Abstract:
A compression release mechanism for use in a single or multi-cylinder engine to make the engine easier to hand start. The assembly includes a compression release shaft disposed substantially within the camshaft. The compression release shaft is formed in at least two segments and can therefore be formed accurately, repeatedly and cost effectively using powder metal technology. Consequently, the weight of the flyweight member that is attached to the compression release shaft can be accurately controlled, thereby allowing the compression release mechanism to disengage at a precisely known rotational velocity of the camshaft. The compression release shaft may engage one or more valve actuation devices, which in turn force exhaust valves open during starting engine speeds. The compression release mechanism is conveniently contained within the housing by a housing wall bearing against the flyweight member and a cam bearing against an end of the compression release shaft. These bearing surfaces also hold the compression release shaft segments together.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to compression release mechanisms for internal combustion engines. 
     It is often desirable to relieve the pressure in an engine combustion chamber during starting so that it is easier for the piston to reciprocate in the engine and thus easier for the operator to manually pull the starter rope. Known compression release mechanisms lessen the pull force required to start the engine, and minimize operator fatigue during starting. 
     One typical compression release mechanism is disclosed in U.S. Pat. No. 3,381,676 issued May 7, 1968 to Campen. The Campen compression release mechanism includes a centrifugally-responsive flyweight, a torsional spring attached to the flyweight, and a central pin which engages a valve tappet at engine starting speeds. At higher engine speeds, the flyweight moves radially outwardly so that the pin disengages the valve tappet when the engine is running. 
     It is known to use a compression release mechanism for multi-cylinder engines. For example, U.S. Pat. No. 5,809,958 issued Sep. 22, 1998 to Gracyalny discloses a centrifugally-responsive flyweight to which is connected a compression release shaft disposed externally of the camshaft. The compression release shaft is connected at one end to the flyweight and extends through respective bores in two cams lobes. The release shaft includes two D-shaped cross-sectional portions which engage two respective lift members. One disadvantage of such an arrangement is that the bores for the release shaft must be drilled subsequently to heat treating the cams. Consequently, the drilling operation is more difficult, time consuming and expensive because the heat treated cams are much harder. Another disadvantage of such an arrangement is that the drilling operation is more difficult in that two separate bores must be drilled. This introduces the possibility of mislocating the bores with respect to one another. Another disadvantage of such an arrangement is that the release shaft is supported by a minimum bearing surface, viz., the two bores in the cams. Consequently, the material from which the release shaft is made must be sufficiently strong. 
     Japanese No. 2-67409(A) to Yoshiharu Isaka also discloses a compression release mechanism for use with multiple cylinders. A flyweight is disposed on the internal side of the cam gear and has a compression release shaft connected thereto. The compression release shaft is disposed internally of the camshaft and includes two D-shaped cross sectional portions therealong, each of which engages a separate lift member, which in turn engage separate valve tappets. 
     It is desirable to further reduce the cost and at the same time, simplify the assembly of a compression release mechanism. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low cost, easy to assembly mechanical compression release for a single or multi-cylinder engine. Specifically, the compression release assembly of the present invention comprises a compression release shaft having at least two segments disposed substantially within a bore in the camshaft. Such an arrangement is easier to assemble and allows production from lower cost parts. 
     In one form thereof, the present invention provides a compression release mechanism for relieving compression during engine starting in an internal combustion engine having a camshaft rotatably disposed within a housing. The mechanism comprises a compression release shaft disposed substantially within the camshaft and comprising first and second compression release shaft segments. A flyweight member is connected to the compression release shaft. A lift member is reciprocably disposed in the camshaft. The lift member engages the compression release shaft so that the lift member extends outwardly from the camshaft and is adapted to engage a valve actuation device. 
     In a preferred form, the inventive compression release mechanism includes the first and second compression release shaft segments being axially non-interlocking and rotationally interlocking. In other words, rotation of one of the segments necessarily produces rotation of the other segment therewith. However, the connection between the two separate segments are not held together axially where they interface within the bore in the camshaft. Instead, one end of the release shaft is engaged by a side surface of a cam whereas the housing engages the flyweight member which is connected to the other shaft segment. It is thus the bearing surfaces of the housing and the cam that hold the two segments together within the bore. 
     In another preferred embodiment, the first compression release shaft segment is integrally formed with the flyweight member, both of which are manufactured using powder metal technology. 
     One advantage of the present invention is that the bore in the camshaft which contains the compression release shaft can be drilled in a simple one step drilling operation without interruption. By contrast, certain prior art devices require drilling through a first cam lobe and then a second cam lobe. This multiple step prior art drilling operation results in burrs on the outside of the cam surface that have to be smoothed and also introduces the possibility that the drill point becomes mislocated after it exits the first cam lobe and enters the second cam lobe. 
     Another advantage of the present invention is that the bore for the compression release shaft is disposed sufficiently within the surface of the camshaft so that the cams can be heat treated after drilling the compression release shaft bore in the camshaft. Advantageously, the camshaft metal is softer and therefore easier to drill prior to the heat treating. 
     Another advantage of the present invention is that the compression release shaft and/or the flyweight member can be formed using powder metal technology. By making the flyweight member from a metal powder, its weight can be adjusted by infiltrating copper or other dense metal into the pressed powder, which in turn allows the speed at which the compression release mechanism disengages to be finely tuned. Furthermore, expensive stamping and machining is avoided. Further still, the process of forming the parts from powder metal is reliable and consistently repeatable. 
     Still another advantage of the present invention is that no fasteners are needed to hold the two segments of the compression release shaft together. Yet, because the compression release shaft is disposed within the camshaft, a large bearing surface is provided therefor so that the two segments rotationally interlock one another without being fastened together. Such an arrangement would not be possible with the compression release shaft disposed externally of the camshaft as in prior art configurations. 
     Yet another advantage of the present invention is that the compression release shaft formed of separate segments is easier to install as part of the engine assembly process. 
     Yet another advantage of the present invention is that a two-piece compression release shaft can be made more cost effectively. Further advantageously, one of the compression release shaft segments can be formed integral with the flyweight member using powder metal technology. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an exploded perspective view of the compression release assembly of an embodiment in accordance with the present invention; 
     FIG. 1A is an exploded perspective view of an embodiment of the present invention showing the two-piece compression release shaft and yoke; 
     FIG. 1B is a perspective view of an embodiment in accordance with the present invention depicting the compression release shaft, yoke and lift members; 
     FIG. 2 is a perspective view of the compression release assembly of an embodiment of the present invention shown at engine operating speeds wherein the lift members are disengaged; 
     FIG. 3 is a perspective view of the compression release assembly of an embodiment in accordance with the present invention depicting slow speed start-up conditions of an engine wherein the lift members are extended; 
     FIG. 4 is a side elevational view of the assembly shown in FIG. 3; 
     FIG. 5 is a cross sectional view taken along lines  5 — 5  of FIG. 4; 
     FIG. 6 is a cross sectional view taken along lines  6 — 6  of FIG. 4; 
     FIG. 7 is a side elevational view of a lift member in accordance with the illustrated embodiment; 
     FIG. 8 is a plan view of a sub-part of the compression release shaft; 
     FIG. 9 is a cross sectional view taken along line  9 — 9  of FIG. 8; 
     FIG. 10 is a cross sectional view taken along line  10 — 10  of FIG. 8; 
     FIG. 11 is a cross sectional view taken along line  11 — 11  of FIG. 8; 
     FIG. 12 is an exploded perspective view of the compression release assembly of a second embodiment in accordance with the present invention; 
     FIG. 12A is an exploded perspective view of the second embodiment of the present invention showing the two-piece compression release shaft and yoke; 
     FIG. 12B is a perspective view of the second embodiment in accordance with the present invention depicting the compression release shaft, yoke and lift members; 
     FIG. 13 is a perspective view of the compression release assembly of the second embodiment of the present invention shown at engine operating speeds wherein the lift members are disengaged; 
     FIG. 14 is a perspective view of the compression release assembly of the second embodiment in accordance with the present invention depicting slow speed start-up conditions of an engine wherein the lift members are extended; 
     FIG. 15 is a side elevational view of the assembly shown in FIG. 14; 
     FIG. 16 is a cross sectional view taken along lines  16 — 16  of FIG. 15; 
     FIG. 17 is a cross sectional view taken along lines  17 — 17  of FIG. 15; 
     FIG. 18 is a side elevational view of a lift member in accordance with the second embodiment; 
     FIG. 19 is a plan view of a sub-part of the compression release shaft; 
     FIG. 20 is a cross sectional view taken along line  20 — 20  of FIG. 19; 
     FIG. 21 is a cross sectional view taken along line  21 — 21  of FIG. 19; and 
     FIG. 22 is a cross sectional view taken along line  22 — 22  of FIG.  19 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one exemplary embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, compression release assembly  20  includes camshaft  22  having cams  24  thereon as is known in the art. Cam gear  26  which engages a gear of the crankshaft (not shown) is attached to camshaft  22 . Valve tappets  28  are shown in phantom and are vertically displaced by cam lobes  30  as camshaft  22  rotates at normal operating speeds. 
     With further reference to FIG. 1, the compression release includes compression release shaft  32  which is further comprised of two segments disposed end to end, first segment  34  and second segment  36 . A centrifugally responsive flyweight member  38  is connected to compression release shaft  32 . First segment  34  and flyweight member  38  are integrally formed from a powder metal using powder metal technology that is known in the art. Advantageously, powder metal technology allows fine adjustments in the weight of flyweight member  38 , which in turn allows fine adjustments in the speed at which the compression release mechanism of the present invention disengages. The weight adjustments are accomplished by varying the amounts of copper in the powder mix before flyweight member  38  and first segment  34  are integrally formed. 
     Lift members  40 , in the shape of plungers, are reciprocably disposed in holes  42  in camshaft  22 . Torsional spring  44  attaches to cam gear  26  and biases flyweight member  38  to the position shown in FIG.  3 . Support collar  46  supports flyweight member  38  in its most inward position as shown in FIG.  3 . 
     With reference to FIGS. 1A and 1B, the structural details of the compression release shaft  32  and flyweight member  38  of the illustrated embodiment can be better appreciated. Flyweight member  38  is shaped in a boomerang configuration so that when the camshaft rotates above a minimum speed, flyweight member  38  is biased outwardly and shaft  32  rotates therewith. With reference to FIG. 1B, second segment  36  includes flat surfaces  48  and  50  thereon which operably engage lift members  40 . With reference to FIGS. 8-10, it can be seen that compression release shaft  32  comprises a D-shaped cross section in areas of flat surfaces  48  and  50 . As also shown with respect to FIGS. 9 and 10, flat surfaces  48  and  50  are angularly offset relative to one another. Such is particularly adaptable to the two cylinders of a V-twin engine. However, the orientation of flat surfaces  48  and  50 , and accordingly, lift members  40  could be modified for a different engine configuration. It can thus be appreciated that, as shaft  32  rotates, it engages bulbous portions  52  of lift members  40  at flat surfaces  48  and  50 , thereby allowing lift members  40  to disengage the respective exhaust valve tappets. 
     With reference to FIG. 1A, the “rotationally interlocking” and “axially non-interlocking” features of the respective segments of shaft  32  can be appreciated. First segment  34  includes scalloped portion  54  and tongue  56  having a substantially semicircular cross sectional shape. Similarly, second segment  36  includes tongue  58  which also has a substantially semi-circular cross section as shown in FIG.  1 A and in more detail in FIG.  11 . Tongue  58  includes flat end  60  which abuts against flat portion  62  of first segment  34 . In assembled form, the forces holding segments  34  and  36  of shaft  32  together are supplied at the ends of shaft  32 . As can be seen in FIG. 5, bearing surface  65  of camshaft housing  64  abuts against a portion of flyweight member  38  proximate to the integral connection of flyweight member  38  and first segment  34 , thereby maintaining shaft  32  within shaft bore  66 . Side surface  68  of cam  24  abuts against and provides a bearing surface for the other end of shaft  32  thereby securing it within bore  66 . 
     It can now be appreciated that segments  34  and  36  of compression shaft  32  are axially non-interlocking. That is, the mating surfaces of segments  34  and  36  are held together axially by forces exerted on each end of shaft  32 , namely, by side surface  68  and bearing surface  65  of camshaft housing  64 . Thus, “axially non-interlocking” for purposes of this specification means that the connection between segments  34  and  36  need not include fasteners, welding, epoxy or the like. Instead, if the force provided by either side surface  68  or camshaft housing  64  were removed, compression release shaft  32  would be free to separate axially into segments  34  and  36 . 
     On the other hand, segments  34  and  36  are “rotationally interlocking.” That is, when one of the segments rotates within bore  66 , the other segment rotates therewith. This rotationally interlocking feature of segments  34  and  36  comprising shaft  32  in the illustrated embodiment is possible because shaft  32  is disposed internally in bore  66  within camshaft  22 . Consequently, shaft  32  is surrounded by a large bearing surface provided by bore  66 , which in turn maintains the mating engagement between flat surfaces  70  and  72  of tongues  56  and  58 , respectively (FIG.  1 A). Thus, rotational movement can be effectively communicated from segment  34  to segment  36 . In general, the rotationally interlocked segments comprise each of segments  34  and  36  including tongue portions  56  and  58  extending therefrom, respectively. The tongue portions have corresponding shapes which interfit with one another. In the illustrated embodiment, the corresponding shapes include flat surfaces  70  and  72  and end  60  and flat portion  62 . However, it is to be understood that one of ordinary skill in the art would be able to substitute other tongue configurations, tongue and groove configurations, etc. which interfit with one another. 
     The particulars of how the compression release mechanism fits within housing  64  can be understood with references to the order in which the respective parts are assembled. Lift members  40  are first placed within holes  42 . Segment  36  is then inserted into bore  66 . Next, segment  34  having flyweight member  38  integrally formed therewith is inserted into bore  66  in such an orientation so that flat surfaces  70  and  72  of tongues  56  and  58 , respectively, rotationally interlock as shown in FIG.  1 B. Thus, compression release shaft  32  extends from flyweight member  38  through cam gear  26  and further extends into bore  66 . Camshaft  22  can then be installed into housing  64 . As shown in FIG. 5, housing member  64  provides bearing surface  65  which abuts against cam gear  26  and flyweight member  38 . Thus, compression release shaft  32  and flyweight member  38  are contained by bearing surface  65  of housing  64  and side surface  68  of a cam  24 . Thus, surfaces  65  and  68  prevent segments  34  and  36  from separating. It can also be appreciated that flyweight member  38  is captured between cam gear  26  and housing  64 , thereby eliminating the need for other parts to secure flyweight member  38  to cam gear  26 . 
     The remaining structural details of the compression release assembly of the illustrated embodiment can be better understood with reference to a description of operation. At start-up operating speeds, such as when an operator is manually pulling on a starter rope (not shown), camshaft  22  is moving at a low rate of speed. During such low rates of camshaft speed, torsional spring  44  biases flyweight member  38  to the position shown in FIGS. 3 and 4. As can be seen in FIG. 4, torsional spring  44  has one of its ends inserted in hole  74  of flyweight member  38 , whereas the other end of spring  44  is inserted in hole  76  of cam gear  26 . Coil  78  of spring  44  pivots freely as flyweight member  38  moves outwardly as shown in phantom lines in FIG.  4 . As shown in FIG. 5, at low camshaft rotational speeds, lift member  40  is fully extended and engages a valve actuation device such as valve tappets  28  such that exhaust valves  80  are open, thereby allowing the gases to escape from the cylinder, which in turn results in the starter cord providing less resistance to being pulled. While the valve actuation devices in the illustrated embodiment are shown as valve tappets  28 , it is to be understood that the principles embodied by the present invention can be applied to engage other valve actuation devices, depending upon the type of engine in which the present invention is employed. Other valve actuation devices include push rods, rocker arms, valves and the like. 
     Upon camshaft  22  obtaining a minimum rotational speed, flyweight member  38  is centrifugally biased outwardly toward the position shown in FIG.  2  and in phantom in FIG.  4 . As noted above, the camshaft rotational speed at which flyweight member  38  begins to move outwardly can be pre-determined by adjusting the weight of flyweight member  38  utilizing powder metal technology. 
     As shown in FIGS. 2 and 4, as the rotational speed of the camshaft reaches a minimum value, flyweight member  38  is biased outwardly, and as a result, lift members  40  retract inwardly and disengage from the valve tappets. As a result, cams  24  control the opening and closing of the exhaust valves, the mechanism by which being widely known in the art. The lift members are biased inwardly into enlarged portion  82  (FIGS. 5 and 6) of holes  42  by the centrifugal force on bulbous portion  52  from the rotation of camshaft  22 . Thus, when shaft  32  rotates from the position shown in FIGS. 1B and 5 to a position wherein surfaces  48  and  50  engage bulbous ends  52 , lift members  40  retract inwardly into camshaft  22  so that cams  24  thereafter operate the opening and closing of the valves (not shown). 
     FIGS. 12-22 show a second embodiment of the present invention. The embodiments are similar in overall concept and function with the reference numbers for similar elements increased by 100 for the second embodiment, i.e., camshaft  22  in FIGS. 1-11 is camshaft  122  in FIGS. 12-22. Major differences between the second embodiment and the discussion above involve the spring, the location of one of the flat surfaces on the compression release shaft, and the size of the bulbous portion of the lift member. 
     As shown in FIGS. 12 and 15 an end of torsional spring  144  is attached to cam gear  126  with rivet  186 , whereas in the first embodiment that end of torsional spring  44  is inserted in hole  74  of cam gear  26 . The end of spring  144  has a loop that goes around pressed in rivet  186 . 
     Referring to FIGS. 12A and 12B, flat surface  150  on second segment  136  of compression release shaft  132  is disposed adjacent tongue  158  providing maximum separation between flat surfaces  148  and  150 . The separation between flat surfaces  148  and  150  is dependent on the separation between lift members  140 . The increased separation between the lift members is due to the moving of the lift member nearest the cam gear to the other side of its cam as shown in FIGS. 13 and 14. Also this embodiment includes support bosses  188  in the area of the camshaft around the two lift members. 
     Referring now to FIG. 18, the size of bulbous portion  152  of lift member  140  has increased over the size of bulbous portion  52  of lift member  40 . The centrifugal force on the enlarged bulbous portion is greater than on its smaller counterpart. The center of gravity of the lift member is on the bulbous side of the lift member such that when the camshaft is turning and the flyweight is opened, the centrifugal force on the center of gravity of the lift member causes the lift member to retract into the camshaft and not make contact with the valve tappet. Without a sizable bulbous on the lift member, the lift member would not retract and would make contact with the valve tappet at engine operating speed causing a wear failure between the valve tappet and the lift member. 
     While an exemplary embodiment of this invention has been described, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.