Patent Publication Number: US-11047267-B2

Title: Variable valve lift system

Description:
FIELD OF THE INVENTION 
     The invention relates to a valvetrain system for an internal combustion engine, and in particular to a system providing variable valve lift. 
     BACKGROUND 
     Traditionally, internal combustion engines use a single cam profile to enable gases to enter or exit the combustion chamber. More modern engines are able vary the valve lift profile depending on multiple factors, such as engine speed and load, to enable greater efficiency. A variable valve lift system utilising a summation rocker system may be used to combine two different cam profiles to produce the desired valve lift profile. The valve lift profile can be modified to suit prevailing engine operating conditions by changing the timing of the two cam profiles relative to each other. 
     Summation rocker systems are known from prior art, the one believed to be closest to the present invention being described in EP1426569. They function using two rockers, each acted upon by a respective one of two cam profiles. The two rockers are connected using a pivot shaft allowing the rockers to rotate relative to each other. 
     A first of the two rockers pivots about the pivot shaft and acts between a first cam profile and the stem of a poppet valve, to open and close the valve. The second of the two rockers is mounted in the engine on a fixed rocker shaft and acts between the second cam profile and the pivot shaft of the first rocker. This raises and lowers the pivot shaft supporting the first rocker in accordance with the profile of the second cam profile. The movement of the pivot shaft changes the position of the first rocker, thereby changing the valve lift. It follows that the valve lift at any point is determined by a combination of both the first and second cam profiles. In each case, a cam profile may be defined by a single cam lobe or, to avoid unbalanced forces, by two identical but axially spaced lobes. 
     For optimum performance, the two rockers should have the same mechanical advantage and the forces applied to the rockers should act to exert only a torque to cause the rocker to rotate about the pivot shaft or the rocker shaft, as the case may be. A further consideration in designing the geometry of the valvetrain is that the space available in the engine to accommodate it may be limited. As these different considerations create conflicting demands, it has hitherto been necessary to compromise and settle for a configuration that is less than optimal in terms of motion geometry and valvetrain packaging. 
     OBJECT OF THE INVENTION 
     The present invention therefore seeks to provide a valvetrain that employs cam summation but offers greater freedom in the relative positioning of the different components of the valvetrain. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a variable valve lift system as hereinafter set forth in claim  1  of the appended claims. 
     Preferred features of the invention are set out in the appended dependent claims. 
    
    
     
       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 a    is a front view of a first embodiment of a variable valve lift system, wherein a cut-out, in the form of a hole is provided in a first rocker for a rocker shaft, 
         FIG. 1 b    shows a detail of an alternative design of the rocker in  FIG. 1 a    in which the cut-out is in the form of a slot that extends to a boundary of the first rocker, 
         FIG. 2  is an isometric view of a second embodiment of the variable valve lift system, wherein a hole in the first rocker and the rocker shaft passing through the hole with clearance each have more than one diameter, 
         FIGS. 3 a  and 3 b    are different views of the first rocker of a third embodiment of the variable valve lift system, wherein the first rocker is manufactured of a single formed piece of sheet metal, 
         FIG. 4  is an isometric view of an alternative first rocker which is made from two sheet metal parts and assembled using a pivot shaft and a cam follower shaft, 
         FIG. 5  is an isometric view of a valvetrain, wherein one of the intake and exhaust valve is operated using a cam summation system while the other valve is operated by a conventional rocker and a single cam profile, 
         FIG. 6  is an isometric view of a valvetrain similar to that of  FIG. 5 , but in which an eccentric bushing is used to optimise the position of the pivot axis of the conventional rocker, 
         FIG. 7  is an isometric view of a valvetrain similar to that of  FIG. 6  showing an alternative way of retaining the eccentric bushing, and 
         FIG. 8  is a view of a further valvetrain similar to that of  FIG. 7  but in which a control spring is mounted on the second rocker rather than the first rocker. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the description below of embodiments of the invention, in order to avoid unnecessary repetition, like parts of different embodiments have been allocated reference numerals with the same last two digits. Hence numerals XX, 1XX, 2XX, 3XX etc. will be to designate identical components, or possibly modified components fulfilling the same function. 
       FIG. 1 a    illustrates a summation rocker system  10  for acting on two poppet valves  16  in dependence upon the combined lifts of first and second cam profiles defined by different cam lobes  14 ,  22  of a concentric camshaft  46 . A first rocker  12  of the system  10  is mounted about a pivot shaft  18  and has a first cam follower  44 , in the form of a roller, in contact with the profile of the first cam lobe  14 , The other end of the first rocker  12  acts on the valve stem via a bridge piece  27  to displace the valves  16  in dependence upon the lift of the profile of the first cam  14 . A second rocker  20  is pivotable about a rocker shaft  24  which is fixedly mounted to an engine, the second rocker  20  having a second follower  48 , again in the form of a roller, in contact with the profile of the second cam  22  (more clearly seen in  FIG. 2 ). The second rocker  20  acts to displace the pivot shaft  18  of the first rocker  12  in dependence of the lift of the profile of the second cam  22 . 
     As can be seen from  FIG. 1 a   , the rocker shaft  24  intersects a plane that passes through the axis of the pivot shaft  18  and the end of the first rocker  12  applying a downward force (as viewed) to the valves  16 . Such positioning of the rocker shaft  24 , while enabling the geometry and packaging of the valvetrain to be optimised, would not be possible conventionally because a solid first rocker  12  and the rocker shaft  24  would be competing to occupy the same space. 
     To accommodate the rocker shaft  24  in such a position, the first rocker  12  of the embodiments of the invention shown in  FIGS. 1 a  and 1 b    is provided with a cut-out  26  configured and dimensioned to prevent the rocker shaft  24  from interfering with movement of the first rocker  12 . In the case of  FIG. 1 a   , the cut-out  26  is in the form of a hole within which the rocker shaft  24  is received with clearance, while in  FIG. 1 b    the cut-out  26   a  is a slot that extends to the boundary of the first rocker  12 . 
     While a circular hole of sufficiently large diameter may be used, it is preferred to minimise the amount of material removed from the first rocker by providing a hole that is elongated in the direction of relative movement. The direction of relative movement may be curved or relatively straight, depending on the geometry of the valvetrain. 
     The first rocker  212 , of the embodiment shown in  FIGS. 3 a  and 3 b   , comprises two inner surfaces  213 ,  215  axially straddling the second rocker  20 , the two surfaces  213 ,  215  defining between them a pocket  217  within which the second rocker  20  is received with clearance. The pivot shaft  18 , which is received in aligned holes  219  in the surfaces  213  and  215  of the first rocker  12  is pivotably connected to the second rocker  20  within the pocket  217 . 
     A control spring  28 , shown in  FIG. 1 a   , is used in order to maintain contact between the first follower  44  and its corresponding cam lobe  14  throughout its rotation. The control spring  28  acts on the first rocker  12  and is mounted to a fixed point on the engine. 
     The optimum position of the control spring  28  creates a force vector through the pivot shaft  18  perpendicular to a line created between the pivot shaft  18  and the fixed rocker shaft  24 . However, it is often a greater priority to minimize the height of the valvetrain, in which case the spring  28  may be moved from this optimum position. 
     In a second embodiment of the invention as shown in  FIG. 2 , the rocker shaft  124  has reduced diameters  121  and  123  in regions where it passes through the first rocker  112 . The regions  121 , 123  of reduced diameter of the rocker shaft  124  allow at least part of the cut-out  126  in the first rocker  112  to be of a reduced diameter, allowing the first rocker  112  to benefit from increased stiffness characteristics due to less material being removed. 
     It would equally be possible to form the reduced regions of the rocker shaft  124  with one or more slots rather than a reduced diameter in order to reduce the size of the cut-out  126  in the first rocker  112 . 
     If formed as a hole, a portion of the cut-out  126  should remain of a diameter to provide a clearance fit for the larger diameter regions of the rocker shaft  124 . The diameter of the rocker shaft  124  is usually specified for a journal bearing of the second rocker  120  and so cannot be directly modified in the region that passes through the second rocker  120 . The position of this larger diameter portion of the hole may be positioned anywhere along its swept range in order to maximize stiffness. 
     The first rocker  212  of  FIGS. 3 a  and 3 b    may be formed from sheet metal. Any holes required, such as those for the rocker shaft, the pivot shaft and a cam follower shaft may be stamped or cut out while the material is still in its unfolded sheet state, with the outer profile of the rocker then stamped or cut out, and then finally folded to form the rocker  212 . Using sheet metal automatically creates a pocket in which the second rocker may be positioned, avoiding the need for machining operations. 
     Alternatively, as illustrated in  FIG. 4 , the first rocker  712  may be assembled from two formed sheet metal parts  712   a  and  712   b . The two parts may be held together by the pivot shaft  718  and the cam follower shaft  730 . Production of the cut-out  726  for the rocker shaft  724  and the pocket for the second rocker may again take place before any forming. It should be noted that the first rocker may be assembled from more than two parts, and it is not essential that all parts be formed of sheet metal. 
     It is common for a summation rocker system to be used on only one of the intake or exhaust valves, the valve being operated using a conventional system with a single cam profile. Such a valvetrain is shown in each of  FIGS. 5 to 7 . In the embodiment of  FIG. 5 , a conventional third rocker  332  is used to operate a valve not shown in the drawing, the rocker  332  acting between the other valve and a third cam lobe  334  mounted to the camshaft  346 . This valve is opened and closed by the third rocker  332  in dependence of a single third cam profile. The rocker  332  pivots about the rocker shaft  324  as shown in  FIG. 5 . While such a configuration minimises the impact of the summation rocker system on the cylinder head design, the rocker shaft  324  may not provide an optimal position for the pivot of the third rocker  332 . 
     The embodiments of the invention shown in  FIGS. 6 and 7 , provide a solution to mitigate this problem. The position of the pivot axis of the rocker shaft  424  in these embodiments is modified by fitting eccentric components such as bushes  436  to the rocker shaft  424 , the third rocker  432  being mounted to the eccentric bush  436 . The bushing  436  must be prevented from rotating on the rocker shaft  424 .  FIG. 6  illustrates the bushing being fixed to the rocker shaft  424  by a machine screw  438 . Alternative fixtures may be used. 
     An alternative approach for preventing rotation of the bushing  436  is adopted in the embodiment illustrated in  FIG. 7 . Rotation and axial movement in a first direction are constrained by a rocker shaft support  440  with a shaft mounting bolt  450  fixing the rocker shaft  424  to the support  440 . Axial movement in the second direction is constrained by a bushing retaining washer  442 , and ultimately the first rocker  412  of the summation rocker system. This solution removes the need for any extra fixings and only requires minimal design changes to the rocker shaft support  440  to accept the bushing  436 . 
     As previously disclosed, the control spring  28  can sometimes be mounted in a less than optimal orientation in order to minimize the overall height of the valvetrain. Moving the control spring  28  from its optimal position requires the spring  28  to produce a higher force. Designing a control spring which exerts sufficient force but still fits into the packaging space of the cylinder head may be difficult or costly. 
       FIG. 8  illustrates an alternative arrangement for the control spring  528 . Using the two-piece formed rocker described in the embodiment of  FIG. 4 , the control spring  528  can be directly mounted to the second rocker  520  instead of the first rocker  512 . As the second rocker  520  is mounted lower than the first, mounting the control spring  528  to the second rocker  520  reduces the valvetrain height whilst maintaining an optimal or near optimal angle of the control spring  528 . In this position the force vector created by the control spring  528  acts through the pivot shaft  518  perpendicular to the line created between the pivot shaft  518  and fixed rocker shaft  24 . The spring  528  is therefore required to produce less force and is easier to package. 
     It will be appreciated that the embodiments described above may be combined where technically possible. For example, the control spring may act on the second rocker independent of the design of the first rocker. If the first rocker were to be constructed in any other way than illustrated in  FIG. 8 , then the first rocker may require a through hole for the control spring to pass through. Alternatively, the control spring may be seated in an indent or hole in the first rocker. 
     Furthermore, it is alternatively possible for the control spring to be arranged to act between the two rockers in order to maintain the desired contact with one of the cam profiles, rather than acting between one of the rockers and a fixed point on the engine. 
     Although the summation rocker system is, in the above embodiments, related to varying the lift of the valve, the duration that the valves are open and the timing of the valves may be varied depending on the phase of the cam lobes with respect to either each other, the crankshaft of the engine, or both. 
     The invention may be used with any number of intake or exhaust valves in the engine, or indeed any engine configuration or number of cylinders. Where more than one valve per rocker is acted upon, the valves may be synchronized through a valve bridge connecting them to the rocker.