Patent Publication Number: US-8534245-B2

Title: Assembly of a valve operating system incorporating a cam summation mechanism

Description:
This invention relates to a valve operating system for an internal combustion engine that uses two coaxial cams to actuate a valve by way of a summation mechanism. In particular, the invention is concerned with assembling such a valve operating system in a cylinder head and its adjustment. 
     BACKGROUND OF THE INVENTION 
     Examples of valve operating systems that incorporate a summation mechanism are shown in the Applicants&#39; earlier EP 1417399, EP 2142768, EP 2257697 and EP 2242912 which are imported herein by reference. Cam summation mechanisms need to have a clearance between at least one of the cam followers and its associated cam at some points in the camshaft rotation cycle, and it is known to provide a control spring to hold the summation lever in contact with one of the cam profiles so that its position is fully defined when there is clearance in the system. The amount of clearance needs to be adjusted in order to ensure that the valve lift is well matched between the different cylinders of the engine and to eliminate manufacturing tolerance variations in the various valvetrain components of the system, and variations in the cylinder head machining. 
     Conventional valvetrain systems also need to compensate for manufacturing variations and in many cases this is achieved by using a hydraulic lash adjuster that increases in length until the cam follower is held in contact with the cam lobe. Hydraulic lash adjusters have the advantages of allowing the system to compensate for temperature changes while the engine is running, compensating for any wear that occurs over the life of the engine, and eliminating the need for any manual adjustment of the system at the time of assembly. 
     A cam summation system using a hydraulic lash adjuster requires some form of stop in order to limit the expansion of the lash adjuster and to maintain the correct amount of clearance. In the absence of such a stop, the lash adjuster would continue to inflate until it has removed all of the clearance from the system. EP 1417399 shows a variety of different methods for maintaining and adjusting the amount of clearance in the system when a hydraulic lash adjuster is used. 
       FIGS. 5 and 6  of the latter patent show that the clearance can be controlled by setting the distance between the centre of the camshaft and the centre of the pivot shaft connecting the summation lever to the valve actuating rockers. Setting the distance between the centre of the camshaft and the centre of the pivot shaft is a particularly advantageous way of controlling the clearance because it still allows the hydraulic lash adjuster to compensate for the effects of thermal expansion in the cylinder head and to compensate for any variation in valve tip position due to tolerances, or due to wear over the life of the engine. Consequently, it is possible to adjust the clearance of the system as soon as a particular cam summation mechanism becomes associated with a particular group of cam lobes, and this adjustment can be completed before the system is fitted to the cylinder head. 
     Alternative designs to those disclosed in EP 1417399 have been proposed in order to achieve the same result by providing a cylindrical contact surface on the camshaft that contacts a corresponding surface either on the summation lever or on the valve actuating rocker(s). 
     A still further earlier proposal has been to rely on an eccentric pivot shaft or an eccentric cam follower axle as disclosed in FIGS. 4D and 5B of EP 2257697. 
     The method by which the clearance adjustment is achieved is not important to the present invention, which can be applied with any of the alternative designs discussed above. 
     The task of assembling of a complete valve operating system that incorporates a camshaft and one or more cam summation mechanisms is complicated because it is necessary to align each of several independently movable cam actuating rockers with a valve stem at one end and a pivot element, such as a lash adjuster, at the other as the camshaft is lowered into position. Furthermore, it is necessary to secure in position the control springs connected to the cam summation levers to urge the cam followers against their respective cam surfaces. 
     SUMMARY OF THE INVENTION 
     With a view to simplifying the above task, the present invention provides in accordance with a first aspect a method of assembling a valve operating system for opening and closing a poppet valve mounted on an engine cylinder head, the system comprising a camshaft having two coaxial cams for operating the valve, a summation lever coupled to followers of the two cams, a control spring acting on the summation lever to urge one of the cam followers into contact with the associated cam, and a valve actuating rocker pivotably connected to the summation lever and acting on the valve to open and close the valve in dependence upon the sum of the instantaneous lifts of the two cams, the method comprising the steps of journaling the camshaft in a carrier, securing the summation lever to the carrier by means of the control spring, and securing the carrier to the engine cylinder head, the control spring and the carrier serving to maintain the position of the cam summation lever during assembly such that the valve actuating rocker is aligned correctly to engage with the valve. 
     In accordance with a second aspect of the invention, there is provided a valve operating system for an engine valve mounted in a cylinder head, comprising two cams mounted coaxially on a camshaft, a summation lever coupled to followers of the two cams and movable in proportion to the instantaneous sum of the lifts of the respective cams, a valve actuating rocker pivotably coupled to the summation lever and operative to open the engine valve in dependence upon the movement of the summation lever and a control spring serving to urge one of the followers into contact with the associated cam, wherein a carrier is provided for supporting the camshaft, the summation lever and the valve actuating rocker in a suitable orientation for the valve actuating rocker to align correctly with the tip of the engine valve as the valve operating system is mated with the engine cylinder head, thereby enabling the valve operating system to be assembled to the engine cylinder head in a single operation. 
    
    
     
       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 perspective view of a valve operating system using two cams and a cam summation mechanism, 
         FIG. 2  is an exploded perspective view of the cam summation mechanism of  FIG. 1 , 
         FIG. 3  is an end view showing the required relative positions of the components of the valve operating system during assembly, 
         FIG. 4  is a section through the valve operating system of  FIG. 3  passing through the centre of the control spring, 
         FIG. 4   a  a detail of  FIG. 4  contained within a dotted circle drawn to an enlarged scale. 
         FIG. 5  is a section similar to that of  FIG. 4  after completion of the assembly, 
         FIG. 5   a  is a detail of  FIG. 5  drawn to an enlarged scale, 
         FIG. 6  is a view similar to that of  FIG. 4  of an alternative embodiment of the invention showing a section passing through the plane of the valve actuating rocker, and 
         FIG. 7  is a view similar to that of  FIG. 5  showing the embodiment of the invention of  FIG. 6  in its assembled state. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a valve operating system  10  is shown which acts in unison on two valves  12  mounted in an engine cylinder head (not shown). The valve operating system  10  comprises two cams  14 ,  16  mounted coaxially on a common camshaft  18 . For the purpose of force balancing and creating an arrangement that is symmetrical, the cam  16  is formed in two halves  16   a  and  16   b  that have identical profiles and straddle the first cam  14 . The purpose of using two cams to act on the same engine valve  12  is that it enables the lift and duration of the valve event to be changed by rotating the cams relative to one another. 
     The cams  14  and  16  act on the valves  12  by way of a cam summation mechanism shown in an exploded view in  FIG. 2 . The cam summation mechanism comprises a summation lever  20  that is coupled to three cam follower rollers  24 ,  26   a  and  26   b . The single roller  24  is associated with the cam  14  and the two rollers  26   a  and  26   b  are associated with the cams  16   a  and  16   b . In this way, the pivot axis of the summation lever  20  moves in proportion to the sum of the instantaneous lifts of the two cams  14  and  16 . 
     Two valve actuating rockers  30  are pivotably mounted one on each side of the summation lever  20 . Each actuating rocker  30  acts at one end on a respective one of the two valves  12  and rests at its other end on one of two hydraulic lash adjusters  32 . The actuating rockers  30  have spacer shims  34 , which may be formed separately from the rockers  30 . The shims  34  maintain a minimum distance between the axis of the rockers  30  and the centre of camshaft  18  by contacting a cylindrical surface on the camshaft  18  when pushed upwards by the lash adjusters  32 . In this way, the shims  34  set the clearance between the cam follower  24  and its associated cam  14  when either of the cam followers are on the base circle of their cams and the associated valves  12  are fully closed. 
     A control spring assembly  40  is connected between a cylinder head cover  50  (see  FIG. 3 ) and one end of the summation lever  20  to pull the end of the lever  20  causing the rollers  26  to contact the cams  16  and creating a clearance between the roller  24  and the cam  14 . Because tension springs are less reliable than compression springs, the spring assembly  40 , as better shown in the exploded view of  FIG. 2 , uses a compression spring  40   a  of which the upper end exerts an upwards force (as viewed) on the summation lever  20  by way of an outer coupling element  40   b  that straddles the spring  40   a  and is connected to the summation lever  20  by a pin  40   c . The spring  40   a  is compressed between the outer coupling element  40   b  and an inner coupling element  40   d  of which the upper end is connected by a pin  40   f  to a plunger  40   e  which, when the operating system is fully assembled, is retained in a bore in an engine cover  50 . In this way, the lower end of the spring  40   a  is fixed in relation to the engine and the upper end acts upwards on the summation lever  20  to emulate the action of a tension spring. 
       FIG. 3  shows an end view of the cover  50  in which the camshaft  18  is journalled and having one or more summation mechanisms assembled to it. The clearance within this assembly can be adjusted before it is fitted to the cylinder head provided that summation mechanism remains associated with a particular group of cam lobes. During assembly, each summation mechanism is temporarily held in place by a clip  42  holding it to the engine cover  50  so that it will remain attached after the clearance adjustment process has been completed. 
     In order to fit the cover  50  to the top of the cylinder head, the rocker orientation needs to be accurately defined such that the valve actuating rockers  30  on all cylinders of the engine will engage with their respective hydraulic lash adjusters  32  and valves  12  as the cover  50  is lowered vertically into position. 
     It can be appreciated from  FIG. 3  that the illustrated desired position of the summation mechanism is not the one that the various components would naturally adopt under the influence of gravity. Rather, the pivoting joints at each end of the control spring assembly  40  would naturally rotate such that the centre of mass of the summation mechanism would lie below the retaining clip  42  holding the plunger  40   e  in the cover  50 . 
     In the illustrated embodiments of the invention, steps are taken to ensure that the cam summation mechanism automatically adopts the correct orientation to align the valve actuating rockers  30  with the valves  12  and the lash adjusters  32  as the cover assembly  50  is lowered into position to mate with the cylinder head. 
     In order to position the summation mechanism correctly, it is necessary to control the orientation of the pivot joints at both ends of the control spring assembly. In the embodiment of the invention shown in  FIGS. 1 to 5 , the position of the summation mechanism is determined by the design of the inner and outer coupling elements  40   d  and  40   b  of the control spring assembly  40 . 
       FIGS. 3 ,  4  and  4   a  illustrate the position that the coupling elements will take up when the spring  40   a  is allowed to expand freely. The outer coupling element  40   b  is moved upwards by the spring  40   a  until it contacts an angled lower face of the plunger  40   e  that locates the summation mechanism in the engine cover. Although, the upper end of the outer coupling element  40   b  is contacting the lower face of the plunger  40   e , it may still pivot about its connection pin  40   f , although in order to do so it must compress the control spring  40   a . Hence the spring  40   a  will act to bias the pivot joint towards the position shown in  FIGS. 3 ,  4  and  4   a.    
     At the lower end of the control spring assembly  40 , the inner coupling element  40   d  has a pair of locating tags  40   g  that engage with stop shoulders  20   a  machined on the summation lever  20  and locate the summation lever in the position shown in  FIGS. 3 and 4  under the action of gravity. The stop shoulders  20   a  on the summation lever  20  remain outside of the normal range of working positions and may move out of contact with the tags  40   g  as the summation lever moves towards its assembled position. 
     Thus, as the cam summation mechanism comes into contact with the hydraulic lash adjuster  32  and the tip of the valve  12 , and as the engine cover  50  is located into its final position relative to the cylinder head, the summation mechanism is able to move to the correct position within its working range of motion. However in the free state, it will always move back to the assembly position shown in  FIGS. 3 and 4  under the action of gravity and the control spring  40   a.    
     It is also necessary to control the orientation of the valve actuating rockers  30  with respect to the summation lever  20  during assembly to make sure that the correct features will engage with the valve  12  and the hydraulic lash adjuster  32 . This is achieved simply in the illustrated embodiment by a pin  38  engaged with a flat or a hole in the summation lever  20  rocker as shown in  FIG. 3 . In this way the valve actuating rocker  30  is restricted to a range of rotation angles slightly larger than its working range. 
     Once the engine cover has been secured in place, as shown in  FIG. 5 , the final assembly operation is to tension the control spring assembly  40  such that it will maintain contact between one of the cam lobes and its followers. This is achieved by securing a fixing screw  40   h  into the top of the plunger  40   e  such that the plunger is held at the top of its bore in the engine cover. An oil seal  40   i  is located around the plunger  40   e , as shown, if it is necessary to prevent leakage around its outer surface. 
     Tensioning the control spring  40   a  has the effect of pulling the top face of the outer coupling element  40   b  away from the angled flat on the underside of the plunger  40   e,  and pulling the tags  40   g  on the lower end of the inner coupling element  40   d  out of engagement with the stop shoulders  20   a  on the summation lever  20 . These location features are now unable to make contact with each other under any circumstances whilst the rocker system is operating. The tensioned control spring assembly  40  is therefore able to pivot freely about its connection pins  40   c  and  40   f.    
     If the valve operating system is disassembled, the removal of the fixing screw  40   h  from the top of the plunger  40   e  will allow the control spring  40   a  to expand, re-engaging the location features. Hence as the engine cover is removed with the camshaft  18  and the summation mechanisms from the cylinder head, the summation levers and valve actuating rockers will naturally return to their assembly position under the action of the return spring and the force of gravity. 
     An alternative embodiment of the invention is shown in  FIGS. 6 and 7 . In describing this embodiment, components equivalent to those described in connection with the first embodiment have been allocated like reference numerals but in the 100 series. In this embodiment, instead of being mounted in a cylinder head cover, the camshaft  118  and the cam summation mechanism are assembled into a camshaft carrier that includes mounting bores for the hydraulic lash adjusters  132 . Because each summation lever  120  is located on one side by the control spring assembly  140  and on the other side by a lash adjuster  132 , its valve actuating rockers  130  will always be in a position to engage with the tips of the engine valves  112  as the camshaft carrier  150  is secured into place inside the cylinder head. 
     As with the previous embodiment, the plunger  140   e  is used to secure the top of the control spring assembly  140  to the carrier  150 . This plunger  140   e  may again be loosely retained in its bore using a clip, as shown in the previous embodiment, the fixing screw  140   h  being secured into the plunger  140   e  to tension the spring  140   a  only after the camshaft carrier has been secured to the head. In this embodiment, the plunger fixing screw  140   h  may alternatively be secured in position before the carrier is assembled to the cylinder head, such that the force of the control spring assembly  140  acts to hold the cam summation mechanism firmly in contact with the camshaft  118 . 
     It would further be possible to machine the hydraulic locating bores for the lash adjusters  132  such that they pass completely through the camshaft carrier  150  and the base of the hydraulic lash adjusters  132  sit on a machined surface in the cylinder head. This would result in the position of the hydraulic lash adjuster perpendicular to its axis being defined by the camshaft carrier and its axial position being defined by the cylinder head. 
     In such a design of camshaft carrier  150 , the action of gravity and the control spring assembly  140  would tend to cause the hydraulic lash adjusters to sit lower in their locating bores than their finally assembled position, which they would take up when the camshaft carrier is secured to the cylinder head. It is therefore possible to use the operation of assembling the camshaft carrier to the cylinder head for increasing the preload on the control spring to the required level for the rocker system to operate correctly. 
     An important feature of both described embodiments is that they offer the potential of integrating the camshaft and rocker system into a pre-adjusted sub assembly that can be produced in isolation from the cylinder head/engine to which it is fitted. As the camshaft may be mounted directly into the engine cover or the camshaft carrier, there is no need to machine this part as an assembly with the cylinder head—a flat mating face on the cylinder head is all that is required to secure the assembly into position. This significantly reduces the complexity of the cylinder head casting and its subsequent machining process.