Patent Application
20120160199
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.
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.
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'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.
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:
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.
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.
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.
The objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings.
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.
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
Each cam 14 has a main portion 14a with a concavo-convex cross-section, and a peripheral portion or shoulder 14b with a circular cross-section. The concavo-convex cross-section of the main portion of cam 14 is readily apparent in
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 12a) 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 14a 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 14a are designed such that, at any two diametrically opposed points X and Y on their respective surfaces (see
r
x
+r
y
=c
The cams are thus complementary. With the above example dimensions, the sum of the radius of cam 12 and the radius of cam portion 14a at such points X and Y is 1.5″. For example, the outermost point on lobe 12a is diametrically opposed to the center of concave part 14c of cam 14, and the respective radii at those points are 1.0″ and 0.5″, the sum of which is 1.5″.
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.
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.
The basket also includes a reinforced upper portion or flange 16a 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
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 14b when the valve is closed (
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
In operation, starting from the valve-closed position shown in
The circular peripheral portions 14b 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 14c of cam 14 moves toward one side of the basket and opens up a significant gap. By virtue of their fixed 1″ radius, portions 14b maintain a minimum of 2″ spacing between the opposed sides of the basket at least where they make contact with it. Portions 14b (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 14b may be provided on either or both sides of central cam 12, and such a disc may help with camshaft balancing.
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.
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
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
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 12a 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.
One example of such a side cam is cam 14a′ in
Depending on the rigidity of the basket, portion 14b may be made with a greater axial width (along the camshaft axis) than portion 14b in the first embodiment, for purposes of structural integrity. Alternatively, a cam 14 may have a part 14a′ (as in
Another embodiment 110 of the invention is depicted in
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
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
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.
