RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS 
   This patent application claims the benefit of U.S. Provisional Application No. 60/847,784, filed Sep. 28, 2006. 


   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The present invention was supported in part by U.S. Government Contract No. DE-FC26-05NT42483. The United States Government may have rights in the present invention. 


   TECHNICAL FIELD 
   The present invention relates to valvetrains of internal combustion engines; more particularly, to devices for controlling the open duration of valves in such valvetrains; and most particularly, to a system for selectively varying the point at which the intake valves close in an internal combustion engine. 
   BACKGROUND OF THE INVENTION 
   Many advances have been made recently toward reducing the emissions and increasing the efficiency of Diesel engines. One such advancement has been the development of Homogeneous Charge Compression Ignition (HCCI) systems. HCCI is a process wherein an initial premixed, homogeneous charge of Diesel fuel and air is compressed and partially burned by high temperature and pressure in a flameless process, followed by one or more post injections of fuel, as opposed to classic Diesel ignition wherein a charge of air is compressed and then injected with Diesel fuel, resulting a stratified mixture of fuel and air. HCCI has yielded many benefits including extremely low emissions of NO x  and particulate matter (soot) because of lower ignition temperatures and the use of a leaner fuel/air mixture. 
   However, HCCI has its challenges. For example, with a compression ratio in the range of 9:1 to 14:1, starting an HCCI-ignited engine in cold weather can be difficult. This challenge can be addressed by selectively varying the point at which the intake valves close during the engine cycle to controllably reduce the compression ratio from that of a higher designed value, optimized for cruising conditions. By selectively keeping the intake valve open for a portion of the compression stroke, a portion of the volume of air that would otherwise be compressed in the cylinder by the up-moving piston, is instead bled back through the open intake valve, thereby effectively reducing the compression ratio on the engine. 
   Mechanization of an HCCI strategy that can selectively vary the compression ratio has been proposed in the past with limited success because of system and hardware complexity. For example, mechanization has been achieved by using two separate cam phasers to operate two intake valves at each cylinder so that intake valve opening and intake valve closing can be controlled by the phasers independently. Although effective, the use of two cam phasers is costly, adds weight to an engine and vehicle and often cannot be fitted into available space. 
   What is needed in the art is a simplified mechanism for selectively varying the closing point of the intake valves of an HCCI engine that is relatively easy to manufacture and assemble, has few parts, and requires minimal packaging space in an engine envelope. 
   It is a principal object of the present invention to provide variable closing timing of intake valves of an internal combustion engine. 
   It is a further object of the invention to simplify the manufacture and assembly of a system for such variable closing timing. 
   SUMMARY OF THE INVENTION 
   Briefly described, a Continuously Variable Valve Duration (CVVD) system in accordance with the invention includes a rocker assembly acted upon by first and second cam lobes disposed on first and second off-spaced intake camshafts, respectively, for selectively varying the closing point of the poppet valves, for example, the intake valves, in an internal combustion engine. As disclosed below, the present invention is described in terms of the engine intake valves, but it should be understood that the invention is also applicable to engine exhaust valves as well, or to both intake and exhaust valves as may be desired. 
   The opening camshaft is rotatably driven by the engine crankshaft and controls conventionally the opening point of the valves through a novel rotatable rocker assembly disposed on a fixed rocker pivot shaft. 
   The closing camshaft is rotatable and is connected to the engine crankshaft through a cam phaser device driven preferably by a gear train from the opening camshaft. The closing cam lobe is poised to take over control of the closing event of the valves through the same rotatable rocker assembly. By changing the rotational phase of the closing camshaft relative to the opening camshaft via the cam phaser, the phase of the closing camshaft can be advanced or retarded relative to the opening camshaft to take over the closing event of the valves. By doing so, the point at which the valves close in the cycle can be selectively varied over a wide range of open times. Additional embodiments for adapting the CVVD system to various valvetrain types are provided. 
   An important advantage of the present device is its simplicity. With the rocker assembly being rotationally mounted on a fixed rocker shaft and the need for only one cam phaser to variably control the open duration time of the valves, the present invention accrues significant manufacturing, mechanical, and cost advantages over prior art arrangements. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIGS. 1 and 1   a  are isometric and exploded isometric views, respectively, of a first embodiment of a CVVD system in accordance with the invention, shown in an assembly for operating a set of intake or exhaust valves in a four-cylinder engine head; 
       FIGS. 2 and 2   a  are end elevation and isometric views, respectively, of the first embodiment shown in  FIGS. 1 and 1   a , showing two intake valve camshafts and lobes operative on a first rocker assembly for a pair of combustion valves; 
       FIG. 3  is a graph showing a family of lift curves for a valvetrain equipped with a CVVD system in accordance with the invention, showing the degree to which the point of intake valve closing can be changed by the system; 
       FIG. 4  is a graph of a composite lift curve taken from  FIG. 3 , showing the transition point at which the second intake valve camshaft takes over control of the lift event to effect a later valve closing point; 
       FIG. 5  is an end elevation view of a second embodiment of a second embodiment of a cam lobe in accordance with the invention; 
       FIG. 6  is a family of lift curves for a valvetrain equipped with the cam lobe shown in  FIG. 5 ; 
       FIGS. 7 and 7   a  are end elevation and isometric views, respectively, of a third, embodiment of a CVVD system in accordance with the invention; and 
       FIGS. 8 and 8   a  are end elevation and isometric views, respectively, of a fourth embodiment of a CVVD system in accordance with the invention. 
   


   Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate currently-preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 ,  1   a ,  2 , and  2   a , a CVVD system  100 , in accordance with the invention, includes opening intake camshaft  102  mounted in base plate  103  for attachment to a cylinder head  104  of internal combustion engine  106 . In the exemplary arrangement, engine  106  is a straight line 4-cylinder engine. 
   Opening intake camshaft  102  includes a plurality of cam lobes  108  equal in number to the number of engine cylinders and spaced along the longitudinal axis  110  of the camshaft. Each cam lobe  108  is defined by a profile having a base circle portion  112 , an ascending flank  114 , a nose portion  116 , and a descending flank  118 . Journals  101  (in this case five) are also spaced along the longitudinal axis of opening intake camshaft  102  and rest on mating bearing surfaces  122  of base plate  103 . Dowels  123  are provided to align base plate  103  with head  104 . First end  124  of opening intake camshaft  102  further includes gear  126  fixed to its end. Gear  126  is rotatably coupled to the engine crankshaft (not shown) through a belt, chain, or gear mesh and is sized to rotate the opening camshaft a single revolution for every two revolutions of the crankshaft, as is known in the art. Second end  128  of opening intake camshaft further includes output gear  130  fixed to its end. Both gears  126 ,  130  are secured to ends  124 ,  128  to rotate with opening intake camshaft  102 . While gear  130  is shown as a spur gear ( FIG. 1   a ), it is understood that it could be, for example, a helical gear ( FIG. 1 ), as well. 
   CVVD system  100  further includes closing intake camshaft  132  off-spaced from and parallel to opening intake camshaft  102 . Closing intake camshaft  132  further includes a plurality of cam lobes  134  equal in number to the number of engine cylinders and spaced along the longitudinal axis  136  of the camshaft. Each cam lobe  134  is defined by a profile having a base circle portion  138 , an ascending flank  140 , a nose portion  142 , and a descending flank  144 . Journals  105  are also spaced along the longitudinal axis  136  of closing intake camshaft  132  and rest on mating bearing surfaces  148  of carrier modules  149 . First end  150  of closing intake camshaft  132  terminates at the first bearing surface  148  in the row of bearing surfaces. Second end  152  of closing intake camshaft  132  further includes a cam phaser  154  having input gear  156  in meshing engagement with output gear  130  of opening intake camshaft  102 . 
   The geared relationship between the opening camshaft and the phaser of the closing camshaft is only exemplary. Obviously, the phaser may be driven directly by the engine crankshaft in a manner similar to that just described for the opening camshaft, or by any other suitable means, for example, by an electric motor, to perform the same valve-closing phasing function. 
   Cam phaser  154  may be of a variety of types of phasers known in the art including a type known in the prior art as a “vaned” cam phaser. As such the phaser is used to selectively alter the phase angle between cam lobes  108 ,  134  of the opening intake camshaft and closing intake camshaft, respectively. 
   CVVD system  100  further includes a plurality of rocker subassemblies  160 , equal in number to the number of cylinders in head  104 , and pivotably mounted on elongate rocker pivot shaft  162 . Axis  164  of rocker pivot shaft  162  is off-spaced from, but parallel to, and disposed between opening intake camshaft  102  and closing intake camshaft  132 . The position of rocker pivot shaft  162  relative to head  104  is fixed. That is, unlike prior art variable valve actuating systems, the pivot point (axis  164 ) of rocker assembly  160  is not moved in order to achieve the desired variation in valve actuation. 
   Rocker subassembly  160  further includes rocker lever  165  having opening input arm  166 , closing input arm  168  and output arm  170 . Roller  172  is rotatably fastened to an end of opening input arm  166  for engagement with an associated opening intake camshaft lobe  108 . Roller  174  is rotatably fastened to an end of closing input arm  168  for engagement with associated closing intake camshaft lobe  134 . Rollers  172 ,  174  are preferably formed of hardened steel as is known in the art. 
   At a distal end of output arm  170  is actuating paddle  176 . Paddle  176  is preferably formed into a compound arcuate shape. Preferably each paddle contact surface  178  is ground for smooth contact with center roller  180  of each roller finger follower  182 . Roller finger follower  182 , as is well known in the art, pivots at first end  184  about hydraulic lash adjuster  186  as a downward force is applied to center roller  180  of roller finger follower  182  to open intake valve  188  against valve return spring  190 . As can be readily seen in  FIG. 2 , rotation of rocker subassembly  160  in a clockwise direction about rocker pivot shaft  162  causes paddle  176  to be shifted leftward relative to center roller  180  of roller finger follower  182 . Because of the progressively increasing radius  191   a ,  191   b  of contact surface  178  as measured from axis  164 , as rocker subassembly assembly  160  rotates clockwise from the closed valve position shown in  FIG. 2 , valve  188  moves axially from its closed position (shown) to a full open position (not shown). It can also be readily seen that rotation of opening intake cam lobe  108  in clockwise direction  189  causes paddle  176  to shift leftward first, then back to control the movement of valve  188 . Likewise, rotation of closing intake cam lobe  134  in counterclockwise direction  193  causes paddle  176  to shift leftward first, then back to control the movement of valve  188 . 
   Torsional return spring  194  is connected at end  194   a  to rocker lever  165  and at end  194   b  to carrier module  149  to thereby bias rocker lever  165  in a counterclockwise direction so as to maintain contact between rollers  172 ,  174  and one or both of cam lobes  108 ,  134  during operation of the engine. While return spring  194  is shown as a torsional spring in  FIG. 2 , it is understood that the return spring may alternately be any type spring such as, for example, a compression coil spring or leaf spring disposed between lever  165  and module  149  for the same purpose. 
   As mentioned previously, gear  126  of opening intake camshaft  102  is driven by a chain, belt or gearing from an engine&#39;s crankshaft in a 2:1 rotational ratio (2 revolutions of the crankshaft for every one revolution of the camshaft  102 ). Since output gear  130  of opening intake camshaft  102  and input gear  156  of cam phaser  154  are of the same diameter and have the same number of teeth, closing intake camshaft  132  is driven from opening intake camshaft  102  in a 1:1 rotational ratio. And, since gears  126 ,  130  are in direct mesh, closing intake camshaft  132  rotates in a counter direction  193  from the direction  189  of opening intake camshaft  102 . Recalling that opening intake camshaft  102  is rotationally connected to closing intake camshaft  132  through cam phaser  154 , a signal received by the cam phaser from a controller (not shown) causes the angular position of lobe  134  of the closing intake camshaft to shift relative to lobe  108  of the opening camshaft, from its default position shown in  FIG. 2  (reference line  196 ), to a more retarded position (reference line  197 ). This shift results in a delay of the closing point of the intake valve as will now be described. 
   Referring now to  FIGS. 2 and 3 , curve  198  represents an exemplary intake valve opening, lift, and closing characteristic profile of CVVD system  100  when the cam phaser is in its default position. That is, the entire valve event is controlled by lobe  108  of the opening intake camshaft contacting roller  172 . During that event, while lobe  134  of the closing intake camshaft rotates from the meshing of gears, its phasing relative to the rotation of lobe  108  does not permit its ascending flank  140 , nose portion  142 , or descending flank  144  to contact roller  174 . That is, as lobe  134  rotates as if to make contact with roller  174  by its ascending flank, roller  172  has already made contact with and started up the ascending flank of lobe  108 . As lobe  134  continues its rotation as if to make contact with roller  174  by its nose portion  142 , roller  172  has already made contact with and started across the nose portion of lobe  108 . Finally, as lobe  134  continues its rotation as if to make contact with roller  174  by its descending flank  144 , roller  172  has already started down the descending flank  118  of lobe  108 . Thus, in the default mode of the phaser, lobe  108  of opening intake camshaft  102  remains in full control of rocker subassembly  160  to open and to close valve  188  in a normal lift profile as though closing lobe  134  did not exist. 
   Following curve  198  in  FIG. 3 , as roller  172  moves up the ascending flank  114  of the lobe  108 , intake valve  188  opens. When the contact point moves through the nose portion  116  of lobe  108 , valve  188  reaches its full open lift  199  of, typically, about 9 mm. Then as roller  172  moves down descending flank  118 , it remains in contact with lobe  108  until the valve closes at about 120 cam angle degrees. 
   When delaying of the closing event of intake valve  188  is desired, a signal from the engine controller directs cam phaser  154  to retard the rotational position of closing intake camshaft  132  relative to opening intake camshaft  102 . This causes a portion of descending flank  144  of closing intake cam lobe  134  to come into contact with roller  174  and to take over control of the movement of rocker sub assembly  160  from opening intake cam lobe  108 . As can be seen by resulting curve  198 , roller  172  remains in contact with lobe  108  almost through the entire opening and closing event. However, since the phasing of closing intake camshaft  132  has been retarded by the cam phaser, roller  174  comes in contact with descending flank  144  of lobe  134  at Point A (phaser retards lobe  134  by 20°) and, from that point until valve  188  is closed (curve  198   a ), closing intake cam lobe  134  takes over from opening intake cam lobe  108  and keeps intake valve  188  open to about 140 cam angle degrees (point  195  in  FIG. 3 ). Curves  198   b  and  198   c  show additional amounts that intake valve  188  can be kept open, in cam angle degrees, by retarding the relative rotational position of closing intake camshaft  132  even further. 
   For clarity of presentation,  FIG. 4  shows parent curve  198  and composite curves  198   a ,  198   b ,  198   c  derived from  FIG. 3 , again showing the increased duration of valve opening. 
   Referring again to  FIG. 3 , in the most retarded mode of closing intake camshaft  132  (composite curve  198   c ), the transition point when closing intake cam lobe  134  takes over occurs at a point when the valve is at a lift of approximately 5 mm, and rocker subassembly  160 , roller finger follower  182 , valve  188  and spring  190  are moving together in the closing direction at a relatively high velocity. At that point, closing intake cam lobe  134  and roller  174  come together, in an attempt to catch the moving mechanism to slow it down in order to extend valve duration. 
   Referring to  FIGS. 5 and 6 , second embodiment  200  in accordance with the invention is identical to CVVD system  100 , having the same components as shown in  FIGS. 1 ,  1   a ,  2 , and  2   a  except for revised opening/closing cam lobes  208 , 234 . Embodiment  200  reduces or eliminates the velocity of the intake valve  188  at the transition point (point A in  FIG. 3 ) between the opening cam lobe  108  and roller  172  and the closing cam lobe  134  and roller  174  by changing the contours of nose portions  216 , 242  to produce flat portions  299 , 299   a  of family curves  298 , 298   a  at peak valve lift. Thus, the transition point C in  FIG. 6 , at which closing cam lobe  234  takes control of rocker sub assembly  160  from opening cam lobe  208 , occurs when the oscillating components of CVVD  200  are near or at zero velocity. Note that the extended point  295  of intake valve closing of CVVD  200  approximately 140 cam angle degrees), as shown in  FIG. 6 , is identical to the extended point  195  of intake valve closing of CVVD  100  (approximately 140 cam angle degrees), as shown in  FIG. 3 . 
   Referring to  FIGS. 7 and 7   a , a third embodiment  300  of a CVVD in accordance with the invention is shown. CVVD  300  is similar to CVVD  100  in that second rocker subassembly  360  acts upon roller finger follower  182  that pivots on a hydraulic lash adjuster  186  to provide a downward force on valve  188  to move valve  188  in an opening direction. Rocker assembly  360  includes an opening input arm  366 , a closing input arm  368  and an output arm  370 ; however, rocker assembly  360  is more compact than rocker assembly  160 , thus lowering the packaging height  367  over that of CVVD  100 . Also, paddle  376  and its contact surface  378  are generally flat, greatly reducing the cost of manufacture. 
   Referring now to  FIGS. 8 and 8   a , a fourth embodiment  400  of a CVVD in accordance with the invention is also a variation of CVVD  100 . Like CVVD  300 , CVVD  400  is a more compact design thereby lowering the package height  467  over CVVD  100  and CVVD  300 . Rocker assembly  460  includes an opening input arm  466 , a closing input arm  468 , and output arm  470 . CVVD  400  is adaptable to a Type 2 (end pivot rocker arm, overhead cam) valve train system, in that output arm  470  engages the stem of valve  488  directly; thus, the hydraulic lash adjuster  186  and roller finger follower  182  required for embodiments  100 , 300  are obviated, thereby simplifying the mechanism even more. 
   While the invention has been described as applicable to intake valves, it is understood that the invention&#39;s application need not be so limited. 
   While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
A continuously variable valve duration system including a rocker assembly acted upon by two off-spaced camshafts for selectively varying the closing point of a valve in an internal combustion engine. An opening camshaft is rotatably driven by the engine crankshaft and controls at least the opening and point of the valve through a rocker assembly disposed on a fixed pivot shaft. A closing camshaft, rotatably connected to the opening intake camshaft through a cam phaser, is poised to take over control of the valve closing event through the same rocker assembly. By changing the rotational phase of the closing camshaft relative to the opening camshaft via the cam phaser, the valve closing event can be either retarded or advanced so as to override the opening camshaft and thus selectively vary the valve event duration.