Abstract:
A continuously variable valve lift system for actuating a combustion valve of an engine includes a camshaft having a camshaft lobe rotatable about an camshaft axis of rotation. A rocker assembly is pivotable for providing reciprocating motion to the combustion valve. A control shaft is rotatable about a control shaft axis of rotation such that rotation of the control shaft about the control shaft axis of rotation changes the position of the rocker assembly to vary the lift of the combustion valve. An actuator selectively rotates the control shaft between a minimum lift position and a maximum lift position. A bias spring surrounding the control shaft axis of rotation biases the control shaft only from the minimum lift position to a predetermined position which is intermediate the minimum lift position and the maximum lift position upon failure of the actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of U.S. provisional patent application Ser. No. 61/554,550 filed Nov. 2, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD OF INVENTION 
     The present invention relates to a continuously variable valve lift system for varying the lift of a combustion valve in an internal combustion engine; more particularly to a continuously variable valve lift system with a mechanism for providing a default lift of the combustion valve. 
     BACKGROUND OF INVENTION 
     Internal combustion engine manufacturers have been developing continuously variable valve lift (CVVL) systems to actuate combustion valves (intake valves and/or exhaust valves) of an internal combustion engine in an effort to increase fuel economy, decrease emissions, and otherwise improve the performance of the internal combustion engine. These CVVL systems may be applied to only the intake valves, to only the exhaust valves, or to both the intake valves and the exhaust valves depending on the need of the internal combustion engine. CVVL systems are used to vary the magnitude of lift of the combustion valves where valve lift is commonly understood to be the distance the combustion valve is moved from its valve seat. One CVVL system is shown in United States Patent Application Publication No. US 2011/0061618 which is commonly assigned and is incorporated herein by reference in its entirety. In this CVVL system, an engine camshaft with an engine camshaft lobe is rotated about an engine camshaft axis as is customary in the internal combustion engine art. A rocker assembly receives input from the engine camshaft lobe via a roller on the rocker assembly where rotational motion of the engine camshaft lobe causes the rocker assembly to pivot in a reciprocating manner. An output portion of the rocker assembly acts on a roller of a finger follower which pivots on a hydraulic lash adjuster. When the finger follower pivots about the hydraulic lash adjuster, a combustion valve is opened and closed. In order to vary the valve lift of the combustion valve, a control shaft is provided which is rotatable about a control shaft axis by an actuator. Rotation of the control shaft about the control shaft axis changes the position of the rocker assembly which results in a change of valve lift of the combustion valve. In the event of a failure of the actuator, it may be desirable for the CVVL system to default to a predetermined valve lift which allows the internal combustion engine to start and to run satisfactorily until a repair can be made. 
     U.S. Pat. No. 7,886,703 teaches a CVVL system with a default mechanism for providing a default valve lift in the event of a failure of the actuator which is an electric motor. In this arrangement, rotary motion of the electric motor is converted into linear motion by a ball screw. The linear motion created by the ball screw is converted into rotational motion of the control shaft by linkage attached to the ball screw and the control shaft. The default mechanism includes two compression springs which act in opposing directions to provide a default valve lift. One drawback to this default mechanism arrangement is that the actuator must work against at least one of the compression springs over the full range of motion of the control shaft during operation which increases the capacity requirements of the actuator. Another drawback to this default mechanism is that it must be used in a system where rotational motion of the actuator is converted into linear motion. 
     U.S. Pat. No. 7,418,933 teaches a CVVL system with a default mechanism for providing a default valve lift in the event of a failure of the actuator which is an electric motor. In this arrangement, the electric motor has an output shaft with a driving gear which meshes with a driven gear. The driven gear is connected to a shaft of a worm gear which meshes with a sector gear of the control shaft of the CVVL system. The default mechanism includes a large diameter gear and a small diameter gear which move with the shaft of the worm gear. A first default spring surrounds the control shaft to bias the control shaft from a maximum lift position to a default position intermediate the maximum lift position and a minimum lift position. A second default spring acts on a gear set, which includes a large diameter gear and a small diameter gear, to bias the control shaft from the minimum lift position the default position. One drawback to this default mechanism arrangement is that the actuator must work against at least one of the default springs over the entire range of motion of the control shaft during operation which increases the capacity requirements of the actuator. Another drawback to this default mechanism is the cost and complexity that are added by the gears that are needed only for the second default spring to bias the control shaft from the minimum lift position to the default position. 
     What is needed is a CVVL system with a default mechanism which minimizes the size requirements of an actuator of the CVVL system. What is also needed is a CVVL system with a default mechanism that adds minimal components and complexity to the CVVL system. 
     SUMMARY OF THE INVENTION 
     Briefly described, a continuously variable valve lift system is provided for actuating a combustion valve of an internal combustion engine. The continuously variable valve lift system includes an engine camshaft having an engine camshaft lobe rotatable about an engine camshaft axis of rotation. The continuously variable valve lift system also includes a rocker assembly that is pivotable for providing reciprocating motion to the combustion valve. The rocker assembly includes a rocker assembly input member for receiving motion from the engine camshaft lobe and a rocker assembly output member for transmitting motion to the combustion valve. The continuously variable valve lift system also includes a control shaft rotatable about a control shaft axis of rotation such that rotation of the control shaft about the control shaft axis of rotation changes the position of the rocker assembly, thereby varying the lift of the combustion valve. The continuously variable valve lift system also includes an actuator for selectively rotating the control shaft between a minimum lift position and a maximum lift position. The continuously variable valve lift system also includes a bias spring surrounding the control shaft axis of rotation for biasing the control shaft only from the minimum lift position to a predetermined position which is intermediate the minimum lift position and the maximum lift position upon failure of the actuator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       This invention will be further described with reference to the accompanying drawings in which: 
         FIG. 1A  is a cross-sectional elevation view, from the front of an internal combustion engine, of a CVVL system in accordance with the present invention, with the CVVL system in a high engine load mode and the input rocker subassembly on the base circle of the engine camshaft lobe; 
         FIG. 1B  is the cross-sectional elevation view of  FIG. 1A  now with the input rocker subassembly on the nose portion of the engine camshaft lobe; 
         FIG. 2A  is the cross-sectional elevation view of  FIG. 1A  now with the CVVL system in a low engine load mode and the input rocker subassembly on the base circle of the engine camshaft lobe; 
         FIG. 2B  is the cross-sectional elevation view of  FIG. 2A  now with the input rocker subassembly on the nose portion of the engine camshaft lobe; 
         FIG. 3  is an isometric view of the CVVL system at a representative camshaft angle; 
         FIG. 4  is a family of representative camshaft timing, lift, and duration curves for a CVVL system in accordance with the present invention; and 
         FIG. 5  is an isometric view, from the back of an internal combustion engine, of an actuator and bias spring of the CVVL system in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In  FIGS. 1A ,  1 B,  2 A, and  2 B; CVVL system  10  in accordance with the present invention is shown at one combustion valve  12  of internal combustion engine  14  as viewed from the front of internal combustion engine  14 . Combustion valve  12  may either be an intake valve or an exhaust valve. CVVL system  10 , when applied to intake valves, manages the internal combustion engine&#39;s intake gas exchange process with changes in the angular position of control shaft  16  which is rotatable about control shaft axis of rotation  17  (shown in  FIG. 3 ) which is the geometric center of control shaft  16 . Similarly, CVVL system  10 , when applied to exhaust valves, manages the internal combustion engine&#39;s exhaust exchange process with changes in the angular position of control shaft  16 . In  FIG. 1A  and  FIG. 1B , CVVL system  10  is shown in a high engine load mode, and in  FIG. 2A  and  FIG. 2B , CVVL system  10  is shown in a low engine load mode. In each of these pairs of figures, a view of CVVL system  10  with input roller  18  on base circle portion  20  of engine camshaft lobe  22  appears to the left ( FIG. 1A  and  FIG. 2A ), and a similar view with input roller  18  on nose portion  24  of engine camshaft lobe  22  (point of maximum lift) appears to the right ( FIG. 1B , and  FIG. 2B ). Engine camshaft lobe  22  is part of engine camshaft  26  which rotates in a conventional manner about engine camshaft axis of rotation  28 . 
     Control shaft  16  is eccentrically fixed to control shaft disc  30  such that control shaft disc  30  rotates eccentrically about control shaft axis of rotation  17  when control shaft  16  is rotated. High engine load events as shown in  FIGS. 1A and 1B  are produced whenever control shaft  16  is rotationally positioned such that input rocker pivot center  32 , which is also the geometric center of control shaft disc  30 , and output cam pivot center  34  are coincidental. Input roller  18 , which is the input member of rocker assembly  36 , is preferably formed of hardened steel and is free to rotate about a steel pin  38  which is staked in place within input rocker clevis  40 . As engine camshaft  26  rotates clockwise, as represented by arrow A, opening flank  42  of engine camshaft lobe  22  pushes input roller  18  upward, causing input rocker subassembly  44  of rocker assembly  36  to rotate in a clockwise direction about control shaft disc  30  and about input rocker pivot center  32 . As rocker subassembly  36  rotates, it turns about input rocker pivot center  32  of control shaft disc  30 . As input rocker subassembly  44  pivots clockwise about input rocker pivot center  32 , output rocker subassembly  48  of rocker assembly  36  is caused to rotate clockwise about output rocker pivot center  34  which is fixed in position. Clockwise rotation of output rocker subassembly  48  advances output cam profile  50  of output rocker subassembly  48  which acts on finger follower  52 . Output cam profile  50  is the output member of rocker assembly  36 . The right end of finger follower  52  pivots about hydraulic valve lash adjuster  54 . In this way, output rocker subassembly  48  pushing down on finger follower  52  transmits lift to combustion valve  12 . The further that output rocker subassembly  48  is rotated, the greater the lift imparted on combustion valve  12  through finger follower  52 . 
     When control shaft disc  30  is in the high engine load mode, as shown in  FIGS. 1A and 1B , maximum lift is imparted to combustion valve  12  whenever input roller  18  reaches nose portion  24  of engine camshaft lobe  22 . At this point, input rocker subassembly  44  and output rocker subassembly  48  cease to move in the clockwise direction. As engine camshaft lobe  22  rotates further in the clockwise direction, nose portion  24  of engine camshaft lobe  22  slips past input roller  18 , and lash spring  56  (shown in  FIG. 3 ) urges input rocker subassembly  44  and output rocker subassembly  48  to rotate counter-clockwise. This counter-clockwise rotation, in turn, reduces lift produced between output cam profile  50  and finger follower  52 . Eventually, as engine camshaft  26  continues to rotate clockwise, input roller  18  reaches base circle portion  20  of engine camshaft lobe  22  where lift remains at zero until the next valve opening event occurs. The motion just described produces a peak lift profile similar to peak lift profile  58  shown in  FIG. 4 , to maximize gas flow through combustion valve  12 . 
     Referring now to  FIGS. 2A ,  2 B, and  5 ; an actuator, shown as electric motor  60  (shown only in  FIG. 5 ), is operationally connected to control shaft  16  through gear set  62  in order to change the angular position of control shaft  16 . It should be stressed that  FIGS. 1A ,  1 B,  2 A, and  2 B are viewed from the front of internal combustion engine  14  while  FIG. 5  is viewed from back of internal combustion engine  14 . As a result, a clockwise rotation of control shaft  16  in  FIGS. 1A ,  1 B,  2 A, and  2 B corresponds to a counter-clockwise rotation of control shaft  16  in  FIG. 5 . When control shaft  16  is rotated significantly clockwise, as viewed in  FIGS. 2A and 2B  or counter-clockwise as viewed in  FIG. 5 , relative to its high engine load mode position as described previously, CVVL system  10  produces lower lift events (see region  64  of  FIG. 4 ) with reduced duration, corresponding to lower engine loads. When this happens, input rocker pivot center  32  of control shaft disc  30  moves inward toward engine camshaft  26 , away from output rocker pivot center  34  of output rocker subassembly  48 . Thus, when engine camshaft lobe  22  induces angular motion to input rocker subassembly  44 , output rocker subassembly  48  pushes down on finger follower  52  to a lesser magnitude than compared to the high engine load mode of operation, thereby transmitting lesser lift to combustion valve  12 . When control shaft  16  is in the lowest engine load mode, CVVL system  10  can generate a short and shallow lift event as represented by curve  66  of  FIG. 4  which is suitable for the lightest of all engine loads. While not shown, CVVL system  10  may also prevent combustion valve  12  from opening, which may be required when it is desired to deactivate some cylinders of internal combustion engine  14  as is known to those skilled in the art of cylinder deactivation. 
     It will be observed that displacement (i.e. rotation) of control shaft  16  from the position shown  FIGS. 1A ,  1 B to that shown in  FIGS. 2A ,  2 B serves to a) change the position of input roller  18  on engine camshaft lobe  22 , thereby advancing the start of valve opening and b) to change the contact point of output cam profile  50  with finger follower  52 , thereby reducing the potential valve lift. More concisely stated, displacement of control shaft  16  displacement changes the position of rocker assembly  36 . Thus, varying the angular position of control shaft  16  between the high engine load position (maximum valve lift position) illustrated in  FIGS. 1A ,  1 B and the low engine load position (minimum valve lift position) illustrated in  FIGS. 2A ,  2 B produces the entire lift curve family depicted in  FIG. 4 . 
     Reference will now be made to  FIG. 5  which, for clarity, has had all elements of CVVL system  10  removed except for control shaft  16 , electric motor  60 , gear set  62 , and other elements that will be described herein. Gear set  62  includes drive gear  68  which is fixed to the output shaft of electric motor  60  to rotate with the output shaft in a one-to-one relationship when an electric current is applied to electric motor  60 . Gear set  62  also includes driven gear  70  which is fixed to control shaft  16  to rotate with control shaft  16  in a one-to-one relationship. Gear set  62  also includes intermediate gear  72  which is operationally disposed between drive gear  68  and driven gear  70 . Intermediate gear  72  includes intermediate large diameter gear  74  which meshes with drive gear  68 . Intermediate gear  72  also includes an intermediate small diameter gear (which is not shown because it is hidden behind intermediate large diameter gear  74 ) which is fixed to intermediate large diameter gear  74  in order to rotate with intermediate large diameter gear  74  in a one-to-one relationship. The intermediate small diameter gear meshes with driven gear  70 . In this way, rotation of drive gear  68  by electric motor  60  causes rotation of control shaft  16 . 
     A default mechanism, shown in part as bias spring  76 , is provided in order to move control shaft  16  to a predetermined position intermediate of the minimum lift position and the maximum lift position in the event of a failure of electric motor  60 . Bias spring  76  is a torsional spring which surrounds control shaft axis of rotation  17 . Bias spring stationary end  78  of bias spring  76  is grounded to internal combustion engine  14  while bias spring moveable end  80  of bias spring  76  applies a biasing force to driven gear  70  only from the minimum lift position to the predetermined position. Driven gear  70  includes cutaway sector  82  which provides reaction surface  84  for bias spring moveable end  80  to act upon from the minimum lift position to the predetermined position. In an alternative not shown, an arcuate slot may be substituted for cutaway sector  82 . When control shaft  16  reaches the predetermined position, bias spring moveable end  80  is prevented from moving further by spring stop  86  which is fixed to internal combustion engine  14 .  FIG. 5  shows control shaft  16  in the predetermined position, and as can be seen, bias spring moveable end  80  is in contact with spring stop  86  and reaction surface  84 . If electric motor  60  were to be actuated to rotate driven gear  70  in a clockwise direction as viewed in  FIG. 5 , spring stop  86  would prevent bias spring moveable end  80  from moving and reaction surface  84  would no longer be in contact with bias spring moveable end  80 . As a result, bias spring  76  would no longer have an effect on electric motor  60  and control shaft  16 . Conversely, if electric motor  60  were to be actuated to rotate driven gear  70  in a counter-clockwise direction as viewed in  FIG. 5 , reaction surface  84  would cause bias spring  76  to wind up and bias spring moveable end  80  would no longer be in contact with spring stop  86 . As a result, bias spring  76  would provide a biasing force on control shaft  16  via driven gear  70  that would urge control shaft  16  to the intermediate position if a failure of electric motor  60  were to occur. 
     While bias spring  76  will urge control shaft  16  to the predetermined position if electric motor  60  fails when control shaft  16  is positioned between the minimum lift position and the predetermined position, bias spring  76  does not urge control shaft  16  to the predetermined position if electric motor  60  fails when control shaft  16  is positioned between the maximum lift position and the predetermined position. Instead only the forces generated by engine camshaft lobe  22  and combustion valve  12  acting on rocker assembly  36  will urge control shaft  16  to the predetermined position. When control shaft  16  reaches the predetermined position, reaction surface  84  will come into contact with bias spring moveable end  80 . Bias spring  76  is selected to provide a spring force that will resist the forces generated by engine camshaft lobe  22  and combustion valve  12  acting on rocker assembly  36 . As a result, the forces generated by engine camshaft lobe  22  acting on rocker assembly  36  are unable to wind up bias spring  76  and control shaft  16  is maintained at the predetermined position. 
     Since electric motor  60  only needs to work against one bias spring for only a portion of the total range of motion of control shaft  16 , electric motor  60  does not need to be increased in load capacity to overcome the forces of two springs which would be needed if a bias spring were provided to bias control shaft  16  toward the predetermined position from both the minimum lift position and the maximum lift position. Furthermore, since there is only one bias spring, there are fewer CVVL system components and the design of the CVVL system is simplified and more compact. 
     While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited.