Abstract:
A four-stroke internal combustion engine includes a cylinder block having a cylinder therein and a piston reciprocally disposed within the cylinder. The piston is operably engaged with a crankshaft. At least one intake valve and one exhaust valve is reciprocally driven by a camshaft. A vacuum release mechanism includes an operating member rotationally supported by the camshaft and has an operator disposed thereon. A centrifugally actuated flyweight member is attached to the operating member, wherein rotation of the camshaft above engine cranking speeds causes the flyweight member to rotate the operating member. A vacuum release member is reciprocally supported by the camshaft and in engagement with the operator wherein rotational movement of the operating member causes radial translation of the vacuum release member through the operator. The operating member and flyweight member are urged to a first position at engine cranking speeds and rotated by the flyweight member through centrifugal force to a second position at engine running speeds. The vacuum release member is in lifting engagement with one of the valves at the first position during a portion of the power stroke and out of lifting engagement with the valve at the second position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to internal combustion engines, and more particularly to a compression release and vacuum release mechanism for four-stoke cycle engines. 
     2. Description of the Related Art 
     Compression release mechanisms for four-stroke cycle engines are well known in the art. Generally, means are provided to hold one of the valves in the combustion chamber of the cylinder head slightly open during the compression stroke while cranking the engine. 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 engine may achieve fall performance. It is normally advantageous 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 numerous and share a common principle which includes activating a valve displacement feature at low crankshaft speeds, i.e., at startup, and deactivating the same at significantly higher crankshaft speeds i.e., run mode. 
     Presently, conventional four-stoke engines require a significant amount of torque to turn the engine over during the power stroke when combustion is not taking place. This is so because the piston is then moving downwardly against a pressure difference due to increasing suction resulting from the partial discharge of gas from the cylinder 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 that pressure difference. 
     In response to the torque developed by suction, one prior art combustion engine suggests using a contoured cam lobe which acts to hold the valve open longer between the compression and power strokes. Starting torque was decreased by this mechanism, however compression and accordingly engine power would significantly decrease compared to conventional engines which employ the traditional “pear-shaped” cam lobes. Yet another prior art mechanism employed a light spring placed on the stem side of the exhaust valve to hold the valve open during start up. However, significant intake and exhaust manifold pressures would be required to close the exhaust valve and thus longer times and increased user effort is required to start the engine. 
     Another device which compensates for torque caused as a result of suction force during the power stroke is disclosed in provisional Patent Application No. 60/231,818, filed Sep. 11, 2000, and assigned to the assignee of the present application, the disclosure of which is expressly incorporated herein by reference. This device utilizes a saddle member pinned to an accessible area of the camshaft and includes a pair of auxiliary cams to sequentially relieve compression and vacuum by lifting the exhaust valve during appropriate portions of the compression and power stroke at engine cranking speeds. Although effective, this device is not readily adaptable to some existing engine designs. Traditionally used engine crankcase designs require casting and machining modifications before this release may be implemented. 
     Accordingly, it is desired to provide a release mechanism that addresses the significant torque developed by both the compression and power strokes and one that is effective in operation and relatively simple in construction. It is further desired to provide a release mechanism which addresses this significant torque, and is retrofittable to a substantial number of existing engine crankcases without significant modification to the engine. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of prior internal combustion engines by providing a mechanical compression and vacuum release, of simple construct, including an operating member rotationally supported by a camshaft and attached to a centrifugally activated flyweight wherein movement of the centrifugal flyweight causes radial translation of a vacuum release member through an operator attached to the operating shaft and the vacuum release member is in lifting engagement with one of the intake or exhaust valves. 
     A four-stroke internal combustion engine is provided and includes a cylinder block having a cylinder therein and a piston reciprocally disposed within the cylinder. The piston is operably engaged with a crankshaft. At least one intake valve and exhaust valve are reciprocally driven by a camshaft. A vacuum release mechanism includes an operating member rotationally supported by the camshaft and has an operator disposed thereon. A centrifugally actuated flyweight member is attached to the operating member, wherein rotation of the camshaft above engine cranking speeds causes the flyweight member to rotate the operating member. A vacuum release member is reciprocally supported by the camshaft and in engagement with the operator wherein rotational movement of the operating member causes radial translation of the vacuum release member through the operator. The operating member and flyweight are urged to a first position at engine cranking speeds and rotated by the flyweight member through centrifugal force to a second position at engine running speeds. The vacuum release member is in lifting engagement with one of the valves at the first position during a portion of the power stroke of the piston and out of lifting engagement with the valve at the second position. 
     The present invention further provides a compression release mechanism. A compression release member is attached to the operator and urged to radially extend in response to rotation of the operating member. The compression release member and the vacuum release member successively attain lifting engagement with an intake or exhaust valve at the first position. The lifting engagement of the compression release member coincides with at least a portion of the compression stroke and the lifting engagement of said vacuum release member coincides with at least a portion of the power stroke. The compression and vacuum release members are out of lifting engagement with the valve at the second position. 
     An object of the present invention is to provide an engine having a mechanical vacuum release mechanism that overcomes substantial operator or starter force caused by suction forces acting on the piston during the power stroke at engine cranking speeds. 
     Another object of the present invention is to provide a compression and vacuum release mechanism easily retrofittable with existing engines crankcases wherein the release mechanism is disposed within the profile of the existing camshaft assembly. These and other objects, advantages and features are accomplished according to the devices, assemblies and methods of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention 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 a sectional view of a single cylinder four-stroke internal combustion engine that incorporates a mechanical compression and vacuum release device in accordance with the principles of the present invention; 
     FIG. 2 is an exploded view of the camshaft and mechanical compression and vacuum release device of FIG. 1; 
     FIG. 3 is a perspective view of the camshaft and mechanical compression and vacuum release device of FIG. 1, illustrating the positioning of the mechanical compression and vacuum release device corresponding to engine startup; 
     FIG. 4 is a perspective view of the camshaft and mechanical compression and vacuum release device of FIG. 1, illustrating the positioning of the mechanical compression and vacuum release device corresponding to an engine run position; 
     FIG. 5A is a fragmentary sectional view of the engine shown in FIG. 1, illustrating the compression and vacuum release assembly in the startup position, depicting a compression release member in an extended position to relieve pressure formed in the cylinder; 
     FIG. 5B is a fragmentary sectional view of the engine shown in FIG. 1, illustrating the compression and vacuum release assembly in the startup position, depicting a vacuum release member in an extended position to relieve vacuum formed in the cylinder; 
     FIG. 6 is a fragmentary sectional view of the engine shown in FIG. 1, illustrating the compression and vacuum release assembly in the run position, depicting compression and vacuum release members recessed below the surface of the cam lobe; 
     FIG. 7 is fragmentary view of the operating shaft illustrated in FIG. 4, depicting the compression release member and the operator; and 
     FIG. 8 is an end view of the operating shaft of FIG. 7 viewed along line  8 — 8  of FIG.  7 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent an embodiment of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIG. 1, there is shown a single cylinder, four-stroke internal combustion engine  10  including a mechanical compression and vacuum release mechanism  12  according to the present invention. Engine  10  includes cylinder block  14 , crankshaft  16  and piston  18 , the piston being operatively connected to crankshaft  16  through connecting rod  20 . Piston  18  coacts with cylinder block  14  and cylinder head  22  to define combustion chamber  24 . Spark plug  26 , secured in cylinder head  22 , ignites the fuel/air mixture after it has been drawn into combustion chamber  24  through an intake valve (not shown) during the intake stroke and has been compressed during the compression stroke of piston  18 . The spark is normally timed to ignite the fuel/air mixture just before piston  18  completes its ascent on the compression stroke. The fuel/air mixture is drawn into combustion chamber  24  from the carburetor of the engine through an intake passage controlled by the intake valve, and the products of combustion are expelled from the cylinder during the exhaust stroke through exhaust port  28  controlled by poppet-type exhaust valve  30 . Although either exhaust or intake valve may be opened to vent compression and vacuum during start-up, it is recognized that preferably exhaust valve  30  cooperates with the compression and vacuum release mechanism  12  in a manner to be discussed hereinafter. 
     Other conventional parts of the valve operating mechanism include timing gear  32  mounted on crankshaft  16  for rotation therewith, and camshaft assembly  36  which includes lobed camshaft  35  and circular camshaft gear  34  rotatably driven by timing gear  32  to thereby rotate camshaft  35  at one-half crankshaft speed. Camshaft  35  comprises conventional pear-shaped exhaust and intake camshaft lobes  38  and  40 , respectively, (FIGS. 1 and 2) which rotate with camshaft  35  to impart reciprocating motion to the intake and exhaust valves via intake or cam follower (not shown) and exhaust cam follower  42 , respectively. Although FIG. 1 illustrates the compression and vacuum release mechanism in a side valve engine, this is but one engine type, and it is envisioned that the compression and vacuum release mechanism is amenable to other engine types, such as OHV and OHC engines, for example, and either vertical or horizontal shaft orientations. Additionally, multiple compression and vacuum releases according to the present invention may be employed on an engine having multiple cylinders, such as a twin cylinder engine, for example. 
     The exhaust valve train is shown in FIG.  1  and includes exhaust cam follower  42  having face  44  adapted to bear tangentially against, and remain in a continuous tracking relationship with, peripherally located bearing surface  46  of exhaust camshaft lobe  38 . Cam follower  42  slides in guide boss  48  of block  14 , and its upper end pushes against tip  50  of valve  30 . In operation, cam follower  42  lifts stem  52  of exhaust valve  30  which lifts face  54  of valve  30  from valve seat  56 . Valve spring  58  encircles stem  52  between valve guide  60  and spring retainer  62 . Spring  58  biases valve  30  closed and also biases cam follower  42  into tracking contact with surface  46  of exhaust lobe  38 . 
     Referring to FIGS. 2-3, camshaft assembly  36  includes annular camshaft gear  34  and elongate camshaft  35  extending axially through camshaft gear  34 . Camshaft  35  includes first end  64  axially extended from a lateral surface of camshaft gear  34  and second end  66  extended in a direction opposite to that of first end  64 . First and second ends  64 ,  66  of camshaft  35  are rotatably supported by engine block  14  through respective bearing assemblies, as is customary. Referring to FIG. 2, camshaft gear  34  and camshaft  35  are typically a single powder metal, forged, or injection molded component which has axis of rotation  68 . First end  64  of camshaft  35  includes the pear-shaped exhaust and intake lobes  38 ,  40 . Exhaust and intake lobes  38 ,  40  are provided with respective bearing surfaces  46 ,  70  which are in a continuously engaged relationship with respective followers (exhaust valve follower  42  shown in FIG.  1 ). Exhaust and intake lobes  38 ,  40  include axially extending through holes  72 ,  74 , radially aligned relative to one another and have respective diameters slightly larger than the diameter of operating shaft  76 , extending therethrough (FIG.  3 ). 
     Referring to FIG. 3, operating shaft  76  is rotatably supported by camshaft  35 . Particularly, first end  78  of operating shaft  76  extends through hole  72  of exhaust lobe  38  and second end  80  extends through intake lobe  40 . First end  78  of operating shaft  76  includes an operator in the form of a cylindrical eccentric  82  and radially extending compression relief projection  84 . Second end  80  of operating shaft  76  is attached to sickle-shaped centrifugal flyweight  86 . Centrifugal flyweight  86  includes cylindrical boss  88  which provides a base for engagement with second end  80  of operating shaft  76 . Second end  80  of operating shaft  76  may be fixed with boss  88  of flyweight  86  through an interference fit or crimping engagement, for example. As best illustrated in FIGS. 2 and 7, operating shaft  76  includes groove  90  which is engaged by retaining ring  92  to prevent excessive movement of operating shaft  76  along axis of rotation  77  in a direction moving away from camshaft gear  34  of camshaft assembly  36 . To prevent operating shaft  76  from excessive axial movement along axis  77  toward camshaft gear  34 , lateral surface  94  (FIG. 2) of compression release projection  84  abuts transverse face  96  of exhaust cam lobe  38 . End face  98  of camshaft  35  is provided with notch  100  to allow operating shaft  76  to be assembled with camshaft assembly  36 . 
     As best shown in FIG. 2, camshaft gear  34  of cam assembly  36  includes a dished recess  102  which encloses centrifugal flyweight  86 . Recess  102  includes side wall  104  and end wall  106 . Referring to FIG. 4, side wall  104  of recess  102  provides a rotational “stop” for operating shaft  76  by contact with outer surface  108  of centrifugal flyweight  86 . When the camshaft assembly  36  attains a significant rotational velocity, coinciding with the engine in a run position, outer surface  108  of centrifugal flyweight  86  contacts side wall  104  of recess  102 . At startup, as illustrated in FIG. 3, flyweight  86  includes an inner surface  110  which contacts outer surface  112  of camshaft  35  to provide a stop for the flyweight at rest. Therefore, it may be seen that mechanical compression and vacuum release  12  is substantially recessed into existing and surrounding structure provided by the camshaft assembly  36 . Consequently, many different engine types may be adapted with the mechanical compression and vacuum release  12  without altering current and proven engine structures. 
     Referring to FIGS. 2,  5 A and  5 B, outboard end  64  of camshaft  35  is fitted with vacuum release member or slider  114  to relieve suction forces acting on piston  18  (FIG. 1) as hereinafter described. First shaft end  64  includes a notched or stepped portion  116  formed in its periphery to facilitate engagement with complimentary stepped portion  118  of slider  114 . Slider  114  is L-shaped and includes a slot  120  located within an outer portion  122  of a first segment  124  of L-shaped slider  114 . Second segment  126  of L-shaped slider  114  includes vacuum release projection  128  outwardly extended from outer portion  130  of second segment  126 . Referring to FIG. 5B, stepped portion  118  includes step surfaces  132  and  134  of slider  114  in sliding engagement with respective step surfaces  136 ,  138  of camshaft  35 . Through step surfaces  132 ,  134  of slider  114 , it may be seen that slider  114  is reciprocally supported by step surfaces  136 ,  138  of camshaft  35 . Surface  140  of slider  114  is substantially perpendicular relative to step surfaces  132 ,  134  of slider  114  and engages complementary surface  142 , provided by stepped portion  116  of camshaft  35 , when the engine is in the run position (FIG.  6 ). 
     As best shown in FIGS. 3 and 4, eccentric  82  extends into slot  120  in slider  114 . Referring to FIG. 8, eccentric  82  is offset a distance “d” relative to axis of rotation  77  (FIG. 2) of operating shaft  76  such that centerline  144  (FIG. 7) of eccentric  82  “orbits” relative to axis of rotation  77  of operating shaft  76 . Referring to FIG. 5B, operating shaft  76  has been positioned by torsional spring  154  (FIG. 2) such that eccentric  82  has urged slider  114  radially outward. In this position, eccentric  82  is in contact with front edge  146  of slot  120  causing movement of slider  114  such that surfaces  140 ,  142  of respective camshaft  35  and slider  114  are parted (FIGS. 5A,  5 B). Conversely, and with particular reference to FIG. 6, counterclockwise rotation of operating shaft  76 , illustrated by arrow  148  in FIG. 8, causes eccentric  82  to contact rear edge  150  (FIG. 4) of slot  120  urging slider  114  toward axis of rotation  68  (FIG. 2) of camshaft  35 . Therefore, rotation of operating shaft  76  urged in a counterclockwise direction  148  by outwardly swinging flyweight  86  (FIG. 4) causes both compression and vacuum projections  84 ,  128  to recede beneath the bearing surface  46  of cam lobe  38 . Accordingly, rotation of operating shaft  76  urged in a clockwise direction, illustrated by arrow  152  in FIG. 8, by inwardly swinging flyweight  86  causes both compression and vacuum projections  84 ,  128  to extend beyond bearing surface  46  of cam lobe  38  in preparation for engagement with cam follower  42  (FIGS. 5A,  5 B). 
     As best illustrated in FIGS. 3 and 4, torsional spring  154  encircles the circumference of sleeve  88  of flyweight  86 . Spring  154  includes first leg  156  anchored to flyweight  86  and second leg (not shown) in contact with camshaft  35 . Spring  154  applies a bias to operating shaft  76 , to assist in returning compression and vacuum release projections  84 ,  128 , outwardly extended beyond surface  46  of lobe  38  as engine crankshaft speed, and associated camshaft speed, significantly slows corresponding to engine shutdown. At engine start-up, which corresponds with the mechanical compression and vacuum release  12  in positions depicted in FIGS. 3,  5 A and  5 B, flyweight  86  is in its retracted position and in contact with camshaft  35 . Compression release member  83  comprises projection  84 , located at first end  78  of operating shaft  76  and projects over bearing surface  46  of exhaust cam lobe  38  to interrupt the tracking relationship between follower  42  and cam lobe surface  46 . Referring to FIG. 5A as cam lobe  38  rotates, compression release projection  84  is shown as having displaced cam follower  42  relative to bearing surface  46  of cam lobe  38 . Consequently, face  54  of exhaust valve  30  is displaced relative to its seat  56  and the compressed air-fuel mixture in cylinder  24  (FIG.  1 ), during the compression stroke, is released. 
     Referring to FIG. 5B, subsequent to compression release projection  84  displacing valve  30  at engine startup, camshaft  35  continues to rotate and vacuum release projection  128  engages and displaces cam follower  42 . Vacuum release projection  128  is outwardly extended in response to eccentric  82  urging slider  114  away from axis of rotation of camshaft  68  (FIG.  3 ). Similar to the compression release projection  84  displacing cam follower  42 , vacuum release projection  128  displaces cam follower  42  and exhaust valve  30  is lifted from its seat  56  to alleviate the vacuum formed in the cylinder  24  during the power stroke. 
     Referring to FIG. 6, once camshaft  35  gains a significant rotational velocity, centrifugal flyweight  86  swings outwardly (FIG.  4 ). Consequently, operating shaft  76  rotates in a counterclockwise direction causing compression release projection  84  to pivot and recede beneath bearing surface  46  of lobe  38 . Contemporaneously, eccentric  82  moves in an upwardly and counterclockwise motion causing slider  114  to move inwardly and vacuum release projection  128 , affixed thereto, is accordingly receded beneath bearing surface  46  of cam lobe  38 . As the engine slows, prompting a decrease in camshaft velocity, torsion spring  154  (FIG. 2) urges flyweight to swing inwardly and projections  84 ,  128  move toward, and eventually beneath, bearing surface  46  of cam lobe  38 . 
     OPERATION 
     While device  12  is in its inoperative position (FIGS.  4  and  6 ), which is designated as the “run” position of the engine, the rotation of exhaust lobe  38  with camshaft  35  at “running speed” causes normal operation of valve  30 , so that valve  30  opens and closes in timed and periodic relation with the travel of piston  18  according to conventional engine timing practice. Thus, exhaust lobe  38  is adapted to open valve  30  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  38  continues to rotate, spring  58  forces cam follower  42  downwardly and valve  30  is reseated. Valve  30  is held closed during the ensuing intake, compression and power strokes. Intake camshaft lobe  40  is likewise of conventional fixed configuration to control the intake valve (not shown) 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. 
     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. However, by incorporating the compression and vacuum release mechanism 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 so that there is no compression loss to decrease the efficiency of the engine when it is running under its own power. 
     Compression and vacuum release mechanism  12  affects the lift of exhaust valve  30  relative to rotation of crankshaft  16  as hereinafter described. Referring to FIG. 1, a four-stroke cycle internal combustion engine  10  is shown and provides four strokes of piston  18  to complete a cycle of operation of the engine, coinciding with 720° of rotation of crankshaft  16 . On the intake stroke, piston  18  moves downwardly from the top of its travel (referred to as top dead center or TDC) to the bottom of its travel (referred to as bottom dead center or BDC). Intake valve (not shown) is opened and exhaust valve  30  is closed during the intake stroke. During the intake stroke, and at crankshaft running speed, a charge of air/fuel mixture is drawn into cylinder  24  above the head of piston  18  and through the intake valve (not shown). Following the intake stroke both the intake and exhaust valves close and the compression stroke is started. Toward the middle of the compression stroke, approximately 110° of crankshaft rotation before TDC, for example, mechanical compression release projection  84  lifts exhaust valve  30  to relieve cylinder pressure and then closes at about 60° before TDC. Following the compression stroke, piston  18  is urged toward BDC in the power stroke, which coincides with both intake and exhaust valves substantially closed. At approximately 60° of crankshaft rotation following TDC toward the end of the power stroke, vacuum release projection  128  lifts exhaust valve  30  off of its seat and suction forces due to vacuum formed in cylinder  24  is relieved. 
     For instance, in an exemplary embodiment of the compression and vacuum release  12 , the intake valve may have a lift of 0.2 inches during the intake stroke and exhaust valve may be lifted 0.03 inches, and held open for 50° of camshaft rotation, by mechanical compression release projection  84  during the compression stroke. Specifically, the mechanical compression release opens the exhaust valve  30  at a crankshaft rotation of 110° prior to TDC and holds open exhaust valve  30  until crankshaft  16  is approximately 60° from TDC. The vacuum release activated by vacuum release projection  128  opens exhaust valve  30  a distance of 0.02 inches at a crankshaft rotation of 60° after TDC to vent suction caused by cylinder vacuum during the power stroke. Thus, the energy of the compressed air/fuel mixture is used to assist moving the piston during the power stroke. Projection  128  holds open exhaust valve  30  at 60° after TDC for a duration of 50° of crankshaft rotation. 
     The disclosed embodiment is not intended to be exhaustive or limit the invention to the precise forms disclosed in the detailed description. While the present invention has been described as having an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. 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.