Abstract:
Mechanical compression and vacuum release mechanisms which are of simple construction and which significantly reduce the effort required to start an internal combustion engine. In several embodiments, the compression and vacuum release mechanisms include a centrifugally responsive flyweight pivotally mounted to the camshaft, the flyweight coupled to a pair of compression and vacuum release pins which include respective compression and vacuum release cams that are in lifting engagement with the valve actuation structure of one of the intake or exhaust valves of the engine during engine starting to relieve compression and vacuum within the combustion chamber and thereby facilitate easier engine starting. After the engine is started and reaches running speed, the flyweight pivots responsive to centrifugal force and in turn pivots the compression and vacuum release cams out of engagement with the valve actuation structure of the intake or exhaust valve to allow the engine to operate normally.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under Title 35, U.S.C. §119(e) of U.S. Provisional Application Ser. No. 60/688,023, entitled MECHANICAL COMPRESSION AND VACUUM RELEASE, filed on Jun. 7, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention.  
         [0003]     The present invention relates to internal combustion engines of the type used with lawnmowers, lawn and garden tractors, snow throwers, generators, other small utility implements, and sport vehicles, and more particularly, relates to a compression and vacuum release mechanism for small four-stoke cycle engines.  
         [0004]     2. Description of the Related Art  
         [0005]     Compression release mechanisms for four-stroke cycle engines are well known in the art. Generally, means are provided to hold one of the intake and exhaust valves in the combustion chamber of the cylinder head slightly open during the compression stroke of the piston while cranking the engine during starting. This action partially relieves the force of compression in the cylinder during starting, so that starting torque requirements of the engine are greatly reduced. When the engine starts and reaches running speeds, the compression release mechanism is rendered inoperable so that the four-stroke cycle of the engine may function normally and the engine may achieve full performance. It is typical for the compression release mechanism to be associated with the exhaust valve so that the normal flow of the fuel/air mixture into the chamber through the intake valve, and the elimination of spent gases through the exhaust valve is not interrupted, and the normal direction of flow through the chamber is not reversed. Examples of compression release mechanisms for four-stroke engines are shown in U.S. Pat. Nos. 3,381,676; 3,496,922; 3,897,768; 4,453,507; 4,977,868; 5,150,674 and 5,184,586. Although known compression release mechanisms are generally effective for relieving compression in the cylinder during cranking the engine, these mechanisms are typically designed to provide compression relief and do not remedy the significant torque established by vacuum in the combustion chamber during the power stroke.  
         [0006]     Conventional four-stoke engines may require a significant amount of torque to turn the engine over during the power stroke when combustion is not taking place, because the piston is moving downwardly against a pressure difference due to increasing suction or vacuum in the combustion chamber resulting from the partial discharge of gas from the combustion chamber during the immediately preceding compression stroke. The increase of torque required corresponds to a substantial operator or starter force required to drive the piston downwardly against such pressure difference.  
         [0007]     Accordingly, it is desired to provide a release mechanism that addresses the significant torque developed by both the compression and power strokes, is effective in operation, and is relatively simple in construction.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides mechanical compression and vacuum release mechanisms which are of simple construction and which significantly reduce the effort required to start an internal combustion engine. In several embodiments, the compression and vacuum release mechanisms include a centrifugally responsive flyweight pivotally mounted to the camshaft, the flyweight coupled to a pair of compression and vacuum release pins which include respective compression and vacuum release cams that are in lifting engagement with the valve actuation structure of one of the intake or exhaust valves of the engine during engine starting to relieve compression and vacuum within the combustion chamber and thereby facilitate easier engine starting. After the engine is started and reaches running speed, the flyweight pivots responsive to centrifugal force and in turn pivots the compression and vacuum release cams out of engagement with the valve actuation structure of the intake or exhaust valve to allow the engine to operate normally.  
         [0009]     In one form thereof, the present invention provides an internal combustion engine, including an engine housing; a crankshaft rotatably supported within the engine housing; a piston coupled to the crankshaft for reciprocation within a cylinder bore between top dead center and bottom dead center positions; a combustion chamber defined between the piston and the engine housing, the combustion chamber having a relatively smaller volume when the piston is in the top dead center position and a relatively larger volume when the piston is in the bottom dead center position; a camshaft driven from the crankshaft, the camshaft including a pair of cam lobes periodically engaging valve actuation structure associated with a pair of intake and exhaust valves; and a compression and vacuum release mechanism, including a flyweight coupled to compression and vacuum release pins, the pins extending along the camshaft and including compression and vacuum release cams, respectively; the flyweight movable responsive to centrifugal forces between a first position corresponding to engine cranking speeds in which the compression and vacuum release cams are each positioned for operative engagement with the valve actuation structure and a second position corresponding to engine running speeds in which the compression and vacuum release cams are each positioned out of operative engagement with the valve actuation structure, and wherein in the first position, the compression release cam engages the valve actuation structure as the piston moves toward the top dead center position and the vacuum release cam engages the valve actuation structure as the piston moves toward the bottom dead center position.  
         [0010]     In another form thereof, the present invention provides an internal combustion engine, including an engine housing; a crankshaft rotatably supported within the engine housing; a piston coupled to the crankshaft for reciprocation within a cylinder bore between top dead center and bottom dead center positions; a combustion chamber defined between the piston and the engine housing, the combustion chamber having a relatively smaller volume when the piston is in the top dead center position and a relatively larger volume when the piston is in the bottom dead center position; a camshaft driven from the crankshaft, the camshaft including a pair of cam lobes periodically engaging valve actuation structure associated with a pair of intake and exhaust valves; and a compression and vacuum release mechanism, including a flyweight movably mounted to the camshaft, the flyweight coupled to a pair of respective compression and vacuum release pins, the pins extending substantially parallel with the camshaft and including compression and vacuum release cams, respectively; the flyweight movable responsive to centrifugal forces between a first position corresponding to engine cranking speeds in which the compression and vacuum release cams are each positioned for operative engagement with the valve actuation structure and a second position corresponding to engine running speeds in which the compression and vacuum release cams are each positioned out of operative engagement with the valve actuation structure, and wherein in the first position, the compression release cam engages the valve actuation structure as the piston moves toward the top dead center position and the vacuum release cam engages the valve actuation structure as the piston moves toward the bottom dead center position.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     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:  
         [0012]      FIG. 1  is a partial sectional view of an exemplary single cylinder, four-stroke internal combustion engine including a mechanical compression and vacuum release mechanism in accordance with the present invention;  
         [0013]      FIG. 2  is a first perspective view of the camshaft and cam gear assembly of the engine  FIG. 1 ;  
         [0014]      FIG. 3  is a second perspective view of the camshaft and cam gear assembly of the engine of  FIG. 1 , showing components of a mechanical compression and vacuum release mechanism according to a first embodiment;  
         [0015]      FIG. 4  is an end view of the cam gear, showing the components of the mechanical compression and vacuum release mechanism of the first embodiment in a first or start position;  
         [0016]      FIG. 5  is an elevational view of the camshaft and cam gear, showing the components of the mechanical compression and vacuum release mechanism in the first or start position;  
         [0017]      FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 5 .  
         [0018]      FIG. 7  is an end view of the cam gear, showing the components of the mechanical compression and vacuum release mechanism of the first embodiment in a second or run position;  
         [0019]      FIG. 8  is an elevational view of the camshaft and cam gear, showing the components of the mechanical compression and vacuum release mechanism in the second or run position;  
         [0020]      FIG. 9  is a perspective view of the camshaft and cam gear assembly of the engine of  FIG. 1 , showing components of a mechanical compression and vacuum release mechanism according to a second embodiment;  
         [0021]      FIG. 10  is an end view of the cam gear of  FIG. 9 , showing the components of the mechanical compression and vacuum release mechanism of the second embodiment in a first or start position;  
         [0022]      FIG. 11  is an end view of the cam gear of  FIG. 9 , showing the components of the mechanical compression and vacuum release mechanism of the second embodiment in a second or run position;  
         [0023]      FIG. 12  is a perspective view of the camshaft and cam gear assembly of the engine of  FIG. 1 , showing components of a mechanical compression and vacuum release mechanism according to a third embodiment;  
         [0024]      FIG. 13  is an end view of the cam gear of  FIG. 12 , showing the components of the mechanical compression and vacuum release mechanism of the third embodiment in a first or start position;  
         [0025]      FIG. 14  is an end view of the cam gear of  FIG. 12 , showing the components of the mechanical compression and vacuum release mechanism of the third embodiment in a second or run position;  
         [0026]      FIG. 15  is a perspective view of the camshaft and cam gear assembly of the engine of  FIG. 1 , showing components of a mechanical compression and vacuum release mechanism according to a fourth embodiment;  
         [0027]      FIG. 16  is an end view of the cam gear of  FIG. 15 , showing the components of the mechanical compression and vacuum release mechanism of the fourth embodiment in a first or start position; and  
         [0028]      FIG. 17  is an end view of the cam gear of  FIG. 15 , showing the components of the mechanical compression and vacuum release mechanism of the fourth embodiment in a second or run position. 
     
    
       [0029]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate several preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention any manner.  
       DETAILED DESCRIPTION  
       [0030]     Referring to  FIG. 1 , there is shown a vertical crankshaft, single cylinder, four-stroke internal combustion engine  10  including a compression and vacuum release mechanism according to one embodiment of the present invention. Other compression and vacuum release mechanisms are disclosed in U.S. Pat. Nos. 6,394,094, 6,536,393 and 6,539,906, each assigned to the assignee of the present invention, the disclosures of which are expressly incorporated herein by reference.  
         [0031]     As is customary, engine  10  includes cylinder block  11 , crankshaft  12  and piston  14 , the piston being operatively connected to crankshaft  12  via connecting rod  16 . Piston  14  cooperates with cylinder block  11  and cylinder head  18  to define combustion chamber  20 . Spark plug  22  secured in cylinder head  18  ignites the fuel/air mixture after it has been drawn into combustion chamber  20  through the intake valve (not shown) during the intake stroke and has been compressed during the compression stroke of piston  14 . The spark is normally timed to ignite the fuel/air mixture just before piston  14  completes its ascent on the compression stroke toward its top dead center (“TDC”) position. The fuel/air mixture is drawn into combustion chamber  20  from the carburetor of the engine through an intake passage controlled by a conventional intake valve (not shown), and the products of combustion are expelled from the cylinder during the exhaust stroke through exhaust port  24  controlled by poppet-type exhaust valve  26 . Although either the intake valve or exhaust valve  26  may be opened to vent compression and vacuum during start-up, it is recognized that preferably exhaust valve  26  functions as the compression and vacuum release valve in a manner to be discussed hereinafter.  
         [0032]     Other conventional parts of the valve operating mechanism, or valve assembly, include timing gear  27  mounted on crankshaft  12  for rotation therewith, and camshaft gear  28  mounted on camshaft  30  and rotatably driven by timing gear  27  to thereby rotate camshaft  30  at one-half crankshaft speed. Camshaft  30  includes conventional pear-shaped intake and exhaust camshaft lobes  32  and  34 , respectively, ( FIGS. 1 and 2 ) which rotate with camshaft  30  to impart reciprocating motion to the intake and exhaust valves via tappets or cam followers  36  (not visible in  FIG. 1 ) and  38 , respectively. Although  FIG. 1  illustrates the compression and vacuum release mechanism in a side valve engine, this is but one engine type, and the compression and vacuum release mechanisms disclosed herein are useable with other engine types, such as overhead valve (“OHV”) and overhead cam (“OHC”) engines of a vertical or horizontal crankshaft type, for example. In the exemplary side valve engine of  FIG. 1 , the valve actuating structures are shown in form of cam followers; however, as discussed below, in engines having other types of valve trains, the valve actuating structures may include lifters, push rods, rocker arms, bucket tappets, etc.  
         [0033]     Referring to  FIG. 2 , intake lobe  32  is shown as the outboard lobe furthest removed relative to camshaft gear  28 , and exhaust lobe  34  is shown inboard with respect to camshaft gear  28  and lobe  32 . The exhaust valve train is shown in  FIG. 1  and includes cam follower  38  having face  42  adapted to bear tangentially against, and remain in a continuous abutting relationship with, peripheral surface  44  of the base circle of exhaust camshaft lobe  34 . Referring to  FIG. 1 , cam follower  38  slides in guide boss  48  of crankcase  50 , and its upper end pushes against tip  46  of valve  26 . In operation, cam follower  38  lifts stem  52  of exhaust valve  26  which lifts face  53  from valve seat  55 . Valve spring  54  encircles stem  52  between valve guide  56  and spring retainer  58 . Spring  54  biases valve  26  closed and also biases cam follower  38  into tracking contact with exhaust lobe  34 . Although the valve train or valve assembly shown in  FIGS. 1 and 2  includes a camshaft having lobes which directly actuate the intake and exhaust valves, other engines in which the present invention may be used may include different valve trains or valve assemblies, such as, for example, an overhead camshaft driven from the crankshaft via linkage and including lobes for opening and closing the intake and exhaust valves; a camshaft driven from the crankshaft and including lobes for actuating push rods connected to rocker arms which in turn open and close the intake and exhaust valves; or a camshaft having a single cam lobe actuating rocker arms which in turn open and close the intake or exhaust valves. Other valve train or valve assemblies are also possible in engines in which the present invention may be used.  
         [0034]     To aid in starting engine  10 , several embodiments of mechanical compression and vacuum release mechanisms, described below, are provided. Generally, while the mechanisms are in their second or inoperative position, which is designated as the “run” position of the engine, the rotation of outboard lobe  34  with camshaft  30  at “running speed” causes normal operation of valve  26 , so that valve  26  opens and closes in timed and periodic relation with the travel of piston  14  according to conventional engine timing practice. Thus, exhaust lobe  34  is adapted to open valve  26  near the end of the power stroke and to hold the same open during ascent of the piston on the exhaust stroke until the piston has moved slightly past top dead center. As camshaft lobe  34  continues to rotate, spring  58  forces cam follower  38  downwardly and valve  26  is reseated. Valve  26  is held closed during the ensuing intake, compression and power strokes. Intake camshaft lobe  32  is likewise of conventional fixed configuration to control the intake valve such that it completely closes shortly after the piston begins its compression stroke and remains closed throughout the subsequent power and exhaust strokes, and reopening to admit the fuel mixture on the intake stroke.  
         [0035]     Since in a conventional engine the intake and exhaust valves are normally closed for the major portion of the power stroke, cranking of the engine is impeded because the piston must pull against a vacuum in the combustion chamber. Such vacuum may be created in the combustion chamber by the operation of a conventional compression release mechanism during engine starting. However, by incorporating any of the compression and vacuum release mechanisms of the present invention, compression and vacuum relief is automatically obtained at cranking speeds to greatly reduce cranking effort and thereby facilitate starting. Moreover, a conventional engine need not be physically altered to effect compression and vacuum release with the mechanism of the present invention incorporated therein. The compression and vacuum release mechanism is responsive to engine speed such that it is automatically rendered inoperative at engine running speeds to prevent compression loss or loss of efficiency of the engine when it is running under its own power.  
         [0036]     Referring to  FIGS. 2 and 3 , a first embodiment of a mechanical compression and vacuum release mechanism of the present invention is shown. Compression and vacuum release mechanism  60   a  includes a hub  62  preferably formed as an integral portion with camshaft gear  28 , and which extends therefrom on opposite sides of camshaft gear  28  as shown in  FIGS. 2 and 3 . Referring to  FIG. 3 , flyweight  64  is pivotally mounted to camshaft gear  28  and generally includes body portion  66 , head portion  68 , and extension portion  70 . Body portion  66  comprises most of the mass of flyweight  64  and includes radial inner surface  72  and radial outer surface  74  having stop projection  76 . Head portion  68  includes a vacuum release pin  78  extending substantially parallel to camshaft  30  and closely yet rotatably fitted within a bore  80  in hub  62 , and flyweight  64  is pivotally mounted to camshaft gear  28  about vacuum release pin  78 . Extension portion  70  extends from head portion  68  and includes a pin  82 .  
         [0037]     Mechanical compression and vacuum release mechanism  60   a  also includes compression release lever  84 , which includes compression release pin  88  extending rotatably through bore  90  in hub  62  via a close fit and aligned substantially parallel to camshaft  30  and vacuum release pin  78 . Compression release lever  84  also includes coupling portion  92  extending orthogonally from compression release pin  88  and including slot  94  therein in which pin  82  of extension portion  70  of flyweight  64  is slidably received to operably couple flyweight  64  and compression release lever  84 . Flyweight  64  and compression release lever  84  may each be formed from a rigid plastic or suitable metal, for example, and preferably each comprise single components including vacuum and compression release pins  78  and  88 , respectively, integrally formed with the remainder of their structures. Referring to  FIG. 3 , hub  62  includes recesses  96  and  98  to accommodate vacuum and compression release pins  78  and  88 , respectively and, as shown in  FIG. 2 , exhaust cam lobe  34  includes recess  100  in which vacuum and compression release cams  102  and  104  at the ends of vacuum and compression release pins  78  and  88 , respectively, are disposed. Vacuum and compression release cams  102  and  104  each include flat portions, as shown in  FIG. 2 .  
         [0038]     Referring to  FIG. 3 , a tension spring  106  includes coil portion  108  mounted to camshaft gear  28  by fastener  110 , such as a rivet or screw, for example, and also includes first arm  112  in engagement with flyweight  64 , and second arm  114  extending through aperture  116  of camshaft gear  28  to anchor second arm  114  to camshaft gear  28 . Spring  106  normally biases flyweight  64  to the start position shown in  FIG. 4 , in which inner radial surface  72  of flyweight  64  abuts hub  62 .  
         [0039]     With reference to  FIGS. 4-9 , operation of compression vacuum release mechanism  60   a  will now be described. Compression and vacuum release mechanism  60   a  is shown in a first or start position in  FIGS. 4 and 5 , which corresponds to engine  10  being stopped or to engine  10  being cranked for starting during which a minimal amount of centrifugal force is imposed upon camshaft  30 , camshaft gear  28 , and mechanical compression and vacuum release mechanism  60   a . As shown in  FIG. 4 , in the start position, spring  106  biases flyweight  64  towards a radially inward position in which inner radial surface  72  of flyweight  64  abuts hub  62 , and vacuum and compression release pins  78  and  88  are rotatably oriented within bores  80  and  90  of hub  62  such that vacuum and compression release cams  102  and  104  each extend beyond the base circle of exhaust cam lobe  34 , as best shown in  FIGS. 5 and 6 . In this position, upon cranking of engine  10 , vacuum and compression release cams  102  and  104  will each contact surface  42  of cam follower  38  of exhaust valve  26  to slightly open exhaust valve  26  as piston  14  is retreating from, and extending toward, its TDC position, respectively, in order to vent combustion chamber  20 . In this manner, engine  10  may be more easily cranked for starting. Advantageously, contact loads from the contact between surface  42  of cam follower  38  and vacuum and compression release cams  102  and  104  is transferred through vacuum and compression release pins  78  and  88  to hub  62  due to the close fit of vacuum and compression release pins  78  and  88  within bores  80  and  90  of hub  62 .  
         [0040]     After engine  10  starts and the rotational speed of camshaft  30  and camshaft gear  28  rapidly increases, a much greater amount of centrifugal force is imposed upon flyweight  64 , thereby urging flyweight  64  against the bias of spring  106  centrifugally outwardly to the position shown in  FIG. 7 , in which radial outer surface  74  is disposed adjacent rim  118  of camshaft gear  28  and stop projection  76  of flyweight  64  is in engagement with rim  118 . In this position, vacuum release pin  78  is rotated along with flyweight  64 , and compression release pin  88  is rotated concurrently with vacuum release pin  78  via the sliding engagement of pin  82  of flyweight extension portion  70  within slot  94  of compression release lever  84  to the positions shown in  FIG. 8 , in which the flat surfaces of vacuum and compression release cams  102  and  104  are oriented such that same do not extend beyond the base circle of exhaust cam lobe  34 . In this manner, the vacuum and compression release effects are terminated after engine  10  starts and, at engine running speeds, engine  10  operates according to a conventional four-stroke timing sequence.  
         [0041]     Referring to  FIGS. 9-11 , a second embodiment of a mechanical compression and vacuum release mechanism of the present invention is shown. Mechanical compression and vacuum release mechanism  60   b  includes several components which are identical or substantially identical to those of mechanical compression and vacuum release mechanism  60   a  of the first embodiment, and the same reference numerals have been used to identify identical or substantially identical components therebetween. In addition, except as described below with respect to  FIGS. 9-11 , the operation of mechanical compression and vacuum release mechanism  60   b  of the second embodiment is substantially similar to that of mechanical compression and release mechanism  60   a  of the first embodiment described above with reference to  FIGS. 1, 2 ,  5 ,  6 , and  8 .  
         [0042]     Referring to  FIG. 9 , flyweight  64  is pivotally mounted to camshaft gear  28  and generally includes body portion  66 , head portion  68 , and extension portion  70 . Head portion  68  includes a vacuum release pin  78  extending substantially parallel to camshaft  30  and closely yet rotatably fitted within a bore  80  in hub  62 . Extension portion  70  extends from head portion  68  and is engaged by one end of rod-linkage member  120 . Rod-linkage member  120  is pivotally mounted in aperture  122  located near end  124  of flyweight extension portion  70 . Mechanical compression and vacuum release mechanism  60   b  also includes compression release lever  84  having compression release pin  88  that includes coupling portion  92  extending orthogonally from compression release pin  88 . Release lever  84  is engaged by the opposite end of rod-linkage member  120  to operably couple flyweight  64  and compression release lever  84 . The end of rod-linkage member  120  is pivotally mounted in aperture  126  position near end  128  of compression release lever  84 .  
         [0043]     Flyweight  64  has a start position shown in  FIG. 10  and an operating position shown in  FIG. 11 , in which vacuum and compression release pins  78  and  88  are rotatably disposed within bores  80  and  90  of hub  62  such that vacuum and compression release cams  102  and  104  each extend beyond the base circle of exhaust cam lobe  34 , as best shown in  FIGS. 5 and 6 . After engine  10  starts, flyweight  64  is urged against the bias of spring  106  centrifugally outwardly to the position shown in  FIG. 11 . As flyweight  64  moves centrifugally outwardly, vacuum release pin  78  is rotated along with flyweight  64 , and compression release pin  88  is rotated concurrently with vacuum release pin  78  via the rod-linkage engagement of linkage member  120  with flyweight extension portion  70  and compression release lever  84  to the positions shown in  FIG. 8 , in which the flat surfaces of vacuum and compression release cams  102  and  104  are oriented such that same do not extend beyond the base circle of exhaust cam lobe  34 .  
         [0044]     Referring to  FIGS. 12-14 , a third embodiment of a mechanical compression and vacuum release mechanism of the present invention is shown. Mechanical compression and vacuum release mechanism  60   c  includes several components which are identical or substantially identical to those of mechanical compression and vacuum release mechanisms  60   a  and  60   b  of the first and second embodiments, and the same reference numerals have been used to identify identical or substantially identical components therebetween. In addition, except as described below with respect to  FIGS. 12-14 , it is understood that the operation of mechanical compression and vacuum release mechanism  60   c  of the third embodiment is substantially similar to that of mechanical compression and release mechanisms  60   a  and  60   b  of the first and second embodiments described above with reference to  FIGS. 1, 2 ,  5 ,  6 , and  8 .  
         [0045]     Referring to  FIG. 12  and as with the previously described embodiments of mechanical compression and vacuum release mechanisms  60   a  and  60   b , flyweight  64  is pivotally mounted to camshaft gear  28  and generally includes body portion  66 , head portion  68 , and extension portion  70 . Head portion  68  includes a vacuum release pin  78  extending substantially parallel to camshaft  30  and closely yet rotatably fitted within a bore  80  in hub  62 . Mechanical compression and vacuum release mechanism  60   c  also includes compression release lever  84  having compression release pin  88  that includes coupling portion  92  extending orthogonally from compression release pin  88 . Extension portion  70  of flyweight  64  extends from head portion  68  and abuttingly and slidably engages longitudinal side surface  130  of compression release lever  84  to operably couple flyweight  64  and lever  84 .  
         [0046]     Flyweight  64  has a start position shown in  FIG. 13  and an operating position shown in  FIG. 14 , in which vacuum and compression release pins  78  and  88  are rotatably oriented within bores  80  and  90  of hub  62  such that vacuum and compression release cams  102  and  104  each extend beyond the base circle of exhaust cam lobe  34 , as best shown in  FIGS. 5 and 6 . In the start position shown in  FIG. 13 , compression release lever  84  is normally positioned by a spring (not shown) similar to spring  106 , in the position shown, in which the radially outward portion thereof abuts extension portion  70  of flyweight  64 . After engine  10  starts, flyweight  64  is urged against the bias of spring  106  centrifugally outwardly to the position shown in  FIG. 14 . As flyweight  64  moves centrifugally outwardly, vacuum release pin  78  is rotated along with flyweight  64 , and compression release pin  88  is rotated concurrently with vacuum release pin  78  via the abutting relationship between flyweight extension portion  70  and compression release lever  84  to the positions shown in  FIG. 8 , in which the flat surfaces of vacuum and compression release cams  102  and  104  are oriented such that same do not extend beyond the base circle of exhaust cam lobe  34 . The abutting engagement between flyweight  64  and compression release lever  84  allow flyweight extension portion  70  to slide along lever surface  130  facilitating rotation of compression release pin  88 .  
         [0047]     Referring to  FIGS. 15-17 , a fourth embodiment of a mechanical compression and vacuum release mechanism of the present invention is shown. Mechanical compression and vacuum release mechanism  140  includes a number of components which are identical or substantially identical to those of the mechanical compression and vacuum release mechanisms  60   a ,  60   b , and  60   c  of the first, second, and third embodiments, respectively, described above with reference to  FIGS. 1, 2 ,  5 ,  6 , and  8 , and the same reference numerals have been used to identify identical or substantially identical components therebetween.  
         [0048]     Compression and vacuum release mechanism  140  includes hub  62  preferably formed as an integral portion with camshaft gear  28 , and which extends therefrom on opposite sides of camshaft gear  28  as shown in  FIGS. 2 and 15 . Referring to  FIG. 15 , flyweight  142  is pivotally mounted to camshaft gear  28  and generally includes body portion  144  and extension portion  146 . Body portion  144  comprises most of the mass of flyweight  142  and includes radial inner surface  148  and radial outer surface  150  having stop projection  152 . Body portion  144  includes a first actuation pin  156  fixedly mounted thereto. Extension portion  146  extends from body portion  144  and includes a second actuation pin  154  fixedly mounted thereto.  
         [0049]     Mechanical compression and vacuum release mechanism  140  also includes vacuum release lever  158 , including vacuum release pin  160  extending substantially parallel to camshaft  30  and closely yet rotatably fitted within a bore  80  in hub  62 . Mechanism  140  also includes compression release lever  162 , including compression release pin  164  extending rotatably through bore  90  in hub  62  via a close fit and aligned substantially parallel to camshaft  30 . Vacuum and compression release levers  158  and  162  each include coupling portion  166  extending orthogonally from vacuum and compression release pins  160  and  164 . Slot  168  is formed in each coupling portion  166  in which actuation pins  154  and  156  of flyweight  142  are slidably received to operably couple flyweight  142  and vacuum and compression release levers  158  and  162 . Referring to  FIGS. 15-17 , hub  62  includes recesses  96  and  98  to accommodate vacuum and compression release pins  160  and  164 , respectively. As with previous embodiments and as shown in  FIG. 2 , exhaust cam lobe  34  includes recess  100  in which vacuum and compression release cams  102  and  104 , located at the ends of vacuum and compression release pins  160  and  164 , respectively, are disposed.  
         [0050]     Referring to  FIG. 15 , a tension spring  170  includes coil portion  172  mounted to camshaft gear  28  by fastener  174 , such as a rivet or screw, for example, and also includes first arm  176  having coil end  178  in engagement with flyweight  142 , and second arm  180 , or reaction arm, in abutting engagement with hub  62  of camshaft gear  28 . Spring  170  normally biases flyweight  142  to the start position shown in  FIG. 16 , in which inner radial surface  148  of flyweight  142  abuts hub  62  of compression and vacuum release mechanism  140 .  
         [0051]     With reference to  FIGS. 5, 6 ,  16 , and  17 , operation of compression vacuum release mechanism  140  will now be described. Compression and vacuum release mechanism  140  is shown in a first or start position in  FIGS. 5, 6 , and  16 , which corresponds to engine  10  being stopped or to engine  10  being cranked for starting during which a minimal amount of centrifugal force is imposed upon camshaft  30 , camshaft gear  28 , and mechanical compression and vacuum release mechanism  140 . As shown in  FIG. 16 , in the start position, spring  170  biases flyweight  142  towards a radially inward position in which inner radial surface  148  of flyweight  142  abuts hub  62 , and vacuum and compression release pins  160  and  164  are rotatably oriented within bores  80  and  90  of hub  62  such that vacuum and compression release cams  102  and  104  each extend beyond the base circle of exhaust cam lobe  34 , as best shown in  FIGS. 5 and 6 . In this position, upon cranking of engine  10 , vacuum and compression release cams  102  and  104  will each contact surface  42  of cam follower  38  of exhaust valve  26  to slightly open exhaust valve  26  as piston  14  is retreating from, and extending toward, its TDC position, respectively, in order to vent combustion chamber  20 . In this manner, engine  10  may be more easily cranked for starting.  
         [0052]     After engine  10  starts and the rotational speed of camshaft  30  and camshaft gear  28  rapidly increases, a much greater amount of centrifugal force is imposed upon flyweight  142 , thereby urging flyweight  142  against the bias of spring  170  centrifugally outwardly in the direction of arrow  182  ( FIG. 16 ) to the position shown in  FIGS. 15 and 17 , in which radial outer surface  150  is disposed adjacent rim  118  of camshaft gear  28  and stop projection  152  of flyweight  142  is in engagement with rim  118 . During rotation of flyweight  142 , actuation pins  154  and  156  slide within slots  168  in the directions of arrows  184  and  186  of  FIG. 16 , respectively. In this position, vacuum release pin  160  and compression release pin  164  are rotated concurrently along with flyweight  142  via the sliding engagement of actuation pins  154  and  156  of flyweight  142  within slots  168  of vacuum and compression release levers  158  and  162 , respectively, to the positions shown in  FIG. 8 , in which the flat surfaces of vacuum and compression release cams  102  and  104  are oriented such that same do not extend beyond the base circle of exhaust cam lobe  34 . In this manner, the vacuum and compression release effects are terminated after engine  10  starts and, at engine running speeds, engine  10  operates according to a conventional four-stroke timing sequence.  
         [0053]     In alternate embodiments, the compression and vacuum release mechanisms  60   a ,  60   b , and  60   c  could be configured such that compression release pin  88  is formed as a portion of flyweight  64  and vacuum release pin is formed as a portion of lever  84 . Also, compression and vacuum release mechanisms  60   a ,  60   b ,  60   c , and  140  could be configured such that vacuum and compression release pins  78 ,  160  and  88 ,  164  are operably associated with the intake valve of engine  10 , or further, by varying the length of vacuum and compression release pins  78 ,  160  and  88 ,  164 , one pin could be associated with the exhaust valve and the other with the intake valve, if desired.  
         [0054]     While this invention has been described as having preferred designs, 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.