Compression relief mechanism

Abstract

An automatic compression relief mechanism for an internal combustion engine with a yoke pivotally mounted on a camshaft by a plunger extending transversely through the cam shaft and underlying an exhaust valve cam follower of the engine. The plunger is extended and retracted by pivotal movement of the yoke in response to spring and centrifugal forces acting thereon to partially open the exhaust valve and thereby reduce the cranking effort required to start the engine and to allow the exhaust valve to function in a conventional manner when the engine is running under its own power.

Description

This invention relates to internal combustion engines and more particularly to automatic compression relief during cranking for starting of internal combustion engines with cam operated cylinder valves.

Mechanisms for significantly reducing the cranking effort required to start an internal combustion engine by partially relieving the compression thereof during cranking are known. For example U.S. Campen Pat. No. 3,381,676, issued May 7, 1968 discloses an automatic compression relief mechanism for four-stroke single-cylinder engines in which a poppet-type exhaust valve is partially opened during the compression stroke only during starting of the engine by a plunger projecting radially outwardly of the exhaust cam to engage a follower operably associated with the exhaust valve. This plunger is retracted during running and projected during starting of the engine by a yieldably biased fly-weight received on the camshaft of the engine and responsive to centrifugal force acting thereon.

Objects of this invention are to provide an improved automatic compression relief mechanism of simplified design which can be economically manufactured and readily assembled and which has a long, maintenance free, useful life.

These and other objects, features and advantages of this invention will be apparent from the following detailed description and the accompanying drawings in which:

FIG. 1 is a fragmentary vertical section of a single cylinder four-stroke internal combustion engine embodying this invention.

FIG. 2 is an isometric view of the camshaft of the engine of FIG. 1 with the compression relief mechanism of this invention assembled thereon.

FIG. 3 is an exploded isometric view of the component parts of the compression relief mechanism and a portion of the camshaft on which they are assembled.

FIG. 4 is a fragmentary enlarged side view of some of the component parts of the engine of FIG. 1 showing in assembled relation the camshaft and compression relief mechanism associated with the valve push rods of the engine.

FIG. 5 is a semi-schematic view of the positions assumed by the component parts of the automatic compression relief mechanism when the engine is running and being cranked for starting in phantom and solid lines respectively.

FIG. 6 is a fragmentary view partially in section of the camshaft of the engine of FIG. 1 with a modified compression relief mechanism embodying this invention assembled thereon.

FIG. 7 is an exploded isometric view of the modified compression relief mechanism and a portion of the camshaft of FIG. 6.

FIG. 8 is a sectional view on line 8--8 of FIG. 6.

Since this invention is primarily concerned with, but not necessarily limited to, single cylinder four-stroke cycle internal combustion engines, the drawings illustrate the invention as embodied in such an engine. As is customary the engine shown in FIG. 1 has a cylinder 10, a crankshaft 12 and a piston 14 operatively connected with the crankshaft through a connecting rod 16. The piston coacts with the cylinder and a cylinder head 18 to define a combustion chamber 20. A spark plug 22 secured in the cylinder head ignites the fuel charge after it has been drawn into the combustion chamber during the intake stroke and then compressed during the compression stroke of the piston, the spark normally being timed to ignite the fuel charge just before the piston completes its ascent on the compression stroke. The fuel charge is drawn into the combustion chamber 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 an exhaust port 24 controlled by a poppet-type exhaust valve 26.

The conventional parts of the valve operating mechanism include a timing gear 27 mounted on crankshaft 12 for rotation therewith and a timing gear 28 mounted on a camshaft 30 and rotatably driven by gear 27 to thereby rotate the camshaft at one-half crankshaft speed. Camshaft 30 carries conventional pear-shaped intake and exhaust cams 32 and 34 (FIGS. 2 and 4) which rotate with the camshaft to impart reciprocating motion to the intake and exhaust valves via flat-footed push rods 36 and 38 respectively. The complete exhaust valve train is shown in FIG. 1 and includes push rod 38 which has a circular follower 40 with a flat underface 42 adapted to bear tangentially against and track upon the periphery 44 of cam 34. A stem 46 of the push rod slides in a guide boss 48 of crankcase 50 and butts its upper end against the stem 52 of exhaust valve 26. A valve spring 54 encircles stem 52 between a valve guide 56 and a spring retainer 58 carried on the stem, spring 54 biasing valve 26 closed and also biasing push rod 38 into tracking contact with cam 34.

The above-described engine and valve train parts are conventional, as is the fixed profile of exhaust cam 34 which consists of a base circle 60 and a lobe 62. When the compression relief mechanism described hereinafter is in its inoperative or run position, rotation of cam 34 with camshaft 30 causes normal operation of valve 26 so that it opens and closes in timed relation with the travel of the piston 14 according to conventional engine timing practice. Thus cam lobe 62 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. When follower 40 contacts base circle 60, spring 58 reseats valve 26 and holds the same closed during the ensuing intake, compression and power strokes. Intake cam 32 is likewise of conventional fixed configuration to control the intake valve such that it closes completely shortly after the piston begins its compression stroke and remains closed through the subsequent power and exhaust strokes, 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 compression stroke, cranking of the engine would be difficult unless some provision is made to vent the combustion chamber 20 during part or all of the compression stroke during cranking of the engine. However, by modifying a conventional engine to incorporate the improved compression relief mechanism in accordance with the present invention, compression relief is automatically obtained at cranking speeds to greatly reduce cranking effort and thereby facilitate starting. In addition, the 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.

As shown in FIGS. 2 and 4, compression relief mechanism 70 is received on camshaft 30 between exhaust cam 34 and an integral flange 72 which limits the axial displacement of relief mechanism 70 relative to camshaft 30. Relief mechanism 70 has a yoke 74 and a tension coil spring 76 received on a cylindrical portion 78 of camshaft 30, the yoke being retained thereon by a plunger 80 extending transversely through the cylindrical portion. Plunger 80 is slidably received in a bore 82 with a counterbore 84 adjacent one end thereof. Bore 82 extends radially through camshaft 30 closely adjacent exhaust cam 34 and underlies cam follower 40. Plunger 80 has a shank 85 with an enlarged head 86 at one end thereof and preferably a chamfer 87 on the other end thereof. To provide an adequate flow of lubricating oil through an axial lubricating passage 88 in camshaft 30, the diameter of shank 85 of plunger 80 should be substantially smaller than the diameter of axial bore 88.

Yoke 74 has a generally C-shaped retainer 90 mounted on camshaft 30 by plunger 80 for pivotal movement in a plane generally transverse to the axis of rotation of exhaust cam 62. Retainer 90 is preferably a one piece sheet metal stamping with an integral generally axially extending flange 92 thereon with a tang adjacent one end thereof having a slot 94 therethrough forming a pair of circumferentially spaced tabs 96 and 96'. As shown in FIG. 3, prior to assembly, tab 96 extends generally radially outwardly relative to the axis of the camshaft and in assembly is bent to extend generally axially with plunger 80 received in slot 94 between tabs 96 and 96' to pivotally mount yoke 74 on camshaft 30. Adjacent the other end of retainer 90, flange 92 forms axially extending generally circumferentially and radially spaced seats 98 and 100 for receiving the head 86 of plunger 80 thereon with a cam surface 102 therebetween for axially shifting plunger 80. The extent of pivotal movement of retainer 90 is limited by a generally radially extending tab 104 and an abutment portion 106 of flange 92. The axial movement of retainer 90 on camshaft 30 is limited by tangs 107 and tabs 96, 96' on flange 92, which abut on an adjacent side face of exhaust cam 34, and the distal side face of the retainer which abuts flange 72 of the camshaft. The free ends of retainer 90 have holes 108 therethrough for connection of coil spring 76 thereto. Yoke 74 has a fly-weight provided by two crescent shaped plates 110 and 112 which are preferably sheet metal stampings located on retainer 90 by projections 114 and brazed thereto.

Preferably, compression relief mechanism 70 is subassembled on camshaft 30 prior to assembly thereof into the engine. Plunger 80 is inserted into bore 82 in the camshaft and then the open throat of C-shaped retainer 90 of yoke 74 with tab 96 thereof extending radially (FIG. 3) is slipped generally radially or transversely over cylindrical portion 78 of the camshaft. In slipping retainer 90 over the camshaft, seat 100 is first lapped with head 86 of the plunger and then the retainer with tab 104 engaging the far side of head 86 is pivoted into position so the plunger bears on tab 96'. Tab 96 is then bent (to the position shown in phantom in FIG. 3) to extend axially to retain the plunger in slot 94 between the tabs. As thus assembled, retainer 90 is mounted on camshaft 30 for limited pivotal movement by the cooperation of plunger 80 therewith and the plunger is entrapped in the camshaft by the retainer. The hooks on the free ends of tension spring 76 are inserted into holes 108 in the free ends of retainer 90 so that, in assembly, the spring extends generally circumferentially between the free ends of the retainer around cylindrical portion 78 of camshaft 30 generally opposite the retainer.

In operation of compression relief mechanism 70, yoke 74 is retained by spring 76 immediately adjacent cylindrical portion 78 in the position shown in solid lines in FIG. 5 when the engine is rotated at a relatively low cranking speed which is usually less than 600 revolutions per minute. In this position, the head 86 of plunger 80 bears on seat 98 of yoke 74 and the other end of the plunger projects radially outward of base circle 60 of exhaust cam 34 to partially open exhaust valve 26 when it would normally be closed if the engine were running. Thus, the free end of plunger 80 will bear on the underface 42 of follower 40 and lift it a predetermined distance (which preferably is a fraction of the full or maximum lift provided by cam 34) thereby lifting exhaust valve 26 off its seat for a portion of each compression stroke to relieve compression during starting as long as the engine speed does not exceed a predetermined maximum cranking speed.

As soon as the engine starts and obtains a running speed under its own power the rotational speed of camshaft 30 increases above a predetermined maximum cranking speed and centrifugal force acting on yoke 74 (including the flyweight thereof) overcomes the bias of spring 76 and pivots the yoke clockwise (as viewed in FIG. 5) so that the head 86 of plunger 80 can bear on seat 100, as shown in phantom in FIG. 5. When the plunger 80 assumes the position shown in phantom in FIG. 5, its free end is retracted so that it does not project beyond base circle 60 of cam 34 and thus valve 26 is not partially opened by the plunger and functions in the conventional manner when the engine is running under its own power. The point at which yoke 74 moves pivotally outward is dependent on the mass thereof and particularly the mass of the flyweight provided by plates 110 and 112 which can be selected to produce this pivotal movement at a predetermined maximum cranking speed, which is typically in the range of 750 to 1000 engine revolutions per minute. This predetermined maximum cranking speed is above the normal engine cranking speed and below the usual idling speed of 1200 to 1500 engine revolutions per minute for a single-cylinder, four-cycle engine. When the running engine is shut down, yoke 74 is pivoted inwardly toward camshaft 30 by spring 76 as the engine slows down below the predetermined maximum cranking speed in coasting to a stop. This pivotal movement of yoke 74 toward camshaft 30 causes cam surface 102 between seats 98 and 100 to engage head 86 of plunger and shift the free end of the plunger radially outward so that head 86 bears on seat 98. This repositions plunger 80 to relieve the compression of the engine when it is next rotated at a cranking speed for starting. Plunger 80 is counterweighted by head 86 so that centrifugal force acting on the plunger tends to urge it into engagement with seats 98 and 100 of yoke 74 whenever shaft 30 is rotated. This assures that plunger 80 will remain firmly engaged with the seats whenever the camshaft is rotating.

FIG. 6 shows a modified form of a compression relief mechanism 70' with a modified yoke 74' assembled on camshaft 30. As shown in FIG. 7, yoke 74' is the same as yoke 74 except that tab 96" extends generally axially rather than radially and tab 104' extends generally radially outwardly rather than inwardly. To limit the extent to which yoke 74' can pivot about pin 80 generally radially outwardly from the camshaft, tab 104' abuts on a positive stop provided by a roll pin 116 which is received with an interference fit in a hole 118 through cam lobe 34.

Compression relief mechanism 70' is assembled on camshaft 30 by inserting plunger 80 through bore 82 in the camshaft and then manipulating yoke 74' to slip the shank end of plunger 80 through slot 104 between tabs 96" and 96'. Yoke 74' is then pivoted generally radially inwardly so that seat 98 or 100 underlies head 86 of the plunger and roll pin 116 is pressed into hole 118 through cam lobe 34 to retain yoke 74' on the camshaft as shown in FIGS. 6 and 8. The ends of spring 76 are then hooked through holes 108 in yoke 74' to yieldably bias the yoke toward the start position as shown in FIG. 8.

In use compression relief assembly 70' operates in the same manner as compression relief assembly 70 with tab 104' abutting on roll pin 116 when yoke 74' is in the run position to limit the extent of generally pivotal movement of the yoke away from the camshaft. Compression relief assembly 70 is believed to be more economical to manufacture although compression relief assembly 70' is somewhat easier to assemble on the camshaft without any special assembly fixtures since tab 96" does not have to be bent during assembly after the yoke is received on the camshaft.

This invention provides a compression relief mechanism of simplified design in which the plunger and yoke retainer cooperate to pivotally mount the yoke on the camshaft and to retain the plunger therein. The compact design with a minimum number of component parts of this invention provides a compression relief mechanism of economical manufacture particularly since the yoke can be made of sheet metal stampings. The use of a retainer in this invention which can be manually slipped over the plunger and camshaft and pivotally mounted on the camshaft by simply bending down one of the tabs provides a compression relief mechanism which can be easily and economically assembled. The simplified design with the minimum number of component parts of this invention also provides a rugged and durable compression relief mechanism with a long maintenance-free, useful life.

Claims

I claim:

1. In an internal combustion engine having a cylinder, a piston reciprocable in the cylinder and defining therewith a combustion chamber, a camshaft with a fixed cam thereon having a fixed profile for operating an exhaust valve, the camshaft and the exhaust cam being driven in timed relation with the speed of reciprocation of the piston, a poppet-type exhaust valve yieldably biased to a normally closed position and operable to control the flow of gases from the combustion chamber to an atmosphere external of the engine, and a follower operatively associated with the exhaust valve in tracking relation with the cam such that rotation of the cam by the camshaft opens and closes the valve in timed relation with reciprocation of the piston, a compression relief mechanism comprising in combination; a passage extending transversely through the camshaft adjacent the cam, a plunger received in said passage with an end underlying at least a portion of the follower tracking the cam, a yoke adjacent and partially surrounding at least a portion of the camshaft, said plunger in assembly continuously extending through a portion of said yoke to mount said yoke on the camshaft for pivotal movement in a plane generally transverse to the axis of rotation of the cam, first and second radially and circumferentially spaced seats adapted to receive the other end of said plunger, said first and second seats being carried by said yoke to be generally transverse to the axis of rotation of said cam, bias means operably connected with said yoke to yieldably urge said yoke to a first position wherein said first seat underlies said other end of said plunger when said camshaft is rotated below a predetermined maximum engine cranking speed, and a flyweight carried by said yoke to pivotally move said yoke against the bias of said bias means in a plane generally transverse to the axis of rotation of the cam to a second position wherein said second seat underlies said other end of said plunger when the camshaft rotates at a speed above said predetermined maximum cranking speed, said plunger having an axial length such that when said other end thereof is received on said first seat said one end thereof extends radially beyond the cam to engage the follower to open the exhaust valve when it would normally be closed during cranking of the engine, and said second seat being sufficiently generally radially offset with respect to said first seat so that when said other end of said plunger is received on said second seat said one end of said plunger does not extend radially beyond the cam so that when the engine is running at a speed above said predetermined maximum cranking speed the exhaust valve is not opened by the plunger when it should be normally closed.

2. The combination of claim 1 wherein said plunger has a counterweight adjacent said other end thereof.

3. The combination of claim 1 wherein said transverse passage through the camshaft has a counterbore in one end thereof and said plunger has a shank and an enlarged integral head adjacent said other end thereof with said shank being received in said passage and said head being received at least partially within said counterbore.

4. The combination of claim 1 wherein said yoke has a retainer with a generally C-shaped configuration with a generally axially extending integral tang thereon having a slot therethrough forming in assembly a pair of axially extending tabs with said plunger extending through said slot between said tabs to pivotally mount said yoke on the camshaft.

5. The combination of claim 4 wherein at least one of said tabs is bendable from a position extending generally transversely of the axis of the cam to a position extending generally axially, whereby said yoke can be readily assembled on the camshaft with said plunger already received in said passage.

6. The combination of claim 1 wherein said yoke has a body partially surrounding a generally axially extending portion of the camshaft adjacent the cam thereof and terminating in a pair of spaced apart free ends and said biasing means comprises a spring connected to said body adjacent said free ends thereof and extending between said free ends so that together said body and spring completely extend around said portion of the camshaft adjacent the cam thereof.

7. The combination of claim 6 wherein said spring comprises a tension coil spring.

8. The combination of claim 4 wherein said retainer partially surrounds a generally axially extending portion of the camshaft adjacent the cam thereof and terminates in a pair of spaced apart free ends, and said biasing means comprises a spring connected to said retainer adjacent said free ends thereof and extending between said free ends so that together said retainer and spring completely extend around said portion of the camshaft adjacent the cam thereof.

9. The combination of claim 1 wherein said yoke comprises a sheet metal retainer partially surrounding a generally axially extending portion of the camshaft adjacent the cam and terminates in a pair of spaced apart free ends, said sheet metal retainer having an integral flange adjacent one free end thereof with generally axially extending radially and circumferentially spaced portions thereof providing said first and second seats.

10. The combination of claim 9 wherein said sheet metal retainer also has a generally axially extending integral tang thereon adjacent the other free end thereof with said tang having a slot therethrough forming in assembly a pair of axially extending tabs with said plunger extending through said slot between said tabs to pivotally mount said yoke on the camshaft.

11. The combination of claim 4 wherein at least one of said tabs is bendable from a position extending generally transversely of the axis of the cam to a position extending generally axially, whereby said yoke can be readily assembled on the cam with said plunger already received in said passage.

12. The combination of claim 9 wherein said sheet metal retainer comprises a stamping.

13. The combination of claim 9 wherein said flyweight comprises at least one sheet metal plate fixed to said sheet metal retainer of said yoke.

14. The combination of claim 10 wherein said flyweight comprises at least one sheet metal plate fixed to said sheet metal retainer of said yoke.

15. The combination of claim 4 wherein the camshaft also comprises a generally transversely extending flange thereon axially spaced from the cam with said retainer received between said flange and the cam to limit generally axial displacement of said retainer with respect to the camshaft such that when said plunger and retainer are assembled on the camshaft they will both be retained thereon.

16. The combination of claim 9 wherein the camshaft also comprises a generally transversely extending flange thereon axially spaced from the cam with said retainer received between said flange and the cam to limit generally axial displacement of said retainer with respect to the camshaft such that when said plunger and retainer are assembled on the camshaft they will both be retained thereon.

17. In an internal combustion engine having a cylinder, a piston reciprocable in the cylinder and defining therewith a combustion chamber, a camshaft with a fixed cam thereon having a fixed profile for operating an exhaust valve, the camshaft and the cam being driven in timed relation with the speed of reciprocation of the piston, a poppet-type exhaust valve yieldably biased to a normally closed position and operable to control the flow of gases from the combustion chamber to an atmosphere external of the engine, and a follower operatively associated with the exhaust valve in tracking relation with the cam such that rotation of the cam by the camshaft opens and closes the exhaust valve in timed relation with reciprocation of the piston, a compression relief mechanism comprising in combination; a yoke partially surrounding at least a portion of the camshaft adjacent the cam and terminating in a pair of spaced apart free ends, said yoke being pivotally mounted adjacent one free end thereof on the camshaft adjacent the cam for pivotal movement in a plane generally transverse to the axis of rotation from a first position to a second position in response to a centrifugal force acting on said yoke when the engine operates a a running speed in excess of a predetermined maximum cranking speed, a spring having generally opposed ends and being connected adjacent said opposed ends thereof to said yoke adjacent said free ends of said yoke and extending between said free ends such that together said yoke and said spring completely surround said portion of the camshaft adjacent the cam, said spring yieldably retaining said yoke in said first pivotal position when the engine is driven at a speed below said predetermined maximum cranking speed, and an actuator carried by the camshaft for engagement with the follower and responsive to movement of said yoke to said first position to lift the follower and partially open the exhaust valve when it would otherwise normally be closed during the compression stroke of the engine and to movement of said yoke to said second position to move the actuator out of operable engagement with the follower.

18. The combination of claim 17 wherein said spring comprises a tension coil spring.

19. The combination of claim 17 wherein said actuator comprises a plunger underlying at least a portion of the follower tracking the cam and being slidably received in a passage extending transversely through the camshaft adjacent the cam with said plunger in assembly continuously extending through a portion of said yoke adjacent one free end thereof to pivotally mount said yoke on the camshaft.

20. The combination of claim 19 wherein said yoke comprises a sheet metal body with a generally axially extending integral tang thereon having a slot therethrough forming in assembly a pair of axially extending tabs with said plunger extending through said slot between said tabs to pivotally mount said yoke on the camshaft.

21. The combination of claim 20 wherein at least one of said tabs is bendable from a position extending generally transversely to the axis of rotation of the cam to a position extending generally axially, whereby said yoke can be readily assembled on the camshaft with said plunger already received in said passage.

22. The combination of claim 17 wherein said yoke comprises a sheet metal stamping.

23. The combination of claim 17 wherein said yoke also comprises a flyweight fixed thereto.

24. The combination of claim 19 wherein said yoke comprises a body having an integral flange adjacent one end thereof with axially extending generally radially and circumferentially spaced portions thereof providing first and second seats with said first seat being generally axially aligned with said plunger when said yoke is in said first pivotal position and said second seat being generally axially aligned with said plunger when said yoke is in said second position, and said plunger having an axial length such that when one end thereof is received on said first seat the other end thereof extends radially beyond the cam to engage the follower to open the exhaust valve when it would otherwise normally be closed during the compression stroke of the engine and said second seat being sufficiently generally radially offset with respect to said first seat so that when the one end of said plunger is received on said second seat, the other end of said plunger does not extend radially beyond the cam so that when the engine is operating at a running speed above said predetermined maximum cranking speed the exhaust valve is not opened by the plunger when it should be normally closed during the compression stroke of the engine.

25. The combination of claim 20 wherein said sheet metal body has an integral flange with generally axially extending radially and circumferentially spaced portions thereof providing first and second seats with said first seat lapping one end of said plunger when said yoke is in said first pivotal position and said second seat lapping said one end of said plunger when said yoke is in said second position, and said plunger having an axial length such that when one end thereof is received on said first seat the other end thereof extends radially beyond the cam to engage the follower to open the exhaust valve when it would otherwise normally be closed during the compression stroke of the engine and said second seat being sufficiently generally radially offset with respect to said first seat so that when the one end of said plunger is received on said second seat the other end of said plunger does not extend radially beyond the cam so that when the engine is operating at a running speed above said cranking speed the exhaust valve is not opened by the plunger when it should be normally closed during the compression stroke of the engine.

26. The combination of claim 1 which also comprises an abutment on said yoke adapted to bear on said plunger when the camshaft rotates at a speed exceeding said predetermined maximum cranking speed to limit the extent of the pivotal movement of said yoke away from the camshaft.

27. The combination of claim 17 which also comprises an abutment on said yoke adapted to bear on said actuator when the camshaft rotates at a speed exceeding said predetermined maximum cranking speed to limit the extent of the pivotal movement of said yoke away from the camshaft.

28. The combination of claim 1 which also comprises a stop carried by the camshaft and an abutment carried by said yoke to bear on said stop when the camshaft rotates at a speed exceeding said predetermined maximum cranking speed to limit the extent of pivotal movement of said yoke away from the camshaft.

29. The combination of claim 17 which also comprises a stop carried by the camshaft and an abutment carried by said yoke to bear on said stop when the camshaft rotates at a speed exceeding said predetermined maximum cranking speed to limit the extent of pivotal movement of said yoke away from the camshaft.

30. The combination of claim 28 wherein said stop comprises a pin fixedly carried by the exhaust cam of the camshaft.

31. The combination of claim 29 wherein said stop comprises a pin fixedly carried by the exhaust cam of the camshaft.

32. The combination of claim 17 wherein a portion of said spring intermediate said ends thereof bears on said portion of the camshaft adjacent the cam.

33. In an internal combustion engine having a combustion chamber, an exhaust valve yieldably biased to a normally closed position and operable to an open position to permit the flow of gases from the combustion chamber to an atmosphere external of the engine, and a camshaft with a cam thereon having a profile for operating the exhaust valve to its open and closed positions, a compression relief mechanism comprising in combination: a passage extending transversely through the camshaft adjacent the cam, a plunger received in said passage, a yoke adjacent the cam, said plunger in assembly continuously extending through a portion of said yoke to mount said yoke on the camshaft for pivotal movement in a plane generally transverse to the axis of rotation of the cam from a first position to a second position in response to a centrifugal force acting on said yoke when the engine operates at a running speed in excess of the cranking speed of the engine, means yieldably retaining said yoke in said first position when the engine is driven at said cranking speed, and means operably connecting said plunger and said yoke and responsive to movement of said yoke to its first position to move said plunger to a first position to at least partially open the exhaust valve when the cam would otherwise allow it to close and responsive to movement of said yoke to its second position to move the plunger to a second position permitting the exhaust valve to close when allowed to do so by the cam.

34. The combination of claim 33 wherein said plunger has a counterweight adjacent one end thereof.

35. The combination of claim 33 wherein said yoke comprises a sheet metal body with a generally axially extending integral tang thereon having a slot therethrough forming in assembly a pair of axially extending tabs with said plunger continuously extending through said slot between said tabs to pivotally mount said yoke on the camshaft.

36. The combination of claim 35 wherein at least one of said tabs is bendable from a position extending generally transverse to the axis of rotation of the cam to a position extending generally axially, whereby said yoke can be readily assembled on the camshaft with said plunger already received in said passage.

37. The combination of claim 33 wherein said yoke comprises a body and said last-mentioned means comprises radially and circumferentially spaced first and second seats carried by said body with said first seat being generally axially aligned with said plunger when said yoke is in its first position and said second seat being generally axially aligned with said plunger when said yoke is in its second position, said plunger having an axial length such that when one end thereof is received on said first seat the other end thereof extends radially beyond the cam to open the exhaust valve when it would otherwise normally be closed during the compression stroke of the engine when driven at said cranking speed and said second seat being sufficiently generally radially offset with respect to said first seat such that when said one end of said plunger is received on said second seat the other end of said plunger does not extend radially beyond the cam so that when the engine is operating at a running speed the exhaust valve is not opened by said plunger when it should be normally closed during the compression stroke of the engine.

38. The combination of claim 33 which also comprises an abutment on said yoke adapted to bear on said plunger when the engine is operating at a running speed to limit the extent of the pivotal movement of said yoke away from the camshaft.

39. The combination of claim 38 wherein said plunger has a counterweight adjacent said one end thereof.