechanism assembled inside the pivot shaft. 
         [0016]    Referring now to  FIGS. 2 and 3 , there are shown portions of the valve train  10  of  FIG. 1 .  FIG. 2  shows an exploded view illustrating components of the internal locking mechanism in greater detail. In addition to the active rocker arm  14  and pivot shaft  30 ,  FIG. 2  shows a first locking element  32  and a first biasing spring  34 .  FIG. 3  shows a cross section of the pivot shaft  30 , the first locking element  32  and a second locking element  36 , and the first and second biasing springs  34  and  38 . The center pivot portion  40  of the active rocker  14  is shaped like a sleeve, where an oil groove  42  located inside the sleeve registers with a transfer passage  44  inside the pivot shaft  30 . 
         [0017]    In the embodiment shown in  FIGS. 1-3 , the locking elements  32  and  36  are polygon-shaped. The polygon-shaped male locking element  32 , with a chamfered end  46 , is hydraulically actuated to slide along the axis of the pivot shaft  30  and engage into the matching female cavity  48  integral with the sleeve of the center pivot portion  40 . This female cavity  48  has a periphery shape complimentary to the polygonal shape of the locking elements  32  and  36 , and each of rocker arms  14 ,  18 , and  20  contains a similar female cavity. The polygon-shaped locking elements  32  and  36 , shown here as having three lobes, can have other profiles, as long as torque-carrying capacity is maintained and axial engagement is easily achieved. Those skilled in the art will recognize that, within the scope of the claimed invention, other locking element profiles can be used. Other possible locking element profiles include, without limitation: shaft keys, splines, et cetera. 
         [0018]    Placement of axially-sliding locking elements inside the rocker arms and around the pivot shaft enables a compact and lighter-weight rocker design. This embodiment also avoids the need for carrying pins, springs, machined holes, and oil-feed galleries located on the outer structures of the rocker arms, which can add mass and complexity to the rocker arms and actuation mechanism. 
         [0019]    Referring now to  FIGS. 1 and 3 , a hydraulic fluid passage  50  runs the axial length of pivot shaft  30  and carries actuation oil to the first locking element  32 , which, in this embodiment, is the low lift to lowest lift locking element. A parallel hydraulic fluid passage  52  carries actuation oil to the second locking element  36 , which, in this embodiment, is the high lift to low lift locking element. A third passage  54  (not shown in  FIG. 3 ) also runs parallel to the other two passages  50  and  52 , and delivers lubrication and lash-adjusting oil to the active rocker  14  and lash adjusters (not shown) via transfer passage  56 . These passages  50 ,  52 , and  54  may be in fluid communication with pressure source  49  via fluid conduit  55  or in fluid communication with the engine oil system (not shown), to supply oil or some other hydraulic fluid to the passages  50 ,  52 , and  54  in order to actuate the locking elements  32  and  36 . In a preferred embodiment, the engine oil system is used to selectively pressurize hydraulic fluid  51  and  53  to the passages  50  and  52 , respectively. The third passage  54  (lubrication channel) communicates with the engine oil circuit. These axial hydraulic fluid passages need not extend throughout the full length of the pivot shaft; other embodiments could include axial passages that end after oil is delivered for rocker actuation or lubrication purposes. Such an embodiment would remove the need for fluid couplings or other hardware for each channel at both ends of the pivot shaft. 
         [0020]    Referring to  FIG. 3 , the valve train  10  is shown in default mode. In this mode, passages  50  and  52  do not contain sufficient pressure in hydraulic fluid  51  and  53  to move locking elements  32  and  36  against their respective biasing springs  34  and  38 . Methods for controlling the pressure of hydraulic fluid  51  and  53  are described in greater detail below. In operation, to select alternative locking modes, the pressure of hydraulic fluid  51  or  53 , or both, can be increased to overcome the force of biasing springs  34  and  38 . In the embodiment shown in  FIG. 3 , three additional locking modes are available in addition to the default mode shown. In a first alternative locking mode, the pressure of hydraulic fluid  51  is increased to the actuation level. Pressure is communicated from passage  50  through transfer passage  44  and into oil groove  42 , and generates sufficient force on locking element  32  to overcome the force of biasing spring  34 ; causing locking element  32  to move leftward (as viewed in  FIG. 3 ). In this actuated state, locking element  32  no longer locks the active rocker arm  14  to the secondary arm  18 , and the two are free to pivot separately. Therefore, the active rocker arm  14  follows the profile of cam lobe  22  and transfers that profile to the valves  11 . 
         [0021]    In a second alternative locking mode, the pressure of hydraulic fluids  51  and  53  in both passages  50  and  52  is increased to actuation level. The pressure in hydraulic fluid  53  is communicated from passage  52  through a transfer passage  45  and into an oil groove  43 , and generates sufficient force on locking element  36  to overcome the force of biasing spring  38 ; causing locking element  36  to move leftward (as viewed in  FIG. 3 ). In this actuated state, locking element  36  locks the active arm  14  to secondary arm  20 , causing the two to pivot commonly. As described above, locking element  32  is in its actuated state, and the secondary arm  18  is disengaged from active rocker arm  14 . Therefore, the active rocker arm  14  and secondary arm  20  pivot commonly, and follow the profile of cam lobe  26 , transferring that profile to the valves  11 . Note that a third alternative locking mode exists, where locking element  32  is not actuated and locking element  36  is actuated. In this third alternative mode, all three rocker arms  14 ,  18 , and  20  are locked together and pivot commonly. This causes the active arm  14  and the valves  11  to follow the profile of which ever cam lobe is the highest, which is cam lobe  26  in the embodiment shown in  FIGS. 1 and 3 . 
         [0022]    In order to regulate the pressure of hydraulic fluid  51  and  53 , the actuation-oil channels  50  and  52  communicate with the engine-oil circuit, possibly through a three-position, four-way control valve (not shown) that directs pressurized oil to one channel while connecting the other channel to the sump, which is a low pressure area. In the third position of the control valve, neither channel  50  nor  52  is pressurized; which is the fail-safe default mode. In this default mode, as shown in  FIG. 3 , the first locking element  32  is engaged making the low lift cam  24  the default lift. If the lowest lift cam profile is not zero (zero being the de-activation state), then in the default mode this first locking element  32  could be designed to stay disengaged by reversing the direction of oil pressure force and the force of biasing spring  34 . 
         [0023]    In an alternate embodiment, two simpler control valves (not show) can be used; each having a two-position, two-way function, one control valve being associated with each actuation channel. In the de-energized mode, each control valve connects the respective channel to the sump. Energizing one or the other valve will connect the respective actuation channel to the high-pressure oil circuit. 
         [0024]    In another embodiment (not shown), a third actuation strategy would eliminate one of the three axial fluid passages  50 ,  52 , or  54 . In this strategy, lubrication and one of the two actuations is done using the same feed and axial fluid passage. As long as the lubrication oil pressure in the passage is regulated to stay below a set value, which is likely to be lower than the engine oil pressure level, that locking element will remain in the un-actuated position. To actuate the shared passage, one control valve will switch the feed pressure from that regulated (low) value to the engine oil pressure. The function of the other control valve controlling the other actuation line remains the same as above. The drawback of combining one actuation channel with the lubrication channel is the resulting regulated (lowered) oil pressure for journal lubrication and lash adjusting. 
         [0025]      FIG. 4  shows an alternate embodiment of a locking mechanism, an end-pivoted valve train  60  driving a single engine valve  11 , and employing a sliding pad  62  at the active rocker arm  14 . The active rocker, shown as having a sliding pad  62  contact with its respective cam  22  could also have a roller (like the rollers  28 ) at the cam end. This roller would lower friction, especially if the lowest valve lift desired is not the zero-lift, deactivation case. The operational characteristics as to the lobe switching, lash adjusting, and oil routing features discussed above for the center-pivoted architecture ( FIGS. 1-3 ) apply to this configuration as well. Locking and actuation strategies are also the same. 
         [0026]    In additional embodiments (not shown) a cam may be provided having two symmetric outer lobes. These symmetric outer lobes would provide the high lift profiles, while the remaining inner lobe would be the low lift profile. A single feed line will actuate both locking elements simultaneously. The low lift center lobe is the default mode of operation, corresponding to either low pressure levels or no oil pressure—such as during a failure in the oil pressure system. When the single feed line is pressurized sufficiently to overcome the force of the biasing springs, the locking elements would lock the inner rocker to both of the two outer rockers (corresponding to the symmetric high lift lobes) and place the valve train in the high lift mode. The single feed line to the locking elements can be a separate line from the lubrication line, or can be shared with the lubrication line by using a regulated pressure line, as described above. 
         [0027]    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.