Abstract:
A crank driven VVT mechanism includes single or dual cranks for actuating oscillating cam drive mechanisms. The crank drive positively actuates the mechanisms in both valve opening and valve closing directions and thus avoids the need to provide return springs as are generally required in cam driven mechanisms to bias the mechanisms toward a valve closed position. However, the crank driven mechanisms of the invention require the oscillating cams to pivot onto the base circle portion during a dwell period in order to provide periods of valve closed engine operation even when the valves are set for maximum opening stroke. Thus, increased motion of the actuating mechanism or a smaller angular extent of the valve lift portions of the oscillating cams is required as compared to a cam driven mechanism. A variable ratio slide and slot control lever drive as well as a back force limiting worm drive for the control shaft are combined with the crank mechanism to provide additional system advantages comparable to those of cam actuated mechanisms.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/136,988, filed Jun. 1, 1999. 
    
    
     TECHNICAL FIELD 
     The invention relates to variable valve timing mechanisms and, more particularly, to valve actuating mechanisms for varying the lift and timing of engine valves. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,937,809, issued Aug. 17, 1999, discloses cam driven variable valve timing (VVT) mechanisms which are relatively compact, and are applicable for operating individual or multiple valves. In these mechanisms, an engine valve is driven by an oscillating rocker cam that is actuated by a linkage driven by a rotary eccentric, preferably a rotary cam. The linkage is pivoted on a control member that is, in turn, pivotable about the axis of the rotary cam and angularly adjustable to vary the orientation of the rocker cam and thereby vary the valve lift and timing. The rotary cam may be carried on a camshaft. The oscillating cam is pivoted on the rotational axis of the rotary cam. 
     U.S. patent application Ser. No. 09/129,270, filed Aug. 5, 1998, now 6,019,076, discloses a similar cam actuated VVT mechanism having various additional features, including a variable ratio pin and slot control member drive providing advantageous control characteristics and a worm drive for the control shaft designed to prevent backdrive forces from overcoming the actuating force of the small drive motor. A particular embodiment of flat spiral mechanism return springs is also disclosed. 
     SUMMARY OF THE INVENTION 
     The present invention provides crank driven VVT mechanisms wherein single or dual cranks are provided for actuating the oscillating cam drive mechanisms. The crank drive is desmodromic in that it actuates the mechanisms in both valve opening and valve closing directions and thus avoids the need to provide return springs as are generally required in cam driven mechanisms to bias the mechanisms toward a valve closed position. However, the crank driven mechanisms of the invention require the oscillating cams to pivot onto the base circle portion during a dwell period in order to provide periods of valve closed engine operation even when the valves are set for maximum opening stroke. Thus, increased motion of the actuating mechanism or a smaller angular extent of the valve lift portions of the oscillating cams is required as compared to a cam driven mechanism. 
     The advantages of control by a variable ratio slide and slot control lever drive as well as a back force limiting worm drive for the control shaft may be combined with the crank mechanism to provide additional system advantages comparable to those of cam actuated mechanisms. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a pictorial view of a first embodiment of crank driven VVT mechanism according to the invention having a single crank drive; 
     FIG. 2 is a side view of the embodiment of FIG. 1 illustrating the mechanism in an intermediate valve lift control position with the valves closed and oscillating cams in a dwell mode; 
     FIG. 3 is similar to FIG. 2 but shows the control shaft in a maximum valve lift position with valves closed and oscillating cams in extreme dwell; 
     FIG. 4 is similar to FIG. 3 but shows the valves fully open and oscillating cams in the maximum valve lift position of the mechanism; 
     FIG. 5 is similar to FIG. 2 but shows the control shaft in a minimum valve lift position with valves closed and oscillating cams in extreme dwell; 
     FIG. 6 is similar to FIG. 5 but shows the valves slightly open and oscillating cams in the minimum valve lift position of the mechanism; 
     FIG. 7 is a cross-sectional view of a worm drive for actuating the control shaft of the mechanism; and 
     FIG. 8 is a pictorial view similar to FIG. 1 but showing an alternative embodiment of mechanism according to the invention having dual cranks on either side of a central crankshaft bearing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIGS. 1-6 of the drawings, numeral  10  generally indicates a portion of an internal combustion engine including a valve actuating mechanism  12  operative to actuate dual inlet valves  14  for a single cylinder of an engine. Mechanism  12  includes a rotary valve actuating crankshaft  16  which extends the length of a cylinder head, not shown, of a multi-cylinder engine, of which the mechanism for only a single cylinder is illustrated. The valve actuating crankshaft  16  may be driven from the engine crankshaft by a chain or any other means which would be suitable for driving a conventional valve actuating camshaft. 
     The valve actuating crankshaft  16  carries a rotary crank or eccentric  18  having an eccentric axis  20  which orbits about a primary axis  22  of the crankshaft  16 . Rotation of the crankshaft  16  is optionally counterclockwise as shown by the arrow  23  of FIG. 2, but an opposite rotation could be used if desired. A connecting rod  24  is connected with rotary crank  18  for a purpose to be subsequently described. Control members (or frames)  26  are mounted on the crankshaft  16  for pivotal motion about the primary axis  22 . If desired, the control members could be mounted other than on the crankshaft. In FIGS. 2 and  3 , the closer control member  26  is not shown so that the positioning of the connecting rod  24  may be clearly shown. 
     The control members  26  each include an outer end  28  connected with a pivot pin  30  disposed on a first pivot axis  32 . A rocker lever, or primary lever,  34  is pivotally mounted at one end to the pivot pin  30  which connects it with the control members  26 . A distal end  35  of the rocker lever  34  is pivotally connected by pins to links  36 . Between its ends, rocker lever  34  is connected with connecting rod  24  which connects the rocker lever  34  with the crank  18  of the crankshaft  16  to positively oscillate the rocker lever  34  upon rotation of the crankshaft  16 . 
     Dual links  36  extend from opposite sides of the rocker lever  34  to outer ends  38  of a pair of secondary levers  40  to which the links  36  are pinned. Levers  40  have inner ends  42  which are mounted on the crankshaft  16  and pivotable about the primary axis  22 . These inner ends define oscillating cams  44 , each having a base circle portion  46  and a valve lift portion  48 . The base circle and valve lift portions are similar in concept to those discussed in the previously mentioned U.S. Pat. No. 5,937,809, although the angular extent and lift angles of the two portions are varied to accommodate the crank driven mechanism. 
     The oscillating cams  44  are engaged by rollers  50  of roller finger followers  52 , each having inner ends  54  which are pivotally seated on stationary hydraulic lash adjusters  56  mounted in the engine cylinder head, not shown. Outer ends  58  of the finger followers  52  engage the stems of valves  14  for directly actuating the valves in cyclic variable lift opening patterns as controlled by the mechanism. Valve springs, not shown, are conventionally provided for biasing the valves in a closing direction. 
     In order to provide the variable valve lift and timing which are results of the mechanism, a control shaft  60  is provided that is pivotable about a secondary axis  62  parallel with and spaced from the primary axis  22 . If desired, the control shaft  60  could be connected to the control members  26  by a gear and tooth connection as shown in previously mentioned U.S. Pat. No. 5,937,809 to vary the mechanism between maximum and minimum valve lift positions. However, in the present embodiments, a preferred pin and slot connection is used as shown in FIGS. 2-6. The control shaft  60  mounts a pair of control levers  64 , only one being shown. Each of the control levers mounts a drive pin  66  which preferably carries a flat sided bushing  68 . Each bushing  68  acts as a slider and is slidable within a slot  70  provided in an arm  72  of an associated one of the frame elements or control members  26 . The slots  70  of the arms are angled with respect to a radius from the primary axis  22  in order to provide a variation in ratio of the movement between the control shaft  60  and the control member  26 , as will be subsequently more fully described. 
     In operation of the mechanism so far described, rotation of the valve actuating crankshaft  16  orbits the crank  18  about the primary crankshaft axis  22 , preferably in a counterclockwise direction as shown by the arrow  23  in FIG.  2 . The crankshaft  16  always rotates in phase with the engine crankshaft, not shown, regardless of variations in the valve lift and timing events. Thus, the crank  18  oscillates the rocker lever  34  around its pivot pin  30  with a cyclic angular oscillation that is a constant function of engine crank rotation. As the rocker lever  34  is pivoted outward away from the primary axis  22 , it draws the link  36  with it, in turn oscillating the secondary levers and associated oscillating cams  44  through a predetermined constant angle with each rotation of the camshaft. 
     FIGS. 3 and 4 illustrate the position of the mechanism  12  with the control member  26  pivoted clockwise to the full valve lift position. FIG. 3 shows the valves  14  closed with the crank  18  rotated to oscillate cams  44  to contact follower roller  50  at the maximum dwell positions of their base circles  46 . FIG. 4 shows the valves  14  fully opened with the crank  18  rotated so the noses of the oscillating cam valve lift portions  48  are engaging the rollers  50 . In the full valve lift position of the control member  26 , pivoting of the oscillating cams  44  by the mechanism forces the finger followers  52  downward as the oscillating cams move from their base circle locations clockwise until the nose of each cam  44  is engaging its associated follower roller  50  in the full valve lift position (FIG.  4 ). This causes the finger follower to pivot downward, forcing its valve  14  into a fully open position. 
     As the crank  18  rotates further from the full open position of the valves, the mechanism rotates the oscillating cams  44  counterclockwise, returning the finger follower rollers  50  to the base circles of the oscillating cams and thereby allowing the valves  14  to be closed by their valve springs, not shown. Continued rotation of the crank  18  rotates the cams  44  further along their base circles during a dwell condition in which the associated engine valves remain closed. Upon reaching maximum eccentricity of the crank in the valve closed position (FIG.  3 ), the oscillation of cam  44  is reversed. However, the valves remain in a dwell condition until the crank again reaches a position at which the cams  44  are moved off their base circles to the beginning of the valve lift portions of the cams and the valves are again opened as previously described. A useful advantage of the present crank actuated mechanism over prior cam actuated VVT mechanisms is that the mechanism cycle is completed without requiring return springs. Instead, the crank and connecting rod positively move the mechanism in both directions of oscillation, avoiding the need for springs other than the usual valve springs. 
     To reduce valve lift and at the same time advance the timing of peak valve lift, the control shaft  60  is rotated clockwise, as shown in FIGS. 2-4 to the position shown in FIGS. 5 and 6 where the control member  26  is rotated fully counterclockwise. FIG. 5 shows the cams  44  in their maximum dwell position at the extreme ends of the base circles while FIG. 6 shows the cams  44  in minimum valve lift position. In this minimum valve lift position of the control shaft  60 , actuation of the rocker lever  34  by the rotary crank  18  is prevented from opening the valves more that a preset minimum because the finger follower rollers  50  are in contact primarily or only with the base circle portions  46  of the oscillating cams. To accomplish this, the angular movement of the control member  22  from its full lift position of FIG. 3, must approximate the angular displacement of the oscillating cams during the valve lift portion of the stroke of the rocker lever caused by the rotary crank so that the finger follower rollers never or only slightly contact the valve lift portion  48  of the oscillating cams. 
     The position of the mechanism  10  about the primary axis  22  is determined by rotation of the control shaft  60  as previously described. Since the engine charge mass flow rate has a greater relative change in low valve lifts than in high valve lifts, the slider and slot connection between each control lever  64  and its control member  26  is designed so that the angled slot provides a variable angular ratio such that, at low lifts, the control shaft must rotate through a large angle for small rotation of the control member. This is accomplished by positioning the angle of the slot relative to a radial line from the primary axis  22  in order to obtain the desired change in angular ratio. With appropriate design, the ratio may be varied from about 5:1 at low lifts with a relatively rapid change toward middle and high lift positions to a ratio of about 2:1. The result is advantageous effective control of gas flow through the inlet valves over the whole range of valve lifts. 
     Because of the requirement of periodic valve opening and valve spring compression of each cylinder, the control shaft in a multi-cylinder engine is required to operate against cyclically reversing torques applied against the control members or frames. If the actuator was required to change the mechanism position during all of the control shaft torque values, including peak values, the actuator would need to be relatively large and expensive and consume excessive power to obtain a reasonable response time. 
     To avoid this, FIG. 7 illustrates a worm gear actuator  74  applied for driving the control shaft  60  to its various angular positions. Actuator  74  includes a small electric drive motor  76  driving a worm  78  through a shaft that may be connected with a spiral return spring  80 . The worm  78  engages a worm gear  82  formed as a semi-circular quadrant. The worm gear is directly attached to an end, not shown, of a control shaft  60  for rotating the control shaft through its full angular motion. The pressure and lead angles of the teeth of the worm and the associated worm gear are selected as a function of the friction of the worm and the worm gear, so that back forces acting from the worm gear against the worm will lock the gears against motion until the back forces are reduced to a level that the drive motor  76  is able to overcome. 
     Thus, in operation, when a change in position of the mechanism control member is desired, drive motor  76  is operated to rotate the worm  78  and the associated worm gear  82  in the desired direction. A spiral torque biasing spring  84  is applied to the worm gear  82  (or the control shaft  74 ) to bias the drive forces so as to balance the positive and negative control shaft torque peaks so that the actuator is subjected to equal positive and negative torques. The biasing spring  84  will thus balance the system time response in both directions of actuation. 
     When the torque peaks are too high in the direction against the rotation of the motor, the worm drive will lock up, stalling the motor until the momentary torques are reduced and the motor again drives the mechanism in the desired direction with the assistance of torque reversals acting in the desired direction. The result is that a relatively low powered motor is able to provide the desired driving action of the control shaft and actuate the mechanisms with a relatively efficient expenditure of power. If used, the return spring  80  is installed so as to cause the actuation system to default to a low lift position during engine shutdown. 
     Referring now to FIG. 8, there is shown an engine  85  with an alternative embodiment of valve actuating mechanism  86  similar in many respects to mechanism  12  of FIGS. 1-6 and wherein like numerals indicate like parts. The embodiment of FIG. 8 differs from that of the first embodiment primarily in the provision of a central crankshaft bearing  88  and the use of dual eccentric cranks  18  connected with dual connecting rods  24  located laterally on either side of the crankshaft bearing  88 . The outer or distal ends of the connecting rods  24  each connect with the rocker lever  34  intermediate its ends as does the single connecting rod  24  of the first described embodiment. 
     In addition, the pictorial view of FIG. 8 differs from that of FIG. 1 in that the dual control members or frames  26  are positioned outward of the oscillating cams  44  in the mechanism  86 , whereas, in the embodiment of FIG. 1, the frames  26  are inside the cams  44 . 
     An advantage of the embodiment of FIG. 8 is that the crankshaft  16  is supported at the center of the mechanism  86  where the main loads are transmitted between the cranks  18  and the connecting rods  24  so that the structure is better able to support the varying loads at their source rather than on bearings spaced completely outward of the mechanism itself, as is the case in the embodiment of FIG.  1 . 
     In other ways, the construction and operation of the embodiment of FIG. 8 is like that of the embodiment of FIGS. 1 and 2 so that further description is believed unnecessary. 
     It should be apparent that the mechanisms illustrated, and many of their features, could take various forms as applied to other engine applications. For example, a single VVT mechanism could be applied to each finger follower or to direct acting followers of an engine, so that the valves could be actuated differently. Alternatively, dual actuators could be installed in a single bank of valves that could allow separate inlet valve control between two inlet valves of each cylinder. In another alternative, one actuator per bank of valves could be applied, but different profiles on the individual oscillating cams of each cylinder could allow one valve to have a smaller maximum lift than the other, so that the valve timing between the two valves could be changed as desired. Such an arrangement would enable low speed charge swirl while still maintaining a single computer controlled actuator. If desired, the mechanism of the invention could also be applied to the actuation of engine exhaust valves or other appropriate applications. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.