[0001] 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.
[0002] 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. Patent No. 6,295,959, assigned to the assignee of the present invention, the
disclosure of which is expressly incorporated herein by reference.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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:
[0014] 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;
[0015] Fig. 2 is a longitudinal sectional view of the engine of Fig. 1;
[0016] 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;
[0017] 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;
[0018] Fig. 5A is a perspective, exploded view of the compression release mechanism of Figs.
3 and 4;
[0019] 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;
[0020] Fig. 6 is an end view of the compression release mechanism in the first operational
position of Fig. 3; and
[0021] Fig. 7 is an end view of the compression release mechanism in the second operational
position of Fig. 4.
[0022] 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.
[0023] 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.
Patent 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.
[0024] 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.
[0025] 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 L
1-L
1. Piston 25 (Fig. 2) is slidably received in cylinder bore 15 within cylinder block
16 along axis L
2-L
2 perpendicular to crankshaft axis L
1-L
1. 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.
[0026] 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.
[0027] Timing shaft 42 is disposed parallel to crankshaft axis L
1-L
1, 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.
[0028] 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 L
1-L
1. 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.
[0029] 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.
[0030] 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.
[0031] 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 A
2 of cam pivot shaft 100 is spaced from, and therefore is eccentric with respect to,
the longitudinal axis A
1 of body portion 92 of cam member 90.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 LA
1-LA
1 associated with flyweight 102, as defined between flyweight pivot shaft 106 and cam
pivot shaft 100, is much longer than a second lever arm LA
2-LA
2 associated with cam member 90, as defined between cam pivot shaft 100 and the longitudinal
axis A
1 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.
[0041] 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 Θ, defined between the first and second positions
of lever arm LA
1-LA
1, 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.
[0042] 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 this invention pertains.
1. An internal combustion engine, including a camshaft (50) having at least one cam lobe
(58a, 58b), said cam lobe including a portion projecting beyond a base circle of said
cam lobe for periodically engaging a valve (62a, 62b); and a compression release mechanism
(70), characterized by a flyweight (102) having a first end (102a) attached at a flyweight pivot (106) to
said camshaft, and a second end (102b) opposite said first end, said flyweight pivotally
movable between first and second positions; a cam member (90) rotatable with respect
to said camshaft, said cam member having a body portion (92) with an axis of rotation
(A1) and a head portion (94) eccentric to said body portion axis, said head portion coupled
to said second end of said flyweight, wherein a first lever arm (LA1-LA1) defined between said flyweight pivot and said head portion of said cam member is
longer than a second lever arm (LA2-LA2) 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; said cam member including
a cam surface (96), 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, characterized in that said first lever arm (LA1-LA1) has a first length and said second lever arm (LA2-LA2) has a second length, 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, characterized in that said first lever arm (LA1-LA1) has a first length and said second lever arm (LA2-LA2) has a second length, 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, characterized in that said first lever arm (LA1-LA1) has a first length and said second lever arm (LA2-LA2) has a second length, 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, characterized in that said flyweight (102) extends around said camshaft (50) between opposite first and
second sides of said camshaft, said flyweight pivot (106) located proximate said camshaft
first side and said second end (102b) of said flyweight located proximate said camshaft
second side.
6. The internal combustion engine of Claim 1, characterized in that said camshaft (50) includes a recess (74) within which said body portion (92) of
said cam member (90) is closely supported for rotation, wherein contact forces between
said valve (62a, 62b) and said cam surface (96) of said cam member are transferred
through said cam member to said camshaft when said flyweight (102) is in said first
position.
7. The internal combustion engine of Claim 1, characterized in that said body portion (92) of said cam member (90) includes a first longitudinal axis
(A1) and said head portion (94) of said cam member includes a pivot shaft (100) coupled
to said flyweight (102), said pivot shaft having a second longitudinal axis (A2) eccentric to said first longitudinal axis.
8. The internal combustion engine of Claim 7, characterized in that said pivot shaft (100) is positively retained by said flyweight (102) in each of
said first and second flyweight positions.
9. The internal combustion engine of Claim 1, characterized by a spring (112) connecting said camshaft (50) and said flyweight (102), said spring
biasing said flyweight to said first flyweight position.
10. The internal combustion engine of Claim 1, characterized in that said camshaft (50) includes a plate (76) disposed perpendicular to said camshaft
and rotatable therewith, said plate including an aperture (78) through which said
cam member (90) is received.
11. The internal combustion engine of Claim 10, characterized in that said flyweight (102) includes a side surface (103b) facing said plate (76), said
side surface spaced from said plate at said second flyweight end (102b) to provide
a clearance space (105) between said second flyweight end and said plate in which
said head portion (94) of said cam member (90) is disposed.
12. The internal combustion engine of Claim 1, characterized in that said camshaft (50) and said flyweight (102) include stop surfaces (86, 116) engagable
with one another in said second flyweight position to limit pivotal movement of said
flyweight.
13. The internal combustion engine of Claim 1, characterized in that said flyweight (102) pivots between about 5 degrees and about 10 degrees between
said first and second flyweight positions.
14. The internal combustion engine of Claim 1, characterized in that said cam member (90) rotates up to 90 degrees between said first and second flyweight
positions.