Abstract:
A system for varying valve timing; i.e. the rotational angle of the crankshaft during which an intake or an exhaust valve of a cylinder of a reciprocating internal combustion engine is open which results in varying valve overlap, and for varying valve lift of such intake and exhaust valves. A desmodromic cam and cam follower convert rotation of a cam shaft to reciprocating rotation, or oscillation, of the cam follower. The reciprocating rotation of the cam follower is converted by the interaction of a secondary cylindrical cam, a cylindrical control ring, and a reciprocating member to linear motion of the reciprocating member which reciprocating member is operatively connected to a poppet valve. Timing and lift of the reciprocating member and valve are variable over predetermined limits as a function of engine rpm and load by rotation of the cylindrical control ring. Each valve train positively closes as well as opens its associated valve.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of valve train or valve operating mechanisms for internal combustion engines and more particularly variable valve timing and valve lift mechanisms. 
     2. Description of the Prior Art 
     The desirability of varying valve timing and valve overlap; i.e., the angle of rotation of the crankshaft of a reciprocating internal combustion engine during which both exhaust and intake valves of a cylinder are open, as a function of speed and load to optimize the efficiency of a reciprocating internal combustion engine under any given operating condition has been recognized. The prior art solutions teach the use of a valve train including a single lobed cam on a rotating cam shaft with a tappet engaging the lobed cam to convert circular motion to reciprocating linear motion of a tappet which is operatively connected directly to a valve as in an overhead cam shaft type engine or through a rocker arm. A heavy duty spring biases the valves closed and the tappet against the cam surface. Changes in timing are accomplished by changing the length of an element in the valve train, the location of the cam shaft lobe to the tappet, or by varying the axis of rotation of rocker arms. 
     SUMMARY OF THE INVENTION 
     The present invention provides a valve train that does not use a lobed cam surface on a rotating cam shaft but rather a cam surface in the form of a groove of constant radius in the shaft which causes a cam follower engaging said groove to convert rotation of the cam shaft into reciprocal rotation, or oscillation, of the cam follower. The reciprocal rotation of the cam follower is converted by a secondary cam reciprocally rotated by the cam follower and a control ring into reciprocal linear motion of a reciprocating member which is operationally connected directly to a poppet valve or through a rocker arm. Movement of the control ring changes the valve timing as measured by crankshaft position and valve lift so that optimum valve timing and valve lift for substantially all operating conditions of a reciprocating internal combustion engine can be achieved. 
     It is, therefore, an object of this invention to provide a valve train which provides optimum valve timing and valve lift over all engine speeds and loads for which a reciprocating internal combustion engine is designed which will reduce engine emissions below that experienced with conventional valve trains. 
     It is another object of this invention to provide a valve train that requires significantly less power to operate a valve. 
     It is still another object of the invention to provide a valve train which is relatively quiet. 
     It is a further object of this invention to provide a valve train in which the mass of the valve train in reciprocating linear motion is minimized. 
     It is another object of the invention to provide a valve train which can be manufactured economically. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which: 
     FIG. 1 is a fragmentary view in section of a conventional internal combustion engine of the overhead cam type showing the valve trains or valve operating mechanisms for one cylinder of an engine; 
     FIG. 2 is a fragmentary side view of a portion of the engine illustrated in FIG. 1; 
     FIG. 3 is a fragmentary plan view taken on line 3--3 of FIG. 1; 
     FIG. 4 is a plan view of the cam shaft illustrated in FIG. 1; 
     FIG. 5 is an enlarged fragmentary view disclosing details of the valve train of my invention; 
     FIG. 6 is a section taken on line 6--6 of FIG. 5; 
     FIG. 7 is an exploded view of the components of one embodiment of a valve train assembly; 
     FIG. 8 is a graph of valve lift vs. crankshaft angle produced by an embodiment of a valve train of this invention in a reciprocating internal combustion engine; and 
     FIG. 9 is a fragmentary view partially in section of a modification of the valve train of my invention in which a valve is connected to the valve train through a rocker arm. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, reciprocating internal combustion engine 10 has a cylinder block 12, a cylinder head 14 which defines a combustion chamber 16. Piston 18 is mounted for reciprocating movement in cylinder 20 formed in block 12. A connecting rod 22 is pivotally secured to piston 18 by means of a conventional wrist pin 24. The lower end of connecting rod 22 is connected to a conventional crankshaft, which is not illustrated. Cam shaft 26 is mounted on engine 10 for rotation about its axis of rotation 28 by conventional journal bearings, which are not illustrated, and is driven by the crankshaft, for example, by means of a timing belt and pulley which are also not illustrated since they are conventional and well known to those skilled in this art. Cam shaft 28 normally completes one revolution for every two revolutions of the crankshaft of engine 10. 
     Cam shaft 26 is provided with a plurality of grooves 30, or cam surfaces, one for each valve of engine 10 driven by cam shaft 26, for example. It should be noted that cam surface 30 is a circular cam of constant radius with respect to the axis of rotation 28 of cam shaft 26 compared with lobed cams conventionally used in the valve trains of reciprocating internal combustion engines in which the radius to various points on the surface of a lobe varies as a function of the angle. 
     Cam follower 32 has a central cylindrical portion 34 and a disk portion 36 rigidly attached to cylindrical portion 34. A plurality of notches 38 are formed in the perimeter of disk 36. Cam follower 32 has an axis of rotation 40. Pin 42 is mounted on the upper end of cylinder 34 remote from disk 36. As can be seen in FIG. 7, the longitudinal axis 44 of cylindrical pin 42 is substantially parallel to but offset from axis 40 of cam follower 32 by a fixed amount. Follower bushing 46 fits on pin 42 and its outer surfaces are designed to engage cam surfaces 30 formed on cam shaft 26. 
     As can be seen in FIGS. 1, 2 and 5, cam follower 32 is journaled in a bore 48 in support member 49 which is illustrated in FIG. 2 as being bolted to cylinder head 14. Cam follower 32 while mounted in bore 48 is free to rotate reciprocally, or oscillate, about its axis of rotation 40 and is mounted so that follower bushing 46 engages groove 30 in cam shaft 26. 
     Secondary cam 50 is comprised of hollow cylindrical member 52 in the cylindrical walls of which are formed a pair of cam slots 54. A plurality of lugs 56 are located on the upper end of member 52 and are shaped and located to fit into notches 38 of cam follower 32. The shape and dimension of secondary cam slots 54 are chosen to provide a range of valve timing and valve lift for optimum engine operation over the entire range of operating conditions of the engine in which the valve trains are installed. 
     Control ring 58 consists of a hollow cylindrical member 60 with a cylindrical disk 62 located near the bottom of member 60. The dimension of cylinder member 60 is such that it has a sliding low friction, fit within cylindrical member 52. The axis 64 of cylindrical member 52 and the axis 66 of cylindrical member 60 substantially coincide when the cylinder 60 is located within cylinder member 52. A pair of slots 68 are formed in the cylindrical walls of member 60 substantially parallel to axis 66. A drive pin 70 is located near the outer perimeter of disk 62. Pin 70 is operatively connected by shaft 69 to a conventional engine rpm or vacuum control, or servo, 72 so that the position of control ring 58 with respect to cylinder head 14, or any similar element of engine 10 that is fixed, can be varied as a function of engine speed, or rpm, and load, intake manifold pressure, as will be described below. 
     Reciprocating member, or valve keeper, 74 has a portion 76 which has a sliding fit within control ring 58. Transverse bore 78 is formed in member 74 into which cross pin 80 can be inserted. Projecting means, or cross pin 80, when the valve train is assembled, as is illustrated in FIG. 5, for example, passes through slots 68 in control ring 58 and engage cam slots 54 of secondary cam 50. 
     Poppet valve 82, which is provided with a groove 84 located toward the upper end of its stem 86 is mechanically, or fixedly, secured to reciprocating member 74 by a pair of keys 88 which are held in place in vertical bore 89 by pin 80 when pin 80 is mounted in bores 78 in reciprocating member 74. 
     Cam follower 32, secondary cam 50 and control ring 58 are held together by the force of coil spring 90 which is illustrated in FIG. 5, for example, as being compressed between the underside of disk 62 of control ring 58 and cylinder head 14. 
     In FIG. 4 the shape of the grooves 30 in cam shaft 26 are best illustrated. Each groove 30 has a substantially straight section 92 and a curved portion 94 in which the groove 30 deviates from a circular path in a plane normal to the axis of rotation 28 of cam shaft 26. As shaft 26 rotates in the direction of arrow 96 in FIG. 4 the follower bushing 46 mounted on cam follower 32 engages the surfaces of groove 30 as seen in FIG. 1. Rotation of shaft 26 one complete revolution will cause cam followers 32 to oscillate through an angle alpha as illustrated in FIG. 3. The excursion of cam surfaces 30 from a true circle, the distance between the axis of rotation 40 of follower 32 and axis 44 of pin 42 on which bushing 46 is mounted and the position of the center of shaft 26 with respect to axis 40 of cam follower 32 as seen in FIG. 2 determine the magnitude of alpha. In a preferred embodiment alpha is 90°. Since cam follower 32 is forced to oscillate about its axis by cam surfaces 30, this type of cam and cam follower is called a desmodromic cam. 
     Reciprocal rotation of follower 32 about its axis of rotation 40 causes a similar rotation, or oscillation, of secondary cam 50 about its axis 64. The engagement of lugs 56 of cam 50 into notches 38 in cam follower 32 provides the mechanical coupling of the two devices which when assembled results in axis 64 of cylinder 52 substantially coinciding with axis 40. The hollow cylindrical member 60 of control ring 58 fits inside hollow cylindrical member 52 of secondary cam 50 with axis 66 of cylindrical member 60 substantially coinciding with axis 64 of secondary cam 50 and the axis of rotation 40 of cam follower 32. The bottom surface of secondary cam 50 engages the top surface of disk 62 of control ring 58. Reciprocating member 74 is located within cylindrical member 60 with projecting means 80 passing through slots 68 of control ring 50 and engaging cam slots 54. Oscillation of secondary cam 50 causes the reciprocating member 74 to move in a reciprocating, or oscillating, linear path along axis 64, for example, since rotation of reciprocating member 74 is prevented by vertical guide slots 68 in control ring 58 which is held in place by shaft 69 which is operatively connected to servo 72. 
     Coil spring 90 biases control ring 58 toward cam follower 32. When engine 10 is cold, a gap of a few thousandths of an inch exist between the top surfaces of control ring 58, secondary cam 50, and the bottom surface of disk 36 of cam follower 32 to provide for expansion of valve 82 mounted in reciprocating member 74 as well as the other components of a valve train such as are illustrated in FIG. 7, for example. 
     The shape of cam slots 54 in secondary cam 50 is chosen so that the timing of the valve 82 of a given cylinder of a reciprocating internal combustion engine will fall within the curves illustrated in FIG. 8. In a preferred embodiment the slots 54 extend through or determine an angle beta of 120° with the portion of the slot extending over an angle gamma, in a preferred case gamma equals 30°, being substantially horizontal as seen in FIG. 7, for example, and with a smaller flat, or horizontal, portion at the other end. The curved portion is chosen to provide optimum valve timing and valve lift as is well known to those skilled in the art. 
     Servo mechanism 72 and the linkage between it and control ring 58 is designed, in a preferred embodiment, to rotate control ring 58 through an angle delta, in a preferred embodiment delta is 30°, as a function of the rpm of engine 10 .[.and its.]. .Iadd.which may vary with .Iaddend.load. Valve timing and valve lift of the valves of a given cylinder vary for exhaust valves from those for minimum engine speed, curve 98, to those for maximum engine speed, curve 100, and for intake valves from those for minimum speed, curve 102, to those for maximum speed, curve 104. The extent of valve overlap at maximum speed in the embodiment illustrated is approximately 168°. .Iadd.Thus, timing will change depending on the initial position of projecting means 80 along the portion of the slot 54 extending over the angle gamma. The amplitude or lift will vary if the final position of the projecting means varies along the diagonal portion of slot 54. If this final position is along the lower horizontal position of slot 54 with each initial position of projecting means 80, there will be no amplitude variation..Iaddend. 
     In FIG. 9 there is illustrated a modification of my invention in which poppet valve 82 is operatively connected to reciprocating member 74 by means of rocker arm 106 which is pivotally mounted on cylinder head 14. Valve 82 is held between the jaws 108, 110 of rocker arm 106 with jaw 110 which is pivotally mounted on rocker arm 106 forced toward jaw 108 through the action of torsion spring 112. The significant changes between the embodiment illustrated in FIG. 1 and that of FIG. 9 is that the cylindrical portion 34 of cam follower 32 is extended and what would be the bottom surface of control ring in FIG. 5 is provided with a cover 114 against which the closing force of valve 82 and rocker arm 106 react. 
     From the foregoing it is clear that the desmodromic cam drive of my invention positively closes as well as opens poppet valves. Thus, it is not necessary for the desmodromic valve train of my invention to overcome the resistance of coil springs conventionally used to bias valves closed and to keep tappets in contact with a lobed cam. This results in a considerable reduction in the amount of energy consumed by the engines and thus increases engine efficiency. 
     The valve train of my invention also significantly reduces the mass of material undergoing linear oscillation since rotational motion of the cam follower is translated to linear motion in close proximity to the valve to be driven which reduces the forces acting on the components of a drive train. 
     The secondary cam and the control ring as a result of their design can be fabricated by stamping which minimizes their cost of production. Further the valve operating mechanism varies valve timing and valve lift of a valve in a reciprocating internal combustion engine as a function of engine rpm or speed and load which greatly enhances engine efficiency. 
     It should be obvious that various modifications can be made to the embodiments of my invention as disclosed herein without departing from the scope of the present invention.