Abstract:
A desmodromic valve system which provides direct bidirectional displacement of a valve stem of an internal combustion engine without the aid of a rocker arm, utilizing a semirigid basket operating in conjunction with a plurality of cams for each valve. The basket is disposed about the camshaft of the engine and secured to the valve stem by an integral retainer on a bottom portion of the basket, and is constrained to motion along the valve stem axis. The basket has a pair of downwardly oriented cam followers in the upper portion thereof, spaced apart from the valve stem axis. A central cam and a parallel pair of side cams are fixedly mounted on the camshaft so as to rotate therewith, the cams substantially surrounded by the basket and cooperating therewith to provide reciprocating valve action with positive bidirectional drive. The central cam is aligned with the valve stem axis, and the side cams are spaced apart from the valve stem axis, parallel to the central cam and respectively aligned with the cam followers. During a first part of a valve cycle, the central cam pushes the valve stem down so as to positively open the associated valve, and the valve stem pulls said basket down with it via the retainer. During a second part of the valve cycle, the side cams push the basket up via their respective cam followers and thereby cause the basket to pull the valve stem so as to positively close the valve.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to desmodromic valve systems, and more particularly to desmodromic valve systems which provide direct bidirectional displacement of a valve stem without the aid of a rocker arm. 
         [0002]    A desmodromic valve system positively opens and closes a valve in an internal combustion engine. This is in contrast to the conventional system in which the valve is positively opened with a cam but closed with a return spring. 
         [0003]    The main benefit of a desmodromic system is the prevention of valve float. In traditional spring valve actuation, as engine speed increases, the inertia of the valve tends to overcome the spring&#39;s ability to close the valve completely before the piston reaches TDC (Top Dead Center). In severe cases, the piston contacts the open valve and causes damage to both engine parts. More generally, if a valve does not completely return to its seat before combustion begins, it can allow combustion gases to escape prematurely, leading to a reduction in cylinder pressure which causes a major decrease in engine performance. This can also overheat the valve, possibly warping it and leading to catastrophic failure. The traditional remedy for valve float is to use a stiffer return spring. This increases the seat pressure of the valve, i.e., the static pressure that holds the valve closed, and reduces valve float at higher engine speeds. However, the engine has to work harder to open the valve. The higher forces between spring and cam cause higher stress on the parts resulting in higher temperature and faster wear or failure in the valve drive system. A desmodromic system can avoid the problem to some extent because, although it has to work against the inertia of the valve opening and closing, it does not have to overcome the energy of the spring. 
         [0004]    Despite their advantages, desmodromic valve drive systems have had limited success in commercial application for various reasons such as design complexity, poor reliability, and valve train binding. Numerous approaches to the various problems have been taken since the earliest days of engine development, more than a hundred years ago, as evidenced by the following patents: 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Pat. No. 
                 Inventor(s) 
                 Issue Date 
               
               
                   
               
             
             
               
                 1,644,059 
                 Holle 
                 Oct. 24, 1927 
               
               
                 1,937,152 
                 Junk 
                 Nov. 28, 1933 
               
               
                 3,183,901 
                 Thuesen 
                 May 18, 1965 
               
               
                 3,430,614 
                 Meacham 
                 Mar. 4, 1969 
               
               
                 4,711,202 
                 Baker 
                 Dec. 8, 1987 
               
               
                 4,763,615 
                 Frost 
                 Aug. 16, 1988 
               
               
                 4,887,565 
                 Bothwell 
                 Dec. 19, 1989 
               
               
                 5,048,474 
                 Matayoshi et al. 
                 Sep. 17, 1991 
               
               
                 5,058,540 
                 Matsumoto 
                 Oct. 22, 1991 
               
               
                 6,276,324 
                 Adams et al. 
                 Aug. 21, 2001 
               
               
                 6,487,997 
                 Palumbo 
                 Dec, 3, 2002 
               
               
                 6,948,468 
                 Decuir 
                 Sep. 27, 2005 
               
               
                 6,951,148 
                 Battlogg 
                 Oct. 4, 2005 
               
               
                   
               
             
          
         
       
     
         [0005]    However, presently, all known desmodromic valve designs have drawbacks which make them undesirable for use in several significant applications, such as production automobiles, and there is no obvious path to a better solution. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a rockerless desmodromic valve system comprising a first cam rotating on a camshaft and cyclically pushing a valve stem, a second cam on the camshaft, and a band extending circumferentially around the second cam and engaging the valve stem, the second cam rotating within the band and causing it to reciprocate so as to cyclically lift the valve stem. The system preferably but not necessarily has a wide band in the form of a basket large enough to encompass multiple cams and to extend completely around them circumferentially. 
         [0007]    Another aspect of the invention is a desmodromic valve system comprising a semirigid band, which may be in basket form, disposed about a camshaft of an internal combustion engine, the semirigid band attached to a valve stem and constrained to motion along the valve stem axis. The system includes rotatable cam means mounted on the camshaft and disposed within the band for coacting with it without substantially changing its shape to positively drive the valve stem in both directions along its axis and thereby provide reciprocating valve action with positive bidirectional drive. 
         [0008]    The objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIGS. 1-5  depict a first embodiment of a rockerless desmodromic valve system according to the present invention. This embodiment includes a semirigid band or “basket” that substantially surrounds a set of cams on a camshaft and engages paired side cams and an associated valve stem so as to pull the valve stem after it is pushed by a central cam. The basket and cams, which are drawn to scale for a nominal ½ inch valve lift, cooperate to provide reciprocating valve action with positive bidirectional drive. 
           [0010]      FIG. 1  is a side view of the assembly with the valve closed, and with the basket and two side cam followers shown in longitudinal cross-section. 
           [0011]      FIG. 2  is a transverse cross-section along line  2 - 2  of  FIG. 1 . 
           [0012]      FIG. 3  is a transverse cross-section along line  3 - 3  of  FIG. 4 . 
           [0013]      FIG. 3A  is a transverse cross-section like that of  FIG. 3  but with the central cam partially cut away to show the shape of an alternative side cam. 
           [0014]      FIG. 4  is a longitudinal cross-section of the assembly with the valve open. 
           [0015]      FIG. 5  is a bottom view of the basket alone, taken along line  5 - 5  of  FIG. 1 . 
           [0016]      FIG. 6  shows reference points on the central cam and one side cam pertaining to the relationship between the cam radii in a preferred embodiment of the present invention. 
           [0017]      FIGS. 7-10  depict a second embodiment of a rockerless desmodromic valve system according to the present invention. Like the first embodiment, this embodiment includes a semirigid band or “basket” and a set of cams including a central cam and a pair of side cams. The basket and cams in this case are drawn to scale for a nominal ¼ inch valve lift. 
           [0018]      FIG. 7  is a side view of the assembly with the valve closed, and with the basket and two side cam followers shown in longitudinal cross-section. 
           [0019]      FIG. 8  is a transverse cross-section along line  8 - 8  of  FIG. 7 . 
           [0020]      FIG. 9  is a transverse cross-section along line  9 - 9  of  FIG. 10 . 
           [0021]      FIG. 10  is a longitudinal cross-section of the assembly with the valve open. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
         [0023]      FIGS. 1-5 , wherein like numerals represent like parts throughout the several views, depict a first embodiment  10  of a rockerless desmodromic valve system according to the present invention. In this embodiment, which is to be understood as one example of a desmodromic valve system according to the present invention, a central cam  12  and a parallel pair of side cams  14  are fixedly mounted on a camshaft  15  so as to rotate therewith, and are substantially surrounded by a semirigid band or “basket”  16  which does not rotate with the camshaft and is constrained by the cams and by its attachment to the stem  20  of a valve  22 . Except as described herein, the camshaft and valve may be conventional parts mounted in a conventional manner in the cylinder head of an internal combustion engine in which each valve has an associated port  24  and has a valve guide (not shown) which closely surrounds the valve stem. 
         [0024]    Basket  16  engages paired side cams  14  and the associated valve stem  20  so as to pull the valve stem after it is pushed by central cam  12 . The basket and cams cooperate to provide reciprocating valve action with positive bidirectional drive. That is, the system positively drives the valve from its closed position, illustrated in  FIGS. 1 and 2 , to its open position, illustrated in  FIGS. 3 and 4 , by conventional cam action by means of cam  12  in contact with the valve stem, and positively drives the valve back to its closed position using the basket, which is secured to the valve stem and raised by paired cams  14  acting through associated cam followers  18 . 
         [0025]    Each cam  14  has a main portion  14   a  with a concavo-convex cross-section, and a peripheral portion or shoulder  14   b  with a circular cross-section. The concavo-convex cross-section of the main portion of cam  14  is readily apparent in  FIGS. 2 and 3 . In  FIGS. 1 and 2 , the concave part  14   c  of main portion  14   a  of cam  14  is below the camshaft axis (and axially separated from the valve stem), and the convex portion engages the associated cam follower  18  and thereby holds the basket in its raised position. Cams  12  and  14  operate in coordinated fashion such that, at this point in the cycle, lobe  12   a  of cam  12  is oriented away from the valve stem and thereby allows the valve to be lifted and thus closed by the basket. Conversely, in  FIGS. 3 and 4 , part  14   c  of cam  14  is above the camshaft axis and lobe  12   a  of cam  12  is oriented toward the valve stem, whereby cam follower  18 —and thus basket  16 —is in its lowest position and the valve is open. A cam follower, such as a snug-fitting cup (not shown), is preferably also provided on the upper end of the valve stem for contact with cam  12 . 
         [0026]    As one example of a set of suitable dimensions for valve system  10 , cam  12  may have a maximum radius of 1 inch (at the outermost point on lobe  12   a ) and a minimum radius of ½ inch, thereby producing a valve lift—the valve displacement between open and closed positions—of ½ inch. Cam portion  14   a  has the same maximum and minimum radii as cam  12 , and its radius at any given point is a function of the radius of cam  12  at a diametrically opposed point. Specifically, cam  12  and cam portion  14   a  are designed such that, at any two diametrically opposed points X and Y on their respective surfaces (see  FIG. 6 ), 
         [0000]    
       
      
       r 
       x 
       +r 
       y 
       =c  
      
     
         [0027]    The cams are thus complementary. With the above example dimensions, the sum of the radius of cam  12  and the radius of cam portion  14   a  at such points X and Y is 1.5″. For example, the outermost point on lobe  12   a  is diametrically opposed to the center of concave part  14   c  of cam  14 , and the respective radii at those points are 1.0″ and 0.5″, the sum of which is 1.5″. 
         [0028]    Basket  16  has a retainer  26  integrally formed in a reinforced bottom portion thereof. The retainer cooperates with a plurality of keys or keepers  28  to secure the basket to the valve stem. The retainer has a downwardly tapered hole and the keepers are likewise downwardly tapered such that the retainer and associated keepers together form a valve stem lock. The keepers are shaped so as to extend into the groove of the valve stem and are held therein by wedging action of the cooperatively tapered portion of the retainer. The retainer may alternatively be formed as a separate part fitted into a hole in the basket. Examples of retainer/keeper sets are disclosed in U.S. Pat. Nos. 4,327,677 and 4,922,867, which are incorporated herein by reference. 
         [0029]    In an alternative embodiment suited for valves in which the groove is closer to the tip of the stem than in the first embodiment, the retainer is formed in the top of a hollow conical member extending up from the bottom of the basket enough to enclose the groove. The system may also provide an extension of the valve guide as additional lateral support for the stem in retrofit applications involving removal of a return spring. In cases with replaceable valve guides, a longer valve guide may be installed which extends into the space formerly occupied by the return spring. In other cases, e.g., heads with cast guides, the guide may be drilled and tapped to receive a threaded cylindrical extension, preferably with an oil seal and/or a roller guide on top. 
         [0030]    The basket also includes a reinforced upper portion or flange  16   a  adjacent each axial end for a cam follower  18 , the flange and cam follower having complementary shapes for retaining the cam follower as shown in  FIGS. 2 and 3 . A hole  17  is provided in the top of the basket for insertion of the cam followers. The cam follower may comprise a roller. 
         [0031]    The basket preferably has a unitary, or monocoque, construction, with solid side walls and open ends, and is semirigid, i.e., slightly flexible but sufficiently rigid that it experiences less than 1% elongation in response to forces applied to it during a cycle of operation of the valve to which it is connected—including in particular the forces applied in the process of returning the valve to its closed position—at camshaft speeds from zero to 5,000 RPM. For example, a basket with a nominal height of 2.5″ experiences elongation of less than 0.025″ as it pulls the valve stem to close the valve at camshaft speeds up to 5,000 RPM. Basket elongation is the primary contributor to the dynamic lash of the valve, which is understood to be the variable lash occurring in operation, i.e., the clearance between the valve stem and cam  12  during operation. Basket elongation of up to 0.100″ may be suitable with certain engine designs, but the basket is preferably sufficiently rigid that it limits the dynamic lash to 0.020-0.030″, more preferably less than 0.010″ and, most preferably, 0.005″ or less. One suitable material is thin-wall cast titanium. There is preferably a gap between the bottom portion of the basket and shoulder  14   b  when the valve is closed ( FIG. 2 ) and, likewise, a gap between the top portion of the basket and shoulder  14   b  when the valve is open ( FIGS. 3 and 4 ). The basket is preferably dimensioned to provide a gap of at least 0.001″ at substantially all points between it and cam  14  at rest. 
         [0032]    The assembly process begins by mounting the baskets on the camshaft before the camshaft is installed in the head. The baskets are moved axially over the cams on the camshaft to their respective cams  12  and  14 . When all the baskets are so mounted, the camshaft is placed in the bearing blocks in the head and secured. Each valve is then installed by sliding its stem through a valve guide and through the hole in the bottom of an associated basket. With the cams oriented as shown in  FIG. 2 , the stem is advanced and the basket is lowered as necessary for the groove in the stem to pass beyond the retainer in the basket, and the keepers are then inserted through one or both open ends of the basket and placed in the groove, after which the retainer is moved into place surrounding the keepers, thereby holding them in the groove. Each cam follower  18  is then inserted through the hole  17  in the top of the basket and slid into a flange  16   a , where it is preferably secured in place with a fastener, e.g., screw  19 , extending into the flange through the top of the basket. Screws  19  are preferably aircraft bolts with anti-rotation features, e.g., drilled heads having a common safety wire through them. The top of the cam follower may be tapered in the direction away from hole  17  to provide a wedge shape to facilitate insertion into the flange. Insertion of cam follower  18  pre-loads the valve stem and tensions, i.e., slightly stretches or elongates, the basket. Cam follower  18  is suitably dimensioned to perform this function. 
         [0033]    In operation, starting from the valve-closed position shown in  FIGS. 1 and 2 , the camshaft rotates nearly 135° to a point at which cam lobe  12   a  begins to engage the valve stem and cam follower  18  simultaneously begins to engage a smaller-radius portion of cam portion  14   a . Cam lobe  12   a  then exerts a downward force on the valve stem until the camshaft has rotated 180°, to the valve-open position shown in  FIG. 3 . The valve stem is free to move down because cam follower  18  engages the smaller-radius portion of cam portion  14   a , including concave part  14   c , during this part of the cycle, and the valve stem pulls the basket down with it as shown in  FIG. 3 . Further camshaft rotation causes cam lobe  12   a  to rotate away from the valve stem and correspondingly brings cam follower  18  into contact with points of progressively larger radius on cam portion  14   a . Cam portion  14   a  thereupon exerts an upward force on cam follower  18  which lifts the basket, which in turn pulls the valve stem up. When the camshaft has rotated a little more than 45° from the position shown in  FIG. 3 , cam follower  18  again bears against the maximum-radius portion of cam portion  14   a  and the valve is closed. The valve and basket positions at this point are as shown in  FIGS. 1 and 2  and remain so for the remainder of the cycle. It will be understood that the above-mentioned angles of 135° and 45° are mere examples and that the angles at which cam lobe  12   a  engages and disengages from the valve stem are functions of desired cam action for a desired valve application. 
         [0034]    The circular peripheral portions  14   b  of cam  14  are provided to resist flexing of the basket and thereby limit its maximum elongation as the valve closes, at which time the concave part  14   c  of cam  14  moves toward one side of the basket and opens up a significant gap. By virtue of their fixed  1 ″ radius, portions  14   b  maintain a minimum of 2″ spacing between the opposed sides of the basket at least where they make contact with it. Portions  14   b  (shoulders) may be on either or both sides of each side cam  14  on the camshaft axis, i.e., the side closer to the central cam, the opposite side, or both. The side closer to the central cam is closer to the line of force (tension) between stem  22  and cam follower  18  during valve closure. Alternatively, a constant-radius disc such as portion  14   b  may be provided on either or both sides of central cam  12 , and such a disc may help with camshaft balancing. 
         [0035]    In an alternative embodiment, the desmodromic valve system has a parallel pair of rings or bands instead of the basket described above. The bands are preferably joined at the bottom by a bridge which includes a retainer such as described above, in a unitary construction or as separate parts. A single band with a single cam  14  is also contemplated. 
         [0036]    The basket with cam follower(s)  18  is effectively a clamp. In cooperation with cam(s)  14 , it clamps the central cam (cam  12 ) to the valve stem, whereby the valve stem is virtually an ideal cam follower throughout the valve cycle. It is strongly preferred to have the clamp extend completely around the central cam circumferentially as shown in the drawings and described above. However, in some applications, it may be adequate for the clamp to extend around the cam on only one side of the camshaft, i.e., the left or right side as viewed in  FIG. 2 , akin to a C-clamp, with curved or straight vertical and horizontal segments. The clamp may comprise one half of the basket described above, i.e., the left or right half as viewed in  FIG. 2 , but including the full retainer and keys and the cam followers as described above. The cam followers may be fixed in position in supporting flanges as described above, or may be vertically adjustable by means of a threaded connection to the top of the clamp or otherwise. Alternatively, the cam followers may be integral parts of the clamp. Such a clamp is provided, if necessary, with suitable means to keep it aligned with the valve stem. For example, a horizontal support bar or guard rail may be provided on the head so as to abut the back side of the half basket (the side opposite the cam) at the level of the camshaft axis. The support bar may, for example, be bolted or otherwise secured to adjacent bearing blocks. 
         [0037]    As an alternative to the half basket just described, a clamp in the form of a half ring akin to a C-clamp may be adequate in some applications. This clamp may have approximately the same width along the camshaft axis as cam  14 , and be aligned with that cam, but have an axial projection rigidly connecting it to the valve stem. It may have the same general cross-sectional shape as the left or right half of the basket as viewed in  FIG. 2 . If necessary, a horizontal support bar or guard rail, as described above, is provided which includes a vertical guide, such as a slot to receive the back of the clamp, to keep the clamp vertically aligned. 
         [0038]    Cams  12  and  14  have complementary shapes as described above, and they are preferably complementary around their entire circumferences, but may be partially complementary in certain applications. It is particularly advantageous for cam  14  to complement cam  12  for the valve-closing portion of the valve cycle, so as to generate a lifting force via the basket or other clamp as soon as the maximum-radius portion of cam lobe  12   a  is past the valve stem. However, an upward force is not necessarily required from the basket during every part of the valve cycle, e.g., during the compression stroke and power stroke of a four-stroke engine, and so, in some applications, the side cam may have a relatively small radius for a significant part of its circumference corresponding to such parts of the cycle (and thus have less rotating mass), provided that the basket is suitably secured to the valve stem and kept aligned with it. The basket may be secured by means of a cap screwed over the keys to keep them in place, or, for some applications, a threaded connection without keys may be adequate. A horizontal support bar or guard rail as described above may be provided on each side of the basket for alignment purposes if necessary. 
         [0039]    One example of such a side cam is cam  14   a ′ in  FIG. 3A . Cam  14   a ′ is designed for clockwise rotation. It extends approximately 120° around the camshaft as illustrated, and it has the same radius as cam  14   a  of the first embodiment for approximately 90°, in the circumferential range from point A to point B, which includes the valve-closing portion of the valve cycle. Those skilled in the art will appreciate that cam  14   a ′ and cam  12  are complementary for that part of the valve cycle. This embodiment preferably includes a circular portion  14   b  joined to cam  14   a ′ and having a constant  1 ″ radius as in the first embodiment. A counterweight  30  is optionally provided on the opposite side of the camshaft from cam  14   a ′ for balancing purposes, and may be mounted on portion  14   b  as shown. Camshaft balance can also be achieved by removing weight, e.g., by machining away areas of portion  14   b  adjacent to cam  14   a ′, and/or by initially forming such adjacent areas and cam  14   a ′ itself with apertures therein, such as in a spoked wheel. Camshaft balance can be achieved by adding or deleting material or a combination of the two. 
         [0040]    Depending on the rigidity of the basket, portion  14   b  may be made with a greater axial width (along the camshaft axis) than portion  14   b  in the first embodiment, for purposes of structural integrity. Alternatively, a cam  14  may have a part  14   a ′ (as in  FIG. 3A ) with the axial width of original part  14   a  (see  FIG. 1 ), and also include the remainder of original part  14   a  but with half its width, whereby some part of cam  14  engages cam follower  18  throughout the cycle, thus maintaining the pre-load on the valve stem and reinforcing portion  14   b.    
         [0041]    Another embodiment  110  of the invention is depicted in  FIGS. 7-10 , wherein like numerals represent like parts throughout the several views. This embodiment and variations thereof may be the same as the embodiment of  FIGS. 1-5  and its variations as discussed above, with exceptions as discussed below. A central cam  112  and a parallel pair of side cams  114  are fixedly mounted on a camshaft  115  so as to rotate therewith, and are substantially surrounded by a semirigid band or “basket”  116  which does not rotate with the camshaft and is constrained by the cams and by its attachment to the stem of a valve  122 . 
         [0042]    The primary difference with this embodiment is that the basket and cams are designed for a ¼ inch valve lift. Basket  116  engages paired side cams  114  and the associated valve stem so as to pull the valve stem after it is pushed by central cam  112 . The basket and cams cooperate to provide reciprocating valve action with positive bidirectional drive. That is, the system positively drives the valve from its closed position, illustrated in  FIGS. 7 and 8 , to its open position, illustrated in  FIGS. 9 and 10 , by conventional cam action by means of cam  112  in contact with the valve stem, and positively drives the valve back to its closed position using the basket, which is secured to the valve stem and raised by paired cams  114  acting through associated cam followers  118 . 
         [0043]    Each cam  114  has a main portion with a concavo-convex cross-section, and a peripheral portion or shoulder with a circular cross-section. In  FIGS. 7 and 8 , the concave part of cam  114  is below the camshaft axis (and axially separated from the valve stem), and the convex portion engages the associated cam follower  118  and thereby holds the basket in its raised position. Cams  112  and  114  operate in coordinated fashion such that, at this point in the cycle, the lobe of cam  112  is oriented away from the valve stem and thereby allows the valve to be lifted and thus closed by the basket. Conversely, in  FIGS. 9 and 10 , the concave part of cam  114  is above the camshaft axis and the lobe of cam  112  is oriented toward the valve stem, whereby cam follower  118 —and thus basket  116 —is in its lowest position and the valve is open. 
         [0044]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.