U.S. patent application number 09/801924 was filed with the patent office on 2001-12-06 for handheld type four-cycle engine.
Invention is credited to Ito, Keita, Nishida, Takao, Watanabe, Sei.
Application Number | 20010047788 09/801924 |
Document ID | / |
Family ID | 26588016 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010047788 |
Kind Code |
A1 |
Ito, Keita ; et al. |
December 6, 2001 |
Handheld type four-cycle engine
Abstract
With regard to a handheld type four-cycle engine including, in a
side wall of a cylinder block, an intake valve, an exhaust valve
and a valve operation mechanism operable in association with a
crankshaft so as to open and close the above-mentioned valves and,
an oil tank is formed in one side wall of the cylinder block, and
the oil tank houses the valve operation mechanism and oil mist
generation means for generating an oil mist from stored oil. It is
thus possible to reliably lubricate the valve operation mechanism
regardless of the operational position of the engine while allowing
the size and weight of the engine main body to be reduced.
Inventors: |
Ito, Keita; (Wako-shi,
JP) ; Nishida, Takao; (Wako-shi, JP) ;
Watanabe, Sei; (Wako-shi, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Family ID: |
26588016 |
Appl. No.: |
09/801924 |
Filed: |
March 9, 2001 |
Current U.S.
Class: |
123/196R |
Current CPC
Class: |
F01M 9/06 20130101; F01M
11/065 20130101; F02B 63/02 20130101; F02F 1/002 20130101; F02B
2275/30 20130101; F02B 2275/34 20130101; F01M 1/04 20130101; F02F
1/305 20130101; F02B 2075/027 20130101; F02B 67/06 20130101; F02B
2275/22 20130101; F02F 1/06 20130101; F02B 75/16 20130101 |
Class at
Publication: |
123/196.00R |
International
Class: |
F01M 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 2000 |
JP |
2000-79307 |
Mar 16, 2000 |
JP |
2000-79308 |
Claims
1. A handheld type four-cycle engine including: a crankcase having
a crank chamber housing a crankshaft; a cylinder block having a
cylinder bore; and an intake valve, an exhaust valve and a valve
operation mechanism provided in a side wall of the cylinder block,
the valve operation mechanism being operable in association with
the crankshaft so as to open and close the intake valve and the
exhaust valve; wherein an oil tank for storing oil is provided so
as to be connected to one side wall running the length of the
crankcase and the cylinder block, and the oil tank houses the valve
operation mechanism and oil mist generation means for generating
oil mist from the stored oil.
2. A handheld type four-cycle engine according to claim 1 wherein
an outward route and a return route are provided for communication
between the oil tank and the crank chamber above the oil stored in
the oil tank, and valve means is provided in the return route, the
valve means introducing the positive pressure component of pressure
pulsations generated in the crank chamber into the side of the oil
tank.
3. A handheld type four-cycle engine according to claim 1 or 2
wherein a breather chamber is communicated with a return route
between valve means and the oil tank, and the breather chamber is
communicated with an air cleaner of an intake system on one side
and with a suction chamber communicated with the oil tank via an
orifice on the other side.
4. A handheld type four-cycle engine including: a crankcase having
a crank chamber housing a crankshaft; a cylinder block having a
cylinder bore; and an intake valve, an exhaust valve and a valve
operation mechanism provided in a side wall of the cylinder block,
the valve operation mechanism being operable in association with
the crankshaft so as to open and close the intake valve and the
exhaust valve; wherein an oil tank for storing oil that is separate
from the crank chamber is provided in one side wall running the
length of the crankcase and the cylinder block, the oil tank houses
the valve operation mechanism and oil mist generation means for
generating oil mist from the stored oil and is arranged so that the
oil mist generated inside the oil tank can be supplied to the crank
chamber, a cooling fan driven by the crankshaft is placed so as to
adjoin the oil tank, and a power output mechanism is provided on
the crankshaft so that the crankcase is interposed between the
power output mechanism and the oil tank.
5. A handheld type four-cycle engine according to claim 1 or 4
wherein a carburettor and an exhaust muffler are placed along a
direction perpendicular to the axes of both the cylinder bore and
the crankshaft on the two sides of the cylinder block.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to improvement of handheld
type four-cycle engines which can desirably be used mainly as a
power source for machines for portable operation such as
trimmers.
[0003] 2. Description of the Relevant Art
[0004] With regard to conventional handheld type four-cycle
engines, one in which an oil mist generated from the oil stored in
an oil reservoir is circulated inside the engine in order to
lubricate the inside of the engine regardless of the operational
position of the engine is already known in Japanese Patent
Application Laid-open No. 10-288019.
[0005] However, the lubrication system disclosed in the
above-mentioned patent publication has a comparatively long and
complicated circulation route in which the oil mist is circulated
from the oil tank to the crank chamber, then to the valve operation
chamber and is then returned to the oil tank. Such a long and
complicated circulation route for the oil mist causes an increase
in the size of the engine main body, which includes the circulation
route, and makes it difficult to reduce the weight thereof. Weight
reduction is one of the important issues when considering
improvements in the operability of, in particular, handheld type
four-cycle engines.
SUMMARY OF THE INVENTION
[0006] The present invention has been carried out in view of the
above-mentioned circumstances, and it is an object of the present
invention to provide a handheld type four-cycle engine which can
reliably lubricate the valve operation mechanism regardless of the
operational position of the engine while making the engine main
body more compact and lighter.
[0007] In accordance with a first characteristic of the present
invention in order to achieve the above-mentioned objective, there
is proposed a handheld type four-cycle engine including a crankcase
having a crank chamber housing a crankshaft; a cylinder block
having a cylinder bore; and an intake valve, an exhaust valve and a
valve operation mechanism provided in a side wall of the cylinder
block, the valve operation mechanism being operable in association
with the crankshaft so as to open and close the intake valve and
the exhaust valve, wherein an oil tank for storing oil is provided
in one side wall running the length of the crankcase and the
cylinder block, and the oil tank houses the valve operation
mechanism and oil mist generation means for generating oil mist
from the stored oil.
[0008] In accordance with the above-mentioned first characteristic,
since the oil tank can be filled with the oil mist generated by the
oil mist generation means, each part of the valve operation
mechanism disposed inside the oil tank can be lubricated reliably
by the above-mentioned oil mist regardless of the operational
position of the engine. It is therefore unnecessary to provide the
engine main body with a special oil supply passage for lubricating
the valve operation mechanism, and the size and weight of the
engine can thus be reduced. Moreover, since the oil tank is
provided in one side wall running the length of the crankcase and
the cylinder block, it is unnecessary to provide an oil reservoir
in the lower part of the crankcase, and the total height of the
engine can thus be reduced and the size of the engine can be
further reduced.
[0009] In accordance with a second characteristic of the present
invention, in addition to the above-mentioned first characteristic,
there is proposed a handheld type four-cycle engine wherein an
outward route and an return route are provided for communication
between the oil tank and the crank chamber above the oil stored in
the oil tank, and valve means is provided for introducing the
positive pressure component of pressure pulsations generated in the
crank chamber into the side of the oil tank.
[0010] The above-mentioned valve means corresponds to the one-way
valve 51 described in the embodiment of the present invention
below.
[0011] In accordance with the above-mentioned second
characteristic, since the oil mist generated in the oil tank is
circulated between the oil tank and the crank chamber by utilising
the pressure pulsations of the crank chamber, the crankshaft and
the piston can be lubricated regardless of the operational position
of the engine. Moreover, since it is sufficient for the oil mist to
merely circulate between the two chambers, that is to say, the oil
tank and the crank chamber, the outward and return routes for
providing communication between the two chambers can be made
comparatively short and simple, and the size and weight of the
engine can be further reduced.
[0012] In accordance with a third characteristic of the present
invention, in addition to the above-mentioned first or second
characteristics, there is proposed a handheld type four-cycle
engine wherein a breather chamber is communicated with the return
route between the valve means and the oil tank, and the breather
chamber is communicated with an air cleaner of an intake system on
one side and with a suction chamber communicated with the oil tank
via an orifice on the other side.
[0013] In accordance with the above-mentioned third characteristic,
the blowby gas generated in the crank chamber is transferred to the
breather chamber together with the oil mist, and separated into gas
and liquid. Then, the blowby gas is discharged to the air cleaner,
and the liquefied oil flows into the suction chamber via the
orifice and can be returned to the low pressure oil tank.
[0014] Furthermore, in accordance with a fourth characteristic of
the present invention, there is proposed a handheld type four-cycle
engine including a crankcase having a crank chamber housing a
crankshaft; a cylinder block having a cylinder bore; and an intake
valve, an exhaust valve and a valve operation mechanism provided in
a side wall of the cylinder block, the valve operation mechanism
being operable in association with the crankshaft so as to open and
close the intake valve and the exhaust valve, characterised in that
an oil tank for storing oil that is separate from the crank chamber
is provided in one side wall running the length of the crankcase
and the cylinder block, the oil tank houses the valve operation
mechanism and oil mist generation means for generating oil mist
from the stored oil and is arranged so that the oil mist generated
inside the oil tank can be supplied to the crank chamber, a cooling
fan driven by the crankshaft is placed so as to adjoin the oil
tank, and a power output mechanism is provided on the crankshaft so
that the crankcase is interposed between the power output mechanism
and the oil tank.
[0015] In accordance with the above-mentioned fourth
characteristic, since the oil tank is provided in one side wall
running the length of the crankcase and the cylinder block, it is
unnecessary to provide an oil reservoir in the lower part of the
crankcase, the total height of the engine can thus be reduced and
the engine can be made compact. Moreover, since the oil tank is
filled with the oil mist generated by the oil mist generation means
and the oil mist is supplied to the crank chamber, each component
inside the crank chamber as well as the valve operation mechanism
placed inside the oil tank can be lubricated reliably regardless of
the operational position of the engine.
[0016] Furthermore, since the cooling fan is placed so as to adjoin
the oil tank, the oil tank and, therefore, the stored oil and the
oil mist inside the oil tank can be cooled effectively by the
cooling air generated by the cooling fan, thus efficiently carrying
out lubrication and cooling of each component.
[0017] Moreover, since the valve operation mechanism and the power
output mechanism are linked to the two ends of the crankshaft, the
loads arising from the valve operation mechanism and the power
output mechanism, during operation of the engine, are applied to
the two separate ends of the crankshaft, it is thus possible to
prevent the load applied to the crankshaft, its bearings and the
side wall of the crankcase from being localised and the durability
thereof can be enhanced.
[0018] In accordance with a fifth characteristic of the present
invention, in addition to the above-mentioned first or fourth
characteristic, there is proposed a handheld type four-cycle engine
wherein a carburettor and an exhaust muffler are placed on the two
sides of the cylinder block along a direction perpendicular to the
axes of both the cylinder bore and the crankshaft.
[0019] In accordance with the above-mentioned fifth characteristic,
the carburettor and the exhaust muffler can be easily arranged
without any obstruction from the oil tank, the cooling fan or the
power output mechanism thus helping to make the engine more
compact.
[0020] The above-mentioned oil mist generation means corresponds to
the oil slingers 36 to 41 described in the embodiment of the
present invention below, and the above-mentioned power output
mechanism corresponds to the centrifugal clutch 31 described in the
embodiment below.
[0021] The above-mentioned objectives, other objectives,
characteristics and advantages of the present invention will become
apparent from an explanation of preferable embodiments which will
be described in detail below by reference to the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is an oblique view showing one embodiment of the
handheld type four-cycle engine of the present invention in
practical use.
[0023] FIG. 2 is a vertically sectioned view of the above-mentioned
four-cycle engine.
[0024] FIG. 3 is a cross-sectional view at line 3-3 in FIG. 2.
[0025] FIG. 4 is a cross-sectional view at line 4-4 in FIG. 3.
[0026] FIG. 5 is a cross-sectional view at line 5-5 in FIG. 4.
[0027] FIG. 6 is a cross-sectional view at line 6-6 in FIG. 5.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] As shown in FIG. 1, a handheld type four-cycle engine E to
which the present invention is applied is fitted as the source of
power to the driving section of, for example, a powered trimmer T.
Since the powered trimmer T is used in a manner in which a cutter C
is positioned in various directions according to the operational
conditions, the engine E is also tilted to a large extent or turned
upside-down as a result, and the operational position is
unstable.
[0029] As shown in FIGS. 2 and 3, the engine main body 1 of the
above-mentioned handheld type four-cycle engine E includes a
crankcase 6 having a crank chamber 6a, a cylinder block 7 having
one cylinder bore 7a, and a cylinder head 8 having a combustion
chamber 8a, and a large number of cooling fins 11 are formed on the
outer peripheries of the cylinder block 7 and the cylinder head
8.
[0030] The crankshaft 12 housed in the crank chamber 6a is
supported in right and left side walls of the crankcase 6 in a
rotatable manner via ball bearings 14 and 14' and is also connected
to a piston 15 fitted in the cylinder bore 7a via a connecting rod
16.
[0031] An oil tank 13 is provided as a continuation of the left
side wall running the length of the crankcase 6 and the cylinder
block 7. The oil tank 13 includes a tank inner half 13a and a tank
outer half 13b, the tank inner half 13a being linked integrally to
the crankcase 6 and the cylinder block 7, and the tank outer half
13b being boltjoined to the tank inner half 13a. The left-hand end
of the crankshaft 12 runs through and projects out of the oil tank
13, and an oil seal 17 in close contact with the outer
circumference of the crankshaft 12 is fitted in the tank outer half
13b.
[0032] A flywheel 24 having a large number of cooling vanes 25 is
fixed to the left-hand end of the crankshaft 12 projecting out of
the oil tank 13, the fly-wheel 24 functioning also as a cooling
fan, and a recoil type starter 26 is placed outside the flywheel
24.
[0033] An oil seal 17' is fitted in the right side wall of the
crankcase 6 so as to adjoin the outside of the bearing 14', the
right-hand end of the crankshaft 12 runs through and projects out
of the oil seal 17', a drive plate 27 is fixed to the right hand
end of the crankshaft 12, and a plurality of centrifugal shoes 28
(one thereof is shown in the figure) are pivotally supported on the
drive plate 27 in a rockable manner. These centrifugal shoes 28,
together with a clutch drum 30 connected to the drive shaft 29 for
driving the aforementioned cutter C, form a centrifugal clutch 31.
When the rotational rate of the crankshaft 12 exceeds a
predetermined value, the centrifugal shoes 28 are pressed onto the
inner periphery of the clutch drum 30 due to the centrifugal force
of the shoes, thereby transmitting the output torque of the
crankshaft 12 to the drive shaft 29.
[0034] The cylinder block 7 and the cylinder head 8 share an
overhang section 19 which overhangs on the side of the oil tank 13,
and the lower part of the overhang section 19 forms a part of the
upper wall of the tank inner half 13a. The combustion chamber 8a is
formed in the part of the cylinder head 8 corresponding to this
overhang section 19, and an intake port 9i and an exhaust port 9e
communicated with the combustion chamber 8a are formed in the
cylinder block 7. An ignition plug 10 is screwed into the cylinder
head 8, the electrodes of the ignition plug 10 projecting into the
combustion chamber 8a.
[0035] An intake valve 18i and an exhaust valve 18e are mounted in
the overhang 19, that is to say, the upper wall of the tank inner
half 13a, in a freely ascending and descending manner so that their
valve heads project into the oil tank 13, the intake valve 18i and
the exhaust valve 18e opening and closing the intake port 9i and
the exhaust port 9e respectively. A valve operation mechanism 23
for operating the intake valve 18i and the exhaust valve 18e so as
to open and close them is disposed inside the oil tank 13.
[0036] The valve operation mechanism 23 includes a drive gear 32
fixed to the crankshaft 12, a pair of driven gears 36 and 37
supported in a rotatable manner on a pair of support shafts 33 and
34 provided in the oil tank 13 above the crankshaft 12 and driven
by the drive gear 32 with a reduction ratio of 1/2, an intake cam
21i and an exhaust cam 21e formed integrally with the driven gears
36 and 37 respectively, an intake cam follower 22i pivotally
supported in the oil tank 13 in a rockable manner and provided
between the intake cam 21i and the intake valve 18i, an exhaust cam
follower 22e pivotally supported in the oil tank 13 in a rockable
manner and provided between the exhaust cam 21e and the exhaust
valve 18e, and valve springs 20i and 20e forcing the intake valve
18i and the exhaust valve 18e towards the closing direction. The
engine E is thus arranged to be of the side valve type.
[0037] The driven gear 36 and the intake cam 21i, and also the
driven gear 37 and the exhaust cam 21e are placed distant from each
other in the axial direction so that they closely adjoin the left
and right side walls respectively of the oil tank 13. Gear type oil
slingers 39 and 40 are formed integrally with the intake cam 21i
and the exhaust cam 21e respectively so as to adjoin them.
[0038] Another support shaft 35 is provided in the oil tank 13 at a
position beneath the crankshaft 12, and this support shaft 35
supports a driven gear 38 and a vane type oil slinger 41 in a
rotatable manner, the driven gear 38 being driven by the drive gear
32 and the oil slinger 41 being integral with the driven gear 38.
The driven gear 38 and the oil slinger 41 are positioned distant
from each other in the axial direction so that they closely adjoin
the left and right inner walls respectively of the oil tank 13.
[0039] A predetermined amount of lubricating oil O is stored in the
oil tank 13, at least one of the three driven gears 36 to 38 or the
three oil slingers 39 to 41 is partially immersed in the stored oil
O regardless of the operational position of the engine E, and the
stored oil O is scattered by the rotation thereof so generating an
oil mist. The three driven gears 36 to 38 therefore also function
as oil slingers, and in fact, the three support shafts 33, 34 and
35 support a pair of oil slingers 36 and 39; 37 and 40; and 38 and
41 respectively, the oil slingers of each pair being distant from
each other in the axial direction.
[0040] As shown in FIG. 3, the intake port 9i is connected to a
carburettor 2 and an air cleaner 4 in that order, and the exhaust
port 9e is connected to an exhaust muffler 3. The carburettor 2 and
the exhaust muffler 3 are placed along a direction perpendicular to
the axes of both the crankshaft 12 and the cylinder bore 7a.
[0041] The route for the circulation of oil mist between the oil
tank 13 and the crank chamber 6a is explained below by reference to
FIGS. 2 and 4 to 6.
[0042] An outward route 45 and an return route 46 for circulating
the oil mist generated inside the oil tank 13 between the oil tank
13 and the crank chamber 6a are provided between the
above-mentioned two chambers 13 and 6a. The outward route 45
includes a through hole formed in the crankshaft 12 and providing
communication between the oil tank 13 and the crank chamber 6a. The
return route 46 includes a valve hole 47 in the bottom wall of the
crankcase 6, a valve chamber 48 communicated with the crank camber
6a via the valve hole 47, and a first return pipe 49 providing
communication between the valve chamber 48 and the oil tank 13. A
one-way valve 51 is provided in the valve chamber 48, the one-way
valve 51 including a reed valve for blocking and unblocking the
valve hole 47. The one-way valve 51 opens so as to unblock the
valve hole 47 when a positive pressure is generated in the crank
chamber 6a accompanying the ascending and descending movement of
the piston 15, and the one-way valve 51 closes so as to close the
valve hole 47 when a negative pressure is generated.
[0043] The open ends of the above-mentioned outward route 45 and
the first return pipe 49 inside the oil tank 13 are arranged so
that they are always above the liquid level of the stored oil 0
regardless of the operational position of the engine E.
[0044] A breather chamber 53 is provided on one side of the valve
chamber 48, the breather chamber 53 being communicated with the
valve chamber 48 via a link hole 52. The breather chamber 53 is
partitioned into a plurality of expansion chambers 53a, 53b (two
chambers in the figure) by means of a wall 54. These expansion
chambers 53a and 53b are communicated with each other via throttle
passages 55 and 55 on both sides of the wall 54, and the expansion
chamber 53b furthest from the link hole 52 is communicated with the
above-mentioned air cleaner 4 via a breather pipe 56.
[0045] A horizontally flattened chamber 57a is provided immediately
below the breather chamber 53, a vertically flattened chamber 57b
is provided in the side wall of the breather chamber 53 on the side
of the oil tank 13, the vertically flattened chamber 57b rising at
one end of the horizontally flattened chamber 57a, and a suction
chamber 57 is formed by the two flattened chambers 57a and 57b. The
horizontally flattened chamber 57a is communicated with the
breather chamber 53 via orifices 58, 58 . . . separately placed at
several positions, and the vertically flattened chamber 57b is also
communicated with the breather chamber 53 via orifices 58, 58 . . .
provided close to the ceiling of the breather chamber 53.
[0046] The suction chamber 57 is communicated with the inside of
the oil tank 13 via a second return pipe 59. In the same way as for
the first return pipe 49, in this case the second return pipe 59 is
arranged so that the open end thereof inside the oil tank 13 is
always above the liquid level of the stored oil O regardless of the
operational position of the engine E.
[0047] The above-mentioned valve chamber 48 and the breather
chamber 53 are formed between a surrounding wall 60 and a gasket
61, the surrounding wall 60 being formed integrally with the lower
part of the crankcase 6 and the gasket 61 being joined to the lower
end of the surrounding wall 60. The horizontally flattened chamber
57a is formed between the gasket 61 and a base plate 62 joined to
the surrounding wall 60 in such a way that the periphery of the
gasket 61 is interposed between the base plate 62 and the
surrounding wall 60. The vertically flattened chamber 57b is formed
in the dividing wall present between the breather chamber 53 and
the oil tank 13.
[0048] Referring again to FIG. 2, an engine cover 65 is fixed to
the engine main body 1 so as to cover it, a recoil type starter 26
is supported in the cover 65, and an air inlet 66 is provided in
the engine cover 65 around the recoil type starter 26 so as to face
cooling vanes 25 of the flywheel 24.
[0049] The action of the embodiment is explained below.
[0050] When the drive gear 32 rotates together with the crankshaft
12 during operation of the engine E, this rotation makes the three
sets of oil slingers 36 and 39; 37 and 40; 38 and 41 supported in
the three support shafts 33, 34, 35 rotate together. Since at least
one of the oil slingers reliably scatters the stored oil O so as to
generate an oil mist regardless of the operational position of the
engine E, the inside of the oil tank 13 can always be filled with
oil mist. The valve operation mechanism 23 is provided inside such
an oil tank 13, the intake cam 21i and the exhaust cam 21e rotating
together with the upper driven gears 36 and 37 operate the intake
valve 18i and the exhaust valve 18e so as to open and close them
with appropriate timing via the cam followers 22i and 22e while
working co-operatively with the valve springs 20i and 20e, and each
component of the valve operation mechanism 23 can therefore be
lubricated effectively by the above-mentioned oil mist. It is
therefore unnecessary to provide the engine main body 1 with a
special oil supply passage for lubricating the valve operation
mechanism 23, and the size and weight of the engine E can thus be
reduced. Moreover, since the oil tank 13 is provided in one side
wall running the length of the crankcase 6 and the cylinder block
7, it is unnecessary to provide an oil reservoir in the lower part
of the crankcase 6, the total height of the engine E can be reduced
and the size of the engine can further be reduced.
[0051] A negative pressure and a positive pressure are generated
alternately in the crank chamber 6a accompanying the ascending and
descending movement of the piston 15 so causing pressure
pulsations; when a negative pressure is generated, the one-way
valve 51 closes so as to block the valve hole 47 and at the same
time the oil mist inside the oil tank 13 is drawn up into the crank
chamber 6a through the through hole of the crankshaft 12, that is
to say, the outward route 45 thus lubricating the crankshaft 12 and
the piston 15. At this stage, the pressure of the oil tank 13
becomes negative due to the oil mist drawn up into the crank
chamber 6a.
[0052] When a positive pressure is generated, the one-way valve 51
opens so as to unblock the valve hole 47, and the oil mist inside
the crank chamber 6a is returned to the oil tank 13 having a low
pressure through the return route 46, that is to say, the valve
hole 47, the valve chamber 48 and the first return pipe 49. It is
thus sufficient for the oil mist to circulate merely between the
two chambers, namely the oil tank 13 and the crank chamber 6a, the
outward route 45 and the return route 46 providing communication
between the above-mentioned two chambers 13 and 6a can be made
comparatively short and simple, and the size and weight of the
engine main body 1 can be reduced.
[0053] Meanwhile, the blowby gas generated in the crank chamber 6a
is transferred to the valve chamber 48 together with the oil mist
and further to the breather chamber 53 through the link hole 52,
and separated into gas and liquid while passing through the first
and second expansion chambers 53a and 53b, and the blowby gas alone
is discharged to the air cleaner 4 via the breather pipe 56.
[0054] The oil separated and liquefied in the breather chamber 53
flows into the suction chamber 57 via the orifice 58, 58 . . .
Since the first suction chamber 57a and the second suction chamber
57b are communicated with the inside of the oil tank 13 having a
low pressure via the second return pipe 59, the oil transferred to
the suction chamber 57 is drawn up into the oil tank 13 through the
second return pipe 59. Since the suction chamber 57 includes, in
particular, the horizontally flattened chamber 57a and the
vertically flattened chamber 57b rising at one end of the
horizontally flattened chamber 57a as mentioned above, and each of
the two chambers 57a and 57b is communicated with the breather
chamber 53 via the orifices 58, 58 . . . , the oil stored in the
breather chamber 53 can flow into the suction chamber 57 and return
to the oil tank 13 even in the case where the engine E is tilted
horizontally so positioning the vertically flattened chamber 57b
downwards, as well of course in the case where the engine E is
upright so positioning the horizontally flattened chamber 57a
downwards.
[0055] Furthermore, since the flywheel 24 having the cooling vanes
25 driven by the crankshaft 12 is placed outside the oil tank 13 so
as to adjoin it, cooling air drawn in through the air inlet 66 of
the engine cover 65 by rotation of the cooling vanes 25 is directly
applied to the oil tank 13, the oil mist and the oil O stored
inside the oil tank can be cooled effectively and the lubrication
and cooling of each part can be carried out efficiently.
[0056] Moreover, since the valve operation mechanism 23 and the
centrifugal clutch 31 are linked to the two ends of the crankshaft
12 with the engine main body 1 disposed therebetween, the loads
arising from the valve operation mechanism 23 and the drive shaft
29 during operation of the engine E separately work on the two ends
of the crankshaft 12. It is therefore possible to prevent the load
from being localised on the crankshaft 12, its bearings 14 and 14'
and the side wall of the crankcase 6, and the durability thereof
can thus be enhanced. Furthermore, since it is unnecessary to
specially increase the load capacity of these parts, the engine E
can be made more compact.
[0057] Moreover, since the carburettor 2 and the exhaust muffler 3
are placed along a direction perpendicular to the axes of both the
crankshaft 12 and the cylinder bore 7a, the carburettor 2 and the
exhaust muffler 3 can easily be arranged without any obstruction
from the oil tank 13, the flywheel 24 or the centrifugal clutch 31
which are arranged along the axis of the crankshaft 12 thus helping
to make the engine E more compact.
[0058] The present invention is not limited to the above-mentioned
embodiment and can be modified in a variety of ways without
departing from the spirit and scope of the invention. For example,
a rotary valve operable in association with the crankshaft 12 and
operating so as to unblock the outward pipe 45 when the piston 15
descends, and to block the outward pipe 45 when the piston 15
ascends can be provided instead of the one-way valve 51.
* * * * *