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.

What is claimed is:  
     
         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.