Abstract:
A valve operating mechanism is described for an internal combustion engine in which at least one valve ( 14 ) of an engine cylinder is operated by two cams ( 10,12 ). The cams ( 10,12 ) have respective cam followers ( 38,36 ) which are resiliently biased to remain in contact with the cams ( 10,12 ) at all times and which act on the valve ( 14 ) by way of a summation linkage ( 20,24 ) in such a manner that the displacement of the valve ( 14 ) at any instant is determined by a combination of the displacements of the two cam followers ( 38,36 ). In the invention, movement of each of the two cam followers ( 38,36 ) is transmitted to the summation linkage ( 20,24 ) by way of a respective one of two pushrods ( 28,30 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 of United Kingdom Patent Application No. 0426352.1 filed Dec. 1, 2004. 
   FIELD OF THE INVENTION 
   The present invention relates to a valve operating mechanism for an internal combustion engine in which at least one valve of an engine cylinder is operated by two cams, wherein the cams have respective cam followers which are resiliently biased to remain in contact with the cams at all times, the cams acting on the valve by way of a summation linkage mounted on the engine cylinder head in such a manner that the displacement of the valve at any instant is determined by a combination of the displacements of the two cam followers. 
   BACKGROUND OF THE INVENTION 
   EP 1426569 discloses such a valve operating system having overhead cams and the relative phasing of the two cams is used to adjust, amongst other things, valve event duration. 
   The aim of the present invention is to implement such a system in a pushrod engine (i.e. an engine in which the cams are arranged within the engine cylinder block) where movement of the cam followers is transmitted to the valves through pushrods and rockers. 
   The difficulty that such a valve operating system presents when using pushrods is that there is inevitably a significant clearance in the system when a valve is closed and both of its cam followers are on the base circles of their respective cams. Steps must therefore be taken to ensure that the pushrods always remain within their sockets in the cam followers and in the valve operating rockers. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a valve operating mechanism for an internal combustion engine in which at least one valve of an engine cylinder is operated by two cams, wherein the cams have respective cam followers which are resiliently biased to remain in contact with the cams at all times, the cams acting on the valve by way of a summation linkage mounted on the engine cylinder head in such a manner that the displacement of the valve at any instant is determined by a combination of the displacements of the two cam followers, characterised in that movement of each of the two cam followers is transmitted to the summation linkage by way of a respective one of two pushrods. 
   U.S. Pat. No. 5,555,860 describes an engine in which a valve is operated by two cams arranged within the engine block. In the latter patent, a summation lever is arranged adjacent the cams in the engine block and a single pushrod is used to transmit the motion of the summation lever to the associated valve by way of a rocker. The control mechanism of the latter patent differs from that of the present invention in that it is not used to achieve variable event duration. Instead variable valve lift is achieved by arranging for the summation lever to be in permanent engagement with one of the cams and spaced from the base circle of the second cam by a gap. Such a gap would be totally inadmissible in the present invention. 
   The present invention offers the advantage of bringing to pushrod engines the advantages of a variable valve operating mechanism that have hitherto only been achievable in an overhead camshaft (OHC) engine, in which the cams are mounted in the cylinder head. 
   In one embodiment of the invention, the two cams are mounted on separate camshafts that are spaced from, and extend parallel to, one another. 
   Alternatively, the two cams may be mounted coaxially with one another as part of a single assembled camshaft. 
   The summation linkage may comprise a rocker mounted on a fixed pivot, one side of the rocker acting on the valve and its opposite side pivotally supporting a summation lever acted upon by the two pushrods. Alternatively, the summation linkage may consist of a rocker mounted on a fixed pivot, one side of the rocker being acted upon by one of the pushrods and its opposite side pivotally supporting a lever which engages the valve and is acted upon by the other pushrod. 
   In order to ensure that each of the pushrods remains permanently in contact at one end with the summation linkage and at the other end with its cam follower, the summation linkage may be resiliently biased by a torsion spring or a compression spring. Alternatively, one of the cam followers or one of the pushrods may be formed of two parts that are resiliently biased apart. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic perspective view of a valve operating mechanism using two spaced camshafts and employing a first configuration of summation linkage, 
       FIG. 2  is a schematic perspective view of a valve operating mechanism using two spaced camshafts and employing a second configuration of summation linkage, 
       FIGS. 3 and 4  are schematic perspective and sides views, respectively, of a valve operating mechanism using coaxial cams and employing the same configuration of summation linkage as shown in  FIG. 2 , 
       FIG. 5  is a view similar to that of  FIG. 2  showing an embodiment of the invention in which the summation linkage is biased by a compression spring, 
       FIGS. 6 and 7  show sections through the embodiment illustrated in  FIG. 5  in different positions of the cams, 
       FIGS. 8 and 9  are views similar to the sections of  FIGS. 6 and 7  of a further embodiment of the invention in which a spring is mounted in one of the cam followers in place of the spring acting on the summation linkage, 
       FIG. 10   a  shows a section through a hydraulic cam follower incorporating a spring, 
       FIG. 10   b  is a section through a fixed cam follower, 
       FIGS. 10   c  and  10   d  are sections through a spring biased cam follower in its extended and fully collapsed position, respectively, 
       FIG. 11   a  is a side view of a spring biased collapsible pushrod in its collapsed state, 
       FIG. 11   b  is a section along the section plane X—X in  FIG. 11   a,    
       FIG. 11   c  is a side view of the pushrod of  FIG. 11   a  in its extended position, and 
       FIG. 11   d  is a section along the section plane Y—Y ins  FIG. 11   c.    
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a valve operating mechanism having two camshafts  10  and  12  mounted in an engine having two mutually inclined banks of cylinders, commonly referred to as a ‘V’ engine. The engine cylinder block and cylinder heads have all been omitted from the drawings in the interest of clarity, only two intake poppet valves  14   a ,  14   b  and two exhaust poppet valves  15   a ,  15   b  being shown in the drawing. The exhaust poppet valves  15   a  and  15   b  are each operated in a conventional manner by a single cam on the camshaft  12  and their operation need not be described further. The intake valves  14   a  and  14   b , on the other hand, are each operated by combining the effect of two cams, one mounted on the camshaft  12  and the other on the camshaft  10 . 
   At least one of the camshafts  10  and  12  is coupled for rotation with the crankshaft by way of a phaser (not shown in the drawings) to allow the phase of the camshafts  10  and  12  to be varied relative to one another. As is well known, a phaser is a coupling that rotates the camshaft in synchronism with the crankshaft (at half the speed in the case of a four-stroke engine) but allows some degree of rotation of the camshaft relative to the crankshaft to vary to the phase of the cams mounted on the camshaft in relation to the engine operating cycle. Several phasers are disclosed in the prior art, suitable examples being hydraulically operated vane-type phasers that can be incorporated in the cogs or pulleys driving the camshafts. 
   Each of the intake valves  14   a ,  14   b  is operated by a rocker  20  mounted in one of the two cylinder heads on a stationary rocker shaft  22 . One end of each rocker  20  acts on the tip of the stem of the associated valve  14  to open and close the valve. The other end of each rocker  20  carries a double ended summation lever  24  which is pivotable relative to the rocker  20  about a pin  26 . 
   In the case of the valve designated  14   a , its summation lever  24  is acted upon at one end by a pushrod  30   a  whose other end is in contact with a cam follower  36   a  riding on a cam  12   a  of the camshaft  12 . The other end of the same summation lever is acted upon by the cam  10   a  of the camshaft  10  by way of a cam follower  38   a  and a pushrod  28   a.    
   Similarly for the valve designated  14   b , its summation lever  24  is acted upon at one end by a pushrod  30   b  whose other end is in contact with a cam follower  36   b  riding on a cam  12   b  of the camshaft  12  and the other end of the same summation lever is acted upon by the cam  10   b  of the camshaft  10  by way of a cam follower  38   b  and a pushrod  28   b.    
   Thus, for each of the valves  14 , the associated summation lever  24  is acted upon at its opposite ends by two pushrods each associated with a cam on a respective one of the two camshafts  10 ,  12 . 
   A torsion spring  32  acts on each rocker  20  and one of the cam followers  36 ,  38  is of the type shown in  FIG. 10   a  which includes a hydraulic lash adjuster, the other being a fixed cam follower of the construction shown in  FIG. 10   b . An adjustable stop  34  limits the maximum clearance. 
   The cam follower of  FIG. 10   a  has a main body  50  carrying a roller  52 . A piston  54  reciprocable within the main body is biased by a spring  56  and forms the movable wall of a hydraulic working chamber  58  into which engine oil is admitted through a non-return valve  59 . The cam follower of  FIG. 10   b  has a body  60  carrying a roller  62  but its piston  64  does not move and for this reason the cam follower is termed a fixed cam follower. The spring  32  and the hydraulic cam follower together ensure that the ends of the pushrods remain at all times in their sockets in the summation lever  24  and in the cam followers. 
   The embodiment shown in  FIG. 2  operates on a similar principle to that of  FIG. 1  but relies on an alternative summation linkage for combining the two cam follower motions transmitted via the pushrods. The embodiment of  FIG. 2  replaces the rocker  20  and the summation lever  24  by a first rocker  20 ′ having a fixed pivot point  22 ′ and a second rocker  24 ′ pivotable relative to the first rocker  22 ′ about a pivot  26 ′ carried by the first rocker  20 ′. One of the pushrods  30   b  acts on the free end of the rocker  20 ′, the other pushrod  28   b  acts on one end of the rocker  24 ′ and the opposite end of the rocker  24 ′ acts on two intake valve  14   b   1  and  14   b   2  by way of a bridge  40  which overlies the tops of the stems of both valves. 
   The operation of this summation linkage, which is believed to be clear from the foregoing description is further explained within the context of an OHC engine in EP 1426569. 
   The embodiment of the invention shown in  FIGS. 3 and 4  uses concentric cams  410 ,  412  with followers  436 ,  438  and separate pushrods  428 ,  430  to operate a summation linkage similar to that shown in  FIG. 2 . The use of similar reference numerals in the 400 series is used to avoid repeating the description of the summation linkage. 
   In the embodiments of  FIG. 2  and of  FIGS. 3 and 4 , a torsion spring and a hydraulic cam follower may once again be used to take up free play and to ensure that ends of the pushrods do not come away from their sockets at any time. 
     FIGS. 5 ,  6  and  7  show an embodiment operating in the same manner as that of  FIG. 2  and, to avoid repetition, like parts are designated by like reference numerals but in the 100 series.  FIG. 5  shows a perspective view of the valve operating mechanism while  FIG. 6  shows a section through the mechanism when both cams are on their base circles and the spring  132  has opened a clearance between the valve  114  and the rocker  124 .  FIG. 7  shows the same section when one of the cams is at maximum lift, bringing the rocker  124  back into contact with the valve  114  at the point of valve opening. The essential difference in this embodiment of the invention is that the torsion spring  32  has been replaced by a helical compression spring  132  which, as shown in  FIG. 6 , biases both the summation lever  124  and the rocker  120  counter-clockwise to open a gap between the summation lever  124  and the valve  114  while maintaining contact with the pushrods  128  and  130  at both ends. In this embodiment also, the rocker  120  has been fitted with a manual adjuster  121  for controlling the clearance in the system. The manual adjuster removes the need for either of the cam followers to be fitted with a hydraulic lash adjuster. 
   In the case of the embodiment of  FIGS. 8 and 9 , which use like reference numerals in the 200 series to designate like parts, the springs  32  and  132  of the previously described embodiments, which act on the summation linkage, are replaced by a spring  76  arranged in one of the cam followers  238 , which is constructed in the manner shown in  FIGS. 10   c  and  10   d . The other cam follower  236  is of the same fixed design as used in the previously described embodiments and shown in  FIG. 10   b.    
   The sprung cam follower shown in  FIGS. 10   c  and  10   d  comprises a body  70  carrying a follower roller  72 . A piston  74  slidable in the main body  70  is biased by a spring  76  so that the cam follower can be extended, as shown in  FIG. 10   c  or contracted, as shown in  FIG. 10   d . In this case, the summation lever  224  remains in contact with the valve stem  214  at all times and the clearance “C” in the system appears within between the main body  70  and the piston  74  of the cam follower. 
   The embodiment of  FIGS. 8 and 9  may use an extendable pushrod in place of an extendable cam follower to achieve the same effect. Such an extendable pushrod  528528 , which would replace the fixed length push rod  228  is shown in its collapsed state in  FIGS. 11   a  and  11   b . The pushrod is formed in two parts  528   a  and  528   b  which can slide relative to one another and are maintained in alignment by means of a sleeve  528   c  which is permanently attached to the lower part  528   a  of the pushrod. A spring  528   d  acts in a direction to separate the two parts and extend the pushrod into the position shown in  FIGS. 11   c  and  11   d.    
   It will be clear from the various embodiments described above that the invention does not reside in the design of the summation linkage employed to combine the actions of the two cams but in the fact that the combining of the action of two cams is carried out within the context of a pushrod engine. 
   In operation, a phaser is attached to each of the two cams to allow the phase of the cams to be adjusted relative to the engine crankshaft. By altering the relative phase of two cams acting on the same intake valve it is possible to vary the valve event duration and the valve lift. Furthermore, when both cams can be independently phased relative to the crankshaft, it is possible to modify the timing of the valve event with the engine cycle.