Compression release mechanism

Abstract

A compression release device for a small internal combustion engine, including a flyweight operatively coupled to a cam member via a cam pivot shaft, wherein a lever arm associated with the flyweight is longer than a lever arm associated with the cam member, such that a relatively small component of pivotal movement of the flyweight is translated into a relatively large component of rotation of the cam member. At engine cranking speeds, a spring biases the flyweight to a first position, such that a cam surface of the cam member extends beyond the base circle of the exhaust cam lobe to partially open the exhaust valve and allow release of pressure within the combustion chamber of the engine. At engine running speeds, the flyweight is moved under centrifugal force to a second position, rotating the cam member to a corresponding second position in which the cam surface is disposed within the base circle of the exhaust cam lobe, such that combustion may occur in a conventional manner.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under Title 35, U.S.C. §119(e) of U.S. Provisional Patent Application Serial No. 60/394,445, entitled COMPRESSION RELEASE MECHANISM, filed on Jul. 8, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to compression release mechanisms for small internal combustion engines of the type used in a variety of applications, such as lawnmowers, generators, pumps, tillers, pressure washers and other lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like.

[0004] 2. Description of the Related Art

[0005] Generally, the intake and exhaust valves of small internal combustion engines are actuated directly by a camshaft located in the cylinder head, or indirectly through the use of rocker arms, tappets, or other similar means. For example, in many existing L-head and overhead valve engines, the crankshaft drives a camshaft which is parallel to the crankshaft and located in the crankcase, and lobes on the camshaft actuate push rods and rocker arms to open and close the valves. One type of overhead cam engine, in which a camshaft in the cylinder head directly actuates the valves, is discussed in U.S. Pat. No. 6,295,959, assigned to the assignee of the present invention, the disclosure of which is expressly incorporated herein by reference.

[0006] At engine cranking speeds during engine starting, the intake and exhaust valves are both closed as the piston rises toward its top dead center position, and substantial pressure is built up in the combustion chamber which resists movement of the piston toward the top dead center position. This pressure must be overcome to crank the engine for starting, and typically requires a substantial amount of force to be exerted by the operator, such as by pulling on the rope of a recoil starter. Therefore, small internal combustion engines typically include a type of compression release mechanism to aid in engine starting.

[0007] Compression release mechanisms for small internal combustion engines are usually operable at cranking speeds to prevent the exhaust valve from fully closing as the piston reaches its top dead center position, thereby allowing venting of pressure from the combustion chamber. In this manner, cranking of the engine is much easier and requires less force to be exerted by the operator. When the engine reaches a predetermined speed after starting, the compression release mechanism is automatically rendered inoperative, such that the exhaust valve fully seats or closes as the piston approaches its top dead center position to allow combustion to proceed in a conventional manner.

[0008] A problem in many known compression release devices is that such devices incorporate a large number of parts, and are often mechanically complex. Further, such devices typically take up an undesirably large amount of space around the camshaft of the engine.

[0009] What is needed is a compression release device for small internal combustion engines which includes a relatively few number of parts, is durable, and which is compact in construction.

SUMMARY OF THE INVENTION

[0010] The present invention provides a compression release device for a small internal combustion engine, including a flyweight operatively coupled to a cam member via a cam pivot shaft, wherein a lever arm associated with the flyweight is longer than a lever arm associated with the cam member, such that a relatively small component of pivotal movement of the flyweight is translated into a relatively large component of rotation of the cam member. At engine cranking speeds, a spring biases the flyweight to a first position, such that a cam surface of the cam member extends beyond the base circle of the exhaust cam lobe to partially open the exhaust valve and allow release of pressure within the combustion chamber of the engine. At engine running speeds, the flyweight is moved under centrifugal force to a second position, rotating the cam member to a corresponding second position in which the cam surface is disposed within the base circle of the exhaust cam lobe, such that combustion may occur in a conventional manner.

[0011] Advantageously, the flyweight has a thin profile, reducing the overall size of the compression release device. Further, the construction of the compression release mechanism allows a relatively small pivotal movement of the flyweight to be transferred into a relatively large rotational movement of the cam member, thereby reducing the amount of space around the camshaft in which the flyweight must operate to actuate the compression release mechanism.

[0012] Further, the body of the cam member is closely received for rotation within a recess in a support hub of the camshaft, such that forces from the contact between the cam surface of the cam member and the exhaust valve are transferred directly through the cam member to the support hub, and are not transferred to the other moving parts of the compression release mechanism, thereby increasing the operational life of the compression release mechanism.

[0013] A pivot shaft of the cam member is closely received within a slot of the flyweight such that, in each of the first and second positions of the flyweight, the pivot shaft is positively retained by the flyweight. Therefore, rotational movement of the cam member independent of corresponding pivotal movement of the flyweight is prevented, such that pivotal movement of the flyweight is accurately transferred to corresponding rotational movement of cam member.

[0014] In one form thereof, the present invention provides an internal combustion engine, including a camshaft having at least one cam lobe, the cam lobe including a portion projecting beyond a base circle of the cam lobe for periodically engaging a valve; and a compression release mechanism, including a flyweight having a first end attached at a flyweight pivot to the camshaft, and a second end opposite the first end, the flyweight pivotally movable between first and second positions; a cam member rotatable with respect to the camshaft, the cam member having a body portion with an axis of rotation and a head portion eccentric to the body portion axis, the head portion coupled to the second end of the flyweight, wherein a first lever arm defined between the flyweight pivot and the head portion of the cam member is longer than a second lever arm defined between the body portion axis and the head portion of the cam member, such that a relatively small pivotal movement of the flyweight translates into a relatively large rotational movement of the cam member; and the cam member including a cam surface, the cam surface projecting outwardly of the base circle when the flyweight is in the first position to engage and at least partially open the valve, the cam surface not projecting outwardly of the base circle when the flyweight is in the second position and not engaging the valve.

[0015] In another form thereof, the present invention provides an internal combustion engine, including a camshaft having at least one cam lobe, the cam lobe including a portion projecting beyond a base circle of the cam lobe for periodically engaging a valve; and a compression release mechanism, including a flyweight attached at a flyweight pivot to the camshaft for pivotal movement between first and second positions, the flyweight including an opening therein; a cam member rotatable with respect to the camshaft, the cam member having a body portion and a pivot shaft eccentric to the body portion and extending therefrom, the pivot shaft closely received within the flyweight opening, wherein a first lever arm defined between the flyweight pivot and the pivot shaft of the cam member is longer than a second lever arm defined between the body portion and the pivot shaft of the cam member, such that a relatively small pivotal movement of the flyweight translates into a relatively large rotational movement of the cam member; and the cam member including a cam surface, the cam surface projecting outwardly of the base circle when the flyweight is in the first position to engage and at least partially open the valve, the cam surface not projecting outwardly of the base circle when the flyweight is in the second position and not engaging the valve.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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:

[0017] FIG. 1 is a perspective cutaway view of an overhead cam engine, showing the crankshaft, timing shaft with timing gear and timing pulley, timing belt, camshaft and camshaft pulley, with the crankcase, cylinder block and cylinder head partially shown;

[0018] FIG. 2 is a longitudinal sectional view of the engine of FIG. 1;

[0019] FIG. 3 is an elevational, partially cut away view of the camshaft, showing the compression release mechanism of the present invention in a first operational position in which the exhaust valve is partially opened;

[0020] FIG. 4 is a elevational, partially cut away view of the camshaft, showing the compression release mechanism of FIG. 3 in a second operational position in which the exhaust valve is allowed to fully close;

[0021] FIG. 5A is a perspective, exploded view of the compression release mechanism of FIGS. 3 and 4;

[0022] FIG. 5B is a top view of the flyweight of the compression release mechanism of FIG. 5A viewed along line 5B-5B of FIG. 5A;

[0023] FIG. 6 is an end view of the compression release mechanism in the first operational position of FIG. 3; and

[0024] FIG. 7 is an end view of the compression release mechanism in the second operational position of FIG. 4.

[0025] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

[0026] Referring to FIGS. 1 and 2, part of the drive train 10 of an overhead cam (“OHC”) engine, which can be either a vertical crankshaft or horizontal crankshaft engine, is shown. The engine is described in further detail in the above-incorporated U.S. Pat. No. 6,295,959. The present compression release mechanism 70, described further below, may be used with such overhead cam engines, or alternatively, may also be used in other types of small internal combustion engines, such as overhead valve (“OHV”) engines and side valve or L-head engines.

[0027] In the engine shown in FIGS. 1 and 2, drive train 10 is generally enclosed within crankcase 8, mounting flange or crankcase base 14, cylinder block 16, cylinder head 18, and cylinder head cover 19. Crankcase 8 generally comprises crankcase base 14 and crankcase upper casing 12, where the upper portion of casing 12 defines a first side 12a of crankcase 8, and the lower portion of casing 12 and crankcase base 14 defines a second side 12b of crankcase 8 opposite the first side of crankcase 8. Crankcase casing 12 includes upper crankshaft bearing 20 and upper timing shaft bearing 24. Crankcase base 14 includes lower crankshaft bearing 22, lower timing shaft bearing 26, mounting flange 23, and oil sump 28. Crankcase casing 12 and crankcase base 14 are attached to one another in a conventional manner.

[0028] Journals 32a and 32b of crankshaft 32 are rotatably carried in upper and lower crankshaft bearings 20 and 22, respectively, and crankshaft 32 is disposed along an axis L1-L1. Piston 25 (FIG. 2) is slidably received in cylinder bore 15 within cylinder block 16 along axis L2-L2 perpendicular to crankshaft axis L1-L1. Cylinder block 16 has integral supports 43 for mounting an electric ignition module (not shown) thereon, and integral cooling fins 34 for dissipating heat. As shown in FIG. 2, connecting rod 37 is rotatably connected to piston 25 by wrist pin 39, and is also rotatably connected to crankshaft 32 between throws 36 in a conventional manner.

[0029] Drive gear 38, secured to crankshaft 32 between upper and lower crankshaft bearings 20 and 22 and disposed in the second or lower side 12b of crankcase 8, is driven by crankshaft 32, and drive gear 38 drives timing gear 40, which is twice the diameter of drive gear 38. Timing gear 40 is secured to timing shaft 42, which is rotatably carried in upper and lower timing shaft bearings 24 and 26, respectively, and extends substantially completely across crankcase 8. Timing gear 40 is disposed in the lower side of crankcase 8. Therefore, crankshaft 32 is directly rotatably coupled to timing shaft 42, via drive gear 38 and timing gear 40, at a location in crankcase base 14, and crankshaft 32 drives timing shaft 42 at half the speed of crankshaft 32 through a gear set including drive gear 38 and timing gear 40.

[0030] Timing shaft 42 is disposed parallel to crankshaft axis L1-L1, and extends externally out of crankcase first or upper side 12a at one end, on which is secured a drive member in the form of toothed timing shaft pulley 44 held in place by snap ring 45 (FIG. 2). Timing shaft pulley 44 drives toothed timing belt 46 and toothed camshaft pulley 48 (FIG. 2) which is secured to an end of camshaft 50 which extends externally of cylinder head 18. Alternatively, other endless loop drives can be employed, such as a chain and sprocket mechanism (not shown). Belt guard 52 substantially covers timing belt 46, and is fixed to cylinder head 18 and crankcase casing 12. As shown in FIG. 1, a portion of timing belt 46 around timing shaft pulley 44 is not covered by belt guard 52 but rather is exposed.

[0031] Camshaft 50 is carried in upper and lower camshaft bearings 54 and 56, respectively, within cylinder head 18, and is disposed along an axis substantially parallel to crankshaft axis L1-L1. Camshaft 50 has spaced intake and exhaust cam lobes 58a, 58b, which periodically actuate tappets 60 as camshaft 50 rotates. Tappets 60 are connected to intake and exhaust valves 62a, 62b extending through valve guides 64 within the cylinder head 18. Valves 62a, 62b seat against valve seats 66 which are press-fitted into cylinder head 18.

[0032] As piston 25 reciprocates, crankshaft 32 drives timing gear 40 at half crankshaft speed, which in turn drives timing shaft 42, timing pulley 44, timing belt 46, camshaft pulley 48, and camshaft 50 at a rotational speed equal to timing gear 40. Rotational camshaft 50 rotates intake and exhaust lobes 58a, 58b, which engage tappets 60 to actuate intake and exhaust valves 62a, 62b in a conventional manner.

[0033] Referring first to FIG. 5A, the components of compression release mechanism 70 will now be described. Camshaft 50 includes support hub 72 disposed adjacent exhaust lobe 58b, and support hub includes curved recess 74. Disposed adjacent support hub 72 is collar 76, which includes cam member hole 78 therethrough in alignment with recess 74, and which also includes pivot shaft aperture 80 and spring aperture 82. Support hub 84 is disposed adjacent collar 76, and includes stop ridge 86 projecting therefrom. Exhaust lobe 58b, support hub 72, collar 76, and support hub 84 may each comprise separate pieces secured individually to camshaft 50. Alternatively, each of the foregoing components may be integrally formed into a single component such as a rigid plastic, for example, which is secured to camshaft 50 in a suitable manner.

[0034] Cam member 90 generally includes body portion 92 and head portion 94. Body portion 92 includes cam surface 96 and flat surface 98, and head 94 includes cam pivot shaft 100 extending therefrom. As may be seen in FIG. 5A, the longitudinal axis A2 of cam pivot shaft 100 is spaced from, and therefore is eccentric with respect to, the longitudinal axis Al of body portion 92 of cam member 90.

[0035] Flyweight 102 is curved in shape to generally complement the outer surface of camshaft 50, and includes first end 102a disposed proximate a first side of camshaft 50, and second end 102b opposite first end 102a which is disposed proximate a second side of camshaft 50. First end 102a of flyweight 102 includes pivot shaft aperture 104 for receipt of flyweight pivot shaft 106 therein. Flyweight pivot shaft 106 is also received within pivot shaft aperture 80 in collar 76 to pivotally attach flyweight 102 to collar 76. Alternatively, flyweight pivot shaft 106 may be integrally formed with either flyweight 102 or collar 76.

[0036] Flyweight 102 includes first side 103a which is disposed along a plane which is perpendicular to camshaft 50, and second side 103b opposite first side 103a which, as shown in FIG. 5B, is disposed along a plane which is slightly oblique with respect to camshaft 50. In this manner, the thickness of flyweight 102 decreases, or tapers, from a maximum thickness at first end 102a of flyweight 102 to a minimum thickness at second end 102b of flyweight 102. With further reference to FIG. 3, a clearance area 105 is defined between second end 102b of flyweight 102 and collar 76. When cam member 90 is inserted through cam member hole 78 of collar 76, head portion 94 of cam member 90 is disposed within clearance area 105, and cam pivot shaft 100 of cam member 90 is closely received within open-ended slot 110 in flyweight 102. Flyweight 102 further includes finger 116 for engaging stop ridge 86 to limit the extent of rotational movement of flyweight 102 with respect to support hub 84, as described below.

[0037] Spring 112 includes opposite ends which are respectively received within spring aperture 114 in flyweight 102 and within spring aperture 82 of collar 76. Washer 118 is received about camshaft 50, and with reference to FIG. 2, it may be seen that washer 118 is sandwiched between flyweight 102 and camshaft bearing 54.

[0038] As shown in FIGS. 1 and 2 and described below, compression release mechanism 70 is disposed on camshaft 50 adjacent exhaust cam lobe 58b such that compression release mechanism 70 acts upon exhaust valve 62b to vent pressure from the combustion chamber of engine 10 at cranking speeds. However, it should be understood that, with minor variations readily apparent to one of ordinary skill in the art, compression release mechanism 70 may also be disposed on camshaft 50 adjacent intake cam lobe 58a such that compression release mechanism 70 acts upon intake valve 62a to vent pressure from the combustion chamber of engine 10 at cranking speeds.

[0039] In operation, and referring additionally to FIGS. 3 and 6, at low speeds, such as engine cranking speeds, spring 112 biases flyweight 102 to a first rotational position in which cam member 90 is held in a corresponding first rotational position such that cam surface 96 extends above the base circle of exhaust cam lobe 58b. Cam surface 96 periodically engages tappet 60 of exhaust valve 62b to partially unseat exhaust valve 62b from valve seat 66 (FIG. 2), thereby partially opening exhaust valve 62b to allow venting of pressure within the engine combustion chamber as piston 25 approaches its top dead center position. In this manner, crankshaft 32 of the engine may be more easily rotated by an operator through the recoil starter mechanism of the engine (not shown), for example, to ease engine starting.

[0040] Referring to FIGS. 4 and 7, after the engine is started, the increased centrifugal force acting on flyweight 102 due to the increased speed of rotation of camshaft 50 causes flyweight 102 to rotate upon flyweight pivot shaft 106 outwardly of support hub 84, overcoming the bias of spring 112, until finger 116 of flyweight 102 engages stop ridge 86 to limit the extent of outward movement of flyweight 102. The foregoing rotation of flyweight 102 also rotationally translates cam pivot shaft 100 of cam member 90 within slot 110 of flyweight 102, thereby rotating cam member 90 such that flat 98 of cam 90 is exposed to tappet 60 of exhaust valve 62b and cam surface of cam member 90 is rotated within recess 74 of support hub 72. In this second position of compression release mechanism 70, flat 98 of cam member 90 does not extend outwardly of the base circle of exhaust cam lobe 58b, such that exhaust valve 62b is only opened when tappet 60 is engaged by the eccentric lobe portion of exhaust cam lobe 58b and combustion may proceed in a conventional manner.

[0041] Advantageously, as shown in FIGS. 3, 4, and 5B, flyweight 102 has a thin overall profile, and further, flyweight 102 has a particularly thin profile at second end 102b thereof, such that clearance area 105 is provided between flyweight 102 and collar 76 in which head portion 94 of cam member 90 is received, thereby reducing the overall width of compression release mechanism 70. Additionally, as shown in FIG. 3, body portion 92 of cam member 90 is closely supported for rotation within recess 74 of support hub 72. Therefore, in the first operational position of compression release mechanism 70 shown in FIG. 3, when tappet 60 of exhaust valve 62a engages cam surface 96 of cam member 90, the contact force is distributed directly through cam member 90 to support hub 72, and is not distributed through the other moving parts of compression release mechanism 70, such as flyweight 102 and spring 112. In this manner, the moving parts of compression release mechanism 70 are less prone to fatigue, increasing the operational life of compression release mechanism 70.

[0042] Also, pivot shaft 100 of cam member 90 is closely received within slot 110 of flyweight 102 such that, in each of the first and second positions of flyweight 102 shown in FIGS. 6 and 7, respectively, pivot shaft 100 is positively retained by flyweight 102 in the positions which are shown in FIGS. 6 and 7. Therefore, rotational movement of cam member 90 independent of corresponding pivotal movement of flyweight 102 is prevented, and pivotal movement of flyweight 102 is accurately and precisely transferred to corresponding rotational movement of cam member 90.

[0043] Referring to FIGS. 6 and 7, it may be seen that a relatively small component of rotational movement of flyweight 102 is transferred into a relatively large component of rotational movement of body portion 92 of cam member 90. Specifically, a first lever arm LA1-LA1 associated with flyweight 102, as defined between flyweight pivot shaft 106 and cam pivot shaft 100, is much longer than a second lever arm LA2-LA2 associated with cam member 90, as defined between cam pivot shaft 100 and the longitudinal axis Al of body portion 92 of cam member 90. As shown in FIGS. 6 and 7, the ratio of the length of the first lever arm to the length of the second lever arm is about 8:1; however, such ratio may be from about 10:1 to about 3:1 to enable a relatively small pivotal movement of flyweight 102 to translate into a relatively large rotational movement of cam member 90.

[0044] Further, as shown in FIGS. 6 and 7, when flyweight 102 moves from its first operational position (FIG. 6) to its second operational position (FIG. 7), cam pivot shaft 100 rotationally translates within slot 110 of flyweight 102. Further, flyweight 102 rotates through a relatively small angle &PHgr;, defined between the first and second positions of lever arm LA1-LA1, which may be about 5-10° for example, whereas cam member 90 rotates through a much greater angle of up to about 90°. Therefore, the overall extent of movement of flyweight 102 is minimized during operation of compression release mechanism 70, conserving space within the cylinder head of the engine.

[0045] While the present invention has been described as having a preferred design, 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 the invention pertains.

Claims

1. An internal combustion engine, comprising:

a camshaft having at least one cam lobe, said cam lobe including a portion projecting beyond a base circle of said cam lobe for periodically engaging a valve; and

a compression release mechanism, comprising:

a flyweight having a first end attached at a flyweight pivot to said camshaft, and a second end opposite said first end, said flyweight pivotally movable between first and second positions;

a cam member rotatable with respect to said camshaft, said cam member having a body portion with an axis of rotation and a head portion eccentric to said body portion axis, said head portion coupled to said second end of said flyweight, wherein a first lever arm defined between said flyweight pivot and said head portion of said cam member is longer than a second lever arm defined between said body portion axis and said head portion of said cam member, such that a relatively small pivotal movement of said flyweight translates into a relatively large rotational movement of said cam member; and

said cam member including a cam surface, said cam surface projecting outwardly of said base circle when said flyweight is in said first position to engage and at least partially open said valve, said cam surface not projecting outwardly of said base circle when said flyweight is in said second position and not engaging said valve.

2. The internal combustion engine of claim 1, wherein said first lever arm has a first length and said second lever arm has a second length, and wherein the ratio between said first length and said second length is between 10:1 and 3:1.

3. The internal combustion engine of claim 1, wherein said first lever arm has a first length and said second lever arm has a second length, and wherein the ratio between said first length and said second length is between 10:1 and 5:1.

4. The internal combustion engine of claim 1, wherein said first lever arm has a first length and said second lever arm has a second length, and wherein the ratio between said first length and said second length is between 10:1 and 8:1.

5. The internal combustion engine of claim 1, wherein said flyweight extends around said camshaft between opposite first and second sides of said camshaft, said flyweight pivot located proximate said camshaft first side and said second end of said flyweight located proximate said camshaft second side.

6. The internal combustion engine of claim 1, wherein said camshaft includes a recess within which said body portion of said cam member is closely supported for rotation, wherein contact forces between said valve and said cam surface of said cam member are transferred through said cam member to said camshaft when said flyweight is in said first position.

7. The internal combustion engine of claim 1, wherein said body portion of said cam member includes a first longitudinal axis and said head portion of said cam member includes a pivot shaft coupled to said flyweight, said pivot shaft having a second longitudinal axis eccentric to said first longitudinal axis.

8. The internal combustion engine of claim 7, wherein said pivot shaft is positively retained by said flyweight in each of said first and second flyweight positions.

9. The internal combustion engine of claim 1, further comprising a spring connecting said camshaft and said flyweight, said spring biasing said flyweight to said first flyweight position.

10. The internal combustion engine of claim 1, wherein said camshaft includes a plate disposed perpendicular to said camshaft and rotatable therewith, said plate including an aperture through which said cam member is received.

11. The internal combustion engine of claim 10, wherein said flyweight includes a side surface facing said plate, said side surface spaced from said plate at said second flyweight end to provide a clearance space between said second flyweight end and said plate in which said head portion of said cam member is disposed.

12. The internal combustion engine of claim 1, wherein said camshaft and said flyweight include stop surfaces engagable with one another in said second flyweight position to limit pivotal movement of said flyweight.

13. The internal combustion engine of claim 1, wherein said flyweight pivots between about 5 degrees and about 10 degrees between said first and second flyweight positions.

14. The internal combustion engine of claim 1, wherein said cam member rotates up to 90 degrees between said first and second flyweight positions.

15. An internal combustion engine, comprising:

a camshaft having at least one cam lobe, said cam lobe including a portion projecting beyond a base circle of said cam lobe for periodically engaging a valve; and

a compression release mechanism, comprising:

a flyweight attached at a flyweight pivot to said camshaft for pivotal movement between first and second positions, said flyweight including an opening therein;

a cam member rotatable with respect to said camshaft, said cam member having a body portion and a pivot shaft eccentric to said body portion and extending therefrom, said pivot shaft closely received within said flyweight opening, wherein a first lever arm defined between said flyweight pivot and said pivot shaft of said cam member is longer than a second lever arm defined between said body portion and said pivot shaft of said cam member, such that a relatively small pivotal movement of said flyweight translates into a relatively large rotational movement of said cam member; and

said cam member including a cam surface, said cam surface projecting outwardly of said base circle when said flyweight is in said first position to engage and at least partially open said valve, said cam surface not projecting outwardly of said base circle when said flyweight is in said second position and not engaging said valve.

16. The internal combustion engine of claim 15, wherein said pivot shaft is positively retained within said opening of said flyweight in each of said first and second flyweight positions.

17. The internal combustion engine of claim 15, wherein said flyweight includes a first end at which said flyweight is attached via said flyweight pivot to said camshaft, said flyweight further including a second end opposite said first end in which said opening is disposed.

18. The internal combustion engine of claim 15, wherein said body portion of said cam member includes a first longitudinal axis, and said pivot shaft includes a second longitudinal axis eccentric to said first longitudinal axis.

19. The internal combustion engine of claim 15, wherein said opening in said flyweight comprises an open-ended slot, said pivot shaft closely received within said openended slot.

20. The internal combustion engine of claim 15, wherein said first lever arm has a first length and said second lever arm has a second length, and wherein the ratio between said first length and said second length is between 10:1 and 3:1.

21. The internal combustion engine of claim 15, wherein said first lever arm has a first length and said second lever arm has a second length, and wherein the ratio between said first length and said second length is between 10:1 and 5:1.

22. The internal combustion engine of claim 15, wherein said first lever arm has a first length and said second lever arm has a second length, and wherein the ratio between said first length and said second length is between 10:1 and 8:1.

23. The internal combustion engine of claim 15, wherein said camshaft includes a recess within which said body portion of said cam member is closely supported for rotation, wherein contact forces between said valve and said cam surface of said cam member are transferred through said cam member to said camshaft when said flyweight is in said first position.

24. The internal combustion engine of claim 15, further comprising a spring connecting said camshaft and said flyweight, said spring biasing said flyweight to said first flyweight position.

25. The internal combustion engine of claim 15, wherein said camshaft includes a plate disposed perpendicular to said camshaft and rotatable therewith, said plate including an aperture through which said cam member is received.

26. The internal combustion engine of claim 25, wherein said flyweight includes a side surface facing said plate, said side surface spaced from said plate at said second flyweight end to provide a clearance space between said second flyweight end and said plate in which said cam member is at least partially disposed.

27. The internal combustion engine of claim 15, wherein said camshaft and said flyweight include stop surfaces engagable with one another in said second flyweight position to limit pivotal movement of said flyweight.