Abstract:
An automatic compression release mechanism for in an internal combustion engine, includes a camshaft assembly including a cam gear, a cam lobe with a notch positioned along a first side of the gear, a tube passing through the cam gear and aligned with the notch, and a support on a second side of the cam gear. An actuator assembly includes a contoured shaft that extends through the tube and resides in the notch. The actuator assembly is rotatable between two operating orientations and a step formed in the surface of the notch prevents the actuator from becoming disabled during engine shut down.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional patent application No. 60/496,433, filed on Aug. 20, 2003. 

   FIELD OF THE INVENTION 
   The present invention relates to internal combustion engines and, more particularly, to automatic compression release mechanisms employed in internal combustion engines. 
   BACKGROUND OF THE INVENTION 
   Automatic compression release mechanisms are employed in internal combustion engines to provide for improved engine performance at a variety of engine speeds. Such mechanisms typically include a component which is actuated based upon engine speed, that varies an exterior surface characteristic of a cam lobe along which mating valve train components actuate exhaust and/or intake valves of the engine. When the engine is cranking, a protrusion is created on the cam lobe such that the exhaust valve opens slightly during the compression stroke of the engine. The reduced compression caused by this “low speed orientation” reduces the effort to start the engine. However, when engine speeds are higher, such as during normal operation or idling, the protrusion is eliminated such that the exhaust valve remains closed during the compression stroke of the engine. This “normal speed orientation” maximizes engine power. 
   Automatic compression release mechanisms of this type often employ a weight assembly that is rotatably affixed to a portion of the camshaft such as a cam gear. As the camshaft rotates, centrifugal forces acting on the weight cause the weight to move radially outwards, away from the camshaft axis. However, the weight is typically biased by a spring towards the camshaft so that when the engine is at low speeds, the weight is pulled inward toward the camshaft. Because the movement of the weight is dependent upon the rotational speed of the camshaft, the movement of the weight can be used to govern components associated with the cam lobe to produce the desired speed-dependent variation in cam lobe shape. Commonly these components include a contoured shaft having a recessed side and an unrecessed side, which is coupled to the weight. The contoured shaft is disposed in a notch formed in the surface of the cam lobe, and when the weight is disposed radially inwards at low engine speed, the unrecessed side of the contoured shaft extends outward beyond the exterior surface of the cam lobe producing a protrusion. When the weight is rotated outwards at higher engine speeds, the recessed side of the contoured shaft faces outward and the protrusion on the cam lobe is largely or entirely eliminated. 
   In many engines, it is desirable to employ an automatic compression release mechanism having as few components as possible, in order to simplify and consequently reduce the costs of the mechanism. This can be achieved to some extent by integrally forming as a single piece assembly the weight and the contoured shaft such that rotation of the weight directly causes rotation of the contoured shaft. For similar cost-related reasons, it often is desirable for engines to employ simply-formed and inexpensive components throughout the cam shaft assembly. For example, the cam gear can be molded out of plastic or die cast as a single piece. Also, the cam lobe can be integrally formed as part of the cam gear, or at least fixedly attached to the cam gear. 
   When shutting down any engine, its rotation is slowed both by friction and by the work of the piston against gasses in the cylinder during the compression stroke. During this shut down the contoured shaft rotates to the low speed orientation in which the protrusion is exposed on the cam surface. If at the final moments of rotation there is insufficient angular momentum to accomplish the compression event, however, the compressed gas will work against the piston to cause a small amount of reversed rotation. This small reversed rotation of the engine can cause the cam follower to bear against the recessed, or flat side of the contoured shaft and rotate it against the bias spring force to its normal speed orientation. The automatic compression release mechanism thus becomes disabled for the subsequent starting event, thus making it difficult to restart the engine due to the high compressive forces. 
   SUMMARY OF THE INVENTION 
   The present invention is an improvement to an automatic compression release mechanism which prevents it from becoming disabled during engine shut down. More specifically, the improvement is a step formed in the notch which rotatably supports the contoured shaft along the surface of the cam lobe. This step blocks or prevents, the contoured shaft from being rotated by the cam follower when the engine rotates in reverse direction during shut down. 
   In particular, the present invention relates to an improvement in an automatic compression release mechanism having a weight assembly for rotating a contoured shaft in a notch of a cam lobe between a low speed orientation in which the contoured shaft presents a first surface that protrudes above a cam lobe surface and a normal speed orientation in which the contoured shaft presents a second surface that is substantially flush with the cam lobe surface. The improvement includes a step formed in the notch of the cam lobe which interacts with the contoured shaft to resist rotation of the contoured shaft from the low speed orientation to the normal speed orientation when the cam lobe moves in a first direction of rotation during engine shut down that is opposite a second direction of rotation of the cam lobe during normal engine operation. 
   The present invention additionally relates to a camshaft assembly that includes a cam lobe having a recess, a cam gear coupled to the cam lobe, and an actuator assembly including a weight and a shaft coupled to one another. The actuator assembly is supported in relation to the cam lobe so that the shaft extends into the recess. The shaft of the actuator assembly is configured so that during low speed rotation of the cam lobe a protuberance formed by a portion of the shaft extends out of the recess beyond a perimeter of the cam lobe, and during normal speed rotation of the cam lobe the protuberance is at least one of reduced and eliminated. Further, the recess includes two curved surfaces that are connected by a step surface, and the step surface restricts rotational movement of the shaft at least some of the time. 
   The present invention further relates to a method of operating a camshaft assembly. The method includes decelerating a rotational speed of the camshaft assembly from a first speed to a second speed, where the camshaft assembly is rotating in a first rotational direction and, as the camshaft assembly is decelerating, rotating a shaft of an actuator assembly of the camshaft assembly within a recess of a cam lobe of the camshaft assembly, so that a protuberance appears on the cam lobe. The method additionally includes receiving an axially extending edge of the shaft adjacent to an axially extending step formed in the recess, where in at least one operational situation the shaft is prevented from rotating in a manner that would cause the edge to pass by the step. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a first perspective view of a single cylinder engine, taken from a side of the engine on which are located a starter and cylinder head; 
       FIG. 2  is a second perspective view of the single cylinder engine of  FIG. 1 , taken from a side of the engine on which are located an air cleaner and oil filter; 
       FIG. 3  is a third perspective view of the single cylinder engine of  FIG. 1 , in which certain parts of the engine have been removed to reveal additional internal parts of the engine; 
       FIG. 4  is a fourth perspective view of the single cylinder engine of  FIG. 1 , in which certain parts of the engine have been removed to reveal additional internal parts of the engine; 
       FIG. 5  is fifth perspective view of portions of the single cylinder engine of  FIG. 1 , in which a top of the crankcase has been removed to reveal an interior of the crankcase; 
       FIG. 6  is a sixth perspective view of portions of the single cylinder engine of  FIG. 1 , in which the top of the crankcase is shown exploded from the bottom of the crankcase; 
       FIG. 7  is a top view of the single cylinder engine of  FIG. 1 , showing internal components of the engine; 
       FIG. 8  is a perspective view of components of a valve train of the single cylinder engine of  FIG. 1 ; 
       FIG. 9  is a perspective view of a camshaft, cam gear and automatic compression release (ACR) mechanism implemented in the engine of  FIG. 1 ; 
       FIG. 10  is a perspective view of the camshaft, cam gear and ACR mechanism of  FIG. 9 , with the ACR mechanism exploded from the cam gear; 
       FIG. 11  is a view in cross-section through the cam lobe showing the ACR mechanism in its normal engine speed orientation; 
       FIG. 12  is a view in cross-section through the cam lobe showing the ACR mechanism in its low speed orientation; 
       FIG. 13  is a view in cross-section through the cam lobe showing the ACR mechanism during engine shut down; and 
       FIG. 14  is a perspective view of the cam lobe showing the recess which receives the ACR. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 and 2 , a single cylinder, 4-stroke, internal combustion engine  100  includes a crankcase  110  and a blower housing  120 , inside of which are a fan  130  and a flywheel  140 . The engine  100  further includes a starter  150 , a cylinder  160 , a cylinder head  170 , and a rocker arm cover  180 . Attached to the cylinder head  170  are an air exhaust port  190  shown in  FIG. 1  and an air intake port  200  shown in  FIG. 2 . As is well known in the art, during operation of the engine  100 , a piston  210  (see  FIG. 7 ) moves back and forth within the cylinder  160  towards and away from the cylinder head  170 . The movement of the piston  210  in turn causes rotation of a crankshaft  220  (see  FIG. 7 ), as well as rotation of the fan  130  and the flywheel  140 , which are coupled to the crankshaft. The rotation of the fan  130  cools the engine, and the rotation of the flywheel  140 , causes a relatively constant rotational momentum to be maintained. 
   Referring specifically to  FIG. 2 , the engine  100  further includes an air filter  230  coupled to the air intake port  200 , which filters the air required by the engine prior to the providing of the air to the cylinder head  170 . The air provided to the air intake port  200  is communicated into the cylinder  160  by way of the cylinder head  170 , and exits the engine by flowing from the cylinder through the cylinder head and then out of the air exhaust port  190 . The inflow and outflow of air into and out of the cylinder  160  by way of the cylinder head  170  is governed by an input (intake) valve  240  and an output (exhaust) valve  250 , respectively (see  FIG. 8 ). Also as shown in  FIG. 2 , the engine  100  includes an oil filter  260  through which the oil for the engine  100  is passed and filtered. Specifically, the oil filter  260  is coupled to the crankcase  110  by way of incoming and outgoing lines  270 ,  280 , respectively, whereby pressurized oil is provided into the oil filter and then is returned from the oil filter to the crankcase. 
   Referring to  FIGS. 3 and 4 , the engine  100  is shown with the blower housing  120  removed to expose a top  290  of the crankcase  110 . With respect to  FIG. 3 , in which both the fan  130  and the flywheel  140  are also removed, a coil  300  is shown that generates an electric current based upon rotation of the fan  130  and/or the flywheel  140 , which together operate as a magneto. Additionally, the top  290  of the crankcase  110  has a pair of lobes  310  that cover a pair of cam gears  320  (see FIGS.  5  and  7 – 8 ). As shown in  FIG. 4 , the fan  130  and the flywheel  140  are above the top  290  of the crankcase  110 . Additionally,  FIG. 4  shows the engine  100  without the rocker arm cover  180 , to more clearly reveal a pair of tubes  330  through which extend a pair of respective push rods  340 . The push rods  340  extend between a pair of respective rocker arms  350  and a pair of cams  360  (see  FIG. 8 ) within the crankcase  110 , as discussed further below. 
   Turning to  FIGS. 5 and 6 , the engine  100  is shown with the top  290  of the crankcase  110  removed from a bottom  370  of the crankcase  110  to reveal an interior  380  of the crankcase. Additionally in  FIGS. 5 and 6 , the engine  100  is shown in cut-away to exclude portions of the engine that extend beyond the cylinder  160  such as the cylinder head  170 . With respect to  FIG. 6 , the top  290  of the crankcase  110  is shown above the bottom  370  of the crankcase in an exploded view. In this embodiment, the bottom  370  includes not only a floor  390  of the crankcase, but also all four side walls  400  of the crankcase, while the top  290  only acts as the roof of the crankcase. The top  290  and bottom  370  are manufactured as two separate pieces such that, in order to open the crankcase  110 , one physically removes the top from the bottom. Also, as shown in  FIG. 5 , the pair of gears  320  within the crankcase  110  are integrally formed as part of, or at least supported by, respective camshafts  410 , which in turn are supported by the bottom  370  of the crankcase  110 . 
   Referring to  FIG. 7 , a top view of the engine  100  (with the top  290  of the crankcase  110  removed) is provided in which additional internal components of the engine are shown. In particular,  FIG. 7  shows the piston  210  within the cylinder  160  to be coupled to the crankshaft  220  by a connecting rod  420 . The crankshaft  220  is in turn coupled to a rotating counterweight  430  and reciprocal weights  440 , which balance the forces exerted upon the crankshaft  220  by the piston  210 . The crankshaft  220  further is in contact with each of the gears  320 , and thus communicates rotational motion to the gears. In the preferred embodiment, the camshafts  410  upon which the cam gears  320  are supported are capable of communicating oil from the floor of the crankcase  110  upward to the gears  320 . The incoming line  270  to the oil filter  260  is coupled to one of the camshafts  410  to receive oil, while the outgoing line  280  from the oil filter is coupled to the crankshaft  220  to provide lubrication thereto.  FIG. 7  further shows a spark plug  450  located on the cylinder head  170 , which provides sparks during power strokes of the engine to cause combustion to occur within the cylinder  160 . The electrical energy for the spark plug  450  is provided by the coil  300  (see  FIG. 3 ). 
   Referring to  FIG. 7  and  FIG. 8 , elements of a valve train  460  of the engine  100  are shown. The valve train  460  includes cam gears  320  driven by camshafts  410  and also includes the cam lobes  360  disposed underneath the respective gears  320  and around respective camshafts  410 . Cam follower arms  470  are rotatably mounted to the crankcase  110  and extend to rest upon the respective cam lobes  360 . The push rods  340  in turn rest upon the respective cam follower arms  470  and as the cam lobes  360  rotate, the push rods  340  are forced outward away from the respective camshafts  410  by the cam follower arms  470  as they follow the contour of their respective cam lobes  360 . This causes the rocker arms  350  to rock or rotate, and consequently causes the respective valves  240  and  250  to open and close at the proper times during the engine cycle. A pair of springs  480 ,  490  positioned between the cylinder head  170  and the rocker arms  350  apply a bias force to the rocker arms in a direction tending to close the valves  240 , 250 . As a result of this bias force upon the rocker arms  350 , the push rods  340  are also forced against the cam follower arms  470  and hence against the cam lobes  360 . 
   The engine  100  is a vertical shaft engine capable of outputting 15–20 horsepower for implementation in a variety of consumer lawn and garden machinery such as lawn mowers. In alternate embodiments, the engine  100  can also be implemented as a horizontal shaft engine, be designed to output greater or lesser amounts of power, and/or be implemented in a variety of other types of machines, e.g., snow-blowers. Further, in alternate embodiments, the particular arrangement of parts within the engine  100  can vary from those shown and discussed above. For example, in one alternate embodiment, the cam lobes  360  could be located above the gears  320  rather than underneath the gears. 
   As shown in  FIGS. 9 and 10 , each cam gear  320  is disposed directly beneath the top cover  290  of the crankcase. A central hub  640  supports each cam gear  320  with respect to its respective cam shaft  410  for rotation about a vertical cam shaft axis  645 . A web  649  extends radially outward from the hub  640  and supports a circular ring of gear teeth  700 . The hub  640  and the ring of gear teeth  700  form an annular-shaped recess on the top side of each cam gear  320 . 
   As shown in  FIGS. 9 and 10 , an automatic compression release (ACR) mechanism is mounted to each of the cam gears (or, in alternate embodiments, one of the cam gears)  320  and disposed in the respective recesses of the cam gears. The ACR mechanism associated with each cam gear includes an actuator assembly  510  comprised of an arc-shaped weight  530  and an integrally formed contoured shaft  540 . In one embodiment, the assembly  510  is formed of powdered metal, although it may also be molded from plastic or other materials, or it may be die cast. The assembly  510  is rotatably mounted to the cam gear  320  by extending the contoured shaft  540  into and through a hollow tube  550  formed through the cam gear web  649 . The contoured shaft  540  rotates about an axis  647  that is parallel to the cam shaft axis  645 . 
   The top end of the contoured shaft  540  is circular in contour and connects to one end of the weight  530 . It extends downward through the tube  550  and into an axially directed notch, or recess  580  formed in the cam lobe  360 . The cam lobe  360  is located beneath the cam gear  320  and the lower end of the contoured shaft  540  is shaped to form a flat recessed surface  620  in its cylindrical surface. This flat surface  620  extends over the axial extent of the cam lobe recess  580  and the contoured shaft  540  has a “D-shaped” cross-section in the recess  580  as shown in  FIGS. 11–13 . 
   As shown best in  FIG. 11 , when the assembly  510  is rotated to a normal engine speed orientation, the flat surface  620  on the contoured shaft  540  faces radially outward and it is substantially flush with the outer surface of the cam lobe  360 . On the other hand, as shown in  FIG. 12 , when the assembly is rotated to a low engine speed orientation, the contoured shaft  540  is rotated within the recess  580  such that a portion of its D-shaped surface protrudes above the surface of the cam lobe  360 . It is this protuberance which pushes upward on the push rods  340  through the cam followers  470  to open the valves  240  and  250  at low engine speed and thereby facilitate easier starting. 
   Referring again to  FIGS. 9 and 10 , the actuator assembly  510  is biased in its low engine speed orientation by a spring  600 . One end of the spring  600  wraps around the weight  530  and its other end bears against a pin (not shown) formed on the cam gear  320 . The spring action produced by two wraps around the top of the contoured shaft  540  biases the weight  530  against the hub  640 . After the engine is started and engine speed builds, the rotation of the cam gear  320  causes the actuator assembly  510  to rotate about its axis  647  and move radially outward from the cam shaft axis  645  against the bias spring force to its normal engine speed orientation. This results from the centrifugal force produced by the rotating weight  530  which swings the arcuate-shaped weight about the axis  647 . When engine speed is reduced, this centrifugal force drops and the bias spring  600  rotates the assembly  510  back to its low engine speed orientation adjacent the hub  640 . 
   Referring still to  FIGS. 9 and 10 , the actuator assembly  510  is retained in place by an annular-shaped spacer  654 . The spacer  654  encircles the cam shaft  410  and it fills the gap between the top of the actuator assembly  510  and the bottom surface of the crankcase cover  290 . The actuator assembly  510  is thus axially retained by the spacer  654  from moving upward. It is trapped in the supporting tube  550  and constrained to rotational movement between its two operating orientations. 
   Referring particularly to  FIGS. 11–14 , an important aspect of the present invention is the shape of the axially directed recess  580  in the surface of the cam lobe  360 . The recess  580  extends axially a substantial distance and it forms a trough having two curved surfaces  582  and  583 . Each curved surface  582  and  583  is shaped to mate with the circular surface of the contoured shaft  540 , however, they are offset from each other to form a step  584 . As shown in  FIG. 13 , when the contoured shaft  540  is in its low engine speed orientation, one edge of its flat surface  620  engages this step  584  and inhibits its rotation to the high speed orientation. This is particularly effective when the engine reverses direction at shut down, as indicated by arrow  588 . The downward pressure of the cam follower  470  acting against the opposite edge of the flat surface  620  attempts to rotate the contoured shaft, but this same downward pressure keeps the contoured shaft  540  seated against the recessed surface  583  and keeps it from lifting over the step  584  and rotating to the normal speed orientation depicted in  FIG. 11 . 
   While the step  584  is effective in blocking rotation of the actuator assembly to the normal engine speed orientation during engine shut down, it does not hinder the transition to normal engine speed during engine start up. During start up the contoured shaft  540  engages the step  584  as shown in  FIG. 12  and the protruding shaft  540  relieves compression to assist starting as described above. As engine speed builds, a torque is applied to the contoured shaft  540  by the weight  530  which rotates the shaft  540  against the edge  584 . In addition, the centrifugal force acting on the actuator assembly as a whole lifts the edge of the contoured shaft  540  over the step  584 . To enable this to occur, the axial opening in the tube  550  (see  FIG. 10 ) must be large enough to allow the contoured shaft  540  to align radially with both curved surfaces  582  and  583 . 
   The interaction of the step  584  in the cam lobe recess  580  and the edge formed on the contoured shaft  540  by the flat surface  620  thus use the very pressure produced by the cam follower  470  which is the cause of the problem during engine shut down to solve the problem. During engine start up, however, this pressure is not applied for a large portion of each revolution of the cam lobe  360  and normal operation of the automatic compression release mechanism is allowed to occur. The present invention thus uses the force which causes the shut down problem to solve the problem. 
   While the foregoing specification illustrates and describes the preferred embodiments of this invention, it is to be understood that the invention is not limited to the precise construction herein disclosed. The invention can be embodied in other specific forms without departing from the spirit or essential attributes of the invention. For example, the present invention is applicable generally to the modification of the exterior surface of cam lobes, whether relating to the exhaust valve, intake valve, or other valves of an engine. The present invention also extends to other aspects of the design of the present camshaft assembly. For example, another aspect of the invention is the above-described means for fastening a weight and contoured shaft actuator assembly to the cam gear, where the contoured shaft extends through an opening formed in the cam gear and into the aligned notch formed in the cam lobe, and where the weight is free to rotate the contoured shaft about an axis through this opening and is axially constrained therein by a spacer disposed around a cam gear hub and extending radially outward therefrom to intercede between the cover and the weight assembly. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.