Abstract:
An overhead camshaft internal combustion engine is disclosed having a valve mechanism which comprises an intake or exhaust poppet valve  14  having a valve stem operated by two cams  16  and  18  mounted for rotation about a common axis. A first rocker  20  mounted on a pivot shaft  26  acts between the first cam  16  and the valve  14.  A second rocker  28  is mounted in the engine on a fixed pivot shaft  30  and acts between the second cam  18  and the pivot shaft  26  of the first rocker  20,  to raise and lower the pivot point  26  of the first rocker  20  cyclically in synchronism with the rotation of the second cam  18.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an overhead camshaft internal combustion engine having a valve mechanism which enables the valve event duration to be varied.  
         BACKGROUND OF THE INVENTION  
         [0002]    The closest prior art to the present invention is believed to be WO03/016684 which is incorporated herein by reference. In the latter patent specification, a summation lever following the movements of two cams is pivotably mounted on a rocker to open a poppet valve by an amount equal to the sum of the lifts occasioned by the separate cams. By varying the phase of the two cams relative to one another, the event duration and the valve lift can be modified and by simultaneously varying the phase of both cams in relation to the engine crankshaft, the valve timing can be modified.  
           [0003]    A disadvantage of this earlier proposal is that it requires the cylinder head architecture to be redesigned.  
         OBJECT OF THE INVENTION  
         [0004]    The present invention seeks to provide an alternative installation package better suited to engines with a rocker operated valve train which avoids the need to remodel the cylinder head while still permitting the event duration to be modified.  
         SUMMARY OF THE INVENTION  
         [0005]    According to the present invention, there is provided an overhead camshaft internal combustion engine having a valve mechanism which comprises an intake or exhaust poppet valve having a valve stem, two cams mounted for rotation about a common axis, a first rocker mounted on a pivot shaft and acting between the first cam and the valve stem to open and close the poppet valve in synchronism with the rotation of the first cam, and a second rocker mounted in the engine on a fixed pivot shaft and acting between the second cam and the pivot shaft of the first rocker, to raise and lower the pivot point of the first rocker cyclically in synchronism with the rotation of the second cam.  
           [0006]    It is preferred for the valve mechanism to be symmetrical so as to avoid any twisting moment on the pivot shaft of the first rocker about an axis transverse to the axis of rotation of the cams.  
           [0007]    In an engine having two valves per cylinder, the valve mechanism may comprise two first rockers following the movements of two first cams which are arranged symmetrically on opposite sides of a single second cam and second rocker.  
           [0008]    In an engine having a single valve per cylinder, the pivot shaft of the first rocker may be carried by two second rockers following two second cams which are symmetrically arranged one on each side of the first cam and first rocker.  
           [0009]    If a phase changing mechanism is provided to vary the phase of the first cam relative to the second cam then the event duration will be adjustable. While altering the relative phase of the first and second cams to vary the event duration, the valve lift achieved during an event will also be varied.  
           [0010]    A further phase change mechanism is preferably provided to vary the phases of both the first cam and the second cam simultaneously in relation to the phase of the engine crankshaft, to allow the timing of the valve event to be set independently of the event duration and valve lift. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention will now be described further, by way of example, with reference to the accompanying drawings, in which  
         [0012]    [0012]FIG. 1 is an isometric view of part of an engine cylinder head fitted with a valve mechanism,  
         [0013]    [0013]FIG. 2 is an exploded view separately showing the components of the valve mechanism,  
         [0014]    [0014]FIG. 3 is an isometric view of the assembled valve mechanism with the parts of the cylinder head omitted, and  
         [0015]    [0015]FIGS. 4 and 5 are front views of the valve mechanism in different positions. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    The engine shown in part in FIG. 1 has a cylinder head generally designated  10 . A camshaft  12  is journalled in the cylinder head  10  and has two sets of cams that are mounted for a limited degree of angular movement relative to one another. For example, the camshaft may comprise a central shaft secured for rotation with one set of cams and surrounded by a sleeve that is rotatable with a second set of cams.  
         [0017]    A phase change mechanism such as shown in FIG. 10 of the above referenced PCT/GB2002/003804 is used to vary the phase of the sets of cams relative to one another. Another such phase change mechanism may be used to adjust the phases of both sets of cams simultaneously in relation to the phase of the engine crankshaft. The construction and operation of the phase change mechanism will not be described herein in detail as it is in itself well known. Furthermore, numerous alternative constructions of phase change mechanism, as disclosed in the prior art, may be used to vary the phases of the cams in the present invention.  
         [0018]    The illustrated engine has two poppet valves  14  per cylinder. These may be either intake or exhaust valves, the other valve(s) of the cylinder not being shown. The valve mechanism now to be described opens and closes the poppet valves  14  and allows the valve event duration and valve lift to be varied by varying the relative phase of the cams on the camshaft  12 .  
         [0019]    In the illustrated embodiment, the camshaft  12  has two first cams  16  arranged one on each side of a second cam  18 . Two first rockers  20  carry roller followers  22  and are pivoted about a common shaft  26 . The opposite ends of the first rockers  20  act by way of respective hydraulic tappets  24  on the ends of the valve stems of the poppet valves  14 .  
         [0020]    The pivot shaft  26  of the first rockers  20  is carried by, or forms part of, a single second rocker  28  that is pivotable about a stationary pivot pin  30  and has a roller follower  32  held in permanent contact with the cam  18  by means of a spring  34 . The pivot pin  30  is received in a bore  36  in the second rocker  28 . The pivot pin  30  secured by means of a bolt  38  to a cross bar  42  which is itself bolted to the cylinder head  10 . The bolt  38  passes through a spacer block  40  which is received in an opening  44  in the second rocker  28 . A pin  46  prevents rotation of the spacer block  40  relative to the cross bar  42 .  
         [0021]    Two arms  48  projecting from the cross bar  42  are fitted with adjustable stop screws  50  which serve to prevent over expansion of the hydraulic lash adjusters  24 .  
         [0022]    In common with PCT/GB 2002/003804, the illustrated valve mechanism operates by adding the profiles of the two cams  16  and  18  in order to generate the valve motion. However, the function of the summation lever is performed in the present invention by the interaction of the two rockers  20  and  28 .  
         [0023]    [0023]FIGS. 4 and 5 show the cams  16  and  18  with the necessary relative phase to achieve maximum valve lift.  
         [0024]    [0024]FIG. 4 shows the valve  14  fully closed and FIG. 5 shows it fully open.  
         [0025]    At the beginning of valve lift, as shown in FIG. 4, the middle cam  18  contacts its follower  32  on the maximum lift dwell portion of the profile, that is to say the top of the cam lobe. This holds the moving pivot shaft  26  in its lowest possible position. As the outer cams  16  now move from their base circle radii to the lift profile, the valve is lifted from its seat.  
         [0026]    Maximum valve lift occurs, as shown in FIG. 5, when the cam followers  22  and  32  are both on the cam lobes. The maximum possible valve lift will therefore occur if the cams  16  and  18  are phased such that both of the profiles contact their followers at maximum lift at the same time.  
         [0027]    The valve  14  will be closed if either of the cam followers  22  and  32  is on the base circle radius of its associated cam. If the cam follower  22  comes off its cam lobe while the follower  32  remains on its cam lobe, the rocker  20  pivots about the pivot shaft  26  to close the valve. On the other hand, if the cam follower  22  remains on the cam lobe while the cam follower  32  comes off its cam lobe, the rocker  20  will close the valve as it pivots about its follower  22  on account of the pivot shaft  26  being raised by the rocker  28 .  
         [0028]    Once the valve  14  has closed, the outer cams  16  return to their base circle and the middle cam  18  returns to its maximum lift dwell ready to start the next valve lift. In this portion of the cycle, the rockers  20  and  28  move even though there is no valve lift, so the control spring  34  is required to hold the follower  32  of the rocker  28  in contact with the cam  18  and the adjustment screws are required to prevent the hydraulic lash adjusters  24  of the outer pair of rockers  20  from over-expanding when there is clearance in the system.  
         [0029]    During this portion of the cycle, the outer cam followers  22  lose contact with their cam lobes  16  and are brought back into contact by the start of the opening ramp on the outer cams  16 .  
         [0030]    The effect of the described valve mechanism is to separate the control of the valve opening and valve closing times of each valve event. It is convenient to view the cams  16  as being the valve opening cams and the cam  18  as the valve closing cam but of course the converse is equally valid.  
         [0031]    The valve mechanism would normally be designed with a particular maximum valve lift and duration in mind. The duration of the event is reduced by advancing the phase of the closing cam  18  relative to the opening cams  16  and this will be accompanied by a reduction in the valve lift because the cam follower  32  of the closing cam  18  will come off the cam lobe before the cam followers  22  of the opening cams  16  reach the lobes of the cams  16 .  
         [0032]    If the closing cam  18  is retarded relative to the opening cams  16 , lift and duration will increase until the maximum valve lift is produced by both cams  16  and  18  being at their maximum lift at the same time. If the closing cam  18  is retarded still further, the valve lift will remain constant at its maximum value, and the event duration will increase by the addition of a dwell at maximum valve lift.  
         [0033]    The valve mechanism described has two valves  14  per cylinder but it will be appreciated that the invention can be applied to an engine with a single intake or exhaust valve per cylinder. In this case, it is desirable to maintain symmetry by providing a single opening cam  16  acting on the valve  14  by way of a rocker  20  and to pivot the rocker  20  on two rockers  28  in contact with two closing cams  18  arranged on opposite sides of the opening cam  16 .