Four-stroke cycle internal combustion engine

A four-stroke cycle internal combustion engine comprises a connecting rod which has an oil dipper at a big-end thereof, a crankcase, a partition wall which surrounds the connecting rod to provide a separation between the crankcase and an oil reservoir and an outer wall which is outward of and surrounds the partition wall and is connected to the partition wall at upper ends thereof so as to define the oil reservoir below the crankcase. A slit is formed in the partition wall below the connecting rod to allow the oil dipper to protrude into and be withdrawn from the oil reservoir to splash oil in the oil reservoir for lubrication by the oil dipper through a swinging motion of the connecting rod. The depth of the oil reservoir is shallower at least in a part of an outer edge area thereof than that below the oil dipper so that a tip of the oil dipper reaches the oil level even when the oil flows into the outer edge area of the oil reservoir.

BACKGROUND OF THE INVENTION
 The present invention relates to a four-stroke cycle internal combustion
 engine preferably used for driving a portable working machine such as a
 portable trimmer, though not limited thereto, and in particular, to a
 four-stroke cycle internal combustion engine which is lubricated by oil in
 an oil reservoir provided below a connecting rod, which oil is splashed up
 by an oil dipper formed on a big-end of the connecting rod.
 DESCRIPTION OF THE PRIOR ART
 It has been known to use a portable working machine such as a portable
 trimmer driven by an electric spark ignition type internal combustion
 engine. Formerly, a two-stroke cycle internal combustion engine was
 generally used as an internal combustion engine for a portable working
 machine, but recently, there has been an increasing demand for utilizing a
 four-stroke cycle internal combustion engine in order to improve the air
 pollution problem caused by exhaust gas. Such four-stroke cycle internal
 combustion engine has an oil reservoir (oil pan) below a crankcase, which
 accommodates oil for lubricating each part of the engine. The oil in the
 oil reservoir is splashed up causing oil-mist by an oil dipper formed on a
 big-end of a connecting rod and the oil-mist is supplied to the crankcase,
 a chamber for a valve gear or the like to lubricate various components.
 In the portable working machine, for example, in a portable trimmer, a
 power transmitting shaft is coupled with a front end of a crankshaft of an
 internal combustion engine via a centrifugal clutch, and a rotary blade
 for trimming is coupled with a front end of the power transmitting shaft
 via a gear box. An operator holds such a portable trimmer by hand and cuts
 weeds on the ground or trims branches of a tree above the operator's head
 with the rotary trimming blade driven by the internal combustion engine.
 Accordingly, the internal combustion engine of the portable working
 machine is forced, during operation, to assume a horizontal position, or
 an upside-down position, or some position other than a cylinder-upright
 position. A substantially U-shaped oil reservoir surrounding both sides
 and a lower side of the crankcase is disclosed, for example, in Japanese
 Patent Laid-open Disclosure Nos. Hei 8-260926 and Hei 10-231717. When a
 four-stroke cycle internal combustion engine takes various positions as
 described above, this type of oil reservoir allows the oil to be
 accommodated in oil recess areas located on the side of the crankcase to
 prevent the oil from flowing into the crankcase or into a cylinder area
 and thereby to prevent the piston from soaking in the oil or the breather
 from being clogged up.
 However, when the internal combustion engine takes various positions as
 described above, the oil in the oil reservoir flows into the oil recess
 areas or other recess areas to prevent the oil from flowing into the
 crankcase or the like, while at the same time there may occur a condition
 where the oil dipper cannot reach the oil and thereby no oil mist is
 supplied to each part of the four-stroke cycle internal combustion engine.
 Accordingly, the object of the present invention is to provide a
 four-stroke cycle internal combustion engine which can be used to drive
 the portable working machine, that is, which ensures the oil to be
 splashed up by the oil dipper even under a tilted condition or the like.
 SUMMARY OF THE INVENTION
 The object of the present invention described above can be achieved by a
 four-stroke cycle internal combustion engine comprising: a connecting rod,
 an oil dipper provided at a big-end of the connecting rod; a crankcase; an
 oil reservoir below the crankcase; a partition wall which surrounds the
 connecting rod to provide a separation between the crankcase and the oil
 reservoir; and an outer wall which is outward of and surrounds the
 partition wall and is connected to the partition wall at upper ends
 thereof so as to define an oil reservoir below the crankcase; a slit
 formed in the partition wall below the connecting rod to allow the oil
 dipper to protrude into and be withdrawn from the oil reservoir to splash
 oil in the oil reservoir for lubrication by the oil dipper through a
 swinging motion of the connecting rod; the depth of the oil reservoir
 being shallower at least in a part of an outer edge area thereof than that
 below the oil dipper so that a tip of the oil dipper reaches the oil level
 of the oil reservoir even when the oil flows into the outer edge area of
 the oil reservoir.
 In the present invention, regarding the depth of the oil reservoir along
 the direction of the center line of the connecting rod, at least a part
 thereof located on an outer edge area is shallower than that located below
 the oil dipper. Accordingly, when the four-stroke cycle internal
 combustion engine is rotated and tilted about an axis intersecting with
 the crankshaft and thereby the oil flows into the outer edge area of the
 oil reservoir, the tip of the oil dipper reaches the oil level and the oil
 is assured to be splashed up by the oil dipper to supply the oil mist to
 each component in the engine.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to the drawings attached herein, embodiments of the present
 invention shall be described by explaining a portable trimmer as one
 example of portable working machines.
 As shown in FIG. 1, a portable trimmer 2 includes a supporting tube 4 which
 has a power transmitting shaft 4a inserted therein, a four-stroke cycle
 internal combustion engine 6 at its rear end, and a working section 8 at
 its front end. The working section 8 is equipped with a cutting blade 8a
 rotatably mounted to cut weeds in a direction indicated by an arrow. A
 rotational force from the four-stroke cycle internal combustion engine 6
 is transmitted via a centrifugal clutch or the like, though not shown, to
 the power transmitting shaft 4a to rotate the cutting blade 8a. An
 operator holds a handle section 10 provided at the middle portion of the
 supporting tube 4 by both hands to perform a trimming operation.
 The four-stroke cycle internal combustion engine 6 of the first embodiment
 shown in FIG. 2 is of an air-cooled type and of an OHC type. As can be
 seen in FIG. 2, the rotation of a crankshaft 12 is transmitted to a
 camshaft 14 through an appropriate gearing mechanism 16. The gearing
 mechanism 16 is disposed on the front side of a cylinder block 18 of the
 four-stroke cycle internal combustion engine 6, that is, on the side
 toward the cutting blade 8a and is accommodated in a gear chamber 20
 defined vertically along an upright cylinder block 18. A cam chamber 23
 which accommodates a valve drive mechanism 21 and is enclosed by a
 liquid-tight detachable cover 22 is disposed on the cylinder block 18.
 As best shown in FIG. 5a, the four-stroke cycle internal combustion engine
 6 has a partition wall 30 which surrounds a connecting rod 24 on both the
 left and right sides and the lower side thereof to define a crankcase 28
 and provides a separation between the crank case 28 and an oil reservoir
 26 disposed therebelow. An outer wall 32 surrounds the partition wall 30
 and is connected to the partition wall 30 at the left and right upper ends
 32a, 32a thereof so as to form the oil reservoir 26 on an outer side of
 the crankcase 28. The oil reservoir 26 includes an oil reserving area 34
 disposed below the crankcase 28 and oil recess areas 36, 36 disposed on
 both sides of the crankcase 28. Each of the oil recess areas 36, 36 has a
 volume capable of accommodating all oil accommodated in the oil reservoir
 area 34.
 Two oil dippers 38, 38 are formed on a big-end 24a of the connecting rod 24
 directed downwardly so as to form a V-shape therebetween in order to
 splash up the oil in the oil reservoir 26 forced by a swinging motion of
 the connecting rod 24 and to supply the oil mist to the crankcase 28 and
 to each part of the cam chamber 23. Each of the oil dippers 38, 38 extends
 straight toward each of the oil recess areas 36, 36 in an obliquely
 downward direction. In detail, each of the oil dippers 38 extends in the
 obliquely downward direction so as to be bilaterally symmetrical with
 respect to a center line O--O of the connecting rod 24 located at top or
 bottom dead center thereof with an angle of about 45 degrees therefrom
 toward each of the oil recess areas 36, 36, respectively. Referring again
 to FIG. 2, each of the oil dippers 38, 38 extends straight in a downward
 direction along the center line O--O of the connecting rod in the side
 elevational view.
 In addition, a slit 40 is formed in the partition wall 30 for allowing the
 oil dippers 38, 38 to protrude into and withdraw from the oil reservoir 26
 by an up-and-down motion of a piston 35 of the four-stroke cycle internal
 combustion engine 6 so as to splash up the oil from the oil reservoir 26.
 The slit 40 is formed to be of minimum size for allowing the oil dipper to
 pass through without interference therebetween so as to prevent the oil
 from flowing into the crankcase 28 while the four-stroke cycle internal
 combustion engine 6 is tilted. As also can be seen in FIG. 2, a first oil
 mist passage 42 is formed in the crankshaft 12, which extends
 longitudinally and communicates with the crankcase 28 at one end thereof.
 A second oil mist passage 44 is formed in the front side portion of the
 cylinder block 18 which extends vertically and communicates with the other
 end of the first oil mist passage 42 at a lower end thereof and also
 communicates with the cam chamber 23 at an upper end thereof. In addition,
 the cam chamber 23 communicates with the crankcase 28 through a third oil
 mist passage 45 formed in the cylinder block 18 extending downward, and
 thereby a circulating channel of the oil mist is defined.
 Referring to FIG. 2, the outer wall 32 is formed approximately into an arc
 defined by a center P located at the center line of the connecting rod 24
 or the oil dippers 38, 38 and a radius R. Further, referring to FIG. 5a,
 the outer wall 32 is formed into a substantially arc-like shape defined by
 the center P and the radius R. That is, the outer wall 32 is formed into a
 hemisphere or bowl shape about the center P, which surrounds the oil
 dippers 38, 38 and is curved upwardly. On the other hand, referring to
 FIG. 2, the partition wall 30 extends horizontally straight below the
 crankcase 28 in the vertical cross-sectional view including the axial line
 O'--O' of the crankshaft 12. In the cross-sectional view shown in FIG. 5a,
 the partition wall 30 is drawn as an arc defined by the center P and a
 radius "r" extending concentrically with the outer wall 32. That is, the
 partition wall 30 is formed into a cylindrical shape which extends
 laterally surrounding the crankshaft 12 and is open upwardly. The outer
 wall 32 is connected to the partition wall 30 at the upper ends 32a, 32a
 thereof to form the oil reservoir 26. That is, as shown in FIG. 5, in the
 cross-section intersecting with the crankshaft 12 at a right angle at a
 center line of the connecting rod 24, the oil reservoir is formed into a
 C-shape opening directed upwardly or a semi-ring-shape having a uniform
 width Y around the connecting rod 24.
 Referring to FIG. 2 again, as described above, the oil reservoir 26 is
 generally formed into a crescent-like shape enclosed by the partition wall
 30 horizontally extending substantially straight and the outer wall 32
 extending as an arc in the vertical cross-section including the crankshaft
 axial line O'--O', and thereby a front outer edge area 26a and a rear
 outer edge area 26b of the oil reservoir 26 are made narrower than a
 downside area 26c thereof located below the oil dipper 38 or made to be
 rather flat. That is, a depth "d" of the oil reservoir 26 measured
 vertically along the longitudinal center line O--O of the connecting rod
 24 takes a maximum value D below the dipper 38. The depth "d" decreases
 gradually with increasing distance from the oil dipper 38 since the outer
 wall 32 is upwardly inclined toward the oil dipper 38 to raise the bottom
 up with increasing distance from the oil dipper 38.
 Referring to FIGS. 2 and 4, the operation of the first embodiment will
 hereafter be described. The four-stroke cycle internal combustion engine 6
 of the portable trimmer 2 in accordance with the first embodiment is used
 in various tilted positions when an operator uses the cutting blade 8a to
 cut weeds located at a height less or more than the operator's waist or
 branches above the operator's head. Among these positions, FIG. 3
 illustrates a forwardly tilted position which occurs when the operator
 uses the cutting blade 8a mounted on a front end of the portable trimmer 2
 to cut weeds or the like located at a height less than the operator's
 waist, that is, when the four-stroke cycle internal combustion engine 6 is
 tilted forward around a lateral line intersecting horizontally with the
 crankshaft 12.
 Further, FIG. 4 illustrates a backwardly tilted position which occurs when
 the operator uses the cutting blade 8a mounted on the front end of the
 portable trimmer 2 to cut the weeds, branches or the like located at a
 height more than the operator's waist, that is, when the four-stroke cycle
 internal combustion engine 6 is tilted backward around the lateral line.
 As can be seen in FIGS. 3 and 4, the oil in the reservoir 26 flows into
 the front outer edge area 26a (in the case of FIG. 3) or the rear outer
 edge area 26b and is accommodated therein. At that time, the height of an
 oil level OL just below the oil dipper 38 is approximately a height H in
 any case, since the front outer edge area 26a and the rear outer edge area
 26b are made narrower, that is, it is approximately the same with the
 height H of the oil level in case of the upright position shown in FIG. 2.
 Since especially the outer wall 32 is formed into a sphere or arc shape in
 the cross-section of FIG. 2, the height H of the oil level OL is kept
 constant to be approximately the same with that of the cylinder-upright
 position shown in FIG. 2 even if the four-stroke cycle internal combustion
 engine 6 is tilted forward or backward, without regard to the magnitude of
 the tilted angle thereof. Each of the volumes of a front portion and a
 rear portion of the oil reservoir 26 with respect to the connecting rod
 center line O--O has a volume capable of accommodating the oil in the oil
 reservoir 26 when the four-stroke cycle internal combustion engine 6 is in
 the forwardly tilted position or in the backwardly tilted position.
 The oil mist in the crankcase 28 is sent to the cam chamber 23 through the
 first oil mist passage 42 and the second oil mist passage 44 by positive
 and negative pressure generated by the up-and-down motion of the piston
 35. Surplus oil mist and liquid oil in the cam chamber 23 is returned to
 the crankcase 28 through the third oil mist passage 45 and then is
 returned to the oil reservoir 26 through the slit 40.
 FIG. 5 shows a schematic vertical cross-section intersecting with the
 crankshaft 12 at a right angle at a position of the connecting rod 24,
 wherein FIG. 5a shows a case where the four-stroke cycle internal
 combustion engine 6 is in the upright position, FIG. 5b shows a case where
 the four-stroke cycle internal combustion engine 6 is in the laterally
 tilted position rotated laterally to the right around the crankshaft 12,
 and FIG. 5c show a case where the four-stroke cycle internal combustion
 engine 6 is in a horizontal position after being rotated further to the
 right up to a horizontal direction.
 The operation of the first embodiment will be described further with
 reference to FIGS. 5a to 5c. As the four-stroke cycle. Internal combustion
 engine 6 is tilted from the upright position shown in FIG. 5a to the
 position tilted toward the right shown in FIG. 5b and then further to the
 horizontal position shown in FIG. 5c, the oil accommodated in the oil
 reserving area 34 gradually flows into the right oil recess area 36. At
 that time, since the outer wall 32 is formed into a spherical shape, a
 height "h" of the oil level OL is kept approximately constant and a
 minimum distance S between the oil level OL and the partition wall 30 is
 also kept approximately constant irrespective of the position of the
 four-stroke cycle internal combustion engine 6. Thereby, even if the
 four-stroke cycle internal combustion engine 6 is in the tilted or even in
 the horizontal position, at least one of the oil dippers 38, that is, at
 least one of the oil dippers 38 extending toward the oil reserving area 36
 where the oil is accommodated will certainly come in contact with the oil
 to certainly produce the oil mist.
 The second embodiment shown in FIGS. 6 and 7 differs from the first
 embodiment shown in FIGS. 2 to 4 in that a rear portion of an oil
 reservoir 26' with respect to the center line O--O of the connecting rod
 24 is formed into a rectangular or box shape in a section thereof. Since a
 front portion of the oil reservoir 26' is configured to be the same as
 that of the first embodiment shown in FIGS. 2 to 4, the description
 thereof will be omitted. If this type of configuration is employed, the
 same operational effect as that of the first embodiment described with
 reference to FIG. 3 can be obtained in the forwardly tilted position which
 occurs when the operator uses the cutting blade 8a to cut weeds or the
 like located at a height less than the operator's waist. Since, usually in
 the trimming operation, a percentage of time consuming for cutting weeds
 or the like at a height less than the operator's waist is maximum while
 that consumed for cutting branches above the operator's head is minimum,
 it is reasonable to form only the front portion into a spherical shape.
 On the other hand, since the rear portion of the oil reservoir 26' is
 formed into a rectangular or box shape, it has the same width W as that of
 the front portion but has a greater volume than that. That is, the rear
 portion has a volume capable of accommodating all oil accommodated in the
 oil reserving area 34, and therefore, for example, as shown in FIG. 7, the
 oil is prevented from flowing into the crankcase 28 through the slit 40
 even if the operator keeps the portable trimmer 2 in an upright position
 with the cutting blade 8a directed upwardly.
 As can be seen in FIG. 8, the oil reservoir 26' of the second embodiment
 differs from that of the first embodiment in that, though an outer wall
 32' in a portion thereof located below the crankcase 28 is formed into an
 arc shape defined by the center P and the radius R similar to that of the
 first embodiment, the outer wall 32' in a portion forming left and a right
 oil recess areas 36', 36' extends straight vertically. As shown in FIGS.
 8b and 8c, each of the oil recess areas 36', 36' has enough volume to
 accommodate all oil so as to prevent the oil from flowing into the
 crankcase 28 through the slit 40. In the second embodiment, since the side
 portions of the outer wall 32' are straight, a distance (width) "w"
 between the outer walls 32', 32' is narrower than the distance (width) W
 between the outer walls 32, 32 of the first embodiment, so that the
 four-stroke cycle internal combustion engine can be made more compact.
 Though an oil reservoir 26" shown in FIG. 9 is configured similar to the
 oil reservoir 26' shown in FIG. 8, the third embodiment differs from the
 second embodiment in that a single oil dipper 38" is formed on a
 connecting rod 24" extending straight along a center line thereof. The oil
 dipper 38 of the first or the second embodiment may be configured in this
 form.
 According to the first embodiment, even if the four-stroke cycle internal
 combustion engine 6 is in any of the cylinder-upright position, forwardly
 tilted position, backwardly tilted position, laterally tilted position or
 horizontal position, the height H of the oil level OL is kept
 approximately constant, so that the oil dipper 38 can always come into
 contact with the oil, effectively splash up the oil and certainly generate
 the oil mist, and whereby each part of the engine can be lubricated
 appropriately even if the operator uses the working machine under any of
 the positions described above for a long time.
 Further, in the first embodiment, since the minimum distance S between the
 oil level OL and the partition wall 30 is kept approximately constant even
 in various tilted positions and each of the oil recess areas 36, 36 has
 enough volume to accommodate all of the oil, the oil is prevented from
 flowing into the crankcase 28 through the slit 40.
 According to the second embodiment, the four-stroke cycle internal
 combustion engine 6 can be made relatively compact in its size, which
 allows the portable trimmer 2 to be designed compact in size as a whole.
 Furthermore, in the first and the second embodiments, since there are
 provided two oil dippers 38, 38 extending toward each of the oil recess
 areas 36, 36 respectively, at least one of the oil dippers 38, 38 can come
 in contact with the oil and splash it up to produce the oil mist even if
 the four-stroke cycle internal combustion engine 6 is in the laterally
 tilted position.
 The present invention has thus been shown and described with reference to
 specific embodiments. However, it should be noted that the present
 invention is in no way limited to the details of the described arrangement
 but changes and modifications may be made without departing from the scope
 of the appended claims.
 For example, a sectional form of the oil reservoir 26 of the four-stroke
 cycle internal combustion engine 6 in accordance with the first embodiment
 shown in FIG. 5 can be modified into that shown in FIG. 8. Even if such
 modification is employed, an operational effect described with reference
 to FIGS. 2 to 4 can be obtained and that described with reference to FIG.
 8 can also be obtained. On the other hand, a sectional form of the
 four-stroke cycle internal combustion engine in accordance with the second
 embodiment shown in FIG. 8 can be modified into that shown in FIG. 5. Even
 if such modification is employed, an operational effect described with
 reference to FIGS. 6 and 7 can be obtained and that described with
 reference to FIG. 5 can also be obtained. Further, sectional forms of the
 four-stroke cycle internal combustion engine in accordance with the first
 and the second embodiments shown in FIGS. 5 and 8 respectively can be
 modified into that shown in FIG. 9. Even if such modification is employed,
 operational effects described with reference to FIGS. 2 to 4 and FIGS. 6
 and 7 can be obtained.
 Though the outer wall 32 of the oil reservoir 26 shown in FIGS. 2 to 4 and
 the front portion of the outer wall 32' of the oil reservoir 26' shown in
 FIGS. 6 and 7 are formed into substantially a spherical shape, they need
 not necessarily be spherical. In the outer edge areas 26a, 26b, 26a' which
 are distant from the center line O--O of the oil dipper 38, the outer
 walls 32, 32' may be formed, for example, by a flat surface inclined
 toward the oil dipper 38 or by a curved surface. In that case, the outer
 walls 32, 32' are not necessarily required to be generally or gradually
 inclined or curved in a range between a portion just below the oil dipper
 38 and the outer edge area 26a, 26b or 26a', but are merely required to be
 inclined or curved at least in the outer edge areas 26a, 26b, 26a'.
 In the front portions 26a, 26a' of the oil reservoir 26 shown in FIGS. 2 to
 4 and of the oil reservoir 26' shown in FIGS. 6 and 7, the outer walls 32,
 32' are curved upwardly so as to surround the oil dipper 38. However, to
 keep the height H of the oil level OL approximately constant, the only
 requirement is that the outer edge areas 26a, 26b, 26a' of the oil
 reservoir 26, 26' are made narrower or flat. Therefore, instead of the
 curved outer walls, the outer edge area of the partition wall 30, for
 example, may be inclined downwardly in the direction to get near the outer
 wall 30.
 Further, though the partition wall 30 is formed into a semi-cylindrical
 shape in each embodiment, the partition wall 30 may be formed into other
 shapes if the outer walls 32, 32' are formed into a spherical, arc-shaped
 or inclined surface.
 Still further, though each of the oil dippers 38, 38' extends straight from
 the connecting rod 24 in the first, the second and the third embodiments,
 it may be formed into other shapes, for example, it may be curved toward
 oil recess area 36 or 36' in order to be certainly brought into contact
 with the oil.
 Furthermore, though two oil dippers 38, 38 are provided in the first and
 the second embodiments, three or more oil dippers 38 may be provided for
 the purpose of certainly producing the oil mist, if necessary.